Structural Polymer Composites for Energy Storage Devices PhD Thesis · PDF fileStructural Polymer Composites for Energy Storage Devices PhD Thesis Atif Javaid December 2011 Department

StructuralPolymerCompositesforEnergyStorageDevices

PhDThesis

AtifJavaid

December2011

DepartmentofChemicalEngineering&ChemicalTechnologyImperialCollegeLondon,SouthKensingtonCampus,London,SW72AZ,UnitedKingdom

StructuralPolymerCompositesforEnergyStorageDevices

Adissertationby

ATIFJAVAID

SubmittedtotheImperialCollegeLondoninpartialfulfilmentoftherequirementsforthe

degreeof

DOCTOROFPHILOSOPHY

Andthe

DIPLOMAOFIMPERIALCOLLEGELONDON

DepartmentofChemicalEngineering&ChemicalTechnologyImperialCollegeLondon,SouthKensingtonCampus,London,SW72AZ,UnitedKingdom

Supervisors

ProfAlexanderBismarckDrEmileSGreenhalghProfMiloSPShafferDrJoachimHGSteinke

3

“In the name of Allah, Most Gracious, Most Merciful”

Dedication

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Dedicated to my supervisor Professor Alexander Bismarck.

Thank you for all your help during my PhD.

I could not have done it without your help.

Declaration

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Declaration

This dissertation is a description of the work carried out by the author in the Department of Chemical Engineering and Chemical Technology, Imperial College London between October 2007 and September 2010 under the supervision of Prof Alexander Bismarck, Dr Joachim Steinke, Dr Emile Greenhalgh and Prof Milo Shaffer. Except where acknowledged, the material is the original work of the author and includes nothing, which is the outcome of work in collaboration, and no part of it, has been submitted for a degree at this or any other university. Keywords: Multifunctional, Composites, Polymer electrolytes, Structural, Supercapacitors

Abstract

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Abstract Multifunctional composites have attracted a great deal of attention as they offer a way to cut

down the parasitic weight in vehicles which not only reduces the operational costs but also

reduces the fuel consumption in vehicles. Current engineering design is increasingly

sophisticated, requiring more efficient material utilisation; sub-system mass and volume are

crucial application determinants. This dissertation contributes to the fabrication of composites

that can store electrical energy and are known as structural supercapacitors. The key in the

fabrication of structural supercapacitors was not simply to bind two disparate components

together, but to produce a single coherent material that inherently performed both roles of a

structural composite and a supercapacitor. This design approach is at a relatively early stage,

and faces significant design and material synthesis challenges. Disparate material

requirements, such as structural and electrochemical properties, have to be engineered and

optimised simultaneously.

This study investigates on structural supercapacitors fabricated by using as-received as well

as activated carbon fibre cloths as reinforcement and electrodes; multifunctional resin as

electrolyte and matrix; and glass fibre cloths, filter papers or polymer membranes as

insulators. Such a system should deliver electrical energy storage capacity as well as bear

mechanical loads. Different liquid electrolytes, such as ionic liquids and salts based on Li+

and NH4+, were studied in order to optimise the multifunctionality of polymer electrolyte.

Mesoporous silica particles were also introduced into polymer electrolytes in order to

enhance the mechanical and electrochemical performance of polymer electrolytes. Nano-

structured/multifunctional resin blends were cured in cylindrical form and were examined by

compression testing as well as impedance spectroscopy. An ionic conductivity of 0.8 mS/cm

and a compression modulus of 62 MPa have been synthesised for the polymer electrolyte in

the current study. By varying the separators, multifunctional resins and the electrodes,

different structural supercapacitor configurations were manufactured using a resin infusion

under flexible tooling (RIFT) method and were characterised to study the electrochemical

performance by using charge/discharge method and mechanical performance by using ±45°

laminate shear testing. The improved structural supercapacitors showed an energy density of

0.1 Wh/kg, a power density of 36 W/kg and a shear modulus of 1.7 GPa.

Acknowledgements

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Acknowledgements This expedition would not have been as fascinating without the interesting people I met along

the way. I would first like to thank my supervisors, Prof Alexander Bismarck, Dr Emile

Greenhalgh, Dr Joachim Steinke and Prof Milo Shaffer for giving me the opportunity to work

on this extraordinary project. I am very thankful for the time, support and the liberty that my

supervisors provided during this course of research work. They also helped me to improve

my working skills and clarify my fundamentals. I am also grateful for the many

brainstorming sessions as well as the leisurely chats we had. I would especially like to thank

Alexander for his kind support and encouragement throughout my PhD work. He always had

a time to listen to my problems with patience and is always a source of inspiration for me.

He always made me smile no matter how tense the situation was. I really enjoyed the

conversations that we had either on the project or on the social matters including the political

system in Pakistan. His jokes on Taliban always brought a smile on my face. I am indeed

fortunate to have him as one of my supervisors.

My sincere appreciation also goes to the University of Engineering and Technology (UET),

Lahore, Pakistan for providing me the financial support during my PhD. It was not possible

for me to carry out my research work in a world renowned institution without the grant that

UET had provided me. I would also like to thank Prof Ghulam Mustafa Mamoor and Prof

Mehmood Ahmad for all their support and help during my undergraduate studies as well

during my lectureship in Polymer Engineering department of UET, Lahore. Special thanks to

Prof Bismarck for supporting me for my 3rd year tuition fees. I would also like to thank Dr

Greenhalgh (MAST Project Coordinator), Ministry of Defence, UK and BAE Systems for

their financial support during the last year of my research work. Thank you for becoming

wind under my wings.

My heartfelt gratitude goes out to my parents, Mr and Mrs Javaid Iqbal, who provided love,

affection and encouragement throughout my PhD. This dissertation would not have been

possible without the support, guidance and sacrifices that they made from all the hard times

growing up to this very day. They were always there when I needed them. Thank you very

much for everything. I would also like to thank my sisters, uncles, aunties and cousins for all

their support and encouragement. Thank you Annie, Shanza, Mishal, Aimen, Kashan, Umair,

Zaid, Ahmar, Farzeen, Shaza, Zimal, Kinza, Minahil, Ryan, Linta, Meerab, Mahad, Maham,

Acknowledgements

8

Laiba, Anosh, Wamiq, Haider, Zaini. Thanks Asifa Khala for your support and

encouragement. Thanks Ammi for all your wishes and support. Unfortunately, you did not

have a chance to see the completion of my PhD today but I am sure, wherever you are, you

must be very proud of me.

I am also grateful to Dr Kingsley Ho for always being there when I needed him. He always

helped me whenever I had problems. He taught me a lot during the past four years and for

which I am sincerely thankful for him. I am most thankful for his support in preparing my

first conference presentation. The succeeding nerve-wracking experience of presenting in

front of a large audience at CSCST Conference, Oxford was somewhat decreased by knowing

that Dr Ho was also there to back me up. A special thank you goes to Dr Hui Qian (Sherry)

for providing the activated carbon fibres. I really enjoyed the fruitful discussions about the

project that we had after she joined the project during my 3rd year. I wish to also thank Prof

Anthony Kucernak and Dr John Hodgkinson for their many insightful recommendations and

their continuous support throughout my research.

At the end, I would like to continue by expressing my sincere obligations to the many friends

and colleagues who made my stay at Imperial unforgettable. I would like to thank Sheema,

Humera, Rose, Ali, Muddassir, Faisal, Ghiyas, Ammar, Ilyas and many more. I would also

like to thank other members of PaCE, Steinke and Nano groups notably Dr Charnwit (Jo)

Tridech, Dr Steven Lamoniere, Dr Johny Blaker, Dr Angelika Menner, Dr Natasha

Shirshova, Dr Emilia Kot, Nadine Graeber, KoonYang Lee, Dr Ivan Zadrazil, Dr Anthony

Abbott, Henry Maples, Su Bai, Jing Li, Hele Diao, Edyta Lam, Dan Cegla, Sally Ewen, Bryn

Monnery, Wei Yuan, Stephen Hodge and many more. I am sure that I have forgotten many

others and for that I apologise. You all have made my research work exciting and fun. My

sincere appreciation is also for the tremendous support provided by the technical staff of

Chemical and Aeronautical engineering departments. A special thanks to Sarah Payne,

Patricia Carry, Richard Wallington, Gary Senior, Joseph Meggyesi, Keith Walker, Susi

Underwood, Rayner Simpson, Jon Cole and other technical staff.

List of Publications and Presentations

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List of Publications and Presentations Current and Future Journal Publications

[1] K. K. C. Ho, S. Shamsuddin, S. Riaz, S. Lamorinere, M. Q. Tran, A. Javaid, A.

Bismarck, Wet impregnation as route to unidirectional carbon fibre reinforced

thermoplastic composites manufacturing, in 2011, 100.

[2] A. Javaid, E. S. Greenhalgh, M. S. P. Shaffer, J. H. G. Steinke, A. Bismarck, Ionically

conductive and mechanically robust crosslinked polymer electrolytes for

multifunctional structural supercapacitor applications, (in preparation).

[3] A. Javaid, A. Bismarck, E. S. Greenhalgh, M. S. P. Shaffer, J. H. G. Steinke,

Improving the ionic conductivity and compression properties of crosslinked polymer

electrolytes through mesoporous silica particle reinforcements for use in structural

supercapacitors, (in preparation).

[4] A. Javaid, K. K. C. Ho, A. Bismarck, J. H. G. Steinke, M. S. P. Shaffer, E. S.

Greenhalgh, Exploring the design parameters of multifunctional structural

supercapacitors with improved mechanical and electrochemical performance for

energy storage applications, (in preparation).

[5] A. Javaid, K. K. C. Ho, H. Qian, E. S. Greenhalgh, J. H. G. Steinke, M. S. P. Shaffer,

A. Bismarck, Multifunctional Structural supercapacitors for energy storage devices,

(in preparation).

[6] A. Javaid, K. K. C. Ho, A. Bayley, E. S. Greenhalgh, J. H. G. Steinke, M. S. P. Shaffer,

A. Bismarck, Improving the multifunctionality of structural supercapacitor through

addition of high surface area carbon black, (in preparation).

Conference Proceedings

[1] A. Bismarck, E. S. Greenhalgh, K. K. C. Ho, A. Javaid, A. Kucernak, M. S. P. Shaffer,

N. Shirshova, J. H. G. Steinke, "Structural power composites as energy storage devices",

List of Publications and Presentations

10

presented at 18th International Conference on Composite Materials, Jeju Island, Korea,

2011.

[2] A. Bismarck, P. T. Curtis, E. S. Greenhalgh, K. K. C. Ho, A. Javaid, A. Kucernak, H.

Qian, M. S. P. Shaffer, N. Shirshova, J. H. G. Steinke, "Structural power composites for

energy storage devices", presented at 14th European Conference on Composite materials,

Budapest, Hungary, 2010.

Poster Presentation

[1] A. Bismarck, E.S. Greenhalgh, K.K.C. Ho, A. Javaid, M.S.P. Shaffer, J.H.G. Steinke,

"Structural polymer composite for power storage", presented at Macro Group Young

Researchers Meeting, University of Nottingham, 2010.

1st prize won in poster presentation.

Conference Presentations



presented at The 18th International Conference on Composite Materials, Jeju Island,

Korea, 2011.



presented at The 17th Joint Annual Conference of CSCST and SCI, Oxford, United

Kingdom, 2010.

Table of Contents

11

Table of Contents

Abstract ..................................................................................................................................... 6

Acknowledgements .................................................................................................................. 7

List of Publications and Presentations ................................................................................... 9

List of Figures ......................................................................................................................... 18

List of Tables .......................................................................................................................... 26

List of Abbreviations ............................................................................................................. 31

List of Notations ..................................................................................................................... 34

Chapter 1 Introduction.......................................................................................................... 38

1.1 Motivation ............................................................................................................. 39

1.2 Methodology ......................................................................................................... 41

1.3 Aims and objectives .............................................................................................. 43

1.4 Thesis Outline ....................................................................................................... 44

Chapter 2 Literature Review ...................................................................................... 46

2.1 Traditional carbon fibre reinforced thermoset composites ................................... 47

2.2 Energy storage devices .......................................................................................... 48

2.3 Multifunctional composites................................................................................... 51

2.3.1 Structural batteries .................................................................................. 52

2.3.1.1 What are batteries? ........................................................................ 52

2.3.1.2 Research trends in structural batteries .......................................... 54

2.3.1.3 Other multifunctional energy storage materials ............................ 59

2.3.1.4 Challenges in structural batteries .................................................. 60

2.3.2 Structural fuel cells .................................................................................. 60

2.3.2.1 What are fuel cells? ....................................................................... 60

2.3.2.2 Research trends in structural fuel cells ......................................... 61

2.3.2.3 Challenges in structural fuel cells ................................................. 63

2.3.3 Structural capacitors ............................................................................... 64

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2.3.3.1 What are capacitors? ..................................................................... 64

2.3.3.2 Research trends in structural capacitors ........................................ 65

2.3.3.3 Challenges in structural capacitors ............................................... 67

2.4 Structural Supercapacitors .................................................................................... 67

2.4.1 Historical background of supercapacitors .............................................. 68

2.4.2 Working principle of supercapacitor ....................................................... 70

2.4.3 Types of supercapacitors ......................................................................... 72

2.4.4 Research trends in supercapacitors ......................................................... 73

2.4.5 Structural polymer electrolytes ................................................................ 75

2.4.6 Activated carbon fibre electrodes ............................................................ 79

2.4.7 Challenges in structural supercapacitors ................................................ 81

Chapter 3 Experimental Section ................................................................................ 83

3.1 Materials................................................................................................................ 84

3.1.1 Uncured epoxy materials ......................................................................... 84

3.1.2 Crosslinker ............................................................................................... 84

3.1.3 Electrolyte salt ......................................................................................... 85

3.1.4 Solvents .................................................................................................... 85

3.1.5 Silica precursor ........................................................................................ 86

3.1.6 Block copolymer surfactant ..................................................................... 86

3.1.7 Woven fibre mats...................................................................................... 86

3.1.8 Paraffin oil ............................................................................................... 87

3.1.9 Separators ................................................................................................ 87

3.2 Mesoporous silica ................................................................................................. 87

3.2.1 Preparation of mesoporous silica monoliths [161] ................................. 87

3.2.2 Preparation of mesoporous silica particles [15] ..................................... 88

3.2.3 Surface area analysis-BET method .......................................................... 88

3.2.4 Particle size analyses- Light scattering method ...................................... 90

3.2.5 Scanning electron microscopy (SEM) ...................................................... 91

3.3 Polymer electrolytes .............................................................................................. 91

3.3.1 Preparation of crosslinked PEGDGE polymer electrolytes .................... 91

3.3.1.1 Preparation of crosslinked PEGDGE electrolytes using TBAPF6

salt…….. ...................................................................................................... 91

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3.3.1.2 Preparation of crosslinked PEGDGE electrolytes using LiTFSI

salt…….. ...................................................................................................... 92

3.3.1.3 Preparation of crosslinked PEGDGE electrolytes using EMITFSI

ionic liquid ................................................................................................... 92

3.3.2 Preparation of crosslinked DGEBA electrolytes ..................................... 93

3.3.2.1 Preparation of crosslinked DGEBA electrolytes using LiTFSI

salt…….. ...................................................................................................... 93

3.3.2.2 Preparation of crosslinked DGEBA electrolytes using EMITFSI

ionic liquid ................................................................................................... 94

3.3.3 Preparation of PAN gel based polymer electrolytes................................ 94

3.3.4 Preparation of crosslinked PEGDGE/DGEBA electrolytes .................... 95

3.3.4.1 Preparation of crosslinked PEGDGE/DGEBA electrolytes using

10 wt% LiTFSI salt ..................................................................................... 95


10 wt% EMITFSI ionic liquid ..................................................................... 95


50 wt% EMITFSI ionic liquid ..................................................................... 96

3.4 Composite polymer electrolytes............................................................................ 97

3.4.1 Preparation of MSP/PEGDGE composite polymer electrolytes ............. 97

3.4.1.1 Preparation of crosslinked MSP/PEGDGE composite polymer

electrolytes using LiTFSI salt ...................................................................... 97


electrolytes using EMITFSI ionic liquid ..................................................... 98

3.4.2 Preparation of crosslinked MSP/DGEBA composite polymer

electrolytes.. .......................................................................................................... 99

3.4.3 Preparation of crosslinked MSP/PEGDGE/DGEBA composite polymer

electrolytes .......................................................................................................... 100

3.4.3.1 Preparation of crosslinked MSP/PEGDGE/DGEBA composite

polymer electrolytes using LiTFSI salt ..................................................... 100


electrolytes using EMITFSI ionic liquid ................................................... 100

3.5 Chemical Activation of carbon fibre mats .......................................................... 101

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3.6 Electrochemical impedance spectroscopy of polymer electrolytes .................... 102

3.7 Mechanical characterisation of polymer electrolytes .......................................... 103

3.7.1 Rheological characterisation of PAN gel polymer electrolytes ............. 103

3.7.2 Mechanical characterisation of solid polymer electrolytes (Compression

testing).. ............................................................................................................... 104

3.8 Composite fabrication using Resin Infusion under Flexible Tooling (RIFT) ..... 105

3.9 Electrochemical characterisation of structural supercapacitors .......................... 108

3.9.1 Cyclic voltammetry ................................................................................ 108

3.9.2 Potential square-wave voltammetry (Charge/discharge) ...................... 109

3.9.3 Electrochemical impedance spectroscopy ............................................. 109

3.10 Mechanical characterisation of composites (±45° laminate tensile test) ... 110

3.11 Fibre volume fraction of structural supercapacitors by acid digestion ...... 112

Chapter 4 Polymer Electrolytes ............................................................................... 114

4.1 Selection of salts for inclusion into polymers ..................................................... 115

4.2 Polyacrylonitrile gel polymer electrolytes .......................................................... 116

4.3 Crosslinked PEGDGE polymer electrolytes ....................................................... 121

4.3.1 Effect of different ionic salts on ionic conductivity and compression

properties of crosslinked PEGDGE electrolytes ................................................ 121

4.3.2 Effect of increasing EMITFSI concentration on ionic conductivity and

compression properties of crosslinked PEGDGE electrolytes ........................... 123

4.4 Crosslinked DGEBA polymer electrolytes ......................................................... 125

4.5 Crosslinked PEGDGE/DGEBA polymer electrolytes ........................................ 126

4.5.1 Crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% [LiTFSI]

in PC…. ............................................................................................................... 126

4.5.2 Crosslinked PEGDGE/DGEBA electrolytes containing 10wt%

EMITFSI… .......................................................................................................... 127

4.5.3 Crosslinked PEGDGE/DGEBA electrolytes containing 50wt%

EMITFSI… .......................................................................................................... 129

4.6 Multifunctionality of polymer electrolytes ......................................................... 131

Table of Contents

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Chapter 5 Polymer Composite Electrolytes ............................................................ 135

5.1 Mesoporous silica ............................................................................................... 136

5.1.1 Surface characterisation of mesoporous silica monoliths (MSMs) and

mesoporous silica particles (MSP) ..................................................................... 137

5.2 Effect of mesoporous silica on the mechanical and electrochemical properties of

polymer composite electrolytes .................................................................................... 145

5.2.1 Ionic conductivity and compression properties of crosslinked

PEGDGE/MSM composite electrolytes containing TBAPF6/PC ........................ 146


MSP/PEGDGE composite electrolytes containing TBAPF6/PC ........................ 147


MSP/PEGDGE composite electrolytes containing LiTFSI/PC .......................... 148


MSP/PEGDGE composite electrolytes containing EMITFSI ............................. 151


DGEBA/MSP composite electrolytes containing LiTFSI/PC ............................. 153

5.2.6 Ionic conductivity and compression properties of crosslinked PEGDGE/

DGEBA/MSP composite electrolytes containing LiTFSI/PC ............................. 153


DGEBA/MSP composite electrolytes containing 50 wt% EMITFSI ................... 155

5.3 Multifunctionality of polymer composite electrolytes ........................................ 156

Chapter 6 Structural Supercapacitors ..................................................................... 160

6.1 Influence of glass fibre separators on the specific capacitance of structural

supercapacitors ............................................................................................................. 161

6.2 Influence of varying charging time on the specific capacitance of structural


6.3 Influence of different types of electrolyte salts on the electrochemical and

mechanical performance of structural supercapacitors ................................................ 166

6.4 Influence of separator type on the specific capacitance and shear properties of

structural supercapacitors ............................................................................................. 169

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6.5 Influence of the polymer electrolyte composition on the electrochemical and

mechanical performance of structural supercapacitors ................................................ 172

6.6 Influence of EMITFSI concentration on electrochemical and mechanical

performance of structural supercapacitors .................................................................... 175

6.7 Influence of the connectivity of copper tape and copper wire on the

electrochemical performance of structural supercapacitors ......................................... 179

6.8 Influence of charge-discharge cycles on the specific capacitance of structural


6.9 Influence of applied potential difference on the energy density of structural

supercapacitor ............................................................................................................... 183

6.10 Influence of addition of MSP on the electrochemical and mechanical


6.11 Configuration of structural supercapacitors .............................................. 187

6.12 Influence of CF activation on the electrochemical and mechanical


6.12.1 Structural supercapacitors with a crosslinked PEGDGE matrix

containing 10 wt% EMITFSI .............................................................................. 190

6.12.2 Structural supercapacitors with a crosslinked 40:60 PEGDGE/DGEBA

blend matrix containing different EMITFSI concentrations ............................... 195

6.12.3 Structural supercapacitors with a crosslinked MSP/PEGDGE matrix

containing 10 wt% EMITFSI .............................................................................. 198

6.13 Multifunctionality of structural supercapacitors........................................ 202

Chapter 7 Conclusions and Suggestions for Future Work .................................... 205

7.1 Conclusions ......................................................................................................... 206

7.1.1 Developments of the polymer electrolytes ............................................. 206

7.1.2 Developments of the polymer composite electrolytes ............................ 208

7.1.3 Developments of the structural supercapacitors ................................... 210

7.2 Suggestion for future work ................................................................................. 213

7.2.1 Improvements in the multifunctionality of polymer electrolytes ............ 213

7.2.2 Improvements in the energy density of structural supercapacitors ....... 214

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7.2.3 Improvements in the power density of structural supercapacitors ........ 215

7.2.4 Improvements in the mechanical performance of structural

supercapacitors ................................................................................................... 215

Appendix A Result tables of polymer electrolytes and polymer composite electrolytes217

Appendix B Instructions of measuring the ionic conductivity of polymer electrolytes and

composite polymer electrolytes ........................................................................................... 225

Appendix C Instructions of measuring the machine compliance for determining the

compression modulus of polymer electrolytes ................................................................... 227

Appendix D Microscopic Evaluation on the MSP reinforced polymer electrolytes ...... 228

Appendix E Shear stress and straincurves for the ±45º laminated structural

supercapacitor specimens .................................................................................................... 233

Appendix F Nuclear magnetic resonance spectroscopy (NMR) of diglycidylether of

bisphenol-A epoxy and 4,4’ methylene bis(cyclo hexyl amine) crosslinker .................... 234

Reference List ....................................................................................................................... 236

List of Figures

18

List of Figures Chapter 1

Figure 1.1 BAE systems Mantis UAV that will employ structural energy composites

(Courtesy of BAE systems). .................................................................................................... 40

Figure 1.2 Spare-wheel floor [11] of a Volvo car replaced with a multifunctional composite

to be developed in the STORAGE project. .............................................................................. 41

Figure 1.3 Cross sectional view of proposed multifunctional structural supercapacitors. ...... 42

Chapter 2

Figure 2.1 Schematic of different electrical energy storage devices by Sels et al. [28]. ......... 49

Figure 2.2 Ragone plot showing energy storage delivery performance for different storage

devices by Kotz et al. [31]. ...................................................................................................... 50

Figure 2.3 Schematic diagram of battery by Goodenough et al. [27]. ..................................... 53

Figure 2.4 Cross sectional view of structural lithium ion battery fabricated by Thomas et al.

[50] ........................................................................................................................................... 54

Figure 2.5 Layup schematic of an embedded thin film lithium energy cells on CF reinforced

epoxy composites by Pereira et al. [51] ................................................................................... 55

Figure 2.6 Schematic (a) and geometry (b) of PowerFibre invented by Neudecker et al. [53]

.................................................................................................................................................. 56

Figure 2.7 Schematic (a) and cross-sectional view (b) of the integrated battery on CF

reinforced epoxy composites by Kim et al. [54] ...................................................................... 56

Figure 2.8 Schematic of a model geometry of a Li-ion battery cell by Kim et al. [41] ........... 57

Figure 2.9 Schematic of the cross-section of structural battery described by Wong. [59] ...... 58

Figure 2.10 Schematic of a structural battery developed by and taken from Liu et al. [60] ... 59

Figure 2.11 Schematic of an autophagous structure-power system for an unmanned air

vehicle by Thomas et al. [61] ................................................................................................... 59

Figure 2.12 Schematic diagram of a fuel cell by Goodenough et al. [27]. .............................. 61

List of Figures

19

Figure 2.13 Pultruded fuel cell panel developed by Peairs et al. [14] ..................................... 62

Figure 2.14. Schematic of a structural fuel cell by South et al. [62]. ...................................... 63

Figure 2.15 Schematic of a capacitor. ...................................................................................... 65

Figure 2.16 Schematic of a structural capacitor by O’Brien et al. [46] ................................... 66

Figure 2.17 Schematic of piezoelectric fibre also acting as structural capacitor by Lin et al.

[69] ........................................................................................................................................... 67

Figure 2.18 Electrolytic capacitor patented by General Electric Company, New York [76]. . 68

Figure 2.19 Electrical energy storage apparatus patented by the Standard Oil Company,

Cleveland, Ohio [77]. ............................................................................................................... 69

Figure 2.20 Schematic of a supercapacitor by Halper et al. [95]. ............................................ 71

Figure 2.21 Schematic of the types of supercapacitors by Haler et al. [95]. ........................... 73

Figure 2.22 Schematic of supercapacitor assembly by Tien et al. [100] ................................. 74

Figure 2.23 History of improvements in ionic conductivity of the polymer electrolytes by

Murata et al. [114]. ................................................................................................................... 77

Chapter 3

Figure 3.1 Chemical structures of PEGDGE (a), DGEBA (b) and PAN (c). .......................... 84

Figure 3.2 Chemical structures of TETA (a) and MCHA (b). ................................................. 85

Figure 3.3 Chemical structures of LiTFSI (a), TBAPF6 (b) and EMITFSI (c). ...................... 85

Figure 3.4 Chemical structure of PC........................................................................................ 86

Figure 3.5 Chemical structure of TEOS. ................................................................................. 86

Figure 3.6 Chemical structure of Pluronic P123 (x = 20, y = 70, z = 20). ............................. 86

Figure 3.7 Adsorption isotherms I to VI classified after IUPAC 1984 (image taken from P.

Somasundaran, 2006). .............................................................................................................. 89

Figure 3.8 Schematic of light scattering through laser diffraction by Malvern [166]. ............ 90

Figure 3.9 An oscillating shear strain and the stress response for viscoelastic materials [171].

................................................................................................................................................ 104

Figure 3.10 Schematic of a RIFT process. ............................................................................. 106

List of Figures

20

Figure 3.11 Vacuum bag during RIFT process (a) Rift setup, (b) sandwiched CF and GF mats

before RIFT process, (c) Structural supercapacitors after RIFT process. .............................. 107

Figure 3.12 Schematic of a ±45° laminated structural supercapacitor during tensile test in

accordance with ASTM D 3518. ........................................................................................... 111

Figure 3.13 Tensile testing of a structural supercapacitor specimen (a) pre tensile test

specimen, (b) post tensile test specimen. ............................................................................... 112

Chapter 4

Figure 4.1 Temperature (a) and frequency (b) sweep tests of PAN1-3D, PAN1-6M and

PAN2 gel based polymer electrolytes (G/ = storage modulus and G// = loss modulus). ....... 117

Figure 4.2 Cyclic voltamograms (a) and impedance spectroscopy plots (b) of PAN1-3D,

PAN1-6M and PAN2 gel polymer electrolytes at room temperature. ................................... 120

Figure 4.3 Ionic conductivity ҡ as function of storage modulus G/ (peak maximum) of three

PAN gel polymer electrolytes by varying PAN/plasticiser concentration at 25°C. .............. 121

Figure 4.4 Effect of increasing EMITFSI concentration on crosslinked PEGDGE electrolyte,

(a) EMITFSI concentration vs. ionic conductivity ҡ and compression modulus E; and ....... 124

Figure 4.5 Ionic conductivity ҡ and compression modulus E (a) and ionic conductivity ҡ and

compression strength σ (b) as a function of increasing concentration of DGEBA xDGEBA in

crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% LiTFSI/PC. ....................... 127



crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% EMITFSI. ......................... 128

Figure 4.7 Stress strain curves of crosslinked PEGDGE/DGEBA blend polymer electrolytes

containing 10 wt% EMITFSI ................................................................................................. 129



crosslinked PEGDGE/DGEBA electrolytes containing 50 wt% EMITFSI. ......................... 130

Figure 4.9 Photograph of what on tissue paper showing phase separation of EMITFSI from

crosslinked PEGDGE/DGEBA electrolyte with xDGEBA of 0.8 containing 50 wt% EMITFSI.

................................................................................................................................................ 131

List of Figures

21

Figure 4.10 Compression modulus E (a) and compression strength σ (b) of different

crosslinked PEGDGE (P) and crosslinked DGEBA (B) electrolytes containing either

LiTFSI/PC (Li) or EMMITFSI (E) as a function of ionic conductivity ҡ at room temperature.

................................................................................................................................................ 133

Chapter 5

Figure 5.1 Schematics of the proposed structural evolution of silica network showing

micelles with PEO-co-PPO corona (thin lines), TEOS molecules (thick lines)and embedded

PEO-co-PPO chains in silica, adopted from Rodriguez-Abreu et al. [191]. .......................... 136

Figure 5.2 BET nitrogen adsorption/desorption isotherms (a) and BJH pore size distribution

(b) of mesoporous silica from samples with varying reaction layer thickness. ..................... 137

Figure 5.3 BET nitrogen adsorption/desorption isotherm (a) and BJH pore size distribution

(b) of mesoporous silica monoliths by varying the curing temperature. ............................... 139

Figure 5.4 Mesoporous silica monolith having internal cracks after ethanol washing. ......... 141

Figure 5.5 Particle size distributions of MSP and NSP obtained by dynamic light scattering.

................................................................................................................................................ 142


(b) of mesoporous (MSP) and non-porous (NSP) silica particles. ......................................... 143

Figure 5.7 SEM images of crushed mesoporous silica monoliths (a) and mesoporous silica

particles (b and c). .................................................................................................................. 145

Figure 5.8 Ionic conductivity ҡ, compression modulus E (a) and compression strength σ (b)

of crosslinked MSP/PEGDGE composite electrolytes, containing 0.8 wt% TBAPF6/PC, as a

function of increasing MSP concentration. ............................................................................ 148


of crosslinked MSP/PEGDGE composite electrolytes containing 0.8 wt% LiTFSI/PC as a

function of increasing MSP concentration. ............................................................................ 149


of crosslinked PEGDGE/NSP composite electrolytes, containing 0.8 wt% LiTFSI/PC, as a

function of increasing non-porous silica particles NSP concentration. ................................. 150

List of Figures

22


of crosslinked MSP/PEGDGE composite electrolytes containing 10 wt% EMITFSI as a

function of increasing MSP content. ...................................................................................... 152


of 40/60 weight ratio of PEGDGE/DGEBA composite electrolytes containing 50 wt%

EMITFSI, as function of increasing MSP concentration. ...................................................... 156

Figure 5.13 Compression modulus E (a) and compression strength σ (b) of different MSP (M)

or NSP (N) reinforced crosslinked PEGDGE (P) and crosslinked DGEBA (B) composite

electrolytes containing TBAPF6/PC (A), LiTFSI/PC (Li) or EMITFSI (E) as a function of

ionic conductivity ҡ at room temperature. ............................................................................. 158

Chapter 6

Figure 6.1 Optical micrographs of various commercially available glass fibre fabrics, (a)

ACG 1, (b) ACG 2, (c) Tissa 1, (d) Tissa 2 and (e) Tissa 3 (microscopic images taken by Dr.

Hui Qian). .............................................................................................................................. 162

Figure 6.2 Charge discharge curves of investigated structural supercapacitors with various

charging times of (a) 10s showing high charge loss and incomplete discharge, (b) 100 s, (c)

250 s and (d) 500 s. ................................................................................................................ 164

Figure 6.3 Specific capacitance Cg as a function of charging time during charge-discharge

experiment.............................................................................................................................. 166

Figure 6.4 Charge-discharge curves of as-received CF reinforced crosslinked PEGDGE

composites containing (a) 0.8 wt% LiTFSI/PC and (b) 10 wt% EMITFSI and two layers of

glass fibre mats as separator. Charging time = 600 s. ............................................................ 167

Figure 6.5 Charge-discharge curves for the as-received CF reinforced crosslinked PEGDGE

composites containing 10 wt% EMITFSI electrolyte with (a) filter paper, (b) polypropylene

(PP) membrane and (c) two layers of glass fibre mat as separators. ..................................... 170

Figure 6.6 Charge-discharge curves for the as-received CF and GF reinforced crosslinked

PEGDGE/DGEBA composites containing 10 wt% EMITFSI as a function of the PEGDGE to

DGEBA weight ratio.............................................................................................................. 173

Figure 6.7 Photographs of post-test in-plane shear specimens of CF and GF reinforced

polymer electrolytes containing 10 wt% EMITFSI with varying content of PEGDGE and

List of Figures

23

DGEBA in the crosslinked matrix (a) Pure DGEBA, (b) 20P:80B, (c) 40P:60B, (d) 60P:40B,

(e) 80P:20B and (f) Pure PEGDGE. ...................................................................................... 175

Figure 6.8 Charge-discharge curves for supercapacitors made using as-received CF and GF

with: (a) 100 wt% EMITFSI, (b) 50 wt% EMITFSI in 40:60 PEGDGE/DGEBA blend matrix,

or (c) 10 wt% EMITFSI in 40:60 PEGDGE/DGEBA blend matrix as electrolyte. .............. 176

Figure 6.10 Nyquist plots for the as-received CF and GF reinforced crosslinked 40:60

PEGDGE/DGEBA blend matrix containing increasing amounts of EMITFSI. .................... 177

Figure 6.11 Photographs of supercapacitor specimens made from CF and GF reinforced

crosslinked 40:60 PEGDGE/DGEBA blend matrix with (a) 10 wt% EMITFSI and (b) 50

wt% EMITFSI after in-plane shear testing. ........................................................................... 179

Figure 6.12 Different configurations of CF based electrodes during fabrication of structural

supercapacitors. ...................................................................................................................... 180

Figure 6.13 Charge-discharge curves of different CF electrode configurations in as-received

CF and GF reinforced crosslinked PEGDGE supercapacitors with 10wt% EMITFSI. ........ 180

Figure 6.14 Evolution of specific capacitance measured at 150 s of charging time for as-

received CF and GF reinforced crosslinked PEGDGE supercapacitors containing 10 wt%

EMITFSI as function of number of charge/discharge cycles. ............................................... 182


PEGDGE supercapacitors containing 10wt% EMITFSI at cycle number (a) 1, (b) 500 and (c)

1000 in charge-discharge experiment. ................................................................................... 183

Figure 6.16 Complex impedance plots for structural supercapacitors made from as-received

CF and GF reinforced crosslinked PEGDGE matrix (a) or crosslinked PEGDGE matrix with

7.5 wt% MSP (b) containing 10 wt% EMITFSI. ................................................................... 185

Figure 6.17 Charge-discharge curves for structural supercapacitors made from as-received CF

and GF reinforced crosslinked PEGDGE matrix (a) or crosslinked PEGDGE matrix with 7.5

wt% MSP (b) containing 10 wt% EMITFSI. Charging time = 600 s .................................... 185

Figure 6.18 Photographs of CF and GF reinforced crosslinked PEGDGE containing 10 wt%

EMITFSI composites after in-plane shear testing with (a) crosslinked PEGDGE and (b)

crosslinked PEGDGE/7.5 wt% MSP. .................................................................................... 187

List of Figures

24

Figure 6.19 Schematic of (a) series, or (b) parallel lay-up combination of two structural

supercapacitors. ...................................................................................................................... 188

Figure 6.20 Charge/ discharge curves for the as-received CF and GF reinforced PEGDGE

containing 10 wt% EMITFSI (a) baseline, (b) three supercapacitors laid-up and tested in

series, or (c) three supercapacitors laid-up and tested in parallel. ......................................... 189

Figure 6.21 Impedance plot for CF and GF reinforced crosslinked PEGDGE composites with

10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes. ............................. 191

Figure 6.22 Charge/ discharge curves for CF and GF reinforced crosslinked PEGDGE

composites with 10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes.

Charging time = 600 s. ........................................................................................................... 192



Charging time = 1500 s. ......................................................................................................... 192

Figure 6.24 Photographs of CF and GF reinforced crosslinked PEGDGE composites

containing 10 wt% EMITFSI after in-plane shear testing with (a) as-received carbon fibre, or

(b) activated carbon fibre (ACF) reinforcements. .................................................................. 194

Figure 6.25 Nyquist plot for the ACF and GF reinforced crosslinked 40:60

PEGDGE/DGEBA blend matrix composites containing various concentrations of EMITFSI.

................................................................................................................................................ 195

Figure 6.26 Charge-discharge curves for the ACF and GF based supercapacitors with (a) 100

wt% EMITFSI, (b) 50 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix,

and (c) 10 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix. ............... 196

Figure 6.27 Photographs of ACF and GF reinforced crosslinked 40:60 PEGDGE/DGEBA

blend matrix with increasing amounts of EMITFSI after shear testing; (a) 10 wt% EMITFSI

and (b) 50 wt% EMITFSI. ..................................................................................................... 198

Figure 6.28 Complex impedance plots for CF and GF reinforced crosslinked MSP/PEGDGE

matrix containing 10 wt% EMITFSI with (a) as-received CF electrode, or (b) ACF

electrodes. .............................................................................................................................. 199

Figure 6.29 Charge/discharge curves for (a) as-received CF, or (b) ACF electrodes and GF

reinforced crosslinked MSP/PEGDGE composites containing 10 wt% EMITFSI. .............. 200

List of Figures

25


reinforced crosslinked MSP/PEGDGE composites containing 10 wt% EMITFSI. .............. 200

Figure 6.31 Photographs of structural supercapacitors consisting of crosslinked

MSP/PEGDGE matrix containing 10 wt% EMITFSI, GF separator and (a) as-received CF or

(b) ACF reinforcements after in-plane shear testing. ............................................................. 202

Figure 6.32 Ragone plot relating energy density E to the power density P of studied structural

supercapacitors in comparison to other energy storage devices. ........................................... 203

Figure 6.33 Multifunctional plot of studied structural supercapacitors relating shear modulus

G12 to the specific capacitance Cg. ......................................................................................... 204

List of Tables

26

List of Tables Chapter 1

Table 1.1 Contributions of the individual components in the proposed multifunctional

composites................................................................................................................................ 42

Chapter 2

Table 2.1 Comparison of battery, capacitor and supercapacitor (values taken from NuinTEK

[36]).......................................................................................................................................... 51

Table 2.2 Capacitance range C, working potential range V and estimated specific energy Eest

of commercially available supercapacitors. ............................................................................. 70

Chapter 3

Table 3.1 Summary of relevant properties of fibre mats. ........................................................ 87

Table 3.2 Composition of PEGDGE polymer electrolytes by increasing the concentrations of

EMITFSI. ................................................................................................................................. 93

Table 3.3 Composition of DGEBA polymer electrolytes by increasing the concentrations of 1

M LiTFSI/PC. .......................................................................................................................... 93

Table 3.4 Composition of PEGDGE:DGEBA blend polymer electrolytes with 10wt% of 1 M

LiTFSI/PC by varying the PEGDGE and DGEBA concentrations. ........................................ 95

Table 3.5 Composition of PEGDGE:DGEBA blend polymer electrolytes with 10wt% of

EMITFSI by varying the PEGDGE and DGEBA concentrations. .......................................... 96

Table 3.6 Composition of PEGDGE:DGEBA blend polymer electrolytes with 50 wt% of

EMITFSI by varying the PEGDGE and DGEBA concentrations. .......................................... 97

Table 3.7 Composition of MSP/PEGDGE composite polymer electrolytes with 0.8wt% of 1

M LiTFSI/PC by varying the MSP concentrations. ................................................................. 98

Table 3.8 Composition of MSP/PEGDGE composite polymer electrolytes with 10wt% of

EMITFSI by varying the MSP concentrations......................................................................... 99

Table 3.9 Composition of DGEBA/MSP composite polymer electrolytes with 20wt% of 1 M

LiTFSI/PC by varying the MSP concentrations. ..................................................................... 99

List of Tables

27

Table 3.10 Composition of PEGDGE/DGEBA/MSP composite polymer electrolytes with

20wt% of 1 M LiTFSI/PC by varying the MSP concentrations. ........................................... 100


50wt% of EMITFSI by varying the MSP concentrations. ..................................................... 101

Table 3.12 Single fibre diameter df, BET surface area As, specific capacitance Cg, tensile

modulus ET and tensile strength σT of as-received and activated carbon fibre mats (results

courtesy of Dr. Hui Qian). ..................................................................................................... 102

Chapter 4

Table 4.1 Ionic conductivity of different salts. ...................................................................... 115

Table 4.2 Specific capacitance Cg and ionic conductivity ҡ of the PAN gel polymer

electrolytes. ............................................................................................................................ 119

Table 4.3 Ionic conductivity ҡ, compression modulus E and compression strength σ of

crosslinked PEGDGE electrolytes by varying different salts (A-0.1 M TBAPF6/PC, Li-1.0 M

LiTFSI/PC, Na-1.0 M NaClO4/PC, E- EMITFSI). ................................................................ 122


crosslinked DGEBA electrolytes as function of increasing LiTFSI/PC concentrations. ....... 125

Chapter 5

Table 5.1 Surface areas, SBET and SLangmuir, pore volume VP, pore width dP, full width at the

half maximum of the BJH pore size distribution FWHM and bulk density of mesoporous

silica monoliths MSM synthesised at 90°C with increasing monolith thickness hsample. ....... 138

Table 5.2 Surface areas, SBET and SLangmuir, pore volume VP, pore width dP, full width at the

half maximum of the BJH pore size distribution FWHM and bulk density of mesoporous

silica monoliths MSM cured at different temperatures TCuring. .............................................. 140

Table 5.3 Surface areas, SBET and SLangmuir, pore volume VP, pore width dP, mass-median

particle diameter d50 and bulk density of mesoporous (MSP) and non-porous silica particles.

................................................................................................................................................ 144

List of Tables

28

Table 5.4 Ionic conductivity ҡ compression modulus E and compression strength σ of

PEGDGE (0.1 M TBAPF6/PC) polymer electrolyte with increasing crushed MSM

concentration. ......................................................................................................................... 146


crosslinked DGEBA/MSP composite electrolytes containing 20 wt% LiTFSI/PC as a function

of increasing MSP concentration. .......................................................................................... 153


crosslinked PEGDGE/DGEBA/MSP composite electrolytes containing 10 wt% LiTFSI/PC as

a function of increasing MSP concentration. ......................................................................... 154


crosslinked PEGDGE/DGEBA electrolytes containing 20 wt% LiTFSI/PC as a function of

increasing MSP concentration. .............................................................................................. 155

Chapter 6

Table 6.1 Thickness and areal weight of various commercially available glass fibre woven

mats studied. .......................................................................................................................... 161

Table 6.2 Charging and discharging capacity, charge loss and specific capacitance Cg of

structural supercapacitors manufactured using as-received carbon fibre mat, crosslinked

PEGDGE containing 0.8 wt% LiTFSI/PC and various glass fabric separators. .................... 163

Table 6.3 The specific capacitance Cg as determined by charge-discharge experiment of

structural supercapacitor as function of varying charging time. ............................................ 165

Table 6.4 Discharge capacity, charge loss ∆ and specific capacitance Cg of CF and GF

reinforced crosslinked PEGDGE composites containing LiTFSI or EMITFSI as electrolyte.

Charging time = 600 s. ........................................................................................................... 168

Table 6.5 CF volume fraction Vf, maximum shear strength τ12m, shear strength at 5000 µε and

shear modulus G12 of structural supercapacitors with crosslinked PEGDGE containing

LiTFSI/PC or EMITFSI. ........................................................................................................ 169

List of Tables

29

Table 6.6 Discharge capacity, charge loss ∆, specific capacitance Cg and density ρ of as-

received CF reinforced crosslinked PEGDGE or DGEBA composites containing 10 wt%

EMITFSI and various separators. Charging time = 600 s. .................................................... 171

Table 6.7 Carbon fibre volume fraction Vf, maximum shear strength τ12m, shear strength at

5000 µε and shear modulus G12 of structural supercapacitors with crosslinked PEGDGE and

crosslinked DGEBA containing 10wt% EMITFSI electrolyte and filter paper (FP), glass fibre

mats (GF) or PP membrane separators. ................................................................................. 172

Table 6.8 Discharge capacity, charge loss Δ, specific capacitance Cg and bulk density ρ of as-

received CF and GF reinforced crosslinked PEGDGE/DGEBA composites as function of

PEGDGE to DGEBA ratio. Charging time = 600 s. .............................................................. 174


shear modulus G12 of structural supercapacitors with crosslinked PEGDGE and DGEBA

polymer electrolytes containing 10wt% EMITFSI as function of PEGDGE to DGEBA ratio.

