Nanostructured Electrochemically Active Electrodes for Applications … · 2017-10-06 · Nanostructured Electrochemically Active Electrodes for Applications in Energy Generation

Nanostructured Electrochemically Active

Electrodes for Applications in Energy

Generation and Storage Devices

Peter Lynch

A thesis submitted for the degree of

Doctor of philosophy in physics

Supervised by Prof. Jonathan N. Coleman

Chemical Physics of Low Dimensional Nanostructures Group

School of Physics

Trinity College Dublin

2016

To Mum. Dad and Declan Casey

DECLARATION

I declare that this thesis has not been submitted as an exercise for a degree at this or

any other university and it is entirely my own work.

I agree to deposit this thesis in the University’s open access institutional repository or

allow the library to do so on my behalf, subject to Irish Copyright Legislation and Trinity

College Library conditions of use and acknowledgement.

Elements of this work that have been carried out jointly with others or by

collaborators have been duly acknowledged in the text wherever included.

Peter Lynch

1

Abstract

In this thesis various materials with dimensions in the nanoscale have been

investigated for use in electrochemical systems. For dye-sensitized solar cells the challenge

of replacing the expensive platinum in the catalytic counter electrode with graphene was

investigated. The influence of size of the flakes with a mass of 0.1mg/cm2 with efficiencies as

a percentage of the reference platinum cell going from 15% for large flakes to 35% for the

smallest flakes. This proved to be less effective for improving the efficiency than increasing

the film thickness where the efficiency as a percentage of platinum increase from 45% to

80% as thicknesses increased from 50 to 1000nm. However, creating films with efficiencies

in excess of 80% that were comparable to platinum remained elusive. As such addition of

other materials into the graphene film to bridge the gap between graphene and platinum was

attempted. By enhancing conductivity, particularly in the vertical direction which was limited

due to the anisotropy of charge transport in graphene networks, with carbon nanotubes the

efficiency became comparable to platinum at 96%. The addition of a more catalytic material,

MoS2, also produced similar results (efficiency compared to platinum reference cell of 95%)

with an additional advantage of the material being cheaper due to its presence in nature. On

investigation of the performance of the electrodes using percolation theory it was revealed

that while the edges of MoS2 are more catalytically active the main advantage of using the

MoS2 was that the nanosheets were on average smaller using the same processing conditions.

The smaller sheets in the lateral dimensions allowed for a higher length to area ratio

increasing the percentage mass of the particles contributing to the catalytic activity of the

material. The relative reactivity of MoS2 edges to graphene edges was a factor 1.5.

Supercapacitor electrode materials in the form of PEDOT:PSS films prepared by a

variety of methods and treated with formic acid to increase conductivity were studied as well

in this thesis. Two models were compared to describe the effect of increasing thickness on the

capacitance per unit area. Both models returned capacitances per unit area within 15% of

each other allowing both models to be considered accurate for investigation of that property.

This capacitance represented a capacitance per unit mass of 33-38.5 F/g depending on the

model used. This corresponds to the capacitance of the PEDOT in the film as the PSS

component does not contribute to the capacitance. When looking at the current-voltage

2

characteristics or the time constant however, the Pell & Conway model which accounts for an

initial charge on the electrode performs better than the Higgins & Coleman model which

assumes no charge initially. The role of diffusion affects the capacitance per unit area at

higher scan rates. Neither of the models account for it and from this the diffusion coefficient

was estimated to be sm /103.63.5 210 which is about 30-40% that of the ions in water.

When accounting for time constant and scan rate the effect of the electrical properties of the

film with length and thickness were found to have a greater effect on film performance than

diffusion. For completeness freeze dried foams of the same material were fabricated to both

reduce the effects of diffusion and increase internal surface area of the films. The benefits of

freeze drying are modest (only increasing the capacitance per unit mass from 33 to 38F/g)

and when compared to the disadvantages like additional processing steps and electrode

thickness (which increases by two orders of magnitude) lead to a conclusion of this method

not being viable for this material system.

Optically transparent supercapacitor electrodes represent a solution to energy storage

for transparent electronics. PEDOT:PSS has a combination of good electrical properties at

high transparencies that allow for application as transparent conductors. In addition

PEDOT:PSS also has an appreciable capacitance which led to it being demonstrated as a

transparent supercapacitor electrode material. In an attempt to improve the electrical

properties of the film a 4 layer sheet of graphene was used as a current collector. The effect

on performance of the addition of the graphene was negligible at comparable transparencies

of PEDOT:PSS only electrodes. In order to provide a significant improvement to these

electrodes the conductive layer needs to be at least of the same sheet resistance as the

capacitive layer. While this will lead to a lower capacitance at low rates the performance at

higher rates could exceed the capacitance of the PEDOT;PSS only electrode.

3

Publications

Aspects of this thesis have been published previously in the following publications. Research

containing major contributions from this thesis is indicated with “ * “.

‘Graphene-MoS2 nanosheet composites as electrodes for dye sensitised solar cells’. Peter

Lynch, Umar Khan, Andrew Harvey, Iftikhar Ahmed and Jonathan N Coleman. Materials

Research Express, Volume 3, Number 3.*

‘Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond

electronics’ Damien Hanlon et al. Nature Communications 6, Article number: 8563

4

Acknowledgements

First and foremost I would like to thank Jonathan Coleman for giving me an opportunity to

pursue my studies. Without his mentorship and support this thesis would not have been

possible.

Thanks to Paul and Tom who helped me with my final year project, providing me

with a taste of the good life as a research student. Paul put up with many a nervous mishap as

I progressed with my graduate studies and Tom was always around to answer my queries on

electrochemistry.

Umar Khan, who served as a bottomless pit of knowledge when it came to producing

dispersions and films and saved me weeks of lost time, is gone but not forgotten. The rest of

the group provided morale support, companionship and occasionally entertainment. The lads

in the office: Peter, Graeme, Conor, Seb and Adam were vital to my sanity. To the rest:

Ronan, Damien, Andrew, Dave, Sonia and JB without whom the labs would have been a

much duller place. Cheers lads for the proof reading and making this into something of

passable readability.

To those outside of Trinity who with or without intending to made these last four

years fly by: Sam, George, James, Eoin. And the housemates who kept me feed and help me

develop my culinary talents: Eoghan, Brian and Harry. My family have been supportive of

me without fail these last four years and never been shy of telling me.

Finally a big thank you is due to Declan Casey. His belief in me from a tender age and

his instruction in maths, physics and applied maths for the leaving cert set me on the path that

resulted in this work but does not end there.

5

Contents Thesis Outline ............................................................................................................................ 8

Dye-Sensitized Solar Cells ........................................................................................................ 9

2.1 Solar Energy......................................................................................................................... 9

2.2 Path to Liquid Junction Solar Cells ................................................................................... 10

2.3 Dye Sensitized Solar Cells (DSSC) – Principles of Operation .......................................... 12

2.4 Working Electrodes ........................................................................................................... 15

2.5 Dyes and Sensitizers ...................................................................................................... 16

2.6 Electrolytes ........................................................................................................................ 17

2.7 Counter Electrodes ........................................................................................................... 18

2.7.1 Platinum Electrodes .................................................................................................... 18

2.7.2 Carbon Electrodes ........................................................................................................ 19

2.7.3 Inorganic Electrodes ................................................................................................... 23

2.7.4 Composite and Hybrid Electrodes .............................................................................. 26

Electrochemical Capacitors ..................................................................................................... 31

3.1 Introduction ........................................................................................................................ 31

3.2 General Characteristics ...................................................................................................... 33

3.3 Non-Faradaic Electrodes ................................................................................................ 35

3.4 Faradaic Electrodes ........................................................................................................ 37

3.5 Electrolytes ........................................................................................................................ 40

3.6 Device Design .................................................................................................................... 41

Methods.................................................................................................................................... 46

4.1 Sample Preparation ............................................................................................................ 46

4.1.1 Liquid Phase Exfoliation by Ultrasonication .............................................................. 46

4.1.2 Stabilisation................................................................................................................. 48

4.1.3 Centrifugation ............................................................................................................. 49

4.1.4 Film Formation ........................................................................................................... 50

4.2 Sample Characterisation .................................................................................................... 53

4.2.1 UV-Vis Specrophotometry .......................................................................................... 53

4.2.2 Profilometry ................................................................................................................. 54

4.2.3 Electron Microscopy .................................................................................................... 54

6

4.2.4 Electrical Charaterisation ............................................................................................. 53

4.2.5 Electrochemcial Charaterisation .................................................................................. 58

4.2.5.1 Voltametry ........................................................................................................... 59

4.2.5.2 Impedance Spectrocopy ........................................................................................ 62

4.2.6 Solar Simulation.......................................................................................................... 63

Materials .................................................................................................................................. 65

5.1 Graphene ............................................................................................................................ 65

5.2 Carbon Nanotubes .............................................................................................................. 68

5.3 MoS2 .................................................................................................................................. 71

5.4 Poly(3,4-ethylenedioxythiophene) ..................................................................................... 72

Graphene Based DSSC Counter Electrodes ............................................................................ 77

6.1 Introduction ........................................................................................................................ 77

6.2 Experimental Procedure ..................................................................................................... 79

6.2.1 Materials ..................................................................................................................... 79

6.2.2 Film Production .......................................................................................................... 79

6.2.3 Electrochemical Charaterisation ................................................................................. 79

6.3 Results and Discussion ...................................................................................................... 82

6.3.1 Graphene Film Thickness Dependence ...................................................................... 82

6.3.2 Addition of Carbon Nanotubes ................................................................................... 88

6.3.3 Addition of MoS2 ........................................................................................................ 90

6.3.4 Graphene Flake Size Dependence .............................................................................. 97

6.4 Conclusions ...................................................................................................................... 100

Thickness Dependence of Capacitance of PEDOT:PSS Supercapacitor Electrodes ............. 101

7.1 Introduction ...................................................................................................................... 101

7.2 Experimental Procedure ................................................................................................... 102

7.2.1 Sample Preparation ................................................................................................... 102

7.2.2 Electrical Charaterisation .......................................................................................... 104

7.2.3 Electrochmical Charaterisation ................................................................................. 104

7.3 Results and Discussion .................................................................................................... 104

7.3.1 Electrical Properties ............................................................................................... 104

7.3.2 Cyclic Voltammetry of Thin Films ....................................................................... 106

7.3.3 Impedance Spectroscopy ....................................................................................... 118

7.3.4 Analysis of Diffusion............................................................................................. 121

7

7.3.5 Limitations due to Electrode Dimensions ............................................................. 124

7.3.5 Freeze Dreied Foams ............................................................................................. 126

7.4 Conclusions ...................................................................................................................... 130

Transparent PEDOT:PSS Supercapacitors with Graphene Current Collectors ..................... 132

8.1 Introduction ...................................................................................................................... 132

8.2 Experimental Procedure ................................................................................................... 133

8.2.1 PEDOT:PSS and PEDOT:PSS on Graphene Film Preparation............................. 133

8.2.2 Optical and Electrical Charaterisation ................................................................... 134

8.2.3 Electrochemical Charaterisation ............................................................................ 134

8.3 Results and Discussion .................................................................................................... 134

8.3.1 Optoelectronic Properties ...................................................................................... 134

8.3.2 Scan Rate Dependence of Capacitance ................................................................. 138

8.3.3 Length Dependence of Capacitance ...................................................................... 141

8.3.4 Impedance Spectroscopy ....................................................................................... 146

8.4 Conclusions ...................................................................................................................... 150

Conclusions and Future Work ............................................................................................... 150

9.1 Conclusions ...................................................................................................................... 150

9.2 Future Work ..................................................................................................................... 151

9.2.1 DSSC Counter Electrodes ..................................................................................... 151

9.2.2 Supercapacitor Electrode Materials ....................................................................... 153

Bibliography .......................................................................................................................... 156

8

1

Thesis Outline

The rise of technology especially personal devices has led to an increased demand for energy.

This leads to a search for alternative energy sources as fossil fuels are not renewable and with

the increasing demand for energy will only become more expensive as time passes. Solar

energy is an abundant energy source waiting to be tapped into on a larger scale. Chapter 2

will introduce the Dye-Sensitized Solar Cell (DSSC) and chapter 6 will explore the

replacement of an expensive component, platinum, with cheaper materials.

These materials are designed to have smaller dimensions to access unique properties

that become available when the particle’s size is reduced. Chapters 4 and 5 will introduce the

materials used in this thesis and how they are processed and tested respectively.

Storing this energy has also become an issue. As devices get more complicated with

more components and better, brighter screens the need to store energy in a compact manner

becomes more pressing. While conventional energy storage uses lithium ion batteries,

supercapacitors with fast charge-discharge times are being used as alternatives or in concert

with them. In this thesis Chapter 7 will look at thicker non-transparent charge storage

materials for more conventional uses. While Chapter 8 will investigate transparent charge

storage materials due to an increase in interest in transparent and flexible devices.

9

2

Dye-Sensitized Solar Cells

2.1 Solar Energy

Due to an increasing global population and the proliferation of technology, the demand for

energy has never been higher and will continue to increase. The decreasing availability of

fossil fuels, combined with the fact that in the process of releasing energy they introduce

carbon dioxide and other toxins into the atmosphere, has made alternative and renewable

energy sources more attractive in recent times.

In Figure 2.1 the graphic shows the relative sizes of the various energy sources, both

renewable and non-renewable, compared to the worldwide energy consumption in 2009 and

the projected energy consumption for 2050. It is clear that the sun is the most abundant

source of renewable energy. The sun provides approximately 1kWh of energy to the surface

of the earth per meter squared on average and capturing even a fraction of a percent of that

would go a long way to catering for mankind’s energy needs.

10

Figure 2.1 Relative sizes in TW of various renewable and non-renewable sources1

Solar energy today is a growing industry with many avenues for generating energy.

Solar water heating is a method of providing hot water and is particularly useful in hot

climates. Concentrating solar light to operate heat engines is another avenue to harvest the

sun’s energy. The main thrust of current research however is in the photocatalytic production

of fuel by water splitting and photovoltaics.

2.2 Path to Liquid Junction Solar Cells

The photovoltaic effect was first observed by Becquerel in 1839. The cell consisted of two

different metal plates separated by an ionic solution, known as an electrolyte. The discovery

of quantum mechanics and the subsequent application to condensed matter theory paved the

way for the first practical solid state solar cell which was announced by Bell Labs in 1954

with an efficiency of about 6%. Research into monocrystalline silicon solar cells has now

advanced to reaching an efficiency of up to 25%2.

11

The original liquid based solar cell took a back seat though a dye sensitized solar cell

had been reported in 1887 by James Moser and in 1964 the enhancement of

photoconductivity of zinc oxide by dye was demonstrated3. In the seventies, interest in

practical liquid junction solar cells began in earnest. These cells consisted of a highly

crystalline semiconductor with a suitable bandgap as a working electrode in an electrolyte

containing a redox couple and a catalyst on the counter electrode to replenish the electrolyte.

Figure 2.3 is a schematic of a liquid junction solar cell showing the various reaction

processes at the electrodes. Incident light is captured by the semiconductor and excites an

electron from the valence band to the conduction band to produce an electron-hole pair. The

hole then moves to the semiconductor-liquid interface where it under goes a reaction to

oxidize the electrolyte molecule as in equation 2.1.

Figure 2.2 Silicon Solar Cell

12

Figure 2.3 Energy Diagram and processes of a Liquid Junction Solar Cell4

Equation 2.1

The electron travels through the circuit and is reintroduced to the electrolyte through a

catalyst, usually platinum or graphite, in the opposite reaction to equation 2.1

Equation 2.2

These solar cells produced reasonably high efficiencies using single crystal semiconductors

of various Transition Metal Dichalcogenides (TMDs)5, Gallium Arsenide

6 and Cadmium

Selenide/Telluride7. The importance of the single crystal was that defects and grain

boundaries acted as electron traps causing recombination losses. Various methods from

pressing8 to chemical treatment

9 of these materials resulted in solar cells with comparable

efficiencies to the single crystal counter parts. However, the base cost of these materials and

the required high purity made them undesirable for commercial application.

2.3 Dye Sensitized Solar Cells (DSSC) – Principles of Operation

A dye sensitized solar cell is essentially a liquid junction solar cell with a different type of

working electrode. Currently, the prevalent working electrode consists of a mesoporous nano-

cystalline titanium dioxide electrode on the order of 10 microns thick covered in a light-

sensitive dye. While the concept had been around for a while in planar oxide electrodes,

13

Gratzel introduced the porous structure to increase dye loading per unit area to produce a

practical solar cell10

.

In Figure 2.4, the schematic of a DSSC is shown with the principle reactions. The dye

captures light in a similar way to the semiconductor in a liquid junction solar cell. In this

case, however, the electron is excited from the highest occupied molecular orbital (HOMO)

to the lowest unoccupied molecular orbital (LUMO) of the dye as in equation 2.3.

Equation 2.3

Where S represents the dye molecule, h is planks constant, ν is frequency and the asterisk

indicates an excited molecule. This is represented by the green line in Figure 4(b). The

electron is then quickly transferred from the excited dye molecule to the titanium dioxide

structure and passed through the circuit. This leaves a positively charged dye molecule in

contact with the electrolyte, usually a tri-iodide couple (I-/I3

-). An ion in the electrolyte loses

an electron to the dye to restore it to the ground state by a reaction similar to equation 2.1.

Equation 2.4

Figure 2.4(a) Schematic of DSSC (b) Energy Level Diagram with electron transfer

processes109.

14

The electron initially produced travels through the circuit and undergoes a similar reaction at

the catalytic counter electrode as in equation 2.2. The transfer of the electrons is indicated by

blue lines in figure 4 (b) and red lines indicate pathways of reverse reactions which result in a

reduction in efficiency analogous to recombination in semiconductor solar cells.

Solar cell efficiency is defined by the short circuit current (Jsc), the open circuit voltage (Voc),

a measure of ideallity known as the fill factor (FF) and the input power (Pin).Efficiency is

given by the following equation:

in

ocsc

P

FFVJ Equation 2.5

To maximize the efficiency of a cell all parameters for a given Pin should be optimized. The

short circuit current density is the current at which there is no applied potential. This is

determined by the incident photon-to-electron conversion efficiency as well as the

recombinations present in the system. The open circuit voltage is the voltage at which no

current flows in the cell. The difference between the Fermi level of the semiconductor and the

redox potential of the electrolyte limits the maximum open circuit potential. The fill factor is

the ratio between the maximum power produced by the cell and the product of the open

circuit voltage and short circuit current.

Currently, the state-of-the-art liquid dye-sensitized solar cell performs with an

efficiency of 12.3%, produced by Yella et al11

. An ideal solid state solar cell has a theoretical

efficiency in excess of 30% however Henry Snaith accounts for losses that occur in the dye-

sensitized solar cell and predicts a maximum theoretical efficiency of 13.4% using the

technologies as of 2010 but describes a pathway to optimizing the maximum efficiency to

20.25%12

. The losses are primarily associated with processes in the working electrode, dye

and electrolyte these include (a) incomplete light harvesting, which can be tackled with

working electrode geometries and dyes with suitable absorption onsets, (b) inefficient photo

induced electron transfer to dye as well as the energetic favourability of such transfers, this

can be tackled by an improvement in the matching of energy levels in the working electrode

oxide and dye, (c) conformational charges in dye due to generation of excitons, (d) and most

importantly the dye regeneration process which is the most energy intensive, due to the over

potential to oxidize the electrolyte and to conversion of the electrolyte species in solution.

15

This can be tackled with alternative electrolyte/dye couples or even replacing the dye with a

hole conductor. (e) Resistive losses throughout the cell also have an impact. The focus of

work in this thesis will be regarding the counter electrode and the resistive losses associated

with the charge transfer process at that electrode.

2.4 Working Electrodes

The design of the working electrode in Grätzel’s seminal paper was 10 microns thick

consisting of 15nm TiO2 particles which were sintered at 450 degrees Celsius to improve the

electrical transport properties of the film. The factors most important in the design of the

working electrode are high surface area for maximum dye adsorption and sufficient porosity

to allow diffusion of the electrolyte.

Since then a significant amount of work has been done to optimise the efficiency of

the cells by adjusting the parameters of the working electrode. For example, increasing

thickness should increase the amount of dye available for light harvesting. However, the

further through the film the light travels, the more light will have already been absorbed by

the previous dye particles. This would lead to a sub-linear relationship with thickness.

Furthermore, optimisation via thickness becomes problematic when one accounts for

transport issues associated with thick films. Optimum thicknesses have been found in the

range of 7 to 14 microns13

.

Particle size is another route towards altering the properties of a working electrode.

Smaller particles have a higher specific surface area allowing for increased loading of dye in

an electrode of similar thickness. A disadvantage of small particles however is the effect on

the electrical properties due to an increased number of inter-particle junctions and as such

must be balanced with the dye loading to produce optimum working electrodes. Work by

Chou et al. demonstrate this effect and show excellent efficiency at 22.5nm14

. This is close to

the commercial 25nm Degussa |TiO2 particles used in multiple studies15–18

. Another aspect of

particle size is scattering effects. Larger particles have found applications as a scattering layer

on top of a transparent layer of smaller particles to improve overall efficiency of the TiO2

working electrodes by passing the light back through the film for further absorption13,19,20

.

16

There are some losses associated with contact between the current collector and the

electrolyte. To prevent this, a thin layer of TiO2, known as a blocking layer, is applied by

various means, the most common being TiCl4 treatment13,18,21

. An added advantage of the

blocking layer is that it improves adhesion of the TiO2 particles to the substrate. As well as

pre-treating the substrate to form a blocking layer, a secondary treatment is carried out after

the film is deposited to improve particle connectivity and surface roughness.

Alternative allotropes of TiO2 have also been considered. One-dimensional nanotubes

have been employed due to the advantage of the path to the current collector being through

one continuous piece of TiO2 as opposed to a series of sintered junctions22,23

. Two-

dimensional titania nanosheets have been utilised also due to the excellent specific surface

area of two-dimensional nanomaterials allowing excellent loading of dye15,17

. They also

demonstrate preferable light scattering properties.

Charge transport improvement has also been investigated through the addition of

conductive particles, especially carbon nanotubes18

and graphene24,25

. These particles provide

a more direct route to the current collector than travelling through the porous TiO2 structure

in addition to having better electrical properties. Ideally, if the sintering process could be

removed due to enhanced conductivity, it would have a considerable impact on the cost

associated with the production of the working electrode.

In addition to TiO2, there are alternative working electrodes. Since TiO2 remains the

state of the art and is the most commonly used working electrode material, the other materials

will only get a brief mention. The n-type semiconductors used are TiO2, ZnO26–28

, SnO228–30

,

Al2O330

and Nb2O531,32

. In addition, there are p-type semiconductors such as NiO33

but these

have more limited applications due to low open circuit voltages.

2.5 Dyes and Sensitizers

The function of the dye in the DSSC is to capture light. The dye is required because the

bandgap of TiO2 is too large so the material does not absorb visible light. The dye initially

used was a ruthenium compound which gave a reddish brown colour. Since then the most

popular dyes that are commercially available are the ruthenium based N3, N719, and ‘black’

dyes shown in figure 2.5.

17

Figure 2.5 molecular structures of common dyes34

The dye needs to satisfy a number of criteria. Firstly, it needs to have an appropriately spaced

HOMO and LUMO to absorb visible light and have a high absorbance coefficient in that

range. It must also adhere suitably to the semiconductor via covalent bonds with the

anchoring groups and be stable in the electrolyte under illumination. The HOMO and LUMO

of the dye molecule must also match the conduction band of the TiO2 to promote efficient

electron transfer while preventing reverse reactions. The same molecular orbital matching is

important with the electrolyte to ensure efficient replenishment of electrons and as such

electrolyte and dye research is sometimes performed in tandem.

Alternative sensitizers are quantum dots and enough research has been performed in

this field to coin the acronym QDSSC or quantum dot sensitized solar cell. In these cells, the

quantum dot replace the dyes. Quantum confinement can alter the optical properties of

particles to produce efficient absorbers in the visible range. CdS, CdSe and PbS have been

popular materials for introduction as quantum dot sensitizers35

.

2.6 Electrolytes

The electrolyte carries the charge transport from the catalytic counter electrode to the

working electrode. It consists of a redox couple in a suitable solvent. The first Grätzel cell

used the iodide/tri-iodide couple and as such it is the most commonly researched couple and

is available commercially.

To ensure high efficiencies, there are numerous criteria for the electrolyte to fulfil.

The redox potential of the redox couple should be negative relative to the oxidation potential

18

of the dye and efficiently regenerate the dye. A high conductivity in the order of 10-3

S/cm is

desirable to ensure good electrical transport between the electrodes. The electrolyte should

not have any adverse reactions with either the dye or the sealant to prevent undesirable losses

due to the electrolyte absorbing light in the visible before it interacts with the dye. Most

importantly, it should be highly stable so as not to lose functionality over time and nor should

it degrade at temperatures below 80⁰C34.

Liquid electrolytes contain an organic solvent, redox couple and occasionally

additives to enhance performance. Currently, the best performing solvent for the electrolyte

has proven to be a mixture of acetonitrile and valeronitrile. As mentioned previously, the

most popular redox couple is the iodide/tri-iodide couple. This is achieved using dissolving

iodine(I2) and a range of iodide salts in the solvent. However others include bromine redox

couples but the state of the art for liquid based DSSCs is with a Cobalt(II/III) couple11

.

Ionic liquids are a way of combining the solvent and redox couple as one component.

The ionic liquids display low vapour pressure leading to better stability when compared to

other electrolytes. Ionic liquids display high viscosities which limits diffusion-assisted

transport however an alternative exchange mechanism allows electron hopping between the

iodide and tri-iodide ions34,36

.

Due to the difficulty working with liquid electrolytes, there has been interest in

developing solid state equivalents of the DSSC. One approach is using additives to gel the

electrolyte. These gelators form a three dimensional matrix in which the liquid electrolyte can

still work as a mobile phase but have a reduced volatility34,37

. Another solid state method

involves using p-type materials to act as the electrolyte and remove the need for a catalytic

counter electrode. This requires a material with a valence band well matched with the HOMO

of the dye molecule. Conducting polymers have been the most promising solid state

electrolytes, with Spiro-OMeTAD showing the highest performance34,38

. Limitations of solid

state electrolytes include poor filling of the working electrode and low carrier mobility.

2.7 Counter Electrodes

The role of the counter electrode is to reintroduce the electrons extracted from the dye back

into the electrolyte after passing through the external circuit. The counter electrode is made of

19

a catalytic material which, since the introduction of the DSSC, has been predominantly

platinum. The reaction carried out is a variation on equation 2.2 which for the case of the

iodide/tri-iodide couple is

Equation 2.6

2.7.1 Platinum Electrodes

Platinum has been used in many electro-catalytic systems including the hydrogen and oxygen

evolution reactions and even the initial liquid junction solar cells. Platinum has been chosen

as a catalyst due to its chemical stability and excellent electrical and thermal conductivity.

Sputtered platinum on FTO has found application as the counter electrode for DSSCs

achieving excellent properties at a thickness range from 0.2 – 2 micron39

. This is not only due

to the electrical and catalytic properties of the platinum but also the reflectance of the film

allowing more light to be captured by the working electrode.

To lower the cost, thin film platinum must be considered. In addition thin films allow

transparency allowing illumination through the counter electrode or application of a reflective

material. These low thicknesses require mass per unit area of platinum in the range of 10-

100µg/cm2. This can be achieved by thermal deposition of chloroplatinic acid which is very

common. A deposition time dependence for sputtering of platinum by Mukherjee et al.

resulted in best performance at 50 nm Pt which was approximately 100µg/cm2 however

reasonable performance at a mass loading of 0.1 µg/cm2 resulted in approximately 78% that

of a film 1000 times the mass40

.

To further enhance platinum electrodes in thin films, exploration of the various

properties of nanostructured platinum was targeted. As mentioned previously with the

working electrodes, smaller particles lead to higher specific surface areas essential for

increasing the number of active sites available for reaction. Other nanostructures have also

been investigated to increase surface area of platinum in thin films41

.

Platinum, however, is a scarce metal and could become prohibitively expensive. Also

sputtering requires specialist equipment with high energy requirements and choloroplatinic

acid is not good for the nvironment. On top of this there some debate as to the stability of

platinum in the electrolyte42

. As such, much research has been undertaken to identify

alternatives.

20

2.7.2 Carbon Electrodes

An obvious option for the replacement of platinum is carbon, which is one of the most

abundant elements on the surface of the Earth. Carbon has been well studied and there are

many forms ready for application as counter electrodes in DSSCs: carbon black, activated

carbon, mesoporous carbon, graphite/ene and carbon nanotubes.

Carbon black is produced by the incomplete combustion of heavy hydrocarbons. It

has a favourable surface area-to-volume ratio which is desirable for catalytic activity.

Murakami et al. developed thick, carbon black-based counter electrodes in excess of 20

microns to achieve an efficiency of 9.1% which, while very high, did not include a platinum

based-cell as reference43

. However, Lin et al. produced thin, transparent, carbon black-based

DSSCs with an efficiency of 7.28% compared with the platinum based DSSCs of 7.12% with

the requirement of a high temperature anneal44

.

Mesoporous carbon is a synthesized carbon material with well-defined mesoscopic (2-

50nm) pores and high surface area making it a good candidate for counter electrode

materials. While higher surface areas should be desirable and can be achieved using smaller

pore diameters, the ability of the molecules of the electrolyte to diffuse into the pores should

be considered. Work by Ramasamay et al. demonstrates this where a high surface area (1400

m2/g)/low pore diameter (3 nm) counter electrode results in a lower efficiency of 6.75%

while a lower surface area (894 m2/g)/higher pore diameter (22 nm) results in a efficiency of

8.18% comparable to that of the platinum counter electrode with an efficiency of 8.85%45

.

