study of flexible multi-wall carbon nano- tubes ... · ii study of flexible multiwall carbon...

STUDY OF FLEXIBLE MULTI-WALL CARBON NANO-

TUBES / CONDUCTIVEPOLYMER COMPOSITES FOR

SUPERCAPACITOR APPLICATIONS

By

Ka Yeung Terence Lee

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

© Copyright by Ka Yeung Terence Lee In 2014

ii

STUDY OF FLEXIBLE MULTIWALL CARBON NANO-TUBES /

CONDUCTIVEPOLYMER COMPOSITES FOR SUPERCAPACITOR

APPLICATIONS

By Ka Yeung Terence Lee

Master of Applied Science

Department of Mechanical and Industrial Engineering

University of Toronto

2014

Abstract

Conductive polymers are promising pseudo capacitive materials as they feature both good

conductivity and high capacitance. Formation of composite between conductive polymers and

carbon nanotubes is a proven technique in enhancing the material electroactivity.

In-situ polymerization of conductive polymers includes polyaniline, polypyrrole and PEDOT:

PSS and composite with MWCNT has been successfully achieved. Composites fabricated by

using different dopants and their performance were studied. Excellent achieved capacitive

performance is due to the combination of pseudo capacitance and double layer capacitance. The

MWCNTs content has significant influence on the morphology and structure of the polymerized

ECP in the composite. And therefore affects the material conductivity and the charge storage

performance. Two electrodes cell performance shows that Ppy/MWCNT composite shows a

more promising performance as electrode materials for EC applications in contrast to

PANI/MWCNT and PEDOT: PSS/MWCNT composites.

iii

Acknowledgement

I would like to give my gratitude to my supervisors, Professor Hani Naguib and Professor

Keryn Lian for their generous guidance for the project. Also I would like to give my thanks to

NSERC for providing financial support for this project.

Special thanks to Professor Keryn Lian and the fellow lab members from the Flexible

electronic lab, especially Han and Matt who kindly provide their expertise and knowledge on the

field on electrochemistry and their kindly help on my works.

Also, I am very thankful for everyone in the SAPL lab who helps me with almost

everything. Lastly, I would like to dedicate this work to my parents for their great support,

motivation and encouragement.

iv

Table of Contents

Abstract ........................................................................................................................................... i

Table of Contents .......................................................................................................................... iii

List of Tables ............................................................................................................................... viii

List of Figures ..................................................................................................................................x

List of Abbreviations ................................................................................................................... xiv

Chapter 1 Introduction

1.1. Preamble .............................................................................................................................1

1.2. Thesis Objective..................................................................................................................2

Chapter 2 Background and Literature Review

2.1. Charge Storage Mechanism of Electrochemical Capacitors ....................................8

2.1.1. Electrochemical Double Layer Capacitor ..............................................8

2.1.2. Electrode Materials for EDLC ...................................................................10

2.1.2.1.Carbon Nanotubes ..........................................................................11

2.1.3. Pseudocapacitance .....................................................................................14

2.1.4. Common pseudocapacitive materials ........................................................15

2.1.4.1. Conductive polymers .........................................................16

2.1.4.2. Polyaniline .........................................................................18

2.1.4.3. Polypyrrole .........................................................................20

2.1.4.4. Poly(3,4-ethylenedioxythiophene) ....................................21

2.2. Hybrid Supercapacitors .........................................................................................24

2.2.1. Carbon/conductive polymer composites electrode/supercapacitors ..........25

2.3. Conductive Polymer Synthesis ..............................................................................28

2.4. Modification Technique of Carbon Nanotubes with Conductive Polymers ........31

2.5. Supercapacitors Applications.................................................................................32

v

2.6. Electrochemical Techniques ..................................................................................33

2.6.1. Cyclic Voltammetry ...................................................................................34

2.6.2. Electrochemical Impedance Spectrometry ................................................35

Chapter 3 Experimental

3.1. Methodology and Approach ...................................................................................41

3.2. Materials .................................................................................................................41

3.3. Sample Fabrications ...............................................................................................42

3.3.1. Composite material synthesis ....................................................................42

3.3.2. Composite electrode and two electrodes cell preparation ..........................43

3.4. Materials Characterization .....................................................................................44

3.4.1. Physical characterization ...........................................................................45

3.4.2 Electrochemical Characterization ..............................................................46

Chapter 4 Polyaniline/MWCNT Composites Study

4.1. Introduction .................................................................................................................52

4.2. DBSA, HCl doped PANI/MWCNT Composite study

4.2.1. Composite Fabrication .................................................................................52

4.2.1.1. HCl doped PANI/MWCNT composites .......................................53

4.2.1.2. DBSA doped PANI/MWCNT composites ...................................54

4.2.2. Physical characterization .............................................................................55

4.2.2.1. Fourier transforms infrared spectrometry (FTIR) ........................56

4.2.2.2. Scanning Electron Microscopy (SEM) .........................................58

4.2.3. Half -cell electrochemical characterization .................................................60

4.2.3.1. Cyclic Voltammetry ......................................................................60

4.2.3.2. Electrochemical impedance spectrometry ....................................62

vi

4.3. Polyaniline/MWCNT composite patrametric study ....................................................64


4.3.2. Physical Characterization.............................................................................67

4.3.2.1. Scanning Electron Microscopy (SEM) ........................................67


4.3.3. Half cell electrochemical characterizations .................................................69

4.3.3.1. Cyclic Voltammetry (CV) ............................................................71

4.3.3.2. Electrochemical impedance spectrometry (EIS) ..........................73

Chapter 5 PEDOT: PSS/MWCNT Composite Study

5.1. Introduction .................................................................................................................76

5.2. PEDOT: PSS/MWCNT / PEDOT/MWCNT study ....................................................76

5.2.1. Composite Materials Fabrication .................................................................76

5.2.1.1. PEDOT: PSS/MWCNT composite fabrication .............................76

5.2.1.2. PEDOT/MWCNT composite fabrication .....................................77


5.2.2.1. Scanning Electron Microscopy (SEM) .........................................79

5.2.2.2. Fourier Transforms Iinfrared Spectrometry (FTIR) ....................81

5.2.3. Half Cell electrochemical characterizations ................................................84


5.2.3.2. Electrochemical Impedance Spectrometry (EIS) .........................86

5.3. PEDOT: PSS/MWCNT composite parametric study .................................................88




5.3.2.2. Fourier Transform infrared spectrometry (FTIR) ........................89

vii

5.3.3. Half Cell Electrochemical Characterization ................................................92


5.3.3.2. Electrochemical Impedance Spectrometry (EIS) .........................94

Chapter 6 Polypyrrole/MWCNT Composite Study

6.1. Introduction .................................................................................................................97

6.2. Study of Dopant/Oxidant Effect on Ppy/MWCNT composites ..................................97

6.2.1. Composites Fabrication ...............................................................................97

6.2.1.1. HCl doped Ppy/MWCNT .............................................................97

6.2.1.2. DBSA doped PPy/MWCNT .........................................................98




6.2.3. Half cell electrochemical characterizations ...............................................103

6.2.3.1. Cyclic Voltammetry (CV) ..........................................................103

6.2.3.2. Electrochemical Impedance Spectrometry (EIS) .......................105

6.3. PEDOT: PSS/MWCNT composite parametric study ...............................................107

6.3.1. Composite Fabrication ...............................................................................107

6.3.2. Physical Characterization...........................................................................108

6.3.2.1. Scanning Electron Microscopy (SEM) ......................................108

6.3.2.2. Fourier Transform infrared spectrometry (FTIR) ......................111

6.3.3. Half Cell Electrochemical Characterization ..............................................112

6.3.3.1. Cyclic Voltammetry (CV) ..........................................................112

6.3.3.2. Electrochemical Impedance Spectrometry (EIS) .......................113

viii

Chapter 7 Two electrodes Cell Electrochemical Evaluation

7.1. Introduction ..............................................................................................................118

7.2. Ppy/MWCNT two electrodes Cell Electrochemical Evaluation ...............................118

7.2.1. Cyclic Voltammetry ..................................................................................119

7.2.2. Electrochemical Impedance Spectrometry ...............................................109

7.2.3. Galvanostatic charge/discharge ................................................................124

7.2.4. Cycling stability test ..................................................................................126

7.3. ECP/MWCNT Composite Two Electrodes Cell Electrochemical Comparison study

7.3.1. Cyclic Voltammetry ..................................................................................129

7.3.2. Electrochemical Impedance Spectrometry ...............................................132

7.3.3. Galvanostatic charge/discharge ................................................................135

7.3.4. Cycling stability test ..................................................................................137

7.3.5. Comparison with reported values from the literature ................................138

Chapter 8 Conclusions and Recommendations ................................ 141

References .............................................................................................................................144

Appendices

Appendix A: Electric Double Layer Theory ................................................................................147

Appendix B: Sample of simulation on Z-view to obtain the fitting parameters for the generalized

circuit of the composite supercapacitor electrodes for (0.02M) Ppy/MWCNT composite cell ..148

Appendix C. Two electrodes PANI/MWCNT electrochemical Evaluation ................................161

Appendix D. Two electrodes PEDOT:PSS/MWCNT electrochemical Evaluation ....................171

ix

LIST OF TABLES

Table 1. 1 comparison of important characteristics between between state of the art supercapacitors and

electrochemical batteries [3, 15 ] ............................................................................................ - 4 - Table 2. 1Comparison of EDLC and Pseudo capacitance [9, 194] ........................................................... - 9 - Table 2. 3 properties and structure of different carbon materials used in EDLCs [13, 16, 54,194] ....... - 12 - Table 2. 4 Electrical and physical properties of various conductive polymers [30] ............................... - 19 - Table 2. 5 Specific capacitance and respective fabrication method for various ECP/carbon composite - 29 - Table 2. 6 Comparison of chemical and electrochemical polymerization [135] .................................... - 31 - Table 5. 1 Weight of aniline monomers and Ammonium persulfate in different compositions ............. - 66 - Table 5. 2Polyaniline to MWCNT weight ratio at different compositions ............................................. - 66 - Table 5. 4 Measured mean tube diameter of the PANI/MWCNT composite fibers ............................... - 67 - Table 5. 5 Parameters of the equivalent circuit model for each electrode (1 cm

2) derived from numerical

fitting of experimental data acquired from nyquist plots ...................................................... - 74 -

Table 7. 1 The Ppy and MWCNT composition at various compositions ............................................. - 110 - Table 7. 2 Measured mean tube thickness and the deposited PANI layer thickness on MWCNT surface .... -

112 - Table 7. 3 Parameters of the equivalent circuit model for each cell (1 cm

2) derived from numerical fitting

of experimental data acquired from Nyquist plots .............................................................. - 116 - Table 7. 5 Weight composition between Ppy to MWCNT at various compositions ............................ - 119 -

Table 8. 1 Parameters of the equivalent circuit model for different composites devices (1 cm2) derived

from numerical fitting of experimental data acquired from nyquist plots .......................... - 125 - Table 8. 2 Summarized important findings from electrochemical impedance ..................................... - 126 - Table 8. 3 Coulomb efficiency and specific capacitance value @0.001A/cm

2..................................... - 127 -

Table 9. 1 Composite electrode material breakdown and weight of PANI/ MWCNT composite in single

electrodes ............................................................................................................................ - 130 - Table 9. 2 Composite electrode material breakdown and weight of Polypyrrole/ MWCNT composite in

single electrodes .................................................................................................................. - 131 - Table 9. 3 Composite electrode material breakdown and weight of PEDOT: PSS/MWCNT composite in

single electrode: .................................................................................................................. - 131 - Table 9. 4 Parameters of the equivalent circuit model for different composites devices (1 cm

2) derived

from numerical fitting of experimental data acquired from nyquist plots .......................... - 136 - Table 9. 5 Summarized important findings from electrochemical impedance ..................................... - 136 - Table 9. 6 Summarized charge time, discharge time, coulomb efficiency% and specific capacitance F/g at

0.001A/cm2 ......................................................................................................................... - 138 -

Table 9. 7 Summary of some reported specific capacitance of solid- state devices from the literature- 142 - Table 9. 8Specific capacitance and respective fabrication method for various ECP/carbon composite . - 143

-

x

Table C. 1 Parameters of the equivalent circuit model for different composites devices (1 cm2) derived

from numerical fitting of experimental data acquired from nyquist plots .......................... - 174 - Table C. 2 Summarized findings from electrochemical impedance data .............................................. - 175 - Table C. 3 Coulomb efficiency and specific capacitance value @0.001A/cm

2 .................................... - 177 -

Table D. 1 Parameters of the equivalent circuit model for each cell (1 cm2) derived from numerical fitting

of experimental data acquired from nyquist plot ................................................................ - 184 - Table D. 2 Summarized important findings from electrochemical impedance study for PEDOT:

PSS/MWCNT ..................................................................................................................... - 187 - Table D. 3 summarized charge time, discharge time, coulomb efficiency% and specific capacitance

(F/cm2) @ 0.001A/cm

2 ........................................................................................................ - 188 -

xi

LIST OF FIGURES

Figure 1. 1 Ragone plot of power density against energy density for various energy storage devices [12] .. - 3 -

Figure 1. 2 Taxonomy of supercapacitor materials [12] ........................................................................... - 4 - Figure 2. 1 Diagrams showing EDLC type capacitors charged (right) and discharged (left) state [11] . - 10 - Figure 2. 2 Schematic representation of A) single walled-CNT and B) Multi-Walled-CNT [195] ........ - 13 - Figure 2. 3 Approaches for functionalizing CNT with TETA start with carboxylic acid functionalization

of MWCNT [82] ................................................................................................................... - 14 - Figure 2. 4 Schematic showing oxidation/reduction process that create pseudocapacitance. ................ - 15 - Figure 2. 5 Schematic which illustrate the process of P-doping and dedoping process of conductive

polymers [134] ...................................................................................................................... - 17 - Figure 2. 6 General chemical formula of PANI [99] .............................................................................. - 19 - Figure 2. 7 Chemical structure of PANI at various oxidation states [99, 117] ....................................... - 20 - Figure 2. 8 Prontonic acid doping of PANI Emeraldine base by HCl to conductive Emeraldine salt state

with two polarons [99,117] ................................................................................................... - 20 - Figure 2. 9 Chemical structure of polypyrrole neutral aromatic and quinoid forms and in oxidized polaron

and bipolaron forms [121] .................................................................................................... - 21 - Figure 2. 10 Structure of the ground state of a) Poly (3, 4-ethylenedioxythiophene) PEDOT and b)

polythiophene respectively ................................................................................................... - 22 - Figure 2. 11 Chemical structure of neutral and doped PEDOT chain [140] ........................................... - 23 - Figure 2. 12 Chemical structure of PEDOT doped with PSS [145] ........................................................ - 25 - Figure 2. 13 Generalized scheme of preparative methods for polymer-CNT composited [146] ............ - 33 - Figure 2. 14 Equivalent circuit of the 3 electrodes setup [155] .............................................................. - 35 - Figure 2. 15 Typical charge/discharge CV characteristic of an electrochemical capacitor .................... - 36 - Figure 2. 16 A sample nyquist plot with impedance vector [158] .......................................................... - 39 - Figure 2. 17 Sample bode plot with one time constant [158] ................................................................. - 39 - Figure 2. 18 An example of nyquist plot for an activated carbon electrode [158] .................................. - 39 - Figure 2. 19 A Randles circuit model with mixed kinetic and charge transfer control (Left) and the

corresponding nyquist plot [153] .......................................................................................... - 41 - Figure 2. 20 Typical charge discharge curve for an efficient system (left) and a system showing built up

ohmic response (right) [159] ................................................................................................. - 42 - Figure 3. 1 In-situ polymerization scheme applied for composite fabrication ..........................................-45- Figure 3. 2 ECP/MWCNT Composite electrode fabrication process ..................................................... - 46 - Figure 3. 3 A sample of Ppy/MWCNT composite electrode with conductive composite material casted

onto stainless steel substrate (Left) and a two electrodes test cell (Right) ..............................-48- Figure 3. 4 Generalised equivalent circuit model used for the supercapacitor cells in this study, consist of

two resistors R1 and R2, and two constant phase elements CPE1, CPE2 ............................ - 50 -

Figure 4. 1 FTIR spectrum of transmittance against wave numbers for pristine polyaniline ................. - 55 -

Figure 4. 2 FTIR spectrum of HCl doped and DBSA doped Ppy/MWCNT composites ....................... - 56 -

Figure 4. 3 HR-SEM micrographs of a) MWCNTs b) HCl doped, b) DBSA doped PANI and c) HCl

doped, e) DBSA doped PANI /MWCNT composite ............................................................ - 59 -

xii

Figure 4. 4 CV comparison of pure MWCNT, PANI, DBSA doped and HCl doped PANI/MWCNT

composite electrodes at 5mV ................................................................................................ - 61 -

Figure 4. 5 Averaged specific capacitance F/cm2 for DBSA and HCl doped PANI/MWCNT .............. - 61 -

Figure 4. 6 Cyclic Exchange of oxidation states of PANI at different potential [99] ............................. - 61 -

Figure 4. 7 Nyquist plot of –Z” vs Z’ of DBSA and HCl doped PANI/MWCNT composite electrodes. B)

The inset enlarged the high frequency portion (100 kHz to 10mHz) ................................... - 63 -

Figure 4. 8 SEM images of A) MWCNT, B) Pure PANI ES, C) 0.05M PANI/MWCNT , D) 0.1M

PANI/MWCNT, E) 0.3M PANI/MWCNT, F) 0.5M PANI/MWCNT, G) 1M PANI/MWCNT . -

67 -

Figure 4. 9 FTIR spectra comparing Ppy /MWCNT composites to pristine Polypyrrole. The highlighted

regions shows the characteristic bands of Ppy displayed on the composites spectra. The inset

compares the Quinoid /Benzenoid vibration intensity. A reduced difference between the

vibration bands intensity in the composites was observed. ................................................... - 70 -

Figure 4. 10 Cyclic Voltammetry at 5mV/s for Pure PANI, MWCNT and PANI/MWCNT composites - 71

-

Figure 4. 11Calculated specific capacitance (F/cm2) for MWCNT, pure PANI ES and PANI/MWCNT

composites............................................................................................................................. - 73 -

Figure 4. 12 a) Nyquist plot of –Z” vs Z’ of, PANI, MWCNT and PANI /MWCNT composite electrodes.

b) Shows the enlarged high frequency portion (100 kHz to 10 MHz). ................................. - 74 -

Figure 5. 1 SEM images of A.1) PEDOT:PSS and A.2)PEDOT(AN), B) PEDOT: PSS /MWCNT C)

PEDOT/MWCNT composite with PEDOT controlled at 0.05M concentration ................... - 80 -

Figure 5. 2 FTIR spectrum of transmittance against wave numbers for PEDOT: PSS .......................... - 82 -

Figure 5. 3 FTIR spectra of transmittance against wave numbers for PEDOT/MWCNT (AN), PEDOT:

PSS/MWCNT, PEDOT: PSS and MWCNT ......................................................................... - 83 -

Figure 5. 4 CV voltammograms of pure PEDOT: PSS and PEDOT (AN) electrodes at 10mV/s and

100mV/s respectively ............................................................................................................ - 85 -

Figure 5. 5 CV voltammograms of PEDOT: PSS/MWCNT and PEDOT/MWCNT (AN) and MWCNT

electrodes at 10mV/s and 100mV/s respectively (1M H2SO4) ............................................. - 85 -

Figure 5. 6 Averaged specific capacitance F/cm2 for PEDOT: PSS/MWCNT and PEDOT/MWCNT

@10mV/s .............................................................................................................................. - 86 -

Figure 5. 7 a) Nyquist plot of –Z” vs Z’ of, PEDOT: PSS/MWCNT and PEDOT/MWCNT (AN)

composite electrodes. b) Shows the enlarged high frequency portion (100 kHz to 10 MHz) - 87

-

Figure 5. 8 SEM images of A) Pure PEDOT: PSS, B) 0.025M PEDOT: PSS/MWCNT, C) 0.05M PEDOT:

PSS/MWCNT, D) 0.1M PEDOT: PSS/MWCNT ................................................................. - 90 -

Figure 5. 9 Chemical structure of PEDOT: PSS. The “dot” and “plus” represent the unpaired electron and

positive charge respectively [181] ........................................................................................ - 91 -

Figure 5. 10 FTIR spectra comparing PEDOT: PSS/MWCNT composites to pristine PEDOT: PSS. The

interaction between PEDOT: PSS and MWCNT in the composite were studied ................. - 92 -

Figure 5. 11 Cyclic Voltammetry at 10mV/s for MWCNT, Pure PEDOT:PSS and PEDOT:PSS/MWCNT

composite electrodes ............................................................................................................. - 94 -

xiii

Figure 5. 12 Calculated specific capacitance (F/cm2) for MWCNT, pure PEDOT: PSS/MWCNT and

PEDOT: PSS/MWCNT composites (1M H2SO4, Scan rate :10mV/s) ................................. - 94 -

Figure 5. 13 a) Nyquist plot of –Z” vs Z’ of, PEDOT: PSS, MWCNT and PEDOT: PSS /MWCNT

composite electrodes. b) Shows the enlarged high frequency portion (100 kHz to 10 MHz). - 95

-

Figure 6. 1 HR-SEM micrographs for A ) Ppy (FeCl3) and B) Ppy(DBSA) C1,C2) FeCl3 doped

Ppy/MWCNT D1, D2) DBSA doped PPy/MWCNT composite ....................................... - 101 -

Figure 6. 2 FTIR spectrum of transmittance against wave numbers for FeCl3 doped polypyrrole ....... - 102 -

Figure 6. 3 FTIR spectrum of FeCl3 doped and DBSA doped Ppy/MWCNT composites ................... - 103 -

Figure 6. 4 Structure of Ppy-DBSA [186] ............................................................................................ - 104 -

Figure 6. 5 CV for Pure Ppy electrodes doped with FeCl3 and DBSA respectively. (1M H2SO4, 10mV/s) . -

105 -

Figure 6. 6 CV for Pure Ppy, MWCNT and Ppy/MWCNT composites doped with FeCl3 and DBSA

electrodes. (1M H2SO4, 10mV/s) ........................................................................................ - 106 -

Figure 6. 7 Calculated capacitance for Pure Ppy, MWCNT, PPy/MWCNT composites doped with FeCl3

and DBSA electrodes .......................................................................................................... - 106 -

Figure 6. 8 Nyquiz plot of –Z” vs Z’ for Ppy/ MWCNT (FeCl3) and Ppy/ MWCNT (DBSA) composite

electrode (100 kHz to 10 MHz) .......................................................................................... - 107 -

Figure 6. 9 SEM images of a) Pure Polypyrrole powder, b) Ppy (0.02M)/MWCNT composite powder and

individual Ppy grown MWCNT tube morphology of c) Ppy (0.02M)/MWCNT, d) Ppy

(0.05M)/MWCNT, e) Ppy (0.1M)/MWCNT and f) Ppy (0.3M)/MWCNT ........................ - 111 -

Figure 6. 10 FTIR spectra comparing Ppy/MWCNT composites to pristine Ppy. Highlighted bands

indicates the Ppy characteristic bands in the composites. Wave shift of pyrrole vibration bands

in the composite spectra was observed. .............................................................................. - 113 -

Figure 6. 11 Cyclic Voltammetry at 10mV/s for Pure Ppy, MWCNT and Ppy/MWCNT composites- 114 -

Figure 6. 12 Calculated specific capacitance (F/cm2) for pure Ppy, MWCNT and MWCNT/Ppy

composites........................................................................................................................... - 114 -

Figure 6. 13 Nyqust plot of –Z” vs Z’ for MWCNT, Ppy and the Ppy/MWCNT electrodes (100 kHz to 10

MHz) ................................................................................................................................... - 115 -

Figure 6. 14 HR-SEM images of a)Ppy/MWCNT (100), b) Ppy/MWCNT (300) and c) Ppy/MWCNT

(500) composites ................................................................................................................. - 118 -

Figure 6. 15 CV plot for pure Ppy, MWCNT, Ppy/MWCNT (100), Ppy/MWCNT (300) and

Ppy/MWCNT(500) (10mV/s, 1M H2SO4) .......................................................................... - 119 -

Figure 6. 16 Calculated specific capacitance (F/cm2) for pure Ppy, MWCNT and Ppy/MWCNT

composites........................................................................................................................... - 119 -

Figure 7. 1 CV plot of Pure Ppy, MWCNT and MWCNT/Ppy composites devices from A)10mV/s,

B)50mV/s,C)100mV/s,D) 500mV/s respectively ............................................................... - 122 -

Figure 7. 2 Specific capacitance (F/cm2) as a function of scan-rate from 10mV/s to 1V/s .................. - 122 -

xiv

Figure 7. 3 Nyquizt plot of –Z” vs Z’ for MWCNT, pure Ppy and Ppy/MWCNT cells (100 kHz to 10

MHz) ................................................................................................................................... - 124 -

Figure 7. 4 Bode plot of negative phase vs frequency (Hz) for MWCNT, Ppy/MWCNT, and pure Ppy cell.

The specified frequency @ -45o indicates the point where resistance to capacitance transition

occur .................................................................................................................................... - 124 -

Figure 7. 5 Specific capacitance C” (F/g) vs frequency (Hz) with time constant values indicated for

MWCNT, Ppy and Ppy/MWCNT cell ............................................................................... - 125 -

Figure 7. 6 Specific capacitance C’ (F/g) vs frequency (Hz) for MWCNT, Ppy and Ppy/MWCNT cell .... -

125 -

Figure 7. 7 Charge/discharge curve of Pure MWCNT, pure Ppy and Ppy/MWCNT supercapacitor at

0.001A/cm2 current density ................................................................................................. - 127 -

Figure 7. 8 CV plot for MWCNT/Ppy (0.02M) (Left) and pure Ppy cell (right) at the 1st cycle and the

1000th cycle ......................................................................................................................... - 128 -

Figure 7. 9 Capacitance change over 1000 cycles for MWCNT/Ppy (0.02M) (left) and pure Ppy cell (right)

respectively ......................................................................................................................... - 129 -

Figure 7. 10 Cyclic Voltammetry at A)10mV/s, B) 50mV/s, C)100mV/s, D)500mV/s, E)100mV/s for

MWCNTs, PAN/MWCNT, PEDOT:PSS/MWCNT and Ppy/MWCNT composites

supercapacitors .................................................................................................................... - 132 -

Figure 7. 11 Variation of the capacitance of MWCNT, PANI/MWCNT, PEDOT: PSS/MWCNT and

Ppy/MWCNT supercapacitor devices as a function of potential scan rates ....................... - 133 -

Figure 7. 12 Nyquiz plot of –Z” vs Z’ for MWCNT, PANI/MWCNT Ppy/MWCNT, PEDOT: PSS

/MWCNT cell (100 kHz to 10 MHz) .................................................................................. - 135 -

Figure 7. 13 Bode plot of negative phase vs frequency (Hz) for MWCNT, PANI/MWCNT Ppy/MWCNT,

PEDOT: PSS /MWCNT cell. The specified frequency indicates the point where resistance to

capacitance transition occur ................................................................................................ - 135 -

Figure 7. 14 .Specific capacitance C” (F/g) vs frequency (Hz) with time constant values indicated for

MWCNT, PANI/MWCNT Ppy/MWCNT, PEDOT: PSS /MWCNT cell. ......................... - 135 -

Figure 7. 15 Specific capacitance C’ (F/g) vs frequency (Hz) for MWCNT, PANI/MWCNT,

Ppy/MWCNT, PEDOT: PSS /MWCNT cell ...................................................................... - 135 -

Figure 7. 16 Charge/discharge curve of PANI/MWCNT, PEDOT: PSS/MWCNT and PANI/MWCNT

composites supercapacitors at current density of a) 0.001A/cm2 b) 0.01A/ cm

2 and c) 0.1A/cm

2

(0V to +0.8V) ...................................................................................................................... - 138 -

Figure 7. 17 Discharge specific capacitance against charge/discharge cycles ...................................... - 140 -

Figure 8. 1 Discharge specific capacitance against charge/discharge cycles - 140

-

xv

Figure A. 1 Electric double layer models a) Helmholtz model, b) GouyChapman model, c) Stern and d)

Bocris, Devanathan, and Muller (BDM) [27,30] ................................................................ - 169 -

Figure C. 1 Cyclic Voltammetry at A)10mV/s, B) 50mV/s,C)100mV/s, D)500mV/s, E)1V/s for MWCNT,

pure PANI, and PANI/MWCNT composites ...................................................................... - 181 - Figure C. 2 Variation in the capacitance PANI and PANI /MWCNT devices as a function of potential

scan rates for a) charge and b) for discharge....................................................................... - 182 - Figure C. 3 Nyquist plot of –Z” vs Z’ of 0.1M PANI/MWCNT supercapacitor. 0kHz to 10mHz) .... - 184 - Figure C. 4 Bode plot of negative phase vs frequency (Hz) for MWCNT, PEDOT: PSS /MWCNT and

pure PEDOT:PSS cells. The specified frequency @ -45o indicates the point where resistance to

capacitance transition occur ................................................................................................ - 184 - Figure C. 5 Specific capacitance C” (F/g) vs frequency (Hz) for MWCNT, PANI and the PANI

/MWCNT composite cells. ................................................................................................. - 186 - Figure C. 6 Specific capacitance C’ (F/g) vs frequency (Hz) for MWCNT, PANI and the PANI

/MWCNT composite cells .................................................................................................. - 186 - Figure C. 7 Charge/discharge curve of PANI, MWCNT and PANI /MWCNT composites supercapacitors.

