development, characterization, and prototyping of ultra ... · sherif ramadan, adam pearson, kyle...

Development, Characterization, and Prototyping of Ultra

Flexible Thin-Film Electrochemical Energy Storage Devices

by

HaoTian Shi

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Department of Mechanical and Industrial Engineering

University of Toronto

© Copyright by HaoTian Shi 2016

ii

Development and Characterization of Ultra Flexible Thin-

Film Electrochemical Energy Storage Devices

HaoTian Shi

Master of Applied Science

Department of Mechanical and Industrial Engineering

University of Toronto

2016

Abstract

With the rapid development flexible electronics, there is a demanding need for fully flexible,

lightweight energy storage systems with high energy and power densities. Herein, two major types

of novel hybrid composite electrodes for electrochemical capacitors (ECs) were studied, namely

substrate supported composite powder electrodes and fully flexible standalone electrospun core-

shell fiber electrodes. For hybrid composite powder electrodes, graphene nanoplatelets

(GnPs)/polyaniline (PAni), multi-wall carbon nanotubes (MWCNT)/PAni, aluminum oxide

(Al2O3)/PAni, and titanium dioxide (TiO2)/PAni, were created with in-situ chemical

polymerization for optimized electrochemical performance. The electrospun core-shell structured

electrodes were made using polyethylene terephthalate (PET) from recycled beverage bottles as

the core material in combination with PAni and GnPs shells in forming high specific surface area

three-dimensional networks to facilitate efficient ion transfer ensuring ideal charge storage

behaviours along with improved mechanical properties. Finally, working EC prototypes were

constructed and characterized with various electrolyte systems and novel composite current

collectors.

iii

Acknowledgments

I would like to first acknowledge my thesis supervisor Dr. Hani E. Naguib for his continued

support and guidance he has provided me during my MASc. studies. His unparalleled scientific

knowledge in polymer composites and smart materials were invaluable to the completion of my

thesis. It was a an exciting two years that have given me a chance to experience working with

companies on real-world challenges, in addition to gaining a foothold in the scientific community

by presenting my work in several conferences. Dr. Naguib has guided me through some of the

most difficult decisions both in my studies and life in general. He was both a great supervisor and

a wonderful teacher. I would like to thank Natural Sciences and Engineering Research Council of

Canada (NSERC), Canada Research Chairs (CRC), Canada Foundation for Innovation (CFI),

Ontario Centres of Excellence (OCE), and Barbara and Frank Milligan Foundation for the financial

support they have provided.

I would also like to say thanks to my amazing colleagues at the Smart Polymers and Composites

Lab (SAPL), namely Gary Sun, Nazanin Khalili, Eunji In, Sharon Li, Farooq Al Jahwari,

Mohammad Anwer, Ali Anwer, Mohammed Kshad, Anastasia Wickeler, Arturo Reza Ugalde,

Sherif Ramadan, Adam Pearson, Kyle Eastwood, Reza Rizvi, and Shahrazad Ghaffari. Everyone

here has such positive energy that constructed a progressive environment that fostered potential

scientific breakthroughs. They have provided me with important insights into solving difficult

problems. I would also like to thank Dr. Olivera Kesler and the members of her Fuel Cell Materials

and Manufacturing Laboratory for providing access to their labs for product characterization. In

addition, I would like to thank Dr. Keryn Lian, and her student George Wu for providing me with

additional advices for my project. I would also want to acknowledge exchange students and

undergraduate thesis students who worked with me on this project, namely Justine Dumas, Marine

Bonnevide, Catherine Solis, Akshay Seshadri, and Toby Zhou.

Special thanks goes to my fantastic colleagues and best friends, Gary, Nazanin, and Eunji for being

there for me every inch of the way. Last but not least, a special thank-you to my parents Jing Sun

and Scott Shi for providing me with support and encouragement every day of my life. Also I would

like to thank my grandparents and other family members whom provided tremendous guidance for

me.

iv

TABLE OF CONTENTS

.....................................................................................................................1

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

1.2 Problem Statement & Motivation ........................................................................................2

1.3 Thesis Objectives .................................................................................................................3

1.4 Thesis Organization .............................................................................................................4

...................................................................................5

2.1 Introduction ..........................................................................................................................5

2.2 The Theoretical Basis of Electrochemical Capacitors .........................................................5

2.2.1 Faradaic vs. Electrostatic Charge Storage ...............................................................6

2.2.2 Cell Representation ..................................................................................................7

2.2.3 EC Compared to Batteries .......................................................................................8

2.2.4 A Comparison of ECs to other Energy Storage Systems .......................................11

2.3 EC Electrodes Materials ....................................................................................................13

2.3.1 Electric Double Layer Capacitance (EDLC) Electrodes .......................................13

2.3.2 EDLC Materials .....................................................................................................14

2.4 Pseudocapacitance .............................................................................................................16

2.4.1 Overview ................................................................................................................16

2.4.2 Materials for Pseudocapacitance ............................................................................17

2.5 Hybrid Electrochemical Supercapacitors ...........................................................................21

2.5.1 Asymmetrical Hybrid EC Cell ...............................................................................22

2.5.2 Composites Hybrid EC Electrodes ........................................................................22

2.6 Pores & Capacitance Relationships ...................................................................................23

2.7 Energetics of Kinetics in Electrode Processes ...................................................................24

2.7.1 Faradaic REDOX Reactions Involving Electron Transfer.....................................24

2.7.2 Ionic Energy ...........................................................................................................24

2.8 3D Structures for Improved Capacitance ...........................................................................25

2.8.1 Carbon Nanotubes (CNT) as Growth Template ....................................................25

2.8.2 Electrospinning ......................................................................................................26

2.9 Electrochemical Capacitor Evaluation Methods ................................................................27

2.9.1 Cyclic Voltammetry (CV)......................................................................................27

2.9.2 Electrochemical Impedance Spectroscopy (EIS) ...................................................28

v

2.9.3 Charge/Discharge Behaviour .................................................................................29

2.9.4 2-Electrodes & 3-Electrodes Construction ............................................................29

2.10 Electrolyte and Charge Transfer Kinetics .........................................................................30

2.10.1 Overcharging Behaviours of Electrochemical Capacitors .....................................30

2.10.2 Polymer Electrolyte Overview ...............................................................................30

2.11 Proton-Conducting Polymer Electrolytes .........................................................................31

2.11.1 Proton-Conducting Mechanisms ............................................................................31

2.12 Polymeric electrolytes and enabled supercapacitors .........................................................33

2.13 Flexible EC Electrodes ......................................................................................................33

2.13.1 Compressible Electrodes ........................................................................................33

2.13.2 Paper-based Electrodes ..........................................................................................35

2.13.3 Flexible Substrate Supported Electrodes ...............................................................37

2.14 Supercapacitor Devices and Its Current Applications ......................................................38

..................................................40

ABSTRACT ..............................................................................................................................40

3.1 Introduction ........................................................................................................................41

3.2 Experimental ......................................................................................................................41

3.2.1 Materials ................................................................................................................41

3.2.2 Methodology ..........................................................................................................42

3.2.3 Characterization .....................................................................................................43

3.3 Results & discussion ..........................................................................................................44

3.3.1 Electrochemical Performance ................................................................................44

3.3.2 Morphology............................................................................................................50

3.3.3 Composition Analysis ............................................................................................52

3.4 Parametric Study on GnPs/PAni Electrode Compositions ................................................54

3.5 Conclusion .........................................................................................................................56

......................................57

ABSTRACT ..............................................................................................................................57

4.1 Highly flexible binder-free core-shell nanofibrous electrode for lightweight

electrochemical energy storage using recycled water bottles ............................................58

4.1.1 Introduction ............................................................................................................58

4.1.2 Materials and Methods ...........................................................................................60

4.1.3 Results & Discussion .............................................................................................64

vi

4.1.4 Conclusions ............................................................................................................73

4.2 Novel Superflex Electrodes with Intercalated Nanoplatelets for High-Performance

Supercapacitor Applications ..............................................................................................74

4.2.1 Introduction ............................................................................................................74

4.2.2 Materials ................................................................................................................76

4.2.3 Characterization .....................................................................................................76

4.2.4 Experimental ..........................................................................................................77


4.2.6 Conclusions ............................................................................................................83

4.3 Layered Conductive Polymer on Nylon Membrane Templates for High Performance,

Thin-Film Supercapacitor Electrodes ................................................................................84

4.3.1 Introduction ............................................................................................................84

4.3.2 Experimental ..........................................................................................................85


4.3.4 Conclusion .............................................................................................................90

...........91

ABSTRACT ..............................................................................................................................91

5.1 Study on the Effect of Melt Blended Nano-fillers on Polypropylene Hybrid Composite

Electrical Properties for EC Applications ..........................................................................92

5.1.1 Introduction ............................................................................................................92

5.1.2 Materials ................................................................................................................94

5.1.3 Experimental ..........................................................................................................94


5.1.5 Conclusion ...........................................................................................................100

5.2 Preliminary Study on PVA-H2SO4 Electrolyte System for Fiber Electrodes ..................101

5.2.1 Experimental ........................................................................................................101

5.2.2 Results and Discussion ........................................................................................102

5.2.3 Conclusions ..........................................................................................................104

5.3 Functional EC Prototyping ..............................................................................................104

.......................................................................107

6.1 Conclusions ......................................................................................................................107

6.2 Challenges and Opportunities of EC Electrodes ..............................................................109

References ....................................................................................................................................111

vii

List of Tables

Table 2-1: Comparison of various materials in terms of charge storage mechanisms [5, 11] ........ 7

Table 2-2: Comparison of performance parameters between ECs and batteries [10] .................. 11

Table 2-4: Decomposition Voltages for the Two Types of Electrolyte ........................................ 30

Table 4-1: PET Film Physical Characteristics with respect to Deposition Time ......................... 64

Table 4-2: A comparison table of FTIR characteristic bands between pure PET fibers and

PAni@PET core-shell fibers ......................................................................................................... 68

Table 5-1: A comparison between the ESR of different electrolyte types tested ....................... 103

viii

List of Figures

Figure 1-1: The market forests for units of wearable devices sold around the world from 2013 to

2020 according to Analysys Mason Consultants, by device type. .................................................. 1

Figure 2-1: A Simple Circuit Model of a Supercapacitor Device .................................................. 7

Figure 2-2: Charging and Discharging Process and Voltage Comparison for Ideal Capacitors and

Batteries ........................................................................................................................................ 10

Figure 2-3: Current Energy Storage Technologies with respect to Energy and Power Densities 11

Figure 2-4: Schematics of Charging and Discharging Double Layer Capacitors ......................... 12

Figure 2-5: EDLC charge storage mechanisms shown with the Helmholtz double layer at the

electrode/electrolyte interface and the potential gradient resulting from the diffuse solvated ions

....................................................................................................................................................... 13

Figure 2-6: LightScribe Technology used in the laser reduction of graphite oxide resulting in the

formation of graphene layers on top of a flexible substrate, allowing high specific capacitance and

electrochemical performance ........................................................................................................ 15

Figure 2-7: Schematics of Pseudocapacitive Charge Storage Mechanism ................................... 17

Figure 2-8: PAni Emeraldine Base and Salt Transition with HCl ................................................ 19

Figure 2-9: Resonance forms of aniline radical cations................................................................ 20

Figure 2-10: PAni powder polymerized in-situ on PET surface shown under SEM .................... 20

Figure 2-11: Electro-polymerized PPy layer SEM images and chemical structure specification 21

Figure 2-12: Overview of the formation of hybrid electrochemical capacitors by combining the

EDLC and pseudocapacitive materials to serve as electrodes ...................................................... 21

Figure 2-13: Design of Asymmetric Supercapacitor Cells ........................................................... 22

Figure 2-14: In-situ Polymerization of Conductive Polymer Chains onto Graphene Surfaces to

Create a Hybrid Electrode ............................................................................................................. 22

Figure 2-15: Relationship between Pore Sizes and Pseudocapacitive Behaviour ........................ 23

Figure 2-16: Ideal In-Situ Polymerization of PAni onto High Specific Surface Area Substrates

such as Carbon Nanotubes ............................................................................................................ 25

Figure 2-17: Electrospinning setup for the nanofiber fabrication ................................................. 26

Figure 2-18: Cyclic Voltammetry Potential Ramping of a Typical CV Experiment ................... 27

Figure 2-19: An example electrochemical impedance spectroscopy diagram showing the

electrolyte resistance or series resistance, the charge transfer resistance, as well as resistive and

capacitive behaviour regions......................................................................................................... 28

Figure 2-20: Schematics of a 2-Electrode Supercapacitor Cell Configuration ............................. 29

Figure 2-21: Illustration of Proton-Hopping Mechanism for Proton Transport ........................... 32

Figure 2-22: Polymer Electrolyte System Design with Cross-linkers for Proton Conduction ..... 33

Figure 2-23: Potential Applications and Development Cycle for Supercapacitors ...................... 38

ix

Figure 3-1: Experimental procedure for the in-situ chemical polymerization of polyaniline on GnPs

via ultrasonication ......................................................................................................................... 42

Figure 3-2: The design of the two-electrode test cell consisting of current collectors constructed

from grade 304 stainless steel; active electrode materials fabricated with in-situ polymerization on

nanoparticle scaffolds; electrolyte is constructed with a filter paper soaked in 1M sulfuric acid. 43

Figure 3-3: A) Cyclic voltammetry (CV) measured at 10 mV/s scanning rate for two-electrode test

cell constructed with varying compositions; B) Calculated gravimetric specific capacitance

comparison between the nanocomposites materials, showing the best performance was obtained

with the GnPs/PAni composite. .................................................................................................... 44

Figure 3-4: Nyquist plots obtained from the electrochemical impedance spectroscopy with

frequency varying from 100 kHz to 0.02 Hz, showing the equivalent resistance (ESR) and charge

transfer resistance (Rct) of the test cells. ....................................................................................... 45

Figure 3-5: Galvanostatic charge discharge (GCD) test for symmetric two-electrode cell setup with

a current of 0.01 A and a potential window of 0 V to 1.0 V for varying electrode compositions.47

Figure 3-6: Electrochemical characterization of pristine PAni composite electrodes, for A)

Comparisons of CVs at varying scanning rates; B) The general shape of the CV curves at 10 mV/s;

C) The specific capacitance change with respect to scan rates. .................................................... 47

Figure 3-7: Electrochemical characterization of Al2O3/PAni composite electrodes, for A)



Figure 3-8: Electrochemical characterization of TiO2/PAni composite electrodes, for A)



Figure 3-9: Electrochemical characterization of MWCNT/PAni composite electrodes, for A)



Figure 3-10: Electrochemical characterization of GnPs/PAni composite electrodes, for A)



Figure 3-11: Conductive polymer being in-situ polymerized onto the surface of the graphene

nanoplatelets. ECPs would utilize the GnPs as a template for polymerization and therefore leading

to large increase in specific surface area values ........................................................................... 50

Figure 3-12: SEM images of pure nanoparticles: a) Pristine GnPs; b) Pristine MWCNT; c) Pristine

in-situ polymerized PAni; ............................................................................................................. 51

Figure 3-13: SEM images of nanocomposite powder electrodes as fabricated: a) Al2O3/PAni; b)

GnPs/PAni; c) TiO2/PAni; d) MWCNT/PAni .............................................................................. 51

Figure 3-14: A comparison of TGA behaviours of composite powders and pristine particles .... 53

Figure 3-15: FTIR spectra of electrode materials fabricated with varying compositions as

indicated. ....................................................................................................................................... 53

Figure 3-16: TGA analysis of GnPs/PAni composite electrodes ramping 20°C to 900°C ........... 54

x

Figure 3-17: A) The specific capacitance measurement showing a decreasing trend in the charge

storage performance with higher GnPs percentages; B) The rate capabilities of the varying

composition of the powder electrode; ........................................................................................... 55

Figure 3-18: SEM images showing: a) 32GnPs/68PAni; b) 16GnPs/84PAni; c) 7GnPs/93PAni 56

Figure 4-1. Fabrication process of the nano-sized PET polymer core fibers from the recycled

PETE1 recyclable beverage bottles. ............................................................................................. 61

Figure 4-2. a) The PETE1 recyclable beverage bottles cut into strips; b) The PET strips were

further cut into rectangular pieces prepared for dissolution in the 1:1 DCM/TFA solvent; c) The

clear viscous solution with 10 wt.% PET resin in 1:1 DCM/TFA solvent for electrospinning .... 61

Figure 4-3. The rigorous stirring process allows the surface to become somewhat hydrophilic, in

that the fiber mat sank in the solution. Polymerization process took place at the liquid-solid

interface and allowed better adhesion between the PAni shell and the PET core; after 4 hours of

polymerization, a PAni@PET core-shell structure was formed ................................................... 62

Figure 4-4. a) SEM Morphology of Pure PAni formed from in-situ chemical polymerization

process with PET solid film as substrate; b) & c) Morphology of the Pure PET fibrous core: the

electrospun PET fibrous structure was from the recycled PET bottles, these fibers have an average

diameter of 121 nm, and observed to have a smooth surface; d) & e) PAni@PET core-shell

structure: after the PAni coating has been applied, it is evident that an even layer of PAni shell has

been adhered on the PET surface; f) At high magnifications, some exposed fiber core shows the

internal core-shell structure as predicted during fabrication. ........................................................ 65

Figure 4-5. a) Fiber sample diameter distribution with varying diameter bins; b) sample of the

flexible thin pure PET fiber mat; c) SEM image of the pure PET electrospun fiber; d) the flexible

PAni@PET core-shell fiber shown under bending; e) The SEM image of the PET fiber coated with

an average of 69 nm of PAni active shell layer ............................................................................ 66

Figure 4-6: Additional SEM images of a) & c) The pure electrospun PET fiber from the recycled

PET bottles at 30,000× and 100,000× magnification; and b) & d) The PAni@PET core-shell

fibrous electrodes at 30,000× and 100,000× magnification .......................................................... 66

Figure 4-7: Thermogravimetric analysis (TGA) of pristine PAni and pure PET fibers in comparison

with the PAni@PET core-shell fiber mat; it can be extracted from the graph that PAni@PET core-

shell structure contains 22.7% PAni shell and 77.3% PET core. The degradation temperature was

observed to be shifted 41°C upwards with the PAni@PET composite fibers in comparison with

the pure PET fibers. ...................................................................................................................... 67

Figure 4-8: A comparison of the Fourier transform infrared spectroscopy(FTIR) absorbance

spectra of Pure PET fibers and the PAni@PET core-shell fibers with the wavenumbers for the

peaks labelled in their perspective colors, for details in regards to the band assignments, please

refer to ........................................................................................................................................... 68

Figure 4-9: a) Atomic Force Microscopy (AFM) image of pure PET fiber mat surface, the surface

roughness factor Ra was measured to be 30.5 nm; b) AFM image of PAni@PET core-shell

composite fiber mat surface, the surface roughness factor Ra was measured to be 37.7 nm; ...... 69

Figure 4-10: a) & b) The water contact angle measurements for the pure electrospun PET and the

PAni@PET core-shell fiber structures. It was evident that the PAni@PET core-shell structure has

a much better wetting and therefore indicating a higher surface energy; c) & d) Schematics

xi

showing the difference in terms of ion accessibilities in relation to the wetting parameters of the

fiber surfaces, with pure electrospun PET film, the aqueous electrolytic ions were not able to

readily access all surfaces ............................................................................................................. 70

Figure 4-11. a) CV scans with variation of scan rates from 5 mV/s to 2000 mV/s; b) CV graphs

with scan rates varied between 5 mV/s to 100 mV/s; c) The calculated specific capacitance with

varying scan rates;......................................................................................................................... 70

Figure 4-12. a) The CV scans at 10 mV/s comparing between pristine PAni electrodes and the

PAni@PET core-shell nanofibers, it is evident that the capacitance was increased dramatically

from the pristine PAni powder electrode with carbon binders; b) The charge/discharge curves

comparing between pristine PAni and PAni@PET nanofibers; c) The presentation of 4 cycles of

charge/discharge at 1.2 A/g current. ............................................................................................. 71

Figure 4-13. a) Electrochemical impedance spectroscopy (EIS) nyquist plot for the PAni@PET

nanofiber structure with measurements from 100kHz to 0.5Hz shown; b) The Bode plot of the

impedance magnitude; c) The Bode plot of the imepdance angles; ............................................. 72

Figure 4-14. a) Cycling behaviour displayed with the comparison of CV curves at 0 cycles and

1500 cycles; b) Comparison of the charge/discharge cycles at 0 cycles and 1500 cycles; c) The

specific capacitance measured at various cycle numbers ............................................................. 73

Figure 4-15: A) Electrospinning setup used in the PET nanofiber film preparation; B)&C)

Scanning electron microscopy of pure electrospun PET film, at a 30,000x and 50,000x

magnification, with a scale bar of 3μm and 2μm; D) Actual flexible pure PET substrate as

fabricated; E) Diameter distribution of PET electrospun nanofibers with ~500 samples using image

analysis tools; ................................................................................................................................ 78

Figure 4-16: A) Large agglomerates formation on top of the PET film surface after dipping in

ultrasound-dispersed GnPs suspension; B) Ultrasonication-assisted method to induce better

adhered GnPs on the PET surface. The average thickness of the coating was measured to be 10 ±

4.2nm; ........................................................................................................................................... 79

Figure 4-17: Flexibility testing showing the GnPs@PET fibers' ability to A) bend or B) twist

without permanent strain deformations; C) TGA analysis of the GnPs@PET fiber electrodes

compared with pure polymer and GnPs ........................................................................................ 81

Figure 4-18: A) Mostly rectangular CV graphs measured at different scan rates from 1000mV/s to

5mV/s, with ideal capacitor behaviours; B) A comparison between the pristine GnPs and

GnPs@PET electrode in terms of the specific current calculated based on the weight of the GnPs

only; C) The Nyquist plot of the EIS measurement of GnPs@PET fibers with an inset of zoomed-

in graph showing the charge transfer impedance to be around 0.4Ω; D) The GCD test of the

GnPs@PET fiber electrodes; E) The CV graph comparison of the electrode cycling after 0 cycles

and 3000 cycles, showing very similar performances; F) The measured specific capacitance drop

of the GnPs@PET fibers after 3000 cycles was 5.8%; ................................................................. 81

Figure 4-19: Schematics showing the change in observed morphology of the nylon membrane

electrode, before and after the chemical in-situ polymerization took place. The large porous

networks in the nylon template allowed monomers and oxidants to fully utilize the nylon surface

for the PAni coating formation. After polymerization, the PAni layer still retained large amount of

pores that continues to aid the electrolytic ion transportation in the charging/discharging processes.

....................................................................................................................................................... 86

xii

Figure 4-20: A) SEM image of the pure nylon membrane template, it is shown to have large porous

networks; B) SEM image of the as-fabricated PAni@Nylon membrane system, the pores were

much smaller in diameter and it clearly shown that the PAni layer was well-attached to the surface;

C) AFM DMT modulus mapping of the pure nylon membrane confirming the existence of large

pore openings; D) AFM DMT modulus mapping of the PAni coated PAni@Nylon membrane

electrode, showing different top surface layer with lower modulus. ............................................ 87

Figure 4-21: Flexibility of the PAni@Nylon electrodes was demonstrated with A) Bending of the

electrode; and B) Twisting of the electrode. In both cases, no permanent deformation was

observed. ....................................................................................................................................... 88

Figure 4-22: A) A comparison between the CV graphs at varying scanning rates ranging from 1000

mV/s to 5 mV/s. At higher scanning rates, it is observed that the CV diagrams becomes less

symmetrical and indicative of electrolytic ions’ movement hindered during the charging and

discharging processes. B) A CV diagram scanned at 10 mV/s showing distinctive REDOX peaks

and more symmetrical behaviour. ................................................................................................. 89

Figure 4-23: A) The specific capacitance measured at varying scanning rates, showing a dramatic

drop in specific capacitance at increasing scanning rates; B) The cycling performance evaluation

of the PAni@Nylon membrane electrode after 3000 cycles. ........................................................ 89

Figure 4-24: Electrochemical impedance spectroscopy (EIS) of the PAni@Nylon Membrane

electrodes, showing a low charge transfer resistance of around 0.5Ω, indicative of efficient

electrode/electrolyte interactions at the contact surface. .............................................................. 90

Figure 5-1: Fabrication Schematic for the Twin-Screw Melt Blending Process .......................... 95

Figure 5-2: a) Electrical Conductivity Measurements for differing wt. % of PAni added to PP

matrix; b) Electrical Conductivity Measurements for different carbon particle and PAni

compositions with PP; c) Electrical Conductivity Comparison for Various Compositions

Containing MWCNT and PAni at 0.1Hz frequency ..................................................................... 96

Figure 5-3: SEM Micrographs of Pure Particles of a) PAni powder; b) Pure PP; c) Pure GnPs; d)

Pure MWCNT ............................................................................................................................... 97

Figure 5-4: SEM Micrographs of 20wt.% PAni with PP showing phase separated PAni particles

embedded ...................................................................................................................................... 98

Figure 5-5: SEM Micrographs of particles embedded into the PP Matrix. a) 5wt. % GnPs + PP; b)

5wt. % GnPs + 10 wt.% PAni + PP; c) 5wt. % GnPs + 20 wt.% PAni + PP; d) 5wt. % MWCNT +

PP; e) 5wt.% MWCNT+10 wt.% PAni + PP; f) 5wt.% MWCNT+20 wt.% PAni + PP; ............. 99

Figure 5-6: AFM images of nanocomposites a) Pure PP; b) 20wt.% PAni + PP; c) 5wt.%

GnPs+10wt.% PAni + PP; d) 5wt.% MWCNT+10wt.% PAni + PP .......................................... 100

Figure 5-7: a) TGA Analysis for PAni/PP Samples for Composition Verification; b) TGA Analysis

for GnPs/PP Samples for Composition Verification .................................................................. 100

Figure 5-8: Process used for the fabrication for PVA-H2SO4 solid-state gel electrolyte for the

construction of ECs ..................................................................................................................... 102

Figure 5-9: Flexibility demonstration of the PVA-H2SO4 electrolyte and its appearance before

depositing onto the PAni/Nylon membrane electrodes. ............................................................. 103

xiii

Figure 5-10: a) An EIS Nyquist Plot comparison between various electrolyte systems; b) A

comparison of cyclic voltammogram between H2SO4 aqueous electrolyte system and the PVA gel

electrolyte system; c)A comparison of cyclic voltammogram between PVA gel electrolyte with

varying addition of PySH; .......................................................................................................... 103

Figure 5-11: a) A concentric design of EC cell for smart textile and wearable energy storage

applications; b) Planer cell design for preliminary prototyping and testing purposes. ............... 105

Figure 5-12: EC prototypes as developed: a) Thick stainless steel current collector with powder

electrodes, and aqueous electrolyte, wrapped in polyimide film; b) Thin stainless steel current

collector with PVA-H2SO4 electrolyte, PAni@Nylon flexible electrodes and PVC packaging; c)

Pure activated carbon film, with PVA-H2SO4 electrolyte, wrapped in polyimide films; and d)

Completely sealed EC cell with flexible current collector, PVA-H2SO4 electrolyte, and

GnPs@PET fiber electrodes. ...................................................................................................... 105

Figure 5-13: A demonstration of the series connection of the 4V EC cell in order to power a while

LED light; a) Schematics showing the connection mechanism and that 4-cell system was utilized

for a 4V system; b) An image of the real-life setup during the testing of the 4V cell performance;

..................................................................................................................................................... 106

1

Introduction

In the recent years, there is a strong demand for wearable electronics, such as smart watches and

activity trackers. According to a market survey and forecast by Analysys Mason Consultants, the

growth trend is likely to continue in the next decade. As shown in Figure 1-1, smart watches and

transformable wearable electronics will occupy a large portion of the future wearable electronics

market. Integrated flexible and lightweight energy storage devices plays a key role in powering

these devices.

Figure 1-1: The market forests for units of wearable devices sold around the world from 2013 to 2020 according to

Analysys Mason Consultants, by device type.

While lithium-ion batteries are well-suited for various electronics applications, the inherent risk of

thermal runoffs makes it difficult to be used for wearable devices that require direct body contact.[1]

In fact, safety is still a major issue even with the introduction of lithium insertion materials as the

electrode.[2] The application of lithium batteries have been severely limited by the narrow

operating window with respect to temperature and voltage, and therefore makes the usage of

additional battery management systems necessary.[1]

2

Electrochemical capacitor (EC) or supercapacitor (SCs) is a new class of capacitors that are

capable delivering high power and energy densities, without the undesired safety concerns.[3-5] The

promising cycling life and electrochemical performance of ECs allows this type of energy storage

devices to be attractive for charging future flexible wearable electronics.[6] However, the

commercially available technology suffers from rigidity and heavy weight, therefore, it is desired

to introduce lightweight, flexible, high performance all-solid-state ECs that can easily be

integrated with wearable electronics. These flexible ECs can be applicable for powering wearable

electronics[7, 8] or miniature implantable biomedical devices.[9] There are also opportunities for

device performance improvements such as reduction in personal electronics charging times and

more efficient automotive start-stop energy storage systems.

Most of conventional capacitor devices currently suffer from low energy density and therefore are

seldom used for energy storage applications in comparison with batteries, in which much cheaper

dollar/Wh stored can be achieved [10, 11]. The continued usage of liquid electrolyte in combination

with the metallic electrodes in battery systems severely limited its application for flexible

electronics, as rigid components are necessary to ensure the safe operations of such devices. ECs

would effectively bridge that gap in terms of energy and power densities, and at the same time

their design simplicity would allow significant weight reductions of the integrated devices.

However, currently available ECs on the market lacked flexibility and rarely are used for personal

wearable electronic devices. One of the major issue with conventional ECs is the low energy

density derived from conventional electrode materials such as graphite or activated carbon. To

improve the energy density, current research efforts have largely focused on precious metal oxide,

such as ruthenium oxide (RuO2), and carbon nanoparticle based electrodes, which suffer from

brittleness, high material and fabrication costs [12, 13]. Difficulties in large scale manufacturing also

pose a challenge for commercialization of such technologies. Novel flexible, lightweight electrode

materials often suffer from poor electrochemical performances and cycling capability. Therefore,

the motivation is to explore low-cost, recyclable materials and fabricate novel hybrid electrodes to

fully utilize the charge storage capabilities in creating an efficient route for ion transfer and storage.

3

The objective for this thesis is to explore novel composite materials for electrochemical energy

storage purposes and ultimately creating a high performance lightweight, flexible EC prototype

utilizing novel composite electrode materials to demonstrate the possibilities of creating ultra

flexible small-scale energy storage devices targeted for future wearable electronics applications.