Carbon can be found in the Earth’s crust in the form of graphite, a layered conductive

material. Much work has been done on mechanically and chemically exfoliated layers of

graphite to produce materials for use as counter electrodes due to the high conductivity and

surface area of such materials. Pristine graphite/ene displays electrochemical catalytic

activity at edge sites while the basal planes tend to be significantly more inert46–48

. Reports

of mechanically exfoliated graphite as counter electrodes show this to be a promising method

for producing viable solar cells. Veerappan et al. produced thick electrodes (6-9 microns)

resulting in a cell with an efficiency of 6.2%, comparable to that of their platinum cell at

6.8%49

. Kavan et al. improved on this by producing a thin, transparent graphene-based

21

counter electrode with an efficiency of 5% compared to the platinum cell of 6.89%. Thinner

films are preferable as they require less material, thus reducing the cost50,51

.

While graphite is useful as a catalytic material for DSSCs, functionalised graphene is

even more suitable. Graphite oxide is commonly prepared by Hummer’s method and can be

exfoliated to produce graphene oxide which is stable in water52,53

. This introduces oxide

functional groups into the basal plane of the graphene which are catalytically active. The

presence of these oxide functional groups also reduces the conductivity and thus reduction of

the graphene oxide is commonly performed. Once reduced, either thermally or chemically,

the graphene retains some functional groups while exhibiting suitable conductivities54,55

.

Graphene oxide has been integrated into DSSCs much more often than pristine graphene due

to the ease at which it can be dispersed in water.

Wan et al. produced thin films (~100nm) of chemically reduced graphene oxide to

operate as counter electrodes for DSSCs. These films, however, did not perform favourably

when compared to platinum demonstrating an efficiency of 0.74% compared to 3.7%56

. The

use of thicker films as done by Zhang et al., with a combination of chemical reduction and

thermal annealing, produced an efficiency of 6.81%, relatively close to that of the platinum at

7.59%57

. Further, work by Roy-Mayhew et al. produced equivalent performance with 6

microns of functionalized graphene with platinum (6.8%) in the tri-iodide couple and

superior performance in the cobalt and sulphur based-couples58

.

Substituting the oxide functional groups of graphene oxide with nitrogen functional

groups, as done by Hou et al., resulted in a higher efficiency of the N-doped reduced

graphene oxide (5.4%). This was greater than both the efficiencies of the undoped reduced

graphene oxide film (4.0%) and the platinum counter electrode (5.1%)59

.

While exfoliation is a common way of producing graphene in liquid phase, it can also

be chemically synthesised as demonstrated by Wang et al. It includes oxygen functionalities

and a three dimensional structure of the 20 micron film increases the surface area available

for reactions. This resulted in a performance efficiency of 7.8% with no comparison to

platinum60

.

Outside of the liquid phase, there have been reports of chemical vapour deposition

(CVD) produced graphene being used in DSSCs. Seo et al. use CVD to produce ‘graphene-

like’ films of thickness 500-600nm as a counter electrode with efficiency of 4.3% compared

22

to 5.9% for a platinum counter electrode61

. Pan et al. use CVD to produce vertically

orientated graphene with the advantage of having edges exposed to electrolyte and good

charge transport to the current collector at 30 micron thickness. This yields an efficiency of

7.63% relative to 8.48% for a platinum based cell62

.

Carbon nanotubes (CNTs) are another allotrope of carbon, although unlike graphite, it

does not occur abundantly in nature and as such needs to be synthesised. The synthesized

tubes can be dispersed in a liquid or paste for deposition. Ramasay et al. produced transparent

CNT films of various thicknesses by spray-coating the CNTs dispersed in ethanol. This

resulted in efficiencies of up to 7.59% with no platinum for comparison63

. Screen printed

electrodes produced by Lee et al resulted in a cell efficiency of 7.67% close to their 7.83%

platinum reference64

.

However, the electrodes produced by depositing CNTs in a dispersion or paste are

randomly aligned and require an additional processing step after synthesis. Nam et al.

produced CVD grown-CNTs directly on an FTO current collector. For comparison, a screen

printed CNT electrode of randomly aligned tubes with a similar thickness (500nm) was

fabricated. The directly synthesised electrode had an efficiency of 10.04% which was greater

than that of the screen printed electrode (8.03%) and even the platinum reference (8.80%)65

.

Doping of CNTs is another viable route to enhancing electro-catalytic activity. Lee et

al. doped vertically aligned CNTs with nitrogen. The nitrogen doped-CNTs (of length 20

microns) provided superior catalytic activity evident in the better current density

characteristics resulting in an efficiency of 7.04% compared to that of a platinum cell of

7.34%66

.

Outside basic carbon materials are a class of material called conductive polymers.

One such material is PEDOT, commonly produced as PEDOT:PSS for stability in solution.

Pringle et al. use electrodeposition of PEDOT directly onto FTO as a counter electrode.

Interestingly, no thickness dependence was observed in the range of thicknesses tested (0.03-

2 microns). This is useful for minimising required material to produce effective counter

electrodes. The PEDOT counter electrode resulted in cells with efficiency of 8%, in excess of

the platinum based cell of 7.9%67

.

Variations on PEDOT namely ProDOT and ProDOT-Et2 were compared by Lee et al.

The platinum reference cell used had an efficiency of 7.77%. Electrodepositing each polymer

23

resulted in films with varying degrees of surface roughness leading to better surface area with

increased roughness. PEDOT had the lowest surface area and demonstrated an efficiency of

3.93%. ProDOT and ProDOT-Et2 had greater surface areas resulting in efficiencies of 7.08%

and 7.88% respectively68

.

Polypyrrole is another conductive polymer utilized as a counter electrode in DSSCs.

Jeon et al. produced polypyrrole nanoparticles and deposited these on FTO as a counter

electrode. The initial electrodes, with no further treatment, gave efficiencies of 5.28% and

treatment in HCl vapour improved this to 6.83%. Further adjustments to the electrolyte

allowed the best possible efficiency of 7.73% with respect to an 8.2% efficiency platinum

reference cell69

.

Polyaniline (PANI) was used to create a partially transparent counter electrode by Tai

et al. via a dip-coating method to produce the PANI films on FTO. The transparency enabled

them to test the counter electrode by both front (photoanode) and back (counter electrode)

illumination. The back illumination had a relatively high efficiency of 4.26% when compared

to that of front illumination (6.54%) which itself nearly matched that of the platinum cell

(6.69%)70

.

2.7.3 Inorganic Electrodes

Transition metal compounds make up the bulk of research into alternative materials for

counter electrodes in DSSCs due to the wide variety in possible compounds. Extensive

research has been undertaken to identify the practicality of these materials. Besides the

multitude of transition metals, there are secondary elements in the compounds to alter the

properties of the material. As such there has been work done on carbides, nitrides, oxides and

chalcogenides.

Molybdenum (Mo2C) and tungsten carbides (WC) were assessed by Wu et al. as

alternatives to platinum. Purchased powders of particles less than 300nm were sprayed onto a

conductive FTO (Flourine doped Tin Oxide) substrate with sheet resistance of 15Ω/sq.

Compared to a platinum cell with 7.89% efficiency, these materials produced efficiencies of

5.35% (WC) and 5.7% (Mo2C)71

. Jang et al. assessed polymer assisted- and microwave

24

assisted-formation of tungsten carbide nanoparticles with diameters of 200 and 30nm

respectively. The smaller particles exhibited higher surface areas and a corresponding higher

efficiency of 7.01%, approximately 85% that of the platinum reference72

.

For nitrides, Li et al. used the nitridation of metal oxide precursors to produce

molybdenum nitride (MoN), tungsten nitride (WN), and iron nitride (Fe2N). While there were

differences in the morphologies of the particles, the thicknesses of each electrode was 13

microns. The efficiencies of the cells were 5.67% (MoN), 3.67% (WN) and 2.65% (Fe2N), as

such the closest to platinum (6.56%) was the MoN electrode73

. Vandium Nitride was

investigated by Wu et al. A variety of morphologies were fabricated by modifying the

synthesis parameters revealing an increase of efficiency with surface area. This resulted in an

efficiency of 7.29% compared to 7.68% for platinum in the tri-iodide couple while exhibiting

better performances in a thiolate/disulphide electrolyte74

.

To improve access of the electrolyte to the material, Song et al. compared MoN

nanorods and particles. The one-dimensional rods resulted in a more porous electrode

allowing for better diffusion of the electrolyte which was observed as a decrease in diffusion

resistance in the EIS spectra. The MoN particles displayed an efficiency of 6.48%. The

enhanced diffusion properties resulted in an efficiency of 7.29% for the MoN nanorods

compared to 7.42% efficiency for the platinum reference75

. Titanium Nitride (TiN) nanotube

arrays on a titanium foil current collector have also been demonstrated as a counter electrode

by Jiang et al. The electrode exceeded that of the platinum reference cell (7.45%) with an

efficiency of 7.73%76

.

Transition Metal Oxides have also been demonstrated as potential catalysts for

DSSCs. Various niobium oxides were synthesised and tested by Lin et al. Three crystalline

structures of Nb2O5 and one of NbO2 were analysed. The most promising candidate was the

NbO2 displaying an efficiency of 7.88% greater than the 7.65% demonstrated by the platinum

based-cell77

. Ruthenium Oxide nanocrystals synthesized by a hydrothermal method by Hou et

al. also displayed an efficiency (7.22%), greater than that of a platinum cell (7.17%)78

. A

broad study by Hou et al. based on DFT calculations of surface energies identified certain

materials within an ideal range of adsorption energies. Iron Oxide (α-Fe2O3) was one of the

candidates that displayed good values and as such was synthesised and tested. The counter

electrode material was screen printed and produced an efficiency of 6.96% comparable to the

7.32% efficiency of platinum79

.

25

One dimensional tungsten oxide (WO2) nanorods of diameter 20nm were synthesised

by Wu et al. and compared to WO3 particles ranging in size from 0.05 to 2 microns. In

addition to the higher surface area of the nanorods, the redox peaks as observed by cyclic

voltammetry of the nanorods were more similar to platinum than the particles indicating

more desirable electrochemical properties. As such, the nanorods outperformed the particles

with an efficiency of 7.25% to 4.67% compared with a platinum counter electrode with


.

Wu et al. produced a study of a wide variety of transition metal carbides, nitrides and

oxides of the transition metals titanium, zirconium, vanadium, niobium, chromium and

molybdenum. All materials were synthesised by a reaction of urea, an oxygen source, carbon

and nitrogen, and a metal chloride precursor. The efficiencies were compiled in a graph

(Figure 2.6) by the author. According to this study, vanadium based-compounds look to be

the best candidates for counter electrode materials81

.

Figure 2.6 Power Conversion efficiencies for various counter electrode materials.81

Transition metal chalcogendides (the chalcogen being sulphur, selenium and tellurium

in these cases) have found applications as counter electrode materials for DSSCs. Cobalt

sulphide, synthesised to have a honeycomb structure to maximise surface area, produced by

Lin et al. resulted in an efficiency of 6.01%, in excess of the 5.71% efficiency of the platinum

reference82

. Gong et al. produced micron size nickel selenide particles for use as counter

electrode material which also outperformed platinum with an efficiency of 8.69% to 8.04%83

.

26

A variety of molybdenum, tungsten and tantalum diselenide were synthesised and

tested by Guo et al. MoSe2 had the highest surface area and yielded an efficiency of 6.71%

while WSe2 had a marginally lower surface area but a superior efficiency of 7.48%.The

platinum reference in this work had an efficiency of 7.91%84

.

Further into the chalcogenides, Guo et al. also studied telluride-based, transition metal

compounds. In this case, cobalt and nickel telluride exhibited efficiencies of 6.92% and

7.21% respectively while the platinum cell reported an efficiency of 7.04%.

Many of these materials, especially the transition metal dichalcogenides (TMDs),

have 2 dimensional configurations leading to high theoretical surface areas. This should make

these materials ideal candidates for counter electrode materials. Cobalt sulphide synthesised

in 2 dimensional nanosheets by Tai et al. outperformed platinum by 6.39% to 6.06%. In

addition, the material was also highly transparent over the visible spectrum70

. MoS2 and WS2

films were tested as counter electrode materials by Wu et al. The performance of the

materials was similar with MoS2 producing an efficiency of 7.59% and WS2 producing an

efficiency of 7.73%, which was approximately that of the platinum cell with 7.64%

efficiency85

.

Tin sulphide (SnS) and tin disulphide (SnS2) nanosheets were the subject of a study

by Chen et al. The SnS composition proved to be the better of the two with an efficiency of

6.56% to 5.14%. One dimensional SnS nanowires were also tested and achieved a lower

efficiency of 5.00% providing evidence that the 2 dimensional configuration was superior86

.

This however is not always the case, a study by Lei et al. on monolayered, few-layered and

nanoparticulate MoS2 revealed that the nanoparticles exhibited superior performance under

illumination with the efficiency of the nanoparticles being 5.41% and the monolayered flakes

being 2.92%. This was explained by an increase in the diffusion impedance suggestive of

possible reaggregation preventing sufficient access of the electrolyte to the catalytic sites87

.

2.7.4 Composite and Hybrid Electrodes

The counter electrode materials discussed so far have been largely of a singular material.

However, there are strategies to enhance electro-catalytic activity by forming hybrids and

composites of two materials. For the purposes of this work, hybrids refers to materials

27

synthesised simultaneously, that form single structure and composites refers to the mixture of

already synthesised or produced materials. In these cases, one material usually provides

electro-catalytic activity while the other provides at least one other property such as

conductivity or porosity to the film.

Due to the diverse properties of the various allotropes and structures of carbon,

studies have been conducted into the possible synergies between alternate carbon materials.

Activated carbon has been demonstrated as a catalytic material for DSSCs due to its high

surface area. Graphene, while working as a catalyst at edge sites, has greater conductivity

enabling better charge transport to the current collector. Wu et al. produced activated

carbon/graphene composites using electrophoretic deposition on FTO. The cell using

activated carbon displayed an efficiency of only 6.66%. The activated carbon/graphene

composite formed by electrophoretic deposition achieved vertically aligned sheets in the

electrode resulted in an efficiency of 7.50%. However, an electrode formed by spin-coating

resulted in more horizontally orientated graphene sheets which led to a reduced efficiency of

5.99% due to less direct conductive paths and increased difficulty of diffusion88

.

Reduced graphene oxide (rGO) has been shown to be a sufficiently catalytic material.

However, due to the tendency of planar sheets to stack in solid films reducing surface area

and the anisotropy of conductivity, significantly lower in the out-of-plane direction, a

material that could increase porosity and conductivity would be an advantage. Carbon

nanotubes (CNTs) provide such an opportunity. Zhu et al. used electrophoretic deposition to

produce such counter electrodes containing various ratios of material. The best observed

efficiency (6.17%) was observed at 60%wt CNTs compared to a platinum cell with an

efficiency of 7.88%. Zheng et al. produced similar counter electrodes by gel coating and

achieved a higher efficiency than platinum (7.79%) with 20%wt CNTs (8.37%)89

. Battumur

et al. combined pristine graphene nanosheets with multi-walled carbon nanotubes to improve

the efficiency of the graphene-only cell achieving 4% compared to 5% for the platinum

based-cell90

.

Conductive polymers have also been used as both a conductive additive and a

provider of electro-catalytic sites in carbon material-based counter electrodes. Carbon

nanotubes were used to provide high surface areas and enhance the conductivity of PEDOT

with the hybrid material producing an efficiency of 4.62%, higher than that of either material

individually91

. Graphene was used to enhance the conductivity of a transparent PEDOT:PSS

28

film increasing the efficiency from 2.3% to 4.5% compared to the platinum reference

(6.3%)92

. PEDOT was also deposited on a pressed graphite substrate by Nagarajan et al. This

was compared to PEDOT on an FTO substrate to reveal that the effect of the graphite was

also providing catalytic activity on top of the role of a current collector. The efficiency of the

composite electrode was 5.78%, in excess of both the PEDOT on FTO counter electrode and

the platinum reference electrode93

. The performance of a polypyrrole counter electrode was

enhanced by the presence of 10%wt graphene quantum dots resulting in an efficiency of

5.27% compared to the platinum counter electrode of 6.02%94

.

A wide range of inorganic materials have demonstrated catalytic activity in DSSCs

but many of these particles are either semiconducting or insulating. Particles not in direct

contact with the current collector are less efficient due to poor charge transport. Conductive

additives, such as carbon nanotubes, graphene and conductive polymers, have the potential to

improve such counter electrodes.

Conductive carbon paste was used to provide conductivity to a wide range of

materials by Gao et al. The most promising of these is cadmium, in which the composite

counter electrode gave an efficiency of 6.71%, comparable with the platinum cell with an


. Wu et al. produced a range of inorganic materials as mentioned

previously. The most promising material, vanadium carbide, was added to mesoporous

carbon further improving the electrical and surface properties of the counter electrode81

.

MoS2 was directly synthesised on reactive sites of functionalised carbon nanotubes by

Yue et al. to improve upon the surface area and conductivity of a MoS2 counter electrode

(with initial efficiency of 5.42%). The hybrid counter electrode exhibited an efficiency of

7.92% greater than that of the platinum cell of 7.11%96

. MoN was produced on carbon

nanotubes using a similar method by Song et al. resulting in an efficiency of 6.74%

comparable to the platinum counter electrode (7.35%)97

.

Guo et al. used CNTs as a porous foundation for the growth of platinum

nanoparticles. The purpose of this counter electrode is not to replace platinum but to improve

upon the standard pyrolysis platinum counter electrode. The hybrid counter electrode resulted

in an efficiency of 7.69%, greater than that of the platinum only-electrode with an efficiency

of 6.31%98

.

29

The bulk of the graphene hybrids are created by a similar method to the CNT hybrids

using functional group sites to promote anchoring for the synthesised inorganic material. As

such, most of the work is done using graphene oxide and reduced graphene oxide. This has

also been done with MoS299,100

, Nickel Phosphide101

, Nickel Oxide102

and NiS2103

. The graph

below (Figure 2.7) illustrates the various efficiencies of the counter electrodes compared to

their platinum counterparts.

4 6 8

3

4

5

6

7

8

9

MoS2

NiP

NiO

NiS2% E

ffic

ien

cy C

E (

%)

% Efficiency Platinum (%)

Figure 2.7 Efficiency of various counter electrode materials synthetically grown on graphene compared to the platinum reference. Black line represents equivalent efficiency with

platinum.

Yue et al. used commercial MoS2 and graphene flakes to create a composite which

was printed with acetylene black and PVDF to form a counter electrode. This produced a

performance of 5.98% close to that of the platinum based cell of 6.23%104

. Cobalt sulfide,

previously mentioned as a good catalytic material, was also incorporated into CVD grown-

graphene on FTO by Das et al. The graphene on FTO alone had an efficiency of 1.27% but

the addition of the CoS increased the efficiency by more than a factor of 2 to 3.42%105

.

Platinum nanoparticles were deposited on graphene films by pulsed laser deposition

by Bajpai et al. The effect of the graphene increased the efficiency of the platinum compared

to platinum deposited onto FTO directly by the same method with the composite counter

electrode displaying an efficiency of 2.91% compared to 2.11% for the platinum alone106

.

30

Sudhajar et al. combined cobalt sulphide with PEDOT:PSS by dispersing CoS

nanoparticles in an aqueous PEDOT:PSS dispersion before spin-coating. This resulted in a

counter electrode with an efficiency of 5.4% compared to 6.1% for the platinum counter

electrode107

.

Yeh et al. produced a titanium nitride/PEDOT:PSS composite film by a similar

method on titanium foil. Various weight percentages of TiN nanoparticles were tested with

the 20%wt composite providing the best performance of 6.67%, slightly higher than the

efficiency of the sputtered platinum cell at 6.57%108

.

Polypyrrole was used in a metal-PPy-carbon composite by Liu et al. Cobalt, iron and

nickel were entrapped in the polypyrrole matrix and each showed improvement in the

catalytic effect of the counter electrode. Cobalt proved to be the most suitable metal for this

purpose with an efficiency of 7.64%109

.

Many of the above methods involve counter electrodes of high thickness, high

temperature processing or complicated methods for synthesis and deposition. Platinum

counter electrodes are usually produced in the thickness range of 20-100nm, via sputtering or

pyrolysis, involving temperatures in excess of 400⁰C and come from an expensive raw

material. The ideal replacement requires an up-scalable method of production combined with

reduced cost. The simplest way to achieve this is by reducing the amount of material

necessary and lowering processing temperatures. This work will resolve to provide some

manner of approaching these criteria.

31

3

Electrochemical Capacitors

3.1 Introduction

The generation of electricity from renewable sources can be infrequent and so to provide for

the constant demand for electrical energy by society, the excess must be stored for later use.

On top of this, portable devices have become essential to modern life and require appropriate

sources of both power and energy.

The charging and discharging of electrical energy for a range of processes have

different power requirements. Currently there are four devices for storing energy each with

different power and energy storage characteristics. These devices are the fuel cell, the battery,

the electrochemical capacitor, and the dielectric capacitor. A Ragone plot as shown in fig. 3.1

shows the energy and power per unit weight of some of these devices.

32

Figure 3.18 Ragone Plot including various devices including capacitors, electrochemical capacitors and a range of batteries110.

The electrochemical capacitor also known as the supercapacitor or ultracapacitor is

the main focus of this thesis. The electrochemical capacitor uses a similar method of energy

storage to the dielectric capacitor by charging surfaces but in construction it bares more

resemblance to a battery. This is due to the presence of two electrodes in an electrolyte. As

such the electrochemical capacitor occupies the space in the Ragone chart between these two

devices. With energy storage capability in the range of 1-10 Wh/kg and power storage in the

range of 500-10000 W/kg110–113

. In addition to the high power capability of electrochemical

capacitors they can be made of environmentally benign materials. The lifetime of

electrochemical capacitors in cycles lies between that of dielectric capacitor and that of a

battery which is in the range of thousands.

33

3.2 General Characteristics

A typical dielectric capacitor has two parallel plates separated by a dielectric. The

capacitance (C) is a property of the material system and given by the formula 3.1:

Equation 3.1

Where ε is the permittivity of the material between the electrodes, A is the common area of

the electrodes, d is the distance between the electrodes, Q is the charge stored on the

electrodes and ΔV is the potential difference between the electrodes. In a dielectric capacitor

the area corresponds to the geometrical area of the plates and the distance is on the order of

microns.

The electrochemical capacitor however uses a liquid-solid interface the solid being the

electrode material and the liquid being an electrolyte instead of a metal-dielectric interface.

This phenomenon was first observed by Helmholtz in 1853. Since then the field of

Figure 3.2 Structure of the Helmholtz Double Layer model114.

34

electrochemistry has gained understanding of this interface and the structure of this interface

is shown in fig. 3.2114

.

At the surface of the solid there exist molecules of the solvent and possibly some

specifically adsorbed ions. This is considered the inner Helmholtz plane. After the inner

Helmholtz plane solvated ions form the outer Helmholtz plane, this is the distance of closest

approach for the solvated ions and is approximately equivalent to twice the diameter of the

solvent molecule. Due to thermodynamic agitation these ions extend into the liquid which is

known as the diffuse layer. The charge density (σ) due to these ions is described as a sum of

both contributions of the ions as in equation 3.2:

Equation 3.2

Where the subscripts S, i and d stand for solution, inner Helmholtz plane and diffuse layer

respectively.

The nature of this interface affects the capacitance of the electrochemical capacitor

relative to the dielectric capacitor. Firstly, by reducing the distance between the interfaces to

the order of a nanometre about a factor of 1000 less than that of the dielectric capacitor.

Design of the electrode in electrochemical capacitors can result in high surface areas leading

to more area between the interfaces than the typical geometric area of the electrode. Current

commercial electrodes have surface areas in excess of 1000m2/g

115.

However due to the presence of two electrodes in the electrolyte the electrochemical

capacitor is in practice two capacitors in series. Considering two identical electrodes this

means an electrochemical capacitor has half the capacitance of a single electrode. The

dielectric constant of the electrochemical capacitor should be that of the solvent molecule.

However, due to the high electric fields and the compact nature of the Helmholtz layer the

dielectric constant of water near charged surfaces has been in the range of 5-25 as opposed to

approximately 80 for the bulk116–119

.

The two key metrics of electrochemical capacitors are the energy and power densities.

These are commonly reported in terms of mass but have also been reported in terms of

volume and area. When taking into account device construction volume is most likely to be

the governing limitation.

35

In the charged state the voltage across an electrochemical capacitor is across the

interfacial layer of ions on either electrode. As well as considering the electrodes, there is a

potential drop associated with the resistance of the cell. This resistance known as the

equivalent series resistance (ESR) is due to the charge transport properties of the electrodes

and the electrolyte. The maximum voltage of an electrochemical capacitor is dependent on

the breakdown potential of the solvent in the electrolyte causing electrolysis. This is

analogous to the breakdown of the dielectric in conventional capacitors. The potential

window for water based electrochemical capacitors is 1.2V while common organic based

commercial electrochemical capacitors have operation voltage windows of 2.7V120

.

The energy of an electrochemical capacitor (E) is governed by the capacitance of the

electrode material (C) and the voltage window over which it operates (ΔV). The formula

describing this relationship is as follows:

Equation 3.3

The maximum possible power based on a load with the same ESR of the electrochemical

capacitor is given by the formula:

Equation 3.4

It must be noted however that the loads electrochemical capacitors work with usually exceed

this resistance. However this is a good metric for comparing devices.

3.3 Non-Faradaic Electrodes

An ideally polarisable electrode is an electrode in which no charge transfer occurs between

the electrode and the electrolyte. While many electrode materials are not entirely ideally

polarisable the bulk of the capacitance comes from the electrical double layer. These

constitute the non-faradaic electrodes.

To operate as a high performance non-faradaic electrode in a super capacitor the

material needs to be reasonably conductive, have a high surface area, good chemical and

36

thermal stability, and cost effective. Various carbon structures have been identified as having

these desirable traits with activated carbons being popular in commercial devices115,121

.

Activated carbons are porous carbon architectures with many methods of fabrication

but essentially involving the combustion of organic compounds. Good activated carbons for

non-faradaic electrodes will have a combination of mesoporous (2-50nm) and microporous

(<2nm) pores115

. The micropores allow the best possible gravimetric capacitance of the

material, up to 15-20µF/cm2 for carbon

122, to be utilized using high surface areas of activated

carbons up to 2500m2/g

115,123,124. While the mesopores ensure the transport of the electrolyte

throughout the material. The micropores should be optimized to accommodate the ions and

solvent molecules of the electrolyte. If the pore is too small to accommodate the ion and the

shell of solvent molecules an increase in resistance occurs due to the energy required for the

removal of solvent molecules. A patent by Okamura et al. tuned the pore size of the positive

and negative electrodes accounting for the difference in cation and anion sizes to produce

enhanced capacitance at low internal resistance125

. Activated carbons have capacitances of

around 100-200 F/g in aqueous electrolytes115,122,123,126,127

.

The random array of micro and mesopores in activated carbons may not lead to the

optimum arrangement of the pores to facilitate charge transport and capacitance. Templating

the growth of carbon to can be achieved using nanostructures as the template and then

etching the template material once the carbon is grown. The mesopores created by the

template provide access to the micropores present in the carbon127–129

.

Conductive low dimensional allotropes of carbon, namely graphene and carbon

nanotubes, have excellent conductivity and high surface areas130

. Films of carbon nanotubes

have a mesoporous structure due to the disordered morphology of the film making them an

excellent material for non-faradaic electrodes131–135

with the possibility for not requiring a

current collector reducing the equivalent series resistance that occurs from the boundary

between a current collector and the active material. They also have excellent mechanical

properties allowing the fabrication of flexible devices136,137

. Capacitances of carbon

nanotubes have reached as high as 180F/g138

.

While graphene possesses superior theoretical surface areas and conductivity to

carbon nanotubes the tendency to restack in films reduces the surface area and prevents the

access of electrolyte. Thin films display capacitances in excess of 100F/g139,140

and thicker

37

films get similar capacitive performance by employing strategies to prevent re-aggregation of

the sheets maximizing the surfaces areas141–146

.

Due to the excellent conductive properties of these carbon nanotubes relative to

activated carbon it is common to see composite materials benefiting from the excellent

porosity and capacitance of the activated carbon. These composites exhibit lower equivalent

series resistance and as a result better power density147–149

.

Graphene and carbon nanotubes have also been used in concert. While graphene has

excellent conductivity it is highly anisotropic with reduced conductivity in the direction

perpendicular to the sheet. In addition the carbon nanotubes prevent stacking of the graphene

sheets allowing improved electrolyte penetration. The result is improvements in capacitance

and resistance improving both energy and power densities of the electrodes124,150,151

.

3.4 Faradaic Electrodes

Carbon electrodes based on non-faradaic electrical double layer capacitance are limited to

specific capacitances of 50µF/cm2

or less152

. For higher capacities faradaic processes must be

considered. A faradic process involves charge transfer between the electrode and the

electrolyte. Batteries operate under faradaic processes where the charge exchange is limited

by solid diffusion processes152,153

. The distinction between battery-like processes and

pseudocapacitance-like processes is that the charge transfer occurs at the surface of the

material producing an electrical response similar to that of a capacitor, hence

pseudocapacitance, while in batteries the ions have to diffuse into the bulk of the material and

the surface has less of a contribution. In general faradaic electrodes are not ideally capacitive

and have a current behaviour like:

Equation 3.5

Where k1 and k2 are rate constants of the various processes and v is the scan rate154

. Semi-

infinite linear diffusion as process associated with batteries results in a current dependent on

v1/2

while the capacitive processes ungoverned by diffusion have current dependent on v.