................................................................................................................................................ 174

Table 6.10 Charge loss Δ, specific capacitance Cg, equivalent series resistance ESR, energy

density E and Power density P of as-received CF and GF reinforced crosslinked 40:60

PEGDGE/DGEBA blend matrix composites as function of decreasing EMITFSI

concentration. Charging time = 600 s. ................................................................................... 178

Table 6.11 Maximum shear strength τ12m, shear strength at 5000 µε and shear modulus G12 of

structural supercapacitors made using as-received CF and GF with crosslinked 40:60

PEGDGE/DGEBA blend matrix as function of decreasing EMITFSI concentration. .......... 178

Table 6.12 Discharge capacity, charge loss ∆ and specific capacitance Cg for different CF

electrode configurations of as-received CF and GF reinforced crosslinked PEGDGE

supercapacitors containing 10 wt% EMITFSI. ...................................................................... 181

Table 6.13 Influence of applied potential difference on the discharge capacity, charge loss Δ,

specific capacitance Cg and energy density E of structural supercapacitors made from CF and

GF reinforced crosslinked PEGDGE containing 10wt% EMITFSI. ..................................... 184

Table 6.14 Influence of MSP addition on the charge loss Δ, specific capacitance Cg,

equivalent series resistance ESR, energy density E and power density P of structural

supercapacitors made using as-received CF and GF reinforced crosslinked PEGDGE

composites containing 10 wt% EMITFSI. ............................................................................. 186

List of Tables

30

Table 6.15 Effect of MSP additions on the maximum shear strength τ12m, shear strength at

5000 µε and shear modulus G12 of as-received CF and GF reinforced crosslinked PEGDGE

matrix composites. ................................................................................................................. 186

Table 6.16 Discharge capacity, charge loss Δ, specific capacitance Cg and theoretical specific

capacitance of structural supercapacitor assembly made using as-received CF and GF

reinforced crosslinked PEGDGE containing 10 wt% EMITFSI connected either series or

parallel combinations. Charging time= 600 s ........................................................................ 189


density E and power density P of structural supercapacitors made from as-received CF or

ACF and GF reinforced crosslinked PEGDGE matrix containing 10 wt% EMITFSI at a

charging time of 600 s or 1500 s. ........................................................................................... 193

Table 6.18 Influence of CF activation on the maximum shear strength τ12m, shear strength at

5000 µε and shear modulus G12 of CF and GF reinforced crosslinked PEGDGE matrix



density E and power density P of structural supercapacitors made from ACF and GF

reinforced crosslinked 40:60 PEGDGE/DGEBA blend matrix composites containing various

concentrations of EMITFSI. .................................................................................................. 197


ACF and GF based supercapacitors with crosslinked 40:60 PEGDGE/DGEBA blend matrix

containing increasing amounts of EMITFSI. ......................................................................... 197


density E and power density P of structural supercapacitors made using CF or ACF and GF

reinforced crosslinked MSP/PEGDGE matrix containing 10 wt% EMITFSI at a charging

time of 600 s or 1500 s. .......................................................................................................... 201


5000 µε and shear modulus G12 of CF and GF reinforced crosslinked MSP/PEGDGE matrix


List of abbreviations

31

List of Abbreviations ACN acetonitrile

ACF activated carbon fibre

AFC alkaline fuel cell

AC alternating current

Al2O3 aluminium oxide

Aq. HCl aqueous hydrochloric acid

ASTM American society for testing and materials

ARL army research laboratory

BT/BaTiO3 barium titanate

BJH Barrett-Joyner-Halenda

BET Brunauer Emmett Teller

CF carbon fibre

DEC diethyl carbonate

DGEBA diglycidylether of Bisphenol-A

DMC dimethyl carbonate

DC direct current

DMFC direct methanol fuel cell

EDL electrochemical double layer

EDLC electrochemical double layer capacitors

EIS electrochemical impedance spectroscopy

EDR equivalent distributive resistance

ESR equivalent series resistance

EtOH ethanol

EC ethylene carbonate

EMITFSI 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

FP filter paper

FWHM full width at the half maximum

GPE gel polymer electrolyte

GF glass fibre

ILSS interlaminar shear strength


32

ISO international standards organisation

PMn-PT lead magnesium niobate-lead titanate

LVE linear viscoelastic

LiTFSI lithium bis(trifluoromethylsulfonyl)imide

LiCoO2 lithium cobalt oxide

LiPF6 lithium hexafluorophosphate

LiFePO4 lithium iron phosphate

LiPON lithium phosphorus oxynitride glass

MEA membrane electrode assembly

MS mesoporous silica

MSM mesoporous silica monoliths

MSP mesoporous silica particles

MCHA 4,4’-methylenebis(cyclohexylamine)

MW molecular weight

MCFC molten carbonate fuel cell

NiMH nickel metal hydride

NSP non-porous silica particle

PDA personal digital assistant

PAFC phosphoric acid fuel cell

PAN polyacrylonitrile

PEEK poly(etheretherketone)

PEGDGE polyethylene-glycol-diglycidylether

PEGDMA poly(ethylene glycol) dimethacrylate

PEO poly(ethylene oxide)

PPO poly(propylene oxide)

PMMA poly(methyl methacrylate)

PPy polypyrrole

PETF poly(ethylene terephthalate) film

PVDF-HFP poly(vinyldifluoride-co-hexafluoro propene)

PaCE polymer and composite Engineering

PSD pore size distribution

KOH potassium hydroxide

PC propylene carbonate


33

PEMFC proton exchange membrane fuel cell

RIFT resin infusion under flexible tooling

RT room temperature

SEM scanning electron microscopy

SiO2 silica/silicon dioxide

SiC silicon carbide

SWCNT single wall carbon nanotube

SDS sodium dodecyl sulphate

SOFC solid oxide fuel cell

Surfactant surface active agent

TBAPF6 tetrabutylammoniumhexafluorophosphate

TEATFB tetraethyl ammonium tetrafluoroborate

TEOS tetraethyl orthosilicate

TiO2 titanium dioxide

TETA triethylenetetramine

UAV unmanned air vehicles

VARTM vacuum assisted resin transfer moulding

20P:80B weight ratio of 20% PEGDGE and 80% DGEBA




List of Notations

34

List of Notations Notation Description Unit

° degree of angle -

% per cent -

vol% per cent by volume -

wt% per cent by weight -

L Length meter [m]

centimetre [cm]

milli meter [mm]

micro metre [µm]

nano metre [nm]

m mass kilogram [kg]

grams [g]

milli gram [mg]

t time seconds [s]

minutes [min]

hours [h]

milli seconds [ms]

ҡ ionic conductivity Siemens/centimetre [S/cm]

E compression modulus mega Pascal [MPa]

ET tensile modulus mega Pascal [MPa]

σ compression strength mega Pascal [MPa]

σT tensile strength mega Pascal [MPa]

τ12m maximum in-plane shear stress mega Pascal [MPa]

τ120.5 in-plane shear stress at 0.5% strain mega Pascal [MPa]

γ12 i shear strain at i-th data point -

ϵχi longitudinal normal strain at i -

ϵyi transverse normal strain at i -

G12 unidirectional shear modulus mega Pascal [MPa]

R resistance ohm [Ω]

E specific energy watt hours/kilogram [Wh/kg]

List of Notations

35

P specific power watt/kilogram [W/kg]

π pi =3.1416 -

I current ampere [A]

T temperature degree Celsius [°C]

V voltage volt [V]

cm2 unit of area square centimetre [cm2]

cm3 unit of volume cubic centimetre [cm3]

df fibre diameter micrometre [µm]

ε0 permittivity of vacuum -

εr dielectric constant of electrolyte -

C capacitance farads [F]

Eest estimated specific energy watts/kilogram [W/kg]

F force newton [N]

Fmax maximum force newton [N]

GPa unit of stress Giga Pascal [GPa]

h thickness of specimen millimetre [mm]

I electrical current amperes [A]

J unit of energy Joule [J]

kN unit of force kilo Newton [kN]

MPa unit of stress mega Pascal [MPa]

MHz unit of frequency mega hertz [MHz]

ml unit of volume milli liter [mL]

mmol milli moles - [mmol]

mV unit of voltage milli volts [mV]

μA current micro ampere [µA]

N Newton - [N]

η viscosity Pascal second [Pa.s]

P power watt [W]

Pa Pascal - [Pa]

rpm revolutions per minute -

ρ density gram/cubic centimetre [g/cm3]

v specific volume cubic centimetre/gram [cm3/g]

Tg glass transition temperature degree Celsius [°C]

List of Notations

36

T Temperature Kelvin [K]

t Time seconds [s]

τ Shear stress mega Pascal [MPa]

ω Frequency hertz [Hz]

Tm Melting temperature degree Celsius [°C]

tan δ loss or dissipation factor -

δ Phase lag angle degrees [°]

ε Strain -

C BET parameter -

€ European currency euros

Fmax maximum force newton [[N]

Q charge coulombs [C]

E electric field newton/coulomb [N/C]

d plate separation centimetre [cm]

A surface area square centimetre [cm2]

AS specific surface area square centimetre/gram [cm2/g]

AS BET BET surface area square centimetre/gram [cm2/g]

Cg specific capacitance farads/cubic centimetre [F/cm3]

MW molecular weight gram/mole [g/mol]

ρa areal density gram/square metre [g/m2]

P/PO relative pressure -

na nitrogen gas molecules adsorbed cubic centimetre / gram [cm3/g]

nm specific monolayer amount of adsorbate cubic centimetre / gram [cm3/g]

L Avogadro’s number (= 6.022×1023) -

Am molecular cross sectional area squared metre/moles [m2/mol]

d0.5 mean diameter micrometre [µm]

Ф phase angle - [°]

E potential difference volts [V]

Z impedance ohm [Ω]

Z’ real part of impedance ohm [Ω]

Z” imaginary part of impedance ohm [Ω]

pH potential of hydrogen, measure of

acidity/basicity -

List of Notations

37

strain rate -

τ0 maximum stress mega Pascal [MPa]

γ0 maximum strain -

G* complex modulus Pascal [Pa]

G/ elastic or shear modulus Pascal [Pa]

G// loss or viscous modulus Pascal [Pa]

FCmax maximum compression force newton [N]

dE/dt voltage sweep rate volts/second [V/s]

Vf fibre volume fraction -

Mi initial mass of specimen grams [g]

Mf final mass of fibres after acid digestion grams [g]

ρc density of specimen grams/cubic centimetre [g/cm3]

ρf density of fibres grams/cubic centimetre [g/cm3]

∆ charge loss per cent [%]

Chapter 1 Introduction

38


This chapter provides an introduction to research into structural power composites for energy

storage devices. The chapter starts with the motivation and focuses on the importance of

multifunctionality in the design of an engineering material. This is followed by the

methodology used in the current research. The aims and objectives are discussed afterwards

followed by the brief description of the structure of the thesis.


39

1.1 Motivation

The motivation of the current research is the premise that the weight and volume is

considered to be a primary concern in engineering design of a load-carrying product [1].

Any material that does not contribute to load-carrying is structurally parasitic. Conventional

design practices pursue optimisation by maximising the efficiency of individual

subcomponents by using advanced materials with higher specific properties or by utilising

new performance technologies. Increasingly complex requirements for numerous applications

require a corresponding increase in the efficiency with which these systems utilise their mass

and volume [2]. Another approach is to design multifunctional materials that can perform two

or more functions simultaneously. Multifunctional designs not only improve the system

efficiency through weight and volume savings but also reduce the complexity of the system.

However, on the other hand, the functions of multifunctional materials are conflicting i.e. the

improvement in one of the properties of a multifunctional material results in the reduction of

another [3]. Therefore, optimisation is required between different properties of the

multifunctional material. There is numerous research underway in the field of multifunctional

materials [4]. One such multifunctional design concept that has attracted a great deal of

attention is energy storage. Structural power composites which exhibit simultaneously

structural and electrical energy storage functions are one such example.

Structural power composites have many possible military as well as civilian applications. For

example, a substantial volume of a laptop or mobile phone is the battery. However, by using

multifunctional composites, the casing of the laptop or mobile phone could also store

electrical energy along with the battery and thus reduce the final system weight and volume.

The range and speed of current unmanned aerial vehicles (UAVs) as well as ground electric

vehicles is limited by the performance of batteries [5]. The outer body of the UAVs and

ground electric vehicles does not contribute to energy generation/storage and therefore, the

replacement of the outer body by multifunctional composites could further improve the

performance of UAVs and ground electric vehicles. Soldiers are required to wear protective

clothing, sensors, communication tools and power sources which are becoming more

complex with the advancement in defence technology [2]. If a soldier will carry heavy items

then this may cause considerable decrease in his/her performance. Therefore, the

development of multifunctional systems having certain levels of mechanical integrity as well

as an electrical energy storage capacity could prove valuable for an extensive variety of

applications.


40

Figure 1.1 BAE systems Mantis UAV that will employ structural energy composites (Courtesy

of BAE systems).

The current study will focus on composite multifunctionality and potential for the

development of structural supercapacitor technology. A structural supercapacitor, a

supercapacitor which in addition to storing electrical energy can carry structural loads, may

offer substantial benefits in many systems including UAVs, ground electric vehicles, mobile

phones, laptops as mentioned above. In the fabrication of structural supercapacitors, two

different roles, including the structural and energy storage, are bound together in a single

coherent material. Different material requirements, such as structural and electrochemical

properties, need to be engineered and optimised simultaneously. The focus of this study is to

manufacture materials that can simultaneously carry mechanical loads whilst storing (and

delivering) electrical energy. The versatility of composite materials means that they provide

an ideal opportunity to develop novel multifunctional materials which can store electrical

energy required to power systems, whilst meeting the demands of the mechanical loading [6].

Although the current study is in its embryotic stage, the concept has received huge attention

in the scientific world. BAE Systems have worked with Imperial College to demonstrate the

concept on the development of a component from Mantis UAV (Figure 1.1). Imperial


41

College is also leading a € 3.3m EU 7th Framework research programme (STORAGE) for

the development of structural power composites to be used in hybrid cars [6]. These

multifunctional composites will be employed in Volvo cars by replacing the spare-wheel

floor of the car saving 15% of its system weight. Different newspapers and science

magazines including Daily Mail [7], CNN [8], Daily Mirror [9], The Economist [10] and

Materials World [11] etc. have published articles on the development of structural energy

storing composites.

Figure 1.2 Spare-wheel floor [11] of a Volvo car replaced with a multifunctional composite

to be developed in the STORAGE project.

1.2 Methodology

In the recent past, research efforts have targeted the development of other multifunctional

storage systems including structural batteries [12], structural capacitors [13] and structural

fuel cells [14] (Chapter 2). The paradigm of a novel multifunctional structural supercapacitor

was adopted in this work in order to develop a low weight multifunctional composite

possessing specified electrical and mechanical properties. Carbon fibre reinforced polymer

composite, having a laminated architecture as shown in Figure 1.3, was the main focus of this

work. Glass fibres, along with filter papers and polymer membranes, were used as separators.

Different polymer matrices were used as polymer electrolytes, including polyacrylonitrile

(PAN), diglycidylether of bisphenol-A (DGEBA) and polyethylene glycol diglycidylether

(PEGDGE). Mesoporous silica particles (Section 5.1) were used as reinforcements embedded

into the polymer matrix (Section 5.2). Table 1.1 shows the contribution of each component of


42

a structural supercapacitor to the multifunctionality of the final composite and its

requirements.

Mechanical Electrical

conductivity

Ionic

conductivity Requirements

Polymer

matrix

Light weight, decent

mechanical properties and

ionic conductivity

Carbon fibre

mats

High surface area, Excellent

mechanical properties

Glass fibre

mats

Good mechanical properties,

porosity, Electronic insulator

and sufficiently dense

Mesoporous

silica

High surface area, narrow

particle size distribution, high

porosity, mechanical properties

Table 1.1 Contributions of the individual components in the proposed multifunctional

composites.

Figure 1.3 Cross sectional view of proposed multifunctional structural supercapacitors.


43

The research into the optimisation of mechanical and electrochemical functionalities of

structural supercapacitors is a challenge because the requirements tend to be contradictory to

each other. The improvement in one functionality leads to the loss in the other functionality.

The interactions between these two functionalities are not immediately understandable

without further exploration of the performance of individual components of structural

supercapacitors. This involves the implementation of a holistic research approach, embracing

the optimisation of the mechanical and electrochemical performance of the individual

subcomponents of structural supercapacitors, including the CF based electrodes, separator

and solid polymer electrolytes.

1.3 Aims and objectives

The central objective of this research is to develop carbon fibre reinforced composites that

simultaneously act as a structural component as well as an energy-storing supercapacitor. The

goal led to the following objectives for this work;

Formulate solid polymer electrolytes [Chapter 4] to achieve both electrical and

mechanical performance (targets: Young’s modulus in compression around 1 GPa and

ionic conductivity around 10-3 S/cm);

Characterise the mechanical and electrochemical properties of solid polymer

electrolytes;

Synthesise mesoporous silica (MS) (with a surface area > 500 m2/g and an average

pore size of 6-7 nm [15]) and characterise the mechanical as well as electrochemical

properties of mesoporous silica filled solid polymer electrolytes (Chapter 5);

Develop composite materials that can be used for energy storage device and structural

performance simultaneously;

Characterise the mechanical and electrochemical performance of the resulting

composites. Electrochemical characterisation involved measuring the specific

capacitance, charging/discharging, internal resistance, equivalent series resistance and

energy and power density characteristics using methods, such as impedance

spectroscopy and charge-discharge cycling methods. Mechanical characterisation

involved shear properties of structural supercapacitors.

Thus, in conclusion, structural supercapacitors should,

o Be light in weight;


44

o Have high mechanical properties, strength (e.g. shear strength of around 100

MPa) and stiffness (e.g. shear modulus of around 2 GPa), to meet the structural

requirements;

o Have high specific capacitance (around 1 F/cm3) and ionic conductivity

(approximately 10-3 S/cm), to deliver sufficient electrical energy when required;

o Be low in cost;

o Perform at operational temperatures (-30°C to 80°C);

o Have long cycle life (at least 15 years [16]);

o Amenable to system integration.

1.4 Thesis Outline

The dissertation is divided into seven chapters. Chapter 1 describes the motivation,

methodology, aims and objectives of the research project. Chapter 2 consists of a review of

the relevant literature providing the background for the following chapters. It starts with the

background on the carbon-fibre reinforced thermoset composites and different energy storage

devices followed by the concept of multifunctional composites focussing on the mechanical

and electrochemical functionalities of the composites. The concept, research trends, and the

current challenges of different structural energy storage devices including structural batteries,

structural fuel cells and structural capacitors are discussed in detail. Particular focus is given

to the concept of structural supercapacitors and the research trends devoted to the

development of the individual subcomponents, including CF based electrodes, polymer

electrolytes and separators.

Chapter 3 contains the materials and experimental methods which were used throughout this

study. It begins by listing all raw materials followed by the formulation of mesoporous silica

reinforcements, polymer electrolytes, composite polymer electrolytes as well as the

fabrication of structural supercapacitors by resin infusion under flexible tooling (RIFT)

process. Characterisation techniques for the evaluation of mechanical and electrochemical

performance of polymer electrolytes as well as structural supercapacitors are also explained

in this chapter. Chapter 4 presents the results and discussion on the characterisation of

polymer electrolytes. Different matrices for polymer electrolytes studied in this work are

polyacrylonitrile (PAN) gel, polyethylene glycol diglycidylether (PEGDGE) and


45

diglycidylether of bisphenol-A (DGEBA). The compression modulus, compression strength

and ionic conductivity measurements of PEGDGE and DGEBA polymer electrolytes are also

discussed in Chapter4.

Mesoporous silica is characterised and discussed in Chapter 5. Different forms of

mesoporous silica including mesoporous silica monoliths (MSMs) and mesoporous silica

particles (MSP) were added as reinforcement to polymer electrolytes and the results of the

structural and electrochemical characterisation of composite polymer electrolytes are

presented. Chapter 6 discusses the characterisation and analysis of various structural

supercapacitors containing different separators, polymer electrolytes, and CF based

electrodes. Chapter 7 summarises the major findings and presents an outlook for future work.

Chapter 2 Literature Review

46


In this chapter, a literature survey, on carbon fibre reinforced thermoset composites as well as

energy storage devices, is first presented followed by a review of the specific topic of

multifunctional composites and supercapacitors. Subsequently, an introduction of a very

promising multifunctional composite technology identified as “structural supercapacitors” is

presented. The development of other energy storage devices having structural properties is

also discussed. Focussing on the prospects of augmenting composite systems with

supercapacitor technology in military and civil applications, the survey then highlights the

literature on the development of individual components of structural supercapacitors, i.e. the

electrodes and electrolytes as well as supercapacitors in order to draw attention to the

research objectives and challenges of this work.


47

2.1 Traditional carbon fibre reinforced thermoset composites

A composite is a structural product fabricated from two or more distinctive materials that are

disparate in nature with widely differing properties (fibre and matrix usually) but when

combined possess an engineering performance exceeding that of the individual components

[17]. Polymer composites play an important role in wide variety of civil and military

applications. In particular, carbon fibre (CF) reinforced epoxy based composites have gained

prime interest because of their outstanding properties, ease of fabrication, low shrinkage after

curing and good thermal resistance [18]. In CF reinforced epoxy based composites, the epoxy

matrix binds the reinforcing carbon fibres together resulting in a coherent structure in which

applied stresses are transferred from matrix to the fibres via the interface [18].

Usually, carbon fibres have the largest volume fraction in CF reinforced epoxy composites.

Most widely used volume fractions of carbon fibre content in CF reinforced composites range

from 50-60 vol% [18, 19]. The main factors affecting the performance of CF reinforced

composites are volume content of all constituents (usually carbon fibres, polymer matrix and

voids), fibre orientation, fibre aspect ratio as well as the strength-moduli characteristics of

both fibres and matrix [20]. Carbon fibres have excellent structural properties and good

electrical conductivity. The electrical resistivity of CFs is very low [21] (of the order of

µΩm) and, therefore, when these fibres, protected by the matrix, are used in fabricating CF

reinforced composites, there exists a good fibre to fibre contact resulting in a composite

having good electrical conduction. The longitudinal electrical resistivity of carbon fibres is

reported to be about 80 µΩm and the transverse electrical resistivity is approximately 4000

µΩm in a CF reinforced composite having CF volume fraction of 55% [21].

The matrix of CF reinforced composites also plays an important role in the overall

performance of CF reinforced composites. The most important issues in selection of matrices

for composites include reinforcement-matrix compatibility in terms of bonding, processing

temperature and processing time. Epoxy resins are the most widely used class of polymer

matrix in the fabrication of CF reinforced composites because of their superior mechanical

properties, thermal resistance, and high tolerance to alkaline conditions [22]. Although

significant interest is now growing in the utilisation of some other thermosets (e.g. polyester,

polyimides, phenolics), as well as thermoplastic (e.g. nylon, PEEK) polymer matrix systems,

in composite fabrication, epoxy resins [23] heavily dominate as matrices in high performance

CF reinforced composite applications.


48

CF reinforced thermoset composites are now widely used in sectors such as wind energy,

aerospace, marine and automotive industries. CF reinforced thermoset composites are used to

manufacture products such as surgical and sports equipment, civil infrastructure and dentistry

equipment. CF reinforced composites are superior as compared to traditional metallic

materials due to their strength-weight ratio, low thermal expansion and high fatigue

resistance [18]. Although composite materials are now used to make a significant part of

aircraft structure (e.g. Boeing 787 Dreamliner aircraft [24, 25]), one of the main focus now in

composite technology is that these materials should serve multiple functions, behave

intelligently, and be greener [26]. Conventional materials such as epoxy resins are now taking

on a new life with the development of more focused and effective processing methods.

Merging conventional materials in unconventional and novel ways is opening a new era of

opportunities.

2.2 Energy storage devices

Development of energy storage devices is another promising field of interest. The availability

of inexpensive energy has become a primary focus of modern economy. Globally, it is

accepted that in the coming years, electrical energy related problems will become of

increasing interest [27]. Electrical energy related problems influence both technical and

economic aspects of modern society. One of the biggest problems regarding electrical energy

is the voltage variation, such as voltage sags and momentary interruptions [27]. Industrial

machinery is mainly affected by these voltage sags. This may result in potential damage or

complete loss of automated production units. Not only factories but residential electricity

consumers are also affected by these voltage sags. Domestic electrical appliances and

personal computers are also very sensitive to these momentary interruptions of electrical

energy. Therefore, a certain energy reservoir is required that can inject electrical energy into

the electrical grid during voltage sags. Great interest has been developed in making and

refining more efficient energy storage devices. A wide variety of energy storage devices is

schematically shown in Figure 2.1.


49

Figure 2.1 Schematic of different electrical energy storage devices by Sels et al. [28].

The selection of the correct device for energy storage depends on various factors such as total

cost, environmental conditions and restrictions and, most important, power and energy

density of the energy storing devices. Different electrical energy storage devices, such as

batteries, supercapacitors, capacitors or fuel cells have different power-energy capabilities.

These electrical energy storage devices are usually performance rated on the basis of the

“Ragone plot” [29, 30]. The Ragone plot (Figure 2.2) shows the discharge rate of specific

power as a function of the specific energy being available for a load. Ideally, the storage

device should have a high energy density as well as high power density, but in reality

compromises have to be made between energy density and power density. Generally, if the

energy is discharged quickly then the energy delivery will be low. Thus, the energy storage

device which provides the most energy at the maximum power discharge rates will be

considered the best in terms of electrical performance [30].

Energy Storage

Indirect Storage Direct Storage

Natural

Reservoir

Artificial

ReservoirMagnetically Electrically

Capacitors

Supercapacitors

Super Magnetic Storage

Systems (SMES)

Batteries

Fuel cells

Flywheels

Pumped hydro

Heat

Compressed air

Hydrogen


50

Figure 2.2 Ragone plot showing energy storage delivery performance for different storage

devices by Kotz et al. [31].

The Ragone plot (Figure 2.2) discloses the current status of the energy storage performance

in which batteries have a high specific energy (approx. 250Wh/kg) but low specific power

(below 1000 W/kg), capacitors have rather high specific power (approximately 107 W/kg) but

low specific energy (below 0.06 Wh/kg) and fuel cells have high energy density (above 1000

Wh/kg) but low power density (below 200 W/kg). Supercapacitors possess intermediate

power density and energy density and also have long life cycles due to the absence of

chemical reactions. Conway [32] has given a comprehensive review on properties and

principles of supercapacitors. Supercapacitors exhibit several advantages over

electrochemical batteries and fuel cells including rapid charge-discharge processes (within

seconds), longer cycle life, and longer shelf life. Large-scale supercapacitors can even

perform power quality regulation of the electrical grid that can avoid costly industrial power

shutdowns [27].

In comparison to batteries that store electrical energy as a result of chemical reactions,

supercapacitors store energy in a different way. In batteries, ions move from one electrode to

the other through an electrolyte and a chemical reaction takes place at the electrodes (or at

least at one of the electrodes). Supercapacitors (Section 2.4), however, store energy

physically without any chemical reaction (Figure 2.20). That is why the process is reversible


51

and the charge-discharge cycle can be repeated almost without limit (hundreds of thousands

of cycles). Supercapacitors store charge in an electrochemical double layer at the electrolyte-

electrode interface. Due to the high surface area of electrodes and extremely low double layer

thickness, supercapacitors have exceptionally high specific and volumetric capacitances. The

only limitation of supercapacitors is their low energy density as compared to batteries and

fuel cells [32]. Due to intermediate power and energy densities of supercapacitors as

compared to batteries, capacitors and fuel cells, applications of supercapacitors have

increased from vehicles and cell phones to large industrial drive systems [31-33]. A

comparison of properties and performance of capacitors, batteries and supercapacitors is

shown in Table 2.1 [34]. A brief overview of the research trends and working of

supercapacitor is presented in the following section 2.4.

Parameters Capacitor Supercapacitor Battery

Charge time 10-6 ~ 10-3 s 1 ~ 30 s 0.3 ~ 3 h

Discharge time 10-6 ~ 10-3 s 1 ~ 30 s 1 ~ 5 h

Energy Density (Wh/kg) < 0.1 1~10 20 ~ 100

Power Density (W/kg) >1,000 1,000 ~ 2,000 50 ~ 200

Cycle life >500,000 > 100,000 500~ 2,000

Charge/Discharge efficiency ~1.0 0.90~0.95 0.70~0.85

Table 2.1 Comparison of battery, capacitor and supercapacitor (values taken from NuinTEK

[34]).

2.3 Multifunctional composites

In recent years, multifunctional composites have attracted a great deal of attention [4, 12, 35-

38]. Multifunctional composite systems are used for material development and thus, act as a

structural material and also exhibit at least one additional performance-linked function such

as electrical, thermal, optical, chemical or electromagnetic functions. Composite systems are

ideally suited for multifunctional performance as the best features of different materials can

be integrated to form a new material that behaves as a homogeneous entity.


52

Generally, composite materials are optimised for improvement in structural performance

[35]. However, in multifunctional materials, two or more different, but useful, functionalities

are inherently available. Ideally, multifunctional materials should have low weight with

desirable mechanical, chemical, thermal, electrical, magnetic and optical properties. The

feasibility of a multifunctional composite depends on the physical/chemical compatibility of

the individual constituents as well as the internal/external interfacial capability of the desired

combination of the constituents’ functions.

In recent years, several multifunctional composites have been designed for various

applications. Self-healing composite materials are one such example in which a

microencapsulated healing agent is embedded into the structural composite matrix containing

a catalyst that polymerises the healing agent upon contact, resulting in healing/repairing of

the damaged region [39]. Structural batteries, for the skin of micro air vehicle, are another

example in which flight time is increased while maintaining the total aircraft weight [35].

Another example of a multifunctional composite is a structural supercapacitor (Section 2.4)

that carries, by nature of its application, mechanical loads as well as storing electrochemical

energy and thus could replace the traditional static load bearing components to reduce the

volume and mass of the overall system. Previous studies have utilized different approaches

for optimising the electrical and mechanical properties in a composite system, including

structural batteries [12, 40-43], structural capacitors [13, 44, 45] and structural fuel cells [14,

46].

2.3.1 Structural batteries

2.3.1.1 What are batteries?

Electrical energy storage, in the form of chemical energy in batteries, is the most

conventional and oldest approach [47]. A battery contains one or more electrochemical cells

connected in series or parallel to give a required power and voltage. Electrons are generated

from the anode as a result of chemical reaction and migrate through an external electrical

circuit to the cathode delivering electrical energy to the load en route. In a lithium-ion battery

cell, the anode contains lithium ions, commonly intercalated within graphite (Figure 2.3).

Positive ions, e.g. lithium ions, migrate inside the cell through an electrolyte from one

electrode to another. The electrolyte does not allow the flow of electrons (only ions) and

should be compatible with both electrodes. The current collectors (anode and cathode),

typically metals, allow electron flow to and from electrodes. Typically, copper and


53

aluminium are used for the anode and cathode, respectively, as the lighter weight aluminium

cannot be used for lithium-based anodes due to its reactivity with lithium. Cell voltage is

defined by the chemical reaction energy occurring in the cell. Usually, the anode and cathode

of batteries are complex composites as they contain polymeric binders, besides the active

material, to hold the powder structure [27]. The anode and cathode also contain conductive

diluents (e.g. carbon black) to give the whole structure electronic conductivity so that

electrons can be transported to the active material [27]. The anode and cathode components

are combined in order to allow a liquid electrolyte to penetrate the structure and the ions to

reach the reacting sites.

Figure 2.3 Schematic diagram of battery by Goodenough et al. [27].

Different battery energy storage applications consist of lead acid, lithium ions, nickel,

cadmium, sodium sulphur, and sodium nickel chloride as electrolytes [27]. Generally, lithium

ion batteries are considered the most powerful batteries offering the same energy as nickel

metal hydride (NiMH) batteries at 20-30% less mass [27]. Li-ion batteries are more

expensive than older technology batteries but are valued for high power portable applications

such as laptops, cell phones and PDAs (personal digital assistant).


54

2.3.1.2 Research trends in structural batteries

At present, the major on-going research in the development of structural batteries is to reduce

the volume and weight of a battery and to enhance its energy density. In order to develop a

multifunctional structure, the battery should be an integral part of a load bearing structure.

Different approaches have been adopted to fabricate multifunctional structural batteries,

including the embedding of batteries on composites (multifunctional structures) as well as the

fabrication of composites acting as a battery itself (multifunctional materials). Pioneering

work in developing such multifunctional structural batteries was done by Thomas et al. [48]

who fabricated a structural battery using electrodes made from active particles bonded by

lithium ion containing poly(vinyl difluoride) and hexafluoropropene (PVDF-HFP) polymer

matrix, current collectors made from metal meshes, and a separator made of micro porous

polyolefin. Structural batteries were manufactured using hot press lamination as shown in

Figure 2.4. They highlighted plastic lithium-ion structure battery materials and examined the

multifunctional potential of commercial cells acting as shear panels and spar caps (a load

bearing structure) [5]. They studied three different configurations of structural batteries and

were able to obtain a specific energy of 95.2 Wh/kg and a stiffness index of 56

(MPa)1/2/(g/cm3) from a carbon-epoxy reinforced structural battery.

Figure 2.4 Cross sectional view of structural lithium ion battery fabricated by Thomas et al.

[48]

Pereira et al. [49, 50] physically embedded solid-state thin film lithium energy cells into a CF

reinforced epoxy based composite without deteriorating the structural properties of composite

and electrochemical properties of lithium energy cells as shown in Figure 2.5.

Charge/discharge baseline performance remained unchanged when the composite was

exposed to uniaxial tensile loading up to 450 MPa. The maximum tensile stress applied to the


55

composite without affecting the performance of lithium energy cells was recorded to be

approximately 50% of the ultimate tensile strength of the composites.

Figure 2.5 Layup schematic of an embedded thin film lithium energy cells on CF reinforced

epoxy composites by Pereira et al. [49]

Another novel approach of developing thin film batteries around fibre substrates was

introduced by Neudecker et al. [51] They fabricated a thin layered structure of battery around

a fibre substrate and used a Lipon (lithium phosphorus oxynitride glass) based electrolyte.

The schematic of a thin layered fibre battery termed “PowerFibre” is shown in Figure 2.6.

The PowerFibre combined the energy storing capability with structural properties and has a

diameter of 33-150 µm. A patch consisting of 1000 PowerFibres delivered a 9 W of power at

3V and 3 A while supplying 0.1 Wh of energy.

(a)


56

(b)

Figure 2.6 Schematic (a) and geometry (b) of PowerFibre by Neudecker et al.[51]

Kim et al. [52] also described another approach by amalgamating a thin film lithium ion

battery and an amorphous silicon solar cell on a printed circuit board and then integrating this

conducting circuit board into a CF reinforced epoxy composite by inkjet printing technique.

A cross-sectional image of the integrated battery cells and composites is shown in Figure

2.7b. The integrated battery composite was fabricated using a vacuum bag moulding process

in an autoclave. The integrated energy storing composite was able to perform as a power

laminate until 0.45% applied static strain after which the battery stopped working within the

integrated structure.

(a) (b)

Figure 2.7 Schematic (a) and cross-sectional view (b) of the integrated battery on CF

reinforced epoxy composites by Kim et al. [52]


57

Hossain et al. [53] suggested carbon-carbon composite anode materials for batteries showing

good mechanical performance and achieving a specific energy of more than 200 Wh/kg.

Some of the most popular work of this type was performed by Kim et al. [40] They

developed three dimensional Li-ion battery cells with networked electrodes that had

improved power delivery capabilities. A schematic of a three-dimensional Li-ion battery cell

is shown in Figure 2.8.

Figure 2.8 Schematic of a model geometry of a Li-ion battery cell by Kim et al. [40]

Although structural batteries were fabricated by embedding batteries into composites, the

above mentioned studies were multifunctional structures instead of multifunctional materials.

A truly multifunctional material was fabricated by a research team at U.S. ARL (Army

Research Laboratory) led by Wetzel. They designed a load bearing battery i.e. a

multifunctional structural battery [54]. If designed with sufficient structural and energy

efficiency, these structural batteries exhibited a significant decrease in system level weight

and volume. Snyder et al. [12] have explored new structural resin electrolytes [55], structural

anode and structural cathode layered metal meshes [56]. They proposed that multifunctional

structural materials can be realized through the focussed development of new materials,

material architectures and low cost scalable fabrication routes [12]. They chose a carbon

fabric based anode, metal mesh cathode (LiFePO4 with acetylene black for electrical

conduction and poly(ethylene oxide) as a binder) and a vinyl ester random copolymer as

polymer electrolytes for their multifunctional Li ion composite batteries [57]. The polymer

electrolyte, used in the fabrication of a structural battery, had a compression modulus of 25


58

MPa and an ionic conductivity of 4 µS/cm at a density of 1.1 g/cm3. The schematic of the

structural battery proposed by Wong et al. is shown in Figure 2.9. The structural battery was

fabricated using VARTM (Vacuum Assisted Resin Transfer Moulding) process. The

structural battery had an electrical resistance of 4.5Ω and a tensile specific stiffness of 3.6

GPa/(g/cm3).

Glass fabric separator

Glass fabric separator

Carbon fabric anode

Carbon fabric anode

Stainless steel cathode substrate

Figure 2.9 Schematic of the cross-section of structural battery described by Wong. [57]

More promising work in the field of structural batteries was reported by Liu et al. [58] who

developed structural batteries with tuneable mechanical properties. These structural batteries

had elastic or potential structural load bearing capabilities. The 100 µm thick cathode of the

structural battery was manufactured by dissolving N-methyl pyrrolidone, carbon black,

carbon nanofibres and LiCoO2 in a high molecular weight PVDF. The anodes were also

manufactured in a similar way to the cathode, with the difference that LiCoO2 was replaced

with coke. The separator used in the structural battery was a polymer blend of poly(vinyl

difluoride) hexafluoropropene (PVDF-HFP) and poly(ethylene glycol) dimethacrylate

(PEGDMA) with 1 M LiPF6 in ethylene carbonate (EC), propylene carbonate (PC), diethyl

carbonate (DEC) and dimethyl carbonate (DMC). Aluminium and copper grids were used as

current collectors. The schematic of the structural battery is shown in Figure 2.10. A specific

capacity of 90 Ah/kg and a modulus of 650 MPa were measured for the electrodes. A flexural

modulus of 3.1 GPa and an energy density of 35 Wh/kg was achieved for the structural

battery.


59

Figure 2.10 Schematic of a structural battery developed by and taken from Liu et al. [58]

2.3.1.3 Other multifunctional energy storage materials

Another novel approach in the structural-battery field was adopted by Thomas et al. [59] at

Naval Research Laboratory, USA who developed an autophagous (“self-consuming”)

GasSpar system that used the vapour pressure of a two-phase liquid-gaseous butane or

propane fuel for strengthening an inflatable composite beam. Autophagous GasSpar system

prototype showed a specific energy of 20 Wh/kg, specific power of 2.9 W/kg, burning time of

7 h and a total usable electrical energy of 8.4 Wh. An autophagous GasSpar system for UAVs

is shown in Figure 2.11 in which GasSpar constitutes the main structural element of the

aircraft wing and combustion thermoelectric conversion process converts the two-phase

hydrocarbon fuel into electrical energy.

Figure 2.11 Schematic of an autophagous structure-power system for an unmanned air

vehicle by Thomas et al. [59]


60

2.3.1.4 Challenges in structural batteries

The current state of the art structural batteries are facing many challenges. The biggest

challenge is in the fabrication of structural batteries. A structural battery having an energy

density equivalent to conventional battery cells will become quite expensive. Also, relative to

the currently available batteries, structural batteries offer very small energy and power

densities [12]. Therefore, the main goal in the development of structural batteries is to

improve the energy and power densities and to reduce the manufacturing costs. Structurally,

multifunctional batteries should allow good load transfer without affecting their performance

and long term durability. Electrochemically, these structural batteries should attain high

power density while maintaining high energy density. Therefore, a major effort is required to

tailor and optimise the multifunctionality of individual constituents of structural batteries, i.e.

anode, cathode, separator and polymer electrolyte.

2.3.2 Structural fuel cells

2.3.2.1 What are fuel cells?

A fuel cell is an electrochemical energy conversion device that converts chemicals, for

example hydrogen and oxygen, into water and in the process produces electricity. Chemicals

constantly flow into the cell and as a consequence, electrical energy is produced (Figure

2.12). The conversion of the fuel (e.g. hydrogen) to energy takes place without combustion

and therefore, the process is efficient, clean and quiet.

A fuel cell consists of two electrodes separated by electrolyte. Usually, hydrogen and oxygen

(air) are fed into the anode and cathode of the fuel cell, respectively. The hydrogen splits into

protons and electrons in the presence of a catalyst. The protons pass through the electrolyte

but the electrons must take the long way around, creating a separate current that can be

utilized before they return to the cathode, to be combined with the hydrogen and oxygen to

form a water molecule. Individual cells are “piled” together to generate useful quantities of

energy.


61

Figure 2.12 Schematic diagram of a fuel cell by Goodenough et al. [27].

The five major types of fuel cells, depending on the type of electrolytes used, are: Alkaline

(AFC), Proton Exchange Membrane (PEMFC), Molten Carbonate (MCFC), Phosphoric Acid

(PAFC), and Solid Oxide (SOFC) fuel cells. Direct Methanol Fuel Cells (DMFCs) are a type

of PEMFC that directly uses methanol as the fuel. The use of expensive catalyst materials,

such as platinum, makes fuel cells costly. Fuel cells are usually classified on the basis of

operating temperature and the type of electrolyte used. Fuel cells are different from batteries

as batteries store chemical energy in a closed system but fuel cells consume reactants. Also,

electrodes within a battery react and change as a battery is charged or recharged. Fuel cells

range in size from hand-held systems to megawatt power stations and operate most efficiently

over a narrow range of performance parameters and at elevated temperature (approximately

100°C - 1000°C). However, fuel cells are not suitable for high power demands.

2.3.2.2 Research trends in structural fuel cells

Peairs et al. [14] described the manufacturing of a structural methanol fuel cell, through

pultrusion process, which serves both to provide an electrical power source as well as to

oppose mechanical loads that the system may experience during its operation. A conceptual

diagram of a pultruded fuel cell is shown in Figure 2.13. The pultruded structural methanol

fuel cell achieved a current density of 4 mA / cm2 at 0.1 V while the VARTM structural

methanol fuel cell achieved a current density of approximately 5.5 mA /cm2 at 0.1V.


62

Figure 2.13 Pultruded fuel cell panel developed by Peairs et al. [14]

The research team at ARL led by Wetzel is also working on structural multifunctional fuel

cells. In ARL, South et al. [60] developed a structural fuel cell by using aluminium foam of

very high porosity as the anode and cathode current collectors, carbon cloth as a gas diffusion

layer, a standard membrane electrode assembly (MEA) and a CF-reinforced epoxy composite

skin for structural support. A schematic of this structural fuel cell is shown in Figure 2.14.

The highly porous aluminium foam, served as an electron conductor, improved the structural

performance of the multifunctional fuel cell and also acted as a distributing medium by

permitting fuel and air reactants to reach the MEA. The structural fuel cell was fabricated

using a hand layup process in a continuous 0°/90° configuration. The ARL structural fuel cell

showed a power density of 12.5 mW/cm2 and an average bending stiffness of 2.3 GPa. High

porosity and high density aluminium foam had the best overall multifunctional performance

in the structural fuel cell.


63

Figure 2.14. Schematic of a structural fuel cell by South et al. [60].

Leading more towards the practical implementation of fabricating structural fuel cells,

Kaempgen et al. [61] introduced a highly conductive single wall carbon nanotube (SWCNT)

based multifunctional electrode which showed a similar performance as amorphous carbon

based regular electrodes. SWCNT based electrodes offered required functions for fuel cell

operation such as current collection, catalyst support, gas diffusion and electrolyte contact.

Commonly used amorphous carbon based electrodes in fuel cells require binders as well as a

structural support (e.g. carbon cloth or metal mesh) at the back side within the fuel cell.

SWCNT based electrodes form a self-supporting film without any binder or structural

support. However, SWCNT based fuel cell turned out to be more expensive than amorphous

carbon based fuel cell.

2.3.2.3 Challenges in structural fuel cells

The development of structural fuel cells faces many challenges. Structurally, the cores of the

fuel cell assembly should be engineered in order to reduce the effect of the midplane

membrane on the shear properties, as the midplane membrane provides poor shear stiffness

and strength. Electrochemically, structural fuel cells need to attain a high power density (at

least an order of magnitude increase) while maintaining high energy density. Also, the cost of

electrodes, catalyst and fuel required for the fuel cell operation is very high. Fuel and waste

product/ stream management is another challenge. Therefore, new potential materials should

be explored for the advancement of structural fuel cells. In addition, further consideration is

required on material architectures as well as fabrication techniques.


64

2.3.3 Structural capacitors

2.3.3.1 What are capacitors?

A capacitor stores energy in the form of an electrostatic field on electrically conducting plates

that are placed very close together but separated with an insulator (called dielectric), shown

in Figure 2.15. Usually a capacitor has more than two plates depending on the capacitance or

dielectric type. The dielectric materials are ceramic, paper, polymer or other insulating

materials and the conducting plates are metals in foil, thick film or thin film form [44]. In

general, the energy density E and power density P can be calculated from the following

equations 2.1 and 2.2,

12

(2.1)

4 (2.2)

Where C is the capacitance, V is the applied potential difference and R is the equivalent

series resistance (ESR). ESR is the undesired internal resistance of a capacitor that appears

with the desired capacitance in series at a specified frequency. Usually, the ESR of a

capacitor is just a small fraction of an Ohm for a low voltage, high capacitance capacitor (e.g.

1000µF, 16V), and can be as high as 2-3 Ohm for a high voltage, low capacitance capacitor

(e.g. 1uF, 450V).

The bulkiness and high frequency performance of capacitors remains an issue. The bulkiness

is specially a major concern when high capacitance is required. Capacitors have very low

energy density (equation 2.1) in comparison to batteries and supercapacitors but the power

density (equation 2.2) is very high as shown in Ragone plot (Figure 2.2). This means that

capacitors are able to deliver or accept high currents, but only for extremely short periods.

For achieving high capacitance in capacitors, the dielectric should be very thin, the

conducting plates should be large in area and several dielectric layers and conducting plates

should be alternatively stacked.


65

Figure 2.15 Schematic of a capacitor.

2.3.3.2 Research trends in structural capacitors

Small, et al. [62] have developed gas dielectric capacitors of 5 and 10 pF with Zerodur (a

glass-ceramic made by Schott AG) as the structural material which are claimed to have small

temperature and voltage coefficients and are stable with time [63].

Windlass et al. [64] have demonstrated that the dielectric properties of polymer matrices can

be improved by the addition of ceramic fillers such as lead magnesium niobate-lead titanate

(PMN-PT) and barium titanate (BT). Colloidal processing of nanoparticle filled epoxy was

used to obtain up to 2 µm thin dielectric films. The results showed that a dielectric constant

of more than 135 was achieved in a PMN-PT filler based epoxy system. A capacitance of 35

nF/cm2 was achieved for the structural capacitor having the thinnest films (2.5-3.0 µm) of

PMN-PT/epoxy dielectric.

Research is being carried out in the US army research laboratory on structural capacitors that

can carry mechanical loads by intercalating glass fibre reinforced polymer dielectric layers

with metalized polymer film electrodes [45]. Researchers are trying to maintain the high

dielectric strength and high capacitance in polymer composites. A simple structural capacitor

can be constructed by placing electrically conductive electrode layers between composite

dielectric plies (Figure 2.16). The dielectric strength and the energy density of a structural

capacitor, with the thinnest woven glass fibre (40 µm) as a separator, was in the range of 100-


66

150 V/µm and 0.2-0.5 J/cm3 respectively. The authors have also devised a multifunctional

metric based on the electrical and structural properties of the structural capacitor through

which they were able to determine the overall multifunctional efficiency of each capacitor

design.

Figure 2.16 Schematic of a structural capacitor by O’Brien et al. [45]

Luo et al.[13] have proposed a continuous carbon fibre / epoxy-matrix composites, with a

paper interlayer (0.04 mm thickness after composite fabrication), as structural capacitors. The

prototype had a capacitance of 1.2×10-7 mF /cm2 and a resistance of 1.89×106 Ω at 2 MHz.

The authors also showed that the composite was conducting in the through-the-thickness

direction without an interlayer or with a more porous paper interlayer. The carbon fibre

epoxy matrix composite having an epoxy impregnated paper of thickness 0.1 mm showed the

capacitance of 0.21×10-7 mF /cm2 due to increase in dielectric thickness.

Carlson et al. [65] developed structural capacitors from CF-reinforced pre-pregs woven

lamina separated by a paper or polymer film dielectric. Different polymer films including

polyamide, polyester and polycarbonate were used as dielectric. A maximum capacitance of

2.5×10-7 mF/cm2 and an inter-laminar shear strength (ILSS) of 21.8 MPa was obtained by

using 80 g/m2 paper as dielectric in structural capacitors. The ILSS of the structural capacitor

with 80 g/m2 paper as dielectric was comparable to ILSS of a standard composite material

(23 MPa).