The main objective of the thesis has been divided into the following three sub-objectives:

1. Explore and study the morphology, electrochemical performance, and composition properties

of a combination of various EDLC and pseudocapacitive materials in creating a composite

powder electrode for hybrid ECs. Experiments mainly focuses on low-cost, high-surface-area

nanoparticles fillers such as aluminum oxide (Al2O3), titanium oxide (TiO2), graphene

nanoplatelets (GnPs), and multi-wall carbon nanotubes (MWCNT) with active

pseudocapacitive polyaniline (PAni) as the matrix.

2. Explore and investigate the use of flexible substrates materials, such as polyethylene

terephthalate (PET) fibers and nylon membranes, for creating a core-shell structured 3D

fibrous electrodes for energy storage applications. Construct and optimize the electrospinning

conditions in creating high-surface-area flexible core substrates from low-cost, recycled PET.

Fully characterize the novel electrodes with respect to material composition, morphology,

specific surface area, 2-electrode symmetrical electrochemical performance, degradation, and

ion transfer efficiencies.

3. Utilizing existing technologies and novel electrode materials, create a working all-solid-state

prototypes for demonstrating the possibilities of creating high energy density, highly flexible,

lightweight symmetrical EC cells. Investigate and explore the possibilities of creating

conductive composites for current collector applications and various optimization routes in

aiding the charge storage capabilities for the EC electrolyte systems. Demonstrate the

electrochemical capabilities of the prototypes by utilizing constructed EC prototypes for

powering LEDs.

4

This thesis has been divided into five main chapters.

Chapter 2 conducts a thorough and relevant background and literature review of the recent

scientific progress on the development of flexible EC electrodes. This chapter provides an

overview of the scientific fundamentals of the charge storage behaviours in ECs to better

understand the mechanisms behind the electrode performance. In addition, the recent scientific

progress section covers the findings in hybrid composite electrodes, flexible 3D electrodes,

compressible electrodes, and paper-based electrodes.

Chapter 3 explores the possibilities of integrating low-cost, high-surface-area nanoparticles

aluminum oxide (Al2O3), titanium oxide (TiO2), graphene nanoplatelets (GnPs), and multi-wall

carbon nanotubes (MWCNT) with pseudocapacitive polyaniline (PAni) in creating a hybrid

powder electrode system. This study focuses on using electrochemical characterization to rule out

certain compositions and place the focus on the most promising combination of materials and

optimize the electrochemical, morphological, and electrical behaviours by varying the

composition, namely for GnPs/PAni composite powder electrode.

Chapter 4 introduces a novel core-shell structured lightweight, ultra flexible electrode for EC

applications, utilizing a polyethylene terephthalate (PET) core nanofiber and nylon membranes,

along with a thin PAni coating. This chapter outlines the construction of the electrospinning

environmental chamber built in-house and discusses the fabrication process in the manufacturing

of the novel flexible electrodes. The morphology, electrochemical performance, hydrophobicity,

specific surface area analysis, and cycling and degradation behaviours are also described in detail.

Chapter 5 discusses the creation process of the all-solid-state ECs utilizing the novel electrodes

created in the previous chapters and outlines the attempts made to create novel electrolytes and

flexible current collectors to replace the currently existing materials for optimized mechanical

properties and reduced weights. This chapter also details the testing process and results for

applying the EC cell for LED lighting applications. Chapter 6 delivers a comprehensive discussion

of the results and conclude the thesis by offering insights into the future development of EC

components.

5

Background & Literature Review

This chapter serves to provide a fundamental understanding of the subject matter in this thesis,

namely the electrochemical capacitor (EC) electrode charge storage behaviours. This chapter

mainly focuses on two areas: background information on the theoretical description of the charge

storage capabilities in EC devices and literature review on the existing electrode materials and

other core components of the EC cell.

Charge storage phenomenon in hybrid ECs is fundamentally a result of the combination of

electrochemical double layer capacitance (EDLC) and pseudocapacitance effects. Electrical

Double Layer model was first introduced by Helmholtz in 1874 and was revised and improved by

Gouy, Chapman, Stern, and Grahame in the following years. [11, 14, 15] The double layer existed at

the electrode/electrolyte solution boundary, which governs adsorption phenomena and influences

charge transfer reaction rates and how electrostatic energy is stored in ECs. [11]

New materials with highly extended active surface areas are very interesting in the development

of ECs, namely treated carbons, metal oxides, conducting polymers, etc.[16-18] The interfacial

capacity is further increased with Faradaic charge storage related to bi-dimensional redox reactions

or 3D intercalation processes [11, 19, 20]. A different charge storage mechanism, pseudocapacitance,

has been developed by Conway at the University of Ottawa between 1975 and 1981.[21, 22] The

capacitors took advantage of energy storage abilities of electrodeposited monolayer of some base

metals or with REDOX systems where RuO2 films with H2SO4 electrolyte.[11, 15] This system

allowed for improved charge storage behaviour, along with better charge/discharge performance

even after 50,000 cycles.[23]

6

A hybrid system where the charge storage capabilities from the double layer capacitance created

by carbon-based electrode material and pseudocapacitance from the REDOX reactions with RuO2

films are combined is called a hybrid supercapacitor.[24] The large capacitance is achieved with

relatively small volume is due to the large specific surface area for the electrode materials such as

porous carbon [19, 25]. Ion adsorption in the case of EDLCs and fast surface REDOX reactions in

the case for pseudocapacitance, can both contribute to the overall performance of the

electrochemical supercapacitor [4]. It is essential to note that the improvement in the capacitance

can be attributed to the advancement in nanostructure research and further understanding of charge

storage mechanisms. [26] The finding that ion desolvation occurs in pores smaller than the solvated

ions has led researchers to discover higher capacitances using carbon electrodes with sub-nano-

sized pores [4, 27]. EDLCs use carbon based active materials with high surface areas, while

pseudocapacitance use metal oxides and ECPs with fast surface or near surface redox reactions for

energy storage. Since the 1950s, there has not been significant improvement on the energy density

of ECs [12]. The fast development in the last few years can be attributed to the discovery of novel

electrode and electrolyte materials, such as new ECPs, carbon nanoparticles such as graphene, as

well as other types of metal oxides [13].

Faradaic charge storage involves electron transfers that take place across the double layer, where

reduction and oxidation occurs at the electrode/electrolyte interface [5]. In capacitors, even though

double-layer electrostatic charge storage dominates, there may still be partial electron transfer

occurring, creating possibilities for pseudocapacitance [11, 22, 24]. Pseudocapacitance charge storage

mechanism takes place when the chemisorption of electron-donative anions occurs. It is this type

of adsorption process at the electrode/electrolyte interface that assist with the electron transfer and

ultimately result in the storage of charge.

Non-Faradaic charge storage occurs when the charge is stored electrostatically and ideally no

electron transfer takes place across the electrode interface [21, 26]. For example, this is the type of

charge storage mechanism that dominates in the case of conventional capacitor devices.

7

Table 2-1: Comparison of various materials in terms of charge storage mechanisms [5, 11]

Typical EC cell is constructed with two electrodes in which one is positively charged and the other

is negatively charged with respect to each other [11, 28]. And the electrodes are separated by an

electrolyte material and a separator [11, 29]. The macroscopic representation of the cell is two

capacitances linked in series indicating charge storage at the electrode/electrolyte interfaces, while

an ohmic resistance is added to represent the resistance of the electrolyte and separator

combination [11, 30]. For electrodes with large surface area contact and porous matrices, further

equivalent circuit elements are needed for the microstructural interfacial interactions [31, 32].

Figure 2-1: A Simple Circuit Model of a Supercapacitor Device

𝑅𝑠 – 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑡𝑒 (𝑆𝑒𝑟𝑖𝑒𝑠) 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒

𝐶𝑑𝑙 – 𝐷𝑜𝑢𝑏𝑙𝑒 𝐿𝑎𝑦𝑒𝑟 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒

𝑅𝑐𝑡 – 𝐶ℎ𝑎𝑟𝑔𝑒 𝑇𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒

𝐶𝜑 – 𝑃𝑠𝑒𝑢𝑑𝑜𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒

In contrast, for equivalent circuit models in the case of batteries, simply a Faradaic resistance

element (RF) which represents the potential dependence of the reciprocal of the rate of oxidation

and reduction charge transfer process, it is in parallel with the double layer capacitance (Cdl) that

is always significant in the charge storage systems [21, 24]. For both electrochemical capacitors and

batteries, a solution resistance (Rs) in series with the Faradaic impedance, ZF, is necessary in order

to fully represent the charge/discharge behaviours of the cell [33]. Rct is a very good indication for

the performance of the electrode/electrolyte interface for high-rate discharge applications. For

Type Basic Mechanisms Example Materials Vacuum Electrostatic ---

Dielectric Electrostatic Glass, Plastics

Oxide Electrolytic Electrostatic Al2O3

Double-layer Electrostatic Carbon-based powders, fibres

Redox oxide film Faradaic (Pseudocapacitance) RuO2, MnO2

Redox polymer film Faradaic (Pseudocapacitance) PAni, PPy, PEDOT

8

polymer-based electrolyte systems, Rs is also a good indicator of the proton-conducting

performance of the electrolyte [11, 29]. Without self-discharge processes, or parallel

pseudocapacitance, the macro-equivalent circuit for the capacitor would include only a solution

resistance (Rs) and a double-layer capacitance (Cdl) [31, 34]. However, Rs and Cdl have smaller

components with their micro-equivalent circuitry for high-area porous electrodes [32, 34].

The charge and discharge process in a battery often involves irreversible conversion process of

chemical electrode reagents, and therefore the cycle life of batteries is restricted to several

thousand cycles [11, 35]. By contrast, capacitors can have almost unlimited number of cycles, due to

the fact that no chemical process is involved [11, 15, 20]. Conventional supercapacitors typically have

low energy density, where the double layer capacitances is about 16-50 μF/cm2 [10, 12]. And

therefore, it is immediately evident that higher surface areas of contact would lead to increase in

specific capacitances. Sufficiently large accessible electrode areas can be realized with novel

materials such as carbon powders, felts, and aerogels, where very large (on the order to 100 F/g)

specific capacitances can be achieved [11, 36-38]. Double-layer capacitors depend on the electrostatic

charge storage mechanisms and therefore recyclability can be improved significantly. [12, 39] Only

electrons are transported to and from the electrode surfaces through the external wiring, while the

cations and anions are transported within the solution to the charged surfaces. CV curves of double

layer capacitors show charge and discharge curves to be mirror images, while for batteries, this is

not common [10, 12, 38, 40].

The battery processes involves usually one or two valence electron charges per atom or molecule

of electroactive reactant are involved. An EC usually has only 10% - 20% of electron charges per

atom of that in batteries, which would explain the fact that capacitors have much less energy

density in comparison to batteries [11, 26]. Since the energy density of a capacitor increases with V2,

a significant increase can be attained using the non-aqueous electrolyte materials, as the

decomposition voltage can be increased to 3 V or 3.5 V.

9

For a capacitor electrode of 1000 m2/g operating at 1 V and have a specific capacitance of

30μF/cm2, the specific capacitance is 300 F/g. At 1 V, the storage energy can also be calculated

with the formula shown below, which gives 150 J/g, in turn can be converted to 41.7 Wh/kg.

𝐺 =1

2𝐶𝑉2

In practice, this cannot be easily attained due to the inaccessibility of electrolyte solution to the

fine pores of the electrode [19, 23, 41]. In addition, the weight of the structure and electrolyte must

also be considered for real-life applications [42]. In the case of a Ni-Cd battery, it can be seen that

by using the Ernst equation for Gibb’s free energy as shown below, the energy density is 225

Wh/kg [43].

∆𝐺 = −𝜁𝐹𝐸

Where, 𝜁 − 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑇𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑

𝐹 − 𝐹𝑎𝑟𝑎𝑑𝑎𝑦′𝑠 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (𝐶

𝑚𝑜𝑙 𝑒−)

𝐸 − 𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 (𝑉)

When comparing batteries to the electrochemical capacitors, the typical energy density is much

higher for the battery systems. In two-electrode electrochemical capacitor systems, the energy

density is about (½)2 of that original energy density, due to each electrode losing about ½ of its

charge after being fully charged [11].

For ECs, the electrostatic free energy of charging a parallel plate device is determined by 𝐺 =

1

2𝐶𝑉2. For batteries, 𝐺 = 𝑄ΔE between the potential of the two electrodes, but for capacitors, for

accumulated charge 𝑄, 𝐺 =1

2𝑄V. This reflects the fact that with the same stored charge and same

operating voltage, the energy stored in capacitors is half of that stored in batteries [10, 15]. The

difference is understood by the fact that for double-layer capacitors, additional elements of charge

must perform work against the charges that has already been stored in the plate [12, 44]. In the case

of batteries, the thermodynamic potential is independent of the charge Q added, as long as the

oxidation and reduction go on without interruption. The amount of reactants is typically plentiful,

but the process reaches terminal when the decomposition voltages of the electrolyte are reached

[11]. A charge/discharge curve comparisons between batteries and capacitors is shown below [11]:

10

Figure 2-2: Charging and Discharging Process and Voltage Comparison for Ideal Capacitors and Batteries

Ideal capacitors are charging and discharging at different voltages, meaning that the capacitor

voltage experiences a linear trend with the extent of the charge, while the ideal batteries would

have a constant charging/discharging voltage behaviour [10]. The decline in the voltage of the

capacitor would arise from the fundamental descriptive equation of the double layer 𝐶 =𝑄

𝑉 and

that 𝑑𝑉

𝑑𝑄=

1

𝐶 . A battery experiences no continuous voltage drops during the charge and discharge

processes, however, the decline in the magnitude can be attributed to the cathodic and anodic

polarization or ohmic potential drop due to internal resistance [45, 46]. The area under the curve

correspond to the energy of charging, and that from the slope of the capacitor charging curve, it is

apparent that the energy stored in capacitors to the terminal voltage with charge Q is half of that

for charging batteries with the same charge and voltage [11].

∫ 𝑉𝐶𝑑𝑄 =1

2∫ 𝑉𝐵𝑑𝑄

Where, 𝑉𝐶 − 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑜𝑟 𝑓𝑖𝑛𝑎𝑙 𝑐ℎ𝑎𝑟𝑔𝑒𝑑 𝑣𝑜𝑙𝑡𝑎𝑔𝑒

𝑉𝐵 − 𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝑓𝑖𝑛𝑎𝑙 𝑐ℎ𝑎𝑟𝑔𝑒𝑑 𝑣𝑜𝑙𝑡𝑎𝑔𝑒

Under conditions where the response is reversible to positive and negative cyclic voltammetry

(CV) sweeps, the resulting voltammograms is the mirror image of the other direction of sweep, if

there is no involvement of diffusion control [13, 31, 38]. This fact is often a good criterion in evaluating

the reversibility in a charge and discharge process. It distinguishes the capacitor and battery

behaviour in a fundamental manner [27, 31]. However, there are currently researchers working on

ways to integrate batteries and capacitors, coming with a so called hybrid system that synthesizes

the benefits of both batteries and capacitors in terms of charge storage characteristics [6, 47]. There

11

were research conducted on integrating anthraquinone (AQ) with high surface area graphene

macro-assemblies to create a hybrid battery/supercapacitor electrode system, which demonstrates

excellent power performance, showed remarkable long-term cycling stability, and good

mechanical behaviour [48].

Table 2-2: Comparison of performance parameters between ECs and batteries [10]

Function Electrochemical Capacitor Lithium-ion (general)

Charge time 1–10 seconds 10–60 minutes

Cycle life 1 million or 30,000h 500 and higher

Cell voltage 1.0 to 4.2V 3.6 to 3.7V

Specific energy (Wh/kg) 5 (typical) 100–200

Specific power (W/kg) Up to 10,000 1,000 to 3,000

Cost per Wh $20 (typical) $0.50-$1.00 (large system)

Service life (in vehicle) 10 to 15 years 5 to 10 years

Charge temperature –40 to 65°C 0 to 45°C

Discharge temperature –40 to 65°C –20 to 60°C

The graph below shows the placement of various currently available energy technologies in terms

of the energy density and power density as indicated [49, 50].

Figure 2-3: Current Energy Storage Technologies with respect to Energy and Power Densities

Fuel cells contains very high energy density meaning they are able to provide consistent power for

a long period of time, however, the power output may not meet the demand of certain high-power

applications [45, 51, 52]. Fuel cells also require the extra fuel storage component, which is used to

supply the reactants for the anode reactions. This component introduces unwanted extra weight

and causes other issues in terms of transportability [51]. The conventional capacitors have low

energy density, but very high power density, thus are typically used to stabilize the power in the

circuit by injecting compensation voltages as required [53]. The energy density for most

12

commercially available ECs is in the range of 18-29 J/cm3 (5-8 Wh/L), while the best commercial

dielectric capacitor such as biaxial oriented polypropylenes (BOPP) is only about 1.2 J/cm3 (0.3

Wh/L) [54]. The dielectric material that separates the opposite static charges between two electrodes

governs the energy storage capability and is given by the formula [55]:

𝑈 = ∫ 𝐸𝑑𝐷

For linear dielectrics, the energy density becomes as follows:

𝑈 =1

2𝐷𝐸 =

1

2𝐾𝜀0𝐸2

Where 𝐾 is the dielectric constant, 𝜀0 is the vacuum permittivity. A typical charged conventional

capacitor has the structure as follows.

Figure 2-4: Schematics of Charging and Discharging Double Layer Capacitors

Researchers have shown that the dielectric system can be modified with 2D hexagonal boron

nitride nanosheets (BNNSs) to improve the Eb and barium titanate to promote K, which would lead

to higher energy storage capacity [54]. With the composition of 12 wt.% BNNSs and 15wt% BT in

the P(VDF-CTFE) system, the energy density was 21.2 J/cm3 which is significantly larger when

compared to other dielectric capacitor systems [54]. ECs effectively bridges the gap between

batteries and conventional capacitors in terms of both power and energy densities, making it suited

for various potential applications.

13

The adsorptions of ions of the electrolyte onto active materials that are electrochemically stable

and have high accessible specific surface area to produce the Helmholtz layer as described [4, 14].

For conventional electrostatic double layer capacitors, the capacitance is calculated as follows:

𝐶 =𝜀𝑟𝜀0𝐴

𝑑

Where 𝜀𝑟 is the electrolyte dielectric constant, 𝜀0 is the dielectric constant for vacuum, 𝑑 is the

effective thickness of the double layer, and 𝐴 is the specific surface area. The difference in the case

of EDLC is the extremely high surface area and the very small effective thickness of the Helmholtz

layer that lead to much higher capacitance values. The energy stored in an EDLC is calculated as

follows:

𝐸 =1

2𝐶𝑉2

Figure 2-5: EDLC charge storage mechanisms shown with the Helmholtz double layer at the electrode/electrolyte

interface and the potential gradient resulting from the diffuse solvated ions

Higher operating voltage can ensure higher energy stored. However, electrochemical systems with

aqueous electrolytes often suffer from the limitation of low operating voltage due to the

decomposition of electrolytes at high potentials (electrolysis) [4, 27]. With purely EDLC charge

storage, there is no Faradaic reaction at the electrodes and therefore eliminating the swelling in the

active material that batteries may experience during charge/discharge. This is also the reason for

14

electrochemical capacitors to undergo millions of cycles while batteries can only achieve a few

thousand. Double layer capacitance for activated carbon with organic electrolyte can range from

100-120 F/g, while with aqueous electrolyte, the capacitance can increase to 150-300 F/g [44].

Activated carbon fibres can have the similar capacitance in comparison to activated carbon

powders, but the high pricing of these materials is unsuitable for practical applications.

For optimized double layer capacitance values to be feasible, the surface area of the

electrode/electrolyte contact must be maximized. This is typically done by selecting high specific

area electrode materials for the construction of the supercapacitor [56]. These surface area

measurements typically reflects the capacitance performance of these active materials when

utilized in electrodes.

By using activated carbon materials with a specific surface area reaching an upwards of 3000 m2/g,

specific capacitance values of 300 F/g have been reported for the investigated experimental

supercapacitors [57]. With the high microporosity, the carbon material is generally used for various

different applications ranging from biomedical overdose management, gas storage and separation,

to water and air filtration purposes [58-60] . Activated carbon is carbon produced from carbonaceous

source materials such as wood, coir, coal, and petroleum pitch [60, 61]. Activated carbon is produced

from the following two processes: physical or chemical reactivation. One way for physical

reactivation is by carbonization of source carbon to pyrolyze at a temperature of 600-900°C in the

absence of oxygen [61, 62]. Another way is the oxidation method where the carbon material is

exposed to oxidizing atmosphere to a temperature range of 600-1200°C [62]. Chemical activation

involves impregnating carbon source with certain an acid, strong base, or a salt, and then apply

physical activation methods at a lower temperature [63, 64]. Chemical activation is preferred as a

result of the lower energy requirements and shorter activation time. Currently, commercially

available supercapacitors with a capacitance value with the 5000 F range are often constructed

using activated carbon materials [65]. Maxwell Technologies Inc. has also developed

complimenting fabrication techniques for the continuous manufacturing of such device [65].

Nesscap Energy has been manufacturing these type of ultracapacitors for commercial usage, in

which an example is shown below.

15

Graphene is a monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb

lattice [66]. The mechanical and electrical properties of graphene materials are astonishing [67]. The

exceptionally thin plate of graphene is believed to be able to offer very high specific area values

and contribute to a number of industrial applications [68, 69]. Large number of composite materials

have been created using graphene as an additive, while others have used pure graphene

nanoplatelets as free-standing electrode materials for energy storage in supercapacitors [52].

Graphene nanoplatelets are graphene bundles with a few to fifty layers of graphene structures, as

shown in the figure above. The structure can be further broken down to finer graphene particles

with ultrasonication. Thin layered nature allowed for the high specific surface area values.

Researchers have also created miniature supercapacitor electrodes of graphene printed onto

flexible substrates using the LightScribe technology to create a supercapacitor with desirable

properties such as lightweight and flexibility [70]. These micro-supercapacitors demonstrated

power density of 200 W/cm3, which is among the highest values achieved for supercapacitors. This

fabrication technique also proved to be scalable for commercialization. Pure graphene materials

for supercapacitor electrode have also been achieved for successful prototyping [71]. This allowed

a maximum capacitance of 205 F/g and a power density of 10kW/kg at an energy density of 28.5

Wh/kg. Other groups reported even better energy densities for graphene-based ultracapacitors,

where a specific energy of 85 Wh/kg has been achieved [72]. Further studies are conducted for

graphene based nanocomposite materials to be utilized for supercapacitor prototyping, which is

discussed in the later sections.

Figure 2-6: LightScribe Technology used in the laser reduction of graphite oxide resulting in the formation of graphene

layers on top of a flexible substrate, allowing high specific capacitance and electrochemical performance

16

Carbon nanotubes are the 3D variation of carbon structures. It is distinguished by the number of

shells surrounding the structure. Single-walled carbon nanotubes are very difficult to produce and

it is very expensive, however, they also contain the most desirable mechanical, thermal, and

electrical properties [73]. It is formed arc discharge process and many other techniques, which most

of the time requires large energy input [74]. Carbon nanotubes possess superb mechanical and

physical properties due to their strong carbon-carbon covalent bonds and unique atomistic

structures [75]. Carbon nanotube fiber, spun directly from gas phase as an aerogel combines high

strength and high stiffness [76]. Some researchers even achieved a toughness that is considerably

greater than that of any commercial high-strength fiber [77]. Due to the material’s high surface

areas, carbon nanotubes are also a desirable material for the energy storage applications [29].

Improved EDLCs can be formed with the incorporation of these nanotube materials into the

electrode matrix, effectively increasing the surface contact between the electrode and electrolyte

[78]. Carbon nanotubes are gaining momentum to be used in many different applications and

therefore the cost is continuously reduced [79]. Carbon nanotubes also have very high electrical

conductivities with can be beneficial to the design of the electrodes providing seamless contact for

ion/electron flow between electrolyte and electrode systems [57, 80]. Functionalization of carbon

nanotubes can also provide additional desirable properties such as higher electronic conductivity

[81]. Thin carbon nanotube fibers can be engineered to manufacturing nanoscale devices that may

be capable of suitable applications with comparable efficiency [82].

Pseudocapacitance refers to the phenomenon of storing energy electrochemically via fast and

reversible Faradaic reactions at the electrode/electrolyte interface [11, 45, 83]. This effect arises from

the several different electrolytic processes, including electrosorption, REDOX reactions, and

intercalation systems [21, 22]. As shown previously, the double layer capacitors store energy via

electrostatic interactions, where a deficit or excess of conduction-band electrons accumulated at

the electrode surface, with the counterbalancing electrons in the electrolyte [5, 14, 41].

Pseudocapacitance is called “pseudo” because its origin is different from the conventional

electrostatic charge storage mechanisms for capacitors. It is associated from the origin of Faradaic

17

charge storage, where no actual chemical reactions take place. They do not involve making or

breaking chemical bonds. The de-solvated ions adsorb to the atomic structure of the electrode and

charges are attached on surfaces physically.

Figure 2-7: Schematics of Pseudocapacitive Charge Storage Mechanism

In batteries, the main energy storage process is resulting from the REDOX reactions, however, the

major difference is that the electrode in the case of the supercapacitors will not undergo phase

change in the REDOX reactions, whereas in the battery systems, phase change occurs [10, 24].

Precious or transition metal oxide materials such as RuO2, IrO2, and Co3O4 experience significant

pseudocapacitance phenomenon at the electrode/electrolyte interface. Trasatti has first proposed

the concept of using RuO2 for construction of electrochemical supercapacitors [84]. The group has

reported desirable electrochemical electrode features from the compound. However, much more

research has gone into RuO2, which presented an extremely interesting pseudocapacitive material

ideal for fabrication of supercapacitor electrodes [11, 84]. A novel symmetric RuO2 supercapacitor

with a high operating voltage of 1.6 V was built using the nanocrystalline hydrous RuO2. The

device exhibited an energy density of 18.77 Wh/kg at a power density of 500 W/kg based with

excellent cycling stability and power capability [85]. The energy density is extremely high and is

very difficult to achieve with other pseudocapacitive materials such as electrically conductive

polymers. However, the high cost of RuO2 at around $100 per gram offsets the potential benefits.

Many other metal oxides such as TiO2 or Al2O3 have not been studied extensively in the field of

18

energy storage but could potentially bring benefits in terms of providing scaffold for

polymerization of ECPs in order to obtain larger surface areas for better charge storage [86].

Electrically conductive polymers are a class of polymers where electrons can flow relatively freely

across [11, 21, 22]. Polyacetylene was one of the first polymers that have been discovered to be highly

conductive and it has been studied intensively for the applications in electronics [87]. Chemical and

physical characterization studies revealed the intrinsic properties that allowed the transfer of

electrons, which is often attributed to the large degree of π-orbital conjugation [11]. Polyacetylene,

along with polyaniline, polypyrrole, and polythiophene materials have been found to be highly

electrically conductive and offered insight into various interesting properties that can be potentially

utilized in different fields of applications [88, 89]. The electric properties of this grade of polymers

allow them to be oxidized or reduced electrochemically with the removal and injection of

electrons, which is the process taking place at the electrode/electrolyte interface. The electrical

conductivity of these polymers can reach the range of 1-100 S/cm, or even higher with properly

oriented forms [90]. The electrochemical processes associated with ECPs in a supercapacitor cell is

based on the REDOX reactions taking place with the sequential Lewis acid- or Lewis Base-

producing steps, involving electron withdrawal or electron donation [11]. Even though these

conducting polymers offer high pseudocapacitance through these Faradaic REDOX reactions, they

can also be characterized as double layer capacitance, since they are electrically conductive and

behaves effectively as metal plates [91]. The cycle life is not as high as for metal oxides, however,

they still can achieve substantial cycle life without too much degradation [92]. The usage of PAni

and PPy in electrochemical capacitors has been suggested as they offer excellent reversibility of

electrochemical charge and discharge over a potential range of 0.8-0.9 V [93]. This phenomenon

was demonstrated by the nature of mirror images in CV graphs.

Polyaniline or PAni is the polymer form of monomer aniline and it has received significant

attention in recent years of research to have desirable properties for the construction of

electrochemical supercapacitor electrodes [94]. The analytical pure base form of PAni which is the

emeraldine oxidation state can be converted from an insulating material to effectively a metal in

19

the form of emeraldine salt (emeraldine hydrochloride) as its electrical conductivity increases from

10-10 S/cm to 5 S/cm with the treatment of 1M HCl [94].

Figure 2-8: PAni Emeraldine Base and Salt Transition with HCl

The polymerization process for PAni can be carried out in several techniques. One of which is the

in-situ polymerization method, where the monomer is oxidized with oxidant such as ammonium

persulfate (APS) [95]. This in-situ polymerization can produce powders in significant quantities and

used for solvent casting in the fabrication of supercapacitor electrodes. The chemical

polymerization of aniline in aqueous solutions was a function of a wide variety of synthesis

parameters, such as pH, relative concentration of reactants, polymerization temperature and time,

etc. However, it was found that the synthesized polymer was not sensitive to most variables [95].

Polyaniline was also used as the main material for electrochemical supercapacitor electrodes in the

recent years, gaining momentum to serve as a suitable pseudocapacitive material [96, 97]. Energy

and power densities of approximately 3.5 Wh/kg and 1300 W/kg has been reported for pure aniline

based supercapacitors [98]. Typical values for specific capacitances for pure aniline based electrode

systems can reach 200 F/g, whereas asymmetrical cells can achieve even higher capacitances [96].

Studies have utilized the variability of the polyaniline structures to come up with smaller nanoscale

polyaniline structures or thin films in which much higher specific capacitance can be achieved at

around 600 F/g [99]. There are also researchers using nanofibers made from polyaniline to achieve

the higher capacitance values due to an increase in surface area contact [100]. Electropolymerization

of PAni can also be achieved which would offer better dispersion and more uniform electrode

construction aiding the charge storage process [11]. The formation of aniline radical cation is the

elementary step in these electropolymerization processes giving rise to a range of different

resonance forms as shown below.

20

Figure 2-9: Resonance forms of aniline radical cations

Following the polymerization or simultaneously, the deprotonation takes place to yield different

forms of PAni, with varying colours [101]. The entire electropolymerization process can be carried

out using cyclic polarization where a potentiostat cycles the potential for effective polymerization

onto metal substrates such as stainless steel plates [102]. The different peaks in the CV diagrams

can also attribute to the various oxidation states that the polymer is in, which helps to explain some

of the large capacitance values from literature. The oxidation peaks also depend on the varying

activation voltage of the oxidation states of PAni.

Figure 2-10: PAni powder polymerized in-situ on PET surface shown under SEM

Polypyrrole or PPy is another type of ECPs, which has comparable characteristics as PAni, which

also has been extensively studied for its desirable properties to be utilized as supercapacitor

electrodes. Films from electropolymerization of oxidized polypyrrole can have an electrical

conductivity of about 10-100 S/cm at room temperature [103]. Researchers have shown promise in

using PPy films for supercapacitor prototyping purposes, which demonstrated ideal capacitive

characteristics with a very high specific power of 110.9 kW/kg in 3M HCl when its specific energy

reaches 18.4 Wh/kg [104]. A specific capacitance of 480 F/g has been observed in the highly porous

PPy electrode configured symmetrically [105].