Pseudocapacitive materials show a combination of the processes.

38

The surface based reactions for charge storage can be broken down into various

mechanisms: underpotential deposition, surface redox reactions, intercalation and reversible

electrochemical doping of conducting polymers152–154

. These processes tend to cause

morphological changes in the charge/discharge cycle causing device lifetimes to be shorter

than those of the electrical double layer based devices.

These processes occur at the surfaces and thus the capacitance can be determined

from the percentage of surface sites that have undergone said process. The percent of surface

sites reacted for these processes is determined by the thermodynamic relation152

:

Equation 2.6

Where θ is the percentage of surface sites, K is the electrochemical equilibrium constant, C is

the concentration of the species involved in the charge transfer, V is the applied potential, F is

the faraday constant, R is the gas constant and T is temperature. Pseudocapacitance, Cϕ, is

given by the charge transferred multiplied by the derivative of surface coverage with respect

to potential as is described by the following formula152

:

Equation 2.7

Noble metals like platinum, rhodium or iridium can adsorb metal atoms to the surface

accompanied by a transfer of charge. The transfer of charge follows the reaction:

Equation 2.8

Where M is the metal ion in solution, S is the surface lattice site and z is the charge of the ion

in terms of the charge of an electron. For single state processes the capacitance associated

with such a reaction for example hydrogen adsorbed on platinum has a maximum value of

approximately 2200µF/cm2. However single state processes have narrow potential windows.

A system with more states represented by multiple charging peaks such as lead adsorbed on

Gold gives capacitance over approximately 0.6V. Due to the narrow potential windows and

expensive material like platinum and gold in addition to the difficulty of reaching surface

areas to be competitive with carbon, pseudocapacitors based on this process are less attractive

than others152

.

39

Redox active sites in materials can be accessed by ions on readily available surface

sites or intercalation sites within the material. This mechanism involves charge exchange

with a redox active site with variable oxidation state and an electrolyte ion. The redox

reaction is described by the equation:

Equation 2.9

Where Ox is the oxidized state, C is the cation in the electrolyte, Red is the reduced state and

z is the charge difference between the two states. Ruthenium oxide (RuO2) was the first

material in which surface redox pseudocapacitance was thoroughly investigated155

. Hydrous

RuO2.nH2O is commonly used as the electrode material. The redox reaction of hydrous RuO2

can be expressed as:

zyzxyx OHRuOzezHOHRuO

)( Equation 2.10

In the equation H represents the cation. RuO2 has multiple redox processes involving redox

states from Ru(II) to Ru(VI)156

, this and the stability over a 1.2V window in aqueous

electrolytes make for an excellent pseudocapacitve material. The maximum theoretical

capacitance of RuO2 is 1450F/g154

. Ruthenium however is very expensive and so other

materials with pseudocapacitive properties were identified for capacitor applications.

These materials can be divided by chemical composition and also the

intrinsic/extrinsic nature of the capacitance. Materials that have intrinsic pseudocapacitance

display capacitive properties regardless of the size or morphology of the material. Materials

with extrinsic pseudocapacitance, however, require modification. Namely, reduction of size

to access the capacitive like behaviour154

.

Transition metal oxides have a range of faradaic materials suitable for replacing RuO2

(MnO2157–159

, TiO2160

, Fe3O4161

, Nb2O5162

, WO3163

and LiCoO2162,164

). Of which MnO2 is the

most extensively studied due to its abundant availability and low impact on the environment.

MnO2 has a theoretical capacitance of 1370F/g over a potential window of 0.8V

corresponding to the redox reaction between the Mn(III) and Mn(IV) redox states158

.

Other transition metal compounds have also been identified as having significant

pseudocapacitance. Transition metal layered double hydroxides (Co(OH)2165

and Ni(OH)2166

)

has also displayed pseudocapacitance. Sulfides of transition metals like MoS2167

and CoS2168

40

are also pseudocapacitive. Nitrides such as vanadium nitride169

and carbides in the two

dimensional MXene variety (Ti3C2170

) have been reported as pseudocapacitive also.

Hetero atoms namely oxygen and nitrogen in normally non-faradaic carbon materials

are also potential redox active sites. In addition to adding pseudocapacitance these

heteroatoms can improve the wettability of the electrode materials171–173

.

Conducting polymers that undergo redox reactions which change the doping state of

the polymer chain have also been used in electrochemical capacitors111,174

. This class of

materials include PANI175

, PPy176

and PEDOT177

as the most common examples. These

materials provide good conductivity and flexibility on top of their pseudocapacitive

properties.

Many pseudocapacitve materials have low conductivity which has a severe impact on

the power of the device due to increasing the equivalent series resistance. In order to counter

this issue composite electrodes are used to make functional electrodes. Many electrodes are

constructed from pastes of the active material, carbon black (for conductivity) and PVDF (as

a binder)157,158,164,178

. Other conductive materials used for composite materials include carbon

nanotubes179–181

, graphene182–184

and conductive polymers185–187

. This approach is particularly

important as the film thickness increases as many of the high capacitance values are reported

for low-mass and thickness films on a current collector. Lee et al. produced 20 micron thick

films of only MnO2 resulting in a capacitance of 0.13 F/g and addition of carbon black

increased this by three orders of magnitude157

. While analysing ultra-thin films is good for

characterising a material it does not facilitate device applications.

On top of the series resistance faradaic materials exhibit a charge transfer resistance

due to the crossing of ions through the electrical double layer and interacting with the surface.

This manifests itself as an additional resistance component in the circuit in such a way that

faradaic materials generally have worse power densities than non-faradaic materials.

In addition the design of electrodes made from these pseudocapacitve materials must

take into account the requirement of high surface areas to maximize the area accessible by

electrolyte.

3.5 Electrolytes

41

The electrolyte contains the electrolyte ions and a solvent. As discussed earlier the potential

window of liquid electrolytes is limited by the electrolysis of the solvent. The maximum

potential windows are 1.2V for water, 2.5-2.8 for organic electrolytes and 3.4-3.7 for ionic

liquids120

. With commercial electrochemical capacitor manufacturers favouring organic

electrolytes. The maximum potential window is not the only important property of an

electrolyte however.

Figure 3.10 shows the many facets in which the electrolyte affects the performance of

a electrochemical capacitor. Ion properties such as conductivity, size and electrode material

interaction are important as are the solvent properties of viscosity and salt solubility. An

extensive review on this topic has been produced by Zhong et al120

.

Figure 3.9 Relationship between electrolye properties and electrochemical capacitor properties taken from reference120.

Solid and quasi-solid electrolytes offer the convenience of the absence of liquid. This

abolishes the possibility of leakage facilitating the integration of electrochemical capacitors

into wearable and flexible technologies. Most of the research in this field involves polymer

gels188,189

.

42

3.6 Device Design

Essentially all devices have two electrodes on current collectors, with a separator between

them and filled with an electrolyte as seen in figure 2.4. Due to the wide range of applications

for electrochemical capacitors there is no one size fits all style of device. Requirements for

different energy and power characteristics as well as physical properties like size, flexibility

and transparency govern the design of devices.

The thickness of the active electrode materials depends on whether the device is

designed for high power (around 10 microns) or high energy (around 200 microns)190

as

energy will increase with more capacitive material while the increase in thickness results in

charge transport issues. While most research focusing on material characterisation focuses on

the capacitance of the electrode material the overall energy and power densities with respect

to both mass and volume are affected by the other components of the cell. Minimizing the

contributions of these components is helpful to realizing effective devices.

Figure 3.10 Schematic of a electrochemical capacitor

Current collectors in commercial devices are usually metal foils such as aluminium or

copper in the range of 20 to 200 microns190–192

. These materials contribute little to the

capacitance of the device and can have thicknesses comparable to the active materials

especially in pseudocapacitve electrodes with thickness limited by conductivity. There is

also a transfer resistance associated with the current collector active material boundary192–194

.

An ideal solution would be to have the electrode material work as its own current collector.

43

The advantages are double, as it removes the contact resistance and increases the percentage

volume of active material in the device. This is often the case for ultra-thin film

electrochemical capacitors based on carbon nanotubes and conductive polymers139,195–197

.

Quintero et al. used carbon nanotubes as the current collector in a 200 micron thick

composite electrode with activated carbon198

.

Separator materials tend to be porous with low resistance to ion diffusion. The contribution of

separators to the equivalent series resistance is proportional to the separator thickness. To

reduce cost and volume and to improve power capability the separator should be as thin as

possible, however the minimum thickness is limited by the possibility of shorting due to free

electrode particles creating contact between electrodes. The materials for separators include

cellulose, glass fibre and various polymers. The thickness range in commercial

electrochemical capacitors is between 15 and 50 microns199

.

While sandwich style electrochemical capacitors have been fabricated, roll type

electrochemical capacitors are the industry norm. The roll type requires mechanical stability

and flexibility in all internal materials. To facilitate this there has been much focus of flexible

electrode materials. The electrical double layer materials, carbon nanotubes and graphene

were combined to make a flexible electrode with a capacitance of 0.1 mF/cm2 by Lu et al

200.

The addition of pseudocapacitive materials have been used to increase performance. Flexible

films using a carbon material and nanostructured MnO2201,202

have been demonstrated with

capacitances per unit area in excess of 1 F/cm2 up to 3.2 F/cm

2. A reduced graphene oxide/

Polyaniline composite electrode fabricated by Wu et al.203

reached a capacitance of 160

F/cm3.

For flexible devices a solid state and flexible electrolyte in conjunction with flexible

electrode materials is required. Carbon nanotubes on paper using an ionic liquid gel

electrolyte by Kang et al.204

and an activated carbon cloth with a polymer gel electrolyte by

Wang et al.205

produced flexible solid state capacitors with capacitances in the order of

10mF/cm2. Le et al.

206 used carbon nanotubes wrapped in a carbon fibre paper to create fibre-

like electrochemical capacitors with 86.8F/cm2. Better non-faradaic based devices using

polymer gel electrolytes include a cotton paper embedded with carbon nanotubes by Hu et al.

creating a 1.08 F device207

and a graphene hydrogel electrode by Xu et al.147

with a

capacitance of 402 mF/cm2.

44

Faradaic materials have also been integrated into all solid state electrochemical

capacitors. Peng et al. constructed an all solid state device using a graphene MnO2 hybrid

film of 40 nm thickness with a polymer gel electrolyte208

. The capacitance based on active

materials was 267 F/g. However low mass loading due to the low thickness of the films limits

device applications. Using carbon fibre as a current collector and mechanical support for

carbon nanoparticles and MnO2 and a polymer gel electrolyte Yuan et al.201

produced a

device with a capacitance of 109mF/cm2. Meng et al.

209 using carbon nanotubes and

polyaniline produced individual 30 micron electrodes of 0.8 F/cm2 incorporated into a device

with polymer gel electrolyte the capacitance was 37 F/cm3 taking into account all device

components.

Transparency is another ideal pursued by portable device designers. This requires

transparent energy and power storage also. King et al discussed the effect of percolation

theory on the production of transparent electrodes with carbon nanotubes195

. This resulted in

a capacitance of around 0.1 mF/cm2 at 90 % transmittance. Many transparent devices have

been fabricated based on carbon materials from carbon nanotubes, graphene and other carbon

materials136,197,210

. These devices range in transparencies of 50 to 70 percent and capacitances

from 0.1 to 1 mF/cm2. Liu et al.

70 used cobalt oxide to from a pseudo capacitive device with

51% transmittance at 550nm wavelength with a capacitance of 6.03 mF/cm2. Higgins and

Coleman produced thin film doped PEDOT:PSS to get capacitances as high as 1 mF/cm2 with

transparencies of 80% for a single electrode196

. In addition, these thin films have high

resistances and thus the dimensions of the device affect the capacitance per unit area.

Comparison of these devices however need to carefully done as internal resistances and

difference in voltage windows affect the energy and power densities.

A popular trend in electrochemical capacitor research has been to combine electrical

double layer electrodes with pseudocapacitor or even battery type electrodes in asymmetric

devices seeking to benefit from the high energy density of the faradaic electrode and the high

power density of the non-faradaic electrode112

. Due to the total capacitance of the electrodes

in series being 1/C = 1/Cf +1/Cnf the upper limit of the capacitance is determined by the lower

capacitance. While the increase in capacitance of the device will not reflect the superior

capacitance of the pseudocapacitance the two materials can have different ranges of working

potentials the device can have a larger potential window than the standard 1.2V associated

with aqueous symmetric devices211–213

. This has a significant impact on energy and power

densities.

45

For the production of commercial materials for electrochemical capacitors, methods

need to be compatible with scalable industrial processes. Doctor blading of pastes onto

substrates is a standard method for producing commercial devices199

. For scalable processes,

the prepared materials must be compatible with paste formation for doctor blading, screen

printing or drop casting. Inkjet214

printing or spray-coating215,216

are excellent methods for

producing thin film electrodes for transparent applications.

In this work PEDOT:PSS will be used to fabricate a range of charge storage devices

with a particular focus on the effect the electrode dimensions have on electrical and physical

transport properties.

46

4 Methods

In this thesis I fabricate a variety of DSSC counter electrodes and supercapacitor electrodes.

The aim of the work on DSSCs was to produce a counter electrode with comparable

performance to platinum with cheaper materials. For the supercapacitor electrodes the aim

was to analyse how the electrical properties associated with the materials used as well as

device dimensions impacted on electrode performance with a view to streamlining device

design.

4.1 Sample Preparation

In order to produce films for use in DSSCs and supercapacitiors raw materials need to be

processed in such a way that mechanically stable films can be reliably produced on a

substrate. The following section deals with methods pertaining to the formation of these

films.

4.1.1 Liquid Phase Exfoliation by Ultrasonication

With solid starting materials either naturally occurring or synthesized, the material is unlikely

to be in a state where the material will have the desired properties required for a device. For

examples, graphite consists of layers of graphene which has superior electrical properties and

carbon nanotubes tend to form bundles217

. Transition metal Dichalcogenides undergo band

structure changes with increasing degree of exfoliation218

.

To separate aggregated materials the attractive force between particles (usually Van

der Waals) must be overcome. The application of sonic energy in excess of 20kHz to liquid

solutions can be used to both separate these aggregated materials and disperse the particles in

a liquid. The process of dispersing materials in a liquid environment is referred to as liquid

phase exfoliation (LPE).

47

The device that provides this energy is commonly known as a sonicator. The sonicator

has an electronic signal generator that drives a piezoelectric converter to provide the

longitudinal mechanical vibrations of the probe. These vibrations can be on the order of 100

microns allowing high power densities of 100W/cm2 to be created.

When applied to powdered material in an appropriate solvent the vibration of the

probe creates waves in the liquid which result in the formation of cavitation bubbles.

Collapse of these cavitation bubbles result in high local temperatures and pressures resulting

in the formation of jets. These jets can drive aggregate particles apart themselves as well as

causing particle collisions which also assist disaggregation.

The use of sonication has been used to produce dispersion of carbon nanotubes, graphene

and other layered materials217,219–222

. This process does not produce monodisperse dispersions

of isolated nanoparticles. Usually a range of bundle thicknesses (for nanotubes) or layers for

two-dimensional stacked materials are observed. In addition differing degree of exfoliation

provided by sonication the particles dimensions are also affected by the process. Due to

‘sonication induced scission’ the covalent bonds of the material being sonicated can also be

broken221,223

. The average size of the particles decays as the inverse square root of sonication

time. This creates extra edge sites in the nanomaterial but otherwise does not cause the

material properties to change as basal plane defects would217

. Depending on the application,

the time of sonication can be adjusted to provide particles of suitable size distribution.

48

Figure 4.11 UP200S sonicator as used in the work in this thesis

Recently an alternative to sonication has been developed. Shear mixing has been

shown to generate sufficient shear forces to cause exfoliation of layered materials. The main

advantage of this is that it can be scaled up to larger volumes allowing a higher

throughput224,225

.

4.1.2 Stabilisation

Once the particles have dispersed by sonication or an alternative method they will re-

aggregate and sediment out of dispersion if they are not stabilized. One method of stabilizing

a dispersion is to choose an appropriate solvent. N-Methyl-2-pyrrolidone has been identified

as an excellent solvent for a wide range of materials217,221,226

due to its good matching of the

surface energy with the nanoparticles. The main disadvantage of the solvents which are

particularly good for stabilizing these nanoparticles tend to be toxic and have high boiling

points. This raises health and safety issues as well as difficulties in processing.

Analysis of the surface energies of solvents using Hansen and Hildebrand parameters

have been carried out in an effort to identify alternative low boiling point solvents. Isopropyl

Alcohol (IPA) with a low boiling point of 83⁰C has been used to disperse two dimensional

materials222,227

.

49

Water is cheap, non-toxic and has a relatively low boiling point. However water is

entirely unsuitable as a solvent due to poor surface energy matching. To produce stabilized

dispersions in water additives must be used to prevent re-aggregation of the nanoparticles.

Surfactants have been used to stabilize dispersions.

Surfactants can be split into ionic and non-ionic surfactants. Ionic surfactants, such as

sodium dodecyl sulphate (SDS) and sodium cholate (SC), bond to the surface of the

nanoparticle with a hydrophobic group and create a charged surface using the ionic tail

group. This charge surface acts as a repulsive layer between dispersed particles. The non-

ionic surfactants such as Triton X bind to the nanoparticles but rather than a charged tail the

steric hindrance of the tail groups prevent re-aggregation228

. Polymers have also been

reported to show this behaviour229

. Polyvinyl alcohol (PVA) in water has been used as a

stabilizing agent230

. As part of this thesis Ethyl Cellulose (EC) dissolved in IPA has been

used to form high concentration dispersions.

The use of additives raises challenges of their own however. The presence of these

surfactants can adversely affect the properties of the film and as such the use of surfactants

has to be administered with caution and may require further treatment steps to remove excess

material.

4.1.3 Centrifugation

Once the sonication process is completed there is a wide range of particle sizes present in the

dispersion. While the larger material in the dispersion will sediment out over time this

process is slow and does not offer a large degree of control over the particle size in the

dispersion. Centrifugation provides a way of removing unnecessary particles from a

dispersion quickly.

Centrifugation involves rotating the liquid sample about an axis at high speeds. The

centripetal force acts as an analogue to gravity with a much higher magnitude causing the

particles of larger sizes to sediment out at higher rates. Multiple steps can be used to isolate

particles in different size ranges231–233

. This is useful due to the suitability of different size

nanosheets to different applications. For example: Long lengths correspond to high aspect

ratios which are desirable for mechanical reinforcement applications and low lengths

correspond to an increased number of edge sites which can be of use in electrochemical

applications.

50

Determination of the size of particles can be done using atomic force microscopy

(AFM) or transmission electron microscopy (TEM). This requires gathering statistics over a

large number of flakes to provide accurate information. Recently use of various spectroscopic

methods have allowed the characterisation of flake dimensions of dispersions allowing for

quicker characterisation232,234

.

4.1.4 Film Formation

Many methods of preparing thin films were used over the course of this thesis. The

production of thin films from dispersion is vital to realising the applications of these

materials. The films used in this thesis range from less than 100 nanometres to in excess of

100 microns. The thickness of the film is an important property which is affected by charge

and mass transport properties in electrochemical applications. The use of liquid dispersion

allows the mixing of material systems provided the solvents are compatible to realise

composite films with a range of compositions. To produce these thin films in a wide range of

thicknesses multiple methods of production have been used.

Vacuum filtration allows the fabrication of uniform thin films by passing the

dispersion through a membrane with a pore size smaller than the size of the particles desired

in the film. The dispersion is pulled through the film using a negative pressure created by a

pump. The local pressure on the membrane is affected by the amount of material deposited

over a certain area allowing for film uniformity over the filtration area.

Figure 4.2 Vacuum Filtration Setup

51

By filtering various masses of dispersed particles films of thicknesses as low as 200nm up to

several microns can be fabricated, knowledge of dispersion concentration allows accurate

determination of film mass per unit area and thickness (provided the density of the material is

known). The films must then be transferred to a desired substrate for application and

measurement. This is an extra processing step. Also the membranes cannot be reused. These

issues, and the fact that the deposition area is limited to that of the filtration set up, limit the

industrialisation of this process. As such other methods of film formation were explored in

this thesis.

Drop-casting is the simplest method for creating films. It involves dropping the

dispersion on a substrate. The solvent can then be evaporated off under a variety of

conditions. Slow evaporation leads to reasonably uniform films but has the drawback of the

“coffee-ring” effect. This causes material to be deposited at the edges of the film. This

method is commonly used for the fabrication of freestanding polymer films. Due to the effect

of ambient conditions on dropcasting it lacks the reproducibility of other methods.

Spin-coating is a technique for creating uniform thin films on a substrate. The

dispersion is placed in the centre of the substrate while the substrate is stationary or rotating

at a low rate. The rate of rotation is then increased which allows the material to spread

causing a thin layer of the dispersion to coat the surface which can then quickly evaporate.

The thickness of the film can be altered by increasing the concertation of the dispersion (to

produce thicker films) and increasing rotation rate (to produce thinner films). One problem

arising from this method is that material can be lost by being flung from the substrate at high

speeds.

Spray-coating is a scalable method for producing films. It involves using an airbrush

gun and a driver gas to aerosolise the dispersion and drive it towards the substrate235

. A

needle controls the amount of dispersion to be aerosolised which can be used to adjust the

rate of deposition. The substrate is heated to a suitable temperature such that on contact the

droplets of dispersion evaporate quickly. This prevents the coffee stain effect and using a

robot to raster the airbrush gun across the substrate allows for excellent uniformity. The area

of the film is determined by the stage area which is easily scaled up which translates into

facile industrialisation.

52

Figure 4.3 Spray-coating onto substrate adapted from source235

Other methods of film deposition used in this work include doctor blading and screen

printing. These methods require a more viscous material to produce films. To increase

viscosity the concentration of the material can be increased as done in Chapter 8. While for

producing pastes of the required viscosity for screen-printing titanium dioxide nanoparticles

an additive ethyl-cellulose (EC), which can later be burned off is added13

.

Doctor blading involves the use of a blade to drag a paste across a substrate after

which excess solvent is evaporated. For supercapacitors the paste is commonly formed using

a slurry formed by mixing the active material with a conductive additive and a binding agent.

Doctor blading allows film formation of various thickness by changing the distance between

the blade and the substrate. While handheld doctor blade apparatus are common for

fabricating devices in the lab it is possible to scale up doctor blading for industrial processes.

Screen printing is similar to doctor blading in that a paste it pulled across a substrate

to produce a thin film. However in this case the material is dragged across the surface of a

screen with a mesh to allow the material to pass through. The thickness of the film is

determined by the thickness of the screen and the thickness and density of the fibres that

make up the mesh. An advantage of screen printing is that patterns with details in the range of

50 microns are possible and multiple layers can be printed allowing printable electronics to

be realised.

53

4.2 Sample Characterisation

Once the dispersions have been stabilised or the films been fabricated it is critical to

characterise them in order to ascertain the suitability of the material for various applications.

4.2.1 UV-Vis Specrophotometry

UV-Vis spectrophotometry uses light in a range of 200-800nm which is passed through a

sample. The intensity of the light source (I0) is then compared with the intensity at the

detector (I). Reduction in intensity is due to absorption and scattering of light by the sample.

Absorption occurs when light of energy in excess of the band gap causes the excitation of an

electron from one optical band to another. These can be associated with HOMO (highest

occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) for molecules

and the conduction and valence bands in solids. Scattering is caused by interaction of the

light with the material that causes the direction of the light to change. This is dependent on

particle size among other factors.

This method is used to characterise both films and dispersions in this work. For

characterising films the transparency was the property of interest. The transparency is the

ratio of the intensity at the detector to the intensity at the light source (T = I/I0). Transparency

is often used to characterise films for applications where high transparency is desirable.

For liquid samples such as dispersion transparency is not as useful for determining the

concentration. The property used in this case is the extinction which varies linearly with

concentrations within a certain concentration range. The relationship between the extinction

(E) and transparency is given by the following equation:

)log(TE Equation 4.1

The concentration is then related to the extinction by the Beer-Lambert law which states:

clE Equation 4.2

54

Where ε is the extinction coefficient, c is the concentration and l is the path length. For

measuring liquid samples the liquid is held in a cuvette of known path length. The

concentration can be found by filtering a known volume of dispersion. Then a dilution series

with known concentrations and extinctions can be used to calculate the extinction coefficient.

Once an extinction coefficient is known this enables quicker determination of the

concentration of subsequent samples. Of further interest the extinction can be broken into

absorption and scattering components. This can be of use to more accurately ascertain the

optical properties of the materials and has been used for determination of average particle

sizes of dispersions232,234

.

4.2.2 Profilometry

In this thesis to determine sample thicknesses contact profilometry was used. This involves

the dragging of a stylus across a surface. The stylus can move vertically using a feedback

loop in such a way that a constant force is maintained. The requirement to use force can be

destructive to some surfaces. The averaging of at least three measurements over the sample

gave the thicknesses for samples.

4.2.3 Electron Microscopy

To analyse topography of films and the dimensions of individual particles of a dispersion

optical microscopy is not sufficient. This is due to the resolution limitation which is

dependent on the wavelength of the particle probing the sample. High energy electrons have a

suitably low wavelength to probe the topology with high resolution.

Electrons are excited using either a thermal gun or a field effect gun and focused

using magnetic lenses and aperatures. Impact with the sample there are multiple interactions

that can happen. Electrons penetrate the surface and can be scattered. The incident electrons

can be scattered and can also scatter electrons in the sample. The result of removing

electrons from sample atoms causes the emission of both X-rays and Auger electrons236

.

Of primary use for determining topology are low energy secondary electrons as only

the electrons generated close to the surface can be detected. The angle of incidence of the

beam with the surface also effects the intensity of secondary electrons making them very

55

sensitive to topology. Secondary electrons are caused by the emission of valence electrons.

Back scattered electrons originate in the electron beam and tend to have higher energies than

secondary electrons. The back-scattering of electrons is dependent on the atomic number of

the sample nuclei and can be used to give compositional information. Auger electrons are

cause by the emission of an electron after another electron fills a hole in the core shells these

and X-rays can be used to give compositional information for atoms of low atomic number.

Transmitted electrons include electrons that have not interacted with the sample237

.

These pass through with no deflection from the beam path. And can be used to detect the

morphology of the sample. Scattered electrons can also be detected. Elastic scattered

electrons can be used to probe lattice properties and generate higher resolution images.

Inelastic scattered electrons can be analysed using electron energy loss spectroscopy (EELS)

to determine composition, bonding and valency of the sample. For transmission electron

microscopy the sample needs to be less than 100nm thick to allow sufficient transmission of

electrons for signal generation. In this thesis electron microscopy is done on flakes collected

on a grid. All transmission microscopy and data analysis was carried out by Andrew Harvey.

56

Figure 4.4 The incident electron beam creates multiple types of signals (A) transmitted electrons (B) Ineleastically scattered electrons (C) eleastically scattered electrons (D) back-

scattered electrons (E) secondary electrons (F) Auger electrons and (G) X-rays

4.2.4 Electrical Characterisation

To characterise the electrical properties of films such as sheet resistance and conductivity 4-

wire IV measurements are performed. This involves attaching four linear electrodes to a film.

A current is passed through the outer electrodes while the voltage drop is measured across the

two inner electrodes. The electrical characteristics of the system can then be analysed. The

advantage of the four wire setup is that it eliminates sources of error such as resistances

associated with electrode contact and the leads.

57

Figure 4.5 The 4-wire experimental setup. Current is passed from electrodes 1 to 4 and the voltage drop across electrodes 2 and 3 is measured.

The films analysed in this thesis display ohmic behaviour (V = IR). The resistance of the film

is determined and related to the sheet resistance by the length between the two inner

electrodes and the width of the film.

l

wRRs Equation 3.3

Where Rs is the sheet resistance, R is the resistance as measured from the four-wire

measurement and w and l are the width and length of the film respectively. Sheet resistance is

a measure used for films of uniform thickness and the unit is the Ohm per square (Ω/sq.).

Sheet resistance can also be used to determine the conductivity of a film as the relationship

between them is:

tRs

1 Equation 3.4

Where σ is film conductivity and t is the film thickness. Conductivity is particularly useful

when considering composites of materials with differing conductivity or very thin films

where conductivity varies with thickness195

.

58

4.2.5 Electrochemical Charaterisation

The properties of electrochemical solar cells and supercapacitors can be assessed using

various electrochemical techniques. The behaviour of electrodes in a solution under potential

can be used to assess charge and mass transport behaviours associated with chemical

reactions and processes.

To measure current and voltage characteristics of films in solution a potentiostat is

used. The potentiostat is capable of generating various voltage and current signals to probe

the properties of the film. In the simplest experimental setup two electrodes, a working

electrode and a counter electrode, can be used and the current-voltage characteristics can be

analysed. The working electrode is the film which is being examined. The counter electrode

is to complete the circuit as two electrodes are required. Common counter electrode materials

are graphite and platinum. In some cases a two electrode measurement with a symmetrical

configuration (working electrode and counter electrode are identical) can be used.

However, in the two electrode setup the potential across the cell is dependent on both

the working electrode and the counter electrode. While the behaviour of the working

electrode with us the fluctuation of potential of the counter electrode can obscure the actual

potential of the working electrode. To counter this a three electrode setup is used. The third

electrode called the reference electrode. The reference electrode is an electrode with a

electrochemical potential that is invariant in the voltage. In this thesis the silver-chloride

reference electrode is used though a range of reference electrodes exist. The silver chloride

electrode is a silver electrode coated in a porous silver chloride layer in a saturated sodium

chloride solution. The redox reaction associated with this system is:

ClsAgesAgCl )()( Equation 4.5

The Nerst equation114

can be used to determine the potential of the electrode.

