ABSTRACT KIM, HYUNGJOO. Functionalized Buckypapers For

ABSTRACT

KIM, HYUNGJOO. Functionalized Buckypapers for Supercapacitor Electrodes: Effect of Intrinsic Chemical Interactions on Electrochemical Performance. (Under the direction of Dr. Philip D Bradford).

Carbon nanotubes (CNTs) and their composites have been widely applied to many fields due to their unique electrical and mechanical properties. In the field of energy storage devices, they have been recognized as promising materials to develop supercapacitor electrodes with outstanding electrochemical performance such as high power density, fast charge/discharge time, low level of heating, safety and long cycle life. Ternary composites, consisting of CNT, conductive polymer (CP) and metal (oxide), have been actively studied to develop supercapacitor electrodes because improved electrochemical performance can be achieved as compared to binary composite or pristine forms. Many researches demonstrated that this improved performance was induced by synergistic effects among the three components in the composite. However, in most of reported researches, all three materials were simply mixed to induce this synergistic effect. To further improve electrochemical performance of ternary composite- based supercapacitors, three different strategies to further improve and understand these synergistic effects were utilized in this study.

The first method studied was the introduction of epoxide functional groups on the buckypaper. Raman, XPS and FT-IR were used to analyze the structure of functionalized buckypaper, indicating that epoxide functional groups were well formed on the surface of buckypaper. In addition, to further investigate advantages of epoxide functionalized buckypaper

(EBP) as a supercapacitor electrode, key factors affecting electrochemical performance were also studied. According to the result, EBP exhibited wider surface area, higher surface energy and enhanced electrical conductivity when compared with purified pristine buckypaper (PPBP). Hence, EBP showed higher electrochemical performance than PPBP. Specific capacitances of

EBP were calculated based on Galvanostatic Charge/Discharge (GCD) curve, resulting in 41.12

F/g, about 3time higher than that of PPBP (15.94 F/g).

Second, chemical interactions among all three different components in the composites were intentionally formed. In this study, both hydrogen bond and metal bonding formations were generated to develop chemically connected composite-based supercapacitor electrode. For instance, hydrogen bond was formed between oxygen in EBP and amine in CPs. On the other hand, silver nanoparticles (AgNPs) were bind with nitrogen in CPs (Polypyrrole and Polyaniline) due to tis high affinity toward AgNP. In addition, nickel oxide nanoparticles (NiO) were also applied and chemically interacted with carboxylic acid in CP (Poly(meta-anthranilic acid)).

These interactions were clarified by XPS, Raman and FT-IR. Furthermore, their effect on electrochemical performance was also determined, revealing that electrochemical performance was improved when compared with control groups with no chemical interactions. This phenomenon can be explained by chemical interactions since they enhanced both charge-transfer efficiency and electrical conductivity.

At last, crystalline CP was applied in the prepared composite. Among CPs selected to develop a supercapacitor electrode in this research, aniline could be polymerized with EBP because epoxide functional groups were acted as oxidants and involved in polymerization.

Therefore, epitaxial growth phenomenon could be occurred, resulting in crystalline polyaniline formation. Due to this crystalline nature, more organized and better packed polyaniline structure was constructed and accordingly high electrical conductivity was obtained leading to improved electrochemical performance.

by Hyungjoo Kim

A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Fiber and Polymer Science

Raleigh, North Carolina 2018

APPROVED BY:

______Philip D Bradford Yuntian Zhu Committee Chair

______Xiangwu Zhang Wei Gao

BIOGRAPHY

Hyungjoo Kim was born in Seoul and grew up in Daejeon, Republic of Korea. He studied advanced organic materials & textile system engineering as an undergrad at Chungnam

National University and became interested in organic/inorganic materials. After obtaining his

B.E in 2012, he continued to pursue M.E in advanced organic materials and textile system engineering at Chungnam National University. His research topic was design and synthesis of organic/inorganic materials. Through that research, he acquired the ability to develop new organic/inorganic materials and analyze their optical/electrochemical properties. After he graduated in 2014, he decided to apply for Ph.D. programs and started his research at North

Carolina State University in August 2015. During his Ph.D. training, his research was focused on organic/inorganic composites, especially functionalized carbon nanotube/conductive polymer/metal (oxide) based composites, to develop energy storage materials with excellent electrochemical performance. After a successful dissertation defense, he will be obtaining a

Ph.D. in Fiber and Polymer Science.

ACKNOWLEDGMENTS

I would like to sincerely thank my advisor, Dr. Philip Bradford and lab advisor, Dr.

Yuntian Zhu for giving me an opportunity to start my graduate career. In addition, due to your guidance, I learned about research ethics, hard work and how to develop at a scientific researcher. Also, I would like to thank to my advisory committee members- Dr. Xiangwu

Zhang and Dr. Wei Gao.

I would also like to say thank to my lab mates (Haotian Deng and Xiaotian Fang) since you help me a lot though out my time in the lab.

At last, I would like to thank my parents and sister for their love, encouragement and support. I really want to say “I love you”.

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TABLE OF CONTENTS

LIST OF SCHEMES ...... vii LIST OF TABLES ...... viii LIST OF FIGURES ...... ix

Chapter 1: Introduction and Motivation ...... 1

Chapter 2: Literature Review ...... 4

2.1 Advantages of Supercapacitor ...... 5 2.2 Different Types in Supercapacitors ...... 6 2.3 Separators...... 10 2.4 Electrolyte ...... 10 2.4.1 Comparison between Aqueous and Organic Electrolytes...... 11 2.5 Electrode Materials...... 11 2.5.1 Carbon Materials ...... 13 2.5.1.1 Activated Carbon ...... 14 2.5.1.2 Carbon Nanotubes ...... 16 2.5.1.2.1 Functionalized Carbon Nanotube ...... 18 2.5.1.3 Carbon Aerogels ...... 20 2.5.1.4 Templated Carbons ...... 22 2.5.2 Conductive Polymers ...... 24 2.5.2.1 Polyaniline ...... 26 2.5.2.2 Polypyrrole ...... 28 2.5.2.3 Polythiophene ...... 29 2.5.3 Noble Metal (Silver) ...... 30 2.5.4 Metal Oxides ...... 31 2.6 Composite ...... 32 2.6.1 Carbon-Conductive Polymer Based Composite ...... 33 2.6.2 Carbon-Metal Oxide Based Composite ...... 34 2.6.3 Metal Oxide-Polymer Based Composite ...... 34 2.7 Electrochemical Performance ...... 36 2.7.1 Three Electrode System ...... 36 2.7.2 Two Electrode System ...... 37 2.7.3 Electrochemical Performance Evaluation ...... 37 2.8 Applications ...... 40

Chapter 3: Chemically Interconnected Ternary AgNP/Polypyrrole/Functionalized Buckypaper Composites as High Energy Density supercapacitor Electrodes ...... 42

3.1 Introduction ...... 43 3.2 Experimental Section...... 46 3.2.1 Materials and Synthetic Procedures ...... 46 3.2.2 Synthetic method for purified pristine buckypaper (PPBP) ...... 46 3.2.3 Preparation of Functionalized Buckypaper (EBP) ...... 47 3.2.4 Synthetic Method of Polypyrrole-Functionalized Buckypaper composite ...... 47

3.2.5 Silver Nanoparticle Coatings ...... 47 3.3 Characterization ...... 48 3.4 Results and Discussion ...... 50 3.4.1 Characterization of the Prepared Ternary Composites ...... 50 3.4.2 Evaluation of Electrochemical Performance for the Prepared Ternary Composites as Supercapacitor Electrodes...... 57 3.4.2.1 Cyclic Voltammetry (CV) ...... 57 3.4.2.2 Galvanostatic Charge-Discharge (GCD) Curve ...... 63 3.4.2.3 Cyclic Stability ...... 68 3.4.2.4 Ragone Plot ...... 70 3.5 Conclusions ...... 73

Chapter 4: AgNP/Crystalline PANI/EBP Composite Based Supercapacitor Electrode with Internal Chemical interaction ...... 74

4.1 Introduction ...... 75 4.2 Experimental Section...... 77 4.2.1 Materials and Synthetic Procedures ...... 77 4.2.2 Synthetic Method for Purified Pristine Buckyaper (PPBP) ...... 78 4.2.3 Preparation of Functionalized Buckypaper (EBP) ...... 78 4.2.4 Synthesis of Polyaniline-Functionalized Buckypaper composite ...... 78 4.2.5 Silver Nanoparticle Coatings ...... 79 4.3 Characterization ...... 79 4.4 Results and Discussion ...... 81 4.4.1 Chemical and Structural Analysis ...... 81 4.4.2 Raman Study ...... 84 4.4.3 Electrochemical Performance Evaluation ...... 87 4.5 Conclusions ...... 98

Chapter 5: Chemically Linked EBP/P(m-ANA)/NiO Based Composite and Their Supercapacitor Performance ...... 99

5.1 Introduction ...... 100 5.2 Experimental Section...... 102 5.2.1 Synthetic Method for Purified Pristine Buckypaper (PPBP) ...... 102 5.2.2 Preparation of Functionalized Buckypaper(EBP) ...... 102 5.2.3 Synthetic Method of Poly(meta-anthranilic acid)-Functionalized Buckypaper composite ...... 102 5.2.4 Nickel Oxide Nanoparticle Coatings ...... 103 5.3 Characterization ...... 103 5.4 Results and Discussion ...... 105 5.4.1 Structural Characterization ...... 105 5.4.1.1 Chemical and Structural Analysis ...... 105 5.4.1.2 FT-IR Study...... 108 5.4.2 Electrochemical Performance Evaluation as Supercapacitor Electrodes ...... 110 5.4.3 Conclusions ...... 122

Chapter 6: Overall Conclusion and Future Work ...... 124

6.1 Overall Conclusions ...... 124 6.2 Future Works ...... 126

References...... 128

Appendix ...... 150

LIST OF SCHEMES

Scheme 3.1 Fabrication method to develop PPBP-PPY-AgNP and EBP-PPY-AgNP ...... 46

Scheme 4.1 Fabrication method to develop EBP-PANI-AgNP and its control groups...... 77

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LIST OF TABLES

Table 3.1 Assignment of Raman peaks for Pure PPY, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP ...... 52

Table 3.2 Specific capacitance values of Pure PPY, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP from CV curves in 10mV/s ...... 62

Table 3.3 ESR values of PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP ...... 66

Table 3.4 Specific capacitance values of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP from GCD curves in 0.25 A/g ...... 68

Table 3.5 A comparison of the energy density of reported electrode materials ...... 72

Table 4.1 Assignment of Raman peaks for Pure PANI, PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP ...... 86

Table 4.2 ESR values of PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP ...... 90

Table 4.3 A comparison of the energy density of reported electrode materials ...... 97

Table 5.1 Assignment of FT-IR peaks for Pure P(m-ANA), PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO ...... 110

Table 5.2 Specific capacitance, IR-drop and ESR values of PPBP, EBP, PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO ...... 113

Table 5.3 A comparison of the energy density of reported electrode materials ...... 122

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LIST OF FIGURES

Figure 2.1 Ragone plot of energy and power densities for existing energy storage and conversion devices ...... 4

Figure 2.2 Classification of different types in supercapacitors ...... 6

Figure 2.3 Scheme of electrochemical double-layer capacitor type ...... 7

Figure 2.4 Scheme of electrochemical Pseudo-capacitor type ...... 8

Figure 2.5 Scheme of electrochemical hybrid capacitor type ...... 9

Figure 2.6 (a) Cyclic voltammetry curves of the supercapacitor cell at different scan rates (10 mV s−1, 40 mV s−1, 80 mV s−1 and 160 mV s−1); (b) galvanostatic charge/discharge curve of the supercapacitor cell at the constant specific current of 0.5 A g−1 ...... 14

Figure 2.7 Normalized capacitance change vs the pore size of the CDC samples prepared at different temperatures; normalized capacitance ...... 15

Figure 2.8 Schematic representation of structures of carbon nanotubes ...... 16

Figure 2.9 Schematic illustration of the spaces in a carbon nanotube bundle for the storage of electrolyte ions ...... 17

Figure 2.10 Reaction mechanism for the synthesis of resorcinol–formaldehyde (RF) wet gels, aerogels, and carbon aerogels ...... 21

Figure 2.11 (a) Macroscopic templating by using muffin mold and microscopic templating by using (b) silica spheres, (c) SBA-15 and (d) zeolite Y ...... 23

Figure 2.12 Commonly used conducting polymers ...... 25

Figure 2.13 Schematic diagram showing the chemical structure, synthesis, reversible acid/base doping/de-doping, and redox chemistry of polyaniline ...... 26

Figure 2.14 Schematic of three-electrode configuration ...... 37

Figure 2.15 Cyclic voltammogram curves of (a) ideal capacitor, (b) EDLC, and (c) pseudocapacitive materials...... 38

Figure 2.16 Ideal schematic representation showing measurement of capacitance of supercapacitor devices and electrodes from charge–discharge curve and electrochemical impedance spectroscopy (EIS) ...... 38

Figure 3.1 Raman Spectra of Pure PPY, PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP...... 51

Figure 3.2 XPS spectra of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP...... 53

Figure 3.3 Binding energy shifts in C1s (a) and N1s (b) spectra...... 55

Figure 3.4 SEM Images of PPBP (A), EBP 3hrs (B), PPBP-PPY (C), EBP-PPY (D) PPBP-PPY-AgNP (E) and EBP-PPY-AgNP (F) ...... 56

Figure 3.5 CV curves of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP at 10mV/s scan rate...... 58

Figure 3.6 CV curves of PPBP (a), EBP (b), PPBP-PPY (c), EBP-PPY (d), PPBP-PPY-AgNP (e) and EBP-PPY-AgNP (f) under different scan rates ...... 60

Figure 3.7 Specific capacitance values of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP under different scan rate ...... 63

Figure 3.8 Charge-discharge curves of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP in 0.25A/g ...... 65

Figure 3.9 GCD curves of PPBP (a), EBP (b), PPBP-PPY (c), EBP-PPY (d), PPBP-PPY-AgNP (e) and EBP-PPY-AgNP (f) under different current densities ...... 67

Figure 3.10 Cyclic stability of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP during 2,500 cycles in 100mV/s ...... 69

Figure 3.11 Ragone plot of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP ...... 71

Figure 4.1 SEM Images of PPBP (A), EBP (B), PPBP-PANI (C), EBP-PANI (D), PPBP-PANI-AgNP (E) and EBP-PANI-AgNP (F) ...... 82

Figure 4.2 XPS spectra of PPBP, EBP, PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP ...... 84

Figure 4.3 Raman spectra for PPBP, EBP, Pure PANI, PBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP ...... 86

Figure 4.4 CV curves of all prepared samples at 10mV/s scan rate (a) and GCD curves of all prepared samples at the current density of 0.25A/g (b) ...... 89

Figure 4.5 XRD patterns of PPBP, EBP, PPBP-PANI and EBP-PANI ...... 92

Figure 4.6 CV curves of PPBP-PANI (a), EBP-PANI (b), PPBP-PANI-AgNP (c) and EBP-PANI-AgNP (d) under different scan rates ...... 93

Figure 4.7 GCD curves of PPBP-PANI (a), EBP-PANI (b), PPBP-PANI-AgNP (c) and EBP-PANI-AgNP (d) under different current densities ...... 94

Figure 4.8 Capacitance Retention of PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP during 2,500 cycles in 200mV/s ...... 95

Figure 4.9 Ragone plot of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP ...... 96

Figure 5.1 SEM Images of PPBP (A), EBP (B), PPBP-P(m-ANA) (C), EBP-P (m-ANA) (D), PPBP-P(m-ANA)-NiO (E) and EBP-P(m-ANA)-NiO (F) ...... 106

Figure 5.2 XPS spectra of PPBP, EBP, P(m-ANA), PPBP-P(m-ANA), EBP-P (m-ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO ...... 107

Figure 5.3 FT-IR spectra of PPBP, EBP, P(m-ANA), PPBP-P(m-ANA), EBP-P (m-ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO; (a) PPBP and EBP, (b) comparison of all prepared electrodes with inset graph of Pure P(m-ANA)...... 109

Figure 5.4 CV (a) and GCD (b) Curves of PPBP, EBP, PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO ...... 112

Figure 5.5 CV curves of PPBP (a), EBP (b), PPBP-P(m-ANA) (c), EBP-P(m-ANA) (d), PPBP-P(m-ANA)-NiO (e) and EBP-P(m-ANA)-NiO (f) under different scan rates ...... 115

Figure 5.6 Specific capacitance values of PPBP, EBP, PPBP-P(m-ANA)-NiO and EBP-P (m-ANA)-NiO under different scan rate ...... 116

Figure 5.7 GCD curves of PPBP (a), EBP (b), PPBP-P(m-ANA) (c), EBP-P(m-ANA) (d), PPBP-P(m-ANA)-NiO (e) and EBP-P(m-ANA)-NiO (f) under different current densities ...... 118

Figure 5.8 Cyclic stability of PPBP, EBP, PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-P (m-ANA)-NiO and EBP-P (m-ANA)-NiO during 2,500 cycles in 200mV/s ...... 119

Figure 5.9 Ragone plot of PPBP, EBP, PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO ...... 121

CHAPTER 1

Introduction and motivation

With rapid industrialization and development of the world there has been a significant increase in energy consumption [1]. In particularly, fossil fuels are the main energy source that world heavily depends on. However, accompanied problems such as high cost, pollution, global warming and geopolitical concerns also have been seriously considered, prompting the development of cleaner energy and the ability to store electrical energy in new devices [2,3].

Among these devices, lithium-ion batteries in rechargeable batteries have been widely adopted due to high energy density and acceptable cycle life. However, their limitations such as low power density, relatively low cycle life and high internal resistance are still regarded as problems

[4]. On the other hand, supercapacitors typically have the opposite set of limitations. They have relatively low energy density although they exhibit higher power density and longer cycle life than batteries [5]. Nevertheless, due to much higher power delivery or uptake, supercapacitors have been considered for energy storage device to complement or replace batteries [6].