21

Figure 2-11: Electro-polymerized PPy layer SEM images and chemical structure specification

Hybrid electrochemical supercapacitors are utilize a combination of electric double layer and

pseudocapacitance as the charge storage mechanisms in the cell [11]. This can be achieved in several

different ways. Typically the hybrid capacitors would be able to perform better compared to those

electrodes with only EDLC or pseudocapacitance as their charge storage strategy [5, 47]. Researchers

have seen a dramatic increase in the supercapacitor cell performance parameters such as the

retention rates. For example, a carbonaceous shell coated PANI and PPy electrodes exhibited very

high capacitance retentions of ∼95 and ∼85% after 10 000 cycles, which are significantly in

comparison to the retention rate of pure PANI (∼20%) and PPy (∼25%) obtained under the same

conditions [106].

Figure 2-12: Overview of the formation of hybrid electrochemical capacitors by combining the EDLC and

pseudocapacitive materials to serve as electrodes

22

An asymmetrical supercapacitor utilizing a two-electrode cell configuration can be constructed

using different active material on the electrodes, effectively taking the advantages of both EDLC

and pseudocapacitive charge storage mechanisms for energy storage [10]. These supercapacitors

were developed in which the positive electrode was based on a pseudocapacitive metal oxide

electrode, and the negative electrode on an EDLC such as carbon-based electrode [105]. A potential

advantage of this type of supercapacitors is their higher voltage and correspondingly their higher

specific energy [107].

Figure 2-13: Design of Asymmetric Supercapacitor Cells

Wang’s group has achieved very high energy and power density asymmetrical supercapacitor

construction using Ni(OH)2/graphene and RuO2/graphene electrodes. An energy density of ∼48

Wh/kg and a high power density of ∼21 kW/kg at an energy density have been achieved with their

design [108]. Many other research has focused on various combinations of electrodes such as

MnO2/Graphene and activated carbon combination, achieving high energy and power densities in

the range of 51.1 Wh/kg, which is much higher than that of MnO2/CNT cells [109].

Many studies have focused on the design and fabrication of nano-composite or micro-composite

materials combining both EDLC carbon-based electrostatic and pseudocapacitive ECPs

electrochemical energy storage mechanisms to achieve optimized active electrode layer [110].

Figure 2-14: In-situ Polymerization of Conductive Polymer Chains onto Graphene Surfaces to Create a Hybrid

Electrode

23

In the case as demonstrated above, for a composite active electrode layer created by using graphene

sheets and conductive polymer PAni, a specific capacitance of as high as 480 F/g can be achieved

[111]. Studies also looked at three types of electrically conducting polymers (ECPs), i.e. PAni, PPy

and poly-(3,4-ethylenedioxythiophene) (PEDOT) in combination with multiwalled carbon

nanotubes as the electrode active material [39].

There is currently no study showing that there is a direct linear relationship with high specific

surface area and the capacitance [4, 5]. Some studies have suggested that pores smaller than 0.5 nm

were not accessible to hydrated ions. Pores with 1nm diameter would have difficulties allowing

the movement of ions in organic electrolytes, since these ions have a diameter larger than 1 nm. A

pore size ranging from 2 nm to 5 nm is the optimized pore size for capacitance improvements.

Partial ion desolvation may occur, allowing access to small pores (< 2 nm), and ultimately lead to

improved capacitive performances. These effects are shown in the figures below.

Figure 2-15: Relationship between Pore Sizes and Pseudocapacitive Behaviour

There is still a lack of understanding in the case of double layer charge storage in confined space

of microspores, where there is no space for Helmholtz layer formation [4]. But according to

researchers, if proper pore sizes were engineered in the case of electrode surfaces, increase in the

overall capacitor performance is achievable [112, 113].

24

Energetics of the electrode processes is used to describe the interaction between the ions in the

solution and the electrode surface itself. It is used to explain several different phenomena in the

electrochemical capacitor systems [21, 114]: 1.Overcharge of the double-layer type when

decomposition of electrolyte happens; 2.The charge and discharge of carbon-based electrode,

which experiences pseudocapacitance; and 3.Self-discharge process in an open circuit situation

following charging

The electrons involved in the process are transferred from or to the Fermi level of the delocalized

conduction band electrons of the metal electrode [78, 115]. The electrons in the metal obey the

“Fermi-Dirac statistics”, where there is 50% chance that the level is occupied by an electron [11,

116]. The energy of the electron in this level is different from the electrons in vacuum with zero

kinetic energy by the electronic work function Φ [117]. The energy required to remove an electrode

in a cathodic process is compensated by the energy gained when it has been transferred to an

electron acceptor, and vice versa for the anodic process from an electron donor [11, 15].

The electron donors or acceptors that are in direct contact with the electrolyte solution will interact

with the ions within the electrolytic solution, and that interaction will depend on the solvation

energy of the ion and also the energy of ionization [118]. The ionic interactions (generation and

consumption of protons) will achieve an overall charge balance in the reaction.

The interactions between electrolyte ions and metal electrodes can be described using an energy

cycle or a Born-Haber cycle based on the first law of thermodynamics.[119] It must be noted that

the voltage a single electrode cannot be measured and therefore, only the difference in voltage is

experimentally accessible. In this case, the work function is cancelled out in the metal contact of

the external measuring device [5, 11]. For the charge transfer to occur at significant rates, a match

between the energy levels of the Fermi level of the metal and suitable LUMO and HOMO must

exist [11]. Therefore, by engineering the electrode-electrolyte interface to ensure the match happens

is essential for the improvement in performance for the supercapacitor devices. This can also allow

the development of mathematical optimization models that can adequately describe the charge

25

transfer and energy phenomenon, contributing to the overall performance of the supercapacitor

prototype.

The metal oxides (RuO2, MnO2, etc.) and electrically conductive polymers (PAni, PPy, etc.)

possess good potential for pseudocapacitance charge storage in the development of

supercapacitors, however, the surface areas of these materials are limited by the dispersion, as well

as the low specific surface areas [120]. However, researchers have focused on increasing the surface

area contact between electrode and electrolyte by engineering 3D charge storage hybrid structures

at the interfacial layer of the electrodes [16, 20, 121, 122]. An et al. directly used multi-walled carbon

nanotubes as the electrode material for electrochemical characterization, with targeted applications

in energy storage systems [123]. They have achieved a high specific capacitance of around 180 F/g,

leading to a reported power density of 20 kW/kg [123]. Baker et al. utilized sulfur-functionalized

thiophene-modified resorcinol–formaldehyde (RF) aerogels to create large surface area electrode

system for electrolytic applications including fuel cells and supercapacitors [121]. Niu et al.

discovered the ability to take advantage of the high specific surface area of carbon nanotubes, thus

creating a possibilities for nanotubes to be applied in the development of supercapacitor electrodes

[124]. They have reported specific capacitances of 102 F/g with a single cell device using 38 wt.%

H2SO4 as the electrolyte[124]. The cell had a power density of >8 kW/kg [124]. However, this study

focused mainly on the creation of this structure with pure carbon based materials. Almeida et al.

took this approach further by polymerizing polyaniline onto carbon fibre and carbon nanotube

surfaces to increase the specific area contact dramatically while adding pseudocapacitance to the

energy storage mechanisms [40].

Figure 2-16: Ideal In-Situ Polymerization of PAni onto High Specific Surface Area Substrates such as Carbon

Nanotubes

26

The idea is demonstrated above. The specific capacitance reported was around 400 F/g, making

the material viable for supercapacitor prototyping purposes [40]. In the same way, by using hydrated

RuO2 particles, and dispersing them onto the surface of multi-walled carbon nanotubes, Sugimoto

et al. have achieved extremely high specific capacitance of around 1300 F/g for the nanosheet they

have created [125].

Electrospinning is a simple, scalable technique of fabricating high aspect ratio nanofibers with

controllable diameters using electrostatic force [126, 127]. Electrospun fibers can have a number of

physical properties such as guided electron transport, high mechanical strength, high degrees of

flexibility, and large surface area, which are all properties that are desired for the electrode material

in a supercapacitor [128]. In fabricating pseudocapacitive materials, electrospinning can be

effectively utilized. Researchers have achieved successful fabrication of PAni-Nanofiber (PAni-

NF) web by electrospinning and was used as electrode materials for supercapacitor systems [126].

The fibers had high aspect ratio of >50, average length of 30 um, and average diameter of 200 nm.

PAni-NF electrodes are shown to have improved specific capacitance with H2SO4 electrolytes,

with 267 F/g compared to chemically in-situ polymerized PAni powder of around 208 F/g. The

PAni-NF was shown to be able to retain 86% of its charge storage abilities after 1000th cycle,

comparing to only 48% for as-is PAni powder. The advantages of interconnected web with the

nanofibers is that it allows easy access of electrolyte through the active polymer material as well

as optimal for the fast ion diffusion and migration in the polymer by increasing active reaction

sites for the Faradaic reaction. The electrospinning setup is shown below.

Figure 2-17: Electrospinning setup for the nanofiber fabrication

27

CV is a type of potentio-dynamic electrochemical measurement. In a cyclic voltammetry

experiment the working electrode potential is ramped linearly versus time and the current response

is measured by the potentiostat [129]. The rate at which the device ramps the applied potential is

called the scan rate.

Figure 2-18: Cyclic Voltammetry Potential Ramping of a Typical CV Experiment

The oxidation and reduction or REDOX reactions giving rise to pseudocapacitance in

supercapacitor systems can also be characterized using CV. The peaks can be attributed to the

different activating voltages of various oxidations states [101]. The capacitance values are typically

calculated from these CV diagrams using the derived formulae as outlined below [11].

Where the effective specific capacitance is calculated from the average current response over the

scanning rate and then taking into account the mass of the active electrode layer applied on the

28

electrodes. It is typically desirable to have a system with CV response in mirror images indicating

that the charge and discharge processes are similar in nature and that the system is capable of

storing charge effectively [21].

The EIS graphs often show a small arc in the high frequency region and a sloppy region in the

middle to low frequency regions. This is due to the charge transfer kinetics at high frequency

region (20 kHz to 1 kHz) and ionic diffusion (mass transfer) rules the low frequency range (1 kHz

to 0.1 Hz).

Rs is the series resistance representing the combination of electrolyte resistance, the inter-particle

resistance of electrode, and contact resistance. Cdl is the electrical double layer capacitance, Rct is

the charge transfer resistance, Zw is the frequency dependent Warburg impedance, and Cp is the

pseudocapacitance. For supercapacitors, the majority of the capacitance is available in the low

frequency region, so focus is placed on these regions [32].

Figure 2-19: An example electrochemical impedance spectroscopy diagram showing the electrolyte resistance or

series resistance, the charge transfer resistance, as well as resistive and capacitive behaviour regions

29

Many parameters descriptive of the performance of the supercapacitor cell can also be measured

from the graph [30]. The Nyquist plot, in combination with the Bode plots for magnitude and angle

gives information about the various representative parameters in the circuit model [31].

The charge and discharge voltage trend can also be measured in the electrochemical experiment,

yielding useful information such as charge/discharge intervals, charge storage efficiency, as well

as the specific capacitance. A symmetrical looking charging discharging graph would indicate a

well-developed electrochemical system capable of reversible charge storage [11]. The

charge/discharge test can be performed with a certain current settings and typically the result can

be dramatically different with varying currents. However, for the ideal case, the current would

correspond linearly to the time it would take to charge and discharge a supercapacitor, giving

symmetrical curves.

Three-Electrode configuration is typically used in an aqueous beaker of abundant electrolyte. The

testing is done against a reference electrode and only the working electrode is the active material

[130]. This way is not an accurate representative of the actual cell performance since the weight of

the liquid as well as the other electrodes are often not taken into weight considerations when the

specific capacitance is calculated.

Figure 2-20: Schematics of a 2-Electrode Supercapacitor Cell Configuration

As shown, the Two-Electrode configuration is a simple construction of a standalone electrode

which is capable of storing charge on its own and the working performance can be accurately

characterized and therefore is geared more towards actual prototyping [115].

30

Solvent of the electrolyte is not involved in the charge storage process, unlike Li-ion batteries

where it contributes to the solid-electrolyte interphase when graphite anodes or high-potential

cathodes are used. The key to increasing the charge storage abilities is to utilize high SSA, making

the electrode completely blocking while electronically conductive [4, 11]. Therefore the objective

function in the optimization of supercapacitors is vastly different from batteries, with only certain

similarities.

Overvoltage of the capacitor can cause the decomposition of the solvent to occur [131]. The double

layer capacitance will continuously increase while with carbon materials; some electrochemical

oxidation of the active material can take place [130]. The current is divided into two different types,

the charging current, 𝑖𝑐, and the Faradaic leakage current, 𝑖𝐹. The leakage current will increase

exponentially after reaching the solution’s decomposition voltage limit. At sufficiently high

potential, 𝑖 → 𝑖𝐹. The optimum capacitor performance is when 𝑖𝐹 is minimized. If 𝑖𝐹 is large at the

end of the half-cycle for capacitor charging, there will be a corresponding self-discharge current

following charge termination [131].

Table 2-3: Decomposition Voltages for the Two Types of Electrolyte

Electrolyte Type Aqueous Organic

Decomposition Voltage 1.2V 3.5 - 4.0V

Polymer electrolytes are highly promising for applications such as rechargeable batteries, fuel

cells, supercapacitors, etc. [132] For electrolyte materials, current research has primarily focused on

the design of solid supercapacitor electrolyte systems with protons as conducting mechanisms [42,

133, 134]. These electrolyte systems are categorized into two categories: polymeric proton-

conducting electrolytes and inorganic/polymer proton-conducting electrolytes [135]. These

electrolyte systems differ in composition and also proton conducting mechanisms [136].

Traditionally, electrolytes are composed of an ion-permeable and electrically insulating separator

film sandwiched in between two electrodes in a liquid electrolyte [115]. Solid state supercapacitors

31

is suitable for a vast variety of applications including consumer electronics, microelectronics,

wearable and printable electronics, due to their lightweight, as well as mobility. However, the

commercialization of these devices still remains challenging and this can be mainly contributed to

a lack of high-performance solid electrolytes. One main factor to consider in the design of solid-

state supercapacitors that affects the series resistance and rate performance is the thickness of both

the electrolyte and the electrode. Polymer electrolytes are produced as flexible freestanding films

which enables integration by sandwiching into the cell design [137].

Proton-conducting polymer electrolytes are applicable in the supercapacitor applications as they

have high ionic conductivity. This is due to the fact that pseudocapacitive electrodes require

protons exchange for their REDOX reactions and effective proton transfers can increase the cell

performance dramatically [96]. Polymer electrolytes are used extensively in fuel cells, but have not

been studied thoroughly for supercapacitor applications. The major difference in the operating

conditions is the temperature, in which, the fuel cell operates at high temperature ranges while

supercapacitors work under ambient conditions typically [52, 135]. Polymer electrolytes should have

high ionic conductivity, low electronic conductivity, good mechanical properties or dimensional

stability, high chemical and environmental stability, and thin-film processability [134]. Polymer

electrolytes conduct via the movement of protons, Li-ions, hydroxide ions, or the ionic specifies

in ionic liquids. Due to the intensive research progress in batteries and fuel cells, proton and Li-

ion conducting systems are the most mature [42].

Proton-conducting solid-state electrolytes have the highest ionic conductivity, but due to their

narrower potential window and their reliance on the presence of water, the usage of them has been

limited [134]. Polymeric proton-conducting electrolytes acquired their proton conducting abilities

from the presence of functional groups on the polymer chains. Researchers have taken advantage

of this to create an inorganic/polymer proton-conducting electrolytes blend inorganic proton

conductors with a polymeric matrix to form gels or composites.

The three distinct mechanisms for protons transfer in polymer electrolytes are namely, proton

hopping or Grotthuss mechanism, diffusion or vehicle mechanism, and direct transport via

32

polymer chain segmental motions [5]. At high relative humidity, Grotthuss mechanism dominates,

as it is the fastest in well-hydrated environments. However, when the environment is relatively dry

vehicle mechanism takes over as the dominant mechanism for proton conduction [136]. Proton

conductivity, therefore, typically increases with temperature and relative humidity.

The Grotthuss mechanism or proton hopping is the mechanism by which an excess proton diffuses

through the hydrogen bonding of water molecules or other liquids involving hydrogen bonds

through the formation of covalent bonds [138]. This effect occurs when there are different sites with

local rearrangement and reorientation characterized by two potential wells, which are separated by

low activation energy [42]. This occurs only in systems with strong hydrogen bonding and therefore

the solid-state electrolyte must be well hydrated.

Figure 2-21: Illustration of Proton-Hopping Mechanism for Proton Transport

Diffusion of protons involves ion diffusion through a proton concentration gradient [139]. This

mechanism is significantly slower in comparison to proton hopping and requires higher activation

energy and lower proton mobility.

Direct transport of protons involves the segmental motion of the polymer chains. Direct transport

only occurs above the glass transition temperature since it is only occurring in the amorphous

phase [42]. The movement of the polymer chains allowed movement of the protons in the matrix.

33

Pure ionomers (mainly -SO3H, -PO4H2, -COOH) has high proton conductivity as long as the

temperature has been kept under 100 °C and they are used extensively for EC purposes. These

polymer materials often contain both hydrophobic and hydrophilic regions.

Figure 2-22: Polymer Electrolyte System Design with Cross-linkers for Proton Conduction

Research has been underway on several fronts in order to improve the flexibility and mechanical

properties of EC electrodes, while creating a hybrid system that takes advantage from both EDLC

and pseudocapacitance storage mechanisms.[140-142]

Hydrogel electrodes are typically prepared via a simple chemical method and can be manufactured

into very thin film electrodes that still retains a large ion-accessible surface area.[143, 144] The main

advantages of such electrodes are the high electrical conductivity, the desirable mechanical

properties, as well as the formation of hierarchical micro-porous 3D network structures[145]. The

flexibility can also be maintained through the integration with aqueous electrolyte systems, as the

micro pores are allowed to fully swell and interact with the electrolytic ions[146]. The high areal

gravimetric specific capacitance values are obtained from the formation of hierarchical 3D

structures that prevent graphene from restacking and expose the active surfaces for electrolytic

interaction. The free movement of the electrolytic ion inside the electrode’s porous networks

further enhances the rate and cycling capabilities. However, hydrogel electrodes tend to suffer

34

from aqueous electrolyte leakages if the cell is not packaged properly, leading to reduced cycle

life.[143] Scaling production that ensures the repeatability of hydrogel-based electrodes is also a

challenge looking forward.

Supercapacitor performance improves dramatically from the increase in electrode surface areas as

a result of promoted EDLC, as well as improved Faradaic REDOX interaction of the electroactive

species. A number of studies have utilized high-surface-area particles such as graphene or multi-

walled carbon nanotubes (MWCNT) to create electrodes for ECs, however, these typically

required binder materials.[16, 123] The binder material does not actively participate in the charge

storage process, and therefore would negatively impact the overall specific capacitance of the EC.

Therefore it became attractive to construct 3D networked structures without the additional binder

or inactive substrate materials.

Hydrogel represents a class of polymeric materials that contain hydrophilic 3D structural networks

that allows the absorption of water molecules.[147] Some of the benefits that hydrogel EC electrodes

offer are the high flexibility and surface area that allows the ions electrolyte solution to fully

interact with the active material within the three-dimension network. Free-standing hydrogels are

typically fabricated through cross-linking the polymeric network chains.[147] Researchers utilized

both EDLC and pseudocapacitive materials in constructing hydrogel electrodes for EC

applications and have reported good electrochemical behaviours.[144, 145, 148, 149]

Early works by Ghosh et al. demonstrated the possibilities of extending the use of conductive

polymer hydrogels to the field of energy storage.[146] Shi et al. constructed ECs with polypyrrole

(PPy) based hydrogel electrodes.[148] These PPy hydrogels had an internal structure that is similar

to foams, that forms a network of interconnected PPy microspheres. Flexibility was ensured via

the use of carbon cloth as current collectors. The cyclic voltammetry showed a high specific

capacitance range in between 300-400 F/g at a current density of 0.2 A/g.[148]

With purely EDLC materials, Xu et al. has successfully fabricated graphene hydrogel based

flexible electrodes that can be applied as electrode for ECs.[143, 144, 150] The graphene hydrogel was

made through a simple one-step hydrothermal process at 180 °C.[150] Their group demonstrated

that flexible ECs with 120 μm thick graphene hydrogel thin film electrodes achieved a high

35

specific capacitance of 186 F/g, an aerial specific capacitance of 372 mF/cm2, and excellent cycling

stability, while maintaining ideal mechanical flexibility.[143]

Researchers have employed various strategies in improving the hydrogel performance since then.

Chen et al. reported a nitrogen-doped graphene hydrogel structure that obtained a high power

density of 205.0 kW/kg even at very high charge/discharge rate of 185.0 A/g, while retaining

95.2% capacitance after 4000 cycles.[151] Nickel foams were also used as a substrate material for

graphene hydrogels in order to improve the ionic access to the graphene hydrogel for improved

EDLC performance.[152] However, the rigidity of the metallic foams restricted the electrode’s

desired flexibility. Hybrid hydrogels became an attractive option in the recent years as the

combination of EDLC and pseudocapacitive materials proved to be an optimal synthesis strategy

in creating high performance ECs. Hao et al. constructed a hybrid hydrogel system with PAni,

polyacrylamide, and α-cyclodextrin, leading to an increase in the specific capacitance at much

lower active material loadings.[149] Chen et al. created a novel graphene-nickel hydroxide

(Ni(OH)2) hybrid hydrogel that contains nanostructured pores that facilitates the ion transfer

within the electrode’s networked structure, thus leading to improved electrode kinetics.[153]

Paper products have supreme flexibility and has been made mechanically stronger and more

resistant to wear and tear in the recent years. Researchers began to mimic the mechanical nature

of paper, and started to experiment with paper-like electrodes made of active charge storage

materials. To ensure flexibility, many researchers began integrating high-surface-area

nanoparticles such as vertically aligned CNT or graphene particles with paper products in order to

create EC electrodes with exceptionally high surface areas, and ultimately charge storage

performances.[154-156] Unlike the typically paper products, the wear and tear issues in paper-based

EDLC electrodes can be effectively addressed with the superior mechanical properties of CNT or

graphene. In order to further improve the electrode/electrolyte interface, Zang et al. presented a

simple strategy to fabricate crumpled graphene paper EC electrodes that showed exceptionally

high uniaxial strain at ~300%, while demonstrating a reasonable specific capacitance at 196

F/g.[157] The graphene paper would be pre-stretching the graphene paper and let it relax bi-axially

to create the folding patterns to enhance the electrode performance.[157]

36

Other groups have looked at other ways of creating this type of lightweight, paper like hybrid

structures for EC electrodes using EDLC materials, such as graphene or partially reduced graphene

oxide, in combination with pseudocapacitive materials, such as PAni or VO2.[154, 158]

Xiao et al

recently constructed a unique sandwiched PAni/graphene/PAni composite paper electrode that

achieved desirable properties such as high electrical conductivity of 340 S/cm, lightweight, while

retaining good mechanical properties.[154] Lee et al suggested a method of integrating large sheets

of graphene oxide and vanadium oxide (VO2) nano-belts that greatly reduces the sheet resistance

of the electrode, thus improving the overall electrode performance.[158]

Various fabrication methods aimed at increasing charge storage for paper electrodes have been

reported. For example, Sun et al successfully fabricated flexible graphene paper fabricated via a

reduction process for EC electrodes induced by flame and have achieved a high surface area of

274.9 m2/g that can facilitate ion transfer within the electrode structure.[159] Zhao et al created

porous graphene paper electrode by producing in-plane vacancy defects in the graphene sheets,

thus allowing electrolytic ions to efficiently transport within the layered graphene systems.[160]

This method utilizes ultrasonication to create high temperature cavitation bubbles that attacks and

penetrate the surface to create facile defects.[160] Even though this method has only been applied

for batteries applications, it is yet to be implemented for supercapacitor applications. Wang et al

recently introduced a type of flexible graphene paper that utilized carbon black nanoparticles as

pillars.[161] The carbon black particles served as spacers that prevents graphene layer from

restacking, this way, the maximum exposed active surface can be ensured.[161] Even at a high

scanning rate of 500 mV/s, the specific capacitance was found to be a reasonable 138 F/g in

aqueous electrolytes with only a 3.85% degradation after 2000 test cycles.[161]

With the increasing research efforts on paper-based electrodes for ECs, this type of electrodes has

shown great promise in producing scalable, low-cost, flexible, high performance energy storage

products. The major advantages delivered by this type of paper electrode are namely the sustained

electrical conductivity, the mechanical strength and elasticity, as well as the increased availability

of active surfaces through the utilization of spacer nanoparticles.[67, 154, 157, 159-161] When combined

with gel-based electrolyte systems, these flexible paper electrodes provide an attractive method of

creating all-solid-state flexible lightweight EC cells that will potentially revolutionize the future

of energy storage. However, the challenge comes with the fabrication of hybrid paper electrodes

37

that allows for high rate capabilities while ensuring the high energy density. There is still space

for improvement when it comes to the energy density of paper-based flexible EC electrode.

Research and development in the field of smart textiles have triggered research interests in fabric

based electrodes for ECs that can be integrated with sensors and actuators smart textiles. In many

studies, various fabric materials were utilized as the flexible substrate, and then by applying a

charge storage coating on the fabric surface in order to enable the functionalities of the electrode.

Liu et al recently used a simple “brush-coating and drying” process to coat a thin layer of graphene

oxide on cotton fabric sheets, which obtained a satisfactory 81.7 F/g as the specific capacitance.[162]

The cotton fabric sheets ensured the flexibility of electrode and provided a facilitated ionic

transport platform for efficient electrode/electrolyte interactions. In another study, polypyrrole

(PPy) nanorods were coated onto the cotton fabric via an in-situ polymerization process. The PPy

coated cotton fabric electrode achieved very high specific capacitance of 325 F/g but suffered from

high cycling losses after only 500 cycles.[163] Even though cotton fabric sheets have been

researched extensively as a flexible substrate, it does not actively contribute to the overall electrode

performance. The electrical conductivity of the electrode material can be significantly improved

via the use of graphene fabrics. Zang et al have reported a hybrid electrode with PAni coated on a

graphene woven fabric material.[164] The flexibility was ensured while the specific area capacitance

reached 23 mF/cm2, a 12 time increase from with the graphene fabric alone.[164] The composite

showed ideal cycling stability with 100% capacitance retention rate after 2000 cycles.[164]

It was deduced that the significant charge storage enhancements originate from the surface area

increases and improved facilitation of ion transfer within the electrode’s networked 3D structure.

And therefore, foams of various types have been considered extensively to be employed as EC

electrodes in the recent years. Meng et al prepared a highly flexible porous graphene free-standing

composite electrode by growing PAni nanowire arrays on a 3D graphene scaffold surface.[165] The

porous graphene structure was created using CaCO3 as a sacrificial template and reducing graphene

oxide. The 3D graphene/PAni nanowire electrode demonstrated superior electrochemical

performance of up to 385 F/g, while retaining 90% of charge storage capacity after 5000 cycles.

Graphene foam created via chemical vapor deposition (CVD) coated with polypyrrole obtained

specific capacitance of 660 F/g, and a specific energy of 71 Wh/kg.[166] The ideal EC electrode

38

behaviour was attributed to the 3D interconnected networks of the composite foam structures.

MWCNT have also been grown onto graphene foams in order to increase the specific surface area

while ensuring the cycling capability is not affected by the introduction of pseudocapacitive

materials.[167, 168] The few-layer graphene/MWCNT foams made by Wang et al as a densely packed

hierarchical nanostructures possessed a high specific surface area of 743 m2/g that contributed to

a 233 F/g specific capacitance.[167] In order to ensure the ions are capable of travelling through the

active network, as well as allowing the maximum active surface to be exposed, it was thought that

a core-shell structural network involving nano-sized fibers and active conductive polymer coating

can be fabricated to optimize EC electrode performance.

Figure 2-23: Potential Applications and Development Cycle for Supercapacitors

Supercapacitors provide higher specific power than batteries and higher specific energy, than

conventional dielectric capacitors because of the high capacitance of the electrode materials.

Recent interests in supercapacitors are stimulated by their potential applications such as power-

storage devices operating in parallel with batteries in hybrid electric and electric vehicles [50]. High

power requirement applications such as back-up energy storage device for micro-grids, or

defibrillators for biomedical applications can be promising routes towards supercapacitor

commercialization. Flexible smart textile is also interesting in that they offer the possibility of

39

integrating energy storage with people’s clothing, giving rise to battery-free future. These are some

of the potential outcomes from the research topic of supercapacitors.

40

Polyaniline-Based Nanocomposite Powder Electrodes Fabricated for

Lightweight Hybrid Supercapacitor Applications

The performance of symmetrical supercapacitor devices based on various polyaniline (PAni)

nanocomposite electrodes has been evaluated with a two-electrode electrochemical setup. The

specific capacitance of the active electrode material has been significantly improved via in-situ

chemical polymerization on nanoparticles possessing high specific surface areas and good

adhesion properties. Electrodes under study are namely, graphene nanoplatelets (GnPs)/PAni,

multi-walled carbon nanotubes (MWCNT)/PAni, aluminum oxide (Al2O3)/PAni, and titanium

dioxide (TiO2)/PAni. Electrode compositions were confirmed via Fourier transform infrared

spectroscopy (FTIR) and thermal gravimetric analysis (TGA). Scanning electron microscopy

(SEM) was used to characterize the morphologies of the nanostructures. From cyclic voltammetry

(CV) at a scan rate of 10 mV/s, as high as a 326.9 F/g specific capacitance has been achieved for

symmetrical devices composed of two GnPs/PAni electrodes. Electrochemical Impedance

Spectroscopy (EIS) showed the charge transfer impedance (Zct) was lowest in the case of

MWCNT/PAni systems. Galvanostatic charge discharge (GCD) tests verified the charge storage

capabilities of all electrodes and confirmed that GnPs/PAni electrodes had the best electrochemical

cell performance. Three-dimensional nanocomposite morphology has been observed in the

micrographs of these nanocomposites, indicating a relationship between the material surface area

and the charge storage ability. Then, a parametric study on the varying composition of the selected

PAni/GnPs electrode has been conducted, namely with GnPs percentage ranging 7%, 12%, 16%,

32%, in order to find the optimized composition for electrode performance. It was concluded that

the 7GnPs/93PAni was the highest performing ratio among the compositions, while it also

outperformed others in capacitance degradation even at high scanning rates.