Another previously developed SiC/BaTiO3 piezoelectric structural fibre [66] was also

reported by Lin et al. [67] to act as a structural capacitor, The schematic of this structural

capacitor is shown in Figure 2.17. This structural capacitor benefited from the dielectric

nature of the BaTiO3 coating applied to the silicon carbide (SiC) core fibre as a cylindrical

capacitor and mechanical reinforcement. The best fibres for energy storage were found to

have an aspect ratio of 0.23 and showed an energy density in the range between 0.0253 and

0.0325 mWh/cm3.


67

Figure 2.17 Schematic of piezoelectric fibre also acting as structural capacitor by Lin et al.

[67]

2.3.3.3 Challenges in structural capacitors

There is an increasing demand for lightweight and compact multifunctional structural

capacitors that are easily adaptable for varying electrical requirements. Unlike structural

batteries and structural fuel cells, structural capacitors provide a quick discharge for short

time periods. The mechanical performance of conventional capacitors is improved by using

composites as dielectrics [68-70] but there has been very little research on structural

capacitors. The requirement of thin separators in a capacitor also limits the structural

properties of the system. Electrically, structural capacitors should attain high energy density

(equation 2.1) while maintaining high power density (equation 2.2). Therefore, new potential

dielectric materials, with high dielectric strength and good structural properties, along with

improved fabrication techniques, are being explored in order to improve the performance of

structural capacitors.

2.4 Structural Supercapacitors

Supercapacitors have matured significantly over the last decade with the potential to facilitate

major advances in energy storage. This dissertation will focus on multifunctional composites

and its potential in supercapacitor technology. A structural supercapacitor is a supercapacitor

which, in addition of storing electrical energy, is also capable of bearing mechanical loads.

Supercapacitors, also called ultra-capacitors or electrochemical double layer capacitors,

utilize high surface area electrodes and thin electrolytic dielectrics (i.e. thin electrical double

layer) to achieve very high capacitance [32]. The performance of a supercapacitor is

dependent on charge accumulation from an electrolyte solution at the electrode/electrolyte

interface through electrostatic attraction by polarized electrodes [71]. Supercapacitors have


68

been used as energy storing devices for nearly a century [32]. Recent developments in

supercapacitor technology have allowed supercapacitor to store greater amounts of energy

(e.g. 30 Wh/kg [72]) as compared to conventional capacitors (e.g. 0.06 Wh/kg [30]) and

greater amounts of power delivery ( e.g. 3200 W/kg [72, 73]) as compared to conventional

batteries (e.g. 1000 Wh/kg [30]). In addition of filling the gap of energy and power densities

between capacitors and batteries, supercapacitors also offer several other promising

advantages including a large number of charge/discharge cycles and the ability of operating

over a wide temperature range.

2.4.1 Historical background of supercapacitors

Although chemists studied the electrical charge storage at the interface between a metal and

an electrolytic solution since the 19th century [32], Becker [74] from General Electric

Company, Inc. New York was the first to patent a low voltage electrolytic capacitor

comprising of porous carbon electrode and electrolytic solution in 1954, as shown in Figure

2.18. The electrolytic capacitor stored electric charge in the pores of carbon and showed a

capacitance of 6 F at 1.5 V. Later, another device was patented in 1966 by Rightmire from

The Standard Oil Company, Cleveland, Ohio and was named as “Electrical Energy Storage

Apparatus” [75]. The device (Figure 2.19) stored energy in the double layer interface and had

a maximum storage capacity of 22 Wh/kg. The Standard Oil Company, Cleveland, UK also

filed another patent [76] on a disc shaped capacitor in 1970 which was comprised of carbon

paste soaked in an electrolyte. Due to subsequent lack of sales, The Standard Oil Company

gave the licence of the capacitor technology to NEC in 1971 who developed its first

commercial double layer capacitor under the name of “supercapacitor” which was primarily

used for memory backup [77].

Figure 2.18 Electrolytic capacitor patented by General Electric Company, New York [74].


69

Figure 2.19 Electrical energy storage apparatus patented by the Standard Oil Company,

Cleveland, Ohio [75].

In 1978, Matsushita Electric Industrial Co. (also known as Panasonic) developed a

supercapacitor under the trade name of “Gold Capacitor” primarily for backup applications.

ELNA Corporation Ltd. manufactured supercapacitors under the trade name of “Dynacap”.

Various other companies including Maxwell Technologies and PRI also started

manufacturing supercapacitors under different trade names in late 1980s. Initially

supercapacitors were designed for military applications, i.e. for missile guidance systems;

laser guided weapons, arms, power conditioners and electromagnetic launchers. More than a

decade later, supercapacitor devices were used in vehicle application [78]. Currently,

supercapacitors, with high power densities, are being manufactured by a number of

companies around the world. A comparison of the performance of various currently available

commercial supercapacitors is presented in Table 2.2.


70

Device Name Company C (F) V (volt) Eest (Wh/kg‡)

Supercapacitor [77] NEC-TOKIN, Japan 0.013-100 2.7-12 0.04-4.22

Supercapacitor [79] CAP-XX, Australia 0.075-2.40 2.3-5.5 0.26-1.40

Boostcap [80] Maxwell Tech. Inc. USA 5-3000 2.5-130 1.38-5.96

Gold Cap [81] Panasonic, Japan 0.015-10 2.1-5.5 0.23-2.80

Dynacap [82] ELNA Co. Ltd., Japan 0.047-300 2.5-6.3 ---**

Powercap [82] ELNA Co. Ltd., Japan 500-1500 2.5 ---**

Bestcap [83] AVX Corporation, USA 0.015-1.0 3.6-16 0.03-0.18

Capacitor modules[84] ESMA, Russia 107-8000 16-52 1.74-7.30

PowerStor [85] Cooper Bussman Electronics, USA 0.47-110 2.3-5.0 0.68-4.34

ESCap [86] Tavrima Canada Ltd., Canada 2.0-160 14.0-300 0.06-0.66

Ultracapacitor [87] LS Mtron Ltd., Korea 3-5400 2.5-84 2.14-6.14

EDLC [88] Nesscap Co., Ltd., Korea 5000 2.7 5.80

Capattery [89] Evans Capacitor Company, USA 0.033-1.5 5.5-25 0.01-0.83

KAPower [90] Kold Ban Int’l, USA 1000 3.0-14.5 4.29

Superfarad [33] Superfarad, Sweden 250 50 5.4

Saft (Gen 3) [33] Saft, France 132 3.0 6.8

Premlis [72] Advanced Capacitor Tech., 2000 4.0 15

Table 2.2 Capacitance range C, working potential range V and estimated specific energy Eest

of commercially available supercapacitors.

‡ Eest ; ** Mass of device not available in literature.

2.4.2 Working principle of supercapacitor

The key components of supercapacitors are electrodes, electrolyte and separator [32] [Figure

2.20]. Energy storage is associated with the build-up and separation of electric charge

accumulated in the electric double layer at the interface between the surface of an electrode

and an electrolyte. When an electrode (electrical conductor) is immersed into an electrolyte,

there is a spontaneous organisation of charges which creates an electrochemical double layer

at the electrode/electrolyte interface with one layer at the surface inside the electrode and the

other layer in the electrolyte as shown in Figure 2.20. This electrochemical double layer

(EDL) behaves as a physical capacitor with the charges in the electrode and electrolyte

separated by a distance of the order of nanometres. The formation of EDL is dependent on

the structure of electrode surface, composition of electrolyte and the potential difference


71

applied between the charges at the electrode/electrolyte interface [91]. At the

electrode/electrolyte interface, the EDL forms and relaxes almost instantaneously (time of

formation/relaxation is ~10-8 s [91]). Thus, the double layer responds rapidly to the potential

changes. There is only a charge rearrangement (no chemical reaction) taking place in the

process. The working voltage of the supercapacitor is determined by the electrolyte

decomposition voltage and is dependent on the current density, operational temperature and

the required lifetime [92].

The characteristics of the electrode materials for supercapacitors include long term stability,

high surface area, high cyclability, and resistance to electrochemical redox reactions [56].

The separator acts as a spacer to prevent the opposing electrodes from touching one another

and causing a short circuit. Thus, a separator should be ionically conducting but

electronically insulating. The voltage window of a supercapacitor is dictated by the operating

pH and the thermodynamic stability of various species in the electrolyte [93]. Energy density

of the supercapacitor (E = ½ CV2) mainly depends on the voltage applied and therefore

voltage window is very important. The electrolyte can be either aqueous or organic (although

non-aqueous are better because of high voltage applications).

Figure 2.20 Schematic of a supercapacitor by Halper et al. [94].

This dissertation will focus on multifunctional composites and their potential in

supercapacitor technology. There are a number of reasons for choosing a supercapacitor


72

approach in this work. Supercapacitors consist of two electrodes, a separator and an

electrolyte. Thus, a relatively straightforward embodiment can be imagined by using

activated carbon fibres (Section 2.4.6) as the electrodes, possessing high surface area and

suitable pore size, combined with an electrolyte (Section 2.4.4) that is low in resistivity but

high in stability. Since high surface area electrodes are required, carbon fibres were activated

in order to introduce mesopores on fibres without compromising their mechanical properties.

Glass fibre was chosen as a separator between the electrodes as it is electronically insulating

and also possesses mechanical properties as compared to other commonly used separator e.g.

glass fibre, porous polypropylene or filter paper [32].

2.4.3 Types of supercapacitors

Supercapacitors can be divided into three general types: pseudo-capacitors, electrochemical

double layer supercapacitors and hybrid capacitors [91]. Each type has its unique charge

storage mechanism which are Faradic, non-Faradic and the combination of the two,

respectively. Faradic processes (e.g. redox process) involve the charge transfer between

electrolyte and electrode. Non-Faradic processes do not involve the chemical mechanism.

However, charges are distributed on surface by physical mechanism without making or

breaking any chemical bond. Types of supercapacitors are schematically shown in Figure

2.21.

In pseudo-capacitors, charge transfer between electrode and electrolyte is accomplished

through electro-sorption, redox-reactions and intercalation processes [32]. Thus, pseudo-

capacitors have greater capacitances and energy densities than electrochemical double layer

capacitors. Electrode materials, used for pseudo-capacitors are metal oxides and conducting

polymers [31]. Conducting polymers have higher conductivity and capacitance than carbon

electrode materials but their reduced cycling stability has hindered the development of

conducting polymer pseudo-capacitors [33]. The most commonly-used metal oxide in

pseudo-capacitors is ruthenium oxide [33] as it incorporates higher conductivity, higher

energy and power densities than similar electrochemical double layer capacitors; the main

limitation is its high cost.

Hybrid capacitors store charge by utilizing both Faradic and non-Faradic processes in order

to get better performance characteristics. Composite hybrid capacitors combine carbon-based

materials with conducting polymer or metal oxide materials in a single electrode [31].

Asymmetric hybrid-capacitors integrate activated carbon electrodes with a conducting


73

polymer anode [31]. Battery type hybrid capacitors couple a supercapacitor electrode with a

battery electrode [32]. Enormous increases in the performance characteristics have been

observed by employing these hybrid capacitors, delivering higher average specific power (0.5

kW/kg) and maximum specific power (9 kW/kg) at 5mA/cm2 in the potential region of 1.0-

3.0 V than the individual double layer carbon supercapacitors that deliver 0.335 kW/kg

average specific power and 5 kW/kg maximum specific power in the potential range of 0.0-

2.8 V [95].

.

Figure 2.21 Schematic of the types of supercapacitors by Haler et al. [94].

2.4.4 Research trends in supercapacitors

Although supercapacitors have been fabricated in the form of composites by various

researchers, the structural properties of the fabricated supercapacitors have not been reported

yet. One such supercapacitor was developed by Zhang et al. [96]. The authors employed

ZnO-CNT (zinc oxide and carbon nanotube) as electrodes and PVA-PMA (poly vinyl alcohol

and phosphomolybdic acid) as a gel polymer electrolyte. The resultant supercapacitor showed

a specific capacitance of 126.3 F/g. However, the specific capacitance was reduced by

increasing the amount of deposited ZnO in CNT due to the decrease in the specific area of

electrodes through ZnO agglomeration.

Supercapacitors

Electric double layer capacitors

Activated Carbons

Carbon Nanotubes

Carbon Aerogels

Hybrid capacitors

Composite Hybrids

Asymmetric hybrids

Battery type hybrids

Pseudocapacitors

Conducting polymers

Metal Oxides


74

Another promising supercapacitor, having electrodes made of polyacrylonitrile (PAN) based

carbon nano-fibre paper, was fabricated by Ra et al. [97]. The PAN based nano-fibre paper

was activated in order to increase the surface area up to 705m2/g. A capacitance of 100 F/g

was obtained when the activated nano-fibres and organic electrolyte was used to prepare a

supercapacitor.

Hu et al. [98] developed a supercapacitor based on polyaniline (PANI) and tin oxide (SnO2)

nano-composite. The nanostructured SnO2 particles were embedded within the netlike PANI

and thus, increase the overall specific surface area of the nano-composite. The authors

reported a specific capacitance of 305.3 F/g and an energy density of 42.4 Wh/kg for the

PANI/SnO2 nano-composite based supercapacitor. However, the supercapacitor showed a

4.5% decrease in the available capacity after 500 cycles.

Copolymer of poly (ethylene oxide) and poly (propylene oxide) and graphite oxide based

composites were used as high performance electrodes for supercapacitor fabrication by Tien

et al. [99]. The graphite oxide was well dispersed in polymer resulting in high accessibility of

graphene oxide sheets to the electrolyte ions. The supercapacitor showed a double layer

specific capacitance of 130 F/g. The schematic of the supercapacitor assembly is shown in

Figure 2.22.

Figure 2.22 Schematic of supercapacitor assembly by Tien et al. [99]

Lewandowski et al. [100] used activated carbon cloth as electrodes and three different

electrolytes (aqueous, organic and ionic liquids based) to manufacture their supercapacitors.

The authors showed that the supercapacitor filled with ionic liquid as electrolyte worked at a

potential difference of 3.5 V and thus showed the highest energy density of 215 kJ/kg.


75

Ionic liquid based electrolyte and mesoporous activated carbon fibre based electrodes were

also used by Xu et al. [101]. The activation of carbon fibre increased the surface area as high

as 3291 m2/g with a pore volume of 2.162 cm3/g. At room temperature, the specific

capacitance of 187 F/g was obtained and was further increased to 196 F/g by increasing the

operating temperature to 60°C.

A team of researchers at U.S. Army Research Laboratory led by Wetzel [102] developed

structural supercapacitors, using different electrodes, which can store electrochemical energy

as well as can carry structural loads. The structural supercapacitors showed a specific

capacitance of 35 mF/g, a tensile modulus of 10 GPa and a lap shear strength of 0.75 MPa.

The authors also suggested of using PPy (polypyrrole) coatings on structural fabric electrodes

in order to further improve the multifunctionality of structural supercapacitors.

2.4.5 Structural polymer electrolytes

The electrolyte can be solid state, aqueous or organic. Aqueous electrolytes are usually

H2SO4 or KOH possessing a dissociation voltage of 1.23 V. Organic electrolytes are prepared

by dissolving alkali metal salts in organic solvents possessing a dissociation voltage greater

than 3 V and therefore resulted in high energy density (E = ½ CV2) when employed in a

supercapacitor. Polymer electrolytes are the most commonly used ionic conductors as they

combine the advantages of solid state electrochemistry with the ease of processing inherent to

plastic materials. Initially, crystalline domains in polymer electrolytes were assumed to be

responsible for the ionic transport; however, it was soon established that the amorphous phase

is solely responsible for the ionic transport in polymer electrolytes [103]. In polymer

electrolyte (not to be confused with the polyelectrolyte in which polymer repeating unit is

covalently bonded with either cation or anion), both the cations and anions contribute to the

ionic conductivity. In polymer electrolytes, the main conductivity controlling parameter is the

segmental motion of polymer host matrix. Therefore, plasticisers are used as internal

lubricants in order to improve the conductivity [104]. By the addition of plasticisers

(excellent solvents for Li salts themselves), polymer electrolyte networks, possessing low to

moderate crosslink density, swell to form gel polymer electrolytes (GPEs). Therefore, the

addition of plasticisers negatively affects the structural properties of polymer electrolytes.

Depending on molarity, Mori et al. [105] found an ionic conductivity of 60 mS/cm from

TEATFB (tetra-ethyl-ammonium-tetra-fluoro-borate) in acetonitrile. In the literature,


76

optimisation of the electrolyte has always been highlighted as the key step towards improving

supercapacitors. The ideal electrolyte should:

a) enter the pores of porous electrodes, i.e. wet the electrodes

b) show a broad electrochemical stability range at the electrode interface (> 5V) [42]

c) produce high ionic mobility for a very fast charge/discharge rate (<1 s) [27]

d) be stable over a broad temperature range (-30°C to 100°C) [27]

e) possess high ionic conductivity (at the level of ≥10-4 S/cm) [42]

f) emit no vapours

The major limitations of aqueous solutions as electrolytes are their narrower operating

temperature window, higher corrosion activity, and lower discharge voltage. Non aqueous

liquid electrolytes have a broader operating temperature, lower corrosion activity and higher

decomposition voltage (> 2.3 V) but they have some limitations including lower electro-

conduction [32] and higher cost [31]. Therefore, very significant research effort has now

been directed to solid or gel polymer electrolytes which possess higher decomposition

voltages, reduced current leakage, lower cost, broader operating temperatures, low

flammability as well as the possibility for thin layer applications. Moreover, their limitations

include relatively low ionic conductivity and poor penetration into pores [31]. But these

limitations can be overcome by utilizing organic plasticisers [104]. Solid or gel polymer

electrolytes not only prevent liquid leakage but also provide vibration shock resistance.

Typically, a solid polymer electrolyte is composed of a salt complex and a polymer having

electron-donor atoms (e.g. nitrogen, oxygen or phosphorous etc). Different polymer

electrolytes have been proposed due to variations in their chemistries and compatibilities with

electrodes that can affect both the mechanical and electrochemical properties of the polymer

electrolytes. These polymer electrolytes include poly(vinyl-sulfone) (ionic conductivity of

374 µS/cm [106]), polyacrylonitrile (ionic conductivity of 2880 µS/cm [107]), poly(vinyl-

chloride) [108], poly(methyl-methacrylate) (gel, ionic conductivity of 1200 µS/cm [109]),

poly(vinylidene-fluoride) [41], poly(ethylene-oxide) (ionic conductivity of 0.1 µS/cm [110]),

polyaniline [98], and diglycidylether-of-bisphenol-A (ionic conductivity of 1.7 µS/cm [55]).


77

Figure 2.23 History of improvements in ionic conductivity of the polymer electrolytes by

Murata et al.[111].

Engineering mechanical properties of polymer electrolytes can be achieved by improving the

robustness of polymer electrolytes as a separator layer between electrodes. Different

approaches have been employed for augmenting structural properties in polymer electrolytes

by taking either a rigid polymer matrix (high mechanical but low electrochemical properties)

and then incorporating salt/solvent and thus making it conductive (compromising

electrochemical properties) [55] or relatively soft polymer matrix (low mechanical but high

electrochemical properties) and then adding ion conducting fillers (electrically insulating to

avoid short circuit) to enhance mechanical properties (approach followed here). The

mechanical properties can be improved by cross linking [112], clay fillers [113], fibre fillers

[114], block copolymers [115], hybrid organic/inorganic polymers [116] and ceramic particle

fillers [98, 117-120].

Among gel polymer electrolytes, polyacrylonitrile possess a high dipole moment and high

ionic conductivity (10-3 S/cm) as compared to PMMA (ionic conductivity of 10-4 S/cm) [92].

Vinyl ester derivatives of polyethylene glycol are another promising class of polymers that

are currently employed in structural solid polymer electrolytes. PEGDGE (Section 4.3) and

DGEBA (Section 4.4) belong to the group of vinyl ester derivatives and possess high


78

compression stiffness (~ 0.1 GPa to 1.2 GPa) and also show reasonable ionic conductivity (~

10-5 S/cm) when a suitable amount of ionic salt is added. These structural polymer

electrolytes are mixed with lithium bis (trifluoro methyl sulfonyl) imide (LiTFSI), tetra butyl

ammonium hexafluorophosphate (TBAPF6) or 1-ethyl-3-methylimidazolium bis (trifluoro-

methyl-sulfonyl) imide (EMITFSI). A lithium salt has been selected because of the

compatibility with polymer electrolytes [55]. Compression stiffness is used as a metric of

mechanical performance as it can be quantified easily for small amounts of materials.

Currently investigated polymer gel electrolytes possess ionic conductivities as high as 10-3

S/cm (e.g. PAN gel containing plasticisers [107]) and currently investigated structural epoxy

resins possess a compression stiffness as high as 4 GPa [55]. However, these investigated

polymer gel electrolytes and structural epoxy resins are not multifunctional. Even though

these results are valuable targets, it is not necessary to achieve them in order to attain overall

weight reduction within multifunctional materials systems and can be attained only through

collaborative increase in conductivity and stiffness.

Reinforcement of nano and micron size inorganic fillers in polymer electrolytes is the most

extensively used recent approach to reduce the natural tendency of crystalline spherulite

formation in polymer host matrix. Reinforcement by inorganic fillers (SiO2 [121], TiO2 [118],

Al2O3 [122], K-LiAlO2 [123], BaTiO3 [124], MgO [125]) in polymer electrolytes is claimed

to be responsible for the enhancement of ambient temperature ionic conductivity and

mechanical properties by about 2-3 orders of magnitude. Filler size (nano better than micron

[121]), surface area and surface nature (type of surface functionality, acidic/basic) are the

main parameters that have been shown to affect the ionic conductivity and morphology of

composite polymer electrolytes [126]. Among inorganic fillers, mesoporous silica (particles

or monoliths) has the potential to provide the desired electrical and mechanical properties to

the structural polymer electrolyte. Mesoporous silica allows the conduction of ions within the

polymer electrolyte. Generally, a significant increase (one to two orders of magnitude) in

ionic conductivity as well as mechanical performance is obtained by the addition of

mesoporous silica in polymer electrolytes [41, 117, 121]. Enhancement of ambient

temperature ionic conductivity is attributed to the stable amorphous phase formation due to

high surface area inorganic filler interactions with the ethylene oxide units which prevent

chain reorganisation and maintain disorder [126]. In addition to crystallization suppression, a

phenomenological model based on effective medium theory has been developed in which

ionic conductivity is assumed to be associated with the existence of a highly conducting layer


79

at or near the filler/polymer interface [127]. Another explanation of increased ionic

conductivity by filler reinforcement in polymer electrolytes is the suppressed ion coupling

and the formation of lithium ion conducting pathways at the filler surface due to Lewis acid-

base interactions between the filler/ion and polymer/filler in the interfacial region. Thus, there

is a great potential of using mesoporous silica, not only from the multifunctional aspect, but

also from the ability to tune the mechanical and electrical performance of a composite by

optimising the filler content in the structural electrolyte.

Poly(ethylene glycol) diglycidylether (PEGDGE), polyacrylonitrile (PAN) and bisphenol-A-

diglycidylether (DGEBA) based polymer electrolytes are employed in this work [Section

4.2]. DGEBA has a clear structural advantage over PEGDGE and PAN but possesses low

ionic conductivity (Section 4.2). PAN possesses high ionic conductivity (~ 10-3 S/cm) but at

the same time possesses negligible mechanical properties as compared to crosslinked

DGEBA and crosslinked PEGDGE. Since PEGDGE has no structural properties (liquid at

room temperature), crosslinking and the incorporation of mesoporous silica particles are

employed to enhance the mechanical properties. Since the ionic conductivity of polymer

electrolytes is mainly dependent on the interactions between monomer chains of the polymer

electrolyte and organic solvent and/or ions. There exists a little interaction in acrylonitrile but

in PEGDGE and DGEBA, ions are almost held by ether-oxygen due to strong interaction

between ions and ether-oxygen resulting in decreased ionic conductivity at ambient

temperature. Thus, long range ionic transport is supported by the ether oxygen in the

amorphous domains [55], coupled to the segmental mobility of the chains, with ions hopping

between coordination sites, whereas structural properties are enhanced by the crosslinking

and inorganic filler (mesoporous silica particles and monoliths).

2.4.6 Activated carbon fibre electrodes

Usually three types of carbon based electrode materials are used for electrochemical double

layer capacitors, (i) high surface area activated carbons, (ii) carbon aerogels and (iii) carbon

nanotubes. Activated carbons are very porous and can exhibit surface areas of up to 3000m2/g

[128]. The porous structure is composed of differently sized nano pores (< 20 Å wide),

mesopores (20-500 Å) and macropores (> 500 Å). In modern power-generating/energy-

storing technologies, carbon is the most widely used material for electrodes. Metal oxides and

conducting polymers are also used as electrodes but high surface area carbon materials

optimise the double layer effect and thus enhance the electrochemical performance of


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supercapacitors [56, 129]. There are numerous reasons for using carbon as electrode,

including low cost, high surface area, low weight, high efficiency, availability and established

electrode production technologies [31]. The capacitance of carbon based electrodes increases

linearly with the surface area and may reach the capacitance of 250 F/g [31]. Carbon based

supercapacitors come close to what one would call the pure electrochemical double layer

capacitor but the presence of surface functional groups on activated carbons can give rise to

pseudo-capacitance [32]. Pseudo-capacitance can be further enhanced by the application of

conducting polymer coatings on activated carbon fibres that result in improved energy

density at the expense of lower mechanical properties (as conductive polymers are not known

for their mechanical properties), slower response times, and creating a more complex system.

Energy density is improved as the electrodes undergo redox reactions whilst electrolyte

counter ions accumulate [56]. Carbon nanotubes, as an electrode material for supercapacitors,

have received enormous interest [96, 130]. Due to lower mechanical properties than carbon

fibres and difficulty in organising CNTs, it cannot be solely used as electrode in

multifunctional composites for supercapacitor applications. However, Baughman et al. [131].

have developed a coagulation-based CNT spinning method to prepare CNT fibres with a

Young’s modulus of 80 GPa and a tensile strength of 1.2 GPa and used these spun CNT

fibres to fabricate a supercapacitor.

Considerable effort has been directed towards the electrochemical properties of different

forms of carbon e.g. fibres [132], carbon black [133], carbon aerogel [134], skeleton carbon

[135], microbeads [136], nano-tubes [137] and nano-foams [138] (carbonised product of

polymer aerogels containing chopped carbon fibres). The problem with the black carbon is

the slow oxidation as well as high equivalent series resistance. Carbon fibres are the most

important component in high performance structural composites (five times stronger than

grade 1020 steel for structural parts, yet still five times lighter) [139]. Since the ultimate

objective of this work is the preparation of multifunctional composites, so carbon fibres are

used as electrodes in this work. Commercial carbon fibres typically have a disordered core

surrounded by a graphitic sheath and are often not available without sizing [56]. Carbon

fibres are sized by applying an uncured epoxy resin on the surface in order to enhance

fibre/matrix interaction and to facilitate handling. Due to low graphitic content and low

surface area (approx. 0.4-1 m2/g), carbon fibres are not, generally, used as electrodes without

activation [56, 132]. Activation of carbon fibre increases the specific surface area by

introducing mesopores at the surface of fibres. Introduction of mesopores enhances the


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electrochemical performance of supercapacitors by increasing the capacity of ions to be

stored at the electrode/electrolyte interface. However, it is unfortunate that a significant part

of the surface area contains nanometre-sized pores (<2 nm) which are not accessible by the

electrolyte ions and, thus, do not take part in overall capacitance [56]. Thus, in order to

achieve better results, it is important to consider electrolyte as well as electrode

characteristics at the same time. Two different general activation methods, physical and

chemical activation, are usually applied to carbon fibres for achieving high surface area and

good microporosity. In physical activation, fibres are, initially, carbonised in nitrogen

environment and then, finally, controlled high temperature (600-1000°C) gasification of these

fibres is performed in an oxidizing gas (e.g. steam, carbon dioxide, air, gas mixture) [140]

atmosphere. Various other physical activation methods are under research including cold

plasma treatment [141], copper electrodeposition [142], laser treatment [143] and oxygen

treatment [144]. In chemical activation, carbon fibres are, initially, impregnated with reactive

reagent (e.g. NaOH [145], KOH [146], HNO3 [147], and H2SO4 [148], H3PO4 [149]) and then,

finally, heat treated (<800OC) in a nitrogen environment. These modifications enhance the

surface area and microporosity but significantly reduce the structural performance of the

original carbon sources and thus an optimisation is required in enhancing surface area so that

mechanical properties are not compromised.

2.4.7 Challenges in structural supercapacitors

The current state of the art supercapacitors are facing many challenges. Although research

and development of supercapacitors is showing significant progress, there is very little

research yet available on structural supercapacitors. For vehicle applications, the structural

supercapacitor should have high energy density (greater than 5 Wh/kg [33]), high power

density, long cycle life as well as good structural performance. Electrically, structural

supercapacitors are facing different challenges, including poor ionic conductivity of the

polymer electrolyte, thicker and lower surface area electrodes, high contact resistance

between electrode and electrolyte, poor bonding of current collectors and electrodes,

fabrication techniques and thickness/porosity of separator. Mechanically, structural

supercapacitors should employ electrolytes with good structural performance, high surface

area electrodes with good mechanical properties and good adhesion of electrode/electrolyte

by enhanced mechanical interlocking and/or chemical bonding. In comparison to structural

batteries, capacitors and fuel cells, structural supercapacitors have distinctive benefits

including high energy to power ratio, large modularity with respect to capacitance and


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voltage, long cycle life, low self-discharge, environmental friendly (do not utilise hazardous

materials), do not require any servicing and do not require any cooling or other auxiliary

installations [31].

Chapter 3 Experimental Section

83


This chapter describes the chemicals utilised as well as the experimental methods adopted in

the preparation of structural supercapacitors. Starting from the preparation of mesoporous

silica and polymer electrolytes (using different electrolyte systems), a nanostructured

polymer electrolyte system was then developed by incorporating mesoporous silica in

polymer electrolytes. This chapter then describes the fabrication of structural supercapacitors

using the RIFT process. The different electrochemical as well as mechanical techniques

employed to characterise the polymer electrolytes and structural supercapacitors are also

discussed in brief.


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3.1 Materials

3.1.1 UncuredepoxymaterialsPoly(ethylene glycol) diglycidylether (PEGDGE, Lot MKBG7915V, Sigma Aldrich, UK,

[150]) and the diglycidylether of bisphenol-A (DGEBA, Lot MKBD2770V, Sigma Aldrich,

UK, [151]) were used as the uncured polymer epoxy resins and are shown in Figure 3.1. The

number averaged molecular weight (Mn) of PEGDGE was 526 with a density of 1.14 g/mL

(at 25°C) and a flash point of 197°C (data from [150]). The degree of polymerisation of

PEGDGE on this basis has been calculated to be ~ DP = 9. The molecular weight of DGEBA

was 340.41 g/mol with an epoxide equivalent weight of DGEBA between 172 to176 (see

NMR in Appendix F) and a density of 1.16 g/mL (at 25°C) (data from [151]).

Polyacrylonitrile (PAN) having a molecular weight of 150,000 was purchased from

Polysciences Inc. (UK) (data from [152]).

Figure 3.1 Chemical structures of PEGDGE (a), DGEBA (b) and PAN (c).

3.1.2 CrosslinkerTriethylenetetramine (TETA, technical grade, purity ≥ 70%, [153]) was used as crosslinker

for PEGDGE having a molecular weight of 146.2 g/mol and a density of 0.98 g/mL (at

25°C). TETA has a melting point of 12°C and a closed cup flash point of 129°C (data from

[153]). 4,4'-Methylenebis(cyclohexylamine) (MCHA, technical grade, 95%, [154]) was used

as a curing agent for DGEBA having a molecular weight of 210.4 g/mol and a density of 0.95

g/mL (25°C) (Please see Appendix F for the NMR of MCHA). MCHA (Figure 3.2b) has a

melting point of 45°C and a closed cup flash point of 159°C (data from [154]). Both

crosslinkers were purchased from Sigma Aldrich (UK).


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Figure 3.2 Chemical structures of TETA (a) and MCHA (b).

3.1.3 ElectrolytesaltLithium bis(trifluoromethyl sulfonimide (LiTFSI, purity ≥ 99.0 wt%, [155]), tetrabutyl

ammonium hexafluorophosphate (TBAPF6) (battery grade, ≥ 99.0 wt%, [156]) and 1-ethyl-3-

methylimidazolium bis(trifluoromethyl sulfonyl)imide (EMITFSI, purity ≥ 98%, [157]) were

used as ionic salts (Figure 3.3) and were purchased from Sigma Aldrich (UK). LiTFSI has a

molecular weight of 287.1 g/mol, a density of 1.334 g/mL and a melting point of 234°C (data

from [155]). TBAPF6 has a molecular weight of 387.4 g/mL and a melting point of 244°C

(data from [156]). EMITFSI is an ionic liquid having a molecular weight of 391.3 g/mol, a

density of 1.524 g/mL (at 20°C) and a melting point of -9°C (data from [157]).

Figure 3.3 Chemical structures of LiTFSI (a), TBAPF6 (b) and EMITFSI (c).

3.1.4 SolventsPropylene carbonate (PC, HPLC grade, 99.7%, [158]) was used as a solvent for ionic salts

(LiTFSI and TBAPF6) and was purchased from Sigma Aldrich (UK). PC (Figure 3.4) has a

molecular weight of 102.1 g/mL, a density of 1.20 g/mL and a boiling point of 240°C (data

from [158]).


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Figure 3.4 Chemical structure of PC.

3.1.5 Silica precursor Tetraethyl orthosilicate (TEOS, >99%, [159]) was used as a silica source and was purchased

from Sigma Aldrich (UK). TEOS (Figure 3.5) has a molecular weight of 208.3 g/mol, a

density of 0.93 g/mL (at 25°C) and a boiling point of 168°C (data from [159]).

Figure 3.5 Chemical structure of TEOS.

3.1.6 Blockcopolymersurfactant Pluronic P123 (Figure 3.6) was kindly supplied by BASF Corporation, USA (Batch number

WPDA622B, [160]) and was used as a block copolymer surfactant in mesoporous silica

preparation. Pluronic P123 has an average molecular weight of 5750 g/mol and a density of

1.01 g/mL (at 25°C). It has a pour point of 31°C (data from [160]).

Figure 3.6 Chemical structure of Pluronic P123 (x = 20, y = 70, z = 20).

3.1.7 Woven fibre mats Polyacrylonitrile (PAN) based woven carbon fibre (CF) mat, with a fibre diameter of 7 µm,

was used as an electrode in the structural supercapacitors. The glass fibre (GF) mat was used

as a separator in the fabrication of structural supercapacitors. Summary of the relevant

properties of the CF and GF mats is shown in Table 3.1.


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Fibre mat

Characteristics HTA Carbon fibre mat Glass fibre mat

Supplier Tissa Glasweberei AG (Oberkulm, Switzerland)

Fibre mat code 862.0200.01 842.0200.01

Weave plain weave plain weave

Areal density (g/m2) 202 200

Mat thickness (mm) 0.21 0.16

Warp Thread count (Fd/cm) 4.90 17.0

Weft Thread count (Fd/cm) 5.00 12.0

Conductivity (S/cm) 656 N/A

Table 3.1 Summary of relevant properties of fibre mats.

3.1.8 Paraffin oil Paraffin oil was used in the preparation of mesoporous silica monoliths and was purchased

from Sigma Aldrich, UK. Paraffin oil has a density of 0.83 g/mL (at 20°C) and a closed cup

flash point of 215°C.

3.1.9 Separators In addition to glass fibre mats, two other separators were also used in the fabrication of

structural supercapacitors including the filter paper and polypropylene membrane. Filter

paper (1001-917, Grade 1) was purchased from Whatman, UK. Polypropylene membrane

(3500, 0.064 µm pore size, 55% porosity, 25 µm thickness) was provided by Celgard, UK.

All other chemicals were used as received.

3.2 Mesoporous silica

3.2.1 Preparationofmesoporoussilicamonoliths[161]Pluronic P123 (2.0 g, 0.020 mmol) was dissolved in a mixture of EtOH (10 g, 11 mmol) and

1.0 M aq. HCl (0.40 g, 0.55 mmol) and stirred for 45 min to prepare a homogeneous solution.

While still stirring, TEOS (4.16 g, 1.00 mmol), weighed in a separate beaker, was then added

immediately to the solution under stirring and the mixture was stirred for 10 min in open air.

The homogeneous solution was then transferred into a porcelain dish (75mm rim diameter,

33mm height and 100 ml capacity) and the dish was covered with perforated aluminium foil.

The dish was kept in a refrigerator for 5 days at 14°C. The silica mixture was then covered

with liq. paraffin oil (Section 3.1.9) layer (3-4 mm). The dish was then placed into an oven at


88

70°C for 12 h for complete ethanol removal. The paraffin oil was then removed from the

surface using filter paper (Whatman, Qualitative grade 1). For the removal of the templating

surfactant (Pluronic), the product was washed with EtOH (20 mL) for 24 h. For this purpose,

the product was slowly (to avoid breaking) placed in a beaker containing ethanol for 24 h.

The silica monolith was then dried in an oven at 50°C to constant weight. Once weight

constant, the BET surface area measurements of mesoporous silica monoliths were carried

out on the following day (see section 5.1.1 for measurement details and BET data).

3.2.2 Preparationofmesoporoussilicaparticles[15]Pluronic P123 (4.0 g, 0.020 mmol) was added to 2.0 M HCl (120 g, 81 mmol) and distilled

water (30 g, 41 mmol) and the mixture was stirred for 1 h at 35°C in an oil bath until a clear

solution was formed. TEOS (8.50 g, 1.00 mmol) was then added and the mixture was left for

14 h at 35°C. The mixture was then sonicated at 60˚C for 1 h using a Transonic T570/H

sonicator (Camlab, UK). Silica particles were separated through gravity filtration (sintered

glass funnel of porosity grade 5) and then washed with ethanol (97 wt%, 50g) in order to

remove the Pluronic. Silica particles were dried in a vacuum oven at 1 bar and 80°C for 24 h

and were stored afterwards in a sealed plastic container. Different characterisation techniques

including BET analysis (Section 5.1.2), particle sizing through light scattering technique

(Section 5.1.2) were also conducted. SEM images (Section 5.1.2) were also recorded for the

mesoporous silica particles.

3.2.3 Surfaceareaanalysis‐BETmethodFor the determination of the specific surface area (AS) and pore size distribution of

mesoporous silica from nitrogen adsorption isotherm at 77.35K (BET method), a

Micromeritics ASAP 2010 analyser (Micromeritics Ltd. UK) was used and the specific

surface area, AS, determined according to the industrial standard ISO 9277 [162].

The BET method is named after Brunauer, Emmet and Teller [163] who developed it in 1938

while working on ammonia catalysts. This was the first method to measure the specific

surface of finely divided and porous solids. Applications of the BET method range from

pharmaceuticals to catalysts, projectile propellants to medical implants, filters to cement etc

[162]. The BET method is a cheap, fast and reliable method and is very well understood and

applicable in many fields [162]. During the measurement, nitrogen molecules are weakly

adsorbed at a given pressure onto the mesoporous silica until saturation. The amounts

adsorbed are measured in equilibrium with the adsorptive gas pressure, p, and plotted against


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relative pressure, P/PO, to give an adsorption isotherm (Figure 3.7). An isotherm, in this case,

is the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or

concentration (if liquid) at constant temperature. The quantity adsorbed is nearly always

normalized by the mass of the adsorbent to allow comparison of different materials (Section

5.1).

The specific surface area, AS, is related to the total number of nitrogen gas molecules

adsorbed to mesoporous silica surface at a given pressure. Specific surface area, AS, is

calculated from the following equation,

1 1∙ (3.1)

where P/PO is the relative pressure, C is the BET constant, na is the number of nitrogen gas

molecules adsorbed, and nm is the specific monolayer amount of adsorbate.

Figure 3.7 Adsorption isotherms I to VI classified after IUPAC 1984 (image taken from P.

Somasundaran, 2006).

0.50 g of mesoporous silica was filled into a glass vessel and was degassed at 110°C for 12 h

in order to remove moisture. The specific surface area, AS, was determined by plotting

vs. . The intercept ( ) and slope ( ) of the resultant curve was used to

derive the value of nm and C. The specific surface area, AS, of mesoporous silica (Section 5.1)

can then be calculated from the equation,

(3.2)


90

where Am is is molecular cross sectional area (for nitrogen Am=0.162 nm2 at 77.3 K) and L is

the Avogadro’s number (= 6.022×1023) [162]. The specific surface area of mesoporous silica

is discussed in Section 5.1 and the tabulated data for the mesoporous silica samples can be

found in table 5.3.

3.2.4 Particlesizeanalyses‐Lightscatteringmethod

The size distribution of mesoporous silica particles was confirmed by light scattering

measurements using a Malvern Mastersizer 2000 particle size analyser (Malvern Instruments

Ltd., UK) that determined the average particle size (d0.5) according to the industrial standard

ISO 13320 [164].

During particle size distribution (PSD) measurements employing a laser diffraction

technique, particles scatter light in different directions with an intensity pattern. The light

scattering pattern is dependent on particle size, particle shape and the optical properties of the

particulate material [165]. In a light scattering apparatus, a light source, capable of emitting

laser light (or other narrow-wavelength source of light), generate a monochromatic consistent

parallel beam of light that is scattered by the particles at different angles. The scattered light

is then traced by focal plane detectors as shown in Figure 3.8.

Figure 3.8 Schematic of light scattering through laser diffraction by Malvern [166].

A silica suspension was prepared by dissolving dodecyl trimethyl ammonium bromide (0.020

g, 0.20 wt% ) in deionised water (10 g, 95 wt%) followed by the addition of mesoporous

silica particles (see Section 3.2.2 for their preparation/or characterisation data) (0.5 g, 4.8

wt%). The mixture was stirred at room temperature using a magnetic stirrer overnight to

ensure that a homogenous silica suspension was obtained. For a statistical average value, all


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measurements were repeated 5 times per condition and are discussed in Section 5.1.2. Particle

size distribution experiments were repeated five times and the mean diameter of the

maximum volume of MSP in the suspension is reported as d0.5 with an accuracy of ± 1% in

Section 5.1.2.

3.2.5 Scanningelectronmicroscopy(SEM)Scanning electron microscopy (SEM) is a method to obtain high resolution pictures of

surfaces. The SEM uses a beam of energetic electrons as compared to a typical optical

microscope that uses visible light. An SEM can provide much higher magnification

(>100,000 times) and much larger depth of field (>100 times) [167] than an optical

microscope.

Scanning electron microscopy was used as a qualitative tool for the mesoporous silica

particles. The surface morphology of mesoporous silica particles was observed using a LEO

Gemini 1525 FEG-Scanning Electron Microscope (Oberkochen, Germany). Samples were

attached directly to the SEM stubs with double sided carbon tape and were sputter coated

with gold (15 nm thickness, 1.5 min, 20 mA, Edwards, UK) as silica particles are electrically

insulating. The SEM micrographs of the MSP are shown in Figure 5.7.

3.3 Polymer electrolytes All polymer electrolyte systems were tested for mechanical compression performance and

ionic conductivity. PEGDGE and DGEBA polymer electrolytes were cut using Lathe

machine (Colchester Student, England) into cylinders with a 13 mm diameter and 25 mm

height for mechanical characterisation and 4 mm height for ionic conductivity measurements.

All the polymer electrolyte specimens were stored in air seal bags and characterisation

measurements were carried out within 2 days of preparation of the polymer electrolytes. The

results for the compression and ionic conductivity measurements of the polymer electrolyte

systems (reported in Chapter 4) were averaged from five individual measurements and the

error was reported as standard deviation. The ionic conductivity and compression properties

of polymer electrolyte systems can be found in Appendix A.

3.3.1 PreparationofcrosslinkedPEGDGEpolymerelectrolytes

3.3.1.1 PreparationofcrosslinkedPEGDGEelectrolytesusing TBAPF6 salt The synthesis of PEGDGE based polymer electrolyte involves PC as a solvent, PEGDGE and

TBAPF6 as conducting electrolyte and TETA as a hardener (crosslinker). TBAPF6 (0.020 g


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of 0.050 wt%, 0.040 mmol, 0.010 cm3) was dissolved in PC (0.36 g of 0.73 wt%, 3.6 mmol,

0.30 mL) and while still stirring, PEGDGE (46 g, 91 wt%, 86 mmol, 40 mL) was added and

stirred continuously until a homogeneous solution was obtained (approx. 5 min). TETA (4.2

g, 8.3 wt%, 28 mmol, 4.2 mL) was then added to the solution and the mixture was further

stirred for 10 min. The solution was then transferred to four 10 ml syringes (BD Plastipak, 13

mm inner diameter, no needle). The filled syringes were finally placed in an oven for 24 h at

80°C.

3.3.1.2 PreparationofcrosslinkedPEGDGEelectrolytesusing LiTFSI salt The synthesis of PEGDGE based polymer electrolyte involves PC as a solvent, PEGDGE,

LiTFSI as conducting electrolyte and TETA as a hardener (crosslinker). LiTFSI (0.090 g,

0.18 wt%, 0.32 mmol, 0.070 cm3) was dissolved in PC (0.30 g, 0.60 wt%, 3.0 mmol, 0.25

mL) and while still stirring, PEGDGE (46 g, 93 wt%, 88 mmol, 41 mL) was added and

stirring continued until a homogeneous solution was obtained (approx. 5 min). TETA (3.3 g,

6.6 wt%, 23 mmol, 3.4 mL) was then added to the solution and the mixture was further

stirred for 10 min. The solution was then transferred to four 10 ml syringes. The filled

syringes were finally placed in an oven for 24 h at 80°C.

3.3.1.3 PreparationofcrosslinkedPEGDGEelectrolytesusing EMITFSI ionic liquid The synthesis of PEGDGE based polymer electrolyte involves PEGDGE, EMITFSI as

conducting electrolyte and TETA as a hardener (crosslinker). EMITFSI (5.0 g, 10 wt%, 13

mmol, 3.3 mL) was mixed in PEGDGE (41 g, 83 wt%, 78 mmol, 36 mL) and was stirred on

magnetic stirrer until a homogeneous solution was obtained (approx. 5 min). TETA (3.7 g,

7.4 wt%, 26 mmol, 3.8 mL) was then added to the solution and the mixture was further

stirred for 10 min. The solution was then transferred to four 10 ml syringes. The filled

syringes were finally placed in an oven for 24 h at 80°C. PEGDGE based polymer electrolyte

samples were also prepared by increasing the wt% of EMITFSI from 10 wt% to 20 wt%, 30

wt%, 40 wt%, 50 wt% and 60 wt% (Section 4.3.2). The compositions of the PEGDGE

polymer electrolyte samples by increasing the EMITFSI concentrations are listed in Table

3.2.


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Sample code EMITFSI EMITFSI PEGDGE TETA

wt% (g) / (mmol) (g) / (mmol) (g) / (mmol)

E20P80 20% 10.0 / 25.6 36.6 / 69.6 3.40 / 23.2

E30P70 30% 15.0 / 38.3 32.1 / 61.0 2.90 / 19.8

E40P60 40% 20.0 / 51.1 27.5 / 52.3 2.50 / 17.1

E50P50 50% 25.0 / 63.9 22.9 / 43.5 2.10 / 14.4

Table 3.2 Composition of PEGDGE polymer electrolytes by increasing the concentrations of

EMITFSI.

