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i
Metal Sulfides and their Composites for Electrochemical
Energy Storage Applications
By
Humera Sabeeh
Roll No. FA17S2PA113
Ph.D. Chemistry
Session (2017-2021)
Supervisor
Dr. Muhammad Farooq Warsi
Associate Professor
Department of Chemistry
The Islamia University of Bahawalpur Pakistan
ii
―In the name of Allah, the Most Gracious, the Most
Merciful‖
iii
Student’s Declaration
I, Humera Sabeeh daughter of Elahi Bakhsh, PhD Scholar of Department of
Chemistry, The Islamia University of Bahawalpur, hereby declare that the research
work entitled, “Metal Sulfides and their Composites for Electrochemical Energy
Storage Applications” is done by me. I also certify that nothing has been incorporated
in this research work without acknowledgement and that to the best of my knowledge
and belief it does not contain any material previously published or written by any
other person or any material previously submitted for a degree in any university
where due reference is not made in the text.
Humera Sabeeh
iv
Supervisor’s Declaration
It is hereby certified that work presented by Humera Sabeeh in the thesis entitled
“Metal Sulfides and their Composites for Electrochemical Energy Storage
Applications‖ is based on the results of research conducted by candidate under my
supervision. No portion of this work has been formerly been offered for higher degree
in this university or any other institute of learning and to best of author‘s knowledge,
no material has been used in this thesis which is not her own work, except where due
acknowledgement has been made. She has fulfilled all the requirements and qualified
to submit this thesis in partial fulfillment for the degree of PhD in Chemistry, at The
Islamia University of Bahawalpur.
Dr. Muhammad Farooq Warsi
(Supervisor)
Associate Professor
Department of Chemistry
The Islamia University of Bahawalpur
v
DEDICATED
To
My Grandparents and Parents
vi
ACKNOWLEDGEMENT
In the Name of Allah, the Most Merciful, the Most Compassionate all praise is to
Allah Almighty, the Lord of the world. A robust pray is “Oh my lord, expand for
me my chest and ease for me my task and untie the knot from my tongue that
they may understand my speech” (Holy Quran: Surah: Ta-Ha: Verses: 25-28).
First and foremost, I must acknowledge my limitless thanks to Allah Almighty, the
Ever-Magnificent; the Ever-Thankful, for His helps and bless. I am totally sure that
this work would have never become truth, without His guidance. Many prayers for the
Holy Prophet Muhammad ( صلى الله عليه وسلم ).
I offer my sincerest gratitude to my research supervisor Dr. Muhammad Farooq
Warsi, who supported me with his immense knowledge and patience, while allowing
room for independent scientific thinking. His commitment to his work is absolutely
uncompromising. One simply could not wish for a better or friendlier mentor. His
guidance and challenging approaches made us more active and confident. His advices
in my research tenure have been one of the main causes of my accomplishment of
doctoral degree.
I would also like to express deep gratitude to Dr. Muhammad Shahid (Department
of Chemistry, College of Science, University of Hafr Al Batin, P.O.Box 1803, Hafr
Al Batin, 31991, Saudi Arabia) for his guidance, encouragement, and gracious
support throughout my research work, for his expertise in the field that motivated me
to work in this area and for his faith in me at every stage of his research. His
mentoring and profound knowledge has encouraged and boosted me to address the
hindrances in my research career at several stages. I gratified to all my respected
teachers including my honorable father for valuable discussions, knowledgeable
guidelines and illuminating advices at every stage of my studies.
I gratefully acknowledge the Chairman, Department of Chemistry, Prof. Dr.
Muhammad Ashraf, for being a pillar of support from the very beginning. I would
like to express my warmest thanks to Dr. Sonia Zulfiqar, Department of Chemistry,
School of Sciences and Engineering, the American University in Cairo, New Cairo,
11835, Egypt for sincere co-operation for applying field-emission scanning electron
microscopy (FE-SEM), energy dispersive X-ray (EDX), Brunauer-Emmett-Teller
(BET) techniques on my prepared samples and all results providing me in time. I pay
vii
special gratitude to Material chemistry laboratory (Department of Chemistry), the
Islamia University of Bahawalpur (Pakistan) is acknowledged for XRD analysis. For
financial support, I acknowledge Higher Education Commission of Pakistan via grant
No. 6276/Punjab/NRPU/ R&D/HEC/2016.
I was lucky to be surrounded by a friendly and ebullient group of M.Phil and Ph.D
scholars. All the students were very co-operative and friendly towards me. I am also
obliged to my class fellows. I specifically acknowledge and pay my gratitude to my
fellow Muhammad Aadil khan for his productive support, caring, helpful attitude,
joint endeavor and moral strength. My progress in the experimental/scrutiny process
through whole research period can be attributed directly to him.
I could not express enough appreciation to my family, I am afraid words could not do
justice to what their support, appreciation and well wishes mean to me. My beloved
Father and my loving Mother who sent me to university and stood by me in every
situation, their love and prayers took me to the place where I am now. Thanks to my
parents for all the moral support and the amazing chances they‘ve given me over the
years May Almighty Allah bless them with good health and prosperous long lives and
be a source of prayer for me.
My dearest uncle Advocate Malik Imam Bakhsh, who helped me through my tough
time with his light hearted talk and well, meant criticism and the joy of my heart, my
loving brothers Muhammad Salah-ud-din and Muhammad Rumman. My sisters
are packet of energy for me and a massive thanks to them for their prodigious love,
encouragement, and lots of prayers. I am gratified to all my beloved persons for being
with me. I also acknowledge my fellows, seniors, juniors and loving friends. Their
support, care and love mean a lot to me.
Humera Sabeeh
viii
Table of Contents
Student‘s Declaration ................................................................................................... iii
Supervisor‘s Declaration ............................................................................................... iv
ACKNOWLEDGEMENT ............................................................................................ vi
Table of Contents ....................................................................................................... viii
List of Tables ............................................................................................................... xii
List of Figures ............................................................................................................ xiii
List of Abbreviations ................................................................................................. xvii
List of Publications .................................................................................................. xviii
Abstract ....................................................................................................................... xix
CHAPTER 1 .................................................................................................................. 1
INTRODUCTION ......................................................................................................... 2
1.1 Overview of energy storage ................................................................................. 2
1.1.1 Capacitors ...................................................................................................... 3
1.1.1.2 Energy stored in a capacitor ....................................................................... 4
1.1.1.3 Lithium-ion batteries .................................................................................. 5
1.1.2 Fuel cells ........................................................................................................ 6
1.2 Important factors of energy storage devices ........................................................ 7
1.2.1 Energy density ............................................................................................... 8
1.2.2 Power density ................................................................................................ 9
1.2.3 Lifespan ......................................................................................................... 9
1.2.4 Cost ................................................................................................................ 9
1.3 Evaluation of capacitance..................................................................................... 9
1.3.1 Specific capacitance from current-voltage (CV) ......................................... 10
1.3.2 Specific capacitance by charge/discharge curve (CDC) .............................. 10
1.3.3 Energy density (E) ....................................................................................... 10
1.3.4 Power density (P)......................................................................................... 11
1.4 Electrochemical capacitors (ECs) ...................................................................... 11
1.4.1 Classification of electrochemical capacitors (ECs) ..................................... 12
1.5 Applications of electrochemical capacitors ........................................................ 20
1.5.1 Hybrid electric vehicles ............................................................................... 21
1.5.2 Consumer electronics .................................................................................. 21
ix
1.5.3 Energy harvesting ........................................................................................ 21
1.5.4 Railway ........................................................................................................ 21
1.5.5 Military and defence applications................................................................ 21
1.5.6 Computer and memory backup chips .......................................................... 21
1.5.7 Security applications.................................................................................... 22
1.5.8 Applications in public sector ....................................................................... 22
1.6 Electrolytes ......................................................................................................... 22
1.6.1 Aqueous electrolyte ..................................................................................... 23
1.6.2 Ionic electrolytes .......................................................................................... 24
1.6.3 Organic electrolyte ...................................................................................... 24
1.6.4 Solid polymer electrolytes (SPEs) ............................................................... 25
1.7 Electrode materials for electrochemical capacitors ............................................ 26
1.7.1 Carbon materials (porous carbon, CNT, graphene) ..................................... 26
1.7.2 Conducting polymers (CPs)......................................................................... 30
1.7.3 Transition metal oxides (TMOs) ................................................................. 31
1.7.4 Metal sulfides .............................................................................................. 31
1.8 Electrode Material: Structural Effect ................................................................. 32
1.8.1 One-dimensional electrode material ............................................................ 32
1.8.2 Two-dimensional electrode material ........................................................... 33
1.8.3 Three-dimensional electrode material ......................................................... 34
1.9 Electrode Material: Compositional Effect.......................................................... 35
1.9.1 Carbon-metal oxide composite .................................................................... 35
1.9.2 Compositional effect of carbon-conducting polymer composites ............... 36
1.9.3 Heteroatoms-doped carbon materials .......................................................... 37
1.10 Metal sulfides for electrochemical capacitors .................................................. 38
Lithium ion storage ability, supercapacitor ................................................................... 40
Aim and Objectives .................................................................................................. 41
CHAPTER 2 ................................................................................................................ 42
LITERATURE REVIEW ............................................................................................ 43
CHAPTER 3 ................................................................................................................ 66
EXPERIMENTAL WORK .......................................................................................... 67
3.1 Chemicals ........................................................................................................... 67
3.2 Instrumentation................................................................................................... 67
x
3.3 Synthesis methods .............................................................................................. 68
3.3.1 Hydrothermal method .................................................................................. 68
3.3.2 Microemulsion method ................................................................................ 69
3.3.3 Reverse microemulsion method .................................................................. 69
3.3.4 Sol-gel method ............................................................................................. 69
3.3.5 Co-precipitation method .............................................................................. 70
3.3.6 Chemical vapor deposition (CVD) .............................................................. 70
3.4 Preparation of graphite oxide (GO) and reduced graphite oxide (r-GO) ........... 70
3.4.1 Preparation of graphite oxide (GO) ............................................................. 71
3.4.2 Reduction of graphite oxide (r-GO) ............................................................ 72
3.5 Synthesis of MoS2 nano-flakes .......................................................................... 72
3.5.1 Synthesis of (MoS2/r-GO) nanocomposites ................................................ 73
3.6 Synthesis of copper sulfide ................................................................................ 74
3.6.1 Synthesis of CuS/CNTs nanocomposites .................................................... 75
3.7 Nickel foam (NF) treatment ............................................................................... 76
3.7.1 Fabrication of NiS/CNTs@NF and NiS/NF electrodes............................... 76
CHAPTER 4 ................................................................................................................ 78
RESULTS AND DISCUSSION .................................................................................. 79
4.1 X-ray diffraction (XRD)..................................................................................... 79
4.1.1 Bragg‘s Law ................................................................................................ 79
4.1.2 XRD parameters .......................................................................................... 80
4.1.3 X-ray diffraction of MoS2 and its nanocomposites ..................................... 80
4.1.4 X-ray diffraction of CuS and its nanocomposites ....................................... 82
4.1.5 X-ray diffraction of NiS and its nanocomposites ........................................ 83
4.2 Fourier transform infrared spectroscopy (FT-IR) .............................................. 84
4.2.1 FT-IR analysis of MoS2 and its nanocomposites......................................... 85
4.2.2 FT-IR analysis of CuS and its nanocomposites ........................................... 86
4.2.3 FT-IR analysis of NiS and its nanocomposites ........................................... 87
4.3 Morphological analysis ...................................................................................... 88
4.3.1 Field Emission Scanning Electron Microscopy ........................................... 88
4.3.2 FE-SEM analysis of MoS2 and its nanocomposites .................................... 89
4.3.3 FE-SEM analysis of CuS and its nanocomposites....................................... 90
4.3.4 FE-SEM analysis of NiS and its nanocomposites ....................................... 91
xi
4.4 Elemental analysis .............................................................................................. 92
4.4.1 Energy Dispersive X-ray ............................................................................. 92
4.4.2 EDX analysis of MoS2 and its nanocomposites .......................................... 93
4.4.3 EDX analysis of CuS and its nanocomposites............................................. 94
4.4.4 EDX analysis of NiS and its nanocomposites ............................................. 94
4.5 Brunauer–Emmett–Teller (BET) analysis .......................................................... 95
4.5.1 Brunauer-Emmett-Teller (BET) analysis of MoS2 and its nanocomposites 96
4.6 UV-Visible spectroscopic analysis..................................................................... 96
4.6.1 Beer Lambert Law ....................................................................................... 97
4.6.2 Ultraviolet/Visible spectroscopic analysis of GO and r-GO ....................... 97
4.7 Electrical conductivity measurements ................................................................ 98
4.7.1 Current-voltage (I-V) measurements ........................................................... 98
4.7.2 Electrical conductivity measurement of MoS2 and its nanocomposites ...... 99
4.7.3 Electrical conductivity measurement of CuS and its nanocomposites ...... 100
4.7.4 Electrical conductivity measurement of NiS and its nanocomposites ....... 101
4.8 Electrochemical measurements of MoS2 and MoS2/r-GO nanocomposites .... 102
4.8.1 Preparation of the working electrode (WE) ............................................... 102
4.8.2 Electrochemical Characterizations of MoS2 and MoS2/r-GO
nanocomposites .................................................................................................. 102
4.8.3 Electrochemical measurements of MoS2 and MoS2/r-GO nanocomposites
............................................................................................................................ 103
4.9 Electrochemical measurements of CuS and CuS/CNTs nanocomposites ........ 108
4.9.1 Preparation of the working electrodes ....................................................... 108
4.9.2 Electrochemical Characterizations of bare CuS and CuS/CNTs
nanocomposites .................................................................................................. 108
4.9.3 Electrochemical measurements bare CuS and CuS/CNTs nanocomposites
............................................................................................................................ 108
4.10 Electrochemical measurements of NiS and NiS/CNTs nanocomposites ....... 112
4.10.1 Preparation of the working electrodes ..................................................... 112
4.10.2 Electrochemical Characterizations of bare NiS and NiS/CNTs
nanocomposites .................................................................................................. 113
4.10.3 Electrochemical measurements of bare NiS and NiS/CNTs
nanocomposites .................................................................................................. 113
CONCLUSION .......................................................................................................... 119
REFERENCES .......................................................................................................... 121
xii
List of Tables
Table 1.1: Properties of electrolytes used in electrochemical capacitors. ............. 24
Table 1.2: Contrast of carbon, metal oxide and metal oxide–carbon composite
electrodes. ............................................................................................ 36
Table 1.3: Summary of the main findings of studied metal sulfides. ................... 39
Table 3.1: List of chemicals/precursors used. ....................................................... 67
Table 4.2: The comparison of the electrochemical performance of the flake-like
MoS2/r-GO NCs, CuS/CNTs NCs and NiS/CNTs NCs with the closely
related literature. ................................................................................ 118
xiii
List of Figures
Figure 1.1: Charge separation and storage in a parallel-plate capacitor (Bhunia,
Editors. 2019,). .......................................................................................... 3
Figure 1.2: Different types of capacitors (A) fixed, (B) polarized, (C) variable
(Bhunia, Editors. 2019,) ............................................................................ 4
Figure 1.3: Schematic of a battery during discharge (a) and charge (b) (Palacín,
2009). ........................................................................................................ 5
Figure 1.4: Schematic of a Li-ion battery during discharge (a) and charge (b)
(Palacín, 2009). ......................................................................................... 6
Figure 1.5: Illustration and working mechanism of a fuel cell (Kwan et al., 2020). .... 7
Figure 1.6: The relationship between the electrochemical properties of energy storing
devices (Kularatna, 2015). ........................................................................ 8
Figure 1.7: Illustration and working mechanism of electrochemical capacitors (Joshi
and Sutrave, 2019). ................................................................................. 12
Figure 1.8: Types of Supercapacitors (Pandolfo et al., 2013)..................................... 13
Figure 1.9: Schematic illustration of EDLC (Simon and Gogotsi, 2010). .................. 14
Figure 1.10: Charge storage mechanism of EDLC (Ou et al., 2014). ........................ 15
Figure 1.11: Typical (a) CV of EDLC and (b) GCD curves of EDLC materials (Van
Aken et al., 2015). ................................................................................... 16
Figure 1.12: Schematic illustration of pseudocapacitors (Joshi and Sutrave, 2019). . 17
Figure 1.13: Various types of reversible redox mechanisms: (a) underpotential
deposition, (b) redox pseudocapacitance and (c) intercalation
pseudocapacitance (Augustyn et al., 2014). ............................................ 18
Figure 1.14: Typical (a) cyclic voltammogram at various sweep rates and (b) CDC at
various current densities of pseudocapacitive materials (She et al., 2018).
................................................................................................................. 19
Figure 1.15: Schematic diagram of hybrid electrochemical capacitors(Chen et al.,
2017). ...................................................................................................... 20
Figure 1.16: Applications of electrochemical capacitor (Conway, 2013). ................. 22
Figure 1.17: (a) Scheme illustrating the preparation of the GPE for supercapacitors
(b) electrochemical performance of prepared samples (Vijayakumar et
al., 2017). ................................................................................................ 25
Figure 1.18: Carbon based materials (Namisnyk and Zhu, 2003a). ........................... 27
xiv
Figure 1.19: Graphene nanosheets(Namisnyk and Zhu, 2003a). ................................ 28
Figure 1.20: (a) PPy coating is expected to facilitate the electronic transport and
prevents dissolution of vanadium in electrolyte. (b) Schematic sketch
illustrate the growth of PPy on V2O5 nanoribbon surface (Qu et al.,
2012). ...................................................................................................... 32
Figure 1.21: (a) Schematic illustration of the crystal structure of TiS2. (b) CVs for
2D-TiS2 nanocrystals (Muller et al., 2015). ............................................ 34
Figure 1.22 : (a) SEM image of the CuS interconnected nanoparticles (b) CV curves
of the CuS networks (c) Schematic graphics of the asymmetric
supercapacitor (Fu et al., 2016). .............................................................. 35
Figure 3.1: Schematic representation of synthesis of graphite oxide. ........................ 71
Figure 3.2: Schematic representation of synthesis of reduces graphene oxide. ......... 72
Figure 3.3: Schematic representation of synthesis of molybdenum disulfide. ........... 73
Figure 3.4: Schematic representation of synthesis of MoS2/r-GO nanocomposites. .. 74
Figure 3.5: Schematic representation of synthesis of copper sulfide. ........................ 75
Figure 3.6: Schematic representation of synthesis of CuS/CNT nanocomposites. .... 76
Figure 3.7: Schematic scheme for the fabrication of NiS/NF and NiS/CNTs@NF
electrodes. ............................................................................................... 77
Figure 4.1: Bragg's law and diffraction of X-ray (Dorset, 1998). .............................. 80
Figure 4.2: X-ray diffraction patterns of pure MoS2, MoS2 /r-GO nanocomposites and
XRD pattern of r-GO (Inset). .................................................................. 81
Figure 4.3: X-ray diffraction patterns of bare CuS and CuS /CNTs nanocomposites.82
Figure 4.4: X-ray diffraction profiles of NiS and NiS/CNTs nanohybrid. ................. 83
Figure 4.5: Fourier transform infrared spectroscopy setup (Larkin, 2011). ............... 85
Figure 4.6: Fourier transform infrared spectroscopy analysis of pure MoS2 and MoS2
/r-GO nanocomposite. ............................................................................. 86
Figure 4.7: Fourier transform infrared spectroscopy analysis of bare CuS and
CuS/CNTs nanocomposites. ................................................................... 87
Figure 4.8: Fourier transform infrared spectroscopy analysis of NiS and NiS/CNTs
nanohybrid. ............................................................................................. 88
Figure 4.9: Principle of field emission scanning electron microscopy (Nuspl et al.,
2004). ...................................................................................................... 89
Figure 4.10: Scanning electron microscopic image of (a) pure MoS2, (b) MoS2 /r-GO
nanocomposites. ...................................................................................... 90
xv
Figure 4.11: Scanning electron microscopic image of (a) bare CuS, (b) CuS/CNTs
nanocomposite. ....................................................................................... 91
Figure 4.12: Scanning electron microscopic image of (a) bare NiS, (b) NiS/CNTs
nanocomposites. ...................................................................................... 92
Figure 4.13: Function of energy dispersive x-ray spectroscope(Goldstein et al., 2003).
................................................................................................................. 93
Figure 4.14: EDX of (a) pure MoS2 and (b) MoS2 /r-GO nanocomposites. ............... 93
Figure 4.15: The EDX spectra of CuS and CuS/CNTs nanocomposites. ................... 94
Figure 4.16: The EDX profiles of NiS and NiS/CNTs nanohybrid. ........................... 95
Figure 4.17: Nitrogen physic-sorption isotherms of (a) pure MoS2 (a) and (b) MoS2
/r-GO nanocomposites. ........................................................................... 96
Figure 4.18: Absorbance of light by the material in cuvette (Perkampus, 1992). ...... 97
Figure 4.19: UV-Visible spectra of graphene oxide and reduce graphene oxide. ...... 98
Figure 4.20: Current-Voltage measurements of pure MoS2 and MoS2 /r-GO
nanocomposites. .................................................................................... 100
Figure 4.21: Current-Voltage measurements of bare CuS and CuS/CNTs
nanocomposites. .................................................................................... 101
Figure 4.22: The I-V profiles of NiS and NiS/CNTs nanohybrid. ........................... 101
Figure 4.23: The CV profile of the (a) pure MoS2 based electrode, (b) MoS2 /r-GO
nanocomposites based electrode at different sweep rates (c) CV profile
of MoS2, MoS2/r-GO and r-GO at 10 mVs-1
......................................... 104
Figure 4.24: CV profile of (a) pure MoS2 based electrode at 1st cycle and after 1000
cycles (b) MoS2 /r-GO nanocomposites based electrode at 1st cycle and
after 1000 cycles (c) The% capacitance retention of pure MoS2 electrode
and MoS2 /r-GO nanocomposites based electrode after various number
of cycles. ............................................................................................... 105
Figure 4.25: (a) Cyclic charge discharge curves of pure MoS2 electrode and MoS2 /r-
GO nanocomposites based electrodes at 1 Ag-1
current density (b) Effect
of current densities on the specific capacitance of pure MoS2 based
electrode and MoS2 /r-GO nanocomposites based electrode ................ 106
Figure 4.26: Nyquist plots of pure MoS2 based electrode and MoS2 /r-GO
nanocomposites based electrode in three electrode system using 1 M
Na2SO4 electrolyte. ............................................................................... 107
xvi
Figure 4.27: The CV profile of the (a) bare CuS electrode, (b) CuS/CNTs
nanocomposites based electrode at various scan rates. ......................... 109
Figure 4.28: (a) The effect of scan rate on specific capacitance of bare CuS and
CuS/CNTs nanocomposites electrodes (b) The % capacitance retention
of bare CuS and CuS/CNTs nanocomposites electrodes after several
numbers of cycles. ................................................................................ 110
Figure 4.29: CV profile of (a) bare CuS based electrode at 1st cycle and after 1000
cycles (b) CuS/CNTs nanocomposite based electrode at 1st cycle and
after 1000 cycles. .................................................................................. 110
Figure 4.30: (a) Cyclic charge discharge curves of bare CuS electrode and CuS/CNTs
nanocomposites based electrodes at 1 A/g current density (b) Effect of
current densities on the specific capacitance of bare CuS electrode and
CuS/CNTs nanocomposites based electrodes. ...................................... 111
Figure 4.31: Nyquist plots of bare CuS based electrode and CuS/CNTs
nanocomposites based electrode in three electrode system using 3M
KOH electrolyte (b) circuit fitted Nyquist plot of CuS/CNTs NCs...... 112
Figure 4.32: The CV profiles of (a) NiS@NF with inset comaritive CV profiles, (b)
NiS/CNTs@NF, AND(c &d) cyclic performance of NiS/CNTs@NF and
effect of number of cyclic tests on the perccentage capacitance retention
(inset). ................................................................................................... 114
Figure 4.33: Cyclic charge/discharge (CCD) profiles of (a) NiS@NF and
NiS/CNTs@NF electrode at 1 A/g (b) Relation between the specific
capacitance and applied current density for both samples electrodes. .. 116
Figure 4.34: Nyquist plots of NiS@NF and NiS/CNTs@NF nanohybrid. .............. 117
xvii
List of Abbreviations
LIB Lithium ion batteries
CV Current voltage
Csp Specific capacitance
CDC Charge/discharge curve
ECs Electrochemical capacitors
EDLCs Electric double layer capacitors
HECs Hybrid electrochemical capacitors
ASIC Application specific integrated circuit
RTILs Room temperature ionic liquids
SPEs Solid polymer electrolytes
CNTs Carbon nanotubes
CPs Conducting polymers
TMOs Transition metal oxides
TMDCs Transition metal dichalcogenides
CCPs Carbon conducting polymers
GO Graphite oxide
rGO Reduced graphite oxide
XRD X-ray diffraction
FTIR Fourier transform infrared spectroscopy
FE-SEM Field emission scanning microscopy
EDX Energy dispersive x-ray spectroscopy
BET Brunauer-Emmett-Teller analysis
WE Working electrode
CV Cyclic voltammetry
CCD Cyclic charge/discharge
EM Electrode material
EIS Electrochemical impedance spectroscopy
xviii
List of Publications
1. Humera Sabeeh, Sonia Zulfiqar, Muhammad Aadil, Muhammad Shahid, Imran
Shakir, Muhammad Azhar Khan, Muhammad Farooq Warsi Flake-like MoS2 Nano-
architecture and its Nanocomposite with Reduced Graphene Oxide for Hybrid
Supercapacitors Applications Ceramics International, 46(13):21064-21072.