Depending on different charge storage mechanisms, supercapacitors are typically classified into two types such as electrochemical double layer capacitors (EDLCs) and pseudo- capacitors. In EDLCs made of carbon-based materials, charges are physically stored by ion- adsorption or desorption process at electrode-electrolyte interface. Whereas, pseudo-capacitors composed of transition metal oxides or conducting polymers can chemically store charges through redox reaction on the surface [7,8]. However, in EDLCs and pseudo-capacitors, low power density and low energy density have been observed, respectively. To overcome these disadvantages, development of hybrid capacitors, a combination of both EDLCs and pseudo- capacitor mechanisms, has been the subject of many research studies because the advantages of

EDLCs and pseudo-capacitors can be maintained while their disadvantages can be mitigated

[9,10]. Among hybrid-capacitors, ternary hybrid composites have shown enhanced specific capacitance, cyclic stability and energy density when compared with single or binary composites.

Thus, they are expected that they will play an important role in prospective future commercial electrodes [9,11]. Carbon materials, conductive polymers and metal/oxides are the main set of components that have been used to develop high performance ternary hybrid composites [12-19].

In the field of supercapacitors, carbon materials have been acknowledged as one of the most popular elements due to their low cost and various forms such as powders, fiber, felts, composites, mats, monoliths and foils. In particularly, requirements to develop excellent EDLCs can be fulfilled by carbon materials because of high surface area and proper pores toward size of ions [20]. Moreover, functionalized carbon nanotubes have been widely used due to their positive effect on wettability, point of zero charge, electrical contact resistance, adsorption of ions (capacitance) and self-discharge characteristics [21].

In pseudo-capacitor materials, conducting polymers have been widely used due to their excellent properties including good intrinsic conductivity, low band gap and fast charge- discharge kinetics. Such properties can be obtained by conjugated bond system along the polymer backbone [22,23]. Typically, polyaniline, polypyrrole and polythiophene have been regarded as good conductive polymers.

To further improve electrochemical performance, silver showing high electrical conductivity is frequently incorporated into supercapacitor electrodes [24-32]. In addition, metal oxides such as Ruthenium Oxide, Manganese Oxide, Nickel Oxide, Cobalt Oxide, Tin Dioxide,

Zinc Oxide, Titanium Oxide, Vanadium Oxide, Copper Oxide, Iron Oxide and Tungsten

Trioxide have also been used in the development of supercapacitors because specific capacitance and energy density can be improved [33-42].

In this work, ternary composites composed of epoxide functionalized buckypaper (EBP), conductive polymers and metal/metal oxides were synthesized to develop an electrode active material for supercapacitors. Unlike other published papers, to maximize the synergistic effect between each material used in this work, the research aim was mainly focused on introduction of interactions such as hydrogen bonding and ligand bonding. Hydrogen bonding can be formed by interaction between epoxide groups in EBP and NH in conductive polymers. At the same time,

NH or functional groups in conductive polymers can also derive ligand bonding with metals and metal oxides. Through these interactions, improved binding forces are expected resulting in excellent electrochemical performances such as higher capacitance. This phenomenon can be caused by stable and fast charge transfer between individual components through hydrogen bonding. In this regard, according to papers published by Wang et al. and Xinming et al. [43,44], composite composed of graphene oxide and polyaniline was prepared for supercapacitor development, resulting in excellent capacitance and enhanced cycling life due to synergistic effect (π-π stacking, electrostatic interactions and hydrogen bonding) between graphene oxide and polyaniline. To our knowledge, such interactions rarely have been investigated. Therefore, we anticipate that this study will advanced the field and provide critical insight into how bonding interactions modify the performance of supercapacitor electrodes.

CHAPTER 2

Literature Review

In general, capacitors can be classified into three types such as electrostatic, electrolytic and electrochemical capacitors. Among them, electrochemical capacitors, also known as supercapacitors, showed more excellent capacitance per unit volume due to use of porous electrode [45]. In addition, electrochemical capacitors also possessed many advantages in comparison with different energy storage types such as capacitors, batteries and fuel cells. As shown in Figure 2.1, Ragone plot of current developed energy storage and conversion devices was visualized. According to this Figure, the gap between batteries and conventional capacitors can be filled by electrochemical capacitors [46]. In this regard, the development of electrochemical capacitors has been started to solve the problem such as low energy capacity of conventional capacitor (condenser or electrostatic capacitor). In addition, when compared with fuel cells and batteries, higher power densities also can be obtained by electrochemical capacitors [46,47].

Figure 2.1 Ragone plot of energy and power densities for existing energy storage and

conversion devices [45]. 4

2.1 Advantages of supercapacitor

(1) High power density: electrochemical supercapacitor exhibited higher power density than lithium ion batteries because electrical charges in supercapacitor are stored both at the electrode surface and in the bulk near the surface of the solid electrode. In addition, when compared with electrochemical redox reactions in batteries, faster charge-discharging rates can be another reason. This is because ionic conduction into the electrode did not hinder charge- discharge reaction [48].

(2) Long life cycle: Unlike batteries which store electrochemical energy via faradaic reactions, supercapacitor stored energy without chemical charge transfer reactions and phase changes, resulting in longer cyclability. This is because irreversible interconversion of chemical electrode reagent and phase changes often occurred during energy storage process in batteries

[48].

(3) Long shelf life: When rechargeable batteries were unused for long time, self- discharge and corrosion were occurred due to unstable chemical electrode and impurities in electrolyte. Thus, batteries are degraded and consequently useless. On the other hand, supercapacitors can maintain their capacitance after long time for nonuse because of relatively stable. Therefore, energy in supercapacitor can be restored similar as original condition [48].

(4) High efficiency: supercapacitor have reversibility for charging-discharging in operation range of voltage. This reversibility comes from faradaic reaction of pseudo-capacitor electrode materials such as chemically modified carbon, metal oxide and conductive polymer

[49]. Moreover, energy loss by heat during each cycle is small or none. Thus, high efficiency of supercapacitor can be accomplished [48].

(5) Wide range of operating temperatures: the function of supercapacitor can be kept effectively in regardless of high or low temperatures. This is because supercapacitors can be designed to maintain their functions in wide range of temperatures by selecting proper electrolyte that can tolerate high or low temperature conditions [50,51]. According to this, supercapacitors have been considered for military application.

(6) Environmental friendliness and safety: supercapacitors are not usually made of hazardous or toxic materials while containing disposable waste materials. Moreover, supercapacitors are also known as safer energy storage device than batteries such as lithium-ion batteries [48].

2.2 Different Types in Supercapacitors

Depending on different components and charge storage mechanisms, the supercapacitors can be divided by three broad classes as shown in figure 2.2. Electrochemical double layer capacitor, pseudo-capacitors and hybrid capacitors are included in typical supercapacitor types

[47].

Figure 2.2 Classification of different types in supercapacitors [52].

Electrochemical double-layer capacitors are made of two electrodes, one electrolyte and one separator as shown in Figure 2.3. In particularly, these two electrodes are composed of carbon-based materials such as activated carbon, carbon nanotubes, etc.

In case of charge storage mechanism for electrochemical double-layer capacitors, charges are stored by non-Faradaic process because charges are not transferred between electrolyte and electrode. In accordance with Figure 2.3, positive and negative charges are firstly separated and accumulated on the surface of different electrodes. Then, opposing ions in electrolyte are also attached to pores in already formed charged electrode. Finally, double-layered charges are achieved in each electrode resulting in high surface area and low distance between electrodes.

Therefore, higher energy densities can be achieved when compared with conventional capacitors

[10,48,49].

Figure 2.3 Scheme of electrochemical double-layer capacitor type [53].

In pseudo-capacitors composed of conductive polymers or metal oxides, charges are transferred between electrode and electrolyte, resulting in Faradaic reactions including electro- sorption, reduction-oxidation reaction and intercalation processes. Due to these three reactions, better capacitance and energy densities can be obtained when compared with electrochemical double-layer capacitances. In addition, the first two reactions are deeply related to surface area of the electrode while the last reaction is relatively unaffected by surface area [10,48]. The principle of pseudo-capacitor is described in Figure 2.4 [49].

Figure 2.4 Scheme of pseudo-capacitor type [53].

The other type of supercapacitor fabricated by two or more materials is called hybrid capacitor. In other words, electrochemical double-layer materials and pseudo-capacitor materials are combined. The emphasis of hybrid capacitor is the advantages that both electrochemical double layer and pseudo-capacitors have can be maximized while disadvantages can be mitigated

[10]. For example, by combining carbon and conducting polymer in one single electrode, both physical and chemical charge storage mechanisms can be accomplished. In particularly, conductive polymer can enhance its contact area with electrolyte since carbon based materials can provide high surface area backbone to conductive polymer [55]. In this regard, according to

supercapacitor developed by Jurewicz [56], composite based on polypyrrole/carbon nanotube showed better capacitance and cycling ability when compared with pure polypyrrole or carbo nanotube. This experimental result well supports the positive synergistic effect between individual component in one composite to develop hybrid supercapacitor.

In accordance with charge storage mechanism of hybrid capacitors as shown in Figure

2.5, both Faradaic and non-Faradaic reactions are occurred at the same time, resulting in higher energy and power densities than electrochemical double-layer capacitor. In addition, stable cycling stability and proper affordability also can be achieved when compared with pseudo- capacitors [10,53,54].

Figure 2.5 Scheme of hybrid capacitor type [53].

2.3 Separators

In supercapacitor, the role of separators is to allow ionic charge transfer while prohibiting electrical contact between the two electrodes. Depending on electrolyte types used in supercapacitor, different types of separators can be applied. For instance, polymer or paper separators are used in organic electrolyte. On the other hand, when aqueous electrolyte was applied, ceramic as well as glass fiber are used as separators [45, 57].

2.4 Electrolyte

Electrolyte is also one of the most important factors to determine electrochemical performance of supercapacitors. In this regard, key properties of electrolyte to affect the performance are including high concentration, temperature coefficient, high conductivity, wide voltage window, high electrochemical stability, high ionic concentration, low solvated ionic radius, low viscosity, low volatility, low toxicity, low cost and availability at high purity [45,52].

Among them, effect of temperature coefficient and the conductivity on electrochemical performance have been mainly considered due to their relationship with low equivalent series resistance (ESR) of supercapacitor. Also, Electrolyte concentration should be sufficiently high because low concentration of electrolyte brought depletion problem when supercapacitor was charged. This phenomenon is also called as starvation effect. In addition, when compared with large electrode surface, amount of accumulated electrolyte is smaller, resulting in inferior electrochemical performance.

These electrolytes can be classified into two categories such as aqueous and organic electrolyte. These two different electrolytes have been most commonly used in supercapacitor researches [52].

2.4.1 Comparison between Aqueous and Organic Electrolytes

In aqueous electrolyte, cell voltage is usually below 1V because water can be decomposed at 1.23V. However, better conductivity and lower minimum pore size requirements were exhibited in comparison with organic electrolyte. On the other hand, organic electrolyte brought the higher cell voltage than 2.7V while exhibiting low power capability due to high specific resistance [45,52].

2.5 Electrode Materials

To select suitable electrode materials for development of supercapacitors, following factors are important [58].

(1) High specific surface area of electrode enhances contact possibility with electrolyte ions, leading to the large capacitance. In this regard, according to research progressed by Barbieri et al. [59], gravimetric capacitance was enhanced as much as specific surface area increased.

However, the capacitance value was irregularly changed regardless of value of specific surface area (over 1200m2/g). This capacitance limitation is contributed from saturated pore walls for charge accommodation. Thus, proper specific surface area for electrode may be the most effective to develop excellent electron double-layer capacitor.

(2) Nitrogen or oxygen containing functional groups are important components to develop promising electrode materials because these functional groups can bring additional faradaic redox reaction and improved wettability, resulting in improved capacitance [50]. This phenomenon can be supported by previously published papers [60-63]. According to Li et al.

[60], mesoporous carbon sphere containing in-frame incorporated nitrogen was successfully obtained by facile polymerization induced colloid aggregation method. The prepared supercapacitor based on this carbon sphere showed 211 F/g as excellent specific capacitance

value. In addition, Kodama et al. [61] also prepared nitrogen enriched mesoporous carbon by using other method such as quinoline-polymerized pitch and consequently exhibit high capacitance (200-300F/g). In addition, carbon sphere@ZnO core-shell nanocomposites developed by Xiao et al [62] revealed 630F/g as specific capacitance value because oxygen functionalized carbon sphere was used in this work. This value is 32-fold higher than pure ZnO.

Moreover, according to carbonaceous material prepared by Pinero et al. [63], high capacitance value was also achieved with good conductivity due to presence of oxygen functional group.

In case of improved wettability, functional groups are also effective since they can prompt rapid electrolyte ion diffusion in electrode material [50]. This effect can be proved by former research progressed by Fang et al. [64]. According to this work, to improve wettability of carbon aerogel toward propylene carbonate solvent, surface treatment (grafting method) was introduced to change hydrophilic carbon aerogel to hydrophobic. Therefore, grafted carbon aerogel could improve wettability with propylene carbonate-based electrolyte, resulting in low internal resistance and high specific capacitance.

(3) Pore structures such as large pore size, short pore length, and simple pore connectivity played an important role to facilitate the ions diffusion with a high speed. For example, paper published by Braun et al [65] addressed that the penetration of electrolyte into the pores could be promoted when wider pores were occupied. In this work, this result was proved by the SAXS and EIS.

According to this, activation process could made pores wider and consequently enhancement of the volumetric double-layer capacitance could be observed.

(4) Effect of internal electric resistance on electrochemical performance of supercapacitors should be importantly considered. This is because, according to equation (1) to calculate power density, its value is inversely proportional to internal resistance.

Power Density = ····················· (1) where V is the potential window, and 2 is the equivalent series resistance.

Therefore, low internal electric resistance is required to obtain fast charging-discharging and low ohmic resistance. For instance, in the research progressed by Cheng et al. [66], CNTs were blended with graphene to develop high performance supercapacitor electrode because internal resistance could be reduced by introducing CNTs exhibiting higher electrical conductivity than reduced graphene. Such decrease of internal resistance could be obtained since pathways for the ions and electrons could be formed by CNTs.

(5) Above these, low volume and weight, low price are also important to apply supercapacitor electrodes to various areas.

(6) To develop supercapacitor electrodes, use of environmentally friendly materials should be significantly considered since seriousness of environmental pollution have been regarded as problems for solutions.

2.5.1 Carbon materials

In various supercapacitor electrodes, carbon-based materials including activated carbon, carbon nanotube, carbon aerogel and templated carbon have been recognized as one of the promising electrode materials due to their own advantages such as abundance, lower cost, easy processing, non-toxicity, higher specific surface area, good electronic conductivity, high chemical stability and wide operating temperature range [48,58]. Especially, electrochemical performances are mainly affected by specific surface area, pore-size distribution, pore shape and structure, electrical conductivity and surface functionality. In this regard, carbon materials can be appropriate materials to develop supercapacitor electrode. Within the framework of charge storage mechanism, carbon-based electrode mainly exhibited electrochemical double-layer

performances. In addition, when cyclic voltammetry and galvanostatic charge-discharge curves are measured as shown in Figure 2.6, rectangular shape and triangular symmetrical curves are obtained, indicating that carbon-based electrode exhibited good capacitance behaviors [48,67].

2.5.1.1 Activated carbon

Activated carbon have been recognized as an excellent electrode material because of large surface area (commercial activated carbon: 1000-3500 m2/g [68]) and moderate cost

(1,399$/ton). Due to these reasons, since more than 20years ago, activated carbon was introduced as an electrode material to develop supercapacitor. In this regard, commercial activated carbon fiber cloth-based supercapacitor cell was developed by Nawa et al [69].

However, limited energy storage capability has been still indicated as a drawback resulting in the limitation of applications based on activated carbon. According to the paper reported by Chen et al. [70], this drawback came from low meso-porosity of activated carbons although they have high surface area. Thus, low electrolyte accessibility was appeared, resulting in limited capacitance. In addition, the paper published by Manocha et al. [71] also pointed out difficulty of pore size distribution control as a disadvantage of activated carbon material even though high

surface area effective to improve electrochemical performance can be obtained by activated carbon. To solve these problems, by considering effect of pore size distribution pore structure and surface functionality on electrochemical performance, researchers have been challenging to design activated carbon to obtain high energy density while maintaining high power density and cycle life [68].

In related to this, the relationship between pore size of activated carbon and electrolyte ion size for electric double-layer capacitor was studied by Largeot et al [72]. According to this work, the highest capacitance value was achieved when both pore size and electrolyte ion size were similar as shown in Figure 2.7.

Figure 2.7 Normalized capacitance change vs the pore size of the CDC samples prepared at different temperatures; normalized capacitance [72].

In case of effect of surface functionality on electrochemical performance of activated carbon, according to the research done by Seredych and co-workers [73], the surface functionalized activated carbon was successfully obtained by melamine and urea treatment.

Through this process, wettability of activated carbon toward electrolyte ions could be improved due to presence of nitrogen and oxygen on the surface of the activated carbon. In addition, additional pseudo-capacitance could appear and consequently good electrochemical performance could be obtained.

2.5.1.2 Carbon Nanotubes

Carbon nanotubes are one of the carbon allotropes including fullerenes and graphite [74].

The structure of carbon nanotube is 1-dimensional (1D) hollowed cylinder based on carbon

unlike graphite (2D) or diamond (3D). In this regard, through clear transmission electron

microscopy (TEM) images, the structure analysis of carbon nanotube was completed by Sumio

Iijima in 1991 [75]. Since then, carbon nanotubes have been attracted and studied, resulting in

advances of science and engineering. In addition, as shown in figure 2.8, the carbon nanotubes

can be classified into zig-zag, chiral and armchair. Furthermore, they can be broadly categorized

into two broad ranges such as single-walled and multi-walled formed [68,76].

Figure 2.8 Schematic representation of structures of carbon nanotubes [68,76].

Depending on how hybridization occurs, carbon atoms in carbon nanotubes can be

hybridized into three possible options such as Sp1, Sp2 and Sp3. In addition, according to

different options, the structural arrangements of carbon atoms can be different, resulting in

different materials. In particularly, Sp2 is the vital option for bonding in carbon nanotubes.