(0.001A/cm2 current density, 0V to +0.8V) ........................................................................ - 188 -

Figure C. 8 CV plot of PANI 0.05M/ MWCNT (Left) and pure PANI cells (Right) at the 1st cycle and the

2000th cycle (100mV/s, 1m H2SO4) .................................................................................... - 190 -

Figure C. 9 Capacitance change over 2000 cycles for PANI 0.05M /MWCNT cell (Left) and PANI (Right)

cell respectively .................................................................................................................. - 190 -

Figure D. 1 Cyclic Voltammetry at A)10mV/s, B) 50mV/s,C)100mV/s, D)500mV/s, E)1V/s for MWCNT,

pure PEDOT: PSS, and PEDOT: PSS/MWCNT composites ............................................. - 181 - Figure D. 2 Variation in the capacitance PEDOT:PSS and PEDOT:PSS/MWCNT devices as a function of

potential scan rates for a) charge and b) for discharge ........................................................ - 182 - Figure D. 3 Nyquist plot of –Z” vs Z’ of 0PEDOT:PSS/MWCNT supercapacitor. 0kHz to 10mHz) - 184 - Figure D. 4 Bode plot of negative phase vs frequency (Hz) for MWCNT, PEDOT: PSS /MWCNT and

pure PEDOT:PSS cells. The specified frequency @ -45o indicates the point where resistance to

capacitance transition occur ................................................................................................ - 184 - Figure D. 5 Specific capacitance C” (F/g) vs frequency (Hz) for MWCNT, PEDOT:PSS and the PEDOT:

PSS /MWCNT composite cells. .......................................................................................... - 186 - Figure D. 6 Specific capacitance C’ (F/g) vs frequency (Hz) for MWCNT, PEDOT:PSS and the PEDOT:

PSS /MWCNT composite cells ........................................................................................... - 186 - Figure D. 7 Charge/discharge curve of PEDOT/PSS, MWCNT and PEDOT: PSS/MWCNT composites

supercapacitors. (0.001A/cm2 current density, 0V to +0.8V) ............................................. - 188 -

Figure D. 8 CV plot of PEDOT: PSS 0.05M/ MWCNT (Left) and pure PEDOT: PSS cells (Right) at the

1st cycle and the 2000

th cycle (100mV/s, 1m H2SO4) .......................................................... - 190 -

Figure D. 9 Capacitance change over 2000 cycles for PEDOT: PSS 0.05M /MWCNT cell (Left) and

PEDOT: PSS (Right) cell respectively ............................................................................... - 190 -

xvi

List of Abbreviations

A = surface area [cm2]

A- = counter anions

AC = Activated carbon

APS = Ammonium Persulfate

BDM = Bocris, Devanathan, and Muller model

CA = Carbon aerogels

CNTs = Carbon nanotubes

C = capacitance (F)

CHm= Capacitance due to the Helmoltz layer

CGC = Capacitance due to the diffusive layer

C+ = counter cations

C’= Real Complex Capacitance

C” = Imaginary Complex capacitance

C/dC = Charge /Discharge

CPE = Constant phase element

CE = Counter Electrode (CE)

CV = Cyclic Voltammetry

D = Electrodes distance [nm]

DI = deionised

dV/dt = scan rate [mV/s]

DBSA = Dodecylbenzenesulphonic acid

EDOT = 3, 4 -ethylene dioxythiophene

xvii

EIS = Electrochemcial Impedance Spectrometry

EC = Electrochemcial capacitors

ECPs = Electroactive Conductive polymers

ESR = Equivalent total internal resistance

EDLC = Electrochemical Double Layer Capacitors

Ed = Ionization energy

F = Farad

HR-SEM = High resolution scanning electronmicroscopy

FTIR = Fourier Transform Infrared Spectroscopy

FeCl3 = Iron Chloride

HCl = Hydrochloric acid

I = current [A]

IHP = Inner Helmoltz plane

IR = Internal resistance [Ω]

MnO2 =Manganese dioxide

OHP = Outter Helmoltz plane

PANI = Polyaniline

Ppy = Polypyrrole

PEDOT: PSS = Poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate)

Pmax = Maximum power

TETA = Triethylenetetramine

PVA = Polyvinyl Alcohol

PTFE = Polytetrafluroethylene

Q = Charges [C]

RE = Reference Electrodes

xviii

RuO2 = Ruthenium oxide

Rct = Charge transfer resistance [Ω]

RC = Resistance-Capacitance [Ω]

SDS = Sodium dodecyl sulfate

V = Applied potential [Volts]

WE = Working electrode

Z’ = Real Impedance

Z” = Complex Impedance

ɛo =Dielectric permittivity [eV-1

m-1

]

ɛr, = Dielectric permittivity of free space

= Time constant

σ = Conductivity[S/m]

tc = Charge time [s]

td = Discharge time [s]

Chapter 1 Introduction

1.1 Preamble

Energy storage is the primary focus of the major world powers and scientific community.

Great interest has been driven in developing more efficient energy storage devices.

Electrochemcial capacitors (EC), which is also known as “supercapacitor” has been actively

investigated as a potential energy storage solution to many systems where sustainability is a great

concern. Capacitor is a fundamental device that occasionally store and release electrical energy.

Conventional electrolytic capacitors consist of two conducting electrodes plates and separated by

insulating dielectric materials. Charge separation (Q) is achieved at the two plates separated by

vaccum or a dielectric of relative permittivity. Under the applied potential (V), the capacitance

(C) is defined by the ratio of stored positive charge (Q) to the applied potential (V) to the system:

(1)

Capacitance is also directly proportional to the dielectric permittivity of free space ɛo and

ɛr, which is the dielectric constant of the insulating material between the two electrodes. Also, the

quantity of the stored charge is also directly proportional to the surface area “A” of the electrode

plate and inversely proportional to the electrode distance “D”. [1-10,17,15,18,19,99]

(2)

- 2 -

The two primary attributes are the energy density and the power density. The energy

density of a capacitor cell is directly proportional to the capacitance (C) and the voltage during

discharge (V).

(3)

For power density, the maximum power is defined as:

Pmax =

(4)

Whereas ESR is the equivalent total internal resistance of the capacitor [1-10, 17,

15,18,19,99, 158]. The voltage during discharge (V) is determined by these resistances. This

shows that the power of a capacitor is highly dependent on the internal resistance. When

compared to other energy storage devices such as batteries and fuel cells. Batteries achieved

energy storage associated with chemical reaction between electrode (e.g. lead, Nickel and

lithium based) and electrolyte [5, 8, 25, 24]. Whereas fuel cells generate ions/ electrons during

the reduction or oxidation of fuel (e.g. Combines hydrogen fuel with oxygen to produce

electrical energy with water and heat as by-products) through internal electrolyte, which is a

direct energy conversion from a chemical to an electrical one [5, 7, 25, 26]. These systems

generate energy through REDOX processes is able to create a much higher energy density then

capacitors. However, the power densities of such devices are relatively low as the energy cannot

be delivered as fast as capacitors. On the other hand capacitor stores relatively less energy per

unit mass, but the stored energy can be discharged rapidly to produce a huge power. In addition,

the operation cost for supercapacitor is very expensive. The runtime cost for supercapacitors is

about ~$100-300 US/kW for only couple of seconds. However for the same cost, the run time

for battery is in hours [3].

- 3 -

Table 1.1 summarized the rating between supercapcitors and the state of the art lithium

ion battery. Despite the intrinsically low energy density of conventional capacitor, supercapacitor

was designed in order to solve the low energy problem by incorporating high energy pseudo

capacitance. A comparable energy density to battery and fuel cell is aimed to be achieved. Figure

1.1 shows a ragone plot of energy density against power density for different energy storage

systems. It can be seen that electrochemical capacitors EC are aimed to overlap the energy gap

between batteries and capacitors, while maintaining high power characteristic.

EC exhibits superior energy storage characteristic by combining both high energy and

power density. In general supercapactiors can be categorized into different charge storage

mechanisms. Figure 1.2 shows the taxonomy of supercapcitors material. The material types are

mainly categorized into electrochemical double layer capacitors and pseudo capacitor. The

combination between the two mechanisms forms a hybrid capacitor [2, 4, 7, 12, and 18]

Figure 1.1 Ragone plot of power density against energy density for various energy storage

devices [14]

- 4 -

Table 1.1 comparison of important characteristics between between state of the art

supercapacitors and electrochemical batteries [3, 15]

Figure 1.2 Taxonomy of supercapacitor materials

Electrode materials for super-Caps

EDLC materials

Carbon fibres

Carbon Aerogel

(CA)

Activated Carbon (AC)

Carbon nanotubes Graphene

Pseudo capacitive materials

Conductive Polymer

Metal Oxides

Composites

Carbon materials with Metal Oxides

Carbon with Conductive polymers

Characteristic Supercapacitor Lithium ion battery

Charge time (s) ~1 second 3-5 minutes

Discharge time (s) ~1 second 3-5 minutes

RC time constant 0.07-0.5s >1000s

Service life >1 Milllion <5000

Specific energy

(Wh/kg)

5 70-100

Specfic power

(kW/kg)

5-10 0.5-1

Cycle efficiency >95% to <75% >90% to <50%

Cost/Wh $1-2/Wh $10-20/Wh

Cost/kW $75-150/kW $25-50/KW

- 5 -

Electrode materials play an important role in defining the supercapacitor energy storage

and delivery capability. Materials including high surface area carbon base materials, metal

oxides and conductive polymers which feature different storage mechanisms have been actively

studied.

Electroactive conductive polymers (ECPs) are very promising materials to be applied as

supercapacitor materials due to the highly achieved pseudo capacitance than the EDLCs due to

the REDOX activities. However, their poor cycling stability limits ECPs in industrial application

[5,7,9,12,17,31]. This problem can be compensated by fabricating composites with carbon base

materials such as carbon nanotube, graphene and active carbon [4, 6, 9, 10, 12,13,14,16, 17, 20,

31, 37, 41, 54-58, 61]. Extensive studies have been focused on electrochemically polymerizing

ECP or their composites onto metallic substrate in electrode fabrication [4, 6, 9, 10, 12, 13, 14,

and 16]. While the chemical polymerization approach is more attracted due to the ease of

synthesis and fast processing. Moreover in electrode design, it is necessary to increase the

capacitance while controlling the resistance of the material in order to achieve high energy

without sacrificing the high power density nature of a capacitor. In this thesis, the three most

popular conductive polymers, namely polyaniline (PANI), polypyrrole (Ppy) and poly (3, 4-

ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS) and their composites with

carbon nanotubes electrochemical performance as supercapacitor electrodes were studied. Their

electrochemical characteristic and capacitive performance were compared.

- 6 -

1.2 Thesis Objectives

In the past few decades, significant studies have been done to understand the electronic

properties of ECP/MWCNT composites. Studies focused on improving the bulk composite

materials electronic properties has been actively reported from the literature. The most

commonly studied polymers are polyaniline (PANI), polypyrrole (Ppy) and PEDOT: PSS. Their

composite with carbon based materials on various applications has been actively explored.

From different studies, parameters such as the polymerization condition (e.g. dopants,

oxidants, temperature and polymerization duration) as well as the electrode fabrication methods

varies from one to each study. And therefore leads to huge variations in the reported capacitance

value. In addition, a thorough comparison study of the capacitive performance among these

three polymers composite was seldom reported from the literature.

For this thesis, the objective is to modify multied-walled carbon nanotubes (MWCNTs)

with polyaniline (PANI), polypyrrole (Ppy) and PEDOT: PSS. The composite is aimed to be

optimized by varying the weight ratio between the ECPs and MWCNTs. Information regarding

polymerizing polymers with high quality and conductivity for respective polymers was

incorporated in materials fabrication process. An-insitu oxidative polymerization technique was

employed with the ease of material fabrication concern. Parametric studies focused on the ECP

contents effect to the electrochemical properties as a supercapacitor electrode for these

ECP/MWCNT composites were seldomly reported. Therefore, an insight on this issue was also

focused on these three polymers. By comparing the performance of the best samples for each of

the polymers, the best candidate for potential supercapacitor application could be deduced based

on the designated fabrication strategy.

- 7 -

In this thesis, the following properties were aimed to be achieved: i) an improved

electrochemical property from the composite, ii) ideal capacitive behaviour with fast charging/

discharging characteristic, iii) improved cycling life and iv) easy fabrication. With the purpose

of accomplishing these objectives, the following approaches were investigated:

To investigate the influence of different dopants and oxidants to the capacitive

performance and morphology of PANI/MWCNT and Ppy/MWCNT. For PEDOT/

MWCNT, the influence of PSS to the composite electrochemical properties was accessed.

To study the influence of polymer to MWCNTs content in the composite to the in general

morphology and electrochemical properties.

Perform two electrodes cell evaluation to the developed ECPs/MWCNT composites

electrode in order to access the potential performance in EC device configuration.

To deduce potentially the best ECP/MWCNT composite among the three based on the

designed experimental criteria

- 8 -

Chapter 2 Background and literature review

2.1. Introduction

In this chapter, a brief overview on supercapacitors charge storage mechanisms and the

respective materials were given. Materials of interest including carbon nanotubes and conductive

polymers (Ppy, PANI, PEDOT: PSS) were described in detail. Various ECPs / CNT composites

fabrication technique were illustrated. The oxidative chemical polymerization approach was

detailed. A literature review was conducted on various ECP/Carbon composite electrode

electrochemical performances. The reported capacitance values were summarized. An

introduction on basic electrochemical characterization techniques was also presented.

2.2. Charge Storage Mechanism of Electrochemical Capacitors

The two most notable mechanisms are electric double-layer capacitance and pseudo

capacitance. Double-layer capacitance is aroused from the separation of charge at the interface

between a solid electrode and an electrolyte [1-10,17,15,18,19], whereas pseudo capacitance is

caused by the fast and reversible faradic reactions that occurs at the solid electrode surface over

an appropriate range of potential [4,5,7,8,9,11,15,18,19,22]. A comparison between the

properties of EDLC and pseudo capacitance is shown in Table 2. 1.

- 9 -

Table 2.1Comparison of EDLC and Pseudo capacitance [9, 194]

2.2.1. Electrochemical Double Layer Capacitors

Electrochemical Double Layer Capacitors (EDLC) is constructed by two electrodes being

immersed in an electrolyte with a separator [4, 6, 10, and 18]. When a charged conductive

material is being immersed in electrolyte solution, in order to achieve a neutral system, counter

charges from the liquid phase will be accumulated close to the charged surface while repelling

the ions of same charge. As suggested by German physicist Helmholtz, this solid –liquid

interface would form double charged layers with opposite polarity which can achieve dielectric

electrostatic charge storage [27, 28 ,30]. The theory of double layer formation is described in

Appendix A.

Electric double layer capacitance

Pseudo capacitance

Higher voltage operation Lower voltage operational range. Limited by

electrochemistry and decomposition of solvent

High power operation (90o phase angle) Phase angle function of frequency

Low ESR for gases or vacuum dielectric Kinetic limitation for high charge/discharge rates,

therefore limited the power capability

Finite ESR and frequency dependence of

phase angle for electrolytic capacitor

Highly reversible

Capacitance constant with voltage

Capacitance dependent on voltage

20-50uF/cm2 Up to 2000uF/cm

2

- 10 -

V

Separator

Electrolyte

Cu

rre

nt

Co

llect

or

Ele

ctro

de

mat

eria

l

Positive electrode

Negative electrode

electrolyte

Charges

Ions

Figure 2.1 Diagrams showing EDLC type capacitors charged (right) and discharged (left)

state

EDLCs utilize the electrochemical double layer to store charge electro-statically at the

electrode /electrolyte interface. As presented on Figure 2.1, when voltage was applied across the

electrodes, electrical charges will be separated and attached to oppositely charged electrode

surfaces. Oppositely charge electrolyte ions will then diffuse across the separator into the porous

electrodes surfaces. Each side of the electrodes forms an interface with the electrolyte. There is

no charge transfer due to faraday process between electrode and electrolyte. The double layer of

charges on the electrode surface forms and relaxes instantly (about ~10-8

s) [9]. Therefore the

current respond to potential change is very rapid [4, 5, 7, 9, and 16]. The governing principal of

the capacitance for this system is same as conventional electrostatic capacitor as shown in

equation 3. [7, 16, 20]

- 11 -

(3)

In this case, the “d” on the governing equation represents the effective thickness of the

double layer, which is now the Helmholtz layer (Dd) instead of the parallel plates distance. The

distance is about half the diameter of the absorbed solvated ions which is in nanometer range [7,

9, and 16].Therefore, in contrast to conventional capacitors, EDLCs capacitance is typically

measured in F, whereas conventional capacitor is only rated in uF or pF in range [4,5,7,11,19].

Moreover, the highly porous electrode surface can greatly increase the effective charge storage

surface area “A”. As a result, the amount of achieved electrical energy is greatly enhanced.

Moreover, since the process is completely electrostatic as there is no faradic charge transfer

process involved. A durable and highly reversible charge –discharge cycling life can be achieved

(>100000 cycles) [4, 9, 18].

2.2.2. Electrode Materials for EDLC

In EDLC, highly porous carbon-based material in nanometer range is commonly utilized

as electrode materials [4-9, 16]. As according to equation 3), it is suggested that the EDLC is

highly proportional to the surface area of the electrode and electrolyte interface. Carbon

materials are highly attracted candidates due to their high specific surface area and promising

conductivity. Studied materials including Activated Carbon (AC), Carbon Aergel (CA),

graphene as well as carbon nanotubes etc. The following Table 2. summarized the major

properties and structure of these carbon materials.

- 12 -

Table 2. 2 properties and structure of different carbon materials used in EDLCs [13, 16, 54,194]

Materials Carbon Nanotubes Graphene Activated Carbon Carbon Aerogel

Dimension 1-D 2-D 3-D 3-D

Surface Area (SA)

m2/g

> 2000 (market) ~2600 1000-3000

(market)

400–800

Conductivity (S/cm) High

5000S/cm

High

~ 5000 S/cm

low Low (50-100S/cm)

Specific capacitance

(F/g)

~80-130 F/g ~100F/g

25 uF/cm3

50uF/cm3,

50-100F/g

50-100F/g

Cost Moderate toHigh Moderate Low Moderate

Structure

2.2.2.1. Carbon Nanotubes

Carbon nanotubes is one of the most studied materials in developing high performance

supercapacitor owning to their novel properties such as high electrical conductivity, high specific

surface area, high charge transport capability [4,9]. Depends on the preparation method, CNTs

appears in two forms, which are single walled carbon nanotubes (SWCNTs) and Multi-Walled

carbon nanotubes (MWCNTs). The schematic of the structure were presented on Figure 2.2

respectively. SWCNT is a single rolled graphene sheet; the tube diameter is from 1-2nm.

Whereas, MWCNTs consist of multiple layers of graphene rolled into a tube. The spacing

between the tubes is about 0.3 nm. The tube diameter is about 2-25nm [195].

- 13 -

Figure 2.2 Schematic representation of A) single walled-CNT and B) Multi-Walled-CNT [195]

CNTs tend to form large bundles as stabilized by the van der waals force and physical

entanglement between individual strands due to the sp3 hybridization on the outer surface. This

aggregated nature lead to low solubility and poor dispersion of the CNTs in solvents such as

water and organic solvents. In order to solve this problem, chemically functionalizing the CNT

sidewalls with functional groups can improve the dispersion ability and the interaction with the

host polymer. This is desirable for further functionalization of the surface. This approach can

introduce chemical functional group covalently bonded onto the sp2

hybridization [195]. These

functional groups can be used as anchoring sites for polymer chains and therefore to improve the

interaction with the polymer matrix [147]. Two major covalent surface functionalization

processes are solution based surface treatment and plasma surface treatment of CNTs.

Process such as acid oxidation would then induce oxygenated functional groups such as

carboxylic acid, ketone, alcohol and ester groups [147]. The most common practice is chemically

treating MWCNTs in a mixture of concentrated acid such as H2SO4 and HNO3. Incorporating

- 14 -

reflux technique under high temperature was also reported [82,83]. Further alteration of

functional group can be achieved by reacting MWCNT-COOH with other chemicals. For

example, Figure 2.3 presents the step by step functionalization of MWCNT with

Triethylenetetramine (TETA) from MWCNT-COOH [82].

H2SO4/HNO3 C

O

OH SOCL2

C

O

Cl

C

O

Cl

TETA

C

O

N N

H

N

H

N

H

H

Figure 2.3 Approaches for functionalizing CNT with TETA start with carboxylic acid

functionalization of MWCNT [82]

The material pore size plays an important role in determining the electrolyte ions

accessibility, and therefore the capacitive properties of the electrode. Conventional material,

activated carbons (AC) possess porous structure resides in the micro range (< 2nm), which has

limited electrolyte ions accessibility [1, 4, 6, 7]. In contrary, due to the high aspect ratio of

MWCNTs strands, the entangled MWCNTs network possesses very high population of

mesoporous structure. The interconnected mesopores (2-50nm) allow continuous charge

distribution and easy diffusion of wide range of electrolyte ions to the MWCNT electrode

surface [8]. Therefore MWCNTs achieves capacitance comparable to those of activated carbon

even though they have lower surface area than activated carbon [10]. It is reported that MWCNT

supercapacitor is able to achieve specific capacitance of 108 F/g at a scan -rate of 10mV/s [5].

From the literature, a better capacitance was achieved from SWCNTs. A study shows that on

average about 2.5 times higher capacitance can be achieved in both aqueous and organic

- 15 -

electrolyte due to the higher intrinsic conductivity [203]. Supercapacitor fabricated from SWNT

electrodes and KOH electrolyte showed a promising power density of 20 kW/kg with a

maximum energy density of ~10 Wh/kg [4]

2.2.3. Pseudocapacitance

Figure 2.4 Schematic showing oxidation/reduction process that create pseudocapacitance.

For an ideal EDLC, the derivative of charge Q over applied Voltage V (dQ/dV) is

constant and independent of voltage [4, 7, and 18]. In contrary, for pseudocapacitance, charge is

stored at the surface or in the bulk of the solid electrode. The transfer of charge is voltage

dependent. Therefore results in pseudo-capacitance determined by the (C= dQ/dV) [4, 7, 18]. In

pseudo capacitance, charge is transferred via fast and reversible faradic interaction between the

electrode and electrolyte. Pseudo-capacitance can be aroused due to three types of

electrochemical processes, such as surface adsorption of ions from the electrolyte; REDOX

reactions involve ions from the electrolyte, and doping/de-doping of active conducting polymer

material [4, 8, 10, 12, 31]. The first two processes are highly surface area dependent while the

third one is mainly a bulk process. The faradic electrochemical processes aroused from the

REDOX or doping/ dedoping process of ECP is very similar to charging and discharging of

- 16 -

batteries [31]. Moreover, the charges transfer process is also voltage dependent. In contrast to

EDLC, the time responds of REDOX reactions are slower, which is in the range of (10-2

to 10-4

)

due to the impedance of the reaction [5]. But then, due to the faradic nature of the reaction, the

systems can significantly generate much larger capacitance (10-100 times) than double layer

capacitors [9]. In any of these cases, the electrodes are essential to have high electronic

conductivity in order to distribute and collect the current [31].

2.2.4. Common Pseudocapacitive Materials

Popular materials including transition metal oxides as well as electronically conducting

polymers have been extensively studied. The use of transition metal oxides have been widely

explored for supercapacitor applications due to their unique layered structure and the wide

variety of oxidation states [9,11,22, 84, 85,86]. The pseudo capacitance of metal oxides is due to

a highly reversible surface chemical reactions or extremely fast and reversible intercalation of

metal ions into the lattice (e.g. H+, Na+, K+) [11, 86] One of the commonly investigated metal

oxides is ruthenium oxide (RuO2); the capacitance is achieved through the insertion and removal,

or intercalation of protons into its amorphous structure [22, 83, 84]. Ruthenium oxide

pseudocapacitors achieve high energy and power densities in its hydrous form [83, 84].

Fairly high specific capacitance, up to 700 F/g could be achieved [84]. However, its

potential application in the industry is severely limited by its high cost [7, 84]. Another metal

oxide being actively researched is manganese dioxide [11, 85, 86]. Manganese dioxide has two

advantages over ruthenium oxide which makes this material a promising one in this field [86].

- 17 -

This material is low production cost and highly compatible with various environmentally

friendly aqueous electrolytes.

2.2.4.1. Conductive polymers

Cp -> Cp n+

(A-)n + ne

- (p-doping)

Cp +ne- -> (C

+)n Cp

n- (n-doping) [134]

++

++

++

+ +

+

+

-

+

-

-

-

- e-

Polymer chain

Current collector

Neutral polymer

chain

+ +

+

+

-

+

-

-

--

+ charged polymer

chain

+

-

-

-

P-dedoping

P-doping

Figure 2.5 Schematic which illustrate the process of P-doping and dedoping process of

conductive polymers

Another common pseudo capacitive material is conductive polymers. Conducting

polymers possess good intrinsic conductivity from semi-conducting to metallic range [30]. They

exhibit fast charge/discharge kinetics owing to the fast doping and de-doping characteristic [7, 9,

18, and 31]. Conductivity is achieved through the conjugated π bond system on the polymer

backbone. Conductivity is ranged from a few S cm−1

to 500 S cm−1

in the doped state [30]. In

order for the polymer to be conductive, a charge carrier counter ions (e.g. Cl-) is required to

cause delocalization of electrons and create conductivity. The process is known as doping.

Conducting polymers can be either p-doped which the polymer chain become oxidized

- 18 -

(positively charged) intercalated with counter anions (A-), or n-doped with counter cations (C+)

when reduced (negatively charged) [18, 30, 31]. The process is illustrated at Figure 2.5.

The discharge reactions are the reverse of the equations. The doping levels (no. of

electrons per unit) in this p-type conducting polymer is typically below 1 (approximately 0.3–0.5

i.e., 2–3 monomer units per dopant) [30]. One drawback of this mechanism is the relatively low

power due to the slow diffusion of ions within the bulk of the electrode causing the slow

charge/discharge rate. Nevertheless, the high energy density achieved from this kinetics can

significantly bridge the energy gap between batteries and double-layer supercapacitors [7, 8, 10,

18, 31-32, 53]. Conducting polymers are attractive as they have high charge density. Moreover,

they are relatively cheap compared to metal oxides [7, 90, and 91]. It is possible to develop

devices with low equivalent series resistance (ESR) and therefore high power, and high energy.

From the literature, carbon based supercapacitor devices can achieve a specific power of 3–4

kWkg−1

and a specific energy of 3–5 Whkg−1

[31], while for conducting polymer based

supercapacitor achieves a slightly lower power at 2 kWkg−1

but the specific energy is doubled

(10Whkg−1

) [31].

In terms of specific capacitance, ECP based materials can achieve up to 400 - 500 F/g in

contrast to about 100F/g for double layer capacitor. However, the application of conductive

polymer in supercapcitors is limited by their poor cycling stability. The ECP electrode would

start to degrade within less than a thousand cycles due to the physical structure changes caused

by the doping/de-doping of electrolyte ions [31-33]. This volume change, or swelling which

eventually causes mechanical failure of the polymer backbone under prolonged charge/

discharge cycles. And therefore degrades the faradic transfer process [7, 8, 30, 52]. Although a

- 19 -

higher specific energies can be achieved by increasing the doping level. However it leads to a

consequence that the higher degree of counter ions insertion and de-insertion would worsen the

volume change effect [8, 30]. Moreover another hindrance is the limited voltage window, it is

reported that the voltage window for conductive polymer is within 1V [30], therefore application

in a higher voltage range is restricted. Table 2.3 summarizes the theoretical capacitance achieved

by the ECPs polyaniline (PANI), Polypyrrole (Ppy) and Poly 3,4 dioxiphiophene (PEDOT) .

Table 2.3 Electrical and physical properties of various conductive polymers [30]

Conductive

polymers

Mw (g mol-1

) Dopant

level

Potential

range (V)

Theoretical

capacitance (F/g)

Conductivity

(S/cm)

PANI 93 0.5 0.7 750 0.1-5

Ppy 67 0.33 0.8 620 10-50

PEDOT 142 0.33 1.2 210 300-500

2.2.4.2. Polyaniline

Figure 2.6 General chemical formula of PANI [99]

Polyaniline (PANI) has the general structure as shown in Figure 2.6. [99]. One unique

feature of PANI is that it exists in three different oxidation states. They are the Leucoemeraldine

oxidation state (fully reduced), the Emeraldine oxidation state (half-oxidized), and the

Pernigraniline oxidation state (fully oxidized) as shown in Figure 2.7 [99]. Moreover, these

oxidation states are highly pH sensitive. Another interesting feature of PANI is that, besides

doping induced by partial oxidation or partial reduction, oxidation states can be converted

through proton exchange in contact with protonic acid [99, 136]. Non-conducting Emeraldine

base (EB) form of PANI can be doped to a highly conductive Emeraldine salt (ES) without

- 20 -

changing the total number of electrons associated with it [99,136]. Such doping process (Figure

2.8) is achieved by protonation of the –NH– group of EB by inorganic protonic acids and is

known as ‘acid doping’ [99,117,118,136]. The acid doping process increases the conductivity of

PANI by more than eight orders of magnitude [117,118]. Positive charges accumulated on the

polymer backbone during protonation of PANI are neutralized by the negatively charged counter

ions of the dopant. The protonation is also accompanied by the drastic change in the electronic

structure, crystallinity, solubility, etc. [118]. The degree of protonation and the resulting

conductivity can be controlled by changing the pH of the dopant acid solution. Color changes

from dark blue to emerald green can be observed upon the doped state is reached.

(i) Leucoemeraldine base: the fully reduced form (ii) Pernigraniline base: the fully oxidized form

(iii) Emeraldine base: the intermediate oxidation state of polyaniline. It is composed

of equal amounts of alternating reduced base and oxidized base unit

Figure 2.7 Chemical structure of PANI at various oxidation states [99, 117]

Figure 2.8 Prontonic acid doping of PANI Emeraldine base by HCl to conductive Emeraldine

salt state with two polarons [99,117]

- 21 -

For polyaniline, similar to polypyrrole, APS and FeCl3 can both be employed as oxidant.

It is claimed that APS under acidic medium (1M HCl) in generally yield the highest conductivity.

Other oxidants such as ceric ammonium sulfate, potassium dichromate, and hydrogen peroxide

are also reported. The solvent for polyaniline polymerization is generally carried out in aqueous

solvent. Addition of acetone, tetrahydrofuran and ethanol has effect on the reaction duration of

the APS/HCl system. Solvent also have effect on morphology and conductivity. It is reported

that, the polymerization medium with 20% ethanol can lead to a uniformed morphology with an

improved conductivity [137]. Polyaniline exhibits many desirable properties in supercapacitor

application; it has high electroactivity, a high doping level (0.5). High theoretical specific

capacitance (up to 400–500 F g−1

in an acidic medium) can be achieved [31]. In addition, it has

good environmental stability, controllable electrical conductivity (Ranged from 0.1 to 5Scm−1

)

[138]

2.2.4.3. Polypyrrole

Figure 2.9 Chemical structure of polypyrrole neutral aromatic and quinoid forms and in

oxidized polaron and bipolaron forms [121]

- 22 -

Polypyrrole is a compound assembled by oxidation of pyrrole rings or substitute of

pyrrole monomers. The structure was presented on Figure 2.9 [117,121]. It is attracted due to its

high conductivity and good environmental stability [117,]. Doping leads to formation of highly

conductive bipolaron state from the neutral aromatic structure. For polypyrrole, wide range of

oxidants can be employed. Including APS (NH4)2S2O8), and transition metal salt (e.g. Fe3+

, Ce4+

,

Cu2+

, Cr6+

and Mn7+

). The use of H2O with FeCl3 (with Fe3+

as catalyst) is attracted. FeCl3 is one

of the common one electron oxidants. A portion of the FeCl3 is contributed to oxidative doping

of the neutral Ppy product into its conducting form which carries a positive charge on about

every third pyrrole unit [117]. Polymerization of Ppy under protonic acid midium as dopant with

APS (NH4)2S2O8) as oxidant was also reported [133]. Polymerization is incorporated with a color

change from initially colorless to blue and eventually black. As an electrically conductive

polymer, Ppy is attractive mainly due to its ease of synthesis procedure while achieving very

promising conductivity. Highly conductive bipolaron structure can be resulted. Polypyrrole has

high doping level (0.5). It exhibits very high conductivity (1—50Scm-1

) and theoretic

capacitance of 620 F/g [31].

2.2.4.4. Poly(3,4-ethylenedioxythiophene) PEDOT

Figure 2.10 Structure of the ground state of a) Poly (3, 4-ethylenedioxythiophene) PEDOT and b)

polythiophene respectively

a) b)

- 23 -

Poly (3, 4-ethylenedioxythiophene) (PEDOT) is a substitute of polythiophene with two

ether groups (3, 4-crown ether) [139]. The structure of PEDOT and polythiophene was presented

on Figure 2.10.a) and Figure 2.10.b) respectively. The ether group is able to introduce an

electron-donating characteristic to PEDOT [16]. Upon doping (either removal (p-doping) or

addition (n-doping) of electrons of electrons), vacancy in the conduction band is created. Radical

(un-paired) cations or anions are formed on the backbone results as a charged unit polaron. The

movement of a pairs (bipolarons) as a unit induces conductivity in PEDOT backbone chain [140].