2. Muhammad Aadil, Sonia Zulfiqar, Humera Sabeeh, Muhammad Farooq Warsi,
Muhammad Shahid Ibrahim A. Alsafaric, Imran Shakird, Enhanced electrochemical
energy storage properties of carbon coated Co3O4 nanoparticles-reduced graphene
oxide ternary nano-hybrids In press: https://doi.org/10.1016/j.ceramint.2020.04.090
3. Nabeela Ashraf, Muhammad Aadil, Sonia Zulfiqar, Humera Sabeeh, Muhammad
Azhar Khan, Imran Shakir, Philips O Agboola, and Muhammad Farooq Warsi Wafer-
Like CoS Architectures and Their Nanocomposites with Polypyrrole for
Electrochemical Energy Storage Applications In press: doi.org/ 10.1002
/slct.202001305
4. Abdur Rahman, Humera Sabeeh, Sonia Zulfiqar, Philips Olaleye Agboola, Imran
Shakir,
Muhammad Farooq Warsi Structural, optical and photocatalytic studies of trimetallic
oxides nanostructures prepared via wet chemical approach, Synthetic Metals 259
(2020) 116228
5. Bushra Bashir, Abdur Rahman, Humera Sabeeh, Muhammad Azhar Khan,
Mohamed F. Aly Aboud, Muhammad Farooq Warsi, Imran Shakir, Philips Olaleye
Agboola, Muhammad Shahid Copper substituted nickel ferrite nanoparticles anchored
onto the graphene sheets as electrode materials for supercapacitors fabrication
Ceramics International 45 (2019) 6759–6766
xix
Abstract
In current study, the metal sulfides such as molybdenum disulfide (MoS2), copper
sulfide (CuS) and nickel sulfide (NiS) were synthesized by facile hydrothermal route
and their nanocomposites with conducting carbon scaffolds (reduced graphite oxide
and carbon nanotubes) were prepared by simple ultrasonic exfoliation approach. The
synthesized materials were successfully studied by various characterization
techniques. The structural, purity, surface morphological study and conductivity of
obtained materials have studied by physicochemical techniques such as X-ray-
diffraction (XRD), fourier transform infrared (FT-IR) spectroscopy, transmission
electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM),
energy dispersive X-ray (EDX), Brunauer-Emmett-Teller (BET) analysis and current-
voltage (I-V) measurements. Furthermore, the electrochemical properties of all
prepared materials were explored by performing electrochemical measurements
including cyclic voltammetry, cyclic charge/discharge and electrochemical
impedance. To improve the electrical and electrochemical properties of MoS2
nanoarchitecture, we formed its nanocomposite (MoS2/r-GO) with 10% r-GO. After
the addition of 10% r-GO, the nanocomposite showed the electrical conductivity of
1.24 × 10-1
Sm-1
that is higher than the pure MoS2 (2.2 × 10-7
Sm-1
). The prepared
nanocomposite also showed higher specific capacitance (441 Fg-1
at 1 Ag-1
) than the
pure MoS2 nanoarchitecture (248 Fg-1
at 1 Ag-1
). The 2-D flake-like structure of the
electrode increased its contact area with the r-GO matrix and electrolyte. The higher
electrical conductivity and specific surface area of the nanocomposite facilitated the
faradic and non-faradic charge storage mechanism. As the nanocomposite showed CV
and CCD profiles in the negative potential window (-1 V to -0.53 V), therefore it has
the potential to be used as a negative electrode material for hybrid supercapacitors
applications. The observed results revealed the potential of the (MoS2/r-GO)
nanocomposite-based cathode for hybrid supercapacitor applications. The
nanocomposite of CuS with CNTs improved the electrical and electrochemical
properties of CuS nanoarchitecture. The electrical conductivity of bare CuS was
relatively less 1.85 10-4
Sm-1
compared to CuS/CNTs nanocomposite (2.34 104
Sm-1
). The CuS/CNTs nanocomposite exhibited higher specific capacitance (422 F/g
at 1 A/g) than the bare CuS nanoarchitecture (285 F/g at 1 A/g). The CuS/CNTs
nanocomposite is missing 16.8% of its initial capacitance after 1000 charge-discharge
xx
cycles. To enhance the electrical conductivity and electrochemical properties of NiS
nanoarchitecture were prepared the nanocomposites with conductive CNTs. The
calculated electrical conductivity (σ) value for the NiS/CNTs sample comes to be
superior (1.42 105 Sm
-1) than that of the NiS sample (4.53 10
-3 Sm
-1), indicating a
positive interaction among the NiS nanoparticles and CNTs. The NiS/CNTs
nanocomposites revealed greater specific capacitance (732 Fg-1
at 1 Ag-1
) than that of
NiS@NF blank electrode (405 F/g). The NiS/CNTs nanocomposites have too much
high cyclic stability and lost just 4.9% of its initial capacitance after 3000 charge-
discharge cycles.
1
CHAPTER 1
INTRODUCTION
2
CHAPTER 1
INTRODUCTION
1.1 Overview of energy storage
Energy is needed for human activities, since in this age of technology we are highly
dependent on energy driven devices. Energy is extensively used in the field of
transportation, agriculture, environmental remediation and cutting-edge medical
paraphernalia. The dependence on energy has increased very much since the industrial
revolution. Prosperity and quality of life across the globe is dependent on energy
growth. Meeting the energy growth in a safe and environmental friendly means has
emerged as a vital challenge. Reliable, affordable and eco sociable energy sources are
capable of enhancing trade, expanding industrialization and improving transportation
especially for developing countries. Thus, these can advance the life quality and
breakout people from dearth. Mandatory to encounter the interminably-growing
energy needs, energy production must be increased. For this all energy sources must
be increased their efficiency. Energy consumption is a major concerning issue in the
modern world. To overcome this serious issue, the various electrode materials
transition metal chalcogenides, metal oxides, MXenes, metal hydroxides, metal
nitrides, metal sulfides, conducting polymers and carbon scaffolds such as carbon
nanotubes, activated carbon, carbon aerogels and graphene are introduced (Miller et
al., 2018). All these materials possess high conductivity, excellent specific
capacitance, good stability and inclusive surface area. The criteria for selection of
electrode materials for energy stowing strategies are excellent conductivity, large
surface area, high chemical stability, good electrochemical interactions between
electrode and electrolyte ions, extraordinary porosity and multifarious oxidation states
(Iro et al., 2016; Theerthagiri et al., 2018). In this age of technology portable devices
have become part and parcel of life. Portable devices like laptops, mobile phones and
tablets require the energy storage system to have a fast charging and long discharging
time. They also require long cycle life with high energy density and high-power
density. For better and uninterrupted energy supply, energy transformation devices
along with energy storage devices are needed to be modified. For industries and heavy
vehicles to operate instant energy supply is required. Energy storage devices which
3
can supply energy rapidly when needed have become a field of interest(González et
al., 2016).
Here, we will discuss different energy storage devices.
1.1.1 Capacitors
Capacitors are the electrical instruments used to store energy between two terminals
separated by dielectric materials shown in Figure 1.1. Current is starts flowing
through the capacitor when DC voltage is applied to the negative terminal of the
capacitor as a consequent of potential difference between terminals and keep flowing
until the potential difference becomes equal to DC voltage supply.
Figure 1.1: Charge separation and storage in a parallel-plate capacitor (Bhunia,
Editors. 2019,).
Capacitors are used in different appliances for different purposes. In electronic
circuits they allow only alternating current (AC) to flow and block the flow of direct
current (DC). In homes and hospitals, they found their application as an output
smoother. They found their use in resonant system as converter of radio frequency to
other frequency (Bhunia and Tehranipoor, 2019). Different types of capacitors are
4
shown in Figure 1.2. A capacitor is usually characterized by its capacitance, which is
ratio between charge and voltage between the terminals.
Mathematically;
C = Q / V (1.1)
Figure 1.2: Different types of capacitors (A) fixed, (B) polarized, (C) variable
(Bhunia, Editors. 2019,)
1.1.1.2 Energy stored in a capacitor
The capacitors hoard energy on the surface of electrodes by a physical process. The
capacitance can be calculated using following expression:
C = εS/4πkd (1.2)
Where, C is the capacitance, S is overlying surface area of electrodes, ε is the
dielectric constant, k is electrostatic constant and d is distance between electrodes.
Capacitors have too high power density and low energy density in comparison with
supercapacitors and conventional batteries. 1.1.2 Batteries
Batteries are made up of three main parts; a positive electrode, negative electrode and
electrically insulating electrolyte. The voltage produced when a chemical reaction
occurs during charging and discharging processes. The batteries stored potential
energy in the chemical form. During charging and discharging process the oxidation
states of the working electrode materials are amended. The active material oxidized at
the anode and reduction takes place at the cathode. Electricity is instinctively
produced when the electrodes are connected and discharging takes place. The reverse
reactions occur and the battery is fully recharged when the external current is applied.
The mechanism of the working principle of a battery is shown in Figure.1.3. The
5
discharging of batteries is very fast at high temperature. Batteries possess low power
density and poor cycling stability.
Figure 1.3: Schematic of a battery during discharge (a) and charge (b) (Palacín,
2009).
1.1.1.3 Lithium-ion batteries
The Lithium-Ion Batteries (LIB) is deliberated as the utmost innovative
electrochemical energy storage device. The LIB owns many advantages such as
amazing energy density, extensive cycle life approximately more than thousand
cycles and inexpensive. The LIB is composed of graphite (acts as negative electrode)
and LiCoO2 (acts as positive electrode) shown in Figure 1.4. The 1M LiPF6 here used
as an electrolyte. The rocking chair mechanism takes place in LIB, because the Li
ions move forward and reverse directions towards the electrodes in the charging and
discharging process. The overall reactions occurring at respective electrodes are
shown in following equations:
C6 + xe- + xLi
+ ↔ LixC6 (1.3)
Li1-xCoO2 + xe- + xLi
+ ↔ LiCoO2 (1.4)
In the above equations the ‗x‘ is the number of moles. The LIB has decreased power
density and energy density owing to the anode (graphite) low irreversible capacity
during cycling. These results are obtained also because of the establishment of a solid
6
electrolyte interphase during the cycling process. The electrode material used here has
some drawbacks such as high cost, thermally not feasible, stumpy power density and
short cycle span.
Figure 1.4: Schematic of a Li-ion battery during discharge (a) and charge (b)
(Palacín, 2009).
1.1.2 Fuel cells
A fuel cell is a unique device that is used to harvest electrical energy from chemical
energy of a fuel in the presence of oxygen. Fuel cells need the continuous supply of
fuel and get oxygen from air. Fuel cells have found their application in hospitals,
automobiles, grocery stores and data centers. Unlike batteries fuel cells are not needed
to charge they will generate electricity as long as there will be supply of fuel. Fuel cell
is a clean, efficient and sociable way of generating electricity as there will be no toxic
by-products (Kwan et al., 2020). A fuel cell made up of negative electrode (anode),
positive electrode (cathode) and an electrolyte. Hydrogen as a fuel is delivered to the
fuel cell through the anode. Oxygen is supplied through the cathode. A catalyst splits
the hydrogen fuel into electrons and protons. Protons are allowed to pass through the
membrane while electrons are allowed to pass through an external circuit. At the
cathode hydrogen and oxygen combine to yield water. A schematic view of a typical
fuel cell is shown in Figure 1.5.
7
Figure 1.5: Illustration and working mechanism of a fuel cell (Kwan et al., 2020).
Fuel cells have wide applications in portable electronics, back-up power system,
automotive, distributed generations, military, aircrafts and auxiliary power (Ferreira-
Aparicio et al., 2018). Fuel cells possess high energy density but very stumpy power
density compared to other energy storage devices. Fuel cells possess limited
applications in portable energy storage devices due to the production and storage of
fuel (hydrogen) and safety issues. The fuel cell certainly not discharged like
conventional batteries and supercapacitors.
1.2 Important factors of energy storage devices
There are some main factors that affect the efficiency and commercial applications of
energy storage devices such as supercapacitors, conventional capacitors and batteries.
The capacitor stores potential energy in the form of an electric field. The batteries
store greater energy per weight called energy density while the capacitors possess
greater charging and discharging rate called power density. The power density and
energy density are two prime factors that are effects the lifetime of energy storage
devices.
Energy density
Power density
Cost
8
Lifespan
1.2.1 Energy density
The quantity of energy stores per volume of a storing device is the energy density.
The Figure 1.6 illustrates the rapport between the electrochemical properties of energy
storage devices. The supercapacitors positioned between several energy stowage
devices such as conventional capacitors, conventional batteries and fuel cells.
Figure 1.6: The relationship between the electrochemical properties of energy storing
devices (Kularatna, 2015).
The conventional capacitors have truncated energy density among all ordinarily used
storage devices. In spite of the electrochemical capacitors, they possess very short
energy density than batteries and fuel cell. The energy density (E = 1/2 CV2) is
directly related with capacitance (C) and voltage (V2). When both the parameters
capacitance and voltage, increases the energy density also increases (Zhong et al.,
2015). This problem can be elucidated using high capacitance electrode materials and
electrolytes with a maximum range of potential gap. The energy density directly
enhanced when cell voltage increased by using electrolytes with a wide range of
potential window because energy is direct related to the V2. The cell voltage increases
resulting capacitance of material increases, both boost up the energy density of
ultracapacitors. Moreover, the efficiency of ultracapacitors manifolds increases when
9
suitable interactions present between electrode materials and electrolytes. Batteries
have superior energy density compared to capacitors.
1.2.2 Power density
The power density is the measure of time that‘s take a device to charge and discharge
energy. The electrochemical capacitors possess astonished power density compared to
batteries. Because the chemical reactions in supercapacitors take short time to release
electrons in the electrical discharge medium compared to batteries. The higher power
density of supercapacitors makes promising for applications in present stabilization
when accessing sporadic renewable energy sources (Chmiola et al., 2006). The
supercapacitors have much attention towards devices such as electric or hybrid
electric vehicles, aircraft, smart grids and portable electronics etc. where high power
density required.
1.2.3 Lifespan
The supercapacitors have extensive life span compared to batteries. Batteries have a
lower number of cycles than ultracapacitors. The batteries involve chemical reaction
between electrodes in electrolyte. Batteries are sojourns working when electrolytes
used up (because no reaction takes place) and have no any fast discharging or charge
storing ability. Such type of reactions are not occur in capacitors for storage energy
so, have vast life span.
1.2.4 Cost
The ultracapacitors are much expensive than batteries. Comparing the lifetime of
batteries with ultracapacitors, the use of supercapacitors may be extensive even too
much initial price. All depends on the lifetime requirement of the explicit application
of energy storage devices.
1.3 Evaluation of capacitance
The electrochemical enactment of ECs can be measured by the foremost
electrochemical properties such as specific capacitance, power density and energy
density. The following four crucial factors are essential for the augmented specific
capacitance for supercapacitors:
Extensive surface area
10
Doping of metals and composite formation with conductive scaffolds to
enhance redox activity and conductivity
Maximum charge/discharge rate
Select wide range of potential window
1.3.1 Specific capacitance from current-voltage (CV)
The specific capacitance (Csp) of electrochemical capacitors is defined as:
(1.5)
Where, Q is the quantity of charge stored and V is the potential window. The
electrochemical double layer capacitance can be calculated using equation 1.6.
Csp = εr εo A/d (1.6)
Where, εr is the relative permittivity (medium), εo is the permittivity (vacuum), A is
the surface area and d is the width of double layer.
The pseudocapacitance of electrode material (EM) can be calculated by equation 1.7.
(1.7)
Where, n is the number of transferred electrons during redox reaction, F is faraday
constant, M is the molecular mass of electrode material and V is the potential
window.
From the CV, the Csp of EM can be calculated as:
(1.8)
Where, I is the current, m is mass of electrode material, ʋ is sweep rate and ΔV is
operating window.
1.3.2 Specific capacitance by charge/discharge curve (CDC)
The Csp of EM is evaluated from CDC analysis by following expression:
(1.9)
Where, I is discharging current, t is the time, m is the mass of EM and ΔV is the
voltage difference.
1.3.3 Energy density (E)
The energy density of ECs is calculated using following equation:
E = 1/2 CV2 (1.10)
11
Where, C is the capacitance and V is the voltage window. The energy density directly
relates with capacitance and square root of voltage window.
1.3.4 Power density (P)
The power density of electrochemical capacitors can be calculated by following
expression:
P = V2/4R (1.11)
Where, V is the voltage window and R is the series resistances of all the components
of the electrochemical device.
1.4 Electrochemical capacitors (ECs)
In the last few decades, the electrochemical capacitors are being considered as
excellent energy storing devices owing to their simple working principle, good cyclic
stability and higher energy density than conventional capacitors and greater power
density than batteries. The ECs or ultracapacitors are the cutting-edge technology in
the field of energy storing. Electrochemical capacitors have many advantages over
other energy storage devices like fuel cells, capacitors and batteries. Electrochemical
devices such as capacitors, fuel cells, batteries and electrochemical capacitors (ECs)
are used in storage and energy conversion applications. ECs have astonished
advantages among them like long life cycle, low cost, environmental friendliness and
rapid charge/discharge process. Notably, the metal sulfides presuppose the reversible
faradic reactions not capacitive. The reactions shown in equations (1.12) and (1.13)
are involved in charging and discharging duration. The reaction takes place in
aqueous media and is completed in two steps. In the first step the metal sulfides
changed into sulfo-hydroxides and in the second step they converted into sulfoxide
(Yu and Lou, 2018).
MS + OH- ⇌ MSOH + e
- (1.12)
MSOH + OH- ⇌ MSO + H2O + e
- (1.13)
ECs like a typical capacitor consist of a two electrode material which is porous shown
in Figure 1.7. Due to porous electrode materials supercapacitors offer higher surface
area to store energy than conventional capacitors. The electrodes are dipped in an
electrolyte unglued by a thin separator. The electrolyte consists of positive and
negative ions dissociated in aqueous solution (Joshi and Sutrave, 2019).
12
Figure 1.7: Illustration and working mechanism of electrochemical capacitors (Joshi
and Sutrave, 2019).
1.4.1 Classification of electrochemical capacitors (ECs)
The ECs have two types. The one is symmetric and the other asymmetric
supercapacitors. The symmetric type of supercapacitors has both electrodes similar.
The anodic and cathodic electrodes are prepared by same material. Although, the
asymmetric electrochemical capacitors in which anode and cathodes are designed
with different materials. The asymmetric electrochemical capacitors have huge
applications in anticipated energy storage device (Borenstein et al., 2017).
Furthermore, supercapacitors based on charge storing mechanism can be classified as
following;
Electric double layer capacitors (EDLC)
Pseudo capacitors
Hybrid electrochemical capacitors (HECs)
13
Figure 1.8: Types of Supercapacitors (Pandolfo et al., 2013).
1.4.1.1 Electric double layer capacitors (EDLC)
The EDLC are the capacitors that store and release the energy by physical adsorption
and desorption no Faradaic reaction takes place (Simon and Gogotsi, 2010). When a
V source is applied to EDLC, the ions from electrolyte are drifted towards their
counter electrodes. Consequently, a double layer of ions is developed at electrode-
electrolyte interface Figure 1.9. Due to formation of double layer of charged species
at interface, the supercapacitors are termed as electric double layer capacitors.
14
Figure 1.9: Schematic illustration of EDLC (Simon and Gogotsi, 2010).
In the mechanism of EDLC there is no exchange of ions or charge transfer occurs
between the electrode and electrolyte interface. For charge storing only the surface of
electrode is accessible for electrolyte ions so, use constant concentration of electrolyte
while studying the electrochemical measurements. The electrode material should have
optimized pore size, pore structure, high conductivity and surface properties for
EDLC. During massive charging and discharging cycles the EDLC storage
mechanism provides high stability and fast energy endorsement and transfer (Ou et
al., 2014).
15
Figure 1.10: Charge storage mechanism of EDLC (Ou et al., 2014).
When the voltage supply is connected to EDLC, the supercapacitors starts charging
and remains in the charged state until the external load is supplied to EDLC. As the
supply voltage is increased more ions are forced to be attached to their counter
electrodes. Surface area and conductive nature of electrodes are crucial for
determining their efficiency. Most commonly used electrode materials EDLCs are
carbon with high surface area, CNTs and r-GO due to their economic friendly nature,
good stability and high conductivity (Lu et al., 2013). As a consequent of non-Fredric
reactions EDLCs unlike conventional capacitors possess long cycle life. The EDLC
have less energy density and greater power density then pseudocapacitors. The EDLC
have low specific capacitance but possess much cycle life (Xu et al., 2014). The
electrodes with extremely small pores hindered the fast diffusion of ions through the
porous matrix during electrochemical process. The specific capacitance of EDLC
enhanced when carbonaceous electrode material possesses compatible pore size for
electrolyte ions. Nowadays, exposed surface carbonaceous materials (CNTs aerogels,
graphene aerogels and onion like morphology of carbon) have great attention towards
EDLC electrode due to entirely open immense surface area for the fast diffusion of
electrolytic ions. The CNTs aerogels gains much attention in EDLC due to high
16
mechanical properties, enormous surface area, low density and electrical conductivity
(Kim et al., 2013). The typical cyclic voltammetry and charging discharging plots of
EDLC material are presented in Figure 1.11. The CV curves of EDLC electrode
material was obtained at various scan rates Figure 1.11 (a). At low sweep rate the
rectangular shape obtained represents the excellent EDLC performance. At high scan
rates the non-rectangular shape of CV plot obtained which showed the more
resistance and leading decrease in specific capacitance. The galvanostatic charging
discharging plot is shown in figure 1.11 (b). The material showed less resistance at
low current densities and leading fast charging discharging process (Van Aken et al.,
2015).
Figure 1.11: Typical (a) CV of EDLC and (b) GCD curves of EDLC materials (Van
Aken et al., 2015).
1.4.1.2 Pseudocapacitors
Unlike EDLCs this type of supercapacitors funtions on the basis of reversible fradaric
redox reactions Figure 1.12. Charging of pseudo capacitors is done by the electron
transfer. As a consequent of fast faradaric reactions pseudo capacitors have higher
energy density than EDLCs (Joshi and Sutrave, 2019). When power is supplied to
pseudo capacitor, the electrolyte ions start drifting towards their respective electrode.
Two layers are obtained at the electrode-electrolyte interface alienated by electrolyte
molecules. The adsorbed ions transfer their charge with electrode without any
chemical reaction. As no chemical reaction occures unlike batteries, thus electrode is
not effected.
17
Figure 1.12: Schematic illustration of pseudocapacitors (Joshi and Sutrave, 2019).
Faradaic capacitors involve reversible and fast redox reaction on the electrode.
According to Conway, the pseudocapacitive mechanism classified into three types
such as a) under potential deposition, b) redox faradaic capacitance and c)
intercalation faradaic capacitance shown in Figure 1.13. In the under potential
deposition type, the metal ions are adsorbed and desorbed on the surface of other
metal electrode under high redox potential. In the redox faradaic capacitance, the
faradaic charge transfer takes place when metal ions electrochemically adsorbed
surface of electrode. In intercalation faradaic capacitance, the energy storage is
comprehended faradaic intercalation of ions into the layers/shafts on the surface of
electrode to form the reversible composite of ions during charging and desorbed when
discharging process occurs (Augustyn et al., 2014).
18
Figure 1.13: Various types of reversible redox mechanisms: (a) underpotential
deposition, (b) redox pseudocapacitance and (c) intercalation pseudocapacitance
(Augustyn et al., 2014).
The various types of faradaic reactions occur on the pseudocapacitive electrode that
are reversible adsorption of electrolyte ions and active species on the surface of
electrode, redox reaction of metal oxides (MOs) and metal hydroxides and reversible
doping/reverse-doping of electroactive polymer based electrodes. The specific
capacitance of pseudocapacitors increases because both the bulk and surface of the
electrode implicates in the electrochemical process. Hence, the pseudocapacitors have
greater value of energy density than EDLC. The faradaic reactions occurs with less
speed compared to EDLC (non-faradaic) reactions and have poor electrical
conductivity, so they have very low power density. Moreover, the faradaic capacitors
sustain with poor cycling stability (Snook et al., 2011). The MOs and CPs are best
known electrode materials for pseudocapacitors. The typical cyclic voltammetry and
charging discharging plot of faradaic capacitor material are presents in Figure 1.14.
The CV curves of Faradaic capacitor electrode material studied at various sweep rates
(5 to 30 mVs-1
) Figure 1.14 (a). In the faradaic reactions the cyclic voltammogram
showed clear pair distinctive of redox peaks. At high sweep rating the oxidation peak
19
shifts towards high voltage. This type of peak shifting implies high electronic
conductivity, fast electron or ion transfer and excellent electrochemical reversibility.
The galvanostatic charging discharging plot is shown in Figure 1.14 (b). The GCD
curves showed distinctive and almost regular plateau region during
charging/discharging process at various current densities. The GCD curves showed
negligible internal ohmic drop (resistance) (She et al., 2018).
Figure 1.14: Typical (a) cyclic voltammogram at various sweep rates and (b) CDC at
various current densities of pseudocapacitive materials (She et al., 2018).
1.4.1.3 Hybrid electrochemical capacitors (HECs)
The HECs can also be called asymmetric electrochemical capacitors. It is actually a
combination of EDLC and pseudocapacitor. Here two electrodes are made up of
different materials. One electrode works on the charge storage mechanism based on
EDLC type and the other functions on the basis of pseudocapacitance. The HECs
have high capability of charge storage due to both faradaic (pseudocapacitive) and
non-faradaic (EDLC) reactions. Non-faradaic reactions provide high cyclic stability,
high power performance while the Faradaic reactions offer greater energy density and
high capacitance. The principle of HECs is governed by the storage principle of both
the EDLC and pseudocapacitors. Both the faradaic and non-faradaic electrodes are
overwhelming the limitations of respective components with enhanced the
capacitance and cycling life. The schematic illustration of HECs made up of lithium
electrode and carbon electrode is shown in Figure 1.15. The HECs are of three types:
symmetric, asymmetric and battery types. HECs composed of different types of
electrodes show excellent electrochemical properties compared to distinct electrodes.
20
The HECs provide improved cycling stability in the Faradaic capacitors which limits
the advantages in energy storage. In the symmetrical hybrid system the EDLC has
enhanced specific capacitance at high voltage. The association of two identical
electrodes categorized in symmetric hybrid capacitors. The symmetric hybrid
supercapacitors are formed by similar pseudocapacitive and EDLC components.
Figure 1.15: Schematic diagram of hybrid electrochemical capacitors(Chen et al.,
2017).
1.5 Applications of electrochemical capacitors
The ECs almost have 5 times higher specific power than conventional batteries. Due
to this higher specific power supercapacitors are widely used in the systems where
high power is required. ECs combined with batteries can enhance the life of the
storage system as well as can fulfill the power requirements (Da Silva et al., 2020;
González et al., 2016). ECs due to their rapid charging ability are best in portable
devices. Another advantage of supercapacitors over batteries is the operating
temperature range (Winter and Brodd, 2004). Non-degradable character of electrodes
during charging discharging in ECs make them superior to batteries (Conway, 2013).
ECs are used in the following fields.
21
1.5.1 Hybrid electric vehicles
The ECs are best choice to enhance the competency of hybrid electric vehicles.
Hybrid electric vehicles can easily be started using energy stored in ECs. The science
behind the application of ECs in hybrid electric vehicle is that energy demand spikes.
The ECs are also used in trains and track switching.
1.5.2 Consumer electronics
The ECs are widely used in the devices like laptops, portable devices and solar cell
systems to stabilize the fluctuation of power supply. Digital cameras and flashlights
are charged with supercapacitors in a very short time.
1.5.3 Energy harvesting
The ECs are best to store the energy harvested from a renewable energy source like
sun. This stored energy is then supplied to power ASIC circuit.
1.5.4 Railway
The ECs are used in railway to utilize the braking energy of trains. The energy taken
from braking is powered to start the diesel engine of trains. Economical maintainance
and sociable character make supercapacitors a good choice.
1.5.5 Military and defence applications
The torpedoes, electromagnetic pulse weapons, navigation, communication tools,
sensors, missiles, radar system and GPS all are required high specific power and
funtioned using appropriate assembly of hybrid electrochemical capacitors.
1.5.6 Computer and memory backup chips
In electronic devices, the memory backup process is the foremost function of
electrochemical capacitors. They not only serve as stabilizer of power supply but also
protect the memory of devices in contradiction of power drop. In the solid state
drives, the hybrid electrochemical capacitors also served as backup memory and
power supplier.
22
Figure 1.16: Applications of electrochemical capacitor (Conway, 2013).
1.5.7 Security applications
The electrochemical capacitors extensivley used in security sector because they
perform functions akin to uninterrupted power supply (UPS) in computers because
they supply power within no time.
1.5.8 Applications in public sector
The electrochemical capacitors are widely used in photovotaic power line system,
street lighting in Japan, stabilizer in wind energy system, LED lamps, ventilator
backup and imaging equipment in medical sector, electronic grids so on. Hence,
electrochemical capacitors show priority over prevailing energy storage systems due
to advantageous features such as higher lifespan, source of clean energy, stabilized
power supply and higher sustainability.