Cl

aF

RTEE ln0 Equation 4.6

Where E is the electrode potential, E0 is the standard cell potential, R is the gas constant, T is

temperature and F is the faraday constant (at 25⁰C RT/F can be treated as 25.693mV) and aCl-

is the activity of the chloride ions in the solution. As the electrode is saturated with NaCl

59

solution the activity of the chloride is constant. This gives a reference electrode potential of

+0.197 vs the standard hydrogen electrode. Assuming the distance between the reference

electrode and the working electrode is sufficiently small the potential of the working

electrode is that between the working electrode and the reference.

Figure 4.6 The setup for a three electrode system. The current between the working electrode and the counter electrode is recorded and the potential difference between the

working electrode and reference electrode is recorded. Image adapted from source114

4.2.5.1 Voltammetry

Voltammetry involves the observation of the behaviour of current in electrochemical cells

over a range of potentials. In this thesis both linear sweep and cyclic voltammetry are

used. Linear Seep voltammetry uses the potentiostat to ramp the potential from one value

to another while cyclic voltammetry ramps the potential from one value to another and

back to the original potential often repeatedly. The decision to use linear sweep

voltammetry versus cyclic voltammetry is determined by the electrochemical system and

the information required from the system. If the reaction of interest is irreversible or no

more information of value can be obtained from performing cyclic voltammetry linear

sweep voltammetry may be used.

60

In this thesis linear sweeps are performed in a two electrode symmetrical setup. The

current and voltage characteristics of the symmetrical cell are described by the Tafel

equation:114

.

EnF

RTII

)ln()ln( 0 Equation 4.7

Where I is the current, n is the number of electrons exchanged in the reaction, F is the

Faraday constant, R is the gas constant, T is temperature and ΔE is overpotential. I0 is the

exchange current density which is characteristic of the electrode process related to the

rate constant and the concentrations of the electrolyte species. As such exchange current

can be used to assess the suitability of dye-sensitized solar cell electrodes. The exchange

current is achieved by extrapolating the linear region of the Tafel plot to zero

overpotential as shown in Fig 7.

Figure 4.7 Characteristic Tafel Plot with extrapolation of linear region to show exchange current

Cyclic voltammetry is used in this thesis to observe redox reaction in potential

counter electrodes for dye-sensitized solar cells and the charge storage in supercapacitor

electrodes. For a cyclic voltammetry experiments mentioned in this thesis the three

electrode setup was used. The phenomena observed by cyclic voltammetry in these cases

is the current flow associated with the formation of the electrical double layer described

in chapter 2 and peaks associated with redox reactions such as the one described in

chapter 1.

61

The electrical response due to the formation of a double layer is that of a capacitor. As

such it can be shown that the current under cyclic voltammetry can be described as:

RC

tsCI exp1 Equation 4.8

where I is the current, s is the scan rate, C is the capacitance of the double layer, t is time

and R is the resistance of the system. The current response of the double layer then looks

like Fig. 8(b).

The presence of redox active species can cause the appearance of peaks in the cyclic

voltammogram. These redox peaks appear on superimposed on the current associated

with the double-layer formation. As such if the double layer current is comparable to the

faradaic current then for current analysis the double layer current should be removed.

The current characteristics of a redox system is shown in Fig 4.9. For the purposes of this

thesis the analysis of the current was qualitative and more attention given to the voltages

at which the peaks appeared.

Figure 4.8 (a) The change of potential with time for a cyclic voltammetry cycle the slopes of the lines are the scan rate. (b) The current response due to double layer capacitance the maximum current is the product of the scan rate and the capacitance.114

The midpoint between the peaks is the standard potential of the redox couple. The distance

between the peaks is dependent on the rate constant of the reaction. A high rate constant

would correspond to a pair of peaks with a low potential difference, for the purposes of

62

testing counter electrodes for DSSC this is a suitable method for comparison of different

electrodes.

Figure 4.9 The current response due to faradic redox reactions. The peak is centred on E0 which is the standard potential of the redox couple114.

4.2.5.2 Impedance Spectroscopy

Impedance spectroscopy is a useful technique for analysing the variation of impedance with

frequency. Potentiostatic impedance spectroscopy involves oscillating a voltage at low

amplitude around a fixed voltage. The low amplitude is used in order to operate within a

linear region of the system which can then be examined using Ohm’s law. The voltage signal

looks like:

)sin(0 tVV Equation 4.9

where V is the Voltage, V0 is the amplitude of the oscillation, ω is frequency and t is time.

This produces a current response which looks like:

)sin(0 tII Equation 4.10

where I represents current. The new term, ϕ, is the phase which represent a delay in the

response of the system. An ideal resistor is completely in phase (0⁰) while a capacitor is

completely out of phase (90⁰). When Ohm’s law is applied the impedance is retrieved which

result in the impedance being expressed in terms of real and imaginary components.

63

When an impedance spectrum is recorded there are two common ways of viewing the

data. The Bode plot plots the phase and the magnitude of the impedance against frequency

while the Nyquist plot plots the imaginary component of the impedance against the real part

of the impedance.

Figure 4.10 Point in Nyquist plot represented by magnitude of impedance and phase. These quantities are plotted against frequency in a Bode plot114.

Usually analysis of impedance spectra involves using a circuit diagram to visualise

what the system looks like before fitting the data to the model to get values of the individual

components. These include resistor and capacitor components as well as components that

model ion diffusion in the electrolyte among others. Common models involve the

transmission line model for supercapacitor pores238

and physical dimensions196

and the

Randel’s circuit239

used to analyse charge transfer reactions such as the one in dye-sensitized

solar cells.

4.2.6 Solar Simulation

The test of solar cells requires the use of light to cause the cell to produce a voltage. For

experimental purposes it is unreliable and impractical to rely on the sun to provide the light

source. However since the light under which these cell will be exposed is the sun’s light a

simulation of the spectrum of the sun is necessary for testing.

Solar simulators provide a light supply approximating that of the sun. They employ a

light source which is commonly a zenon lamp but LED slight sources are also available. An

air mass filter is also used to simulate the absorption of light by the earth’s atmosphere. The

output of the lamp and filter of the simulator used in this thesis is shown in Fig 4.11.

64

Figure 4.11 the spectral irradiance of a solar simulator from manufacturer’s website

65

5

Materials

5.1 Graphene

Graphite is a layered material consisting on hexagonal lattices of sp2 hybridised carbon

atoms each about 0.142nm apart. The interlayer spacing is 0.335nm and the layers are

adhered to each other by Van Der Vaals forces. Individual layers were used to theoretically

examine the electronic properties of graphite but were thought to be unstable240

. These layers

were given the name ‘graphene’. As such the graphene lattice was used to describe the

formation of carbon nanotubes and fullerenes.

Figure 5.1 Graphene sheet and the construction of various carbon allotropes fullerene (green) nanotube (red) and graphite (blue) adapted from reference241

66

Monolayer graphene produced by mechanical exfoliation was first observed by

Novoselov & Geim in 2004 using scotch tape to mechanically isolate few layered to mono-

layered samples for electronic measurements. The few layered flakes with a thickness of less

than 3nm could be isolated with 10µm in the lateral dimensions242

.

These few layered graphenes were able to be transferred for a free standing

measurement of electronic properties such as charge carrier density and mobility of

approximately 1013

cm-2

and 10,000cm2V

-1s

-1 corresponding to sub-micrometre ballistic

transport. Free standing is critical for ascertaining the properties of extremely thin films as

substrate interaction can have an effect. On top of the excellent electrical properties, the

mechanical properties of graphene are phenomenal, making it one of the strongest known

materials243

. It is also very strongly absorbing of light for an atomically thin sample with a

transparency of 97.3%244

. Absorbance is related to transparency by a negative logarithmic

relationship as shown in equations 4.1 and 4.2.

With these excellent properties came the desire to produce graphene in reasonable

quantities to actualise devices. Unfortunately mechanical cleavage of graphite is not

compatible with industrial scaling. As such alternative methods to produce large quantities of

defect free graphene are required. The most popular methods are silicon carbide growth,

chemical vapour deposition and liquid phase exfoliation.

Silicon carbide growth involves heating a silicon carbide crystal in an ultra-high

vacuum to evaporate silicon on the surface leaving a layer of sp2 carbon atoms.

Approximately 30% of these carbon atoms are bonded covalently to silicon atoms in lower

layers degrading the graphene-like properties. However hydrogen intercalation can separate

the carbon from the substrate245

. Domain sizes and graphene properties vary depending on the

face of the crystal used with the carbide face having domains of 200nm compared to 100nm

of the silicon face. This results in better electronic properties for the carbide face246,247

. The

advantage of using silicon carbide as a substrate is that it is widely used in the electronics

industry.

Chemical Vapour Deposition (CVD) uses hydrocarbons such as methane, acetylene or

ethylene vapours at high temperatures on a variety of substrates. The most popular substrates

are metals, in particular copper248

, though semiconducting and insulating substrates have

been examined also with limited success. Particularly for deposition on metallic substrates,

the substrate needs to be removed for testing of electronic properties. This is done by

67

adhering a PMMA stamp over the graphene and etching away the metal in an acid249

. The

graphene can then be transferred to an arbitrary substrate. Domain size for CVD deposited

sample is also critical for producing the best possible electronic properties of the film250

. As

well as requiring good electronic properties, high area films are highly desirable. Bae et al.

produced 30 inch graphene by CVD using a roll to roll method which produced excellent

sheet resistances for the transparency251

.

Liquid phase exfoliation as it is applied in this thesis is described in the previous

chapter. The production of few layer and multilayer graphene in solution is a necessary step

for applications in printed electronics and for application in composites. However the lateral

dimension of the graphene layers is not significantly greater than a micron221

. Outside of the

methods included in this thesis there are other methods to exfoliate graphite in solution.

These are the oxidation of graphite to produce hydrophilic graphite oxide which produces

stable layers in aqueous media and graphite intercalation followed by exfoliation.

The oxidation of graphite into graphite oxide (GO) is commonly done by a modified

Hummers method52,252

. The graphite oxide can then be exfoliated using sonication, stirring or

thermal expansion. The graphene layers produced in this manner tend to have larger lateral

sizes than those produced using direct liquid exfoliation of graphite. These layers are heavily

oxidised and have much lower conductivity than graphene flakes54,55

. To combat this the

graphene oxide can be reduced chemically in solution with anhydrous hydrazine which also

prevents reaggregation55

. Thermal reduction in vacuum or under inert gas of films fabricated

from a GO dispersion is also possible54,253,254

.

Exfoliation of intercalated graphite compounds is facilitated by the increased distance

between the graphite layers due to the presence of the intercalation atoms/molecules. These

tend not to be stable in air and as such do not experience the popularity that the other liquid

phase methods enjoy255

.

The production of graphene at the large scale for applications can be split between the

formation of continuous graphene sheets and graphene flakes in dispersion. There is no one-

size-fits-all for application. Continuous graphene films see application as transparent and

flexible electrodes in a wide variety of devices. Doping of large area films can produce sheet

resistances of 30 Ω/sq. at 90% transparency251

. These continuous sheets can be used as

current collectors especially where transparency is required such as polymer and dye-

sensitized solar cells.

68

With excellent physical properties graphene is suitable for the reinforcement of

materials. Polyvinyl alcohol and polyurethane, among other polymers, can be cast from

composite dispersions of polymer stabilised graphene resulting in superior physical

properties230,256

. Similarly, conductive plastic composites can be made using a graphene filler

to provide conductivity257

.

High surface areas also require flakes of smaller sizes which can be produced by

liquid exfoliation. This is useful for applications where high surface areas are required, such

as supercapacitors. As such, graphene flakes have seen widespread application in thin film

and porous electrodes for supercapacitors139,140,145

and batteries258

. Also, due to the high

conductivity of graphene, the addition of graphene to less conductive but electrochemically

active materials to form enhanced composite electrodes is a common practice92,104,105,184,208

.

As well as having high surface areas the flakes also have edges. While the basal plane of

graphene is chemically inert, it has been shown that the edges are much more reactive46,48,259

.

This opens the possibility of using graphene as a catalyst and a sensor. While planar CVD

graphene can be used as a sensor, the presence of defects enhances the adsorption of gas,

allowing production of sensors from more economically produced graphene flakes260

. The

edges of graphene flakes also provide catalytic sites viable for the reduction of complexes in

DSSC electrolytes261

.

Graphene can also be functionalised, as demonstrated in the production of graphene

oxide. This functionalisation affects the chemical properties of the material as well as

improving the wettability of the surface262

. Graphene oxide has seen application in

supercapacitors due to the pseudocapacitance associated with these oxide groups as well as in

DSSCs. Beyond oxygen functionalisation, graphene can also be functionalised with nitrogen

groups. This can be achieved by introducing ammonia after the deposition of graphene by

CVD or additional chemistry on graphene oxide263,264

. There is a multitude of processes that

can be used to alter graphene and tune the properties of graphene for a range of

applications265

5.2 Carbon Nanotubes

The presence of carbon nanotubes (CNTs) were discovered among fullerenes by Iijima266

.

Fullerenes were produced by the arc discharge of graphite in the presence of a catalyst. In

69

comparison to the fullerenes the carbon nanotubes consisted of at least two concentric tubes

of rolled graphene. The spacing between layers is that of graphite. These tubes had high

aspect ratios with diameters of 4-40nm and lengths up to 1µm. The realisation of single

walled carbon nanotubes (SWCNTs) was reported in 1993 with a range of diameters from 0.4

to 3nm267

.

These tubes formed with a range of different chiralities. The chirality of a nanotube

corresponds to the orientation of the graphitic plane relative to the axis of the tube. The

chirality of the nanotube can be armchair, zigzag, or described by a vector of the lattice. The

chirality of the tube can affect properties including diameter and electronic properties with

one third of carbon nanotubes being conducting with the rest having a semiconductor

nature268

. Separating carbon nanotubes of different chiralities remains a challenge to

researchers.

The production of carbon nanotubes from the arc discharge method produces a large amount

of carbonaceous impurities and, as such, alternative methods to produce nanotubes in an

economically viable manner have been pursued. CVD is the most prominent method for mass

producing CNTs. Similar to the production of graphene the process involves the passing of

hydrocarbon vapours over a catalytic substrate. In the case of carbon nanotube formation, the

catalyst is in the form of particles for production of nanotubes, as opposed to being a flat foil

to promote growth of graphene269

. As such, a degree of control of length and diameter is

possible. The presence of a catalyst in the formation requires removal which can be achieved

by reflux in acids though this can reduce tube length and introduce surface groups on the

tube270,271

. As such, treatment of tubes should be done with great care.

Carbon nanotubes are available commercially in powder form, in which the tubes tend to

bundle via Van der Waals forces. To prepare nanotubes for applications they must be

individualised to access their excellent properties. Liquid phase exfoliation is an excellent

method for individualising tubes and allows for solution processing. Much of the work on

liquid exfoliation of graphene was a direct follow on from the dispersion of carbon nanotubes

as the material is similar. As such, dispersions of carbon nanotubes have been reported in

solvents272

, aqueous surfactant solutions273

and polymers274

including biopolymers such as

DNA275

.

Carbon nanotubes have a wide range of applications which can be achieved once exfoliated

in a liquid. Due to the excellent mechanical properties of carbon nanotubes the use a filler to

70

reinforce polymers is a common application276

. This can be done by direct exfoliation of

polymers and nanotubes in a solvent which is then cast and evaporated to form a film. For

other polymers melt processing is required to allow the introduction of nanotubes. The

creation of conductive composites is also possible277

.

Due to the excellent electrical properties of carbon nanotubes, networks of nanotubes can be

deposited from solution through a variety of methods to form conductive thin films. A carbon

nanotube randomly deposited from solution provides a conductive path across the film. The

variety of chiralities present as well as junctions between nanotubes means that the excellent

conductivities associated with individual metallic nanotubes cannot be realised, however

these films are still sufficiently conductive to be of interest. These networks are also

mechanically stable under bending and can be applied to flexible applications. Reducing

thickness leads to transparent electrodes, which could be of great importance in transparent

and flexible displays. At high transparencies however the percolation of the carbon nanotubes

has significant bearing on the film conductivity and presents challenges in producing films

that can achieve the requirements set by industry278

. Longer nanotubes are better for

conductivity and this is a problem as sonication induced scission occurs in dispersion

formation. To avoid this, dissolving nanotubes in chorosulfonic acid demonstrated by Mirri et

al. allowed a solution of carbon nanotubes with a length of 10µm to be produced. This

resulted in a sheet resistance of 140Ω/sq. at 88%279

. This is better than films produced from

sonicated dispersions278,280,281

. Nanotube films have been used in solar cells as transparent

current collectors compatible with the flexible organic solar cells.

Due to a high surface area, corresponding to one side of a graphene sheet (1315m2/g)

282

,carbon nanotubes see applications where high surface area is important. This is particularly

important for electrochemical systems such as supercapacitors134

and DSSCs64

, among others.

In many electrochemical systems carbon nanotubes are used as a conductive filler96,97,179,181

.

Due to a lower percolation threshold than graphene, effective conductivities can be reached at

lower volume fractions of nanotubes. This allows for more active material per electrode. As

such, composite electrodes have been produced for almost every electrochemical application.

The functionalisation of nanotubes can be achieved to allow dispersion in aqueous media and

to introduce other properties. The most common functionalization is oxidation of CNTs in

acid283

. The introduction of these oxygen functionalities allows hydrogen bonding, which

helps stabilize the nanotubes in water at the expense of the nanotubes electrical properties.

71

Many other functionalities exist and can be used to produce nanotubes effective in catalysis,

supercapacitors and batteries284

.

The application of nanotubes to electronics is an exciting field as the variety of electronic

properties available from nanotubes can allow nanotube-only logic elements. However, while

these applications have been developed, the reliably producing nanotubes of the correct

chirality in both sufficient amounts and with sufficient accuracy to enable wafer scale

development remains a significant challenge to researchers.

5.3 Molybdenum Disulfide (MoS2)

The discovery of the stability of graphene led to an increased interest in the possibility of

other two dimensional materials. Transition metal dichalcogenides (TMDs) were known to

have a structure of covalently bonded MX2 planes. Where the M represents the transition

metal and the X represents the chalcogenide. The planes have the transition metal layer

sandwiched between two layers of chalcogenide. These layers can have different symmetries

namely 1T, 2H and 3R. Monolayer TMD’s only have two symmetry groups 1T and 1H

reflecting the triangular and octahedral morphologies of the layers285

.

Figure 5.2 top-down, side and individual view of 1T TMD structure (left), Top-down, side and individual view of 1H TMD structure (right). Image adapted from source285.

72

Of primary interest in this thesis is Molybdenum Disulphide (MoS2). MoS2 in the 2H

symmetry is naturally occurring which is advantageous for liquid phase exfoliation as the

starting material does not need to be synthesised. MoS2 in the bulk state is an indirect-

bandgap semiconductor of 1.29eV. As the material approaches monolayer however, the band

structure undergoes changes such that it eventually becomes a direct-bandgap semiconductor

of 1.8eV286

. This leads to the possibility of using MoS2 in optoelectronic devices.

Naturally occurring MoS2 has long been subjected to intercalation chemistry and the

discovery of graphene led to renewed interest in these intercalation compounds287

. As such,

intercalation proved to be helpful in exfoliating this material in a liquid environment. The

stabilisation of the 1T phase is also possible using this method288

. The 1T symmetry displays

a higher conductivity than the 1H symmetry and as such is of interest for devices that require

high conductivities289

. Due to difficulties processing intercalated compounds, methods to

exfoliate MoS2 similar to the way graphene has been exfoliated have been developed224,290,291

.

In fact the surface tension range of solvents that exfoliate graphene and MoS2 overlap due to

the similar surface energies of the materials290,292

.

Liquid phase exfoliation, as mentioned earlier, is ideal where large quantities of

nanostructured material is required. Due to the ability to form intercalation compounds, MoS2

has been integrated into lithium ion batteries293

and supercapacitors167

. MoS2 is not as

chemically inert as graphene (though as with graphene the chemical activity derives from the

edges) and, as such, it has seen application in electrochemical catalytic systems, such as

DSSCs85

, the hydrogen evolution reaction294

and gas sensors295

. Reinforcement of polymers

is also possible but graphene is both cheaper and a better reinforcement material291

. Due to

the band-gap being in the visible spectrum MoS2 films can be used as light detectors due to

the photoconductivity effect296,297

.

MoS2 can also be synthesized via a solvothermal method. While reports of MoS2 nanosheets

produced by this method exist, they tend to have small lateral dimensions298,299

. As such, the

sheets would be equally well described as quantum dots. Much of the synthesis of MoS2

using the solvothermal route uses another material such as GO100

or TiO2300

to provide anchor

sites to promote growth.

73

For electronic applications, prototype devices can be fabricated using the scotch-tape method.

In order to provide suitable material of sufficient quantity, a synthetic method is required.

Physical and chemical vapour deposition is used to produce MoS2 from a variety of

Molybdenum precursors such as elemental molybdenum301

, (NH4)2MoS4302

and Molybdenum

Oxides296,303,304

. A traditional CVD process using vaporised MoCl5 with H2S has also

produced MoS2 on a variety of substrates305

. These materials can be used to fabricate

transistors302,303

, photo-transistors296

, photovoltaics306

and gas sensors301

.

5.4 Poly(3,4-ethylenedioxythiophene), PEDOT

Conductive polymers became the focus of intense research in the 1980s after the synthesis of

polyacetylene by Shirakawa et al307

. The potential for applications of these materials was

limited by the poor environmental stability and difficulties in processing and film formation.

Poly(3,4-ethylenedioxythiophene), or PEDOT, is a polymer that displays significant

conductivity and is sufficiently stable in air to be utilised in a range of applications.

The synthesis of PEDOT is commonly done using either an electrochemical polymerisation

or a chemical polymerisation of the EDOT monomer. Electrochemical polymerisation can be

used to form films on substrates308

. The requirement for the substrate as well as the scalability

of electrochemical deposition limits the practicality of electrophoretically produced PEDOT.

As such, for producing PEDOT in sufficient volume, chemical polymerisation is the most

efficient synthetic route.

Chemical polymerisation involves introducing the EDOT monomer to an oxidizing agent.

This is commonly either an Iron (III) compound (FeCl3 or Fe(OTs)3) or Na2S2O8309,310

.

PEDOT is initially formed in an undoped state which has a bandgap of 1.5eV and strong light

absorption in the visible range. Further doping shifts the maximum absorption into the infra-

red, producing a transparent blue material at low thicknesses and opening up the possibility

for transparent electrical applications311

. The conductivity of the undoped state is also

significantly lower (<10-5

S/cm)311

while doped PEDOT has a conductivity in the range of 1-

1000S/cm312,313

. The PEDOT formed by chemical polymerisation forms short chained

polymer cations with approximately 2 positive charges per 6 monomers.

74

Figure 5.3 Structure of PEDOT and PSS

This leads to the PEDOT being insoluble in practical solvents. To circumvent this, the

monomer/oxidant solution can be applied directly to the substrate314

or the oxidant can be

applied to the substrate and then be exposed to an EDOT vapour315

.

Liquid processing allows a range of deposition methods to be used and also the material to be

mass produced for commercial use. To produce a dispersed material, poly(strenesulfonate)

(PSS) is added to the reaction mixture316

. The deprotonated PSS molecule acts as a counter

ion to which the PEDOT can adhere to. PSS is water soluble and forms globules with

PEDOT on the inside and the PSS in contact with the water. It is worth noting that the

PEDOT polymer chain is much shorter than that of the PSS polymer chain, allowing the PSS

to envelope the PEDOT. This copolymer PEDOT:PSS is stable in water, with globules of size

less than 100nm and is commercially available from Heraeus GmBH with a PSS to PEDOT

mass ratio of 2.5317

.

75

Figure 5.4 PEDOT in a PSS chain as formed and stable in water (top) PEDOT is depicted as thicker lines while the PSS is longer and thinner, after secondary doping the PEDOT is

liberated from the PSS globule allowing for improved conductivity (bottom) image adapted from source313.

The resultant PEDOT:PSS films have low conductivities due to the presence of the insulating

PSS and the tendency for the PEDOT chains to be enclosed in the PSS chains. The

conductivity is usually lower than 1S/cm which is not suitable for many applications. To

further improve the electrical conductivity of these films, doping these films is attempted.

Since the PEDOT in these films is already in a doped state, this process is referred to as

secondary doping.

Addition of compounds to aqueous dispersions of PEDOT:PSS can be used to enhance the

conductivity of the resultant films. This is usually done by altering the aqueous medium in

such a way that the hydrophobic PEDOT can emerge from the PSS globule. Surfactants and

Ionic liquids313,318

have been used but the most effective additives tend to be high boiling

points solvents such as DMSO, DMF, THF319

, EG312

and NMP320

. These high boiling point

solvents alter the energetics of the liquid medium and as the water evaporates during film

formation the emergence of the PEDOT from the PSS becomes more energetically

favourable.

Treatment of films after deposition is an alternative method. The mechanism of these

treatments is a phase segregation of the PEDOT and the PSS.313

This can be achieved with

the high boiling point solvents mentioned previously as dispersion additives. Salt solutions

have also been used but the conductivity enhancement is modest and the migration of ions

presents an issue321

. Methanol and other alcohols have also been demonstrated as suitable

dopants of PEDOT:PSS films322

. A range of organic and inorganic acids have also been used

to enhance the conductivity323

. Repeated treatments in sulfuric acid at 160⁰C result in a

conductivity of 3065S/cm324

.

76

Due to the high transparency and conductivity of PEDOT thin films, the most obvious

application is as a transparent conductor. Formic acid treatment, as done by McCarthy et al

and used extensively in this thesis, produced a transparent conducting film with a sheet

resistance of 80Ω/sq. at a transparency of 90%, well within the requirements of industry325

.

The conductivity was approximately 900S/cm The sulfuric acid treatment mentioned

previously produced a film with 67Ω/sq. at 90% transparency324

These transparent films see application in solar cells such as polymer solar cells324

and

DSSCs67

. For DSSCs, this material can double as a current collector and a catalyst for the

electrolyte couple. In polymer solar cells, they can act as the current collector and a hole

injection layer. PEDOT has also found applications in many electrochemical devices. Even

had it not significant electrochemical activity, it would have found application as an

alternative to the insulating polymer binders used in many devices. The electrochemical

properties of PEDOT allow charge transfer to control the doping of the film. Doping of the

monomer site occurs according to the following equation:

CSSEDOTCeSSEDOT nn

0 Equation 5.1

Here SS is the monomer styrenesulfonate and C+ is the cation. The cation allows for charge

transfer between the electrode and the electrolyte allowing donation of an electron to the

PEDOT monomer. When the EDOT monomer carries no charge (as in the right hand side of

the equation) it is said to be undoped and is insulating310

. This allows PEDOT to be active as

a pseudocapacitor. The specific capacitance of PEDOT is over 100F/g and, as such, PEDOT

has been used as both the sole electrode material and in a variety of composites326

. The

undoped PEDOT film also absorbs more strongly in the visible and, as a result,

electrochromic devices can be fabricated using PEDOT311

.

77

6

Graphene Based DSSC Counter Electrodes

6.1 Introduction

In 1990 the Gratzel cell or Dye-Sensitized Solar Cell (DSSC) was established as a potential

electrochemical solar cell10

. While the efficiency was relatively low, this cell type displayed

considerable advantages in that it was potentially cheap to manufacture and did not require

the material purity associated with silicon based solar cells. Also the cells could be made in

normal ambient conditions and did not require clean room facilities. Dye-Sensitized Solar

Cells all employ a counter electrode which operates as a catalyst for the tri-iodide couple in

the following reaction327

:

IeI 323 Equation 6.1

As with many electrochemical reactions, platinum is the most commonly used and

effective catalysts for this reaction. However, for a cell type that stands out for its low cost

and ease of manufacture, the platinum counter electrode has always been problematic. The

high price of platinum has encouraged much research into alternative materials for use as

catalytic counter electrodes in DSSCs328

.

Much of this work has focused on producing carbon-based counter electrodes. This

interest is due to the high surface area and conductivity of the various forms of carbon such

as activated carbon and carbon aerogels329

, carbon nanotubes65

and graphene57,58,61,330–334

.

With graphene the catalytic activity is associated with active sites on the nanosheet edges47,48

.

The simplest way to maximise the overall catalytic performance of a graphene counter

electrode is to increase the number of catalytically active sites by either reducing the

nanosheet size (i.e. increasing total edge length) or increasing the electrode thickness. While

78

the second approach is relatively simple, above some critical thickness the overall catalytic

activity will saturate as the process becomes limited by mass and charge transport effects335

.

In addition, from a purely economic standpoint, one would like to use the minimum graphene

mass possible. The importance of thickness in these studies is often overlooked. While many

of these carbon electrodes have been in excess of a micron in thickness, Kavan et al. have

produced thin catalytic graphene films with high transparencies50

. However, what is really

needed is a comprehensive study on the dependence of DSSC cell performance on graphene

counter electrode thickness.

Addition of carbon nanotubes to enhance the efficiency of graphene counter

electrodes is a way of both increasing the internal porosity of the film allowing diffusion of

the electrolyte ions into the internal surface of the film and of improving the conductivity of

the film. This has been done with both reduced graphene oxide and pristine graphene with

reasonable effectiveness90,336

.