Figure 2.11 Chemical structure of neutral and doped PEDOT chain [140]

One of the disadvantages of EDOT monomer is the poor solubility in water. The use of

volatile and toxic organic solvent (e.g. Acetone Nitrite) is usually required in polymerizing

EDOT [141]. In order to improve the solubility of EDOT in aqueous solvent, (sodium

polystyrene sulfonate) PSS is usually coupled with the processing of PEDOT. PSS serve two

roles in PEDOT: PSS composite. PSS is a water soluble polymer which serve as a good water

dispersant to enhance PEDOT polar group compatibility [142]. Moreover PSS acts as a charge

compensating counter ion to stabilize the p-doped PEDOT in the PEDOT: PSS complex [142].

- 24 -

For the PEDOT: PSS system. The PEDOT segment is oligeromic rather than polymeric

[118]. High molecular weight of PEDOT is seldom occurred. PEDOT: PSS has very high

stability. The PEDOT+ and PSS

- ionic species is hardly separated from each others. Oligomeric

PEDOT segments are electrostatically attached to PSS chain. The high conductivity of

PEDOT:PSS can be attributed to the stacked arrangement of the PEDOT chains within a large,

entangles structure. PEDOT: PSS allows conduction of both electrons and ions. In this

composite, PEDOT is doped by PSS. Figure 2.12 show the chemical structure of neutral and

doped PEDOT: PSS respectively. PEDOT provides electronic conductivity, while PSS provides

both enhanced electronic conductivity and cationic conductivity. Equation 4 and 5 show the

oxidation and reduction process of PEDOT: PSS respectively [143]. The electronic charge

transferred during reduction is balanced by an equal amount of ionic charge (M+) transported

through the over-oxidized polymer channel [143]. The ion transport efficiency is determined by

parameters such as the net-charge, physical size and chemical structure [143]. PEDOT can be

both p-doped and n-doped. It is electron rich and consequently has low oxidation potential with

wide potential window (1.2-1.5V). Has low band gap (1-3eV) and high conductivity in the p-

doped state (300-500 S/cm) [5, 9].

PEDOT to PSS ratio affects the conductivity and the film forming property. Various

proportions of PEDOT and PSS were employed for different applications. In conductive coating

application, PEDOT: PSS is 1:2.5 which yield about 1.3% of solid content with conductivity

upto10S/cm. While for passive matrix OLED display, PEDOT: PSS is about 1:20. Solid

contented up to 3%. However conductivity was dropped to 10-5S/cm. Therefore, increasing PSS

content would lead to decrease of conductivity of the PEDOT: PSS composite [118]. Most

- 25 -

polythiophene derivatives have low capacitance in the n-doped state relative to the p-doped state.

PEDOT can be used in both symmetric and asymmetric capacitors [31] on negative side, it has

low theoretical specific capacitance due to the large molecular weight and relatively low doping

level (0.33) compared to other conductive polymer such as polyaniline and polypyrrole [31].

Figure 2.12 Chemical structure of PEDOT doped with PSS [145]

PEDOT0 + M+PSS- PEDOT+ PSS-+ M+ + e-

PEDOT+ PSS-+ M+ + e- PEDOT0 + M+ + PSS-

45

- 26 -

2.3. Hybrid Supercapacitors

Hybrid capacitors attempt to utilize both faradic and non-faradic processes in charge

storage. Integrating carbon-based materials and pseudo capacitive conducting polymer / metal

oxide materials can generate hybrid energy storage system by combining the two different

charge storage mechanisms. Hybrid supercapacitors utilize both faradic and non-faradic

mechanisms for charge storage; therefore a greater energy density can be achieved due to the

faradic activities. The limited cycling stability of conductive polymer can also be compensated

by forming complex with the carbon nanotubes matrix [4, 10, 18, and 31].

These composite electrodes incorporate both physical and chemical charge storage

mechanisms together in a single electrode. Highly porous carbon network facilitate a capacitive

double-layer of charge. Moreover it provides a high surface-area backbone that increases the

contact between the deposited pseudo capacitive materials and electrolyte. The pseudo capacitive

conductive polymer is able to further increase the capacitance of the composite electrode through

faradic reactions [12, 31, 39, and 40].

2.3.1. Carbon- Conductive polymers Composite Electrodes / Supercapactors

Composite electrodes integrate two or more materials together as active charge storage

materials. Most popular approach is to fabricate composites between carbon base materials with

pseudo capacitive (ECP or metal oxides). While CP-CP or metal oxides composites had also

been studied [31, 37, 68, 92-108]. For carbon based composites, carbon materials such as carbon

nano-fibers, activated carbon, graphene and carbon nanotubes were actively studied [10, 12, 16,

- 27 -

54, 55, 56, and 57]. The porous structure provided by the carbon base material network provides

a high surface area which facilitates double layer charge storage and the deposition of the pseudo

capacitive material. Moreover, this large surface area can also increase the contact with the

electrolyte for effective pseudo capacitive charge storage.

In generally, an improved electroactivity of the carbon/ECP composite materials can be

obtained [62, 98,169]. Studies discovered that the interaction between the two materials causes

overlapping between the π-bonds surface of the MWCNTs and the π conjugates of the polymer

chain. And therefore improves the electron mobility. Further, it is reported that the carbon

material can also provide doping effect to the conductive polymers by forming a charge transfer

complex. This can induced higher degree of protonation to the polymer chain and hence, the

conductivity [62, 98, 169]. In addition, the carbon materials can also provide mechanical

reinforcement to the conductive polymer in the composite. The swelling and physical shrinkage

problem resulted from the doping and dedoping process can be compensated. The mechanical

properties of the ECP can be reinforced by the MWCNTs. The interaction between the two

materials causes micro-mechanical interlocking at the interface between the two materials. With

carboxylic acid functionalization on the MWCNTs surface, chemical bonds can be formed

between the two materials, and therefore facilitate stress transfer from the ECP to the MWCNTs

[202]. From a study of Mariana et.al, suggested that 10 wt% of CNT in Ppy/CNT composite can

lead to 170% increase of stiffness of the polymer matrix [201] As a result, it is believed that the

cycling stability of conductive polymer can be improved by forming composite with MWCNTs.

[20, 38, 94-96,106,111].

From the literature, Lota et al. has reported capacitive performance of PEDOT/MWCNT

composite prepared via different fabrication method including i) In-situ chemical polymerisation

- 28 -

on ultrasonically dispersed CNTs (ii) Direct blending of PEDOT with CNTs and (iii)

Electrochemical deposition of PEDOT onto the CNTs. Improved cycling life from the composite

was due to adaptation to volume changes upon insertion and removal of counter-ions from the

MWCNTs [106]. Study from Hong fang An et.al reported an improvement of capacitance from

174F/g to 433 F/g achieved by carbon aerogel /Polypyrrole composite electrode [37]. Nano-

composite formed between polyaniline (PANI)/Graphene composites was found to have

improved cycling stability by Chun Li et.al. Their study found that a laminated architecture of

graphene can support the PANI fibers in place to suppress the swelling and shrinking mechanism.

An increase of capacitance from 147F/g to 210F/g was observed [112]. Furthermore, vapor

grown carbon fibers /Ppy composite provides high power capacity with specific capacitance of

550Fg−1

at 200mVs−1

scan rate in 6M KOH electrolyte [94]. Table 2.4 summarized some of the

composites electrode performance from the literature

- 29 -

Table 2.4 Specific capacitance and respective fabrication method for various

ECP/carbon composite

Polymeric

materials

Carbon based

Electrode

Material

Fabrication Method Electrolyte Specific

Capacitance F/g

Reference

PEDOT Activated

carbon

Direct blending of AC

with 5wt% PEDOT

1 M

TEABF4/PC

solution

Device: 120F/g @

10mV/s

[108]

Graphene Chemical oxidative

polymerization

2M HCl and

2M H2SO4

electrolyte

HCl: 304F/g

@1mV/s

H2SO4: 261F/g

@1mV/s

[63]

MWCNT 1. Chemical

polymerization

2. Direct blending

3. electro-chem

deposition

1M H2SO4 1. 130F/g

2. 120F/g

3. 150F/g

@1mV/s

[106]

Polyaniline

(PANI)

SWCNT Electrochemical

polymerization

1M H2SO4 190.6F/g @5mV/s [98]

Carbonized

PANI

Carbonization of PANI

fibers under ultimate

temperature

30wt% kOH 163F/g@ 5mV/s [113]

MWCNT Chemical

polymerization:

: K2Cr2O7 in 50 ml of 1

M HCl

1M H2SO4 320 F/g@1mV/s [12]

Graphene Chemical oxidation of

PANI onto graphene

1M H2SO4 210F/g @ 5mV/s [112]

Polypyrrole

(Ppy)

Carbon Aerogel Chemical

polymerization:

FeCl3

6M KOH 477F/g @2mV/s [37]

MWCNT Chemical

polymerization:

FeCl3 +0.1M/L HCl

1M H2SO4 200F/g @ 1mV/s [12]

MWCNT Electrochemical

deposition:

p-toluene sulphonic

acid /tiron

0.5M

Na2SO4

310F/g @ 2mV/s [114]

SWCNT Mini emulsion

polymerization:

SDS and

1-pentanol

1 M LiClO4

solution

134F/g @ 20mV/s [95]

- 30 -

2.4. Conductive Polymer Synthesis

In General, conductive polymers are classified as cationic or anionic [31,117,118]. Most

ECPs are cationic polymers which can be polymerized through oxidation from the monomers.

Numbers of synthetic methods for conductive polymers fabrication have been proposed.

Electrochemical polymerization, chemical polymerization, photochemically initiates

polymerization and enzyme catalysed polymerization [117]. Among these methods, chemical

polymerization and electrochemical polymerization are most commonly incorporated.

The advantage of electrochemical polymerization over chemical polymerization is the

ability to produce thin film and the easy control of polymer thickness and morphology.

Parameters that determine the polymerized polymers including solvent medium, working

potential, current density and temperature [117].

In contrast to the electrochemical method, the chemical polymerization approach is

benefited by its simplicity. In the chemical process, no complex electrochemical set up is

required. It can be easily scaled up in batch production. Polymers in form of powders or

colloidal dispersion can be prepared. Moreover, the process exhibit high flexibility since the

polymerization can be carried out in neutral solution (e.g. water). However, the control over the

morphology is very limited [117,118]. Similarly, parameters including monomers to oxidant

ratio, temperature, and solvent used during the polymerization process would significantly affect

the property of the polymer.

- 31 -

Table 2.5 Comparison of chemical and electrochemical polymerization [135]

Polymerization methods Advantages Disadvantages

Chemical polymerization Easier functionalization of

polymer backbone

Does not require

complicated electrochemical

setup

Aqueous solution can be

used as polymerization

medium

Large scale batch production

is possible

Thin film fabrication

Relatively poor control of

polymer’s morphology

Required binder materials

in film fabrication

Electrochemical

polymerization

Easier control of morphology

Simultaneous polymerization

and film formation

Allows thin film synthesis

Requires electrochemical

setup

Complication in mass

fabrication

Cannot separate film from

the substrate

For the oxidative chemical polymerization approach, parameters including oxidant

concentration, solvent medium, dopant, as well as temperature would cause significant effect to

the physical and electrical properties of conductive polymers. An oxidation strength too high

would cause a faster rate of polymerization, therefore results as a low conductive polymer with

large aggregates [127]. For example, for PANI, when the APS oxidant content is too high; the

condition becomes unfavorable for high molecular weight polyaniline generation. Moreover, the

excessively oxidized aniline monomers would causes the conjugated π electronic system being

destroyed. And therefore the number of charge carried, and hence the conductivity will be

reduced [128].

The solvent also have influence to the quality of the polymerized CP. For example, water

and alcohol medium favour the polymerization more than solvent such as acetonitrile,

tetrahydrofuran, chloroform and benzene in Ppy [117]. Study also reported that the conductivity

of Ppy increases as the polarizability of the carrier solvent increase. Although the conductivity

- 32 -

decreases in non-aqueous media due to increase stabilizer adsorption, but as a result lead to an

increase in the polymerization rate in the polar organic media [128]

For the working temperature, polymerization in room temperature was claimed to create

defect on the surface such as branching. The most widely adopted temperature range is from 1-

5oC with resulted high molecular weight polymer (~ 30000- 60000g mol

-1). Molecular weight is

inversely proportional to the temperature. Temperature as low as -30oC and -40

oC can lead to

Mw > 400000. This polymerization process would take up to 48 hours to complete. Reports

shows that ~ 0oC is the most ideal condition with high yield and conductivity (>80 %), while the

polymerization process only requires 3-5 hrs to complete [117].

The dopant counter ion plays an important role in determining the conductivity in the

chemically polymerized polymers. Dopant ions in conducting Ppy are positioned interstitially

between polymer chains. Dopant ions can be incorporated from the oxidant. For example FeCl3

provides Cl- whereas (NH4)2S2O8 provides HSO4

-/SO4

2- [102,104,117,121,131,132]. For the

polymerization incorporating acidic medium, the concentration of the acid medium has

significant effect on the physical chemical properties and molecular weight. On the influence of

duration, for example PANI doped with HClO4 results as twice as much polymerization time

than doped with HCl, H2SO4. Moreover a more compact morphology was also resulted. The pH

of acid has significant effect on the PANI morphology; pH > 4 would causes formation of

oligomeric and non-conducting species. Moreover, acidic strength of the protonic acid would

affect the number of quinoid ring formation. Therefore affects the conductivity [133]. Table 2.5

summarized the comparison between chemical and electrochemical approaches.

- 33 -

2.5. Modification Technique of Carbon Nanotubes with Conductive

Polymers

Polymer (powder)

Carbon materials (powder)

Mechanical blending (screw mixing)

Polymer-Carbon solid (Solid)

Polymer (powder)

Carbon materials (powder)

Stir/ Sonication mixing in solution

Polymer-carbon dispersion (liquid)

Monomer(liquid)

Carbon materials

(Dispersion)

Stir/ Sonication + oxidant

Polymer-carbon (Solid)

1. Direct mixing

1 b). Solution mixing mixing

1a). Solid-state mixing

2. In situ chemical polymerization

3.Electrochemical polymerization

Monomer dispersion

V vs reference electrode (e.g. SCE)Electrode-CNT Polymer film on

carbon surface

Monomer + CNT dispersion

V vs reference electrode (e.g. SCE)Electrode Polymer-carbon

composite film

a)

b)

Figure 2.13 Generalized scheme of preparative methods for polymer-CNT composited

Attempts to combine CPs and CNTs in producing functional composite materials with

improved properties for various applications have been actively studied. When the two materials

interact with each others, stacking of the π-bonds between the two components which lead to

synergy in the electroactivity of the composite was suggested [98, 102, 127 and 146]. Several

- 34 -

studies proposed an enhanced delocalization of electron in the composite as well as a doping

effect from the CNTs to CP matrix due to charge transfer complex (e- donor/accepter) formation

results as enhanced conductivity of the ECP/ CNT composites [102].

Various synthesis methods for the preparation of ECP/CNT composites includes: (i)

Direct mixing of the ECPs with CNTs; (ii) Chemical polymerization of the corresponding

monomer in the presence of CNTs; and (iii) Electrochemical synthesis of ECPs on a CNT

electrode. Figure 2.13 summarized the scheme for composite fabrication methods. Various types

of composites have been reported, such as solid/ solution blending, in situ-chemical

polymerization in CNT dispersion, as well as electrochemical depositions.

2.6. Supercapacitor Applications

ECs have potential applications in wide range of fields and industries. For example in the

application of electric vehicles and hybrid electric vehicles, supercapacitors can supply pulse

power for batteries or fuel cells in engine starting, acceleration, and also can store energy during

braking within a short time [14, 15]. The stored energy can be reused when the vehicle starts

moving again. In particular, supercapacitors may also replace or be combined with batteries as

the electric power source of electric vehicles and hybrid electric vehicles [15] due to their high

power delivery or uptake and relatively high energy stored [16].

On top of the outstanding electrochemical performance, another advantage of

supercapacitors is the great reduction in size. This allows promising application in portable

electronic devices where miniature size is required [48]. ECs can function as short-term power

back up lasting 15 to 30 seconds in various electronic devices such as laptops computer, digital

camera and cell phones etc [51].

- 35 -

The application can also be extended to the medical sector. The design of an EC device

also features physical advantages such as paper thin and high flexibility. This also favours a high

compliance with human body and easy installation onto garments [26]. Therefore it is very

suitable to serve as sustainable energy storage system for portable medical devices/ electronics

such as portable health monitoring system (e.g. e-textiles) and various implants [26, 52].

In spite of the advantages of supercapacitor discussed above, they are currently prevented

from fully commercialized for long duration applications mainly due to their high cost and very

limited energy density. Hence, discovering new electrode materials with reduced cost and

increased energy density are the urgent issues for the future research work [53].

2.7. Electrochemical Techniques

2.7.1. Cyclic Voltammetry

Cyclic voltammetry (CV) is a useful technique for studying macroscopic electrochemical

surface reaction of electrode materials. The charges response with regard to a changing voltage,

and therefore the capacitance can be measured. In cyclic voltammetry experiment for single

electrode, a half-cell three electrodes setup is incorporated. A schematic of the setup is presented

on Figure 2.14.

A

WE RE CE

Figure 2.14 Equivalent circuit of the 3 electrodes setup

- 36 -

In this three electrodes cell, the working electrode (WE), which is the tested electrode

makes contacts to the electrolyte and a desired DC potential is applied to the system in order to

facilitate transfer of charge between the electrodes and electrolyte. The reference electrode (RE)

acts as the other half of the cell with a known potential. It is placed close to the polarization layer

of the WE in order to measure the voltage difference between the WE and the RE [155]. The

counter electrode (CE) takes on the roles of supplying electrons to allow flow of current between

working and counter electrode, it provides current required to balance the current observed at the

working electrode [152,156].

The measured charge current from CV is plotted against the applied voltage to generate a

cyclic voltammogram plot. The following information can be obtained from a CV plot: 1) The

reversibility of the charge/discharge process, 2) Indication of any noticeable stages in charge /

discharge of the electrodes. 3) Total charge accumulated over a potential range and 4) the

dynamic electrode charge discharge behaviour with increasing scan rates [196]

Figure 2.15 Typical charge/discharge CV characteristic of an electrochemical capacitor [196]

- 37 -

Figure 2.15 represent various CV profiles. An ideal EDLC exhibit a highly rectangular

shape. Maximum charge current is reached and maintains constant as there is no leakage.

Moreover the charge current is independent of the potential as the sign of the current switch right

away as the applied potential becomes negative. For a resistive system, the current becomes

dependent to the potential, the tilted plot with increasing slope meaning that the higher the

resistance of the system. Usually this can be observed from switching from a rectangular shape

into a parallelogram shape. For a faradic system, the CV curve deviate from rectangular shape

due to Oxidation /Reduction peaks. This indicates the presence of REDOX species in the system.

The current change with respect to time can be calculated as follow.

Since dQ/dt is the current “i” and dE/dt is potential scanrate “ ”. The equation then becomes :

From this derivation, the charge current at steady state in respond to ramping voltage is presented.

By measuring the charging current at the given scan rate, the capacitance can be calculated by

dividing the net charge by the “dQ” by the Voltage window dE [99,152,153]. The main

advantage of CV is its simplicity. The area under the curve is also proportional to the capacitance.

- 38 -

2.7.2. Electrochemcial Impedance Spectrometry

Electrochemcial impedance spectrometry (EIS) provides a more thorough understanding

of a supercapacitor system than any other electrochemical technique. In EIS the application of

sinusoidal signal in wide range of frequency can be used to study the electrochemical behaviour

at the high and low frequency end of a single electrode or a device. Moreover, the capacitance

can also be measured [158]. In EIS, impedance is measured to assess the ability of a system to

resist the flow of current. The excitation potential Et and the respond signal It have the following

form:

Et = Eo sin (ω t) It = Io sin (ω t + ɸ)

According to ohm’s law, the expression of the impedance is calculated as follows.

The impedance is expressed in terms of magnitude Zo and the phase shift ɸ. According to Euler

relation, the potential and the current signals can be expressed as follow.

Exp (j ) = cos ( ) + jsin ( )

E = Eo exp (j t)

I = Io exp (j t- )

Therefore, the impedance is then expressed as:

|Z ( = Z’+ jZ”

The expression of impedance is then composited of a Z’ real and an imaginary part Z”. If

the real part is plotted on the X-axis and the imaginary part is plotted on the Y-axis of a chart, we

obtain a "Nyquist Plot" (Figure 2. 16). The Y-axis is negative and that each point on the nyquist

plot is the impedance at one frequency.

- 39 -

From left to right, the impedance was presented from high to low frequency. On the

nyquist plot the impedance can be represented as a vector (arrow) of length |Z|. The angle

between this vector and the X-axis, commonly called the “phase angle ( ”, another presentation

is the bode plot which the impedance is plotted with negative phase on Y-Axis against the log

frequency on the X-axis. A sample bode plot is shown on Figure 2. 17. Unlike the nyquist Plot,

frequency information was indicated on the bode plot [153].

Figure 2.18 An example of nyquist plot for an activated carbon electrode [158]

Figure 2. 16 A sample nyquist plot with

impedance vector [158] Figure 2. 17 Sample bode plot with one

time constant [158]

- 40 -

Supercapacitors oscillate between two states, as a resistor at high frequencies and a

capacitor at low frequencies. [157,158]. A nyquist plot of an activated carbon electrode is shown

on Figure 2.18. At high frequencies ~10 kHz, supercapacitors behave as a resistor with resistance

intersected with the real axis. However when approach towards low frequency (0.013Hz), the

imaginary part of the impedance sharply increases and the plot tends to be a vertical line which

indicates a highly capacitive behavior [157,158]. In the middle frequency range, factors such as

electrode porosity and electrode thickness affect the ions migration rate from the electrolyte to

electrode [153]. The point where the low-frequency vertical line takes off defines the ‘‘knee

frequency’’. Below this frequency knee point, the whole capacitance is reached. This frequency

which indicates the resistance to capacitance transition can also be evaluated from the bode plot.

This frequency appears at where the plot intersects at -45o phase. Capacitive behaviour started to

display below this frequency [153,158].

The data from nyquist plot is generally analysed in terms of equivalent circuit model. A

model which matches the impedance data is required to generate a correct fitting in order to

accurately estimate the parameters such as the double layer capacitance and any resistance due to

charge transfer and diffusion associated with the double layer capacitance and charge transfer

[62, 99,158]. The ideal capacitor model does not exist. Different models are used to describe the

investigated system. For example, the most common and simplest model is the Randles cell as

shown on Figure 2.19.

- 41 -

Figure 2.19 A Randles circuit model with mixed kinetic and charge transfer control (Left) and

the corresponding nyquist plot [153]

2.7.3. Galvanostatic Charge /Discharge

Galvanostatic method consists of applying a constant current pulse to the cell and

measuring voltage response over time. Cyclic Charge Discharge is a standard technique for

testing the performance of EDLCs and batteries. A repetitive loop of charging and discharging is

called a cycle. The measurement is conducted under constant current until a set voltage is

reached. The charge of each cycle is measured and the capacitance C, in farad (F) can be

calculated. Since

, and

. Therefore the capacitance equals to

. Where V is

the cell potential in volts (V), I is the cell current in amperes (A), and (Q) is the charge in

coulombs (C) or ampere-seconds (As). Efficiency of each cycle is indicated by the coulomb

efficiency ( which is calculated from the ratio of discharge time td to charge time tc [29]. A

typical high efficiency charge/ discharge plot of a high efficiency supercapacitor display a highly

symmetrical triangular shape with a constant slope (dV/dt). Which indicates a low ohmic drop of

the system as presented on Figure 2.20 a).

- 42 -

Figure 2.20 Typical charge discharge curve for an efficient system (left) and a system showing

built up ohmic response (right) [159]

When the system undergoes increasing self-discharge at high current density. The plot

displays an exponential shape. A higher equivalent series resistance (ESR) leads to a large

voltage drop (IR-drop) at each half-cycle and therefore dramatically reduces the power capability.

And therefore greatly damages the efficiency of this EDLC [159].

- 43 -

Chapter 3 Experimental

3.1. Methodology and Approach

In order to fabricate MWCNT and ECPs composites, an oxidative chemical

polymerization approach was selected with ease of fabrication consideration. The entire

polymerization process is conducted in solution medium. The general approach is to disperse

MWCNTs into an appropriate solvent. Then the monomers along with oxidant were added into

the CNT solution in order to allow the monomers to be polymerized and being grafted onto the

MWCNT lattice. In this study, the MWCNTs used were functionalized with carboxylic acid

(COOH) group on the surface. This allows the polymers to be covalently bonded to the

MWCNTs and therefore improve interaction between the two materials [39, 107].

3.2. Materials

3.2.1. Multi-Walled Carbon Nanotubes

The Multi-walled carbon nanotube in powder form were purchased from NanocylTM

.

The CNTs are produced via Chemical Vapor Deposition (CVD). The model NC3100 grade is

functionalized with COOH (4%). The average diameter of the tube is 9.5nm with average length

of 1.5um.

3.2.2. Polyaniline

The aniline monomer (ACS reagent, > 99.5%) in liquid form was purchased from Sigma-

Aldrich. Ammonium persulfate (APS) in powder form was used as oxidant for polymerization.

- 44 -

Hydrochloric Acid (1M HCl) and dodecyl benzene sulfonic acid (DBSA) were applied as dopant

for the polyaniline system. Detail fabrication process is illustrated in later chapter.

3.2.3. Polypyrrole

Pyrrole monomer (Reagent grade, 98%) in liquid form was purchased from Sigma-

Aldrich. The pyrrole monomers were subjected to distillation prior fabrication. Iron Chloride

(FeCl3) was applied as oxidant and dopant during the fabrication process.

3.2.4. PEDOT / PEDOT: PSS

EDOT monomer (3, 4-Ethylenedioxythiophene) in liquid form was purchased from

Sigma-Aldrich. For polymerization of PEDOT, acetonitrile was used as solvent since EDOT is

not soluble in water. While for PEDOT: PSS, poly styrene sulfonate (PSS) allow polymerization

of EDOT under aqueous medium. A mixture of FeCl3 and APS were applied as oxidant and

dopant. The fabrication process is detailed in later chapter.

3.3. Samples Fabrication

3.3.1. Composite Material Synthesis

An in-situ polymerization method was used for composite fabrication for all the

ECP/MWCNT composite. The general step by step fabrication scheme is demonstrated in figure

3.1. Weighted MWCNT powder was first dissolved into aqueous solvent along with weighted

monomers. Continuous stirring (~5 minutes) and ultrasonication using sonication probe was

applied to improve the dispersion of the mixture. Upon addition of oxidant, the polymerization

process will then be initiated in the CNT medium. It is believed that the monomers will then be

polymerized and deposited on top of the MWCNTs after allowable duration; the precipitate is

- 45 -

then filtered and dried. The bulk composite can then be extracted. The chunk of material was

then grinded down into powder form. The detailed process for each conductive polymer

composite fabrications is detailed in the following chapters

100mg MWCNT

Adding MWCNT and weighted Monomers

Ultrasonication (5minutes)

Adding oxidant , polymerization was allowed

for 5 hours (@ 0oC)

ECP/MWCNT composite

Suction filtration

Dried under 80 oC over night

Figure 3.1 In-situ polymerization scheme applied for composite fabrication

3.3.2. Composite electrode and two electrodes cell preparation

In electrode preparation, additives such as carbon black and graphite, along with a

polymeric binder were used to create a good adhesion and conductivity between the composite

materials and the current collector [154]. Metal foil (e.g. stainless steel, titanium) can be used as

- 46 -

current collector. In constructing a two electrodes cell, a separator is required to allow ionic

conductivity while preventing electronic current to flow between the electrodes [154].

Slurry was first prepared by mixing the weighted composite powder with

polytetrafluroethylene (PTFE) (10wt% of the composite) and weighted graphite conductive ink

(Alfa aesar) with carbon base ingredients including graphite (20%) and carbon black (2%). From

previous studies, PTFE powder was fixed at low content (5-10wt% of the composite) to avoid

building up resistance. Isopropanol was used as solvent of PTFE. After stir mixing, a

homogeneous conductive paste was then casted onto stainless steel sheet in dimension of 1cm x

1cm. The process is described on figure 3.2. The details of total mass for each composite

materials presence in the electrode will be detailed in the following chapters. In fabricating a test

cell, a device was formed by pressing two electrodes together with a piece of filter paper soaked

with 1M H2SO4 electrolyte being sandwiched in between.

ECP/MWCNT composite powder

PTFE powder

Graphite conductive

ink

Isopropal Alcohol

Stir mixing

Conductive mixture (1cm x 1cm) was being casted onto Stainless steel

sheet

Figure 3.2 ECP/MWCNT Composite electrode fabrication process

- 47 -

Figure 3.3 A sample of Ppy/MWCNT composite electrode with conductive composite material

casted onto stainless steel substrate (Left) and a two electrodes test cell (Right)

3.4. Materials Characterizations

3.4.1. Physical Characterizations

3.4.1.1. Scanning Electron Microscopy (SEM)

Morphology of the synthesized ECP/MWCNT composites strongly influences the

electrochemical properties. Hitachi S2-5200 HR-SEM machine was used to provide images with

detailed information of the composite material morphology. The high magnification (>x10000) is

able to study the micro, nano-structure. The uniformity of ECP polymerization onto MWCNTs

can be observed. The surface morphology of the composite can be studied to correlate the study

of material electrochemical performance.

3.4.1.2. Fourier Transform Infrared Spectroscopy (FTIR)

Fourier Transform Infrared Spectroscopy (FTIR) method allows one to characterize

vibrations in molecules by measuring the degree of Infrared energy absorption of that correspond

to the excitation of the molecules [163, 164]. From the excitation at different wavelength of a

FTIR spectrum, the presence of certain characteristic bonds confirms the successful

- 48 -

polymerization of ECP in the composite. Moreover, the possible interaction between the ECP

and MWCNT could also be observed from any distortion of the characteristic bands and new

bands formations. FTIR machine (Bruker: Alpha) was used in this study.

3.4.2. Electrochemical Characterization

3.4.2.1. Cyclic Voltammetry (CV)

Single electrode electrochemical evaluation was incorporated with a potentiostat (Model:

CHI 760C) in a half-cell three electrodes setup. In this study, the reference electrode (RE) used

is Silver/Silver Chloride (Ag/AgCl, 1M KCl) with a known potential of 0.2 V vs hydrogen.

Platinum (Pt) electrode was used as counter electrode (CE). 1M H2SO4 electrolyte is

incorporated as electrolyte medium. The voltage window is set within 1V [39]. For two

electrodes device evaluation, the composite electrodes act as both WE and CE. Capacitance is

calculated by dividing the recorded net charge from the system (Q) by the voltage window (V).

Cyclic voltammogram of current density (A cm-2

) was plotted against voltage (V) vs Ag/AgCl.

Specific capacitance is calculated per surface area and per gram of active material

(ECP/MWCNT) in the electrode.

In studying the performance in a supercapcitor cell, a two electrodes test cell was

incorporated. It is noted that in the two electrodes system, the lower capacitance value of both

electrodes connected in series will determine the total capacitance according to the following

equation (6) and (7).