3.3.2 PreparationofcrosslinkedDGEBAelectrolytes

3.3.2.1 PreparationofcrosslinkedDGEBAelectrolytesusing LiTFSI salt The synthesis of DGEBA based polymer electrolyte involves DGEBA, PC as a solvent,

LiTFSI as conducting electrolyte and MCHA as a hardener (Section 3.1.2). LiTFSI (1.16 g,

2.33 wt%, 4.06 mmol, 0.870 mL) was dissolved in PC (3.84g 7.67 wt%, 37.6 mmol, 3.20

mL) and the mixture was stirred for 15 min. DGEBA (35.3 g, 70.7 wt%, 104 mmol, 30.5 mL)

was added and the mixture was further stirred with a magnetic stirrer (300 rpm) in open air at

room temperature for 1 h. MCHA (9.66 g, 19.3 wt%, 45.9 mmol, 10.2 mL) was added to

mixture and was further stirred for 2-3 minutes and was then immediately shifted to four 10

mL syringes. The mixture was left for crosslinking for 2 days at room temperature inside the

syringes. DGEBA based polymer electrolyte samples were also prepared by increasing the

wt% of 1.0 M LiTFSI/PC from 10 wt% to 20 wt%, 40 wt%, 60 wt% and 80 wt% and the

compositions are reported in Table 3.3.

Sample code 1 M LiTFSI/PC LiTFSI PC DGEBA MCHA

wt% (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)

Li20B80 20% 2.33 / 8.12 7.67 / 75.1 32.3 / 94.9 7.70 / 36.6

Li40B60 40% 4.66 / 16.2 15.3 / 150 24.2 / 71.1 5.84 / 27.8

Li60B40 60% 6.99 / 24.3 23.0 / 225 16.2 / 47.6 3.81 / 18.1

Li80B20 80% 9.32 / 32.5 30.7 / 301 8.08 / 23.7 1.90 / 9.03

Table 3.3 Composition of DGEBA polymer electrolytes by increasing the concentrations of 1

M LiTFSI/PC.


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3.3.2.2 PreparationofcrosslinkedDGEBAelectrolytesusing EMITFSI ionic liquid The synthesis of DGEBA based polymer electrolyte involves DGEBA, EMITFSI as a

conducting electrolyte and MCHA as a hardener (crosslinker). EMITFSI (5.00 g, 10.0 wt%,

12.8 mmol, 3.28 mL) was mixed in DGEBA (35.3 g, 70.7 wt%, 104 mmol, 30.5 mL) and the

mixture was stirred through magnetic stirrer (300 rpm) in open air at room temperature for 1

h. MCHA (9.66 g, 19.3 wt%, 45.9 mmol, 10.2 mL) was added in mixture and was further

stirred for 2-3 minutes and was then shifted to four 10 mL syringes and was left for

crosslinking for 2 days at room temperature.

3.3.3 PreparationofPANgelbasedpolymerelectrolytes

The preparation of PAN gel electrolyte involves PAN as polymer matrix, EC as a plasticiser,

TBAPF6 as conducting electrolyte, and PC as a solvent. PAN (3.60 g, 7.20 wt%) was dried in

a vacuum oven (1 bar) for 24 h at 75°C. TBAPF6 (1.60 g, 3.27 wt%, 4.22 mmol) was

dissolved at room temperature in a mixture of EC (26.8 g, 53.5 wt%, 304 mmol, 20.3 mL)

and PC (18.0 g, 36.0 wt%, 176 mmol, 2.2 mL) and the solution was stirred for 30 min to

make it homogeneous. PAN was then added to this homogeneous solution. This mixture was

then placed on a preheated hot plate (110°C); attached with thermocouple, under slow

stirring. After about 3-4 min, a very viscous solution was formed. This PAN gel was named

as PAN1. PAN1 was stored in bottle at room temperature before using it for rheological as

well as electrochemical characterisation measurements (Section 4.2). Oscillatory rheological

tests were conducted for these PAN gel based polymer electrolyte (Figure 4.2).

For the preparation of a second PAN gel electrolyte (PAN2, rheological characterisation after

3 days of preparation), PAN (13.0 g, 26.0 wt%) was dried in a vacuum oven (1 bar) for 24 h

at 75°C. TBAPF6 (10.2 g, 20.4 wt%, 26.3 mmol) was dissolved at room temperature in a

mixture of EC (18.3 g, 36.6 wt%, 208 mmol, 13.9 mL) and PC (8.50 g, 17 wt%, 83.2 mmol,

7.08 mL) and the solution was stirred for 30 min to make it homogeneous. PAN was then

added slowly to this homogeneous solution in order for complete dissolution. This mixture

was then placed on a preheated hot plate (110°C), attached with thermocouple, under slow

stirring. After about 3-4 min, a very viscous solution was formed. This PAN gel was named

as PAN2. PAN2 was stored in bottle at room temperature before using it in rheological as

well as electrochemical characterisation measurements (Table 4.2). The reported results for

all the characterisation measurements for PAN gel polymer electrolytes (Section 4.2) were

averaged from five individual measurements and the error was reported as standard deviation.


95

3.3.4 PreparationofcrosslinkedPEGDGE/DGEBAelectrolytes

3.3.4.1 Preparationofcrosslinked PEGDGE/DGEBA electrolytes using 10 wt% LiTFSI salt The synthesis of crosslinked PEGDGE/DGEBA electrolyte involves PEGDGE and DGEBA

as polymer matrix, PC as a solvent, LiTFSI as a conducting electrolyte, combined TETA and

MCHA as hardeners (crosslinkers). The preparation was identical for all polymer electrolytes

(other than quantities) and the following procedures describe the preparation of the

PEDGE/DGEBA 80:20 polymer electrolyte.

LiTFSI (1.17 g, 2.33 wt%, 4.07 mmol, 0.870 mL) was dissolved in PC (3.83 g, 7.67 wt%,

37.5 mmol, 3.20 mL) and the mixture was stirred for 15 min and then DGEBA (7.07 g, 14.1

wt%, 20.8 mmol, 6.09 mL) and PEGDGE (33.0 g, 66.0 wt%, 62.7 mmol, 28.9 mL) was

added and the mixture was further stirred in magnetic stirrer (300 rpm) in open air at room

temperature for 1 h. MCHA (1.93 g, 3.86 wt%, 9.18 mmol, 2.03 mL) and TETA (3.02 g, 6.05

wt%, 20.7 mmol, 3.08 mL) were added in mixture and further stirred for 2-3 minutes. The

mixture was transferred quickly to a 10 mL syringe and left to crosslink for 2 days at room

temperature. The crosslinked polymer was then named as Li10:80P:20B polymer electrolytes

blend. Other weight ratios of crosslinked PEGDGE to crosslinked DGEBA were also

prepared by varying the PEGDGE to DGEBA weight ratio i.e. 60P:40B, 40P:60B, 20P:80B

with P referred to PEGDGE and B referred to DGEBA respectively; the compositions of the

remaining polymer electrolytes are tabulated in Table 3.4.

Sample code

Ionic Electrolyte Polymer 1 (P) Polymer 2 (B)

LiTFSI PC PEGDGE TETA DGEBA MCHA

(g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)

Li10:60P:40B 1.17 / 4.08 3.83 / 37.5 24.7 / 47.0 2.30 / 15.7 14.1 / 41.4 3.90 / 18.5

Li10:40P:60B 1.17 / 4.08 3.83 / 37.5 16.5 / 31.4 1.50 / 10.2 21.2 / 62.3 5.80 / 27.6

Li10:20P:80B 1.17 / 4.08 3.83 / 37.5 8.25 / 15.7 0.75 / 5.13 28.3 / 83.1 7.70 / 36.6

Table 3.4 Composition of PEGDGE:DGEBA blend polymer electrolytes with 10wt% of 1 M

LiTFSI/PC by varying the PEGDGE and DGEBA concentrations.

3.3.4.2 Preparationofcrosslinked PEGDGE/DGEBA electrolytes using 10 wt% EMITFSI ionic liquid The synthesis of crosslinked PEGDGE/DGEBA electrolyte involves PEGDGE and DGEBA

as polymer matrices, EMITFSI as a conducting electrolyte, combined TETA and MCHA as


96

hardeners (crosslinkers). EMITFSI (5.00 g, 10.0 wt%, 12.8 mmol, 3.28 mL) was mixed in

DGEBA (7.07 g, 14.1 wt%, 20.8 mmol, 6.09 mL) and PEGDGE (33.0 g, 66.0 wt%, 62.7

mmol, 28.9 mL) and the mixture was stirred on magnetic stirrer (300 rpm) in open air at

room temperature for 1 h. MCHA (1.93 g, 3.86 wt%, 9.18 mmol, 2.03 mL) and TETA (3.02

g, 6.05 wt%, 20.7 mmol, 3.08 mL) were added in mixture, further stirred for 2-3 minutes, and

then immediately transferred to the 10 mL syringe. The mixture was left for crosslinking for

2 days at room temperature. The crosslinked polymer was then named as E10:80P:20B

polymer electrolytes blend. The specimens were stored in air seal bags before all the

characterisation measurements. Other weight ratios of PEGDGE to DGEBA were also

prepared, using the identical procedure described above, by varying the PEGDGE to DGEBA

weight ratio i.e. 60P:40B, 40P:60B, 20P:80B with P referred to PEGDGE and B referred to

DGEBA respectively and the compositions of polymer electrolytes are tabulated in Table 3.5.

Sample code


EMITFSI PEGDGE TETA DGEBA MCHA

(g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)

E10:60P:40B 5.00 / 12.8 24.7 / 47.0 2.30 / 15.7 14.1 / 41.4 3.90 / 18.5

E10:40P:60B 5.00 / 12.8 16.5 / 31.4 1.50 / 10.3 21.2 / 62.3 5.80 27.6

E10:20P:80B 5.00 / 12.8 8.35 / 15.9 0.85 / 5.81 28.0 / 82.2 7.80 / 37.1

Table 3.5 Composition of PEGDGE:DGEBA blend polymer electrolytes with 10wt% of

EMITFSI by varying the PEGDGE and DGEBA concentrations.

3.3.4.3 Preparationofcrosslinked PEGDGE/DGEBA electrolytes using 50 wt% EMITFSI ionic liquid The synthesis of crosslinked PEGDGE/DGEBA electrolyte involves PEGDGE and DGEBA

as polymer matrix, EMITFSI as a conducting electrolyte, combined TETA and MCHA as

hardeners (crosslinkers). EMITFSI (25.0 g, 50.0 wt%, 63.9 mmol, 16.4 mL) was mixed in

DGEBA (3.93 g, 7.85 wt%, 11.5 mmol, 3.38 mL) and PEGDGE (18.3 g, 36.6 wt%, 34.8

mmol, 16.1 mL) and the mixture was stirred on magnetic stirrer (300 rpm) in open air at

room temperature for 1 h. MCHA (1.07 g, 2.15 wt%, 5.10 mmol, 1.13 mL) and TETA (1.67

g, 3.34 wt%, 11.4 mmol, 1.71 mL) were added in mixture, further stirred for 2-3 minutes and

then immediately transferred to the 10 mL syringe. The mixture was left for crosslinking for

2 days at room temperature. The crosslinked polymer was then named as E50:80P:20B

polymer electrolytes blend. Other weight ratios of crosslinked PEGDGE to crosslinked


97

DGEBA were also prepared, using the identical procedure described above, by varying the

PEGDGE to DGEBA weight ratio i.e. 60P:40B, 40P:60B, 20P:80B with P referred to

PEGDGE and B referred to DGEBA respectively and the compositions of polymer

electrolytes are tabulated in Table 3.6.

Sample code


EMITFSI PEGDGE TETA DGEBA MCHA

(g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)

E50:60P:40B 25.0 / 63.9 13.8 / 26.2 1.20 / 8.21 7.85 / 23.1 2.15 / 10.2

E50:40P:60B 25.0 / 63.9 9.15 / 17.4 0.83 / 5.68 11.8 / 34.7 3.22 / 15.3

E50:20P:80B 25.0 / 63.9 4.58 / 8.70 0.43 / 2.94 15.7 / 46.1 4.29 / 20.4

E50:0P:100B 25.0 / 63.9 N/A N/A 19.6 / 57.6 5.40 / 25.7

Table 3.6 Composition of PEGDGE:DGEBA blend polymer electrolytes with 50 wt% of

EMITFSI by varying the PEGDGE and DGEBA concentrations.

3.4 Composite polymer electrolytes All composite polymer electrolyte systems were tested for mechanical compression

performance and ionic conductivity. The samples were produced as cylinders having flat and

parallel ends by cutting the composite polymer electrolyte systems (24 mm height and 13 mm

diameter for the mechanical characterisation measurements and 4 mm height and 13 mm

diameter for the electrochemical measurements) on Lathe machine (Colchester Student,

England). The samples were stored in a sealed plastic bag at room temperature and

characterisation measurements were carried out within 2 days of preparation of composite

polymer electrolytes. The results for the compression and ionic conductivity measurements of

the composite polymer electrolyte systems (reported in Chapter 5) were averaged from five

individual measurements and the error was reported as standard deviation. The

electrochemical and mechanical characterisation results of the composite polymer electrolyte

systems can be found in Appendix A.

3.4.1 PreparationofMSP/PEGDGEcompositepolymerelectrolytes

3.4.1.1 Preparationofcrosslinked MSP/PEGDGE composite polymer electrolytes using

LiTFSI salt

LiTFSI (0.100 g, 0.180 wt%, 0.348 mmol, 0.0610 mL) was dissolved in PC (0.300 g, 0.590

wt%, 2.94 mmol, 0.240 mL) and MSP (Section 3.2.2) (1.25g, 2.50 wt%) were added as a


98

powder. While still stirring on magnetic stirrer (500 rpm), PEGDGE (45.1 g, 90.2 wt%, 85.7

mmol, 39.3 mL) was added and stirring continued for 5 h at room temperature at which point

a homogeneous solution was obtained. TETA (3.25 g, 6.51 wt%, 22.2 mmol, 3.30 mL) was

added to the solution and stirring continued for 15 min. The solution was transferred into a

mould of 10 ml syringe and the syringes were finally placed in an oven for 24 h at 80°C. The

reported results for the electrochemical (Section 3.5) and mechanical (Section 3.6.2)

characterisation measurements of MSP/PEGDGE based polymer electrolytes with 0.80 wt%

of 1 M LiTFSI/PC (see Table A.7 of Appendix A) were averaged from five individual

measurements. An identical procedure was adopted by increasing the wt% of MSP from 2.50

wt% to 5.00 wt%, 7.50 wt%, 10.0 wt% and 12.5 wt% in composite PEGDGE based polymer

electrolytes and were characterised mechanically as well as electrochemically (Table A.7 of

Appendix A). The compositions of other PEGDGE polymer electrolytes by increasing the

MSP concentration are reported in Table 3.7.

Sample code MSP LiTFSI PC MSP PEGDGE TETA

wt% (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)

Li0.80P100M5.0 5.00 0.10 / 0.35 0.30 / 2.94 2.50 / 41.7 43.9 / 83.5 3.20 / 21.9

Li0.80P100M7.5 7.50 0.10 / 0.35 0.30 / 2.94 3.75 / 62.5 42.8 / 81.4 3.05 / 20.9

Li0.80P100M10 10.0 0.10 / 0.35 0.30 / 2.94 5.00 / 83.4 41.6 / 79.1 3.00 / 20.5

Li0.80P100M12.5 12.5 0.10 / 0.35 0.30 / 2.94 6.25 / 104 40.4 / 76.8 2.95 / 20.2

Table 3.7 Composition of MSP/PEGDGE composite polymer electrolytes with 0.8wt% of 1 M

LiTFSI/PC by varying the MSP concentrations.

3.4.1.2 Preparationofcrosslinked MSP/PEGDGE composite polymer electrolytes using

EMITFSI ionic liquid

EMITFSI (5.00 g, 10.0 wt%, 12.8 mmol, 3.28 mL) was mixed in mesoporous silica particles

(1.25 g, 2.50 wt%, 20.8 mmol, 0.690 mL) and the mixture was stirred for 30 min followed by

the addition of PEGDGE (40.1 g, 80.2 wt%, 76.2 mmol, 35.2 mL). The mixture was stirred

again on magnetic stirrer (300 rpm) in open air at room temperature for 5 h. TETA (3.67 g,

7.35 wt%, 25.1 mmol, 3.75 mL) was added in mixture and was further stirred for 2-3

minutes. The solution was then transferred into 10 ml syringes and was finally placed in an

oven for 24 h at 80°C. Similar procedure was adapted by increasing the wt% of MSP from

2.50 wt% to 5.00 wt%, 7.50 wt%, 10.0 wt% and 12.5 wt% in composite PEGDGE based

polymer electrolytes and were characterised mechanically as well as electrochemically (see


99

Table A.9 of Appendix A). The compositions of other PEGDGE polymer electrolytes by

increasing the MSP concentration are reported in Table 3.8.

Sample code MSP EMITFSI MSP PEGDGE TETA

wt% (g) / (mmol) (g) / (mmol) (g) / (mmol) (g) / (mmol)

E10P100M5.0 5.00 5.00 / 12.8 2.50 / 41.7 38.9 / 74.0 3.60 / 24.6

E10P100M7.5 7.50 5.00 / 12.8 3.75 / 62.5 37.8 / 71.9 3.45 / 23.6

E10P100M10 10.0 5.00 / 12.8 5.00 / 83.4 36.6 / 69.6 3.40 / 23.3

E10P100M12.5 12.5 5.00 / 12.8 6.25 / 104 35.5 / 67.5 3.25 / 22.2

Table 3.8 Composition of MSP/PEGDGE composite polymer electrolytes with 10wt% of

EMITFSI by varying the MSP concentrations.

3.4.2 PreparationofcrosslinkedMSP/DGEBAcompositepolymerelectrolytesLiTFSI (2.33g, 4.66 wt%, 8.12 mmol, 1.75 mL) was dissolved in PC (7.67 g, 15.3 wt%, 75.1

mmol, 6.39 mL) and after that, MSP (Section 3.2.2) (1.25 g, 2.50 wt%) were added. While

still stirring, DGEBA (30.4 g, 60.9 wt%, 89.4 mmol, 26.2 mL) was added and stirring

continued for 5 h at room temperature at which point a homogeneous solution was obtained.

MCHA (8.31 g, 16.6 wt%, 39.5 mmol, 8.75 mL) was then added to the solution and was

further stirred for 15 min. The solution was then transferred into 10 ml syringes and was

finally placed in an oven for 24 h at 80°C. Identical procedure was adopted by increasing the

wt% of MSP from 2.50 wt% to 5.00 wt%, 7.50 wt%, 10.0 wt% and 12.5 wt% in composite

DGEBA based polymer electrolytes and were characterised mechanically (Section 3.6.2) as

well as electrochemically (Section 3.5) and are tabulated in Table A.10 of Appendix A. The

composition of the remaining composite polymer electrolytes are reported in Table 3.9.

Sample code

MSP MSP LiTFSI PC DGEBA MCHA

wt% (g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

Li20B100M5.0 5.00 2.50 / 41.7 2.33 / 8.12 7.67 / 75.1 29.4 / 86.4 8.10 / 38.5

Li20B100M7.5 7.50 3.75 / 62.5 2.33 / 8.12 7.67 / 75.1 28.5 / 83.7 7.75 / 36.8

Li20B100M10 10.0 5.00 / 83.3 2.33 / 8.12 7.67 / 75.1 27.5 / 80.8 7.50 / 35.6

Li20B100M12.5 12.5 6.25 / 104 2.33 / 8.12 7.67 / 75.1 26.5 / 77.8 7.25 / 34.4

Table 3.9 Composition of DGEBA/MSP composite polymer electrolytes with 20wt% of 1 M

LiTFSI/PC by varying the MSP concentrations.


100

3.4.3 PreparationofcrosslinkedMSP/PEGDGE/DGEBAcompositepolymerelectrolytes

3.4.3.1 Preparationofcrosslinked MSP/PEGDGE/DGEBA composite polymer electrolytes using LiTFSI salt LiTFSI (2.33 g, 4.66 wt%, 8.12 mmol, 1.75 mL) was dissolved in PC (7.67 g, 15.3 wt%, 75.1

mmol, 6.39 mL) and after that, mesoporous silica particles (1.25 g, 2.50 wt%) were added

(Section 3.2.2) under stirring followed by PEGDGE (14.2 g, 28.4 wt%, 27.0 mmol, 12.4 mL)

and DGEBA (18.3 g, 36.5 wt%, 53.6 mmol, 15.7 mL). The mixture was stirred again on

magnetic stirrer (300 rpm) in open air at room temperature for 5 h. TETA (1.30 g, 2.60 wt%,

8.90 mmol, 1.33 mL) and MCHA (4.99 g, 9.97 wt%, 23.7 mmol, 5.25 mL) was added in

mixture and was further stirred for 2-3 minutes. The solution was then transferred into 10 ml

syringes and was finally placed in an oven for 24 h at 80°C. Similar procedure was adapted

by increasing the wt% of MSP from 2.5 wt% to 5 wt%, 7.5 wt%, 10 wt% and 12.5 wt% in

composite PEGDGE based polymer electrolytes and were characterised mechanically as well

as electrochemically (Table A.11). The composition of the remaining composite polymer

electrolytes are reported in Table 3.10.

Sample code

Samples Electrolyte MSP

Polymer 1 (P) Polymer 2 (B)

MSP LiTFSI PC PEGDGE TETA DGEBA MCHA

wt% (g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

Li20:40P:60B:M5.0 5.00 2.33 /

8.12

7.67 /

75.1

2.50 /

41.7

13.7 /

26.0

1.26 /

8.62

17.7 /

52.0

4.84 /

23.0

Li20:40P:60B:M7.5 7.50 2.33 /

8.12

7.67 /

75.1

3.75 /

62.5

13.3 /

25.3

1.21 /

8.27

17.1 /

50.2

4.64 /

22.1

Li20:40P:60B:M10 10.0 2.33 /

8.12

7.67 /

75.1

5.00 /

83.4

12.8 /

24.3

1.19 /

8.14

16.5 /

48.5

4.51 /

21.4


20wt% of 1 M LiTFSI/PC by varying the MSP concentrations.

3.4.3.2 Preparationofcrosslinked MSP/PEGDGE composite polymer electrolytes using EMITFSI ionic liquid EMITFSI (25.0 g, 50.0 wt%, 63.9 mmol, 16.4 mL) was mixed in mesoporous silica particles

(Section 3.2.2) (1.25 g, 2.50 wt%, 20.8 mmol, 0.690 mL) and the mixture was stirred for 30

min followed by the addition of PEGDGE (8.71 g, 17.4 wt%, 16.6 mmol, 7.64 mL) and


101

DGEBA (11.2 g, 22.4 wt%, 32.9 mmol, 9.65 mL). The mixture was stirred again on magnetic

stirrer (300 rpm) in open air at room temperature for 5 h. TETA (0.790 g, 1.59 wt%, 5.43

mmol, 0.810 mL) and MCHA (3.06 g, 6.12 wt%, 14.5 mmol, 3.22 mL) was added in mixture

and was further stirred for 2-3 minutes. The solution was then transferred into 10 ml syringes

and was finally placed in an oven for 24 h at 80°C. Similar procedure was adapted by

increasing the wt% of MSP from 2.50 wt% to 5.00 wt%, 7.50 wt%, 10.0 wt% and 12.5 wt%

in composite PEGDGE based polymer electrolytes and were characterised mechanically as

well as electrochemically (Table A.13). The composition of the remaining composite

polymer electrolytes are reported in Table 3.11.

Sample code

MSP EMITFSI MSP Polymer 1 (P) Polymer 2 (B)

PEGDGE TETA DGEBA MCHA

wt% (g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

(g) /

(mmol)

E50:40P:60B:M5.0 5.00 25.0 / 63.9 2.50 / 41.7 8.25 / 15.7 0.75 / 5.13 10.6 / 31.1 2.90 / 13.8

E50:40P:60B:M7.5 7.50 25.0 / 63.9 3.75 / 62.5 7.79 / 14.8 0.71 / 4.86 10.0 / 29.4 2.75 / 13.1

E50:40P:60B:M10 10.0 25.0 / 63.9 5.00 / 83.4 7.33 / 13.9 0.67 / 4.58 9.42 / 27.7 2.58 / 12.3

E50:40P:60B:M12.5 12.5 25.0 / 63.9 6.25 / 104 6.87 / 13.1 0.63 / 4.31 8.83 / 25.9 2.42 / 11.5


50wt% of EMITFSI by varying the MSP concentrations.

3.5 Chemical Activation of carbon fibre mats

Activated carbon fibre mats were provided by Dr Hui Qian and were used as received.

Chemical activation of carbon fibre involves impregnation of carbon fibres with thick

potassium hydroxide (KOH) slurry followed by heat treatment. A 30×21 cm sized carbon

fibre mat was placed in Pyrex® tray while slurry of 1.85 M KOH in water (typical KOH

loading was approximately 6.5 wt% after drying) was transferred onto the fibre mat. The

sample was left inside a fume cupboard for 3 hours before the excess KOH was dispensed

away and the cloth was placed on a PET (PolyEthylene Terephthalate) film to dry overnight.

Subsequently, the dried carbon fibre fabric was transferred onto a perforated stainless steel

frame and positioned into the retort (Lenton ECF 12/30) where it was heated at 5˚C/min to

800˚C under nitrogen (flow rate of 0.5 L/min) and held at temperature for 30 min. The system

was allowed to cool to room temperature while in inert nitrogen atmosphere over

approximately 14 h. The resultant activated carbon fibres mat was then washed extensively


102

with deionised water to eliminate any residual alkali until pH7, and was finally dried in a

vacuum oven at 80 ˚C. The typical mass loss was around 6 wt%. The characteristics of

activated carbon fibre are shown in Table 3.12.

CF mats df

(μm)

As

(m2/g)

Cg

(F/g)

ET

(GPa)

σT

(MPa)

As received 7 0.21 0.06 3.29 ± 0.09 204 ± 4

Activated 6.9 21.4 2.63 3.96 ± 0.13 207 ± 4

Table 3.12 Single fibre diameter df, BET surface area As, specific capacitance Cg, tensile

modulus ET and tensile strength σT of as-received and activated carbon fibre mats (results

courtesy of Dr. Hui Qian).

3.6 Electrochemical impedance spectroscopy of polymer electrolytes

Electrochemical impedance spectroscopy (EIS) is a useful technique for the investigation of a

wide range of bulk and interfacial electrical properties associated with materials where ionic

conduction is in prime consideration e.g. ionic salts, liquid electrolytes, solid and gel polymer

electrolytes and ionic conductive glasses [168]. EIS is widely employed in the study of

rechargeable batteries, supercapacitors and fuel cells. EIS is usually determined by applying

an AC potential to an electrochemical cell and measuring the current through the cell [169].

The AC potential penetrates into the bulk pores of the electrochemical cell and shows the

amount of solvated ions at a specific frequency that reach the pore surface [170].

Electrochemical impedance is defined as,

exp Φ Φ Φ (3.3)

where Z0 is the magnitude of Z ( / and Ф is the phase angle of current I

versus potential difference E.

The real part of impedance is usually plotted against the imaginary part of impedance to get a

complex plot called Nyquist plot.

For the electrochemical characterisation of polymer electrolytes (Section 3.4), an Ivium-n-

Stat Multichannel potentiostat (Ivium Technologies, The Netherlands) was used. Ionic

conductivity for liquid electrolytes was measured using JenWay 4330 conductivity and pH

metre (UK). The impedance measurements were conducted in frequency window of 10-1 Hz

to 105 Hz and the amplitude of sinusoidal voltage was 0.5 V. Detailed procedures for the


103

calculation of ionic conductivity of polymer electrolytes (Section 4.3) are given in Appendix

B. Ionic conductivity of all polymer electrolytes (Section 3.3 and Section 3.4) can be found in

Appendix A.

3.7 Mechanical characterisation of polymer electrolytes

3.7.1 RheologicalcharacterisationofPANgelpolymerelectrolytes

In order to mechanically characterise the polyacrylonitrile gel based polymer electrolyte

(Section 3.3.3), oscillatory rheological tests were conducted at room temperature (PAAR

Physica UDS200 Universal Dynamic Spectrometer (Germany), by using a concentric,

cylinder type (Z3 DIN with bob and cup gap of 1.06 mm) measuring system. Frequency

sweep tests of PAN gel polymer electrolytes (see Section 3.3.3 for preparation) were

conducted in a frequency window of 0.01 to 100 Hz at a strain rate of 0.1% and temperatures

of 25°C, 40°C and 90°C. Temperature sweep tests on these PAN gel polymer electrolytes

were conducted in a temperature window of 25°C to 120°C at a frequency of 0.1 Hz and a

strain rate of 0.1%.

At constant frequency, ω, and strain amplitude, γ0, the polyacrylonitrile gel based polymer

electrolyte was deformed sinusoidally (γ = γ0sinωt). As a result, the stress, τ, started

oscillating sinusoidally at the same frequency, ω, (radial frequency, ω= 2πf) after few start-up

cycles (generally it is shifted by a phase angle, δ, with respect to strain wave [171]). This can

be represented by two wave forms as shown in Figure 3.9. Figure 3.9 shows that the

oscillating stress, τ, is produced as a result of oscillating strain, γ, but is shifted by phase

angle, δ, as at lower frequencies, response is of viscous liquid [172]. The stress wave is

broken down into two waves. τ/ is the wave in-phase with the strain wave and τ// is the wave

90° out-of-phase of the strain wave. However, τ// wave is in-phase with the strain rate wave,

, (i.e.γ dγ ⁄ dt). The maximum stress, τ0, divided by the maximum strain, γ0, is a constant

for a given frequency ω and is called complex modulus, G*.

∗ / (3.4)

where G* is a complex number and has a real part G/ and imaginary part G//.


104

Figure 3.9 An oscillating shear strain and the stress response for viscoelastic materials

[171].

The stress wave can be analyzed by decomposing it into two waves of same frequency. Thus,

the resulting stress has following two dynamic moduli:

τ =γ0 [G/sinωt+ G//cosωt], where G/ is the elastic or shear modulus and is in-phase with the

deformation. G// is the loss or viscous modulus and is 90° out-of-phase with deformation. G//

is measure of energy dissipated per cycle of deformation per unit volume. Loss or dissipation

factor tan δ is the ratio of the loss component (G//) to the storage component (G/). The value

of the loss or dissipation factor (tan δ) characterises the ratio of viscous-to-elastic properties

in viscoelastic materials. By decreasing δ, and consequently decreasing viscous losses,

material transits from pure viscous to pure elastic.

(3.5)

3.7.2 Mechanicalcharacterisationofsolidpolymerelectrolytes(Compressiontesting)

Compression properties are important in engineering practise to determine the compression

modulus and compression strength while the material is in service. Compression testing of

polymer electrolytes was conducted because the testing is relatively simple and require small

amount of specimen material with relatively simple geometry [173]. Unlike tensile testing,

compression testing does not require expensive grips [173]. Compression tests were

conducted between the plates of a compression testing machine (Easy 50, Lloyds

Instruments, UK) having 50 kN load cell and 50 kN frame in accordance to the ASTM

standard D695 [173]. The samples were prepared as cylinders (see section 3.3) of


105

approximately 13 mm in diameter and 25 mm in length having parallel flat ends. Specimens

were cut using scalpel and ends were made parallel flat by using abrasive sheet. Specimens

were placed between two PTFE plates (50 mm diameter, 15 mm height, RS Components,

UK) in order to avoid slippage and compression force was applied in an axial direction to the

faces of the specimen. The compression strength σ, maximum stress supported by the

samples, was calculated from the following equation 3.6.

(3.6)

where is the maximum compression force carried by a polymer electrolyte sample

during the test and A is the original minimum cross-sectional area of polymer electrolyte

sample. Cross-head deflection speed of testing was 1 mm/min. Compression stiffness was

determined by recording the force-displacement graph and linearly fitting tangent to the

steepest part of the plot (0.02 %-0.25 % of the strain applied). Five tests were conducted on

nominally identical polymer electrolyte samples (Specimens prepared from the same polymer

electrolyte preparation but cut out from a different mould (syringe) (containing cured

polymer electrolyte) prepared in a same batch (Section 3.3) and the results were reported as

an average value with errors as standard deviation. The test machine compliance was

accounted for when determining the compressive modulus values (see Appendix C for

details).

3.8 Composite fabrication using Resin Infusion under Flexible Tooling (RIFT)

Composites were fabricated using a resin infusion under flexible tooling (RIFT) process.

During a RIFT process, resin is allowed to impregnate into a dry fibre pack loaded into a

vacuumed bagging flexible film. The flow of resin was due to vacuum drawn under the film.

The vacuum bag had two ends connected with tubing in which one of the tubing delivered the

resin while the other maintained a negative pressure in the vacuum bag. The negative

pressure removed the air from the stack of dry laminates and thus resulted in a minimise

voids. A schematic of a RIFT process is shown in Figure 3.10.


106

Hot PlateResin

Inlet OutletLaminate 1 Laminate 2

VacuumPump

Vacuum bag

Resin Diffusion membrane

Release fabric

Melinex film

Flashtape 1

Vacuum bagging sealent

FEP tube

Figure 3.10 Schematic of a RIFT process.

During the composite fabrication by RIFT process, 800 mm×430 mm of polyester based

Melinex® film (50 µm thickness, PW 122-50-RL, PSG group, UK) was first attached on a

heating plate (920mm×460 mm, Wenesco, Inc., USA) using a high temperature resistant

polyester based adhesive tape (Silicon adhesive, Flashtape 1, Aerovac, UK). The heating

plate was coupled to a temperature controller (Wenesco, Inc., USA). 700 mm×330mm PTFE

coated glass release fabric (FF03PM, Aerovac, UK) was placed on top of the Melinex® film

followed by a 500mm×330mm polyester flow media based resin diffusion membrane (warp

knitted diffusion knap, 15087B, Newbury engineer textile, UK). Another PTFE coated glass

release fabric (700 mm×330 mm) was placed on top of the resin diffusion membrane. Two

carbon fibre mats (180 mm×140 mm) were cut at ±45° through a 45°angled square set

protractor (RS number 663-768, RS Components, UK) and copper tape (copper foil coated

with an electrically conductive acrylic adhesive, 0.035 mm thickness, 25 mm width, 542-

5511, RS components, UK) was applied around the corners of the carbon fibre mats

(specifications of the carbon fibre mats can be found in Section 3.1.8). Carbon fibre mats

were then placed in hot press (Carver, UK) at 100°C for 6 h under pressure of 1 ton. Two

glass fibre mats (220 mm×180 mm) were cut at ± 45° through a 45° angled square set

protractor and were sandwiched between two ± 45° cut carbon fibre mats (180 mm×140

mm). The laminate was laid up by hand ensuring that the lay-up was balanced (i.e. the crimp


107

lines of the CF plies were mirrored around the midplane). Two copper wires (RS No. 177-

0621, 7 strands/0.1 mm, RS components, UK) were also placed in laminate assembly such

that each copper wire was placed between carbon fibre and glass fibre mats (see Section 6.7

for the effect of copper tape). Another release fabric and resin diffusion membranes were

placed on top of the laminate assembly. Two 700mm long Legris fluoropolymer FEP tubes

(RS components, UK) possessing an inner diameter of 6 mm were connected on each end of

the setup using a vacuum bag sealant tape (SM5127, Aerovac, UK). One of the tubes was

used as resin inlet and the other one connected to the vacuum pump (Island Scientific, UK).

The whole setup was then covered with a 600 mm×1000 mm Capran-518 heat stabilised

Nylon 6 blown tubular vacuum bagging film (Aerovac, UK) which was sealed using a

vacuum bag sealant tape. Composite fabrication by RIFT process is shown in Figure 3.11.

Figure 3.11 Vacuum bag during RIFT process (a) Rift setup, (b) sandwiched CF and GF

mats before RIFT process, (c) Structural supercapacitors after RIFT process.


108

The resin inlet tube was blocked by bending it several times and then wrapping it with

Flashtape-1. Vacuum was applied from the second tube connected to a vacuum pump (Rotary

pump, Model EDM6, Island Scientific, UK). Laminates were left under vacuum for 20 min at

1 bar (pressure determined using pressure gauge attached to RIFT setup). After 20 min of

vacuum, the resin inlet tube was unblocked and was dipped in polymer electrolyte container

so that polymer electrolyte started flowing through the laminate assembly. The resin inlet

tube was blocked again when the polymer electrolyte (approximately 350 g in total was

needed for the fabrication of 4 supercapacitors) completely went through the laminate

assembly. The laminates were left to cure at 80°C (temperature reading from controller of the

RIFT setup) for 24 h in the RIFT setup. The structural supercapacitors were removed from

the RIFT setup after 24 hr and were used for electrochemical and mechanical testing. The

structural supercapacitors were stored in air-tight plastic bags. The electrochemical (see

Section 3.8) and mechanical (see Section 3.9) testing was conducted within 7 days of

composite fabrication.

3.9 Electrochemical characterisation of structural supercapacitors

For the electrochemical characterisation of structural supercapacitors, an Ivium-n-Stat

Multichannel potentiostat (Ivium Technologies, The Netherlands) was used. The most widely

used electrochemical analyses for the characterisation of energy storage devices are:

1) Cyclic voltammetry method

2) Charge-discharge method

3) Electrochemical impedance spectroscopy

3.9.1 Cyclicvoltammetry

Cyclic voltammetry is a relatively quick and simple method as it can give directly the

accessible capacitance (C) as a response current (I), given by,

(3.7)

where dE/dt is the voltage sweep rate (s). Typical sweep rates are of the order of 3mV s-1 to 1

V s-1 but the values can be larger. The maximal energy density of structural supercapacitors

can be obtained from the capacitance as shown in equation 3.8,

12

(3.8)


109

where E is the maximal energy density (J/kg), C the capacitance (F), V the potential

difference applied in (V) and m the mass of structural supercapacitor. Cyclic voltammetry

tests for the structural supercapacitors were conducted using sweep rates of 3 mV s-1, 20 mV

s-1 and 50 mV s-1 in the voltage range of -0.5 V to 0.5 V as in this voltage range water

decomposition can be avoided. The capacitance [Chapter 6] was calculated from the

following equation 3.9,

(3.9)

where I is the discharge current density for the discharge time t.

3.9.2 Potentialsquare‐wavevoltammetry(Charge/discharge)

Potential square-wave voltammetry test of structural supercapacitors [Chapter 6] was

performed by applying step voltage of 0.1 V for 10 min. The capacitance (C) value (in F)

was obtained from the equation,

∆

∆ (3.10)

where I was the discharge current and ∆E the change in voltage in time ∆t. The specific

capacitance was calculated as capacitance per volume unit (F/cm3).

3.9.3 Electrochemicalimpedancespectroscopy

Impedance spectroscopy is a useful technique for the investigation of wide range of bulk and

interfacial electrical properties associated with the supercapacitors. Electrochemical

impedance spectroscopy (EIS) is usually determined by applying AC potential to

supercapacitor cell and measuring the current through the cell [169]. The AC potential

penetrates into the bulk pores of the electrochemical cell and shows the amount of solvated

ions at a specific frequency that reach the pore surface of the electrodes. Electrochemical

impedance is defined as,

exp Φ Φ Φ (3.11)

where Z0 is the magnitude of Z ( / and Ф is the phase angle of current I

versus potential difference E.

The real part of impedance is usually plotted against the imaginary part of impedance to get a

complex plot called Nyquist plot. The two copper wires of the structural supercapacitors were


110

connected to the two electrodes of the Potentiostat. The impedance measurements were

conducted in frequency window of 1 Hz to 105 Hz and the amplitude of sinusoidal voltage

was 500 mV. Impedance method was used to calculate the equivalent series resistance (ESR)

of structural supercapacitors (Chapter 6). ESR was measured (measurement was identical to

the procedure described in Appendix B) from the x-intercept of high frequency curve in the

Nyquist plot.

3.10 Mechanical characterisation of composites (±45° laminate tensile test)

In-plane shear response of ±45° laminated structural supercapacitors was investigated by

tensile test in accordance with ASTM D3518 [174]. The tensile test of a ±45° laminate is a

matrix dominated property as during application of a tensile force on ±45° laminate, the

carbon fibres tried to align themselves resulting in the damage of polymer electrolyte that is

supporting the fibre ±45° alignment, as shown in Figure 3.12. In the tensile test of a ±45°

laminate, transverse and longitudinal strains were recorded by applying a uniaxial tension.

The ±45° laminated structural supercapacitor was made symmetric about the mid plane by

using two glass fibre mat based separators. Therefore, the shear strain relationship was

developed by calculating the maximum in-plane shear stress for the ±45° laminated structural

supercapacitor from the equation 3.12,

2

(3.12)

where is the maximum in-plane shear stress in MPa, is the maximum load at or

below 5% shear strain in N, and A is the cross sectional area in mm2.

The shear strain was also calculated at each required data point using Equation 3.13,

(3.13)

where is the shear strain at i-th data point, is the longitudinal normal strain at i-th data

point and is the transverse normal strain at i-th data point


111

y

x

tensile force

fibre direction

Figure 3.12 Schematic of a ±45° laminated structural supercapacitor during tensile test in

accordance with ASTM D 3518.

The unidirectional shear strength (translaminar i.e. through thickness), G12, was obtained by

taking the maximum load, in plane shear strain, taken from the initial linear portion of the

unidirectional shear stress and shear strain curve by using the equation 3.14:

∆∆

(3.14)

An Instron 4505 machine (Bucks, UK) was used for the tensile test of ±45° laminated

structural supercapacitors. For PEGDGE based structural supercapacitors, a 5kN load cell

was used and for the DGEBA based structural supercapacitors, a 10 kN load cell was used.

The test specimen was in a rectangular shape with a length of 160 ± 2.50 mm, a width of 25 ±

0.50 mm and a thickness of 0.80 ± 0.020 mm. The ends of each specimen were grit blasted

and adhesively bonded with fibre glass composites end tabs, with a gauge length of 100 mm.

Two strain gauges (Type FLA-10-11, Tokyo Sokki Kenkyujo Co. Ltd, Japan) were attached

to the each face of the bar to measure longitudinal strain as well as transverse strain. The

crosshead speed was 2 mm/min. The tensile force applied to the specimen was recorded

every 0.5 s through the test until the specimen failed as shown in Figure 3.13.


112

Figure 3.13 Tensile testing of a structural supercapacitor specimen (a) pre tensile test

specimen, (b) post tensile test specimen.

3.11 Fibre volume fraction of structural supercapacitors by acid digestion

The fibre volume fraction of all types of structural supercapacitors was also determined using

ASTM D3171 [175]. Acid digestion of structural supercapacitors was carried out using a

mixture of sulphuric acid (95-97% purity, Sigma Aldrich) and hydrogen peroxide (50 wt%

solution in water, Sigma Aldrich) as mentioned in Procedure B of ASTM D 3171 [175].

Procedure B was selected because the method is suitable for epoxy resin based composites

and does not require any reflux condenser as compared to other procedures mentioned in

ASTM D 3171. Each specimen (3cm×2cm) from different structural supercapacitors was

weighed and pre-digestion weight was recorded to the nearest 0.0001 g. The density of each

pre-digested specimen was also measured through helium pycnometry using Accupyc 1330

(Micromeritics, USA). The specimen was then placed in a 100-mL conical flask and 25mL of

sulphuric acid (95-97% purity) was added. The beaker was placed on hot plate at 120°C for

10 min until the mixture started to fume and the solution changed its colour to dark brown. 35

mL of hydrogen peroxide (50 wt% in H2O) was added slowly in the mixture in order to


113

oxidise the polymer electrolyte until the solution changed to transparent. The carbon fibre and

the glass fibre started floating to the top of the solution. The conical flask was removed from

the hot plate and the solution was allowed to cool. Carbon fibre and glass fibre were then

separated through gravity filtration (sintered glass funnel of porosity grade 4) and were then

washed with excess deionised water for 30 min followed by acetone (99.5% purity, GPR

RECTAPUR, VWR, UK) washing (20 mL for 5 min) in order to improve drying times. The

specimens were placed in oven for 24 h at 100°C. The mass and density of the dried fibres

were measured using the balance and Accupyc 1330 (Micromeritics, USA) respectively. The

fibre volume fraction was calculated using equation 3.15,

100 (3.15)

where is fibre volume fraction, is the initial mass of specimen, is the final mass of

fibres after acid digestion, is the density of specimen and is the density of fibres.

Chapter 4 Polymer Electrolytes

114


This chapter covers the results and discussion of polymer electrolytes used for the preparation

of structural supercapacitors. The ionic conductivity results of different salts in propylene

carbonate are first presented followed by the results as well as discussion of the

multifunctionality of PEGDGE matrix containing different salts. Subsequently, LiTFSI/PC

and EMITFSI (ionic liquid) were selected for various other PEGDGE, DGEBA and

PEGDGE/DGEBA formulations. A PAN gel based polymer electrolyte was also prepared

and characterised mechanically and electrochemically. The ionic conductivity and

compression properties of different polymer electrolytes are discussed to draw attention to the

research goals and challenges of this work.


115

4.1 Selection of salts for inclusion into polymers

In electrochemical devices, operating at ambient temperatures, the electrolyte salt should,

ideally, dissolve and dissociate in the solvent and the solvated ions should be able to move in

polymer medium with high mobility. Ions should be inert to electrolyte solvent and should be

non-toxic and thermally stable. The degree of dissociation of salts dissolved in the polymer

host depends on the total concentration of salt in matrix. Generally, the degree of dissociation

decreases with increasing salt concentration and thus, at optimal salt concentration, the

electrolyte possesses maximum total concentration of free ions. In the present work, the ionic

conductivity of six different salts were measured. Lithium ions and sodium ions based salts

had been selected because of the high energy density and compatibility with polymer

electrolytes [55]. Tetrabutyl ammonium hexafluorophosphate (TBAPF6) was selected as it is

less hygroscopic as compared to lithium and sodium based salts. Since lithium, sodium and

ammonium based salts are solid at room temperature, propylene carbonate (PC) was used as

solvent. PC was selected as a solvent for salts because of its low melting point [176], high

dielectric constant [176], low vapour pressure [176], low toxicity [176] and low

decomposition due to oxidation [177]. Ionic liquid from the imidazolium family i.e. 1-ethyl-

3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) was also selected

because of its high ionic conductivity, large electrochemical window and non-flammability

[178]. EMITFSI is also less hygroscopic as compared to lithium bis(trifluoromethyl

sulfonyl)imide (LiTFSI) and sodium perchlorate (NaClO4) which made it an ideal candidate

to be used as a salt for the study of the selected polymer electrolytes. Table 4.1 shows the

ionic conductivity of different salts in PC and of the ionic liquid studied in this work.

No. Salt/PC ҡM† ҡL‡

mS/cm mS/cm

1 1.0 M LiClO4/PC 5.65 ± 0.05 5.45 [93]

2 1.0 M LiPF6/PC 5.91 ± 0.09 5.80 [93]

3 1.0 M LiTFSI/PC 7.82 ± 0.03 8.50 [179]

4 0.1 M TBAPF6/PC 2.49 ± 0.04 ---*

5 1.0 M NaClO4/PC 6.46 ± 0.02 ---*

6 EMITFSI 9.73 ± 0.06 10.0 [178]

Table 4.1 Ionic conductivity of different salts.