41

This study focuses on novel energy storage solutions with desirable properties such as lightweight,

flexibility, as well as high energy and power densities. In this study, the energy storage potential

of composite electrodes based on polyaniline (PAni) with carbon-based nanoparticles graphene

nanoplatelets (GnPs) and carbon nanotubes (CNT) were explored. GnPs and CNT can effectively

improve the electrode performance by reducing internal resistance and increasing contact surface

area. Metal oxides, such as titanium dioxide (TiO2) and aluminum oxide (Al2O3), were also

incorporated with PAni matrix in hope to create novel supercapacitor electrode surface layers as

part of the study. The limited cycling stability of conductive polymer can also be compensated by

forming complex structures with nanoparticles.[169] A highly efficient hybrid supercapacitor

electrode was made possible utilizing both EDLC and pseudocapacitance as charge storage

mechanisms. It has been noted that three-dimensional charge storage structures deposited onto the

current collector surface can lead to larger contact surface area in the electrode-electrolyte interface

leading to improved electrode performance. [170] With results obtained from this study, combined

with previous findings, it is possible to examine the key parameters in the capacitive behaviour of

polymeric nanocomposite based supercapacitors to further enhance our understanding of these

novel devices, as well as to help open up additional possibilities for future energy storage research.

Large surface area to volume ratio can allow much higher charge storage capabilities via electrode-

electrolyte interfacial Faradaic redox reactions. [16, 171] Polymer nanoparticle interactions were

found to have a significant effect on the morphology behaviour of the nanocomposite material

leading to changes in electrode performance. [141]

Four different compositions of PAni-based nanocomposite active electrode materials were

fabricated for the purpose of this study. All samples were fabricated using the ultrasonicated, in-

situ chemical polymerization process with ammonium persulfate (APS) as the oxidant.

Aniline (ACS reagent grade, ≥99.5%) was acquired from Sigma-Aldrich. The monomer was

received in a dark brown color and was distilled to a golden clear appearance. The distillation was

42

carried out in a sealed environment, the temperature of the heating plate was set to around 250°C.

Ammonium persulfate (APS) (ACS reagent grade, ≥98.0%), TiO2 (Nanotubes, 25nm average

diameter), Al2O3 (<50nm Particle Size) were also acquired from Sigma-Aldrich. GnPs were grade

2 nanoplatelets, purchased from Cheaptubes Inc. Multi-walled carbon nanotubes (MWCNT) were

acquired from Nanocyl, grade NC7000. The graphite conductive ink was purchased from Alfa

Aesar.

The aniline monomer was first distilled and stored in a dark environment at 4°C. A weighted

amount of nanoparticles was added to an aqueous mixture of 20 mL of HCl, 4 mmol of APS.

Another solution with 20mL of distilled water with 4 mmol of aniline monomer. The solution

containing the nanoparticles was then ultrasonicated in an ice bath at 0°C for 1 hour at an amplitude

15 with Misonix ultrasonic liquid processor model S4000 with a CL-5 type converter. The solution

with aniline monomer was then ultrasonicated under the same condition to an opaque white

colored solution, and then was added and ultrasonicated with the solution in an ice bath for another

30 minute at the same amplitude. 4 hours were allowed for the polymerization process in an ice

bath under the acidic environment. The schematics of the above stated procedure is outline in

Figure 3-1. After the polymerization reached 4 hours, the color of the solution becomes dark green,

indicating the polyaniline emeraldine base formation. The precipitate was filtered, washed with

deionized water and dried overnight in a vacuum oven at 50°C.

Figure 3-1: Experimental procedure for the in-situ chemical polymerization of polyaniline on GnPs via ultrasonication

The dried composite powder were then crushed in a mortar to fine powder forms and mixed with

polytetrafluoroethylene (PTFE), graphite conductive ink and isopropanol. The resulting paste was

then casted onto stainless steel sheet (Grade 304) in dimensions of 1 cm by 1 cm. Graphite

conductive ink and polytetrafluoroethylene (PTFE) were used as the binder materials with the

43

composite powder to fabricate an active electrode layer. Since a two-electrode cyclic voltammetry

is used to characterize the capacitance performance of the system, a symmetrical two-electrode

symmetric cell was then constructed. This specific methodology has been described elsewhere.

[172] A piece of filter paper was soaked in 1M sulfuric acid serving as both the electrolyte and

separator in the cell. The overall design of the test cell is shown in the schematics in Figure 3-2.

Figure 3-2: The design of the two-electrode test cell consisting of current collectors constructed from grade 304

stainless steel; active electrode materials fabricated with in-situ polymerization on nanoparticle scaffolds; electrolyte

is constructed with a filter paper soaked in 1M sulfuric acid.

The electrochemical performance characterization was carried out with Solartron 1470E multi-

channel potentiostat via two-electrode electrochemical testing method. The electrochemical tests

conducted were cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and

galvanostatic charge discharge (GCD) tests. CVs were performed at varying scan rates, ranging

from 10 mV/s to 100 mV/s, with a voltage window from -0.2 V to 1.0 V which is selected with

regards to the optimum operating condition for polyaniline-based electrodes, as well as to prevent

the decomposition of water molecules in the electrolyte. The specific capacitance was calculated

based on the average of the charging capacitance and discharging capacitance values due to the

unsymmetrical CV observations. The EIS was performed from a high frequency of 105 Hz to a low

frequency of 0.02 Hz. The GCD was performed at a current density of 0.01 A/cm2, with the voltage

cycling between 0 V and 1.0 V. The weight of the active layer materials was calculated based on

the weight of the electrode before and after the casting of the active material. The morphology of

the electrode surface structures was analyzed using scanning electron microscopy (SEM) with JSM

6060, JEOL Inc. The Fourier transform infrared spectroscopy (FTIR) was performed with Bruker

ALPHA FT-IR spectrometer. The thermal gravimetric analysis (TGA) was carried out with TA

Instrument Model Q50 at a temperature scan rate of 10°C/min, from 20°C to 900°C.

44

With the two-electrode CV results, as shown in Figure 3-3 & Figure 3-7, it was found that pristine

GnPs or PAni, underwent the same treatment, have performed relatively poorly in comparison to

the nanocomposite materials fabricated. As a baseline, the pristine PAni electrodes yielded a

specific capacitance of 173.7 F/g, which falls in the range of typical conductive polymer

electrodes. The best performance was shown with the GnPs/PAni composition, reaching a specific

capacitance of 326.9 ± 21.4 F/g, while the Al2O3/PAni samples were also promising, showing a

specific capacitance of 216.0 ± 37.7 F/g. These results shows the charge storage abilities of

supercapacitor electrodes can be significantly improved via in-situ polymerization onto nanoscale

structures.

Figure 3-3: A) Cyclic voltammetry (CV) measured at 10 mV/s scanning rate for two-electrode test cell constructed

with varying compositions; B) Calculated gravimetric specific capacitance comparison between the nanocomposites

materials, showing the best performance was obtained with the GnPs/PAni composite.

The capacitance increase in the case of GnPs/PAni composite is attributed to the 3D structural

formation that is explained in the later sections that allows better contact at the electrolyte and

electrode interface. The REDOX peaks of PAni active layer can also be clearly observed in the

CV diagram, contributing to the overall capacitance. For the Al2O3/PAni, the specific capacitance

increase is attributed to the inclusion of Al2O3 nanoparticles, which can aid the storage of charge

electrostatically. The poor performance of TiO2/PAni electrodes can be attributed to the fact that

the titania particle is not electrochemically active and does not participate in the charge storage

mechanism. In addition, it does not provide a suitable substrate that allows the PAni active layer

to evenly attach at surface, therefore, leading to low surface exposure to electrolyte system. The

45

excess PAni was thought to agglomerate without exposing active surfaces, leading to poor

electrochemical performances. It was also interesting to note that the MWCNT/PAni composite

electrode led to a decrease in the specific capacitance, which may be attributed to the

agglomeration of MWCNT particles and PAni was not able to properly attach onto the surface

even after prolonged ultrasonication in an effort to disperse the nanoparticle.

EIS was performed to characterize the impedance frequency response of the test cell to gain

insights into the equivalent series resistance (ESR) and the charge transfer resistance (Rct) of the

electrolyte and electrode systems. In all cases of the test cells, 1M sulfuric acid was used as the

electrolyte. And this can be verified by the similar ESR values of the various test cells obtained

from the Nyquist plots as shown in Figure 3-4. The ESR and Rct values of each test cell with

varying compositions are summarized in Table I.

Figure 3-4: Nyquist plots obtained from the electrochemical impedance spectroscopy with frequency varying from

100 kHz to 0.02 Hz, showing the equivalent resistance (ESR) and charge transfer resistance (Rct) of the test cells.

Table I. Equivalent series resistance (ESR) and charge-transfer resistance (Rct) for test cells with varying electrode

compositions, and the equivalent circuit diagram for the system, where Cdl is the double layer capacitance and CΦ is

the pseudocapacitance.

Composites ESR (Ω) Rct (Ω)

Al2O3/PAni 0.631 1.789

GnPs/PAni 0.399 1.601

TiO2/PAni 0.665 7.125

MWCNT/PAni 0.478 1.122

Pristine GnPs 0.283 N/A

Pristine PAni 0.319 2.201

- -

46

It is clear that the aqueous sulfuric acid electrolyte is a good ionic conductor with fairly low ESR

as shown in the Nyquist plot, where each composition shared an ESR of around 0.5 Ω. The Rct or

the size of the semi-circle in the Nyquist plot is an important parameter in characterizing the

interfacial charge transfer resistance of the electrode. From the graphs, it is clear that the pristine

GnPs does not experience this phenomenon, which indicates the material has very high electrical

conductivity, therefore leading to negligible Rct. MWCNT/PAni and GnPs/PAni samples were also

found to have charge transfer resistances within the desired range, indicating good charge transfer

behaviours at the electrolyte and electrode interface. Their attained values are lower compared to

pristine PAni nanoparticle electrodes and this observation can be explained by the highly

conductive behaviour of these carbon-based structures, forming a 3D conductive network that aids

the effective reduction in charge transfer resistance. The system can be described using a simplified

circuit model as shown in the circuit diagram above. At low frequencies, the test cells experience

capacitive behaviour as Zc = 1/jωC, which is indicated in the Nyquist plot as the fast increase in

imaginary impedance values. At this frequency region, the EIS behaviour characterized by the

Warburg resistance is a result of slower processes such as ion diffusion and mass transport. It is

observed in the Nyquist diagrams that Al2O3/PAni and GnPs/PAni composite electrodes attained

the best performance in terms of ion diffusion and mass transport phenomenon, as indicated by the

slope at the low frequency regions. The poor performance of TiO2/PAni electrodes was again

verified with the EIS graph, showing a high charge transfer resistance of 7.125 Ω, which also

corresponds to a larger semi-circle, indicating resistive behaviours. This can be explained by the

fact that the TiO2 nanoparticles are not electrically conductive and due to the poor attachment of

the PAni active layer, the surface conductivity was severely impeded by the addition of this type

of nanoparticles.

The GCD tests, as shown in Figure 3-5, were conducted with a current density 0.01 A, over a

voltage window from 0 V to 1.0 V for all the test cells. The pure GnPs electrodes were not suitable

for the GCD test due to its low specific capacitance as found earlier in the CV diagrams. The pure

GnPs electrode curve is shown on the leftmost part of the graph, showing almost instantaneous

charge and discharge, which is undesired in an energy density perspective. This test allowed for

the practical verification of the electrochemical performance of the test cell, under a specific

condition. The specific capacitance calculated from the GCD graph can be matched to the results

obtained from the CV graphs, confirming the findings. The GnPs/PAni composite experienced the

47

most desirable charge retention behaviour as shown in Figure 3-5, with the longest discharge time

interval. The GnPs/PAni composite charge/discharge curve was also shown to be more

symmetrical in comparison with the other types of composites, indicative of good charge retention

and low charge transfer resistance.

Figure 3-5: Galvanostatic charge discharge (GCD) test for symmetric two-electrode cell setup with a current of 0.01

A and a potential window of 0 V to 1.0 V for varying electrode compositions.

Figure 3-6: Electrochemical characterization of pristine PAni composite electrodes, for A) Comparisons of CVs at

varying scanning rates; B) The general shape of the CV curves at 10 mV/s; C) The specific capacitance change with

respect to scan rates.

As shown in Figure 3-6, the pure PAni electrode CV diagrams were very representative of a typical

conductive polymer electrode system, with REDOX peaks apparent at voltages representative of

the transition between PAni oxidation states. With the increase of scanning rates, as with typical

conductive polymer electrodes, the specific capacitance decreases dramatically, reaching a specific

capacitance of 84.3 F/g after the scanning rate increased to 100 mV/s, a 51.5% decrease from the

original value. This can be attributed to the lack of a porous 3D framework in the case of pure in-

situ polymerized PAni, with a low surface area exposure to the electrolytic ions.

48

Figure 3-7: Electrochemical characterization of Al2O3/PAni composite electrodes, for A) Comparisons of CVs at



Figure 3-8: Electrochemical characterization of TiO2/PAni composite electrodes, for A) Comparisons of CVs at



With the addition of Al2O3 and TiO2, as shown in Figure 3-7 and Figure 3-8, respectively, the

composite electrodes showed a less symmetrical CV curves, which can be attributed to the fact

that the electrical conductivity of Al2O3 and TiO2 nanoparticles is fairly low and therefore,

contributing to an increased resistance behaviour, highlighted by the CV shape. The rate capability

was also severely limited in both cases, with a 73.3% drop in the case of TiO2/PAni electrodes,

when the scan rate was increased from 10 mV/s to 100 mV/s, and a similar 74.4% decrease in the

case of Al2O3/PAni electrode over the same range.

When comparing between the TiO2/PAni and Al2O3/PAni electrodes, it is clear that the

electrostatic charge storage capabilities of Al2O3 actively contributed to the overall capacitance as

shown in the CV with a much higher charge retained during the discharge cycle. Al2O3/PAni

system achieved a higher than expected specific capacitance of 215.9 F/g. In addition, the

incompatibility between TiO2 and PAni also became apparent when examining the CV curves,

showing almost no REDOX peaks characteristic of PAni based active surface layers.

49

Figure 3-9: Electrochemical characterization of MWCNT/PAni composite electrodes, for A) Comparisons of CVs at



Figure 3-10: Electrochemical characterization of GnPs/PAni composite electrodes, for A) Comparisons of CVs at



The scan rate also affected the specific capacitance of MWCNT and GnPs, but not as significant,

indicative of less hindrance in the ion diffusion at the electrode/electrolyte interface. In the case of

the MWCNT/PAni composition, the specific capacitance decreased from 139.9 F/g to 70.5 F/g as

a result of increased scanning rate, corresponding to a 49.6% capacitance drop. The rate capability

of GnPs/PAni electrode showed the best performance among the powder composite electrodes

tested, with a 24.3% drop from 326.9 F/g to 247.4 F/g over the similar range. It is believed that the

contact can be further improved if the electrodes were to be more porous, allowing more effective

ion transfer between the electrolyte and electrode materials.

This result has raised the possibility of polymer-nanocomposite interactions leading to increased

specific surface area and in turn led to the improved charge storage ability. For the pristine GnPs,

even though they have an extremely large specific surface area, the poor capacitance characteristics

have led to the low specific capacitance of single digits. However, with the aniline monomer

polymerized onto the GnPs template, as shown in Figure 3-11, the surface area exposing PAni to

the electrolyte has increased dramatically, which directly influence the pseudocapacitance

50

characteristics of the electrode, leading to much improved capacitance behaviour. Graphene

nanoplatelets used in the experiment have an average specific surface area of 500 - 700 m2/g, with

4-5 layers, and an average thickness of 8 nm, according to the data provided by the supplier. These

dimension parameters allows aniline monomer to have enough space to penetrate and grow on top

on the nanoplatelets, with optimized distribution. Al2O3/PAni electrodes have also shown

promising increases in the specific capacitance value from the CV graphs. This increase can also

be attributed to the increase in surface area contact for the composite material.

Figure 3-11: Conductive polymer being in-situ polymerized onto the surface of the graphene nanoplatelets. ECPs

would utilize the GnPs as a template for polymerization and therefore leading to large increase in specific surface area

values

However, in the case of the MWCNT/PAni composite material, the specific capacitance was

measured to be 139.9 ± 28.4 F/g, which is even lower than the specific capacitance value of pristine

PAni at 173.7 ± 16.0 F/g. From this result, it can be deduced the conductive polymer was not

polymerized on to the entire surface of the MWCNT due to space constraints. This can be caused

by the fact that micro-sized agglomerates of MWCNT still exist while the polymerization was

carried out in-situ. TiO2/PAni electrodes also performed poorly in the CV measurements with a

specific capacitance of 81.88 ± 12.3 F/g, which indicates a 53% drop from the specific capacitance

of pristine PAni. In comparison between the cyclic voltammograms of pristine GnPs and the

GnPs/PAni composite electrodes, it is observed that the pristine GnPs experiences only one

significant pair of redox peaks while the GnPs/PAni composite shows two pairs of redox peaks

due to the fundamental material behaviour. The transition is characterized as the redox transition

from leucoemeraldine to emeraldine state and the Faradaic transformation between emeraldine and

pernigraniline forms.

The strong correlation between the specific surface areas of the composite materials and the

electrode performance was verified via scanning electron microscopy where the three-dimensional

structure formation of many composite powders were observed, leading to increased exposed

surface to electrolyte.

51

Figure 3-12: SEM images of pure nanoparticles: a) Pristine GnPs; b) Pristine MWCNT; c) Pristine in-situ polymerized

PAni;

It is shown in Figure 3-12, the SEM images of pristine nanoparticles and ECPs namely, GnPs,

MWCNT, and PAni. These nanoparticles theoretically serve as templates for the in-situ

polymerization process and it allows for the integration of pseudocapacitance from ECPs and

EDLC from metal oxides and carbon-based materials.

By dispersing the nanoparticles using ultrasonication, large agglomerates as seen in the Al2O3 case,

can be broken down to nano-sized particles in solution, allowing for optimal utilization of their

high specific surface areas as the polymerization template. The completion of in-situ

polymerization led to the formation of three-dimensional nanostructures that aided the charge

storage behaviour as a result of increasing specific surface area.

Figure 3-13: SEM images of nanocomposite powder electrodes as fabricated: a) Al2O3/PAni; b) GnPs/PAni; c)

TiO2/PAni; d) MWCNT/PAni

52

From the SEM images shown in Figure 3-13, it became somewhat apparent to how these powder

electrodes behaved. In the case of Al2O3/PAni composite, the PAni was polymerized onto the

Al2O3 particles, and therefore, creating somewhat porous structures, and this has allowed better

penetration of ions within the electrolyte at the surface, allowing better interfacial interactions

between the electrolyte and the electrode, aiding the charge storage ability of the cell. From the

GnPs/PAni image, it can be deduced that the nanoplatelet shaped graphene was broken down into

micro-sized plates with ultrasonication and the PAni particles were polymerized on it with good

dispersion. Since GNP is a good material for EDLC, this allowed further integration of both charge

storage mechanisms for better supercapacitor behaviours. In the case of TiO2/PAni composite, the

TiO2 nanotubes were not acting to supporting the PAni particles, but instead looks to be phase

separated during the in-situ polymerization process. As the PAni tends to polymerize in bulk, the

polymerization of PAni may have been blocked by the TiO2 nanotubes, leading to reduced

electrochemical activity. And therefore, this formation of additional 3D structure did not lead to

enhanced electrochemical cell performance, but rather resulting in a poor specific capacitance

value. For the MWCNT/PAni composite, it is shown that even though the MWCNT particle

appears to be well-dispersed, they tend to wrap around the larger PAni agglomerates, effectively

reducing the pseudocapacitive activity offered by PAni, leading to reduced electrochemical

performance of the cell.

The TGA was performed under a nitrogen environment at 10°C/min, with the temperature going

from 20°C to 900°C for all pristine powder samples, as well as composite powder samples. As

shown in Figure 3-14, the TGA results have confirmed the composition of all samples fabricated

with the in-situ polymerization process. The end weight % for pristine PAni is 30.44%. The

degradation behaviour of all PAni based composites followed the same trend while the

nanoparticles retained an average of around 95-100% of their initial weights.

This confirmed the amount of nanoparticles retained after the fabrication process. The weight

percentage of the nanoparticles within the composite was controlled at 12.0 – 13.5 wt. %. The

GnPs/PAni composite retained 38.13% of its original weight, which would be attributed to the

weight loss of GnPs gradually until 900°C, as it retained 93.36% of its original weight.

53

Figure 3-14: A comparison of TGA behaviours of composite powders and pristine particles

Figure 3-15: FTIR spectra of electrode materials fabricated with varying compositions as indicated.

The FTIR spectra for all PAni composites are shown in Figure 3-15. For pristine PAni FTIR

spectrum, the peak at 1588.6 cm-1 corresponds to the C-H stretching in aromatic compounds. The

1493.7 cm-1 peak is characteristic for the C=N stretching in aromatic compounds. The band

observed at 1300.9 cm-1 confirms the C-N stretching in primary aromatic amines. [173] C-H bending

vibrations were also observed in the peak located at the 1164.1 cm-1. The peak at 837.2 cm-1 can

be attributed to the out of plane C-H in benzene ring. [174] These peaks have agreed well with PAni

values previously reported in literature. In comparison, the FTIR spectra for GnPs/PAni composite

offered similar peaks, however, there was significant difference in the 1658.2 cm-1 and 1400.2 cm-

1 peaks which indicate quinoid and benzenoid ring C=C stretching bands, respectively. This was

indicative of the GnPs presence in the composites. For the Al2O3/PAni composite, the peak at

1006.2 cm-1 is representative of Al-OH bonding for octahedrally coordinated aluminum. [175] Peaks

54

around 557.1 cm-1 to 677.5 cm-1 bands can be attributed to Bands due to the Ti-O bonds vibrations

were observed in the range of 491.51 cm-1 to 573.8 cm−1 in the case of TiO2/PAni composite

electrodes, while the other peaks were characteristics of PAni bonds. [175, 176] For the

MWCNT/PAni samples, the band around 1405.7 was characteristic of C=C bonds and indicative

of MWCNT presence.

From the previous survey of materials, it was observed that the best performing electrode was the

GnPs/PAni composite powder electrode, which has achieved a significant increase in specific

capacitance from 173.7 F/g of pristine PAni to a value of 326.9 F/g, representative of an 88%

increase in the charge storage behaviours. In addition, it was also found that the rate capability was

also improved to around 30.9% decrease in specific capacitance at a higher scanning rate of 100

mV/s. Using TGA thermal analysis, it was shown that the GnPs nanoparticles accounted for 12.2%

of the total mass of the composite. This section discusses the effect of varying the GnPs/PAni

composition by varying the percentage of the GnPs particles within the composite. The

experimental procedure was kept consistent as the previous section, but varied the amount of GnPs

particles initially placed in the solution. Due to the polymerization process of PAni, and the

formation of additional oligomers that is not effectively attached on the GnPs surface, the

percentage composition is determined using the TGA. In this study, GnPs percentages were varied

at 7%, 12%, 16%, and 32%, as determined with thermal analysis, as shown in Figure 3-16.

Figure 3-16: TGA analysis of GnPs/PAni composite electrodes ramping 20°C to 900°C

55

As shown in Figure 3-17, it was found that the 7GnPs/93PAni performed the best in terms of the

specific capacitance, achieving 357.1 F/g at 10 mV/s scanning rate, with reasonable rate

capabilities. The 12GnPs/88PAni also showed encouraging performance of 326.9 F/g. As the GnPs

content increases, there was a sudden decrease, however, the trend was stable even at higher GnPs

percentages. With more GnPs added, it was found that the adhesion of the powder composite

became less, and it was more difficult to fully attach the powder electrode onto the current

collector, with the same amount of PTFE and graphite ink binder.

Figure 3-17: A) The specific capacitance measurement showing a decreasing trend in the charge storage performance

with higher GnPs percentages; B) The rate capabilities of the varying composition of the powder electrode;

The rate capability of each composition of PAni/GnPs composite was also analyzed. It is found

that the 16GnPs/84PAni composition showed the best rate performance at 39.2% drop in

capacitance, while the 7GnPs/93PAni composite, despite with the highest specific capacitance,

showed very large decrease in specific capacitance as the scanning rate was increased, a 54.2%

decrease. The shape of the CV curves are similar to the GnPs/PAni composite electrodes shown

in the earlier sections and therefore are not discussed further here.

The morphology of each PAni/GnPs sample with varying compositions was very much alike, as

shown in Figure 3-18. However, with some careful examination, it was found that with less GnPs

added to the composite, the PAni was able to fully utilize the large surface area offered by the

GnPs platelets as locations for polymerization, for which the powder electrode demonstrated a

layered effect, where the openings allowed for the free penetration of electrolytic ions for increased

electrochemical interactions with the PAni active layer.

56

Figure 3-18: SEM images showing: a) 32GnPs/68PAni; b) 16GnPs/84PAni; c) 7GnPs/93PAni

From this study, it has been found that the nanostructure formation and appropriate choice of

composition in the fabrication of PAni-based nanocomposites can significantly improve the two-

electrode device performance of the supercapacitor electrode. A significant increase in specific

capacitance has been achieved, from 173.7 F/g of pristine PAni to a value of 326.9 F/g in the case

of GnPs/PAni composite. In the case of Al2O3/PAni composite, a quantifiable improvement in

specific capacitance was also observed, achieving a specific capacitance value of 215.9 F/g, giving

rise to the possibilities of incorporating conductive polymers with metal oxides. From the SEM

images, the increase in capacitance performance was mainly attributed to the 3D structure

formation as a result of desirable interactions between the composite material components. The

increase in specific surface area led to increase in electrode-electrolyte interfacial interactions,

which resulted in the device performance improvements. The reduction in charge transfer

resistance as determined by impedance spectroscopy has also shown an improvement in

supercapacitor performance with the GnPs/PAni composites. GCD test verified the specific

capacitance results as determined by the cyclic voltammograms. TGA and FTIR results have

confirmed the material composition of the nanocomposites. When varying the GnPs/PAni

compositions, it was found that the 7GnPs/93PAni performed the best in terms of the specific

capacitance, achieving 357.1 F/g at 10 mV/s scanning rate, with reasonable rate capabilities.

57

Highly flexible binder-free core-shell nanofibrous electrode for

lightweight electrochemical energy storage

This chapter discusses a novel approach to constructing lightweight, highly flexible electrodes

using fiber templates and assisted by ultrasonication. The first flexible substrates investigated was

polyethylene terephthalate (PET) obtained from recycled plastic bottles (PETE1 grade). Active

materials were PAni and GnPs, coated independently.

The PAni@PET fiber electrode achieved a specific capacitance was found to be 347 F/g with

aqueous H2SO4 electrolyte, at a scanning rate of 10 mV/s, with excellent rate capabilities and

resistance to degradation. The core-shell structure formation allowed the electrolytic ions to

transfer into the electrode network without excessive impedance. The GnPs@PET fiber electrode

was successfully fabricated with a similar ultrasonication assisted, which has achieved a very

symmetrical and rectangular CV diagram with extremely high capacitance retention even at very

high scan rates. It has achieved 72.1 F/g capacitance with aqueous H2SO4 electrolyte at 100 mV/s

scanning rate, if the GnPs weight was considered the active mass. The GnPs@PET fiber composite

electrode also experienced very little degradation after 3000 charge/discharge cycles, highlighting

the composite’s resistance to degradation even after continued usage. The adhesion of the GnPs

on top of the PET fiber has also proved to be stable after repeated washing.

The second material utilized as the flexible substrate was a type of nylon membrane, used for west

blotting. PAni particles were successfully coated onto the porous structure, thus, allowing charge

to be stored on the flexible surface. GnPs was too wide to be inserted into the small pores offered

by the nylon membrane and therefore, was not successfully fabricated.

58

Herein, we investigate the creation of a novel flexible nanocomposite fiber with conductive

polymer polyaniline (PAni) coating on a polyethylene terephthalate (PET) substrate allowed for

increased electrochemical performance while retaining the ideal mechanical properties such as

very high flexibility. The binder-free PAni-wrapped PET (PAni@PET) fiber with a core-shell

structure was successfully fabricated through a novel technique. The PET nanofiber substrate was

fabricated through optimized electrospinning method, while the PAni shell was chemically

polymerized onto the surface of the nanofibers. The PET substrate can be made directly from

recycled PETE1 grade plastic water bottles. The resulting nanofiber with an average diameter of

121 nm, with a specific surface area of 83.72 m2/g, led to better ionic interactions at the

electrode/electrolyte interface. The PAni active layer coating was found to be 69 nm in average

thickness. The specific capacitance was found to have increased dramatically from pure PAni with

carbon binders. The specific capacitance was found to be 347 F/g at a relatively high scan rate of

10 mV/s. The PAni/PET fiber also experienced very little degradation (4.4%) in capacitance after

1500 galvanostatic charge/discharge cycles at a specific current of 1.2 A/g. The mesoporous

structure of the PAni@PET fibrous mat also allowed for tunable capacitance by controlling the

pore sizes. This novel fabrication method offers insights for the utilization of recycled PETE1

based bottles as a high performance, low cost, highly flexible supercapacitor device.

With the introduction of flexible wearable electronics devices such as smartwatches, medical

sensors, curved smartphones, and smart textiles, there is an immediate need for flexible

components such as active matrix organic light-emitting diode (AMOLED) digital displays,

printed circuit boards (PCBs), and lightweight energy sources. Novel materials for these

applications have been studied intensively in the past decade [70, 177-182]. With the demanding energy

1 Shi, H. H., & Naguib, H. E. (2016). Highly flexible binder-free core–shell nanofibrous electrode for lightweight electrochemical

energy storage using recycled water bottles. Nanotechnology, 27(32), 325402. https://doi.org/10.1088/0957-4484/27/32/325402,

© 2016 IOP Publishing, Reproduced with permission. All rights reserved.

https://doi.org/10.1088/0957-4484/27/32/325402

59

consumption from these wearable devices, energy sources with increasing power and energy

densities are continuously being developed. Supercapacitor or electrochemical capacitor (EC)

represents a new class of energy storage devices capable of delivering high power and energy

density simultaneously [3-5, 183]. The high power rating of ECs at around 10 kW kg–1 allows this

type of energy storage devices to be attractive for fast charging of personal electronics [177, 184].

Concurrently, researchers have been looking for new ways to reduce production cost, enhance the

cycling performance while preserving the flexibility and lightweight of the EC cells [70, 182, 185-187].

The charge storage mechanisms of electrochemical capacitor are mainly electrical double layer

capacitance (EDLC) and pseudocapacitance [5]. Recent progress made on EDLC materials focuses

on high-surface-area carbon based materials such as graphene, graphene oxide, and multi-walled

carbon nanotubes (MWCNT), which utilizes the charge separation in the Helmholtz layer formed

at the electrode electrolyte interface to store energy [44, 70, 72, 188-190]. However, due to the low

specific capacitance obtained from these carbon materials, EDLC type materials are typically

integrated with pseudocapacitive materials that employ fast faradaic interactions to store charge.