6)

- 49 -

Assuming C1 and C2 are capacitance of the two electrodes of the supercapacitor and they

are closed (C1 = C2), the capacitance of single electrode is about two times of a two electrodes

device [99]. Therefore, in two electrodes devices study, the obtained capacitance value for entire

cell was than divided by two to obtain specific capacitance F/cm2.

3.4.2.2. Electrochemical Impedance spectrometry (EIS)

In EIS, AC potential was applied to an electrochemical cell and then the current was

measured through the cell. The negative of the imaginary impedance on Y-axis is plotted against

the real part on the X-axis to generate a nyquist Plot of –Z” vs Z’ from high to low frequency

[157]. The capacitor behaviour as well as the total equivalent resistance (ESR) and the ionic

transfer resistance (Rct) at the electrode /electrolyte interface can be evaluated at the low and high

frequency region respectively [157,158]. Bode plot of phase against frequency and complex

capacitance plot were used to further evaluate the capacitive performance in device setup [158].

The C’ value at the low frequency end equals to the frequency independent capacitance

value. Moreover, the RC time constant which is calculated from the can be deduced

from the imaginary capacitance plot. Where fo is the corresponding frequency where max C” was

located. The lower the time constant simply indicates a faster discharge time and therefore a

7)

8)

- 50 -

higher discharge power. A promising capacitor device should have a time constant of not higher

than 2s [18,158]

Figure 3.4 Generalised equivalent circuit model used for the supercapacitor cells in this

study, consist of two resistors R1 and R2, and two constant phase elements CPE1, CPE2

ω

Z-view software was used to model the system from the data retrieved from the nyquist

plot. The charge transfer resistance (Rct) and equivalent circuit resistance (ESR), as well as the

capacitive behaviour can be evaluated from the generalized circuit model as illustrated on Figure

3.4.

The constant phase element (CPE) is governed by the following equation 9. For n = 1, the

ZCPE behaves as an ideal capacitor with capacitance TCPE. While for n = 0, the ZCPE becomes a

resistor with resistance value equals to 1/TCPE [99]. While ~0.5 denotes diffusion dominated

behaviour which leads to Warburg impedance [99]. From the model, R2 represents the Rct, where

R1 indicate the ESR of the system. CPE1-T and CPE2-T are expressed as a capacity at the non-

homogeneous electrode surfaces and the ionic diffusion process respectively [99]. CPE2-P

values determine the capacitive nature of the system. The value at ~0.9 would indicate a very

highly capacitive performance. The corresponding parameters are estimated by Chi-squared

fitting the data to the corresponding nyquist plots.

9)

- 51 -

3.4.2.3. Galvanostatic charge/discharge test

Charge/discharge experiment was performed on the electrochemical cell in two

electrodes device with the electrodes applied as both WE and CE. The cell is charged to a desired

voltage (0.7V) under constant current, and then discharged until the cell voltage between WE

and CE decreases to 0V. The ohmic respond at different applied constant current was analysed.

The specific capacitance was evaluated from the linear part of the discharge curves using the

following equation 10) [99,108]. Where ‘C’ is the specific discharge capacitance (F/cm2), ‘I’ is

the discharge current density (A/cm2). dt/dV = (T2 - T1)/ (Vmax - ½Vmax) where T2 - T1 is the time

difference between Vmax and 1/2Vmax. The efficiency of charge discharge was indicated by the

coulomb efficiency, which was calculated from the ratio of discharge time td to charge time tc.

[158].

- 52 -

Chapter 4 Polyaniline/MWCNT Composite Study

4.1. Introduction

Dodecylbenzenesulphonic acid (DBSA) and hydrochloric acid (HCl) are two of the most

common dopants incorporated in PANI fabrication [97,99,129,130,161]. Their physical

properties as well as electrical properties in pellet samples were commonly studied. However, a

comparison of the performance as supercapacitor electrode utilizing these two dopants was

seldom reported. Therefore, a study was carried out in comparing the capacitive performance

between the two PANI/MWCNT composite prepared by doping with DBSA and HCl to evaluate

the effect of dopants to the PANI/MWCNT composite morphology and electrochemical

performance. For the 2nd

part of the study, a parametric study was conducted to study the

influence of polymer content to the electrochemical performance of the PANI/MWCNT

composite. Finally, Evaluation was performed in two electrodes cell configuration to study the in

general PANI/MWCNT composite performance as a supercapacitor under designated conditions.

4.2. DBSA, HCl doped PANI/MWCNT study

4.2.1. Composite Material Fabrications

Aniline was first purified through distillation. The functionalized MWCNT is able to

facilitate better dispersion in aqueous solution and improve interfacial interaction with the

polymer [31]. Ammonium Peroxydisulfate (APS, (NH4)2S2O8, ACS reagent >98.5%, Sigma

Aldrich) was used as oxidant [98, 105]. Polymerization was carried out in aqueous medium at 0-

4Oc in melting ice bath [98,105,117]. Both hydrochloric acid (HCl) and dodecylbenzenesulphonic

acid (DBSA, 70wt% in isopropanol, Adrich) were used as dopant for the polymerized PANI. For

the case of HCl used as dopant, Sodium dodecyl sulfate (SDS) was used as surfactant to improve

- 53 -

the MWCNT dispersion in the solution. While for DBSA doped process, SDS was not required

as DBSA also function as surfactant [165].

4.2.1.1. HCl doped PANI/MWCNT composites

100mg of functionalized MWCNT powder was first dissolved into 100ml of 1M HCl with

1.4g of SDS surfactant in 100ml of deionised water with 20% ethanol. The solution was then

subjected to ultrasonication to well disperse the MWCNT in the solution. Weighted aniline

monomer was first dissolved into HCl to form Aniline hydrochloride [98, 105, 130, and 146].

The solution was then added to the MWCNT solution and being stirred for 1 hour. APS in 100ml

1M HCl was first pre-cooled for one hour. The ratio of oxidizing species APS to aniline has been

reported to be oxidant/aniline 1.25 [166]. APS solution was then added drop wise into the

stirring solution to initiate the polymerization process. The polymerization was then allowed for

5 hours. The composite material precipitate was then filtered out and washed with methanol to

coagulate the dispersion [137]. After that it was then further washed with HCl and lastly washed

with acetone to remove low molecular weight polymer in order to keep the precipitate in fine

powder form [167]. The precipitate is then filtered and dried under 80oC

4.2.1.2. DBSA doped PANI/MWCNT

The fabrication for DBSA doped PANI/MWCNT composite was conducted in a similar

fashion. 100mg of MWCNT powder was first dissolved into 100ml of deionised (DI) water with

weighted DBSA. The solution was then subjected to stirring and ultrasonication using sonication

probe for 5 minutes to well disperse the MWCNT in the solution. Weighted aniline monomer

was first dissolved into DI water. The solution was then added to the MWCNT solution and

- 54 -

being stirred for 1 hour. APS was dissolved in 100ml DI water and then pre-cooled for one hour.

APS solution was then added drop wise into the stirring solution to initiate the polymerization

process. The polymerization was the allowed for 5 hours. The filtered out composite material

was washed with methanol, DI water and acetone. The precipitate was then filtered and dried

under 80oC.

Table 4. 1 Summary of weighted amount of aniline monomers, APS and DBSA dopant

respectively

Aniline APS :Aniline

(1.25:1)

DBSA:Aniline

(4:1)

Molarity

(100ml)

Weight

(g)

Molarity

(100ml)

Weight

(g)

Molarity

(100ml)

Weight

(g)

0.1M

0.93

0.125M

2.85

0.4M

13.06

Table 4. 2 Summary of weighted amount of aniline monomers, APS and HCl dopant respectively

Aniline APS :Aniline

(1.25:1)

HCl (aq)

Molarity

(100ml)

Weight

(g)

Molarity

(100ml)

Weight

(g)

Molarity

(100ml)

0.1M

0.93

0.125M

1.15

1M

Table 4. 3 Polyaniline to MWCNT weight ratio in the composites

Sample names: Total

PANI/MWCNT

Composite weight:

PANI/CNT

composition

CNT weight

%

PANI/MWCNT

(HCl)

0.603g ~84:16 16.6%

PANI/MWCNT

(DBSA)

0.714g ~86:14 14%

- 55 -

4.2.2. Physical characterization

4.2.2.1. Fourier Transforms Infrared Spectrometry (FTIR)

From the FTIR spectrum, each peak shows various molecular activities as respond to

infrared light absorption. By studying the findings from the literatures, the following summarized

bands shows significant characteristic of the emeraldine salt (ES) structure of PANI. Moreover,

the interaction between PANI and MWCNT in doped with different dopants can be studied from

the altered band activities.

Figure 4.1 FTIR spectrum of transmittance against wave numbers for pristine polyaniline

0

0.2

0.4

0.6

0.8

1

1.2

200 700 1200 1700 %

Tra

nm

itta

nce

Wave number (cm-1)

798

1148

1296

1466

671

1559

- 56 -

Figure 4.2 FTIR spectrum of HCl doped and DBSA doped Ppy/MWCNT composites

0

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000 2500 3000 3500 4000 4500

PANI ES

HCl doped

PANI/MWCNT

MWCNT

DBSA doped

PANI/MWCNT

3398

2897

657

551

1559

1466 1296 1148 798

671

- 57 -

Table 4.4 Characteristic FTIR bands for polyaniline

FTIR bands (cm-1

) Assignments:

~1559

C=C stretching vibration of the quinoid

structure

~1466 C-C stretching vibration of the benzenoid

structure

~1296 C–N stretching mode of the aromatic amines

benzenoid units

~1148 plane bending vibration of C–H

~798

N-H wag of secondary amines (1 hydrogen &

2 aromatic group)

~671 out-of-plane bending of C–H

Figure 4.1 shows the FTIR spectrum of PANI ES. The following bands were summarized

which indicates the existence of the conductive emeraldine salt form. 798 cm-1

shows N-H wag

of secondary amines (1 hydrogen & 2 aromatic groups) [166]. The band at 1296 cm-1

is assigned

to the C–N stretching mode of the aromatic amines benzenoid units [163, 166, and 168].

Characteristic bands at 1559 cm−1

and 1466 cm−1

corresponding to the C=C and C-C stretching

vibration of the quinoid and benzenoid structure respectively [168]. It should be noted that the

quinoid vibration intensity is appeared to be less than that of the benzenoid vibration in pure

PANI ES [169]. The peak at 671 cm-1

is attributed to the out-of-plane bending of C–H [170]. The

band at ~1125 -1148 cm−1

is assigned to plane bending vibration of C–H [131], which was

formed during protonation of the PANI chain. This characteristic band represents the

delocalization of electrical charges [131,166].

On Figure 4.2 for both DBSA and HCl doped PANI/MWCNT composites, all of the

characteristic bands of PANI were observed from the spectra. For the quinoid and benzenoid

vibration band. Both of these bands were broadened in the composites, which might indicate the

increase amount of the two vibration activities [164]. Moreover, the difference between the

- 58 -

quinoid and benzenoid intensities was reduced. The minor increase in the quinoid stretch

intensity suggests that PANI/MWCNT interactions promote and/or stabilize the quinoid structure.

This suggests the π bonded surface of MWCNT interact strongly with the π conjugated structure

of polyaniline, especially at the quinoid ring [102].

Moreover, at the wide band of ~ 3354- 3398 cm−1

indicates the stretching of the N-H.

This peak intensity appears higher in the PANI/MWCNT than the pure PANI ES. This possibly

indicates the proton transfer from PANI to the MWCNT network which causes the hydrogen

atoms in the amino group being substituted. The sp2

carbons compete with PANI for the Cl- ion

and thus would disturb the N-H bond. And therefore lead to the increase in N–H stretch intensity

[98, 102]. Moreover, the shift and widening of the conductive band (1155cm-1

to 1115 cm-1

) in

both of the composites possibility suggests a change of doping level in the composite [171].

This two could be evident showing a formation of charge transfer complex between the PANI

and the MWCNT which causes the doping effect from MWCNT to PANI. [102].

On the DBSA doped PANI/MWCNT composite spectra, the following additional bands

were observed. The band at 2897 cm-1

indicates S=O and C–H stretching of the benzenoid ring in

DBSA [172]. Strong bands of 551 cm-1

and 657 cm-1

represent C-S stretching vibration and

characteristic vibration of DBSA [172]. These bands indicated the presence of DBSA in the

PANI chain.

- 59 -


Figure 4.3 HR-SEM micrographs of a) MWCNTs b) HCl doped, b) DBSA doped PANI and c)

HCl doped, e) DBSA doped PANI /MWCNT composite

Figure 4.3 presents the HR-SEM micrograph of the bare MWCNTs, HCl doped and

DBSA doped PANI and PANI/MWCNT. From Figure 4.3A, the well interconnected MWCNTs

A

B C

D E

- 60 -

strands plays an important row in providing the mesoporosity for an easy access of ions for

double layer charge storage. For pure PANI powders, HCl doped PANI powder (Figure 4.3B)

displays more surface features than the DBSA doped PANI. For the DBSA doped powder as

displayed on Figure 4.3C, it shows a chunkier bulk structure with a smoother surface.

For the PANI/MWCNT composites, HCl doped PANI/MWCNT composites displayed

formations of fibrous structure with high entanglement. In contrast to pure MWCNT, the

increase in surface roughness of the fibrous shaped structure indicates the formation of PANI

over the MWCNTs network. However, from the high PANI to MWCNT weight ratio (84:16). It

suggests that the composite is mainly occupied by the PANI polymer. Therefore an explanation

to the formation of fibrous structure is that MWCNTs provide a framework for the

polymerization of PANI into fibrous structure. The well interconnection of the composite fibers

is very important in providing the porosity necessary for high surface charge storage. In contrast,

for the case of DBSA doped composite, the PANI was failed to polymerize into fibrous structure.

The DBSA doped composite consists of aggregated and agglomerated chunk of PANI particles.

While along the PANI agglomerates, there are MWCNTs strands connected in between. The

PANI: MWCNT weight ratio is (~86:14). The formation of this flake like aggregates is due to

the hydrogen bonds between the free DBSA molecules surrounding the polymerized PANI chain

at the aniline-DBSA complex [173]. It shows that CNT act as PANI interparticle bridges in this

composite. The morphology shows a relatively non-homogeneous dispersion of doped PANI

formation among the MWCNT. The interconnected MWCNT network is less visible. The

surface morphology of the differently doped PANI/MWCNT was studied to correlate the

materials electrical conductivity and electrochemical performance.

- 61 -

4.2.3. Half cell electrochemical characterization

4.2.3.1. Cyclic Voltammetry

Figure 4.4 CV comparison of pure MWCNT,

PANI, DBSA doped and HCl doped

PANI/MWCNT composite electrodes at 5mV

Figure 4.5 Averaged specific capacitance

F/cm2

for DBSA and HCl doped

PANI/MWCNT

+2e- , -2SO42- -2e- , +2SO4

2-

+2e- , -2SO42- -2e- , +2SO4

2-

LB

ES

PNS

Figure 4.6 Cyclic Exchange of oxidation states of PANI at different potential [99]

- 62 -

Figure 4.4 presents the CV voltammograms for DBSA doped and HCl doped

PANI/MWCNT composite electrodes in a half cell three electrodes set up. Both the HCl doped

and DBSA doped composite displayed improved charge storage properties compared to bare

MWCNT and PANI. For both PANI/MWCNT composites, they both exhibits much higher

capacitive performance with higher REDOX current. This shows an improved conductivity from

the composite. For the HCl doped PANI/MWCNT composite, the CV plot displays a relatively

more rectangular in shape which is a sign of low resistivity of the system which is associated

with the fast ionic transfer as well as a highly ions accessible surface [7, 98, 99, 152]. Moreover,

the existence of the distinct pairs of REDOX peaks also implies the functionality of PANI which

takes part in the pseudo capacitive charge storage [7, 98, 99]. These peaks have been assigned to

the transition from Emeraldine state to Leucoemeraldine state (A), 2nd

peak represents transition

from Emeraldine to Pernigraniline transition for the 2nd

oxidation step (B) [99]. These two pairs

of peaks appear at around at (AA’) 0.28V, 0.09V and (BB’) 0.55V, 0.45V vs. Ag/AgCl

respectively [99]. In contrast, for the DBSA doped composite, only the 1st oxidation peak is

observed on the CV plot. The 2nd

peak which is associated with Emeraldine to Pernigraniline

transition is less pronounced in this composite. This is possibly due to the cluster morphology of

the doped PANI which limited the 2nd

oxidative transition [138]. Moreover, the less

rectangularity of the curve and the increase of slope at the discharge cycle also indicate a higher

resistivity at the electrode /electrolyte interface [98, 99 and 152]. As observed from the SEM

image, this is due to the aggregation of the PANI-DBSA complex and non-homogeneous

dispersion of PANI. This would constricts the porosity provide by the porous network. And

therefore limits the surface area for charge storage at the double layer. Also, the observed higher

achieved charge current density of the REDOX peaks indicates higher pseudo capacitive charge

- 63 -

storage dependant kinetics for this composite electrode. The calculated specific capacitance is

shown on Figure 4.5. On average HCl doped PANI/MWCNT composite electrode achieves a

relatively higher capacitive performance.

4.2.3.2. Electrochemical impedance spectrometry

Figure 4.7 Nyquist plot of –Z” vs Z’ of DBSA and HCl doped PANI/MWCNT composite

electrodes. B) The inset enlarged the high frequency portion (100 kHz to 10mHz)

Table 4.5 Parameters of the equivalent circuit model for each electrode (1 cm2) derived from

numerical fitting of experimental data acquired from Nyquist plots

Electrodes: ESR CPE1-T CPE1-P Rct CPE2-T CPE2-P

DBSA doped 3.47 0.00024 0.79 2.8 0.2 0.69

HCl doped 3.79 0.00056 0.82 2.3 0.27 0.78

EIS was conducted to evaluate the capacitive and resistive response at varying frequency

range of the DBSA and HCl doped PANI/MWCNT composites in the half cell set up. The

nyquist plot is presented on Figure 4.7. At the high frequency region, HCl doped composite

shows a slightly smaller diameter of the extended semicircle then the DBSA doped composite.

This indicates a relatively minor charge transfer hindrance at the electrode /electrolyte interface

for the HCl doped composite electrode than the DBSA doped composite. This was also indicated

A B

- 64 -

by the lower achieved Rct value from HCl doped PANI/MWCNT as listed on Table 4.5. At the

low frequency region, the capacitive performance was evaluated. The low frequency straight line

of DBSA is less parallel to the imaginary axis. The inclination indicates possible non-

homogeneity of the electrode surface. Moreover, the both higher CPE2-P and CPE2-T values of

HCl doped PANI/MWCNT also indicates a higher capacitance than the DBSA doped composite.

In this part of the study, the electrochemical performance including capacitive and

resistance behaviour of PANI/MWCNT composites doped with DBSA and HCl were evaluated.

The synergistic effect on the composite electrochemical performance was owing to the

improvement in electroactivity due to interaction between PANI and MWCNT in the composites.

The CO-OH sites on the functionalized MWCNTs are likely to interact with the aniline monomer

[98,169]. As a results of forming a charge transfer (e- donor/ accepter) complex between

MWCNT and the PANI [98,138,128,129]. Since MWCNT is a well electron accepter and

therefore tends to accept electron donated from the PANI. This transfer process promotes the

protonation of PANI from the doping effect of MWCNT [62, 98]. Moreover, the promoted

delocalization of electron might be caused by π - π stacking between the PANI conjugated π

system and the large π bonds surface from the MWCNT. Therefore results in an improved

conductivity [62, 98, and 129]. These phenomenons were proved from the increasing quinoid

vibration and N-H stretch intensities as indicated from FTIR study.

The major reason for the relatively higher capacitive performance achieved from HCl

doped is owing to the uniform and homogeneous deposition PANI morphology and the

entangled network formation. This provides a better electrochemical accessible electrode surface

and a better interfacial interaction between the two materials. In addition, the higher ions

accessibility of HCl doped PANI in the composite is due to the presence of porous structure, it

- 65 -

could also be seen from the existence of the 2nd

oxidation peaks. Moreover, the type of counter

ions would also influent the mobility during ions doping and dedoping [8, 9, 31,174]. Presented

results indicate HCl doped PANI/MWCNT composites results in a higher conductive PANI

chain formation in the composite.

4.3. Polyaniline /MWCNT composite parametric study

Previously, it was found that HCl doped PANI/MWCNT composite yields better charge

storage performance than the DBSA doped composite. Therefore, HCl was chosen as dopant for

the parametric study. In this study, the aniline monomer concentration during fabrication was

varied from 0.1M to 1M to control the polymerization yield of PANI in the composite. The

influence of PANI content to the material morphology and electrochemical properties was

investigated.

4.3.1. Composites Fabrication Process

100mg of MWCNT powder was first dissolved into 100ml of 1MHCl solution with 1.4g of

SDS surfactant with 20% ethanol [105]. The solution was then subjected to ultrasonication to

improve the MWCNT dispersion in the solution. Weighted aniline monomer was first dissolved

into 1M HCl to form Aniline hydrochloride [117,105,166, 171]. The solution was then added to

the MWCNT solution and being stirred for 1 hour. APS in 100ml 1M HCl was first pre-cooled for

one hour. The ratio of oxidizing species APS to aniline 1.25:1 [166]. APS solution was then

added drop wise into the stirring solution to initiate the polymerization process. The

polymerization was then allowed for 5 hours. The composite material was then filtered out and

washed with methanol to coagulate the dispersion [105]. The precipitate was then washed with

- 66 -

HCl and acetone to keep the precipitate in fine powder [171]. The precipitate is then filtered and

dried under 80oC. The weighted components required for different composition was presented on

Table 4.6. The weighted proportion between MWCNT and PANI of each composition as well as

the MWCNT weight % are listed on table 4.7.

Table 4.6 Weight of aniline monomers and Ammonium persulfate in different compositions

Assigned sample

names:

Aniline M

concentration

(100ml)

Aniline

monomers

weight

APS M

concentration

(100ml)

APS

weight

0.05M PANI/MWCNT 0.05M 0.46g 0.0375M 0.85g

0.1M PANI/MWCNT 0.1M 0.93g 0.075 M 1.71g

0.3M PANI/MWCNT 0.3M 2.79g 0.225 M 5.13g

0.5M PANI/MWCNT 0.5M 4.65g 0.375 M 8.56g

1M PANI/MWCNT 1M 9.31g 0.75 M 17.11g

Table 4.7 Polyaniline to MWCNT weight ratio at different compositions

Sample names: PANI/MWCNT

Composite weight:

PANI/CNT

composition

CNT weight %

0.05M PANI/MWCNT 0.31g 68.75:31.25 31.25%

0.1M PANI/MWCNT 0.62g 84:16 16.6%

0.3M PANI/MWCNT 1.56g 93.51:6.49 6.49%

0.5M PANI/MWCNT 2.18g 95.43:4.5 4.58%

1M PANI/MWCNT 4.36g 97.7:2.29 2.29%

- 67 -

4.3.2. Physical Characterization


Figure 4.8 SEM images of A) MWCNT, B) Pure PANI ES, C) 0.05M PANI/MWCNT , D) 0.1M

PANI/MWCNT, E) 0.3M PANI/MWCNT, F) 0.5M PANI/MWCNT, G) 1M PANI/MWCNT

Table 4.8 Measured mean tube diameter of the PANI/MWCNT composite fibers

Samples: PANI:CNT

composition

Average tube

thickness (nm)

PANI layer thickness (nm)

MWCNT ~8

0.05M PANI/MWCNT 68.75:31.25 ~20.4 5.7

0.1M PANI/MWCNT 84:16 ~23.4 6.6

0.3M PANI/MWCNT 93.51:6.49 ~38.2 14.6

0.5M PANI/MWCNT 95.43:4.5 ~49.5 20.25

1M PANI/MWCNT 97.7:2.29 ~64.4 27.7

A B C

G

D E F

- 68 -

Figure 4.8 displayed selected SEM images of the MWCNT, PANI (ES) and various

compositions of PANI/MWCNT composites. MWCNT SEM image shows a well entangled

network as presented previously. The effective utilization of MWCNTs in the composite is

greatly dependent on the dispersion homogeneity in the polymers [140].

Polyaniline powders with flake like non-homogeneous morphology was observed. At

0.05M (68% PANI), 0.1M (85% PANI) (figure 4.8. C and D), the image reviews formation of

interconnected tubular structured composite which allows formation of high porosity. Since the

composite is mainly occupied by PANI, therefore the explanation of this tubular structure

formation is that MWCNT act as template which allows the PANI to be polymerized into fibrous

shape. As the PANI concentration increases from 0.3M (PANI 93%) to 1M (PANI 97%),

increase in surface roughness and aggregation of PANI agglomerates indicates that the

polymerization becomes out of controllable at very low CNT contents (< 7%). The PANI

polymer continues to grow without following any ordered pattern. The result shows that

MWCNT (>15%) in the composite allows an ordered formation of PANI in tubular form with

uniform surface morphology. The measured average tube thickness of the composites was shown

on table 4.8. An increase of tube thickness with increasing PANI content was observed.

- 69 -

0

0.5

1

1.5 4.3.2.2. Fourier transforms infrared spectrometry (FTIR)

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000 2500 3000 3500 4000 4500

0

0.5

1 0

0.5

1

1.5 0

0.5

1 -1

0

1

3398 1466 671 798 1148 1296

1559

Pure PANI

0.05M PANI/MWCNT

0.1M PANI/MWCNT

0.3M PANI/MWCNT

0.5M PANI/MWCNT

1M PANI/MWCNT

- 70 -

Figure 4.9 FTIR spectra comparing Ppy /MWCNT composites to pristine Polypyrrole. The

highlighted regions shows the characteristic bands of Ppy displayed on the composites spectra.

The inset compares the Quinoid /Benzenoid vibration intensity. A reduced difference between the

vibration bands intensity in the composites was observed.

The PANI/MWCNT composites FTIR spectra were presented figure 4.9, all of the

characteristic bands of PANI were observed from the spectra. The Quinoid (1559 cm-1

) and

Benzenoid (1466 cm-1

) vibration bands was again analysed. Both of these bands become board in

the composites, again indicates the possible increase of the two vibration activities [48].

Moreover, as compared to pristine PANI, the difference between the Quinoid and Benzenoid

intensity was reduced. A minor increase in the quinoid stretch intensity suggests that

MWCNT/PANI interactions tend to promote and/or stabilize the quinoid structure. This suggests

that the π bonded surface of MWCNT interact strongly with the π-conjugated structure of

polyaniline, especially at the quinoid ring [102]. Moreover, at the wide band of ~ 3354- 3398

cm−1

, this shows the stretching of the N-H bond. This peak is more pronounced in the

PANI/MWCNT than pure PANI ES. This possibly indicates the proton transfer from PANI to

the MWCNT network which causes the hydrogen atoms in the amino group being substituted [98,

131]. The interaction causes the Sp2

carbons compete with the ions and thus would disturb the

N-H bond. And therefore lead to the increase in N–H stretch intensity [98, 131]. In addition, the

- 71 -

shift and widen of the conductive band (1155cm-1

- 1115 cm-1

) in the composites possibility

suggests a change (increase) of doping level in the composite [171]. This two could be evident

showing a formation of charge transfer complex between the PANI and the MWCNT which

causes the doping effect from MWCNT to PANI [98].

4.3.3. Half cell electrochemical characterization


Figure 4.10 Cyclic Voltammetry at 5mV/s for Pure PANI, MWCNT and PANI/MWCNT

composites

B’

B

A’

A

- 72 -

The CV voltammogram of MWCNT, PANI ES and the PANI/MWCNT composites at

scan-rate 5mV/s were shown on figure 4.10. MWCNT electrode shows a pair of carbon REDOX

peaks with highly rectangular shape due to the charge storage at the double layer. PANI

electrodes CV show a pairs of distinct REDOX peaks which indicate the doping/dedoping

process. Once again these peaks represents transition from Emeraldine state to Leucoemeraldine

state (A), 2nd

peak represents transition from Emeraldine to Pernigraniline transition for the 2nd

oxidation step (B) [7, 98, 99,152]. These pairs of peaks appear at around at (AA’) 0.28V, 0.09V

and (BB’) 0.55V, 0.45V vs. Ag/AgCl respectively [99]. For the PANI/MWCNT composite

electrodes, the CV figures demonstrated a sharper and more pronounced peaks with higher

charge currents indicates an improved material conductivity which lead to a faster charge

respond from applied voltage [41, 99]. Initial increase of capacitance was observed as PANI

content increases from 68% (0.05M) to 85% (0.1M). As the PANI content further increases from

to 93% (0.3M) and onwards to 97% (1M), a drop in specific capacitance was observed. The

calculated specific capacitance per cm2 of electrode surface is shown in the bar chart on figure

4.11. Among the composites, 0.1M PANI/MWCNT yielded the best capacity from the largest

area coverage of the CV curve. Maximum specific capacitance is achieved at 0.353 F/cm2, which

shows a significant improvement in contrast to 0.157 F/cm2

and 0.122 F/cm2

for pure MWCNT

and pure PANI (ES) electrode respectively. Capacitance value is then dropped to 0.181F/cm2 at

1M PANI concentration.

An explanation is that at higher PANI concentration leads to a thicker deposited polymer

layer on the MWCNTs network as reviewed from SEM micrographs. This highly compacted

PANI layer minimizes the effective utilization of MWCNT porous network. Since the thicker the

PANI layer requires more time for the ions to diffuse through. Therefore this would inhibit the

- 73 -

high rate charge storage performance [175]. The lower achieved charge current in respond to

voltage increase suggest a poor conductivity in composite with a higher PANI content. This is

attributed to the non-homogeneity of the deposited PANI layer at high PANI concentration [128].

In addition, the decrease in capacitive performance at high PANI content in the composite could

also be due to a less effective interaction with the MWCNTs due to the higher compactness of

the deposited PANI [14]. Therefore, the capacitive performance becomes more contributed to the

PANI in the composite. Result suggests that lower PANI content in the composite is able to

optimize the composite capacitive performance.

Figure 4.11Calculated specific capacitance (F/cm2) for MWCNT, pure PANI ES and

PANI/MWCNT composites

- 74 -

4.3.3.2 Electrochemical Impedance spectrometry (EIS)

Figure 4.12 a) Nyquist plot of –Z” vs Z’ of, PANI, MWCNT and PANI /MWCNT composite

electrodes. b) Shows the enlarged high frequency portion (100 kHz to 10 MHz).

Table 4.9 Parameters of the equivalent circuit model for each electrode (1 cm2) derived from

numerical fitting of experimental data acquired from nyquist plots

Electrodes ESR CPE1-T CPE1-P Rct CPE2-T CPE2-P

MWCNT 0.54 0.00032 0.99 0.041 0.0034 0.78

PANI 2.21 0.00014 0.86 1.211 0.091 0.61

0.05M PANI/MWCNT 3.10 6.213E-5 0.91 0.851 0.25 0.73

0.1M PANI/MWCNT 2.22 7.223E-5 0.93 0.852 0.25 0.731

0.3M PANI/MWCNT 2.35 0.00018 0.89 1.311 0.22 0.71

0.5M PANI/MWCNT 2.08 0.00012 0.89 1.421 0.18 0.66

1M PANI/MWCNT 3.51 0.0017 0.47 2.211 0.15 0.57

The EIS analysis was performed in frequency range from 100 kHz to 10 mHz. The

operating voltage was controlled at 5mV. The AC impedance spectra are shown on nyquist plot

of Z’ VS –Z” on figure 4.12. The impedance spectra of the composites all consist of a distorted

semicircle in the high frequency region and an inclined straight line in the low frequency end.