† ҡM is the ionic conductivity measured using a conductivity meter

‡ ҡL is the literature value for the ionic conductivity; *Value not available.


116

The ionic conductivity of LiClO4 and LiPF6 in PC were lower as compared to the LiTFSI/PC

but higher than TBAPF6/PC. However, LiClO4 was not used in polymer electrolytes due to its

hazardous nature (can intensify fire as it is an oxidiser [93]) and LiPF6 was not used as it is

thermally unstable at elevated temperatures [93]. LiTFSI was proved to be safe, thermally

and hydrolytically stable, highly conducting and melts at 23°C (decomposition up to 380°C

[93]). However, the only problem with LiTFSI was its hygroscopic nature. LiTFSI had low

chemical stability towards ambient moisture as it crystallised resulting in low ionic

conductivity. Therefore, TBAPF6 was also studied. Although TBAPF6 had two times lower

ionic conductivity than lithium based salts and possesses a high melting point (387.4°C),

however, it is stable at ambient moisture and thus could be used in open air. LiTFSI had to be

mixed with solvent before adding into the polymer matrix. However, the solvent (PC)

negatively affects the mechanical properties of the polymer, since PC acts as a plasticiser as

will be discussed in the following sections (Section 4.3.1). Hence, the ionic liquid EMITFSI

was also studied. EMITFSI has a high ionic conductivity and also does not require any

solvent as it is liquid at room temperature.

4.2 Polyacrylonitrile gel polymer electrolytes

The PAN based polymer electrolyte was a gel at room temperature because of high

concentration of plasticiser. In order to determine the mechanical properties of a PAN gel

polymer electrolyte, oscillatory rheological characterisation was carried out. Although the

mechanical properties of PAN gel based polymer electrolyte will be low (as PAN is a gel), it

does possess higher ionic conductivity than PEGDGE (Table 4.3). Initially, shear strain

amplitude sweep (or for short strain sweep) tests were conducted. Strain sweep tests are

oscillatory tests performed at variable amplitude sweeps, keeping the frequency as well as the

measuring temperature constant. At low amplitude sweep values, the shear moduli (G/ and

G//) were constant. This plateau called the linear viscoelastic (LVE) range. Strain sweep tests

are usually carried out to determine the limit of the LVE range. The dynamic moduli

remained constant as long as the strain rates were below 1.5%. Thus, from the strain sweep

tests, a strain rate of 1.5 % was chosen at which frequency sweep tests were conducted in the

frequency window of 0.1-10 Hz. Frequency sweep tests were carried out for investigating the

time dependent shear behaviour.


117

40 80 120 160

10-3

10-1

101

103

105

G/ , G

// (P

a)

Temperature (C)

PAN1-3D G/

PAN1-3D G//

PAN1-6M G/

PAN1-6M G

PAN2 G/

PAN2 G//

//

(a)

10-2 10-1 100 101 10210-2

100

102

104

106

G/ , G

// (P

a)

Frequency (Hz)

PAN1-3D G/

PAN1-3D G//

PAN1-6M G/

PAN1-6M G//

PAN2 G/

PAN2 G//

(b)

Figure 4.1 Temperature (a) and frequency (b) sweep tests of PAN1-3D, PAN1-6M and PAN2

gel based polymer electrolytes (G/ = storage modulus and G// = loss modulus).

Inspection of Figure 4.1b reveals that both the dynamic moduli G/ and G// of PAN1-6M,

remained constant as long as the frequency was below the limiting value (0.8-1 Hz). The

PAN1-6M gel was stable under this condition. At higher frequencies (> 1 Hz), the limit of the

linear viscoelastic range was exceeded and the structure of the sample had been reformed

already or even completely destroyed molecularly [180]. Figure 4.1b also shows that the

PAN1-3D polymer electrolyte has a liquid character up to the frequency 1 Hz and after 1 Hz,

polymer electrolyte has a gel character. However, PAN2 gel performance was unaffected

over a wide range of frequencies (0.01 to 100 Hz). Figure 4.1a shows the evolution of

dynamic moduli during a temperature scan from 25°C to 150°C performed on different PAN

gel samples. At low temperature (20 to 60°C), the storage modulus G/ and loss modulus G//


118

decreased slowly as the temperature increased and (i.e., at 60°C, G//G//≈10) indicating the

elastic behaviour of a gel and the microstructure remained unchanged over this temperature

range. Such behaviour is characteristic of a strong gel with a three dimensional network

structure [180]. The network responded elastically at small deformations. The magnitude of

G/ ( ≈ 105 Pa) for PAN2 was an indication of a high density of network forming bonds.

Above 60°C, the storage modulus decreased rapidly as temperature increased up to 150°C

indicating a structural change that corresponded to a decreasing elastic response which was

also consistent with visual observation of the gel transforming into a soft viscoelastic state

(weak gel). The storage modulus was greater than the loss modulus which was entirely

different from PAN1-3D showing the typical viscoelastic response for a strong gel network

and did not change the dependence of G/ on frequency gradually.

In PAN1-3D, the storage modulus G/ was greater than the loss modulus G// indicating a gel

character but the magnitude of G/ was lower (~ 103 Pa) as compared to PAN2 gel based

polymer electrolyte (~105 Pa), the PAN/solvent ratio was of 26/74 by mass and in case of

PAN1-3D the PAN/solvent ratio was 7/93 by mass (section 3.3.3). It is clearly evident from

Figure 4.1a that as the temperature increased the storage modulus of the gel decreased. PAN

gel polymer electrolytes behaved more elastically at 25°C than at 60°C which is in

accordance with the observation by Nicotera et al. [180]. PAN based polymer electrolyte

(PAN1-3D and PAN1-6M) exhibited low dynamic moduli G/ and G// (Figure 4.1) at low

frequency (0.1 Hz) which was possibly due to weaker gel interactions and low concentration

of polymer (7 wt% by weight) in the polymer electrolyte.

The PAN gel polymer electrolyte showed an ionic conductivity of 3.8 mS/cm (Table 4.2).

The results were little higher than the room temperature ionic conductivity of 3.20 mS/cm

reported by Nicotera et al. [180] for a similar polymer electrolyte but of slightly different

composition. The ionic conductivity of the studied polymer electrolyte (PAN2) was also little

higher than the room temperature ionic conductivity of 3.16 mS/cm reported by Perera et al.

[181] for a PAN gel polymer electrolyte containing Mg (ClO4)2. PAN1-3D (prepared as

described in Section 3.3.3) had a low polymer concentration (7.7 wt%) as compared to PAN2

and, therefore, had poor rheological properties. The PAN based polymer electrolyte was a

viscous liquid and was not completely transformed into gel which is completely evident from

Figure 4.1; the loss modulus was greater than the storage modulus. However, after 6 months,

the PAN based polymer electrolyte had gelled completely and thus rheological properties


119

were slightly changed as shown in Figure 4.1. However, an optimisation of polymer and

electrolyte concentrations was required in order to get high rheological properties without

compromising the ionic conductivity. Therefore, the polymer concentration was increased

from 7.7 wt% to 26 wt%. 26 wt% of PAN polymer was the maximum concentration limit in

the electrolyte (0.1 M TBAPF6/PC) where the PAN dissolved fully in the solvent. A further

increase of the polymer concentration resulted in incomplete dissolution, polymer aggregates

were observed. The storage modulus of the PAN gel polymer electrolyte increased two orders

of magnitude from 1kPa to 100 kPa.

Gel polymer

electrolyte

Cyclic Voltammetry* Charge/Discharge Impedance spectroscopy

Cg C

g ҡ Cg

(µF/g) (µF/g) (mS/cm) (µF/g)

PAN1-3D 18.71 15.98 1.350 31.17

PAN1-6M 22.38 41.01 1.324 32.51

PAN2 20.78 18.34 3.791 40.37

Table 4.2 Specific capacitance Cg and ionic conductivity ҡ of the PAN gel polymer

electrolytes.

* Cyclic voltammetry test was conducted at a sweep rate of 50 mV/s.

The electrochemical properties of PAN gel polymer electrolytes were studied using cyclic

voltammetry, charge-discharge and electrochemical impedance spectroscopy measurements

to show the possibility of using them as electrolytes in a supercapacitor. The ionic

conductivity was improved three times in PAN2 even though the TBAPF6/PC concentration

was decreased from 92.3 wt% to 74 wt%. The salt TBAPF6 in EC/PC concentration was

increased from 0.1 M to 1.0 M. The increased salt concentration in a given polymer resulted

in an increased number of charge carriers which led to a rise in ionic conductivity.


120

-0.5 0.0 0.5

-2x10-6

0

2x10-6

I (A

)

E (V)

PAN1-3D PAN1-6M PAN 2

(a)

0 2x104 4x104

0

2x104

4x104

-Z//

Z/ (

PAN1-3D PAN1-6M PAN 2

(b)

0 200 400 6000

80

160

240

Figure 4.2 Cyclic voltamograms (a) and impedance spectroscopy plots (b) of PAN1-3D,

PAN1-6M and PAN2 gel polymer electrolytes at room temperature.

Cyclic voltamograms and impedance plots of PAN gel polymer electrolytes are presented in

Figure 4.2a. The impedance curve (Figure 4.2b) can be clearly divided into two parts. At

higher frequencies, the capacitive impedance Z’’ is small, but increases with decreasing

frequency. There is a semicircular shape associated with a parallel combination of capacitive

and resistive components, relating to the parallel plate geometry of a capacitor and net

leakage/ionic resistance, respectively [31]. At lower frequencies, the electrochemical double

layer formation during charging process becomes significant as ions had enough time to

move to the electrode surface which is clear at a critical “knee frequency” [32]. The knee

frequency is the maximum operating frequency at which the majority of the electrochemical

capacitance can be obtained. The capacitance is dependent on the frequency and thus

decreases at higher frequencies. It follows that the device should be operated below the knee


121

frequency. The voltamogram (Figure 4.2a) deviated from the ideal rectangular shape. Figure

4.3 attempts to capture the multifunctional character of the PAN based gel polymer

electrolytes by plotting conductivity as a function of storage modulus (peak maximum). PAN

2 gel polymer electrolyte outperforms the other two PAN gels by three orders of magnitude

in terms of storage modulus and approximately three times in terms of ionic conductivity.

100 101 102 103 104 1051

2

3

4

(m

S/cm

)

G/ (Pa)

PAN1-3D PAN 2 PAN1-6M

Figure 4.3 Ionic conductivity ҡ as function of storage modulus G/ (peak maximum) of three

PAN gel polymer electrolytes by varying PAN/plasticiser concentration at 25°C.

4.3 Crosslinked PEGDGE polymer electrolytes

4.3.1 Effect of different ionic salts on ionic conductivity and compression

propertiesofcrosslinkedPEGDGEelectrolytes

Although PAN gel polymer electrolytes have a high ionic conductivity they do not possess

any significant mechanical properties rendering them a rather poor choice for structural

supercapacitor applications. Although the mechanical properties of PAN gel polymer

electrolyte were improved by increasing the polymer concentration (Figure 4.3) but still it

was gel and thus, not suitable. Thus, possible alternatives to PAN gel polymer electrolyte

were sought. Since PEGDGE is liquid at room temperature and has no structural properties,

crosslinking of PEGDGE was done by adding the amine hardener triethylenetetramine

(TETA) in order to enhance the mechanical properties of polymer. Amorphous PEG unit is

the carrier for ionic species in PEGDGE polymer electrolytes. Thus, a reduction in ionic


122

conductivity by adding the hardener concentration was attributed to the decreased flexibility

of the polymer chains resulting in reduced ion coordination sites.

Different salts in PC as well as ionic liquid were used as electrolyte in crosslinked PEGDGE

polymer electrolyte to optimise the electrochemical and mechanical performance. 0.8 wt% of

electrolyte to the crosslinked PEGDGE was fixed because, in case of TBAPF6/PC electrolyte,

a small increase in concentration of electrolyte to crosslinked PEGDGE led to a very soft

polymer electrolyte. 0.1 M concentration of TBAPF6/PC was used as it was the saturation

concentration of TBAPF6 in PC. Since PC is acting as a solvent for the salt as well as a

plasticiser for the polymer, the increased concentration of PC negatively affected the

mechanical properties (Table 4.3).

Sample code 0.8 wt% of Electrolyte

in PEGDGE

ҡ E σ

(µS/cm) (MPa) (MPa)

A0.80P99.2 0.1 M TBAPF6/PC 12.3 ± 1.23 5.46 ± 0.200 1.86 ± 0.210

Li0.80P99.2 1.0 M LiTFSI/PC 17.3 ± 1.52 10.2 ± 0.240 5.06 ± 0.350

Na0.80P99.2 1.0 M NaClO4/PC 18.3 ± 3.53 11.3 ± 0.230 5.11 ± 0.401

E0.80P99.2 EMITFSI 19.5 ± 3.20 12.2 ± 0.250 5.96 ± 0.241


crosslinked PEGDGE electrolytes by varying different salts (A-0.1 M TBAPF6/PC, Li-1.0 M

LiTFSI/PC, Na-1.0 M NaClO4/PC, E- EMITFSI).

The ionic conductivity of PEGDGE electrolytes, containing different salts in PC or ionic

liquid, were slightly different as 0.1M TBAPF6/PC in crosslinked PEGDGE showed the

lowest ionic conductivity (12.3 ± 1.23 µS/cm) but EMITFSI in crosslinked PEGDGE showed

the highest ionic conductivity (19.9 ± 2.24 µS/cm) and did not require a solvent. A room

temperature ionic conductivity of 36 µS/cm has been reported previously by Liang et al.

[182] for PEGDGE films containing 22 wt% lithium perchlorate in PC cured with α-ω-

diamino-poly(propylene oxide). Overall, the compression modulus of crosslinked PEGDGE

electrolytes increased from 5.46 MPa (using 0.1 M TBAPF6) to 12.2 MPa (using EMITFSI),

but for the creation of structural supercapacitor, the polymer matrix is required to exhibit both

ionic conductivity and appropriate mechanical properties. Currently investigated structural

epoxy resins possess a compression stiffness as high as 4 GPa [55]. Similarly, the ionic

conductivity of ionic liquids have reached values of 0.01 S/cm [178]. Therefore, further


123

increase in compression properties as well as ionic conductivity of polymer matrix is required

in order to use it as a matrix for structural supercapacitors.

4.3.2 Effect of increasing EMITFSI concentration on ionic conductivity and

compressionpropertiesofcrosslinkedPEGDGEelectrolytes

The effect of increasing EMITFSI concentrations from 0.8 wt% to 60 wt% on the ionic

conductivity and compression properties of crosslinked PEGDGE electrolytes were also

investigated. The results (Figure 4.4) showed that ionic conductivity increased with

increasing EMITFSI concentration from 0.8 wt% to 50 wt%. The increasing EMITFSI

concentration generally resulted in an increase of the ionic conductivity and this behaviour

was in accordance with other results which were reported earlier in the literature for other

polymer electrolytes with ionic liquids [183, 184]. The highest ionic conductivity of 176

µS/cm was achieved for a 50 wt% EMITFSI concentration in crosslinked PEGDGE (Figure

4.4a). When the concentration of EMITFSI in crosslinked PEGDGE electrolytes was

increased beyond 50 wt%, the conductivity decreased slightly from the maximum value

possibly due to the formation of ion aggregates within the polymer electrolyte. A similar

trend of ionic conductivity as function of electrolyte concentrations in PEO polymer was

previously reported by Kim et al. [120] who showed that the ionic conductivity increased

from 50 µS/cm to 325 µS/cm and then decreased to 250 µS/cm as the ionic liquid (1-ethyl-3-

methylimidazolium tetrafluoroborate) concentration was increased from 0 to 0.3 mol.

The increase in the concentration of EMITFSI up to 50 wt% resulted in increasing the ionic

strength and, therefore, enhanced ionic mobility within the crosslinked PEGDGE. The ionic

conductivity of crosslinked PEGDGE electrolyte increased to 176 µS/cm at 50 wt%

EMITFSI concentration (Figure 4.4a). However, the decrease in ionic conductivity at higher

EMITFSI concentration (60 wt%) was possibly due to the excessive ions starting aggregate

within the polymer electrolyte [185, 186]. On the other hand, the compression properties

decreased with increasing EMITFSI concentration. It is well known that the stiffness of the

polymer electrolyte drops with increasing ionic conductivity [55, 93, 182]. The compression

modulus gradually decreased from 12.2 MPa to 4.53 MPa with increasing EMITFSI

concentration from 0.8 wt% to 30 wt% (Figure 4.4a). Matsumoto et al. [187] observed a

similar trend of decreasing tensile modulus of a crosslinked DGEBA electrolyte; it decreased

690 MPa to 45 MPa as the EMITFSI concentration was increased from 34 wt% to 50 wt%.

The compression modulus remained almost constant with further increasing EMITFSI


124

concentration up to 60 wt%. Compression strength also gradually decreased with increasing

concentration of EMITFSI (Figure 4.4b). By increasing the EMITFSI concentration, the

crosslinked structure of polymer electrolyte weakened as there was less crosslinked structure

and more liquid phase (i.e. ionic liquid) which decreased the compression strength. Overall,

variations in the EMITFSI concentration showed a large impact on the ionic conductivity and

compression properties of crosslinked PEGDGE electrolytes.

0 20 40 60 80 1000

250

500

750

1000 E

EMITFSI (wt%)

S/c

m)

0

3

6

9

12

E (M

Pa)

0 20 40 60 80 1000

250

500

750

1000

EMITFSI (wt%)

S/cm

)

0

2

4

6

MPa)

Figure 4.4 Effect of increasing EMITFSI concentration on crosslinked PEGDGE electrolyte,

(a) EMITFSI concentration vs. ionic conductivity ҡ and compression modulus E; and

(b) EMITFSI concentration vs. ionic conductivity ҡ and compression strength σ.


125

4.4 Crosslinked DGEBA polymer electrolytes

Another promising polymer resin is the crosslinked diglycidylether of bisphenol-A which is

currently under investigation at USA Army Research Laboratory [55]. Crosslinked DGEBA

was also studied by Matsumoto et al. [187] who reported a tensile modulus of 690 MPa and

an ionic conductivity of 10 µS/cm for crosslinked DGEBA electrolyte samples containing 34

wt% EMITFSI. The effect of increasing LiTFSI/PC concentration on the ionic conductivity

and compression properties was also investigated. Crosslinked DGEBA electrolytes

containing LiTFSI/PC had a higher compression modulus but lower ionic conductivity as

compared to PEGDGE based polymer electrolytes.

Sample code 1.0M LiTFSI/PC ҡ E σ

LixBy (wt%) † (wt%) (µS/cm) (MPa) (MPa)

Li0B100 0 N/A* 3044 ± 155 45.6 ± 1.68

Li10B90 10 1.92 ± 0.240 1583 ± 14.14 113 ± 17.2

Li20B80 20 6.10 ± 0.0301 905 ± 73.4 81.1 ± 0.270

Li40B60 40 11.9 ± 1.06 25.1 ± 0.58 68.5 ± 0.701

Li60B40 60 138 ± 3.58 0.922 ± 0.401 0.631 ± 0.0801

Li80B20 80 1580 ± 13.8 0.211 ± 0.0102 0.120 ± 0.0401

Li100B20 100 7820 ± 30.2 N/A**


crosslinked DGEBA electrolytes as function of increasing LiTFSI/PC concentrations.

† LixBy with x and y being the weight percentages of 1 M LiTFSI/PC (Li) and crosslinked diglycidylether of

bisphenol-A (B) respectively; * No ionic conductivity due to pure resin; ** No compression properties.

The compression modulus and ionic conductivity of crosslinked DGEBA electrolytes with

increasing amounts of 1 M LiTFSI/PC is summarised in Table 4.4. To impart ionic

conductivity, LiTFSI/PC was added into the polymer resin. The compression modulus of

crosslinked DGEBA electrolyte gradually decreased and the ionic conductivity gradually

increased with increasing LiTFSI/PC concentration. The possible explanation of increase in

ionic conductivity of crosslinked DGEBA electrolytes with increasing LiTFSI/PC

concentration is the increased number of ion coordination sites within the polymer matrix. It

is clearly evident from Table 4.4 that Li10B90 polymer electrolyte had excellent mechanical

properties (a compressive modulus of ~1.6 GPa), which was about two orders of magnitude

higher as compared to the crosslinked PEGDGE electrolyte (6.4 MPa). However, the ionic


126

conductivity of Li10B90 electrolyte (1.9 µS/cm) was only about an order of magnitude lower

as compared to the crosslinked PEGDGE electrolyte containing 10 wt% LiTFSI/PC (20.3

µS/cm). Since samples were characterised electrochemically in air and the LiTFSI is very

hygroscopic, it is possible that the hydrated ions forms at the surface of the sample resulting

in increased ionic conductivity. In order to determine the actual ionic conductivity of these

polymer electrolytes, it is required to prepare and characterise the samples in a moisture free

environment (e.g. glove box).

4.5 Crosslinked PEGDGE/DGEBA polymer electrolytes

4.5.1 CrosslinkedPEGDGE/DGEBAelectrolytescontaining10wt%[LiTFSI]inPC

In order to enhance the mechanical properties of PEGDGE based polymer electrolytes,

various stoichiometric amounts 80/20, 60/40, 40/60 and 20/80 of DGEBA were added to

PEGDGE. The addition of DGEBA to PEGDGE provided a material which was significantly

stiffer than crosslinked PEGDGE electrolytes (Figure 4.5). Crosslinked DGEBA/PEGDGE

electrolytes were also prepared and characterised. Compression modulus, compression

strength and ionic conductivity of crosslinked DGEBA/PEGDGE electrolytes with varying

PEGDGE to DGEBA concentrations are summarised in Figure 4.5.

The compression modulus of crosslinked PEGDGE electrolyte increased from 6.42 MPa to

932 MPa as the concentration of DGEBA increased in PEGDGE (Figure 4.5). A similar trend

of increasing compression strength from 2 MPa to 113 MPa was also observed with

increasing DGEBA concentration. The increase in modulus was attributed to the reduced

polymer mobility as DGEBA concentration was increased in PEGDGE. However, the ionic

conductivity of polymer electrolytes reduced by an order of magnitude with the addition of

DGEBA. Polymer electrolytes samples containing PEGDGE concentration xPEGDGE of 0.2 and

0.4, in Figure 4.5, were the points of interest as these samples showed high compression

properties with reasonable ionic conductivity than other combinations of the

PEGDGE/DGEBA blends.


127

0.0 0.2 0.4 0.6 0.8 1.01

10

100 E

xDGEBA

S/c

m)

(a)

1

10

100

1000

E (M

Pa)

0.0 0.2 0.4 0.6 0.8 1.01

10

100

xDGEBA

S/c

m)

(b)

1

10

100

MP

a)



crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% LiTFSI/PC.

4.5.2 CrosslinkedPEGDGE/DGEBAelectrolytescontaining10wt%EMITFSI

In order to improve the ionic conductivity and compression properties of crosslinked

PEGDGE/DGEBA electrolytes over those of previous polymer electrolyte systems, various

stoichiometric amounts of PEGDGE/DGEBA containing 10 wt% EMITFSI were also

prepared. As evident from Figure 4.6, the incorporation of DGEBA to PEGDGE provides a

material which was significantly stiffer than the crosslinked PEGDGE electrolytes.

Crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI was selected as baseline

material for structural supercapacitors because of intermediate ionic conductivity (28 µS/cm),

compression strength (4.9 MPa) and compression modulus (9 MPa) as mentioned in Figure

4.4. Compression modulus and ionic conductivity of crosslinked PEGDGE/DGEBA


128

electrolytes with varying PEGDGE and DGEBA concentration but constant the EMITFSI

concentration of 10 wt% are shown in Figure 4.6a. The compression modulus of the polymer

electrolytes increased when adding proportionally more DGEBA segments into the

crosslinked PEGDGE network (Figure 4.6a). Since the length between the two crosslinking

epoxy groups in DGEBA is smaller as compared to that in the PEGDGE, the increased

concentration of DGEBA in PEGDGE enhanced the degree of crosslinking and thus reduced

the free volume of polymer [182].

0.0 0.2 0.4 0.6 0.8 1.01

10

100

Conductivity Compressive Modulus E

xDGEBA

S/c

m)

(a)

1

10

100

1000

E (M

Pa)

0.0 0.2 0.4 0.6 0.8 1.01

10

100

Conductivity Compressive strength

xDGEBA

S/c

m)

(b)

1

10

100 (MP

a)



crosslinked PEGDGE/DGEBA electrolytes containing 10 wt% EMITFSI.

The ionic conductivity of crosslinked DGEBA/PEGDGE electrolyte decreased from 28

µS/cm for pure crosslinked PEGDGE electrolytes to 3.6 µS/cm for pure crosslinked DGEBA


129

electrolytes containing 10 wt% EMITFSI as shown in Figure 4.6. The compression strength

of polymer electrolyte has a different trend to the ionic conductivity and compression

modulus as shown in Figure 4.6b. The compression strength increased up to 213 MPa in

45/55 weight ratio of PEGDGE/DGEBA in the blend and then started decreasing gradually.

The compression modulus, on the other hand, increased gradually with increasing DGEBA

concentration. This trend was due to the structural change in the polymer electrolyte when

DGEBA segments were added in PEGDGE. The stress strain curves (Figure 4.7) of the

PEGDGE/DGEBA blend electrolytes containing 10 wt% EMITFSI also confirmed the trend.

It can be observed that the slopes of the stress strain graphs clearly increased with the

increasing concentration of DGEBA in PEGDGE. The polymer electrolyte changed from

being a very soft and rubbery matrix (crosslinked PEGDGE electrolyte) to a very plastic

matrix (50/50 weight ratio of PEGDGE/DGEBA in the blend) and then to a very brittle

glassy matrix (crosslinked DGEBA electrolyte).

0.00 0.15 0.30 0.45 0.60 0.750

75

150

225

0.00 0.02 0.04 0.060

1

2

3

4(h)

(g)

(f)

(d)

(c)

(b) (a)

(e)

(h)

(g)

(f)

(e)

(d)

Str

ess

(MP

a)

Strain ()

100P:0B (a) 80P:20B (b) 60P:40B (c) 50P:50B (d) 40P:60B (e) 30P:70B (f) 20P:80B (g) 0P:100B (h)(c)

Figure 4.7 Stress strain curves of crosslinked PEGDGE/DGEBA blend polymer electrolytes

containing 10 wt% EMITFSI

4.5.3 CrosslinkedPEGDGE/DGEBAelectrolytescontaining50wt%EMITFSI

Crosslinked PEGDGE electrolytes containing 50 wt% EMITFSI showed highest ionic

conductivity (176 µS/cm) but low compression modulus (3.8 MPa) among various other

EMITFSI concentrations in crosslinked PEGDGE as discussed previously (Section 4.3.2).

Also, it was demonstrated previously in sections 4.5.1 and 4.5.2 that the mechanical


130

performance of crosslinked PEGDGE electrolytes was improved with the introduction of

DGEBA segments. Therefore, crosslinked PEGDGE electrolyte containing 50 wt% EMITFSI

with increasing DGEBA concentration was also investigated.

The compression modulus of the polymer electrolyte increased from 4 MPa to 32 MPa with

increasing DGEBA weight concentration xDGEBA from 0 to 0.6 respectively in crosslinked

PEGDGE/DGEBA electrolytes (Figure 4.8). However, further increasing the amount of

DGEBA in crosslinked PEGDGE network (xDGEBA = 0.8) resulted in a phase separation of

EMITFSI from the polymer blend as shown in Figure 4.9. This phase separation was even

clearer in the pure DGEBA polymer electrolyte containing 50 wt% EMITFSI.

0.0 0.2 0.4 0.6 0.8 1.010

100

1000

Conductivity Compressive Modulus E

xDGEBA

S/c

m)

(a)

1

10

100

E (M

PA

)

0.0 0.2 0.4 0.6 0.8 1.0

10

100

1000 Conductivity Compressive strength

xDGEBA

S/c

m)

(b)

1

10

100

MPa)



crosslinked PEGDGE/DGEBA electrolytes containing 50 wt% EMITFSI.


131

The compression strength increased from 1.2 MPa of a crosslinked PEGDGE electrolyte

(xDGEBA = 0) to 11 MPa for a crosslinked PEGDGE/DGEBA blend electrolyte (xDGEBA = 0.4)

containing 50 wt% EMITFSI and then remained almost constant (as shown in Figure 4.8b).

This is because of the change in matrix structure from a very rubbery and soft to a very

ductile but plastic material. On the other hand, the ionic conductivity tended to increase five

times by the addition of DGEBA segments in PEGDGE network from DGEBA concentration

of 0 to 0.6 in crosslinked PEGDGE containing 50 wt% EMITFSI (Figure 4.8). However, the

decrease in ionic conductivity in 100% crosslinked DGEBA electrolyte containing 50 wt%

EMITFSI can not only be attributed to the phase separation of polymer matrix and ionic

liquid but also to the formation of ion aggregates and hence affecting the ionic mobility.

Figure 4.9 Photograph of what on tissue paper showing phase separation of EMITFSI from

crosslinked PEGDGE/DGEBA electrolyte with xDGEBA of 0.8 containing 50 wt% EMITFSI.

4.6 Multifunctionality of polymer electrolytes

Different structural polymer electrolytes were prepared using crosslinked PEGDGE and

crosslinked DGEBA electrolytes containing different electrolytes (1.0 M LiTFSI/PC and

EMITFSI). EMITFSI concentration was increased in crosslinked PEGDGE electrolytes.

Crosslinked PEGDGE electrolytes had high ionic conductivity (176 µS/cm) but poor

compression modulus (4 MPa) at an EMITFSI concentration of 50 wt%. Crosslinked

PEGDGE/DGEBA blends with varying weight ratios of PEGDGE and DGEBA were also

prepared. The structural performance of the crosslinked PEGDGE electrolytes was

remarkably increased by the incorporation of DGEBA but at the cost of a reduced ionic

conductivity. EMITFSI was selected as electrolyte for further studies because of its high ionic

conductivity among other salts studied and also, to avoid using the propylene carbonate (PC)

solvent in polymer electrolytes. The addition of PC solvent in polymer electrolyte negatively

affected their compression properties.


132

The multifunctional character of the polymer electrolytes, described in previous sections of

Chapter 4, can be truly captured by plotting the compression modulus or compression

strength, measures of the mechanical performance, as a function of ionic conductivity, a

measure of the electrochemical performance, for the full range of structural polymer

electrolytes (Figure 4.10). Ionic conductivity, compression modulus and compression

strength of various polymer electrolytes were plotted on logarithmic axes as the properties

span over several orders of magnitude. Structural resins, such as crosslinked DGEBA, are

positioned on the y axis of the multifunctional curve due to high compression properties but

negligible ionic conductivity. Similarly, liquid electrolytes, such as EMITFSI or LiTFSI/PC,

are positioned on the x-axis of the multifunctionality curve. Two reference lines were drawn

between the properties of crosslinked DGEBA or crosslinked PEGDGE (structural resins)

and EMITFSI, the ionic liquid electrolyte, in order to create a baseline for the full range of

polymer electrolytes. Another reference line for polymer electrolytes containing LiTFSI/PC

was also drawn between the properties of LiTFSI/PC (liquid electrolyte) and of crosslinked

DGEBA. The polymer electrolytes, positioned well above the reference line in Figure 4.10,

are multifunctional.

Figure 4.10a established a trend for compression modulus versus ionic conductivity of

polymer electrolytes. Figure 4.10a showed that crosslinked PEGDGE/DGEBA blend

electrolytes containing 50 wt% EMITFSI had the best multifunctionality in terms of

compression modulus and ionic conductivity. Some of the crosslinked PEGDGE/DGEBA

electrolytes containing 10 wt% of EMITFSI or LiTFSI/PC were also positioned above the

reference line but had poor ionic conductivity as were more close to the y-axis. All the

remaining polymer electrolytes were either positioned on or below the reference line.

Figure 4.10b detailed the trend in compression strength relative to the ionic conductivity of

polymer electrolytes. The reference lines, chosen here, were also drawn between the

compression strength of crosslinked DGEBA or crosslinked PEGDGE and ionic conductivity

of EMITFSI or LiTFSI/PC. Due to the brittle glassy nature, the compression strength of

reference crosslinked DGEBA was low (45 MPa). Therefore, most of the polymer

electrolytes positioned themselves well above the reference line in Figure 4.10b.


133

100 101 102 103 10410-1

100

101

102

103

104

Increasing multifunctionality

E (

MP

a)

(S/cm)

LiyB

b

Li10

:aP:bB

ExP

a

E10

:aP:bB

E50

:aP:bB

pure crosslinkedPEGDGE

pure LiTFSI/PC

pure EMITFSI

(a)

pure crosslinkedDGEBA

100 101 102 103 10410-1

100

101

102

103


pure LiTFSI/PC

pure EMITFSI


LiyB

b

Li10

:aP:bB

ExP

a

E10

:aP:bB

E50

:aP:bB

(M

Pa)

(S/cm)


(b)

Figure 4.10 Compression modulus E (a) and compression strength σ (b) of different

crosslinked PEGDGE (P) and crosslinked DGEBA (B) electrolytes containing either

LiTFSI/PC (Li) or EMMITFSI (E) as a function of ionic conductivity ҡ at room temperature.

A structural polymer electrolyte with a crosslinked 20:80 PEGDGE/DGEBA blend

containing 10 wt% EMITFSI (E10:20P:80B) had a compression modulus of 1.7 GPa and an

ionic conductivity of 4µS/cm. The ionic conductivity of E10:20P:80B polymer electrolyte was

however 250 times lower as compared to the target value of 1 mS/cm (Section 1.3). Another

promising structural polymer electrolyte with a crosslinked 40:60 PEGDGE/DGEBA blend

containing 50 wt% EMITFSI (E50:40P:60B) exhibited an ionic conductivity of 550 µS/cm


134

and a compression modulus of 35 MPa. The ionic conductivity of E50:40P:60B polymer

electrolyte was just 1.8 times lower as compared to the target ionic conductivity and its

compression modulus was 29 times lower than the target compression modulus of 1 GPa

(Section 1.3). In order to further improve the ionic conductivity and compression modulus of

polymer electrolytes, mesoporous silica particles could be used as reinforcements as

discussed in the following chapter (Chapter 5).

It is clearly evident from the multifunctionality plots (Figure 4.10) that different weight ratios

of PEGDGE/DGEBA blend containing 10 wt% and 50 wt% EMITSFI were clearly

multifunctional and were therefore selected as matrices for structural supercapacitors.

Overall, the improvements in ionic conductivity and compression properties of studied

polymer electrolytes indicate that these structural electrolytes have potential for

multifunctional structural supercapacitor applications.

Chapter 5 Composite Polymer Electrolytes

135

Chapter 5 Polymer Composite

Electrolytes

Results on the compression properties and ionic conductivity of crosslinked polymer

composite electrolytes are discussed in this chapter. As discussed in the literature review

(Section 2.4.5), among different fillers, mesoporous silica was selected as reinforcements in

polymer electrolytes. Two different forms of mesoporous silica i.e. mesoporous silica

monoliths (MSM) and mesoporous silica particles (MSP) were used to prepare polymer

composite electrolytes. Characterisation of the MSM as well as MSP shall be presented first.

Following this, results on the mechanical and electrochemical properties of MSP and crushed

MSM reinforcements in crosslinked PEGDGE will be discussed. MSP were also used as

reinforcements in different crosslinked PEGDGE and crosslinked DGEBA electrolyte

formulations in order to prepare a mechanically and electrochemically optimised polymer

composite electrolyte for structural supercapacitors.


136

5.1 Mesoporous silica

Among inorganic fillers, mesoporous silica, with pore diameters between 2 and 50 nm, has

attracted considerable interest in the last decade from both the technological and scientific

point of view [188]. Surfactant templating in aqueous solutions is employed in the presence

of inorganic precursors so that as-synthesized material contains embedded hydrophobic

domains. These hydrophobic domains are later removed by either calcinations or solvent

extraction. In the present work, high surface area mesoporous silica monoliths and particles

were explored as fillers in polymer electrolytes with the aim to improve their ionic

conductivity and mechanical properties. Mesoporous silica monoliths and particles were

selected due to their high surface area, large pore volume, excellent mechanical and thermal

stability and ability to maximise the adsorption of liquid electrolyte by preserving a porous

structure in polymer electrolytes. The proposed scheme for the structural evolution of a

mesoporous silica network during the sol-gel process is shown in Figure 5.1. Initially, a

reverse hexagonal phase of surfactant was formed by solubilising aqueous HCl (1 M) in the

Pluronic P123 [(EO)20(PO)70(EO)20] [189]. TEOS and PEO-co-PPO copolymer chains were

mutually stable forming a continuous self-assembled medium. After hydrolysis and

condensation reactions, cores containing PEO and non-reacted ethanol form a regular array.

The deposition of silica around the micelles formed walls. During ethanol washing of the

monolith and particles, the PEO-co-PPO chains were extracted from the mesopores. In case

of mesoporous silica particles, the self-assembly of the silica network was continuously

disturbed during the sol-gel process by continuous stirring of the reaction mixture which

impeded the formation of thick silica walls resulting in the formation of silica particles.

Figure 5.1 Schematics of the proposed structural evolution of silica network showing

micelles with PEO-co-PPO corona (thin lines), TEOS molecules (thick lines)and embedded

PEO-co-PPO chains in silica, adopted from Rodriguez-Abreu et al. [189].


137

5.1.1 Surface characterisation of mesoporous silica monoliths (MSMs) and

mesoporoussilicaparticles(MSP)

Nitrogen adsorption-desorption isotherms measured for mesoporous silica monoliths were

typical of a Type IV isotherm with type H2 hysteresis (formerly termed type E) [190]

showing obvious capillary condensation (hysteresis loop) at medium relative pressure which

indicated the presence of mesopores. The relative pressure range corresponding to capillary

condensation became wider with the decrease in thickness and curing temperature of MSM,

which showed the presence of pores and pore-widening in the channel-like mesopores. Large

hysteresis loops, between the adsorption and desorption curves, were typical of mesoporous

silica. Such strong hysteresis was believed to be related either to the capillary condensation

associated with large pore channels or to the modulation of the channel structure of MSM.

0.0 0.5 1.0

0

6

12

18

24

Qua

ntit

y A

dsor

bed

(cm

3 /g)

Relative Pressure (P/P0)

1.45 mm 1.87 mm 2.48 mm 3.11 mm 4.50 mm 5.52 mm

(a)

1 10 100

0.000

0.005

0.010

0.015

Pore

vol

ume

(cm

3 /g)

Pore size (nm)

1.45 mm 1.87 mm 2.48 mm 3.11 mm 4.50 mm 5.52 mm

Figure 5.2 BET nitrogen adsorption/desorption isotherms (a) and BJH pore size distribution

(b) of mesoporous silica from samples with varying reaction layer thickness.


138

In the adsorption/desorption isotherm for mesoporous silica monoliths (Figure 5.2), three

well distinguished regions were evident: (i) monolayer/multilayer adsorption, (ii) capillary

condensation taking place in the mesopores and (iii) multilayer adsorption on the outer

surface of monoliths. Capillary condensation occurred at a higher relative pressure (p/p0 =

0.4-0.7) for the mesoporous silica with pore sizes ranging from 3.78 to 7.23 nm. A delay in

isotherm (i.e. lag between adsorption and desorption isotherms) was seen which could be due

to the interconnectivity of the real pore network or due to phase transition in an isolated pore.

It was found that the variation in MSM thickness by increasing the volume of the reaction

mixture affected the surface area and pore size distribution of the silica monoliths as shown

in Table 5.1.

Sample

code

hsample AS, BET AS, Langmuir VP dP FWHM†† ρ*

(mm) (m2/g) (m2/g) (cm3/g) (nm) (nm) (g/cm3)

MSM1 1.45 14.7 21.5 0.022 5.96 1.53

1.45 ±

0.06

MSM2 1.87 16.0 23.3 0.024 6.07 2.14

MSM3 2.48 31.2 45.6 0.040 5.14 1.48

MSM4 3.11 38.6 54.0 0.036 3.78 2.05

MSM5 4.50 2.73 3.99 0.004 6.27 1.60

Table 5.1 Surface areas, AS, BET and AS, Langmuir, pore volume VP, pore width dP, full width at

the half maximum of the BJH pore size distribution FWHM and bulk density of mesoporous

silica monoliths MSM synthesised at 90°C with increasing monolith thickness hsample.

* bulk density of silica measured through Accupyc.

Figure 5.2b showed Barrett-Joyner-Halenda (BJH) pore size distributions for the mesoporous

silica samples with varying reaction layer thickness, calculated from the adsorption branch of

the isotherm. Around 90 % of the pore volume was made up of mesopores in the 2-10 nm

range. The pore size distribution (PSD) revealed that the silica monoliths had mesopores of

sizes centred at around 4.5 nm. The narrow pore size distribution (full width at the half

maximum FWHM of approximately 1.9 nm) indicated the uniformity in pore structure. There

were also some large mesopores with an average size of approximately 25 nm in the silica

network.


139

0.0 0.5 1.00

60

120

180

Qua

ntit

y A

dsor

bed

(cm

3 /g)

Relative Pressure (P/P0)

90 C 70 C 60 C

(a)

1 10 1000.00

0.04

0.08

Por

e vo

lum

e (c

m3 /g

)

Pore size (nm)

90 C 70 C 60 C

(b)


(b) of mesoporous silica monoliths by varying the curing temperature.

The surface area of the monoliths was low (38 m2/g) as compared to typical mesoporous

silica monoliths with surface areas up to 450 m2/g [161] that could be due to thick pore walls

compared to conventional materials (e.g. SBA-15 [191], MCM-41 [192]). Another possibility

of the low surface area of MSM was the very low fraction of microporosity in the samples

which for conventional mesoporous materials, contributes to most of the specific surface area

[189]. Microporosity in monoliths was related to the PEO-co-PPO chains of Pluronic P123

that were distributed molecularly in the silica network during the sol-gel process and left

micro-pores when taken away during washing with ethanol. However, portions of Pluronic

P123 remained in the core of the templating reverse aggregates [193] and thus, contribute to

the silica network with micro-pores. In addition to the low surface area, another problem with

the silica monoliths was the cracking which occurred during curing. One possible explanation


140

of the cracking of the surface of monoliths was the high temperature (90°C) curing process as

given in the mesoporous silica monolith recipe [161]. Ethanol (boiling point 78.4°C) resided

in the bi-continuous phase of silica network and therefore, vaporised instantly at 90°C.

During evaporation, the ethanol vapour caused the silica walls to rupture and thus changed

the pore structure.

Sample

code

TCuring hsample AS, BET AS, Langmuir VP† dP

‡ FWHM †† ρ*

(°C) (mm) (m²/g) (m²/g) (cm³/g) (nm) (nm) (g/cm³)

MSM2 90

1.88 ±

0.02

16.0 23.3 0.02 6.07 2.84 1.45 ± 0.06

MSM6 70 159 229 0.22 5.49 2.30 1.51 ± 0.03

MSM7 60 226 325 0.30 5.29 1.66 1.53 ± 0.04

Table 5.2 Surface areas, AS, BET and AS, Langmuir, pore volume VP, pore width dP, full width at

the half maximum of the BJH pore size distribution FWHM and bulk density of mesoporous

silica monoliths MSM cured at different temperatures TCuring.

In order to avoid cracking of mesoporous silica monoliths, the silica network was cured at

lower temperatures [161], i.e. 70°C and 60°C (Figure 5.3). Surface area and pore structure

parameters of mesoporous silica monoliths produced at various curing temperatures are listed

in Table 4.2. The BET surface area and total pore volume increased with decreasing curing

temperature. At the curing temperature of 60°C, the monoliths possessed the highest BET

surface area (226 m²/g) and total pore volume (0.3 cm³/g). The pores could not be templated

in the presence of either increased or decreased quantity of ethanol in mixture. The problem

of cracking of mesoporous silica monoliths during curing was also solved as a result of using

low curing temperature of 60°C. A monolith with few internal cracks was obtained (Figure

5.4).


141

Figure 5.4 Mesoporous silica monolith having internal cracks after ethanol washing.

In addition to mesoporous silica monoliths, mesoporous silica particles were also prepared to

be used as reinforcements for polymer electrolytes. These silica particles contained

mesopores within the silica network which was confirmed by BET surface area analysis

(Figure 5.6). Particle size was determined for the mesoporous silica particles as well as non-

porous similar sized silica particles as MSP using light scattering technique (Section 3.2.4).

The mass-median particle diameter (d50) of the non-porous silica particles NSP (d50 of 0.41

µm) was calculated to be approximately four times smaller than for the mesoporous silica

particles MSP (d50 of 1.74 µm). Particle size distribution curves for the MSP and NSP

obtained using dynamic light scattering (DLS) analysis technique are shown in Figure 5.5.

The size distribution of the MSP and NSP are found to be unimodal. NSP had a narrower

particle size distribution with a FWHM of 0.64 µm. Conversely, MSP also showed a narrow

particle size distribution curve with a FWHM of 2.85 µm (Table 5.3).


142

10-2 10-1 100 101 1020

2

4

6

8

10

Vol

ume

(%)

Particle Size (m)

MSP NSP

Figure 5.5 Particle size distributions of MSP and NSP obtained by dynamic light scattering.

Nitrogen adsorption-desorption isotherms of MSP and NSP are shown in Figure 5.6. MSP

showed a typical Type IV isotherm with Type H2 hysteresis and showed a capillary

condensation step at a relative pressure (P/Po) from 0.60 to 0.78. Such strong hysteresis was

believed to be related either to the capillary condensation associated with large pore channels

or to the variations in the MSP channel structure. In the adsorption/desorption isotherm for

the MSP (Figure 5.6), three well distinguished regions were also observed as discussed in

Section 5.1.1. A delay in isotherm of MSP (i.e. lag between adsorption and desorption

isotherms) was observed which could be due to the interconnectivity of the pore network or

due to phase transition in an isolated pore.


143

0.00 0.25 0.50 0.75 1.00

0

200

400

600

Qua

ntit

y A

dsor

bed

(cm

3 /g)

Relative Pressure (P/Po)

MSP NSP

(a)

100 101 102 103

0.0

0.2

0.4

Por

e vo

lum

e (c

m3 /g

)

Pore size (nm)

MSP NSP

(b)


(b) of mesoporous (MSP) and non-porous (NSP) silica particles.

The isotherm for the NSP did not have any hysteresis loop like MSP (Figure 5.6a). NSP

showed a typical reversible type II isotherm which is characteristic of non-porous materials

[190]. From a relative pressure (P/Po) of 0.1, the isotherm started forming an almost linear

section indicating the completion stage of monolayer coverage and the beginning of

multilayer adsorption. Barrett-Joyner-Halenda (BJH) pore size distributions for the MSP and

NSP, calculated from the adsorption branch of the isotherm, are shown in Figure 5.6b. No

porous structure was observed in the NSP as shown in Figure 5.6b. In the MSP, around 90 %

of the pore volume was made up of mesopores in the 6-20 nm range. From the pore size

distribution (PSD) curves, it was clear that the porosity of the MSP was made up of

mesopores of sizes centred at around 8 nm. The narrow pore size distribution indicated the


144

uniformity in pore structure, as expected for surfactant template materials, with the full width

at the half maximum (FWHM) of approximately 2.6 nm (Figure 5.6). There were small

mesopores constituting 0.4 cm3/g of the MSP and also some larger mesopores with an

average size of approx. 40 nm.