However, both quantum confinement and σ-π rehybridization can be occur by circular curvature

of carbon nanotubes. Therefore, to compensate this phenomenon, π orbital delocalization can be

formed from outside of the tube, resulting in stronger strength, higher electrical and thermal

conductivity when compared with graphite [77]. In addition to these properties, good

accessibility of electrolyte ions was also received attention. According to paper reported by

Inagaki et al [78], storage spaces in carbon nanotubes are suitable to enhance the accessibility of electrolyte ions as shown in figure 2.9. At first, outside surface is formed in basal plane without any edge plane where induced electrolyte decomposition and hindered the use of capacitor at high voltage. In addition, interlayer space also can be interacted with electrolyte ions.

Figure 2.9 Schematic illustration of the spaces in a carbon nanotube bundle for the storage of electrolyte ions [78].

Due to excellent properties that carbon nanotubes have, various applications such as electrochemical devices, hydrogen storage, field emission devices, nanometer-sized electronic devices, sensors and probes, etc. have been studied [79]. Among them, supercapacitor electrodes in electrochemical devices have been attracted for carbon nanotube applications. The reason is that, as mentioned above, high strength electrical conductivity and good accessibility of carbon nanotube matches with the requirement of supercapacitor electrode development. For example, according to supercapacitor developed by An et al. [80], heat treated single walled carbon nanotube-based electrode showed 180 F/g, 20 kW/kg and 6.5 Wh/kg as specific capacitance, power density and energy density, respectively. In addition, research progressed by Niu et al.

[81] also fabricated carbon nanotube sheet electrode with showing high power density (over

8000 W/kg). This power density was higher than commercial capacitor based on activated carbon. 17

2.5.1.2.1 Functionalized Carbon Nanotube

Carbon nanotube can be generally functionalized by acid treatment, resulting in introduction of functional groups containing oxygen atoms on the surface of carbon nanotube. In this regard, due to the presence of oxygen based functional groups, properties of carbon nanotube can be enhanced as follows [21,48].

(1) In a composite consisting of polymer and carbon nanotube, interfacial bonding strength between both materials plays an important role to enhance mechanical properties. This interfacial strength can be increased by generating functional groups on the surface of carbon nanotube due to chemical bonding interaction between functionalized carbon nanotube and polymer. This theory can be supported by the research progressed by Liu et al [82]. This work described that interfacial bonding strength between carbon nanotube and epoxy was improved by introducing functionalized groups (hydroxyl or carboxyl groups) on the surface of carbon nanotube film using atmospheric pressure helium/oxygen plasma. This improvement was derived from chemical bonding between OH in the functional groups of carbon nanotube film and epoxy polymers. Therefore, higher peeling and tensile strength of the functionalized carbon nanotube film/epoxy composite were clearly observed in comparison with pure carbon nanotube film/epoxy composite.

(2) Carbon nanotubes have large surface area, resulting in strong tendency to form agglomerates that impedes the applications for carbon nanotubes. Thus, high dispersion of carbon nanotube should be considered as important factor. In this regard, high dispersion can be accomplished by functionalization of carbon nanotubes since polarity can be increased by introducing functional groups such as hydroxyl or carboxylic acid groups. For example, effect of acid treatment on dispersibility of carbon nanotube was estimated by Osorio et al [83]. According to this paper,

surface of carbon nanotube was successfully oxidized by acid treatment. When pure and functionalized carbon nanotubes were dispersed in aqueous media, the functionalized carbon nanotubes were well dispersed while pure one became the sediment. This result indicated that introduced functional groups by acid treatment brought the high efficacy of dispersion for carbon nanotubes.

(3) Strength of carbon nanotube can be improved by introducing functional groups on the carbon nanotube. According to paper reported by Park et al [84], hydroxyl or carboxylic acid groups were introduced on the surface of carbon nanotube fibers by plasma treatment. Then, these oxygen-containing functional groups induced hydrogen bonding between individual carbon nanotubes. Thus, higher tensile strength (0.86 N/tex) was achieved when compared with pure

CNT fibers (0.62 N/tex). Moreover, young’s modulus was also enhanced from 27.9 N/tex to 32.9

N/tex after plasma treatment.

Especially, effect of functionalized carbon nanotube on electrochemical performance have been importantly regarded for the development of supercapacitor. This is because excellent electrochemical performance can be obtained by functional groups on the surface of carbon nanotube in comparison with pristine carbon nanotube. The details of improved properties are described as follow: (1) High wettability (2) Low electrical contact resistance (3) Excellent adsorption of ions (high capacitance) (4) Low self-discharge characteristic [21,48].

Based on these properties suitable to develop supercapacitor electrode, functionalized carbon nanotubes have been studied as electrode materials. For instance, nitric acid treated multi- walled carbon nanotubes-based supercapacitor electrode have was developed by Lee et al. [85].

This prepared supercapacitor electrode exhibited specific capacitance value (68.8 F/g) more than

7times higher than pristine multi-walled carbon nanotubes. This high specific capacitance value

was contributed from the use of functionalized multi-walled carbon nanotubes as supercapacitor electrode material since both hydrophilic nature and large surface area that improve ionic interaction between multi-walled carbon nanotubes and electrolytes can be provided by surface functionalization (hydroxyl, carbonyl and carboxyl group) of multi-walled carbon nanotubes. In addition, He and co-workers also developed supercapacitor electrode based on aniline treated carboxyl group functionalized carbon nanotubes composite [86]. In particularly, when polyaniline was formed on the surface of functionalized carbon nanotubes, chemical linkage can be generated between -COO- in carboxyl group and -NH= in polyaniline. When compared with polyaniline/pristine carbon nanotube composite, sea cucumber like structure was clearly observed by scanning electron microscope and better electrochemical performance was significantly achieved. In this regard, the authors emphasized that these excellent electrochemical properties were induced by surface functionalization of carbon nanotubes since both high electrode/electrolyte contact area and short path length for electrons and electrolyte ions transport could be accomplished by chemical interaction between functional group and polyaniline.

2.5.1.3 Carbon Aerogels

Carbon aerogels are composed of primary particles interconnected like network. Due to the microstructure of carbon aerogels, they exhibit excellent properties such as low mass densities, continuous porosities, high surface areas and high electrical conductivity [87].

Accordingly, carbon aerogels have been considered as promising materials for various applications such as supercapacitors, rechargeable batteries, advanced catalyst supports for fuel cells, gas diffusion electrodes, adsorbents, chromatographic packing, thermal insulators, capacitive deionization and molecular sieves [88].

To fabricate these carbon aerogels, sol-gel process with subsequent pyrolysis of the organic aerogels from organic starting materials such as resorcinol-formaldehyde, melamine- formaldehyde or phenolic-furfural systems was used [89]. The detailed reaction mechanism using resorcinol-formaldehyde as starting material was described in Figure 2.10 [90].

Figure 2.10 Reaction mechanism for the synthesis of resorcinol–formaldehyde (RF) wet gels, aerogels, and carbon aerogels [90].

Through this process, colloidal-like carbon become being interconnected and special porosity is occurred in carbon aerogels [91]. The preparation method of carbon aerogels is important since the micro-texture (particle size and pore distribution) of carbon aerogels can be determined by the gel composition (catalyst, precursor, solid ratio) and the pyrolysis temperature. For example, particle size can be controlled by catalyst concentrations [92]. This phenomenon can be explained by the paper reported by Jun et al [93]. According to this paper,

particle size of carbon aerogels increased with higher RC ratio (molar resorcinol to catalyst ratios).

However, when conventional process was introduced to make carbon aerogels, meso- pores are mostly occupied, resulting in low electrochemical double-layer capacitance [68].

Therefore, introduction of micropores by additional activation process is effective to enhance the specific surface area. In this regard, according to carbon aerogel based electric double layer capacitor developed by Fang et al [91], surface modification of carbon aerogels was effective to not only lower resistance but also increase more accessible surface area. These results were derived from grafting of vinlytrimethoxysilane functional groups on the surface of activated carbon aerogels. The reason is that hydrophobisation and affinity toward propylene carbonate based electrolyte were increased by these functional groups, resulting in the enhancement of wettability in electrolyte. In addition, the supercapacitor developed by Li et al. [94] also indicated the positive effect of nitric acid activation of the carbon aerogels on electrochemical performances without any destruction of constitution and morphology of carbon aerogels. This is because carbon aerogels were changed to hydrophilism by nitric acid treatment, resulting in enhanced wettability in electrolyte. Therefore, the specific capacitance was enhanced from

101F/g (specific capacitance of pristine carbon aerogel) to 141F/g and it was maintained almost

100% after 2,000 cycles.

2.5.1.4 Templated Carbons

Templated carbons have been considered as one of the important candidates to develop supercapacitor electrode materials because templating method lead to excellent nanostructured carbons with well controlled narrow pore size distributions, ordered pore size distributions, ordered pore structures, large specific surface areas and interconnected networks [95]. Based on

these advantages, templated carbons have been studied for supercapacitor electrode. These templated carbons are generally manufactured by using different templates as shown in Figure

2.11 [68].

Figure 2.11 (a) Macroscopic templating by using muffin mold and microscopic templating by using (b) silica spheres, (c) SBA-15 and (d) zeolite Y [68].

According to this fiugre, by using different template and carbon precursors, various carbon structures have been developed. For example, microporous templated carbon material doped with nitrogen and oxygen was prepared by Ania and co-workers [96]. For synthesis of this carbon material, zeolite Y was used as template while both acrylonitrile and propylene were used as precursors. In this work, as the supercapacitor electrode, gravimetric capacitance and cyclability of the prepared carbon material were estimated and showed about 340 F/g and over

10,000 cycles, respectively. These good electrochemical properties were derived from high porosity and surface area, controlled and narrow pore size distribution in microporous range, large heteroatoms and good accessibility to surface groups. Especially, well controlled pore size and ordered straight pore channels were proper to enhance accessibility of electrolyte ions.

Moreover, in charge storage mechanism, additional pseudocapacitive effect was induced by the presence of nitrogen and oxygen functional groups, resulting in electron transfer reactions with acid electrolyte. On the other hand, with using micro-pores as well as macro- and meso-pores, 3- dimensional hierarchical porous graphitic carbon-based supercapacitor electrode was developed by Wang et al. [97]. The analyzed results indicated that both high energy and power densities were achieved due to combination of advantages that individual pores have. For example, ion- buffering reservoirs and excellent electrical conductivity were obtained by macro-pores and meso-pore walls, respectively. Furthermore, charge storage also can be increased by micropores.

2.5.2 Conductive polymers

As shown in Figure 2.12 [100], conductive polymers are composed of conjugated bond system along the polymer backbone and consequently exhibit good conductivity (ex)

Polyaniline: 0.1-5 S/cm; Polypyrrole: 10-50 S/cm; Polythiophene: 300-400 S/cm) [98,99,101]. In addition, good flexibility, low cost and easy synthetic method are also typical properties of conductive polymers. Due to these excellent characteristics, conductive polymers have been actively studied. Especially, as supercapacitor electrode materials, polyaniline, polypyrrole and polythiophene have been widely used because of their high theoretical specific capacitances such as 750, 620 and 485 F/g, respectively [22,23].

Figure 2.12 Commonly used conducting polymers [100].

In details, following three main reasons make conductive polymers apply to supercapacitor electrode: (i) high specific capacitance because the doping process involves the entire polymer mass; (ii) high conductivity in the charged state; and (iii) generally fast doping/un-doping process. Herein, energy storage device with low equivalent series resistance

(ESR) and high specific power can be realized. In addition, the conventional conducting polymers are also cheap and environmental non-toxic. However, low cycle life of the conducting polymer electrodes in supercapacitors have been regarded as the problem due to the volume changes during the ion insertion/de-insertion process in the charge–discharge procedure [102].

2.5.2.1 Polyaniline

Polyaniline is the typical conductive materials polymerized with aniline monomer by either chemical oxidation or electrochemical polymerization methods under mild conditions. In addition, as shown in Figure 2.13 [103], depending on redox and doping/de-doping states, polyaniline can be existed as a salt or base in three isolable oxidation states as follows (1) Leuco- emeraldine: the fully reduced state of polyaniline; (2) Emeraldine: the half-oxidized state of polyaniline; (3) Pernigraniline: the fully oxidized state of polyaniline [104-107].

Figure 2.13 Schematic diagram showing the chemical structure, synthesis, reversible acid/base doping/de-doping, and redox chemistry of polyaniline [103].

In particularly, depending on different states of polyaniline, its conductivity can be different. For instance, emeraldine state exhibited electrical conductivity while rest states are insulators [107]. The conductivity of emeraldine state was contributed by cations occurred during protonation of the polyaniline in the basic emeraldine form. This is because the cations participated in disproportionation reactions forming semiquinone radical cations, resulting in

conductivity enhancement [103]. In addition, conductivity of polyaniline can be controlled by doping level. In this regard, this reversible conductivity brought polyaniline to various applications such as batteries, sensors, actuators, electromagnetic shielding, antistatic coatings, corrosion protection and electro-optic and electrochromic devices [108].

In supercapacitor area, polyaniline has been recognized as one of the important electrode materials for development of pseudo-capacitor electrode because of its desirable properties such as high electroactivity, high doping level, excellent environmental stability, easy process and high specific capacitance. In this regard, by Sivakkumar et al [109], the polyaniline nanofiber- based electrode for aqueous redox supercapacitor was prepared and evaluated for its electrochemical performance. According to the measurement, the synthesized electrode-based polyaniline nanofiber showed that high specific capacitance value (554 F/g at 1.0A/g). On the other hand, cycling stability was poor with showing drastically decrease of specific capacitance value from 554 F/g to 57 F/g after 1000th cycles.

Like above result, poor cycling stability not enough to satisfy requirement of supercapacitor application has been pointed out as disadvantage of polyaniline. The poor cycle stability can be induced by degradation of polyaniline [22,23]. Therefore, to improve cycling stability of polyaniline-based supercapacitor electrodes, researches such as combination with carbon materials or modification of polyaniline structure have been reported. In related to this, according to the research progressed by Dong and co-workers [111], polyaniline/multi-walled carbon nanotubes composite based supercapacitor electrode was prepared by in situ chemical oxidative polymerization method and consequently, 328 F/g was obtained as capacitance value.

This value is much higher than pure polyaniline electrode. Such high capacitance value was accomplished by introduction of multi-walled carbon nanotubes in the synthesized composite

since they provide high surface area and good conductivity. On the other hand, Sivakkumar et al.

[112] reported that polyaniline was modified by forming poly(n-methyl aniline) and resulting in higher stability. The reason is that methyl groups of polyaniline blocked proton exchange sites.

2.5.2.2 Polypyrrole

Polypyrrole has been recognized as one of the most typical conductive polymers due to its simple and easy synthetic procedure because initial monomer (pyrrole) can be easily oxidized, water soluble and commercially available. Furthermore, polypyrrole has excellent advantages including environmental stability, good redox properties and high electrical conductivity [113-

116]. Hence, with such outstanding properties, polypyrrole has been applied to various application areas such as batteries, supercapacitors, electrochemical (bio)sensors, conductive textiles and fabric, mechanical actuators, electromagnetic interference (EMI) shielding, antistatic coatings and drug delivery systems [113,115,117].

Especially, in supercapacitor area, polypyrrole has been accepted as a promising material because the requirements to develop portable and flexible electronic power sources can be satisfied by intrinsic properties of polypyrrole in which greater mass density and better degree of flexibility were found when compared with other conducting polymers [118,119]. This means that higher performance supercapacitor can be developed by polypyrrole even though smaller volume was used. In addition, polypyrrole can be easily adjusted to various supercapacitor forms

[119]. Based on above mentioned properties, polypyrrole based supercapacitor have been reported. For instance, Yang et al. reported that self-assembled free-standing polypyrrole film was prepared by interfacial polymerization, resulting in maximum specific capacitance (261 F/g at 25 mV/s) and good electrochemical stability (75% after 1,000cycles) [120]. In addition, Li and

Yang synthesized polypyrrole based flexible film by chemical oxidation with methylorange-

FeCl3 as a reactive self-degradable template. This prepared film also showed 576 F/g as a high specific capacitance and initial capacitance was retained for 82% after 1000cycles.

However, similar to other conductive polymers, polypyrrole based supercapacitor applications have been also limited due to poor cycling stability. This shortcoming was derived from three main reasons as follows: (1) poor mechanical stability, (2) active material loss, (3) over-oxidative degradation [122]. Therefore, effective approaches to solve these problems have been studied. First, to improve mechanical stability, conductive polymer was combined with carbon materials or metal oxides. Second, inner layer of the polypyrrole also can be utilized by controlling micro-structure and morphology of the polypyrrole. Third, electrolytes that can be used in wide electrochemical windows [123].

2.5.2.3 Polythiophene

The conductive polymer composed of a group of sulfur containing heterocycles is called as polythiophene and can be prepared by various methods such as chemical, electrochemical, ultrasonics assisted electrochemical, photochemical and template synthesis methods [124]. This polythiophene and its derivatives have been applied to various areas [125-131]. This is because their intrinsic properties such as high stability of its (un)doped states, structural modification and solution processability [132].

In addition to abovementioned properties, polythiophene also exhibited high charge carrier mobility, environmental stability and longer wavelength absorption in comparison with other polymers [22, 133]. Hence, polythiophene have been applied to microelectronic devices including electro chromic displays, catalysts, organic field effect transistor, chemical/gas sensors, photo-electrochemical cells and actuators [134]. In particularly, electrochemical performance of supercapacitor based on polythiophene have been estimated, revealing that its high potential

ability for supercapacitor electrode materials. In this regard, by Senthilkumar and co-workers, pure polythiophene was synthesized by chemical oxidative polymerization method and its electrochemical performance was evaluated [124]. The result indicated that 117F/g was obtained as specific capacitance that is suitable value for redox supercapacitor electrode.