- 75 -

The parameters used for fitting to the generalized equivalent model were summarized on table

4.9. From the result, the ESR of all the systems is in a range from 2 to 2.3Ω. While the resistance

for 1M PANI/MWCNT has the highest resistance up to 3.51Ω. For Rct, MWCNT electrode

achieves the lowest Rct (0.041Ω) as expected due to the fast transfer kinetic of highly ions

accessible porous surface. In contrary, pure PANI shows higher Rct due to the slower

doping/dedoping charge transfer kinetic. In PANI/MWCNT composites, a decrease in Rct was

observed. Rct was dropped to 0.851 and 0.852 for 0.05M and 0.1M PANI/MWCNT respectively.

This indicates an improved ionic transfer rate of the PANI/MWCNT composite. As PANI

concentration increases, the charge transfer resistance was increased to 2.211 Ω at 1M PANI

concentration. In addition, the high CPE2-P value (~0.731) of 0.1M PANI/MWCNT among the

composites also indicates the relatively higher capacitive behaviour, which agrees with findings

from CV. Decrease in CPE2-T value as PANI content increases also indicate increasing diffusion

hindrance which leads to built up resistivity and therefore minimized the capacitive performance.

The findings from EIS and CV indicate a high dependency of the resistance in counter

ions transfer at the electrode/electrolyte interface on the deposited polymer thickness in

PANI/MWCNT composite. Increase deposition of PANI leads to build up of a thick and compact

polymer layer as shown from the SEM micro graph, which than reduces the rate of charge

transfer by constricting the pores of MWCNT networks. This shows that by controlling a lower

PANI content in the composites, the ionic transfer resistance of the composite can be minimized.

When comparing 0.05M PANI/MWCNT to 0.1M PANI/MWCNT, the higher capacitance

achieved indicates an optimal weight ratio between PANI and MWCNT exist and therefore

results as an improved charge transfer complex and a more effective utilization of PANI in

pseudo capacitive charge storage. In this study, the ratio was found to be PANI: MWCNT =

- 76 -

85:15. Furthermore, it is believed that this thinner polymer layer can reduces the ions diffusion

distance for effective charge transfer at the interface. Therefore, an optimal deposited polymer

thickness is essential in minimizing the charge transfer resistance at the same time optimizing the

charge storage activity.

- 77 -

Chapter 5 PEDOT: PSS/MWCNT composite

study

5.1. Introduction

The electrochemical properties between PEDOT/MWCNT and PEDOT: PSS/ MWCNT

were first studied. The better performing composite was then chosen to conduct a parametric

study by controlling the PEDOT polymer content and therefore study the influence of the

composites electrochemical properties and charge storage capability. It was found that PEDOT:

PSS /MWCNT results in a much better capacitive performance compared to PEDOT/MWCNT

(AN) under the same controlled fabrication parameters. In the following part of the study, a

parametric study was conducted to study the influence of PEDOT: PSS to MWCNT content in

the composite morphology and investigate the effect to the electrochemical performance.

5.2. PEDOT:PSSMWCNT / PEDOT/MWCNT study

5.2.1. Composite Materials Fabrication

5.2.1.1. PEDOT:PSS/MWCNT composite fabrication

3, 4 -Ethylenedioxythiophene (EDOT, 97%, Sigma Aldrich) was used without further

purification. The PEDOT: PSS/MWCNT composite fabrication procedure was referenced from

the work of Farah et.al [62]. MWCNT (100mg) was first dissolved into in 100ml of 1M HCl

solution with weighted PSS. Ultrasonication was applied to de-bundle the CNT in order to yield

a good dispersion. 0.05M of EDOT (0.7g) was then added into the MWCNT/PSS mixture and

was stirred for 1 hour. EDOT and PSS molar ratio was maintained at 2:1 [177]. The mixture was

then stirred and cooled to about 0~4oC in melting ice bath. Ammonium persulphate ((NH4)2S2O8)

- 78 -

and Iron Chloride (FeCl3.6H2O) (1:1) were applied as oxidant (Oxidant: EDOT = 0.8:1) [62].

The oxidant was then added drop wise to the mixture to initiate the polymerization process.

Polymerization was then allowed for 12 hours. The precipitate was then filtered and washed with

deionised (DI) water, methanol, acetone and diethyl ether was used to eliminate lower molecular

weight polymers [62]. The collected precipitate was then dried overnight at 100oC and then

grinded into powder form.

5.2.1.2. PEDOT/MWCNT composite fabrication

Acetonitrie aprotic solvent was used as PEDOT polymerization medium. MWCNT

(100mg) was first dissolved into 100ml of solution with weighted PSS [106]. Ultrasonication

was again applied to disperse the MWCNTs. EDOT (0.7g) was then added to the MWCNT

solution and stirred for 1 hour. The mixture was then stirred and cooled to about 4oC in ice bath.

Ferric Chloride (FeCl3.6H2O) was applied as oxidant and dopant to the system (FeCl3:

EDOT=0.8:1). The oxidant was then added drop wise to the mixture to initiate polymerization

process. Polymerization was allowed for 12 hours. The precipitate was again filtered and washed

with DI water, methanol, acetone and diethyl ether to eliminate low molecular weight polymers

[62]. The precipitate was then dried overnight at 100oC and then grinded into powder form. The

weighted components were listed on table 5.1 and 5.2.

- 79 -

Table 5.1 Summary of weighted amount of EDOT monomers, APS, FeCl3 dopant and PSS

respectively for PEDOT: PSS/MWCNT fabrication

EDOT APS (APS :EDOT

=0.8:1)

FeCl3(FeCl3: EDOT

=0.8:1)

PSS (PSS : EDOT=2:1)

Molarity

(100ml)

Molarity

(100ml)

Molarity

(100ml)

Weight

(g)

Molarity

(100ml)

Weight

(g)

Molarity

(100ml)

Weight (g)

0.05M 0.025 0.04 0.324 0.04M 0.324 0.025 0.5

Table 5. 2 Summary of weighted amount of EDOT monomers, FeCl3 dopant respectively for

PEDOT/MWCNT fabrication

EDOT FeCl3 (FeCl3: EDOT

=0.8:1)

Molarity

(100ml)

Weight

(g)

Molarity

(100ml)

Weight (g)

0.05M

0.7

0.04M

0.324

Table 5. 3 PEDOT: PSS to MWCNT weight ratio at different compositions

Sample names: Total PEDOT:PSS

/MWCNT

Composite weight:

PANI/CNT

composition

MWCNT

weight

CNT weight %

PEDOT:PSS/MWCNT 0.728g 86.3:13.7 100mg 13.7%

PEDOT:MWCNT 0.483g 79.4:20.6 100mg 20.6%

- 80 -

5.2.2. PEDOT/MWCNT , PEDOT: PSS/MWCNT composites physical

characterization


Figure 5.1 SEM images of A.1) PEDOT:PSS and A.2)PEDOT(AN), B) PEDOT: PSS /MWCNT C)

PEDOT/MWCNT composite with PEDOT controlled at 0.05M concentration

A.1 B.2

B.1 B.2

C.1 C.2

- 81 -

Figure 5.1. displayed the morphology of pure PEDOT, and both PEDOT: PSS /MWCNT

and PEDOT/MWCNT composites respectively. Pure PEDOT (Figure 5.1.A.2) and PEDOT: PSS

(Figure 5.1.A.1) both displayed morphology a flake like cluster. For the PEDOT: PSS/MWCNT

composite (Figure 5.1.B.1&2), the composite shows relatively less agglomeration of PEDOT:

PSS particles within the composite. It is possibly due to the negatively charged PSS can non-

covalently be bonded to the MWCNTs surface [178]. This creates active sites over the MWCNT

surfaces which allow a uniform polymerization of nano-structured PEDOT onto MWCNT

surface. Minor aggregation of MWCNT bundles was observed.

While for the PEDOT/MWCNT composite without the presence of PSS, SEM picture

displayed a more flake like structure of PEDOT among the MWCNT networks. This shows that

the MWCNTs tend to be embedded within the PEDOT polymer matrix. In contrast, the porosity

of the MWCNT network in the composite is more constricted due to the less uniform deposition

of PEDOT over the MWCNTs network. The morphology comparison shows that PEDOT:

PSS/MWCNT composite particles show a relatively more uniform deposition with higher pore

size distribution

- 82 -

5.2.2.2. Fourier Transform Infrared Spectroscopy (FTIR)

Figure 5.2 FTIR spectrum of transmittance against wave numbers for PEDOT: PSS

Table 5.4 Main characteristic FTIR bands for PEDOT:PSS

FTIR bands (cm-1

) Assignments:

~1289

Asymmetric and symmetric vibrations of SO3

groups in PSS chains

~1485 -C-C- stretching quinoidal structure

delocalization of electrical charges caused by

deprotonation

~1586 -C=C- stretching quinoidal structure

~670, 890 C-S-C bond vibration in the thiophene ring

~1173,1135

-C-O-C- vibration

0

0.2

0.4

0.6

0.8

1

1.2

400 600 800 1000 1200 1400 1600 1800 2000

% T

ran

mit

tan

ce

1586

1485 1289

1173 890

670

1135

- 83 -

Figure 5.3 FTIR spectra of transmittance against wave numbers for PEDOT/MWCNT (AN),

PEDOT: PSS/MWCNT, PEDOT: PSS and MWCNT

Figure 5.3. presents the FTIR spectra of Pure PEDOT: PSS. The characteristic bands of

PEDOT were summarized as follows. The band at 1289 cm−1

is attributed to the asymmetric

vibrations of SO3 groups [179]. The vibration at 1485 cm−1

and 1586 cm−1

are caused by the -C-

C- and -C=C- stretching of the quinoidal structure and the stretching of thiophene ring in

PEDOT [62, 179]. The band at about 890 cm−1

is caused by the -C-S-C- bond vibration in the

thiophene ring [179]. The band at 1135 cm−1

, 1173 cm−1

are attributed to -C-O-C- vibration in

PEDOT [62, 180]. For PEDOT: PSS, extra band presented at 1289 cm−1

was attributed to the

asymmetric vibrations of SO3 groups which indicate the presence of PSS [179]. The details were

summarized in Table 5.4.

1586

1485

1289

1173 890

670

1135

1286

3013 2880

PEDOT/MWCNT (AN)

PEDOT:PSS/MWCNT

(AN)

PEDOT:PSS

MWCNT

- 84 -

Figure 5.3.displays the FTIR spectra for PEDOT/MWCNT (AN), PEDOT: PSS/MWCNT

and pure MWCNT. For PEDOT: PSS/MWCNT composite, there is new band introduced at

around 3013 cm-1

. This band is linked to the –OH stretching vibrations of the carboxylic group

from the acid treated CNTs [179]. The peak at about 2880 cm-1

indicates C-H stretching which

shows the presence of sp3

defects on the acid treated MWCNTs surface [179]. These two bands

were observed on the MWCNT spectrum. Moreover, shift of the band at 1289 cm-1

which is

contributed to -SO3- vibration in PSS chain was observed. The band was shifted to 1286 cm-1

. It

is believed that the π-π interactions between MWCNT and PEDOT: PSS would disrupt the SO3-

H bond in the PSS. This would reduce the interaction between PEDOT: PSS, but in turn, causes

formation of interconnected channel between the conductive phases with the MWCNTs [30].

These results indicate the interaction between the PEDOT: PSS and MWCNT between the π

conjugates in the composites. However another peak corresponds to -C-O-C- vibration which is

at 1135 cm-1

was not available as well. This is possibly due to stabilization of the bond due to the

interaction with MWCNT.

However, for PEDOT/MWCNT (AN) composite, the characteristic bands which

indicates PEDOT formation was not pronounced. The low intensity possibility indicates a

weaker formation of PEDOT being interact with MWCNT without the presence of PSS.

Moreover, the strong peak at 1289 cm-1

as attributed to -SO3- vibration in PSS chain is not

observed due to the absence of PSS in the composite.

- 85 -

5.2.3. Half cell electrochemical evaluation


Figure 5.4 CV voltammograms of pure PEDOT: PSS and PEDOT (AN) electrodes at 10mV/s

and 100mV/s respectively

Figure 5.5 CV voltammograms of PEDOT: PSS/MWCNT and PEDOT/MWCNT (AN) and

MWCNT electrodes at 10mV/s and 100mV/s respectively (1M H2SO4)

- 86 -

Figure 5.6 Averaged specific capacitance F/cm2 for PEDOT: PSS/MWCNT and

PEDOT/MWCNT @10mV/s

Cyclic Voltammetry was first conducted to study the charge storage capability of PEDOT:

PSS /MWCNT and PEDOT/MWCNT (AN) electrodes in a three electrodes set up at 10mV/s and

100mV/s. Figure 5.4. hows that a close capacitive performance between both PEDOT: PSS and

PEDOT (AN) electrodes from the high symmetry of the two plots.

From figure 5.5, for the composites, both PEDOT: PSS /MWCNT and PEDOT/MWCNT

(AN) composites displayed an improved charge storage performance compared to bare MWCNT

electrodes and PEDOT at both scan rates. At 10mV/s, a higher rectangular shape indicates the

contribution of charge storage at the double layer. PEDOT: PSS/MWCNT exhibits a much better

performance as compared to PEDOT/MWCNT (AN) from the larger area coverage of the CV

curve. The specific capacitances for both composites were summarized on figure 5.6. PEDOT:

PSS/MWCNT achieves highest capacitance of 0.074F/cm-1

. While that achieved by

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

PEDOT (AN) PEDOT:PSS MWCNT PEDOT/MWCNT (AN)

PEDOT:PSS/MWCNT

Charge

Discharge Sp

ecif

ic c

apac

itan

ce F

/cm

2

- 87 -

PEDOT/MWCNT was only 0.038F/cm2. Compared to that achieved by MWCNT which is

0.033F/cm-1

is just very minimal observable improvement.

5.2.3.2. Electrochemical Impedance Spectrometry

Figure 5.7 a) Nyquist plot of –Z” vs Z’ of, PEDOT: PSS/MWCNT and PEDOT/MWCNT (AN)

composite electrodes. b) Shows the enlarged high frequency portion (100 kHz to 10 MHz)

Table 5.5 Parameters of the equivalent circuit model for each cell (1 cm2) derived from



PEDOT:PSS/MWCNT 1.69 0.00051 0.84 0.085 0.051 0.57

PEDOT/MWCNT (AN) 1.87 3.8E-6 1.31 1.14 0.12 0.47

Figure 5.7 presented the nyquist plot of PEDOT: PSS/MWCNT and PEDOT/MWCNT

(AN) composite electrodes respectively. Figure 5.7 b) shows the enlarged high frequency portion.

At high frequency region, PEDOT: PSS/MWCNT shows a smaller diameter of the semi circle

portion than the PEDOT/MWCNT (AN). This again shows a better charge transfer kinetic with a

lower hindrance at the interface for the PEDOT:PSS /MWCNT. When we look at the low

frequency region, the low frequency end of PEDOT: PSS/MWCNT is more parallel to the

- 88 -

imaginary axis as compared to PEDOT/MWCT (AN). This indicates a higher capacitive

behaviour and a relatively minor diffusion problem due to Warburg impedance at low frequency.

The parameters generated from data fitting to the equivalent circuit model were shown on

table.5.5. Again, the better performance of PEDOT: PSS/MWCNT is indicated by both the lower

ESR and higher CPE2-T and CPE2-P values.

To conclude, PEDOT/MWCNT (AN) displayed a less promising performance as

compared to PEDOT: PSS/MWCT. This is caused by the relatively poor contact between the

PEDOT and MWCNT without PSS as indicated from the inhomogeneous polymerization of

PEDOT over MWCNTs and the weaker intensity from FTIR spectrum. Moreover, this can also

be seen from the low profile of the displayed PEDOT characteristic bands from this composite.

This led to weakened interactions between the PEDOT and MWCNT in the composite. And

therefore leads to a minimized effect from the MWCNTs in improving the in general composite

electroactivity. Moreover, PEDOT: PSS system allows an improved electronic and ionic

conductivity as stated previously. This is shown from the higher achieved charge current by the

PEDOT: PSS/MWCNT composite electrode compared to PEDOT/MWCNT (AN) as indicated

on the CV plots. In sum, in comparing the two composites, PEDOT: PSS/MWCNT is a better

candidate as a supercapacitor electrode.

- 89 -

5.3. PEDOT:PSS/MWCNT composite parametric study

5.3.1. Composite Fabrication

MWCNT (100mg) was first dissolved into in 1M HCl solution with weighted PSS.

Ultrasonicated was again applied to de-bundle the CNTs in order to yield a good dispersion.

Weighted EDOT in different molar concentrations (0.025M, 0.05M and 0.1M) was then added to

the MWCNT/PSS mixture and was stirred for 1 hour. EDOT and PSS molar ratio was

maintained at 2:1. The mixture was then stirred and cooled to about 3-4 oC in ice bath.

Ammonium persulphate ((NH4)2S2O8) (APS) and Iron Chloride ((FeCl3.6H2O)) (1:1) were

applied as oxidant (Oxidant to EDOT was kept at 0.8:1 molar ratio). The oxidant was then added

drop wise to the mixture to initiate polymerization process. Polymerization was allowed for 12

hours. The precipitate was then filtered and washed with DI water, methanol, acetone and diethyl

ether to eliminate low molecular weight polymers [35]. The collected precipitate was then dried

overnight at 100oC and then grinded into powder form. The summarized material contents were

presented on table. 5.6. And the respective material composition for each sample was

summarized on table 5.7.

Table 5. 6 Weight of EDOT monomers, ammonium persulfate, ferric chloride and PSS in

different compositions

Assigned sample names: EDOT

monomers

weight

APS

weight

FeCl3

weight

PSS

weight

0.025M PEDOT:PSS/MWCNT 0.35g 0.228g 0.16g 0.25g

0.05M PEDOT:PSS/MWCNT 0.7g 0.45g 0.32g 0.5g

0.1M PEDOT:PSS/MWCNT 1.4g 0.85g 0.62g 1g

- 90 -

Table 5. 7 PEDOT: PSS to MWCNT weight ratio at different compositions

Sample names: Total PEDOT:PSS

/MWCNT

Composite weight:

PANI/CNT

composition

MWCNT

weight

CNT

weight %

0.025M

PEDOT:PSS/MWCNT

0.34g 70.8:29.2 100mg 29.6%

0.05M

PEDOT:PSS/MWCNT

0.73g 86.3:13.7 100mg 13.7%

0.1M

PEDOT:PSS/MWCNT

1.29g 92.26:7.74 100mg 7.74%

5.3.2. Physical characterization


Figure 5.8 SEM images of A) Pure PEDOT: PSS, B) 0.025M PEDOT: PSS/MWCNT, C) 0.05M

PEDOT: PSS/MWCNT, D) 0.1M PEDOT: PSS/MWCNT

A B

C D

- 91 -

Figure 5.8 presents the SEM pictures of Pure MWCNTs, PEDOT: PSS and the PEDOT:

PSS/MWCNT composites. The effective utilization of MWCNTs in the composite is greatly

dependent on the dispersion homogeneity of the polymers [140]. Again a flake like cluster

morphology was observed for pure PEDOT: PSS. For PEDOT: PSS/MWCNT composites

powder from PEDOT: 0.025M, 0.05M, 0.1M, the SEM pictures were shown on figure 5.8 B) to

D). In contrast to that of PANI/MWCNT and Ppy/MWCNT, uniform deposition of PEDOT:

PSS onto individual MWCNT was rarely observed. Rather, PEDOT: PSS forms a matrix that

covers the MWCNT networks. At low PEDOT: PSS content 0.025M (PEDOT: PSS 70%) and

0.05M (PEDOT: PSS 85%), the visible porosity was due to the interconnected MWCNTs.

However, the MWCNTs were not completely covered by the polymers. The CNTs were rather

loosely connected with some of the aggregation within the polymer matrix. As the PEDOT

concentration reaches 0.1M (PEDOT: PSS 98%), the MWCNTs strands were enwrapped around

the PEDOT: PSS matrix scattered. The interconnections between CNTs are less visible. This is

possibly due to the high film forming property of the PEDOT: PSS which lead to non uniform

PEDOT: PSS formation in powered composition fabrication.

5.3.2.2. Fourier Transform infrared spectrometry (FTIR)

Figure 5.9 Chemical structure of PEDOT: PSS. The “dot” and “plus” represent the unpaired

electron and positive charge respectively [181]

- 92 -

Figure 5.10 FTIR spectra comparing PEDOT: PSS/MWCNT composites to pristine PEDOT:

PSS. The interaction between PEDOT: PSS and MWCNT in the composite were studied

0

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8

1

1.2

400 900 1400 1900 2400 2900

0

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8

1

1.2

% T

ran

mit

tan

ce

PEDOT

PSS

PEDOT: PSS (0.025M) /MWCNT



2988 cm-1 2878 cm-1

cm-1

3013 cm-1 2879 cm-1

1289 cm-1

1284 cm-1

1286 cm-1

1287 cm-1

3001 cm-1 2878 cm-1

- 93 -

Figure 5.10 presents the FTIR spectra for the PEDOT: PSS/MWCNT composites. The

characteristic bands of PEDOT: PSS all appears in the composite which confirms the formation

of PEDOT: PSS on MWCNTs. Compared to PEDOT: PSS, There are new bands introduced at

around 2980-3300 cm-1

(PEDOT 0.025M: 2988cm-1

, PEDOT 0.05M: 3013 cm-1

, PEDOT 0.1M:

3001 cm−1

). This band is due to the –OH stretching vibrations of the carboxylic group as stated

previously. The peak at about 2880 cm-1

indicates C-H2 stretching which shows the presence of

sp3

defects on the acid treated MWCNTs surface [182]. Moreover, certain wave shift was

observed in the composites. A shift of the band at 1289 cm-1

which is contributed to -SO3-

vibration in PSS chain. The band was shifted to 1284 cm-1

, 1286 cm-1

and 1287 cm-1

for 0.025M,

0.05M and 0.1M respectively. It is believed that the π-π interactions between MWCNT and

PEDOT: PSS would disrupt the SO-3H bond in the PSS. This would reduce the interaction

between PEDOT: PSS, but in turn causes an interconnection between the conductive phases with

the MWCNTs [183]. These results indicate the interaction between the PEDOT: PSS and

MWCNT between the π conjugates in the composites. The larger wave shift of the -SO3-

vibration with increasing MWCNT content (lower PEDOT: PSS content) is a possible indication

of the presence of more conductive phase in the composite with higher MWCNT content.

- 94 -

5.3.3. Half Cell Electrochemical Characterization


Figure 5.11 Cyclic Voltammetry at 10mV/s for MWCNT, Pure PEDOT: PSS and PEDOT:

PSS/MWCNT composite electrodes

Figure 5.12 Calculated specific capacitance (F/cm2) for MWCNT, pure PEDOT: PSS/MWCNT

and PEDOT: PSS/MWCNT composites (1M H2SO4, Scan rate: 10mV/s)

CV was performed at 10mV/s scan rate. From figure 5.11, PEDOT: PSS and the

composite electrodes all displayed a pair of REDOX peaks which indicates the REDOX process

of PEDOT: PSS. The anodic and cathodic potentials are 0.51V and 0.48V vs Ag/AgCl

respectively [106]. The rectangular shape of curve indicates an electrochemical accessible

electrode surface [62, 162]. The higher peak current achieved from the composites again

- 95 -

indicates an improved conductivity of PEDOT: PSS matrix with the presence of MWCNT [162].

Furthermore, this proves the doping effect from the interaction between MWCNT and the

PEDOT: PSS matrix as discovered from FTIR. The calculated specific capacitance F/cm2 was

summarized in the bar chart on figure 5.12. Maximum capacitance was achieved at 0.071 F/cm2

for PEDOT (0.05M): PSS /MWCNT. This is due to the preserved porosity due to the MWCNTs

network at this low PEDOT: PSS content.

It was observed that further increase in PEDOT: PSS to 0.1M content leads to

degradation of the charge storage performance of the composite. As reviewed from SEM, this is

possibly due to the loosely connected MWCNT within the non-uniformly polymerized PEDOT:

PSS matrix. Therefore inhibit the formation of a structure with defined porosity. The scattered

MWCNTs in the polymer matrix was unable to effectively form a conducting path which

improves the material conductivity. Capacitance was then dropped to 0.042 F/cm2. The

capacitive property is highly determined by PEDOT: PSS in the composite.

5.3.3.2. Electrochemical impedance spectrometry (EIS)

Figure 5.13 a) Nyquist plot of –Z” vs Z’ of, PEDOT: PSS, MWCNT and PEDOT: PSS /MWCNT

composite electrodes. b) Shows the enlarged high frequency portion (100 kHz to 10 MHz).

- 96 -

In EIS, the electrical conductivity at the electrode/electrolyte interface was evaluated

[181]. The analysis was performed from 100 kHz to 10 mHz. The operating voltage was

controlled at 5mV. The AC impedance spectra are shown on nyquist plot of Z’ VS –Z” on figure

5.13 b). All of the plots of the composites consist of a distorted semicircle in the high frequency

region and a sloped line (~45o) in the low frequency end results in Warburg resistance. It is

believed that the doping and de-doping of the PEDOT is rate determining process in the

composite [98].

Table 5.8Parameters of the equivalent circuit model for single electrodes (1 cm2) derived from



MWCNT 0.54 0.00033 0.99 0.041 0.0034 0.78

PEDOT:PSS 1.45 0.00015 0.86 0.64 0.034 0.61

PEDOT:PSS(0.025M)/MWCNT 1.78 6.21E-5 0.91 0.13 0.070 0.43


PEDOT:PSS(0.1M)/MWCNT 1.56 0.00018 0.89 0.43 0.046 0.54

For capacitive behaviour at low frequency, MWCNT electrode achieves CPE2-P with

0.78 which is the highest among the PEDOT: PSS and the composite electrodes which indicate a

better capacitive behaviour. For PEDOT: PSS and the composites. In generally the CPE2-P

values are all far from 1; this indicates the existing diffusion hindrance of the electrode and a

non- ideal capacitive behaviour. This was also indicated from the inclined low frequency end

from the nyquist plot. For composite with lower PEDOT:PSS content (0.025M and 0.05M), the

less incline of the low frequency end and higher CPE2-T, along with lower Rct (0.13 Ω- 0.23 Ω)

indicates a relatively minor diffusion hindrance for charge transfer between the electrode and

electrolyte, and therefore a higher capacitive performance [98, 108, 184]. Increasing Rct of the

- 97 -

composites with higher PEDOT:PSS content indicates a reduction in rate of charge transfer due

to increasing PEDOT: PSS (minimizing MWCNT contents). This is due to the compacted

PEDOT: PSS matrix and the unorganized arrangement of MWCNTs in the composite as shown

from the SEM. This also forms a higher resistance at the interface between the CNT and PEDOT:

PSS [108]. The system resistance is in the range from 1.4Ω to 1.8Ω. This also shows a relatively

low resistance in the set up.

For the specific capacitance, the estimated capacitance F/cm2

value is indicated by CPE-T

in Table 6.8. With higher MWCNT content in the composite, a higher specific capacitance was

shown. Maximum capacitance is achieved at PEDOT: PSS (0.05M)/MWCNT which is 0.081

F/cm2. This shows a good agreement to the finding from CV. Again, the capacitive performance

then drop as the PEDOT content was further increased.

From CV and EIS results, it shows that with higher MWCNT content in the PEDOT: PSS,

the introduced porous MWCNT network is able to increase the surface area for charge storage at

the double layer. The minimized resistance of the material facilitates ionic transfer which reduces

the system resistance and therefore improves the capacitive performance. Results indicate that

0.05M of PEDOT concentration shows the best electrochemical performance. It is believed that

at this MWCNT to PEDOT: PSS proportion, the MWCNTs content provides interconnected

channels which facilitates an efficient penetration of the ions. Secondly, the π – π interaction

between the MWCNTs and PEDOT: PSS at this composition possibly reduces the energy loss

during the charge/discharge phases by the improved electron transport property [76, 98,105,166].

The capacitance reaches maximum at 0.05M PEDOT concentration then decreases as the

PEDOT concentration increases, the charge storage mechanism is mainly limited by pseudo

- 98 -

capacitive process of PEDOT: PSS [76, 171]. This indicates that 0.05 M PEDOT concentrations

facilitate a better electrochemical utilization of the composite electrode in both double layer and

pseudo capacitive process.

- 99 -

Chapter 6 Polypyrrole/MWCNT Composite

Study

6.1. Introduction

A study was carried out in comparing the capacitive performance between the two

Ppy/MWCNT composite prepared by doping with DBSA and FeCl3 to evaluate the effect of

dopants to the PANI/MWCNT composite morphology and electrochemical performance. For the

2nd

part of the study, a parametric study was conducted to study the influence of polymer content

to the electrochemical performance of the Ppy/MWCNT composite.

6.2. Study of dopant/Oxidant effect on Ppy/MWCNT composite

6.2.1. Composite materials fabrication

6.2.1.1. HCl doped Ppy/MWCNT

Ppy/MWCNT composites were synthesized by oxidative chemical polymerization using

iron chloride (FeCl3.6H2O) as Oxidant [40,185-187]. Oxidant to monomer mole ratio is

controlled to be 0.75 in order to achieve high pyrrole conductivity [188]. MWCNT (100mg) was

first dispersed into de-ionized water with 20% methanol. Addition of 20% methanol has found to

improve conductivity and morphology of the Ppy [129]. Ppy content (0.02M) was added in

MWCNT solution. The suspension was then sonicated for 5 minutes in order to facilitate a good

dispersion. Iron chloride (FeCl3.6H2O) oxidant was added drop wise into the stirring MWCNT

suspension. The polymerization process was allowed for 5 hours. The nano-composite was then

- 100 -

filtered and washed with water and ethanol. The precipitate was then dried in vacuum oven at

60oC over night.

6.2.1.2. DBSA doped Ppy/MWCNT

100mg of MWCNT powder was first dissolved into 100ml of 1M DBSA with 20%

ethanol. The solution was then subjected to ultrasonication to create a well dispersed MWCNT

solution. DBSA was also act as surfactant in this case [173]. DBSA to Ppy molar ratio is (1:1).

Weighted pyrrole monomer was then added to the MWCNT solution and being stirred for 1 hour.

Weighted APS in 100ml deionised water was first pre-cooled for one hour. APS solution was

then added drop wise into the stirring solution to initiate the polymerization process. The

polymerization was then allowed for 5 hours. The composite material was then filtered out and

washed with methanol to coagulate the dispersion. Then washed with HCl to finally dope the

PANI and lastly wash with acetone to keep the precipitate in fine powder. The precipitate is then

filtered and dried under 80oC.