Silica

Particles

AS, BET AS, Langmuir VP† dP d50

†† ρ*

(m²/g) (m²/g) (cm³/g) (nm) (µm) (g/cm³)

MSP 523 728 0.76 5.80 1.74 1.77 ± 0.02

NSP 50.6 70.4 0.11 87.9 0.41 2.19 ± 0.03

Table 5.3 Surface areas, AS, BET and AS, Langmuir, pore volume VP, pore width dP, mass-median

particle diameter d50 and bulk density of mesoporous (MSP) and non-porous silica particles.

SEM images of the MSP and crushed MSM are shown in Figure 5.7. The particle size of

MSP was also confirmed from the images. SEMs of crushed MSMs showed that the particle

size was very irregular. SEM image of the crushed silica monolith (Figure 5.7a) showed the

non-uniform crushing of the monoliths which was necessary to preserve the mesopores of

MSM. This resulted in irregular particle size distribution of MSM. Micrometre sized

mesoporous silica particles provided favourable ion transport properties when these silica

particles were used to reinforce polymer electrolytes as discussed in following Section 5.2.

SEM images of MSP showed that particles were spherical in shape.


145

Figure 5.7 SEM images of crushed mesoporous silica monoliths (a) and mesoporous silica

particles (b and c).

5.2 Effect of mesoporous silica on the mechanical and electrochemical

properties of polymer composite electrolytes

In order to increase the mechanical as well as electrochemical performance of polymer

electrolytes (Chapter 4), mesoporous silica was used as filler. The interaction between silica

and polymer chains leads to the formation of the three-dimensional network and thus

provides mechanical stability to the polymer electrolyte. This exceptional microstructure is

expected to provide the silica with effective interactions towards other components of the

polymer electrolyte. Also, the addition of mesoporous silica preserves the porous structure

(i.e. free volume) of polymer electrolyte resulting in the reduced risk of leakage [194] and

maximum adsorption of liquid electrolyte [195]. Mesoporous silica monoliths (MSM),


146

mesoporous silica particles (MSP) and non-porous silica particles (NSP) were incorporated

into various crosslinked polymer electrolytes and the polymer composite electrolytes were

characterised mechanically and electrochemically as discussed in following sections.

5.2.1 IonicconductivityandcompressionpropertiesofcrosslinkedPEGDGE/MSM

compositeelectrolytescontainingTBAPF6/PC

Mesoporous silica monoliths were crushed into smaller particles and were used as filler for

crosslinked PEGDGE containing 0.1 M TBAPF6/PC as electrolyte. The resulting polymer

composite electrolyte showed very poor ionic conductivity and compression properties. The

ionic conductivity of polymer composite electrolyte dropped to half from 12.3 to 6.12 µS/cm

and the compression modulus decreased by an order of magnitude from 3.51 to 0.42 MPa at

15 wt% loading of crushed MSMs. This was attributed to monoliths having been crushed into

very irregular sized particles forming mesoporous micron-dust. Fine crushing of monoliths

was avoided in order to preserve the mesopores of the monoliths. The ionic conductivity and

compression properties of crosslinked PEGDGE containing TBAPF6/PC as a function of

increasing concentration of crushed MSM loading are shown in Table 5.4.

Sample codes† MSM ҡ E σ

wt% (µS/cm) (MPa) (MPa)

A0.80P99.2S0 0.0 12.3 ± 1.2 3.51 ± 0.04 1.89 ± 0.42

A0.80P96.7S2.5 2.5 9.21 ± 0.6 1.65 ± 0.14 1.88 ± 0.14

A0.80P94.2S5.0 5.0 10.2 ± 0.6 1.42 ± 0.24 1.75 ± 0.24

A0.80P91.7S7.5 7.5 7.40 ± 0.1 1.07 ± 0.22 1.70 ± 0.16

A0.80P88.2S10 10.0 7.32 ± 0.4 0.91 ± 0.02 1.67 ± 0.66

A0.80P86.7S12.5 12.5 13.0 ± 1.0 0.53 ± 0.31 1.57 ± 0.31

A0.80P84.2S15 15.0 6.71 ± 0.1 0.56 ± 0.07 1.52 ± 0.07

A0.80P81.7S17.5 17.5 6.12 ± 0.5 0.42 ± 0.09 0.45 ± 0.11


PEGDGE (0.1 M TBAPF6/PC) polymer electrolytes with increasing crushed MSM

concentration.

† AxPaSb – x wt% of TBAPF6/PC (A), a wt% of crosslinked PEGDGE (P) and b wt% of crushed MSM.


147

5.2.2 IonicconductivityandcompressionpropertiesofcrosslinkedMSP/PEGDGE

compositeelectrolytescontainingTBAPF6/PC

MSP were also used as filler for crosslinked PEGDGE containing 0.8 wt% 0.1 M

TBAPF6/PC as electrolyte. In comparison to crushed MSMs, MSP incorporation into

crosslinked PEGDGE electrolyte resulted in an increase of the ionic conductivity and the

compression properties (Figure 5.8). The addition of MSP to crosslinked PEGDGE

electrolyte may improve the conduction of ions at the polymer-silica interface resulting in an

increased ionic conductivity. The ionic conductivity of the crosslinked PEGDGE composite

electrolyte increased from 12.3 µS/cm to 175 µS/cm following the addition of 7.5 wt% MSP

into crosslinked PEGDGE electrolyte. However, further increasing the MSP loading resulted

in a decreased ionic conductivity which was possibly due to the silica aggregation within the

polymer network resulting in formation of local regions near MSP aggregates having reduced

ionic mobility (please see Appendix D for microscopic evaluation of 12.5 wt% MSP addition

in crosslinked PEGDGE polymer electrolytes).

The results obtained (Figure 5.8) are in accordance with the trends reported elsewhere [118].

Aravindan et al. [118] reported an increase in the ionic conductivity of PVDF-co-HFP

polymer electrolyte for 5 wt% addition of titanium dioxide particles followed by a decrease in

the ionic conductivity with increasing particle concentration. The incorporation of up to 7.5

wt% MSP into crosslinked PEGDGE also improved the compression modulus from 3.5 MPa

to 9.5 MPa (Figure 5.8a). The compression strength of crosslinked MSP/PEGDGE composite

electrolytes improved from 1.9 to 3.3 MPa (Figure 5.8b). Increase in compression properties

of polymer composite electrolytes may be attributed to the effective interactions among the

mesoporous silica particles, polymer and the liquid electrolyte as well as to the addition of

hard filler. MSP may hold a strong coordination with the polymer chains at the amount as

low as 7.5 wt% resulting in the enhancement of compression strength of polymer composite

electrolyte.


148

0 5 10 15 200

50

100

150

200

E

MSP (wt.%)

(

S/c

m)

0

4

8

12

E (M

Pa)

(a)

0 5 10 15 200

50

100

150

200

MSP (wt.%)

(

S/c

m)

0

2

4

(M

Pa)

(b)


of crosslinked MSP/PEGDGE composite electrolytes, containing 0.8 wt% TBAPF6/PC, as a

function of increasing MSP concentration.


compositeelectrolytescontainingLiTFSI/PC

MSP were also used as filler for crosslinked PEGDGE containing 0.8 wt% LiTFSI/PC as

electrolyte. Ionic conductivity and compression properties of crosslinked MSP/PEGDGE

composite electrolytes are shown in Figure 5.9. The mechanical performance of crosslinked

PEGDGE containing 0.8 wt% 0.1 M TBAPF6/PC was poor as compared to the PEGDGE

containing 0.8 wt% 1.0 M LiTFSI/PC due to increased concentration of PC (see also Section

4.3.1). PC is a necessary solvent for both LiTFSI and TBAPF6 as it imparts ion mobility but

it also plasticises the matrix. Therefore, the effect of varying MSP concentration in

crosslinked PEGDGE electrolyte containing 1 M LiTFSI/PC was also studied. The ionic


149

conductivity of the polymer composite electrolytes increased from 17.3 µS/cm to 246 µS/cm

upon the addition of 7.5 wt% MSP into crosslinked PEGDGE (Figure 5.9). However, ionic

conductivity decreased again with further increasing MSP loading which was possibly due to

the silica aggregation as discussed previously (Section 5.2.2).

0 5 10 150

90

180

270

360 E

MSP (wt.%)

(

S/c

m)

0

6

12

18

E (M

Pa)

(a)

0 5 10 150

80

160

240

320

MSP (wt.%)

(

S/c

m)

0

3

6

9

(M

Pa)

(b)


of crosslinked MSP/PEGDGE composite electrolytes containing 0.8 wt% LiTFSI/PC as a


The increase in the ionic conductivity of polymer composite electrolyte by the addition of

MSP may be attributed to the dissociation of ion-pairs in polymer electrolyte. Li+ cations or

TFSI- anions are adsorbed on the silica surface leading to high counter ion concentration in

the vicinity of the silica (space charge layer) [196]. Similar to the previously reported trends

(Section 5.2.2), the compression properties of polymer composite electrolyte also improved


150

with the addition of MSP. The compression modulus of polymer electrolyte was improved by

51% for the polymer composite electrolyte containing 7.5 wt% MSP loading (Figure 5.9a). A

60% improvement in the compression strength of polymer composite electrolyte (Figure

5.9b), from 5.1 MPa to 8.2 MPa, was also seen for crosslinked PEGDGE containing 7.5 wt%

MSP.

Nonporous silica particles (NSP) were also used as control inorganic fillers to separate the

effect of silica addition and mesoporosity. NSP were added to crosslinked PEGDGE

containing 0.8 wt% 1 M LiTFSI/PC as electrolyte. Just like increasing the MSP concentration

in crosslinked PEGDGE (Figure 5.9), increasing the concentration of NSP also improved the

ionic conductivity as well as compression properties (Figure 5.10).

0 5 10 150

40

80

120 E

NSP (wt.%)

(

S/cm

)

0

5

10

15

20

E (M

Pa)

(a)

0 5 10 150

30

60

90

120

NSP (wt.%)

(

S/cm

)

0

4

8

12

(M

Pa)

(b)


of crosslinked PEGDGE/NSP composite electrolytes, containing 0.8 wt% LiTFSI/PC, as a

function of increasing non-porous silica particles NSP concentration.


151

However, only a 500% improvement in ionic conductivity of NSP polymer composite

electrolyte was seen as compared to the 1300% improvement of the ionic conductivity of

MSP polymer composite electrolytes at 7.5 wt% silica particles loading. The much larger

improvement in ionic conductivity obtained by MSP addition revealed that the mesopores of

the MSP (Table 5.3) did help to improve the ionic mobility by possibly allowing the liquid

electrolyte to pass through the silica pores. The compression properties of NSP containing

crosslinked PEGDGE were similar to those of MSP containing crosslinked PEGDGE. The

compression modulus improved from 10.2 MPa to 14.5 MPa at 10% NSP loading in

crosslinked PEGDGE composite electrolyte (Figure 5.10a). The compression strength also

improved from 5.06 MPa to 8.59 MPa at 10 wt% NSP addition. Similar compression

properties of NSP and MSP polymer composite electrolytes suggested that the porous nature

of MSP did not negatively affected the compression properties which were possibly because

of the filling of MSP with the electrolyte.


compositeelectrolytescontainingEMITFSI

In order to further improve the ionic conductivity without negatively affecting compression

properties, crosslinked MSP/PEGDGE composite electrolytes were also prepared using

EMITFSI as liquid electrolyte. The results of ionic conductivity and compression properties

of the polymer composite electrolytes with increasing concentration of MSP are summarised

in Figure 5.11. The ionic conductivity as well as compression modulus increased significantly

by one to two orders of magnitude upon incorporation of MSP into crosslinked PEGDGE

containing 10 wt% EMITFSI as electrolyte. It can be seen that the electrochemical as well as

the mechanical properties of the polymer electrolyte containing EMITFSI outperform those

containing Li+ salt due to the absence of propylene carbonate (see also Section 4.3.1). The

ionic conductivity and compression modulus of the polymer electrolyte containing MSP

increased by 1100% and 50%, respectively, when compared to crosslinked PEGDGE

containing 10 wt% EMITFSI. In addition to the explanations given for the increased ionic

conductivity discussed previously (Section 5.2.3), Wieczorek et al. [127] have proposed

another explanation for the enhancement of the ionic conductivity to be due to Lewis acid

base type interactions among surface centres, ions and ether-oxygen base groups of the

polymer electrolyte and, therefore, the filler/polymer interactions influence the cationic and

anionic transport within the polymer electrolyte.


152

0 5 10 150

90

180

270

360

E

MSP (wt.%)

S/cm

)

(a)

0

5

10

15

20

25

E (M

Pa)

0 5 10 150

90

180

270

360

MSP (wt.%)

S/c

m)

(b)

0

3

6

9

MP

a)


of crosslinked MSP/PEGDGE composite electrolytes containing 10 wt% EMITFSI as a

function of increasing MSP content.

Increasing the MSP concentration to 10 wt% and above may have led to the formation of

aggregates within the polymer matrix resulting in a reduction of compression modulus, which

decreased from 21.9 MPa for 7.5 wt% MSP to 9.24 MPa for 15 wt% MSP (Figure 5.11a), and

the compression strength, which decreased from 6.95 MPa for 7.5 wt% MSP to 3.62 MPa for

15 wt% MSP (Figure 5.11b). Nevertheless, this study has shown that the incorporation of

MSP into the crosslinked PEGDGE containing 10 wt% EMITFSI allowed developing a resin

with improved ionic conductivity and mechanical properties. However, for structural

supercapacitors, further enhancement of the mechanical performance without deteriorating

the electrochemical performance of the polymer composite electrolytes was required.

Therefore, another structural resin (DGEBA) was studied.


153

5.2.5 Ionic conductivity and compressionpropertiesof crosslinkedDGEBA/MSP

compositeelectrolytescontainingLiTFSI/PC

Table 5.5 summarises the ionic conductivity and compression properties of crosslinked

DGEBA with increasing MSP concentration. A similar trend to that observed for the

crosslinked MSP/PEGDGE electrolytes (Section 5.2.3) was observed. Compression moduli

increased by 47% upon the incorporation of 7.5 wt% MSP into crosslinked DGEBA. The

ionic conductivity improved by almost 65% when 7.5 wt% MSP were added to crosslinked

DGEBA electrolytes. Compression strength was also improved by 2% with the incorporation

of 7.5 wt% MSP in crosslinked DGEBA electrolytes. As mesoporous silica particles were

filled with electrolyte by mixing them with the LiTFSI/PC first and then adding to the

monomer, Li+ and TFSI- ions remained in the mesopores of MSP resulting in improved ionic

mobility within the polymer electrolyte.

Sample code MSP ҡ E σ

% (µS/cm) (MPa) (MPa)

Li20B100M0 0 6.10 ± 0.03 905 ± 73.4 81.1 ± 0.27

Li20B100M2.5 2.50 6.54 ± 0.67 1232 ± 54.1 111 ± 1.02

Li20B100M5.0 5.00 8.42 ± 4.10 1278 ± 72.7 101 ± 5.43

Li20B100M7.5 7.50 10.1± 2.11 1328 ± 7.40 82.5 ± 6.31

Li20B100M10 10.0 7.85 ± 1.87 401.4 ± 63.1 85.1 ± 1.26

Li20B100M12.5 12.5 4.22 ± 0.64 859.6 ± 25.4 53.4 ± 4.02

Li20B100M15 15.0 2.14 ± 0.15 496.7 ± 39.1 41.8 ± 2.85


crosslinked DGEBA/MSP composite electrolytes containing 20 wt% LiTFSI/PC as a function

of increasing MSP concentration.


DGEBA/MSPcompositeelectrolytescontainingLiTFSI/PC

The results of MSP reinforcement of crosslinked PEGDGE as well as crosslinked DGEBA

electrolyte clearly showed that both mechanical properties as well as ionic conductivity of the

polymer electrolytes can be improved. Therefore, MSP were also used to reinforce

crosslinked PEGDGE/DGEBA. The results of the ionic conductivity and compression

properties for the composite polymer electrolytes with 10 wt% LiTFSI/PC are summarised in

Table 5.6. A blend of 20% PEGDGE and 80% DGEBA was chosen for MSP incorporation as


154

the blend had high compression properties and intermediate ionic conductivity as discussed in

previous section 4.5.1. A 150% increase in ionic conductivity was observed in the crosslinked

PEGDGE/DGEBA/MSP composite electrolyte containing 10 wt% LiTFSI/PC with the

addition 10 wt% of MSP. Compression modulus and compression strength of the polymer

composite electrolyte, containing 20/80 weight ratio of PEGDGE/DGEBA and 10 wt%

LiTFSI/PC, also improved by 6% and 38%, respectively, upon addition of 10 wt% MSP

(Table 5.6).

Sample code PEGDGE:

DGEBA

MSP ҡ E σ


Li10:20P:80B:M0

20:80

0 3.10 ± 0.70 932 ± 14.9 105 ± 5.86

Li10:20P:80B:M7.5 7.5 7.57 ± 0.54 975 ± 10.4 135 ± 3.41

Li10:20P:80B:M10 10 7.81 ± 0.74 987 ± 11.7 145 ± 0.50

Li10:40P:60B:M0

40:60

0 5.44 ± 0.90 628 ± 34.2 89.5 ± 7.11

Li10:40P:60B:M7.5 7.5 9.75 ± 1.14 642 ± 24.3 81.0 ± 1.14

Li10:40P:60B:M10 10 10.5 ± 0.21 616 ± 33.4 84.5 ± 2.01


crosslinked PEGDGE/DGEBA/MSP composite electrolytes containing 10 wt% LiTFSI/PC as

a function of increasing MSP concentration.

Table 5.6 and Table 5.7 summarise the ionic conductivity and compression properties of

polymer composite electrolytes by the addition of 7.5 wt% and 10 wt% MSP reinforcements

into the crosslinked PEGDGE/DGEBA. DGEBA addition to PEGDGE resulted in a decrease

of the ionic conductivity but increase the compression modulus. Incorporation of 10 w%

MSP into crosslinked PEGDGE/DGEBA containing 20 wt% LiTFSI/PC resulted in a 57%

improvement in ionic conductivity, a 13% improvement in compression modulus and a 27%

improvement in compression strength.


155

Sample code PEGDGE:

DGEBA

MSP ҡ E σ


Li20:20P:80B:M0

20:80

0 6.28 ± 0.73 598 ± 13.7 59.4 ± 2.11

Li20:20P:80B:M7.5 7.5 6.69 ± 0.41 604 ± 14.40 64.4 ± 2.70

Li20:20P:80B:M10 10 9.87 ± 1.21 673 ± 18.70 75.6 ± 5.11

Li20:40P:60B:M0

40:60

0 11.8 ± 0.51 42.4 ± 4.31 6.84 ± 0.44

Li20:40P:60B:M7.5 7.5 27.7 ± 2.71 53.1 ± 2.19 10.2 ± 3.21

Li20:40P:60B:M10 10 17.5 ± 0.24 25.9 ± 1.53 5.72 ± 3.24


crosslinked PEGDGE/DGEBA electrolytes containing 20 wt% LiTFSI/PC as a function of

increasing MSP concentration.


DGEBA/MSPcompositeelectrolytescontaining50wt%EMITFSI

From the previous results of varying crosslinked PEGDGE/DGEBA concentration containing

50 wt% EMITFSI, the 40/60 weight ratio of PEGDGE/DGEBA, respectively, in polymer

electrolyte showed the best performance in terms of ionic conductivity as well as

compression properties (Section 4.5.3). Therefore, this formulation of crosslinked

PEGDGE/DGEBA was selected as a matrix for MSP incorporation. Similar to the previous

results of polymer composite electrolytes containing MSP, the 40/60 weight ratio of

crosslinked PEGDGE/DGEBA composite electrolytes also had two times higher ionic

conductivity and compression properties. The compression modulus increased by 94% upon

7.5 wt% MSP incorporation into polymer composite electrolyte (Figure 5.12). Compression

strength was also increased by 122% for a matrix containing 7.5 wt% MSP. A 93% increase

in ionic conductivity was observed for 10wt% MSP addition.

Increasing the concentration of MSP to 10 wt% and above caused the compression modulus

of the composite polymer electrolyte to decrease by 60% (Figure 5.12) which was possibly

due to the MSP aggregation. Also, a drop of 69% in compression strength was observed

when the MSP loading was increased beyond 7.5 wt%. Overall, this study showed that

incorporation of MSP had a positive impact on the both ionic conductivity and compression

properties of polymer electrolytes.


156

0 5 10 150

300

600

900 E

MSP (wt.%)

S/c

m)

(a)

0

20

40

60

80

E (M

Pa)

0 5 10 150

300

600

900

MSP (wt.%)

S/c

m)

(b)

0

7

14

21

MPa)


of 40/60 weight ratio of PEGDGE/DGEBA composite electrolytes containing 50 wt%

EMITFSI, as function of increasing MSP concentration.

5.3 Multifunctionality of polymer composite electrolytes

In this chapter, three different forms of silica, including the mesoporous silica monoliths

(MSM), mesoporous silica particles (MSP) and nonporous silica particles (NSP) were

investigated as reinforcements for structural polymer electrolytes. MSP reinforcement of

crosslinked PEGDGE electrolytes improved the ionic conductivity much more as compared

to the reinforcement of MSM and NSP. However, incorporation of either MSP or NSP into

crosslinked PEGDGE did not affect the compression properties. Different structural polymer

composite electrolytes were also prepared by incorporating MSP into crosslinked

PEGDGE/DGEBA electrolytes containing LiTFSI/PC or EMITFSI as electrolytes. Figure

5.13 attempts to capture the multifunctional behaviour of the structural polymer electrolytes.


157

The ionic conductivity was plotted as a function of compression modulus and compression

strength. Pure structural resins and pure electrolytes lie upon either the x-axis (structural

resins) or y-axis (liquid electrolytes). Figure 5.13 summarises multifunctionality of the

structural polymer electrolytes containing different concentrations of MSP and NSP. The

incorporation of MSP to crosslinked PEGDGE/DGEBA electrolytes resulted in improved

ionic conductivity and compression properties as discussed in previous sections.

A reference line was drawn between the properties of crosslinked DGEBA or crosslinked

PEGDGE (structural resins) and EMITFSI (ionic liquid) or LiTFSI/PC (liquid electrolyte) in

order to create a baseline for the full range of polymer composite electrolytes. The polymer

composite electrolytes, positioned above the reference line in Figure 5.13, exhibited true

multifunctionality. Figure 5.13a detailed the trend in compression modulus as a function of

ionic conductivity of polymer electrolytes. Also, Figure 5.13b established a trend for

compression strength versus ionic conductivity of polymer electrolytes. It is clearly evident

from Figure 5.13 that most of the polymer composite electrolytes provide multifunctional

benefits. The MSP reinforced copolymer consisting of a 40/60 weight ratio of

PEGDGE/DGEBA in the crosslinked resin and 50 wt% EMITFSI outperforms the other

polymer composite electrolytes, in terms of multifunctionality, as they were positioned well

above the reference line at the centre of the plot (Figure 5.13).

A structural polymer electrolyte with a crosslinked PEGDGE containing 10 wt% EMITFSI

and 7.5 wt% MSP concentration (E10P100M7.5) had an ionic conductivity of 291 µS/cm and a

compression modulus of 21.9 MPa. The ionic conductivity of E10P100M7.5 polymer composite

electrolyte was 3.4 times lower as compared to the target ionic conductivity of 1 mS/cm

(Section 1.3) and its compression modulus was 46 times lower as compared to the target

compression modulus of 1 GPa (Section 1.3). Whereas the structural polymer composite

electrolyte made by curing a 40:60 PEGDGE/DGEBA blend containing 50wt% EMITFSI

and 7.5 wt% MSP exhibited an ionic conductivity of 849 µS/cm and a compression modulus

of 62.0 MPa. Although compression modulus was still 16 times lower as compared to the

target but its ionic conductivity was exceptional. It was also observed that all the polymer

composite electrolytes showed the maximum improvements in ionic conductivity and

compression properties at either 7.5 wt% or 10 wt% MSP additions. A further increase of

MSP concentrations in polymer composite electrolytes resulted in a decrease of their ionic


158

conductivity and compression properties which was possibly due to the silica aggregation as

discussed in previous sections.

10-1 100 101 102 103 104 10510-1

100

101

102

103

104



pure LiTFSI/PC


A0.80

PaM

y

Li0.80

PaM

y

Li0.80

PaN

z

E10

PaM

y

Li20

BaM

y

Li10

:40P:60B:My

Li10

:20P:80B:My

Li20

:40P:60B:My

Li20

:20P:80B:My

E50

:40P:60B:My

E (

MP

a)

(S/cm)

(a)

pure EMITFSI

10-1 100 101 102 103 104 10510-1

100

101

102

103


(M

Pa)

(S/cm)

A0.80

PaM

y

Li0.80

PaM

y

Li0.80

PaN

z

E10

PaM

y

Li20

BaM

y

Li10

:40P:60B:My

Li10

:20P:80B:My

Li20

:40P:60B:My

Li20

:20P:80B:My

E50

:40P:60B:My

(b)



pure LiTFSI/PC

pure EMITFSI

Figure 5.13 Compression modulus E (a) and compression strength σ (b) of different MSP (M)

or NSP (N) reinforced crosslinked PEGDGE (P) and crosslinked DGEBA (B) composite

electrolytes containing TBAPF6/PC (A), LiTFSI/PC (Li) or EMITFSI (E) as a function of

ionic conductivity ҡ at room temperature.

It is clearly evident from the multifunctionality plots (Figure 5.13) that most of the composite

polymer electrolytes including the crosslinked MSP/PEGDGE electrolyte containing 10 wt%

EMITFSI and a crosslinked 40:60 PEGDGE/DGEBA blend containing 50 wt% EMITFSI

showed a clear multifunctionality. Overall, this study had shown that incorporation of MSP


159

has a positive impact in developing a multifunctional resin. Both mechanical and

electrochemical properties of polymer composite electrolytes were improved simultaneously

by the addition of MSP which played a vital role in the enhancement for the

multifunctionality of structural supercapacitors.

Chapter 6 Structural Supercapacitors

160

Chapter 6 Structural

Supercapacitors

A significant effort of this research project has been devoted towards the development of a

low weight structural supercapacitor possessing energy storage capabilities and mechanical

properties. This chapter reports the results of the specific capacitance and shear properties of

structural supercapacitors fabricated using various separators, electrolyte salts at various

concentrations and various solid polymer electrolytes. The influence of varying the charging

time on the specific capacitance of structural supercapacitors is also analysed. Finally, the

effect of reinforcing the polymer electrolytes with MSP as well as carbon fibre activation on

the overall performance of the structural supercapacitors is presented.


161

6.1 Influence of glass fibre separators on the specific capacitance of

structural supercapacitors

The internal resistance of supercapacitors depends on the distance between two electrodes

and, therefore, on the thickness of the separator [32]. The electrochemical performance of the

supercapacitor could be improved by using thinner separators. A separator should be thin

enough to keep the electrodes apart and it should also contribute towards the overall

mechanical performance of the device. Woven glass fibre mats are potential separators as

they are electrical insulators, preventing the electrical contact between the carbon fibre

electrodes, and are available as thin mats (as thin as 35 µm thick) and possess good

mechanical properties.

Glass fibre mat Thickness (µm) Areal weight (g/m2)

ACG 1 30 22

ACG 2 60 49

Tissa 1 90 110

Tissa 2 160 200

Tissa 3 250 300

Table 6.1 Thickness and areal weight of various commercially available glass fibre woven

mats studied.

Different grades of woven glass fibre mats were studied as separators. These glass fabrics

ranged from a lightweight 49 g/m2 fabric with a thickness of 30 µm (ACG 1) to 300 g/m2

fabric with a thickness of 250 µm (Tissa 3). The weight and thickness of the woven glass

fabrics studied are summarised in Table 6.1. It would be expected that the supercapacitor

with the thinnest separator between electrodes would outperform all others in terms of

specific capacitance. However, it was found that the supercapacitor manufactured using the

thinnest available glass fibre mats (ACG 1 (30 µm) and ACG 2 (60 µm)) did not show any

discharge capacity during charge-discharge experiments. As can be seen on the optical

microscopic images (Figure 6.1) of the glass fibre mats, single mats of ACG1, ACG 2 and

Tissa 1 were not densely woven enough to prevent contact of the CF electrodes, which led to

shorting of the electrical circuit. However, both Tissa 2 and Tissa 3 glass fabrics were

completely closed and exhibited the tightest weave packing (Figure 6.1).


162

Figure 6.1 Optical micrographs of various commercially available glass fibre fabrics, (a)

ACG 1, (b) ACG 2, (c) Tissa 1, (d) Tissa 2 and (e) Tissa 3 (microscopic images taken by Dr.

Hui Qian).

The mechanical characterisation of structural supercapacitors requires that the number of

separator layers should always be in an even number in order to fabricate a balanced

composite with balanced crimp lines (i.e. have a symmetric layup). The number of layers of

glass fibre separators strongly influenced the apparent specific capacitance of supercapacitors

at a defined charging time. The specific capacitance of structural supercapacitors decreased

with increasing number of glass fabric layers. However, at the same time, the supercapacitors


163

made with two layers of ACG 1, ACG 2 or Tissa 1 exhibited very high short circuiting which

is possibly due to open fabric windows (Figure 6.1). By increasing the number of glass fibre

layers, the charge loss reduced dramatically which is possibly due to the closing of the tow

gap of glass fibre fabrics. The supercapacitors containing two layers of Tissa 2 glass fabric as

a separator showed high charge loss (87.2%) and intermediate specific capacitance (0.33

mF/cm3). The supercapacitor with two layers of Tissa 3 glass fabric as a separator had a very

small specific capacitance (0.02 mF/cm3) at a charging time of 10 s. Similarly, high charge

loss was also observed for the supercapacitors with six layers of ACG 2 (77.6%) and Tissa 1

(68.2%) but at the same time showed poor specific capacitance (0.06 and 0.14 mF/cm3,

respectively) at 10 s of charging time. Therefore, the study was continued by exploring Tissa

2 glass fabric as a separator in structural supercapacitors.

Separator Charge Discharge Method

GF Type No. of layers Charge (mC) Discharge (mC) ∆† (%) Cg (mF/cm3)

ACG 1

2 No discharging ability

4 42.0 1.25 97.0 1.021

6 18.6 1.40 92.5 0.978

ACG 2

2 47.6 0.96 98.0 0.681

4 15.7 0.43 97.3 0.253

6 0.49 0.11 77.6 0.063

Tissa 1

2 No discharging ability

4 1.79 0.17 90.5 0.159

6 0.63 0.20 68.2 0.135

Tissa 2 2 3.97 0.51 87.2 0.331

Tissa 3 2 0.82 0.05 94.0 0.023

Table 6.2 Charging and discharging capacity, charge loss and specific capacitance Cg of

structural supercapacitors manufactured using as-received carbon fibre mat, crosslinked

PEGDGE containing 0.8 wt% LiTFSI/PC and various glass fabric separators.

Charging time during charge discharge experiments = 10 s;

† ∆ is the percentage difference between the charging and discharging capacity of the composite divided by the

charging capacity.


164

6.2 Influence of varying charging time on the specific capacitance of


Filter paper was also used as separator in the fabrication of structural supercapacitors.

However, from the charge/discharge curves of structural supercapacitors manufactured from

as-received carbon fibres as electrodes, crosslinked PEGDGE containing 0.8 wt% of 1 M

LiTFSI/PC as polymer electrolyte and filter paper as separator, it was seen that the

supercapacitor charged for 10 s exhibited a high charge loss (69.9%) and, therefore, not much

energy was stored within the supercapacitor. A huge charge loss was also observed in the

structural supercapacitors with different GF separators as discussed in previous section 6.1.

The charge loss was estimated from the difference between the charging and discharging

capacity of the structural supercapacitors. In addition to high charge loss, structural

supercapacitor also showed an incomplete discharge (Figure 6.2).

-4x10-5

0

4x10-5

8x10-5

Incomplete discharge

I (A

)

t (s)

10s High charge loss (69.9%)

0 10 20 30 40

(a)

-4x10-5

0

4x10-5

8x10-5

(b)

I (A

)

t (s)

250 s

0 250 500 750 1000

-4x10-5

0

4x10-5

8x10-5 (c) 600 s

I (A

)

0 600 1200 1800 2400t (s)

-4x10-5

0

4x10-5

8x10-5 1000 s

I (A

)

(d)

0 1000 2000 3000 4000t (s)

Figure 6.2 Charge discharge curves of investigated structural supercapacitors with various

charging times of (a) 10s showing high charge loss and incomplete discharge, (b) 250 s, (c)

600 s and (d) 1000 s.


165

It was anticipated that if the charging and discharging time were to increase, then perhaps

more energy could be stored in the structural supercapacitor. It was also observed that the

energy storage capacity was better utilised when the supercapacitors were charged longer

(Figure 6.2). It was shown that the discharge capacity, measured from the area under the

discharging curve in the charge/discharge cycle, of the structural supercapacitor also

increased 600% with charging time from 10 s to 1000 s (Table 6.3). Thus, when increasing

the discharge capacity, the resultant specific capacitance of supercapacitor also increased

with increasing charging time (Figure 6.3). The charge lost was expressed as a percentage of

total charge stored for the studied conditions and decreased with increasing the charging time

(Table 6.3). The summary of the data, described above including the specific capacitance

calculated using charge-discharge method is presented in Table 6.3.

Charge time

(s)

Charge

(mC)

Discharge

(mC)

∆†

(%)

Cg

(mF/cm3)

10 0.331 0.0101 69.9 0.0804

50 1.15 0.602 47.5 0.502

100 1.84 1.25 31.8 1.03

150 2.37 1.80 23.8 1.49

200 2.87 2.30 20.1 1.90

250 3.73 3.26 12.8 2.69

500 4.36 3.75 13.9 3.10

600 5.75 5.01 12.9 3.52

750 5.01 4.41 12.0 3.64

1000 6.68 6.01 9.93 4.97

Table 6.3 Charge capacity, discharge capacity, charge loss ∆ and specific capacitance Cg as

determined by charge-discharge experiment of structural supercapacitor as function of

varying charging time.


charging capacity.

The specific capacitance of structural supercapacitors improved 60 times (from 0.08 mF/cm3

to 4.97 mF/cm3) when the charging time was increased from 10 s to 1000 s. A charging time

of 600 s was selected for further experiments due to a reduced charge loss (12.9%) and high

specific capacitance (3.5 mF/cm3). The high charge loss of structural supercapacitor which


166

resulted in low power density was a serious problem during electrochemical characterisation.

The high charge loss could have a number of reasons including the increase in thickness of

separator (details in Section 6.1), polymer electrolyte resistance (details in Sections 6.5 and

6.6) and poor layup configurations (details in Section 6.11). Another possible reason of the

high charge loss could be the poor electrical connection between the current collectors and

the electrodes due to the formation of a polymer layer around the edges of electrodes during

composite fabrication (details in Section 6.7) as the direct connection of the supercapacitor

electrodes (CF mats) to the potentiostat for electrochemical characterisation could also

introduce an additional contact resistance. All these possible reasons of high charge loss in

structural supercapacitors will be explored further in the following sections.

10 100 10000

1

2

3

4

5

Cg (

mF

/cm

3 )

Charging time (s)

Figure 6.3 Specific capacitance Cg as a function of charging time during charge-discharge

experiment.

6.3 Influence of different types of electrolyte salts on the electrochemical

and mechanical performance of structural supercapacitors

Two different salts, LiTFSI and EMITFSI (Section 3.1.3), were used in order to study their

effect on the electrochemical and mechanical performance of as-received CF reinforced

crosslinked PEGDGE matrix composites using glass fibre mats as an insulator. As discussed

in Section 4.3.1, the LiTFSI salt, when added to a polymer matrix, required a solvent (e.g.

propylene carbonate) in order to dissociate into ions. However, the addition of propylene

carbonate affected the mechanical performance of the polymer electrolytes. EMITFSI, an

ionic liquid, does not require any solvent. For that reason, LiTFSI or EMITFSI were used as


167

an ion source and blended into the resin with the aim to identify, which was the more suitable

to be used in the PEGDGE, DGEBA or PEGDGE/DGEBA resins to be used as matrix for

structural supercapacitors to enhance the energy storage capability. The results of charge-

discharge experiments for the structural supercapacitors using 600 s of charging time are

presented in Figure 6.4. The incorporation of EMITFSI into the crosslinked PEGDGE

resulted in a better specific capacitance of structural supercapacitors.

-5x10-4

0

5x10-4

1x10-3

2x10-3

(b)

I (A

)

t (s)

0.8 wt% LiTFSI/PC (a) 10wt% EMITFSI (b)

(a)

0 600 1200 1800 2400

Figure 6.4 Charge-discharge curves of as-received CF reinforced crosslinked PEGDGE

composites containing (a) 0.8 wt% LiTFSI/PC and (b) 10 wt% EMITFSI and two layers of

glass fibre mats as separator. Charging time = 600 s.

Different concentrations of LiTFSI (0.8 wt%) and EMITFSI (10 wt%) were chosen for this

study as the mechanical performance of crosslinked PEGDGE electrolyte with 0.8 wt%

LiTFSI or 10 wt% EMITFSI was very similar (Section 4.3). The aim of this study was to

improve the specific capacitance of supercapacitors without affecting their mechanical

properties. Therefore as expected, the composites containing 10 wt% EMITFSI had twice the

specific capacitance as the composite containing 0.8 wt% LiTFSI/PC. The specific

capacitance and total discharge capacity calculated from these charge discharge experiments

(Figure 6.4) are presented in Table 6.4. The composites containing EMITFSI (8.8 mF/cm3)

showed a higher specific capacitance compared to the other composite (5.8 mF/cm3).

However, in the case of structural supercapacitor containing EMITFSI; the composite lost

more than 80% of the charge (Figure 6.4 b). This high internal leakage was possibly due to


168

loose carbon fibres forced through the separator during the RIFTing process (Section 6.1) or

poor current collection during characterisation (Section 6.7).

Salt Discharge (mC) ∆†(%) Cg(mF/cm3)

0.8 wt% LiTFSI/PC 9.39 ± 0.97 21.2 ± 4.12 5.807 ± 0.43

10 wt% EMITFSI 15.6 ± 1.02 81.5 ± 6.58 8.808 ± 0.75

Table 6.4 Discharge capacity, charge loss ∆ and specific capacitance Cg of CF and GF

reinforced crosslinked PEGDGE composites containing LiTFSI or EMITFSI as electrolyte.

Charging time = 600 s.


charging capacity; Density of composites was 1.78 ± 0.11 g/cm3.

The original, as well as normalised, in-plane shear properties of structural supercapacitors are

presented in Table 6.5. The shear modulus (353 MPa) of as-received CF/GF reinforced

crosslinked PEGDGE composite containing 10 wt% EMITFSI was similar to one of the

supercapacitor with a crosslinked PEGDGE matrix containing 0.8 wt% LiTFSI/PC (334

MPa). The in-plane shear properties of the structural supercapacitors containing glass fibre

mats as separator were not affected by using a high concentration of EMITFSI (10 wt%).

Fibre volume fractions of structural supercapacitors were measured using acid digestion and

the shear properties normalised to the carbon content. Other separators were also used in the

fabrication of structural supercapacitors including the filter paper and polymer membrane

(Section 6.4). During acid digestion, the filter paper or polymer membrane separators in the

composites were also digested along with the epoxy. However, the glass fibres remained

unaffected during the acid digestion experiment. Therefore, in order to compare the results,

the carbon content was selected for the normalisation of the data as compared to the

reinforcement content.


169

Salt CF Vf

(%)

Shear properties Normalised shear properties to CF Vf = 55%

12m/MPa 12

0.5/MPa G12/MPa 12m/MPa 12/MPa

0.8 wt% LiTFSI/PC 36.8± 1.11 5.44 ± 0.37 1.67 ± 0.20 334 ±20.81 8.13 ± 0.55 499 ± 36.02

10 wt% EMITFSI 38.1± 0.82 6.12 ± 0.24 1.54 ± 0.30 353 ±27.80 8.83 ± 0.35 509 ± 40.11


shear modulus G12 of structural supercapacitors with crosslinked PEGDGE containing

LiTFSI/PC or EMITFSI.

Normalised shear properties to 55 volume % carbon content original shear properties

carbon fibre vf×55; The thickness and density

of composites containing glass fibres were 0.78 ± 0.11 mm and 1.78 ± 0.11 g/cm3 respectively.

The data showed that 10wt% EMITFSI in as-received CF and GF reinforced crosslinked

PEGDGE electrolyte performed best electrochemically as well as mechanically. Therefore,

EMITFSI was used as an electrolyte salt in the matrices for further studies.

6.4 Influence of separator type on the specific capacitance and shear

properties of structural supercapacitors

One of the ways to improve the specific capacitance of structural supercapacitors should be

reducing the distance between the electrodes. Since the requirements for improving the

mechanical and electrochemical performance of a structural supercapacitor were conflicting,

an optimisation of electrochemical and mechanical properties was required. In order to

achieve this aim of improving the internal resistance and specific capacitance, different types

of separators were investigated in this study, glass fibre mats (GF), a filter paper (FP) and a

polypropylene (PP) membrane.


170

-1x10-3

-7x10-4

0

7x10-4

1x10-3

(c)

(a)

I (A

)

t (s)

Filter paper (a) PP membrane (b) Glass Fabric (c)

(b)

600 610 620

-1x10-3

0

1x10-3

1200 1210 1220

-1x10-3

0

1x10-3

0 600 1200 1800 2400

Figure 6.5 Charge-discharge curves for the as-received CF reinforced crosslinked PEGDGE

composites containing 10 wt% EMITFSI electrolyte with (a) filter paper, (b) polypropylene

(PP) membrane and (c) two layers of glass fibre mat as separators.

The composite with PP membrane as a separator showed the best electrochemical

performance of all composites (Figure 6.5). The filter paper containing composite showed the

lowest specific capacitance but least charge loss amongst the other composite

supercapacitors. The glass fibre containing composites had an intermediate specific

capacitance and the charge loss was highest among all the composites studied. The discharge

and specific capacitance data for the structural supercapacitors containing various separators

in as-received CF reinforced crosslinked PEGDGE or DGEBA composites is summarised in

Table 6.6. The charge-discharge curves were very noisy due to the low ion mobility in the

very brittle crosslinked DGEBA matrix.


171

Matrix Separator Discharge (mC) ∆† (%) Cg (mF/cm3) ρ (g/cm3)

PEGDGE

(10wt%

EMITFSI)

FP 10.2 ± 0.42 10.4 ± 0.34 7.06 ± 0.45 1.74 ± 0.01

GF 15.6 ± 1.02 81.5 ± 6.58 8.81 ± 0.75 1.82 ± 0.04

PP membrane 10.4 ± 0.04 14.5 ± 1.04 9.90 ± 0.22 1.69 ± 0.04

DGEBA

(10wt%

EMITFSI)

FP 3.9E-4 ± 3.5E-5 7.37± 1.78 2.7E-4 ± 2.0E-5 1.37 ± 0.01

GF 5.7E-4 ± 1.2E-4 28.2 ± 8.40 3.3E-4 ± 5.0E-5 1.53 ± 0.01

PP membrane 3.7E-4 ± 1.2E-5 6.15 ± 1.18 3.5E-4 ± 2.0E-5 1.32 ± 0.01

Table 6.6 Discharge capacity, charge loss ∆, specific capacitance Cg and density ρ of as-

received CF reinforced crosslinked PEGDGE or DGEBA composites containing 10 wt%

EMITFSI and various separators. Charging time = 600 s.


charging capacity.

As-received CF reinforced crosslinked PEGDGE composites containing 10wt% EMITFSI as

electrolyte and filter paper as separator had the highest shear modulus (420 MPa) as

compared to composites containing PP membrane (306 MPa) or glass fibre (353 MPa). This

was possibly due to the good adhesion between the crosslinked matrix and filter paper in the

structural supercapacitor as no delamination was observed in the structural supercapacitors

with filter paper as separator. Structural supercapacitors, fabricated using PP membrane as a

separator, had the highest electrochemical performance which was due to the lowest separator

thickness among others. It was also observed that the specimens containing PP membrane as

a separator underwent significant delamination upon mechanical testing and also exhibited

the lowest mechanical performance. However, a glass fibre mat containing structural

supercapacitors had an intermediate electrochemical (Table 6.6) as well as mechanical

performance (Table 6.7). Therefore, glass fibre mat was used as a separator in structural

supercapacitors for further studies. It should also be noted that the thickness of the specimens

varied due to the intrinsic thickness of the separators. Shear properties were normalised to a

carbon fibre volume fraction of 55% in order to allow the comparison of shear properties

among the various structural supercapacitors. The carbon fibre volume fraction was used as

standard for normalisation because of the degradation of filter paper as well as PP membrane

during the acid digestion.


172

Polymer

electrolyte Separator

CF Vf

(%)


12m/MPa 12


PEGDGE

(10 wt%

EMITFSI)

FP 45.1±0.77 8.01±0.66 2.64±0.20 420±38.1 9.77±0.80 512±46.5

GF 38.1±0.82 6.12±0.24 1.54±0.30 353±27.8 8.83±0.35 510±40.1

PP 57.4±1.46 4.83±0.32 N/A* 306±26.4 4.63±0.31 293±25.3

DGEBA

(10 wt%

EMITFSI)

FP 42.4±0.82 44.2±7.47 14.9±2.02 2846±381 57.3±9.69 3692±494

GF 32.9±0.80 65.7±5.13 20.1±1.24 3668±229 110±8.58 6132±383

PP 55.4±0.94 45.2±1.38 19.6±2.22 3305±202 44.9±1.37 3281±201

Table 6.7 Carbon fibre volume fraction Vf, maximum shear strength τ12m, shear strength at

5000 µε and shear modulus G12 of structural supercapacitors with crosslinked PEGDGE and

crosslinked DGEBA containing 10wt% EMITFSI electrolyte and filter paper (FP), glass fibre

mats (GF) or PP membrane separators.

†Normalised shear properties at 55 volume % carbon content original shear properties

carbon fibre vf×55;

* Composite failed before 5000 µϵ; the thickness of composites containing glass fibre mats, filter paper and PP

membrane was 0.78 ± 0.02 mm, 0.72 ± 0.03 mm and 0.51 ± 0.03 mm, respectively.

6.5 Influence of the polymer electrolyte composition on the

electrochemical and mechanical performance of structural supercapacitors

Pure crosslinked PEGDGE containing EMITFSI or LiTFSI/PC as electrolyte was used as

matrix in the composite laminates (Section 6.3). However, the mechanical performance of

crosslinked PEGDGE was very poor because of its soft and rubbery nature (Section 4.3).

Therefore, PEGDGE resin was blended with DGEBA (as discussed in Section 4.5) at a fixed

content of EMITFSI (10 wt%) with the aim to improve the mechanical properties while

maintaining the electrochemical properties of the supercapacitors.

Figure 6.6 shows the charge-discharge plots for the studied structural supercapacitors as

function of the PEGDGE to DGEBA ratio. All the studied composites contained 2 layers of

glass fibre mats as the separator and 10 wt% EMITFSI as the electrolyte. The composite with

pure crosslinked PEGDGE containing 10 wt% EMITFSI as polymer electrolyte had the best

charge-discharge performance amongst all the composites while the composite with a pure

crosslinked DGEBA matrix containing 10 wt% EMITFSI had the lowest specific capacitance.