1.6 Electrolytes
The electrolytes have great importance in energy storage devices. The electrolytes for
electrochemical capacitors are highly conductive and provide connection between the
electrodes. The electrolytes contain solvent in which suitable liquid and solid
conductive chemicals are dissolved and produced cations and anions (Gao and Lian,
23
2014). The electrolyte for excellent electrochemical measurements possess following
properties:
High electrochemical stability
Wide potential window
Stumpy volatility
Less flammability
Inexpensive
Electrochemically inert with ultracapacitor components
Extensive range of operating temperature
Efficient compatibility with electrode material
Eco-friendly
The overall performance of energy storage devices enhanced when electrolyte meets
above said requirements. The numbers of aqueous, organic and ionic electrolytes are
used while studying electrochemical performance of electrodes material for energy
storage applications. There are three types of electrolytes mainly used in ECs: (a)
aqueous electrolyte (b) organic electrolyte (c) solid polymer electrolytes.
1.6.1 Aqueous electrolyte
The aqueous electrolyte can perform properly under ambient conditions. The aqueous
electrolyte has no restrictions of moisture and gas during electrochemical study. They
offer fast charge transfer during charging and discharging process due to low
resistance and high conductivity. The KCl, KOH, NaOH, H2SO4, Na2SO4, NH4Cl and
Li2SO4 etc. mainly used as aqueous electrolyte for electrochemical capacitors. The
aqueous electrolytes with smaller ions are leading high capacitance during adsorption
of ions in faradaic system. The ionic conductivity increases by enhancing the ionic
concentration and pH of the electrolyte. The electrodes are corroded when very low
and high pH of electrolytes used. Although, the aqueous electrolytes have limited
potential window (less than 1.3V) that‘s lead the low power density and energy
density due to decomposition effect of solvent.
24
Table 1.1: Properties of electrolytes used in electrochemical capacitors.
Electrolyte Operating
window (V)
Conductivity
(mS/cm)
Thermal
stability (°C)
Reference
Aqueous < 1.25 >405 -20 to 49 (Pandolfo and
Hollenkamp, 2006)
Ionic
Liquid
< 3 <100 -40 to 85 (Galinski et al.,
2006)
Organic < 6 <15 -100 to 400 (Ue, 2007)
1.6.2 Ionic electrolytes
The ionic liquids have great importance in the fabrication of supercapacitors due to
amazing properties such as low flammability, high thermal stability, wide range of
potential window (~ 2.0 to 6.0) and high chemical stability. Ionic liquids are
solvent free electrolytes work at room temperature. Ionic electrolytes improved the
performance of ionic liquid based ultracapacitors due to wide voltage range and high
electrical conductivity. Nowadays, the room temperature ionic liquids (RTILs) are
used because of unique properties such as low flammability, non-volatility, wider
potential window, high conductivity and low viscosity. The cyclic amines, aliphatic
quaternary ammonium salts, quaternary ammonium salts, pyrrolidinium,
immidazolium, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
(EMITFSI) and EMIBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) are the
foremost ionic liquids used in supercapacitors.
1.6.3 Organic electrolyte
The organic electrolytes have advantages such as wide potential window, maximum
conductivity and eco-friendly. The organic electrolytes provides higher potential
window up to 6 V than aqueous and ionic electrolytes. The energy density increases
too much, due to wide range of potential. The commonly used organic solvents for
electrochemical capacitors are acetonitrile, propylene carbonate, propylenecarbonate,
tetraethylammonium tetrafluoroborate (TEABF4), γ-butyrolactone tetraethyl phonium
tetrafluoroborate, ethyl-methyl-imidazolium bis(trifluormethyl sulfonyl)imide, poly
(dimethylsiloxane) (PDMS) and 1-ethyl 3-methylimidazolium tetrafluoroborate
(EMImBF4). The electrochemical properties of ultracapacitors are enhanced by some
precise amount of RTILs into organic solvents (electrolytes). This type of electrolyte
mixture provides wide potential window, less charge transfer resistance and high
conductivity (Palm et al., 2013).
25
1.6.4 Solid polymer electrolytes (SPEs)
Solid polymer electrolytes (SPEs) have great importance among all above said types
of electrolytes. The liquid electrolytes require high cost packaging for safety purpose
to evade probable leakage of electrolyte. So, the volume of device increases and large
size wearable devices are usually annoying. To overcome on these problems the solid
phase electrolytes were introduced in new design flexible energy storage devices
requires high flexibility, high safety, reduce weight, maximum lifespan, excellent
electrode-electrolyte interactions and small size. Now the potential demand of SPEs
for high performance energy storage gadgets increased due to their efficient
electrochemical and mechanical stability. The SPEs are composed of three basic
components polymer, solvent (aqueous or organic) and electrolyte salt. The
commonly used SPEs are polyvinyl alcohol (PVA), polyacrylonitrile (PAN),
polyvinylidene fluoride (PVDF), polyethylene oxide, poly hexafluoropropylene
(HFP), poly methyl methacrylate PMMA, poly-(ethylene glycol) blending poly
acrylonitrile, polyamine ester (PAE) and PVDF-co-HFP. The dry polymers have low
conductivity, mechanical stability and less interaction between electrolyte ions and
electrode. Hence, the conductive quasi-solid gel polymer based electrolytes are
introduced. A distinctive solid state gel polymer electrolyte supercapacitor is shown in
Figure 1.17. The 2-hydroxy-3-phenoxy propyl acrylate (HPA) monomer is best for
the preparation of non-aqueous gel polymer electrolytes. The prepared material has
low charge transfer resistance, high specific capacitance with excellent charge
discharge stability (Vijayakumar et al., 2017).
Figure 1.17: (a) Scheme illustrating the preparation of the GPE for supercapacitors
(b) electrochemical performance of prepared samples (Vijayakumar et al., 2017).
26
1.7 Electrode materials for electrochemical capacitors
The capacitance of a electrochemical capacitors is largely depended on electrode
materials. A good electrode possesses high chemical stability, controlled pore size and
economical nature. Electrode surface area is also crucial for adsorbing charged
species on it. The capacitance of a supercapacitor is greatly enhanced by faradric
redox reaction.The metal oxides, metal hydroxides and conducting polymers are used
for psuedocapacitors. The hybrid capacitors are asymmetric and contains capacitance
of both double layer and pseudocapacitor electrode materials. In hybrid capacitors, the
EDLC electrode show extraordinary specific power and faradic pseudocapacitance
electrode provides amazing specific energy.
1.7.1 Carbon materials (porous carbon, CNT, graphene)
Carbon based electrodes are known to show EDLC behavior. Carbon based electrode
materials are disscuss here.
1.7.1.1 Carbon
Carbon as an electrode for EDLCs is the most parsimonious material. It found it‘s use
as an electrode material in the form of carbon nanotubes, activated carbon and carbon
aerogel etc. Although carbon has low electrical conductivity however, it‘s larger
surface area makes it a favorable electrode material for EDLCs (Namisnyk and Zhu,
2003b).
1.7.1.2 Activated carbon
The activated carbon chiefly shows the EDLC capacitance due to presence of micro-
pores. The main advantage of activated carbon is that they possess very low electrical
resistance. Activated carbon may be moulded into solid electrodes by dispersing in
water (Beck and Dolata, 2001; Niu and Wang, 2008). Activated carbon is moulded
into desired shape having extensive variety of pore sizes.
1.7.1.3 Carbon aerogels
Carbon aerogels due to their characteristics like low density, vibration stability,
mechanical stability, high porosity and larger surface area etc. have emerged as a
decent electrode material for EDLCs. The carbon aerogels are more conductive in
nature compared to activated carbons. The aerogel electrodes are fabricate by
pyrolysis of resorcinol formaldehyde aerogels. The electrodes of aerogels are
27
mechanically stable and reedy and have homogeneous pore size and thickness in
many hundred micrometers. Carbon aeregels are made by the method described in
literature. Carbon aerogel made electrode with high energy and power density was
reported (LaClair, 2003; Lerner, 2004).
Figure 1.18: Carbon based materials (Namisnyk and Zhu, 2003a).
1.7.1.4 Graphene
Graphene is a mono atomic diffuse layer of sp2 hybridized carbon atoms which are
assembled in hexagon lattice structure resemblance with honeycomb. The graphene is
a sheet containing carbon atoms organized in 2D lattice. Graphene is considered the
prime material of other graphitic forms such as carbon nanotubes (CNTs), fullerenes,
and graphite etc. The reason behind the dominance of graphene over other graphitic
forms is that other materials are formed by arranging graphene sheets upon each
other. Carbon nanotubes 1D lattice structure is formed by turning the graphene sheet
round and if three graphene layers are repeated then 3D lattice graphite or lead pencil
is formed. Graphene sheet is folded in zero dimensions to form fullerenes (Yamada,
2014). Graphene possess splendid and exclusive properties such as extensive surface
area, excellent thermal stability, superior electrochemical activity, wide voltage
window and excellent electronic properties make graphene an auspicious applicant for
energy storage applications (Thakur, 2012).
28
Figure 1.19: Graphene nanosheets(Namisnyk and Zhu, 2003a).
1.7.1.4.1 Graphene for electrochemical capacitors
Among carbonaceous materials, the graphene has exclusive theoretical specific surface
area (2630 m2/g) and capacitance of 550 Fg-1
. Due to astonish electrochemical properties,
the graphene is extensively used as electrode material in energy storage devices (ECs)
applications. Nowadays, the modified graphene is use in ECs. The modified graphene
includes various forms such as graphene sheets with corrugatelation, layered structure
graphene, thin sheet of graphene and nitrogen doped graphene. The main purpose of
modifications developed in the structure of graphene is to boost the electrochemical
properties. The specific capacitance increased due to highly extensive surface area of
modified graphene (Kim et al., 2011). More recently, the researchers have much
attention towards ECs applications of nanoporous carbon materials pooled with
graphene. The nanoporous carbon materials are derivatives of carbide and have
distinctive porosity.
1.7.1.4.2 Graphene composites for electrochemical capacitors
The metal oxide, metal hydroxide and metal sulfides all have pseudocapacitive
behavior for storing energy in ECs. They possess least electrical conductivity and
squat specific capacitance. When these materials are combined with carbonaceous
scaffolds such as graphene carbon black and carbon nanotubes showed excellent
electrochemical properties. Because, the carbonaceous scaffolds possesses high
electrical conductivity, unique active morphologies and extensive effective surface
area. Moreover, the electrochemical measurements improved by using nanoscale
29
materials. The nanoscale materials have broad surface area. The extensive surface
area reduces the charge transfer extent and shortens the diffusion during
electrochemical study (Chen et al., 2010).
1.7.1.5 Carbon nanotubes (CNTs)
The carbon nanotubes have unique pore size (mesoporous in nature) then other
micropores activated and large surface area carbonaceous materials. Carbon
nanotubes due to their amazing conductivity and greater ability to maintain contact
(wettability) are considered good electrode materials (Arepalli et al., 2005; Signorelli
et al., 2009). Carbon nanotubes were first time publicized as an emerging electrode
material by Niu et al. (Niu et al., 1997). A few or more graphitic shells are arranged in
the form of hollow cylinder to form carbon nanotubes. SWCNTs and MWCNTs are
grown in a rolled up hexagonal closed structure. It may have a lot of faults in its
structure if prepared at low temperatures (Sinnott and Andrews, 2001). Inner space of
layers in MWCNTs is about 0.4 nm to few nanometers and outer diameter ranges
from 2-20 nm. The SWCNTs have diameter of 0.4 to 2 nm and possess length in
micro meters. MWCNTs have closed ends encapsulated by half fullerene which
facilitate in closing both ends. Spongy characteristic of carbon nanotubes (CNTs)
provide space for electrolyte ions to intercelate into electrode pores. Higher surface
area and low equivalent series resistance make carbon nanotubes a potential candidate
as compared to activated carbon are however they are not economical favorable. It has
wide applications in energy storage gadgets, catalysis, electronic devices, nanoprobes,
tissue engineering and hydrogen storage etc.
1.7.1.5.1Carbon nanotube for electrochemical capacitors
Modern research has been implemented to fabricate advance CNTs electrodes for
high performance energy storage devices with excellent energy density and power
density. For this purpose the CNTs directly aligned on the electrical active surface of
substrate. Furthermore, to improve the flexibility of CNTs electrode for
supercapacitors, the CNTs directly decorated on the flexible nanoporous cellulose
paper substrate. Succeeding both the important parameters such as high power density
and energy density for energy storage devices has significant courtesy to energy
storage community. Batteries have high energy density and low power density
compared to supercapacitors. The main focus is to achieve both foremost factors i.e.
high energy density and high power density in same energy storage device with high
30
safety and long lifespan. To overcome on this problem, fabricate the electrodes of
metal oxides and sulfides on the surface of CNTs. The vertically affiliated CNTs
substrate has better unifications with electroactive materials for high rate and high
capacity electrodes due to more spacing, well alignment, extensive surface area and
more electrolyte accessibility (Kang et al., 2006; Taberna et al., 2006). The CNTs
used as electrode material for supercapacitors due to innovative properties such as
high electrical conductivity, larger effective surface area, high charge transport
ability, better interactions with electrolyte and enhanced mesoporosity (Baughman et
al., 2002). The CNTs incorporated with PANI showed higher value of Csp ( > 185
F/g) (Deng et al., 2005). The specific capacitance (Csp) of CNTs treated within higher
pH medium was 85 F/g. If electrochemically active material was added in electrolyte
than the capacitance of CNTs much enhanced. The hydroquinone (HQ) added into
0.075 M aqueous sulfuric acid electrolyte the capacitance boost up to 3195 F/g at low
scan rate (5 mV/s) (Wang et al., 2014). The polyaniline/vertical-aligned carbon
nanotubes nanocomposite electrode material showed maximum Csp more than 403
F/g in 1 M HClO4 electrolyte. The (PANI/VA-CNTs) nanocomposite material showed
less value of Csp ~ 315 in an organic solvent (EMIBF4) (Wu et al., 2017).
1.7.2 Conducting polymers (CPs)
Conductive polymers (CPs) are widely used for energy storage applications
(supercapacitors) due to high charge density, revocable faradaic redox property and
low cost compared to more expensive transition metal oxides. The CPs also possesses
low values of band gaps, high intrinsic conductivity, fast CDC kinetics, easy synthesis
approach and apt morphologies compared to conventional polymers. The CPs can be
p-doped or n-doped (K.S. Ryu and S.H. Chang, (2004)). When the anions reduced the
p-doped CPs is formed and n-doped CPs obtained when cations reduced. The
charging processes of these are as follows:
Cp → Cpn+
(A−)n + ne
− (p-doping) (1.14)
Cp + ne− → (C
+)n Cp
n− (n-doping) (1.15)
The equations 1.14 and 1.15 are charge reactions, the reverse of both 1.14 and 1.15
reactions are the discharging processes. The CPs conducts electricity through
conjugated bond system laterally the backbone of polymer. The CPs are usually
formed by electrochemical or chemical oxidation of monomers. The heterocyclic
conducting polymers (HCPs) such as polypyrrole, polythiophene, poly- indole (PIn),
31
polyaniline and their derivatives (contains nitrogen atoms) are considered pertinent
electrode materials for supercapacitors. The polypyrrole (PPy) and polyaniline
(PANI) are more primarily used polymers for electrochemical capacitors. The PPy
and PANI prepared by easy synthesis approaches. However, they are in pure forms
reduces the specific capacitance and poor cycling stability of electrochemical
capacitors.
1.7.3 Transition metal oxides (TMOs)
Transition metal oxides (TMOs) are extensively used as electrode materials for
electrochemical capacitors. The TMOs have high capacitance and fast redox reactions
due to variable oxidation states. The TMOs as a pseudocapacitive materials have
number of oxidation states. During oxidation and reduction reactions, the TMOs
change oxidation states and protons easily move in and out from the oxide lattice. The
TMOs enhance the energy density and specific capacitance of the supercapacitors.
The pseudocapacitance is obtained by the interactions of ions with loosely attached
surface ions of electrode material. The TMOs electrode materials showed excellent
redox reactions for charge storage due to presence of surface functional groups, flaws
and proper boundaries of particles. The several TMOs such as NiO, RuO2, V2O5,
CuO2, Fe2O3, WO3, TiO2, MnO2, ZnO, and Co3O4 have been studied as
electrochemical capacitor electrode materials.
1.7.4 Metal sulfides
From the last past decade, the transition metal dichalcogenides (TMDCs) have wide-
ranging deliberations in flexible energy storage devices as well as in industry sector
and thermal management. Metal sulfides as electrode material have much attention to
researchers for energy storage devices due to astonished properties such as distinctive
structure, easy preparation, inexpensive, high porosity and maximum (theoretical)
specific capacitance. The TMDCs includes the atoms of VI A group (S, Se and Te)
and show attraction with transition metals such as Mo, Cu, W, Co, Ni, V and Mn and
formed various layered structure. Metal sulfides are abundant and exist in nature as
minerals such as pyrite (FeS2), heazlewoodite (Ni3S2), chalcocite (Cu2S), molybdenite
(MoS2) and so on.
32
1.8 Electrode Material: Structural Effect
1.8.1 One-dimensional electrode material
The one-dimension materials are fiber shaped materials and having one dimension in
nanometer scale for example nanowires, nanotubes, nanorods, nanobelts, nanoribbons
and nanofibers. They possess excellent electronic transport property, high length-
diameter ratio and low value of impedance, which enhance the kinetics of
electrochemical processes. The 1D nanoarchitectures provide nucleating sites for
introducing many entities in the same structure. The large length diameter of 1D
materials inhibits the self-assembly of undesirable materials into the structure. The
self-aggregation process may decrease by using these materials. Furthermore, 1D
nanoarchitectures augment candidates as electrode materials for flexible energy
storage devices due to exclusive physical and chemical properties. The longitudinal
axis of 1D nanoarchitectures offers proficient transport pathway for both ions and
electrons. Qunting Qu et al. prepared core shell PPy and nanoribbons like
morphology of V2O5. They prepared V2O5@PPy composite electrode material for
supercapacitors. The prepared material showed maximum cycle stability and retains
95% specific capacitance after ten thousand charge/discharge cycles. The physical
appearance of electrolyte was not changed during the reaction when V2O5@PPy sued.
The (a) part of Figure 1.20 shows the expected coating of PPy to facilitate the
electronic transport and prevents dissolution of vanadium in electrolyte and (b) part
shows the schematic sketch of successful growth of PPy on V2O5 nanoribbon surface
(Qu et al., 2012).
Figure 1.20: (a) PPy coating is expected to facilitate the electronic transport and
prevents dissolution of vanadium in electrolyte. (b) Schematic sketch illustrate the
growth of PPy on V2O5 nanoribbon surface (Qu et al., 2012).
33
1.8.2 Two-dimensional electrode material
The two-dimension materials having two dimensions in nanometre scale for example
graphene sheets, nanosheets, nanoplates, nanocoats, nanofilms, nanofibers. The 2D
materials showed exclusive electrochemical properties in supercapacitors due to
astonished properties such as strength, transparency, electrochemical stability,
flexibility, wide surface area and mechanical stability. The transition metal oxides like
RuO2, V2O5, NiO, Co2O3, MnO2, SnO2, IrO2 and MoO2, are considered as imperative
electrode materials for electrochemical capacitors. The layered transition metal
compounds and their hybrids have graphite like properties categorized into two
dimensional materials. In this category the inter-plane atoms are intermingled through
van der Waals interactions and in-planes atoms associated by chemical bonds. The
vanadium, nickel, iron, so on are naturally present in layered form structure. The
transition metal chalcogenides are two dimensional materials having ultrathin layered
morphology. They are considered the best electrode materials for energy storage
applications with amended electrochemical performance due to excellent mechanical
strength, high conductivity, and extensive surface area. The materials having two-
dimensional architecture are extensively used for flexible electrochemical capacitors
owing to excellent mechanical strength, conductivity, surface area and also have
ability to improve the successive charge/discharge processes. Compared to bulk
materials, the 2D morphology offers unique behavior towards supercapacitors
applications. Guillaume A. Muller et al. prepared 2D nanocrystals of titanium
disulfide by the hot injection method. The Figure 1.21 shows the schematic
illustration of the crystal structure of TiS2 (a) and CVs for 2D-TiS2 nanocrystals (b).
The prepared composite electrode showed high pseudocapacitance and rate capability
(Muller et al., 2015).
34
Figure 1.21: (a) Schematic illustration of the crystal structure of TiS2. (b) CVs for
2D-TiS2 nanocrystals (Muller et al., 2015).
1.8.3 Three-dimensional electrode material
The three-dimension materials having three dimensions in nanometer scale for
example polycrystals, graphite, hollow spheres, nanofoams and nanosponges. The
three dimensional materials with various morphologies exhibited excellent
electrochemical performance due to enormous interface area of electrode-electrolyte,
smooth electron/ion transport pathway and extensive surface area. The electrode with
excellent electrochemical performance needs maximum amount of working material
becomes challenge for researchers. To solve this problem the three dimensional
carbon-metal oxide (3D-CMOs) nanostructures are introduced. The stable structures
of 3D-CMOs materials provide high rate capability and cycle stability so, fruitful for
both electrochemical capacitors and batteries. Additionally, they have a dearth of
electron conductive entities, fast transfer of electrons and ions. However, to overcome
this issue, extensively loaded electrodes are fabricated with highly active materials
with a wide surface area (Fu et al., 2016; Xia et al., 2014). Wenbin Fu et al. prepared
copper sulfide nanoparticles. The prepared electrode material have an extensive
interconnected network of nanoparticles, provides high surface area and fast
electron/ion transport path. The CuS nanoparticles showed excellent capacitance,
cycle stability, energy density, power density and rate capability. The SEM image of
the CuS networks, CV curves of the CuS interconnected nanoparticles and schematic
graphics of the asymmetric supercapacitor are shown in Figure 1.22 (a), (b) and (c)
respectively (Fu et al., 2016).
35
Figure 1.22 : (a) SEM image of the CuS interconnected nanoparticles (b) CV curves
of the CuS networks (c) Schematic graphics of the asymmetric supercapacitor (Fu et
al., 2016).
1.9 Electrode Material: Compositional Effect
1.9.1 Carbon-metal oxide composite
TMOs have generally low conductance except RuO2. They have high value of
resistivity. The solution resistance and charge transfer resistance are increases due to
low conductivity of TMOs. They show huge IR loss at a high current density so, the
rate capability and power density drops. The pure TMOs show poor cycling stability
during charge/discharge reactions due to a high value of strain. The high strain creates
cracks in the electrode material. They have dearth surface area, suitable pore size and
porosity. Thus currently, the researchers upgraded the study of electrode materials for
ECs with efficient electrochemical performance. The composites of TMOs with
conductive carbon scaffolds are legendary for enlightening the electrochemical
performance of the electrochemical capacitors (Lokhande et al., 2016). Only TMOs
have great ability to store both charge and energy. The presence of carbon in
conductive carbon-metal oxide composites provides physical support and suitable
channels for charge transport during electrochemical processes. The carbon based
TMOs composite materials with various nanoarchitectures provides suitable electrode
porosity, specific surface area, well pore size distribution, high electronic conductivity
which improves the performance of electrochemical capacitors performance.
Table.1.2 gives a divergence of carbon, metal oxide and metal oxide–carbon
composite electrodes.
36
Table 1.2: Contrast of carbon, metal oxide and metal oxide–carbon composite
electrodes.
Electrode
materials
Csp Conductivity Rate
capability
Cost Stability
Carbon Low High High Low Good
Metal oxide High Low Low High Deprived
Metal oxide-
carbon
composites
High Variable (accordance
of carbon support)
Good Moderate Good
The carbon-metal oxide composite materials have excellent electrochemical
properties such as high power density, excellent rate capability, good cyclic stability
maximum specific capacitance and high energy density. Dan Wu et al reported the
synthesis of manganese dioxide and it‘s composite with carbon. The MnO2/C
composite have improved electrochemical performance (Wu et al., 2020). Jose
Garcia-Torres et al. prepared the manganese dioxide. The composite of MnO2 with
various nanostructured carbon (carbon black, carbon nanotubes) materials was
prepared. The CB/CNT/MnO2 NT showed maximum capacitance (246 F/g) and high
cycle stability(Garcia-Torres et al., 2019).
1.9.2 Compositional effect of carbon-conducting polymer composites
For the purpose of enhancing electrochemical performances, it is necessary to
fabricate carbon-conducting polymer (CCPs) based composites (Shayeh et al.,
2015; Xu et al., 2010; Zhang and Lou, 2013). The composites of CCPs with
conductive scaffolds improved the capacitance, cycling stability, high surface area
and high specific energy. Devalina Sarmah et al. prepared the ternary MoS2-
rGO@PPyNTs nanocomposites. The ternary nanocomposites showed excellent
electrochemical performances compared to bare MoS2-PPyNTs, rGO-PPyNTs and
PPyNTs electrodes. The ternary MoS2-rGO@PPyNTs nanocomposites showed
extraordinary ~ 1562 F/g specific capacitance at current density of 1A/g and high
specific energy and power density (Sarmah and Kumar, 2018). Chuhan Sha et al.
reported the synthesis of MoS2/PANI/rGO aerogels for high performance
supercapacitors. The ternary MoS2/PANI/rGO nanocomposites exhibited high specific
capacitance ~ 620 F/g and high retention in capacitance (78%) after two thousand
charge/discharge cycles (Sha et al., 2016). Mandira Majumder et al synthesized
37
molybdenum disulfide/polyindole/carbon black. The polyindole have fused benzene
and pyrrole ring. The PIn/CB/MoS2 composite possessed high capacitance (513cm-3
)
and excellent coulombic efficiency (~98%) (Majumder et al., 2017).
1.9.3 Heteroatoms-doped carbon materials
Heteroatoms doped carbon materials with 3D interconnected nanostructures
electrodes are auspicious contestant for energy storage applications. The porous
carbon materials reveal desired electrochemical functionalities, high surface area and
good stability so, extensively used in catalysis, gas storage, sensors, adsorbents,
separation and energy storage sectors. In order to multiply the electrochemical
performance of energy storage devices, the more recent research focus on introduction
of heteroatoms to carbon nanoarchitectures. In faraday reaction the pseudocapacitance
originates from the active sites of heteroatoms. The incorporation of heteroatoms like
N, S, P and B into the carbon materials enhances the EDLC efficiency. This type of
strategy improves both the faradaic and non-faradaic capacitance. The N and P both
belong to same family, the P has more ability to donate electrons and possess robust
n-type behavior. Yi Lin et al. prepared HTC-NPG-AC electrode. The prepared
electrode exhibited excellent capacitance of 406 F g-1
at current density of 0.2 Ag-1
.
The fabricated working electrode showed maximum rate capability (78%) (Lin et al.,
2019). Xiaona Yan et al. prepared activated carbon nanofiber networks (ACFNs)
doped with heteroatoms such as N, P and O. The prepared electrode material
possessed high surface area, rate capability, cycling stability and specific capacitance
(182 F/g) (Yan). Furthermore, the electrochemical performance of carbon materials
enhanced by dual-heteroatoms doping approach. Wen Lei et al reported the
preparation of N/S doped carbon microspheres. The material exhibited high specific
capacitance ~ 278 F/g with good rate capability (Lei et al., 2017). Yuanyuan Li et al.
reported the effective synthesis of P and N co-doped porous CNT@carbon
core@shell nano-networks (PN-CNTs) for high performance supercapacitors. The
PN-CNTs electrode material showed astonish specific capacitance ~ 333 F/g, high
rate stability and maximum retention capacitance after successive eight thousand
cycles (Li et al., 2016b).