Table 6.1 weight components for FeCl3 doped and DBSA doped Ppy/MWCNT composite

fabrication

FeCl3 Doped

MWCNT Pyrrole FeCl3

(0.75:1)

SDS H2O (ml)

100mg 1.3g 2.43g 1.4g 200

DBSA Doped MWCNT Pyrrole DBSA

(1:1)

APS

(2.3:1 Ppy)

H2O (ml)

100mg 1.3g 2.61g 1.05g 200

- 101 -

6.2.2. FeCl3 & DBSA doped Ppy/MWCNT physical and electrochemical

characterization:

6.2.2.1. Scanning electron microscopy (SEM)

Figure 6.1 HR-SEM micrographs for A ) Ppy (FeCl3) and B) Ppy(DBSA) C1,C2) FeCl3 doped

Ppy/MWCNT D1, D2) DBSA doped PPy/MWCNT composite

A B

C.1 C.2

D.1 D.2

- 102 -

Figure 6.1.A. and B shows the morphology of Ppy doped with FeCl3 and DBSA

respectively. Completely two morphology were presented here. Ppy using FeCl3 as dopant with

water as polymerization medium shows a flake like structure with globe shape formation on the

surface. Whereas in the case of doped with DBSA in acidic medium, a chucky morphology was

observed from the SEM. For the Ppy/MWCNT composites, with FeCl3 doped system shows a

uniform and homogeneous formation of Ppy with tubular structure. Very minimal spherical

globule formation on top of the surface was observed. A closer look on the composite powder

display high entanglement due to the MWCNT network. However, the DBSA doped composite

leads to the formation of huge chunk and aggregates of the Ppy, and therefore a lower

conductivity [132]. The SEM micrograph displayed that MWCNTs were trapped inside the big

chunk of polymer. In the presents of organic dopant, the chain is organized in an ordered three

dimensional fashion [132]. The scattered CNTs within the Ppy matrix would inhibit the porous

structure formations

6.2.2.2. Fourier transforms infrared spectrometry (FTIR)

Figure 6. 2 FTIR spectrum of transmittance against wave numbers for FeCl3 doped polypyrrole

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

400 900 1400 1900 2400

% T

ran

mit

tan

ce

1529 cm-1

1444 cm-1

1287 cm-1

1144 cm-1

875 cm-1

768 cm-1

657 cm-1

1018 cm-1

- 103 -

Figure 6.3 FTIR spectrum of FeCl3 doped and DBSA doped Ppy/MWCNT composites

Figure 6.2 presents FTIR spectrum for Pure Ppy. The highlighted characteristic bands

indicate the formation of pyrrole ring. The characteristic bands and the bonds activities were

summarized on table 6.2. The peaks at ~1,540 cm-1

and ~1,447 cm-1

are associated with

conjugated C–N and C–C asymmetric and symmetric ring stretching vibrations,

respectively.1290 cm–1

is due to C-N stretching vibration in the ring. The band at 1144cm–1

and

1018cm–1

are caused by C-H in plane deformation and N-H in-plane deformation respectively.

The C-H out of plane vibration is observed at 875 cm-1

. While 768 cm-1

belongs to C-H out-of-

plane ring deformation C-C out-of-plane ring deformation or C-H rocking. [128,133].

0

0.2

0.4

0.6

0.8

1

1.2

% T

ran

mit

tan

ce

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

500 1000 1500 2000 2500 3000 3500 4000

2897 cm-1 657 cm-1

551 cm-1

1153 cm-1

1540 cm-1

1447 cm-1

1290 cm-1

(sharp)

1147 cm-1 1094 cm-1

1022 cm-1

DBSA doped Ppy/MWCNT

FeCl3 doped Ppy/MWCNT

1290 cm-1 (weak)

- 104 -

For the composite spectra as presented on figure 6.3, the appearance of Ppy characteristic

bands in the composite indicates successful formation of Ppy on the MWCNTs. For FeCl3 doped

composite, the peak at 1290 cm-1

is sharper than that of DBSA doped composite. This is

associated with the high conductivity arouse by the in plane deformation vibration of N-H on the

protonated nitrogen [132].

For the DBSA doped composite, the following additional bands were observed. The

peaks at 2897 cm-1

correspond to S=O and C–H stretching. This indicates the presence of

benzenoid ring in the DBSA. The peak at 1153 cm-1

represents the S=O stretching vibration of

sulfonate anions, SO3-, when compensating the positive charges in the Ppy chains [132]. The

peak at 657cm-1

indicates other characteristic vibrations of DBSA [185]. The bands at 551 cm-1

represent the C–S stretching vibrations. This implies the introduction of the sulfonic acid groups

into the polymer backbone. In addition, the peak intensity at 1290 cm-1

is so much smaller

compared to that in FeCl3 doped. This indicates symmetric stretching of N-O due to the existence

of sulphonic acid [164]. Assignment of different wavenumber and respective bonds activities

were summarized on table.6.2.

Figure 6.4 Structure of Ppy-DBSA [186]

- 105 -

Table 6.2 Summary of bands wave number and corresponding molecular activities

6.1.3. Three electrodes half cell infrared spectrometry (FTIR)

6.1.3.1. Cyclic Voltammetry

Figure 6.5 CV for Pure Ppy electrodes doped with FeCl3 and DBSA respectively. (1M H2SO4,

10mV/s)

FTIR bands (cm-1

) Assignments

1,540

Conjugated C–N asymmetric and symmetric ring

stretching vibrations

1,447 C–C asymmetric and symmetric ring stretching

vibrations

1291 C-N stretching vibration

1143 C-H in plane deformation

1018

N-H in-plane deformation respectively

875 The C-H out of plane vibration

768 C-H out-of-plane ring deformation C-C out-of-plane

ring deformation or C-H rocking

2897 S=O and C–H stretching

1153 S=O stretching vibration of sulfonate anions, -SO3-,

551 cm-1

,657cm-1

C–S stretching vibrations

- 106 -

Figure 6.6 CV for Pure Ppy, MWCNT and Ppy/MWCNT composites doped with FeCl3 and DBSA

electrodes. (1M H2SO4, 10mV/s)

Figure 6.7 Calculated capacitance for Pure Ppy, MWCNT, PPy/MWCNT composites doped with

FeCl3 and DBSA electrodes

The CV curve of both pure Ppy (FeCl3) and Ppy (DBSA) were presented figure 6.5. Ppy

(FeCl3) displays a more rectangular shape indicates involvement of surface charge storage

possibility due to the observable surface roughness. Whereas, pairs of defined REDOX peaks

0.032 0.033

0.0453

0.112

0.0844

0.0282

0.0064 0.0088

0.106

0.0623

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

MWCNT Ppy (FeCl3) Ppy (DBSA) Ppy/MWCNT (FeCl3) Ppy/MWCNT (DBSA)

charge

Discharge

- 107 -

were observed on the Ppy (DBSA) indicates the charge storage is purely pseudo capacitive in

nature. Regarding the CV for Ppy/MWCNT composites as displayed on figure 6.6. All of the

amplified REDOX peaks and achieved charge current indicates an improved conductivity and

charge storage property form the composites.

Composite utilized FeCl3 as dopant displays an ideal EDLC performance with high

contribution of the double layer for charge storage. However, DBSA doped displays a more

distinct REDOX peak shows that the process is more pseudo capacitive dominated than double

layer of charges in the composite. From figure 6.6, improvement in capacitance was achieved

from both FeCl3 and DBSA doped composites. The FeCl3 doped Ppy/MWCNT composites

results in a higher specific capacitance than the DBSA doped composites.

6.1.3.2. Electrochemical impedance spectrometry

Figure 6.8 Nyquiz plot of –Z” vs Z’ for Ppy/ MWCNT (FeCl3) and Ppy/ MWCNT (DBSA)

composite electrode (100 kHz to 10 MHz)




DBSA doped 1.865 0.00036 0.75 3.26 0.11 0.59

FeCl3doped 1.598 6.31E-5 0.92 1.65 0.18 0.79

- 108 -

EIS was used to evaluate the capacitive and resistive response at varying frequency range

of the DBSA and FeCl3 doped Ppy/MWCNT composites in the half cell set up. At the high

frequency region on figure 6.8, FeCl3 doped composite shows a slightly smaller diameter of the

extended semicircle than the DBSA doped composite. This indicates a minor charge transfer

hindrance at the electrode /electrolyte interface for the FeCl3 doped composite electrode than the

DBSA doped composite. This was also indicated by the lower achieved Rct value from FeCl3

doped Ppy/MWCNT. At the low frequency region, the low frequency end of DBSA is less

parallel to the imaginary axis. This indicates a higher diffusion dominated process and less

homogeneous electrode surface (A longer kinetics of the Ppy). Moreover, both higher CPE2-P

and CPE2-T values of FeCl3 doped Ppy/MWCNT also indicates a higher capacitive nature than

the DBSA doped composite.

From both of the studies, it can be seen that the degree of uniformity of Ppy surface

morphology significantly affect the conductivity and charge storage performance [128]. For the

FeCl3 doped composites, it generates a uniform Ppy layer onto the MWCNTs network, and

therefore leads to formation of an ordered tubular structure. This favours the construction of

porous structure from the entanglement of the tubular structure. In contrary, the chunky

morphology with low homogeneity of the DBSA doped composites leads to a much lower

utilization of the porosity from the MWCNTs in charge storage. A lower conductivity was also

due to the lack of connection paths formed between individual Ppy chunks. Another reason that

leads to the difference in conductivity achieved could be owing to the polymerization medium

and the type of dopant ion incorporated. Study reported that acidity of the polymerization

medium could also affect the no. of quinoid ring formation. And therefore affects degree of

delocalization of the chain and thus the conductivity of the polymer [133].

- 109 -

System using FeCl3 as oxidant provides FeCl4- and Cl

- ions were incorporated as dopant

ion to the system [121]. Whereas, for the case of DBSA doped sulphonic acid was employed as

dopant ions. The structures also have influence on the ion’s mobility. From the literature, it is

suggested that Ppy system incorporated with FeCl4- achieves higher mobility than other dopant

ions [121]. Moreover, the surfactant could also be the contributed factor to the doped Ppy.

Surfactant was employed to improve dispersion of MWCNT. However, study found that the

anionic surfactant would also enter the Ppy chain as counter ions during the polymerization

process [185]. The structure of anionic surfactant would also affect the in general doping level of

the Ppy chain and therefore affect the conductivity [185].

In the case of FeCl3 doped, SDS was used as surfactant. Study found that at same molar

level, SDS yields a much higher improved conductivity [185]. This could be an explanation in

this case of highly conductive FeCl3 Ppy/MWCNT composites compared to the one doped with

DBSA [185]

6.2. Ppy/MWCNT composite parametric study

6.2.1. Composite Fabrications

MWCNT (100mg) was first dispersed into de-ionized water with 20% methanol.

Addition of 20% methanol has found to improve conductivity and morphology of the Ppy [129].

Various pyrrole contents (0.02M, 0.05M, 0.1M and 0.3M) was added into MWCNT solution.

The suspension was then sonicated for 5 minutes in order to facilitate a good dispersion. Iron

Chloride (FeCl3.6H2O) oxidant was added drop wise into the stirring MWCNT suspension. The

polymerization process was allowed for 5 hours. The nano-composite was then filtered by

- 110 -

suction filtration and washed with water and ethanol. The precipitate was then dried in vacuum

oven at 60oC over night.

Table 6. 4 The Ppy and MWCNT composition at various compositions

Composition name Pyrrole

Molarity

Monomer

weight

Composite Weight

(g)

Ppy/MWCNT

Ppy: CNT MWCNT

weight %

Ppy(0.02M)/MWCNT 0.02M 1.34 0.46 78.3:21.7 21%

Ppy(0.05M)/MWCNT 0.05M 3.35 3.45 90:10 10%

Ppy(0.1M)/MWCNT 0.1M 6.71 6.81 95.91:4.09 4%

Ppy(0.3M)/MWCNT 0.3M 20.1 10.62 98.95:1.05 1%

6.2.2. Physical Characterizations


B C

C.1 B.1

A

- 111 -

Figure 6.9 SEM images of a) Pure Polypyrrole powder, b) Ppy (0.02M)/MWCNT composite

powder and individual Ppy grown MWCNT tube morphology of c) Ppy (0.02M)/MWCNT, d) Ppy

(0.05M)/MWCNT, e) Ppy (0.1M)/MWCNT and f) Ppy (0.3M)/MWCNT

For the Ppy/MWCNT parametric study, the pyrrole concentration was varied from 0.02M

to 0.3M. The SEM micrographs of pure Ppy and the Ppy/MWCNT composites were shown on

figure 6.9. From figure 6.9a), pure Ppy shows a flake like morphology with spherical globule

shaped structure over the surface. Figure 6.9b) displayed morphology of single 0.02M

Ppy/MWCNT composite powder exhibiting high porosity. Figure 6.9 c) to e) reveals

morphology of the Ppy/MWCNT composite from 0.02M to 0.3M Ppy concentration. 0.02M (Ppy

70%) and 0.05M (Ppy 90%) display formations of tubular structure with uniform surface

morphology. From high Ppy to MWCNT weight ratio, again this implies that the polymerization

and the formation of Ppy in fiber shape follow the same fashion as illustrated in the case for

PANI/MWCNT. The MWCNTs provide a framework for the Ppy to be polymerized into tubular

D E

D.1

- 112 -

shape. When having a closer look to the fiber surface, increase formation of globule like

structure was also observed as Ppy content increases from 0.02M to 0.05M. At 0.1M Ppy level

(Ppy 95%), the globe shaped structure formation on the surface becomes more severe which

leads to a non-uniform surface morphology. At 0.3M Ppy level (Ppy 98%) with MWCNT

content drops to only 2%, SEM mainly displays aggregated Ppy structure without following any

ordered formation. Again, the importance of MWCNT contents for the formation of porous

structure with a uniform surface is illustrated. The measured average tube thickness is

summarized on Table 6.5.

Table 6. 5 Measured mean tube thickness and the deposited PANI layer thickness on MWCNTs

Samples Ppy/CNT

composition

Average tube

diameter (nm)

Ppy layer thickness

(nm)

MWCNT ~7

Ppy(0.02M)/MWCNT 78.3:21.7 ~107 ~50

Ppy(0.05M)/MWCNT 90:10 ~173 ~83

Ppy(0.1M)/MWCNT 95.91:4.09 ~303 ~14

Ppy(0.3M)/MWCNT 98.95:1.05 N/A N/A

- 113 -

6.2.2.2. Fourier Transform infrared spectrometry (FTIR)

Figure 6. 10 FTIR spectra comparing Ppy/MWCNT composites to pristine Ppy. Highlighted

bands indicates the Ppy characteristic bands in the composites. Wave shift of pyrrole vibration

bands in the composite spectra was observed.

0

0.2

0.4

0.6

0.8

% T

ran

mit

tan

ce -0.2

0

0.2

0.4

0.6

0.8

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

0.2

0.4

0.6

0.8

1

1.2

500 1000 1500 2000 2500 3000 3500 4000

Ppy

1018 cm-1

Ppy(0.02M) /MWCNT

Ppy (0.05M) /MWCNT

Ppy (0.1M) /MWCNT

Ppy (0.3M) /MWCNT

1287 cm-1

1143cm-1

1293cm-1

1025 cm

-1

1150 cm-1

1292 cm-1

1023 cm

-1 1146cm

-1

1292 cm-1

1021 cm

-1 1149cm

-1

1287 cm-1

1019 cm-1

1144 cm-1

- 114 -

For the Ppy/MWCNT composites, the FTIR spectra are presented on figure 6.10. All of

the composites exhibit characteristic bands of Ppy. This again indicates the presence of

polymerized Ppy in the composites. In comparison to pure Ppy, shifting of the bands of pyrrole

rings vibration and C-N stretching were observed. The peaks at 1287cm-1

, 1143 cm-1

and 1018

cm-1

which represents C-N stretching vibration in the ring C-H in plane deformation and N-H in-

plane deformation respectively, These bands were shifted to 0.02M: (1293 cm-1

, 1150 cm-1

,1025

cm-1

), 0.05M (1292 cm-1

, 1146 cm-1

, 1023 cm-1

), 0.1M : (1292 cm-1

, 1149 cm-1

,1021 cm-1

), 0.3M :

(1287 cm-1

, 1144 cm-1

,1019 cm-1

) This indicates the interaction between MWCNT and Ppy

attributed to the oxygen atom form the carboxylic acid group on the MWCNTs [187, 189].

These bands were shifted to the highest wave number at 0.02M, and then the band shift effect

decreases and goes back to original corresponding positions. This possibly indicates a stronger

complex formed between the two materials at 0.02M Ppy concentration and therefore produce

the most disturbance to these vibration and stretching activities.

6.2.3. Half-cell electrochemical evaluation


Figure 6.11 Cyclic Voltammetry at 10mV/s

for Pure Ppy, MWCNT and Ppy/MWCNT

composites

Figure 6.12 Calculated specific capacitance

(F/cm2) for pure Ppy, MWCNT and

MWCNT/Ppy composites

Voltage (V) vs Ag/AgCl

- 115 -

As shown on figure 6.11, the pair of REDOX peaks indicates doping and dedoping

process of the polypyrrole. The Ppy/ MWCNT composite CV curve exhibit a highly rectangular

in shape which indicates a very ideal capacitive behaviour. Calculated specific capacitance

values for the composites were presented on figure 6.12. At 10mV/s, highest performance was

achieved at Ppy (0.02M)/MWCNT with high capacitance of 0.112F/cm2. Same phenomenon can

be observed, the electrodes capacitance decreases as the Ppy content was increased from 0.02M

to 0.1M. Continuous increase of Ppy content to 0.3M in the composite display performance close

to pure Ppy electrode. This indicates the charge storage mechanism is mainly contributed to Ppy

in the composite. The MWCNT influence becomes less effective. The capacitance value is

decreased to 0.026F/cm2 when Ppy concentration is increased to 3M.

6.2.3.2. Electrochemical Impedance spectrometry (EIS)

Figure 6.13 Nyqust plot of –Z” vs Z’ for MWCNT, Ppy and the Ppy/MWCNT electrodes (100

kHz to 10 MHz

- 116 -




Ppy 2.02 0.00015 0.86 1.65 0.036 0.58

MWCNT 0.54 0.00033 0.99 0.041 0.0034 0.78

MWCNT/Ppy(0.02M) 1.60 6.31E-5 0.92 1.65 0.18 0.79

MWCNT/Ppy(0.05M) 1.33 0.00015 0.88 1.17 0.12 0.72

MWCNT/Ppy(0.1M) 1.71 0.00012 0.89 1.01 0.096 0.64

MWCNT/Ppy(0.3M) 3.28 0.0018 0.58 1.43 0.013 0.56

The nyquist plots were presented on figure 6.13 and the summarized parameters from

curve fitting were listed in table 6.6 From the 45o incline low frequency tail of pure Ppy, it

indicates the high resistance characteristic due to the highly diffusion dependant REDOX

reaction in contrast to the ideal EDLC characteristic of MWCNT (A Longer time required for

insertion desertion of electrolyte ions). Regarding the Ppy/MWCNT composite electrodes,

0.02M and 0.05M achieved the minimal Rct and ESR which indicates the better conductivity and

higher ionic accessibility at the interface with lower Ppy content in the composite. The low

frequency tails for both composites appear to be sharp and parallel to the Y-Axis indicates

improved capacitive behaviour. Similar to the case in PANI composite, both ESR and Rct

increase as the Ppy content in the composite increases to 0.1M and 0.3M. Highest resistance was

observed at Ppy/MWCNT (0.3M) composition. For the capacitive behaviour of the composites,

the decreasing trend of the CPE-T value when Ppy content in the composite rises again indicates

the degradation of capacitive performance. 0.02M Ppy/MWCNT is determined as the best

performing composition by coupling with both low Rct and high capacitance.

Both CV and EIS studies again show a high dependency of the degree of ionic transfer

activity on the Ppy content in the Ppy/MWCNT composite. Increasing Ppy molar concentration

- 117 -

lead to build up of a compact polymer layer and increase in surface roughness as shown from the

SEM micro graph. The high polymer content would restrict the formation of the porous networks

and therefore hinder the effective charge storage at the interface. Moreover, the heterogeneous

surface structure with high roughness due to the globule formation would cause reduction in

conductivity as well [128].

It was found that the polymerization mechanism is similar to the finding from

PANI/MWCNT composite, where the polymer is polymerized into fibrous structure due to the

MWCNTs. However this is different from one of the reported study, a different observation was

reported from B. Zhang et.al. In their study on Ppy composite with MWCNT and carbon fiber

[39]. A thin Ppy layer was found to form a core shell over the MWCNT surface with thickness

about 6nm- 8nm, where the Ppy: MWCNT is 20:80 [39]. Therefore, in this study, the MWCNT

content in the composite was further increased and further investigates the composite

electrochemical properties.

In this part of the study, the MWCNT content was varied from 100mg to 300mg and

500mg while fixing the pyrrole content at 0.02M. The MWCNT content was increased from

23% to 47%. Figure 6.14 shows the SEM images of the composites. The measured average

thicknesses of the composite strands diameter are summarized on table 6.7.

- 118 -

Figure 6.14 HR-SEM images of a)Ppy/MWCNT (100), b) Ppy/MWCNT (300) and c)

Ppy/MWCNT (500) composites

A.1 A.2

B.1 B.2

C.1 C.2

- 119 -

Table 6.7 Weight composition between Ppy to MWCNT at various compositions

Composition name Pyrrole

Molarity

Total

composite

weight (g)

MWCNT

content (g)

Ppy:

CNT

MWCNT

weight %

Average tube

diameter

Ppy/MWCNT (100) 0.02M ~0.45 100mg 77:23 23% ~162nm

Ppy/MWCNT (300) 0.02M ~0.54 300mg 63:37 37% ~47.8nm

Ppy/MWCNT (500) 0.02M ~0.63 500mg 53:47 47% ~26.57nm

Figure 6. 15 CV plot for pure Ppy, MWCNT,

Ppy/MWCNT (100), Ppy/MWCNT (300) and

Ppy/MWCNT(500) (10mV/s, 1M H2SO4)

Figure 6. 16 Calculated specific capacitance

(F/cm2) for pure Ppy, MWCNT and

Ppy/MWCNT composites

However, an interesting result was observed. As presented on figure 6.15. and figure 6.16,

the measured specific capacitance was dropped from 0.141F/cm2 to 0.0526F/cm

2 as the

MWCNT content in the composite was increased from 23% to 47%. In order to explain the

phenomenon, it is believed that in the case with Ppy content > MWCNT content, the higher

polymer content can increase the utilization of Ppy to function in pseudo capacitive charge

storage. At the same time the preserved porosity in the composite also allows effective usage of a

high surface area for charge storage at the double layer.

0.032 0.033

0.141

0.0706

0.0526

0.0282

0.0064

0.133

0.0647

0.04556

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

MWCNT Ppy Ppy/MWCNT(100) Ppy/MWCNT(300) Ppy/MWCNT(500)

Charge

discharge

Specific C

apacitan

ce (F/cm2)

A/cm

^2

- 120 -

Again, this observation further proves that an optimal Ppy to MWCNT in the composite

is essential in optimizing the composite electrochemical performance by effectively utilizing the

Ppy to function in pseudocapacitive charge storage. It is suggested that a higher MWCNT in the

composite does not necessarily give rise to a better capacitance performance.

- 121 -

Chapter 7 Two Electrodes Cell Electrochemical

Evaluation

7.1. Introduction

Three electrodes cell is a very useful mean in investigating single electrode

electrochemical behavior. However the specific capacitance determined does not reflect the

performance in a supercapacitor device. Only a two electrodes system should be taken into

account for a practical evaluation as a supercapacitor device [35, 56]. In this chapter, the two

electrodes cell electrochemical evaluation of Ppy/MWCNT composite was demonstrated. A

comprehensive two electrodes cell study of the three different composite materials is

summarized on chapter 8. Same studies have been done on both PANI/MWCNT and

PEDOT:PSS/MWCNT composites as well. For the details please refer to appendix C and D.

7.2. Ppy/MWCNT two electrodes Cell Electrochemcial Evaluation

7.2.1. Cyclic Voltammetry (CV)

A B

- 122 -

Figure 7.1 CV plot of Pure Ppy, MWCNT and MWCNT/Ppy composites devices from A)10mV/s,

B)50mV/s,C)100mV/s,D) 500mV/s respectively

Figure 7.2 Specific capacitance (F/cm2) as a function of scan-rate from 10mV/s to 1V/s

C D

E

- 123 -

The Ppy/MWCNT composite supercapacitor devices’ performance was first evaluated by

mean of cyclic voltammetry. The performances under varying potential ramp rate from 10mV/s

up to 1V/s were accessed. The CV plots were presented on figure 7.1.

A significantly higher capacitive performance was achieved by the composites device

then bare Ppy and MWCNT devices were again observed. Ppy (0.02M) /MWCNT and

Ppy(0.05M)/MWCNT composite devices shows very high charge propagation characteristic

even at very high potential scan rate of 100mV/s to 500mV/s. Under such scan rate, the highly

rectangular shape of curve was still maintained. The highest performance of 0.02M

Ppy/MWCNT can be seen from the largest area coverage, followed by 0.05M Ppy/MWCNT. For

0.1M Ppy/MWCNT, the parallelogram shape of curve at 100 mV/s indicates the slowly build up

of resistance of the system. While 0.3M device behaves very similar to pure Ppy device with

poor capacitive performance.

Specific capacitance as a function of potential scan rate for each device was shown on

figure 7.2. The high power capacity is indicated by the high linearity of curve with minimal drop

of capacitance with increasing scan rates. A drop of capacitance over scan rates indicates the

charge storage process is limited by the slower doping / dedoping kinetic of the polymer. For

0.02M, 0.05 M concentrations, the capacitance is quite stable shows that the ability for the

capacitor to cycle under high scan-rates up to 500mV/s. 0.02M Ppy/MWCNT device shows the

best performance from the highest achieved capacitance and the relatively high linearity of the

curve. When comparing 0.05M to 0.1M, there is a transition of performance as scan rate

increases. At low scan rate (10mV/s to 50mV/s), 0.1M achieves higher capacitance than 0.05M

with higher energy density. However, when scan rate reaches 100 mV/s and onwards, 0.05M

outperforms 0.1M device with a minor capacitance drop. This indicates that the 0.1M device

- 124 -

achieves higher energy density possibility due to the higher Ppy content and therefore the

capacitance is determined by the higher energy pseudo capacitive activity.

Whereas 0.05M device exhibit higher power characteristic compared to 0.1M. It is

believed that in 0.05M composition, the effect from the double layer of charge is dominated

which allows a more efficient charge/ discharge across the interface. This shows that the power /

energy capability of the composite can be tailored via controlling the composition between the

two materials.

7.2.2. Electrochemical impedance spectrometry (EIS)

Figure 7. 3 Nyquizt plot of –Z” vs Z’ for MWCNT,

pure Ppy and Ppy/MWCNT cells (100 kHz to 10

MHz)

Figure 7. 4 Bode plot of negative phase vs

frequency (Hz) for MWCNT, Ppy/MWCNT, and

pure Ppy cell. The specified frequency @ -45o

indicates the point where resistance to

capacitance transition occur

Figure 7.3. shows the nyquist plot. In this case, the size of the extended semi-circle

portion increases with the Ppy content, which indicates increasing hinderance of ionic transfer

process at the interface in the device. The estimated ESR and Rct as well as all other parameters

generated from fitting to the equivalent circuit model were presented on Table 7.1. Again, the

highly capacitive nature of Ppy (0.02M)/MWCNT and Ppy (0.05M)/MWCNT device was

-45o

- 125 -

indicated from high CPE2-P values (0.763 and 0.719 for 0.02M and 0.05M respectively) and

highest achieved negative phase angles. As the composites Ppy content increases from 0.05M to

0.3M, both increase decline of the low frequency tail and diminish of CPE2-P value indicates the

drop of capacitive property of the supercapacitor devices.

Table 7.1 Parameters of the equivalent circuit model for different composites devices (1 cm2)

derived from numerical fitting of experimental data acquired from nyquist plots

Electrodes: ESR Rct CPE1-T CPE1-P CPE2-T CPE2-P

MWCNT 0.39 0.25 0.00025 1.089 0.0078 0.78

Ppy 0.89 0.85 4.95e-5 0.99 0.013 0.66

MWCNT/Ppy(0.02M) 0.78 0.41 0.00058 0.79 0.052 0.76

MWCNT/Ppy(0.05M) 0.84 0.32 0.0014 0.87 0.045 0.72

MWCNT/Ppy(0.1M) 0.74 0.65 1.29E-5 1.16 0.047 0.43

MWCNT/Ppy(0.3M) 0.62 1.15 1.29E-5 1.09 0.107 0.41

Figure 7. 5 Specific capacitance C” (F/g) vs

frequency (Hz) with time constant values

indicated for MWCNT, Ppy and Ppy/MWCNT

cell

Figure 7. 6 Specific capacitance C’ (F/g) vs

frequency (Hz) for MWCNT, Ppy and

Ppy/MWCNT cell

0.02M: ~0.22s 0.05M: ~0.22s

0.1M: ~1.02s

0.3M: ~ >8.5s

Ppy: ~ >8.5s

MWCNT: ~0.17s

- 126 -

Table 7. 2 Summarized important findings from electrochemical impedance

Electrodes ESR Rct Peak C” Correspondin

g frequency

(Hz)

RC Time

constant (s)

C’ ( Specific

capacitance

F/cm2)

Frequency

at -45o

phase (Hz)

Phase

angle

MWCNT 0.39 0.25 1.83E-03 ~0.118 ~0.17 0.0072 17.3 Hz -75.6o

Ppy 0.89 0.85 5.77E-03 ~0.118 >8.5 0.012 9.77 Hz -64.9o

MWCNT/Ppy(0.02M) 0.78 0.32 1.02E-02 ~4.54 ~0.22 0.027 4.54 Hz -82.2o

MWCNT/Ppy(0.05M) 0.84 0.41 8.32E-03

~4.54 ~0.22 0.024 5.48 Hz -82.6o

MWCNT/Ppy(0.1M) 0.74 0.65 7.75E-03

~0.977 ~1.02 0.025 2.55 Hz -79.3o

MWCNT/Ppy(0.3M) 0.62 1.16 5.47E-03 ~0.118 >8.5 0.016 3.74 Hz -71.1o

For complex capacitance, the real (C’) and imaginary (C”) capacitance against log

(frequency) were presented on figure 7.5 and figure 7.6 respectively. Again, highly capacitive

0.02M and 0.05M displayed very low value which is as low as 0.22s indicates a very fast

discharge characteristic of the devices. The value increases for system with increasing Ppy

content in the composites. goes up to 1.02s at 0.1M Ppy concentration. For 0.3M and pure Ppy,

the peak was not even observed within the frequency range. This indicates a much slower

discharge kinetic for these two devices. Highest capacitance of 0.0286F/cm2 was achieved from

0.02M at low frequency. With the lowest charge transfer resistance (fast ionic transport), it

supports a high power output from the 0.02M Ppy/MWCNT capacitor cell. The trend of

decreasing capacitance with increasing Ppy content was again observed here.