The specific capacitance gradually decreased with increasing concentration of DGEBA in the

crosslinked PEGDGE matrix.


173

0 600 1200 1800 2400-4x10-4

-2x10-4

0

2x10-4

4x10-4

(f)

(e) (d)

(c)

(b)

I (A

)

t (s)

100P:0B (a) 80P:20B (b) 60P:40B (c) 40P:60B (d) 20P:80B (e) 0P:100B (f)

(a)

750 1000 1250

0

5x10-7

1x10-6

1250 1500 1750

-4.0x10-7

-2.0x10-7

0.0


PEGDGE/DGEBA composites containing 10 wt% EMITFSI as a function of the PEGDGE to

DGEBA weight ratio.

Discharge capacity and specific capacitance of these systems after 600 s of charging time in

charge-discharge experiments is given in Table 6.8. It can be seen that the PEGDGE/DGEBA

ratio strongly influenced the specific capacitance of the supercapacitors. That is, as the

DGEBA content in PEGDGE increased, the specific capacitance decreased. The charge-

discharge curves for the structural supercapacitors with either pure crosslinked DGEBA

matrix or crosslinked 20:80 PEGDGE/DGEBA blend containing 10 wt% EMITFSI were very

noisy because of the low ion mobility associated with the very stiff crosslinked matrix and

also had a very small specific capacitance with high charge loss (Table 6.8). The pure

PEGDGE based composite had the highest charge loss which was possibly due to loose

carbon fibres passing through the separator during RIFTing. The high charge loss observed

for the structural supercapacitors with 40:60 and 20:80 PEGDGE/DGEBA blend matrix

composites was possible due to increased mechanical stiffness of the respective matrices

hindering the ionic mobility.


174

Matrix

PEGDGE:DGEBA Discharge (mC) Δ† (%) Cg (mF/cm3) ρ (g/cm3)

100:0 15.6 ± 1.02 81.5 ± 6.58 8.81 ± 0.75

1.784 ± 0.11

80:20 8.57 ± 1.13 20.1 ± 1.72 4.84 ± 0.59

60:40 6.12 ± 1.32 20.2 ± 3.65 3.41 ± 0.75

40:60 0.19 ± 0.09 72.3 ± 10.3 0.10 ± 0.04

20:80 0.10 ± 0.01 57.5 ± 3.54 0.06 ± 0.004

0:100 5.7E-4 ± 1.2E-4 28.2 ± 8.40 3.3E-4 ± 5.0E-5

Table 6.8 Discharge capacity, charge loss Δ, specific capacitance Cg and bulk density ρ of

as-received CF and GF reinforced crosslinked PEGDGE/DGEBA composites as function of

PEGDGE to DGEBA ratio. Charging time = 600 s.

The addition of DGEBA to PEGDGE improved the structural performance of the

supercapacitors. The supercapacitor with a crosslinked matrix consisting of 20% PEGDGE

and 80% DGEBA had 15 times higher shear modulus and strength as compared to pure

crosslinked PEGDGE matrix composites (See shear stress strain curves in Appendix E). The

improvement in the shear moduli and shear strengths of the composites with increasing

DGEBA content in the matrix was indicative of the increased matrix resin modulus (Section

4.5), which in turn led to changes in failure mechanism. Structural supercapacitors with a

pure crosslinked PEGDGE matrix had the smallest shear modulus (353 MPa) among all

structural supercapacitors because of the soft nature of crosslinked PEGDGE matrix.

Matrix

(wt. ratio)

CF Vf

(vol%)


12m/MPa 12


100P:0B 38.1 ± 0.82 6.12 ± 0.24 1.54 ± 0.30 353 ± 27.8 8.83 ± 0.35 510 ± 40.1

80P:20B 36.4 ± 0.74 9.50 ± 0.44 2.08 ± 0.22 438 ± 47.4 14.4 ± 0.66 662 ± 71.6

60P:40B 35.1 ± 0.65 33.1 ± 4.22 2.58 ± 0.14 506 ± 31.1 51.9 ± 6.61 793 ± 48.7

40P:60B 37.1 ± 0.22 82.7 ± 2.23 11.4 ± 0.76 2307 ± 121 123 ± 3.31 3420 ± 179

20P:80B 37.7 ± 0.95 103 ± 6.94 17.2 ± 1.23 3605 ± 195 150 ± 10.1 5259 ± 284

0P:100B 32.9 ± 0.80 65.7 ± 5.13 20.1 ± 1.24 3668±229 110 ± 8.58 6132 ± 383


shear modulus G12 of structural supercapacitors with crosslinked PEGDGE and DGEBA

polymer electrolytes containing 10wt% EMITFSI as function of PEGDGE to DGEBA ratio.

†Normalised shear properties at 55 % Vf original shear properties

carbon fibre vf×55; Composite thickness = 0.78 ± 0.02 mm.


175

Photographs of post-in-plane shear tested specimens of structural supercapacitor specimens

made using as-received CF and GF as reinforcements with increasing concentration of

DGEBA in crosslinked PEGDGE containing 10 wt% EMITFSI are shown in Figure 6.7.

Stress whitening was observed in all the failed specimens. The other major feature, observed

in specimens composed of structural supercapacitors with 60:40, 80:20 and 100:0

PEGDGE/DGEBA blend matrices, was delamination between the CF and GF layers.

Delamination was possibly due to the fibre reorientation (scissoring) of the ±45° CF and GF

mats during testing in the direction of the applied load inducing interlaminar stresses at the

ply interfaces. However, delamination was not observed in the specimens, composed of

structural supercapacitors with 0:100, 20:80 and 40:60 PEGDGE/DGEBA blend matrices,

possibly due to the stiff resin-rich layer present between the CF and GF plies. Matrix

cracking was also observed in specimens with a pure crosslinked DGEBA matrix containing

10 wt% EMITFSI resulting in the detachment of strain gauge. Matrix cracking was due to the

brittle nature of crosslinked DGEBA containing 10 wt% EMITFSI (Figure 4.7).

Figure 6.7 Photographs of post-test in-plane shear specimens of CF and GF reinforced

polymer electrolytes containing 10 wt% EMITFSI with varying content of PEGDGE and

DGEBA in the crosslinked matrix (a) Pure DGEBA, (b) 20P:80B, (c) 40P:60B, (d) 60P:40B,

(e) 80P:20B and (f) Pure PEGDGE.

6.6 Influence of EMITFSI concentration on electrochemical and

mechanical performance of structural supercapacitors

EMITFSI concentration was also varied in structural supercapacitors made from as-received

CF and GF reinforced 40:60 PEGDGE/DGEBA blend matrix to study the influence on the

electrochemical and mechanical performance of the composites. 40:60 PEGDGE/DGEBA

blend matrix containing 50 wt% EMITFSI was selected because the blend showed the best

ionic conductivity and compression properties amongst others (Section 4.5.3). The charge-

discharge curves for the as-received CF and GF reinforced crosslinked 40:60

PEGDGE/DGEBA blend matrix composites as function of EMITFSI concentration are


176

presented in Figure 6.8. It was clear that the supercapacitor containing 10 wt% EMITFSI had

the lowest specific capacitance (0.1 mF/cm3) due to the high stiffness of matrix (Section

4.5.2). There was also an obvious charge loss (72%) underlying the exponential capacitive

charging current of this supercapacitor. The supercapacitor, with the pure EMITFSI

electrolyte, had a highest specific capacitance (12 mF/cm3) and a lowest charge loss (8.8%).

-8x10-4

-4x10-4

0

4x10-4

8x10-4

(c)

(b)

(a)

I (A

)

t (s)

100wt% EMITFSI (a) 50wt% EMITFSI+40P60B (b) 10wt% EMITFSI+40P60B (c)

(c)

0 600 1200 1800 2400

600 610 6200

1x10-6

2x10-6

1200 1210 1220-1x10-6

-5x10-7

0

Figure 6.8 Charge-discharge curves for supercapacitors made using as-received CF and GF

with: (a) 100 wt% EMITFSI, (b) 50 wt% EMITFSI in 40:60 PEGDGE/DGEBA blend matrix,

or (c) 10 wt% EMITFSI in 40:60 PEGDGE/DGEBA blend matrix as electrolyte.

Figure 6.9 shows the complex impedance plot (also called Nyquist plot) of the

supercapacitors made using as-received CF and GF as function of EMITFSI in the blend

matrix. The plot can be broadly divided into two regions in all cases; at high frequencies, the

capacitive impedance Z’’ (= -1/ωC) is small, but increases with decreasing frequency. The

semi-circular shape of the plot was associated with a parallel combination of a resistive

component, due to resistance in ionic mobility, and capacitive component, due to parallel

plate geometry of supercapacitor. This semicircular shape (also called Warburg region [31])

is a consequence of the distributed resistance/capacitance in a porous electrode. At high

frequencies, the resistance and capacitance of the supercapacitor decreases as the accessible

part of the active porous electrode is small. The equivalent series resistance (ESR) was

calculated from the x-intercept of the high frequency curve (see Section 3.9.3). The

supercapacitors made using as received CF and GF with 100 wt% EMITFSI, 50 wt%

EMITFSI and 10 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix had an


177

ESR of 0.6 Ω, 5.5 Ω and 280 Ω respectively (Table 6.10). In the low frequency region, the

electrochemical charging of the double layer surface becomes significant as the ions of the

electrolyte have enough time to settle at the surface of the CF electrodes. The impedance

response approached vertical in the low frequency region which is the purely capacitive

response of the electrodes.

0 100000 200000 3000000

30000

60000

90000

100 Hz100 kHz

100 kHz

(-1)

Z''

()

Z' ()


(a)

1 Hz

(b)

(c)

0 15 30 450

50

100

150

Figure 6.9 Nyquist plots for the as-received CF and GF reinforced crosslinked 40:60

PEGDGE/DGEBA blend matrix containing increasing amounts of EMITFSI.

Frequency range of 105 Hz to 1 Hz. Applied potential of 0.5 V.

The electrochemical performance for all supercapacitors made with increasing EMITFSI

concentration in the matrix determined from charge-discharge and impedance spectroscopy is

shown in Table 6.10. The energy density E and the power density P of the supercapacitors

decreased with decreasing concentration of EMITFSI in the matrix. The decrease in power

density with decreasing EMITFSI concentration was attributed to the decrease in the amount

of ions settling at the surface of the CF electrodes.


178

EMITFSI

(wt%) Matrix Δ (%) Cg (mF/cm3)

ESR

(Ω)

E

(Wh/kg)

P

(W/kg)

100 N/A 8.76 ± 0.65 12.1 ± 0.20 0.63 0.012 838

50 40P:60B

9.55 ± 1.28 10.1 ± 0.94 3.47 0.009 47.4

10 72.3 ± 10.3 0.10 ± 0.04 280 1.2E-4 0.37


density E and Power density P of as-received CF and GF reinforced crosslinked 40:60

PEGDGE/DGEBA blend matrix composites as function of decreasing EMITFSI

concentration. Charging time = 600 s.

E = energy density in Wh/kg .

, U = 3.5 V; P = power density in W/kg.

;

Density measured using helium pyncrometry = 1.78 ±0.11 g/cm3; composite thickness = 0.78±0.03 mm.

Although decreasing concentration of EMITFSI to crosslinked 40:60 PEGDGE/DGEBA

blend matrix deteriorated the electrochemical performance of the supercapacitors (Table

6.10), their structural performance improved (Table 6.11). The shear moduli of the laminates

were improved approximately three times and strength around ten times when EMITFSI

concentration in the supercapacitor matrix was decreased from 50 wt% to 10 wt%. The

increase in the shear moduli and strength of the composites with decreasing EMITFSI

concentration was indicative of the increased matrix resin modulus (Section 4.5.3).

EMITFSI

(wt%) Matrix


12m/MPa 12


100 N/A*

50 40P:60B

7.48 ± 0.34 3.67 ± 0.56 802 ± 24.1 11.1 ± 0.50 1189 ± 35.7

10 82.7 ± 2.23 11.4 ± 0.76 2307 ± 121 123 ± 3.31 3420 ± 179


structural supercapacitors made using as-received CF and GF with crosslinked 40:60

PEGDGE/DGEBA blend matrix as function of decreasing EMITFSI concentration.

Thickness of composites = 0.78 ± 0.03 mm; CF volume fraction in composites = 37.1% ± 0.7 %;

†Normalised shear properties at carbon fibre volume fraction of 55% original shear properties

carbon fibre vf×55;

* No shear properties measured as the electrolyte was liquid.

Failed structural supercapacitor specimens showed stress whitening on the surface which

indicated matrix cracking during in-plane shear loading (Figure 6.10). The other feature


179

observed in the structural supercapacitors containing 50 wt% EMITFSI was delamination of

the CF and GF layers. However delamination was not observed in structural supercapacitor

containing only 10wt% EMITFSI in the matrix possibly due to the relatively high stiffness of

the polymer electrolyte (Section 4.5.2).

Figure 6.10 Photographs of supercapacitor specimens made from CF and GF reinforced

crosslinked 40:60 PEGDGE/DGEBA blend matrix with (a) 10 wt% EMITFSI and (b) 50 wt%

EMITFSI after in-plane shear testing.

6.7 Influence of the connectivity of copper tape and copper wire on the

electrochemical performance of structural supercapacitors

A huge charge loss of around 80% of the charging capacity was observed during the charge-

discharge experiments of the structural supercapacitors with a crosslinked PEGDGE matrix

containing 10 wt% EMITFSI. It was thought that the charge loss was possibly due to a

number of factors including poor ionic conductivity of the resin (Section 6.5), thickness of

separators (Section 6.4) or the open tow gap in separators (Section 6.1) but also poor

electrical contact between the electrochemical testing device and the CF electrodes. In order

to address the problem of high charge loss, CF based electrodes were connected with copper

wire and copper tape.

(a) Baseline (b) Copper tape electrodes (c) Copper wire connected

electrodes


180

(d) Copper tape and copper wire

connected electrodes (e) Copper tape on half electrodes (f) Overlapping of CF electrodes

Figure 6.11 Different configurations of CF based electrodes during fabrication of structural

supercapacitors.

Six different CF electrodes configurations were studied (Figure 6.11). Different CF electrode

configurations were prepared by applying copper tape around edges of the CF electrodes or

connecting silver plated copper wires to the end of each CF electrode during composite

manufacturing using RIFTing process or both. An overlapping configuration was also studied

in which both CF electrodes overlapped by placing GF, half the length of the CF mat, in the

middle. The edges of the CF electrodes were sealed with copper tape and were then

connected with copper wire (Figure 6.11 f). This overlapping configuration is also being used

by a team of researchers at the Army Research Laboratories (ARL), Aberdeen, USA [102].

600 1200 1800 2400-8x10-4

-4x10-4

0

4x10-4

8x10-4

I (A

)

t (s)

Baseline (a) Copper tape (b) Copper Wire (c) Copper tape and Wire (d) Copper tape on half mat (e) Overlapping (f)

600 650 700 7500

4x10-4

8x10-4

1200 1250 1300 1350-8x10-4

-4x10-4

0

Figure 6.12 Charge-discharge curves of different CF electrode configurations in as-received

CF and GF reinforced crosslinked PEGDGE supercapacitors with 10wt% EMITFSI.


181

The charge-discharge curves (Figure 6.12) showed that the structural supercapacitors were

charged more effectively by applying copper tape (Figure 6.11b) or copper wire (Figure

6.11c) or both (Figure 6.11d) and structural supercapacitor with copper wire/copper tape

(Figure 6.11d) showed the best electrochemical performance (Table 6.12). It was also

observed that the charging and discharging capacity of structural supercapacitors were also

affected by the change in CF electrode configurations possibly due to the different available

active area of CF electrodes for charge storage. The specific capacitances from the charge-

discharge experiments are summarised in Table 6.12. The results demonstrated that the

charge loss was small in the overlapping configuration (10% of charging capacity) and the

copper tape/wire configuration (8% of charging capacity) as compared to other CF electrode

configurations. Copper wire/copper tape configuration over CF electrodes was selected for

further studies because of the low charge loss. The specific capacitance of the composites

however was not significantly affected by the change of the electrode connection as the

change in specific capacitance is mainly associated with the change in surface area of the CF

electrodes (Section 6.12).

Configurations Discharge (mC) ∆ (%) Cg (mF/cm3)

Baseline 15.6 81.5 8.81

Copper tape 10.9 23.3 9.84

Copper wire 11.7 17.7 7.75

Copper tape / wire 10.7 8.14 10.3

Copper tape on half mat 9.56 36.8 9.26

Overlapping 7.41 9.95 7.76

Table 6.12 Discharge capacity, charge loss ∆ and specific capacitance Cg for different CF

electrode configurations of as-received CF and GF reinforced crosslinked PEGDGE

supercapacitors containing 10 wt% EMITFSI.

6.8 Influence of charge-discharge cycles on the specific capacitance of


It is well known that energy storage devices (batteries and fuel cells) age faster when exposed

to repeated charge and discharge cycles [91]. The major drawback of energy storage devices,

such as batteries or fuel cells, is their relatively small cycle life due to redox chemical

reaction taking place at the electrode surface [32]. Supercapacitors are claimed to have long

cycle life of tens of thousands of charge-discharge cycles [32]. Therefore, it was necessary to


182

investigate the specific capacitance of the structural supercapacitors over a number of charge-

discharge cycles. The as-received CF and GF reinforced crosslinked PEGDGE

supercapacitors containing 10 wt% EMITFSI was subjected to repeated charge-discharge

cycling. The composite was copper taped around edges and two copper wires were connected

to each CF electrode (Figure 6.11d). The data obtained over about 1000 cycles are shown in

Figure 6.13. A charging time of 150 s was chosen for studying the evolution of specific

capacitance as function of repeated charging/discharging of structural supercapacitors during

a charge discharge experiment. At a 150 s of charging time, the charge loss (15%) was

relatively low . It is clearly evident that even after 1000 cycles of charging and discharging of

the structural supercapacitor, there was only a small change in specific capacitance (15% of

original value of specific capacitance).

0 200 400 600 800 10000

3

6

9

Cg (

mF

/cm

3 )

Cycle number

Figure 6.13 Evolution of specific capacitance measured at 150 s of charging time for as-

received CF and GF reinforced crosslinked PEGDGE supercapacitors containing 10 wt%

EMITFSI as function of number of charge/discharge cycles.

The charge-discharge curves for cycle number 1, 500 and 1000 for the structural

supercapacitor are shown in Figure 6.14. The structural supercapacitor showed best

discharging capacity during the first cycle and the worst during the 500th cycle. However,

after 1000 cycles, the specific capacitance (4.6 mF/cm3) nearly approached the original value

(5.0 mF/cm3). Although, there was a small change, a drop of 15% of its original value, in the

specific capacitance of the structural supercapacitor from cycle number 400 to cycle number

700 (Figure 6.13) but the slight decrease in specific capacitance could be attributed to the


183

number of reasons including the fact that the charge-discharge experiment lasted for 7 days.

However, this small variation in specific capacitance is common and reported elsewhere [97,

197-199]. The results indicated the high reversibility of the charge storage process taking

place on the active surface of CF based electrodes.

-2x10-4

-1x10-4

0

1x10-4

2x10-4

(c)

(b)

I (A

)

t (s)

Cycle 1 (a) Cycle 500 (b) Cycle 1000 (c)

(a)

0 200 400 600


PEGDGE supercapacitors containing 10wt% EMITFSI at cycle number (a) 1, (b) 500 and (c)

1000 in charge-discharge experiment.

6.9 Influence of applied potential difference on the energy density of

structural supercapacitor

The electrochemical characterisation of structural supercapacitors was conducted at 0.1 V of

applied potential difference in the charge-discharge experiments. The influence of applied

potential difference on the electrochemical performance of structural supercapacitors was

further investigated by varying the potential difference from 0.1 V to 3.5 V. The structural

supercapacitors were dried before charge/discharge experiments. The charging and

discharging capacity as well as energy density increased with increasing applied potential

difference but the capacitance (discharge capacity divided by the applied potential, equation

3.10, Section 3.9.2) remained almost constant. The results are reported in Table 6.13 and

suggested using 3.5 V for the calculation of energy densities of the other studied composites.

The potential difference was not increased further above 3.5 V in the charge-discharge

experiment as the EMITFSI was stable only up to 4 V [200].


184

Potential Difference (V) Discharge (mC) Δ† (%) Cg (mF/cm3) E (mWh/kg)

0.1 10.7 8.14 10.3 0.01

0.5 56.3 16.8 10.8 0.22

1.5 175 29.6 11.2 2.02

2.5 283 22.4 10.9 5.42

3.5 357 23.0 9.81 9.59

Table 6.13 Influence of applied potential difference on the discharge capacity, charge loss Δ,

specific capacitance Cg and energy density E of structural supercapacitors made from CF

and GF reinforced crosslinked PEGDGE containing 10wt% EMITFSI.


charging capacity; density of supercapacitor = 1.78 ± 0.11 g/cm3

6.10 Influence of addition of MSP on the electrochemical and mechanical

performance of structural supercapacitors

The addition of DGEBA into the PEGDGE resin of structural supercapacitors resulted in

increased shear properties at the cost of reduced specific capacitance (Section 6.5). Similarly,

increasing the concentration of EMITFSI in structural supercapacitors improved the specific

capacitance but caused a drop in shear properties. Therefore, a different approach was

selected to enhance the mechanical as well as electrochemical performance of composites by

introducing MSP into the polymer electrolyte of structural supercapacitors. The addition of

7.5 wt % MSP into the crosslinked PEGDGE matrix containing 10 wt% EMITFSI resulted in

improved compression properties as well as ionic conductivity (as discussed in Section

5.2.4).

The electrochemical properties of the obtained as-received CF and GF reinforced crosslinked

MSP/PEGDGE composites containing 10 wt% EMITFSI were studied using charge-

discharge experiment and impedance spectroscopy. Figure 6.15 shows the Nyquist plot of the

composites with and without MSP incorporated in the matrix. The equivalent distributed

resistance (EDR), the x-intercept of the low frequency curve, for the composites containing

MSP was around 4 times lower as compared to the EDR for crosslinked PEGDGE matrix

supercapacitors. The equivalent series resistance (ESR), the x-intercept of high frequency

curve, was also reduced to half by the addition of 7.5 wt% MSP to crosslinked PEGDGE

matrix supercapacitors (Table 6.14). The decrease in ESR with MSP addition can be

attributed to the improved ionic conductivity of polymer electrolyte (Section 5.2.4).


185

0 200 400 6000

80

160

1 Hz

100 kHz

(b)

(a)

(-1)

Z''

()

Z' ()

PEGDGE (a) PEGDGE+7.5wt% MSP (b)

100 Hz

Figure 6.15 Complex impedance plots for structural supercapacitors made from as-received


7.5 wt% MSP (b) containing 10 wt% EMITFSI.

Frequency range = 105 Hz to 1 Hz. Applied potential = 0.5 V

The results for the charge-discharge experiments for the same composites are presented in

Figure 6.16. It is clear that the composites containing MSP had a higher discharge capacity as

compared to pure crosslinked PEGDGE composites and thus had a higher specific

capacitance. The charge loss (8%) of the structural supercapacitor with a crosslinked

PEGDGE matrix containing 10 wt% EMITFSI was same as compared to the charge loss

(12%) of the supercapacitor with a crosslinked MSP/PEGDGE containing 10 wt% EMITFSI.

-6x10-4

-3x10-4

0

3x10-4

6x10-4

(b)

I (A

)

t (s)

PEGDGE (a) PEGDGE+7.5wt% MSP (b)

(a)

0 600 1200 1800 2400

Figure 6.16 Charge-discharge curves for structural supercapacitors made from as-received


7.5 wt% MSP (b) containing 10 wt% EMITFSI. Charging time = 600 s


186

The electrochemical properties of the structural supercapacitors are presented in Table 6.14.

Both the energy density and the power density were doubled by the addition of MSP into the

crosslinked PEGDGE matrix. The increase in energy density was possibly due to the porous

structure of MSP which helped more ions settling in the electrochemical double layer of the

supercapacitor. MSP addition into matrix of structural supercapacitors may had limited the

ionic aggregation at the CF electrode surface and therefore, resulted in a 48% improvement in

specific capacitance.

MSP

(wt%) Matrix Δ (%)

Cg

(mF/cm3)

ESR

(Ω)

E

(Wh/kg)

P

(W/kg)

0 PEGDGE /

10wt% EMITFSI

8.14 10.3 8.63 0.010 18.0

7.5 11.7 15.2 5.14 0.019 34.7

Table 6.14 Influence of MSP addition on the charge loss Δ, specific capacitance Cg,

equivalent series resistance ESR, energy density E and power density P of structural

supercapacitors made using as-received CF and GF reinforced crosslinked PEGDGE

composites containing 10 wt% EMITFSI.

Density of supercapacitor = 1.78 ± 0.11 g/cm3.

The shear properties of the same composites are presented in Table 6.15. The addition of

MSP into the crosslinked PEGDGE matrix of the composites resulted in a four times

improvement of shear modulus and six fold improvements in shear strength. It was also

observed during testing that the failure was associated with delamination of the CF and GF

layers in both composites.

MSP

(wt%) Matrix


12m/MPa 12


0 PEGDGE

10wt% EMITFSI

6.12 ± 0.24 1.54 ± 0.30 353 ± 27.8 8.83 ± 0.35 510 ± 40.1

7.5 39.4 ± 3.09 7.12 ± 0.98 1470± 253 56.9 ± 4.46 2122 ± 365

Table 6.15 Effect of MSP additions on the maximum shear strength τ12m, shear strength at

5000 µε and shear modulus G12 of as-received CF and GF reinforced crosslinked PEGDGE

matrix composites.

Thickness of composites was 0.78 ± 0.03 mm and the CF volume content of the composites was 38.1%.


187

Photographs of the structural supercapacitor specimens after in-plane shear testing are shown

in Figure 6.17. Whitening was observed in the middle of the specimens indicating matrix

cracking during shear testing. Delamination of CF and GF layers was also observed in both

structural supercapacitor specimens.

Figure 6.17 Photographs of CF and GF reinforced crosslinked PEGDGE containing 10 wt%

EMITFSI composites after in-plane shear testing with (a) crosslinked PEGDGE and (b)

crosslinked PEGDGE/7.5 wt% MSP.

6.11 Configuration of structural supercapacitors

In order to further improve the specific capacitance of structural supercapacitors, thicker

laminates were also fabricated by laying-up three structural supercapacitors together to form

a supercapacitor “bank”. Copper wires were used to connect each CF mat of the laminate in

such a way that the conjugated supercapacitors were either connected in series or parallel as

shown in Figure 6.18. Connecting supercapacitors in series or parallel has its pros and cons;

the major advantage of connecting supercapacitors in series is that the overall potential

difference is increased (VTotal = V1 + V2 + V3 +…) [32]. However on the other hand, the

overall capacitance is reduced in the series combination of supercapacitors (1/C total = 1/ C1 +

1/ C2 + 1/ C3 …..) [32]. For example, if three supercapacitors having 1 F/cm3 capacitance are

charged at 3.5 V they would produce an output of 10.5 V (approximately) with an overall

capacitance of 0.33 F/cm3 when connected in series. The number of panels connected in

series must be kept low in order to keep the Equivalent Series Resistance (ESR)1 to a

minimum. The increase in ESR, due to increase in number of panels connected in series,

reduces the power density by hindering the movement of ions towards the electrical double

layer on each electrode.

1 The ESR rating of a capacitor is a rating of quality. A theoretically perfect capacitor would be lossless and have an ESR of zero.


188

Series combination Parallel combination

Figure 6.18 Schematic of (a) series, or (b) parallel lay-up combination of two structural

supercapacitors.

When supercapacitors are arranged in parallel the voltage that can be applied is equal to the

lowest voltage rating of an individual supercapacitor. The capacitance equals the sum of all

the individual cells (C total = C1+C2+C3…). The structural supercapacitors tend to have a very

low supply voltage rating. A poor connection between supercapacitors can cause an even

higher charge loss than the internal resistance of the structural supercapacitors themselves.

Three structural supercapacitors were connected together prior to RIFTing and the charge-

discharge curves for different combinations are shown in Figure 6.19. The same conjugated

supercapacitor performed differently when connected in series or parallel combinations. The

charge as well as discharge capacities of series combination of conjugated structural

supercapacitors was around an order of magnitude less as compared to the parallel

combination of conjugated structural supercapacitors.

CF electrodes

GF separators


189

0 600 1200 1800 2400

-2x10-3

-1x10-3

0

1x10-3

2x10-3

(c)

(b)

(a)I

(A)

t (s)

Baseline (a) Series (b) Parallel (c)

600 1200 1800

-2x10-5

0

2x10-5

Figure 6.19 Charge/ discharge curves for the as-received CF and GF reinforced PEGDGE

containing 10 wt% EMITFSI (a) baseline, (b) three supercapacitors laid-up and tested in

series, or (c) three supercapacitors laid-up and tested in parallel.

The specific capacitances of different combinations of conjugated structural supercapacitor

are presented in Table 6.16. The results showed that the capacitance was doubled when

supercapacitors were joined together in parallel. However, when the same supercapacitors

were connected in series, the capacitance was reduced seven times. In the ideal case, the

overall capacitance of series combination should be 2.6 mF/cm3 and of the parallel

combination should be 23 mF/cm3 assuming each of the three conjugated supercapacitors had

equal specific capacitance.

Combinations Discharge

(mC)

Δ†

(%)

Cg

(mF/cm3)

Theoretical

Cg (mF/cm3)

One supercapacitor 11.7 17.7 7.75 N/A

Series 10.9 50.6 1.56 2.58

Parallel 129 57.6 18.6 23.2

Table 6.16 Discharge capacity, charge loss Δ, specific capacitance Cg and theoretical

specific capacitance of structural supercapacitor assembly made using as-received CF and

GF reinforced crosslinked PEGDGE containing 10 wt% EMITFSI connected either series or

parallel combinations. Charging time= 600 s

Density of supercapacitor is 1.78 ± 0.11 g/cm3


190

6.12 Influence of CF activation on the electrochemical and mechanical

performance of structural supercapacitors

Structural supercapacitors were also fabricated using activated carbon fibre mats as

electrodes (Section 3.5). Chemically activated CF mats were provided by Dr H. Qian and

were used as electrodes in supercapacitors because of their high surface area [102]. The

mechanical properties of the activated CF mats were similar to those of as-received CF mats

(Table 3.12). Copper tape and copper wire were used to connect each CF based electrode to

enable in-situ testing of electrochemical properties. The activation of CF mats was necessary

due to the dependence of specific capacitance mainly on the surface area of CF electrode. The

activation of CF mats formed nano-sized pits on fibre surface (as discussed in Section 2.4.6)

and therefore, increased the overall surface area of the electrode from 0.210 m2/g to 21.4 m2/g

(Table 3.12). This led to an increase in the number of sites for charge storage and thus,

increased the specific capacitance of the electrode [146].

6.12.1 Structural supercapacitors with a crosslinked PEGDGE matrix

containing10wt%EMITFSI

Activated CF and GF reinforced PEGDGE polymer electrolyte composites were fabricated

and were tested both electrochemically and mechanically. Figure 6.20 shows the impedance

plot for the structural supercapacitors fabricated by the impregnation of as-received CF and

ACF with PEGDGE polymer electrolyte containing 10wt% EMITFSI. It is evident that the

as-received CF based supercapacitor exhibited a smaller semicircle with low equivalent

distributed resistance (EDR), as compared to the ACF based supercapacitor. EDR arise from

the resistance offered by the ionic diffusion through the pores of the ACF electrodes and

therefore contributes to the overall resistance of the supercapacitor. Increased EDR of the

ACF based supercapacitor also confirmed the increase in the number of pores at the surface

of electrodes as compared to the CF based supercapacitor.


191

0 400 800 12000

80

160

240

(b)

(a)

-Z''

()

Z' ()

as-received CF (a) ACF (b)

100 Hz

1 Hz

100 kHz

Figure 6.20 Impedance plots for CF and GF reinforced crosslinked PEGDGE composites

with 10 wt% EMITFSI containing (a) as-received CF or (b) ACF electrodes.

Frequency range = 105 Hz to 1 Hz; Applied potential = 0.5 V.

The results of the charge-discharge experiment are shown in Figure 6.21. It was evident that

the as-received CF based supercapacitors showed lower charge loss (8% of the charging

capacity) as compared to the ACF based supercapacitors (47% of the charging capacity). The

ACF based supercapacitor had a high charge loss and incomplete charging and discharging

capacity after 600 s. The internal resistance was estimated from the final charging current

after the capacitive component died away. ACF based supercapacitors also showed high

internal resistance as compared to the as-received CF based electrodes which was in

comparison to the impedance study. The increase in the charging and discharging capacity of

the ACF based supercapacitors can be attributed to the high surface area of electrodes due to

nano-sized pit formation during CF activation. This resulted in an increased number of ions

stored at the double layer formed at the electrode/electrolyte interface.


192

-4x10-4

-2x10-4

0

2x10-4

4x10-4

I (A

)

t (s)


600 s Charging time

0 600 1200 1800 2400

(b)

(a)







Although, Figure 6.21 showed a clear advantage of ACF over CF based supercapacitors in

terms of electrochemical performance, the discharging part of the ACF based supercapacitors

was incomplete. Therefore, the charging time of supercapacitors was increased from 600 s to

1500 s in order to allow for a complete discharge and to reduce the charge loss during

-4x10-4

-2x10-4

0

2x10-4

4x10-4

(a)

(b)

I (A

)

CF/PEGDGE/GF (a) ACF/PEGDGE/GF (b)

0 1000 2000 3000 4000

t (s)

1500 s of Charging time


193

charging of ACF based supercapacitor. At 1500 s of charging time, the ACF based structural

supercapacitor exhibited a decreased charge loss from 47% to 8% of the charging capacity

(Figure 6.22).

The specific capacitance data obtained from charge-discharge experiments as well as

impedance spectroscopy for the as-received CF and ACF based supercapacitors at 600 s as

well as 1500 s of charging time is presented in Table 6.17. Irrespectively of the charging

time, the specific capacitance of the ACF based supercapacitor was around eight times higher

than that of the CF based supercapacitor. In comparison to the two orders of magnitude

increase in the BET surface area of ACF electrodes (Table 3.12), the increase in specific

capacitance of structural supercapacitors when using ACF electrodes was low. This may be

attributed to the lower ion content of the polymer electrolyte. However, similar behaviour

was also observed by Snyder et al. [102] who observed 7 times increase in specific

capacitance of structural supercapacitors when using HTA-modified ACF. Not only the

specific capacitance but also the energy density showed a remarkable increase when using

ACF (Table 6.17). However, the power density remained almost the same. Since power

density is dependent on how fast the ions moves to form the double layer at the

electrode/electrolyte interface and thus is mainly dependent on the ionic conductivity of the

polymer electrolyte. On the other hand, the energy density is dependent on number of ions

stored at the electrode/electrolyte interface. The increased number of ions stored at the

electrode surface led to increased charge storage and thus, increased capacitance and energy

density.

Electrodes Charging time (s) Δ† (%) Cg (mF/cm3) ESR (Ω) E (Wh/kg) P (W/kg)

as-received

CF

600 8.14 10.3 8.63

0.010 18.0

1500 4.12 13.1 0.013

ACF 600 47.1 86.1

10.3 0.081

16.1 1500 8.51 96.3 0.091


density E and power density P of structural supercapacitors made from as-received CF or

ACF and GF reinforced crosslinked PEGDGE matrix containing 10 wt% EMITFSI at a

charging time of 600 s or 1500 s.

Density of supercapacitors = 1.78 ± 0.11 g/cm3.


194

Table 6.18 shows the shear properties of the composites reinforced by as-received CF or ACF

and GF separators. The shear modulus as well as shear strength remained almost same

following the activation of CF. The results were normalised to a fibre volume fraction of 55%

in order to make the shear results comparable with the rest of the shear properties of the

studied structural supercapacitors as shown in Table 6.18.

Electrodes


12m/MPa 12


as-received CF 6.12 ± 0.24 1.54 ± 0.30 353 ± 27.8 8.83 ± 0.35 510 ± 40.1

ACF 6.15 ± 0.74 1.34 ± 0.10 292 ± 16.0 8.88 ± 1.07 422 ± 23.1


5000 µε and shear modulus G12 of CF and GF reinforced crosslinked PEGDGE matrix


†Normalised shear properties at carbon fibre volume fraction of 55%original shear properties

carbon fibre vf×55; The thickness of

composites was 0.78 ± 0.02 mm and the carbon fibre volume content of the composites was 38.1%.

Photographs of the failed in-plane shear specimens with CF and ACF reinforcements are

shown in Figure 6.23. Delamination was observed in both specimens possibly due to the soft

rubbery resin-nature of the matrix between the CF and GF plies. Stress whitening was also

observed in the failed specimens due to the cracking of matrix.

Figure 6.23 Photographs of CF and GF reinforced crosslinked PEGDGE composites

containing 10 wt% EMITFSI after in-plane shear testing with (a) as-received carbon fibre, or

(b) activated carbon fibre (ACF) reinforcements.


195

6.12.2 Structural supercapacitorswith a crosslinked 40:60 PEGDGE/DGEBA

blendmatrixcontainingdifferentEMITFSIconcentrations

The ACF based electrodes were also used as reinforcement to fabricate a supercapacitor with

crosslinked 40:60 PEGDGE/DGEBA blend matrix containing 10 wt% EMITFSI. The

EMITFSI concentration in the matrix of structural supercapacitors was raised to study the

influence of CF activation on their electrochemical and shear properties. The polymer

electrolyte based on a crosslinked 40 to 60 weight ratio of PEGDGE to DGEBA containing

50 wt% of EMITFSI performed best in terms of ionic conductivity and compression

properties (Section 4.5.3). The results for the as-received CF reinforced structural

supercapacitor were already presented in the previous section (Section 6.6). The semicircular

shape of the Nyquist plots (Figure 6.25) for all three composites were similar to those

previously studied. An equivalent series resistance (ESR) of 0.7 Ω, 3.6 Ω and 304 Ω was

found for the supercapacitors with 100 wt% EMITFSI, 50 wt% EMITFSI in crosslinked

40:60 PEGDGE/DGEBA blend matrix and 10 wt% EMITFSI in crosslinked 40:60

PEGDGE/DGEBA blend matrix (Table 6.19).

0 100000 200000 3000000

30000

60000

90000

120000

100 kHz

(c)

(b)

-Z''

()

Z' ()


(a)1 Hz

0 20 400

50

100

150

Figure 6.24 Nyquist plot for the ACF and GF reinforced crosslinked 40:60

PEGDGE/DGEBA blend matrix composites containing various concentrations of EMITFSI.


The results of the charge-discharge experiments for the ACF and GF reinforced crosslinked

40:60 PEGDGE/DGEBA blend matrix composites containing various concentrations of

EMITFSI are presented in Figure 6.25. It is clear that the supercapacitor containing 10 wt%

EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix had the poorest charge


196

discharge capacity. There was also an obvious internal resistance underlying the exponential

capacitive charging current of the supercapacitor with 10 wt% EMITFSI in crosslinked 40:60

PEGDGE/DGEBA blend matrix. Structural supercapacitor with crosslinked 40:60

PEGDGE/DGEBA blend matrix containing 10wt% EMITFSI had the highest charge loss

(75% of the charging capacity) whilst as expected the pure EMITFSI based supercapacitor

had the lowest charge loss (4% of charging capacity).

-4x10-3

-2x10-3

0

2x10-3

(c)

(b)

(a)

I (A

)

t (s)


Charging

0 600 1200 1800 2400

600 610 6200

3x10-6

6x10-6

1200 1210 1220-2x10-6

-1x10-6

0

Discharging

Figure 6.25 Charge-discharge curves for the ACF and GF based supercapacitors with (a)

100 wt% EMITFSI, (b) 50 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend

matrix, and (c) 10 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix.

The energy density E and the power density P of the supercapacitor (Table 6.19) decreased

with the decreasing concentration of EMITFSI in the crosslinked 40:60 PEGDGE/DGEBA

blend matrix. The decrease in power density was attributed to the low ionic conductivity of

the polymer electrolyte that resulted in a decrease in the number of ions accessing the pores

of the CF electrodes.


197

EMITFSI (wt%) Matrix Δ (%) Cg (mF/cm3) ESR (Ω) E (Wh/kg) P (W/kg)

100 N/A 4.28 189 ± 3.65 0.69 0.181 765

50 40P:60

B

12.2 76.4 ± 6.22 3.61 0.072 42.4

10 75.1 0.49 ± 0.09 304 4.8E-4 0.52


density E and power density P of structural supercapacitors made from ACF and GF

reinforced crosslinked 40:60 PEGDGE/DGEBA blend matrix composites containing various

concentrations of EMITFSI.

Density and thickness of laminates were 1.78 ± 0.11 g/cm3 and 0.78 ± 0.03 mm, respectively.

The decrease in concentration of EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend

matrix deteriorated the electrochemical performance of supercapacitors (Table 6.10) but at

the same time, the structural performance of the laminates was improved (Table 6.20). The

composites with 10 wt% EMITFSI in crosslinked 40:60 PEGDGE/DGEBA blend matrix had

a three time larger shear moduli and around nine times higher shear strength as compared to

composites containing the same fibres but 50 wt% EMITFSI in crosslinked 40:60

PEGDGE/DGEBA blend matrix. The shear properties for the supercapacitor having pure

EMITFSI as electrolyte were not measured since the liquid electrolyte has no structural

performance. The reduction in the moduli and strength of the composites with increasing

EMITFSI concentration was indicative of the decreased matrix resin modulus (Section 6.6).

EMITFSI

(wt%) Matrix


12m/MPa 12


100 No shear properties measured due to liquid electrolyte

50 40P:60B

7.52 ± 0.66 4.41 ± 0.48 762 ± 21.9 10.9 ± 0.95 1100 ± 31.6

10 64.5 ± 4.24 8.97 ± 0.92 1870 ± 194 93.1 ± 6.12 2699 ± 280


ACF and GF based supercapacitors with crosslinked 40:60 PEGDGE/DGEBA blend matrix

containing increasing amounts of EMITFSI.

Composite thickness = 0.78 ± 0.02 mm; CF Vf = 37.1 vol%

Photographs of the post-in-plane shear tested structural supercapacitor specimens containing

ACF and GF as reinforcements in a crosslinked 40:60 PEGDGE/DGEBA blend matrix with


198

increasing EMITFSI concentrations are shown in Figure 6.26. Delamination was observed in

structural supercapacitor specimens containing 50 wt% EMITFSI. Fibre reorientation

(scissoring) was also observed in both specimens in the direction of applied load. Stretching

of the fibres resulted in a necking behaviour at the middle of the structural supercapacitor

specimens containing 10 wt% EMITFSI was observed. Matrix cracking was also observed in

both specimens.

Figure 6.26 Photographs of ACF and GF reinforced crosslinked 40:60 PEGDGE/DGEBA

blend matrix with increasing amounts of EMITFSI after shear testing; (a) 10 wt% EMITFSI

and (b) 50 wt% EMITFSI.

6.12.3 Structural supercapacitors with a crosslinked MSP/PEGDGE matrix

containing10wt%EMITFSI

In order to further enhance the electrochemical as well as mechanical performance of

structural supercapacitors, the MSP reinforced crosslinked PEGDGE matrix containing 10

wt% EMITFSI was further reinforced with activated carbon fibres. The structural

supercapacitors were copper taped around the edges and were connected with copper wire.

The incorporation of 7.5 wt% MSP into crosslinked PEGDGE matrix containing 10 wt%

EMITFSI and as-received CF resulted in a remarkable improvement in the energy as well as

power densities and shear properties (Section 6.10). Therefore, it was decided to investigate

the effect of CF activation on the performance of MSP reinforced crosslinked PEGDGE

matrix structural supercapacitors with the aim to further improve their energy density.

The Nyquist plots for the as-received CF or ACF and GF reinforced MSP/PEGDGE polymer

electrolyte containing 10 wt% EMITFSI are shown in Figure 6.27. An equivalent series

resistance of 5.07 Ω and 5.14 Ω was measured for the supercapacitors containing as-received

CF and ACF based structural supercapacitors, respectively (Table 6.21). It is worth noting

that the ESR remained unaffected by the activation of CF electrodes. However, the EDR in


199

ACF based structural supercapacitors was increased due to increase in number of pores as

compared to the CF based supercapacitors.

0 50 100 150 2000

15

30

45

60

100 kHz

- Z

'' (

)

Z' ()


(a)

(b)

1 Hz

100 Hz

Figure 6.27 Complex impedance plots for CF and GF reinforced crosslinked MSP/PEGDGE

matrix containing 10 wt% EMITFSI with (a) as-received CF electrode, or (b) ACF

electrodes.


Figure 6.28 shows the charge-discharge curves for the supercapacitors made from activated

CF or as-received CF electrodes, GF separators and MSP/PEGDGE containing 10 wt%

EMITFSI as matrix. The charge-discharge curves showed that the charge loss increased as

the as-received CF electrodes were replaced with ACF based electrodes in the fabrication of

structural supercapacitors. This high charge loss (46% of charging capacity) in ACF based

structural supercapacitor having MSP/PEGDGE matrix was further reduced by increasing the

charging time from 600 s to 1500 s. Figure 6.29 shows the charge-discharge plot for the as-

received CF and ACF based structural supercapacitors characterised at 1500 s of charging

time. The increase in charging time reduced the charge loss of the system from 46% to 12%

of charging capacity (Table 6.21). However, ideally, a supercapacitor should charge fully in

fraction of seconds but due to the low surface of CF electrodes as compared to the

commercially available electrodes and the high internal resistance of the structural

supercapacitors, high charging time was required.


200

-6x10-4

-3x10-4

0

3x10-4

6x10-4

600 s of charging time

(b)I

(A)

t (s)


(a)

0 600 1200 1800 2400


reinforced crosslinked MSP/PEGDGE composites containing 10 wt% EMITFSI.



reinforced crosslinked MSP/PEGDGE composites containing 10 wt% EMITFSI.

Charging time = 1500s.

The electrochemical data from the charge-discharge experiments and impedance

spectroscopy are summarised in Table 6.21. The composites showed a significant

improvement in specific capacitance as well as energy density by introduction of activated

CFs as electrodes during fabrication of structural supercapacitors. However, power density of

-8x10-4

-4x10-4

0

4x10-4

8x10-4

(a)

(b)

I (A

)

CF/MSP/PEGDGE/GF (a) ACF/MSP/PEGDGE/GF (b)

0 1000 2000 3000 4000

t (s)

1500 s of Charging time


201

structural supercapacitors remained unaffected as it is dependent on the ionic conductivity of

polymer electrolyte.

Electrodes Charging time (s) Δ† (%) Cg (mF/cm3) ESR (Ω) E (Wh/kg) P (W/kg)

as-received

CF

600 11.7 15.2 5.07

0.016 35.7

1500 1.02 20.0 0.020

ACF 600 46.1 94.0

5.14 0.092

35.5 1500 11.9 112 0.110


density E and power density P of structural supercapacitors made using CF or ACF and GF

reinforced crosslinked MSP/PEGDGE matrix containing 10 wt% EMITFSI at a charging

time of 600 s or 1500 s.

Density of supercapacitor is 1.78 ± 0.11 g/cm3.

