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

efficiency of 7.57%80

.

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

efficiency of 7.06%95

. 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

(

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

.