- 127 -

7.2.3. Galvanostatic Charge/discharge evaluation

Figure 7.7 Charge/discharge curve of Pure MWCNT, pure Ppy and Ppy/MWCNT

supercapacitor at 0.001A/cm2 current density

Table 7.3 Coulomb efficiency and specific capacitance value @0.001A/cm2

Electrodes Charge

time(s)

Discharge

time(s)

Coulomb efficiency (%) Specific Capacitance @

0.001A/cm2 (F/cm

2)

Ppy 32.5 16 49.2% 0.020

MWCNT 5 4.5 90% 0.0061

MWCNT/Ppy(0.02M) 35.9 29.4 81.9% 0.053

MWCNT/Ppy(0.05M) 29.2 23.8 81.5% 0.042

MWCNT/Ppy(0.1M) 24.7 19.2 77.7% 0.050

MWCNT/Ppy(0.3M) 12.1 7.8 64.5% 0.019

Charge/discharge efficiency of the composite devices were assessed under current densities of

0.001A/cm2. Moreover, the cell capacitance respected to its power was evaluated. The charge

/discharge plots were shown on figure 7.7. The Ppy/MWCNT composites and pure MWCNT

device displays very high linearity and symmetrical triangular shape suggests very low internal

resistance due to highly effective charge storage at the double layer. No observable ohmic drop is

- 128 -

seen from the charge/ discharge switching point. This indicates in generally very low resistance

of the two-electrode cell. In contrast, the lack of symmetry of curve for pure Ppy device indicates

the redox dependent doping and dedoping of the polymer which has a lower efficiency. The

calculated coulomb efficiency as well as the specific capacitance at 0.001A/cm2 was shown on

table 7.3.

In general the performance of supercapacitor device is 0.02M>0.05M>0.1M>0.3M>Ppy.

Ppy (0.02M)/MWCNT device achieved promising performance with efficiency up to 81.8% and

capacitance of 0.037F/cm2. The excellent stability is attributed to the higher electrical

conductivity of this composition and the high surface area due to the porous structure. The worst

performance was discovered on 0.3M Ppy/MWCNT cell. These agree with previous findings

from both EIS and CV studies

7.2.4. Cycling Stability Test

Figure 7.8 CV plot for MWCNT/Ppy (0.02M) (Left) and pure Ppy cell (right) at the 1st cycle and

the 1000th

cycle

- 129 -

Figure 7.9 Capacitance change over 1000 cycles for MWCNT/Ppy (0.02M) (left) and pure Ppy

cell (right) respectively

In the composite, the MWCNTs network provides a large mesoporous surface to be in

contact with the conductive polymer which can facilitate stress transfer from the polypyrrole to

the MWCNTs, and therefore the polymer stability can be improved by compensating the

swelling of the polymer due to ions intercalation during the doping /dedoping process. The

cycling stability of the Ppy (0.02M)/MWCNT and pure Ppy composite device were assessed

over 2000 cycles of CV at 100mV/s scan rate as presented on figure 7.8. The measured specific

capacitance over 2000 cycles was shown on figure 7.9. An improved efficiency achieved from

the Ppy/MWCNT was again observed from the close charge and discharge capacitance as

compared to pure Ppy cell. Pure Ppy cell does not experience a very stable performance as can

be seen from the fluctuating achieved capacitance and the imbalance of charge discharge portion

indicates a poor efficiency.

Ppy/MWCNT composite cell experienced initial drop of 18.6% in capacitance in the first

100 cycles. As the number of cycle increases, steady drop of capacitance was observed. However,

the value reaches steady state at about the 700th cycle. The capacitance value stays around 0.027

- 130 -

F/cm2. From the 1000

th to the 2000

th cycles, result shows that high stability was maintained at

steady capacitance level while maintaining a high efficiency. However, pure Ppy cells show a

poor stability which experience continuous loss of capacitance over the 2000 cycles as shown

from the steady decline of the capacitance value. This shows that the cycling stability of the

polypyrrole was successfully improved due to the introduction of the MWCNTs in the composite.

7.3. ECP/MWCNT Composite Two Electrodes Cell Performance Comparison

study

In this chapter, the electrochemical performance of the best compositions from each

ECP/MWCNT composites: 0.1M PANI/MWCNT, PEDOT: PSS (0.05M)/MWCNT and

Ppy(0.02M)/MWCNT were summarized. In fabricating a test cell, a filter paper soaked with 1M

H2SO4 electrolyte was sandwiched in between the ECP/MWCNT composite electrodes to create

a cell. Cyclic Voltammerty (CV), electrochemical impedance spectrometry (EIS) and galvanic

charge/discharge tests were performed. In the following comparison, the specific capacitance is

measured in per gram of active (ECP/MWCNT composite) materials for comparison purpose.

Information regarding the weight of the active materials per single electrode for the

ECP/MWCNT composites were summarized on Table .7.4. 7.5 and 7.6.

- 131 -

Table 7.4. Composite electrode material breakdown and weight of PANI/ MWCNT composite in

single electrodes

Electrode Materials Weight (g) Weight (%)

PANI/CNT

0.2g 28.38%

Polytetrafluroethylene (PTFE) 0.02g 2.838%

Graphite Conductive Ink 0.487g 68.78%

Average weight of materials on single electrode material (16 samples): ~0.00611g /cm.

PANI/MWCNT = 0.00611g /cm x 28.38%~0.00173g

Table 7.5. Composite electrode material breakdown and weight of Polypyrrole/ MWCNT

composite in single electrodes


Polypyrrole/CNT 0.05g 9.67%



Average weight of materials on single electrode material (16 samples): ~0.00562g/cm.

Polypyrrole/MWCNT = 0.00562g/cm x 9.67%: ~0.000543g

Table 7.6 Composite electrode material breakdown and weight of PEDOT: PSS/MWCNT

composite in single electrode


PEDOT:PSS/CNT

0.05g 18.12%



Average weight of materials on single electrode material (16 samples): ~0.00682g/cm.

PEDOT:PSS/MWCNT = 0.00682g/cm x 18.12%: ~0.00124g

- 132 -

7.3.1. Cyclic Voltammetry

Figure 7.10 Cyclic Voltammetry at A)10mV/s, B) 50mV/s, C)100mV/s, D)500mV/s, E)100mV/s

for MWCNTs, PAN/MWCNT, PEDOT:PSS/MWCNT and Ppy/MWCNT composites

supercapacitors

E

A B

C D

- 133 -

Figure 7.11 Variation of the capacitance of MWCNT, PANI/MWCNT, PEDOT: PSS/MWCNT

and Ppy/MWCNT supercapacitor devices as a function of potential scan rates

CV at varying potential scan-rate from 10mV/s to 1V/s at voltage window from 0V to

+0.9V. Figure 7.10 A) to E) presents the CV plots compares the charge storage performance of

PANI/MWCNT, PEDOT: PSS/MWCNT and PANI/MWCNT electrode respectively from scan-

rate 10mV/s to 1V/s respectively. The CV plots were presented in current density (A/g) against

applied voltage (V). These figures show the gradual change of the device charge storage

performance as scan-rate increases.

In comparison of the three composites, Ppy/MWCNT device clearly shows a much better

performance. The outstanding charge storage performance can be observed from the larger area

coverage of the curve. Moreover, the ability of maintaining well defined rectangular shape up to

high scan-rate (0.1V/s to 0.5V/s) indicates a very high charge propagation characteristic and high

conductivity which results as a fast current in respond to voltage change in the device. This also

shows that the Ppy/MWCNT composite electrode possess a highly ion accessible porous surface.

- 134 -

Electrolyte ions are able to reach the inner pores for charge storage under the fast scan-rate due

to the porosity introduced by the composite.

In contrary, PANI/MWCNT CV displays distortion of the shape at 0.5V/s and 1V/s. This

indicates an increase in ohmic respond due to relatively limiting ionic accessibility of the

electrode from the electrolyte at the high scan-rate [56]. PEDOT: PSS/MWCNT shows the worst

capacitive performance due to the low uniformity of the polymerized PEDOT: PSS in the

composite. The lower performance also indicates the synergy of MWCNT with PEDOT: PSS is

the least comparable to the other two composites due to the poor dispersion of MWCNT within

the PEDOT: PSS matrix as illustrated from previous chapter.

The calculated specific capacitance (F/g) against potential scan-rate is shown on figure

7.11. The power capability of a device is indicated from the linearity of the curve. MWCNT

device displays very high power capability from maintaining constant capacitance from low to

high scan rates. Again, Ppy/MWCNT shows the best performance from the highest achieved

capacitance Maximum capacitance of ~67.8F/g is achieved at 10mV/s scan-rate. In addition, the

high linearity indicates minor decrease of capacitance from high to low scan-rates which shows a

very promising cycling ability from low to high scan-rate. In contrast, PANI/MWCNT displays

an irregular shape indicating a lower power capability at high scan-rate due to the dominance of

diffusion dependent pseudo capacitive activity from PANI. This can also be indicated from the

presence of the more pronounced REDOX peaks from CV.

- 135 -

7.3.2. Electrochemical impedance Spectrometry (EIS)

Figure 7.12 Nyquiz plot of –Z” vs Z’ for

MWCNT, PANI/MWCNT Ppy/MWCNT,

PEDOT: PSS /MWCNT cell (100 kHz to 10

MHz)

Figure 7.13 Bode plot of negative phase vs

frequency (Hz) for MWCNT, PANI/MWCNT

Ppy/MWCNT, PEDOT: PSS /MWCNT cell. The

specified frequency indicates the point where

resistance to capacitance transition occur

Figure 7.14 .Specific capacitance C” (F/g)

vs frequency (Hz) with time constant values

indicated for MWCNT, PANI/MWCNT

Ppy/MWCNT, PEDOT: PSS /MWCNT cell.

Figure 7.15 Specific capacitance C’ (F/g)

vs frequency (Hz) for MWCNT,

PANI/MWCNT, Ppy/MWCNT, PEDOT: PSS

/MWCNT cell

-45o

0.223 s

0.25 s

0.476 s

0.15 s

52.7 F/g

32F/g

19.6 F/g

7 F/g

- 136 -

Table 7.7 Parameters of the equivalent circuit model for different composites devices (1 cm2)


Composite device ESR CPE1-T CPE1-P Rct CPE2-T CPE2-P

MWCNT 0.3904 0.249 0.00025147 1.088 0.0077769 0.77953

Ppy/MWCNT 0.781 0.000298 1.04 0.412 0.054 0.883

PANI/MWCNT 0.81 0.000119 1.267 0.332 0.0487 0.753

PEDOT:PSS/MWCNT 0.69 2.223E-5 1.334 1.23 0.011 0.48

Table 7.8 Summarized important findings from electrochemical impedance

Electrodes ESR

(Ω)

Rct (Ω)

Peak

C”(F/g)

frequency

(Hz) @ C”

max

RC

(s)

C’

( Specific

capacitan

ce F/g)

Phase

Angle

frequency

(Hz) at -45o

MWCNT 0.39 0.002 2.1 1.18E-01 0.15 7 -75.6 o 17.3

Ppy/MWCNT 0.781 0.41 18.8 4.48 0.223 52.7 -82.2o 5.48

PANI/MWCNT 0.81 0.33 8.76 0.664 0.25 32 -82.2 o 4.86

PEDOT:PSS/MWCNT 0.69 1.23 5.77 2.10 0.476 19.6 -74.7 o 2.1

EIS was performed in frequency range from 100 kHz to 10 mHz. The operating voltage

was controlled at 5mV. The AC impedance spectra are shown on nyquist plot of Z’ VS –Z”. The

nyquist plot of the three composites devices was shown in figure 7.12. The parameters generated

from fitting to the nyquist plot were summarized on table 7.7. The impedance behaviour of

Ppy/MWCNT device shows very close to ideal EDLC by coupling a low Rct and highly vertical

line towards low frequency end. Whereas, PEDOT:PSS/ MWCNT shows a larger semi circle

portion indicates very high Rct. PANI/MWCNT shows very minimal Rct but then a higher ESR

indicates a higher internal resisitance and diffusion limitaion existed in the device. The better

capacitive behaviour from Ppy/MWCNT composite device is also indicated from the higher

CPE2- P value which is 0.88 in contrast to those of PANI/MWCNT and PEDOT:PSS/MWCNT

which are 0.75 and 0.48.

- 137 -

The bode plot of phase against log frequency is presented on figure 7.13. Minimal phase

angle of -82.2

was achieved by both Ppy/MWCNT and PANI/MWCNT. That of PEDOT:

PSS/MWCNT composite is -74.7

which again indicates the least capacitive property of this

composite device. Normally the phase is below -90o because the existing deviation which is

caused by the interaction of the electrode between the solvent and the porous electrode surface in

the device [44]. The frequency which corresponds to resistive to capacitive transition is

measured from the frequency point where the phase reaches -45o

[38]. These frequencies were

observed at 5.48Hz, 4.86Hz and 2.1Hz for Ppy/MWCNT, PEDOT: PSS/MWCNT and

PANI/MWCNT respectively. The knee frequency of Ppy/MWCNT is the highest meaning that

Ppy/MWCNT device starts to display capacitive behaviour earlier than the other device.

For complex capacitance plots, the real (C’) and imaginary (C”) capacitance plotted

against log (frequency) were presented on figure 7.14 and figure 7.15.respectively. The findings

from AC impedance, including the specific capacitance values were summarized on table 7.8.

Once again, Ppy/MWCNT achieves highest capacitance of 52.7F/g. The findings agree with

previous studies and showing a good agreement with CV results. For the RC time constant,

highly capacitive Ppy/MWCNT displays very low value which is as low as 0.22s. This

indicating a very fast discharge characteristic and very high power density of the devices.

Whereas PANI/MWCNT shows the slowest discharge kinetic among the three.

- 138 -

7.3.3. Galvanostatic Charge/discharge test

Figure 7.16 Charge/discharge curve of PANI/MWCNT, PEDOT: PSS/MWCNT and

PANI/MWCNT composites supercapacitors at current density of a) 0.001A/cm2 b) 0.01A/ cm

2

and c) 0.1A/cm2

(0V to +0.8V)

Table 7.9 Summarized charge time, discharge time, coulomb efficiency% and specific

capacitance F/g at 0.001A/cm2

Composite: tc td Coulomb

efficiency

Specific capacitance F/g

Ppy/MWCNT

35.9 29.4 81.9% 47.88

PANI/MWCNT 39.5 29.4 74.4% 17.05

PEDOT:PSS

/MWCNT

19.9 14 70.4% 9.154

C )

A)

B)

PANI/MWCNT

PEDOT:PSS/MWCNT

Ppy/MWCNT

PANI/MWCNT

PEDOT:PSS/MWCNT

Ppy/MWCNT

PANI/MWCNT

PEDOT:PSS/MWCNT

Ppy/MWCNT

- 139 -

The charge/ discharge performance of composite supercapacitors at current densities of

0.1A/cm2, 0.01A/cm

2 and 0.001A/cm

2 as a function of time (s) were shown on figure7.16 a) to c)

respectively. At the very low 0.001A/cm2 current densities, the plot presents typical symmetrical

triangular shape with high linearity which indicates a very efficient charge discharge

performance with very low voltage (IR) drop during the process. The coulomb efficiency and the

specific capacitance per gram were summarized at table 7.9. Among the three, Ppy/MWCNT

device again displays best performance at this low charge density increase rate with highest

capacitance of 47.88F/g.

The performance in high current density is interested. Increasing cell discharge at higher

current density (0.01A/cm) causes an exponential shape of charge/ discharge voltage against

time. The performance of Ppy/MWCNT is maintained from the longer C/dc elapse time as well

as the higher achieved charge voltage in contrast to PANI/MWCNT and PEDOT: PSS/MWCNT.

On the other hand at 0.1A/cm2, PANI/MWCNT and PEDOT: PSS/MWCNT device shows a

higher ESR of the device and hence larger voltages (IR) drop [43, 44]. Therefore minimizes the

power and capacity due to limited electrode accessibility by electrolyte ions. The internal

resistance of PEDOT: PSS /MWCT are believed to be very high due to the lack of captured data

at 0.1A/cm high charge density rate.

The ability for Ppy/MWCNT to maintain highly symmetrical shape and the higher

achieved potential shows a very fast rate charge/ discharge performance of this composite device.

The high power capability of Ppy/MWCNT composite device was again demonstrated here. This

agrees with previous findings from CV.

- 140 -

7.3.4. Cycling Stability Comparison

Figure 7.17 Discharge specific capacitance against charge/discharge cycles

Cycling life of a supercapacitor is another important factor in considering potential

device application eligibility. The cells were subjected to charge discharge under cyclic

voltammetry at moderate high scan rate of 100mV/s over 2000 cycles. The specific discharge

capacitance (F/g) over charge/ discharge cycles for different ECP/MWCNT composite devices

were summarized on figure 7.17. It is observed that all of the composite devices experienced a

loss of capacitance for the beginning first 400-700 cycles. Ppy/MWCNT device experienced

initial drop of 18.6% in capacitance in the first 100 cycles. As the number of cycles increase,

steady drop of capacitance was observed. However, the value reaches steady state at about the

700th cycle. The capacitance value stays around 52 F/g. The same observation can be seen from

PANI/MWCNT and PEDOT: PSS/MWCNT devices. The capacitance also stabilizes at 600-700

cycles. The capacitance than remain stable at about 15.6 F/g and 11.6 F/g respectively over the

- 141 -

rest of the cycles. In generally, the effect of the MWCNT in improving the cycling stability in

the three different polymers was observed.

7.3.5. Comparison with reported studies from the literature

The composites achieve highly comparable capacitive performance as compare to the

findings from the literature. As refer to table 7.10 which summarized results from the literature

was again presented here. Findings from this study was also included the capacitance value in

F/g for single electrode reported from this study was included. The reported value from this

study is highly comparable from the values reported from the literature. For Ppy/MWCNT a high

capacitance ~206.3F/g was reported at 10mV/s, which is even higher than the reported

capacitance achieved at lower scan-rates in a range of 1-5mV/s. For PANI/MWCNT composites,

the reported value at 5mV/s is ~230F/g. Which is highly comparable to the capacitance achieved

from the work on PANI composite with graphene [112] and SWCNT [98].

However, the reported capacitance value for PEDOT: PSS/MWCNT from this study is

less than the reported value at the same scan-rate as compared to the composite with activated

carbon. Moreover from the established findings, it is believed that the oxidative polymerization

method in powdered samples might not be suitable for PEDOT: PSS fabrication.

For two electrodes cell capacitance, several performance of solid-state device from the

literature was summarized on table 7.11. In comparison to the reported capacitance from this

study, the device capacitances F/g for Ppy/MWCNT and PANI/MWCNTs at 100mV/s are 62F/g

and ~44F/g respectively. A highly comparable performance to the literature was achieved.

- 142 -

Moreover, the high power characteristic up to 1000mV/s was reported. The capacitance as well

as performance under high potential scan rate was seldomly reported from other related studies.

This shows a very high performance characteristic in a packaged cell environment.

Table 7.10 Summary of some reported specific capacitance of solid- state devices from the

literature

Composite electrolyte Specific capacitance Reference

Flexible PANI/MWCNT H2SO4-PVA gel 31.4F/g (50mV/s) [192]

Cellulose-MWCNT composite 1M : Tetraethylammonium

tetrafluoroborate/ propylene

carbonate

36F/g [190]

SWCNT micro film

6M KOH

50 F/g (100mV/s)

[191]

- 143 -

Table 7.11 Specific capacitance and respective fabrication method for various ECP/carbon

composite

Polymeric materials Carbon based

Electrode

Material

Fabrication Method Electrolyte Specific

Capacitance

(F/g)

Reference

PEDOT (3,4-ethylenedioxythiophene)

Activated carbon

Direct blending of AC with 5wt% PEDOT

1 M TEABF4 120F/g @ 10mV/s

[108]

Graphene Chemical oxidative polymerization

2M HCl and 2M H2SO4

HCl: 304F/g @1mV/s H2SO4: 261F/g @1mV/s

[63]

MWCNT 1. Chemical polymerization

2. Direct blending 3. electro-chem

deposition

1M H2SO4 1. 130F/g 2. 120F/g 3. 150F/g @1mV/s

[106]

* MWCNT Chemical oxidation 1M H2SO4 50 F/g @ 10mV/s

From this study

Polyaniline (PANI) SWCNT Electrochemical polymerization

1M H2SO4 190.6F/g @5mV/s

[98]

Carbonized PANI

Carbonization of PANI fibers under ultimate temperature

30wt% kOH 163F/g@ 5mV/s

[113]

MWCNT Chemical polymerization: : K2Cr2O7 in 50 ml of 1 M HCl

1M H2SO4 320 F/g@1mV/s

[12]

Graphene Chemical oxidation of PANI onto graphene

1M H2SO4 210F/g @ 5mV/s

[112]

* MWCNT Chemical oxidation 1M H2SO4 203.5 F/g @5mV/s

From this study

Polypyrrole (Ppy) Carbon Aerogel

Chemical polymerization: FeCl3

6M KOH 477F/g @2mV/s

[37]

MWCNT Chemical polymerization: FeCl3 +0.1M/L HCl

1M H2SO4 200F/g @ 1mV/s

[12]

MWCNT Electrochemical deposition: p-toluene sulphonic acid /tiron

0.5M Na2SO4 310F/g @ 2mV/s

[114]

SWCNT Mini emulsion polymerization: SDS and 1-pentanol

1 M LiClO4 134F/g @ 20mV/s

[95]

* MWCNT Chemical Oxidation 1M H2SO4 206.61 F/g @10mV/s

From this study

- 144 -

Chapter 8 Conclusions and Recommendations

8.1. Conclusion

In this research, various contents of ECP/MWCNT composites for polypyrrole, PEDOT:

PSS and polyaniline were synthesized successfully through an in-situ polymerization process.

The successful combination of double layer and pseudo capacitance was observed from the

significantly improved capacitance from the composite electrode. Moreover, the fabricated

electrode also exhibits a very low ESR, and therefore is able to maintain a higher specific power.

It is concluded that:

The ECP to MWCNT content have significant influence to the polymerized ECP

structure and morphology. It was found that an optimal weight ratio between the CNT

and ECP can be achieved in order to generate the best charge storage performance. The

maximal achieved charge current is due to an improved interfacial interaction between

the two components.

Further addition of ECP content leads to formation of an aggregated polymer with a

heterogeneous surface morphology. As a result diminishes the polymer conductivity. The

poor interaction between ECP and MWCNT would also weaken the synergy in the

electroactivity from the composite.

A different composite formation was suggested. As reported from the literature, ECP

tends to form a thin core shell around the MWCNTs with thickness in 3-5nm the double

layer is mainly contributed to the MWCNT network. However, this study shows that

polypyrrole is grown into a tubular shape with the MWCNTs act as template for the

polymerization with a layer thickness >100nm. The formation of the porous structure is

- 145 -

due to the entanglement of the tubular shaped composite. With this morphological

structure, an equally promising capacitive performance was achieved.

The influence of dopant/oxidant to the polymerized polymer was also investigated. For

both PANI and Ppy, the performance of the DBSA/APS system is less comparable to the

composite using HCl and FeCl3 for PANI/MWCNT and Ppy/MWCNT composites

respectively.

For the PEDOT: PSS/MWCNT composite, the performance is not as outstanding as the

other two composites. The polymerization of the polymer onto MWCNTs does not form

a controlled porous structure as in PANI and Ppy/MWCNT composites. This is possibly

owing to the high film forming property of PEDOT: PSS. From the literature, PEDOT:

PSS and the composite is usually applied in thin film formation [32, 99, 100 and 117].

Therefore the approach in fabricating composite in powder form might not be a

promising way in processing the PEDOT: PSS and its composite.

Two electrodes cell evaluation shows that Ppy/MWCNT cell displays the best capacitive

characteristic in two electrodes cell in generally by coupling both high capacitive and low

resistive characteristics. A highly rectangular shape on CV up to 0.5V/s scan-rate shows

very fast rate charge propagation characteristic. AC impedance analysis indicates an in

general low resistive nature of the device in terms of materials contact and ionic transfer

properties. Galvanostatic charge/discharge test shows a promising high power capability

from the ideal charge discharge characteristic. A relatively promising cycling stability

was also achieved from this composite device in contrast to the PANI/MWCNT and

PEDOT: PSS/MWCNT composites.

- 146 -

In this study, since the focus of the study is on the electrode material performance.

Therefore, the capacitance of the materials is a great indication of the electrode

performance. Fabrication of a device requires standard packaging procedures. And the

reported energy and power density of commercial devices also account for factors

including the packaging materials, current collector as well as the electrolyte ions.

Moreover, the electrolyte ionic conductivity also plays an important role in determining

the cell resistance as well as the cell ionic conductivity, and hence the power and energy

characteristic. Based on the experimental parameters in this study, the energy and power

densities of the best sample of Ppy/MWCNT composite were estimated at 8.6 Wh/kg and

390 W/kg respectively.

Figure 8.1 Ragone plot of power versus specific energy for various electrochemical energy

storage devices. The red circle indicates the energy and power performance of the 0.02M

Ppy/MWCNT composite cell [14]

- 147 -

As refer to the ragone plot on figure 8.1, the performance of this composite cell is

marked as a red dot. The performance of the cell resides at the higher energy end of EC

region. However, the power performance resides in the middle range. It is believed that

the poor packaging of the test cell would also lead to a higher contact resistance within

the cell, therefore underestimates the true power performance under a cell environment.

8.2. Future works

In this study, from the material engineering perspective, an enhanced capacitance was

achieved through combining conductive polymer and carbon nanotubes in form of composite.

The performance was aimed to be optimized through varying the ECP to CNTS contents.

However, there are areas to be further improved from both composite materials engineering

perspective and device design perspective. In order to further improve the power performance,

and at the same time further increase the capacitance closer to the theoretical limit. The

following areas can be investigated.

8.2.1. CNTs surface manipulation

As previously discussed, parameters such as pore size, pore volumes of the carbon

materials greatly influence the materials ionic conductivity, which is related to the mobility of

ions inside the pores, and therefore the capacitance. The following practices can be employed to

manipulate the porous structure of the CNTs electrodes.

Studies involving manipulating the CNTs alignment have been conducted to improve the

capacitance of MWCNT electrode. A study from Dai .et al found that by vertically aligning the

CNT strands can control the spacing in between the tubes and therefore provide more

mesoporosity. Moreover it was claimed that this aligned structure also allow individual tubes to

- 148 -

participate in charge/ discharge process. And a higher energy density can be achieved [73]. It

would be interesting to fabricate composite through depositing polymer onto aligned CNTS. The

controlled spacing between tubes could hypothetically increase the contact surface between the

polymers with electrolyte and therefore increase the usable pseudocapacitive kinetics to achieve

higher energy density.

Functional groups can be incorporated onto MWCNT surface via various surface

treatments. Incorporation of carboxylic acid group on the CNTs surface can enhance the

capacitance up to seven times of performance was reported. It is believed that the surface treated

CNT with defects built up on the outer wall can create extra microvolumes for the storage of

charges [72 and73]. However excess aggressive acid treatment could lead to structural damage of

the CNTs surface with decreased aspect ratio [197]. However, thermal treatment as well as ozone

treatment technique can effectively improve the conductivity without damaging the structure

[197]. It is believed that the increase presence of functional group can promote electron transfer

between the carbon atoms.

8.2.2 Increasing voltage window limitation

Since the energy is proportional to the capacitance (C) and the Voltage (V). The increase

of capacitance is successfully achieved from this study, whereas the voltage window is still a

limiting factor for both carbon and conductive polymers. The power performance can also be

improved since the power is also proportional to the cell voltage. Commercially available

organic electrolytes were used in supercapacitors device in order to achieve a higher voltage

window up to 2.7V. However this electrolyte requires complicated assembly process and they

are expensive [198]. On the other hand, ionic liquid electrolyte can improve the cell voltage

- 149 -

window up to 3.5V. However, the conductivity of the electrolyte is low and has a very high

viscosity, therefore limits the specific power and high rate capability [198].

Therefore, instead of focusing solely on the electrolyte, the focus was shifted to the

electrode materials itself. A study from Susana et.al suggested that constructing an asymmetric

cell by varying the mass ratio between the cathode and anode can strategically increase the

voltage window of the cell. Her work was conducted of activated carbon supercapacitor cell. The

mass of positive electrode is greater than the negative electrode by a ratio of 2.46. The operating

voltage window was increased from 1.2V to 2V in 0.5M K2SO4 neutral electrolyte [198]. High

cycling stability with retaining more than 75% of capacitance over 10000 cycles was obtained.

Her work demonstrated that the loss of performance of the cell is mainly owing to the aging of

the positive electrode. The asymmetric mass-balancing approach can be applied to study the cell

performance with carbon nanotubes and conductive polymers composite electrodes. These

attempts have not been reported from the literature. Moreover, the investigation of acidic or basic

electrolyte should also be conducted.

8.2.3. Modeling of supercapacitor mechanisms and composite material properties

Quantitative modelling has been exploited to study and predict the performance

characteristic of supercapcitors in order to develop a design that can reach closer to the

theoretical limit. Research includes developing equivalent circuit models to capture porous

electrode behaviour, and developing empirical relationships between material pore size, surface

area and achievable capacitance. From the materials perspective, mathematical modelling can be

used to optimize the charge transfer kinetic of a capacitor cell by predicting the optimal mass

ratio between the two electrodes. In terms of composite design, the most important parameter to

- 150 -

be optimized is the proportion between the CNTs and polymer matrix. This could possibility be

predicted through computational techniques.

8.2.3.1.Empirical model

Research on developing empirical relations of materials pore size, surface area on

the capacitance and resistance properties has been conducted. It was proven that the

capacitance is proportional to specific pore size rather than the entire surface area. It is

suggested that electrolyte ions cannot diffuse into pores beneath a threshold of size. And

therefore some of the porous area is not contributed to charge storage. This helps us

understand the importance in controlling the porous structure and pore size of the

electrode materials. Further provide guidelines to determine the optimal condition for

maximizing ionic accessibility of the electrodes [4]

8.2.3.2.Equivalent circuit model

Mathematical model using fundamental circuit elements were used to model

complex electrochemical behaviours. Simple models were used to predict performance of

double layer capacitance at the porous electrode and electrolyte interface. Recently,

equivalent circuit also developed to include additional faradic effect from

pseudocapacitance to model behaviour of composite capacitive materials. Lists of models

were summarized on table 8.1[4].

- 151 -

C

Simple capacitor model

CRs

Capacitor in series with resistor which

account for the RC behaviour

RL

Rs

Simple circuit that account for capacitance,

leakage resistance as well as cell

resistance.

RL

Rs

Rs

Based on the double layer capacitor model

with addition of pseudo capacitance effect

RL RL RL RL

Rs Rs Rs

Pores

Electrode / Electrolyte interface

A transmission lines model to model a

porous electrode. Lines of C parallel with

R represent one single pore on the

electrode

Table 8.1. Summary of varies supercapacitors model and their characteristic [4]

8.2.3.3.Symmetric/ asymmetric cell capacitance optimization

Since the charge storage performance of the electrode materials is also dependent

on the amount, therefore the mass of the active materials. Mathematical functions can be

implemented to predict the optimal mass ratio between the cathode and anode. A study

was conducted by Grame et.al to study the optimal mass ratio between the cathode and

anode of a capacitor cell within both symmetric and asymmetric cell with activated

carbon AC/AC and AC/ECP (PANI/Ppy) electrode materials. It was not surprised that the

ratio for symmetrical configuration is 1 with both electrodes having equal mass and

- 152 -

therefore the same capacitance. Whereas, in asymmetric configuration, maximum

capacitance was achieved with cathode having 0.65 times higher mass than the anode

[200]. This can be employed in evaluating the system utilizing CNTs as the carbon base

material. This also provides a direction in studying the influence of cathode and anode

mass ratio in composite/ composite cell configuration in order to improve the composite

cell capacitance under an optimal electrolyte condition [200].

8.2.3.4. Atomic modelling of composites to predict composite materials properties

Computational techniques can be conducted to obtain more insights in the

material properties of the composite. Mariana et.al presents computational methods based

on molecular mechanics and dynamic to predict the mechanical properties of Ppy/CNT

composite in order to access the anticorrosion ability of the composite to the carbon steel

[201]. Software Materials Studio was incorporated to construct the 3 dimensional bulk

model of the covalently bonded Ppy/SWCNT composite with varying CNT contents.