In addition to purely carbon counter electrodes, a number of other materials have been

studied. For example polymers, in particular PEDOT:PSS have been demonstrated as

catalysts337

, as well as transition metal compounds usually with oxygen, carbon, sulfur and

other chalcogenides. Many of these compounds are produced as particles with dimensions

ranging from a couple of hundred nanometers to microns81,83,84,338

. In addition, counter

electrodes from arrays of two dimensional (2D) transition metal compounds are being

explored such as molybdenum and tungsten sulfide85

, molybdenum selenide339

, tin sulfide86

and cobalt sulfide70

.

However, many of the more effective 2D catalysts are not good conductors of

electricity285

. This means the cell performance may be limited by the resistance of the counter

electrode, especially for thicker electrodes. To address this problem, a number of researchers

have demonstrated hybrid systems composed of a combination of conductive component,

usually a carbon allotrope or polymer, and an electrocatalytically active material89,92,105,340–342

.

Of particular interest are counter electrodes formed from MoS2/Graphene hybrids. Most of

the work done using MoS2 for counter electrodes has followed a synthetic route for the

production of material which is then combined with pristine graphene104,343

. Alternatively,

reduced graphene oxide has been used with the functional groups on Graphene Oxide acting

as a site for synthesizing the MoS299,100

. However, in many cases these processing routes are

overly complex and not in keeping with the goal of low cost, easily processed cells. This

79

could be addressed by using a simpler route to produce MoS2/graphene composite films. I

believe Liquid Phase Exfoliation (LPE) represents such a route.

Liquid Phase Exfoliation is a very simple and scalable method for exfoliating layered

crystals such as graphite, h-BN or MoS2 to give large quantities of nanosheets stably

suspended in appropriate liquids219,290,292

. The resultant nanosheets have been shown to have

virtually no basal plain defects introduced during the exfoliation process. The only defects are

due to edge sites which tend to be electrochemically active50,344

. Using these methods, it is

simple to produce mixed dispersions of different types of nanosheets (e.g. graphene and

MoS2) or even nanosheets and nanotubes179,290,345

. To our knowledge very little work has

been done on MoS2 counter electrodes prepared by liquid phase exfoliation. While this may

be due to the poor performance of MoS2 nanosheets relative to the smaller synthesized sheets

and nanoparticles87

, the inherent processability of LPE nanosheets should allow significant

performance enhancement via electrode optimisation.

In this work, we used LPE graphene nanosheet dispersions to produce counter

electrodes with a range of thicknesses. This allowed us to identify 400 nm as the optimum

electrode. We then further improved the counter electrode performance by adding small

amounts of carbon nanotubes or MoS2, resulting in a DSSCs with efficiency of up to 96% of

the efficiency of the equivalent platinum cell. With a significant contribution from the MoS2

flakes being due to the relative flake dimensions a further study on graphene flakes of

varying length was conducted to further probe the effect of flake size on efficiency.

6.2 Experimental Procedure

6.2.1 Materials

Graphite was purchased from Future Carbon GmBH. Molybdenum Disulfide, Ethyl Cellulose

and Isopropyl Alcohol was purchased from Sigma Aldrich. P3 Carbon Nanotubes were

purchased from Carbon Solutions Inc. Titania Pastes, a conductive substrate (fluorine doped

tin oxide on glass), Pt Counter Electrodes and Electrolyte Materials were purchased from

Dyesol.

6.2.2 Film Production

80

Dispersions of materials were prepared by probe sonicating, using a GEX600, 48 W, 24 kHz,

flat head probe, 8g of material in 80ml of 10mg/ml Ethyl Cellulose in Isopropyl Alcohol for

48 hours. Dispersions were then centrifuged in a Hettich Mikro 22R at 2000 rpm for 90

minutes. Concentrations were obtained by filtration of known volumes followed by washing

with isopropanol and obtaining the final mass.

Dropcast films were made by dropping known volumes of dispersion on cleaned 2cm ×

2.5cm FTO glass sheets. Different thickness were achieved by changing the deposited

volume. Having to make new films for each measurement resulted in a variation in the

thickness range for electrochemical testing and cell assembly. Spin coated films were

fabricated by droping 200 µL on a cleaned FTO glass and spinning at 1000 rpm. Different

thicknesses were achieved by changing concentration by dilution with isopropyl alcohol for

thinner films or by repeating the process for thicker films. Spraycoated films were fabricated

using a Harddner and Steenbeck Infinity airbrush spray system operated by a Janome

JR2300N robot. The dispersion was diluted to approximately 0.1mg/ml and sprayed onto

cleaned FTO glass at 130⁰C. Compositions were altered by mixing the two dispersions.

Once the films were deposited by dropcasting they were heated in air at 370⁰C for 20

minutes. Thicknesses were measured using a Dektak 3 Surface Profilometer.

Special case for flake size dependence:

Dispersions of materials were prepared by probe sonicating, using a GEX600, 48 W, 24 kHz,

flat head probe, 0.8g of material in 80ml of 1mg/ml Sodium Cholate in deionised water for 48

hours. Dispersions were then centrifuged in a Hettich Mikro 22R at a 12krpm for 90 minutes

the supernatant was collected for film production while the sediment was redispersed and

centrifuged at progressively lower spin rates to achieve different flake sizes. Concentrations

were obtained by filtration of known volumes followed by washing with deionised water and

obtaining the final mass.

The same mass of each dispersion was filtered and washed to obtain films of similar mass

(0.1mg/cm2) on ethyl cellulose membranes for comparison for the various flake sizes. The

films were then transferred to FTO glass sheets

81

0 1000 2000 3000

0

2000

4000

He

igh

t (n

m)

Distance (m)

(b)(a)

Figure 6.1 (a)dropcast graphene counter electrode (b) a sample profiometry scan exhibiting surface morphology

6.2.3 Electrochemical Charaterisation

All electrochemical measurements were carried out using a Gammry 3000 Potentiostat.

Electrodes were confined to an area of 1 cm2 using Surlyn 15µm sealant. Cyclic

Voltammograms were measured in a three electrode setup using a Pt counter electrode and an

Ag/AgCl reference electrode at a scan rate of 50mV/s from -0.2 to 1 V vs. Ag/AgCl. The

electrolyte used was 0.1 M LiClO4, 5 mM LiI, and 0.5 mM I2 in acetonitrile. For Linear

Sweep and EIS measurements symmetrical cells were prepared using the High Performance

Electrolyte provided by Dyesol. Linear Sweep scan range was from -1 to 1 V vs. Ag/AgCl.

EIS was measured at 0.5V vs. Ag/AgCl at 10mV amplitude from 1MHz to 0.1Hz.

6.2.4 Cell Production

FTO glass was cleaned by bath sonication for 15mins in Decon 90 solution, deionised water,

Acetone and stored in Isopropyl alcohol prior to use. FTO glass was immersed in 40 mM

solution of TiCl4 for 30 minutes at 70⁰C prior to screen printing 3 layers of approximately 4

µm of 90T transparent paste followed by 1 layer of WER2-O reflector paste. These were then

sintered in a furnace ramping up to temperatures of 500⁰C over 2 hours as described by Ito et

al.13

82

Figure 6.2 TiO2 photoanodes from left to right: As screen printed, the scattering layer side face up after 24 hours in dye, the transparent layer side face up after 24 hours in dye.

Cells were assembled as open cells separated by 15 nm Surlyn sealant. The working electrode

was immersed in the High Performance Electrolyte provided by Dyesol while the counter

electrode was clipped on to complete the cell. For comparison, the same working electrode

was used for a Platinum counter electrode and a Graphene/MoS2 counter electrode. Electrical

Characteristics of the cells were obtained using a Keithley 2400 Source Meter.

6.3 Results and Discussion

6.3.1 Graphene Film Thickness Dependence

0 300 600 900 12000

5

10

15

20

25

30<L> = 586nm

Count

Length (nm)

(c)(b)(a)

Figure 6.3(a) TEM of Typical Graphene Flake. Scale Bar = 100nm. (b) Histogram from TEM data (c) SEM of Graphene Film. Scale bar = 500nm

LPE was used to produce suspensions of graphene nanosheets in solutions of ethyl cellulose

in isopropyl alcohol (see methods). Shown in Fig 6.6(a) is a transmission electron

83

microscopy (TEM) image of a typical few-layer graphene nanosheet. Statistical TEM

analysis yielded a nanosheet size (L) histogram as shown in Fig 6.3(b) which gave an average

flake size of 586nm these were typically few layer flakes. The larger flake size was used as

there was difficulty in producing sufficient amounts of the smaller nanosheets. However, due

to there only being one centrifugation step the lower size nanosheets are present in this more

polydisperse sample. These nanosheet suspensions were formed into films by dropcasting

onto glass substrates. Shown in Fig 6.3(c) is a scanning electron microscope image of the

surface of such a film. This shows a surface morphology which is rough and porous,

properties which will facilitate electrolyte penetration and access of ions to catalytic sites in

the network interior. This makes these films suitable for use as counter electrodes.

In order to observe the electrochemical activity at different relative potentials, cyclic

voltammetry (CV) was performed for both a 400 nm thick graphene counter electrode

produced by the dropcast method and, for comparison, a platinum counter electrode in an

electrolyte composed of I3-/I

- species in acetonitrile. The pairs of peaks in Figure 6.4(a)

between 0 and 0.4 Volts vs. Ag/AgCl corresponds to the redox of I3-/I

- while the pairs of

peaks between 0.5 and 0.9V vs. Ag/AgCl are the redox couple of I2/I3-.81

In figure 6.4(a) the

voltage separation between the peaks for the graphene electrode is 0.17 V which is smaller

than the platinum separation of 0.24 V. This provides a slight performance advantage to the

graphene counter electrode. However the current densities at the graphene peaks associated

with the I3-/I

- couple are low relative to those of platinum, particularly for the reduction of I3

-,

which will have a negative impact on the graphene counter electrodes performance.

Dropcast graphene electrodes of various thickness (t=85-1100 nm) were fabricated into

symmetric cells using a high performance electrolyte provided by Dyesol and characterised

using linear sweep voltammograms (Figure 6.4(b)). These were compared to a platinum

electrode which is reflected as the shaded area. In each case, the exchange current density

was extracted by fitting the data around -1V to the Tafel equation114

. The exchange current

density (Figure 6.4(c)) increases linearly with electrode thickness from 2 mA/cm2 (t=85 nm)

to 19 mA/cm2 for a thickness of 1100nm. This is a reflection of the expected linear increase

in the number of catalytic sites with electrode thickness and indicates good electrolyte

penetration throughout the internal volume of the electrode. Importantly, by a thickness of

400 nm, the exchange current density surpasses that of platinum electrodes (~8.5 mA/cm2).

84

Electrochemical impedence spectroscopy (EIS) was performed also as shown in Figure 6.4(d)

for electrodes of varying graphene thickness as well as Pt. In all cases, the curves were

dominated by a single semicircle indicating a simplified Randel’s circuit239

, allowing us to

estimate the charge transfer resistance from the circle width along the ZReal axis. The charge

transfer resistances of all electrodes is plotted versus electrode thickness in Figure 6.4(e). We

find an inverse fall off with thickness from 19 (t=85 nm) to <1 for a thickness of

1100nm. This compares to a charge transfer resistance of ~6 Ω for the platinum electrode.

We find the charge transfer resistance to scale inversely with the exchange current density in

Figure 6.4(f) as expected from the low potential approximation to the Butler-Volmer

equation. This confirms the charge transfer resistance to be controlled by the number of

catalytically active sites.

85

-0.3 0.0 0.3 0.6 0.9 1.2-0.5

0.0

0.5

1.0

1.5

(f)(c)

(e)(b)

(d)

J (

mA

/cm

2)

Voltage (V vs. Ag/AgCl)

400nm Graphene

Pt

(a)

-1 0 1-6

-4

-2

Pt

85nm

215nm

400nm

1100nm Lo

g (

J/m

Acm

-2)

Voltage (V)

0 300 600 900 1200

0

5

10

15

20 Graphene

Pt

J0 (

mA

/cm

2)

Thickness, t (nm)

20 30 40 50 60 70

-2

0

2

4

6

8

10

12

14

16 Pt

85nm

215nm

400nm

1100nm

ZIm

(

)

ZRe

()

10 100 1000

1

10

100

Rct (

)

Thickness, t (nm)

0 1 2 3

0

50

100

Rct (

)

1/J0 (cm

2/mA)

Figure 6.4(a) Cyclic voltammograms measured for counter electrodes of both platinum and a 400 nm thick graphene film. (b) Tafel Plots measured for counter electrodes of platinum

(shaded area) and dropcast graphene films of varying thickness. (c) Exchange current density data (extracted from Tafel Plots) for graphene counter electrodes of a range of thickness. (d)

Electrochemical impedance spectra measured for counter electrodes of platinum and dropcast graphene films of varying thickness. (e) Charge transfer resistance plotted versus

film thickness. (f) Charge transfer resistance plotted against the inverse of the exchange current density. In (c), (e) and (f) the red square represents the datum for a Pt electrode.

86

0.0 0.2 0.4 0.60

-2

-4

-6

-8

-10

-12

-14

-16

Pt 5.52%

60nm Gra 2.72%

200nm Gra 3.82%

900nm Gra 4.28%

Curr

ent D

ensity (

mA

/cm

2)

Voltage (V)

Figure 6.5 IV Characteristics of DSSCs with graphene counter electrodes of different thickness. Derived parameters are shown in table 1.

We used graphene films of a range of thickness as counter electrodes in DSSCs (see

methods). Current density-voltage (J-V) curves for cells with different graphene electrode

thicknesses are shown in Fig 6.5. Shown for comparison is data for a cell with a standard Pt

counter electrode. It is clear from these curves that DSSC performance improves as the

graphene thickness is increased (device parameters are given in Table 6.1).

Sample t (nm) Jsc (mA/cm2) Voc (V) FF (%) Eff (%)

Graphene 60 11.66 0.626 37.3 2.72

Graphene 200 12.63 0.655 46.2 3.82

Graphene 900 12.59 0.659 51.6 4.28

Platinum n/a 14.06 0.653 60.1 5.52

Table 6.1: Table containing cell parameters for counter electrodes of varying thickness (t) -

short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency (Eff)

To test this thickness dependence in more detail, we fabricated cells with graphene counter

electrodes which were deposited at a range of thicknesses by three different methods:

87

dropcasting (t = 60nm – 900nm), spin-coating (t = 15nm – 360nm) and spray-coating (t =

80nm – 420nm). We measured the J-V curves in all cases and extracted Jsc, Voc, FF and Eff .

70

80

90

100

(d)

(c)

(b)

Dropcast

Spin Coated

Spray Coated

Jsc/J

sc,P

t (%

)

(a)

0 200 400 600 800 100094

96

98

100

102

Vo

c/V

oc,P

t(%

)

50

60

70

80

90

100

FF

/FF

Pt (

%)

0 200 400 600 800 100040

50

60

70

80

Eff/E

ffP

t (%

)

Thickness (nm)

Figure 6.6(a) Short current density, (b) open circuit voltage, (c) fill factor and (d) efficiency for DSSCs fabricated with graphene counter electrodes plotted as a function of graphene film

thickness. Each parameter is expressed relative to that measured for an equivalent cell with a Platinum counter electrode.

The parameters are plotted against thickness in Fig 6.6(a)-(d). In all cases, the values are

normalised to the values measured for the Pt counter electrode. It is clear from the graphs that

there is little difference between dropcasting, spin-coating and spray-coating especially at the

higher range of thicknesses. For most samples it can be seen that the normalised short circuit

current density of the graphene electrodes is at 80-90% of the platinum electrode with little

dependence on thickness. Increasing the thickness of the graphene layer improves the

normalised open circuit voltage from 95% to 101% of platinum electrode. The most notable

increase however is in the normalised fill factor which increases from ~60% to ~85% as the

88

thickness increases from 15 to 900 nm. Primarily because of the increase in FF, the

normalised efficiency increases from ~50% to ~80% (relative to Pt) over the thickness range.

We note that there is a rapid increase in efficiency as the thickness increases to 400 nm, after

which minimal increases occur. Since the purpose of integrating graphene into the cell is to

be cost competitive, we suggest that 400 nm is the optimum counter electrode thickness.

6.3.2 Addition of Carbon Nanotubes

Carbon nanotube have previously been used to enhance the conductivity and porosity of

various graphene counter electrodes. The SEM image in figure 6.3(a) shows that the

graphene flakes are orientated close to parallel to the substrate plane. This means in the

vertical direction towards the current collector in the cell there are more junctions per micron

than there are parallel to the current collector. The carbon nanotubes can reduce this

anisotropy by weaving between the flakes facilitating contact to the current collector.

To observe the effect of addition of carbon nanotubes on the electrochemical properties of the

graphene film, EIS spectroscopy analysis was performed to extract estimates for the charge

transfer resistance as shown in figure 6.7.

10 20 30 40 500

5

10

15

20(b)

-Zim

()

Zreal

()

0% CNT

2%

5%

10%

15%

100%

(a)

0 20 40 60 80 100

4

6

8

10

12

14

16

Rct (

)

% weight CNT (%)

Figure 6.7 (a) Nyquist plot of impedance for graphene-CNT composite films (b) estimated charge transfer resistance of graphene-CNT composite films

The Nyquist plot in figure 6.7(a) shows a graphene plot similar to the characteristic Randle’s

circuit (black). The carbon nanotube only sample (navy) displays two characteristic semi-

circles imposed upon each other with the centres of the circles being displaced relative to

each other. The charge transfer resistance was estimated by taking the distance between the

start of the first semicircle and the end of the second semicircle. Even a small amount of

89

addition of the carbon nanotube results in a plot resembling the carbon nanotube plot more

closely.

There is minimal effect of the addition of carbon nanotubes on the series resistance of the

films which would be dominated by the current collector. The decrease in charge transfer

resistance as a result of the addition of carbon nanotubes is non-trivial with the charge

transfer resistance of the graphene only film being over 15Ω while addition of 15% carbon

nanotubes reduces this to 3Ω which is lower than that of the carbon nanotube only film of

approximately 6Ω. This could be due to the graphene having more available active sites than

the CNTs, as a result the improved conductivity of the CNTs provides better access to the

electrochemically active sites reducing the charge transfer resistance.

Due to the expense of carbon nanotubes, their addition at large amounts is not economically

feasible currently. To reduce the economic impact of introducing CNTs analysis of the cells a

carbon nanotube weight percent of 5% was chosen as this resulted in a significant decrease in

charge transfer resistance. Two 4µm thick films were compared in figure 6.8 one with 5%

carbon nanotube added and one without.

0.0 0.2 0.4 0.60

-2

-4

-6

-8

-10

-12

-14

Pt = 5.23%

Gra 4m =4.59%

Gra+ 5 CNT 4m =5.03%

Curr

ent D

ensity (

mA

/cm

2)

Voltage (V)

Figure 6.8 IV Characteristics of DSSCs with a graphene counter electrode with a thickness of 4μm (red) and a graphene counter electrode of same thickness with 5% weight carbon

nanotubes (blue) compared to a DSSC with a platinum counter electrode (black). Derived parameters are shown in table 3.

The characteristics of the cells show that there is an increase in short circuit current from

11.03 to 12.18mA/cm2. There is also a small increase of 0.01V in the open circuit voltage due

90

to the addition of the nanotubes. The fill factor is larger for the graphene only film but due to

the significantly higher current the efficiency of the film with the nanotubes is higher. The

efficiencies represented as percentages of the platinum cells are 87.6% for the graphene only

cell and 96.2% for the addition of the carbon nanotubes. This is very close to being

competitive with platinum. However, there is much more mass of graphene/CNT used and

with CNTs costing at least 3 times as much as platinum using conventional methods these

electrodes are not as economically viable.

Sample Jsc (mA/cm2) Voc (V) FF (%) Eff (%)

Graphene 11.03 0.62 67.0 4.58

Graphene:CNT 95:5 12.18 0.63 65.6 5.03

Platinum 13.29 0.6 65.6 5.23

Table 6.2: Table containing cell parameters for counter electrodes of graphene, graphene with

5%wt CNTs and Platinum - short circuit current (Jsc), open circuit voltage (Voc), fill factor

(FF) and efficiency (Eff)

6.3.3 Addition of MoS2

A number of papers have suggested MoS2 nanosheets to be effective catalysts for use in

DSSC counter electrodes with evidence suggesting that they are more effective than

graphene-based catalysts.328

However, networks of MoS2 nanosheets are poor electrical

conductors displaying in-plane conductivities227

of ~10-6

S/m and out of plane conductivities

which are three orders of magnitude lower.297

Thus we would expect counter electrodes

fabricated from networks of MoS2 nanosheets to be limited by their electrical properties. This

implies that mixtures of graphene and MoS2 nanosheets could make effective counter

electrodes. One would expect that while MoS2 might contribute more to the catalytic activity,

graphene would dominate the charge transport. Similar strategies have recently proved

successful for nanosheet-based supercapacitors,179

lithium ion batteries346

and hydrogen

evolution catalysts347

where addition of nanotubes has overcome resistance limitations,

boosting performance.

91

To test this, we prepared MoS2 nanosheets by liquid phase exfoliation. Shown in figure 6.9(a)

is a TEM image of a typical MoS2 nanosheet. Statistical analysis yielded histograms which

gave an average flake size of 156 nm (Fig 6.9(a)). Shown in figure 6.9(c) is an SEM of a

dropcast MoS2 film on glass. Clearly the network morphology is very similar to the graphene

networks.

The MoS2 and graphene suspensions were then mixed in a number of different ratios to give a

set of composite dispersions. These were then formed into MoS2/graphene composite films

by drop-casting for use as counter electrodes (thickness 400 nm in all cases).

0 200 400 6000

10

20

30

40

Count

Length (nm)

<L>= 156nm (c)(b)(a)

Figure 6.9 (a) TEM of Typical MoS2 Flake. Scale bar = 100nm. (b) MoS2 nanosheet size histogram extracted from TEM data. (c) SEM of MoS2 Film. Scale bar = 500nm

To investigate the effect of addition of MoS2 on the electrochemical properties of the

graphene film EIS spectroscopy analysis was performed to extract estimates for the charge

transfer resistance as shown in figure 6.10.

92

10 20 30 40 50 60 70-2

0

2

4

6

8

10

12

14 Gra

10% MoS2

75%MoS2

-Zim

g (

Zreal

(0 20 40 60 80

12

14

16

18

20

22

(b)

Rct (

)

% Weight MoS2 (%)

(a)

Figure 6.10 Charge transfer resistance of graphene-MoS2 composite films

The samples shown in figure 6.10(a) compare the Nyquist plots of a graphene only

symmetrical cell with those of a cell with electrodes with 10% and 75% weight MoS2

respectively. The MoS2 plot is not shown as the resistances were approximately two orders of

magnitude higher. It is sufficient to say that the graphene has a significant impact on the

charge resistance of the MoS2 as a result.

In figure 6.10(b) the charge transfer resistances are compared for films with different weight

percentages of MoS2. While initially the charge transfer resistance is higher than graphene for

small amounts of MoS2 it decreases for further addition of MoS2. 75% weight MoS2 gives a

charge transfer resistance of approximately 12Ω compared to 16Ω for the graphene electrode.

This could be due to the enhanced electrochemical activity of the semiconducting MoS2

benefiting from the enhanced charge transport in the composite provided by the graphene

To observe the effect of charge transfer resistance on cell performance JV curves were

measured for DSSCs prepared using such composite counter electrodes with a subset shown

in figure 6.11. It is clear from this data that the DSSC performance varies strongly with the

compositional balance of MoS2 and graphene (device parameters are given in table 6.3).

93

0.0 0.2 0.4 0.60

-2

-4

-6

-8

-10

-12

-14

Pt 4.40%

Gra 3.62%

90% Gra 4.35%

MoS2 2.53%

Curr

en

t D

en

sity (

mA

/cm

2)

Voltage (V)

Figure 6.11 IV Characteristics of DSSCs with MoS2/graphene composite counter electrodes of different compositions. Derived parameters are shown in table 2.

Sample Jsc (mA/cm2) Voc (V) FF (%) Eff (%)

Graphene 10.7 0.652 51.9 3.62

MoS2 9.14 0.589 47.0 2.53

90:10 Gra:MoS2 11.91 0.646 56.5 4.35

Platinum 13.39 0.657 50.0 4.40

Table 6.3: Table containing cell parameters for counter electrodes of varying composition -

short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency (Eff)

To quantify this trend, the DSSC characteristics were extracted from the JV curves as

described above, normalised to the equivalent value measured for a Pt electrode-DSSC and

displayed in Fig 6.12 (a)-(d) as a function of graphene mass fraction. In all cases we find the

MoS2-only DSSCs to perform poorly with values of Jsc, Voc, FF, and Eff considerably lower

than the equivalent values for devices with graphene-only counter electrodes. The cumulative

effect of this can be seen in the normalised efficiency which was <45% for the MoS2-only

94

device compared to >75% for the graphene-only device. For both Jsc and Voc, the normalised

performance increased monotonically with graphene content in a manner roughly described

by the rule of mixtures (Figure 6.12 (a)-(b)). However, the normalised Fill Factor behaves

differently, increasing from ~75% for the MoS2-only device to ~115% for the counter

electrode containing 90% graphene before falling to approximately 80% for the graphene

only device (Figure 6.12 (c)). As a result of the behaviour of the FF, the normalised

efficiency also displays non-monotonic behaviour, rising from 40% to 95% for the counter

electrode containing 90% graphene before falling to <80% for the graphene only device

(Figure 6.12(d)). This data clearly shows that adding small amounts of MoS2 nanosheets

(~10wt%) to a graphene counter electrode can give a non-trivial increase in efficiency. With

regard to the measurements from EIS this seems to be at odds with the higher percentage

weight films displaying a lower charge transfer resistance.

We can understand these results in a phenomenological manner as follows. We assume that

MoS2 is a more effective catalyst than graphene. In the simplest case the efficiency of a

MoS2/graphene mixed electrode is given by the rule of mixtures:

)1(2 FMoSfGraphene MEffMEffEff Equation 6.2

where EffGraphene and EffMoS2 are the efficiency of graphene- and MoS2-only electrodes and

Mf is the mass fraction of graphene in the mixed electrode. Then the efficiency should fall on

adding graphene to an MoS2-only electrode. However, for electrodes dominated by MoS2, the

electrode resistance will be so high as to prevent the electrode operating to its full capability.

Adding graphene increases the electrode conductivity according to the percolation scaling

law179

which can be expressed approximately as

2

fM Equation 6.3

(here we set the percolation threshold to zero for simplicity and take the percolation exponent

as its universal value348

). By analogy to previous results for MnO2/nanotube based

supercapacitor electrodes, addition of the graphene should increase the effectiveness of the

electrode by minimising any resistance-based limitations. I propose the efficiency of the

95

60

70

80

90

(d)

(c)

(b)

Jsc/J

sc,P

t (%

)

(a)

0 25 50 75 10085

90

95

100

Vo

c/V

oc,P

t(%

)

70

80

90

100

110

120

FF

/FF

Pt (

%)

0 25 50 75 10030

40

50

60

70

80

90

100

110

Eff/E

ffP

t (%

)

Mass Fraction Graphene (%)

Figure 6.12(a) Short current density, (b) open circuit voltage, (c) fill factor and (d) efficiency for DSSCs fabricated with MoS2/graphene composite counter electrodes (t=400 nm) plotted

as a function of graphene mass fraction. Each parameter is expressed relative to that measured for an equivalent cell with a Platinum counter electrode. The dashed lines in (a)

and (b) represent rule of mixtures type behaviour. The line in (d) is a fit to equation 6.4.

composite electrode can be roughly modelled by modifying equation 6.2 by multiplying it by

equation 6.3 to crudely account for the effect of the graphene on the electrode conductivity.

Then we find:

)1(2

2

0 fMoSfGraphenef MEffMEffkMEffEff Equation 6.4

where k is a constant and Eff0 accounts for the fact that even in the absence of graphene, a

thin layer of MoS2 near the current collector can act as an effective catalyst without resistance

limitations. We have applied this expression to the data in figure 6.12(d), finding it to well-

match the overall behaviour of the data. This analysis implies Graphene

MoS

EffEff 2 ~6 and

0

2

EffEff MoS ~10. This suggests that while the MoS2 nanosheets used here may be a far better

catalyst than the graphene nanosheets, only about 10% of nanosheets in an MoS2-only

electrode are close enough to the current collector to be active.

96

These results clearly show that the MoS2 nanosheets studied here are better catalysts for the

tri-iodide reaction than the graphene sheets. In both cases, it is likely that the catalytically

active sites lie on the nanosheet edge.47,48,50

. This is certainly the case for MoS2 nanosheets

when catalysing the hydrogen evolution reaction344

and Ni(OH)2 nanosheets when catalysing

the oxygen evolution reaction.349

This means that two factors come into play, the site

catalytic activity, often expressed via the turn-over frequency, and the number of sites. While

the relative values of the former parameter are unknown, we can say something about the

relative active site densities for MoS2 and graphene. The number of edge catalytic sites per

unit volume of electrode scales inversely with nanosheet length.347

The mean lengths of the

nanosheets used here were 586 nm versus 156 nm for graphene and MoS2 respectively. This

means that, assuming all sites are active and roughly equally spaced, there are about four

times as many active sites per unit volume of MoS2 compared to graphene on a per unit

volume basis. Moreover, the analysis of McAteer et al.347

would suggest that the catalytic

activity should scale as the turnover number, R, divided by mean nanosheet length, <L>.

Assuming that the efficiency scales roughly linearly with catalytic activity would give

2

22

MoS

Graphene

Graphene

MoS

Graphene

MoS

L

L

R

R

Eff

Eff Equation 6.5

Then, using the values given above would suggest that 2 / ~1.5MoS GrapheneR R . This implies

that the MoS2 catalytic activity on a per site basis is only slightly larger than that of graphene.