The objective of many studies was to obtain a composite hybrid that takes advantage of both charge

storage mechanisms to yield an optimal storage regime [40, 142, 191-193]. Pure EDLC systems offer far

superior cycling performance, with typically less than capacitance 5% drop after more than 5000

charging and discharging cycles [56, 72, 183, 184, 189, 194], while pseudocapacitive materials, such as

metal oxides and conductive polymers, boost the energy and power density of the overall

supercapacitor devices. However, the main limitations for these powder composite electrodes were

their lack of flexibility and the need for binder materials.

Many advances in flexible ECs have been made through various fabrication methods with the

deposition of active materials on flexible substrates, such as paper, polymers, etc., in order to

obtain complete functionality while maintaining its structural flexibility [142, 182, 184, 194, 195]. Current

studies began investigating the possibilities of attaching charge storage materials onto flexible

porous surfaces that allows the electrolytic ions to be efficiently transported onto the electrode

surface[196]. Many have suggested the optimal pore sizes to be within the range of 2 nm to 50 nm

or mesoporous that maximize the ion-accessible surface areas, without hindering the transport

phenomenon [56, 122, 128, 197]. The proper pore selection offers the key to achieving ideal specific

capacitance and rate capability for hybrid ECs. Many surface modification techniques were

60

employed to create higher specific surface area for increased electrolyte/electrode interactions [177].

However, the adhesion properties and the issues with tear and wear in the case of paper-based

electrodes remain some of the biggest challenges for flexible implementations.

Polyethylene terephthalate (PET) thermoplastic resin is used extensively for various applications.

Every year, around 43 billion PET bottles have been sold in the United States alone, but only 30%

of these are recycled for future use. The wasted PET resin can cause severe damage to the

environment due to its slow degradation regime. If it was possible to recycle the PET thermoplastic

resin from water bottles for energy storage purposes, it would open up entirely new possibilities

for the disposal of such plastic wastes.

Herein, we examine a novel fabrication route, using recycled PET based water bottles, to create a

porous core-shell fiber structure for high performance EC construction. The structural backbone

is an electrospun nanofibrous PET network with an average diameter of 121 nm. The active

material is a 69 nm pseudocapacitive conductive polymer polyaniline (PAni) shell layer well

attached on the surface of core PET fibers. Electrospinning technique has been employed

previously in constructing polymer nanofibers in the diameter range of 50 to 200 nm for various

mechanical and electrical property enhancements. However, the core-shell structure formation of

recycled PET with an active layer of PAni coating (PAni@PET) has not yet been reported for the

energy storage applications. The specific capacitance for the PAni@PET flexible fibrous binder-

free electrodes measured 10 mV/s showed 347 F/g, indicating a dramatic increase from pure PAni

electrodes with carbon binder. Some of the key improvements over the previously reported flexible

electrodes are the ideal cycling capabilities, extremely flexible characteristics, low charge transfer

resistance, good adhesion, high specific surface area, and high hydrophilicity.

The PET resin for the construction of the PET nanofiber substrate was obtained from the recycled

water or other beverage bottles with identification of PETE1 recyclable logos. The solvents,

namely trifluoroacetic acid (TFA, ReagentPlus, 99%) and dichloromethane (DCM, anhydrous,

≥99.8%) utilized for electrospinning process was purchased from Sigma Aldrich without further

modifications. The aniline monomer (ACS reagent, ≥99.5%) was obtained from Sigma Aldrich,

and underwent distillation to obtain a golden transparent color before usage. Hydrochloric acid

61

(HCl) and ammonium persulfate used in the chemical polymerization of PAni were used without

further modifications.

Figure 4-1. Fabrication process of the nano-sized PET polymer core fibers from the recycled PETE1 recyclable

beverage bottles.

Figure 4-2. a) The PETE1 recyclable beverage bottles cut into strips; b) The PET strips were further cut into

rectangular pieces prepared for dissolution in the 1:1 DCM/TFA solvent; c) The clear viscous solution with 10 wt.%

PET resin in 1:1 DCM/TFA solvent for electrospinning

As shown in Figure 4-1 schematics, and Figure 4-2 experimental pictures, the raw PET resin was

obtained from various recycled beverage bottles by cutting them into small, thin strips. The

obtained PET strips were dissolved in a solvent mixture of 1:1 molar ratio TFA:DCM, as

commonly used for electrospinning PET. The amount of PET resin dissolved was controlled at 10

wt. %. The solution was then transferred to a sealed glass bottle and magnetic stirred at 200 RPM

for 24 hours before the electrospinning process. A viscous, clear PET electrospinning solution can

be obtained. The electrospinning process was carried out using a conventional electrospinning

chamber with controlled temperature and relative humidity (R.H.). The chamber was first pre-

conditioned to a temperature around 30 °C, with the R.H. below 10 %. The metal syringe tip was

1.58 mm in diameter, fitted with a polytetrafluoroethylene (PTFE) syringe at 21.41 mm in

62

diameter. The syringe was filled with the electrospinning solution, then secured onto the syringe

pump (New Era Pump Systems, NE 300). The collector was a grade 304 stainless steel sheet. A

high voltage DC source was connected to the metallic syringe tip, whilst the fiber collector was

grounded. The source DC voltage used for the process was 15 kV, and the distance between the

high voltage needle tip and the fiber collector was set to 10 cm. The pumping rate was set to 0.04

mL/Hr. and the entire process continued for 30 minutes to obtain a thin flexible white film. The

film was then be detached from the collector and placed within the vacuum oven at 60 °C for 12

hours.

Figure 4-3. The rigorous stirring process allows the surface to become somewhat hydrophilic, in that the fiber mat

sank in the solution. Polymerization process took place at the liquid-solid interface and allowed better adhesion

between the PAni shell and the PET core; after 4 hours of polymerization, a PAni@PET core-shell structure was

formed

As shown in Figure 4-3, the formation of PAni was carried out via an in-situ chemical

polymerization process. The oxidant used was ammonium persulfate (APS), and the acidic

environment required for protonation was created with 1M HCl. Upon doping with an acid, PAni

emeraldine base can be converted to emeraldine salt, which is electrically conductive and suitable

for charge storage applications [198]. A solution with 4 mmol of APS and 20 mL HCl was made

and the previously obtained PET core nanofiber was placed in the solution. The PET nanofiber

core was hydrophobic at first when placed in the solution, and therefore needs to be stirred

vigorously for 4 hours before the entire film is under solution surface. This process is very

important in obtaining uniform core-shell structures throughout the film surface. Another solution

with 4 mmol of aniline monomer and 20 mL of distilled water was made and stirred rigorously to

form a white opaque solution. The solution containing aniline was then added dropwise into the

63

oxidant/acid solution. The mixture was then placed in an ice bath at 0 °C for 4 hours to allow for

polymerization, which can be characterized by a significant color change [199]. After the

polymerization process, the PAni@PET film was washed thoroughly with distilled water and

ethanol to remove excess monomers and oligomers. The washed PAni@PET fiber was then dried

overnight in an oven at 50°C.

Electrochemical characterization were performed using a Solartron 1470E multi-channel

potentiostat, with a 2-electrode symmetrical cell setup with 1 cm2 pieces of PAni@PET nanofibers

serving as the electrodes, an aqueous 1M sulfuric acid (H2SO4) as the electrolyte and thin (0.05

mm) flexible stainless steel sheets as the current collector. CV measurements were performed at

varying scan rates, ranging from 5 mV/s to 2000 mV/s, with a voltage window from -0.2 V to 1.0

V, which was selected with respect to the optimum operating condition for PAni-based electrodes.

Electrochemical impedance spectroscopy (EIS) was performed from a high frequency of 105 Hz

to a low frequency of 0.02 Hz. The GCD was performed at a current density of 12 A/g and 1.2

A/g, with the voltage cycling between 0 V and 1.0 V. The specific capacitance was calculated

based on the weight of the entire PAni@PET fiber electrode. The electrochemical measurements

were averaged over five sets of samples and the repeatability has been verified.

The morphology of the electrode surface structures was analyzed using an FEI Quanta FEG 250

environmental scanning electron microscope (ESEM). The water contact angle test was carried

out with standard sessile drop technique with drops of 20 μL of deionized water on the surface,

measured at 25°C. Brunauer-Emmett-Teller (BET) specific surface area analysis was performed

by Quantachrome Instruments Nova 1200e, with CO2 as the adsorbate gas and a degassing

temperature of 200°C. Fourier transform infrared spectroscopy (FTIR) was performed with a

Bruker ALPHA FT-IR Spectrometer, averaged over 25 scans. Thermal gravimetric analysis (TGA)

was carried out with TA Instrument Model Q50 at a temperature scan rate of 20°C/min, from 20°C

to 900°C, in order to verify the composition of the PAni@PET fiber structures. Roughness and

surface characteristics were measured via the usage of an atomic force microscope (AFM), namely

a Bruker MultiMode 8 with PeakForce tapping mode.

64

The PET nanofibers were obtained via conventional electrospinning technique, without the aid of

rotating collectors for alignment. The alignment of the fibers will disrupt the proper size

distribution of the pores that allows the electrolytic ions to travel freely for interfacial charge

storage interactions with the electrode. Previous studies noted that the mesoporous structures with

pore diameters in the range of 2 nm to 50 nm are the most appropriate for supercapacitor electrode

applications [4]. However, it was found in our study that a blend of macro-, micro-, and meso-

porous structures allowed for more efficient charge transfers at the electrode/electrolyte interface,

hence better performance at the fast scanning rates of even up to 500 mV/s. This distribution of

various types of pores gave rise to more accessible surface areas, and therefore, it was deduced

that randomly distributed fiber structures can sufficiently meet the objectives of maximizing

accessible surface areas for electrolytic ions. Via the conventional electrospinning technique, PET

core fibers with an average diameter of 121 nm was produced. The overlapping or crisscross nature

of the PET fibers in the mat depth allowed for formation of high-surface-area 3D structures that

effectively increased the available surface for ion access, without hindering the ion transport, as

the mix of macro-, micro-, and meso- pores allowed for highly efficient transport routes to form.

Table 4-1: PET Film Physical Characteristics with respect to Deposition Time

As shown in Table 4-1, the PET electrospun core mat thickness is a function of time and therefore,

the time allowed for electrospinning process was carefully controlled in obtaining the film with

ideal thickness and flexibility. The observed density was very consistent at around 0.71 g/cm3, and

the porosity was calculated to be around 50%. The availability of porous structures allows the

efficient insertion of oxidant, acid, and the monomer for the in-situ polymerization to take place.

The trade-off here is the formation of thicker films can increase the three-dimensional surface area,

allowing higher per-area capacitance values, but the material flexibility can be jeopardized. Also,

as was observed, thicker PET substrates can impede the in-situ chemical polymerization process,

Deposition Time

(minutes)

Thickness

(µm)

Area-specific Weight

(mg/cm2)

Observed Density

(g/cm3)

Porosity

(%)

10 7 0.5 0.714 48.24%

30 16 1.2 0.750 45.65%

60 33 2.3 0.697 49.49%

90 55 4 0.727 47.30%

65

causing uneven PAni attachment, while hardening the film, making it unsuitable for potential

flexible applications.

Figure 4-4. a) SEM Morphology of Pure PAni formed from in-situ chemical polymerization process with PET solid

film as substrate; b) & c) Morphology of the Pure PET fibrous core: the electrospun PET fibrous structure was from

the recycled PET bottles, these fibers have an average diameter of 121 nm, and observed to have a smooth surface; d)

& e) PAni@PET core-shell structure: after the PAni coating has been applied, it is evident that an even layer of PAni

shell has been adhered on the PET surface; f) At high magnifications, some exposed fiber core shows the internal

core-shell structure as predicted during fabrication.

As shown in Figure 4-4 a), by only using a flat, solid, PET film as the substrate, the aniline

monomer was unable to reach the surface pores of the PET material and can only form large PAni

agglomerates not well-adhered on the PET surface, indicating a weak interaction between PAni

and PET. The PET solid flat film was formed via a solvent casting technique with the same

recycled PET resin and treatment as for the PET core fibers.

Figure 4-4 b) and c) showed the morphology of electrospun pure PET nanofiber core. BET surface

area analysis with CO2 adsorbate gas showed a specific surface area of 73.12 m2/g for the pure

PET electrospun fibers. The increased surface area was created via the interlacing fibrous three-

dimension structure and the interconnected nature throughout the depth fiber membrane.

66

Figure 4-5. a) Fiber sample diameter distribution with varying diameter bins; b) sample of the flexible thin pure PET

fiber mat; c) SEM image of the pure PET electrospun fiber; d) the flexible PAni@PET core-shell fiber shown under

bending; e) The SEM image of the PET fiber coated with an average of 69 nm of PAni active shell layer

As shown in Figure 4-5 a), the average diameter of these fibers is measured to be 121 nm, with

fiber diameters ranging from 24.2 nm to 301.5 nm. This non-woven textile structure offered the

ideal pore sizes from the crisscross patterns formed by the randomly distributed fibers, which

allowed the oxidant and aniline monomer solutions to polymerize at the liquid-solid interface,

which led to the appropriate PAni@PET core-shell formation. The electrospinning parameters

were tuned in order to avoid the formation of beads and other non-uniformity. Figure 4-4 d) and

e) showed the very evenly distributed layer of PAni shell formed via the in-situ chemical

polymerization method as described. The average thickness of the PAni shell layer was measured

to be around 69 nm and gives an overall core-shell structure thickness of around 260 nm, which

was also presented in Figure 4-5 a).

Figure 4-6: Additional SEM images of a) & c) The pure electrospun PET fiber from the recycled PET bottles at

30,000× and 100,000× magnification; and b) & d) The PAni@PET core-shell fibrous electrodes at 30,000× and

100,000× magnification

67

After the core-shell structure formation, as demonstrated by the bending tests and SEM images

shown in Figure 4-5 d) as well as Figure 4-6, the PET backbone continues to provide flexible

structural support and good surface adhesion properties for the active conductive polymer layer to

better interact with the electrolytic ions. Via the thermogravimetric analysis (TGA) measurements,

shown in Figure 4-7, the PAni shell accounted for 22.7% of the overall mass of the core-shell

structure. Figure 4-5 b) and c) also showed the sample and the SEM image of the pure PET

electrospun mat. The low weight and high flexibility were some of the desired features of the

substrate material.

Figure 4-7: Thermogravimetric analysis (TGA) of pristine PAni and pure PET fibers in comparison with the

PAni@PET core-shell fiber mat; it can be extracted from the graph that PAni@PET core-shell structure contains

22.7% PAni shell and 77.3% PET core. The degradation temperature was observed to be shifted 41°C upwards with

the PAni@PET composite fibers in comparison with the pure PET fibers.

Figure 4-5 e) showed the uniformity and the complete coverage of the coating layer on the PET

fiber substrate in the PAni@PET core-shell composite. The specific surface area increased slightly

to 83.72 m2/g from the pure PET fiber, indicating of additional micro-porous structures of less than

2nm in diameters forming on the PAni shell surfaces.

Fourier transform infrared spectroscopy (FTIR) has been performed on both the pure PET

electrospun film, as well as the PAni@PET core-shell fiber structures, as shown in Figure 4-8 and

Table 4-2.

68

Table 4-2: A comparison table of FTIR characteristic bands between pure PET fibers and PAni@PET core-shell fibers

in regards to the perspective bond vibrations; it is observed that the core-shell PAni@PET structure demonstrated both

characteristic peaks for PET fiber and PAni particles;

Figure 4-8: A comparison of the Fourier transform infrared spectroscopy(FTIR) absorbance spectra of Pure PET fibers

and the PAni@PET core-shell fibers with the wavenumbers for the peaks labelled in their perspective colors, for

details in regards to the band assignments, please refer to

The results clearly indicated the difference in the IR spectra between the pure PET fibers and the

PAni@PET fibers, with the additional characteristic peaks of PAni shown in the PAni@PET fiber

composite materials, confirming the composition. The flat surface of the flexible electrode was

also ensured in fabricating the electrodes. The PAni@PET composite fiber mat was very well

adhered onto the stainless current collector using carbon paste. As shown in the Figure 4-9, the

roughness factor Ra of the surface was measured using AFM technique to be 37.7 nm, similar to

that for pure PET fibers at 30.5 nm.

Characteristic Wavenumbers (cm-1) Assignment

Pure PET Fiber PAni@PET Core-Shell Fiber

Common 3100 cm-1 - 2800 cm-1 vibrations Aromatic and aliphatic –C–H

1715 cm-1 1715 cm-1 Ester group stretching

1572 cm-1 Quinonoid ring stretching

1490 cm-1 Benzenoid ring stretching

1300 cm-1 C–N stretching of secondary aromatic amine

1266 cm-1 Mainly due to ester C–O stretching

1245 cm-1 C–N in the polaron lattice of PAni

1101 cm-1 1099 cm-1 Cis isomer of ethylene glycol or C=O stretching

1017 cm-1 1017 cm-1 In-plane bending of C–H bond

873 cm-1 Out-of-plane benzene

729 cm-1 726 cm-1 Out-of-plane bending of C–H bond

602 cm-1 Out-of-plane O–H vibrations of alcohol

506 cm-1 CO2 deformation

69

Figure 4-9: a) Atomic Force Microscopy (AFM) image of pure PET fiber mat surface, the surface roughness factor Ra

was measured to be 30.5 nm; b) AFM image of PAni@PET core-shell composite fiber mat surface, the surface

roughness factor Ra was measured to be 37.7 nm;

One of the key features of the PAni@PET core-shell fiber structure is its high surface energy. The

high surface energy allows better ionic interaction with the typical aqueous electrolyte used for

electrochemical capacitors. The pure PET fiber is shown to have a water contact angle of 131.66°,

while the PAni@PET core-shell structure measured a water contact angle of 8.36°, as shown in

Figure 4-10 a) and b). It has been reported previously, that flat solid pure PET surface has a water

contact angle of 72.4°, indicative of partial wetting.

With the electrospinning treatment, it was found that the modified PET surface led to a

significantly decreased surface energy, indicated by the increase in water contact angle. The

decreased surface energy can be attributed to the formation of overlaying nanofibers, which

affected the surface interfacial interactions. After the formation of the PAni shell layers on the

PET nanofiber surface, the almost complete wetting indicated that the PAni shell was wrapped

uniformly throughout the PET electrospun nanofiber surfaces. This can be readily indicated by the

hydrophilicity of the PAni@PET fibrous core-shell structure in comparison to the hydrophobic

pure PET nanofiber.

As demonstrated with the schematics in Figure 4-10 c) and d), the high surface energy allowed for

the aqueous electrolyte system to readily penetrate into the available electrode surfaces which led

to better charge storage behaviours and lower charge transfer resistance.

70

Figure 4-10: a) & b) The water contact angle measurements for the pure electrospun PET and the PAni@PET core-

shell fiber structures. It was evident that the PAni@PET core-shell structure has a much better wetting and therefore

indicating a higher surface energy; c) & d) Schematics showing the difference in terms of ion accessibilities in relation

to the wetting parameters of the fiber surfaces, with pure electrospun PET film, the aqueous electrolytic ions were not

able to readily access all surfaces

The viability of the PAni@PET core-shell nanofiber structure to be employed in energy storage

system has been examined using a two-electrode symmetrical electrochemical cell setup, which

was selected to present a better estimation of the actual capacitance offered by the electrode. The

specific capacitance calculations from the three-electrode setup quadruples the values shown here

and may overestimate the EC performance in some cases. The CV scan rates were varied between

5 mV/s to 2000 mV/s, and the results are shown in Figure 4-11 a) and b). It was observed that even

at very high scanning rates at 2000 mV/s, the PAni@PET nanofibers still experienced capacitor

behaviours indicating that the electrode three-dimensional matrix allowed electrolytic ions to

travel freely through the thickness of the electrode material and interact with the PAni shell

effectively.

Figure 4-11. a) CV scans with variation of scan rates from 5 mV/s to 2000 mV/s; b) CV graphs with scan rates varied

between 5 mV/s to 100 mV/s; c) The calculated specific capacitance with varying scan rates;

71

When examining the range of CV curves with slower scan rates from 5 mV/s to 100 mV/s, the

redox peaks were apparent with the current peaks in the charge/discharge processes. The

uniformity of the CV curve shapes indicated that by varying the scan rates between 5 mV/s and

100 mV/s, the charge transfer process taking place in the porous electrode surface was not readily

affected. The specific capacitance at 10 mV/s as calculated from Equation 1) was 347 F/g, which

presented a dramatic increase from the pristine PAni powder electrodes with carbon binders. The

efficient contact between the electrode and the electrolytic ions was also demonstrated with Figure

4-11 c), the relatively small decrease in specific capacitance values as the scan rates were

increased. Even at a very high scan rate of 500 mV/s for conductive polymers, the PAni@PET

nanofiber electrode still retained a specific capacitance of 244.9 F/g, which still represented a

relatively high capacitance for electrolytic capacitors. This observation can be attributed to the

optimally sized porous structures that allowed high surfaces of the active PAni shell layer to be

utilized for charge storage without hindering the ion transfer.

Figure 4-12. a) The CV scans at 10 mV/s comparing between pristine PAni electrodes and the PAni@PET core-shell

nanofibers, it is evident that the capacitance was increased dramatically from the pristine PAni powder electrode with

carbon binders; b) The charge/discharge curves comparing between pristine PAni and PAni@PET nanofibers; c) The

presentation of 4 cycles of charge/discharge at 1.2 A/g current.

As show in Figure 4-12 a), the specific capacitance of binder-free PAni@PET nanofibers measured

at 10 mV/s showed a significant increase in comparison to pristine PAni powder electrode with

carbon ink binders. From the charge discharge curves, it is also evident that not only the specific

capacitance increased with the core-shell PAni@PET structures, the IR drop was also much less.

The carbon binder clearly introduced higher internal resistance at the electrode/current collector

interface and the higher thickness of the pristine PAni electrodes were also contributing to the

higher IR drops. The discharging time for PAni@PET nanofiber in a typical discharge cycle is

147.4 s, which showed a higher energy density in comparison to previously reported values. With

a 1.2 V applied voltage window, it can be calculated that PAni@PET nanofiber electrodes offered

72

an energy density of 69.4 Wh/kg, which is comparable to a typical NiCad battery. From the

galvanostatic 4-cycle test, it was observed that the symmetrical shape of the charge/discharge

cycles were also very consistent. This type of electrochemical supercapacitor device can be easily

adapted for flexible energy storage purposes for low power personal or biomedical devices, due to

its high energy density, good cycling capabilities, and good mechanical behaviours.

Figure 4-13. a) Electrochemical impedance spectroscopy (EIS) nyquist plot for the PAni@PET nanofiber structure

with measurements from 100kHz to 0.5Hz shown; b) The Bode plot of the impedance magnitude; c) The Bode plot

of the imepdance angles;

Figure 4-13 a) showed the complex plane Nyquist plot of the impedance of the PAni@PET

nanofibers. The curve intercepted the real-axis at around a 45° angle indicating an efficient porous

electrode behaviour. The ESR was estimated at 1 kHz to be around 223 mΩ, which demonstrated

a very low contact resistance at the electrode/electrolyte interface. The series resistance of around

1.0 Ω was from the aqueous 1M H2SO4 electrolyte system. The almost vertical slope of the Nyquist

plot showed ideal capacitor behaviours as the frequency decreased, which verified the hypothesis

that the porous structures created with the electrospinning process aided the charge storage

efficiency. One of the key benefits delivered by the PAni@PET nanofibers is the cycling capacity.

From Figure 4-14 a), it is clearly shown that the CV measurements taken at 0 cycles and 1500

cycles have been similar in both shape and magnitude. Figure 4-14 b) showed the galvanostatic

charge discharge behaviour comparison between the PAni@PET nanofiber behaviour at 0 cycles

and 1500 cycles. In the 1500 charging cycles, the capacitance degraded only from 347 F/g to 331.9

F/g, which represents only a 4.4 % decrease in charge storage capacities, a significant improvement

over the previously studied PAni-based electrode systems. And the discharging time intervals only

decreased from 147.4 s to 142.8 s, which was also an insignificant drop in capacitance. Therefore,

it can be concluded that the PAni active surface was well attached on the PET substrate surface,

73

and suffered almost no apparent loss of capacitance after 1500 cycles, which is already a high

cycle number for conductive polymer based pseudocapacitor devices.

Figure 4-14. a) Cycling behaviour displayed with the comparison of CV curves at 0 cycles and 1500 cycles; b)

Comparison of the charge/discharge cycles at 0 cycles and 1500 cycles; c) The specific capacitance measured at

various cycle numbers

A novel core-shell PAni@PET nanofibrous structure has been successfully fabricated with

recycled PETE1 type recyclable beverage bottles. The weak mechanical properties of PAni has

been readily compensated with the PET flexible structural support with good adhesion. From the

morphology analysis, the PAni@PET nanofiber has a PET core with 121 nm in average diameter,

while the PAni shell layer has an average thickness of 69 nm. The dispersion property of PAni

shell layer was examined with surface energy analysis to show that the PAni wrapping has been

very uniform throughout the structure. The higher surface energy led to better ionic interactions

between the PAni@PET nanofiber electrode and the electrolytic ions. The specific capacitance

was found to be 347 F/g at a scan rate of 10 mV/s, while even at a very high scan rate of 500 mV/s,

the capacitance only decreased to 244.9 F/g, which represented a very good specific capacitance

value for pseudocapacitive materials. The energy density was measured to be 69.4 Wh/kg,

comparable to a typical NiCad battery system. EIS showed that the charge transfer resistance was

also minimal, along with a BET specific surface area measurement of leading to the conclusion

that the blend of macro-, micro-, and meso-pores aided the charge transfer process, without

hindering their interactions with the electrode. The galvanostatic cycling test showed that after

1500 cycles, the specific capacitance only dropped 4.4%. It was deduced that the formation of

three-dimensional porous structures throughout the electrode contributed to the stability of the

charge storage capabilities and the high energy density offered by the symmetrical two-electrode

device.

74

Herein, we present a novel strategy that allows GnPs to be dispersed evenly and well-adhered to a

nano-sized fibrous PET structure that allowed efficient access by the solvated ions to interact with

the large exposed active surface area. Scanning electron micrographs (SEM) revealed core-shell

structure formation of GnPs on top of the PET nanofibers. The average diameter of the pure

electrospun PET nanofibers is 114 ± 41.3 nm, with the GnPs shell of 10-30 nm in thickness. The

specific capacitance has been measured to be 72.1 F/g at a relatively fast cyclic voltammetry (CV)

scan rate of 100 mV/s, based on the weight of the active GnPs material present on the substrate

surface. This indicates the large exposed carbon surface area to the solvated ions within the

aqueous electrolyte in the supercapacitor cell has reduced the charge transfer resistance at the

interface to only 0.4 Ω, indicating efficient charge transfer taking place. Galvanic charge/discharge

(GCD) tests showed the IR drop at the start of discharge cycles is minimal. Cycling tests verified

the cycling stability of the GnPs@PET electrodes. The specific capacitance was measured to be

70.7 F/g after cycling 1000 times, corresponding to a 98.1% capacitance retention rate. The CV

performed at relatively high scan rate showed that the specific capacitance did not dramatically

decrease even at 100 mV/s scan rate, and the cyclic voltammetry still retained an ideal shape even

at a very high scan rate of 2000 mV/s. The resulting energy density with a voltage window of 1.2

V is calculated to be 14.41 Wh/kg, and the power density is 1.16 kW/kg. The energy density for

the GnPs@PET fibers is comparable to that of Ni metal hydride battery, with a much higher power

density, which corresponds to a decreased charging time.

Graphene nanoplatelet structures have attracted significant scientific attention as a result of their

unique electrical and mechanical properties. Graphene is a monolayer of carbon atoms tightly

packed into a two-dimensional (2D) honeycomb lattice of sp2 carbon atoms bonded in two plane

directions [66, 200]. The mechanical and electrical properties of graphene materials are astonishing

2 Yousaf M., Shi H. T. H., Wang Y., Chen Y., Ma Z., Cao A., Naguib H. E., Han R. P. S. (2016). Novel Pliable Electrodes for

Flexible Electrochemical Energy Storage Devices: Recent Progress and Challenges. Adv. Energy Mater., 6: 1600490. doi:

10.1002/aenm.201600490. Reproduced with Permission.

75

[67]. The exceptionally thin plate of graphene is believed to be able to offer very high specific area

values and contribute to a number of industrial applications [68, 69]. Large number of composite

materials have been created using graphene as an additive, while others have used pristine

graphene nanoplatelets as electrode materials for energy storage in supercapacitors [52]. Single

sheets of graphene also has the highest electrical conductivity among other materials. The high

theoretical specific surface area of up to 2630 m2/g has been reported for single layer graphene [72,

188]. From quantum capacitance measurements, it has been reported that the intrinsic quantum

capacitance per surface area is 21μF/cm2, leading to an ideal capacitance value of 552.3 F/g when

combined with the theoretical graphene specific surface area [188, 201]. However, in practical

settings, this has only allowed pristine graphene to obtain specific EDLC values in the range of

100 F/g to 200 F/g [72]. With the pseudocapacitance contributions from metal oxides such as RuO2

or MnO2 or conductive polymers such as polyaniline (PAni) or polypyrrole (PPy), the specific

capacitance values of these hybrid devices have reached the range of 200 F/g to 1000 F/g [47, 191,

202, 203]. However, pseudocapacitor materials exhibit slow response times and rapid degradation

rates, which are undesired properties in supercapacitor applications.

In contrast, purely carbon based electrode are considered more attractive as a result of its better

cycling stability and higher resistance to degradation [204]. The ideal properties of graphene

deteriorate rapidly with the increase in the number of stacking layers of the 2D carbon sheets, as

the inter-layer contact resistance and oxygen insertion can dramatically reduce the material

performance. In addition, the flexibility of the graphene nanoplatelets decreases considerably with

stacking of multiple graphene layers. Various fabrication techniques have been employed to

produce thin layered graphene sheets that can be used for thin-film supercapacitors without

hindering its ideal properties [142, 170, 184, 205]. One of the strategies has been laser-scribed graphene

(LSG) on flexible substrates, which achieved an excellent specific capacitance of 265 F/g [184].

However, due to the nature of the laser-based fabrication technology, it is difficult to scale up in

LSG production. Additionally, the pore accessibility of the graphene-based electrode system is

also extremely important as it represents the surface area that can be effectively accessed by the

solvated ions within the electrolyte system during electrochemical charging and discharging

processes [11, 72, 187]. With restacking of graphene sheets, it is found that the pore accessibility

decreases dramatically due to the inter-graphene spaces that are not readily accessible by solvated

ions, leading to poor electrochemical performance [206, 207].