However, polythiophene cannot satisfy the requirements of practical applications for supercapacitor electrodes since, when compared with polyaniline and polypyrrole, electrochemical performance of polythiophene is inferior. This disadvantage came from both fast loss of power density and lower specific capacitance [23]. Therefore, to solve these problems,

Alabadi et al. developed binary composite by combining graphene oxide and polythiophene

[135]. According to this work, high specific capacitance (up to 296 F/g) and long-term stability

(over 91.86%) were accomplished due to synergistic effect between graphene oxide and polythiophene. Especially, this effect also provided improved energy density (up to 148 Wh/Kg) at power density of 41.6 W/Kg.

2.5.3 Noble Metal (Silver)

Among various noble metals including gold, platinum and paladium, silver has been widely used for the development of supercapacitor by combining with other electrode materials such as carbon materials, conductive polymers and metal oxides. This is because silver exhibited high conductivity and resistant to oxidation and corrosion. In this regard, according to previous researches [24-31], established specific capacitance of pure electrode materials can be enhanced after silver deposition. For instance, according to pseudo-capacitor electrode synthesized by

Sawangphruk et al [24], silver/polyaniline composite exhibited about 2-fold higher specific capacitance (420 F/g) than pure polyaniline. In addition, better cycling stability was also observed from silver/polyaniline composite when compared with only polyaniline-based

composite. In this regard, these results could be explained by the presence of silver in the composite since both good conductivity and fast ion transportation could be provided by silver.

Furthermore, silver nanoparticles also showed the bridge effect in the work reported by Kim and

Park [29]. Due to this effect, 212 F/g was obtained as the specific capacitance of silver-graphite nanofiber/polyaniline much higher when compared with those of pure polyaniline (80 F/g) and graphite nanofiber/polyaniline (153 F/g). In case of the supercapacitor electrode studied by Patil et al. [30], silver also played an important role to enhance specific capacitance from 285 F/g (for polyaniline) to 512 F/g (for silver/polyaniline) at 0.9 weight percent doping of silver. This is because silver nanoparticles as metallic conductor can mediate the effective charge migration through the polyaniline and consequently, charge hopping can be occurred through them. Hence, electron transfer can be enhanced.

2.5.4 Metal Oxides

Various metal oxides including Ruthenium Oxide [33], Manganese Oxide [34], Nickel

Oxide [35], Cobalt Oxide [36], Zinc Oxide [37], Vanadium Oxide [38], Copper Oxide [39] and

Iron Oxide [40], etc. have been actively studied. The reason is that larger capacitance can be achieved by using metal oxides due to their electrochemical processes that occur both on the surface and in the bulk of the electrode materials [136]. For example, Ruthenium Oxide exhibited high capacitance about 150-260 F/cm and this value is about ten times higher than carbon capacitance [21,137,138].

In particularly, Ruthenium oxide has been considered as the most effective supercapacitor material due to its high specific capacitance, excellent reversibility, and long cycle life.

However, its disadvantages such as high cost, toxic and naturally less abundant hinder the practical application for supercapacitor. Therefore, other metal oxides except Ruthenium Oxide

also have been studied and shown excellent electrochemical properties. In this regard, according to the paper studied by Huang et al. [139], single-crystalline Nickel Oxide nanosheet array on

Nickel foam was developed by facile hydrothermal approach. This prepared supercapacitor electrode showed high specific capacitance (674.2F/g), good rate capability and excellent cycling stability (93.5% after 5000cycles). These excellent properties were derived from high conductivity of the single-crystalline Nickel Oxide nanosheet arrays on the conducting substrate and large surface area induced by the mesoporous Nickel Oxide nanosheets. In addition,

Manganese Oxide based supercapacitor was also studied by Ragupathy and co-workers [140].

According to this research, amorphous nanostructured Manganese Oxide based supercapacitor electrode was fabricated by the mixing Potassium Permanganate with Ethylene Glycol under ambient condition, resulting in high cyclic stability (92% after 2,000cycles). In related to this, authors suggested that good electrochemical performance can be provided by physiochemical properties of nano- Manganese Oxide with high surface area (230m2/g) obtained from secondary meso-pores and proper water content.

2.6 Composite

Based on typical electrode materials such as carbon, conductive polymers, metals and metal oxides, much efforts have been introduced to develop high performance supercapacitors.

Especially, new composite types made of two or more materials have been intensively studied, resulting in better electrochemical performance than conventional electrode materials.

Noteworthy, this superior property is contributed from synergistic effect between different electrode materials because advantages that each material has can be maximized while drawbacks can be resolved [141,142]. In addition, according to published paper written by K. V.

Sankar et al [9], specific capacitance of composites can be enhanced by not only synergistic

effect between individual components but also following reasons such as i) high area occupation of ions into the electrode, ii) large number of active sites, iii) low charge transfer resistance iv) low diffusive resistance and (v) high diffusive coefficient.

2.6.1 Carbon-Conductive Polymer Based Composite

By synthesizing both carbon material and conductive polymer in one composite, the synergistic effect between them can be created, resulting in high potential for supercapacitor application. In particularly, this synergistic effect can be more effective by combining carbon nanotube and conductive polymer since the large pseudo-capacitance of conducting polymers along with the high conductivity and strong mechanical strength of the carbon materials can be obtained [142-144].

For example, in the paper reported by Zhang et al. [145], the supercapacitor based on polypyrrole/carbon composite was fabricated, exhibiting 487 F/g as high specific capacitance and rate capability. To prepare this composite, chemically modified ordered mesoporous carbon was coated by nano-thin polyprryole through in-situ chemical polymerization. In particularly, ordered mesoporous carbon was chemically modified by wet-oxidative method with nitric acid and high content of oxygen-containing functional groups were introduced on the surface of carbon. Due to the presence of these functional groups, electron delocalization on the polypyrrole and low resistance in the composite could be achieved by charge transfer complex between chemically modified carbon and polypyrrole. Therefore, the supercapacitor electrode in this work showed larger specific capacitance and better rate capability when compared with pure polypyrrole. As the other type of carbon/polymer composites, Zhou and co-workers developed the composite consisting of polyaniline and multi-walled carbon nanotubes as a core-shell structure [146]. For the fabrication of this composite, in-situ polymerization was introduced.

According to this research, 560 F/g for the maximum specific capacitance and good cycling stability were obtained. In this regard, this electrochemical performance was achieved by charge transfer such as conjugated interaction between individual components.

2.6.2 Carbon-Metal Oxide Based Composite

High energy density and stable power density can be achieved by combining carbon materials and metal oxides due to both ionic and electronic conductivity of electrode surface.

Furthermore, high surface area and regular pore structures of carbon materials is suitable to easily synthesize with metal oxide [142].

By using such positive effect mentioned above, supercapacitor electrode was prepared by

Kalpana et al. [147]. By mixing zinc oxide and carbon aerogel, this supercapacitor electrode was fabricated, resulting in 500 F/g as specific capacitance and good electrochemical stability. These results were contributed by high surface area and porosity of carbon aerogels. This is because ionic charges could be effectively accumulated in the double layer at the electrode.

Besides, Wang and co-workers fabricated Manganese oxide/carbon nanotube embedded carbon nanofibers nanocomposites for the development of supercapacitor electrode [148]. In this work, when compared with Manganese oxide/carbon nanofiber composite, the prepared electrode showed higher specific capacitance (374 F/g), better rate capability (53.4%) and excellent long-term stability (94% after 1,000cycles). This is because carbon nanotube provided the improved electrical conductivity, higher-specific-surface-area network structure with porosity and interconnectivity and strong mechanical stability.

2.6.3 Metal Oxide-Polymer Based Composite

To develop composite based supercapacitor electrode, metal-oxide have been incorporated into polymer. When compared various metal oxides embedded to polymer,

ruthenium oxide has been exhibited the most excellent electrochemical performance. For instance, Deshmukh and co-workers developed polyaniline-ruthenium oxide composite thin film as supercapacitor electrode [149]. This composite based electrode exhibited 830 F/g for maximum specific capacitance while maintaining 85% of its initial capacitance after 5,000 cycles. These values were higher than pure polyaniline based supercapacitor electrode. These improved electrochemical properties were derived from strong interaction between polyaniline and Ruthenium Oxide in the prepared composite.

However, other kinds of metal oxide also have been studied due to aforesaid disadvantages of ruthenium oxide. In this regard, according to research conducted by Sharma et al. [150], Manganese oxide was embedded to polypyrrole for the development of supercapacitor electrode. In this composite, polypyrrole can provide large surface area for nanocrystalline

Manganese oxide. On the other hand, Manganese oxide got involved in improving the chain structure, conductivity and stability of polypyrrole. Therefore, this interaction between

Manganese oxide and polypyrrole finally brought the higher specific capacitance (620 F/g) than pure Manganese oxide (225 F/g) and pure polypyrrole (250 F/g). In addition, Mujawar and co- workers utilized titanium oxide to develop supercapacitor electrode with polyaniline [151]. In this work, polyaniline/titanium oxide nanocomposite was prepared by electro-polymerization and estimated for its electrochemical performance, resulting in high specific capacitance (740 F/g) and good cycling stability (13% loss after 1,100 cycles). These electrochemical performances were attributed from aligned and open- ended polyaniline/titanium-based nanotube arrays since high surface areas and ordered nanostructures could be provided.

In addition, in composites including metal oxide and conductive polymer, polymer does not only play an important role for synergistic effect with metal oxide, but also for wrapping

metal oxide to prevent dissolution by acid electrolyte. According to the paper reported by Yuan et al. [152], the prepared composite consisting of polyaniline, Manganese oxide and multi-walled carbon nanotube showed large specific capacitance (384 F/g) and good specific capacitance retention (79.9% after 1,000 cycles) in acidic mixed electrolytes. In related to this, polyaniline with good conductivity and capacitive performance could be the important role for protecting

Manganese oxide from reductive-dissolution process while serving as one more electro-active material in the prepared composite.

2.7 Electrochemical Performance

To evaluate electrochemical performance of supercapacitors, cyclic voltammetry (CV), galavanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were typically used. Through these methods, necessary parameters including specific capacitance, cycle life, charge-discharge rate, etc. can be obtained [22,23].

2.7.1 Three Electrode System

For measurement of electrochemical performance, three-electrode system is used. In this regard, according to Figure 2.14, the system is mainly consisting of working electrode (WE), reference electrode (RE), counter electrode (CE) and electrolyte. In case of working electrode, active materials are coated on the working electrode or directly used as the working electrode. In this electrode, reaction of interest can be occurred. On the other hand, reference electrode and counter electrode is composed of saturate calomel electrode and platinum electrode, respectively.

In purpose of use for these two electrodes, reference electrode played a role for potential control

and measurement while counter electrode is utilized for current circuit close. In addition, organic solvent or aqueous solution are used as electrolyte [23,153].

Figure 2.14 Schematic of three-electrode configuration [23].

2.7.2 Two Electrode System

The two electrodes system is containing cathode, anode and separator. According to the structure of the system, short circuits are avoided by placing separator between cathode and anode. In addition, to play a role of current collector, metal plates are attached to the two electrodes and their separator. For the measurement, the prepared cell was soaked in electrolyte and dried in a vacuum oven to remove any trapped air. On the other hand, for synthesis of anode and cathode, inks or pastes were created from the active materials and accordingly prayed/pasted on the stable electrode sheets such as carbon papers or carbon fibers. Finally, prepared cathode and anode are cut into proper dimensions [47].

2.7.3 Electrochemical Performance Evaluation

With two or three-electrode systems, electrochemical performances of supercapacitor from three methods including cyclic voltammetry (CV), galavanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) can be measured. In cyclic voltammogram of a capacitor as shown in Figure 2.15, Ideal shape of capacitor curve is rectangular. On the other

hand, depending on different electrochemical behaviors such as electrochemical double-layer or pseudocapacitive materials, shape of the curve can be close to rectangular or have sharp peaks.

Figure 2.15 Cyclic voltammogram curves of (a) ideal capacitor, (b) EDLC, and (c) pseudocapacitive materials [45].

In addition, charge-discharge curve must be linear and isosceles triangle shape as shown in Figure 2.16. At last, in case of the graph of electrochemical impedance spectroscopy, capacitive region and plot can be obtained. Therefore, based on these three graphs, not only capacitance can be calculated, but also other necessary information such as cycle life, coulombic efficiency, etc. can be achieved [22,47].

Based on results of electrochemical performance measured by above analysis, specific capacitances, equivalent series resistance (ESR), energy density and power density were calculated by using following equations [154-157].

the specific capacitances (C (Fg-1)) of samples were determined by following equations

(1) and (2) depending on CV and GCD bases, respectively.

∫ IdV (1) C (Fg) = vm(V − V)

Where numerator is area under the curve of CV, v is the scan rate, m is the mass of the electrode material, V2 and V1 are the lower and upper voltage limits

2I (2) C (Fg) = m(dv/dt)

Where I is applied current, M is mass of the electrode material, dv/dt is slope of discharge curve.

Form GCD curves, IR drops were also clarified and used to calculate equivalent series resistance (ESR) with equation (3) as shown in below.

(3) R (Ω) = 2

Where RESR is indicating ESR value, IRdrop is IR-drop values measured from discharge curve in

GCD and ICons is constant current density.

In addition, energy density and power density were also estimated by following equations

(4) and (5), respectively.

1 E (4) Energy Density (ED) = CV (5) Power Density (PD) = 2 t

Where C (Fg-1) is specific capacitance, V is Potential window, E is energy density and t (s) is discharge time from GCD.

2.8 Applications

As much as demand of improved energy storage was enhanced for diverse applications, rechargeable lithium ion batteries have been replaced to supercapacitors due to their high-power density, rapid charge/discharge capability and long-life cycle. According to this, current typical applications such as main or alternating power sources are as follows [45-47].

 Video recorders, TV satellite receivers: backup of TV-channel setting, recording times,

and clock time. The backup is provided for a duration of h to weeks.

 Car audio system, taxi meter: backup of radio station, memory, taxi fare programs and

accumulated fare data while the car-radio or the taxi meter is taken out of the car or the

car battery is disconnected. Backup for a few h to a few days.

 Alarm clock radios, process controllers, home bakery, coffee machines: protects clocks

and programmed functions from getting lost in case of temporary power outage. Backup

for min to h.

 Photo and video cameras, programmable pocket calculators, electronic agendas and

organizers, mobile phones, and pagers. The backup is provided during the replacement

of the batteries for s to min.

 Toys: for example, cars with ‘rechargeable motors’ contain an EC which can be

recharged from a battery- or mains-powered charger. The charging takes about 10 s and

power is supplied to the car for several 10 s. Due to compactness and low weight the

cars can accelerate very fast.

 Fail-safe positioning: the EC provides the power for open or close positioning in case of

power failures. In the past, mostly spring systems have been used. The use of electric

actuators with ECs allows to make smaller, cheaper and faster systems.

 Starter applications: the EC provides the main part of the pulse power for starting of, e.g.

Diesel locomotives. It is charged within B1 min from lead-acid batteries. It allows to

start the Diesel engine at very low temperature (Siberia). The size of the battery system

may be reduced by up to 50%. Since the pulse currents drawn from the batteries are

much smaller, the life of the batteries doubles.

 Solar watch: After being completely charged the EC may feed the watch for several

days. The watch does not need any battery replacement during its lifetime.

 Solar lanterns, road marking lanterns, lighting of time-tables at bus stop, illumination of

parking meters, traffic warning signals: the combination of solar panels, LEDs instead of

incandescent bulbs and ECs makes a reliable system with a long lifetime and no needs

for maintenance.

CHAPTER 3

Chemically Interconnected Ternary AgNP/Polypyrrole/Functionalized Buckypaper

Composites as High Energy Density Supercapacitor Electrodes

Hyungjoo Kima, Manivannan Ramalingamb, Qingwen Lic, Yuntian Zhud, Xiangwu Zhanga, Wei

Gaoa, Young-A Sonb* and Philip D Bradforda* a Department of Textile Engineering, Chemistry and Science, North Carolina State University,

Raleigh, NC 27606, USA b Department of Advanced Organic Materials Engineering, Chungnam National University, 220

Gung-dong, Yuseong-gu, Daejeon 305-764, South Korea c Suzhou Institute of Nano-Tech and Nano-Bionics, Suzhou, 215213, China d Department of Materials Science and Engineering, North Carolina State University, Raleigh,

27695, USA

* : Co-corresponding author

Abstract

In this study, a ternary composite was fabricated to develop hybrid supercapacitor electrodes with high performance. This ternary composite consisted of epoxide functionalized buckypaer, pseudo-capacitive polypyrrole (PPY) and highly electrical conductive silver nanoparticles (AgNP). To characterize the prepared electrode, Raman spectroscopy, X-ray photon spectroscopy (XPS) and scanning electron microscopy (SEM) were carried out, showing that PPY and AgNP were uniformly coated on surface of buckypaper. Hydrogen bonding and ligand bonding were found to be present through analysis of XPS binding energy shifts. In addition, cyclic voltammetry (CV) and galvanostatic charge-discharge analysis (GCD) were also conducted to estimate the electrochemical performance of prepared composites. According to the

obtained results, specific capacitance values of EBP-PPY-AgNP were significantly higher than other control groups not chemically interconnected between individual materials. In addition, both IR (Internal Resistance)-drop decrease and enhanced cycling stability (103% capacitance retention after 2,500 cycles) were also observed. Finally, energy density was calculated, showing that higher value than other reported papers which studied composite based electrodes by using similar materials. These results suggested that prepared ternary composite, especially possessing chemical interactions, is promising for high energy density supercapacitor electrode.

3.1 Introduction

Carbon nanotubes and their structures were clarified by Sumio Iijima in 1991 [75].

Beginning with this research, the era of carbon nanotubes began and their excellent properties brought new vistas of advanced science technology including energy storage and energy conversion devices, etc [79,158].