In fact most of the difference between MoS2 and graphene efficiencies is due to the fact that

the MoS2 nanosheets used here are considerably smaller than the graphene ones.

6.3.4 Graphene Flake Size Dependence

Multiple works have suggested that the edges of the flake are responsible for the catalytic

activity of the graphene sheets in DSSCs47,48,51

. There is very little information on the effect

of the size of the flake on the efficiency of a cell. Ahmad et al use various graphene sources

with different surface areas and initial particle sizes which result in different efficiencies350

.

This surface area difference is probably due to the average number of layers of the graphene

particle provided and the particle sizes are in the range of 2-5µm. However since the

graphene is used in a composite with PEDOT:PSS the particle size effect of graphene only is

not investigated.

97

In this work graphene flakes of various average lengths were produced by exfoliation of

graphite in water and surfactant and filtered before transfer to FTO glass. The average length

of the flakes was determined statistically using transmission electron microscopy. The

histograms representing this analysis are shown in figure 6.13.

0

10

20

12krpm <L> = 94nm5krpm <L> = 129nm

2krpm <L> = 202nm

(d)(c)

(b)

Count

1krpm <L> = 446nm

(a)

0

10

20

30

0 200 400 600 800 10000

5

10

15

20

Count

Length (nm)

0 200 400 600 800 10000

5

10

15

20

25

30

Length (nm)

Figure 6.13 (a) Histogram of flake sizes for 1krpm sample with sample transmission electron microscopy image inset (b) histogram of flake size for 2krpm sample (c) histogram of flake

size of 5krpm sample (d) histogram of flake size of 12krpm sample

The length of the flakes decreases from 446nm to 202nm between the spin rates of 1 and

2krpm. To achieve smaller still flakes the spin rate was increased to 5krpm and subsequently

12krpm. The smallest average flake size achieved was 94nm. It is worth mentioning that the

relative concentrations of these dispersions decrease with decreasing flake size as

concentration is inversely proportional to the spin rate. This reduces the cost effectiveness of

the smaller flakes as the yield is significantly lower.

We used thin graphene films (0.1mg/cm2) as counter electrodes in DSSCs. This was done to

limit any possible saturation in efficiency that could be due to film thickness which will be

discussed later. Current density-voltage (J-V) curves for cells with different graphene flake

98

size electrodes are shown in Fig 6.14. Shown for comparison is data for a cell with a standard

Pt counter electrode. It is clear from these curves that DSSC performance improves as the

graphene flake size is decreased (device parameters are given in table 1).

0.0 0.2 0.4 0.60

2

4

6

8

10

12

14

16

18

Pt =6.7%

202nm =1.59%

129nm =2.44%

194nm =2.38%

Curr

ent D

ensity (

mA

/cm

2)

Voltage (V)

Figure 6.14 IV Characteristics of DSSCs with graphene counter electrodes of different flake lengths. Derived parameters are shown in table 1

From figure 6.14 there is a clear difference between the 2krpm sample and the 5krpm sample.

To analyse this difference the J-V characteristics ie. the short circuit density, Jsc, the open

circuit voltage, Voc, the fill factor, FF, and the efficiency, Eff ( /ff sc oc inE J V FF P , where Pin

is the input power) were extracted and represented as a percentage of the corresponding

platinum cell as shown in figure 6.15.

Sample <L> (nm) Jsc (mA/cm2) Voc (V) FF (%) Eff (%)

1krpm 446 9.64 0.626 17.9 1.09

2krpm 202 13.51 0.655 18.7 1.59

5krpm 129 14.73 0.659 25.7 2.44

12krpm 94 13.68 0.653 26.8 2.38

Table 6.4: Table containing cell parameters for counter electrodes of varying flake length

<L> - short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency (Eff)

99

Below a certain flake size the short circuit current seems to be roughly invariant around 83%

that of the platinum cell. The highest flake size has a much lower current density at short

circuit of below 55% that of the platinum counter electrode. The open circuit voltage

increases steadily as flake length decreases. The Fill factor sees a dramatic increase between

a flake length of 200nm and 130nm increasing by one third, thereafter there is not as much

influence on the fill factor. The efficiency as a result of these properties ranges from 14% that

of the platinum counterpart at a flake length of 440nm to 35% at a flake length of 90nm.

While this increase is dramatic the efficiencies with respect to the platinum counterparts are

still very low. In order to pursue parity with platinum based DSSCs increasing the thickness

of the graphene film seems like a more viable strategy. This is because the current methods

cannot produce smaller flakes in sufficient quantities. However some forms of carbon black

have low particle sizes which may be a viable route to efficient thin counter electodes.

55

60

65

70

75

80

85

0.0025 0.005 0.0075 0.0191.5

92

92.5

93

93.5

94

94.5

28

30

32

34

36

38

40

42

44

46

(d)

(c)

(b)

Eff/E

ffP

t (%

)

Vo

c/V

oc,P

t(%

)

FF

/FF

Pt (

%)

(a)

0.0025 0.005 0.0075 0.01

15

20

25

30

35

Jsc/J

sc,P

t (%

)

<L>-1 (nm

-1)

Figure 6.15 (a) Short current density, (b) open circuit voltage, (c) fill factor and (d) efficiency for DSSCs fabricated with graphene counter electrodes plotted as a function of graphene flake length. Each parameter is expressed relative to that measured for an equivalent cell

with a Platinum counter electrode.

100

6.4 Conclusions

In this chapter we have analysed the dependence of the performance of dye sensitised

solar cells on the thickness of a graphene nanosheet counter electrode establishing an

optimum thickness of 400 nm. A thicker film was significantly enhanced by the addition of

carbon nanotubes. Using the optimised thickness of 400nm, we explored the effect of

changing the counter electrode composition by mixing the graphene with MoS2 nanosheets.

We find the performance to be optimised a composition of 90:10 graphene:MoS2 by mass. At

this composition, the efficiency was very close to that measured for a platinum counter

electrode. This data is consistent with the MoS2 nanosheets being somewhat better catalysts

but requiring the graphene nanosheets to render the electrode conductive. More detailed

analysis suggests the better performance of the MoS2 nanosheets to be mostly down to their

smaller size. A graphene flake size dependence was conducted which revealed a sub-linear

dependence on the inverse flake length. This implies that the relationship between edge sites

and efficiency is not as simple as initially thought.

Thin film composites using Graphene and MoS2 derived from liquid phase exfoliation

may represent a cheap alternative, using naturally occurring starting materials, to produce

counter electrodes comparable to those produced via synthetic routes. The potential for

upscaling in liquid phase exfoliation is an added benefit for driving down the price of counter

electrode materials in DSSCs.

101

7

Thickness Dependence of Capacitance of PEDOT:PSS

Supercapacitor Electrodes

7.1 Introduction

In this chapter, the characteristics of thick PEDOT:PSS films as supercapacitor electrodes are

investigated. Recent work has produced transparent supercapacitors using high conductivity

doped PEDOT:PSS films196

. Expanding on this work, we investigate thick, non-transparent

electrodes with a view to analysing the dependence of capacitance per unit area with

thickness.

Using the dimensions of commercial supercapacitor electrodes for energy storage (a

50 micron current collector and a capacitive layer in the order of 100s of microns) as a rough

template, the aim is to produce effective electrodes in that range or lower. The added

advantage of PEDOT:PSS is that it acts as a current collector itself, thus opening up the

possibility of achieving comparable energy storage at lower thicknesses increasing the

percentage mass of a device with capacitive properties.

Two deposition techniques will be assessed for effective deposition of the material

with an emphasis on scalable methods to facilitate production on an industrial scale.

Dropcasting is an obvious scalable method which can produce thick, free standing films in

moulds while doctor blading can be used to produce thinner films on a substrate.

For comparison with theory, we will work from thin electrodes up to higher

thicknesses to analyse difficulties that may arise due to limitations associated with electrical

resistance and, most importantly, diffusion. These limitations lead to a sub-linear dependence

of capacitance per unit area with thickness and as such require analysis to optimise devices.

102

This will be assessed using the rate dependence of the capacitor performance using

previously established theory196,351

.

For long charge/discharge times associated with low scan rates, we can assume that

capacitance will be close to theory defined by resistance limitations. As the scan rate

increases the proportion of the electrode accessible by the electrolyte ions may decrease

resulting in deviation from resistance limited-theory.

Using the knowledge gained from this study, the advantages of increasing the

thickness (increasing intrinsic capacitance per unit area, CA, and decreasing sheet resistance,

Rs) will be balanced against the main disadvantage (diffusion limitations). This will be used

to propose an optimum thickness for certain applications.

7.2: Experimental Procedure

7.2.1: Sample Preparation

Clevios PH1000 PEDOT:PSS in water dispersion was purchased from Hereus. 95% Formic

Acid was purchased from Sigma Aldrich.

To achieve films of thicknesses in the range of 1-20 microns, the doctor blading

technique was used. To produce a dispersion with suitable viscosity for this method, the

initial dispersion had to be concentrated using a Krosflo tangential flow filtration setup

(Figure 7.1). The initial concentration was approximately 1.1% by weight (w/w) and after

processing through the Krosflo this had increased to 2.3% w/w. The deposition was defined

by the height of heat resistant tape (of approximately 50 microns) adhered to a glass slide.

Concentrated dispersion was placed at the top of the slide and guided down the length by a

blade. The dispersion was then dried on a hotplate at 130⁰C for 10 minutes. Thickness control

was provided by multiple deposition steps and using additional layers of tape. The width of

the samples was in the 7-8 millimetre range.

103

Figure 7.1: KrosFlo tangential flow filtration system. Dispersion is pumped from resevoir right and passed through the filtration tube. Solvent passes through the filter into waste container

(right) while more concentrated dispersion returns to resevoir.

Increasing the thickness further using the doctor blading technique was prohibitively

time consuming so drop casting films in petri dishes using the initial dispersion produced

films in the 20-90 micron thickness range. To increase the thickness range, the concentrator

was used to increase the solution to approximately 2.3% w/w resulting in a film thickness of

200 microns.

Freeze dried films were also fabricated in an effort to increase the effectiveness of the

super capacitor electrodes by increasing porosity. Initial attempts to produce freeze dried

electrodes using the given dispersion yielded electrodes of unsuitable mechanical stability.

The concentrator produced a dispersion of 1.6% w/w before freeze drying. The thickness of

these films were in the range of 1-4 millimetres but, due to the sponge-like nature of the

films, high accuracy in thickness was difficult to obtain. Freeze drying was performed by Dr.

Zahra Gholmovand.

All films were then treated in formic acid by being submerged for 10 seconds

followed by drying in air.

104

7.2.2 Electrical Characterisation

Electrical characterisation was carried out via a four-wire measurement of the IV

characteristics of the films. This was done using a Keithley 2400 sourcemeter. The voltage

range was 0-0.1V. Connections were established using alcohol-based silver paint and silver

wires.

7.2.3 Electrochemical Characterisation

Electrochemical characterisation was carried out using a Gamry Ref 3000 potentiostat. All

measurements were taken in a three electrode setup using a silver/silver chloride (Ag/AgCl)

reference electrode and a graphite rod as the counter electrode. The electrolyte used was

0.5M Potassium Sulphate (K2SO4).

Cyclic voltammograms (CVs) were taken at a variety of scan rates from 0.02V/s to 2V/s. The

voltage window was between 0.1V versus Ag/AgCl and 0.8V versus Ag/AgCl. The electrode

was contacted with silver paint at the top which was insulted using varnish as shown in Fig

7.2(a & b). Potentiostatic EIS Spectra were taken at 0.4V versus Ag/AgCl using an AC

voltage perturbation of 10mV.


7.3.1 Electrical Properties

Films of thicknesses in the range of 1-20 microns were produced using doctor blading. A

characteristic sample prepared for measurement is shown in figure 7.2(a). For further

increases in thickness, drop-casting was used to produce samples similar to that shown in

figure 7.2(b).

105

Figure 7.2: Doped PEDOT:PSS electrodes for electrochemical testing. A thin 2.8mm sample (left) compared with a thicker 20mm sample (right) contacted at the top using silver paint.

The paint is insulated with varnish to prevent contact with the electrolyte.

The electrical characteristics of these films is critical in the absence of a current

collector normally present in supercapacitor electrode systems. From previous work, the

conductivity of the untreated films as prepared is of the order of 1 S/cm196,325

. Treating

PEDOT:PSS to improve the conductivity is critical to reduce resistance limitations in

devices. Extensive work has been done to improve PEDOT:PSS film conductivity, especially

for transparent applications. Excellent conductivities have been achieved using alcohols

(1362 S/cm)322

, ethylene glycol (1418 S/cm)352

, and sulfuric acid (3065 S/cm)324

to post-treat

the films. These methods improve the conductivity by removing some of the PSS and by

changing the conformation of the molecules in the film.

Using the same method as McCarty et al325

., which involves a simple one step dip of

the film in formic acid and allowing it to dry in air, the sheet resistance varies with thickness

as shown in figure 7.3. Sheet resistance, which ranges from approximately 2 Ω/sq for the

thinnest film measured (approximately 5 µm) to less than 0.1 Ω/sq for the thickest

(approximately 200 µm), should vary inversely with thickness

assuming a constant

conductivity. This is the case for the majority of the films which have conductivities close to

what would be expected for films with conductivity of 900 S/cm. This shows that the doping

of the films is effective up to thicknesses in excess of 100 microns.

106

10 100

0.1

1

Sh

ee

t R

esis

tan

ce

(

/sq

)

Thickness (m)

S=900S/cm

Figure 7.3: Sheet Resistance versus thickness of Formic Acid Treated PEDOT:PSS films.

For the thicker films, the sheet resistance is lower than the lowest PEDOT supercapacitor

electrodes as produced by Wang et al with a sheet resistance of 1.4 Ω/sq326

.

7.3.2 Cyclic Voltammetry of Thin Films

To ascertain the performance of the doctor-bladed and drop-cast thin films, the electrodes

were examined using cyclic voltammetry from 0.1 to 0.8V versus Ag/AgCl for scan rates in

the range of 0.02 V/s to 2 V/s. The samples were 11 mm in length and approximately 8 mm

in width.

Figure 7.4 displays the cyclic voltammograms of a thin 2.8 µm film (a) and a thick 40

µm film (b). The lowest and highest scan rates were 0.02 V/s (red) and 2 V/s (green)

respectively. Both films are approximately boxlike and symmetrical with the exception of the

high voltages in the anodic cycle and the low voltages in the cathodic cycle which is due to a

slow electrochemical process. The relative size of the voltammogram at 2 V/s to the

voltammogram at 0.02V/s is much smaller for the thicker film. This work seeks to describe

the effect of thickness on the characteristics of supercapacitors.

107

0.0 0.2 0.4 0.6 0.8

-200

-150

-100

-50

0

50

100

150

200

(b)

j/s (

F/m

2)

0.0 0.2 0.4 0.6 0.8

-4000

-2000

0

2000

4000

(a)

Figure 7.4: (a) Cyclic voltammogramms of a 2.8µm PEDOT:PSS film starting with scan rates

ranging from 0.02 V/s (red) to 2 V/s (green). (b) Cyclic Voltammogramms with same scan

rate range for a film of thickness 40 µm.

Previous work by Higgins and Coleman196

used the following equation to describe the

current density, j:

)exp(1

tCs

jA Equation 7.1

where CA is the intrinsic capacitance per unit area which would be achieved by charging

infinitely slowly, s is the scan rate, t is time and τ is the time constant of the capacitor.

Current density divided by scan rate (j/s) is useful as it allows comparison of cyclic

voltammograms over a range of scan rates. This equation assumes that the capacitor starts

with no charge on the electrodes. However, in cyclic voltammetry there is an initial charge on

the electrodes from the previous cycle. Figure 7.4 displays the whole of the cyclic

voltammogram such that the initial current or charge is clearly non-zero.

To account for this, Pell and Conway351

used the boundary conditions equating the

currents and the start of the anodic current (top part of the curve) and the cathodic current

(bottom part of the curve) at the initial voltage (in this case 0.1V) and the final voltage

(0.8V). This produced equation 7.2:

)exp(

)exp(1

21

t

sV

Cs

jA Equation 7.2

108

where ΔV is the voltage window of the cyclic voltammogram and all other variables have

been defined. The anodic part of the current has been plotted for films of various thicknesses

at different scan rates to illustrate the suitability of equations 7.1 and 7.2 to describe the

current in figure 7.5.

0.10 0.12 0.14 0.16 0.18 0.20

0

100

200

j/s (

F/m

2)

(d) 40m 0.5V/s(c) 40m 0.02V/s

(b) 2.8m 0.5V/s

Experimental

Higgins & Coleman

Pell & Conway

(a) 2.8m 0.02V/s

0.0 0.2 0.4 0.6 0.8

-100

0

100

0.1 0.2 0.3 0.4

-2000

0

2000

j/s (

F/m

2)

Voltage vs. Ag/AgCl (V)

0.0 0.2 0.4 0.6 0.8

-1000

0

1000

Voltage vs Ag/AgCl (V)

Figure 7.5: The experimental (black), as predicted by Higgins & Coleman (red) and as predicted by Pell & Conway (green) anodic current density divided by the scan rate for (a) a 2.8mm film at 0.02 V/s, (b) a 2.8mm film at 0.5 V/s, (c) a 40mm film at 0.02 V/s and (d) a

40mm film at 0.5 V/s

For the lower scan rate of 0.02 V/s, the earlier stage of the anodic current is where the

largest difference between the different models occur. With the exception of low thickness

and scan rate, the Pell & Conway model is the better model for fitting the anodic current

density curves due to the negative currents at 0.1 V vs Ag/AgCl.

109

In these models CA, the intrinsic capacitance per unit area, is a characteristic property

of the capacitor and should be invariant under scan rate and time constant. The fitting of the

anodic current density at different scan rates with the Pell & Conway model results in

different values for CA and time constant. This is illustrated in figure 7.6(a).

0.0 0.2 0.4 0.6 0.8

-1000

-500

0

500

1000

(b)

CEXP

= 1512F/m2 = 0.9s

CSIM

= 2074F/m2 = 2.44s

j/s (

F/m

2)

Voltage vs Ag/AgCl (V)

Thickness 40m

Scan Rate = 0.5V/s

(a)

0.01 0.1 1

800

1000

1200

1400

1600

1800

2000

2200

CA (

F/m

2)

Scan Rate (V/s)

1

2

3 C

A

(s

)

Figure 7.6: (a) The variance of intrinsic capacitance per unit area (black, left axis) and time

constant (red, right axis) with scan rate. (b) The experimental anodic current density with

fitting parameters at 0.5 V/s (black) simulated anodic current using the Pell & Conway CA

and time constant values for 0.02 V/s

This decay in CA and time constant may be due to the effect of diffusion. This would

result in only part of the capacitor being active at a given time due to the time it takes the

potassium ions to enter and leave the polymer matrix. Diffusion will be given a more in-depth

discussion later. Since time constant is the product of the capacitance and resistance, the

variance of the two properties is expected to be similar but this is not the case, as shown in

figure 7.6(a), due to the model making the best possible fit without being able to account for

diffusion.

Figure 7.6(b) compares the experimental anodic current density (red) with a simulated

anodic current density at 0.5 V/s (black). The experimental values for CA and time constant

were given by the Pell & Conway fitting the current density at 0.5V/s. The simulated anodic

current uses the Pell & Conway equation for current density but uses the values for CA and

110

time constant for the fit at 0.02 V/s the initial values which should be invariant with scan rate.

Despite the higher CA value for the simulated curve, the current density is lower due to the

higher time constant. This is an example of how diffusion can help produce higher current

densities and hence higher capacitances at higher rates than would be expected.

Plotting the scan rate dependence of the current density divided by the scan rate at

various voltages in the potential window can be done by substituting for time in equation 7.1

and 7.2 where t=(V-Vi)/s where V is the voltage of the electrode compared to the reference

electrode and Vi is the starting voltage of the scan. Figure 7.7 demonstrates the fitting of the

current density divided by the scan rate to the scan rate for; (a) a 2.8 µm film fitted using

equation 7.1, (b) a 40 µm film fitted using equation 7.1, (c) a 2.8 µm film fitted using

equation 7.2 and (d) a 40 µm film fitted using equation 7.2.

0.01 0.1 1-50

0

50

100

150

j/s (

F/m

2)

j/s (

F/m

2)

40m2.8m

(d)

40m

Scan Rate (V/s)

2.8m

0.01 0.1 1

-500

0

500

1000

1500

2000

2500

(b)

Scan Rate (V/s)

(a)

-50

0

50

100

150

0.2V

0.3V

0.4V

0.5V

0.6V

0.7V

-500

0

500

1000

1500

2000

2500

(c)

Figure 7.7 (a & c) Scan rate dependence of current density divided by scan rate for a 2.8 µm thick film at voltages from 0.2 to 0.7V vs. Ag/AgCl. (b & d) Same plot for 40µm thick film. (a

& b are fitted using equation 7.2 while (c & d) are fitted using equation 7.2

Due to the redox peak at the end of the anodic current, there is a contribution to the

current density at the higher voltages, particularly at low scan rates. This manifests as an

increase of the current density divided by the scan rate relative to that predicted by the

111

equations. Since the contribution of this peak is lower at higher scan rates, the fitting has been

constrained to the higher scan rates at the higher voltages.

The fitting using equation 7.1 (Figure 7.7(a & b)) is a particularly bad fit at the lower

voltages. This is due to the initial starting current being below zero. As the voltage increases,

however, the fit improves as can be seen in figure 7.6 where the fitting of the two models

converge at higher voltages. The fitting for equation 7.2 is clearly better thus validating the

equation as the appropriate model for this material system.

For the thinner films, the fits result in an intrinsic capacitance per unit area and time

constant that is constant. Compared to the thinner films, however, the fitting for the thicker

40 µm film produces lower intrinsic capacitances per unit area at the low voltages which can

again be attributed to diffusion as discussed previously.

The current density divided by the scan rate at 0.4 V has been chosen as the most

representative voltage for most films as it is sufficiently high for the effect of diffusion to be

reduced and below the onset of the redox peak. As such, the current density divided by the

scan rate at 0.4 V has been plotted against scan rate for various thicknesses to extract values

for the intrinsic capacitance per unit area and the time constant (figure 7.8).

0.01 0.1 1

j/s @

0.4

V (

F/m

2)

Scan Rate (V/s)

0.01 0.1 1

1

10

100

1000

10000

(b) 2.8m

40m

90m

Scan Rate (V/s)

(a)

Figure 7.8: (a) Scan Rate Dependence of the current density divided by scan rate for thicknesses of 2.8, 22 and 90 µm with fitting according to theory from Higgins (Equation 7.1). (b) Scan Rate Dependence of the capacitance per unit area for thicknesses of 2.8, 40

and 90 µm with fitting according to theory from Pell & Conway (Equation 7.2).

112

At low scan rates, the current density is larger for the thicker films. The effect of

increasing scan rate leads to a decrease in charge density which is more pronounced in the

thicker films. The thinnest film (thickness 2.8 µm) current density divided by scan rate at 2

V/s is approximately 60% that of the current density divided by the scan rate at 0.02 V/s. In

contrast, the thickest film retains less than 0.1% of the current density divided by scan rate at

0.02 V/s at 2 V/s.

The plots were fitted using the models used by Higgins & Coleman - equation 7.1 (a)

and Pell & Conway - equation 7.2 (b). The most noticeable difference between the fits occurs

at scan rates in excess of 0.1 V/s. After 0.1 V/s, the Pell & Conway model predicts lower

current densities than is experimentally observed while the Higgins & Coleman model

predicts higher current densities. This can be explained for the Higgins & Coleman model by

reviewing figure 7.5(d) and is due to the limitation of the model starting with a zero current

density. This phenomenon has been discussed previously for the Pell & Conway model and is

caused by diffusion.

The capacitance of the film can be extracted from the current density divided by the

scan rate by integrating equation 7.1 or 7.2 over the voltage range to get the charge stored and

then dividing by the voltage window. Substituting dV for sdt allows integration of equation

7.3

VV

V

i

i

dVs

j

VA

C 1 Equation 7.3

which gives the capacitance per unit area as equation 7.4 using the Higgins & Coleman

model:

)exp(11

s

VV

sC

A

CA Equation 7.4

The Pell & Conway model is represented as equation 7.5 which is determined by

substituting equation 7.2 for j/s into equation 7.3 and integrating:

)exp(1

)exp(1

21

s

V

sVV

sC

A

CA Equation 7.5

113

These give expressions for the experimentally observed capacitance per unit area in

terms of previously introduced variables. Figure 7.9 plots the experimental capacitance per

unit area against the scan rate for the same films in figure 7.8 and fits the data for (a) the

Higgins & Coleman Model – equation 7.4 and (b) the Pell & Conway model – equation 7.5.

0.01 0.1 1

10

100

1000

10000(b)

C/A

(F

/m2)

Scan Rate (V/s)

(a)

0.01 0.1 1

2.8m

40m

90m

Scan Rate (V/s)

Figure 7.9: (a) Scan rate dependence of the capacitance per unit area for thicknesses of 2.8, 22 and 90 µm with fitting according to theory from Higgins & Coleman. (b) Scan Rate

Dependence of the capacitance per unit area for thicknesses of 2.8, 22 and 90 µm with fitting according to theory from Pell & Conway.

Here the thicker films initially have a higher capacitance per unit area at low scan rates but

where the capacitance decreases as scan rate increases. The relative change in capacitance is

not as significant for thinner films. These figures differ from the figures in 7.8 as these data

are achieved over the entire voltage range as opposed to one point. Therefore, extracting

intrinsic capacitance per unit area and the time constant using this method is more accurate.

Interestingly, the values for intrinsic capacitance and time constant obtained using the Pell &

Conway fit for current density at 0.4V and capacitance per unit area agree strongly and are

shown in figure 7.10. However, this does not occur for the Higgins & Coleman model and as

such the values for capacitance per unit area represent those from fitting the data in figure

7.9(a).

The values for intrinsic capacitance per unit area and time constant are plotted as a

function of mass per unit area of the film in figure 7.10 (a) and (b) respectively. When plotted

against mass per unit area, the intrinsic capacitance per unit area follows a straight line. The

114

slope of this line yields an intrinsic capacitance per unit mass of 38.5 F/g for the Higgins &

Coleman Model and 33 F/g for the Pell and Conway model.

These capacitances per unit mass are lower than the capacitance per unit mass of

PEDOT films in literature which is in the range of 85 to 115 F/g326

. However, there is a

significant mass of PSS in the electrode (the ratio is PEDOT:PSS is 1:2.5 by mass according

to manufacturer specifications). While the doping of the film can remove some PSS, a

significant amount remains. Assuming minimal loss of PSS, the capacitances per unit mass of

the PEDOT-only component is 135 F/g for the Higgins and Coleman model and 116 F/g for

the Pell and Conway model. This suggests the Pell and Conway model is better as it is closer

to the range defined by literature. The formation of PEDOT for these electrodes involves

electrochemical polymerisation and deposition, which requires a conductive substrate and

investment of resources to gain suitably thick films. The advantage of the PEDOT:PSS is that

the catalyst molecules can be removed by a chemical step after polymerisation. This allows

for dispersion of the polymer particles in water which facilitates the simple formation of films

by the methods used in this work and also compatibility with processes like spray coating.

1 10 100

100

1000

10000(b)

CA (

F/m

2)

M/A (g/m2)

(a)

1 10 100

0.1

1

10

Higgins

Pell + Conway

(s)

M/A (g/m2)

Equation y = a + b*x

Weight No Weighting

Residual Sum of Squares

3.71451 0.57272

Pearson's r 0.98978 0.97893

Adj. R-Square 0.97713 0.95309

Value Standard Error

TauIntercept 0.37776 0.27325

Slope 0.06001 0.00306

CIntercept 0.19238 0.10729

Slope 0.01627 0.0012

Figure 7.10: (a) Intrinsic capacitance per unit area vs. mass per unit area as derived from fittings of the scan rate dependence of the current density divided by the scan rate and the

capacitance per unit area vs. the scan rate. (b) Time constant vs. mass per unit area similarly derived

The time constant is plotted against mass per unit area in figure 7.10(b). This

produces a straight line graph. The time constant is the product of the capacitance and

resistance of the system.

115

)( eelectrolytelectrode RRC Equation 7.6

Here C is the capacitance and R represents the resistance components due to the inherent

resistance of the electrode and the electrolyte. The capacitance is given simply by the product

of the intrinsic capacitance per unit area multiplied by the area (L×W, where L is length and

W is width). The resistance of the electrode is given by the sheet resistance (Rs) divided by

the electrode width and multiplied by the electrode length. The electrolyte resistance is given

by the inverse of the product of the electrolyte conductivity (GA) and the area through which

the current travels. The full expression of the time constant is shown in equation 7.7.

A

MM

A

SAG

tC

LC

LWGW

LRLWC

2

)1

( Equation 7.7

This can be further simplified by converting the intrinsic capacitance per unit area to

the intrinsic capacitance per unit mass (CA = CMρt where ρ is the density of the film and t is

the thickness). Rs the sheet resistance is also given by the inverse of the product of the

conductivity and the thickness ((σt)-1

).

This equation can be used with the equation of the line from the linear fit of the data.

This shows that the intercept is dependent on the conductivity of the film while the slope is

dependent on the conductivity of the electrolyte. The conductivities of the doped

PEDOT:PSS film extracted from the Higgins & Coleman model and the Pell & Conway

model are 176 and 297 S/cm respectively. However, there are large standard errors in the

intercept and as such this could partially explain the lower than expected value of 900 S/cm.