76

Herein, we present a novel strategy that allows graphene nanoplatelets (GnPs) to be dispersed

evenly and well adhered to a nano-sized fibrous structure that opens up the possibilities for

efficient access by the solvated ions to interact with the exposed active surface area. Scanning

electron micrographs (SEM) revealed core-shell structure formation of GnPs on top of the PET

nanofibers. The average diameter of the pure PET nanofibers is 114 ± 41.3 nm, with the GnPs

shell of 10 – 30 nm in thickness. The specific capacitance has been measured to be 72.1 F/g at a

relatively fast cyclic voltammetry (CV) scan rate of 100 mV/s, based on the weight of the active

GnPs material present on the substrate surface. This indicates the large exposed carbon surface

area to the solvated ions within the aqueous electrolyte in the supercapacitor cell has reduced the

charge transfer resistance at the interface to only 0.4 Ω, indicating efficient charge transfer taking

place. Galvanic charge/discharge tests showed the IR drop as the cell begins to discharge is

minimal. Cycling tests verified the cycling stability of GnPs-based electrodes, the specific

capacitance was measured to be 70.7 F/g after cycling 1000 times, corresponding to a 98.1%

capacitance retention rate. The CV performed at relatively high scan rate showed that the specific

capacitance did not dramatically decrease even at 100 mV/s scan rate, and the cyclic voltammetry

still retained the ideal shape even at very high scan rates as high as 2 V/s. The resulting energy

density with a voltage window of 1.2V is calculated to be 14.41 Wh/kg, and the power density is

1.16 kW/kg. The energy density for the GnPs@PET fibres is comparable to that of equivalent to

that of a Ni metal hydride battery, with a much higher power density, which can lead to decreased

charging times.

Poly (ethylene terephthalate) (PET) granular pellets (with 30% glass beads as reinforcer) was

purchased from Sigma Aldrich, and used as is. Grade 3 Research grade graphene nanoplatelet

(GnPs) with an average thickness of 8-15 nm, and an average specific surface area of 500-700

m2/g was purchased from CheapTubes Inc. Solvents used for electrospinning, namely

dichloromethane (DCM, anhydrous, ≥99.8%), trifluoroacetic acid (TFA, ReagentPlus grade, 99%)

were purchased from Sigma Aldrich without further modifications.

The electrochemical characterization was performed with a Solartron 1470E multi-channel

potentiostat using a two-electrode cell testing setup. The current collector was grade 304 stainless

77

steel sheeting, with a 0.05 mm thickness. The electrolyte used for the testing is aqueous 1M H2SO4.

Electrochemical testing was performed at room temperature. CVs were performed at varying scan

rates, ranging from 5 mV/s to 2000 mV/s, with a voltage window from 0 V to 1.2 V which is

selected with regards to the optimum operating condition for carbon-based electrodes.

Electrochemical impedance spectroscopy (EIS) was performed from a high frequency of 105 Hz

to a low frequency of 0.02 Hz. The GCD was performed at a current density of 1 A/g, with the

voltage cycling between 0 V and 1.0 V. The specific capacitance was calculated based on the

weight of the active material GnPs adhered to the PET film after ultrasound treatment. The

morphology of the electrode surface structures was analyzed using FEI Quanta FEG 250

environmental scanning electron microscope (ESEM). Brunauer-Emmett-Teller (BET) specific

surface area analysis was performed by Quantachrome Instruments Nova 1200e, with CO2 as the

adsorbate gas. The thermal gravimetric analysis (TGA) was carried out with TA Instrument Model

Q50 at a temperature scan rate of 10 °C/min, from 20 °C to 600 °C.

The PET nanofibers were first fabricated via a conventional electrospinning method, as shown in

Figure 4-15 A). An amount of PET granular pellets were first dissolved with 1:1 DCM:TFA

solution and magnetically stirred for 24 hours to prepare a well dispersed 10 wt.% PET

electrospinning solution. The PET solution was then loaded into a PTFE syringe to be dispensed.

The temperature and relative humidity of the electrospinning chamber was regulated to 30 ˚C and

10%, respectively. The PTFE syringe tip was then connected with a plastic connection tubing to

the metallic syringe needle located 9 cm above the collector. A 20 kV DC voltage was applied by

a high voltage power supply (Gamma High Voltage Research Inc.) to the metallic needle tip

(1.58±0.01mm in diameter). The syringe pump (New Era NE-300) was set to a rate of 1mL per

hour to initiate the electrospinning process. The process was carried out for 30 minutes to obtain a

very strong, yet flexible thin white PET electrospun film. The PET fiber film was quickly stored

in the vacuum oven to avoid any moisture contact.

Then, in order to produce the GnPs@PET fiber, 50mL of deionized water was used to disperse the

GnPs nanoparticles. An amount of GnPs was then added into the deionized water. Ultrasonication

(Misonix ultrasonic liquid processor model S4000 with a CL-5 type converter) was carried out at

an amplitude of 15, for 30 minutes, to fully disperse the GnPs nanoparticle. PET electrospun fiber

78

film was then placed into the GnPs colloidal suspension to be treated with gentle ultrasonication

to open up the porous structure in order to allow infusion of the dispersed GnPs. An amplitude of

5 was used for a duration of 30 minutes to obtain a black, wetted GnPs@PET film. The

GnPs@PET composite fiber was then taken out of the suspension and washed repeatedly with

clean deionized water and ethanol to remove an excess large agglomerates of GnPs attached on

the surface of the PET nanofiber. The GnPs@PET fiber film was then dried in an oven at 50°C for

24 hours.

Figure 4-15: A) Electrospinning setup used in the PET nanofiber film preparation; B)&C) Scanning electron

microscopy of pure electrospun PET film, at a 30,000x and 50,000x magnification, with a scale bar of 3μm and 2μm;

D) Actual flexible pure PET substrate as fabricated; E) Diameter distribution of PET electrospun nanofibers with

~500 samples using image analysis tools;

As shown in Figure 4-15 B), C) & E), the SEM analysis of the pure electrospun PET flexible film

showed characteristics of nano-sized fibers in the diameters ranging from 32.7 nm to 254 nm, with

an average diameter of 114 ± 41.3 nm. The flexibility of the actual sample was demonstrated in

Figure 4-15 D). Previous studies focus mostly on the surface modification on the electrode material

in order to increase the surface area that allows more effective EDLC for charge storage. However,

it is noted in our study that to obtain higher surface area it is more effective to construct open-cell

porous template structures that allows the solvated ions to penetrate through and reach the

interconnected fibers surfaces to achieve improved EDLC. BET analysis showed a specific surface

area of 73.1 m2/g for pure PET fiber films, demonstrating a suitable high surface area scaffold

material that is capable of hosting high performance active materials.

79

With larger accessible platform for charge storage, it was then possible to coat a very thin layer of

active EDLC material, such as GnPs, on top of the three dimensional fibrous structure to increase

the overall EC performance. With the ideal electrical conductivity of GnPs structures, it was also

possible to form interconnected conductive structures that may eliminate the need for metallic

current collectors. Ultrasonication is a conventional process that utilizes ultrasound as the source

of energy to agitate particles within a liquid sample. This technique has been used extensively in

the past for dispersing nanoparticles and to break the agglomerates into nano-sized entities that

can be used for various scientific and industrial purposes. However, even with the GnPs particles

dispersed by ultrasonication, simply dipping the PET nanofiber into the GnPs colloidal suspension

did not allow the GnPs particles to adhere onto the nanofibers, resulting in large agglomerates on

the surface, as shown in Figure 4-16 A).

Figure 4-16: A) Large agglomerates formation on top of the PET film surface after dipping in ultrasound-dispersed

GnPs suspension; B) Ultrasonication-assisted method to induce better adhered GnPs on the PET surface. The average

thickness of the coating was measured to be 10 ± 4.2nm;

As investigated previously, the interfacial adhesion between flat PET film and the monolayer

graphene has been found to be poor.[208] And it was also reported that graphene interfacial adhesion

to flat PET roll is low in strength and therefore can be used for continuous printing or patterning

onto other better adhered substrates.[209] Therefore, it was not expected that the dispersed GnPs

will effectively attach onto the PET nanofibers substrate by simply dipping the film into the GnPs

colloidal suspension. The main mechanism that drives GnPs to attach to the surface of substrates

is via Van der Waal’s interactions. However, it was observed that the interaction was poor, and

the GnPs were more favored to form large agglomerates rather than adhering to the PET nanofiber

surface. Large GnPs agglomerates of sizes in the range of 1-2 μm were observed throughout the

80

PET substrate surface. Without applying ultrasound energy, the surface fibers are more closely

intertwined, blocking the insertion of GnPs particles, even at a dispersed state.

Herein, it was found that the attachment of the GnPs onto the fibers can be significantly improved

with another ultrasonication method. With the ultrasound-assisted fabrication technique, as shown

in Figure 4-16 B), a unique lightweight, flexible, high surface area material GnPs@PET has been

successfully created. By placing the PET nanofiber mat into the colloidal solution and place the

ultrasonication probe directly above the substrate material. With a gentle sonication at an

amplitude of 10, the GnPs particles was observed to attach much better onto the nanofibers to form

layers of thickness ranging from 6-16 nm after repeated washing, with an average of 10 ± 4.2 nm.

GnPs have a typical thickness of 8-15 nm which correspond well to the thickness increase observed

experimentally. The stacked GnPs allowed for gaps that resembled the mesoporous structures (in

the range 2-25 nm), which was shown to be the most effective porous structures that allows the

ions move freely, while the specific surface area is optimized.[11] Therefore, even though the

layered effect still exists with the GnPs, the energy and power density performance of the electrode

is still considerably higher than other types of graphene-based supercapacitors.[52, 70, 72, 188] The

thickness of the obtained GnPs@PET film was controlled at 0.15 ± 0.01 mm. It was deduced that

the energy from the ultrasound waves effectively opened the porous structures in the PET

nanofiber mat that created pathway for the GnPs particles to travel into the inner pores of the

fibrous mat and making better contact with the PET fiber, thus achieving better adhesion. The

sample after treatment was washed repeatedly with deionized water and ethanol without disrupting

the GnPs conductive coating, which shows that the adhesion has indeed improved between the

GnPs and PET substrate. Sonochemistry can also be used to explain some aspects of the better

adhesion achieved after ultrasonication treatment. With a gentle ultrasonication at a frequency of

20 kHz, cavitation is consistently formed throughout the suspension. The formation, expansion,

and implosion of these localized bubble can cause the temperature to rise to 5500°C, which can be

catalytic for the GnPs adhesion process.[210, 211] The temperature of the bubbles can disrupt the PET

fiber surface, causing localized roughening locally, and therefore allows better GnPs attachment

while the ultrasound simultaneously keeps the GnPs dispersed during the fabrication process. Pure

PET electrospun fibers have already shown promising flexibility in previous studies and therefore

was chosen as the ideal substrate for GnPs.[174] The superflex GnPs@PET electrodes allows the

supercapacitor cell to be highly flexible. The flexibility test has showed that the GnPs@PET fiber

81

mat can be bent 360˚ or twisted one complete turn without exhibiting irreversible strain

deformations, as shown in Figure 4-17.

Figure 4-17: Flexibility testing showing the GnPs@PET fibers' ability to A) bend or B) twist without permanent strain

deformations; C) TGA analysis of the GnPs@PET fiber electrodes compared with pure polymer and GnPs

After the superflex GnPs@PET fibers have been fabricated, its performance as a supercapacitor

electrode has been electrochemically characterized to allow a better understanding of the EDL

capacitance in this structure. TGA results, as shown in Figure 4-17 C) have showed that the

adhered GnPs particles on the PET nano-substrate accounted for 18.22% of the total weight of the

GnPs@PET fiber.

Figure 4-18: A) Mostly rectangular CV graphs measured at different scan rates from 1000mV/s to 5mV/s, with ideal

capacitor behaviours; B) A comparison between the pristine GnPs and GnPs@PET electrode in terms of the specific

current calculated based on the weight entire electrode; C) The Nyquist plot of the EIS measurement of GnPs@PET

fibers with an inset of zoomed-in graph showing the charge transfer impedance to be around 0.4 Ω; D) The GCD test

of the GnPs@PET fiber electrodes; E) The CV graph comparison of the electrode cycling after 0 cycles and 3000

cycles, showing very similar performances; F) The measured specific capacitance drop of the GnPs@PET fibers after

3000 cycles was 5.8%;

-

82

Cyclic voltammetry (CV) performed at various scan rates ranging from 5 mV/s to 1000 mV/s has

shown that the charge transfer takes place effectively within the electrode. As shown in Figure

4-18 A), the rectangular shape of the CV was retained even at very high scan rates (i.e. 1000 mV/s).

This indicated that the charge transfer between the electrolyte and the porous electrode was not

hindered, and therefore demonstrated an ideally shaped CV graph. The obtained capacitance at

100 mV/s was 72.1 F/g, indicating that majority of the GnPs surface area was accessible to the

ions in the electrolyte, allowing for very effective charge storage mechanism. The conventional

supercapacitors uses surface modification and other techniques to increase the surface area at the

electrode/electrolyte interface. However, we have created a nano-porous fibrous structure that

allows the electrolyte to access of the surface area available throughout the GnPs@PET film

thickness, thus allowing better EDLC performance. Previous studies have relied on activated

carbon (AC) to provide high surface area for EDLC, however, the difficulty in controlling the

porosity and pore sizes proved to limit the energy and power densities in EDLC-based EC

electrodes. Herein, the porous structures in the GnPs@PET fibers can be effectively controlled by

tuning electrospinning and GnPs sonication-assisted coating parameters. The diameter of the PET

fibers and the thickness of the GnPs coating allows researchers to optimize the charge storage

capabilities, without sacrificing the overall mechanical properties of the electrochemical cell.

The EIS result shown in Figure 4-18 C) indicates that the charge transfer impedance is very low

at around 0.4 . This verified the fact that even with high scan rates, the CV graphs still showed

fairly rectangular ideal capacitor behaviour for the GnPs@PET electrode, indicating that the

electrolytic ions are allowed to travel freely into and out of the porous electrospun GnPs@PET

electrode system. From the GCD curves shown in Figure 4-18 D), the energy density was

calculated to be 14.41 Wh/kg, and the power density is 1.16 kW/kg for the GnPs@PET fibers. The

energy density of this electrode is already comparable to that of Ni metal hydride battery while the

power density is dramatically higher than common batteries. With the utilization of non-aqueous

solid-state electrolyte, the voltage window can be stretched from 1.2 V to 4.0 V, which would

result in dramatic increases in both energy and power densities. Therefore, this type of EC

electrodes can be suitable for many flexible, high power requirement applications. The cycling test

in Figure 4-18 E) & F) showed a common trend with carbon-based EDL capacitors, where the

degradation in specific capacitance is essentially zero in the first 2000 cycles. At 3000 cycles, it

was found that the specific capacitance decreased to around 94.2% of the initial value, which is

83

still considerably higher than most pseudocapacitance materials. It is hypothesized that this

reduction may be due to the detachment of GnPs from the PET substrate after 2000 cycles of

charge cycles. Surface treatment of the PET fiber scaffold structure may provide a key to further

improving the adhesion between the GnPs and the PET nanofibers, allowing better cycling

performance of the GnPs@PET superflex electrode systems.

The GnPs@PET nano-sized electrospun fibrous structure was successfully fabricated via a novel

ultrasound assisted technique, in which the adhesion between the GnPs particles and the PET

nanofibers was improved. The EC performance fabricated using the GnPs@PET electrospun fibers

were measured to be 72.1 F/g at a relatively high scanning rate of 100 mV/s. The CV graphs in

rectangular shapes also showed ideal capacitor behaviour of GnPs@PET electrodes, even at very

high scanning rates of 1000 mV/s. The impedance spectroscopy showed a desirable low charge

transfer capacitance of 0.4 , which indicated that the electrolyte was able to penetrate through

the fibrous structure and that has allowed for better EC overall performance. The cycling tests

showed that after 3000 cycles, the capacitance was retained at 94.2 % of the original capacitance

measured, which was expected for carbon-based electrode systems. The resulting energy density

with aqueous electrolyte is calculated to be 14.41 Wh/kg, and the power density is 1.16 kW/kg.

This novel ultrasound-assisted fabrication strategy showed promise in creating superflex, high-

performance EC devices that can be applicable for energy storage purposes in biomedical or

personal electronic industries.

84

This study focuses on the construction and characterization of ultra-high surface area porous

electrodes based on coating of nano-sized conductive polymer materials on nylon membrane

templates. Herein, a novel nano-engineered electrode material based on nylon membranes was

presented, which allows the creation of supercapacitor devices that is capable of delivering

competitive performance, while maintaining desirable mechanical characteristics. With the

formation of a highly conductive network with the polyaniline nano-layer, the electrical

conductivity was also increased dramatically to facilitate the charge transfer process. Cyclic

voltammetry and specific capacitance results showed promising application of this type of

composite materials for future smart textile applications.

Herein, we present a method of creating a thin layer of polyaniline (PAni) via chemical

polymerization on top of the flexible nylon porous template to create a standalone binder-free

electrode system suited for flexible EC applications. The monomers were able to penetrate into the

pores of the nylon template and chemically polymerize onto the nylon surface. Using 1M sulfuric

acid as the electrolyte, a two-electrode EC cell was fabricated with the PAni@Nylon membrane

electrode systems. From cyclic voltammetry measured at 10 mV/s scan rate, these electrodes

demonstrated a reasonable overall specific capacitance of 151.0 F/g, by factoring the entire weight

of the electrode into the calculations. The charge transfer resistance (Rct) was measured to be only

0.5 Ω, which is within the desirable range for efficient charge transfer behaviours in

electrochemical systems. The morphology of the PAni@Nylon membranes were also studied and

correlated to the improvement in the charge storage mechanisms. It was deduced that the open

porous structures within the nylon template was contributing to the overall increase in the exposed

surface area after the in-situ chemical polymerization of PAni was carried out. However, with the

increasing scanning rate, there were non-idealities with the membrane electrode that contributed

to a decrease in the specific capacitance, partly due to difficulties in ion transport with some porous

networks blocked by the PAni nanoparticles. The flexibility of the PAni@Nylon membranes was

tested in that the electrode are capable of bending and twisting without permanent deformation.

This type of flexible membrane systems can be utilized for a variety of energy storage applications

alike.

85

Aniline monomer (ACS reagent grade, ≥99.5%) was purchased from Sigma-Aldrich. The

monomer was received in a dark brown colour and was distilled to a golden transparent

appearance. The distillation was carried out in a sealed environment, the temperature of the heating

plate was set to around 250°C. Hydrochloric Acid (HCl) (ACS reagent grade, 37%), ammonium

persulfate (APS) (ACS reagent grade, ≥98.0%) were also purchased from Sigma-Aldrich. The

nylon template used as the flexible substrate was purchased from GE healthcare, sold as Hybond™

membrane for western blotting.

An amount of APS was weighed and added to 20 mL of 1M HCl solution to form an acidic oxidant

solution. The aniline monomer was first distilled as described and stored in a dark environment at

4°C. An amount of distilled aniline monomer was then added to another 20 mL of distilled water.

The ratio between the monomer and oxidant is 1:1. Rigorous stirring was used to produce a white

opaque color solution. The nylon membrane was cut into strips and placed in the oxidant acidic

solution. The nylon membrane was subjected to 30 minutes of ultrasonication at an amplitude of

10 in an ice bath. After the mixing, the monomer solution was then added into the oxidant solution

and polymerization was allowed for varying periods of time. The color of the solution becomes

dark green, indicating the PAni emeraldine base formation. The nylon membrane also changed

color to a dark green nature indicating the formation of PAni on nylon template surface. The

PAni@Nylon membrane electrode was then washed repeated with deionized water and ethanol,

before drying overnight in the vacuum oven at 50°C. Since a two-electrode cyclic voltammetry is

used to characterize the capacitance performance of the system, a symmetrical two-electrode

symmetric cell was then constructed. A piece of filter paper was soaked in 1M of sulfuric acid

(H2SO4) serving as both the electrolyte and separator in the cell. No binders were involved in the

cell fabrication process. Thin stainless steel sheeting (grade 304) was used as the current collector

material.

Electrochemical performance characterization was carried out with a two-electrode cell system

using a Solartron 1470E multi-channel potentiostat. Cyclic voltammetry (CV) was performed on

all samples with varying scanning rates from 5mV/s up to 1000 mV/s with a voltage range of -

86

0.2V to 1.0V, based on the suitable operating condition selected for PAni EC systems. The

electrochemical impedance spectroscopy (EIS) was performed to characterize the charge transfer

resistance and the frequency range for the measurement was from 100 kHz to 0.02 Hz. Cycling

test was carried out with 5A/g of charge/discharge cycles for 3000 cycles. The weight of the active

layer materials was calculated based on the weight of the entire electrode, including the substrate

material. The electrode morphology was analyzed using FEI Inspect S50 scanning electron

microscope (SEM) and Bruker Multi-mode 8 atomic force microscope (AFM).

As shown in Figure 4-19, on the pure nylon membrane, it was observed that there were large

porous structures that would allow the monomer to penetrate into the network and fully interact

with the oxidant during in-situ chemical polymerization to form layered PAni nanoparticles on the

surface of the template system. This also serves as a protective layer against acid corrosion of the

nylon template. The as-fabricated PAni@Nylon membranes showed an even coverage of the PAni

particle layer. From the SEM images, it was clear that smaller porous openings were still retained

that allows the transport of electrolyte to take place.

Figure 4-19: Schematics showing the change in observed morphology of the nylon membrane electrode, before and

after the chemical in-situ polymerization took place. The large porous networks in the nylon template allowed

monomers and oxidants to fully utilize the nylon surface for the PAni coating formation. After polymerization, the

PAni layer still retained large amount of pores that continues to aid the electrolytic ion transportation in the

charging/discharging processes.

The SEM micrographs as shown in Figure 4-20 A) and B) clearly demonstrated this observation.

The pores on the active electrode surface were formed during the templated in-situ polymerization

87

process. With better control of the chemical polymerization process, these pores can be modified

and optimized for the ions to pass freely into the internal 3D structures. The surface area exposed

can increase dramatically leading to even better electrochemical performance.

The nylon template itself is a highly flexible membrane material that does not actively participate

in the charge storage mechanism, however plays a major role in the mechanical properties of the

PAni@Nylon membrane electrode. The flexibility of the electrode allows the system to be applied

to flexible personal electronics more effectively, allowing a better integration with smart devices,

where the non-essential components, such as flexible wristbands, can be replaced with this type of

energy sources, eliminating the need for bulky batteries.

The AFM images shown in Figure 4-20 C) & D) allows better observation of the nature of the

electrode surface behaviours. From the modulus mapping, it is clearly shown that the coating of

the PAni material has been applied evenly on the surface of the nylon template, forming a thin

layer of active material. The modulus difference is the material difference between the nylon

membrane and the PAni polymer coating.

Figure 4-20: A) SEM image of the pure nylon membrane template, it is shown to have large porous networks; B) SEM

image of the as-fabricated PAni@Nylon membrane system, the pores were much smaller in diameter and it clearly

shown that the PAni layer was well-attached to the surface; C) AFM DMT modulus mapping of the pure nylon

membrane confirming the existence of large pore openings; D) AFM DMT modulus mapping of the PAni coated

PAni@Nylon membrane electrode, showing different top surface layer with lower modulus.

One of the major advantages of the PAni@Nylon electrode system is the superior mechanical

performance of it. The electrode is highly flexibility, inherited from the nylon membrane itself,

allowing the electrode to bend and twist without suffering permanent deformation. The adhesion

between the PAni coating on the nylon template was also ideal. After the oligomers and large

88

agglomerates were removed from the surface after repeated washing, the PAni does not detach

easily from the electrode surface, indicative of strong physical bonding between the two surfaces.

Figure 4-21: Flexibility of the PAni@Nylon electrodes was demonstrated with A) Bending of the electrode; and B)

Twisting of the electrode. In both cases, no permanent deformation was observed.

The electrochemical performance was characterized via a two-electrode test setup. This type of

testing has been set up to mimic a symmetric two-electrode EC cell, for which both electrodes are

the PAni@Nylon flexible electrodes. At a very high scanning rate of 1000 mV/s, it is clear that the

CV diagram is only somewhat symmetrical, which leading to the prediction that the holes or pores

within the PAni@Nylon membrane electrodes are not interconnected that allows the ions to travel

freely without significant hindrance. With a much slower scanning rate at 10 mV/s, the distinctive

REDOX peaks were apparent and the CV graph was much more symmetrical. The slower scanning

rate is typically suited to conductive polymer based electrode systems due to the nature of their

charge storage mechanisms, i.e. with the reversible Faradaic reactions at the electrode surface. At

10 mV/s scanning rate, the specific capacitance was calculated to be 151.0 F/g, taking into account

of the weight of the entire electrode. The layered nature of the active material on the top surface

was able to achieve higher specific capacitance in comparison to pristine PAni at the

electrolyte/electrode interface.

The efficient transport of electrolytic ions ensures the normal EC operations even at very high

scanning rates. However, due to the existence of only a small number of pores with miniature sizes,

the electrolytic ions was not able to fully interact with the expose active electrode coating material,

and therefore, leading to unsymmetrical CV diagrams. The PAni layer was likely a layered 3D

network that is formed on the template surface, however, due to the nature of the porous structures,

the network was not effectively utilizing the entire exposed surface.

89

Figure 4-22: A) A comparison between the CV graphs at varying scanning rates ranging from 1000 mV/s to 5 mV/s.

At higher scanning rates, it is observed that the CV diagrams becomes less symmetrical and indicative of electrolytic

ions’ movement hindered during the charging and discharging processes. B) A CV diagram scanned at 10 mV/s

showing distinctive REDOX peaks and more symmetrical behaviour.

From the CV graphs, it is possible to use Formula (1) to calculate the specific capacitances for the

PAni@Nylon electrode at varying scanning rates. The findings are reported in Figure 4-23 A). The

specific capacitance dropped from 164.6 F/g measured at 5 mV/s, down to 48.8 F/g at 1000 mV/s

scanning rate. As mentioned, this decrease can be attributed to the lack to freely available transport

pathways for ions to travel within the 3D internal structure of the PAni@Nylon electrode.

Figure 4-23: A) The specific capacitance measured at varying scanning rates, showing a dramatic drop in specific

capacitance at increasing scanning rates; B) The cycling performance evaluation of the PAni@Nylon membrane

electrode after 3000 cycles.

The cycling test utilized 10 A/g high rate charging discharging the EC cell for 3000 cycles. The

specific capacitance showed a moderate drop from 151.0 F/g to 125.1 F/g, a 21.8% drop, which

corresponds to the typical drop in capacitance for conductive polymer based electrodes. However,

this may also be caused by the degradation behaviour from the corrosion of the nylon membranes

with the acidic electrolyte. It was thought that the PAni coating can effectively eliminate the

corrosion caused by the incompatibility between sulfuric acid, but with the porous structure and

90

also the capillary action forces, the acid could eventually reach the nylon template and causing

degradation of the scaffold. From Figure 4-24, the EIS graphs shows a good capacitor behaviour

at lower frequencies. The charge transfer resistance is quite low at 0.5Ω, demonstrating ideal

electrode/electrolyte interactions at the electrolyte/electrode interface. The electrolyte resistance

at 0.9Ω is consistent with typical aqueous electrolyte systems.

Figure 4-24: Electrochemical impedance spectroscopy (EIS) of the PAni@Nylon Membrane electrodes, showing a

low charge transfer resistance of around 0.5Ω, indicative of efficient electrode/electrolyte interactions at the contact

surface.

We have presented a novel method of creating a thin flexible lightweight PAni@Nylon membrane

electrode, consisting of polyaniline (PAni) coating on top of the flexible nylon porous template

suited for various EC applications. The coating of PAni layers was applied with in-situ chemical

polymerization method within the nylon template. From the morphology studies, it was found that

the pore sizes were decreased after the coating process, leading to hindrance in the ion transport

during the charging discharging process leading to decreased performance at higher scanning rates.

The overall specific capacitance of 151.0 F/g was measured at 10 mV/s, taking into account the

weight of the entire PAni@Nylon electrode. The charge transfer resistance was measured to be

only 0.5Ω with EIS. The cycling test showed a 21.8% drop in specific capacitance after 3000 test

cycles. The PAni@Nylon membranes electrodes were capable of bending and twisting without

permanent deformation, showing ideal flexibility characteristics. This type of flexible membrane

electrode systems can be effectively used for powering future flexible electronics and provides an

attractive alternative to bulky battery systems for energy storage applications.

- -

91

Prototype Development and Exploration into Other Flexible

EC Components

This chapter discusses the various perspectives in the overall cell design, including current

collectors and electrolyte considerations in the prototyping process of all-solid-state EC cell.

The current collector typically employed in commercial EC systems are metal films, such as

stainless steel grade 304 that was used in previous sections. However, the heavy weight and less

flexible mechanical characteristics are unfavourable aspects in the overall design. Therefore,

herein, we explored a completely different approach by introducing GnPs and MWCNT into

polypropylene (PP) matrix in hope of constructing a conductive network that allows the

conventional metallic current collectors to be replaced. The highest obtained conductivity was

2.5x10-4 S/cm, which is still significantly lower than the metallic counterpart, but from a

percolation threshold perspective, the addition of nanoparticles allowed the plastic insulator PP to

reach the percolation threshold easily, and that will aid the development of future flexible

conductive thermoplastic polymer networks. The electrolyte was also a major issue in the

construction of a solid-state EC cell. The aqueous electrolyte tend to evaporate under atmosphere

conditions. The use of PVA-H2SO4 system created a network where the H2SO4 electrolyte is

locked in the matrix and the evaporation rate was effectively reduced that offered insights in the

choice of electrolyte systems and its fabrication process.

The next portion focuses on extending the EC electrodes and utilizing the constructed, fully

functional cells in the prototyping process. The 4V cell constructed with 4 pieces of 1cm x 1cm

electrode, along with aqueous electrolyte and thin stainless steel current collectors was able to

satisfy the flexibility and mechanical requirement upon bending and twisting, yet was still able to

allow for ideal electrochemical performance. The constructed cell, charged with a 2AA battery DC

source for 5s, was able to sustain a white LED for 1 minute.

92

The electrical conductivity of polypropylene (PP) was improved via the addition of carbon-based

nanoparticles such as graphene and multi-walled carbon nanotubes (MWCNT). In addition, a

hybrid composite structure consisting of PP matrix, polyaniline conductive polymer, as well as

carbon-based nanofillers. The samples were fabricated via melt blending technique which is

capable for large scale industrial production. Electrical conductivity measurements determined that

the electrical performance of nanocomposite was increased 10 orders of magnitudes to a maximum

value of 2.5×10-4 S/cm, achieved by 20wt.% polyaniline (PAni), 5wt.% MWCNT, and 75wt.% PP

samples. The increase in electrical conductivity was attributed to the formation of conductive

networks observed under SEM and AFM. The samples were further characterized via TGA

analysis technique to verify its content. These nanocomposites are developed for future

supercapacitor current collector applications.