Recently, pure carbon nanotubes have been not only used but also functionalized carbon nanotubes have been widely studied for abovementioned applications since their intrinsic properties of pure carbon nanotubes can be improved. For example, dissolubility of natural carbon nanotubes in typical organic solvents or water have been recognized as the problem that hinders applications. Therefore, functionalizing or modifying carbon nanotubes have been actively studied and consequently their solubility was successfully improved without surfactants

[159-163]. Above and beyond this positive effect, strength of carbon nanotubes can also be enhanced after functionalization [84]. In particularly, as supercapacitor electrode materials, functionalized carbon nanotubes showed better electrochemical performances (ex) wettability; electrical contact resistance; adsorption of ions; self-discharge characteristics) when compared with pristine carbon nanotubes [21,164]. Thus, supercapacitor electrodes based on functionalized

carbon nanotubes have been steadily studied [165-169]. Especially, among various functionalized carbon nanotubes, epoxidized carbon nanotube based buckypaper reported by

George at al. revealed that it can be well combined with polymer matrix to from composite material since large pores were formed by epoxide functional groups in the buckypaper [170].

Among various reported supercapacitor electrode based on functionalized carbon nanotubes, composites types containing both functionalized carbon nanotube and conducting polymers have been noted due to their excellent electrochemical performance attributed from synergistic effect. For instance, supercapacitor electrode made by only conductive polymer have the limitation for practical application because of poor cycling stability [171]. However, this obstacle can be overcome by synthesizing conductive polymer with carbon nanotubes since high surface area and good conductivity can be provided by carbon nanotubes [111]. Furthermore, oxygen containing groups, such as carboxyl groups, obtained by carbon nanotubes functionalization can be chemically connected with conductive polymers, resulting in enhanced electrochemical characteristics This is because high electrical conductivity and low charge transfer resistance can be obtained by chemical interaction such as hydrogen bonding between functionalized carbon nanotube and conductive polymer [172-174].

With the synergistic effect, introduction of silver to various supercapacitor electrodes has been reported to further enhance electrochemical performance [24-31]. According to these studies, specific capacitance of prepared supercapacitor electrodes showed enhancement after silver deposition. This improvement can be explained by the silver because both good electrical conductivity and fast ion transportation can be obtained through the presence of silver in supercapacitor electrode material [24]. In addition, silver has high affinity toward nitrogen atom.

Therefore, coordinate covalent bonded structure between silver and nitrogen containing

conductive polymers such as polyaniline can be built and provide improved electrochemical performance when compared with pure polyaniline supercapacitor electrode. In this regard, according to research progressed by Patil et al. [30,175], in complex structure formed between silver and polyaniline, electron deficient in nitrogen atom occurred by decrease of electron density on the nitrogen that acted as donor atoms. Therefore, conductivity of the resultant complex can be enhanced, resulting that proper nature can be provided to insert/extract electrolyte ions into/from supercapacitor electrode based on silver/PANI. Consequently, specific capacitance can be increased [24,32,176].

In this study, epoxide functionalized buckypaper was prepared to develop supercapacitor electrode. Moreover, polypyrrole and silver were additionally synthesized with the prepared functionalized buckypaper to further enhance electrochemical performance. Especially, synergistic effects between individual materials in the resultant ternary composite were intentionally generated. For example, hydrogen bonding interaction between epoxide group on the functionalized buckypaper and polypyrrole was given while forming coordinate covalent bonded structure between polypyrrole and silver. Through such series of chemical links between each component, electrochemical performance was improved. In this regard, to the best our knowledge, even though, various composites were previously reported to develop supercapacitor electrode, buckypaper mainly containing epoxide functional groups and accordingly induced chemical linkages effect on supercapacitor performance have rarely been studied.

3.2 Experimental Section

3.2.1 Materials and Synthetic Procedures

All chemicals were purchased from Sigma Aldrich and used without further purification.

In addition, all procedures to synthesize ternary composite as supercapacitor electrode were described in Scheme. 3.1.

Scheme 3.1 Fabrication method to develop EBP-PPY-AgNP with its control groups.

3.2.2 Synthetic method for purified pristine buckypaper (PPBP)

To develop ternary composites for supercapacitor electrode materials, pristine buckypaper was selected. The buckypaper was composed of multiwalled carbon nanotube and produced by floating chemical vapor deposition method [177]. For further purification, prepared pristine buckypaper was soaked in 6M HCl solution for overnight. After that, purified buckypaper was dried in vacuum for overnight to remove residual solvent.

3.2.3 Preparation of Functionalized Buckypaper (EBP)

After this, prepared buckypaper was functionalized by epoxide treatment. This functionalization was not only effective to improve its properties related to electrochemical performance, but also introduce hydrogen bonding with other electrode materials. In this regard, meta-chloroperoxybenzoic acid (m-CPBA) was used for functionalization, resulting in epoxide group formed on the surface of buckypaper [178]. The intimate synthetic procedure will be as in the following. Purified buckypaper (PPBP) was immersed in 5wt% meta-chloroperoxybenzoic acid (m-CPBA) / CH2Cl2 solution for 3 hours. After epoxidation treatment, the functionalized

EBP was washed for 5times with CH2Cl2 and 5times with Ethanol, respectively. Finally, the prepared EBP was dried overnight at room temperature to evaporate residue solvent.

3.2.4 Synthetic Method of Polypyrrole-Functionalized Buckypaper composite

0.1mL of pyrrole monomers was dissolved in 0.1M HCl (10mL). To the prepared solution, PPBP or EBP were added and kept in RT for 12hrs. Then, 0.1M HCl (10mL) solution containing FeCl3 (0.24g) was added by dropwise. The mixed solution was kept in RT for 6hrs and washed with distilled water and Methanol several times. Finally, Polypyrrole treated samples

(called as PPBP-PPY and EBP-PPY, respectively) were dried in vacuum for overnight to remove residual solvent.

3.2.5 Silver Nanoparticle Coatings

The prepared PPBP-PPY and EBP-PPY were immersed in solution containing silver nanoparticles (silver nanoparticles (particle size: 20nm / density: 0.02mg/L) were purchased from Sigma-Aldrich) in RT for 12hrs. After that, silver nanoparticles deposited samples were dried in vacuum for overnight. The final products were named as PPBP-PPY-AgNP and EBP-

PPY-AgNP, respectively.

3.3 Characterization

The morphologies of prepared composite samples were characterized by low voltage

field emission scanning electron microscope (Merlin Compact, Zeiss). To clarify the

structure and components of samples, Raman spectra were recorded by high resolution

Raman spectrophotometer (LabRAM HR-800, HORIBA scientific). In addition, X-ray

photoelectron spectroscopy (XPS) were also performed by MultiLab 2000 (Thermo

Scientific).

To assess the electrochemical performances of prepared samples, cyclic

voltammetry (CV) and galvanostatic charge/discharge (GCD) were recorded by Autolab

(PGSTAT128N, Metrohm Autolab Instrument). During the measurements, three-electrode

system was introduced. In the system, Pt and saturated calomel electrode were used as

counter electrode and reference electrode, respectively. In addition, PVA/H3PO4 was used

as electrolyte. This electrolyte was prepared by dissolving PVA (1g) and H3PO4 (1g) in

distilled water (10g). In case of experimental condition, CV curves were measured with the

applied potential in -0.2 ~ 0.8 V at 10, 20, 30, 40, 50 and 100mV/s scan rate. In addition,

GCD curves were measured with a working voltage of -0.2 ~ 0.8 V at a current density of

0.25, 0.5, 0.75, 1.0 and 2.0 A/g.

Based on results of electrochemical performance measured by above analysis,

specific capacitances, equivalent series resistance (ESR), energy density and power

density were calculated by using following equations [154-157].

The specific capacitances (C (Fg-1)) of samples were determined by following

equations (1) and (2) depending on CV and GCD bases, respectively.

∫ C (Fg) = (1) ()

where numerator is area under the curve of CV, v is the scan rate, m is the mass of the

electrode material, V2 and V1 are the lower and upper voltage limits

C (Fg) = (2) (/)

where I is applied current, M is mass of the electrode material, dv/dt is slope of discharge

curve.

Form GCD curves, IR drops were also clarified and used to calculate equivalent

series resistance (ESR) with equation (3) as shown in below.

R (Ω) = (3)

where RESR is indicating ESR value, IRdrop is IR-drop values measured from discharge curve

in GCD and ICons is constant current density.

In addition, energy density and power density were also estimated by following

equations (4) and (5), respectively.

Energy Density (ED) = CV (4) where C (Fg-1) is specific capacitance and V is Potential window.

Power Density (PD) = (5) where E is energy density and t (s) is discharge time from GCD.

3.4. Result and discussion

The ternary composites which consist of epoxide functionalized buckypaper, polypyrrole and silver nanoparticles were successfully prepared to develop supercapacitors. In addition, their morphologies, structures and electrochemical performance were investigated.

3.4.1 Characterization of the prepared ternary composites

To clarify the structure of prepared ternary composites in this work, Raman spectra were measured as shown in Figure 3.1. According to the spectra, the changes of peaks were observed depending on different treatments in synthetic methods.

First of all, intensities of both D (defects and disorder) and G (graphite) bands in PPBP were significantly changed after epoxide treatment. In this regard, the results revealed that intensity of D band in EBP was increased while that of G band was decreased. Therefore, higher

ID/IG ratio was also shown in EBP (0.83) when compared with PPBP (0.43). This phenomenon was contributed from change of crystalline graphitic structure to defects induced by the introduction of oxygen containing epoxide functional groups on the buckypaper [179]. After polypyrrole (PPY) polymerization treatment as a second step to develop ternary composite-based supercapacitors, several new peaks were found in Raman spectra of both PPBP-PPY and EBP-

PPY. As shown in Table 3.1, these new peaks were corresponded to peaks that could be obtained from pure PPY, indicating that it was well-formed in both PPBP and EBP. Lastly, to further improve electrochemical performance by enhancement of electrical conductivity, silver nanoparticles (AgNP) were deposited to PPBP-PPY and EBP-PPY, respectively. In this regard, raman spectra of PPBP-PPY-AgNP and EBP-PPY-AgNP still showed similar peaks toward pure

PPY, PPBP-PPY and EBP-PPY, indicating that structure of polypyrrole was not damaged by silver nanoparticles.

PPBP EBP Pure PPY PPBP-PPY EBP-PPY PPBP-PPY-AgNP EBP-PPY-AgNP Intensity (a.u) Intensity

1000 2000 Raman Shift (cm-1)

Figure 3.1 Raman Spectra of Pure PPY, PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP.

Table 3.1 Assignment of Raman peaks for Pure PPY, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP.

Unit: cm-1 Ring C-H N-H Breathing C-N C=C Pure PPY 925 1042/1073 1234 1363 1483 1590 PPBP-PPY 934 1053/1077 1234 1367 1483 1589 EBP-PPY 935 1050/1073 1237 1363 1477 1584 PPBP-PPY-AgNP 920 1041/1073 1237 1368 1492 1588 EBP-PPY-AgNP 919 1041/1073 1239 1368 1492 1587

To further investigate structures of all the prepared composites in this study, characterization of chemical components was also studied by XPS spectra in Figure 3.2. According to the results, O 1s peak of EBP was shown with higher intensity in comparison with that of PPBP, revealing that oxygen containing epoxide functional groups were well attached to buckypaper after epoxide treatment. In addition, N 1s peaks were newly observed in both PPBP-PPY and EBP-PPY.

This result came from deposited PPY in the composites. Furthermore, Ag 3d peaks were also clearly observed due to presence of loaded AgNP in both PPBP-PPY-AgNP and EBP-PPY-AgNP.

These results obtained from XPS were corresponded with Raman results, indicating that ternary composites composed of functionalized buckypaper, PPY and AgNP were successfully synthesized.

PPBP EBP PPBP-PPY EBP-PPY C PPBP-PPY-AgNP O EBP-PPY-AgNP N Ag3d Intensity (a.u) Intensity

900 800 700 600 500 400 300 Binding Energy (eV)

Figure 3.2 XPS spectra of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY- AgNP.

Similar as above mentioned Raman peak shifts phenomena as shown in Figure 3.1, binding energy shifts of both C 1s and N 1s in XPS were also measured to validate the presence of chemical interactions between individual materials in the composites. Figure 3.3 (a) and (b) represented C 1s and N1s signals, respectively. Hydrogen bonding interaction was proved by comparison of binding energies for PPBP-PPY and EBP-PPPY in C 1s and N 1s. In Figure 3.3

(a), C 1s signal for EBP-PPY was shifted to higher binding energy than that for PPBP-PPY, indicating that electron density of carbon atoms in buckypaper was reduced by withdrawing effect of hydrogen bonding [180,181]. This is because electrons in carbon atoms were shifted to both oxygen atoms of epoxide functional groups and NH of PPY. On the other hand, as depicted

in Figure 3.3 (b), negative shift against pure PPY was observed in N 1s signal for EBP-PPY since electron density was enhanced by hydrogen bonding. This phenomenon was induced by electron dislocation around nitrogen atoms, resulting in electron donation through epoxide functional groups in EBP. After AgNP deposition, binding energies in N 1s for both PPBP-PPY-

AgNP and EBP-PPY-AgNP were shifted to 399.9 eV, revealing that ligand bonding interaction was formed between AgNP and nitrogen in PPY. This shift was caused by electron transfer from nitrogen in PPY to AgNP [30]. Therefore, electron density of nitrogen in PPY was decreased, inducing positive binding energy shifts of N1 s in both PPBP-PPY-Ag and EBP-PPY-Ag when compared to PPBP-PPY and EBP-PPY, respectively.

(a) C1s

PBP-PPY EBP-PPY Intesity (a.u)Intesity

280 281 282 283 284 285 286 287 288 289 290 Binding Energy (eV)

(b) N1s Pure PPY PPBP-PPY EBP-PPY PPBP-PPY-AgNP EBP-PPY-AgNP Intensity (a.u) Intensity

397 398 399 400 401 402 403 Binding Energy (eV)

Figure 3.3 Figure Binding energy shifts in C1s (a) and N1s (b) spectra.

Figure 3.4 (a)~(f) showed SEM images of prepared composites depending on different treatments. As shown in Figure 3.4 (a)~(d), when compared between before and after PPY treatment, thickness of CNT bundles was enhanced, indicating that PPY was well deposited on the both PPBP and EBP. This results also supported that CNT backbone structure was well maintained due to uniformly coated PPY. In addition, after AgNP deposition to both PPBP-PPY and EBP-PPY, it was clearly observed that AgNP was well dispersed and attached to CNT bundles as shown in Figure 3.4 (e) and (f). These SEM images were corresponded to fore mentioned results obtained from Raman, XPS and XRD spectra.

Figure 3.4 SEM Images of PPBP (A), EBP 3hrs (B), PPBP-PPY (C), EBP-PPY (D) PPBP-PPY- AgNP (E) and EBP-PPY-AgNP (F)

3.4.2 Evaluation of electrochemical performance for the prepared ternary composites as

supercapacitor electrodes

Through the previous analyzed results from Raman, XPS, XRD and SEM, structures and morphologies of the ternary composites, synthesized in this work, were obviously clarified. In particularly, EBP based composites revealed the presence of intrinsic chemical interactions including hydrogen bonding and ligand bonding. In this regard, to further estimate the final products as supercapacitor electrodes and effect of such chemical interactions on electrochemical behaviors, both cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were measured by utilizing a three-electrode system in PVA/H3PO4 electrolyte.

3.4.2.1 Cyclic Voltammetry (CV)

According to the obtained result as can be seen Figure 3.5, close rectangular shape was clearly observed in CV curve of PPBP, indicating a typical electrochemical double-layer capacitance behavior (EDLC) [182]. However, after epoxide treatment, faradaic redox peaks were obviously found in EBP, revealing that pseudo-capacitance behavior was induced due to presence of oxygen containing epoxide functional groups [183]. After PPY was synthesized with both PPBP and EBP, corresponding CV curves were significantly deviated from close rectangular shape to an oblate form. This change can be explained by switch of charge storage mechanism according to reduction and oxidation mode of PPY in PPBP-PPY and EBP-PPY electrodes [184]. Their loops of CV curves became wider than those of PPBP and EBP.

Furthermore, when AgNPs were deposited to both PPBP-PPY and EBP-PPY, comparatively rectangular shapes were observed from both PPBP-PPY-AgNP and EBP-PPY-AgNP, indicating that they exhibited excellent electrochemical behavior [185]. In case of their CV curves, wider

integrated areas were achieved than AgNP untreated electrodes. Especially, the widest integrated area was responded from EBP-PPY-AgNP.

PPBP EBP 3 PPBP-PPY EBP-PPY 2 PPBP-PPY-AgNP EBP-PPY-AgNP

-1 Current DensityCurrent (A/g)

-2

-3

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V)

Figure 3.5 CV curves of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY- AgNP at 10mV/s scan rate.

To further investigate electrochemical behaviors of prepared electrodes, CV was measured at different scan rates. In Figure 3.6 (a-f), all the prepared electrode materials showed similar changes as scan rate increased. Especially, CV curves of PPY and AgNP treated electrodes exhibited roughly rectangular mirror image in terms of zero-current line. In addition, they were progressively changed to oblate form as scan rate increased since entering/ejecting and diffusion rates of counter ions were slower than transfer rate of electrons in the prepared electrodes [186].

(b) (a) 10mV/s 2.0 10mV/s 4 20mV/s 20mV/s 30mV/s 30mV/s 1.5 40mV/s 40mV/s 50mV/s 50mV/s 2 100mV/s 1.0 100mV/s

0.5 0 Current (A/g) 0.0 Current Density (A/g)

-0.5 -2

-1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) (d) Voltage (V) (c) 10mV/s 10mV/s 20mV/s 4 20mV/s 30mV/s 5 30mV/s 40mV/s 40mV/s 50mV/s 50mV/s 2 100mV/s 100mV/s

0 0 CurrentDensity(A/g)

Current Density (A/g) Density Current -2

-5

-4

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 (e) 8 Voltage (V) (f) 8 Voltage (V) 10mV/s 10mV/s 20mV/s 20mV/s 6 6 30mV/s 30mV/s 40mV/s 4 40mV/s 4 50mV/s 50mV/s 2 100mV/s 2 100mV/s

0 0

-2 -2 CurrentDensity (A/g) Current Density (A/g) -4 -4

-6 -6

-8 -8 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Voltage (V)

Figure 3.6 CV curves of PPBP (a), EBP (b), PPBP-PPY (c), EBP-PPY (d), PPBP-PPY-AgNP (e) and EBP-PPY-AgNP (f) under different scan rates.