38
1.10 Metal sulfides for electrochemical capacitors
Metal sulfides nanostructured materials have been widely studied due to their
excessive contribution in many devices such as light emitting diodes, fuel cells,
thermoelectric and nonvolatile memory devices, sensors, lithium ion batteries, ECs
and solar cells. The TMDs such as molybdenum disulfide (MoS2), manganese sulfide
(MnS), nickel sulfides (NiS), tungsten disulfide (WS2), copper sulfide (CuS),
vanadium disulfide (VS2), cobalt sulfides (CoS) etc. as electrode materials have great
interest in energy storage devices especially ECs. The metal sulfides are categorized
into two types: layered and non-layered sulfides on the basis of their structure. In
layer metal sulfides, the three atom layers (S-M-S) are joining with each other through
covalent bond and individual layer formed by van der Waals interactions. The layered
metal sulfides are MoS2, SnS2, VS2 and WS2 and so on (Rui et al., 2014). The
morphology of MoS2 is single-layer which is similar to graphene. The MoS2 layers
arrange in a pile through weak interactions and can be exfoliated into mono or
multilayers by chemical and physical methods. The atomic structure of molybdenum
disulfides consists of layer of molybdenum atoms inserted between the layers of
sulfur atoms. For double-layer charge storage, the MoS2 also used as an EM for
capacitor owing to sheet-like structure. The three dimensional porous materials are
used for ECs due to suitable structure for ionic transportation and high
electrochemical reactivity. On the other hand the NiS2, CoS2, MnS, FeS2 and so on are
non-layered metal sulfides and linked together through covalent bond. They gain
much attention towards recent research era due to maximum capacity, abundance,
eco-green and inexpensive (Shao et al., 2018). The metal sulfides or mixed metal
sulfides possess special priority over carbon materials, metal oxides and metal
hydroxides because of presence of more rapid electron transfer specie (sulphur atom)
and superior conductivity. The carbon materials, metal oxides and metal hydroxides
have some drawbacks such as low specific capacitance, stockpiles of materials, low
density of electrode and exertion in energy storage devices. The sulfides are selected
extensively compared to selenides due to low toxicity and massive abundance in
nature (Chandrasekaran et al., 2019). Overall, the dissolution/ recrystallization
process selects to fabricate the metal sulfide that‘s changes the oxides into sulfides.
Furthermore, the improved electrochemical properties of metal sulfides can be
achieved by choosing suitable synthesis method and precise modifications in
39
structure, size, composition and morphology of the metal sulfides. The performance
and lifespan of energy storage devices be improved by making electrodes of transition
metal sulfides with various morphologies. The electrochemical performance of metal
sulfides enhanced by forming conductive composites with conductive carbon
scaffolds such as graphene, active carbon, carbon nanotubes etc. The electrochemical
properties of metal sulfides also boost up by surface modification approach. By
successful carbon coating on the surface of metal sulfides enhanced the cycle
stability, lifetime and capacitance of energy storage device due to high conductivity,
volume expansion and effective surface area of carbonaceous composites. The surface
modifications enhance the porosity of electrode material and reduce the side reactions
during electrochemical study and improve the performance. The number of
preparation methods is used for the fabrication of metal sulfides such as
hydrothermal, atomic layer deposition, co-precipitation, ball milling, microwave,
electrodeposition, chemical vapor deposition, polymerization, exfoliation,
electrochemical preparation and thermal deposition method (Samad et al., 2015; Sami
et al., 2017; Sun et al., 2018; Xu et al., 2017; Yuan et al., 2018). The utmost methods
are exfoliation (at high temperature) and solid state fabrication of metal sulfides.
Nanostructured TMDCs have astonished applications in various fields as modern
electrode materials such as, electrochemical hydrogen generation, batteries,
supercapacitors, and heterogeneous catalysis due to fundamental properties such as
inexpensive, frequently abundant in presence, easy preparation, high capacitance and
extraordinary electrochemical properties. The WS2, WSe2, MoSe2 and MoS2
semiconducting materials possessed great attraction in energy renovation and storage
devices. The TMDCs have astonished applications in sensing, dehydrosulfurization,
biomedicine, hydrogen generation, solar cells, dye-sensitized solar cells, energy
storage and nano-electronics (M. Lu, 2013.; Naoi et al., 2012). The summary of main
findings of some studied metal sulfides is explained in Table 1.3.
Table 1.3: Summary of the main findings of studied metal sulfides.
Sr.No Electrode
Materials
Morphology Specific
Capacitance
Application Reference
1 VS4/CNTs/rGO Nanorods 1004 mF·cm-
2
Supercapacitors (Wang et
al., 2020)
2 VS2@S Flowerlike
nanoparticles
414 mAh g-1
Lithium‐Sulfur
Batteries
(Chen et al.,
2020)
3 VS4/rGO Flowerlike
nanoparticles
492 mA h g-1
Aluminum‐ion
batteries
(Zhang et
al., 2018)
4 V3S4/RGO Porous sheets 499.9 Fg-1
Supercapacitors (Kalam et
40
al., 2018)
5 VS2/C-Fe/PANI Nanosheets 517 F g-1
Supercapacitors (Rantho et
al., 2018)
6 WS2/RGO Nanolayers 351 Fg-1
Supercapacitors (Ratha and
Rout, 2013)
7 Copper tungsten
sulfide (CWS)
Crumpled sheet 2667 F g-1
Supercapacitors (Pazhamalai
et al., 2019)
8 WO3/WS2 Flower like
nanosized plates
509 F g-1
Supercapacitors (Mandal et
al., 2018)
9 WS2 Nanosheets 242 F g-1
Lithium ion
storage ability,
supercapacitor
(Ansari et
al., 2018)
10 G/TS/TO Nanoparticles 149mF cm-2
Supercapacitors (Yang et al.,
2019b)
11 WS2 (QDs) Crystal
structure
456.99 Fg-1
Supercapacitors (Yin et al.,
2019)
12 NiS- PbS
hybrid
Nanoparticles 126 mA h g-1
Supercapacitors (Mun et al.,
2019)
13 PbS Fractal fern-like
architecture
499 Fg-1
Supercapacitors (Dai et al.,
2019)
14 γ-MnS/rGO Flake-like 548 F g-1
Supercapacitors (Zhang et
al., 2017)
15 α-MnS/N-rGO Spheres 934 F g-1
Supercapacitors (Quan et al.,
2016)
16 RGO/CuS
composite
Clusters of
nanoparticles
947 F g-1
Supercapacitors (Zhao et al.,
2018)
17 FeS@Carbon Shell like
nanoparticles
631.99
mAhg-1
Sodium-Ion
Battery
(Bu et al.,
2018)
18 FeS2 Nanospheres 483 F g-1
Supercapacitors (Javed et al.,
2016)
19 CoS/FeS Core–shell 137.99 F g-1
Supercapacitors (Al Haj et
al., 2019)
20 C/ FeS Mesoporous
nanoparticles
268 Fg-1
Supercapacitors (Yu and Li,
2019)
21 CuS and Fe-CuS Mesoporous
spherical
nanoparticles
3283 and
517 Fg-1
Supercapacitors (Brown et
al., 2018)
22 CuS Nanowire 304.99 F g-1
Supercapacitors (Hsu et al.,
2014)
23 CuS Nanosheets 275.98 F g-1
Supercapacitors (Xu et al.,
2016)
24 CuS Nanoparticles 50 mA g-1
Supercapacitors (Fu et al.,
2016)
25 CuS Hollow
microflowers
537 F g-1
Supercapacitors (Liu et al.,
2018)
26 Ni-Co
S/rGO/CNT
Nanosheets 1874.99 Fg-1
Supercapacitors (Chiu and
Chen, 2018)
27 NiS Nanoplates 788 F g-1
Supercapacitors (Gaikar et
al., 2016)
28 MoS2/MWCNT Nanosheets 453 F g-1
Supercapacitors (Huang et
al., 2014)
29 MoS2/PANI/rGO Nanosheets 331 F g-1
Supercapacitors (Chao et al.,
2018)
30 CNT/MoS2 Nanosheets 13.2 F cm-3
Supercapacitors (Lv et al.,
2016)
41
Aim and Objectives
The main aim of this PhD project is to develop the electrode materials for
supercapacitors that exhibit the maximum electrochemical energy storage properties
with good cyclic stability. To meet this aim, following objectives are planned:
To prepare binary nanocomposites of transition metal sulfides.
To prepare metal sulfides with unique morphology.
To prepare various carbon based materials such as graphene, carbon
nanotubes etc. and utilized for making nanocomposites with as-prepared
metal sulfides.
The ultracapacitors should be cut-rate using low cost materials and
fabrication processes.
The ultracapacitors should attain high cyclic stability after maximum (15000
cycles) CV cycles.
To select unique non-hazardous electrolytes that has high ionic conductivity,
large and stable potential window.
42
CHAPTER 2
LITERATURE REVIEW
43
CHAPTER 2
LITERATURE REVIEW
Jie Chao et al. prepared the hierarchical nanosheets of MoS2/polyaniline/reduced
graphene oxide by a facile hydrothermal method. The structural stability and surface
area of MoS2/PANI nanosheets were enhanced by aligning them onto the r-GO
nanosheets. The conductive r-GO in the MoS2/PANI/r-GO provided fast ions and
electrons transport in the electrode material. Moreover, rGO provided hetero-interface
between MoS2, rGO and PANI. With ultrahigh active surface area and excellent
conductivity of MoS2/PANI/rGO HNSs proved as excellent electrode material for
supercapacitors. At first cycle the MoS2/PANI/rGO-300 HNSs showed specific
capacitance more than 330 Fg-1
at 10Ag-1
with 81% retention in capacitance after
40,000 cycles.
The MoS2/PANI/r-GO (300)//MoS2/PANI/r-GO (300) symmetric supercapacitor,
showed specific capacitance 97 Fg-1
at 2Ag-1
. For the assembling of asymmetric
supercapacitor, the active carbon (AC) and MoS2/PANI/rGO-300 HNSs acts as anode
and cathode respectively exhibits high specific capacitance (73 Fg-1
) at 2Ag-1
with
retention of 87% specific capacitance after twenty thousand cycles. From the results
of electrochemical measurements the MoS2/PANI/rGO-300 HNSs acts as excellent
electrode material for supercapacitor (Chao et al., 2018).
Devalina Sarmah et al. synthesized layer-by-layer nanoparticles of MoS2. The MoS2
nanostructures were prepared by the hydrothermal method. The nanocomposites of
MoS2 were prepared with reduced graphene oxide nanosheets through the
hydrothermal method. The ternary nanocomposites of MoS2/rGO with synthesized
nanotubes of polypyrrole were prepared by a facile hydrothermal method. The MoS2-
rGO nanocomposites afford higher electronic conductivity, effective electroactive
sites and larger surface area due to the layered porous structure. Various electrodes of
as-prepared material were prepared (rGO-PPyNTs, PPyNTs and MoS2-PPyNTs) and
compared them with MoS2-rGO@PPyNTs. The ternary nanocomposites exhibits
much more electrochemical measurements with outstanding specific capacitance
(<1560 Fg-1
at 1Ag-1
) with operating window range between -0.3 to 1.3 V. The MoS2-
rGO@PPyNTs nanocomposites showed cycle stability of 72% after ten thousand
44
cycles at 10 Ag-1
and power and current density 800Wkg-1
and <550 Whkg-1
(Sarmah
and Kumar, 2018).
Shouzhi Wang et al. fabricated a 3D network of molybdenum disulphide. The
nanocomposites of MoS2 were prepared with carbon nanotubes (CNTs) and reduced
graphite oxide (r-GO) composites. The as-prepared materials possessed unique
applications in flexible supercapacitor. The physicochemical properties of all
prepared sample material were studied. The obtained material possessed high energy
storage capacity. The MoS2@CNT/RGO showed an interconnected framework and
high porosity. As an electrode in supercapacitors these materials possessed high
specific capacitance of 131 mF cm−2
at 0.1 mA cm−2
. The electrode materials have
long lifetimes. The electrode material exhibited excellent electrochemical
performance. Due to tremendous electrochemical properties these materials were used
in lightweight, small, portable and flexible electronic devices (Shouzhi et al., 2017).
Turki Alamro et al. fabricated chemically MoS2-PEDOT nanocomposites at
controlled conditions using various ratios of precursors. The crystallinity, potential
properties and morphology of as fabricated material were analyzed by XRD, FTIR
spectroscopy, Raman spectroscopy, TEM and SEM techniques. The electrochemical
properties were determined. The power density, energy density and Csp of the
supercapacitors were determined by charging-discharging, nyquist and cyclic
voltammetry (CV) plots. The specific capacitance value of the nanocomposites
electrode was 361 F/g. Charging-discharging of MoS2-PEDOT nanocomposites
showed enhanced pseudocapacitive behavior for energy storage applications (Alamro
and Ram, 2017).
Mandira Majumder et al. synthesized MoS2/polyindole (PIn) by the hydrothermal
method. Ternary composites of MoS2/polyindole (PIn) with carbon black (CB)
prepared by in situ polymerization method. Polyindole used due to unique properties
because of its distinctive structure. The charge transportation increases in the medium
due to the assimilation of carbon black and MoS2 into the polyindole matrix so,
electrochemical measurements were amended. The ternary PIn/CB/MoS2-2 showed
greater values of gravimetric and volumetric capacitance at 1Ag-1
current density. The
retention capacitance of the composites was more than 90 % after five thousand
cycles at 10Ag-1
. Hence, from the interpretation of electrochemical results the
prepared composite was suitable electrode material for supercapacitor application
(Majumder et al., 2017).
45
Chuhan Sha et al. reported the synthesis of molybdenum disulfide. The molybdenum
disulfide was synthesized by hydrothermal method. The graphite oxide prepared by
facile Hummer‘s method. The MoS2/PANI was prepared by polymerization method.
The ternary nanocomposites of MoS2/PANI/r-GO-aerogel were prepared. The ternary
MoS2/PANI/r-GO nanocomposites were prepared by dispersing MoS2/PANI
nanoparticles with graphene oxide and reduced it with urea using hydrothermal
method. The as-prepared material was analyzed by various techniques such as XRD,
XPS, N2-adsorption-desorption isotherms, SEM and TEM. The nanomaterials
possessed flower like structure and enhanced the surface area confirmed from SEM.
The surface area of MoS2/PANI/rGO nanocomposites was greater than the pure MoS2
and MoS2/PANI studied from the BET analysis. The Csp of prepared materials was
less than 620 F/g at 1.0 A/g. The ternary electrode presented good cycle stability, 78
percent retention capacitance after two thousand cycles. From the results the prepared
ternary nanocomposites possessed excellent electrochemical measurements and
beneficial for energy storage devices (Sha et al., 2016).
Anukul K. Thakur et al. prepared binary nanocomposites with varying weight
percent of Molybdenum disulfide (MoS2) and polypyrrole successfully via adopting
in-situ polymerization route. The as-synthesized material was characterized through
FE-SEM, TEM and XRD. The electrochemical measurements of prepared material
were explored by various techniques such as CV, CCD and EIS. The 25 weight
percent composition of MoS2 (MP2) showed maximum value of Csp as compared to
pure polypyrrole and MP1 and MP3 (weight percent of 10.5 and 50 respectively). The
suitable amount of MoS2 in MP2 nanocomposites possessed greater value of
capacitance due to greater interactions between MoS2 and PPY compared to other
compositions. Moreover the MP2 showed decrease in Csp not more than 7.3% after
five thousand cycles and possessed greater values of power density and energy
density compared to PPy (Thakur et al., 2016).
Karthikeyan Krishnamoorthy et al. succeeded few layers of MoS2. They obtained
layered structure of MoS2 by ball milling method. The obtained exfoliated MoS2
sheets were confirmed by XRD, Raman analysis and FESEM. The prepared material
showed thin nanosheets morphology analyzed by SEM. The electrochemical
measurements of MoS2 based wire type solid state supercapacitors (WSCs) were
observed by various electrochemical techniques. The electrochemical measurements
were obtained at potential range of 0 to 1 V at various sweep rates. The MoS2
46
electrode material showed more than 110 mFcm-1
specific capacitance with good
capacitance retention ~ 90% after 2500 cycles and energy density of 8.0nWhcm-1
. The
electrode material showed excellent physicochemical and applicational properties so,
possessed power for energy storage devices applications (Krishnamoorthy et al.,
2016).
Xuan Li et al. reported the synthesis of molybdenum disulfide. The GO was
synthesized by simple Hummer‘s method. The molybdenum disulfide /reduced
graphite oxide composite was prepared via facile hydrothermal method. The
composite of molybdenum disulfide with polyaniline was synthesized by
polymerization process. The as prepared pure and composite materials were studied
by various characterization techniques such as XRD, XPS, BET, SEM, TEM and
Raman spectroscopy. The PANI nanowires successfully embedded on the surface of
molybdenum disulfide /reduced graphene oxide nanosheets observed by SEM and
TEM analysis. The surface area of MoS2/r-GO was greater than MoS2/r-GO@PANI
composite material investigated by BET analysis. The electrochemical measurements
were studied in 1M H2SO4 electrolyte and ~ 2.0 mg material pasted on the surface of
substrate. The superficial surface of MoS2/r-GO@PANI homogeneously covered by
PANI and showed enhanced electrochemical performance. The MoS2/RGO@PANI
electrode material exhibits the highest value of specific capacitance more than 1200
Fg-1
at current density of 1Ag-1
with 82 % retention in capacitance after three
thousand cycles. The electrode material possessed good power and energy density so,
from the extraordinary electrochemical performances the prepared electrode material
be suitable for energy storage devices (Li et al., 2016a).
Lijun Ren et al. prepared 3D tubular molybdenum disulfide (MoS2) and PANI
nanowires with a width of ~ 20 nm by oxidative polymerization of aniline monomer
and hydrothermal method. Three-dimensional tubular hybrid materials were prepared
with various concentrations of polymer. The prepared molybdenum disulfide (MoS2)
and PANI nanowires were characterized by XRD, FESEM, TEM and FTIR
techniques. The MoS2/PANI possessed tubular morphology analyzed by SEM. The
electrochemical measurements were obtained in 1M sulfuric acid electrolyte and
operating window from -0.32 to 0.42 V. The cyclic voltammetry obtained at 6
mVs-1
sweep rate. The growth of PANI nanowires on tubular MoS2 surface showed
excellent performance of the electrodes in electrochemical measurements and good
cycling stability. The MoS2/PANI-60 hybrid electrode when PANI was used as 60%,
47
showed high value of capacitance of 552 F/g at a current density of 0.5 A/g. At
current density 1 A/g the, Csp of two electrodes was 124 F/g. The as-prepared
material was exhibiting astonished capacitance retention of PANI electrode (Ren et
al., 2015).
Gengzhi Sun et al. informed the successful incorporation of MoS2/rGO into well
aligned multi walled carbon nanotube sheet by twisting. The prepared materials were
characterized by various characterization techniques. The prepared samples were
further studied by electrochemical techniques. The electrochemical measurements
studied here were CV, EIS and GCD. For construction of flexible, asymmetric ECs
the rGO/MWCNTs and MoS2/r-GO/MWCNTs twines acts as cathode and anode
correspondingly. The MoS2-rGO/MWCNT fiber based asymmetric ultracapacitor
possessed excellent cycling stability, greater coulombic efficiency and energy density
at 1.4 V potential window. The MoS2-rGO/MWCNT type of fiber like asymmetric
devices might be used in flexible electronics (Sun et al., 2015).
Bingling Hu et al. synthesized successfully porous tubular C/MoS2 nanocomposites.
The tubular C/MoS2 nanocomposites were synthesized using porous anodic aluminum
oxide (AAO) oxidation method. The cylindrical C/MoS2 nanocomposites were
studied by various physicochemical characterization techniques. The cylindrical
C/MoS2 nanocomposites were used as a template for the first time for performing
electrochemical analysis. The tubular C/MoS2 nanocomposites exhibit good EM for
ECs. An as-prepared material was showed higher value of capacitance more than 200
F/g at 1 A/g with long-term cycling stability more than thousand cycles and possessed
electrochemical energy storage applications (Hu et al., 2013).
Ke-Jing Huang et al. prepared molybdenum disulfide. The MoS2 nanosheets were
synthesized by a simple hydrothermal method. The nanocomposites of MoS2 with
multi-walled carbon nanotube were prepared via the hydrothermal method in the
presence of L-cysteine as the sulfur source. The MoS2 and its composites were
characterized by XRD, FE-SEM, TEM, FT-IR and Raman analysis. Furthermore, the
as-prepared materials were subjected to electrochemical study. The all
electrochemical measurements were conceded in 1M Na2SO4 electrolyte. The
potential window selected from 0.2 to 0.16 V. The composite electrode revealed
greater electrochemical measurements compared to MoS2 and MWCNT. The
MoS2/MWCNT composite showed high specific capacitance more than 450 F/g at
1A/g compared to MoS2 and MWCNT electrodes with 85.8 % retention capacitance
48
after thousand cycles. From inspiring electrochemical results the MoS2/MWCNT
electrode suitable for high performance supercapacitors (Huang et al., 2014).
Edney Geraldo da Silveira Firmiano et al. fabricated the molybdenum disulfide
nanoarchitectures. The graphite oxide for nanocomposites formation was synthesized
by facile Hummer‘s method. The molybdenum disulfide was successfully inserted on
reduced graphene oxide by microwave heating. The synthesized materials were
investigated by XRD, SEM, HRTEM, FTIR, XPS and AFM studies. The three
different concentrations of MoS2 were deposited on graphene denoted as low, medium
and high concentration LCMoS2/RGO, MCMoS2/RGO and HCMoS2/RGO
respectively. The specific capacitance values >120 Fg-1
for LCMoS2/RGO, >260 Fg-1
for MCMoS2/RGO and >145 Fg-1
for HCMoS2/RGO concentrations of prepared
working electrode material were obtained at scan rate of 10 mVs-1
in 1M HClO4
electrolyte. For LCMoS2/RGO composite the energy density was more than 60 Whkg-
1 and showed 8% drop in Csp after thousand cycles (da Silveira Firmiano et al., 2014).
Ke-Jing Huang et al. fabricated three dimensional spheres like molybdenum
disulfide. The graphite oxide was prepared by the solution phase method. The MoS2-
graphene composite was prepared by facile hydrothermal method at specific reaction
conditions. The physico-chemical characterization of MoS2-graphene was explored by
XRD, FT-IR, XPS, Raman analysis and TEM. The three dimensional sphere like
morphology of MoS2-graphene was confirmed by transmission electron microscope.
The electrochemical measurements of the obtained composite material were studied
by CV, GCD and EIS techniques. The electrochemical techniques were studied in 0.1
M NaOH electrolyte. The Csp of MoS2-graphene electrode material was more than
240 Fg-1
at 1Ag-1
. The electrode material possessed more than 92 % retention in
specific capacitance after thousand cycles at 1Ag-1
due to communal effect of
conductive scaffold (graphene) and provides effective interaction with electrolyte and
prevents combination of electroactive materials during charge-discharge measurement
(Huang et al., 2013).
Huanwen Wang et al. reported the successfully conversion of multi-walled carbon
nanotubes (MWCNTs) to curved graphene nanosheets (CG) by facile Hummer‘s
method. The MWCNTs
Cut and opened in the crosswise and longitudinal direction. The synthesized materials
were explored by various techniques such as RD, FE-SEM, TEM, Raman analysis,
BET study and FTIR spectroscopy. The as-prepared CGN possessed distinctive
49
nanotubes (1D) and graphene (2D) structure confirmed by XRD. The as prepared
material showed maximum specific capacitance and high cycle stability during the
process of charge-discharge for supercapacitor. The electro-measurements were
carried out in various electrolytes such as 1 M KOH, 1 M H2SO4, and 1 M Na2SO4 in
potential window -0.1 to 1 V. The enhanced electrochemical performances stand due
to prolonged defect density, greater interaction with electrolyte ions and greater active
surface area. Moreover, the unique prepared material suitable for energy storage
devices such as batteries, gas storage, nanoelectronics and sensors (Wang et al.,
2012).
P. Himasree et al. fabricated the CuS@MnS nanostructures on Ni foam substrate via
hydrothermal method. The synthesized (binder free) material was best exploit for
supercapacitors. The prepared electrode material was studied by various
characterization techniques. The obtained material possessed coral reef-like
nanostructure confirmed by SEM images. The excellent adhesion of prepared
nanostructures on substrate, showed many advantages such as improved storage and
charge backlog, enhanced electrochemical interactions between CuS and MnS, better
acceleration of electron and electrolyte ion transfer and greater surface area on
substrate. The electrochemical techniques were carried out in 3 M potassium
hydroxide solution. The cyclic voltammetry CuS@MnS/NF electrode was studied at
various scan rates from 10-50 mVs-1
. The CuS@MnS/NF composites electrode
showed terrific electrochemical performance. The as prepared nanocomposites
electrode possessed good cycling stability, high specific capacity and showed
excellent rate capability compared to that of CuS/NF which was more than 0.04 A h g-
1 and MnS/NF (> 0.004 A h g
-1). The sample electrode was demonstrated the unique
composites material for electrochemical energy storage devices (Himasree et al.,
2019).
Yuerong Ba et al. reported that fiber-shaped supercapacitors have great attention
towards flexible, portable and wearable electronics. The synthesis was carried out
using chemical bath and electrochemical deposition. Commercial poly (ethylene
terephthalate) (PET) thread was changed into an electrochemically and electrically
conductive by mixing copper sulfide (CuS) and polyaniline (PANI). The as-prepared
PANI/CuS/PET electrode showed excellent physical and electrochemical
performance so, used in portable energy storage devices. The PANI/CuS/PET
50
electrode was exhibit first-rate cycling stability with 93.1 % retention (>1000 cycles)
and high value of specific capacitance 0.029 F cm-2
(Ba et al., 2018).
Yanxia Liu et al. successfully prepared copper sulfide with different morphologies
CuS hollow microflowers (HM-CuS), CuS solid microspheres (SM-CuS) and CuS
tubular structures (T-CuS) using different solvents. The physical characterization of
all the synthesized materials was investigated by XRD, SEM and TEM. Furthermore,
the as-material was applicable to electrochemical techniques. All the electrochemical
measurements were obtained in 3 M potassium hydroxide at 5-100 mVs-1
different
sweep rates. The CuS hollow microflowers (HM-CuS) showed excellent specific
capacitance more than 535 F g-1
at 8 Ag-1
among other prepared hierarchical
morphologies. The HM-CuS electrode material also exhibit 16.4 % decrease in initial
capacitance after twenty thousand cycles at 6 Ag-1
. For the concentration of
asymmetric supercapacitor the activated carbon act as cathode and H-CuS as anode
and achieved good power and energy density of 185 W kg-1
and <15 Wh kg-1
. So,
from the electrochemical performance the H-CuS electrode might be best for energy
storage devices (Liu et al., 2018).
Ikkurthi Kanaka Durga et al. prepared coriander leaf like nanostructure copper
sulfide. The CuS nanostructure was prepared by simple cost free and low temperature
solution route. The prepared material was studied by XRD, XPS, EDS FESEM and
TEM. The coriander leaf like morphology was confirmed from the SEM images. The
electrochemical measurements were carried out in potassium hydroxide electrolyte.
The electrode material studied at 15 mVs-1
sweep rate. The 10 mg mass of CuS
nanostructure was pasted on substrate. Copper sulfide widely used in numerous
applications such as energy storage and energy conversion devices due to unique
properties. For supercapacitor the CC-3 h electrode material exhibited good specific
capacitance more than 5000 Fg-1
at 4Ag-1
with 107 % capacitance retention after two
thousand cycles. The enhanced electrochemical performance of the CC-3 h electrode
was due to effective electroactive species, greater electrocatalytic activity, excellent
conductivity and huge surface area and suitable for high performance supercapacitors
(Durga et al., 2018).