Polymers are used in various common applications as a result of their desirable features such as

flexibility, lightweight, and manufacturability. Polymer materials are typically electrically

insulative and are not considered ideal for any electronic applications. In recent years,

nanocomposites that allow electrons to transfer freely have attracted significant scientific attention

[91, 212]. This is aimed at the possibility of flexible, lightweight, and easily fabricated electronic

component alternatives to the currently available technologies [82, 137]. Specifically, conductive

polymer nanocomposites are highly desirable for various applications in the fields of biomedical,

energy harvesting and storage, as well as personal electronics, where lightweight and easily

fabricated conductive materials are highly desired. Currently, research on such novel materials has

primarily focused on the high electrical conductivity achieved with usage of conductive polymers

such as polyaniline (PAni), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT)

[37, 110, 213]. For these polymers, the electrical conductivity ranges from 0.1-5 S/cm for PAni, to as

high as 300-500 S/cm for PEDOT [11, 36], which is considered highly electrically conductive by

many standards. Highly conductive nano-fillers such as graphene and multi-walled carbon

nanotubes (MWCNT) can be used to further enhance the composite performance [80, 214]. However,

even though the progress in such areas have been steady, major problems such as difficulty in

93

manufacturing, as well as undesirable mechanical properties, making the application of such

materials severely limited [11].

Efforts are underway to create the possibility of a novel polymer nanocomposite material that is

easily mold into various products, readily scalable in production, and offers added benefits such

as durability, lightweight, desired electrical performance etc. [28]. In the past studies, researchers

have utilized chemical polymerization, electrochemical polymerization, and melt blending

methods to create electrically conductive nanocomposite with a variety of polymer matrix.

However, with solvent casting and electropolymerization, even though the dispersion is very

uniform and often highly conductive samples can be fabricated, it is difficult to scale up the

manufacturing process. With melt blending, the dispersion of the nanoparticles is not ideal,

however, the process is highly scalable and is often desirable if mass production is required [80, 215].

In this study, with a new melt blending process involving carefully tuned parameters, the gap

between the performance and manufacturability has been bridged.

Herein, the polymer matrix under study is polypropylene (PP) owing to its ideal manufacturability,

low cost, and desirable structural properties which allows for the creation of all-solid-state

conducting materials suitable for commercialization [216, 217]. Highly conductive nano-sized carbon

materials such as graphene and carbon tubes are used in this study to further enhance the electrical

performance of the polymer matrix [67, 215, 218]. In the case of nano-sized conductive particles such

as MWCNT and graphene, the extrusion process has demonstrated its ability to have dispersions

comparable to solvent casting and the measured conductivity was increased further with the

incorporation of conductive polymers such as PAni [219]. The formation of three-dimensional

structures have been studied extensively with scanning electron microscopy (SEM) and atomic

force microscopy (AFM) techniques to confirm that the alignment and dispersion of the

nanoparticles is actively contributing to the increase in electrical conductivity. To further

characterize and verify the nanocomposites, thermal gravimetric analysis (TGA) were utilized.

The successful fabrication of these composite materials can allow systematic replacement of

stainless steel sheets or thin layers of other metal materials that were typically used as current

collectors for supercapacitors [39]. Electrochemical supercapacitors have been extensively studied

for its charge storage abilities via both double-layer capacitance and pseudocapacitance

mechanisms [11]. Often researchers report values of specific capacitance with respect to the mass

94

of the electrode active layer which is extremely thin and cannot be representative of the entire

capacitor mass [18]. Therefore, with the integration of nanocomposite as an alternative to the metal

current collector, total mass of supercapacitors can be further reduced, leading to a lightweight

package which offer large energy and power densities, applicable for aerospace and transportation

industries [28, 38].

The polymer base matrix (Daploy™ WB260HMS) polypropylene used in the study is purchased

from Borealis. It has ideal structural properties and also is designed for melt blending via twin

screw compounding. The MWCNT (90% Purity, Nanocyl™ NC7000) used as conductive filler

was purchased from Nanocyl, with an average diameter of 9.5nm and an average length of 1.5μm.

The thin wall carbon nanotubes offer high electrical conductivity and also a comparably low

percolation threshold. The graphene nanoplatelets (GnPs) (90% Purity, Grade 2 Industrial Grade)

are purchased from CheapTubes Inc. These GnPs particles have an average diameter of less than

2μm and thickness of 8nm. The GnPs’s 2D structure allows high electrical conductivity along both

its length and depth. In order to examine the effect of the creation of hybrid structures with PP and

PAni, PAni powder (Emeraldine salt, Mw>20,000) purchased from Sigma Aldrich were used to

prepare a hybrid material aimed at increasing the electrical conductivity further.

The samples were made from melt blending of PP with varying wt.% of GnPs, MWCNT, and PAni

powder by using a twin-screw compounder (DSM Xplore micro-compounder) at 200°C, 50 rpm

for 10 minutes. Pure PP samples were fabricated for under the same processing parameters. The

extruded composite strips were cooled with water to obtain a solid strip of sample as shown in

Figure 5-1. Water cooling is used as it can cool the extruded sample all around uniformly, allowing

consistent cooling to prevent the extruded nanocomposite from being deformed and also ensures

the desired degree of crystallinity. Then, the strip was pelletized into smaller pieces in order to

process further. The pellets were compression molded into disk samples with 2cm diameter and

1.2mm thickness with a hot press. The treatment temperature for the hot press was set to 200°C.

Teflon sheets and stainless steel plate molds were used in combination to ensure no contamination.

Samples were heated up for 5 minutes to ensure complete melting and then 5 tons of force was

then applied to the sample for additional 8 minutes for shaping. The mold were then water cooled

95

to obtain desired solid polymer nanocomposite. PAni powder was protonated with 1M HCl

overnight to ensure that the PAni powder is functionalized to obtain its desired electrical

conductivity.

Figure 5-1: Fabrication Schematic for the Twin-Screw Melt Blending Process

Before performing electrical conductivity testing, the top and bottom of the samples were coated

with a thin layer of platinum layer to ensure the proper contact between the sample and the

measuring device. The sputter coating process was performed at 5mA with 1 minute on each side.

The sides of the samples were sanded to ensure that platinum is not contributing to the conductivity

measurements. The electrical conductivity was measured with 4-point probe method using the

dielectric analyzer (Alpha-N High Resolution Dielectric Analyzer, Novocontrol Technologies).

TGA (Q50, TA Instrument) was used for particle loading verification ramping at 10°C/min to

600°C using platinum sample pan. SEM (JSM 6060, JEOL Inc.) and AFM (Multimode 8, Bruker

Inc.) for particle dispersion and composite morphology analysis. For SEM, liquid nitrogen was

used to cool and break the sample to expose desired surface, and a thin layer of platinum particles

were sputter coated on top of the sample surface to prepare a conductive surface for proper

scanning. For AFM scanning, the samples were first melted at 200°C and was pressed between

two glass slides in order to obtain a flat surface for surface scanning.

Mixing

Chamber

Twin-Screw

Water

Bath

Valve

Strip of

Nanocomposite

Entrance

96

The electrical conductivity measurements for PP with HCl treated PAni powder melt blended with

the technique described in the experimental section are shown in Figure 5-2. In literature,

researchers have found that the protonated PAni can have an electrical conductivity on the order

of 100 S/cm, which is much higher than typical polymer materials, but still quite small comparing

to metal. The addition of treated PAni did not have a significant effect on the conductivity of the

PP based matrix. The effect is contributing to an increase of a maximum of one order in terms of

electrical conductivity at high frequencies, but at low frequencies, the effect is not significant. This

cannot lead to any conclusive result regarding the effect on electrical conductivity via the addition

of only protonated PAni into PP matrix.

Figure 5-2: a) Electrical Conductivity Measurements for differing wt. % of PAni added to PP matrix; b) Electrical

Conductivity Measurements for different carbon particle and PAni compositions with PP; c) Electrical Conductivity

Comparison for Various Compositions Containing MWCNT and PAni at 0.1Hz frequency

However, the effect of PAni addition is more significant when a hybrid nanocomposite is created

with PP, carbon-based nanoparticles and PAni. The results are shown in the Figure 5-2 b). With

5wt.% GnPs added into the PP matrix, there is a significant increase in electrical conductivity of

about 5 orders. The GnPs nanoplatelets was well-dispersed, however, the melt-blending process

has affected the GnPs structure, leading to GnPs platelets breakage and therefore causes the

conductive network to be disrupted. The addition of 10wt.% PAni into the nanocomposite with

5wt.% GnPs showed an increase in electrical conductivity by a factor of 10 at low frequency

regions, demonstrating that the percolation has been reached. However, further addition of 20wt.%

PAni with the 5wt.% GnPs composite showed a decrease of 2 orders in the electrical conductivity.

This may be attributed to the fact that the change in the morphology of the nanocomposite caused

contact resistance to increase.

97

From Figure 5-2 c), the composition with MWCNT showed more promising results in terms of

electrical conductivity. A 10-order increase was observed with the addition of 5wt.% MWCNT

with PP polymer matrix, as shown in Figure 5-2 b). MWCNT is highly conductive and the

dispersion of MWCNT via melt blending has contributed to the conductivity increase of the

composite material. Measurements have also shown that the addition of PAni powder was

contributing to a further increase in the electrical conductivity. 10wt.% PAni added to the 5wt.%

MWCNT/PP composite, the electrical performance further improved. The composite with 20wt.%

PAni, 5wt.% MWCNT, and 75wt.% PP showed the best electrical conductivity performance,

reaching an electrical conductivity of 2.5×10-4 S/cm. It is deduced that the structural behaviour of

MWCNT contributed to the increase. As PAni is added into the polymer matrix, MWCNT tends

to wrap around the PAni structures leading to a more coherent conductive network.

Figure 5-3: SEM Micrographs of Pure Particles of a) PAni powder; b) Pure PP; c) Pure GnPs; d) Pure MWCNT

The pure particles, PAni powder and PP polymer matrix are shown in the SEM micrographs in

Figure 5-3. It is shown in Figure 5-3 a) that the pure PAni powder in micro agglomerates. The PP

matrix in Figure 5-3 b) showed a uniform flat surface without significant surface features. Pure

GnPs samples are shown in Figure 5-3 c), which indicates a fairly thin collection of 2D

nanoplatelet structure. The MWCNT consist of stranded thin tube structures tangled together in

agglomerates nature as shown in Figure 5-3 d).

For the PAni and PP compositions, SEM images in Figure 5-4 clearly show that the PAni powder

does not break down into micro- or nano-sized particles as previously hoped. The dispersion of

the PAni particle is not good and that would explain the fact that the electrical conductivity has

not improved significantly via the addition of PAni particles alone. The embedded PAni particles,

a) b

c) d

98

even though has conductivities on the orders of 1×100 S/cm, would not form a conductive network,

as particles are separated by the PP matrix. The processability of PAni was a significant issue

preventing the effective utilization of these conductive polymer materials in industries.

Figure 5-4: SEM Micrographs of 20wt.% PAni with PP showing phase separated PAni particles embedded

When examining the morphology of the nanoparticles embedded within the polymer

nanocomposite with SEM, it was observed that the breakage of GnPs platelets and MWCNT was

a common phenomenon for all the samples, which has severely limited the electrical performance

of these composite materials. These micrographs are shown in Figure 5-5. Comparing to solvent

casting, the drawback of melt blending is the fact that 2D GnPs and nanotubes structures breaks

into smaller platelets or short strands of nanotubes and therefore, it is difficult for a conductive

network to form within the matrix. However, despite this common issue for melt blended samples,

the dispersion of GnPs and MWCNT is extremely uniform and weak conductive networks may

still be formed leading to the observation of increase in electrical conductivity described earlier.

In Figure 5-5 a), the 5wt.% GnPs + PP sample showed GnPs breakage and thin layers of GnPs

platelets sticking out of the matrix, suggesting that there is no distinct orientation to the alignment

of the GnPs nanoplatelets. In Figure 5-5 b), the GnPs dispersion was improved via the addition of

PAni particles. The GnPs platelets were also more intact, leading to an increase in the electrical

conductivity. However, with more addition of PAni particles, the structure of the composite has

been disrupted as shown in Figure 5-5 c), where the GnPs platelets have experience breakage,

leading to a decrease in conductivity.

a) b

99

Figure 5-5: SEM Micrographs of particles embedded into the PP Matrix. a) 5wt. % GnPs + PP; b) 5wt. % GnPs + 10

wt.% PAni + PP; c) 5wt. % GnPs + 20 wt.% PAni + PP; d) 5wt. % MWCNT + PP; e) 5wt.% MWCNT+10 wt.% PAni

+ PP; f) 5wt.% MWCNT+20 wt.% PAni + PP;

With the addition of MWCNT, the dispersion is extremely uniform across the surface of the PP

matrix as shown in Figure 5-5 d)–f). With only the MWCNT added to the PP matrix in Figure 5-5

d), it is observed that the MWCNT has been broken down into very short strands of tube shape

structures, which scattered around the PP matrix. Even though the small particles still managed to

form weak conductive links in the composite, the conductivity is able to further increase if these

MWCNT were more intact as long tubular shape structures. The images in Figure 5-5 e),f) clearly

show the addition of PAni has helped preserving length of the MWCNT strands comparing to the

case where only MWCNT was added.

Further AFM was performed to validate the dispersion of nanoparticles within the polymer matrix.

Figure 5-6 a) shows that pure PP has a fairly flat surface, with only small defects created when the

sample was made. Figure 5-6 b) shows the dispersion of PAni powder in a 20wt.% PAni + PP

sample. It can be seen that he particles are in fairly large agglomerates and are not connected with

other PAni fillers, leading to no change in the electrical conductivity. Figure 5-6 c) shows that

GnPs nanoplatelets have been dispersed well through the polymer matrix, but its structure has been

broken down to smaller plate shapes, which would disrupt the formation of conductive networks,

leading to inferior electrical conductivity as hoped. In contrast, the AFM image of MWCNT/PP

composite in Figure 5-6 d) shows very uniform dispersion of MWCNT in short strands on the

surface exposed. This allow the formation of strong conductive networks of CNT structures,

leading to conductivity values as high as 2.5×10-4 S/cm.

a) b

c) d

100

Figure 5-6: AFM images of nanocomposites a) Pure PP; b) 20wt.% PAni + PP; c) 5wt.% GnPs+10wt.% PAni + PP;

d) 5wt.% MWCNT+10wt.% PAni + PP

Figure 5-7: a) TGA Analysis for PAni/PP Samples for Composition Verification; b) TGA Analysis for GnPs/PP

Samples for Composition Verification

Figure 5-7 shows the TGA results for the nanocomposites after fabrication. During the melt

blending process, it is deduced that PAni powders may be difficult to mix properly with the PP

matrix in the twin-screw compounder. The TGA data does not accurately describe the amount of

PAni and PP entered the chamber. However, for the GnPs/PP samples, it is clearly shown that the

weight percentages of these composites is accurate as calculated. It is the same with the other

carbon-based nanocomposite samples and therefore, the results obtained for these samples are

representative for each of the compositions.

The PP hybrid samples with PAni and carbon-based nano-fillers were successfully fabricated

through melt blending process using twin-screw compounder. The electrical conductivity did not

a b

c) d

101

improve significantly with the addition of PAni for various weight percentages. However, when

5wt.% of GnPs and MWCNT were introduced, the electrical conductivity increased dramatically.

With further addition of PAni, highest electrical conductivity was achieved by 20wt.% PAni,

5wt.% MWCNT, and 75wt.% PP samples, at which the electrical performance of nanocomposite

was increased 10 orders of magnitudes to a maximum value of 2.5×10-4 S/cm. This increase was

attributed to how well the particles were dispersed in the nanocomposite samples as examined by

SEM and AFM. It was deduced that the PAni was aiding the dispersion of MWCNT in forming a

conductive network resulting in the high electrical conductivity achieved.

Optimized electrolyte systems are essential for the proper functioning of EC as energy storage

devices. The issue associated with aqueous electrolyte is the need of sealed packaging system in

order to keep the EC cell hydrated, otherwise, water would escape and ion mobility decreases

exponentially, resulting in poor cycling capabilities and energy storage performances. The ideal

electrolyte is capable of locking in the moisture or hydrated electrolyte in the system, or better yet,

utilizes a completely different mechanism in the transportation of electrolytic ions. However, as

discussed previously, the challenge has been to effectively improve on the equivalent series

resistance (ESR) in the electrolyte system. As investigated previously, the aqueous 1M H2SO4

electrolyte has been optimized in the transfer of electrolytic ions and it has been found that the

ESR was around 0.5Ω and due to the freely moving ions in the aqueous system, it was capable of

efficiently traversing through the fiber electrodes with three-dimensional networks. Herein,

another type of electrolyte system, namely PVA-H2SO4 gel electrolyte was investigated and

integrated with the previously studied novel electrodes to form an all-solid-state EC system. Due

to time constraints, the full electrochemical characterization was performed on the PAni/Nylon

membrane electrodes, as investigated previously in chapter 4, subsection 3.

Poly vinyl alcohol (PVA) was obtained from Sigma-Aldrich and used as is. The sulfuric acid

(H2SO4) was the same grade as used previously for comparison purposes. The concentration of the

sulfuric acid used was controlled at 1M. 1 gram of PVA powder was weighted and added into

10mL of deionized water. The solution was then heated to 85 C and keep stirring at 100 RPM

102

using a magnetic stirrer for 1.5 hours, until the PVA was completely dissolved to form a clear

solution. 10 mL of the 1M H2SO4 solution was then added into the mixture and then the solution

was heated and stirred for another 30 minutes. Then, the solution was poured into a petri dish for

drying. After 72 hours of drying at room temperature, the film detaches from the glass petri dish

and the thickness was measured for the ionic conductivity measurements. The dried film was then

sandwiched between as-prepared PAni/Nylon electrodes to perform two-electrode EC cell testing

on the electrolyte performance. In order to increase the pseudocapacitive charge storage

capabilities of the electrolyte system, an electrochemically active compound 2-Mercaptopyridine

(PySH) was added into the solution as varying percentages with respect to the weight PVA powder,

namely 5%, 10%, and 20%.

Figure 5-8: Process used for the fabrication for PVA-H2SO4 solid-state gel electrolyte for the construction of ECs

Electrochemical performance characterization was carried out with a two-electrode cell system

using a Solartron 1470E multi-channel potentiostat. Cyclic voltammetry (CV) was performed on

all samples with varying scanning rates from 5mV/s up to 1000 mV/s with a voltage range of -

0.2V to 1.0V, based on the suitable operating condition selected for PAni EC systems. The

electrochemical impedance spectroscopy (EIS) was performed to characterize the charge transfer

resistance and the frequency range for the measurement was from 100 kHz to 0.02 Hz. The weight

of the active layer materials was calculated based on the weight of the entire electrode, including

the substrate, each electrode was controlled to be around 1cm2 or 1.9 mg in weight.

The fabricated solid-state electrolyte experienced exceptional flexibility and mechanical

properties. From the images shown in Figure 5-9 a), it was demonstrated that the film was able to

experience bending without permanently deforming. It is shown also to be completely transparent

103

and is very difficult to dehydrate, even after 10 days exposed to atmosphere. This feature enables

the EC cell to last longer and requires less specialized packaging materials to prevent the

evaporation of the electrolyte.

Figure 5-9: Flexibility demonstration of the PVA-H2SO4 electrolyte and its appearance before depositing onto the

PAni/Nylon membrane electrodes.

From the electrochemical characterization, it was discovered that the electrolyte experienced

higher ESR as expected, however, the difference was not significant as to cause a resistance-

dominated cyclic voltammetry behaviour. The increased ESR is mainly due to the impedance

imposed by the PVA matrix in hindering the movement of the electrolytic ions in the electrolyte.

However, with the addition of PySH agents, it was discovered that the ESR decreased considerably

and that additional REDOX peaks began to form in the cyclic voltammetry characterization.

Figure 5-10: a) An EIS Nyquist Plot comparison between various electrolyte systems; b) A comparison of cyclic

voltammogram between H2SO4 aqueous electrolyte system and the PVA gel electrolyte system; c)A comparison of

cyclic voltammogram between PVA gel electrolyte with varying addition of PySH;

Table 5-1: A comparison between the ESR of different electrolyte types tested

Electrolyte Type ESR (Ω)

H2SO4 Aqueous 0.93

Pure PVA Gel 9.61

PVA Gel + 5% PySH 8.46



104

The specific capacitance value was not decreased dramatically, dropping from 151.0 F/g using

aqueous H2SO4 electrolyte as previously investigated to a value of 90.6 F/g, corresponding to a

40% decrease in the electrochemical performance of the EC symmetrical cell. However, with

further addition of PySH pseudocapacitive agents, the specific capacitance further decreased as a

result, reaching a value of 82.6 F/g and 89.2 F/g for PVA+5%PySH and PVA+10%PySH samples,

respectively. However, as the percentage increased to PVA+20%PySH, the specific capacitance

increased and have superseded the pure PVA gel electrolyte with a specific capacitance value of

113.2 F/g, with large REDOX peaks contributing to the overall capacitance behaviour.

With the development of PVA gel electrolyte system, the solid-state gel electrolyte was

successfully fabricated, and they have behaved ideally in terms of flexibility and mechanical

properties. In addition, the electrolyte performed relatively well in comparison to its aqueous

counterpart, with a 40% decrease in specific capacitance as determined with the CV tests. The

ESR was increased due to the hindrance of ion movement in the solid-state electrolyte system,

however, it was not significant enough to lead to a resistance-dominated behaviour. With the

addition of pseudocapacitive REDOX agents PySH, the specific capacitance was first decreased

but with the PVA+20%PySH samples, the specific capacitance was improved in comparison to

the pure PVA electrolyte as tested. This serves as a preliminary basis for more in-depth analysis

of electrolyte systems in the future and to provide an optimized prototyping strategy in

development all-solid-state EC prototypes in the next section.

There are several approaches that were brainstormed and also from past literature. To be more

effectively integrated with textiles, or form a novel type of smart skin energy storage layer, a

concentric type of EC, as shown in Figure 5-11 a), can be realized from a combination of chemical

or physical synthesis methods. However, challenges arise from the lack of economic fabrication

methods and the aim for this thesis is to not utilize complicated costly manufacturing preliminary

prototype utilizing a planer design was realized for this purpose, with the structure shown in Figure

5-11 b).

105

Figure 5-11: a) A concentric design of EC cell for smart textile and wearable energy storage applications; b) Planer

cell design for preliminary prototyping and testing purposes.

Previously, thick stainless steel current collectors were typically used for the prototyping process

and therefore leading to inflexible packages. Additionally, when aqueous electrolytes were

employed, the leakage of electrolyte can lead to capacitance drops, poor cycling capability, and

even harmful contaminations the liquid acidic electrolyte.

The improvement on EC cells has been accomplished through the investigation of various

electrode materials, in terms of both electrochemical performance and their mechanical properties.

In addition, after the preliminary studies performed on current collector and electrolyte

components, the flexibility of the EC was further improved. As shown in Figure 5-12 a), the

flexibility of a planer cell developed with previously available technology was bulky, inflexible,

and also suffers from electrolyte leakage issues. The next cell in b) was fabricated using electrodes

as developed previous in this thesis, and combined with a thin stainless steel current collector.

Figure 5-12: EC prototypes as developed: a) Thick stainless steel current collector with powder electrodes, and

aqueous electrolyte, wrapped in polyimide film; b) Thin stainless steel current collector with PVA-H2SO4 electrolyte,

PAni@Nylon flexible electrodes and PVC packaging; c) Pure activated carbon film, with PVA-H2SO4 electrolyte,

wrapped in polyimide films; and d) Completely sealed EC cell with flexible current collector, PVA-H2SO4 electrolyte,

and GnPs@PET fiber electrodes.

+

-

-

+

106

The packaging system was PVC tape as selected to prevent further electrolyte losses. The

flexibility was further improved with the PVA-H2SO4 gel electrolyte as developed earlier with

significantly improved mechanical properties. The image in c) demonstrated a possibility for future

development where the current collector would be completely replaced by the electrode

themselves with adequate electrical conductivity and with PVA-H2SO4 gel as the electrolyte. The

last image demonstrates a prototype where the EC cell was completely sealed within a chemically

resistance envelope and the GnPs@PET fiber electrode along with PVA-H2SO4 gel electrolyte and

a novel current collector coating as developed elsewhere.

Figure 5-13: A demonstration of the series connection of the 4V EC cell in order to power a while LED light; a)

Schematics showing the connection mechanism and that 4-cell system was utilized for a 4V system; b) An image of

the real-life setup during the testing of the 4V cell performance;

The EC cells were still limited with the 1.2 V decomposition voltage of water as the solid-state

electrolyte is still hydrated with water molecules, and therefore to demonstrate the charging and

discharging behaviours of the as constructed cells under real-life testing conditions. A white LED

was used as the energy drain and it can be lit with 3 V or higher voltage supplied. The test showed

that by charging the 4 V EC series connected cell with 2xAA batteries, at 3.2 V, for 5 seconds, the

while LED can be lit for more than 1 minute. Even though with decreasing LED brightness, it

demonstrated that the EC cell sustained at least 3 V voltage for more than 1 minute after a 5 s DC

charging period. This demonstration, not only showed the feasibility using the GnPs@PET fiber

electrodes as potential energy storage materials for future EC development, but also allowed a

simple and practical testing set up to be implemented under real-life operating scenarios for novel

materials to be prototyped.

107

Concluding Remarks and Future Work

The development of flexible, thin film electrochemical capacitor (EC) energy storage purposes has

been intensively studied in the last decade. One of the most important components is the electrode

layer, which needs to provide an effective route in transporting electrolytic ions at the

electrode/electrolyte interface. The major focus on this thesis was the development and

improvements of flexible EC electrodes. This thesis explored various aspects of the fabrication

routes in cost-effective manufacturing of novel composite electrode materials for EC purposes.

Cost-effective nanoparticles was used as nano-fillers to create three-dimensional conductive

networks in association with conductive polymer polyaniline (PAni). Novel core-shell structured

fibrous electrodes utilizing a novel ultrasonication assisted methods were also successfully

fabricated. By utilizing these novel flexible electrodes, several high performance flexible fully

functional EC prototypes were created with ideal mechanical and electrochemical properties

desirable for possible future wearable electronics applications. The feasibility of the flexible EC

electrodes and electrolytes was proved using a 4-cell system connected in series to yield a 4V EC

cell. A white LED was able to be lit for more than 1 minute after only 5 seconds of DC battery

charging process.

The first portion of this thesis discussed the fabrication and characterization of composite powder

EC electrodes composed of aluminum oxide (Al2O3), titanium oxide nanotubes (TiO2), graphene

nanoplatelets (GnPs), and multi-wall carbon nanotubes (MWCNT) with a PAni matrix. It was

found that the nanostructure formation and appropriate choice of composition in the fabrication of

PAni-based nanocomposites can significantly improve device performance of the supercapacitor

electrode. A significant increase in specific capacitance has been achieved. The value was

increased from 173.7 F/g for pristine PAni to a value of 326.9 F/g in the case of GnPs/PAni

composite. As examined with microscopy, it was found that conductive layered formation in the

case of GnPs/PAni composite allowed efficient insertion of electrolytic ions and ultimately

108

resulted in the improved electrochemical performance. The reduction in charge transfer resistance

as determined by EIS has also shown an improvement in EC performance with the GnPs/PAni

composites. In addition, for Al2O3/PAni composite, a notable improvement in specific capacitance

was also observed, achieving a specific capacitance value of 215.9 F/g. This increase was mainly

attributed to the nano-sized porous structural formation as a result of desirable interactions between

the composite material components. It was also observed that by selecting an optimal composition

for the ratio between nanoparticles and conductive polymers in the case of GnPs/PAni composite,

the electrochemical performance can be further improved.

The second portion of the thesis investigated the use of flexible substrates materials, electrospun

PET nanofibers from recycled beverage bottles, for creating a core-shell structured 3D fibrous

PAni@PET electrodes for energy storage applications. PET fibers were fabricated with an average

diameter of 121 nm and was coated with a thin active PAni layer, with an average thickness of 69

nm, using in-situ chemical polymerization method. The weak mechanical properties of PAni has

been readily compensated with the PET flexible structural support with good adhesion. The PAni

shell wrapping has been shown to be very uniform throughout the structure. The specific

capacitance was found to be 347 F/g at a scan rate of 10 mV/s, while even at a very high scan rate

of 500 mV/s, the capacitance only decreased to 244.9 F/g, which represented a very good specific

capacitance value for pseudocapacitive materials. It was deduced that the formation of three-

dimensional porous structures throughout the electrode contributed to the stability of the charge

storage capabilities and the high energy density offered by the symmetrical two-electrode device.

The GnPs@PET nano-sized electrospun core-shell fibrous structure was also successfully

fabricated via similar technique. The EC performance fabricated using the GnPs@PET electrospun

fibers were measured to be 72.1 F/g at a relatively high scanning rate of 100 mV/s. The CV graphs

in rectangular shapes also showed ideal capacitor behaviour of GnPs@PET electrodes, even at

very high scanning rates of up to 1000 mV/s. The impedance spectroscopy showed a desirable low

charge transfer capacitance of 0.4, which indicated that the electrolyte was able to penetrate

through the fibrous structure and that has allowed for better EC overall performance. This novel

ultrasound-assisted fabrication strategy showed promise in creating superflex, high-performance

EC devices that can be applicable for energy storage purposes in biomedical or personal electronic

industries.

109

The last portion of the thesis discussed the prototyping and full-scale testing process of fully

functional ECs. By utilizing existing technologies and novel electrode materials fabricated from

previous sections within the studies, a working all-solid-state prototype was created to demonstrate

the possibilities of creating high energy density, highly flexible, lightweight symmetrical EC cells.

4V EC cells were constructed and proved to be able to power white LEDs. Further experimental

work was conducted in search to alternative composite current collectors and electrolyte systems.

Polypropylene composites with conductive particles such as GnPs and MWCNT were fabricated

using twin-screw compounding technology in hope to create an alternative flexible current

collector. However, even though there were promising increases shown in the electrical

performance, these composites are currently not feasible for implementation as current collectors

for ECs. In addition, solid-state PVA electrolytes with varying amount of pseudocapacitive

particles PySH were also investigated with PAni/Nylon membrane electrodes and was proved to

provide an optimized solid-state alternative to aqueous electrolyte systems.

While there has been tremendous progress in the development of high performance flexible EC

electrodes for all-solid-state electronics, there are still a number of challenges that need to be

addressed before bringing the technology to market. One of the major challenges is to maintain

the rate capabilities while increasing the energy density. With hybrid electrode materials, the

energy density is typically ensured with the inclusion of pseudocapative components, but that also

contributes to decreased rate capabilities as a result of interfacial REDOX reactions.