Based on measured CV curves, specific capacitances of all electrode materials were calculated by using equation (1). In addition, the obtained values were summarized in Table 3.2.

First of all, when compared between PPBP (23.33 F/g) and EBP (50.86 F/g), latter electrode showed more than two times higher specific capacitance. This result was contributed from the additional pseudo-capacitance property provided by oxygen containing functional groups in EBP

[172]. In addition, after PPY was deposited to the PPBP and EBP, their specific capacitances were drastically increased to 185.03 F/g (PPBP-PPY) and 246.12 F/g (EBP-PPY), respectively.

This enhancement was achieved by combination of advantages that CNT and PPY have. For example, redox-active PPY can provide high pseudo capacitance simultaneously with both

EDLC and high conductivity from CNT [180]. In particularly, specific capacitance of EBP-PPY was higher than that of PPBP-PPY. As shown in Scheme 1, this phenomenon can be explained by hydrogen bonding interaction between epoxide functional groups in EBP and NH in PPY since it was responsible for improvement of electrical conductivity while weakening charge transfer resistance [172-174]. Therefore, higher specific capacitance was generated from EBP-

PPY than PPBP-PPY. In addition, when AgNP was finally deposited, 245.83 F/g and 349.87 F/g were achieved as specific capacitance values of PPBP-PPY-AgNP and EBP-PPY-AgNP, respectively. These values were relatively higher than those of AgNP nontreated electrodes, indicating positive effect of AgNPs on enhancement of specific capacitance. This effect was contributed from presence of ligand bonding between N in PPY and AgNP in Scheme 1. The reason is that electron density of nitrogen in PPY can be reduced to form coordinate covalent bonding with AgNP, enhancing electrical conductivity in resultant coordinate covalent bond.

According to this, specific capacitance can be increased [30].

Table 3.2 Specific capacitance values of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP from CV curves in 10mV/s.

Specific Capacitance PPBP 23.33 F/g EBP 50.86 F/g PPBP-PPY 185.03 F/g EBP-PPY 246.12 F/g PPBP-PPY-AgNP 245.83 F/g EBP-PPY-AgNP 349.87 F/g

Effect of scan rate on the changes of specific capacitances for all electrode materials was also determined as depicted in Figure 3.7. All electrode materials revealed that their specific capacitances were diminished as scan rate increased from 10 to 100mV/s. This phenomenon was induced by change of charge storage capacity. If the scan rate was decreased, both charge and discharge process in supercapacitor can be sufficiently progressed, resulting in enough diffusion and migration of electrolyte ions for more charge up in active material. On the other hand, as scan rate increased, electrolyte ions cannot diffuse and migrate. Thus, surface area of active material for charge storage partially becomes inaccessible [187].

350 PPBP EBP 300 PPBP-PPY EBP-PPY 250 PPBP-PPY-AgNP EBP-PPY-AgNP 200

150

Specific Capacitance (F/g) Specific 100

0 0 50 100 Scan Rate (mV/s)

Figure 3.7 Specific capacitance values of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP under different scan rate.

3.4.2.2 Galvanostatic Charge-Discharge (GCD) Curve

Another important factor to evaluate the electrochemical performance is rate capability of electrodes. In this regard, Figure 3.8 described GCD curves of all prepared electrodes. According to the obtained result, all prepared electrodes presented GCD curve form that can be observed from the typical capacitor, by showing linear correlation between charge/discharge time and voltage [185]. However, when compared among measured electrodes, their charge-discharge time was differently observed and order of the time length was EBP-PPY-AgNP > EBP-PPY >

PPBP-PPY-AgNP > PPBP-PPY > EBP > PPBP. In addition, IR-drop should be considered as an important factor for estimation of electrochemical performance in GCD. The reason is that small

IR-drop reflected low internal resistance, indicating that energy involved in occurrence of

unwanted heat during charge-discharge process in energy storage device [185,188]. Especially,

EBP based electrodes showed smaller IR-drop values than PPBP based electrodes. With measured IR-drop values, ESR (Equivalent Series Resistance) was also calculated based on equation (3) and organized in Table 3.3. This ESR value (RESR) is one of the important parameters to evaluate ionic conductivity in electrolyte toward electrode. In other words, low

ESR value means high ionic conductivity [156,157]. In this regard, the result indicated that the lowest ESR value (0.1614 Ω) was obtained from EBP-PPY-AgNP electrode. Interestingly, when compared between PPBP and EBP based electrodes, the latter exhibited lower ESR values. This result revealed that greater compatibility with electrolyte can be achieved from electrode containing EBP than PPBP. Therefore, more excellent electrochemical performance mentioned above also can be explained by this reason.

PPBP EBP 0.8 PPBP-PPY EBP-PPY PPBP-PPY-AgNP 0.6 EBP-PPY-AgNP

0.4 Voltage (V) Voltage 0.2

0.0

-0.2 0 250 500 750 1000 1250 1500 1750 2000 2250 Time (S)

Figure 3.8 Charge-discharge curves of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP in 0.25A/g.

Table 3.3 ESR values of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY- AgNP.

ESR (Ohm) PPBP 0.178 Ω EBP 0.1432 Ω PPBP-PPY 0.2918 Ω EBP-PPY 0.2202 Ω PPBP-PPY-AgNP 0.1812 Ω EBP-PPY-AgNP 0.1604 Ω

To examine effect of different current densities on electrochemical behavior of the electrode, GCD analysis of all prepared electrodes was also carried out at different current densities As can be seen in Figure 3.9 (a~f), GCD curves of all prepared electrodes exhibited longer charge-discharging time span and smaller IR-drop as much as lower current density was applied. These results were derived from slow electrochemical process that can provide enough time for enhancement of possibilities that electrolyte ions can approach more deep pores in electrodes [189].

(a) (b) 0.8 0.25A/g 0.8 0.25A/g 0.5A/g 0.5A/g 0.75A/g 0.75A/g 0.6 0.6 1.0A/g 1.0A/g 2.0A/g 2.0A/g 0.4 0.4 Voltage (V) Voltage (V) 0.2 0.2

0.0 0.0

-0.2 -0.2 0 20 40 60 0 30 60 90 120 150 Time (S) Time (S) (c) 0.25A/g (d) 0.25A/g 0.8 0.5A/g 0.8 0.5A/g 0.75A/g 0.75A/g 1.0A/g 1.0A/g 0.6 2.0A/g 0.6 2.0A/g

0.4 0.4 Voltage (V) Voltage (V) 0.2 0.2

0.0 0.0

-0.2 -0.2 0 250 500 750 1000 1250 1500 1750 0 200 400 600 800 1000 Time (S) (f) 0.25A/g (e) Time (S) 0.25A/g 0.8 0.5A/g 0.8 0.5A/g 0.75A/g 0.75A/g 1.0A/g 1.0A/g 0.6 0.6 2.0A/g 2.0A/g

0.4 0.4 Voltage (V) Voltage (V) 0.2 0.2

0.0 0.0

-0.2 -0.2 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Time (S) Time (S)

Figure 3.9 GCD curves of PPBP (a), EBP (b), PPBP-PPY (c), EBP-PPY (d), PPBP-PPY-Ag (e) and EBP-PPY-Ag (f) under different current densities.

GCD curves at 0.25A/g were also applied to calculate the specific capacitance all the electrode materials with equation (2). According to the result shown in Table 3.4, the highest specific capacitance of 625 F/g was achieved from EBP-PPY-AgNP and followed by EBP-PPY

(500 F/g), PPBP-PPY-AgNP (454.55 F/g), PPBP-PPY (333.33 F/g), EBP (42.21 F/g) and PPBP

(15.94 F/g). This result accorded with the trend of specific capacitances calculated from CV curves, implying that positive effect of chemical interactions such as hydrogen bonding and ligand bonding on electrochemical behavior of prepared electrodes in this study.

Table 3.4 Specific capacitance values of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP from GCD curves in 0.25 A/g.

Specific Capacitance PPBP 15.94 F/g EBP 41.12 F/g PPBP-PPY 333.33 F/g EBP-PPY 500 F/g PPBP-PPY-AgNP 454.55 F/g EBP-PPY-AgNP 625 F/g

3.4.2.3 Cyclic Stability

The cyclic stability, one of the most important electrochemical performance, was measured since it is the parameter to estimate application potential as practical supercapacitor electrode in real field. In this regard, as depicted in Figure 3.10, specific capacitance retention abilities of all prepared electrodes were estimated after 2,500 cycles in 100mV/s. EBP-PPY-

AgNP, final product in this study, still maintained 103% of its initial capacitance. In particularly, when compared between PPBP based and EBP based electrodes, the latter revealed more stable behavior. This is due to synergistic effects (hydrogen bond and ligand bond) and accordingly

enhancement of both electrical and mechanical stability [12].

PPBP 140 EBP PPBP-PPY 120 EBP-PPY PPBP-PPY-AgNP EBP-PPY-AgNP 100

40 Specific Capacitance (F/g) Capacitance Specific

0 0 500 1000 1500 2000 2500 Number of Cycles

Figure 3.10 Cyclic stability of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP- PPY-AgNP during 2,500 cycles in 100mV/s.

3.4.2.4 Ragone Plot

With cyclic stability, energy and power densities, described in Ragone plot, also have been considered as important properties of supercapacitor electrode for practical application.

Through this Ragone plot, both energy storage capability and rate capability of energy delivery of supercapacitor can be determined. In this regard, Figure 3.11 described the Ragone plot of prepared electrodes in this study. According to the result, when compared with other control groups, EBP-PPY-AgNP occupied the highest energy (73.46 Wh/kg) and power (345.77 W/kg) densities. While power density increased to 1527.79 W/Kg, energy density still remained as

25.21 Wh/kg. Especially, in case of energy density for EBP-PPY-AgNP, maximum value was much better than other reported similar materials-based supercapacitor electrodes as shown in

Table. 3.5. In addition, when compared between PPBP and EBP based electrodes, both energy and power density of latter samples exhibited relative high values derived from more excellent electrochemical performance of EBP than that of PPBP. Hence, it can be another evidence emphasizing the important role of chemical interactions in supercapacitor electrode.

100 PPBP EBP PPBP-PPY 80 EBP-PPY PPBP-PPY-AgNP EBP-PPY-AgNP

Energy Density Energy (Wh/Kg) 20

0 500 1000 1500 2000 2500 Power Density (W/kg)

Figure 3.11 Ragone plot of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP.

Table 3.5 A comparison of the energy density of reported electrode materials.

Electrode Material Experimental Conditions Energy Ref. Description Density (Wh/Kg) Electrolyte Reference Potential Range (V)

EBP-PPY-AgNP PVA/H3PO4 SCE -0.2~0.8 73.46 This work

PPBP-PPY-AgNP PVA/H3PO4 SCE -0.2~0.8 52.5 This work

GO-PPY-Ag 1M H2SO4 Ag/AgCl 0.0~0.8 25.5 [190]

MnO2/PPY/CNT 1M Na2SO4 - 0~1.0 38.4 [191]

EBP-PPY PVA/H3PO4 SCE -0.2~0.8 55.01 This work

PPBP-PPY PVA/H3PO4 SCE -0.2~0.8 39.68 This work PPY nanowires/ A-CNT 1.0 M KCl SCE -0.6~0.6 15.1 [192]

PPy/MWCNT 0.5M Na2So4 - -0.5~0.4 4.5 [193]

PPy/MWCNT 0.5M Na2So4 SCE -0.5~0.4 10.7 [194]

PPy/CNT strip PVA/H2SO4 with HQ Ag/AgCl 0~0.8 4.7 [195]

PPy/CNT fiber 1M H2SO4 SCE -0.2~0.6 31.2 [196]

PPy/Oxidized CNT fibers 1M LiClO4 - 0~1.0 42 [197]

PPY/CNT grown over 0.5M H2SO4 - -0.2~0.6 3.9 [198] carbon cloth

PPy/CNT fiber 6M H3PO4/PVA - 0~0.8 3.6 [199]

PPy/TiO2/SWCNT 1M KCl - -0.3~0.3 1.0 [200]

PPY/MnO2 0.5M Na2SO4 - -0.2-0.8 44 [201]

PPY/rGO/CNT 0.5M Na2SO4 Ag/AgCl N/A 14.3 [202]

EBP PVA/H3PO4 SCE -0.2 ~0.8 4.92 This work

PPBP PVA/H3PO4 SCE -0.2 ~0.8 1.84 This work

3.5 Conclusions

In summary, ternary hybrid composite consisting of EBP, PPY and AgNP was designed and fabricated. In this hierarchical structure, EBP was prepared by epoxide treatment toward purified buckypaper and then, PPY was deposited to EBP by in-situ polymerization. Finally,

AgNP was coated by self-assembly method. In particularly, unlike ternary composites reported in most of published papers, chemical interactions were intentionally introduced into this prepared composite as follows; (a) hydrogen bonding interaction between epoxide functional groups in EBP and NH in PPY, (b) ligand bonding formation between N in PPY and AgNP. In addition, for the supercapacitor electrode application of EBP-PPY-AgNP and its control groups

(PPBP, EBP, PPBP-PPY, EBP-PPY and PPBP-PPY-AgNP), their electrochemical performance was also determined by CV and GCD. In this regard, the obtained results revealed that EBP-

PPY-AgNP, final target product, exhibited the highest specific capacitance, most excellent rate capability, long cycling stability and more abundant energy/power densities in comparison with other control groups. Interestingly, EBP based composites always showed more excellent electrochemical performance than PPBP based composites. In addition, when compared with previously reported supercapacitor electrodes consisting of similar components, EBP based composites chemically interconnected showed higher energy density. Through all these results, it was clarified that synergistic effects induced by chemical interactions positively affected the electrochemical performance. Therefore, developed EBP-PPY-AgNP can be suitable and a promising electrode material for high energy density supercapacitor electrode.

CHAPTER 4

AgNP/Crystalline PANI/EBP Composite Based Supercapacitor Electrode with Internal

Chemical Interactions

Hyungjoo Kima, Manivannan Ramalingamb, Qingwen Lic, Yuntian Zhud, Xiangwu Zhanga, Wei

Gaoa, Young-A Sonb* and Philip D Bradforda* a Department of Textile Engineering, Chemistry and Science, North Carolina State University,

Raleigh, NC 27606, USA b Department of Advanced Organic Materials Engineering, Chungnam National University, 220

27695, USA

* : Co-corresponding author

Abstract

In this work, polyaniline was conformally coated on epoxide functionalized buckypaper

(EBP) through in-situ polymerization. Due to presence of epoxide functional groups on the surface of buckypaper, chemical interactions were formed between oxygen in epoxide groups and NH in polyaniline (PANI). This chemical interaction was identified by peak shifts and change in intensity in the Raman spectrum. Additionally, crystalline peaks (21.56o, 23.92o,

26.76o) were also clearly observed in X-Ray Diffraction (XRD), indicating that the epoxide groups on the functionalized buckypaper played a role of oxidants to from crystalline PANI. On the other hand, no peak or intensity changes or crystalline peaks were not observed in the non- functionalized buckypaper (PPBP) based composite. Both the hydrogen bonding and crystalline

nature induced between epoxide functionalized buckypaper and polyaniline enhanced electrical conductivity of the prepared composite (EBP-PANI), resulting in high specific capacitance

(783.9 F/g), which was better than a control group which did not have any interactions except pi- pi stacking (PPBP-PANI: 656.02 F/g). Finally, AgNP were applied to EBP-PANI to further enhance electrical the conductivity to achieve additional electrochemical performance. Due to presence of AgNP and its interaction with N in polyaniline, the specific capacitance of EBP-

PANI-AgNP reached 915.62 F/g. In contrast, PPBP-PANI-AgNP showed slight enhancement of specific capacitance from 656.02 F/g to 690.44 F/g. These results emphasize the positive effect of the chemical interactions among individual materials of prepared composite in this study.

4.1 Introduction

Supercapacitors, also called as electrochemical capacitor, exhibit excellent electrochemical performance such as high-power density, long cycle life (>100,000 cycles), and rapid charging-discharging rates [12,68]. With recent work on increasing their energy densities, supercapacitors have been recently recognized as an alternative energy storage device to batteries, as the demand for mobile power increases [12].

Depending on charge storage mechanism or cell configuration, supercapacitors can be classified into three broad categories: electric double-layer capacitors (EDLCs), pseudo- capacitors, and hybrid capacitors [52]. In EDLCs consisting of carbon materials, charges are stored by non-faradaic reaction. On the other hand, faradaic redox reactions are involved for charge storage in pseudo-capacitors and are usually based on conductive polymers or metal oxides [53]. These mechanisms play an important role to determine performance of both EDLCs and pseudo-capacitors. For example, in case of EDLCs, charges are physically transferred without any chemical and phase changes, thus relatively long cycle lifes can be achieved [203].

However, energy density is smaller than in pseudo-capacitors [54]. Both charge storage mechanisms have their strengths and weaknesses. Drawing on the strengths from both, hybrid capacitors utilizing both Faradaic and non-Faradaic processes have been studied and have resulted in higher energy and power densities than EDLCs while improving the cycling stability which has been pseudo-capacitors main limitation [10]. Such hybrid capacitors have been developed by synthesizing both EDLCs and pseudo-capacitors materials in a single electrode.

The importance of hybrid capacitors is recognized and the amount of work in this area is increasing rapidly.

To develop hybrid supercapacitors, carbon nanotubes have been synthesized with various conductive polymers such as polypyrrole, polyaniline and polythiophene [146,204,205]. Among these conductive polymers, polyaniline have been widely studied because of its environmental stability, ease of synthesis, low cost of the monomer and large conductivity range [206]. The conductivity of polyaniline can be controlled by changing its redox and doping/de-doping states because three different isolable oxidation forms such as Leuco-emeraldine, Emeraldine and

Pernigraniline can be formed in polyaniline [104-108]. In addition, the electrical conductivity of polyaniline can be enhanced as the percent crystallinity is increased [207].