The shear properties of the composites are presented in Table 6.22. As seen before the shear

properties remained unaffected by using the ACF in the structural supercapacitors (Table

6.22). There was minor improvement in the shear modulus from 1.5 GPa to 1.8 GPa as the

as-received CF electrodes were replaced with ACF electrodes in structural supercapacitors.

This minor improvement could be attributed to the improvement of the fibre/matrix interface.

Since the activation of CF introduced nano-sized pits at the fibre surface, the crosslinked

polymer electrolyte formed a stronger bond with rough surfaced CF and thus improved the

mechanical performance of composites.

Electrodes


12m/MPa 12


as-received CF 39.4 ± 3.09 7.12 ± 0.98 1470 ± 253 56.9 ± 4.46 2122 ± 365

ACF 38.6 ± 2.07 8.44 ± 0.55 1763 ± 342 55.7 ± 2.99 2545 ± 494


5000 µε and shear modulus G12 of CF and GF reinforced crosslinked MSP/PEGDGE matrix


Thickness of composites was 0.78 ± 0.02 mm and the carbon fibre volume content of the composites 38.1 vol%.


202

Photographs of the post-in-plane shear tested structural supercapacitor specimens containing

as-received CF or ACF and GF reinforced crosslinked MSP/PEGDGE matrix are shown in

Figure 6.30. As discussed in previous sections (Section 6.10), whitening due to matrix

cracking and delamination of CF and GF layers were also observed in the failed specimens.

The fibres in both CF and GF mats were reoriented in the direction of the applied load

inducing interlaminar stresses resulting in delamination at the ply interfaces.

Figure 6.30 Photographs of structural supercapacitors consisting of crosslinked

MSP/PEGDGE matrix containing 10 wt% EMITFSI, GF separator and (a) as-received CF or

(b) ACF reinforcements after in-plane shear testing.

6.13 Multifunctionality of structural supercapacitors

The Ragone plot (Figure 6.31) correlates the specific energy (energy density) with the

specific power (power density) of structural supercapacitors and is similar to the one plotted

elsewhere [91]. Figure 6.31 demonstrates that all the structural supercapacitors with ACF

electrodes lie in the supercapacitor range. However, all structural supercapacitors made with

CF electrodes were positioned in the capacitor range of the Ragone plot. This is possibly due

to the low surface area of CF electrodes (0.21 m2/g) in comparison to that of the ACF

electrodes (21 m2/g). The supercapacitor fabricated using ACF electrodes, GF separator and

pure EMITFSI ionic liquid outperforms the other supercapacitors but it has no structural

properties. The energy density and power density of the ACF based structural supercapacitors

is about two orders of magnitude below that of commercial supercapacitors. However, the

fabricated structural supercapacitors have an edge in possessing mechanical properties as

compared to the commercial storage devices.


203

0.01 0.1 1 10 100 1000100

102

104

106

CF/Ea:40P:60B/GF

CF/E10

:xP:zM/GF

ACF/E10

:P90

/GF

ACF/Ea:40P:60B/GF

ACF/E10

:xP:zM/GF

P (W

/kg)

E (Wh/kg)

Supercapacitors

Capacitors

Batteries

Fuel cells

Figure 6.31 Ragone plot relating energy density E to the power density P of studied

structural supercapacitors in comparison to other energy storage devices.

CF-as-received carbon fibre mat; ACF- activated carbon fibre mat; GF-glass fibre mat; P-crosslinked PEGDGE;

B- crosslinked DGEBA; E-EMITFSI; M-mesoporous silica particles.

Figure 6.32 summarises the multifunctional performance of the structural supercapacitors

fabricated and characterised, mechanically as well as electrochemically, as discussed in

previous sections. The plot correlates the specific capacitance with an in-plane shear modulus

of the structural supercapacitors. Materials with multifunctional performance in terms of

specific capacitance and shear modulus will lie in the upper right quadrant of the plot.

Reference lines were also drawn in Figure 6.32 between the in-plane shear modulus (24.2

GPa [201]) of commercial structural composite with ±45° fibre orientation and the specific

capacitance of commercial supercapacitors (8 F/cm3 [202]). Another reference line was also

drawn between the in-plane shear modulus of crosslinked DGEBA composites (4.8 GPa) and

the specific capacitance of ACF/pure EMITFSI/GF supercapacitor in order to compare the

results within the limits of the current study. ACF and GF reinforced crosslinked

PEGDGE/DGEBA composites containing 50 wt% EMITFSI outperformed the ones

containing only 10 wt% EMITFSI electrochemically. However, ACF and GF reinforced

crosslinked MSP/PEGDGE electrolyte composites showed a unique combination of specific

capacitance and shear modulus (Figure 6.32) indicating that these composites do deliver

useful multifunctional performance.


204

10-4 10-2 100 102 10410-1

100

101

102

CF/E10

:P90

/S

CF/E10

:B90

/S

CF/E10

:xP:yB/GF

CF/Ea:40P:60B/GF

CF/E10

:xP:zM/GF

ACF/E10

:P90

/GF

ACF/Ea:40P:60B/GF

ACF/E10

:xP:zM/GF

G12

(G

Pa)

Cg (mF/cm3)

Increasingmultifunctionality

Commercial supercapacitor

Structuralcomposite

ACF/E100

/GF

CF/B100

/GF

Figure 6.32 Multifunctional plot of studied structural supercapacitors relating shear modulus

G12 to the specific capacitance Cg.

CF- as received carbon fibre mat; ACF- activated carbon fibre mat; S- different separators including GF mat,

polypropylene membrane and filter paper; GF-glass fibre mat; P-crosslinked PEGDGE; B- crosslinked DGEBA;

E-EMITFSI; M-mesoporous silica particles.

Chapter 7 Conclusion and Future Works

205

Chapter 7 Conclusions and

Suggestions for Future Work

This chapter concludes the dissertation by summarising the major findings of the current

study and presents suggestions to further extend the research. In this dissertation, an effort

has been made to provide a new perspective to the multifunctionality of structural

composites. The current study has defined a broad framework to fabricate novel structural

composites that can store electrical energy and bear mechanical loads simultaneously.


206

7.1 Conclusions

The research presented a novel holistic concept of multifunctional materials based on carbon

and glass fibre reinforced composite systems that have dual functionalities; store electrical

energy and carry mechanical load. The carbon and glass fibre reinforced composites were

fabricated into a supercapacitor. Structural supercapacitors could be conceivably applied to

the load-carrying structures that require electrical energy, such as unmanned aerial vehicles

and ground electrical vehicles, mobile phones and laptops. The research into the optimisation

of mechanical and electrochemical functionalities of structural supercapacitors was

challenging because both functionalities have conflicting requirements; the improvement in

one functionality leads to the loss in other functionalities. Therefore, a thorough study was

conducted to explore the performance of individual subcomponents of structural

supercapacitors. This involved the implementation of a holistic research approach by

embracing the optimisation of the mechanical and electrochemical performance of the

individual subcomponents of structural supercapacitors including the carbon fibre based

electrodes, separator and polymer matrix. The research work is in its early stages and requires

considerable further development. This dissertation demonstrated the basic concepts. The

research demonstrated in this dissertation can be broadly divided into three main aspects

which are summarised in the following sections.

7.1.1 Developmentsofthepolymerelectrolytes

Three different polymer electrolytes, including, poly (ethylene glycol) diglycidylether

(PEGDGE), diglycidylether of bisphenol-A (DGEBA) and polyacrylonitrile (PAN) gel

polymer electrolytes, were formulated using different salts, as the ion source, and the

characterisation of the mechanical and electrochemical performance of these structural

polymer electrolytes was discussed in Chapter 4. Since the PAN based polymer electrolyte

was a gel, oscillatory rheological characterisation of PAN was carried out in order to

characterise its mechanical properties. The mechanical properties of crosslinked PEGDGE

and crosslinked DGEBA electrolytes were tested in compression because the testing required

a small amount of material. Structural supercapacitors require a very ion conductive but

mechanically strong polymer matrix. It was demonstrated that the pure gel electrolyte (PAN)

was clearly unsuitable for the role of polymer matrix in the development of structural

supercapacitors because of poor mechanical performance and therefore, was considered as

the control sample/matrix for a purely electrical device.


207

Crosslinked PEGDGE electrolytes were soft rubber-like matrices (compression modulus of 9

MPa) but showed reasonable electrochemical performance (ionic conductivity of 27.6

µS/cm). Crosslinked DGEBA electrolytes were brittle glass-like matrices with high

mechanical performance (compression modulus of 3068 MPa) but showed very poor

electrochemical performance (ionic conductivity of 3.6 µS/cm). The polymer electrolytes

under consideration contained a complex interlay of variables affecting both ionic

conductivity and compression modulus, including the amount and type of crosslinkers, length

and concentration of polyether, concentration and type of salt as well as degree of

crosslinking of the polymer matrix. Overall, it was found that increase in ion conductivity

resulted in a decreased mechanical performance. This suggested the formulation of polymer

electrolytes with a broad spectrum of mechanical and electrochemical behaviour spanning

from a highly ion conductive but structurally weak polymer electrolyte (PAN) to a highly

structural but poorly ion conductive (DGEBA) polymer electrolyte.

The electrochemical and mechanical performance of PAN and crosslinked PEGDGE polymer

electrolytes were improved by changing different parameters including the type of salt and

salt/polymer weight ratios. The storage modulus of PAN gel polymer electrolyte was

increased from 102 Pa to 105 Pa and the ionic conductivity of PAN based polymer electrolyte

was increased from 1.4 mS/cm to 3.8 mS/cm by using Li+ salt and increasing the polymer

concentration (as discussed in Section 4.2). The compression modulus and ionic conductivity

of crosslinked PEGDGE polymer electrolyte was increased by using different salts (as

discussed in Section 4.3.1). It was also demonstrated that amongst different salts studied for

polymer electrolytes, EMITFSI resulted in the best mechanical and electrochemical

performance. Different other salts studied in this work including LiTFSI and TBAPF6 were

solid and therefore required a plasticiser for the matrix (propylene carbonate). The addition of

propylene carbonate into the PEGDGE matrix, containing LiTFSI salt, resulted in improved

ion conductivity (from 7.9 µS/cm to 17 µS/cm) but negatively affected the compression

modulus (decreased from 14.8 MPa to 10.2 MPa). EMITFSI was an ionic liquid and therefore

did not require propylene carbonate. The addition of 0.8 wt% EMITFSI into crosslinked

PEGDGE matrix resulted in an ionic conductivity of 19.5 µS/cm and a compression modulus

of 12.2 MPa.

In order to further increase the mechanical properties without affecting the ionic conductivity

of polymer electrolytes, DGEBA was added to crosslinked PEGDGE polymer electrolytes


208

with 10 wt% EMITFSI and the compression modulus was increased by 2-4 orders of

magnitude (Section 4.5). An increase in compression modulus and ionic conductivity was

also observed by the increasing the EMITFSI concentration from 10 wt% to 50 wt% in

crosslinked PEGDGE/DGEBA electrolyte. A polymer electrolyte with an ionic conductivity

of 500 µS/cm and a compression modulus of 30 MPa was formulated by optimising the ratio

of PEGDGE, DGEBA and EMITFSI (Section 4.5.3). Overall, the results suggested that a neat

homo-polymer electrolyte will be unlikely to meet the structural as well as electrochemical

needs of the polymer electrolyte. The addition of bisphenol-A functional groups and

reduction of PEG content in the polymer electrolyte result in decreased ion conductivity but,

at the same time, increased compression modulus.

7.1.2 Developmentsofthepolymercompositeelectrolytes

One of the two different approaches used in the development of multifunctional polymer

electrolytes was the addition of high strength brittle matrix (crosslinked DGEBA) into the

low strength soft matrix with reasonable electrochemical performance (crosslinked

PEGDGE) for optimising the multifunctionality of structural polymer electrolytes (as

discussed in previous section 7.1.1). Another approach followed was the addition of

inorganic fillers into the polymer electrolytes with the aim to further improve the

electrochemical and mechanical performance. Mesoporous silica was considered best for the

improvement of the multifunctionality of polymer electrolyte due to the insulating nature of

silica inhibiting the electronic movement but at the same time facilitating the ionic movement

due to the presence of mesopores throughout the silica particles. The introduction of nano-

structured mesoporous silica particle reinforcement into the polymer electrolyte allowed the

matrix to successfully perform dual roles of mechanical and electrochemical functionalities.

Undoubtedly, there is further optimisation required in the pore size distribution and the

particle size in order to further improve the interfacial interactions of polymer electrolyte and

silica particles which is considered to be the controlling factor of stiffness and interfacial

performance.

Mesoporous silica particles and monoliths were prepared (Chapter 3) and characterised

(Chapter 5). Mesoporous silica monoliths had a poor mechanical performance when used as

reinforcements in crosslinked PEGDGE polymer electrolyte due to a poor monolith/polymer

interface. A clear phase separation between the monolith and PEGDGE matrix was observed

(Section 5.2.1.1). Therefore, another approach was chosen i.e. the addition of mesoporous


209

silica particles to crosslinked PEGDGE and crosslinked DGEBA polymer electrolytes as a

means to achieve high ionic conductivity as well as compression modulus. The introduction

of MSP into the polymer electrolyte resulted in an order of magnitude improvement in the

modulus and ionic conductivity. The increase of the ambient temperature ionic conductivity

was attributed to the dissociation of ion-pairs in the polymer matrix and therefore, based on

the surface acid-base property of such particles, these dissociated anions or cations were then

adsorbed on the surface leading to higher counter ion concentration in the vicinity of the

oxide (space charge layer) [196]. Wieczorek et al. [127] had also previously explained that

the enhancement of ionic conductivity of silica reinforced polymer electrolytes was due to the

Lewis acid base type interactions among surface centres, ions and ether-oxygen base groups

of the polymer electrolyte which were indicative of the filler/polymer interactions influence

on the cationic transport within the polymer electrolyte. The increase in compression

modulus was attributed to the addition of hard inclusions (MSP).

The presence of mesopores in the silica particles also facilitated the improvements in

electrochemical and mechanical properties of structural polymer electrolytes. This was

further confirmed by introducing comparable sized non-porous silica particles (NSP) into the

polymer electrolyte. The NSP addition into the structural polymer electrolytes also resulted in

improved ionic conductivity but the MSP incorporation showed much better ionic

conductivity (three times improvement) as compared to the NSP reinforcements without

affecting the mechanical performance (Section 5.2.1.4). A polymer composite electrolyte

with an ionic conductivity of 0.8 mS/cm and a compression modulus of 62 MPa was

formulated by optimising the PEGDGE, DGEBA, EMITFSI and MSP concentrations

(Section 5.2.4).

The major hindrance in the utilisation of MSP for the use of reinforcement in structural

polymer electrolytes and later for the structural supercapacitors was the slow production of

MSP. Currently, a maximum of 10g - 15 g silica particles could be produced in 5 days and a

minimum of 45-50 g MSP was required for the fabrication of a single supercapacitor

composite using a RIFT. Both the mechanical and electrochemical performance of

crosslinked PEGDGE polymer electrolytes were improved by the addition of MSP which

played a vital role in the augmentation of improved electrochemical and shear properties in

structural supercapacitors.


210

7.1.3 Developmentsofthestructuralsupercapacitors

The characterisation of the mechanical and electrochemical performance of structural

supercapacitors was discussed in Chapter 6. The effects of separator type and thickness,

polymer electrolyte and activation of CF electrodes on the mechanical and electrochemical

performance of structural supercapacitors were investigated. A structural supercapacitor with

as-received CF electrodes, crosslinked PEGDGE containing 0.8 wt% LiTFSI/PC as polymer

electrolyte and filter paper as separator was fabricated. The major problem observed during

the electrochemical characterisation of this structural supercapacitor was the huge charge loss

(70% of charging capacity) and incomplete charging (Section 6.1). The huge charge loss was

attributed to the poor ionic conductivity of the polymer electrolytes and the poor current

collection from the CF based electrodes during charging.

Different separators, including woven glass fibre mats and polypropylene membrane, were

also studied to optimise the mechanical and electrochemical properties of structural

supercapacitors. Glass fibres were selected because they are good insulators and have good

mechanical properties, adding to the mechanical performance of supercapacitors. Five

different thicknesses of glass fibre mats were studied (Section 6.1). The specific capacitance

was dependent on the distance between the electrodes [32]. Therefore, the electrochemical

performance of a supercapacitor could be improved by employing a thin separator. However,

a huge charge loss was observed in all the cases studied (more than 70% of charging

capacity). This was due to the relatively open weave of the glass fibre mat allowing the short

circuiting. Among the different separators studied (Section 6.4), the specific capacitance of

structural supercapacitors with a polypropylene membrane as a separator (9.9 mF/cm3) was

best as compared to the ones with glass fibre mats (8.8 mF/cm3) and filter paper (7.0

mF/cm3). However, supercapacitors with PP membrane separators had poor shear properties

resulting in premature delamination and early mechanical failure due to poor

membrane/polymer electrolyte adhesion. Glass fibre mat was best due to the reduced charge

loss as well as reasonable specific capacitance. In shear testing of the structural

supercapacitor specimens, scissoring of the carbon fibres of the ±45° laminates was observed

and therefore, the shear testing was stopped before the start of scissoring.

The effect of increasing charging time on the electrochemical performance of the structural

supercapacitors during charge-discharge experiment was also studied. It was observed that

the charge storage was directly proportional to the charging time. Surprisingly, the charge


211

loss was also reduced with an increase in charging time. The charge loss estimated from the

difference between the charging and discharging capacity during the charge-discharge

experiment of structural supercapacitors, was reduced from 70% of charging capacity to 13%

of charging capacity by increasing the charging time from 10 s to 10 min (Section 6.2).

The mechanical and electrochemical performance of structural supercapacitors with different

salts including LiTFSI and EMITFSI were also studied. It was observed that the composites

containing EMITFSI performed best mechanically as well as electrochemically (Section 6.3).

The influence of the polymer electrolyte composition on the mechanical and electrochemical

performance of structural supercapacitors was also studied. It was observed that the specific

capacitance decreased with an increasing concentration of DGEBA in crosslinked PEGDGE

polymer electrolyte due to reduced ion mobility. However, the shear properties were

improved. A composite based on crosslinked 40:60 PEGDGE/DGEBA blend matrix with

10wt% EMITFSI concentration showed an optimised shear modulus (83 MPa) and specific

capacitance (0.1 mF/cm3) as discussed in section 6.5. In order to further optimise the specific

capacitance, it was tried to increase the EMITFSI concentration from 10 to 50 wt% in the

crosslinked 40:60 PEGDGE/DGEBA blend matrix. The structural supercapacitors showed an

improvement in the power density but the specific capacitance as well as mechanical

performance of the composite remained almost same as the PEGDGE matrix containing

10wt% EMITFSI based structural supercapacitors. The influence of MSP addition to

crosslinked PEGDGE electrolyte based structural supercapacitors was also studied (Section

6.10). The energy density, power density and the shear modulus increased from 0.010 Wh/kg,

16.1 W/kg and 0.35 GPa to 0.016 Wh/kg, 35.5 W/kg and 1.5 GPa respectively.

The specific capacitance of structural supercapacitors was also improved by improving the

electrical connectivity of CF electrodes. This was achieved by attaching copper wires to the

CF mats and by sealing the edges of the CF mats with copper tape. It was observed that the

use of copper wire and copper tape in structural supercapacitors slightly reduced the charge

loss but the problem of charge loss was not completely eliminated (Section 6.7). The

performance of structural supercapacitors during 1000 charge-discharge cycles was also

studied and it was demonstrated that the specific capacitance remained almost constant

throughout the 1000 cycles of charging and discharging (Section 6.8). Charge-discharge

experiments were also conducted at the working potentials of up to 3.5 V (Section 6.9). It

was observed that the specific capacitance of the structural supercapacitors remained almost


212

constant by increasing the working potential of the experiment. In order to further enhance

the performance of structural supercapacitors, thicker laminates were also fabricated by

laying up three structural supercapacitors together to form a supercapacitor “bank”. These

thick laminates were characterised electrochemically by connecting them in series or parallel.

The specific capacitance decreased in series combination and increased in parallel

combination of same laid-up structural supercapacitor.

The influence of the activation of CF based electrodes was also studied. It was observed that,

upon CF activation, the specific capacitance of the structural supercapacitors was improved

by an order of magnitude. The activation of CF based electrodes was necessary due to the

dependence of specific capacitance mainly on the structure of fibre electrode. The activation

of CF mats formed nano-sized pits on fibre surface and therefore, increased the overall

surface area of the electrode from 0.21 to 21 m2/g. This led to an increase in number of sites

for charge storage and thus, increased the specific capacitance of the electrode [146].

However, introduction of increased number of nano-sized pits on the surface of the fibre

electrode during activation could lead to a substantial reduction in mechanical properties.

Therefore, CF mats were activated only up to the extent at which the mats retained their

mechanical strength (Table 3.12). The ACF based composites showed improved charging and

discharging capacity as compared to the as-received CF based composites which was due to

the high surface area of electrodes due to nano-sized pit formation during CF activation. The

energy density also showed a remarkable increase by the CF activation from 0.01 to 0.08

Wh/kg in PEGDGE based composites. However, the activation of CF did not affect the

power density as the power density is dependent on the mobility of ions in the polymer

electrolyte. The shear modulus as well as shear strength remained almost unaffected by the

activation of CF in PEGDGE based composites.

The ACF based electrodes were also used as reinforcement for crosslinked 40:60

PEGDGE/DGEBA blend matrix containing 50 wt% EMITFSI (Section 6.12.2). A

supercapacitor using pure EMITFSI (no polymer) was also fabricated. A supercapacitor with

a crosslinked 40:60 PEGDGE/DGEBA blend matrix containing 10wt% EMITFSI led to the

highest charge loss value (75% of the charging capacity) whilst the pure EMITFSI based

supercapacitor performed best (4% of charging capacity). The laminate containing 10 wt%

EMITFSI in a supercapacitor with a crosslinked 40:60 PEGDGE/DGEBA blend matrix

resulted in an around three times improvement in the shear moduli and an around nine times


213

improvement in strength as compared to 50 wt% EMITFSI in the supercapacitor with a

crosslinked 40:60 PEGDGE/DGEBA blend matrix. The shear modulus as well as shear

strength remained almost unaffected by the activation of CF in a supercapacitor with

crosslinked 40:60 PEGDGE/DGEBA blend matrix.

Electrochemical and mechanical performance of structural supercapacitors was further

enhanced by using MSP reinforced PEGDGE as a polymer electrolyte along with activated

carbon fibre mat to fabricate a structural supercapacitor (Section 6.12.3). The crosslinked

MSP/PEGDGE composites containing ACF electrodes showed a significant improvement in

specific capacitance and energy density because of increase in surface area of electrodes. The

improved structural supercapacitor, having crosslinked MSP/PEGDGE, ACF, GF as polymer

electrolyte, electrodes and separator respectively, had an energy density of 0.1 Wh/kg, a

power density 36 W/kg and a shear modulus of 1.7 GPa.

7.2 Suggestion for future work

This dissertation details the capacity and benefit of using structural supercapacitors in

military and civilian applications. Although the performance of structural supercapacitors

could be improved considerably, e.g. the specific capacitance had been improved from 0.08

mF/cm3 (as-received CF and filter paper reinforced crosslinked PEGDGE polymer

composites containing 0.8 wt% LiTFSI/PC) to 112 mF/cm3 (ACF and GF reinforced

PEGDE/MSP polymer composites containing 10 wt% EMITFSI), there are still many aspects

of structural supercapacitors that require further research work. In order to understand the

relationship between the ionic conductivity and compression properties of the polymer

electrolytes or specific capacitance and the shear properties of structural supercapacitors, a

simple Group Interaction Modelling (GIM) [203, 204] could be a way forward. Other aspects

that require further research work include the following challenges:

7.2.1 Improvementsinthemultifunctionalityofpolymerelectrolytes

An ionic conductivity of 0.8 mS/cm and a compression modulus of 62 MPa for the polymer

electrolyte were achieved in the current study. However further improvement is required in

order to increase the electrochemical performance of structural supercapacitors. Recently,

number of ionic liquids [205, 206] was synthesised exhibiting high ionic conductivity (20

mS/cm). Therefore, other improved ionic liquids should be explored in polymer electrolytes.

The tensile and shear properties of structural polymer electrolytes should also be explored in

future. Further optimisation of ionic liquid and polymer matrix concentration is required.


214

Addition of mesoporous silica particles to polymer electrolytes leads to an increase in ionic

conductivity and compression properties of polymer composite electrolytes. A further

optimisation of mesoporous silica particles is required. A decrease in particle size and

improvement in porosity as well as surface area of the MSP can further improve the

multifunctional performance of the polymer electrolyte.

7.2.2 Improvementsintheenergydensityofstructuralsupercapacitors

The improvement in energy density is mainly dependent on the specific capacitance of the

structural supercapacitors. The specific capacitance of the structural supercapacitors was

improved from 0.08 mF/cm3 to 112 mF /cm3. At a working voltage of 3.5 V, this will be

equivalent to an energy density of around 0.1 Wh/kg. Current state of the art supercapacitors

have energy densities of up to 10 Wh/kg and modern batteries have energy densities of up to

250 Wh/kg. To compare structural supercapacitor technology against these competitors

(current state of the art supercapacitors), the energy density needs to be increased to atleast

two orders of magnitude. A major improvement in energy storage can be achieved by

increasing the surface area of the CF mat based electrodes. The activation of CF mats has

increased the surface area from 0.2 to 22 m2/g (Section 3.5) but a further increase in surface

area is required. This could be achieved by grafting carbon nanotubes [207] or by coating the

carbon fibre electrodes with activated carbon black powder having high surface area.

Structural supercapacitors with activated carbon black coated ACF electrodes were later

studied by MSc student J. Tu [208] and mentioned an improvement in specific capacitance

from 70 to 126 mF/cm3, without any significant change in shear properties, when carbon

black powder was coated at the surface of ACF electrodes.

A small improvement in electrochemical performance of structural supercapacitors was

achieved by reducing the distance between an electrode and insulator (i.e. using thin

insulators). This can be done by switching from either glass fibre mats to polymer

membranes or by using a thinner glass fibre mat which still has to avoid the CF/CF contact

through the separator. Another approach to improving the energy storage will be to move to

hybrid storage device having a combination of supercapacitor and battery or more precisely

pseudo supercapacitors in which charge-transfer pseudo capacitance is generated from

reversible Faradic reactions occurring on the surface of CF mat based electrodes. However,

there are number of hurdles involved in pursuing this idea of hybrid energy storage devices

e.g. the consumption of materials involved in the battery component during the charging and


215

discharging of the devices resulting in an early mechanical degradation or the corrosion of

structural carbon fibres or glass fibres due to the use of alkalis or acids involved in battery

chemistry.

7.2.3 Improvementsinthepowerdensityofstructuralsupercapacitors

At this stage, the structural supercapacitors showed a power density of around 42 W/kg at 3.5

V. Current state of the art supercapacitors exhibit a power density of up to 10,000 W/kg. This

challenge is considered to be the dominant one. The power density can be improved by either

decreasing the equivalent series resistance of the structural supercapacitors or increasing the

applied voltage. Electrical tests of carbon fibre mats suggest that the high resistance values in

structural supercapacitors are not associated with the in-plane weave resistivity (around 0.6

Ω). The most probable cause of high resistance is the poor ionic conductivity of the polymer

electrolyte matrix. The addition of MSP to polymer electrolytes and the optimisation of salt

concentration in the polymer matrix may have improved the ionic conductivity but further

tailoring of the matrix microstructure is required to improve the paths for the ion migration

within the polymer electrolyte matrix. Another way of lowering the internal resistance is to

decrease the polymer electrolyte thickness between two electrodes [32]. However, a portion

of ions are removed from the bulk of polymer electrolyte during the charging of structural

supercapacitor and form a double layer at the electrode/electrolyte interface. This resulted in

a decrease of ionic concentration and eventually the ionic conductivity of the polymer

electrolyte. If the thickness of the polymer electrolyte is reduced then the concentration of

ions will be decreased and, therefore, will limit the power and energy density of the system.

The charge loss of the structural supercapacitors can also be improved by grafting ionic

species (e.g. sulfophenyl groups [209]) to the surface of the CF mat based electrodes. A

simple approach of increasing the power density can be to increase the applied voltage which

can be achieved by simply preparing and then characterising the structural supercapacitors in

an inert atmosphere as well as using ionic liquids with broader voltage window.

7.2.4 Improvementsinthemechanicalperformanceofstructuralsupercapacitors

Delamination and poor shear performance has been seen to be a critical failure mode for

these structural supercapacitors because of the unusual resin, non-optimised fibre surface

chemistry and non-optimised fabrication conditions. The use of thin polymer membranes in

the structural supercapacitor lead to an improved electrochemical performance but resulted in

poor mechanical performance due to early delamination at the electrode/insulator interface.


216

Therefore, further exploration of improving the electrode/insulator interface will be beneficial

by surface modifications of polymer membrane based insulators e.g. grafting or plasma

treatments. Increasing the laminate thickness by stacking multiple structural supercapacitor

laminates is another solution but this layup configuration of structural supercapacitor will

deteriorate the specific capacitance of the system. Different other mechanical properties of

structural supercapacitors including the compression and tensile properties could also be

determined. Although, HTA carbon fibre woven fabrics were studied as potential electrodes

of the structural supercapacitors, it is a possibility to use carbon fibre woven fabrics with

varying thickness, in future work, to see the effect of thickness of the electrodes on the

mechanical performance of structural supercapacitors. Different other structural geometries

of carbon based electrodes, e.g. micro-braided carbon fibres [210, 211], could also be studied

as potential structural supercapacitor electrodes as they may offer additional mechanical

performance without increasing the product cost.

In summary, these materials have enormous potential and, once mature, will have a

significant impact on a wide range of civilian and military applications. However, this field is

still very immature and there are still considerable hurdles to be addressed, most notably the

power density of these materials.

Appendix A

217

Appendix A Result tables of polymer electrolytes and polymer

composite electrolytes

Key

A stands for 0.1MTBAPF6/PC,

Li.stands for 1.0 M LiTFSI/PC,

Na stands for 1.0 M NaClO4/PC,

E stands for EMITFSI,

P stands for crosslinked PEGDGE with TETA

B stands for crosslinked DGEBA with MCHA

M stands for MSP

N stands for NSP

All electrochemical (Section 3.5) and mechanical (Section 3.6.2) characterisation

measurements were repeated 5 times and the error was reported as standard deviation

Table A.1 Ionic conductivity and mechanical characterisation data for crosslinked PEGDGE

polymer electrolytes containing 0.8 wt% of different electrolytes

Sample code 0.8 wt% of Electrolyte in

PEGDGE

ҡ E σ


A0.80P99.2 0.1 M TBAPF6/PC 12.3 ± 1.23 5.46 ± 0.200 1.86 ± 0.210

Li0.80P99.2 1.0 M LiTFSI/PC 17.3 ± 1.52 10.2 ± 0.240 5.06 ± 0.350

Na0.80P99.2 1.0 M NaClO4/PC 18.3 ± 3.53 11.3 ± 0.230 5.11 ± 0.401

E0.80P99.2 EMITFSI 19.5 ± 3.20 12.2 ± 0.250 5.96 ± 0.241

Appendix A

218


polymer electrolytes by varying EMITFSI concentration

Sample code EMITFSI ҡ E σ

(wt %) (µS/cm) (MPa) (MPa)

E0.8P99.2 0.8 19.5 ± 3.20 12.2 ± 0.250 5.96 ± 0.241

E10P90 10 27.6 ± 2.63 9.00 ± 0.330 4.88 ± 0.452

E20P80 20 29.0 ± 2.03 6.34 ± 0.281 4.67 ± 0.304

E30P70 30 38.9 ± 2.97 4.53 ± 0.340 3.58 ± 0.164

E40P60 40 80.2 ± 2.58 4.01 ± 0.302 2.41 ± 0.337

E50P50 50 176 ± 2.95 3.83 ± 0.410 1.16 ± 0.101

E60P40 60 162 ± 2.15 3.91 ± 0.511 1.21 ± 0.100

Table A.3 Ionic conductivity and mechanical characterisation data for crosslinked DGEBA

polymer electrolytes by varying 1.0 M LiTFSI/PC

Sample code 1.0M

LiTFSI/PC ҡ E σ

nE-mB (wt%) (wt%) (µS/cm) (MPa) (MPa)

Li0B100 0 N/A 3044 ± 155 45.6 ± 1.68

Li10B90 10 1.92 ± 0.240 1583 ± 14.14 113 ± 17.2

Li20B80 20 6.10 ± 0.0301 905 ± 73.4 81.1 ± 0.270

Li40B60 40 11.9 ± 1.06 25.1 ± 0.58 68.5 ± 0.701

Li60B40 60 138 ± 3.58 0.922 ± 0.401 0.631 ± 0.0801

Li80B20 80 1580 ± 13.8 0.211 ± 0.0102 0.120 ± 0.0401

Appendix A

219

Table A.4 Ionic conductivity and mechanical characterisation data for crosslinked

PEGDGE/DGEBA electrolytes with 10 wt% of 1.0 M LiTFSI/PC

Sample code ҡ E σ


Li10:100P:0B 20.3 ± 2.42 6.42 ± 0.311 2.18 ± 0.180

Li10:80P:20B 10.7 ± 0.401 15.4 ± 0.302 6.19 ± 0.311

Li10:60P:40B 8.10 ± 0.414 60.7 ± 3.30 9.04 ± 0.351

Li10:40P:60B 5.44 ± 0.902 628 ± 34.2 89.5 ± 7.11

Li10:20P:80B 3.10 ± 0.701 932 ± 14.9 105 ± 5.86

Li10:0P:100B 1.92 ± 0.240 1583 ± 14.14 113 ± 17.2


PEGDGE/DGEBA electrolytes with 10 wt% of EMITFSI.

Sample

code ҡ E σ


E10:100P:0B 27.6 ± 2.63 9.00 ± 0.330 4.88 ± 0.452

E10:80P:20B 19.1 ± 1.28 16.3 ± 0.781 6.06 ± 0.590

E10:60P:40B 7.41 ± 1.07 34.2 ± 2.56 21.1 ± 0.601

E10:50P:50B 5.13 ± 0.460 53.3 ± 0.901 192 ± 29.8

E10:45P:55B 4.93 ± 0.481 68.1 ± 1.54 213 ± 26.0

E10:40P:60B 4.67 ± 0.391 305 ± 7.55 179 ± 7.32

E10:35P:65B 4.59 ± 0.270 740 ± 39.2 180 ± 15.1

E10:30P:70B 4.21 ± 0.331 932 ± 20.9 162 ± 21.2

E10:25P:75B 4.11 ± 0.201 1253 ± 24.4 145 ± 28.1

E10:20P:80B 3.98 ± 0.142 1693 ± 24.3 97.2 ± 7.68

E10:0P:100B 3.58 ± 0.130 3068 ± 40.6 39.2 ± 1.19

Appendix A

220


PEGDGE/DGEBA electrolytes with 50 wt% of EMITFSI.

Sample code ҡ E σ


E50:100P:0B 176 ± 2.95 3.83 ± 0.403 1.16 ± 0.0803

E50:80P:20B 106 ± 3.55 5.15 ± 0.620 2.68 ± 0.342

E50:60P:40B 158 ± 4.89 6.89 ± 0.311 11.4 ± 1.02

E50:40P:60B 538 ± 15.6 32.0 ± 3.04 9.04 ± 0.382

E50:20P:80B 1057 ± 25.4 28.8 ± 2.38 8.05 ± 0.242

E50:0P:100B 253 ± 17.5 91.4 ± 4.65 9.07 ± 0.580


composite electrolytes containing 0.8 wt% of 0.1 M TBAPF6/PC as function of increasing

MSP concentration


(wt%) (µS/cm) (MPa) (MPa)

A0.80P99.2M0 0.0 12.3 ± 1.23 3.51 ± 0.04 1.89 ± 0.22

A0.80P96.7M2.5 2.5 30.9 ± 0.90 3.68 ± 0.37 2.22 ± 0.18

A0.80P94.2M5.0 5.0 61.9 ± 7.60 4.68 ± 0.94 2.75 ± 0.24

A0.80P91.7M7.5 7.5 175 ± 13.8 9.54 ± 0.16 3.29 ± 0.12

A0.80P88.2M10 10 127 ± 12.1 4.52 ± 0.62 1.94 ± 0.19

A0.80P86.7M12.5 12.5 57.7 ± 4.74 4.37 ± 0.18 1.78 ± 0.12

A0.80P84.2M15 15 23.0 ± 3.20 3.53 ± 0.20 1.57 ± 0.13

A0.80P81.7M17.5 17.5 12.9 ± 1.80 1.89 ± 0.42 5.50 ± 0.15

Appendix A

221


composite electrolytes containing 0.8 wt% of 1.0 M LiTFSI/PC as function of increasing

MSP concentration.


(wt%) (µS/cm) (MPa) (MPa)

Li0.80P99.2M0 0 17.3 ± 1.52 10.2 ± 0.240 5.06 ± 0.350

Li0.80P96.7M2.5 2.50 60.9 ± 9.01 12.0 ± 0.370 7.11 ±0.681

Li0.80P94.2M5.0 5.00 81.9 ± 7.60 13.9 ± 0.940 7.68 ± 0.870

Li0.80P91.7M7.5 7.50 246 ± 22.9 15.4 ± 0.660 8.17 ± 0.570

Li0.80P88.2M10 10.0 187 ± 26.1 11.6 ± 0.621 6.84 ± 0.390

Li0.80P86.7M12.5 12.5 27.0 ± 0.741 9.87 ± 0.180 6.55 ± 0.621

Li0.80P84.2M15 15.0 10.4 ± 0.202 6.53 ± 0.201 5.42 ± 0.322


composite electrolytes containing 0.8 wt% of 1.0 M LiTFSI/PC as function of increasing

NSP concentration.



Li0.80P99.2N0 0 17.3 ± 1.52 10.2 ± 0.240 5.06 ± 0.350

Li0.80P96.7N2.5 2.50 57.1 ± 3.56 11.9 ± 0.730 7.22 ± 1.32

Li0.80P94.2N5.0 5.00 73.5 ± 12.5 13.1 ± 0.321 7.14 ± 1.17

Li0.80P91.7N7.5 7.50 96.7 ± 11.8 13.9 ± 0.902 6.78 ± 1.67

Li0.80P88.2N10 10.0 47.3 ± 8.45 14.5 ± 0.411 8.59 ± 1.31

Li0.80P86.7N12.5 12.5 21.6 ± 7.86 11.2 ± 0.354 6.88 ±0.362

Appendix A

222


composite electrolytes containing 10 wt% of EMITFSI as function of increasing MSP

concentration.



E10P100M0 0 27.6 ± 2.63 9.00 ± 0.330 4.88 ± 0.452

E10P100M2.5 2.50 57.9 ± 9.95 15.2 ± 2.24 4.58 ± 1.01

E10P100M5.0 5.00 121 ± 10.8 20.1 ± 1.95 5.14 ± 0.444

E10P100M7.5 7.50 291 ± 54.1 21.9 ± 0.841 6.95 ± 0.640

E10P100M10 10.0 217 ± 48.1 21.6 ± 0.712 6.47 ± 0.360

E10P100M12.5 12.5 42.5 ± 5.65 9.85 ± 0.642 2.14 ± 0.181

E10P100M15 15.0 12.4 ± 1.54 9.24 ± 0.780 3.62 ± 0.0921

Table A.11 Ionic conductivity and mechanical characterisation data for crosslinked DGEBA

composite electrolytes containing 20 wt% of 1.0 M LiTFSI/PC as function of increasing MSP

concentration.



Li20B100M0 0 6.10 ± 0.0301 905 ± 73.4 81.1 ± 0.270

Li20B100M2.5 2.50 6.54 ± 0.670 1232 ± 54.1 111 ± 1.02

Li20B100M5.0 5.00 8.42 ± 4.10 1278 ± 72.7 101 ± 5.43

Li20B100M7.5 7.50 10.1± 2.11 1328 ± 7.40 82.5 ± 6.31

Li20B100M10 10.0 7.85 ± 1.87 401.4 ± 63.1 85.1 ± 1.26

Li20B100M12.5 12.5 4.22 ± 0.641 859.6 ± 25.4 53.4 ± 4.02

Li20B100M15 15.0 2.14 ± 0.150 496.7 ± 39.1 41.8 ± 2.85

Appendix A

223


PEGDGE/DGEBA composite electrolytes containing 10 wt% of 1.0 M LiTFSI/PC as




Li10:20P:80B:M0 0 3.10 ± 0.701 932 ± 14.9 105 ± 5.86

Li10:20P:80B:M7.5 7.50 7.57 ± 0.541 975 ± 10.4 135 ± 3.41

Li10:20P:80B:M10 10.0 7.81 ± 0.740 987 ± 11.7 145 ± 0.502

Li10:40P:60B:M0 0 5.44 ± 0.902 628 ± 34.2 89.5 ± 7.11

Li10:40P:60B:M7.5 7.50 9.75 ± 1.14 642 ± 24.3 81.0 ± 1.14

Li10:40P:60B:M10 10.0 10.5 ± 0.210 616 ± 33.4 84.5 ± 2.01


PEGDGE/DGEBA composite electrolytes containing 20 wt% of 1.0 M LiTFSI/PC as




Li20:20P:80B:M0 0.0 6.28 ± 0.730 598 ± 13.7 59.4 ± 2.11

Li20:20P:80B:M7.5 7.5 6.69 ± 0.410 604 ± 14.4 64.4 ± 2.70

Li20:20P:80B:M10 10.0 9.87 ± 1.21 673 ± 18.7 75.6 ± 5.11

Li20:40P:60B:M0 0.0 11.8 ± 0.510 42.4 ± 4.31 6.84 ± 0.440

Li20:40P:60B:M7.5 7.5 27.7 ± 2.71 53.1 ± 2.19 10.2 ± 3.21

Li20:40P:60B:M10 10.0 17.5 ± 0.240 25.9 ± 1.53 5.72 ± 3.24

Appendix A

224

Table A.14 Ionic conductivity and mechanical characterisation data for crosslinked 40:60

PEGDGE/DGEBA blend composite electrolytes containing 50 wt% of EMITFSI as function

of increasing MSP concentration.



E50:40P:60B:M0 0 538± 15.6 32.0 ± 3.04 9.04 ± 0.382

E50:40P:60B:M2.5 2.50 598 ± 91.0 35.6 ± 1.17 9.07 ± 0.140

E50:40P:60B:M5.0 5.00 697 ± 28.8 38.8 ± 1.32 9.99 ± 0.271

E50:40P:60B:M7.5 7.50 849 ± 50.1 62.0 ± 2.69 19.6 ± 1.08

E50:40P:60B:M10 10.0 1038 ± 65.4 26.4 ± 0.721 6.91 ± 0.431

E50:40P:60B:M12.5 12.5 842 ± 45.2 25.4 ± 0.840 6.15 ± 0.940

Appendix B

225

Appendix B Instructions of measuring the ionic conductivity of

polymer electrolytes and composite polymer electrolytes

Required materials:

Polymer electrolyte (cylindrical disc of 13 mm diameter and 4mm height)

Stainless steel electrodes (cylindrical disc of 13 mm diameter and 2mm height)

Required equipment:

Sample holder

Ivium-n-Stat Multichannel Potentiostat (Ivium Technologies, The Netherlands)

Measurement:

1. Sandwich the cylindrical disk shaped polymer electrolyte between two stainless steel

electrodes to make a cell;

2. Place the sandwiched cell in the sample holder and connect the sample holder with the

Ivium Potentiostat;

3. Run the impedance spectroscopic test at a potential of 0.5 V and a frequency range of

100,000 Hz to 0.1 Hz;

4. Import and open the raw data from the impedance spectroscopic test in ZView software

(Scribner Associates, http://www.scribner.com/zplot-and-zview-for-windows.html).

5. Select the first semi-circular region (high frequency curve) using the moveable two

points on the Nyquist plot;

6. Open Instant Fit option in the Tools menu and fit the semi-circular curve using

different available models. Select the best fit of the curve and note down the RS value

which is the x-intercept of the high frequency curve;

7. The ionic conductivity ҡ can be measured using the following equation

κ

Note: Equivalent series resistance (ESR) of structural supercapacitors was also calculated

using the method described above (Steps 3 to 6). ESR was the x-intercept of high frequency

curve. Equivalent distributed resistance (EDR) was the x-intercept of low frequency curve.

Appendix B

226

Figure A.1 ZView software showing the x-intercept of the Nyquist plot for the 40:60

PEGDGE/DGEBA blend matrix with 50wt% EMITFSI and 7.5 wt% MSP.

Appendix C

227

Appendix C Instructions of measuring the machine compliance for

determining the compression modulus of polymer electrolytes

Measurement:

Compressive force was applied in the axial direction of the two platens of compression

testing machine (Easy 50, Lloyds Instruments, UK) without placing any sample in between.

The compression force against machine extension was recorded during the test (Figure B.1).

The test was stopped as soon as the compression force reached to 12 kN. The compliance test

was repeated four times. The compression force versus machine extension was divided into

following three regions.

1) Compression force (load) between 0 N and 800 N;

2) Compression force (load) between 800 N to 1200 N;

3) Compression force (load) between 1200 N to 12 kN.

0 4000 8000 120000.0

0.1

0.2

0.3

0.4

0.5

Mac

hine

Ext

ensi

on (

mm

)

Load (N)

Figure B.1 Load against machine extension during the compression of plates of test machine

Constants (a0, a1, a2, …., a7) of the polynomial curve (y = a0 + a1x + a2x2 + a3x

3 +….+a7x7) of

three different regions were measured using curve fitting technique in origin software. The

compliance error was calculated using the constants in the polynomial equation and was then

subtracted from the machine extension data recorded during the compression testing of the

polymer electrolytes.

Appendix D

228

Appendix D Microscopic Evaluation on the MSP reinforced polymer

electrolytes

Uniform distribution of 7.5 wt% MSP in crosslinked PEGDGE containing 10 wt%

EMITFSI

Figure D.1 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI and

7.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 200 µm.



Appendix D

229





Appendix D

230



12.5 wt% addition in crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI

showing MSP agglomeration in polymer composite electrolytes



Appendix D

231





Appendix D

232



Figure D.10 SEM image of crosslinked PEGDGE electrolyte containing 10 wt% EMITFSI

and 12.5 wt% mesoporous silica particles (MSP) as reinforcements at a magnification of 10

µm.

Appendix E

233

Appendix E Shear stress and straincurves for the ±45º laminated

structural supercapacitor specimens

0 5000 10000 150000

12

24

36

She

ar S

tres

s (M

Pa)

Shear Strain (

100P:0B (a) 80P:20B (b) 60P:40B (c) 40P:60B (d) 20P:80B (e) 0P:100B (f)

(a)

(b)

(c)

(d)

(f)

(e)

Figure E.1 Shear stress-strain curves of CF and GF reinforced crosslinked

PEGDGE/DGEBA blend polymer electrolyte based structural supercapacitor specimens

containing 10 wt% EMITFSI with varying PEGDGE to DGEBA ratio.

Appendix E

234

Appendix F Nuclear magnetic resonance spectroscopy (NMR) of

diglycidylether of bisphenol-A epoxy and 4,4’ methylene bis(cyclo

hexyl amine) crosslinker

Figure E. 1 NMR of diglycidylether of bisphenol A epoxy

Appendix E

235

Figure E. 2 NMR of 4,4’ methylene bis(cyclo hexylamine) crosslinker

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