Virtual traction test confirms an enhanced stiffness was achieved from the composite.

Highest stiffness was achieved at 10wt% of CNT. A better anti-corrosion behaviour was

also achieved from the more anodic potential achieved from this composition [201].

These results were also approved by experiments. It is believe that the same approach can

be conducted to analytically predict the best ECP/CNT composition in order to yield the

best electrical and electrochemical performance parameters required for charge storage

application. The degree of capacitance improvement from the composite could also

possibility be predicted through this method.

- 153 -

8.2.4. Device design and functionability study

To further conduct studies on device design and function perspectives:

The promising performance of the ECP/MWCNT composite in two electrodes cell was

demonstrated in this study. These composite electrodes can be used to develop flexible

solid state device by employing solid state electrolyte (e.g. with Poly Vinyl alcohol (PVA)

based solid electrolyte (SiWA-H3PO4 –XLPVA Gen3a XL [192])

Incorporate different current collectors to increase design flexibility: For example

titanium foil, flexible graphite sheet as current collector. Polydimethylsiloxane (PDMS)

can be utilized as a polymeric backbone to provide high flexibility as well as compliance

to the electrode film material [190].

Study possible influence due to environmental factors such as temperature, stress to the

cell capacitive performance

8.2.5. Doping nature of conductive polymer investigation

Conductive polymers are classified as cationic and anionic [31], therefore the doping

nature is either p-doped (positively charged) or n-doped (negatively charged). Polypyrrole and

polyaniline are reported to be P-doped type where as PEDOT can exhibits both p-doped and n-

doped [31]. However, the functionability of Ppy in alkaline electrolyte media raised question on

the doping nature of the polypyrrole. There are study reported capacitance achieved from Ppy

electrodes with KOH electrolyte, however, there are also reports that pH have influence on the

polymer structure. In alkaline solution, OH- ions would attract the pyrrole ring and causing

irreversible degradation of π-conjugates of the polypyrrole back bone [199]. Therefore an

investigation can be further conducted to study the performing condition of polypyrrole.

- 154 -

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Appendices

Appendix A: Electric Double Layer Theory

Appendix B: Sample of simulation on Z-view to obtain the fitting parameters

for the generalized circuit of the composite supercapacitor

electrodes for (0.02M) Ppy/MWCNT composite cell on

Appendix C: Two electrodes PEDOT:PSS/MWCNT electrochemical

Evaluation

Appendix D: Two electrodes PANI/MWCNT electrochemical Evaluati

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Appendix A: Electric Double Layer Theory

The very first Helmholtz double layer model suggests an absorbed compact layer of ions

is concentrated on the electrode surface (Fig. A. 1 A). And the stored charge is linearly

dependent to the voltage. However this model neglected factors such as ionic diffusion in the

electrolyte and the interaction beyond the first layer of absorbed ions [27, 29, and 30].

A refined model includes a diffusive layer which takes the ionic concentration and ions

thermal motion into account. In the Gouy-Chapman model, no distinct/ solid layer of charges is

formed between the electrode/ electrolyte interface. The voltage decays exponentially as charges

diffuse away from the electrode surface. However, this model predicts the ions as point charge

which is not the case in real life scenario. Moreover, issue such as hydration force from the

solvent, as well as overlapping problem occurred on a porous surface were neglected [27, 29]. In

Stern and Grahame‘s model, they considered that ions do have finite size with the charge located

at the center. Therefore it is suggested that the double layer is formed by two parts, which

combines a compact layer (Stern layer) and a diffusive layer of charges as suggested from the

Gouy-Chapman model (Fig. A. 1 B) [27,29,30]. Therefore the capacitance is the sum of the two.

(5)

*CHm= Capacitance due to the Helmoltz layer

*CGC = Capacitance due to the diffusive layer

The most commonly used model nowadays is known as the Bocris, Devanathan, and

Muller BDM model (Fig. A. 1 D). The model includes the action of solvent/ water molecules on

the electrode surface. The charged surface causes a fixed dipole within the water molecules and

therefore leads to a fixed alignment. The model is composed of three layers, the inner Helmoltz

- 169 -

plane (IHP) which is filled with the solvent molecules, the second layer is known as the outter

Helmoltz plane (OHP) where the counter ions are located. And the third region is known as the

diffusive layer [27, 29 and 30].

Vo

V

A) Helmholz double layer model

Vo

V

B) Gouy-Chapman double layer model

Vo

V

C) Stern double layer model

OHPIHP

Ions with hydrated shell

Ions with hydrated shell

D) Bocris, Devanathan, and Muller

(BDM)

Figure A. 1 Electric double layer models a) Helmholtz model, b) GouyChapman model, c) Stern

and d) Bocris, Devanathan, and Muller (BDM) [27,30]

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Appendix.B: Sample of simulation on Z-view to obtain the fitting parameters

for the generalized circuit of the composite supercapacitor

electrodes for (0.02) Ppy/MWCNT composite cell

Figure B. 1 showing the nyquist plot (Left) and the bode plot of phase against frequency (Right)

for 0.02M Ppy/MWCNT composite electrode. Red curve represent the curve generated from

original data, whereas the green curve represents the generated from the equivalent circuit as

shown below. The corresponding value of the circuit elements are summarized on the table

above

- 171 -

Appendix C: Two electrodes PANI/MWCNT electrochemical Evalaution

C.1. Cyclic Voltammetry

Figure C. 1 CV plot contrasting performance of PANI/MWCNT composites devices and pure

PANI, MWCNT device at a)10mV/s, b)50mV/s,c)100mV/s,d) 500mV/s & e)1V/s

A B

C D

E

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Figure C. 2 Specific capacitance (F/cm2) as a function of scan-rate from 10mV/s to 1V/s

In two electrodes setup, CV was performed at potential scan- rate from 10mV/s to 1V/s at

voltage window 0V to +0.9V. Figure C.1 presents the CV plots from scan-rate 10mV/s to 1V/s.

These figures show the gradual changes of the current density/ voltage characteristic as scan-rate

increases. The 0.1M PANI/MWCNT composite device shows very outstanding capacitive

performance from the larger coverage area of curve. At 10mV/s (Figure C.1.A), a well

rectangular shape with single pair of peaks was observed. The defined rectangular shape of the

CV curve was maintained as scan-rate rises to 50mV/s (Figure C.1. B). At 100mV/s (Figure

C.1 .C), the parallelogram in shape indicates initial build up of internal resistance at this high rate.

But the distortion is still minimal this shows the capability for the 0.1M PANI/MWCNT

supercapacitor to maintain promising propagation to the porous surface up to 100mV/s high

potential ramp rate environment. Similar performance is displayed from the 0.05M, 0.3M and

0.5M PANI/MWCNT composites. However, as compared to 0.1M PANI/MWCNT, the smaller

- 173 -

size of the CV plot also indicates weaker charge storage ability from these three composite cells.

For instance, 0.05M cells display a higher rectangular in shape at 100mV/s scan rate. This is

again due to the higher contribution of the double layer for charge storage in the composite.

When the scan-rate is raised to high level at 500mV/s and 1V/s (Figure C.1. D), distortion

of the CV curves indicates an increase in resistance of the system as scan-rate increases for all of

the composite cells. This is due to limiting ionic accessibility of the electrode from the

electrolyte at the high scan-rate [99,128]. This phenomenon was shown from the measured

specific capacitance on Figure C.2. A huge drop of capacitance was observed when reaching

500mV/s scan-rate and onwards. However, for 1M PANI/MWCNT, the performance remains

poor from the highly irregular shape of curve regardless of the scan rate.

The specific capacitance against scan rates was plotted on Figure C.2. Again highest

capacitance was achieved from 0.1M PANI/MWCNT composite cell. The high rate performance

can be deduced from the linearity of curve. MWCNT cell is very linearity which resembles high

power deliver ability by maintaining the capacitance as scan rate goes up. When we look at the

composite cell, 0.05M displays very high power capability which is close to pure MWCNT cell.

However from 0.1M to 1M PANI/MWCNT, the drop in the linearity is a big indication of

decreased power performance with increasing PANI content. Moreover, from the appearance of

the REDOX peak on CV and the poor performance at high scan rate (>100mV/s) also indicates

the dominance of pseudo capacitive activity due to the doping/de-doping of PANI in the

composite with PANI content greater than 0.1M [99]. The finding suggested that, the

- 174 -

introduction of PANI at an optimal level can improve the capacitance and therefore energy

density. However, there is a trade off in the device power performance.

C2. Electrochemical impedance Spectrometry

Figure C. 3 Negative Phase angle plot against

frequency in logarithmic scale which shows

phase angle close to 90 at low frequency

Figure C. 4 Bode plot of negative phase vs

frequency (Hz) for MWCNT, PANI/MWCNT and

pure PANI cell. The specified frequency @ -45o

indicates the point where resistance to

capacitance transition occur

Table C. 1 Parameters of the equivalent circuit model for different composites devices (1 cm2)


Composite device R1 (ESR) R2 (Rct) CPE1-T CPE1-P CPE2-T CPE2-P

MWCNT 0.39 0.25 0.00025 1.088 0.0078 0.78

PANI 0.92 1.67 0.00056 0.74 0.0104 0.54

0.05M PANI/MWCNT 0.65 3.07 0.00041 0.39 0.032 0.77

0.1M PANI/MWCNT 0.81 0.33 0.00012 1.27 0.060 0.75

0.3M PANI/MWCNT 0.65 0.48 0.00028 0.84 0.048 0.66

0.5M PANI/MWCNT 0.79 0.53 0.00101 0.82 0.026 0.63

1M PANI/MWCNT 0.69 1.19 0.00078 0.96 0.016 0.64

- 175 -

Figure C. 5 Specific capacitance C” (F/g) vs

frequency (Hz) with time constant values

indicated for MWCNT, Ppy and PANI/MWCNT

cell.

Figure C. 6 Specific capacitance C’ (F/g) vs

frequency (Hz) for MWCNT, Ppy and

PANI/MWCNT cell.

Table C. 2 Summarized findings from electrochemical impedance data

Electrodes ESR (Ω) R2 (Ω) Peak C” Corresponding

frequency (Hz)

RC Time

constant (s)

C’ ( Specific

capacitance

F/cm2)

Frequency

at -45o

phase (Hz)

Phase

angle

MWCNT 0.39 0.249 1.83E-03 1.18e-01 ~0.17 0.0072 17.3 -75.6 o

PANI 0.92 1.67 1.35E-3 0.374 ~2.67 0.0052 12.02 -76.4 o

0.05M PANI/MWCNT 0.65 3.072 6.37E-3 2.1 ~0.48 0.0205 3.09 -79.5 o

0.1M PANI/MWCNT 0.81 0.332 1.34E-2 6.63 ~0.25 0.049 20.8 -82.2 o

0.3M PANI/MWCNT 0.65 0.48 1.02E-2

2.1 ~0.476 0.0393 5.48 -82.6 o

0.5M PANI/MWCNT 0.79 0.53 8.81E-3 3.09 ~0.323 0.0265 2.55 -80.4 o

1M PANI/MWCNT 0.69 1.19 7.59E-3 2.1 ~0.476 0.0253 4.53 -80.4 o

EIS was performed from 100 kHz to 10 MHz at operating voltage of 5mV. The nyquist

plot was shown on Figure C.3. Pure MWCNT cell shows very ideal EDLC behaviour from the

very straight and linear nyquist plot. For the composites, the extended semi-circle portion

increases as the PANI content in the composite increases which indicates the charge transfer

0.1M : 0.25s

MWCNT: 0.17s

PANI: 2.67s

0.05M : 0.476s

0.5M : 0.323s

0.3M : 0.476s

1M : 0.476s

- 176 -

activity due to increasing PANI content. The estimated resistance and CPE parameters from the

generalized circuit were summarized on Table C. 2. The high capacitive performance of the

0.1M PANI/MWCNT device was shown from the small semi circle portion as well as the

straight and less inclined low frequency tail. This is also indicated from the high CPE2-P and

CPE2-T values, as well as the highest achieved phase angle -82.8o. The highest achieved

capacitance by 0.1M PANI/MWCNT at low frequency is 0.0597 F/cm2 (CPE2-T). The increase

in Rct and ESR, as well as further decline of CPE2-T values again indicates the drop of

capacitive performance with PANI content increase from 0.1M to 1M in the composites.

A more in-depth analysis of the low frequency capacitive behaviour was conducted

through the complex capacitance plot on Figure C.5 and Figure C.6. The specific capacitance

measured from complex capacitance was listed on Table C. 2. Once again highest capacitance

was achieved from 0.1M PANI/MWCNT at 0.0489 F/cm2, the decreasing capacitance trend was

again observed here the capacitance was dropped to 0.0253 F/cm2 at 1M PANI/MWCNT. For

the measured time constant, the values were summarized on Table C. 2. The discharge responds

were very close among the composites which are within the range from 0.15 to 0.4 seconds.

Since time constant =RC, where R is the ESR of the cell. Among the composite cells, the

outstandingly high capacitance of 0.1M PANI/MWCNT also implies a very low cell resistance

for this composite device in contrast to the others. The drop of performance with increasing

PANI content demonstrated from the EIS study is very consistent to the finding from CV.

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C3.Galvanostatic Charge/discharge evaluation

Figure C. 7 Charge/discharge curve of pure PANI, MWCNT and PANI/MWCNT supercapacitor

at 0.001A/cm2

Table C. 3 Coulomb efficiency and specific capacitance value @0.001A/cm2

Electrodes Charge

time(s)

Discharge

time(s)

Coulomb efficiency (%) Specific Capacitance @

0.001A/cm2 (F/cm

2)

PANI 5.3 3.1 58.4% 0.0053

MWCNT 5 4.5 90% 0.0061

0.05M PANI/MWCNT 35.8 26.4 73.7% 0.0558

0.1M PANI/MWCNT 39.5 29.4 74.4% 0.059.6

0.3M PANI/MWCNT 36.5 27.45 75.2% 0.0513

0.5M PANI/MWCNT 36.2 26.3 72.65% 0.0498

1M PANI/MWCNT 29.92 22 73.5% 0.0387

The galvanstatic charging and discharging mechanism of the PANI/MWCNT

supercapacitor in 1 M H2SO4 electrolytic media. Figure C.7. displays the C/dC curve under

0.001A/cm2

current density. From the curve, no IR drop at the beginning or at the switching

- 178 -

point was observed. This indicates a very minimal resistance contributed to the ions transport

due to the high conductivity of the electrolyte. Pure MWCNT and 0.05M C/dC curve exhibit

very high symmetry and linearity suggest a very low internal resistance due to highly efficient

EDL and excellent charge propagation across the electrodes [62].

As the PANI content was increased to 0.1M, 0.3M and 0.5M, the longer elapse time of

the charge discharge indicates an enhanced capacitive performance [176]. However, a very

minor decrease of curve linearity was also observed. At the voltage switching point (the peak

point), the minor bending of curve is due to resistance attributed to concentration polarization

aroused from general mass transport of the system [5]. Minimal bending indicates efficient ionic

diffusion characteristic due to promising ionic accessible electrodes surface. At the middle

portion of the curve, any bending of the curve indicates the ESR of the system [5]. Lastly, the

bending of the end point is attributed to activation polarization related to kinetic of any presence

of REDOX reaction [5]. For composites with PANI 0.1M to 0.5M, minor bending at the tail was

observed indicates increased resistance due to the diffusion limited pseudo capacitive process of

PANI.

At 1M PANI/MWCNT device, further reduction of the capacitive performance can be

observed from the huge degradation of curve linearity at the middle portion and the reduced

elapse time. The calculated coulomb efficiency and specific capacitance were summarized on

Table C. 3. Again an improved capacitive performance in contrast to bare MWCNT and PANI

were observed here. Up to 90% efficiency was achieved by MWCNT EDLC as expected. The

PANI/MWCNT composites in generally achieves efficiency between 73% to 75%. With regards

to the capacitance, the performance is 0.1M > 0.05M > 0.3M >0.5M >1M> MWCNT> PANI.

- 179 -

Maximum capacitance was again achieved by 0.1M PANI/MWCNT. The degradation of C/DC

efficiency and charge storage performance as PANI content increases was gain observed from

this study.

C.4. Cycling stability Test

Figure C. 8 CV plot of 0.1M PANI/MWCNT cell and pure PANI cell at the 1st cycle and the

2000th

cycle

Figure C. 9 Capacitance change over 2000 cycles for 0.1MPANI /MWCNT (Left) and pure

PANI cells respectively (Right)

- 180 -

The cycling stability test was done on Pure PANI and the 0.1M PANI/MWCNT

composite cells. The cells were cycled over 2000 cycles from Cyclic Voltammetry at 100mV/s

scan rate. Figure C.8 shows the first and the 2000th

cycles for PANI/MWCNT and PANI cells.

The distortion of both CV curves indicates degradation of charge/discharge performance over

2000 cycles. The charge and discharge capacitance at every 100 cycles was plotted against the no.

of cycles on figure C.9. For pure PANI cell, gradual drop of capacitance over 2000 cycles

indicates the poor stability over time of pure PANI as a supercapacitor device. Regarding 0.1M

PANI/MWCNT composite cell, a drop of capacitance occurs after the first 300 cycles. About a

loss of 20% was observed. But then after the 300th

cycles and onwards, the capacitance becomes

stabilized at about 0.0193F/cm2. This shows that the introduction of MWCNTs in the composites

cell greatly improve the cycling stability by compensating the swelling issue of the ECP due to

the doping process.

- 181 -

Appendix D: PEDOT:PSS/MWCNT Two electrodes Cell Electrochemical

Evaluation

D.1. Cyclic Voltammetry (CV)

Figure D. 1 Cyclic Voltammetry at A)10mV/s, B) 50mV/s,C)100mV/s, D)500mV/s, E)1V/s for

MWCNT, pure PEDOT: PSS, and PEDOT: PSS/MWCNT composites

A B

C D

E

- 182 -

Figure D. 2 Variation in the capacitance PEDOT:PSS and PEDOT:PSS/MWCNT devices as a

function of potential scan rates for a) charge and b) for discharge

The performance of the supercapacitor device was first evaluated by mean of CV. The

evaluation was performed under varying potential scan rates from 10mV/s to 1V/s. Figure D.1.

shows the CV plots for PEDOT: PSS and PEDOT: PSS/MWCNT composites at 10mV/s,

50mV/s, 100mV/s, 500mV/s and 1V/s respectively.

For PEDOT: PSS cell, at low scan rate 10mV/s, a defined oxidation peak occurs at 0.1V.

As scan-rate increases, the peak is less pronounced as the doping and de-doping of conductive

polymer is a highly diffusion dependent process [76, 98]. Without the presents of MWCNT, pure

PEDOT: PSS cell shows a poor charge propagation properties with high internal resistance from

the non-rectangular shape of the CV curve. Moreover, a high imbalance of charge and discharge

portion indicates a significantly low charge/discharge efficiency of the pure PEDOT: PSS device.

- 183 -

For the PEDOT: PSS/MWCNT composites, composite cells at 0.025M and 0.05M

PEDOT: PSS contents resemble a rectangular shape from scan rate 10mV/s up to 100mV/s; this

shows good charge propagation behaviour with fast diffusion of ions into the PEDOT:

PSS/MWCNT composite with low resistance at the interface [76, 98]. However, as the PEDOT:

PSS content was increased to 0.1M in the composite, a degradation of performance was observed.

The CV plots of PEDOT: PSS (0.1M)/MWCNT composites shows characteristics very close to

pure PEDOT: PSS displaying high ohmic response and poor efficiency. With the presents of

MWCNTs, the PEDOT: PSS (0.1M)/MWCNT composite shows a better efficiency and

capacitive performance than the pure PEDOT: PSS cell.

Figure D.2 presents the discharge specific capacitance F/cm2

at varying scan rates from

10mV/s to 1V/s. High power capability was again observed from pure MWCNT device. For the

composites at PEDOT: PSS 0.025M and 0.05 M concentrations. After an initial decrease of

capacitance from 10mV/s to 50mV/s, the capacitance is relatively stable showing the good power

capability for the capacitor to cycle under high scan rates up to 500mV/s. The significantly poor

power property for PEDOT: PSS can be seen from the huge drop of capacitance from 10mV/s to

50mV/s. PEDOT: PSS (0.1M)/MWCNT resemble the same property and showing the poor

power performance .As agreed to the finding from CV test, PEDOT: PSS (0.05M)/MWCNT cell

shows the best performance from the highest discharge capacitance which is 0.029F/cm2

at

10mV/s.

- 184 -

D.2. Electrochemical impedance spectrometry (EIS)

Figure D. 3 Nyquist plot of –Z” vs Z’ of 0.1M

PANI/MWCNT supercapacitor. 0kHz to

10mHz)

Figure D. 4 Bode plot of negative phase vs

frequency (Hz) for MWCNT, PEDOT: PSS

/MWCNT and pure PEDOT:PSS cells. The

specified frequency @ -45o

indicates the point

where resistance to capacitance transition

occur

Table D. 1 Parameters of the equivalent circuit model for each cell (1 cm2) derived from

numerical fitting of experimental data acquired from nyquist plot


MWCNT 0.3904 0.00025147 1.088 0.249 0.0077769 0.77953

PEDOT:PSS 0.631 4.112E-7 1.75 2.4 0.073 0.325




Figure D.3 shows the nyquist plot’s high frequency portion of the plot. The low

frequency tail of all the plots form a 45o incline to the real axis, this indicates the existing

diffusion problem in the device attributed to the frequency dependent ionic diffusion from

electrolyte to the electrode surface due to the PEDOT doping/dedoping [9, 12, 34].

4.53Hz 17.4Hz

-45o

- 185 -

As compared to other composites, PEDOT: PSS (0.025)/MWCNT and PEDOT: PSS

(0.05)/MWCNT low frequency end are more parallel to the imaginary axis indicates a relatively

better capacitive behaviour. While PEDOT: PSS and PEDOT: PSS (0.1M)/MWCNTs cells

shows a relatively poor performance from the flatter slope at the low frequency region. Table D.

1 shows the parameters generated from numerical fitting of impedance data to the equivalent

model. PEDOT: PSS (0.05)/MWCNT display both minimal Rct (1.21Ω) and CPE2-T value

(0.115 F/cm2) which indicates the relatively higher capacitive performance. Both decrease of

CPE2-T and CPE2-P, along with the rise of Rct values as PEDOT:PSS further increases again

indicates build up of resistance and lead to a poor capacitive performance which agrees with

findings from CV. However, the CPE2-P values are all below 0.5. This shows that the composite

devices behave far from ideal EDLC capacitors due to the severe diffusion hindrance.

The bode plot of phase against log frequency is presented on Figure D. 4. MWCNT cell

exhibits high capacitive properties from the highly achieved phase angle which is -75 degree. In

generally all the composites exhibit phase angle lower than pure MWCNTs cell and being far

from reaching -90. This shows that the PEDOT: PSS / MWCNT composite cell does not exhibit

ideal EDLC performance. It is also indicated that PEDOT: PSS (0.05M)/MWCNT cell shows a

relatively better capacitive performance among the composite cells from the highest achieved

phase angle -74.7 degree at low frequency among the composites. The poorer capacitive

performance of PEDOT: PSS (0.1M)/MWCNT cell was also indicated from the low achieved

phase angle which is -62o.

The frequency intersects with -45o

phase indicates the frequency where transition to

capacitive behaviour occurs. MWCNT shows the highest frequency again indicates the highly

capacitive characteristic. For the composite, PEDOT: PSS (0.05M)/MWCNT, the frequency is

- 186 -

the highest (4.53Hz) which again indicates the higher capacitive characteristic among other

compositions.

Figure D. 5 Specific capacitance C” (F/g) vs

frequency (Hz) for MWCNT, PEDOT:PSS and

the PEDOT: PSS /MWCNT composite cells.

Figure D. 6 Specific capacitance C’ (F/g) vs

frequency (Hz) for MWCNT, PEDOT:PSS and

the PEDOT: PSS /MWCNT composite cells

Complex capacitance plot was used to further study the capacitive behaviour at low

frequency end. C’ and C” against frequency were plotted on Figure D. 5 and Figure D. 6. From

figure D.6. The C’ values at the lowest frequency end shows the cell specific capacitance (F/cm2).

From the plot it shows that 0.1M PEDOT: PSS and pure PEDOT: PSS achieves higher

capacitance. However this is not accurate, since from the low CPE2-T value (>0.4), we can

deduce that the capacitance is mainly due to the diffusion of charge carrier. This was proven

from CV study. If we look at C” vs frequency, the peak capacitance did not occur for both

PEDOT: PSS and 0.1M PEDOT: PSS/MWCNT within the frequency range. It means that the

true capacitive behaviour did not occur within the frequency range.

- 187 -

The important parameters were summarized on table D.2. The poor performance is

indicated from the very high RC time constant for both PEDOT: PSS and 0.1M (>8.5s). For the

0.025M and 0.05M PEDOT: PSS/MWCNT composites, although the specific capacitance of

0.05M is slightly lower than that of 0.025M. However, a more promising performance was

achieved from 0.05M as can be seen from the fast time constant (0.48s), lower Rct (1.21 Ω) and

higher CPE2-P value.

Table D. 2 Summarized important findings from electrochemical impedance study for PEDOT:

PSS/MWCNT

Composite

device

ESR(Ω) Rct

(Ω)

Peak

C”(F/g)

Correspondin

g frequency

(Hz) to C”

max

RC time

constant

(s)

C’

( Specific

capacitance

F/g2)

Phase

Angle

frequency (Hz) at

-45o

MWCNT 0.3904 0.249 1.83E-03 5.88 ~0.17 0.00715 -75.6o 15.3Hz

PEDOT:PSS 0.631 2.4 0.032 0.118 >8.5 0.052 -60.8o 0.977Hz

PEDOT:PSS

(0.025M)/MWCNT

0.69 1.232 0.0105 0.118 >8.5 0.029 -70.1o 3.09Hz

PEDOT:PSS

(0.05M)/MWCNT

0.715 1.210 0.00716 2.11 ~0.48 0.024 -74.7o 4.53Hz

PEDOT:PSS

(0.1M)/MWCNT

0.622 2.36 0.0361

0.118 >8.5 0.068 -62o 0.977 Hz

- 188 -

D.3. Galvanostatic charge/discharge test

Figure D. 7 Charge/discharge curve of PEDOT/PSS, MWCNT and PEDOT: PSS/MWCNT

composites supercapacitors. (0.001A/cm2 current density, 0V to +0.8V)

Table D. 3 summarized charge time, discharge time, coulomb efficiency% and specific

capacitance (F/cm2) @ 0.001A/cm

2

Electrode tc (s) td (s) Coulomb efficiency

(%)

Specific capacitance

(F/cm2)

MWCNT 5 4.5 90% 0.0061

PEDOT:PSS 42.6 18.7 43.4% 0.014

PEDOT (0.025M):PSS /MWCNT 19.9 14 70.4% 0.0175

PEDOT(0.05M):PSS /MWCNT 23.4 19.5 83.3% 0.026

PEDOT(0.1M):PSS /MWCNT 30 11.2 37.3% 0.0225

Figure D. 7 shows the charge/discharge characteristic of the MWCNT, PEDOT: PSS and

the PEDOT: PSS/MWCNT composites supercapacitor cells in 1M H2SO4 electrolytic media.

Again, the absent of IR drop indicates the high enough conductivity of the electrolyte. The

MWCNT cell shows a relatively fast charge/ discharge characteristic due to the highly ions

accessible porous surface. But then the capacitance is relatively low which is only 0.006 F/cm2 is.

- 189 -

PEDOT: PSS cells displayed a very low linearity and symmetry between charge and discharge

portion which indicates a very large internal resistance and poor C/dc performance. For PEDOT:

PSS (0.025M) /MWCNT and PEDOT: PSS (0.05M) /MWCNT composites, the high linearity

and symmetry indicate improved charge/discharge efficiency compared to the PEDOT: PSS cell.

No ohmic drop was observed neither at the beginning nor at the switching point. Once again the

C/dc characteristic of PEDOT: PSS (0.1M)/MWCNT is similar to the pure PEDOT: PSS /

MWCNT indicates the drop of efficiency with increasing PEDOT: PSS content. The high

degree of bending at the end of the discharge point indicates a high resistance due to activation

polarization caused by the doping / dedoping kinetics of PEDOT: PSS [5].

The charge/ discharge efficiency was increased from 43.4% for bare PEDOT: PSS cell to

70.4% and 83.3% for PEDOT: PSS (0.025M) /MWCNT and PEDOT: PSS (0.05M) /MWCNT

composite respectively. This indicates the introduction of MWCNTs in the composite effectively

improves the composite charge/discharge and capacitive performance. As the PEDOT: PSS

content increases to 0.1M, it is observed that the linearity of curve greatly decreases. A

significant drop of performance with increasing resistance was observed. The efficiency was

dropped to 37.3%. Optimal performance was achieved at 0.05M. The measured capacitance for

PEDOT: PSS (0.05M)/MWCNT cell is achieved at 0.026F/cm2, which is comparable to the

previous findings from CV at the same current density.

- 190 -

D.4. cycling stability test

Figure D. 8 CV plot of PEDOT: PSS 0.05M/ MWCNT (Left) and pure PEDOT: PSS cells (Right)

at the 1st cycle and the 2000

th cycle (100mV/s, 1m H2SO4)

Figure D. 9 Capacitance change over 2000 cycles for PEDOT: PSS 0.05M /MWCNT cell (Left)

and PEDOT: PSS (Right) cell respectively

From previous study, PEDOT: PSS (0.05M) /MWCNT composite cell shows the best

electrochemical performance. Therefore, the cycling stability test was performed on PEDOT:

PSS (0.05M)/MWCNT and PEDOT: PSS cells to compare the stability performance between the

composite and bare PEDOT: PSS polymer.

CV was performed over 2000 cycles at 100mV/s scan rate. Figure D. 8 shows the CV

plot before and after 2000 cycles for PEDOT: PSS (0.05M)/MWCNT and PEDOT: PSS cells

- 191 -

respectively. The CV of the composite cell shows built up of resistance from the distortion of the

curve after 2000 cycles. While the rectangular shape was still maintained, the degradation of

capacitive performance was relatively minor. For pure PEDOT: PSS cell, in general it achieves

much lower capacitance then the composite cell as expected. A lower capacitive performance

was shown as expected. A steady drop of capacitance over 2000 cycles was observed.

Figure D. 9. presents the specific capacitance (F/cm2) change over 2000 cycles. For

PEDOT: PSS (0.05M): MWCNT composite cell. After 2000 cycles, the capacitance was reduced

from 0.0176 F/cm2 to 0.0143F/cm

2 which is about 17.6% drop of capacitance after 700 cycles.

But then after the 700th

cycles and onwards, the capacitance becomes stabilized at about

0.0143F/cm2

level. This shows that the introduction of MWCNTs in the composites cell

improves the device cycling stability.

While for pure PEDOT: PSS cell, gradual drop of capacitance over 2000 cycles indicates

the poor stability over time of pure PEDOT: PSS as a supercapacitor device. Result shows that

the PEDOT:PSS/MWCNT composite shows a relatively more promising capacitive performance

over time with both higher achieved capacitance and minor loss of capacitance after long

charge/discharge cycle

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