Silver has been also considered as high conductive material that can further enhance electrochemical performance of hybrid capacitors. Hybrid capacitors containing silver have are now quite common [208-210]. In research results published by Sawangphruk et al. [24], silver exhibited good electrical conductivity while providing fast ion transport at the same time.

In this work, both polyaniline and silver were combined with carbon nanotubes to develop new example of a hybrid capacitor. The carbon nanotubes were functionalized by acid treatment (meta-chloroperoxybenzoic acid (m-CPBA)) to introduce epoxide functional groups on

their surfaces. The electrochemical performance of the hybrid capacitor in this study were considered as follow with the three following hypotheses. (1) Epoxide groups on functionalized carbon nanotubes will act as an oxidant when aniline is polymerized, resulting in crystalline polyaniline showing high conductivity [211]; (2) Hydrogen bonding between epoxide groups on the functionalized carbon nanotubes and amine in polyaniline can be formed, leading to improved binding forces in composite [44]; (3) Chemical interactions between silver and nitrogen atoms in polyaniline, can be obtained, providing high conductivity and improving specific capacitance [30,175]. All of these factors should lead to a device with improved electrochemical performance.

4.2 Experimental Section

4.2.1 Materials and Synthetic Procedures

All chemicals were purchased from Sigma Aldrich and used without further purification.

In addition, all procedures to synthesize ternary composites as supercapacitor electrodes were described in Scheme 4.1.

Scheme 4.1 Fabrication method to develop EBP-PANI-AgNP and its control groups.

4.2.2 Synthetic Method for Purified Pristine Buckypaper (PPBP)

4.2.3 Preparation of Functionalized Buckypaper (EBP)

The PPBP was then functionalized using an epoxide treatment [178]. Meta- chloroperoxybenzoic acid (m-CPBA) was used for functionalization, resulting in epoxide group formed on the surface of buckypaper. Specifiically, Pristine buckypaper was immersed in 5wt% meta-chloroperoxybenzoic acid (m-CPBA) / CH2Cl2 solution for three hours. After epoxidation treatment, the functionalized EBP was washed five times with CH2Cl2 and five times with

Ethanol, respectively. Finally, the prepared EBP was dried overnight at room temperature to evaporate residue solvent

4.2.4 Synthesis of Polyaniline-Functionalized Buckypaper Composite

0.5g Aniline dissolved in 1M HCl was added to the PPBP/EBP in petri dish and kept in

RT for 1hr. After 1hr, precooled 1.22g ammonium persulfate (APS) dissolved in distilled water was added by drop wise. Then, prepared solution was kept in 0~ 5o for 12hrs. After this, polyaniline treated samples were washed with distilled water, acetone and Ethanol for five times, respectively. Finally, to remove residual solvent, the final product was dried in vacuum overnight

4.2.5 Silver Nanoparticle Coatings

The prepared PPBP-PANI and EBP-PANI were immersed in solution containing silver nanoparticles (silver nanoparticles (particle size: 20nm / density: 0.02mg/L) in RT for 12hrs.

After that, the silver nanoparticles coated samples were dried in vacuum overnight. The final products were named PPBP-PANI-AgNP and EBP-PANI-AgNP, respectively.

4.3 Characterization

The morphologies of prepared composite samples were characterized using a low voltage field emission scanning electron microscope (Merlin Compact, Zeiss). To clarify the structure and components of samples, Raman spectra were recorded with a high resolution Raman spectrophotometer (LabRAM HR-800, HORIBA scientific). In addition, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) characterization were performed using a

MultiLab 2000 (Thermo Scientific) and X-pert PRO MPD (Bruker), respectively.

To determine the electrochemical performancs of the prepared samples, cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests were recorded using an

Autolab Instrument (PGSTAT128N, Metrohm). To take measurements, a three-electrode system was utilized. In the system, Pt and saturated calomel electrode were used as the counter electrode and reference electrode, respectively. The electrolyte used was PVA/H3PO4. This electrolyte was prepared by dissolving PVA (1g) and H3PO4 (1g) in distilled water (10g). In case of experimental condition, CV curves were measured with the applied potential in -0.2 ~

0.8 V at 10, 20, 30, 40, 50 and 100mV/s scan rate. In addition, GCD curves were measured with a working voltage of -0.2 ~ 0.8 V at a current density of 0.25, 0.5, 0.75, 1.0 and 2.0 A/g.

Based on data collected in the analysis outlined above, specific capacitance, energy density and power density were calculated by using following equations [154-157].

The specific capacitances (C (Fg-1)) of samples were determined by following equations

(1) and (2) depending on CV and GCD bases, respectively.

∫ IdV (1) C (Fg) = vm(V − V)

Where numerator is area under the curve of CV, v is the scan rate, m is the mass of the electrode material, V2 and V1 are the lower and upper voltage limits

2I (2) C (Fg) = m(dv/dt)

Where I is applied current, M is mass of the electrode material and dv/dt is slope of discharge curve.

From the GCD curves, IR drops were also clarified and used to calculate the equivalent series resistance (ESR) with equation (3) as shown below.

(3) R (Ω) = 2

Where RESR is indicating ESR value, IRdrop is IR-drop values measured from discharge curve in

GCD and ICons is the constant current density.

In addition, energy density and power density were also determined using equations (4) and (5), respectively.

1 (4) Energy Density (ED) = CV 2

Where C (Fg-1) is specific capacitance and V is Potential window.

E (5) Power Density (PD) = t

Where E is energy density and t (s) is discharge time from GCD.

4.4 Results and Discussion

4.4.1 Chemical and Structural Analysis

To analyze surface morphologies of all the prepared samples, FESEM images were collected and shown in Figure 4.1 (a-f). According to Figure 4.1 (a) ~ (d), after polyaniline was synthesized with both PPBP and EBP by in-situ polymerization, a coating was clearly established. The CNT bundles became thicker as shown in Figures 4.1 (c) and (d). PANI was conformally coated on the CNT surfaces in both PPBP-PANI and EBP-PANI composites. The interconnected CNT buckypaper structure was retained even as the PANI uniformly coated resulting in both a high surface area structure and an in-tact interconnected CNT network. Parts

(e) and (f) of Figure 4.1 show PPBP-PANI-AgNP and EBP-PANI-AgNP, respectively. These composites were prepared by introducing AgNPs. Through these images, it was clearly observed that AgNPs uniformly coated the CNT-PANI bundles while maintaining the existing underlying structure.

Figure 4.1 SEM Images of PPBP (A), EBP (B), PPBP-PANI (C), EBP-PANI (D), PPBP-PANI- AgNP (E) and EBP-PANI-AgNP (F).

To confirm the state of the chemical components of the samples in this study, XPS was conducted and the results are shown in Figure 4.2. After epoxide treatment, the O1s peak was drastically increased, showing that oxygen containing epoxide groups were significantly introduced on the buckypaper. When aniline coatings were applied to both PPBP and EBP, the

N1s peak appeared in XPS spectra. This result indicated that nitrogen containing polyaniline was well formed in the prepared composites such as PPBP-PANI and EBP-PANI. In addition, both

PPBP-PANI-AgNP and EBP-PANI-AgNP exhibited observable Ag3d signals at 369 and 375 eV in their XPS spectra, revealing that AgNPs were deposited on the composites confirming the

SEM results. The 369 and 375 eV peaks resulted from Ag3d3/2 and Ag3d5/2. According to this,

6eV as binding energy difference between Ag3d3/2 and Ag3d5/2, indicated that metallic silvers

(zero valent state) existed in both PPBP-PANI-AgNP and EBP-PANI-AgNP [212,213]

PPBP EBP PPBP-PANI O EBP-PANI PPBP-PANI-AgNP O KLL EBP-PANI-AgNP Ag3d C N Intensity (a.u)

900 800 700 600 500 400 300 Binding Energy (eV)

Figure 4.2 XPS spectra of PPBP, EBP, PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and

EBP-PANI-AgNP.

4.4.2 Raman Study

Figure 4.3 shows the Raman spectra of all the samples. From the spectra of PPBP and

EBP, both the D band and G band were clearly observed. However, their intensities were different. In the case of the D band, the EBP showed higher intensity than that of PPBP. On the other hand, lower intensity of G band was found in EBP than PPBP. The ID/IG ratio of EBP

(0.83) was about two times higher than that of PPBP (0.43), indicating that epoxide functional groups modified the chemical structure of the CNTs from a more crystalline graphitic form to a structure with more defects [179]. It can be inferred that the epoxide grafting onto the CNT

surface was the cause of the increase in disorder. After in-situ polymerization with both PPBP and EBP, several new peaks were apparent in the Raman spectra of PPBP-PANI and EBP-PANI.

As shown in Table 4.1, these new peaks corresponded to the specific peaks that are obtained from pure PANI. In addition, peaks indicating presence of PANI were also observed in PPBP-

PANI-AgNP and EBP-PANI-AgNP. Through all these results, it was found that PANI was successfully formed in the composite electrodes. The structure of polyaniline was well- maintained even when AgNPs were incorporated in the composites such as PPBP-PANI-AgNP and EBP-PANI-AgNP.

Wavenumber shifts or intensity changes in the Raman spectra were interpreted to be from chemical interactions in the prepared composites When compared with pure PANI, it was observed that the peaks in EBP-PANI were shifted, revealing an interaction between EBP and

PANI. For example, C-N peak in EBP-PANI was shifted to higher wavenumber than pure PANI.

On the other hand, C-N peak in PPBP-PANI was observed in same wavenumber with pure

PANI. We interpret this interaction to be hydrogen bonds between oxygen in epoxide groups on

EBP and NH in PANI [209]. The interaction between nitrogen in PANI and AgNP can be associated with the intensity change of the C=N stretching band of the quinoid ring for both

PPBP-PANI-AgNP and EBP-PANI-AgNP. According to the obtained result in this study, intensities of C=N peaks in both PPBP-PANI-AgNP and EBP-PANI-AgNP were decreased or diminished. This indicated that chemical interaction, also called as coordinate covalent bonding formation, was induced between AgNP and C=N in PANI [209,214].

PPBP EBP Pure PANI PPBP-PANI EBP-PANI PPBP-PANI-AgNP EBP-PANI-AgNP Intensity (a.u)

500 1000 1500 Raman Shift (cm-1)

Figure 4.3 Raman spectra for PPBP, EBP, Pure PANI, PBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP.

Table 4.1 Assignment of Raman peaks for Pure PANI, PBP-PANI, EBP-PANI, PPBP-PANI- AgNP and EBP-PANI-AgNP.

-1 C-C C-H + Unit: Cm C-H C-N C-N C=N C=C (Out of Plane) (Out of Plane) Pure PANI 417/510 735/804 1171 1231 1374 1507 1596 PPBP-PANI 415/512 723/810 1171 1231 1376 1505 1589 EBP-PANI 420/514 720/809 1169 1254 1370 1508 1592 PPBP-PANI-AgNP 418/526 745/841 1169 1221 1376 1503 1592 EBP-PANI-AgNP 419/525 746/833 1161 1217 1373 - 1588

4.4.3 Electrochemical Performance Evaluation

After complete characterization of the structure the electrochemical performance of the supercapacitor electrodes was assessed. Figure 4.4 (a and b) show CV and GCD curves of all the samples in this study. According to the results as shown in Figure 4.4 (a), the CV curves of

PPBP exhibited typical double-layer capacitance behavior with a nearly rectangular shape since only pure electrostatic charges were adsorbed on the surface of CNTs without any reversible redox reactions. Thus, resulting specific capacitance calculated was only 23.33 F/g. On the other hand, after epoxide treatment, the EBP had a specific capacitance of 50.86 F/g, two times higher than that of PPBP. This enhancement was attributed to additional pseudo-capacitance behavior induced by functional groups such as epoxide groups on the surface of CNTs in EBP [183]. After polyaniline was formed in both PPBP and EBP, wide area CV curves were observed in BP-PANI composites (including PPBP-PANI and EBP-PANI), showing that their specific capacitances were dramatically enhanced to 656.02 and 783.90 F/g, respectively. This phenomenon also can be explained by introduction of pseudo-capacitance behavior obtained from polyaniline.

Moreover, when AgNPs were added to both PPBP-PANI and EBP-PANI, their specific capacitances were further enhanced. However, increase efficiency of its specific capacitance for

EBP-PANI-AgNP (915.62 F/g) was much higher than that for PPBP-PANI-AgNP (690.44 F/g).

Similar tendency of electrochemical performance was also found in GCD curves as exhibited in Figure 4.4(b). All of the GCD curves revealed a linear relationship between potential of charge/discharge and time, also indicating both good capacitance behavior and excellent reversibility [215]. In addition, both PPBP and EBP exhibited perfectly triangular shape that can be found from EDCL behavior [8]. However, GCD curves of composites containing PANI or

AgNP, such as PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP, deviated

from linearity due to the contribution of pseudo-capacitance behavior [216]. The longest discharge length was achieved from EBP-PANI-AgNP, followed by PPBP-PANI-AgNP, EBP-

PANI, PPBP-PANI, EBP and PPBP, indicating that specific capacitance can be listed in same order as shown in Table 4.2. These values were in agreement with the results obtained from CV curves. In addition, smaller IR-drop value was also found in EBP-PANI-AgNP (0.0250 V) when compared to PPBP-PANI-AgNP (0.0348 V). Similar to this, EBP-PANI (0.1208 V) showed smaller IR-drop than PPBP-PANI (0.1880 V). Based on these IR-drop values, their ESR (RESR) values, that can clarify ionic conductivity of supercapacitor electrode in electrolyte, were also calculated by using equation (3) [156,157]. The testing showed that EBP-PANI-AgNP had the lowest ESR value of 0.05Ω, indicating the highest ionic conductivity compared to the other samples PPBP-PANI-AgNP (0.07Ω), EBP-PANI (0.24 Ω) and PPBP-PANI (0.38 Ω).

(a) PPBP EBP 8 PPBP-PANI EBP-PANI 6 PPBP-PANI-AgNP EBP-PANI-AgNP 4

-2

CurrentDensity (A/g) -4

-6

-8

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V)

PPBP (b) EBP 0.8 PPBP-PANI EBP-PANI PPBP-PANI-AgNP EBP-PANI-AgNP 0.6

0.4 Voltage (V) 0.2

0.0

-0.2 0 500 1000 1500 2000 2500 3000 Time (S)

Figure 4.4 CV curves of all prepared samples at 10mV/s scan rate (a) and GCD curves of all prepared samples at the current density of 0.25A/g (b).

Table 4.2 ESR values of PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP.

ESR (Ohm)

PPBP-PANI 0.3760 Ω EBP-PANI 0.2416 Ω PPBP-PANI-AgNP 0.0696 Ω EBP-PANI-AgNP 0.0500 Ω

When the PPBP based and EBP based composites are compared, the latter exhibited higher specific capacitance values, showing both wider area CV curves and longer time length in

GCD as well as smaller IR-drop and lower ESR. This phenomenon can be explained by presence of chemical interactions deduced from the Raman spectra in the composite containing EBP. This is because both higher electrical conductivity and lower charge transfer resistance can be achieved when hydrogen bonding interactions are introduced between epoxide functional groups in EBP and NH in PANI [172-174]. Crystalline polyaniline also can be helpful for increasing the electrochemical performance its composites. This is due to the fact that the crystalline structure has a higher electrical conductivity [207]. This phenomenon was resulted from following reasons: (1) The intra-molecular mobility of the charged species along the chain, (2) to some extent the intermolecular hopping of charge carriers (due to better packing) [207]. In Figure 4.5,

XRD patterns of EBP-PANI and PPBP-PANI are displayed. The three peaks centered at 21.56o,

23.92o and 26.76o are from EBP-PANI. These peaks are attributed to periodicity parallel to the

PANI chains, perpendicular to PANI chains and periodicity caused by hydrogen bonding between PANI, respectively [217,218]. The crystalline PANI peaks in EBP-PANI were induced since epoxide groups in EBP may be involved in polyaniline formation as oxidants. In this regard, according to paper reported by Majumdar et al [211], anilinium ions could be arranged along the epoxide groups array in the head to tail fashion with their benzene rings perpendicular

to the surface of the graphene oxide. Then, portion of epoxide rings could be strained and uniformly disassembled. Therefore, oxidation-reduction process could be occurred, resulting in oxidative dimerization of anilinium ions while epoxide groups were changed to >C=C< with the elimination of a neutral water molecule. By repeating this process, crystalline polyaniline could be formed while maintaining hydrogen bonding interaction with epoxide groups on the graphene oxide. Furthermore, introduction of AgNPs were also effective to improve electrochemical performance of prepared electrodes since AgNP coordinate bonding formation with nitrogen in

PANI reduced its electron density and accordingly electrical conductivity in resultant coordinate covalent bond was enhanced [30]. Interestingly, even though AgNPs were coated to both PPBP-

PANI-AgNP and EBP-PANI-AgNP composites, the latter showed much higher increase efficiency than former, implying that successive interactions, including hydrogen bonding and coordinate covalent bonding formation, between three different materials were also effective to improve electrochemical performance of supercapacitor electrode.

PPBP EBP PPBP-PANI EBP-PANI Intensity (a.u) Intensity

20 30 40 50 60 70 80 2 Theta (Degree)

Figure 4.5 XRD patterns of PPBP, EBP, PPBP-PANI and EBP-PANI.

To further investigate the electrochemical behavior, CV and GCD for prepared electrodes, including PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP, were also measured at different scan rate and different current densities, respectively. In this regard,

Figure 4.6 (a -d) describe the change of CV curves depending on different scan rate. The CV curves of all electrodes became closer to oblate with the redox peaks disappearing as the scan rate was increased. This behavior was a result of having slower migration and diffusion rate of counter ions when compared with transfer rate of electrons in the electrodes [186].