The slope is the intrinsic capacitance per unit mass divided by the electrolyte conductivity as

ρt is equivalent to the mass per unit area. This gives conductivities of the electrolyte for the

Higgins & Coleman model and the Pell & Conway model of 64 and 206 mS/cm respectively.

The electrical properties as determined by the Pell & Conway model are superior to those

from the Higgins and Coleman model in both the electrode and electrolyte conductivities.

The overall capacitance per unit area of the electrodes is important for integration into

devices. The capacitance per unit area of the electrodes as shown in figure 7.11. The

behaviour of the capacitance per unit area of these films can be fitted by inserting equation

7.7 into equations 7.4 and 7.5 using the values received for intrinsic capacitance per unit

mass, conductivity of the PEDOT:PSS film and the conductivity of the electrolyte.

116

The fits for both models are suitable at low scan rates of 0.02 V/s (a) and 0.1 V/s (b).

However, as the scan rate increases the models do not provide a good fit. The Higgins and

Coleman model in particular does not fit well after 30 g/m2 for 0.5 V/s (c) the source of this

deviation was initially assumed to be diffusion but is mainly due to the inability of the model

to account for negative current densities at higher scan rates. However, since the Pell &

Conway model fits better beyond 30g/m2 this shows the suitability of the model.

117

1 10 10010

100

1000

10000

(c)

(b)

C/A

H+C

P+C

C/A

(F

/m2)

(a)

1 10 10010

100

1000

10000

C/A

(F

/m2)

1 10 10010

100

C/A

(F

/m2)

M/A (g/m2)

Figure 7.11 Capacitance per unit area for (a) 0.02V/s (b) 0.1V/s and (c) 0.5V/s. Blue line fitted using Higgins and Coleman Model. Red line fitted using Pell and Conway model.

The maximum capacitance per unit area achieved is 6000 F/m2

at 0.02 V/s or 0.6

F/cm2

which is the preferred unit for devices. This is comparable with the capacitances in

literature for PEDOT and PEDOT:PSS films with the highest being 0.92 F/cm2. Due to the

118

relatively low sheet resistance and the ease of fabrication producing supercapacitor electrodes

following this method would be viable.

The analysis of the models show that the Pell & Conway is the better of the models. It

more accurately fits the current densities and hence the values for the capacitances and

conductivity using this model are more accurate than those achieved using the Higgins &

Coleman model. This is due to the negative current densities that arise as thickness and scan

rate increases. Both models fail to account for diffusion which leads to poor fitting of the

model at high scan rates.

7.3.3 Impedance Spectroscopy

Films were also tested using potentiostatic impedance spectroscopy with a voltage amplitude

of 10 mV around 0.4V vs. Ag/AgCl in a frequency range from 1 MHz to 0.1 Hz. Samples of

impedance spectra are given in figure 7.12 (a) and (b) for a thin 2.8 µm film and a thick 40

µm film respectively. This was used to analyse the characteristics of the electrode. The main

components for an electrode system such as the PEDOT:PSS electrode in the range of 1 to

10 15 20

0

100

200

High Frequency

-Zim

g (

)

Zreal

()

Low Frequency

6 7 8 9

0

2

4

6

8

10

12 (b) 40m

Zreal

()

(a) 2.8m

Figure 7.12: Impedance spectra of films with thickness of (a) 2.8 µm and (b) 40µm

200 microns are the resistive elements, the capacitive elements and the diffusive elements114

.

The impedance can be separated into real and imaginary components due to the phase

response of these elements. There is an obvious effect on both the real and imaginary

components of impedance with frequency.

119

The resistive elements are from the connections of the system, the sheet resistance of

the electrode and the resistance of the electrolyte. These resistances are independent of

frequency. The capacitive impedance is frequency dependent, completely out of phase with

the oscillating voltage and follows the equation:

CjZC

1 Equation 7.8

where j is the imaginary number and ω= 2πf with f being the frequency of oscillation in the

experiment. The diffusive element is determined by the Warburg impedance. The Warburg

impedance is frequency dependent with a phase of 45 degrees. It is described by the equation

)11(j

AZ W

W

Equation 7.9

where AW is the Warburg coefficient.

0 5 10

3

4

5

6

7

8

9

10

11

12

Zre

al @

f =

1M

Hz

Rs (sq)

Figure 7.13: Real Impedance at 1MHz vs the sheet resistance of the corresponding electrode.

A simple analysis of these components can be done by looking at regions of low and

high frequency. At high frequency, the imaginary components which are related inversely to

the frequency are minimized leaving the real components independent of frequency isolated.

This gives the resistances due to the electrolyte and the electrode and connections in the

120

experimental setup. Figure 7.13 displays the comparison of the real component of the

impedance at the highest frequency with the sheet resistance of the electrode.

Assuming only contributions due to the sheet resistance of the electrode to the

impedance at high frequencies a straight line through the origin would be expected. The line

in figure 7.13 has an intercept of 3.5 Ω. The contribution due to the electrolyte resistance as

determined in the previous section is in the order of 0.2 Ω, which is far below the value of the

intercept. This means that the rest of the impedance at high frequencies originates in the

connections of the experimental setup. This could also be the source of the scatter in this plot.

At low frequencies, the imaginary component of the impedance goes as 1/ω. This

corresponds to the capacitor element. The impedance at low frequencies is shown in figure

7.14(a)

0.1 1 10 100

0.01

0.1

1

10

100 2.8 m

20 m

40m

90m

1/f

C/A

(F

/m2)

-Zim

g (

)

Frequency (Hz)

0 50 100 150 200 250

0

2000

4000

6000

8000

(b)

EIS

CV

M/A (g/m2)

(a)

Figure 7.14: (a) Imaginary component of the impedance for the lower range of frequencies (b) Capacitance per unit area as obtained from Impedance spectra (black) and cyclic

voltammetry (red)

At the low end of the spectrum linear fits can be made with 1/f and the capacitance can then

be extracted from the slope using equation 7.8. Figure 7.14(b) shows the capacitance per unit

area as found from impedance spectroscopy (black) compared with the capacitance per unit

area as found from cyclic voltammetry at 0.02 V/s. The agreement is suitable for the lower

values of thickness however there is a significant divergence for the thickest film. This could

121

be due to the lower frequencies having a lower equivalent scan rate than 0.2 V/s which would

then result in a higher capacitance per unit area being obtained.

7.3.4 Analysis of Diffusion of Ions through Polymer Network

When designing an electrode for use in a commercial supercapacitor, attention must be paid

to the dimensions of the electrode. For low thicknesses, the energy capability per unit mass is

constant according to equation 2.3 capacitance increases approximately linearly as does mass.

This changes when the thickness, or as the scan rate gets too large, the capacitance starts to

drop, as described by equation 7.5. However, due to diffusion, equation 7.5 returns a lower

capacitance than is observed experimentally as indicated in figure 7.3 where the lower

capacitance and time constant due to diffusion produce a larger current density over the

voltage window and hence a higher capacitance.

By normalising the j/s of the experimental results with respect to that which would be

predicted by equation 7.2 and plotting it against sτ/ΔV, the behaviour of the electrodes in

reality (data points) can be compared to theory (line – Equation 7.2) as shown in figure

7.15(a). The same can be done for capacitance per unit area and equation 7.5. to produce

figure 7.15(b).

1E-3 0.01 0.1 1 10

1E-3

0.01

0.1

1 (b)

(J/s

)/C

A

s/V

1E-3 0.01 0.1 1 10

1E-3

0.01

0.1

1

(C

/A)/

CA

s/V

(a)

Figure 7.15: (a) Master curve of current density divided by scan rate for all samples as a function of the product of the scan rate and time constant divided by the voltage window.

122

The line is the plot of equation 7.2 (b) Master curve of capacitance per unit area as a function of same. The line is the plot of equation 7.5

All the experimental data coalesces into a master curve for direct comparison with

theory. There is a departure from the expected value, defined by the line in the graph, once

sτ/ΔV exceeds a certain value. This value is different for both j/s and C/A due to the fact that

the j/s data is taken at one voltage while the C/A is extracted by integrating j/s over the entire

voltage window. As a result, the point at which diffusion has an effect on the C/A (0.25)

occurs at a lower value of sτ/ΔV than it does for j/s (0.4).

Taking these values and using equation 7.7 for the time constant, the scan rate at

which diffusion has an effect for a given mass per unit area can be resolved. The black line in

figure 7.16(a) is the maximum scan rate as determined by the deviation in figure 7.15(a) and

the red line is the maximum limiting scan rate as defined by the deviation in figure 7.15(b).

The points in figure 7.16(a) are the point at which deviation occurred from the scan rate

dependence according to figures 7.8 and 7.9 with black and red points corresponding to j/s

and C/A respectively.

0 50 100 150 200 250 300

0.1

1

(b)

Lim

itin

g S

ca

n R

ate

(V

/s)

M/A (g/m2)

(a)

0 1x109

2x109

3x109

4x109

0.0

0.5

1.0

1.5

2.0

T-2 (m

-2)

Figure 7.16: (a) The black line represents the scan rate at which diffusion has an effect from the master curves of current density divided by scan rate(figure 7.15(a)). Black dots

represent scan rate point of deviation from individual fits (figure 7.6(a)). Red line represents the scan rate at which diffusion has an impact on capacitance per unit area (figure 7.15(b)).

Red dots represent the scan rate point of deviation from individual fits (figure 7.6(b)). (b) Point of scan rate dependence deviation as determined from fits of current density divided by

scan rate (red) and capacitance per unit area (black) as a function of the inverse thickness squared.

123

The source of the large error bars is due to the sampling of the electrode at different

scan rates. Within error, however, there is agreement between the points and the lines. The

point of deviation was plotted against the inverse square root of the thickness in figure

7.16(b). The relationship is approximately linear which can be explained using one

dimensional linear diffusion. For one dimensional linear diffusion the average distance the

ions will travel is:

Dtx Equation 7.10

where x represents distance, D represents the diffusion coefficient and t represents the time.

The relationship between time and scan rate has been discussed already. Substituting time

and rearranging the equation gives the relationship of electrode thickness to scan rate where

all variables have been previously defined with the exception of T which is the electrode

thickness. Γ has been chosen to avoid confusion with t for time in the previous equation.

)(2 iVV

Ds

Equation 7.11

The diffusion coefficient from the black line is 1210103.5 sm and the diffusion

coefficient corresponding to the red line is 1210103.6 sm . The Stokes-Einstein equation

provides a way to estimate the diffusion coefficient of the ions in a solution.

R

TkD B

6 Equation 7.12

where T is the temperature, η is the viscosity of the solvent and R is the radius of the ion.

Using the value for the radius of a potassium ion in water as calculated by Pau et al.353

, the

diffusion coefficient is 129106.1 sm . This indicates that the diffusion coefficient in the

film is close to that in water. The diffusion coefficient in a porous material is defined as the

diffusion coefficient in the solvent multiplied by the porosity of the film and divided by the

tortuosity of the pores. The diffusion coefficients as calculated from figure 7.16(b) are in the

range of 30-40% of the diffusion coefficient in water. This reduced rate of diffusion has a

significant impact on the rate dependence of these electrodes.

124

7.3.5 Limitations Due to Electrode Dimensions

The capacitance as determined by equation 7.5 starts to decrease before evidence of

diffusion. In figure 7.12(b), the capacitance per unit area is 0.3 times the intrinsic capacitance

per unit area when diffusion takes an effect. The decrease described by equation 7.5 starts to

become significant when sτ/ΔV is approximately 0.1 which is before the point at which

diffusion occurs. This means the electrical properties of the film have a bigger impact on the

decrease of capacitance with electrode dimensions and scan rate than diffusion. Equating 0.1

with sτ/ΔV and substituting in equation 7.3 for the time constant the maximum scan rate is

given by the following equation:

A

MM

G

tCLC

Vs

2max

10

Equation 7.13

Similarly, the maximum mass per unit area before the capacitance per unit area decreases as a

function of sτ/ΔV can be found by manipulating the formula to yield:

M

AM

Max C

GLC

s

V

AM

2

10 Equation 7.14

These equations are plotted in figure 7.17 to work as a model to define the maximum scan

rate (a) and maximum mass per unit area (b) before the capacitance per unit area deviates

significantly from its maximum possible value.

125

1 10 100

0.01

0.1

1(b)(a) L=1cm

L=2cm

L=4cm

L=8cm

sm

ax (V

/s)

M/A (g/m2)

0.01 0.1 1

1

10

100

M/A

ma

x (g/m

2)

s (V/s)

Figure 7.17: (a) maximum scan rate against thickness for various lengths of electrode (b) maximum mass per unit area against scan rate for various electrode lengths.

This graph works as a template for device design. Depending on the rate of charge required

for the device, one can choose an appropriate length and thickness for the electrodes. For

example, if the device will be operated at 0.1 V/s, a 2 cm long electrode with a mass per unit

area of approximately 60g/m2 would be most suitable as maximizing the product of length

and thickness is paramount to getting the highest capacitance per unit area. To find better

materials for supercapacitor electrodes, minimizing the time constant while keeping a high

capacitance is critical. This requires finding materials with higher conductivities for the

electrodes. Films of carbon nanotubes with a range of conductivities from 2000 S/cm for

liquid exfoliated films278,281

to over 12,000 S/cm354

for acid dissolved tubes which are higher

than that of the doped PEDOT:PSS in this work would be an ideal candidate. Especially with

the higher capacitance per unit mass reported for carbon nanotubes (in excess of 100 F/g).

However, carbon nanotubes are currently quite expensive and may only see application as a

conductive additive as a result.

126

7.3.6 Freeze Dried Foams

In order to improve performance per unit geometric area, especially at higher rates, freeze

dried films were fabricated. These foams allow electrolyte to enter the material more easily.

In figure 7.18, the left image shows an edge-on image of a freeze dried film of a similar mass

per unit area to the 90 µm sample. The right image in figure 7.18 shows a torn portion of an

edge revealing thin walls which form the pores.

Figure 7.18: The freeze dried film over 2mm thick similar mass to sample 90mm thick (left) Image of torn edge showing thin wall structure (right).

The porous nature of the electrode should allow quicker access of the electrolyte into

the electrode reducing the effects of diffusion. The freeze dried electrodes were tested by

cyclic voltammetry from 0.1 to 0.8 volts vs. Ag/AgCl with scan rates from 0.02 to 2 V/s. In

figure 7.19(a), cyclic voltammograms of a dropcast film of mass similar to that of the freeze

dried film (figure 19(b)) are displayed for comparison.

127

0.0 0.2 0.4 0.6 0.8

-5000

0

5000

j/s (

F/m

2)

(b) M/A = 92g/m2


(a) M/A = 84g/m2

0.0 0.2 0.4 0.6 0.8

-5000

0

5000


Figure 7.19: CVs of (a) a 84 g/m2 dropcast film and (b) a 92 g/m2 freeze dried film with scan rate range 0.02 V/s (red) to 2 V/s (green)

The red lines are the 0.02 V/s scan and the green lines are the 2 V/s scans. The

performance with increased scan rate for the freeze dried films is improved relative to the

dropcast film. This is more clearly visible if the current density divided by the scan rate and

capacitance per unit area are plotted against scan rate. In figure 7.20, these values are

normalised with respect to the value at 0.02 V/s for both the freeze dried film and the

dropcast counterpart for direct comparison.

0.01 0.1 1

1E-3

0.01

0.1

1

Freeze Dried 92g/m2

Dropcast 84g/m2

No

rma

lise

d j/s

@ 0

.4V

(F/m

2)

Scan Rate (V/s)

(a)

0.01 0.1 1

0.01

0.1

1

No

rma

lize

d C

/A

Scan Rate (V/s)

Figure 7.20: (a) Scan rate dependence of normalised (dq/dV)/A @ 0.4 V of freeze dried (black) and dropcast (red) films. (b) Scan rate dependence of normalized capacitance per unit area of freeze dried (black) and dropcast (red) films.

128

The current density at 2 V/s is approximately an order of magnitude higher for the

freeze dried film compared to the dropcast film. The difference in capacitance per unit area is

not as pronounced. However, consulting figure 7.11 for films in this mass range, diffusion

only has an effect close to 0.2 V/s. The effect of the improvement of freeze dried films are

evident immediately.

The fact that the improvement occurs earlier than expected for diffusion means there

is an improvement in either the intrinsic capacitance per unit area, which could be due to an

increase in surface area per unit mass of the material, or a reduction in the time constant

which would suggest the film allows better charge transport properties.

In order to find the intrinsic capacitance per unit area and time constant, the scan rate

dependence of capacitance per unit area was fitted according to equation 7.5.

0 20 40 60 80 100

0

1000

2000

3000

4000

(b)

CA (

F/m

2)

M/A (g/m2)

Equation y = a + b*x

Weight No Weighting

Residual Sum of Squares

619.70989

Pearson's r 0.9999

Adj. R-Square 0.9997

Value Standard Erro

CaIntercept 155.22804 25.04288

Slope 38.37995 0.38278

(a)

0 20 40 60 80 100

0.0

0.5

1.0

1.5

2.0

2.5

(s

)

M/A (g/m2)

Figure 7.21: (a) Intrinsic capacitance per unit area of the freeze dried films (black) and other films (blue) with a linear fit (red) to determin intrinsic capacitance per unit mass. (b) Time constant of freeze dried films (black) compared to other films (blue) with a linear fit (red).

Plotting the intrinsic capacitance per unit area and time constant against mass per unit

area, as shown in figure 7.21(a) and (b) respectively, the freeze dried films can be compared

with the films from the previous section. The capacitance per unit area of the freeze dried

films has a similar slope to the films from the previous section giving an intrinsic capacitance

per unit mass of 38 F/g. However, there is an appreciable intercept which may have arisen

due to experimental error.

129

The time constant has a much lower slope but a higher intercept. This suggests poorer

conductivity in the film but a better conductivity in the electrolyte possibly due to the

geometry of the electrode. From the intercept, the conductivity of the freeze dried film is 307

S/cm while the conductivity of the electrolyte is 0.42 S/cm2, almost twice as high compared

to the films in the previous section. This results in a time constant that is lower than that of

the dropcast films in the range of masses of the freeze dried films resulting in better rate

performance. The values extracted from the line-fits in figure 7.21 can be used to describe

the behaviour of capacitance per unit area with thickness by substituting for CA and τ in

equation 7.5.

1 10 100 100010

100

1000

10000

0.02V/s

0.1V/s

.5V/s

C/A

(F

/m2)

M/A (g/m2)

Figure 7.22: The dependence of capacitance per unit area with mass per unit area at 0.02 V/s (black), 0.1 V/s (red), 0.5 V/s (blue).

The model predicts the capacitance per unit area of the freeze dried films well,

particularly for the lower scan rates. However, the thickness of the freeze dried electrodes

compared to the dropcast electrodes results in a reduced volumetric capacitance which is

important for devices in which compact energy storage is required.

130

7.4 Conclusions

In this chapter, supercapacitor electrodes were fabricated by various casting methods and

subsequently doped in order to improve the conductivity of the electrode. A wide range of

thicknesses were produced with the aim of understanding the effects of thickness on

conductivity, capacitance and time constant.

Two models were used to describe the scan rate behaviour of capacitance. The

Higgins model fit the J/s-V curves well for low thicknesses and scan rates. However, as both

thickness and scan rate increased, the Pell and Conway model provided better insight into the

performance of the electrodes.

The effect of thickness on sheet resistance of the films was used to find the electrical

conductivity of the doped PEDOT:PSS films which was approximately 900 S/cm.

Comparing experimental data to theory, an intrinsic capacitance per unit mass of 34 F/g was

obtained. This capacitance was comparable to other work on PEDOT films accounting for

the mass of PSS in the film.

The electrical properties of the electrolyte, which were dependent on the electrode

thickness, were analysed by comparing the variation of the time constant, which is dependent

on the resistance of the system, with mass per unit area. Once the dependence of the time

constant was ascertained, the dependence of capacitance per unit area on mass per unit area

was compared to theory with good agreement. The maximum capacitance reached was 0.6

F/cm2

with a sheet resistance of less than 0.1 Ω/sq. The capacitance was comparable to other

work on PEDOT based capacitor electrodes, however, the sheet resistance was an order of

magnitude lower than similar electrodes326

.

Analysis of the theory led to establishing dimensional and rate-based limitations for

the design of the electrodes. This information is critical when designing devices for particular

applications as there is no ‘one size fits all’ for supercapacitors. The effect of diffusion on the

electrodes was found not to be as significant as that of the effect on the electrical properties

of the electrode and the electrolyte.

131

In order to improve charge transport of the electrolyte and limit the effect of diffusion,

the PEDOT:PSS dispersion was freeze dried. This led to an improved intrinsic capacitance

per unit mass of 38 F/g, coupled with an increase in the electrolyte conductivity by a factor

of two. These electrodes, however, were much less dense which would prove problematic in

compact device applications. Freeze drying a more concentrated dispersion may provide

suitable porosity while also keeping the film thickness in an appropriate range.

132

8

Transparent PEDOT:PSS Supercapacitors with Graphene

Current Collectors

8.1 Introduction

In this chapter the merits of adding a current collector to PEDOT:PSS transparent

supercapacitor electrodes will be discussed. Transparent supercapacitors with PEDOT:PSS

have been extensively investigated by Higgins and Coleman resulting in capacitances of 1

mF/cm2 at transparencies of 80 %

196.

Graphene has also seen application in transparent supercapacitors. 20 µF/cm2

is the

theoretical capacitance per unit area of graphitic carbon122

. The high surface area of graphene

allows a maximum capacitance per unit mass of 526 F/g. For transparent supercapacitors the

amount of graphene per unit area has to be constrained to limit light absorption. 4 layers of

graphene would have a transparency below 90 % and a mass per unit area of 3.8 x 10-8

g/cm2.

This leads to a theoretical capacitance per unit area in the order of tens of micro-farads per

centimetre squared.

Chen et al used wrinkled graphene as a stretchable flexible transparent electrode material

using a solid electrolyte. The wrinkling increases the mass per unit area and allows for

stretching. The total transparency of the device was 57 % and the capacitance was 5.8

µF/cm2.355

A combination of CVD graphene as a current collector and graphene oxide

quantum dots for enhanced capacitive properties, fabricated by Lee et al, reported a

capacitance of 9.09 µF/cm2 with an electrode transparency of 92.97 % at 550 nm

356. Yoo et al

report a high capacitance of 80 µF/cm2 for in plane graphene devices comprising of 2

graphene layers357

.

133

The low capacitances of graphene limit its application as the capacitive element in

supercapacitor electrodes. The excellent electrical and mechanical properties of graphene

make the use of graphene feasible as the current collector element of the electrode used in

conjunction with materials with higher capacitances with better transparencies.

Carbon nanotubes have also been demonstrated as a potential material in transparent

capacitors136,195,197,358

. As have composites with carbon nanotubes359–361

. Transition metal

oxides with high capacitances per unit mass have also been demonstrated in transparent

supercapacitors188,362

.

In this chapter PEDOT:PSS will be deposited on both PET (Polyethylene terephthalate) and

graphene substrates. The effect on the optoelectronic and capacitive properties of the

PEDOT:PSS film of the substrate will be investigated to determine the effectiveness of

graphene as a current collector for transparent PEDOT;PSS supercapacitor electrodes. The

dependence of these properties on the length of the electrode will also be investigated in

order optimize device design parameters.

8.2 Experimental Procedure

8.2.1 PEDOT:PSS and PEDOT:PSS on Graphene Film Preparation

Clevios PH1000 PEDOT:PSS dispersion was purchased from Hereus. 95% Formic Acid was

purchased from Sigma Aldrich. 4 layer CVD graphene on PET substrate was provided by

Strupinski et al363

.

The provided PEDOT:PSS solution was diluted to both 0.2 and 0.4 mg/ml. The method of

producing the film whether on a PET (Polyethylene terephthalate) substrate or a graphene-on-

PET substrate is the same as done by McCarthy et al325

. The substrate was heated to 130⁰C

and the diluted PEDOT:PSS solution was fed to a Sud-ko airbrush spray gun using a pipette

tube as a reservoir.

The material was deposited by spraying using a Janome JR2300N robot over an area of 5 by

5 cm2. The robot followed a raster pattern with line spacing of 0.5 cm with the airbrush

substrate distance being approximately 10 cm. The driving gas to the airbrush spray gun was

nitrogen with a pressure in the range of 45-55 psi.

134

Once the material was deposited the films were dipped in formic acid for 5 seconds to

improve the conductivity before being allowed to dry in air.

8.2.2 Optical and Electrical Characterisation

UV-Vis transmission spectra were taken in the wavelength range of 350 to 700 nm using a

Cary 600i spectrophotometer using a PET reference for comparison. The electrical properties

of the film were examined by running IV curves using the four wire method with a Keithley

2400 sourcemeter in the range of 0 to 1V. The film was contacted using an alcohol based

silver paint and silver wires.

8.2.3 Electrochemical Characterisation

Electrochemical characterisation was carried out using a Gammry Ref 3000 potentiostat. All

measurements were taken in a three electrode setup using a silver/silver chloride (Ag/AgCl)

reference electrode and a graphite rod as the counter electrode. The electrolyte used was

0.5M Potassium Sulphate (K2SO4).

Cyclic voltammograms (CVs) were taken at a variety of scan rates from 0.1 V/s to 5 V/s. The

voltage window was between 0.1 V and 0.8 V versus Ag/AgCl. The electrode was contacted

with silver paint at the top which was insulted using varnish. Potentiostatic EIS Spectra were

taken at 0.4 V versus Ag/AgCl using an AC voltage perturbation of 10 mV.


8.3.1 Optoelectronic Properties

Transparent films in a transparency range of approximately 95-80 % at 550 nm were

fabricated on both PET and graphene substrates. The most and least transparent materials

deposited on PET is shown in figure 8.1. At the top of the films, the silver paint for

135

contacting is visible while the varnish to protect the silver paint from the electrolyte is less

visible. Each sample was prepared by depositing a known volume of a known concentration

over 5 x 5 cm2. Then a doping step was undertaken using formic acid to improve the

conductivity of the film. As such, samples will be referenced by the concentration of

PEDOT:PSS in dispersion times the volume sprayed with a tag for substrate as follows

pt(concentration in mg/ml x 10)x(volume sprayed in deposition)-substrate. The tag for the

substrate is given as PET or Gra (for graphene).

Figure 8.1 Sprayed PEDOT:PSS on PET - pt2x4-PET(left) and pt4x6-PET (right) scale bar = 1cm

For transparent electronics good transparency across the visible spectrum is required. To

characterize further the transparency of our deposited PEDOT:PSS on both PET and

Graphene-on-PET, UV-Vis spectrophotometry was analysed in the range from 350 to 700nm.

In figure 8.2. the transparencies of the films are shown in the wavelength range. For low

thicknesses of PEDOT:PSS the transparency increases with decreasing wavelength with few

visible features. The thicker PEDOT:PSS films show a feature around 450-480 nm. The

graphene film which consists of 4 layers has a maximum transparency around 90 % at the

highest wavelength and this decreases steadily as the wavelength decreases.

136

400 500 600 70050

60

70

80

90

100

pt2x4

pt4x4

pt4x6

Gra

Gra+pt2x4

Gra+pt4x4

T (

%)

Wavelength (nm)

Figure 8.1 UV-Vis spectra for a range of thicknesses of PEDOT:PSS on PET and Graphene-on-PET

Theoretically, a single layer of graphene has a transparency of 97.7 %244,364

at 550 nm the

transparency of the 4-layer graphene as measured to be 89.4 %, which is lower than what

would be expected theoretically of 91%.

Once the viability of the material’s transparency has been determined, the electrical

properties must be analysed. ITO has been a leading transparent conductor for some time

with transparencies in excess of 90 % and sheet resistances in the range of 10s of Ω/sq281

.

However, it lacks the mechanical stability to be effective in flexible and stretchable devices,

displaying irreversible loss of conductivity under strains in excess of 1 %. The industry

standard requires a sheet resistance < 100 Ω/sq. and a transparency > 90 % at 550 nm.

To analyse the competitiveness of the PEDOT:PSS films, the graphene film, and the

PEDOT:PSS on graphene films, the transparency of the films was plotted against the sheet

resistance as calculated from a 4-wire measurement as shown in figure 8.3.

137

50 100 150 200 250 300 350 40076

78

80

82

84

86

88

90

92

94

96

PEDOT:PSS

Graphene

Graphene + PEDOT

Tra

nspare

ncy @

550nm

(%

)

Sheet Resistance (/sq)

Figure 8.3 Transparency of PEDOT:PSS, graphene and PEDOT:PSS-on-graphene films compared with sheet resistance. Lines correspond to equation 1.

The transparency of a conductive film at a certain wavelength (λ) can be expressed as a

function of sheet resistance according to equation 8.1.

2

0

2

0)(

21)(

21)(

sDC

op

opR

Zt

ZT

Equation 8.1

where Z0 is the impedance of free space (377 Ω), σop is the optical conductivity of the

material at a given wavelength, and t is the thickness of the film. Sheet resistance (Rs) is

defined as Rs = tDC

1where DC is the DC conductivity of the material. The formic acid

treated PEDOT:PSS samples fit well to a line (black) with a ratio of optical conductivity to

DC conductivity of 48. This ratio is the figure of merit used for transparent conductors, the

higher this figure of merit the more suitable the material is for a transparent conductor. The

blue line corresponds to a figure of merit of 40. The graphene sample provided had a sheet

resistance of 337 Ω/sq. and a transparency of 89.4 %. This is a significantly higher resistance

at comparable transparencies due to the high absorption of light that occurs in graphene

making graphene an inferior transparent conductor.

As far a transparent conductors go, the doped PEDOT:PSS meets the required industrial

criteria for transparent conductors. The criteria are met when the ratio of optical to DC

138

conductivity is 35 according to equation 8.1. The value achieved in this work is consistent

with the work of McCarthy et al. which gives an average figure of merit of 50.