Guoxiang Wang et al. synthesized the 3D porous Copper sulfide. The CuS was
synthesized by hydrothermal method at various specific conditions. The composite of
CuS was formed with active carbon (AC). The CuS and CuS/AC composite were
studied by various characterization techniques. The size and morphology of prepared
51
material were determined by XRD and SEM respectively. The surface area of
CuS/AC composite (539 m2g
-1) was studied from BET analysis. The electrochemical
techniques of as-prepared materials were performed including cyclic voltammetry,
electrochemical impedance spectroscopy and galvanostatic charge-discharge. The
measurements were studied in potential window 0.44 to 0.3 V in 6 M KOH
electrolyte. The cyclic voltammetry was carried out at different 5 to 100 mVs-1
scan
rates. The excellent electrochemical measurements such as specific capacitance,
power and energy density and cyclic durability of CuS-AC porous electrode improved
due to the greater surface area and conductivity of active carbon. The flower like
porous CuS-AC electrode possessed maximum specific capacitance more than 245
F/g at 05 A/g, 9 % capacitance loss after five thousand cycles and good energy
density ~ 25 Wh kg-1
. From the excellent electrochemical results the prepared
electrode material be great attention towards energy storage applications (Wang et al.,
2018).
Tingkai Zhao et al. prepared CuS nanoparticles. The nanoparticles were prepared by
simple facile hydrothermal method. The GO was prepared by easy Hummer‘s
method. The CuS composites with RGO were synthesized using hydrothermal route.
The obtained material was characterized by XRD, FT-IR, Raman analysis, FESEM,
TEM and XPS techniques. The CuS showed good agreement with literature. The
clusters and nanoparticle like morphology of copper sulfide was confirmed from SEM
images. The electrochemical properties of obtained materials were determined. The
electrode material was studied at different scan rates. The CuS/RGO flexible electrode
showed excellent specific capacitance (<940 Fg-1
) at scan rate of 10mVs-1with 89 %
retention capacitance after five thousand successive cycles. Furthermore, prepared
electrode exhibited energy density 105 Whkg-1
and 2.6 kW kg-1
power density. The
CuS/RGO nanocomposites might be appropriate for energy storage devices such as
ultracapacitors (Zhao et al., 2018).
Jinxue Guo et al. synthesized the CuS double-shell hollow nanocages. The reaction
was followed by successive sulfidation of precursor material in NaS2 and etching
carried out by HCL. The physical properties of prepared material were studied by
various characterization techniques. The material was in nanocages morphology
confirmed from SEM. The CuS double shell nanostructures possessed high surface
area compared to single shell copper sulfide. The double shell CuS was studied as EM
for ultracapacitors. The prepared nanoarchitecture revealed best electrochemical
52
performance and showed good cycling stability and maximum Csp. The prepared
electrode material extensively used in energy stowage applications due to unique
hollow double-shell morphology (Guo et al., 2017) .
Sandhya Yadav et al. reported that the CuS nanocrystals yield by the reaction of
copper acetate and Sodium thiosulfate. The material was prepared through co-
precipitation route. The as-prepared material was characterized using various
characterization techniques. The prepared material was studied by XRD, SEM, DSC,
FT-IR and Raman analysis. The pure covellita phase of CuS nanocrystals was formed
at low pH value confirmed by XRD. The optical band gap of as-synthesized samples
at various pH values were from 3.3 eV to 3.7 eV confirmed by FT-IR and Raman
spectroscopy (Yadav and Bajpai, 2017).
Haihua Hu et al. prepared copper sulfides. The CuS with various compositions and
morphologies were prepared by facile hydrothermal process. The various
compositions of copper sulfides were prepared by varying the temperature of reaction.
The prepared material was investigated by various techniques such as XRD, FE-SEM,
TEM HAADF-STEM, SAED and EDS. Moreover, the prepared sample material
showed tremendous electrochemical measurements for supercapacitors. The
electrochemical properties were obtained using three probe electrode system in 3M
KOH electrolyte. Among all compositions of CuS the CuS-140 displayed the premier
specific capacitance <1000 F/g at current density of 1 A/g. The three compositions
CuS-140, CuS-160 and CuS-180 of copper sulfides revealed the outstanding cycle
stabilities and retention capacitance were > 95 %, <90 % and 100 % respectively after
thousand cycles. The as-prepared electrodes material with excellent electrochemical
performances is suitable for energy storage applications (SCs) (Hu et al., 2017).
Chandu V. V. Muralee Gopi et al. meritoriously prepared CoS, PbS, CuS, and NiS
metal sulfides. The proposed materials were prepared using simple and cost free
solution method. The composites were synthesized with carbon nanotubes. The as-
synthesized material was subjected to supercapacitors and quantum dot-sensitized
solar cells (QDSSCs) study. All obtained composites were further investigated by
XRD, XPS and SEM for morphology and structural analysis. Among all CNT/CuS,
CNT/CoS, CNT/NiS and CNT/PbS electrodes, the CNT/NiS possessed high energy
density and cycling stability. As compared to metal sulfides the composites of metal
sulfides with CNTs exhibited excellent electrochemical performance due to high
conductivity and greater surface area of CNTs and porous structure of metal sulfides.
53
Inclusively the metal sulfides/CNTs flexible electrodes might be have great vision for
advanced electrochemical energy storage devices (Muralee Gopi et al., 2017).
Zhen Tian et al. conveyed the synthesis of metal sulfide (CuS). The copper sulfide
was prepared by facile solution method. It‘s composite with 3D graphene was
prepared. The prepared materials were investigated through various techniques such
as XRD, XPS, SEM, Raman spectroscopy and BET analysis. The synthesized
material was subjected to electrochemical studies. The electrochemical measurements
were analyzed in 3 M NaOH electrolyte. The analysis was studied at 100 mVs-1
scan
rate and potential ranges from 0.1 to 1.6 V. The as-prepared materials showed
good Csp more than 240 F g-1
at 4 Ag-1
. The CuS/3DG electrode possessed minor
reduction (5%) in capacitance after successive five thousand cycles was observed
with 5 Whkg-1
at approximately 450 Wkg-1
power density. The CuS with unique
structure provide larger surface area and shorter dispersal distance. Moreover, the
highly conductive 3D graphene lessen the resistance and improved the cycle stability
so, from the performance the prepared electrode material be suitable for
ultracapacitors (Tian et al., 2017).
Yu Jun Yang et al. fabricated copper sulfide nanoparticles. The copper sulfide was
prepared by simple hydrothermal method. The composites of CuS with multiwall
carbon nanotubes (MWCNT) were prepared. The average size of obtained material
was 200 nm determined by XRD study. The thin wire like morphology of prepared
material was observed by SEM. The electrochemical measurements of CuS/MWCNT
electrode was carried out in 1M Na2SO4 electrolyte and 6M NaOH for CuS
nanoparticles suitable because of conversion of CuS to Cu(OH)2. The electrochemical
performance of CuS nanoparticles and CuS/MWCNT electrode showed excellent Csp
and cyclic stability in 1M Na2SO4 electrolyte (Yang, 2017).
Peijian Yao et al. prepared CuS, graphene and PANI. The PANI polymer was
fabricated by polymerization method. The graphene-copper sulfide (G-CuS) on
Terephthalate (PET) as transparent conductive electrode (TCE) was successfully
prepared. The new flexible TCE electrode was prepared by CVD, hydrothermal and
photolithography route. The TEC have ~20 Ω sq-1
sheet resistance with 85 %
transmittance. The PANI/G-CuS working electrode showed remarkable area
capacitance <0.017 Fcm-2
at current density of 0.025 Acm-2
in H2SO4–PVA hydrogel
electrolyte. The as-prepared G-CuS unique flexible transparent conductive electrode
(TCE) with admirable mechanical and chemical stability, transparency and
54
conductivity might be much practical attention towards energy storage devices (Yao
et al., 2017).
Seenu Ravi et al. reported the synthesis of polyimidazole coated copper sulfide by
facile hydrothermal method and its composites with CNTs on nickel foam substrate
using simple dip coating method. The CuS nanoparticles were distributed on CNTs in
the manifestation of distilled water and dimethylformamide solvents. The
physicochemical properties of prepared materials were investigated by various
techniques. The all obtained materials were studied by XRD, SEM, EDX, XPS, BET
and Raman analysis. The electrochemical measurement of prepared material was
studied. The measurements were carried out in 2 M KOH electrolyte. The properties
were studied on three probe setup. The Ag/AgCl and Pt wire were used as reference
and counter electrodes respectively. The PIM/CuS@CNT material showed excellent
capacitance than CuS@CNT electrode and exhibit < 90 % retention in specific
capacitance after successive thousand cycles at 1.2 Ag-1
. From the excellent
performance of PIM/CuS@CNT electrode it might be useful for supercapacitors
(Ravi et al., 2016b).
Wence Xu et al. prepared CuS nanosheets and their different forms by dealloying a
Ti-Cu amorphous alloy. The sulfuric acid was employed in the dealloying process for
determining of properties of different forms of copper sulfide. The spherical cluster
forms of CuS nanosheets were prepared by above said method. The CuS nanoparticles
and nanosheets were formed using minimum concentration of sulfuric acid. The CuS
spherical clusters exhibited maximum specific capacitance more than 275 Fg-1
at 5
mVs-1
sweep rate. The as-prepared electrode material showed excellent
pseudocapacitive behavior due to larger surface area and without changing the
volume of nanosheets. After thousand cycles the electrode material possessed
retention capacitance more than 70 % at specific current density (2 Ag-1
). The
synthesized electrode material might be used as electrochemical energy storage
applications (Xu et al., 2016).
Wenbin Fu et al. reported the synthesis of copper sulfides nanostructures by
hydrothermal method. The as-prepared nanostructures arranged as intersected
nanoparticles, confirmed by XRD and scanning electron microscope. The prepared
sample was characterized by some characterization techniques. The physical
properties of CuS nanoparticles were determined by XRD, XPS, SEM and TEM.
Furthermore, the CuS nanoparticles were studied by electrochemical techniques. The
55
electrochemical techniques including cyclic voltammetry, electrochemical impedance
spectroscopy and galvanostatic charge-discharge was studied. The working electrode
was prepared on Ni foam substrate. All measurements were studied in 6M KOH
electrolyte with potential ranges from 0 to 0.5 V at various (5 to 80 mVs-1
) sweep
rates. The CuS nanoparticles possessed great specific capacitance approximately 50
mA-1
at 1Ag-1
current density and offered high cyclic stability and rate capability. The
electrochemical measurements of CuS nanoparticles enhanced due to porous
interconnected network structure. For the construction of asymmetric supercapacitors,
the activated carbon used as cathode and CuS nanoparticles as anode and showed
good energy density >18 Wh kg-1
at power density 500 Wkg-1
. From the outcomes of
electrochemical performance of the CuS nanoparticles, it might be best for
supercapacitors (Fu et al., 2016).
Sana Riyaz et al. prepared the copper sulfide nanoparticles. The simple sol-gel
method was employed for the synthesis of CuS nanoparticles. The synthesized
nanoparticles were analyzed by XRD, SEM, EDS, FTIR and UV-visible
spectroscopy. The crystallite size was 17.7 nm find out by XRD analysis. There was
no impurity seen in the EDS spectrum of CuS nanoparticles. The obtained CuS
possessed spherical morphology analyzed by SEM. The band gap of CuS was
obtained by performing the UV-visible spectroscopy. The band gap was 2.89 eV
calculated from the Tauc plot (Riyaz et al., 2016).
Ke-Jing Huang et al. reported the preparation of copper sulfide (CuS) which act as
electrode material in supercapacitor. To augment the electrochemical performance of
supercapacitors, reduced graphene oxide (r-GO)-wrapped CuS hollow spheres were
studied by solvothermal method. The synthesized materials were characterized by
Transmission electron microscope, X-ray photoelectron spectrum, Raman spectra,
scanning electron microscope and powder X-ray diffraction techniques. The CuS/
rGO electrode showed tremendous cyclic stability of < 96 % retention after 1200
cycles at a current density of 2.0 A g-1
and maximum specific capacitance more than
2300 Fg-1
due to high conductivity of rGO and hollow spherical structure (Huang et
al., 2015b).
Yang Lu et al. reported the synthesis of porous 3 dimension microsphere copper
sulfide. The copper sulfide was prepared by facile PVP assisted reflux method. The
composite of CuS (flower shaped microsphere) with carbon nanotubes was prepared.
The prepared materials were characterized by various characterization techniques
56
such as XRD, SEM, HRTEM, Raman spectroscopy TG, DSC and EDX analysis. The
CuS/CNT possessed high conductivity and greater specific surface area. Moreover,
the electrochemical measurements of prepared samples were studied. The
measurements were studied on three electrode setup. The working electrode prepared
on Ni-foam and used polyvinylidene fluoride (PVDF) polymer as binder. The
prepared working electrodes were dried at 60 ºC for 11 h. The CV technique was
studied in the 0 to 0.62 V potential ranges at several scan rates. The CuS/CNT
nanocomposite electrode showed high rate capability, cycle stability and Csp (< 1950
Fg-1
at 10 mAcm-2
current density after ten thousand cycles) because of synergistic
effect of conductive carbon nanotubes. The CNTs help to maintain stability during
CCD study keep the volume change and transfer of electrons in the sample. The
CuS/CNT might be great attention for energy storage devices such as supercapacitors
(Lu et al., 2015).
Mou Pal et al. reported the work in which they prepared copper sulfide
nanostructures using simple wet chemical route. The as-prepared material was
characterized by various techniques such as SEM, XRD, EDX, BET analysis, UV–
Vis diffuse reflectance spectroscopy, micro Raman and Fourier transform infrared
spectroscopy. The CuS nanoparticles possessed hexagonal structure confirmed by
both XRD and Raman analysis. Due to porous structure of CuS the band gap was 2.05
eV and showed good Photocatalysis of methylene blue under irradiation of visible
light (Pal et al., 2015).
Karthikeyan Krishnamoorthy et al. synthesized the copper sulfide nanoparticles.
The CuS nanoparticles were prepared using cost free simple sonochemical route. The
structure and morphology of CuS nanoparticles were studied by XRD and SEM. The
as-prepared nanoparticles possessed hexagonal geometry with 40 nm average
crystallite size confirmed by XRD. The spherical shaped morphology of CuS with 50
nm size was confirmed by SEM. The electrochemical measurements of prepared
material were studied. All electrochemical properties such as cyclic voltammetry,
electrochemical impedance spectroscopy and galvanostatic charge-discharge were
carried out in 6 M KOH electrolyte. The cyclic voltammetry was studied under -0.4
– 0.6 V at different sweep rates from 2 to 50 mVs-1
. The CuS nanoparticles exhibited
high specific capacitance more than 62 Fg-1
at sweep rate of 5 mVs-1
. The prepared
electrode material possessed pseudocapacitive behavior confirmed by Nyquist and
Bode plots of electrochemical impedance analysis. The CuS nanoparticles showed
57
excellent electrochemical performance and have great attention towards
pseudocapacitive applications (Karthikeyan Krishnamoorthy, 2015).
Yu-Kuei Hsu et al. prepared CuS nanowires. The CuS nanostructure was synthesized
by simple wet-chemical process. The sulfidation of copper was carried out in the
presence of Na2S. The synthesized material was characterized by physico-chemical
techniques such as X-ray diffraction spectroscopy, SEM, XPS and Raman
spectroscopy. The obtained electrode material exhibited excellent electrochemical
measurements. The electrochemical properties of CuS were obtained in 1M NaOH
electrolyte. The cyclic voltammetry was studied in the range of 0.5 to -0.6 V. The
CuS NW electrode showed greater specific capacitance < 300 Fg-1
at different sweep
rate and more than 70 Wh kg-1
energy density achieved at 2 mA cm-2
. The as-prepared
electrode material possessed 87% retention in capacitance after five thousand cycles
(Hsu et al., 2014).
Ke-Jing Huang et al. reported the synthesis of copper sulfides. The various forms of
copper sulfide were prepared by facile hydrothermal method. The diverse forms of
copper sulfides were formed with or absence of surfactants sodium dodecylbenzene
sulfonate (CuS-SDBS) and (cetyltrimethylammonium bromide (CuS-CTAB). The
copper sulfides nanosheets possessed high value of specific capacitance (< 833 Fg-1
)
at 1Ag-1
compared to CuS-SDBS and CuS-CTAB. The thin layer CuS nanosheets also
exhibited the good cyclic stability as compared to CuS-SDBS and CuS-CTAB.
Hence, the CuS layer with excellent electrochemical performances, promising for
energy storage applications (Huang et al., 2015a).
Ting Zhu et al. synthesized copper sulfide. The needle like nanostructure composites
of copper sulfide with CNT was prepared by simple chemical conversion method.
Firstly the CNT/SiO2 introduced on the substrate and formed 1-dimensional
CNT@SiO2@CuSilicate and then converted into CuS by reacting with NaS2 using
hydrothermal method. The as-prepared material characterized by XRD, TEM,
FESEM and BET analysis. The electrochemical measurements of all prepared
materials were studied. The cyclic voltammetry, electrochemical impedance
spectroscopy and galvanostatic charge-discharge electrochemical properties of
prepared material were studied. The 1M KOH electrolyte was used while studying all
electrochemical measurements. The cyclic voltammetry was carried out in 0 to 0.5 V
potential ranges at different scan rates from 2 to 50 mVs-1
. The CuS@CNT revealed
58
high cycle stability and specific capacitance and selected as best for supercapacitors
(Zhu et al., 2012a).
R.B. Pujari et al. synthesized cubic MnS nanostructures. The MnS thin layer by
chemical bath deposition method. The temperature of reaction was 358 K. The phase
identification of manganese sulfide was analyzed by X-ray diffraction. The elemental
composition was studied by X-ray photoemission spectroscopy. The specific surface
area 120.42 m2 g
-1 of prepared material was explored through Brunauer-Emmett-
Teller. The surface texture of manganese sulfide was confirmed by field-emission
scanning electron microscopy. The galvanostatic charge discharge technique showed
specific capacitance value less than 750 Fg-1
at 1mAcm-2
current rate. After 2000 CV
cycling at 100 mV s-1
scan rate, the MnS thin films was possess 85 % retention
capacity. The microfiber-MnS thin films were fabricated in PVA-KOH gel and less
expensive stainless steel substrate. The MnS electrodes used in supercapacitor
demonstrated red LED illumination was the practical application of MnS thin films
(Pujari et al., 2016).
Xianfu Li et al. successfully fabricated γ-MnS. The γ-MnS was synthesized through
solvothermal method. The graphite oxide was prepared by common Hummer‘s
method. The γ-MnS formed composite with reduced graphene oxide using
solvothermal method. The synthesized γ-MnS/rGO composites were characterized
morphologically and structurally by scanning electron microscopy and X-ray
diffraction. Furthermore, the electrochemical properties of prepared materials were
studied. Electrochemical analysis was done through cyclic voltammetry and
galvanostatic charge-discharge techniques. The nanocomposites showed larger value
of SC 800 Fg-1
at current density of 5 A g-1
and after 2000 cycles there was no
decrease in initial values. The galvanostatic charge-discharge curves of γ-MnS/rGO
composites electrode showed good capacitive behavior (Li et al., 2015).
Alan Meng et al. reported the synthesis of unique material having excellent
electrochemical properties. The presented work is best for energy storage and
conversion devices. The nickel sulfides nanoparticles fastened on N-GNTs (nitrogen-
doped graphene nanotubes). The N-GNTs prepared by facile chemical deposition
method. The N-GNTs@NS nanoparticles were prepared by electrochemical-
deposition method. The prepared material was studied by various characterization
techniques. These techniques included XRD, Raman analysis, SEM and HRTEM. The
prepared material was further subjected to electrochemical analysis. The
59
electrochemical measurements were studied including cyclic voltammetry,
electrochemical impedance spectroscopy and galvanostatic charge-discharge. The
electrochemical measurements were carried out in 3 M potassium hydroxide
electrolyte. The cyclic voltammetry of prepared material was carried out at potential
range from -0.2 to 0.6 V. The synthesized material possessed high specific capacity
and high cyclic stability maintenance capacitance < 95 % after twelve thousand
cycles. For construction of asymmetric ultracapacitors the N-GNTs@NSNs electrode
act as anode and activated carbon act as cathode. The asymmetric supercapacitors
showed excellent energy stability ~ 50 Whkg-1
at 800 Wkg-1
power density (Meng et
al., 2020).
A. Simon Justin et al. synthesized carbon sphere@nickel sulfide composites. The
carbon sphere@nickel sulfide composites were prepared using water-bath method
with definite mole ratios. The as-prepared material was characterized through
different characterization techniques. The material was characterized by XRD, SEM,
TEM and BET analysis. The structure of C@NiS was confirmed by XRD data. The
morphology of nanocomposites was analyzed by TEM. The mesoporous structure
with enhanced surface area confirmed by BET. The electrochemical properties of
prepared materials were studied. The cyclic voltammetry, electrochemical impedance
spectroscopy and galvanostatic charge-discharge electrochemical properties of
C@NiS were studied. The electrochemical studies revealed that C@NiS (0.5:1)
showed specific capacitance greater than 1000 F g-1
at 1 A g-1
. The nanocomposite
electrode displayed excellent charge/discharge cycles ( > 4000 cycles ) with the ∼ 83
% of retention (Simon Justin et al., 2019).
Xinyu Lei et al. prepared nickel sulfide with modified carbon nanotubes (NiS/CNT).
The NiS/CNT nanostructures were prepared by hydrothermal method. The carbon
coated NiS/CNT was also prepared by facile solution method. The as-prepared
materials were studied by various physicochemical characterization techniques. The
prepared materials were studied by XRD, XPS, SEM, TEM, N2 adsorption/desorption
isotherms and FTIR spectroscopy. All synthesized materials were further studied by
electrochemical techniques such as cyclic voltammetry, electrochemical impedance
spectroscopy and galvanostatic charge-discharge. The working electrode material was
mixed with appropriate amount of binder and possessed 0.008 g mass on substrate.
The measurements were carried out in 6M potassium hydroxide electrolyte. The
NST/CNTs@C electrode among NiS2 and NiS2/CNTs showed enhanced
60
electrochemical performance because of incorporation of carbon into the matrix of
NiS2/CNTs. Due to carbon the NST/CNTs@C electrode possessed high surface area,
very less charge transport resistance and also maintain the morphology of material
during charge-discharge study (Lei et al., 2019).
Nandhini Sonai Muthu et al. prepared mesoporous nickel sulfide nanoarchitectures.
The NiS was prepared by facile hydrothermal process. The nickel sulfide
nanoarchitectures studied by various characterization techniques. The morphological,
structural and electrochemical properties of prepared material were affected by
annealing temperature. The Ni3S4 nanostructures (annealed at 205°C) exhibited
extensive surface area and improved electrochemical properties. The nickel sulfide
nanoarchitectures were subjected to electrochemical studies. The electrochemical
measurements were includes CV, EIS and GCD properties. The electrochemical
measurements were investigated in potassium hydroxide (6M) electrolyte. The studied
were obtained in the potential range of 0 to 0.5 V. The cyclic voltammetry was
studied at 30 mVs-1
scan rate. The porous Ni3S4 nanostructures electrode showed
1182 ± 74 to 546 ± 13 Fg-1
at current density of 5-40 Ag-1
. The symmetric
electroelectrode system (N2//N2) fabricated by porous Ni3S4 nanostructures provided
high energy density of 9 Wh kg-1
at current density of 2Ag-1
with 28 % decrease in
initial capacitance value after five thousand cycles. The two symmetric systems used
to light up the red LED so; the porous Ni3S4 nanostructures (annealed at 205 °C)
electrode material gives excellent electrochemical results which are might be
applicable in energy storage devices (Sonai Muthu and Gopalan, 2019).
Liu Yanga et al. prepared the nickel-molybdenum oxide sulfide composites. The
samples were prepared by simple solution sulfidation and hydrothermal method. The
as-prepared material was studied by some important physical characterization
techniques such as XRD, SEM TEM, XPS and N2 adsorption-desorption isotherm.
Furthermore, the Ni-Mo-O-S electrode material subjected to electrochemical analysis
under extensive performance conditions. The measurements were studied on three
probe set up. The 2 M KOH electrolyte was used while studying the selected
electrochemical properties. The cyclic voltammetry was studied at the range of 0 to
0.6 V optimized potential window. The study was done at various 5 to 50 mVs-1
sweep rate. The Ni- Mo-O-S electrode showed maximum specific capacitance
approximately 2178 Fg-1
at 1Ag-1
after successive five thousand charging-discharging
61
cycles. The hybrid supercapacitor formed by Ni- Mo-O-S and activated carbon
electrodes (Ni-Mo-O-S//AC HSC) and delivered maximum < 50.60 Wh kg-1
at 850W
kg-1
power density. The electrode material revealed excellent retention in initial
capacitance (> 94 %) after ten thousand charging-discharging cycles (Yang et al.,
2019a).
S. Nandhini et al. fabricated the nickel sulfide nanostructures. The nickel sulfide
nanostructures were prepared by using three synthesis methods such as hydrothermal
(H), microwave (M) and a combination of both above said methods (MH). The NiS
(H, MH) and Ni9S8 (M) exhibited hexagonal and orthorhombic structures confirmed
by XRD data respectively. The SEM micrographs of as-prepared nickel sulfide
nanostructures confirmed nanoflake (M), spherical (H) and layered structure (MH)
morphologies. The electrochemical study of layered structure provided the largest Csp
of 964 F g-1
in 6M KOH electrolyte. A symmetric supercapacitor provided specific
capacitance less than 120 F g-1
at 1 A g-1
with energy and power density less than 20
Whkg-1
(S et al., 2018).
Jing Zhao et al. prepared hierarchical nickel sulfide. The NiS was prepared by facial
sacrificial template method under different vulcanizing time. The synthesized NiS
was characterized by different characterization techniques like BET, XRD, SEM,
TEM and XPS. By increasing the vulcanizing time the NiS structures was transform
into microflower to microspheres confirmed by results. The NiS-18 (18h reaction
time) microflowers were possessed greater and rough surface area. The
electrochemical measurements of as-obtained material were studied. The the electrode
material was pasted on Ni-foam and dried the obtained electrodes at 60˚C. The
electrochemical performance was carried out in 4M KOH electrolyte. Electrochemical
measurements showed highest specific capacitance with excellent cycling stability
(89.2 %). The asymmetric supercapacitor, achieved high potential window 1.6 V
when positive electrode was NiS-18 and negative electrode NiS//AC used. The
NiS//AC supercapacitor showed excellent Coulombic efficiency (100 %) and
tremendous cycling performance (87.3 %) (Zhao et al., 2017).
Bing Guan et al. synthesized hierarchical NiS microflowers. The NiS microflowers
were obtained by sulfuration process. The sample was thoroughly investigated by
various characterization techniques such as BET, XRD, XRS, SEM and TEM. The
surface of NiS microflowers was very rougher confirmed by SEM results. The NiS
microflowers were also subjected to electrochemical measurements such as cyclic
62
voltammetry, galvanostatic charge-discharge and electrochemical impedance
spectroscopy. The cyclic voltammogram of NiS microflowers was obtained from the
range of -0.12 to 0.65 V potential window at various number of sweep rates from 5 to
100 mVs-1
. The NiS microflowers possessed good cycling stability (97.8 %) after
1000 charge-discharge cycles at current density of 10Ag-1
and specific capacitance
1122.7 F g-1
in 3M KOH electrolyte. Additionally, an asymmetric supercapacitor, NiS
as the positive electrode and AC as the negative electrode, delivered high energy
density (31 Whk g-1
at power density of 0.9 kWk g-1
) with operating window 1.8 V
(Guan et al., 2017).