The lack of thorough theoretical understanding of the electrode/electrolyte interfacial interactions

also contributes to in sufficient control over the overall energy storage performance. To date, most

of the research studies have focused on reporting experimental observations rather than proposing

theoretical predictions of the fundamental EC behaviours. Advanced simulation techniques that

allows better understanding of the electrode surface behaviours can lead to improved control over

the electrical conductivity, energy storage performances, as well as mechanical properties, and

ultimately result in a more robust, flexible electrode system for future flexible energy storage

applications.

110

Scalable and cost-effective production was also a challenge for most EC electrodes. The packaging

of EC cells is one of the key fabrication steps that prevents the electrolyte from escaping and

contributes to prolonged lifetime of the device. Without a properly designed flexible packaging

system, the leakage of the electrolyte can be detrimental to the EC performance. The future of

flexible ECs is extremely bright, however, without the introduction of key innovative solutions

that are able to resolve the above mentioned challenges, it would still be difficult to implement

flexible EC technologies in an industrial scale. With the current research efforts, we are on track

to a future of high energy density flexible ECs that will fundamentally change the way we power

flexible electronics.

111

References

[1] B. B. Garcia, A. M. Feaver, Q. Zhang, R. D. Champion, G. Cao, T. T. Fister, K. P. Nagle, G. T. Seidler,

Journal of Applied Physics 2008, 104, 014305.

[2] A. K. Singh, K. Mandal, Journal of Applied Physics 2015, 117, 105101.

[3] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Advanced Materials 2014, 26, 2283.

[4] P. Simon, Y. Gogotsi, Nature materials 2008, 7, 845.

[5] B. E. Conway, W. G. Pell, Journal of Solid State Electrochemistry 2003, 7, 637.

[6] A. Vlad, N. Singh, J. Rolland, S. Melinte, P. M. Ajayan, J. F. Gohy, Scientific reports 2014, 4.

[7] Y. Meng, Y. Zhao, C. Hu, H. Cheng, Y. Hu, Z. Zhang, G. Shi, L. Qu, Advanced materials 2013, 25,

2326.

[8] Q. Niu, K. Gao, Z. Shao, Nanoscale 2014, 6, 4083.

[9] C. Meng, O. Z. Gall, P. P. Irazoqui, Biomedical microdevices 2013, 15, 973.

[10] A. Du Pasquier, I. Plitz, S. Menocal, G. Amatucci, Journal of Power Sources 2003, 115, 171.

[11] B. E. Conway, Electrochemical supercapacitors: scientific fundamentals and technological

applications, Kluwer Academic/Plenum Publishers, New York 1999.

[12] P. Sharma, T. S. Bhatti, Energy Conversion and Management 2010, 51, 2901.

[13] X. Zhang, W. Shi, J. Zhu, W. Zhao, J. Ma, S. Mhaisalkar, T. L. Maria, Y. Yang, H. Zhang, H. H. Hng,

Nano Res. 2010, 3, 643.

[14] D. C. Grahame, Chemical Reviews 1947, 41, 441.

[15] J. O. M. Bockris, A. K. N. Reddy, Modern electrochemistry: an introduction to an interdisciplinary

area, Vol. 2, Springer, 1973.

[16] Z.-F. Li, H. Zhang, Q. Liu, L. Sun, L. Stanciu, J. Xie, ACS applied materials & interfaces 2013, 5,

2685.

[17] M. Pumera, Energy & Environmental Science 2011, 4, 668.

[18] G. A. Snook, P. Kao, A. S. Best, Journal of Power Sources 2011, 196, 1.

[19] D. W. Wang, F. Li, M. Liu, G. Q. Lu, H. M. Cheng, Angewandte Chemie International Edition 2008,

47, 373.

[20] R. Saliger, U. Fischer, C. Herta, J. Fricke, Journal of Non-Crystalline Solids 1998, 225, 81.

[21] B. E. Conway, E. Gileadi, Trans. Faraday Soc. 1962, 58, 2493.

[22] B. E. Conway, V. Birss, J. Wojtowicz, Journal of Power Sources 1997, 66, 1.

[23] J. P. Zheng, T. R. Jow, Journal of Power Sources 1996, 62, 155.

[24] B. E. Conway, Journal of the Electrochemical Society 1991, 138, 1539.

[25] E. Frackowiak, F. Beguin, Carbon 2001, 39, 937.

[26] P. Simon, A. F. Burke, The electrochemical society interface 2008, 17, 38.

[27] P. Simon, Y. Gogotsi, B. Dunn, Science Magazine 2014, 343.

[28] C. Meng, C. Liu, L. Chen, C. Hu, S. Fan, Nano letters 2010, 10, 4025.

112

[29] L. Wen, D. Liming, Carbon Nanotube Supercapacitors, 2010.

[30] J. R. Macdonald, E. Barsoukov, History 2005, 1, 8.

[31] P. L. Taberna, P. Simon, J.-F. Fauvarque, Journal of The Electrochemical Society 2003, 150, 292.

[32] S. Buller, E. Karden, D. Kok, R. W. De Doncker, "Modeling the dynamic behavior of supercapacitors

using impedance spectroscopy", 2001.

[33] R. L. Spyker, R. M. Nelms, Aerospace and Electronic Systems, IEEE Transactions on 2000, 36, 829.

[34] R. Faranda, M. Gallina, D. T. Son, "A new simplified model of double-layer capacitors", 2007.

[35] G. S. U. George, Applied Science II, Technical Publications, 2007.

[36] Y. Xu, Y. Wang, J. Liang, Y. Huang, Y. Ma, X. Wan, Y. Chen, Nano Res. 2009, 2, 343.

[37] R. Holze, Y. P. Wu, Electrochimica Acta 2014, 122, 93.

[38] Q. Xiao, X. Zhou, Electrochimica Acta 2003, 48, 575.

[39] E. Frackowiak, V. Khomenko, K. Jurewicz, K. Lota, F. Beguin, Journal of Power Sources 2006, 153,

413.

[40] D. A. Almeida, C. P. Fonseca, M. R. Baldan, N. G. Ferreira, ECS Transactions 2012, 41, 13.

[41] G. Salitra, A. Soffer, L. Eliad, Y. Cohen, D. Aurbach, Journal of the Electrochemical Society 2000,

147, 2486.

[42] H. Gao, K. Lian, RSC Advances 2014, 4, 33091.

[43] H. Ogawa, M. Ikoma, H. Kawano, I. Matsumoto, "Metal hydride electrode for high energy density

sealed nickel-metal hydride battery", 1988.

[44] C. Emmenegger, P. Mauron, P. Sudan, P. Wenger, V. Hermann, R. Gallay, A. Züttel, Journal of Power

Sources 2003, 124, 321.

[45] M. Winter, R. J. Brodd, Chemical reviews 2004, 104, 4245.

[46] H. Gualous, D. Bouquain, A. Berthon, J. M. Kauffmann, Journal of power sources 2003, 123, 86.

[47] H. Wang, Z. Xu, Z. Li, K. Cui, J. Ding, A. Kohandehghan, X. Tan, B. Zahiri, B. C. Olsen, C. M. B.

Holt, Nano letters 2014, 14, 1987.

[48] P. G. Campbell, M. D. Merrill, B. C. Wood, E. Montalvo, M. A. Worsley, T. F. Baumann, J. Biener,

Journal of Materials Chemistry A 2014, 2, 17764.

[49] H. Ibrahim, A. Ilinca, J. Perron, Renewable and Sustainable Energy Reviews 2008, 12, 1221.

[50] I. Hadjipaschalis, A. Poullikkas, V. Efthimiou, Renewable and Sustainable Energy Reviews 2009, 13,

1513.

[51] L. Carrette, K. A. Friedrich, U. Stimming, Fuel cells 2001, 1, 5.

[52] H.-J. Choi, S.-M. Jung, J.-M. Seo, D. W. Chang, L. Dai, J.-B. Baek, Nano Energy 2012, 1, 534.

[53] N. S. Choi, Z. Chen, S. A. Freunberger, X. Ji, Y. K. Sun, K. Amine, G. Yushin, L. F. Nazar, J. Cho, P.

G. Bruce, Angewandte Chemie International Edition 2012, 51, 9994.

[54] Q. Li, K. Han, M. R. Gadinski, G. Zhang, Q. Wang, Advanced Materials 2014, 26, 6244.

[55] F. T. Ulaby, M. M. Maharbiz, Circuits, National Technology & Science Press, 2010.

[56] L. Zhang, F. Zhang, X. Yang, G. Long, Y. Wu, T. Zhang, K. Leng, Y. Huang, Y. Ma, A. Yu, Scientific

reports 2013, 3.

113

[57] V. V. N. Obreja, Physica E: Low-dimensional Systems and Nanostructures 2008, 40, 2596.

[58] P. Le Cloirec, C. Brasquet, E. Subrenat, Energy & fuels 1997, 11, 331.

[59] S. Sircar, T. C. Golden, M. B. Rao, Carbon 1996, 34, 1.

[60] M. Suzuki, Carbon 1994, 32, 577.

[61] T. Wigmans, Carbon 1989, 27, 13.

[62] F. Rodriguez-Reinoso, M. Molina-Sabio, Carbon 1992, 30, 1111.

[63] F. Caturla, M. Molina-Sabio, F. Rodriguez-Reinoso, Carbon 1991, 29, 999.

[64] J. i. Hayashi, A. Kazehaya, K. Muroyama, A. P. Watkinson, Carbon 2000, 38, 1873.

[65] C. Nanjundiah, R. P. Braun, R. T. E. Christie, C. J. Farahmandi, Google Patents, 2003.

[66] A. K. Geim, K. S. Novoselov, Nature materials 2007, 6, 183.

[67] H. Chen, M. B. Müller, K. J. Gilmore, G. G. Wallace, D. Li, Advanced Materials 2008, 20, 3557.

[68] A. H. C. Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, A. K. Geim, Reviews of modern physics

2009, 81, 109.

[69] M. A. Rafiee, J. Rafiee, I. Srivastava, Z. Wang, H. Song, Z. Z. Yu, N. Koratkar, Small 2010, 6, 179.

[70] M. F. El-Kady, R. B. Kaner, Nature communications 2013, 4, 1475.

[71] Y. Wang, Z. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen, Y. Chen, The Journal of Physical Chemistry

C 2009, 113, 13103.

[72] C. Liu, Z. Yu, D. Neff, A. Zhamu, B. Z. Jang, Nano letters 2010, 10, 4863.

[73] S. Iijima, T. Ichihashi, 1993.

[74] T. W. Ebbesen, P. M. Ajayan, Nature 1992, 358, 220.

[75] W. Lu, M. Zu, J. H. Byun, B. S. Kim, T. W. Chou, Advanced Materials 2012, 24, 1805.

[76] B. I. Yakobson, P. Avouris, in Carbon nanotubes, Springer, 2001, 287.

[77] R. Saito, G. Dresselhaus, M. S. Dresselhaus, Physical properties of carbon nanotubes, Vol. 4, World

Scientific, 1998.

[78] A. Kongkanand, P. V. Kamat, Acs Nano 2007, 1, 13.

[79] P. M. Ajayan, O. Z. Zhou, in Carbon nanotubes, Springer, 2001, 391.

[80] S. H. Lee, E. Cho, S. H. Jeon, J. R. Youn, Carbon 2007, 45, 2810.

[81] Y.-P. Sun, K. Fu, Y. Lin, W. Huang, Accounts of Chemical Research 2002, 35, 1096.

[82] Z. Li, G. Luo, F. Wei, Y. Huang, Composites Science and Technology 2006, 66, 1022.

[83] M. Jayalakshmi, K. Balasubramanian, Int. J. Electrochem. Sci 2008, 3, 1196.

[84] S. Trasatti, G. Buzzanca, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1971,

29, A1.

[85] H. Xia, Y. S. Meng, G. Yuan, C. Cui, L. Lu, Electrochemical and Solid-State Letters 2012, 15, A60.

[86] Q. Wang, Z. H. Wen, J. H. Li, Advanced Functional Materials 2006, 16, 2141.

[87] H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang, A. J. Heeger, J. Chem. Soc., Chem.

Commun. 1977, 578.

114

[88] C. K. Chiang, C. R. Fincher Jr, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, A. G.

MacDiarmid, Physical Review Letters 1977, 39, 1098.

[89] J. R. Reynolds, C. K. Baker, C. A. Jolly, P. A. Poropatic, J. P. Ruiz, in Conductive Polymers and

Plastics, Springer, 1989, 1.

[90] G. Leising, Polymer Bulletin 1984, 11, 401.

[91] X.-M. Chen, J.-W. Shen, W.-Y. Huang, Journal of Materials Science Letters 2002, 21, 213.

[92] H. H. Kuhn, A. D. Child, W. C. Kimbrell, Synthetic Metals 1995, 71, 2139.

[93] A. Rudge, J. Davey, F. Uribe, J. Landeros Jr, S. Gottesfeld, "Performance evaluation of polyaniline as

an active material for electrochemical capacitors", 1994.

[94] A. G. MacDiarmid, J. C. Chiang, A. F. Richter, A. J. Epstein, Synthetic Metals 1987, 18, 285.

[95] Y. Cao, A. Andreatta, A. J. Heeger, P. Smith, Polymer 1989, 30, 2305.

[96] F. Fusalba, P. Gouérec, D. Villers, D. Bélanger, Journal of the Electrochemical Society 2001, 148, A1.

[97] K. S. Ryu, K. M. Kim, N.-G. Park, Y. J. Park, S. H. Chang, Journal of Power Sources 2002, 103, 305.

[98] D. Bélanger, X. Ren, J. Davey, F. Uribe, S. Gottesfeld, Journal of the Electrochemical Society 2000,

147, 2923.

[99] V. Gupta, N. Miura, Materials Letters 2006, 60, 1466.

[100] S. R. Sivakkumar, W. J. Kim, J. Choi, D. R. MacFarlane, M. Forsyth, D.-W. Kim, Journal of power

sources 2007, 171, 1062.

[101] W.-S. Huang, B. D. Humphrey, A. G. MacDiarmid, Journal of the Chemical Society, Faraday

Transactions 1: Physical Chemistry in Condensed Phases 1986, 82, 2385.

[102] L. Huang, Z. Wang, H. Wang, X. Cheng, A. Mitra, Y. Yan, J. Mater. Chem. 2002, 12, 388.

[103] A. F. Diaz, J. I. Castillo, J. A. Logan, W.-Y. Lee, Journal of Electroanalytical Chemistry and

Interfacial Electrochemistry 1981, 129, 115.

[104] J. Wang, Y. Xu, J. Wang, X. Du, Synthetic Metals 2011, 161, 1141.

[105] L.-Z. Fan, J. Maier, Electrochemistry communications 2006, 8, 937.

[106] T. Liu, L. Finn, M. Yu, H. Wang, T. Zhai, X. Lu, Y. Tong, Y. Li, Nano letters 2014, 14, 2522.

[107] J. Zhang, J. Jiang, H. Li, X. S. Zhao, Energy & Environmental Science 2011, 4, 4009.

[108] H. Wang, Y. Liang, T. Mirfakhrai, Z. Chen, H. S. Casalongue, H. Dai, Nano Res. 2011, 4, 729.

[109] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Advanced Functional Materials 2011, 21,

2366.

[110] C. Peng, S. Zhang, D. Jewell, G. Z. Chen, Progress in Natural Science 2008, 18, 777.

[111] K. Zhang, L. L. Zhang, X. S. Zhao, J. Wu, Chemistry of Materials 2010, 22, 1392.

[112] D. Puthusseri, V. Aravindan, S. Madhavi, S. Ogale, Energy & Environmental Science 2014, 7, 728.

[113] M. Arulepp, J. Leis, M. Lätt, F. Miller, K. Rumma, E. Lust, A. F. Burke, Journal of power sources

2006, 162, 1460.

[114] C. Amatore, B. Fosset, J. Bartelt, M. R. Deakin, R. M. Wightman, Journal of electroanalytical

chemistry and interfacial electrochemistry 1988, 256, 255.

[115] A. K. Shukla, S. Sampath, K. Vijayamohanan, Current science 2000, 79, 1656.

115

[116] S. Chaturvedi, S. Biswas, Resonance 2014, 19, 45.

[117] H. B. Michaelson, Journal of Applied Physics 1977, 48, 4729.

[118] P. Muller, Pure and applied chemistry 1994, 66, 1077.

[119] W. W. Porterfield, Inorganic Chemistry, Academic press, 2013.

[120] Z. Li, Z. Xu, H. Wang, J. Ding, B. Zahiri, C. M. B. Holt, X. Tan, D. Mitlin, Energy & Environmental

Science 2014, 7, 1708.

[121] W. S. Baker, J. W. Long, R. M. Stroud, D. R. Rolison, Journal of non-crystalline solids 2004, 350,

80.

[122] B. Xu, F. Wu, R. Chen, G. Cao, S. Chen, Z. Zhou, Y. Yang, Electrochemistry Communications 2008,

10, 795.

[123] K. H. An, W. S. Kim, Y. S. Park, J.-M. Moon, D. J. Bae, S. C. Lim, Y. S. Lee, Y. H. Lee, Advanced

Functional Materials 2001, 11, 387.

[124] C. Niu, E. K. Sichel, R. Hoch, D. Moy, H. Tennent, Applied Physics Letters 1997, 70, 1480.

[125] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, Y. Takasu, Angewandte Chemie International

Edition 2003, 42, 4092.

[126] S. Chaudhari, Y. Sharma, P. S. Archana, R. Jose, S. Ramakrishna, S. Mhaisalkar, M. Srinivasan,

Journal of Applied Polymer Science 2013, 129, 1660.

[127] S. Ramakrishna, K. Fujihara, W.-E. Teo, T.-C. Lim, Z. Ma, An introduction to electrospinning and

nanofibers, Vol. 90, World Scientific, 2005.

[128] K. Sujith, A. M. Asha, P. Anjali, N. Sivakumar, K. R. V. Subramanian, S. V. Nair, A. Balakrishnan,

Materials Letters 2012, 67, 376.

[129] P. T. Kissinger, W. R. Heineman, Journal of Chemical Education 1983, 60, 702 %@ 0021.

[130] M. A. Anderson, K.-C. Liu, C. M. Mohr, Google Patents, 1999.

[131] B. E. Conway, W. G. Pell, T. C. Liu, Journal of Power Sources 1997, 65, 53.

[132] R. C. Agrawal, G. P. Pandey, Journal of Physics D: Applied Physics 2008, 41, 223001.

[133] L. Malavasi, C. A. J. Fisher, M. S. Islam, Chemical Society Reviews 2010, 39, 4370.

[134] M. Rikukawa, K. Sanui, Progress in Polymer Science 2000, 25, 1463.

[135] B. Smitha, S. Sridhar, A. A. Khan, Journal of Membrane Science 2005, 259, 10.

[136] P. Jannasch, Current opinion in colloid & interface science 2003, 8, 96.

[137] R. Dweiri, J. Sahari, Journal of Power Sources 2007, 171, 424.

[138] C. J. T. de Grotthuss, Galvanique. Ann. Chim 1806.

[139] K. D. Kreuer, A. Rabenau, W. Weppner, Angewandte Chemie International Edition in English 1982,

21, 208.

[140] K. Y. T. Lee, H. H. Shi, K. Lian, H. E. Naguib, Smart Materials and Structures 2015, 24, 115008.

[141] H. Lin, L. Li, J. Ren, Z. Cai, L. Qiu, Z. Yang, H. Peng, Scientific reports 2013, 3.

[142] G. Xin, Y. Wang, X. Liu, J. Zhang, Y. Wang, J. Huang, J. Zang, Electrochimica Acta 2015, 167, 254.

[143] Y. Xu, Z. Lin, X. Huang, Y. Liu, Y. Huang, X. Duan, Acs Nano 2013, 7, 4042.

[144] Y. Xu, Z. Lin, X. Huang, Y. Wang, Y. Huang, X. Duan, Advanced Materials 2013, 25, 5779.

116

[145] L. Zhang, G. Shi, The Journal of Physical Chemistry C 2011, 115, 17206.

[146] S. Ghosh, O. Inganäs, Advanced Materials 1999, 11, 1214.

[147] E. M. Ahmed, Journal of advanced research 2013.

[148] Y. Shi, L. Pan, B. Liu, Y. Wang, Y. Cui, Z. Bao, G. Yu, Journal of Materials Chemistry A 2014, 2,

6086.

[149] G.-P. Hao, F. Hippauf, M. Oschatz, F. M. Wisser, A. Leifert, W. Nickel, N. Mohamed-Noriega, Z.

Zheng, S. Kaskel, ACS nano 2014, 8, 7138.

[150] Y. Xu, K. Sheng, C. Li, G. Shi, ACS nano 2010, 4, 4324.

[151] P. Chen, J.-J. Yang, S.-S. Li, Z. Wang, T.-Y. Xiao, Y.-H. Qian, S.-H. Yu, Nano Energy 2013, 2, 249.

[152] J. Chen, K. Sheng, P. Luo, C. Li, G. Shi, Advanced Materials 2012, 24, 4569.

[153] S. Chen, J. Duan, Y. Tang, S. Zhang Qiao, Chemistry–A European Journal 2013, 19, 7118.

[154] F. Xiao, S. Yang, Z. Zhang, H. Liu, J. Xiao, L. Wan, J. Luo, S. Wang, Y. Liu, Scientific reports 2015,

5.

[155] L. Nyholm, G. Nystrom, A. Mihranyan, M. Stromme, Adv. Mater. 2011, 23, 3751.

[156] V. L. Pushparaj, M. M. Shaijumon, A. Kumar, S. Murugesan, L. Ci, R. Vajtai, R. J. Linhardt, O.

Nalamasu, P. M. Ajayan, Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 13574.

[157] J. Zang, C. Cao, Y. Feng, J. Liu, X. Zhao, Scientific reports 2014, 4.

[158] M. Lee, B. H. Wee, J. D. Hong, Advanced Energy Materials 2015, 5.

[159] D. Sun, X. Yan, J. Lang, Q. Xue, Journal of Power Sources 2013, 222, 52.

[160] X. Zhao, C. M. Hayner, M. C. Kung, H. H. Kung, ACS nano 2011, 5, 8739.

[161] G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu, J. Lian, Small 2012, 8, 452.

[162] W.-w. Liu, X.-b. Yan, J.-w. Lang, C. Peng, Q.-j. Xue, Journal of Materials Chemistry 2012, 22,

17245.

[163] J. Xu, D. Wang, L. Fan, Y. Yuan, W. Wei, R. Liu, S. Gu, W. Xu, Organic Electronics 2015, 26, 292.

[164] X. Zang, X. Li, M. Zhu, X. Li, Z. Zhen, Y. He, K. Wang, J. Wei, F. Kang, H. Zhu, Nanoscale 2015,

7, 7318.

[165] Y. Meng, K. Wang, Y. Zhang, Z. Wei, Advanced Materials 2013, 25, 6985.

[166] S. Chabi, C. Peng, Z. Yang, Y. Xia, Y. Zhu, RSC Advances 2015, 5, 3999.

[167] W. Wang, S. Guo, M. Penchev, I. Ruiz, K. N. Bozhilov, D. Yan, M. Ozkan, C. S. Ozkan, Nano

Energy 2013, 2, 294.

[168] X. Dong, Y. Ma, G. Zhu, Y. Huang, J. Wang, M. B. Chan-Park, L. Wang, W. Huang, P. Chen, Journal

of Materials Chemistry 2012, 22, 17044.

[169] H. Zengin, W. Zhou, J. Jin, R. Czerw, D. W. Smith, L. Echegoyen, D. L. Carroll, S. H. Foulger, J.

Ballato, Advanced Materials 2002, 14, 1480.

[170] X. Zang, P. Li, Q. Chen, K. Wang, J. Wei, D. Wu, H. Zhu, Journal of Applied Physics 2014, 115,

024305.

[171] D. Jenicek, A. McCarthy, J. Kassakian, Journal of Applied Physics 2014, 115, 204309.

117

[172] K. Y. T. Lee, H. Naguib, K. Lian, "Flexible Multiwall Carbon Nano-Tubes/Conductive Polymer

Composite Electrode for Supercapacitor Applications", presented at ASME 2014 Conference on Smart

Materials, Adaptive Structures and Intelligent Systems, 2014.

[173] J. Vivekanandan, V. Ponnusamy, A. Mahudeswaran, P. S. Vijayanand, Archives of Applied Science

Research 2011, 3, 147.

[174] M. Trchová, I. Šeděnková, E. Tobolková, J. Stejskal, Polymer Degradation and Stability 2004, 86,

179.

[175] A. Adamczyk, E. Długoń, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

2012, 89, 11.

[176] P. M. Kumar, S. Badrinarayanan, M. Sastry, Thin Solid Films 2000, 358, 122.

[177] S. L. Candelaria, Y. Shao, W. Zhou, X. Li, J. Xiao, J.-G. Zhang, Y. Wang, J. Liu, J. Li, G. Cao, Nano

Energy 2012, 1, 195.

[178] N.-K. Chang, C.-C. Su, S.-H. Chang, Applied Physics Letters 2008, 92, 063501.

[179] C. Pang, C. Lee, K. Y. Suh, Journal of Applied Polymer Science 2013, 130, 1429.

[180] J.-S. Park, H. Chae, H. K. Chung, S. I. Lee, Semiconductor science and technology 2011, 26, 034001.

[181] A. Sugimoto, H. Ochi, S. Fujimura, A. Yoshida, T. Miyadera, M. Tsuchida, Selected Topics in

Quantum Electronics, IEEE Journal of 2004, 10, 107.

[182] L. Nyholm, G. Nyström, A. Mihranyan, M. Strømme, Advanced Materials 2011, 23, 3751.

[183] X. Li, B. Wei, Nano Energy 2013, 2, 159.

[184] M. F. El-Kady, V. Strong, S. Dubin, R. B. Kaner, Science 2012, 335, 1326.

[185] Y.-Y. Horng, Y.-C. Lu, Y.-K. Hsu, C.-C. Chen, L.-C. Chen, K.-H. Chen, Journal of Power Sources

2010, 195, 4418.

[186] L. Hu, M. Pasta, F. L. Mantia, L. Cui, S. Jeong, H. D. Deshazer, J. W. Choi, S. M. Han, Y. Cui, Nano

letters 2010, 10, 708.

[187] X. Peng, L. Peng, C. Wu, Y. Xie, Chemical Society Reviews 2014, 43, 3303.

[188] M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Nano letters 2008, 8, 3498.

[189] Y. Wang, J. Zhu, Nanotechnology 2015, 26, 055401.

[190] J. M. Boyea, R. E. Camacho, S. P. Sturano, W. J. Ready, Nanotech. L. & Bus. 2007, 4, 19.

[191] W. Wang, S. Guo, I. Lee, K. Ahmed, J. Zhong, Z. Favors, F. Zaera, M. Ozkan, C. S. Ozkan, Scientific

reports 2014, 4.

[192] L. Ka Yeung Terence, S. HaoTian Harvey, L. Keryn, E. N. Hani, Smart Materials and Structures

2015, 24, 115008.

[193] Z.-S. Wu, G. Zhou, L.-C. Yin, W. Ren, F. Li, H.-M. Cheng, Nano Energy 2012, 1, 107.

[194] Y. Sun, R. B. Sills, X. Hu, Z. W. Seh, X. Xiao, H. Xu, W. Luo, H. Jin, Y. Xin, T. Li, Nano letters

2015, 15, 3899.

[195] A. Russo, B. Y. Ahn, J. J. Adams, E. B. Duoss, J. T. Bernhard, J. A. Lewis, Advanced materials 2011,

23, 3426.

[196] Z. Li, M. Shao, L. Zhou, R. Zhang, C. Zhang, J. Han, M. Wei, D. G. Evans, X. Duan, Nano Energy

2016.

118

[197] A. Soffer, M. Folman, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1972,

38, 25.

[198] J.-C. Chiang, A. G. MacDiarmid, Synthetic Metals 1986, 13, 193.

[199] N. Gospodinova, L. Terlemezyan, Progress in Polymer Science 1998, 23, 1443.

[200] K. S. Novoselov, A. K. Geim, S. Morozov, D. Jiang, Y. Zhang, S. a. Dubonos, I. Grigorieva, A.

Firsov, Science 2004, 306, 666.

[201] J. Xia, F. Chen, J. Li, N. Tao, Nature nanotechnology 2009, 4, 505.

[202] G. Zhu, Z. He, J. Chen, J. Zhao, X. Feng, Y. Ma, Q. Fan, L. Wang, W. Huang, Nanoscale 2014, 6,

1079.

[203] H. H. Shi, H. E. Naguib, "Fabrication and characterization of polyaniline-graphene nanoplatelets

composite electrode materials for hybrid supercapacitor applications", presented at SPIE Smart Structures

and Materials+ Nondestructive Evaluation and Health Monitoring, 2015.

[204] C. Liu, F. Li, L. P. Ma, H. M. Cheng, Advanced Materials 2010, 22, E28.

[205] M. H. Ervin, Nanotechnology 2015, 26, 234003.

[206] X. Yang, J. Zhu, L. Qiu, D. Li, Advanced materials 2011, 23, 2833.

[207] Y. Wang, Y. Wu, Y. Huang, F. Zhang, X. Yang, Y. Ma, Y. Chen, The Journal of Physical Chemistry

C 2011, 115, 23192.

[208] C. Xu, T. Xue, J. Guo, Q. Qin, S. Wu, H. Song, H. Xie, Journal of Applied Physics 2015, 117, 164301.

[209] T. Choi, S. J. Kim, S. Park, T. Y. Hwang, Y. Jeon, B. H. Hong, Nanoscale 2015, 7, 7138.

[210] K. S. Suslick, Science 1990, 247, 1439.

[211] K. S. Suslick, Sci. Am. 1989, 260, 80.

[212] T. Kuilla, S. Bhadra, D. Yao, N. H. Kim, S. Bose, J. H. Lee, Progress in Polymer Science 2010, 35,

1350.

[213] Y. Wang, Journal of Physics: Conference Series 2009, 152, 012023.

[214] J. R. Potts, D. R. Dreyer, C. W. Bielawski, R. S. Ruoff, Polymer 2011, 52, 5.

[215] Q. Wang, J. Dai, W. Li, Z. Wei, J. Jiang, Composites science and technology 2008, 68, 1644.

[216] J. Karger-Kocsis, Polypropylene structure, blends and composites, Springer, 1995.

[217] G. Natta, P. Corradini, Il Nuovo Cimento 1960, 15, 40.

[218] F. H. Gojny, M. H. G. Wichmann, B. Fiedler, I. A. Kinloch, W. Bauhofer, A. H. Windle, K. Schulte,

Polymer 2006, 47, 2036.

[219] M. Mastragostino, C. Arbizzani, F. Soavi, Solid state ionics 2002, 148, 493.

development, characterization, and prototyping of ultra ... · sherif ramadan, adam pearson, kyle...

Documents