Other materials which have been used to fabricate transparent conductors include carbon

nanotubes278,280,281

, graphene251,280,364

, metal nanowires215,216,365,366

and conductive

polymers367

.

Carbon nanotubes have been extensively tested as transparent conductors and have a figure of

merit of 13281

for pristine films. However, acid treatments or annealing can improve this to

25-35 which approaches industry requires. The junction resistance in CNT films can be

overcome with small amounts of graphene flakes improving the ratio by 40 %280

.

The limitations of graphene as a transparent conductor were reviewed by De & Coleman364

where a variety of methods of production were investigated and the figures of merit were

compared. The values lie in groups around 0.7, 4.5 and 11. This is lower than even pristine

carbon nanotube films. They stipulate that a sheet resistance of 10 Ω/sq. at a transparency of

90% can be reached using doping to increase the product of mobility and charge carrier

density. Bae et al. use a layer by layer method to stack 4 nitric acid doped graphene sheets to

achieve a sheet resistance of 30 Ω/sq. (an order of magnitude lower than the graphene in this

study) at 90% transmittance251

.

8.3.2 Scan Rate Dependence of Capacitance

To evaluate the energy storage performance of the transparent PEDOT:PSS films on both

PET and graphene of different lengths, the samples were tested using cyclic voltammetry.

The potential range was 0.1 to 0.8 V versus an Ag/AgCl reference electrode. The width of the

electrodes was 9 mm and the length range test was from 4.5 cm to 1 cm.

Figure 8.4 displays the cyclic voltammograms of a 3cm long film of sample pt2x4-PET (a)

and a 3cm long film of pt2x4-Gra (b). The lowest and highest scan rates were 0.1V/s (red)

and 5V/s (green) respectively. Both films are approximately boxlike, representing ideal

capacitor-like behaviour. The relative size of the voltammogram at 5V/s, compared to the

voltammogram at 0.1V/s, is smaller for the film without graphene as a current collector. The

capacitance values are also higher for sample 2x4-Gra. This can be explained by the decrease

139

in time constant due to a decrease in film resistance. The effect of which is described by

equation 7.1 in the previous chapter.

0.0 0.2 0.4 0.6 0.8-6

-4

-2

0

2

4

6

pt2x4-Gra 3cm

pt2x4-PET 3cm

0.0 0.2 0.4 0.6 0.8-6

-4

-2

0

2

4

6

0.0 0.2 0.4 0.6 0.8

-10

-8

-6

-4

-2

0

2

4

6

8

10

j/s (

F/m

2)

j/s (

F/m

2)

pt4x4-PET 3cmpt4x4-PET 3cm


0.0 0.2 0.4 0.6 0.8

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

(d)(c)

(b)


(a)

Figure 8.4 Cyclic voltammograms of (a) 0.2mg/ml PEDOT:PSS x 4ml on PET (b) 0.2mg/ml PEDOT:PSS x 4ml on CVD (4 layer) Graphene (c) 0.4mg/ml PEDOT:PSS x 4ml on PET (d)

0.4mg/ml PEDOT:PSS x 4ml on CVD (4 layer) Graphene. The red line is the voltammogram at 0.1V/s and the green voltammogram is the one at 5V/s

For figure 8.4(c) and (d) corresponding to 3cm in length films of samples pt4x4-PET and

pt4x4-Gra respectively are almost indistinguishable to the eye, with the pt4x4-Gra sample

having a marginally higher capacitance. This is possibly due to the relative resistance of the

graphene film (337 Ω/sq.) being significantly higher than that of the PEDOT:PSS doped film

(50 Ω/sq) compared to the pt2x4 samples where the sheet resistance exceeds 100 Ω/sq.

At first glance it looks like the addition of the current collector only provides significant

improvement on the performance when the resistance of the PEDOT:PSS film is high enough

to be comparable to the graphene film. To inspect the effect on the capacitance per unit area,

the cyclic voltammetry curves were integrated to give capacitances per unit area for the range

of scan rates tested. The integration of the curve can be expressed as equation 7.4.

140

0.1 1

1

4.2cm

3cm

2.5cm

2cm

1.5cm

1cm

pt4x4-Gra pt4x4-PET

pt2x4-Gra pt2x4-PET

C/A

(F

/m2)

0.1 1

1

(d)(c)

(b)(a)

0.1 1

2

4

6

8

10

C/A

(F

/m2)

Scan Rate (V/s)

0.1 1

2

4

6

8

10

Scan Rate (V/s)

Figure 8.5 Scan rate dependence for various lengths ranging from 1cm(magenta) to over 4cm (black) for (a) 0.2mg/ml PEDOT:PSS x 4ml on PET (b) 0.2mg/ml PEDOT:PSS x 4ml on CVD (4 layer) Graphene (c) 0.4mg/ml PEDOT:PSS x 4ml on PET (d) 0.4mg/ml PEDOT:PSS x 4ml on

CVD (4 layer) Graphene

In figure 8.5 the capacitance per unit area of various films as a function of scan rate is plotted.

(a) and (b) compare pt2x4-PET and 2x4-Gra samples respectively. The capacitances are

higher as was clear from the cyclic voltammogram as is the improved performance at higher

rates. Figures 8.5(c) and (d) compare pt4x4-PET and 4x4-Gra samples. The increased

capacitance per unit area is more obvious. The effect on scan rate dependence in particular

for the longer films (black) is also better for films with the graphene substrate provideding

larger C/A values for corresponding rates especially for the large scan rate..

To extract the characteristic features, the intrinsic capacitance per unit area, CA, and time

constant, τ, the scan rate dependence was fitted to equation 7.4. The fit is good across the

range of films returning values for CA that are within about 5% of the average.

141

8.3.3 Length Dependence of Capacitance

In Fig 8.5 the scan rate dependence for various lengths was shown. The fittings of these

capacitance per unit area curves resulted in CA and τ for each length of film. CA should be

invariant with length. However τ is dependent on the electrical properties of the film which

are determined by the dimensions of the film, as seen in equation 7.7. A modified version of

this equation can be used to extract values for sheet resistance and electrolyte conductivity.

A

AsA

G

CLRC 2 equation 8.2

Figure 8.6 shows the time constant as a function of film length squared for the films in figure

8.5. The time constant varies linearly for all electrodes with length squared. By fitting to a

straight line to the data, the sheet resistance can be obtained from the slope and the electrolyte

conductivity can be retrieved from the intercept.

0.0000 0.0005 0.0010 0.0015 0.00200.0

0.5

1.0

pt2x4-PET

pt2x4-Gra

pt4x4-PET

pt4x4-Gra

(s)

L2 (m

2)

Figure 8.6 The time constant of various films: 0.2mg/ml x 4ml PEDOT:PSS on PET, 0.2mg/ml x 4ml PEDOT:PSS on CVD Graphene. 0.4mg/ml x 4ml PEDOT:PSS on PET and 0.4mg/ml x 4ml

PEDOT:PSS on CVD Graphene.

The average electrolyte conductivity for all films analysed is 21 mS/cm2 which is close to the

values established in the previous chapter (64 mS/cm2). Of greater interest to this work is the

142

resistance of the films. The transparency of all films has been plotted against both CA and Rs

derived from the fitting of equation 7.4 to the scan rate dependence in figure 8.7.

2 4 6 8 10 12 147678808284868890929496

T (

%)

CA (F/m

2)

(a) (b)

0 25 50 75 100 125 1507678808284868890929496

Rs (/sq)

Figure 8.7 (a) Transparency against intrinsic capacitance per unit area for all films on PET substrate (black) and on graphene substrate extracted from fits to CV data (red) (b)

transparency against sheet resistance for all films on PET substrate (black solid) and on graphene substrate (red solid)determined from fitting the time constants of the films dependence on length compared with the corresponding values of sheet resistance as

determined by the 4-wire measurement (hollow crossed points)

In figure 8.7(a) the capacitance of the films using graphene as a substrate suffer from lower

transparencies for the same values of CA. Transparency can be fit to CA by modifying

equation 8.1. and letting the thickness of the electrode, t = CA/(CMρ). Where CM is the

intrinsic capacitance per unit mass (34,000 F/kg from previous chapter) and ρ is the film

density (1200 kg/m3).

2

0

2

0)(

21)(

21)(

A

M

op

op CC

Zt

ZT

Equation 8.3

This equation is a good fit to the data for transparent capacitors for the PEDOT:PSS-on-PET

samples. Due to the lack of contribution of capacitance from the graphene layer while still

contributing to the transparency, the fit is significantly worse for the PEDOT:PSS-on-

graphene. By multiplying equation 8.3 by the transparency of the graphene which is 89.4 %

(indicated by the red line) the fit improves to describe the transparency and capacitance well.

For homogenous systems, a figure of merit for transparent capacitors could be the ratio of

optical conductivity to the product of intrinsic capacitance per unit mass and density. This

figure of merit must be below 2.9x10-5

S.m2.F

-1 to achieve intrinsic capacitances of 1 mF/cm

2

143

at 90% transparency. The figure of merit for the PEDOT:PSS only samples is 4.55x10-5

S.m2.F

-1. While PEDOT:PSS is above this threshold, addition of nanoparticles with high

specific capacitance may be an avenue to reaching this number. This, in addition to the metric

for sheet resistance, can be used for design of transparent capacitor electrodes. Figure 8.7(b)

shows sheet resistances agrees well with those found using the 4-wire measurement.

Taking the values for CA and τ and applying them to equation 7.4 the equation can be

compared to the length dependence of the films. In figure 8.8 a sample CVs for various

lengths of films is provided. As length increases the capacitance per unit area decreases. This

is due to the increased resistance that charge has to pass through to the current collector. In

figure 8.8. a CV of the pt2x4-Gra film at different lengths shows a box-like voltammogram at

a length of 1cm(red line). As length increases, the voltammogram deviates from the original

boxlike shape. Since the current is normalized by area, the area of the curve in the CV for

greater lengths is clearly smaller.

0.0 0.2 0.4 0.6 0.8-6

-4

-2

0

2

4

6

j/s (

F/m

2)


pt2x4-Gra

Figure 8.8 Cyclic voltammograms for the pt2x4-Gra for various lengths from 1cm (red) to 3cm (green)

The effect of increased length can be expressed by substituting for τ in equation 7.4 with

equation 8.2.

A

AsA

A

AsAA

G

CLRCs

VG

CLRC

V

sC

A

C

2

2 exp11 Equation 8.4

144

When the values for CA and τ derived from the fitting of the scan rate dependence curves in

figure 8.5. are substituted into this equation, the equation provides a good fit as shown in

figure 8.9.

0.01 0.02 0.03 0.04 0.05

1

C/A

(F

/m2)

0.01 0.02 0.03 0.04 0.05

1

0.01 0.02 0.03 0.04 0.05

1

C/A

(F

/m2)

Length (m)

pt2x4-PET

pt4x4-PET pt4x4-Gra

pt2x4-Gra

0.01 0.02 0.03 0.04

2

4

6

8

10

Length (m)

(a) (b)

(d)(c)

Figure 8.9 Length dependence for various scan rates for (a) 0.2mg/ml PEDOT:PSS x 4ml on PET (b) 0.2mg/ml PEDOT:PSS x 4ml on CVD Graphene (c) 0.4mg/ml PEDOT:PSS x 4ml on PET

(d) 0.4mg/ml PEDOT:PSS x 4ml on CVD Graphene

The comparison with similar amounts of PEDOT:PSS on PET and graphene show in

improvements of electrical and capacitance properties which have a more significant effect

on the time constant. This improvement is more pronounced for lower amounts of

PEDOT:PSS, as the sheet resistance of the graphene substrate is more comparable to that of

the PEDOT:PSS.

While the effect of the addition of the graphene current collector is of interest, direct

comparison between films of similar transparency is more important for this application.

There is a trade off with the addition of graphene as the graphene absorbs as much as the

pt4x4 around 550nm. This means that there is a significant amount of material (in terms of

relative absorbance) in the PEDOT:PSS-on-Graphene films that is effectively not

contributing to the capacitance. Consulting figure 8.7(a) shows a significant gap in intrinsic

145

capacitance per unit area. The films pt4x6-PET and pt4x4-Gra have similar transparencies

and so a direct comparison over length and scan rate is possible.

Figure 8.10 (a) Scan rate dependence of capacitance per unit area of two samples with similar transparency (approximately 79%): pt4x4-Gra(black) and pt4x6-PET(red). Dashed line is the difference between the capacitances according to equation x.4 and dotted line is the ratio of capacitances. (b) Length dependence of capacitance per unit area of two samples with similar transparency (approximately 79%): pt4x4-Gra(black) and pt4x6-PET(red). Dashed line is the difference between the capacitances according to equation x.4 and dotted line is the ratio of capacitances.

Figure 8.10. provides comparison between the pt4x4-Gra and the pt4x6-PET for both

different scan rates when the sample length is 3 cm (a), and different lengths when the scan

rate is 0.5 V/s (b). The fits over the testing range for both samples, show no signs of

converging, with the fits at the extrema seeming to become parallel.

For better comparison, the difference in capacitance per unit area (dashed line) and ratio of

capacitance (dotted line) of the fits were plotted. The ratio decreases slowly for both scan rate

and length. The point at which the ratio would equal one is much larger in both scan rate and

length for practical purposes.

However, adjusting the equation by decreasing the time constant by a factor of 2 (equivalent

to reducing the resistance by half), allows the ratio to equal one for scan rates of 1 V/s at a

length of 3 cm or less than 5 cm at 0.5 V/s. The reducing the resistance by half requires the

transparent conductor layer to have the same resistance as the PEDOT:PSS layer using the

parallel resistors rule. This is a resistance in the range of 35-40 Ω which is achievable for N-

doped graphene. In summary, the graphene used for this study is of insufficient conductivity

and transparency to make a meaningful impact on transparent capacitor performance.

0.1 11

10 pt4x4-Gra

pt4x6-PET

Difference

Ratio

C/A

(F

/m2)

Scan Rate (V/s)

0.01 0.02 0.03 0.04 0.051

10

(b)

Length (m)

(a)

146

8.3.4 Impedance Spectroscopy

To assess the impedance of the electrodes, impedance spectroscopy from 1 MHz to 0.1 Hz at

0.4 V with respect to Ag/AgCl with a perturbation amplitude of 10 mV was performed. The

Nyquist plot in figure 8.11(a) gives lines with two characteristic regions: one with a phase of

approximately 90⁰ and at low frequencies and one with a phase near 45⁰ corresponding to

high frequencies. The frequency at which the electrode changes phase from 90⁰ to 45⁰ as

frequency decreases is shown in figure 8.11(b) and (c). For the shortest film the transition

occurs around 10 Hz and for the longest film the transition occurs around 1.5 Hz.

-100 -50 0 50 100 150 200

0

100

200

300

400

500

-Zim

g (

)

Zreal

()

4.25cm

3cm

2.5cm

2cm

1.5cm

1cm20

40

6080

100

Zim

g (

)

Zre

al (

)

0.1 1 10 100

10

100

1000(c)

(b)

Frequency (Hz)

(a)

-1

Figure 8.11 (a) Nyquist plot of pt4x4-Gra sample for various lengths (b) The real component of the Nyquist Plot against frequency (c) The imaginary component of the Nyquist plot

against frequency.

Using a modified analytical solution to the transmission line model along the length of a thin

film capacitor196,238

the impedance can be described by the resistive and capacitive elements

as well as the device dimensions. The impedance below the transition frequency is:

LWC

iR

W

LZ

A

s

3

Equation 8.5

Where ω is the angular frequency in radians per second and all other variables have been

previously defined.

147

Extrapolating the linear portion below the transition frequency upward/downward gives the

real impedance of the system, which can be plotted against the ratio of length to width of the

electrode which as shown in figure 8.12(a) for various films.

1 2 3 4 5

0

100

200

300

pt2x4-PET

pt2x4-Gra

pt4x4-PET

pt4x4-Gra

pt4x6-PET

Zre

al (

)

L/W

0 100 200 300 400

76

78

80

82

84

86

88

90

92

94

96

PEDOT EIS

Graphene + PEDOT EIS

PEDOT:PSS 4-wire

Graphene + PEDOT 4-wire

(b)

T (

%)

Rs (/sq)

(a)

Figure 8.12(a) The Real Impedance component plotted against the ratio of length to width of various samples (b) The sheet resistance extracted from the fitting of previous figure(solid

points) compared with values obtained from the 4-wire measurement (crossed points).

The slope of these straight line graphs is equivalent to the sheet resistance divided by 3. In

figure 8.12(b) the sheet resistances found using the impedance data is compared to the sheet

resistances as found by the 4-wire measurement. There is not an agreement between the

impedance measurement and the four wire measurement.

Analysis of the low frequency region of the imaginary component of the impedance shows an

inverse relationship with the angular frequency. In figure 8.13(a) the low frequency points

were plotted against the inverse of the angular frequency and fitted to a straight line. The

capacitance of the films was extracted according to the imaginary part of equation 8.5 and

plotted against area for various films to get CA in figure 8.13(b). For comparison,

transparency was plotted against the CA obtained from the impedance data and the

voltammetry to reasonably good agreement.

148

0.2 0.4 0.6 0.8

100200300400500600700800900

10001100

4.5cm

3cm

2.5cm

2cm

1.5cm

1cm

-Zim

g (

)

1/ (s/rad)

0.0001 0.0002 0.0003 0.00040

1x10-3

2x10-3

3x10-3

4x10-3

5x10-3

6x10-3

pt2x4-PET

pt2x4-Gra

pt4x4-PET

pt4x4-Gra

pt4x6-PET

C (

F)

A (m2)

2 4 6 8 10 12 14 167678

8082

848688

9092

9496

PEDOT EIS

Gra + PEDOT EIS

PEDOT CV

Gra + PEDOT CV

T (

%)

CA (F/m

2)

Figure 8.13(a) Imaginary component of the impedance against the inverse angular frequencies in the low frequency linear region of a pt4x4-Gra film. (b) The capacitance of

various films plotted against area (c) a comparison of the transparency against the intrinsic capacitance per unit area obtained from impedance data (solid points) and voltammetry

data (crossed points)

149

8.4 Conclusions

In this chapter CVD graphene was used as a current collector for transparent supercapacitors.

The effect of the current collector was to reduce the sheet resistance of the electrode which

would also reduce the time constant. A reduced time constant would hopefully provide better

performances at higher scan rates and lengths.

The CVD graphene provided for this study had a sheet resistance of 337 Ω/sq. at

approximately 90 % transparency. This compares unfavourably with the formic acid doped

PEDOT:PSS which has a sheet resistance around 60 Ω/sq. for the same transparency.

The electrochemical characteristics of the films as supercapacitor electrodes were analysed

using cyclic voltammetry and impedance spectroscopy. The intrinsic capacitance per unit

area derived from the analysis of the data produced by both methods were in agreement.

As with sheet resistance, the intrinsic capacitance per unit area was lower for the films

employing a graphene current collector. This was expected as graphene has a capacitance per

unit area two orders of magnitude lower than that of PEDOT:PSS. However, analysis of the

time constants revealed that the ratio between the capacitance per unit area did not decrease

sufficiently with either length or scan rate to encourage implementation.

Were the current collector to have a comparable conductivity to the PEDOT:PSS, this could

result in capacitances that exceed the performance of PEDOT:PSS alone for scan rates over 1

V/s at an electrode length of 3 cm or for scan rates of 0.5 V/s for electrode lengths of 5 cm.

This is achievable with chemically doped graphene251

.

Another avenue is to employ other materials as the current collector such as carbon nanotubes

or metal nanowires. These materials have seen successful application as transparent

conductors and may be useful in transparent capacitors also215,280,366

.

150

9

Conclusions and Future Work

9.1 Conclusions

In this thesis various materials with dimensions in the nanoscale have been investigated for

use in electrochemical systems. For dye-sensitized solar cells the challenge of replacing the

expensive platinum in the catalytic counter electrode with graphene was thoroughly

investigated. The influence of size of the flakes proved to be less effective for improving the

efficiency than increasing the film thickness. However, creating films with efficiencies in

excess of 80% of that achievable with platinum remained elusive. As such, addition of other

materials into the graphene film to bridge the gap between graphene and platinum was

attempted. By enhancing conductivity, particularly in the vertical direction which was limited

due to the anisotropy of charge transport in graphene networks, with carbon nanotubes the

efficiency became comparable to platinum. The addition of a more catalytic material, MoS2,

also produced similar results with the advantage of the material being cheaper due to its

presence in nature. On investigation of the performance of the electrodes using percolation

theory it was revealed that while the edges of MoS2 are more catalytically active the main

advantage of using the MoS2 was that the nanosheets were on average smaller using the same

processing conditions. The smaller sheets in the lateral dimensions allowed for a higher

length to area ratio increasing the percentage mass of the particles contributing to the

catalytic activity of the material.

Supercapacitor electrode materials in the form of PEDOT:PSS films prepared by a

variety of methods and treated with formic acid to increase conductivity were alsostudied in

this thesis. Two models were compared to describe the effect of increasing thickness on the

capacitance per unit area. Both models returned capacitances per unit area within 10% of

151

each other allowing both models to be considered accurate for investigation of that property.

When looking at the current-voltage characteristics or the time constant however, the Pell &

Conway model which accounts for an initial charge on the electrode performs better than the

Higgins & Coleman model which assumes no charge initially. The role of diffusion affects

the capacitance per unit area at higher scan rates. Neither of the models account for it and

from this the diffusion coefficient was estimated to be sm /103.63.5 210 which is about

30-40% that of the ions in water. When accounting for time constant and scan rate the effect

of the electrical properties of the film with length and thickness were found to have a greater

effect on film performance than diffusion. For completeness freeze dried foam of the same

material were fabricated to both reduce the effects of diffusion and increase internal surface

area of the films. The benefits of freeze drying are modest and when compared to the

disadvantages like additional processing steps and electrode thickness lead to a conclusion of

this method not being viable for this material system.

Optically transparent supercapacitor electrodes represent a solution to energy storage

for transparent electronics. PEDOT:PSS has a combination of good electrical properties at

high transparencies that allow for application as transparent conductors. In addition

PEDOT:PSS also has an appreciable capacitance which led to it being demonstrated as a

transparent supercapacitor electrode material. In an attempt to improve the electrical

properties of the film a 4 layer sheet of graphene was used as a current collector. The effect

on performance of the addition of the graphene was negligible at comparable transparencies

of PEDOT:PSS only electrodes. In order to provide a significant improvement to these

electrodes the conductive layer needs to be at least of the same sheet resistance as the

capacitive layer. While this will lead to a lower capacitance at low rates the performance at

higher rates could exceed the capacitance of the PEDOT:PSS only electrode.

9.2 Future Work

9.2.1 DSSC Counter Electrodes

In this thesis graphene composite counter electrodes were fabricated which had a

performance almost as good as a platinum counter electrode (96% of Pt cell). These 400nm

152

thick films however were not optically transparent which limits the possible applications. The

reduction in size of the graphene particles resulted in an increase in efficiency. By reducing

the size of graphene further, it may be possible to manufacture transparent graphene counter

electrodes with comparable performance to platinum counter electrodes. Below a certain size

all of the atoms in the graphene sheet will be chemically active limiting the effect of size with

performance. Such a study would be of significant interest. Carbon black can be purchased

with particle sizes as low as 20nm. Comparison of carbon to the smallest possible graphene

(both by performance and economic viability) will determine the better carbon allotrope for

counter electrodes.

As far as other two dimensional materials are concerned the molybdenum and

tungsten dichalcogenides are the most well-known and easily dispersed. A direct comparison

of these materials would be instructive as well as identifying other materials such as gallium

selenide and other III-VI layered semiconductors. However, molybdenum disulfide has an

advantage in being available in nature while the other materials require synthesis. That and

the toxicity of the selenide and telluride elements may lead to molybdenum disulfide being

the best material in this class for many applications.

Size selection for the TMDs (Transition Metal Dichalcogenides) is another avenue for

increasing efficiency. In this thesis it has been shown that MoS2 is 1.5 times more active per

flake length. While reduction of sizes in graphene is difficult the TMDs are regularly

produced in smaller sizes as evidenced by work by Backes et al232–234

. By combining the

enhanced activity per unit length and minimizing the flake size to maximise edge length per

unit mass, reasonable efficiencies could be achieved with minimal material.

In this work the focus has primarily been on graphene due to the relative cheapness of

the starting material. Carbon nanotubes have much better electrical properties in composites

due to a lower percolation threshold. This leads to a lower loading requirement for CNTs

which combined with the possibility of price reduction in the future (due to improved

synthesis methods) could result in CNTs being the dominant conductive additive in films

with non-conductive electrochemically active materials. As such proper analysis of

percolation of CNT composites for counter electrodes should be a priority.

Eliminating the FTO (fluorine doped tin oxide) as well as the platinum is also a priority due

to the increasing price of indium. Much work has been done on replacing this material as

mentioned multiple times throughout this thesis. What is interesting is that many materials

153

used in the transparent conductive electrodes, for example: PEDOT:PSS, CNTs and

graphene, are also catalysts in counter electrodes for DSSCs. There has been considerable

work in replacing either the platinum or the FTO but there has been considerably less in

replacing both simultaneously. This represents a significant research interest.

9.2.2 Supercapacitor Electrode Materials

In this thesis a variety of thicknesses are used to produce supercapacitor electrodes with

different performances and potential applications. One of the issues with the PEDOT:PSS

electrodes is the relatively low capacitance per unit mass of 37F/g. This is much lower than

traditional carbon based double layer materials which can have capacitances in excess of

100F/g. One way of addressing this is the addition of a pseudo capacitive material as

discussed in chapter 2. One of the leading materials for this application is Ruthenium Oxide

RuO2. The use of RuO2 nanoparticles in a PEDOT:PSS matrix produced by spray coating

was done by Zhang et al368

. This material produced capacitances of 1.2mF/cm2 at 93%

transparency which is an improvement on work done by Higgins et al196

.

After receiving some of these RuO2 nanoparticles from Zhang attempts were made to

introduce these nanoparticles to the thicker PEDOT:PSS films. This was achieved by

dispersing the RuO2 nanoparticle powder in the aqueous PEDOT:PSS dispersion via

sonication. Films were produced by dropcasting. The resultant film had a different texture on

either side due to the RuO2 nanoparticles not being stable in the aqueous medium and

precipitating out of the dispersion. The capacitance of these films were measured and

compared to the standard PEDOT:PSS electrodes. Due to a limited amount of material only

two films have so far been measured. However, the results are encouraging with 20% by

weight of RuO2 nanoparticles showing an improvement of almost a factor of 5 as seen in

figure 9.1.

Moving forward, improving the dispersion of the RuO2 particles in the PEDOT:PSS

dispersion is a priority. Once the dispersion is stabilised, further investigation of the variation

of capacitance per unit area with composition and scan rate to optimize the energy storage

properties of the electrode will be conducted. The dependence of the electrical properties will

154

also be in need of investigation but the work by Zhang et al. provide the expected dependence

of conductivity with composition.

10

100

1000

10% RuO2

20% RuO2

PEDOT:PSS only

C/A

@ 5

0m

V/s

(m

F/c

m2)

Thickness (m)

10 100

Figure 9.1 Capacitance per unit area of PEDOT:PSS/RuO2 composite films measure from CVs at 50mV/s

As an alternative to adding nanoparticles to form composites PEDOT:PSS can be

used to synthesise pseudocapacitive nanoparticles. Liu et al. add MnO2 nanoparticles to

PEDOT nanowires by simply soaking the PEDOT in KMnO4 solution369

. The capacitance

was increased by a factor of 4 after a soaking time of 10 minutes. In an attempt to replicate

this on the spray deposited PEDOT:PSS resulted in a film of a slightly different colour and

comparable transparency compared to an untreated PEDOT:PSS sprayed film. Attempts at

electrochemical testing resulted in failure. This was possibly due to a change in conductivity

as a result of the reaction. This was avoided by using a gold current collector in the case of

Liu et al. To compare the PEDOT:PSS to the MnO2 decorated PEDOT:PSS the material was

sprayed onto an ITO (Indium Tin Oxide) substrate which would serve as the current

collector. We used a concentration of 25mM KMnO4 and a treatment time of up to 1 minute.

Figure 2 displays the scan rate dependence of capacitance per unit area for sprayed

PEDOT:PSS films of similar thickness for soak times of up to one minute. At the longest

soak time a capacitance improvement of a factor of 3 is possible. As scan rate increases the

improvement is less significant due to the lower rate capability of the pseudocapacitive

material and a possible decrease in sheet resistance.

155

To improve on this further work on the optical properties is necessary to investigate if

this material can be competitive with the PEDOT:PSS/RuO2 nanoparticle composites

produced by Zhang et al. Also Gui et al used KRuO4 in a similar method to that of Liu et al to

produce RuO2 nanoparticle decorated nanowires this would also be of interest to possible

transparent supercapacitors370

.

100 10000.5

1

1.5

2

2.5

33.5

4

0s

5s

10s

30s

60s

C/A

(m

F/c

m2)

Scan Rate (mV/s)

Figure 9.2 Scan rate dependence of capacitance per unit area of various PEDOT:PSS films on ITO following soaking in 25mM KMnO4 solution

Ideally to make flexible electrodes the ITO film would be removed and replaced with

a flexible transparent conductor such as carbon nanotubes or graphene (as shown in chapter

7). The limitation of the graphene in chapter 7 with sheet resistance in excess of 300 Ω/sq.

needs to be addressed. As mention previously the doped graphene produced by Bae et al with

a sheet resistance of 30 Ω/sq. at a transparency of 90% would be an ideal replacement for the

ITO251

.

156

10

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