Xiaoxian Zang et al. prepared the series of nickel sulfide nanowires. The nickel
sulfide nanowires were prepared under varied reaction conditions by simple chemical
solution method. The synthesized Ni3S2−Ni, Ni3S2−NiS−Ni, and Ni3S2−NiS
nanowires were characterized by some characterization techniques such as XRD,
XPS, SEM, TEM and selected-area electron diffraction (SAED). The electrochemical
measurements of the prepared samples were studied. The measurements were studied
out in 3 mol L-1
potassium hydroxide electrolyte. Among all samples the Ni3S2−NiS
nanowires represented high electrochemical efficiency. The Ni3S2−NiS nanowires
electrode exhibited maximum specific capacitance < 1077 F g-1
at realistic specific
currents of 5 A g−1
. The Ni3S2−NiS nanowires possessed 76.3 % retention capacitance
after ten thousand cycles at 20 Ag-1
but remaining samples showed outstanding 100 %
of cyclic stability after 10,000 cycles (Zang et al., 2016).
Hailong Chen et al. fabricated the nickel sulfide. The nickel sulfide was fabricated by
facile hydrothermal method. The composites of nickel sulfide with graphene and
CNTs were prepared. The nickel sulfide nanosheets successfully fastened on the
conductive surface of graphene and carbon nanotubes. The prepared NiS/GNS/CNT
samples were further studied by electrochemical techniques. The prepared material
was subjected to supercapacitor applications. The measurements were carried out in -
0.1 to 0.35 V potential windows at different sweep rates. The NiS/GNS/CNT
electrode possessed maximum value of specific capacitance 2376.99 Fg-1
at 2mVs-1
sweep rate and high cycle stability ~ 1600 Fg-1
compared to bare CuS nanosheets. The
as-synthesized NiS/GNS/CNT electrode material with excellent electrochemical
performances might be used in supercapacitors for the sea flashing signal system
(Chen et al., 2014).
63
Shengjie Peng et al. reported the preparation of cobalt sulfide and nickel sulfide. The
samples were prepared by simple solution based method. The cobalt sulfide
microspheres assembled by nanosheets showed 3D design with solid, double shell,
yolk shell and hollow cores with varied pore size and surface area confirmed by SEM
and N2 -sorption isotherms. All samples were further studied by electrochemical
measurements. The properties were studied in 3M KOH electrolyte. The CoS2 hollow
spheres showed excellent specific capacitance, high cycling stability and first-rate
charge-discharge stability. The as-prepared electrode material with excellent
electrochemical performances may favorable for heightened supercapacitor
applications (Peng et al., 2014).
Xin-Yao Yu et al. reported the preparation of multi-shells, categorized, non-spherical
structures of metal sulfides. The box-in-box hollow nickel sulfide with double-shells
was synthesized. The proposed sample was prepared by hydrothermal method. The
physical properties of as-prepared material were studied by various characterization
techniques. The structure and morphology of as-prepared material were investigated
by XRD and FESEM respectively. The NiS material was used as electrode material
for electrochemical measurements. The NiS electrode exhibits high specific
capacitance, good cycle stability and rate performance. The specific capacitance was
obtained more than 665 Fg-1
at 1Ag-1
and < 93% retention in initial capacitance after
three thousand cycles at current density of 4Ag-1
. From the astonished results the
prepared electrode material observed best for energy storage devices and other
potential applications (Yu et al., 2014).
Lianbo Ma et al. synthesized carbon coated nickel sulfide (CCNS) nanoparticles. The
GO was prepared by Hummer‘s method. The composites of CCNS nanoparticles with
reduced graphene were prepared using simple hydrothermal method. The average size
of carbon coated Ni3S2 was ~ 26 nm and regularly distributed on the sheets of reduced
graphene oxide investigated by XRD and SEM respectively. The CCNS-RGO
composites showed high electrochemical measurements as compared to RGO and
CCNS electrodes. The CCNS-RGO electrode indicated superb specific capacitance
< 860 Fg-1
at sweep rate of 6 mVs-1
and possessed admirable cycling stability only 1.4
% decrease was observed initial Csp after five hundred cycles. The low-cost and
viable synthesis and excellent electrochemical performance of CCNS-RGO
composites material contributed much attention towards energy storage devices
applications (Ma et al., 2014).
64
Xuejun Liu et al. fabricated the nickel sulfide. The composites of NiS with
electrochemically reduced graphene oxide (ERGO) were synthesized using
electrochemical co-deposition method. During the process the GO sheets dissolved in
solution mixture and precipitate and then deposit on electrode. The as-prepared
material was studied by XRD and SEM. Furthermore, the prepared material studied
by electrochemical measurement. The electrodes were prepared on Ni-foam and dried
at 60 ˚C. All electrochemical techniques were studied in 2M potassium hydroxide
electrolyte. The Csp of NiS/ERGO composite was more than 1392 Fg-1
at 2Ag-1
and
also observed 96 % retention in capacitance after 1500 charge-discharge cycles. The
nickel sulfide/ electrochemically r-GO nanocomposite with exceptional
electrochemical performance selected best for supercapacitors (Liu et al., 2014).
Zhicai Xing et al. prepared successfully nickel sulfide. The NiS nanospheres were
prepared by hydrothermal route. The L-cysteine was used as a source of sulfur. The
composites of NiS nanospheres with reduced graphene oxide nanocomposites were
prepared by hydrothermal method. All prepared materials were studied by various
characterization techniques. The prepared materials were further studied by
electrochemical techniques. The cyclic voltammetry, galvanostatic charge-discharge
and electrochemical impedance spectroscopy properties of as- obtained materials
were studied. The cyclic voltammetry was studied in 2 mol/L KOH electrolyte at
various scan rates. The nanosphere ultrafine particles have fine pores and these
NiS/rGO nanocomposites used in supercapacitors as electrode material and showed
maximum specific capacitance with good cycling stability (Xing et al., 2013).
Shu-Wei et al. reported the formation of electrodes of flaky nickel sulfide
nanostructures deposit on nickel foam by proposed potentiodynamic deposition
method. The as-prepared nickel sulfide nanoflakes characterized by XRD, SEM and
TEM techniques. The unique material was further studied by electrochemical
measurements. The electrochemical techniques were studied in 1 M potassium
hydroxide electrolyte. The prepared electrode exhibited maximum specific
capacitance more than 215 Fg-1
at current density of 2Ag-1
and good retention in
specific capacitance approximately 90 % after thousand successive cycles at 4Ag-1
in
1M KOH electrolyte. The flaky Ni3S2 nanostructure electrode material could be
selected best for auspicious energy storage applications (Chou and Lin, 2013).
Aming Wang et al. fabricated the nickel sulfide. Firstly the graphite oxide was
prepared by Hummer‘s method. The NiS/GO nanocomposite was prepared by L-
65
cysteine assisted facile hydrothermal method. The prepared nanocomposite was
investigated by X-ray diffraction, scanning microscopy and transmission electron
microscopy. The prepared nanoparticles were scattered on graphite oxide sheet and
possessed hexagonal phase with crystallite size of 50 nm confirmed by SEM and
XRD respectively. Furthermore, the prepared materials were studied by
electrochemical techniques. All measurements were studied in aqueous solution of 6
M KOH as electrolyte. The Nis/GO electrode showed astonished electrochemical
measurements because of conductive graphite oxide network. The NiS/GO
electroactive electrode exhibited high Csp 800 Fg-1
at current density of 1 Ag-1
(Wang et al., 2013).
By Ting Zhu et al. prepared nickel sulfide nanospheres. The nickel sulfide
nanospheres were synthesized using template-engaged conversion route. Firstly, the
SiO2@nickel silicate core-shells were prepared and then completely converted into
nickel sulfide by reacted with Na2S by facile hydrothermal method. The obtained
sample material was studied by field emission scanning microscopy, transmission
electron microscopy and X-ray diffraction techniques. The as-synthesized hollow
nanospheres NiS nanosheets electrode showed astonished electrochemical
measurements. The electrochemical properties were studied in 2 mol/L aqueous
solution of potassium hydroxide electrolyte. The measurements were explored in the -
0.15 to 0.55 V potential ranges at various scan rates. The NiS nanosheets electrode
showed excellent electrochemical performance and suggested best for energy storage
devices such as supercapacitors (Zhu et al., 2011).
66
CHAPTER 3
EXPERIMENTAL WORK
67
CHAPTER 3
EXPERIMENTAL WORK
3.1 Chemicals
All precursors with their specification for the synthesis of flake-like MoS2
nanoarchitectures, flake-like CuS nanoarchitectures, NiS nanoarchitectures, graphite
oxide (GO) and reduced graphite oxides (r-GO) were listed below in Table. 3.1.
Table 3.1: List of chemicals/precursors used.
Sr.No Chemical Name Molecular
Formula
Percentage
purity (%)
Supplier
1 Molybdic acid MoO3·H2O ~ 98 Sigma-Aldrich
2 Urea CH4N2O ~99 Sigma-Aldrich
3 Thioacetamide C2H5NS ~ 98 Sigma-Aldrich
4 Copper sulfate CuSO4∙5H2O ~ 98 Sigma-Aldrich
5 Thiourea CH4N2S ~ 99 Sigma-Aldrich
6 Hydrated nickel
chloride
NiCl2.6H2O 99.95 Sigma-Aldrich
7 L-cysteine C3H7NO2S ~ 98 Sigma-Aldrich
8 Graphite powder C ~ 99 Merck
9 Potassium
permanganate
KMnO4 ~ 99 Riedel-deHaen
10 Sodium nitrate NaNO3 ~ 99 Merck
11 Hydrogen peroxide H2O2 33 Merck
12 Ammonia solution NH5O 25 Sigma-Aldrich
13 Hydrazine hydrate N2H4.H2O ~ 80 Sigma-Aldrich
14 Sodium Sulfate Na2SO4 ~ 99 Sigma-Aldrich
15 Potassium
hydroxide
KOH ~ 99.89 Sigma-Aldrich
16 Nafion C7HF13O5S·C2F4 5% Sigma-Aldrich
3.2 Instrumentation
The all proposed metal sulfides and their nanocomposites were characterized by
following characterization techniques.
(i) X-ray diffraction (XRD)
XRD analysis of all prepared samples was done using (i) Philips X‘ Pert PRO
3040/60 Diffractometer with Cu-Kα radiation source (λ =0.15402 nm) (ii) Bruker D8
high resolution LynxEyeXE detector with Cu radiation source (materials chemistry
laboratory, IUB).
68
(ii) Field emission scanning electron microscopy (FE-SEM)
FE-SEM images of prepared metal sulfides and their nanocomposites was recorded
using ZEISS LEO SUPRA 55 electron microscope.
(iii) Elemental analysis
The elemental analysis (EDX) of prepared metal sulfides and their nanocomposites
was logged using JEOL JCM-6000Plus electron microscope.
(iv) UV- Visible absorption spectroscopy
All UV-Visible spectra were obtained using Agilent Carry 60 UV/Visible
spectrophotometer.
(v) Fourier transform infrared spectroscopy (FT-IR)
The FT-IR of all prepared samples was characterized through Alpha Bruker ATR with
OPUS/Mentor software (400-4000cm-1
).
(vi) Brunauer–Emmett–Teller (BET) analysis
Brunauer–Emmett–Teller (BET) method was used to obtained the BET surface area
of prepared samples through micromeritics ASAP 2020 from N2 adsorption isotherms
at 77 K.
(vii) Current-voltage measurements
The current-voltage measurements were completed through 6487 Pico ammeter using
KEITHLEY voltage source.
(viii) Electrochemical measurements
All the electrochemical measurements were completed on the Gamry interface 5000
(06531) through three-electrode setup.
3.3 Synthesis methods
Many specific methods are used to synthesize the nanoscale materials, such as
microemulsion method, reverse microemulsion method, sol-gel method, hydrothermal
method, chemical vapor deposition method, co-precipitation method etc. These
methods are summarized as below:
3.3.1 Hydrothermal method
In hydrothermal method, the assorted chemical reaction takes place in the presence
of aqueous or non-aqueous solvent at high pressure and temperature conditions. This
method is known as solvothermal method when non-aqueous solvents are used. Low
quantity of precursors is required for this method. Varieties of nanostructures
69
(nanorods, nanowires, nanoparticles, nanoribbons, etc.) and desired particle size can
be obtained by hydrothermal method. In this method the reaction proceed in
apparatus known as autoclaves (stain-less steel pressure vessel lined with teflon
material). Hydrothermal autoclave should possess special characteristics such as
inert to high temperature and pressure, leak proof and inactive to bases, acids and
oxidizing agents. This method possesses lot of advantages such as homogeneity in
structure, low energy requirement, single step process, ultra-low solubility of
crystals, less time, and better quality of product (Quan et al., 2010). All desired
metal sulfides and their composites will be synthesized by facile hydrothermal and
simple sonication methods respectively.
3.3.2 Microemulsion method
In the synthesis of nanomaterials the word ―microemulsions‖ used for clear, isotropic,
macroscopically homogeneous and thermodynamically stable solutions.
Microemulsion method involves three main components- a surfactant and two
immiscible liquids water and oil. An interfacial layer formed by surfactant molecules
separated these two immiscible liquids. As a result of separation, two microscopic
structures are formed (Shah, 2015). One microscopic structure contains oil droplets
dispersed in the water phase (o/w microemulsion or normal microemulsions) and
other microscopic structure contains water droplets dispersed in non-polar oil phase
(w/o microemulsion or reverse microemulsion method) (Anwar et al., 2014).
3.3.3 Reverse microemulsion method
For the synthesis of nanoarchitectures with desired properties like size, morphology
and crystallinity, reverse microemulsion technique is used. In reverse microemulsion
method, the single phase with thermodynamically stable system contains water, oil
and surfactant (Sajjad, 2011). The interfacial tension between water and oil is
decreased by surfactant molecules and a transparent uniform solution is formed. Polar
ends of surfactant molecules surround the water and form a spherical shaped
nanodroplet of water which is dispersed in bulk oil phase. This water droplet acts as
reaction medium for the synthesis of nanoparticles(Abazari and Sanati, 2013).
3.3.4 Sol-gel method
The sol-gel method is a low cost, solution phase, low temperature, two step method
used for the synthesis of nanomaterials. The colloidal suspension of solid particles in
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liquid is called ―Sol‖ (Bhat et al., 2013). A solid material containing liquids in its
pores is called ―Gel‖. In sol-gel method, a system from liquid phase which is act as
sol is converted into solid phase ―gel‖. Hydrolysis and condensation reactions take
place in this method. Precursor molecules (metal alkoxides or metal chlorides) are
hydrolyzed by reacting with solvent (water or other organic solvents) to form M-OH
species. A three-dimensional M-O-M network is obtained when the condensation
reaction propagates and the –OH groups attached with metal react with other metal
chlorides or alkoxides to form R-OH, H-Cl or H2O species which are then eliminated
from the reaction mixtures (Shirinparvar et al., 2016).
3.3.5 Co-precipitation method
The Co-precipitation method is used for the synthesis of nanomaterials due to easy
synthesis scheme, less expensive and requires less time and involves the precipitation
of more than one metal ions from the same reaction mixture. In the co-precipitation
method, a mixed solution containing metal precursors is added slowly into the
reaction chamber. The pH of the reaction mixture is maintained by slow addition of
an alkaline solution which leads to the co-precipitation of two metallic salts. The
precipitates obtain then subjected to filtration, washing and drying. This method is
affected by different parameters such as the pH, temperature, time period and ratio of
concentration of metal precursors (Mukhopadhyay et al., 2015).
3.3.6 Chemical vapor deposition (CVD)
Chemical vapor deposition method is usually used for the preparation of thin films. In
the chemical vapor deposition method, vapor-phase precursors (precursor may be
solid, liquid or gas) are used to deposit nanomaterials on a substrate surface (Ashby,
2009). The precursors are broken down into reactive radicals through heating at high
temperatures. These reactive radicals then disperse and adsorb onto the substrate.
Solid thin films are then deposited through surface chemical reactions (Alagarasi,
2011).
3.4 Preparation of graphite oxide (GO) and reduced graphite oxide (r-GO)
Graphite Oxide (GO) was synthesized using the Hummer's method (Krishnamoorthy
et al., 2013). By using natural graphite powder and sodium nitrate raw material, the
graphite oxide (GO) was prepared. The reduction of graphite oxide yield reduced
graphene oxide (r-GO).
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3.4.1 Preparation of graphite oxide (GO)
Graphite oxide was prepared by the facile Hummer‘s method shown in Figure 3.1.
Graphite powder (3.0g) and sodium nitrate (3.0g) were mixed. The black colored
graphite dispersion was obtained by adding 150 cm3
of H2SO4.The black color
obtained mixture was stirred for an hour at room temperature. The mixture was set on
ice-bath. Then KMnO4 (18g) was added gradually for an hour. The mixture was turns
to green color after the addition of KMnO4. The resultant green color mixture was
stirred for 60 minutes. Uninterruptedly the mixture was stirred for 46 hours at room
temperature. The purple brown slurry was succeeded after 46 hours constantly
stirring. After that 280 cm3
of deionized water was added to the slurry and a brown
suspension was obtained. Then brown suspension was stirred for two hours stirring
was then stopped. After this 850 cm3of marginally warm water was added to the
brown suspension. Then 60 cm3 of H2O2 was added to stop the reaction and a yellow
suspension was obtained. The aqueous solution of 6 wt% (in 500 deionized water) of
concentrated H2SO4 and 1 wt% (in 500 deionized water) of H2O2 was prepared and
combined them for the purpose of washing of obtained yellow suspension to increase
the pH value (7.0). The yellow suspension was washed 4-times from the above
prepared solution mixture. Then a dark brown suspension was attained. The
suspension was washed several times with de-ionized (DI) water till the pH reached
equal to neutral level (i.e. 7). Subsequently the, dark brown colored graphite oxide
(GO) was achieved (Song, 2014).
Figure 3.1: Schematic representation of synthesis of graphite oxide.
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3.4.2 Reduction of graphite oxide (r-GO)
After the preparation of GO, the reduced graphite oxide was prepared as shown in
Figure 3.2. The reduction of graphite oxide (GO) with hydrazine formed the reduced
graphite oxide (r-GO) (She, 2013). The as-prepared 5 cm3of GO was added in a clean
beaker and mixed with 10 cm3 of deionized water. The homogeneous suspension was
formed after the centrifugation and sonication of solution mixture. Additional
sonication of the suspension was sustained for 120 minutes. After sonication then
36μL ammonia solution and 6μL of hydrazine were added in mixture. Then stirring of
the mixture was carried out by placing the beaker carefully in a paraffin oil-bath for
60 minutes at 90 ºC. The dark brown color of GO was transformed into a dark black
color solution confirming the fabrication of reduced graphite oxide (r-GO) (Tiwari et
al.).
Figure 3.2: Schematic representation of synthesis of reduces graphene oxide.
3.5 Synthesis of MoS2 nano-flakes
Flake-like molybdenum disulfide nanoarchitectures were prepared as shown in Figure
3.3. The molybdenum disulfide nanoarchitectures were prepared by a urea assisted
facile hydrothermal method. The molybdic acid, urea and thioacetamide were used as
precursor‘s materials. The molybdic acid (24 mg), urea (200 mg) and thioacetamide
(28 mg) were dissolved in 80 mL of deionized water via a magnetic stirrer. The
obtained solution was stirred for 1 hour at room temperature. The resulting
73
homogeneous solution was transferred to 100 mL autoclave and placed it into the
oven at 210 °C for 18 hours. After 18 hours heating, the autoclave was cooled down
to room temperature. The obtained black MoS2 solid was washed with deionized
water and ethanol. The washed sample was dried at 90 °C for 8 hours.
Figure 3.3: Schematic representation of synthesis of molybdenum disulfide.
3.5.1 Synthesis of (MoS2/r-GO) nanocomposites
The nanocomposites of molybdenum disulfide with reduced graphite oxide (MoS2/r-
GO) were prepared via an ultrasonic exfoliation method as shown in Figure 3.4. For a
typical batch synthesis, 90 mg of MoS2 nano-architecture was dispersed in 330 mL
suspension (30 mg / L) of r-GO. The resultant mixture was sonicated for 2 hours to
obtain a homogeneous mixture. Finally, the sonicated solution was dried at 90˚C for
48 hours to get the dried nanocomposites.
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Figure 3.4: Schematic representation of synthesis of MoS2/r-GO nanocomposites.
3.6 Synthesis of copper sulfide
Copper sulfide with unique morphology was prepared as illustrated in Figure 3.5. The
copper sulfide nanostructures were prepared by a facile hydrothermal method. The
copper sulfate and thiourea (as sulfur source) were used as precursor materials. These
reagents were analytically pure in synthesis and were used without any distillation.
The weighed the calculated amount of precursor materials. The CuSO4 (29.0 mg) was
dissolved in appropriate amount (30 mL) of distilled water. The 18 mg of thiourea
was dissolved separately in 40 mL of distilled water and dissolved homogeneously
under a magnetic stirrer. The thiourea solution was slowly poured into the copper
sulfate solution under constant stirring. The resultant mixture was stirred for an hour.
Then, the solution mixture was poured into a 100 mL Teflon cup of autoclave and
placed in an oven. The reaction conditions were applied, 200°C temperature for 13 h.
After completion of reaction the autoclave was removed from the oven and cooled
down at room temperature. The obtained sample was in black color. The sample was
washed with deionized water and ethanol. The washed sample was dried at 50°C for 9
hours.
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Figure 3.5: Schematic representation of synthesis of copper sulfide.
3.6.1 Synthesis of CuS/CNTs nanocomposites
The nanocomposite of copper sulfide with carbon nanotubes (CuS/CNT) was
prepared via the ultrasonic exfoliation method as shown in Figure 3.6. The 10 mg of
CNT nanowires were dissolved in distilled water. For a typical batch synthesis, 90 mg
of CuS nano-architecture was dispersed in 20 mL solution of CNT nanowires. The
resultant mixture was sonicated for an hour to obtain a homogeneous mixture. Finally,
the sonicated solution was dried at 90˚C for 48 hours to achieve the dried
nanocomposites of CuS/CNTs.
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Figure 3.6: Schematic representation of synthesis of CuS/CNT nanocomposites.
3.7 Nickel foam (NF) treatment
The sheets of the nickel foam were cut into small pieces (14 mm × 22 mm). The
pieces of the NF were cleaned with 3 M HCl, ethanol, and ultrapure water in an
ultrasonic bath for 10 minutes each. The purpose of the washing was to remove the
oxide coating from the surface of the nickel foam.
3.7.1 Fabrication of NiS/CNTs@NF and NiS/NF electrodes
The NiS and NiS/CNTs were synthesized and furnished on the nickel foam via a
single-step hydrothermal route. The hydrated nickel chloride (NiCl2.6H2O) and L-
cysteine (as sulfur source) were used as precursor materials. In detail, NiCl2.6H2O
(0.0713 g) was dissolved (via magnetic stirrer) in extra pure water (20 ml) to get a
light green solution. The L-cysteine (0.08 g) was dissolved separately in extra pure
water (20 ml) to get the milky solution. The L-cysteine solution was slowly poured
into the NiCl2.6H2O solution under constant stirring. Approximately 20 ml of CNTs
aqueous suspension (40 mg/L) was sonicated for 45 minutes in a separate beaker and
then mixed with the above precursor solution by magnetic stirring.
The mixed solutions along with two pieces of cleaning nickel foam (14 mm × 22 mm)
were transferred into the Teflon cup (100 ml capacity). Then, the autoclave was
77
heated in the laboratory oven at 160°C for 8 h to complete the hydrothermal reaction.
After an 8 h hydrothermal reaction, the NiS/CNTs@NF electrodes were collected and
washed several times initially with the extra pure water and finally with the ethanol.
Finally, the sample decorated NFs were annealed at the 300°C for 1 h in the
laboratory furnace. For comparison, the pristine NiS@NF electrodes were also
prepared via a similar process in the absence of CNTs. The fabrication scheme for
the NiS/CNTs@NF electrode is shown in Fig.1.
Figure 3.7: Schematic scheme for the fabrication of NiS/NF and NiS/CNTs@NF
electrodes.
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CHAPTER 4
RESULTS AND DISCUSSION
79
CHAPTER 4
RESULTS AND DISCUSSION
4.1 X-ray diffraction (XRD)
X-ray diffraction (XRD) is a valuable technique to determine molecular and atomic
structure of the material, crystal compositions, lattice parameters and spacing of the
crystal planes. Professor Wilhelm Conrad Roentgen, a German professor in 1895
observed X-rays very first time and received noble prize in physics in 1901 for his
splendid discovery. X-rays are the radiation which are electromagnetic in nature with
short wavelengths (10 to 10-3
nm) and high energies between -rays and UV
radiation in electromagnetic spectrum (200eV to 1 MeV). When X-ray beam falls on
atom, either it is scattered by the electrons due to e-field variation or it may absorbed
by the atom results in the emission of electrons. During diffraction, incident x-ray
beam first penetrates and then scattered by the crystal atomic planes. Diffracted beam
leads to the constructive or destructive interference. On constructive interference
diffraction pattern appeared on photographic plate whereas intensity is null in case of
destructive interference. Lattice points in diffraction pattern appeared in the form of
spots. From these diffraction spots information about the crystal planes can be
obtained. Two types of diffractometer (single crystal diffractometer and powder
diffractometer) can be used to get the diffraction pattern of the crystals. One
dimension phase study is discussed by powder XRD while information in three
dimensions can be obtained by single crystal XRD (Dorset, 1998).
4.1.1 Bragg’s Law
In 1913, Sir W.H. Bragg and his son Sir W.L. Bragg presented the X-rays
constructive interference phenomenon and awarded noble prize in 1915. They
explained the law by following expression;
(4.1)
d = interplanar distance
λ = x-ray specific wavelength
θ = angle of beam
n = order of reflection (1,2,3,4,…)
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Figure 4.1: Bragg's law and diffraction of X-ray (Dorset, 1998).
4.1.2 XRD parameters
Debye Scherer equation is used to calculate crystallite size of the crystals. The
expression is as follows;
(4.2)
In this equation:
D = crystallite size
k = Scherer constant = 0.9
λ = wavelength of x-ray used (CuKα = 1.5406 A°, MoKα = 0.7107 A
°)
ß = Full width half maximum (FWHM)
θ = Bragg‘s angle (radians)
4.1.3 X-ray diffraction of MoS2 and its nanocomposites
The XRD patterns of the pure MoS2 and MoS2/r-GO NCs are depicted in Figure 4.2.
The seven characteristic diffraction peaks at 2θ= 14.4°, 34.2°, 39°, 44.5°,49.7°,60.5°,
and 70° are indexed to the (002), (101), (103), (006), (105), (008) and (108) planes of
the pure MoS2. All observed diffraction peaks were matched with data card JCPDS #
037-1492.The existence of only characteristics diffraction peaks of MoS2 and absence
of any extra peak in the diffraction pattern indicated the purity of the synthesized
sample. The XRD pattern of MoS2/r-GO NCs apart from MoS2 diffraction peaks also
contained the additional broad peak at 2θ = 24.5°. This broad peak (as illustrated in
the inset of the Figure 4.2) attributed to the (002) plane of the r-GO sheets
(Gebreegziabher et al., 2019; Stobinski et al., 2014). The occurrence of all diffraction
81
peaks of the MoS2 in the XRD of nanocomposites revealed that nanocomposites
formation does not change the phase purity of the MoS2 (Hassan et al., 2010; Li and
Liu, 2009; Qiu et al., 2017; Yu et al., 2015). The crystallite size (S) of the pure MoS2
was calculated by using equation (1), which is well known Scherer equation.
(4.3)
The calculated crystallite size (S) of the pure MoS2 was 45 nm.
Figure 4.2: X-ray diffraction patterns of pure MoS2, MoS2 /r-GO nanocomposites and
XRD pattern of r-GO (Inset).
cos
9.0D
82
4.1.4 X-ray diffraction of CuS and its nanocomposites
The XRD patterns of the pure CuS and CuS/CNTs nanocomposites are presented in
Figure 4.3. The seven characteristic diffraction peaks at 2θ= 28.8°, 31.5°, 45.9°,
47.6°,52.4°,54.4° and 59.0° are well indexed to the (102), (103), (008), (110), (108),
(114) and (116) planes of the bare CuS. All observed diffraction peaks were matched
with data card JCPDS # 078-0876 (Riyaz et al., 2016). The sharp peaks of the
obtained XRD pattern of bare CuS shows the exclusive crystallinity. The absence of
any impurity and intermediate phase (CuxSx) peaks and presence of only characteristic
peaks in the pattern of bare CuS showed the purity of prepared material. The XRD
pattern of CuS/CNTs nanocomposites apart from CuS diffraction peaks also contained
the additional broad peak at 2θ = 23.8° (Huq et al., 2016). The calculated crystallite
size of the bare CuS was 47 nm.
Figure 4.3: X-ray diffraction patterns of bare CuS and CuS /CNTs nanocomposites.
83
4.1.5 X-ray diffraction of NiS and its nanocomposites
The crystallite size of the NiS and crystal phase of the NiS and its nanohybrid with
CNTs was examined from their PXRD profile. The PXRD profiles (in 2θ = 10-80°) of
the NiS and its nanohybrid with CNTs are presented in Figure 4.4.
Figure 4.4: X-ray diffraction profiles of NiS and NiS/CNTs nanohybrid.
The eight characteristic reflections observed at 2θ = 30.6°, 34.2°, 45.7°, 53.8°, 58.8°,
64.9°, 70.7°, and 74.0° index to the (110), (101), (102), (110), (103), (200), (004), and
(202) diffraction planes of the hexagonal phased NiS, respectively. All observed
diffraction peaks were matched well with data card JCPDS # 002-1280, confirming
the successful fabrication of a single phased NiS sample (Ashraf et al., 2020a). The
presence of intense characteristics diffraction peaks of NiS and non-existence of any
impurity peak in the diffraction pattern demonstrate a higher degree of crystallinity
and good purity of the as-prepared NiS solid sample. In comparison, the PXRD
pattern of NiS/CNTs nanohybrid apart from the NiS diffraction peaks also contains
the additional broad peak at 2θ = 25.2°, indicating the existence of CNTs (Ashraf et
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al., 2020b) The crystallite size (D) of the bare NiS NPs was estimated using Scherer
expression represented in Eq.4.4 (Zafar et al., 2020).
(4.4)
The calculated crystallite size of the bare NiS was 14.45 nm. The percentage porosity
of the NiS solid sample was determined via the following equation (Rehman et al.,
2020):
1001(%)
x
Porosity
(4.5)
In Eq.4.5, ρb and ρx and symbolize the bulk-density and x-ray, respectively. The
observed value of the percentage porosity for the NiS solid sample was 21.3%. The
porous nature of the sample is beneficial for its cyclic activity and also reduces the
mass transfer resistance (Ashraf et al., 2020b).
4.2 Fourier transform infrared spectroscopy (FT-IR)
Albert Abraham Michelson has invented Michelson interferometer which developed
the Fourier transform infrared spectroscopy. FTIR is carried out to get the fingerprints
of the molecules and to illustrate the structure. The first infrared spectrometer was
invented in 1950. Infrared region exhibits lower energy than visible region and higher
energy as compared to microwave region (Perkins, 1986). The range of infrared
region is 14000 cm-1
to 10 cm-1
. It is further sub divided into three regions (near IR,
mid IR and far IR).
near-infrared = 14,000 cm-1
- 4000 cm-1
mid-infrared = 4000 cm-1
- 400 cm-1
far-infrared = 400 cm-1
- 10 cm-1
IR spectrum received by the near IR absorption is fewer and broad and its
interpretation is difficult therefore Mid-IR region is favorable because mostly
compounds vibrations happened in this region. Molecular vibrations are defined
through degree of freedom. Degree of freedom of polyatomic molecules is defined by
―3n rule‖ in which ‗n‘ corresponds to the no of atoms in a molecule and ‗3‘ is the
movement of molecules in X, Y and Z directions. Fundamental vibrations of the
molecule typically depend on the total number of atoms in a molecule. Linear and
cos
9.0D
85
non-linear molecules possess 2n-5 and 3n-6 fundamental vibrations respectively.
Molecules generally own in phase (symmetric) and out of phase (asymmetric)
bending and stretching vibrations. Mostly absorption bands lay below 1500 cm-1
region called ‗fingerprint region‘ (Anderson, 2004; Bertoldo Menezes et al., 2018;
Larkin, 2011; Pavia et al., 2008).
Figure 4.5: Fourier transform infrared spectroscopy setup (Larkin, 2011).
4.2.1 FT-IR analysis of MoS2 and its nanocomposites
The FT-IR transmission spectra of pure MoS2 and MoS2/r-GO NCs were obtained to
determined their chemical composition and elemental atmosphere. Figure 4.6 shows
the FT-IR spectra of the pure MoS2 and MoS2/r-GO NCs at wavenumber range from
400-3000 cm-1
. The FT-IR spectrum of pure MoS2 contained three bands in the
fingerprint region. The 1st band (463 cm
-1) attributed to the vibration of the metal-
sulfur (Mo-S) bond at the apical position while the last two bands (at 514 cm-1
and
546 cm-1
) attribute to the terminal and bridging S-S vibration (Weber et al., 1995).
The FT-IR spectrum of MoS2/r-GO NCs showed two new bands at 1166 cm-1
and
1570 cm-1
along with characteristics bands of the MoS2. The high-intensity band at
1570 cm-1
was observed due to typical C=C vibrations of the r-GO sheet. The
relatively lower intensity band at 1166 cm-1
was due to C-OH vibrations of the
86
residual oxygen atoms at the r-GO sheet. Moreover, the complete absence of the C=O
and other C-O bands showed the complete reduction of the GO into the r-GO
(Andrijanto et al., 2016; Tuz Johra and Jung, 2015).
Figure 4.6: Fourier transform infrared spectroscopy analysis of pure MoS2 and MoS2
/r-GO nanocomposite.
4.2.2 FT-IR analysis of CuS and its nanocomposites
The obtained FTIR spectra of bare CuS and its composite (CuS/CNTs) were studied
to investigate the elemental composition and chemical vibrations with in the prepared
sample. The FTIR spectra were studied in the wavenumber range from 400 to 4000
cm-1
. The figure 4.7 shows the FTIR spectra of bare CuS and CuS/CNTs
nanocomposites. The peak observed at 3371cm-1
indicates the N-H stretchings due to
existence of thiourea. The peaks acquired at 1279 and 727 cm-1
corresponds to the C-
O and O-S stretching due to intermediate product (acetone) and sulphate group
respectively. The C-S group vibrations found at 665 cm-1
. The high intensity metal
sulfide (Cu-S) bond observed at 572 cm-1
(Tank et al., 2017). The FTIR spectrum of
CuS/CNTs nanocomposites revealed two additional bands. The high intensity peak
87
observed at 1675 and small peak at 1729 cm-1
due to presence of C=C vibrations of
CNTs and O-H stretching of water respectively (Mohammadi et al., 2016).
Figure 4.7: Fourier transform infrared spectroscopy analysis of bare CuS and
CuS/CNTs nanocomposites.
4.2.3 FT-IR analysis of NiS and its nanocomposites
The characteristic functional groups and associated bond vibrations of the prepared
NiS and NiS/CNTs nanohybrid were studied by their FTIR analysis. Figure 4.8 shows
the FTIR spectra of NiS and NiS/CNTs nanohybrid. The FTIR spectra were recorded
in the wavenumber range from 4000 to 400 cm-1
. The peaks observed at 3828, 3449
and 3391 cm-1
ascribed to the bending and stretching vibrations of the organic
functional groups such as C-H and O-H. The peak detected at 2921 cm-1
revealed the
influence of the stretching vibrations of the N-H functional group of L-cysteine (a
precursor). The peak at 2844 cm-1
attribute to the bending mode of sulfur. The
symmetric stretching vibrations of C=S were observed at the 739 cm-1
bands. The
characteristic metal- sulfur band was obtained at a wavenumber of 598 cm-1
and
revealed Ni-S stretching vibrations.
88
Figure 4.8: Fourier transform infrared spectroscopy analysis of NiS and NiS/CNTs
nanohybrid.
The FTIR spectrum of NiS/CNTs nanocomposite revealed two prominent additional
bands. The relatively intense absorption band at 1699 cm-1
was observed due to C=C
vibrations of CNTs, while a weaker band at 1255 cm-1
can be assigned to the
stretching vibrations of the absorbed water O-H group (S et al., 2018; Sonai Muthu
and Gopalan, 2019; Yuan and Luan, 2013).
4.3 Morphological analysis
4.3.1 Field Emission Scanning Electron Microscopy
FE-SEM is a versatile technique to study material‘s microscopic structure in
nanometer (nm) range by scanning its surface. FE-SEM technique is used to identify
the material and pollution of the particle and its elimination. It is also used to analyze
the particles‘ morphology and their particle size. The surface of the material is being
scanned by an electron beam to micrometer (1μm) depth. The FE-SEM instrument
consists of an objective lens, electric guns, and a system for electron detection, two
89
types of condenser lens and a set of deflector. Field emission gun and thermionic gun
can be used in a scanning electron microscope. Energy of 1-30 keV is supplied by
electron guns. There is a cross section of smallest beam at electron gun called
crossover. Crossover can be de-magnified by two or three stage system of electron
lens. De-magnification is usually carried out to generate an electron probe on
specimen‘s surface with 1-10 nm diameter and 1-100 pA current. Electron probe
scanning is done through a deflection coil over the area of specimen in raster like
manner. The deflection coil exists in front of final lens and joined the computer
display system (monitor) or a tube of cathode ray. Electron detectors collect the
signals caused by the contact specimens and primary beam (Khursheed, 2011; Nuspl
et al., 2004).
Figure 4.9: Principle of field emission scanning electron microscopy (Nuspl et al.,
2004).
4.3.2 FE-SEM analysis of MoS2 and its nanocomposites
The morphological investigations of the as-prepared pure MoS2 and MoS2/r-GO NCs
were carried out by FE-SEM analysis. The FE-SEM of pure MoS2 at 19,080
magnifications is illustrated in Figure 4.10 (a). The analysis of the FE-SEM image
revealed that the synthesized MoS2 exhibited flake like morphology. The 2D flake-
like MoS2 has an average thickness of 61.3 nm and an average width of 267.8 nm. All
these values were calculated with the help of ImageJ software. The FE-SEM image of
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the MoS2/r-GO (Figure 4.10 b) showed that flake-like MoS2 nano-architecture is
evenly submerged in the r-GO sheets.
Figure 4.10: Scanning electron microscopic image of (a) pure MoS2, (b) MoS2 /r-GO
nanocomposites.
4.3.3 FE-SEM analysis of CuS and its nanocomposites
The surface morphology of CuS and their nanocomposites was investigated through
FE-SEM. The FE-SEM of CuS at 19,080 magnifications is demonstrated in
figure 4.11 (a). The CuS nanoarchitecture is interconnected with together and formed
porous inimitable nanostructure. The CuS possess an average thickness of 38.5 nm
and calculated using ImageJ software. The FE-SEM image of CuS/CNTs
nanocomposites was shown in figure 4.11 (b). The image of CuS/CNTs
nanocomposites represents the consistently distribution of CNTs in the entire network
of bare CuS nanoarchitectures.
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Figure 4.11: Scanning electron microscopic image of (a) bare CuS, (b) CuS/CNTs
nanocomposite.
4.3.4 FE-SEM analysis of NiS and its nanocomposites
The surface morphology of NiS and its nanohybrid with CNTs was examined via their
FESEM micrographs. Figure 4.12 displays the FESEM micrographs of NiS and
NiS/CNTs nanohybrid at different magnifications. The FE-SEM micrograph of a fine
solid NiS at 25 ×103 magnification is illustrated in figure 4.12 (a). Clearly, the NP of
the NiS is almost spherical and their size ranges from 83.4 to 91.7 nm. The NPs in the
NIS are widely dispersed, but few groups of 2-5 NPs can also be seen in the FESEM
micrograph. In fact, the NPs of the NiS possess higher surface energy and are labile to
show agglomeration. The particle sizes were calculated using the ImageJ software.
The FESEM micrograph of the NiS/CNTs at 26.5 ×103 magnification is illustrated in
figure 4.12 (b). It can be seen that the CNTs are uniformly distributed
and entangle with the NiS NPs within the solid NiS/CNTs sample. The CNTs, due to
their excellent conductivity, can provide many conductive channels within the
NiS/CNTs matrix to accelerate the redox reaction.
92
Figure 4.12: Scanning electron microscopic image of (a) bare NiS, (b) NiS/CNTs
nanocomposites.
4.4 Elemental analysis
4.4.1 Energy Dispersive X-ray
An analytical method which is used to characterize the material chemically and to
investigate its elemental composition is called Energy dispersive X-ray spectroscopy.
It may be abbreviated as EDS or EDX or EDAX. The EDX instrument cannot work
individually but works with the integration of scanning electron microscope. The
principle that let the EDS to perform is capability of electromagnetic radiation of high
energy (X-rays) to emit core electrons from the atom. EDS works on the principle of
‗Moseley‘s law‘ which dictates that there was a direct relationship between atom‘s
atomic number and released light frequency. Removal of electrons from the system
creates a hole that can be filled by high energy electron and release energy on
relaxation. The energy which is released during relaxation is different for each
element present in periodic table. The bombardment of the samples on X-rays will
give identification of elements and their proportion present in the sample (Goldstein et
al., 2003).
93
Figure 4.13: Function of energy dispersive x-ray spectroscope(Goldstein et al.,
2003).
4.4.2 EDX analysis of MoS2 and its nanocomposites
The EDX spectra of the prepared samples were obtained to determine their elemental
composition. The EDX spectrum of the pure MoS2 figure 4.14 (a) revealed that the
sample mainly contained Mo and S elements. The EDX spectrum of the
nanocomposite figure 4.14 (b) also contained carbon atoms along with the atoms Mo
and S that confirmed the successful synthesis of the MoS2/r-GO NCs. The small
quantity of oxygen was also found in the EDX spectrum that may be due to the
residual oxygen in the r-GO sheets.
Figure 4.14: EDX of (a) pure MoS2 and (b) MoS2 /r-GO nanocomposites.
94
4.4.3 EDX analysis of CuS and its nanocomposites
The elemental composition of prepared bare CuS and CuS/CNTs nanocomposites
were acquired to ensures the purity of prepared samples. The bare CuS
nanoarchitecture possessed primarily Cu and S elements confirmed by EDX spectrum
of bare CuS figure 4.15 (a). The EDX spectrum of the CuS/CNTs figure 4.15 (b)
revealed that the material sample contained carbon atom along with the elements of
bare CuS. The EDX spectrum confirmed the successful preparation of the bare CuS
and CuS/CNTs nanocomposites with maximum purity.
Figure 4.15: The EDX spectra of CuS and CuS/CNTs nanocomposites.
4.4.4 EDX analysis of NiS and its nanocomposites
The elemental composition of the fabricated samples was also examined via their
EDX analysis. Figure 4.15 displays the EDX profiles of the NiS and NiS/CNTs
nanohybrid. The EDX profile of the NiS sample figure 4.16 (a) exhibits only Ni and S
elemental peaks, indicating the successful fabrication of the desired sample with
higher purity. In comparison, the EDX profile of NiS/CNTs nanohybrid figure 4.16
(b) exhibits the Ni, S, and C elements peaks, confirming the formation of NiS/CNTs
nanohybrid. The additional peak of the carbon element was observed due to the
existence of the CNTs matrix within the nanohybrid (NiS/CNTs).
95
Figure 4.16: The EDX profiles of NiS and NiS/CNTs nanohybrid.
4.5 Brunauer–Emmett–Teller (BET) analysis
Brunauer-Emmett-Teller is a basic theory describes the gas molecules‘ physical
adsorption onto the surface of the solid and helpful to measure specific surface areas
of materials. Generally this theory applies on the systems exhibiting multilayer
adsorption and normally uses the gases that are chemically inert as adsorbate to the
material‘s surface to enumerate the surface area. Nitrogen as gaseous adsorbate is
used commonly to probe the surface of the material. As nitrogen is used as probing
gas therefore, standard BET instruments run on the boiling temperature of nitrogen
(77K or -195.79 °C) (Mishra et al., 2019; Naderi, 2015).
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4.5.1 Brunauer-Emmett-Teller (BET) analysis of MoS2 and its nanocomposites
The specific surface area of pure MoS2 and MoS2/r-GO NCs was determined by
Brunauer–Emmett–Teller (BET) measurements. The N2 adsorption-desorption
isotherms of pure MoS2 and MoS2/r-GO NCs are depicted in Figure 4.17. The BET
surface area of MoS2/r-GO NCs (163.6 m2g
-1) was larger than the pure MoS2 (81.4
m2g
-1). The observed less surface area of the pure MoS2 is due to their agglomeration
and stacking. The r-GO sheets in the nanocomposites itself provided additional
surface and increased the surface area of the MoS2 by controlling their agglomeration
(Atchudan et al., 2018; Xiang et al., 2019; Yang et al., 2016; Yue et al., 2019).
Figure 4.17: Nitrogen physic-sorption isotherms of (a) pure MoS2 (a) and (b) MoS2
/r-GO nanocomposites.
4.6 UV-Visible spectroscopic analysis
UV/Visible spectroscopy provides knowledge about the transitions of electrons of
atoms and molecules. It is associated with the excitations of electrons from low
energy level to high energy level. These excitations are occurred in consequence of
ultraviolet and visible light absorption. The type of spectroscopy caused by the
electronic transitions from lower energy level to higher energy level is recognized as
―electronic spectroscopy‖. As these energy levels are fixed so precise amount of light
is required for electronic transition to occur. In electromagnetic spectrum, ultraviolet
and visible regions range from 400 nm - 10 nm and 800 nm - 400 nm respectively.
Ultraviolet region is subdivided into near (quartz) and far (vacuum) regions range
from 400 nm - 200 nm and 200 nm – 10 nm respectively. Possible electronic
97
transition are σ to π*, σ to σ
*, π to π
*, π to σ
*, n to π* and n to σ
*(Fleming and
Williams, 1966; Perkampus, 1992).
4.6.1 Beer Lambert Law
Beer Lambert law is the basics to measure light and absorption. It is a relationship
between lessening of light when it travels through a substrate and the characteristics
of the substrate. This law states that ―absorbance (A) is directly proportional to the
concentration of the substrate (c)‖.
Figure 4.18: Absorbance of light by the material in cuvette (Perkampus, 1992).
Below is the equation for Beer Lambert law;
(
) (4.6)
A = Absorbance (A)
Io = Incident light intensity
I = transmitted light intensity
ε = extinction coefficient/molar absorptivity (mol-1
dm-3
cm-1
)
c = concentration of substrate (mol/dm3)
l = cell dimension/optical path length (cm)
4.6.2 Ultraviolet/Visible spectroscopic analysis of GO and r-GO
The synthesis of GO and its reduction to r-GO was analyzed by UV-Visible
spectroscopic studies. The UV-Visible spectra of GO and r-GO are depicted in figure
98
4.19. The strong absorption peak at 233 nm (π-π* transition of conjugated carbon
atoms) and relatively weaker absorption peak at 304 nm (n-π* transition of carbonyl
carbon atoms) confirmed the formation of GO (Baykal et al., 2017; Krishna et al.,
2014; Lai et al., 2012; Wong et al., 2015). In the UV-Visible spectrum of r-GO, two
variations are observed. The first, the conjugated carbon peak shifted toward a higher
wavelength (271 nm) that indicated the restoration of the conjugated structure. 2nd
, the
weak absorption peak completely disappears that expresses the removal of oxygen
atoms (Aadil et al., 2016; Al-Nafiey et al., 2017; Andrijanto et al., 2016; Rabchinskii
et al., 2016).
Figure 4.19: UV-Visible spectra of graphene oxide and reduce graphene oxide.
4.7 Electrical conductivity measurements
4.7.1 Current-voltage (I-V) measurements
A basic, fast, cheap and remarkable technique to explore electrical characteristics and
electrical behavior of various materials is its current-voltage (I-V) measurements.
Four or two probe methods are usually used to carry out I-V measurements. In two-
probe method, two electrical points with already known voltage are used to find out
current (I) between two contacts. Whereas, in four-probe technique two inner and two
outer points at equal distance for voltage and current terminals are used respectively
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(Bijl, (1919).; Iranzo et al., 2010). Voltage is applied between the contacts of device
to measure current (I) and Ohm‘s law is used to find out resistance (R) of the material.
The expression for Ohm‘s law is as follows:
(4.7)
V = voltage (V)
I = current (A)
R = resistance (Ώ)
To calculate the resistivity following expression is used;
(4.8)
ρ = resistivity (Ώm)
R = resistance (Ώ)
l = length (m)
A = area (m2)
4.7.2 Electrical conductivity measurement of MoS2 and its nanocomposites
The electrical conductivity (ρ) of the pure MoS2 and MoS2/r-GO NCs was measured
via two probe current-voltage (I-V) measurements within the potential window of -5
V to +5 V. To conduct I-V measurements, the synthesized materials are converted
into pellets of width (w) , ~ 2.2 nm and area (A) ~ 50.21 mm2 using die. The obtained
I-V curves of pure MoS2 and MoS2/r-GO NCs are depicted in figure 4.20. The IV-
curve of the pure MoS2 is non-linear that shows its poor electrical conductivity.
While the I-V curve of MoS2/r-GO NCs is almost linear that indicated its improved
electrical conductivity due to r-GO sheets. The electrical conductivity (σ) of prepared
samples was calculated by using equation (4.7):
RA
w (4.9)
Here 'w' is the width while 'A' is the area of the sample pellets. The values of
resistance (R) were calculated by taking the inverse slop of the obtained I-V curves
(Aadil et al., 2016; Rehman et al., 2019; Shaheen et al., 2016; Yousaf et al., 2019).
The calculated results indicate that MoS2/r-GO NCs (1.24 × 10-2
Sm-1
) exhibited
higher electrical conductivity than the pure MoS2 (2.2 × 10-7
Sm-1
).
100
Figure 4.20: Current-Voltage measurements of pure MoS2 and MoS2 /r-GO
nanocomposites.
4.7.3 Electrical conductivity measurement of CuS and its nanocomposites
The current-voltage (I-V) measurement of bare CuS and CuS/CNTs nanocomposites
was studied. The pellets of samples were prepared using die. The width and diameter
of pellets are 2.25 nm and 50.29 mm2 respectively. The I-V measurements carried out
in the potential window of -6V to +6V. The figure 4.21 shows the I-V curve of bare
CuS and CuS/CNTs nanocomposites. The bare CuS shows poor electrical
conductivity due to non-linearity of the I-V curve. The I-V curve of CuS/CNTs
nanocomposites is almost linear and possessed enhanced conductivity due to presence
of conductive CNTs. The values of resistance (R) were calculated by taking the
inverse slop of the obtained I-V curves. The calculated results indicate that CuS/CNTs
nanocomposites (2.34 104 Sm
-1) showed higher electrical conductivity compared to
the bare CuS (1.85 10-4
Sm-1
).
101
Figure 4.21: Current-Voltage measurements of bare CuS and CuS/CNTs
nanocomposites.
4.7.4 Electrical conductivity measurement of NiS and its nanocomposites
The electrical conductivity (σ) of the NiS and NiS/CNTs nanohybrid was calculated
to examine the effect of CNTs on the conductivity enhancement. For this purpose, the
solid samples were transformed into pellets (diameter = 51.52 mm2, thickness = 2.31 )
and I-V experiments were conducted on a picoammeter at room temperature (32-
35°C) within the voltage window of -5 V to 5 V. The I-V data of the NiS and
NiS/CNTs nanohybrid was used to plot their I-V profiles displayed in figure 4.22.
Figure 4.22: The I-V profiles of NiS and NiS/CNTs nanohybrid.
The I-V profile of the NiS sample‘s pellets figure 4.22 (a) is non-linear with less
slope, indicating its poor voltage to the current response. In comparison, the I-V
profile of the NiS/CNTs nanohybrid is almost linear and exhibits a larger slope,
102
indicating its improved electrical conductivity due to the CNTs matrix. The exact
value of the electrical conductivity (σ) was calculated for NiS and NiS/CNTs samples
from their I-V profiles via the following relationship (Zafar et al., 2020).
The thickness (w) of the sample's pellets was determined directly from the digital
vernier caliper while their area (A) was determined from their diameter using well
know relationship . The value of the resistance (R) for both samples (Nis &
NiS/CNTs) was determined from the I-V profiles by taking the inverse of slop. The
calculated electrical conductivity (σ) value for the NiS/CNTs sample comes to be
superior (1.42 105 Sm
-1) than that of the NiS sample (4.53 10
-3 Sm
-1), indicating a
positive interaction among the NiS nanoparticles and CNTs.
4.8 Electrochemical measurements of MoS2 and MoS2/r-GO
nanocomposites
4.8.1 Preparation of the working electrode (WE)
The prepared samples were mixed with nafion (binder) and ethanol solvent to form
the homogeneous slurry. The slurry was decorated on the electrically conductive side
of the indium tin oxide (ITO) glass. The decorated ITO glass was dried in the oven for
6 hours at 60 °C. The weight of the dry sample on the ITO glass was 1.32 mg. We
used the nafion binder due to its superior electrical and cationic conductivity. The
kinetics of the electrochemical reaction highly depends upon the electron/ionic
transport. Therefore, the nafion binder seems to be more suitable for cathodic
material, as cations from the electrolyte can diffuse into the material to take part in the
redox reaction.
Moreover, the nafion due to its good electrical conductivity also provides an effective
electrical connection between the current collector and active material (Cheng and
Scott, 2014; Lin et al., 2008; Mustapha, 2011; Zhang et al., 2012).
4.8.2 Electrochemical Characterizations of MoS2 and MoS2/r-GO
nanocomposites
All the electrochemical tests were obtained using Gamry interface 5000 (06531) via
three-electrode setup in 1 M Na2SO4 electrolyte. In the three-electrode setup, along
with the sample electrode (working electrode), the Ag/AgCl and Pt wire were used as
a reference and counter electrode respectively. The CV and CCD profiles of the
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samples based working electrodes (WE) were obtained within a potential window of -
1 V to -0.53 V at various sweep rate/current densities. The EIS tests were performed
within the AC frequency range of 100 kHz-100 mHz.
4.8.3 Electrochemical measurements of MoS2 and MoS2/r-GO nanocomposites
The electrochemical activities of the prepared samples were tested via the three-
electrode setup in 1 M Na2SO4 electrolyte. The cyclic voltammetry (CV) profiles of
the pure MoS2 electrodes and MoS2/r-GO NCs electrodes were obtained at sweep
rates 5, 15, 30, 60, 80 and 100 mVs-1
within the potential window from -1 V to -0.53
V.
The obtained cyclic voltammetry (CV) profiles of pure MoS2 electrode at different
sweep rates are depicted in the figure 4.23a. The non-rectangular shape of the CV
profiles with clear oxidation / reduction tilt at -0.64 V and -0.80 V revealed that
mainly faradic (pseudocapacitive) reaction is involving to store charge (Javed et al.,
2015). The CV profile with smaller enclosed area and curvature at extreme potential
reveals the poor capacitive performance and higher resistivity of the pure MoS2
electrode (Xu et al., 2020). While the quasi-rectangular shape CV profiles (Figure
4.23b) with diffused redox peaks showed that both faradic and non-faradic
mechanisms contributed to store charge in MoS2/r-GO NCs based electrode. The
larger enclosed area in the CV profiles of the nanocomposites indicated its enhanced
charge storage capacity due to the synergistic effects of faradic (MoS2) and non-
faradic mechanism (r-GO) (Aadil et al., 2016). The increase in the distance between
redox peaks at a higher sweep rate indicated the time-limited electrode-electrolyte
interaction (Ramachandran et al., 2013; Zhou et al., 2020).
The possible faradic (pseudocapacitance) and non-faradic (EDLCs) reactions at the
working electrode are given as follows:
SNaMoeNaMoS
2 (4.10)
NaMoSeNaMoS surface 22 )( (4.11)
The specific capacitance of the samples electrode was calculated by equation (4.12):
(4.12)
The figure 4.23c illustrates the CV profiles of MoS2, MoS2/r-GO, and bare r-GO at
the sweep rate of 10 mVs-1
. It is evident from the area within the CV loops that
mV
IdVCsp
104
nanocomposites exhibits higher specific capacitance than the MoS2 and r-GO. In fact,
the higher specific capacitance of the nanocomposites is the result of synergistic
effects between the MoS2 (pseudocapacitance) and r-GO (electric double layer
capacitance).
Figure 4.23: The CV profile of the (a) pure MoS2 based electrode, (b) MoS2 /r-GO
nanocomposites based electrode at different sweep rates (c) CV profile of MoS2,
MoS2/r-GO and r-GO at 10 mVs-1
.
The cyclic stability of the samples electrode was also tested at 80 mVs-1
for 1000 CV
cycles. Figure 4.24 (a) illustrates the CV profiles of the pure MoS2 based electrode
obtained after the 1st and 1000
th CV cycles. The decreased area under the CV loop
after 1000th
CV cycles indicated the reduction in the specific capacitance due to the
volume expansion and mass loss at the working electrode.
105
Figure 4.24: CV profile of (a) pure MoS2 based electrode at 1st cycle and after 1000
cycles (b) MoS2 /r-GO nanocomposites based electrode at 1st cycle and after 1000
cycles (c) The% capacitance retention of pure MoS2 electrode and MoS2 /r-GO
nanocomposites based electrode after various number of cycles.
While the CV profiles of the MoS2/r-GO NCs based electrode (Figure 4.24 b) showed
the much better response even after 1000th
CV cycles. In fact, the highly conductive r-
GO matrix in the nanocomposites not only provided the conductive channels for
charge transfer but also stabilized the materials against the volume expansion and
mass loss. The relation between the number of CV cycles and % capacitance retention
is shown in the figure 4.24 c. The MoS2/r-GO NCs based electrode showed 84.2%
capacitance retention after 1000 CV test that is superior to the % capacitance retention
of the pure MoS2 based electrode (68.4%).
Moreover, the electrochemical performance of the prepared samples was further
evaluated under controlled current cyclic charge/discharge (CCD) tests. The CCD
profiles of pure MoS2 based and MoS2/r-GO NCs based electrode at 1 Ag-1
are
106
depicted in Figure 4. 25a. The tilted triangle CDD of the samples indicated their
pseudocapacitive nature (Wang et al., 2019). The expanded CCD profile of the
nanocomposites-based electrode with zero IR drop indicated its higher specific
capacitance and lower resistivity respectively.
The obtained CCD results were found in accordance with the CV results and showed
the major contribution of the faradic mechanism to store charge. The specific
capacitance of the pure MoS2 based and MoS2/r-GO NCs based electrodes was
calculated by using equation (4.13):
V
tiCsp
(4.13)
The calculated Csp of the pure MoS2 based electrode was 248 Fg-1
while MoS2/r-GO
NCs based electrode was 441 Fg-1
. The higher Csp of the nanocomposites-based
electrode was due to EDLC contribution and improved electronic conductivity that
also enhanced the faradic contribution toward the total capacitance.
Furthermore, the CCD test was conducted at various current densities from 1-5 Ag-1
and corresponding Csp capacitance was calculated by equation (6). The observed
relation between the Csp and current density is depicted in the form of a graph in the
Figure 4.25 b. Results indicated that the value of Csp decreased with an increase in
the current density for both working electrodes. This is again due to time-limited
electrode-electrolyte interaction at higher current density as has been discussed by CV
data.
Figure 4.25: (a) Cyclic charge discharge curves of pure MoS2 electrode and MoS2 /r-
GO nanocomposites based electrodes at 1 Ag-1
current density (b) Effect of current
107
densities on the specific capacitance of pure MoS2 based electrode and MoS2 /r-GO
nanocomposites based electrode
The electrochemical impedance spectroscopic (EIS) tests were conducted to check the
electrochemical performance of the prepared samples in the AC circuit. These tests
were completed in 1M Na2SO4 electrolyte within the frequency range of 100 kHz to
100 mHz. Figure 4.26 (a) shows the Nyquist plots of pure MoS2 based and MoS2/r-
GO NCs based electrodes.
Figure 4.26: Nyquist plots of pure MoS2 based electrode and MoS2 /r-GO
nanocomposites based electrode in three electrode system using 1 M Na2SO4
electrolyte.
In fact, the Nyquist plot gives the information about various types of resistances that
were faced by active material in the AC circuit. These resistances decreased the
theoretical voltage window as a result of energy density (Ed) and power density (Pd),
which are merely voltage-dependent, also decreased. The intercept of the Nyquist
curve at the real axis at higher frequency region, a semicircle at a moderate frequency
region and a diagonal line at lower frequency region attribute to the solution
resistance (Rs), charge transfer resistance (Rct) and Warburg resistance (Rw)
respectively. The smaller semicircle and almost vertical line of the nanocomposites-
based electrode indicated its lower charge transfer and lower diffusion (Warburg)
resistance respectively. The exact value of the Rct (23.5Ω) was determined by
applying the circuit to the obtained EIS data in EC-lab software. The modified Randle
circuit that was fitted to the EIS data of the prepared samples electrodes is shown in
the inset of the figure 4.26 (b).
108
4.9 Electrochemical measurements of CuS and CuS/CNTs
nanocomposites
4.9.1 Preparation of the working electrodes
The homogeneous slurry of synthesized CuS and their nanocomposites materials was
prepared by adding appropriate amount of dry finely grinded samples into binder and
ethanol. The nafion used as binder due to excellent electrical conductivity. The
obtained slurry was feast over the clean and dried conductive surface of substrate
(ITO glass) material. The sample loaded electrode was dried overnight at 60 °C in an
oven. The dry working electrode possessed 1.34 mg of prepared sample.
4.9.2 Electrochemical Characterizations of bare CuS and CuS/CNTs
nanocomposites
The electrochemical measurements were studied on the Gamry interface 5000 (06531)
through three-electrode setup. All the electrochemical measurements were explored in
the electrolyte of 3 M KOH. In the three-electrode setup, the sample electrode act
as working electrode, the counter and reference electrode were Pt wire and Ag/AgCl
respectively. The CV and CCD properties of the prepared samples were achieved
within a potential window of 0.0 V to 0.6 V. The EIS tests were done within the AC
frequency (100 kHz to 100 mHz).
4.9.3 Electrochemical measurements bare CuS and CuS/CNTs nanocomposites
The electrochemical measurements of the synthesized materials were studied in 3 M
KOH electrolyte via three-electrode probe system. The CV profiles of bare CuS and
CuS/CNTs nanocomposites were studied within the potential window of 0.0V to 0.6V
at different scan rates (10 to 100mV/s). The CV profile of bare CuS electrode is
illustrated in figure 4.27 (a). The CV profile shows clear pairs of reversible redox
waves at various sweep rates. The existence of redox peaks shows that the prepared
electrode material owns faradaic (pseudocapacitive) behavior. The obtained redox
waves were stores charge mainly through the reversible faradaic redox reactions. This
performance is related to other chalcogenide materials. The redox reaction in the CuS
material ascribed due to replacement of cations between electrode material and
electrolyte (Bodenez et al., 2006).
109
Figure 4.27: The CV profile of the (a) bare CuS electrode, (b) CuS/CNTs
nanocomposites based electrode at various scan rates.
The bare CuS electrode material exhibited poor charge storage proficiency due to
slighter area of CV hoop. The CuS/CNTs nanocomposites showed slightly greater
area of CV hoop with prominent redox peaks due to incorporation of conductive
CNTs into the matrix of CuS shown in Figure 4.27 (b). The CuS/CNTs
nanocomposites electrode improved the pseudocapacitance owing to introduction of
EDLCs capacitance. At higher scanning rates, the fidelity in the shape of CV curves
of CuS/CNTs nanocomposites responsible for excellent reversible redox reactions.
The antithetical effect of sweep rate on the capacitance of as-prepared electrodes of
bare CuS and CuS/CNTs nanocomposites is depicted in figure 4.28 (a). The plot of
specific capacitance versus scan rate clearly shows the prominent deterioration in
capacitance of working electrodes as the scan rate increases. This behavior is
observed due to very time short interactions of electrode and electrolyte ions. At
higher scan rate the electrolyte ions not found appropriate interactions with electrodes.
110
Figure 4.28: (a) The effect of scan rate on specific capacitance of bare CuS and
CuS/CNTs nanocomposites electrodes (b) The % capacitance retention of bare CuS
and CuS/CNTs nanocomposites electrodes after several numbers of cycles.
The retention in capacitance of prepared electrodes was also determined after the 1st
and 1000 CV cycles. The figure 4.29 (a and b) showed the retention in capacitance of
bare CuS and CuS/CNTs nanocomposites electrode respectively at the scan rate of
70mV/s with number of CV cycles.
Figure 4.29: CV profile of (a) bare CuS based electrode at 1st cycle and after 1000
cycles (b) CuS/CNTs nanocomposite based electrode at 1st cycle and after 1000
cycles.
The figure 4.28 (b) showed the % capacitance retention of bare CuS and CuS/CNTs
nanocomposites electrodes after several numbers of cycles. The bare electrode
material holds depressed cycling stability as compared to nanocomposites electrode.
The bare electrode material showed 74 % retention capacitance and CuS/CNTs
111
nanocomposites showed considerable better retention capacitance (83.2%). The
conductive CNTs in the nanocomposites electrode provide the stability against the
volume expansion and extensive active network for better charge transfer during the
cyclic performance.
The electrochemical performance of as-prepared electrodes was explored by cyclic
charge-discharge (CCD) property. The figure 4.30 (a) elucidates the CCD profile of
bare CuS and CuS/CNTs nanocomposites electrodes.
Figure 4.30: (a) Cyclic charge discharge curves of bare CuS electrode and
CuS/CNTs nanocomposites based electrodes at 1 A/g current density (b) Effect of
current densities on the specific capacitance of bare CuS electrode and CuS/CNTs
nanocomposites based electrodes.
The CCD profile also obtained within the potential range of 0.0 to 0.6V at 1 A/g
current density. The non-linear shape of CCD plot indicates the pseudocapacitive
behavior. At same current density (1 A/g) the extended time-plot curve of CuS/CNTs
nanocomposites electrode exhibited greater specific capacitance (422 F/g) than bare
CuS electrode (285 F/g). The figure 4.30 (b) represents the inverse effect of various
current densities on specific capacitance of electrodes(Hsu et al., 2014). As the
current densities increases, the value of specific capacitance decreases due to
excessive charge accumulation on the surface of working electrodes.
Furthermore, the electrochemical impedance spectroscopic (EIS) study of the
prepared electrodes was conducted in the AC circuit. The EIS results were completed
in 3M KOH electrolyte within the frequency range of 100 kHz to 100 mHz. The
112
Nyquist plot of bare CuS and CuS/CNTs nanocomposites based electrodes is shown
in Figure 4.31 (a).
Figure 4.31: Nyquist plots of bare CuS based electrode and CuS/CNTs
nanocomposites based electrode in three electrode system using 3M KOH electrolyte
(b) circuit fitted Nyquist plot of CuS/CNTs NCs.
The EIS plot provides the informations about the various reaction resistances. The
larger diameter of semicircle of bare CuS electrode indicates the greater charge
transfer resistance (Rct) and possesses larger value of Warburg resistance (Rw) (Hou
et al., 2017). In case of nanocomposites the, curve possessed very small semicircle
and shows less Rct value. The CuS/CNTs nanocomposites also have vertical
impedance line at low frequency indicates the very least Rw value. Hence, the
electrolytes ions diffuse quickly and improved the capacitance (Yadav et al., 2018).
The precise value of the Rs and Rct was 24Ω and 21.5Ω respectively obtained. The
resistances were determined using EC-lab software. The adapted Randle circuit that
was fixed to the EIS data of the sample electrodes is shown in the inset of the Figure
4.31(b).
4.10 Electrochemical measurements of NiS and NiS/CNTs
nanocomposites
4.10.1 Preparation of the working electrodes
For fabrication of working electrodes, the homogeneous slurry of synthesized
materials (NiS and NiS/CNTs nanocomposites) was prepared. For slurry, the
appropriate amount of materials was mixed with ethanol and binder. The obtained
slurry was spread over the clean and dried conductive surface of ITO glass substrate.
113
The sample loaded electrode was dried at 60 °C for 12 h in an oven. The dry
working electrode possessed 1.32 mg of prepared sample.
4.10.2 Electrochemical Characterizations of bare NiS and NiS/CNTs
nanocomposites
The electrochemical measurements were obtained on the Gamry interface 5000
(06531) through three-electrode system. The electrochemical measurements were
studied in 3 M KOH. In the three-electrode system, the sample electrode act as
working electrode, the counter and reference electrode were Pt wire and Ag/AgCl
respectively. The CV and CCD properties of the prepared samples were obtained in
the potential window ranges from 0.0 V to 0.6 V. The EIS tests were achieved within
the AC frequency of 100 kHz to 100 mHz.
4.10.3 Electrochemical measurements of bare NiS and NiS/CNTs nanocomposites
The electrochemical measurements of the prepared materials were succeeded in 3 M
KOH electrolyte via a three-electrode probe setup. The CV profiles of NiS@NF and
NiS/CNTs@NF nanocomposites were collected within the potential window of 0.0 V
to 0.55 V at various scan rates like 5, 10, 20, 40, 60, 80, and 100 mV/s. Figure 4.32
(a) presents the CV profile of the NiS@NF sample, while inset of the Fig.6a contains
comparative CV profiles of pristine NF, NiS@NF, and NiS/CNTs@NF sample
electrodes at 70 mV/s.
It is clear from the comparative CV profiles of the three sampling electrodes (@ 70
mV/s) that the CV profile of the pristine NF-based electrode enclosed a minimal area
in its charge/discharge loop. Therefore, the contribution of the pristine NF toward the
total capacitance of NiS@NF and NiS/CNTs@NF sample electrode can be ignored.
The CV loops of the NiS@NF electrode figure 4.32 (a) are non-rectangular and
present a pair of oxidation-reduction bumps suggesting the presence of pure
pseudocapacitive mechaNiSm for storing the charge. The reversible redox reaction
between the NiS and KOH that involved to store charge can be expressed as (Kim et
al., 2018):
(4.14)
114
In comparison, the VC profiles of the NiS/CNTs@NF nanohybrid electrode figure
4.32 (b) were found to surround a comparatively larger area with a higher specific
capacity than the NiS@NF sampling electrode. In addition, the CV loops of the
NiS/CNTs@NF nanohybrid electrode show negligible inclination (at extreme
potential), indicating reduced resistivity (Sabeeh et al., 2020). The consistent shape of
the CV loop, even at a higher scan rate, demonstrates that the redox reaction between
NiS and aqueous KOH is highly reversible (Cao et al., 2017). In fact, the superior
electrochemical properties of the NiS/CNTs@NF nanohybrid are the result of the
synergistic effects between the redox-active NiS NPs and conductive as well as a
capacitive matrix of CNTs.
Figure 4.32: The CV profiles of (a) NiS@NF with inset comaritive CV profiles, (b)
NiS/CNTs@NF, AND(c &d) cyclic performance of NiS/CNTs@NF and effect of
number of cyclic tests on the perccentage capacitance retention (inset).
The cyclic stability of the NIS/CNTs@NF electrode in terms of capacity retention has
also been examined by carrying out 3000 cyclic tests (at 70 mV/s). As shown in
115
figure 4.32 (c), there is a slight change in the area under the NiS/CNTs@NF loop,
even after 3000 tests, which indicates good cyclic stability.
The specific capacitance of the NiS/CNTs@NF electrode was computed from their
CV loops after every 200 cyclic tests and the results are shown in figure 4.32 (d). The
figure shows that the NiS/CNTs@NF sample capacity has increased from its original
value to 102.3% after 1000 initial cyclic tests. The observed increase in the value of
the specific capacity in % may account for the activation of the electrode following
ion percolation. However, after 1000 initial tests, the specific capacity percentage of
the NiS/CNTs@NF electrode gradually decreased to 95.1% when the number of the
cyclic tests rose to 3000. The CNTs matrix, with its innovative structure, good
mechanical and electrical properties, not only increases the nanohybrid's surface and
conductivity but also protects it from volume expansion during cyclic testing (Xie et
al., 2014).
The percentage capacity retention curves for the NiS@NF and NiS/CNTs@NF
samples as a function of the scan rates applied are shown in the insert in figure 4.32
(d). We can see that both sample electrodes experience a capacitance fade by
increasing the applied scan rate from 5 to 100 mV/s. In fact, at a higher scanning rate
(100 mV/s), only a few electrolytes have access to the majority of the electrode due to
the limited time. As a result, the redox reaction between electrode species and
electrolyte ions is also limited, resulting in a lower specific capacity. Conversely, at a
lower scanning speed (5 mV/s), there is enough time for electrolyte ions to penetrate
deeper into the electrode to increase the probability of the reaction to improve the
value of the specific capacitance (Gupta et al., 2019). Yet, the NiS/CNTs@NF
electrode showed superior rate capability as it retains 78.5%, while pristine NiS@NF
electrodes retain just 58.2% of their initial capacitance on increasing the sweep rate
from 5 to 100 MV/s.
The electrochemical performances of NiS@NF and NIS/CNTs@NF electrodes were
also examined and compared using cyclic discharge (CCD) experiments. The CCD
profiles for the NIS@NF and NiS/CNTs@NF electrodes at 1 A/g in the 0 V to 0.55 V
potential window are shown in figure 4.33 (a). The figure clearly shows that the CCD
profile of the NIS/CNTs@NF has a longer discharge time (402 s) compared with the
CCD profile of the NiS@NF electrode (223 s). The very wide discharge profile of the
116
NiS/CNTs@NF electrode reveals a specific capacity exceeding that of the NiS@NF
electrode.
Figure 4.33: Cyclic charge/discharge (CCD) profiles of (a) NiS@NF and
NiS/CNTs@NF electrode at 1 A/g (b) Relation between the specific capacitance and
applied current density for both samples electrodes.
More precisely, the specific capacities of the sample electrodes have been calculated
using their discharge time. At the current density of 1 A/g, the specific capacitance of
the NiS@NF and NiS/CNTs@NF electrodes were come to be 405 and 732 F/g,
respectively. The rate capability of the NIS and NIS/NC samples were also examined
and compared by calculating the specific capacity at different current densities (1 to 5
A/g). Figure 4.33 (b) illustrates the decrease in the specific capacity of the both
sampling electrodes by increasing the applied current density from 1 A/g to 5 A/g.
This can be explained by the fact that at a higher current density, the interaction
between the electrode and the electrode becomes time-limited (Aadil et al., 2020b). In
addition, the NIS/CNTs@NF electrode retains approximately 84.5% of its capacity at
1A/g by increasing the current density to 5A/g.The higher rate capability can be
attributed to the porous nature of the NPs (supported by PXRD) and the buffering
action of the CNTs in the NiS/CNTs matrix.
The superior specific capability of the NIS/CNTs@NF electrode is the result of
synergistic effects between the PsCs (of the NiS) and the EDLCs (of the CNTs). The
excellent electrochemical performance of NIS/CNTs@NF can be attributed to its self-
supporting design, good conductivity, high surface area that not only reduces the
resistance to contact, but also improves the kinetics and above all the specific
117
capacity of the final nanohybrid. The electrochemical activity of the fabricated
NiS/CNTs@NF electrode was compared to the closely bonded electrode material in
Table 4.2.
Additionally, EIS tests were performed to examine and compare the charge transport
aptitude of NiS@NF and NIS/CNTs@NF electrodes. The NiS@NF and CNTs@NF
electrode Nyquist profiles were obtained in a frequency range from 10 Hz to 100 kHz
and are shown in figure 4.34. Usually, a Nyquist profile consists of three important
parts that provide information about three usually, a Nyquist profile consists of three
important parts that provide information about three different resistances that are
offered during EIS testing. The high-frequency X intercept indicates the solution
strength (Rs), the arc or half circle in the middle indicates the load transfer resistance
(Rct), and a low-frequency line indicates the Warburg resistance (Rw) that electrolyte
ions face during their transportation within the electrode (Yousaf et al., 2020). In
addition, the Nyquist pattern of the NiS/CNTs@NF electrode has a more vertical
straight line (parallel to the imaginary axis) with a larger slope value, showing its
superior capacitive behavior than that of the pristine NiS@NF electrode (Aadil et al.,
2020a).
Figure 4.34: Nyquist plots of NiS@NF and NiS/CNTs@NF nanohybrid.
118
In fact, the added CNTs with NiS NPs has reduced the Faraday or Charge Resistance
(Rct) of the NiS/CNTs electrode, as well as improved its ability to store the charge.
Table 4.1: The comparison of the electrochemical performance of the flake-like
MoS2/r-GO NCs, CuS/CNTs NCs and NiS/CNTs NCs with the closely related
literature.
Materials Specific
capacitance
(F/g)
%
Retention
(cycles)
Reference
MoS2-rGO (binder free) 387.6 (1.2 A/g) 100 (1000) (Saraf et al., 2018)
MoS2-rGO 2D sheets 387 (1 A/g) 88 (1000) (Jose et al., 2016)
MoS2-rGO hybrid film 282 F/g (20 mV/s) 93 (1000) (Patil et al., 2014)
MoS2/3D rGO 88.3 F/g (0.1 A/g) 78 (2000) (Zhang et al., 2016)
MoS2/C@rGO
sandwich
340 F/g (1 A/g) 90(1000) (Liu et al., 2019)
MoS2/r-GO NF 441 F/g (1 A/g) 84.2@1000 Present work
CuS 50 mA g-1
70 (1000) (Fu et al., 2016)
CuS/rGO 251 F/g 70 (1000) (You et al., 2020)
CNT@CuS 121 F/g 90 (1000) (Zhu et al., 2012b)
CNT/CuS 120 F/g 93 (1000) (Ravi et al., 2016a)
CuS/CNTs NF 422 F/g (1 A/g) 83.2@1000 Present work
Flaky Ni3S2 410 F g−1
91 (1000) (Chou and Lin, 2013)
NiS/rGO 660 F/g (50 A/g) 72 (700) (Xing et al., 2013)
Ni3S2-NiS 999 F/g (5 A/g) 75 (1000) (Zang et al., 2016) NiS2–Ni(OH)2/10%CNT 650 F/g 80 (1000) (Prabakaran et al., 2020)
NiS/rGO 715 @ 1 A/g 90.9/1000 (Yang et al., 2014)
NiS/CNTs NF 732 F/g ( 1 A/g) 95.1@ 3000 Present work
119
CONCLUSION
The leading objective of the present work was to prepare the stable electrode
materials with manifolds amended electrochemical properties for electrochemical
capacitors. In summary, prepared flake-like MoS2 nanoarchitecture by urea assisted
hydrothermal method and form their nanocomposite with r-GO with help of simple
ultra-sonication technique. The CuS and NiS nanoarchitectures were also prepared by
facile hydrothermal method and obtained their nanocomposites with highly
conductive CNTs using simple ultrasonic exfoliation method. The structural, purity,
surface morphological study and conductivity of obtained materials were studied by
physicochemical techniques such as XRD, FTIR, FE-SEM, EDX, BET analysis and
current-voltage measurements. I-V results confirmed the higher electrical
conductivity of MoS2/r-GO NCs (1.24 × 10-4
Sm-1
) compared with the pure MoS2 (2.2
× 10-7
Sm-1
) due to the presence of highly conductive matrix of r-GO. The prepared
nanocomposites of MoS2 also showed higher specific capacitance which was 441 Fg-1
at 1 Ag-1
than the pure MoS2 nanoarchitecture (248 Fg-1
at 1 Ag-1
). The enhanced
electrochemical activity of the nanocomposites is due to its unique flake-like structure
and its reduced charge transfer resistance (Rct ~ 23.5 Ω). The 2-D flake-like structure
of the electrode increased its contact area with the r-GO matrix and electrolyte. The
higher electrical conductivity and specific surface area of the nanocomposites
facilitated the faradic and non-faradic charge storage mechanism. The r-GO matrix
not only acted as a capacitive supplement but also facilitated the redox reaction
because of its superior electrical conductivity. The better capacitive performance of
the nanocomposites electrode was due to the synergistic effects between the r-GO and
MoS2. The CV, CCD and EIS results recommended the as-synthesized MoS2/r-GO
NCs as a novel cathode material for energy storage applications. The CuS/CNTs
exhibited greater conductivity 2.34 x 104 Sm
-1 than bare CuS material (1.85 x 10
-4 Sm
-
1). The nanocomposites electrode showed higher specific capacitance (422 F/g at 1
A/g) compared to the bare CuS electrode (285 F/g at 1 A/g) due to synergistic effects
of CNTs and CuS. The CuS/CNTs nanocomposites possessed higher cycling stability
and showed 83.2% capacitance retention after 1000 charge-discharge cycles. The
CuS/CNTs nanocomposite provided reduced diffusion and charge transfer resistances
compared to bare CuS electrode so, reducing the path length for electrolyte ions
during electrochemical tests. The calculated electrical conductivity value for the
120
NiS/CNTs sample come to be superior (1.42 105 Sm
-1) than that of NiS sample
(4.53 10-3
Sm-1
), indicating a positive interaction among the NiS nanoparticles and
CNTs. The NiS/CNTs nanocomposites revealed greater specific capacitance (732 F/g
at 1 A/g) than the bare CuS nanoarchitectures (319 F/g at 1 A/g). The NiS/CNTs
nanocomposites have too much high cyclic stability and lost just 4.9% of its initial
capacitance after 3000 charge-discharge cycles. As a result, the nanocomposites
depicted the enhanced electrochemical performance in a sense of capacitance
retention and cyclic stability with excellent rate capability. The and NiS/CNTs
exhibited high specific capacitance (732 F/g at 1 A/g) compared to MoS2/r-GO
nanocomposites and CuS/CNTs (422 F/g at 1 A/g) nanocomposites due to unique
flake-like morphology. The CNTs perform a dual role in the NiS/CNTs nanohybrid.
Firstly, they have improved NiS/CNTs conduction behavior due to their higher
conductivity (106-10
7 S/m). Secondly, they also serve as a capacitive complement,
contributing to the total capacity of the NiS/CNTs. The CNTs owing to their excellent
mechanical strength and hollows structure buffer the noise/CNTs nanohybrid from the
pulverization and volume expansion during the electrochemical tests. Aforementioned
materials showed excellent electrochemical measurements and improved
pseudocapacitance of nanocomposites therefore, nominated as propitious electrode
materials in future for hybrid electrochemical capacitors fabrication.
121
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