(a) 10mV/s (b) 15 10mV/s 20mV/s 20 20mV/s 30mV/s 30mV/s 10 40mV/s 40mV/s 50mV/s 10 50mV/s 5 100mV/s 100mV/s

0 0

-5 Current Density (A/g) Current Current Density (A/g) Current -10 -10

-15 -20 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Volatage (V) Voltage (V)

0 0

-5 -5 Current Density (A/g) Current Density Current Density (A/g) Density Current -10 -10

-15 -15

-20 -20 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Voltage (V)

Figure 4.6 CV curves of PPBP-PANI (a), EBP-PANI (b), PPBP-PANI-AgNP (c) and EBP- PANI-AgNP (d) under different scan rates.

The GCD curves, evaluated at different current densities are seen in Figure 4.7 (a ~ d), It was clearly observed that longer charge-discharging time span and smaller IR-drop were achieved from the electrodes when current densities were lower. This result is related to the slow electrochemical process in low current density. The additional time allows the electrolyte ions to penetrate more deeply within the pores in the electrode [189].

(a) 0.25 A/g (b) 0.25A/g 0.8 0.5 A/g 0.8 0.75 A/g 0.5A/g 1.0 A/g 0.75A/g 1.0A/g 0.6 2.0 A/g 0.6 2.0A/g

0.4 0.4 Voltage (V) Voltage Voltage (V) 0.2 0.2

0.0 0.0

-0.2 -0.2 0 500 1000 1500 0 500 1000 1500 2000 Time (S) Time (S)

0.4 0.4 Voltage (V) Voltage Voltage (V) 0.2 0.2

0.0 0.0

-0.2 -0.2 0 500 1000 1500 2000 0 500 1000 1500 2000 2500 3000 Time (S) Time (S)

Figure 4.7 GCD curves of PPBP-PANI (a), EBP-PANI (b), PPBP-PANI-AgNP (c) and EBP- PANI-AgNP (d) under different current densities.

Cyclic stability of supercapacitor electrodes is also considered as an important parameter for practical application of the electrodes. As shown in Figure 4.8, capacitance retention of the electrodeswere investigated after 2,500 cycles in the scan rate of 200mV/s. EBP-PANI-AgNP possessed 85.67% of its original specific capacitance which was the highest among any of the samples. In this regard, positive effect of chemical interactions, such as hydrogen bonding and coordinate covalent bonding formation, on electrochemical performance was assured, again.

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PPBP-PANI 40 EBP-PANI PPBP-PANI-AgNP EBP-PANI-AgNP

Capacitance Retention (%) Retention Capacitance 20

0 0 500 1000 1500 2000 2500 Cycle Numbers

Figure 4.8 Capacitance Retention of PPBP-PANI, EBP-PANI, PPBP-PANI-AgNP and EBP-PANI-AgNP during 2,500 cycles in 200mV/s.

Finally, energy and power densities of prepared electrodes in this study were plotted in

Ragone plot described in Figure 4.9. Through this Ragone plot, their performance also can be estimated for potential application in real field with cyclic stability. The result revealed that

143.43Wh/Kg and 289.96W/Kg were achieved as the energy and power density of EBP-PANI-

AgNP, respectively. In addition, even though power density was enhanced to 1754.81Wh/kg, energy density of EBP-PANI-AgNP remained to 103.17 Wh.Kg. In this regard, when compared with other control groups, these values were the highest. In particular, as shown in Table. 4.3, the energy density of EBP-PANI-AgNP was compared with those of other previous developed electrodes and exhibited superior performance. With this result, there was another notable

phenomenon. It was that EBP based electrodes showed higher energy and power densities than

PPBP based electrodes. This result can support the importance of chemical interactions, such as hydrogen bond and coordinate covalent bonding formation, in EBP based composites to improve their electrochemical performance.

PPBP-PANI 240 EBP-PANI 220 PPBP-PANI-AgNP 200 EBP-PANI-AgNP 180 160 140 120 100 80 60 Energy Density (Wh/Kg) Density Energy 40 20 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Power Density (W/kg)

Figure 4.9 Ragone plot of PPBP, EBP, PPBP-PPY, EBP-PPY, PPBP-PPY-AgNP and EBP-PPY-AgNP.

Table 4.3 A comparison of the energy density of reported electrode materials.

Electrode Material Experimental Conditions Energy Ref. Description Density (Wh/Kg) Electrolyte Reference Potential Range (V)

EBP-PANI-AgNP PVA/H3PO4 SCE -0.2~0.8 143.43 This work

PPBP-PANI-AgNP PVA/H3PO4 SCE -0.2~0.8 80.89 This work GNS/AgNP/PPY 1M KCl Ag/AgCl -0.7~0.6 81 [19]

GO-PPY-Ag 1M H2SO4 Ag/AgCl 0.0~0.8 25.5 [190] MWCNT/AgNPS 0.5M NaOH Ag/AgCl -1.1~-0.2 60.7 [219]

EBP-PANI PVA/H3PO4 SCE -0.2~0.8 48.81 This work

PPBP-PANI PVA/H3PO4 SCE -0.2~0.8 36.35 This work

CNT/PANI H2SO4/PVA - 0~0.8 7.1 [220]

PANI/rGO H2SO4/PVA SCE -0.2-0.8 29.3 [221] PPY nanowires/ A-CNT 1.0 M KCl SCE -0.6~0.6 15.1 [192]

PPy/Oxidized CNT fibers 1M LiClO4 - 0~1.0 42 [197]

EBP PVA/H3PO4 SCE -0.2 ~0.8 4.92 This work

PPBP PVA/H3PO4 SCE -0.2 ~0.8 1.84 This work

4.5 Conclusions

In this study, EBP-PANI-AgNP and its control groups were designed and synthesized via epoxide treatment, in-situ polymerization and dip-coating methods. Their structures and morphologies were identified by Raman spectroscopy, X-ray photon spectroscopy (XPS),

Scanning electron microscopy (SEM) and X-ray diffraction (XRD). In addition, the composites were evaluated as the supercapacitor electrode with CV and GCD. As we expected, the results showed that EBP-PANI-AgNP, the final target product, exhibited the best electrochemical performance when compared with other control groups. When compared between PPBP based electrodes and EBP based electrodes, latter electrodes showed better performance such as higher specific capacitance, better rate capability as well as smaller IR-drops and lower ESR. These performances can be explained through the following three factors; (1) hydrogen bonding interaction between oxygen on EBP and NH in PANI; (2) Crystalline polyaniline formed by epoxide functional groups in EBP involved in polymerization as oxidants; (3) coordinate bonding formation between N in PANI and AgNP. This is because both charge transfer efficiency and high electrical conductivity can be achieved by three factors. From the above results with three factors, epoxide groups in EBP and its chemical interactions can be used to improve the performance of supercapacitor electrodes.

CHAPTER 5

Chemically Linked EBP/P(m-ANA)/NiO Based Composites and their Supercapacitor

Performance

Hyungjoo Kima, Manivannan Ramalingamb, Qingwen Lic, Yuntian Zhud, Xiangwu Zhanga, Wei

Gaoa, Young-A Sonb* and Philip D Bradforda* a Department of Textile Engineering, Chemistry and Science, North Carolina State University,

Raleigh, NC 27606, USA b Department of Advanced Organic Materials Engineering, Chungnam National University, 220

27695, USA

* : Co-corresponding author

Abstract

Hybrid supercapacitor electrodes were fabricated from composites of Nickel oxide (NiO),

Poly(meta-Anthranilic acid) (P(m-ANA)) and epoxide functionalized bukcypapers (EBP). To maximize the synergistic effect between individual materials, chemical interactions were introduced in the form of (1) hydrogen bonding interactions between the epoxide functional groups on the EBP and amine groups in P(m-ANA) and (2) hydrogen bonding formation between carboxylic acid groups in P(m-ANA) and NiO. Due to the presence of those chemical interactions, both higher electrical conductivity and lower charge transfer resistance were achieved, resulting in enhancement of electrochemical performance. As a result, the specific capacitance (calculated from GCD at a current density 0.25 A/g) of EBP-P(mANA)-NiO reached

to 1,000 F/g which is about 2 times higher than PPBP-P(m-ANA)-NiO (555.56 F/g). An energy density of 120.82 Wh/Kg and power density of 381.48 W/Kg were achieved for the EBP-P(m-

ANA)-NiO, composite.

5.1 Introduction

Increasing demand of renewable energy has spurred the need for energy storage devices with both high energy and power densities. [222-224]. Batteries are well known devices with high energy density and acceptable cycle life [225]. However, some limitations, such as relatively low power density, high internal resistance and long term cycle life, still remain as problems that must be solved [226]. As a result, batteries have seen greatest use in applications where energy density is the primary requirement. To overcome this limitation, the study of supercapacitors has been steadily increasing since they may be able to complement or replace batteries due to their higher power density and ability to charge and discharge at a much greater rate [227].

To develop supercapacitors with high performance in all critical areas, significant effort has been made in studying and developing them. In particular, binary or ternary nanostructured supercapacitors are an active area of research [228-232]. According to reported papers, synergistic effects can be achieved through combination of materials such as carbon, conductive polymers and metal oxides. Carbon materials exhibit relatively low specific capacitance although they possess good electrical conductivity and high cyclic stability [233,234]. In contrast, higher specific capacitance is a typical advantage of conductive polymer or metal oxides while their stability and response during charge-discharge cycling less than desired [70,233,235].

Supercapacitor electrodes consisting of single phase have been limited for practical application.

However, by combining those materials, the individual drawbacks can be minimized. This can be

100

attributed to; (1) the specific capacitance of carbon of carbon based electrodes can be improved through adding a large pseudo capacitance from the of conductive polymer component; (2) high mechanical stability of the carbon phase can help to increase the cyclic stability of the conductive polymer component; (3) metal oxide performance can be improved by utilizing high surface area and electrically conductive carbon materials; and (4) the addition of conductive polymer can play an important role for enhancing pseudo capacitance behavior of metal oxides

[70,142-144,236,237]. All of the above show that the electrochemical performance of supercapacitors can be maximized by the synergistic effect provided by the right combination of multiple components.

In this study, ternary composites composed of carbon nanotubes, Poly(m-Anthranilic acid) and nickel oxide nanoparticles were prepared and analyzed for their electrochemical performance as supercapacitor electrodes. To maximize the synergistic effect among individual component in the composite, the carbon nanotubes (which were the main structural template) were first functionalized with epoxide groups. The added functional group provided additional chemical interactions between the components leading to higher performance. This goal of this work is to elucidate those interactions and determine how they play a role in the improved performance.

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5.2 Experimental Section

5.2.1 Synthetic Method for Purified Pristine Buckypaper (PPBP)

All chemicals were purchased from Sigma Aldrich and used without further purification.

5.2.2 Preparation of Functionalized Buckypaper (EBP)

The purified buckypaper was then functionalized with epoxide groups [178]. Purified buckypaper (PBP) was immersed in 5wt% meta-chloroperoxybenzoic acid (m-CPBA) / CH2Cl2 solution for 3 hours. After epoxidation treatment, the functionalized EBP was washed five times with CH2Cl2 and then five times with Ethanol, respectively. The EBP was then dried overnight at room temperature to evaporate the residual solvent.

5.2.3 Synthetic Method of Poly(meta-anthranilic acid)-Functionalized Buckypaper

composite

Aniline (0.20g) and anthranilic acid (1.37g) were dissolved in a 1.2M HCl (60mL) solution. The PPBP and EBP were soaked in that solution for 30min. Then, 6.8g of APS in was added to 40 mL of 1.2M HCl and this solution was added to the first and held at room temperature for 12hrs. After 12hrs, the prepared samples were collected and washed with distilled water and Ethanol several times. The final products, called as PPBP-P(m-ANA) and

EBP-P(m-ANA), were dried in vacuum for 24hrs.

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5.2.5 Nickel Oxide Nanoparticle Coatings

The prepared PPBP-P(m-ANA) and EBP-P(m-ANA) composites were immersed in a water containing nickel oxide nanoparticles (particl size: 50 nm / density 2.1mg/L) at room temperature for 10 minutes. The samples were then washed with water several times and dried in vacuum overnight. The final products were given the labels PPBP-P(m-ANA)-NiO and EBP-

P(m-ANA)-NiO, respectively.

5.3 Characterization

The morphologies of prepared composite samples were characterized using a low voltage field emission scanning electron microscope (Merlin Compact, Zeiss). To clarify the structure and components of samples, FT-IR spectra were recorded by vacuum Infrared spectrometer (KBr pellet/transmission mode in Vertex 80&80V, Bruker). X-ray photoelectron spectroscopy (XPS) was performed by MultiLab 2000 (Thermo Scientific).

To determine the electrochemical performance of the prepared samples, cyclic voltammetry

(CV) and galvanostatic charge/discharge (GCD) testing were conducted using an Autolab

(PGSTAT128N, Metrohm Autolab Instrument). A three-electrode system was utilized. In the system, a platinum and saturated calomel electrode were used as counter electrode and reference electrode, respectively. PVA/H3PO4 was used as electrolyte. This electrolyte was prepared by dissolving PVA (1g) and H3PO4 (1g) in distilled water (10g). CV curves were acquired with an applied potential in the range of -0.2 ~ 0.8 V at 10, 20, 30, 40, 50 and 100mV/s scanning rates. GCD curves were produced with a working voltage range of -0.2 ~ 0.8 V at current densities of 0.25, 0.5, 0.75, 1.0 and 2.0 A/g.

Specific capacitances, equivalent series resistance (ESR), energy density and power density were calculated using following equations [154-157].

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The specific capacitances (C (Fg-1)) of samples were determined by following equations (1) for

CV and (2) for GCD based testing.

∫ IdV (1) C (Fg) = vm(V − V)

In the above equation, the numerator is area under the CV curve, v is the scan rate, m is the mass of the electrode material, and V2 and V1 are the lower and upper voltage limits.

2I (2) C (Fg) = m(dv/dt)

In this equation, I is the applied current, m is mass of the electrode material and dv/dt is the slope of the discharge curve.

From the GCD curves, IR drop was determined and used to calculate equivalent series resistance

(ESR) with equation (3) as shown in below.

(3) R (Ω) = 2

Where RESR is the ESR value, IRdrop is IR-drop values measured from the discharge curve in

GCD and ICons is the constant current density.

Energy density and power density were also determined using equations (4) and (5), respectively.

1 (4) Energy Density (ED) = CV 2

Where C (Fg-1) is the specific capacitance and V is the potential window.

E (5) Power Density (PD) = t

Where E is energy density and t (s) is the discharge time from GCD.

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5.4 Results and Discussion

5.4.1 Structural Characterization

5.4.1.1 Chemical and Structural Analysis

From observation of SEM images, the morphologies of the samples were investigated and are shown in Figures 5.1(a-f). Each treatment step changed surface characteristics of the samples. According to Figures 5.1 (a) - (d), it was clearly observed that width and surface roughness of CNT bundles were increased and made less uniform after in-situ polymerization, however, it also revealed that the P(m-ANA) was uniformly coated across all of the CNT bundles. Figures 5.1 (e) and (f) show that NiO nanoparticles were well distributed on their surfaces without any changes of the underlying structure.

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Figure 5.1 SEM Images of PPBP (A), EBP (B), PPBP-P(m-ANA) (C), EBP-P (m-ANA) (D), PPBP-P(m-ANA)-NiO (E) and EBP-P(m-ANA)-NiO (F).

All of the sample types in this study were investigated using XPS and the spectra are shown in

Figure 5.2. When comparing PPBP and EBP, it was found that intensity of O1s peak in EBP was drastically increased, resulting from the epoxide treatment. After in-situ polymerization of P(m-

ANA) the N1s peak (which was not apparent in the XPS spectra of PPBP and EBP) showed up in the PPBP-P(m-ANA) and EBP-P(m-ANA) spectra. This new peak was induced from nitrogen in P(m-ANA). The PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO spectra contained the Ni 2p peaks confirming that the particles seen in Figure 5.1 were in fact nickel oxide. 106

PPBP EBP PPBP-P(m-ANA) C EBP-P(m-ANA) O PPBP-P(m-ANA)-NiO EBP-P(m-ANA)-NiO Ni2p N Intensity (a.u) Intensity

900 800 700 600 500 400 300 Binding Energy (eV)

Figure 5.2 XPS spectra of PPBP, EBP, P(m-ANA), PPBP-P(m-ANA), EBP-P (m-ANA), PPBP- P(m-ANA)-NiO and EBP-P(m-ANA)-NiO.

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5.4.1.2 FT-IR Study

FT-IR spectra of all samples in this study are shown in Figure 5.3 (a) and (b). Their assigned peaks were also organized in Table 5.1. In the EBP spectra, the peaks centered at 1055 cm-1 and 1258.45 cm-1 were more significant than in the PPBP material. This result can determine the presence of epoxide functional groups, introduced by epoxide treatment, in EBP

[238,239]. After polymerization, several new peaks were found in the FT-IR spectra of both

PPBP-P(m-ANA) and EBP-P(m-ANA). Those peaks, in both PPBP-P(m-ANA) and EBP-P(m-

ANA) samples have similar positions to pure P(m-ANA) showing that m-anthranilic acid was successfully polymerized in the samples.

FTIR not only helped to identify the materials in the composites but also helped to identify the intrinsic chemical interactions between the materials by examining C-O-C, C=O and

O-H peak shifts in the spectra. The C-O-C peak shifted from 1055cm-1 in EBP to 1066cm-1 in

EBP-P(m-ANA). this peak shift, only observed in EBP-P(m-ANA), were interpreted as chemical interactions such as hydrogen bonding between oxygen in the epoxide functional groups in EBP and hydrogen of amine in P(m-ANA) [240]. On the other hand, in case of chemical interaction between carboxylic acid in P(m-ANA) and nickel oxide nanoparticles, it was determined by shifts of C=O and O-H peaks in both PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO. Both

C=O and O-H peaks shifted to higher wavenumbers after nickel oxide nanoparticles were deposited on the samples. These peak shifts can be also explained by hydrogen bond between carboxylic acid in P(m-ANA) and nickel oxide nanoparticles in the prepared composites

[241,242].

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(a)

C-H

1433 cm-1 (C-O-H)

1262.35 cm-1 (C-O)

1258.45 cm-1 (C-O) Transmittance (a.u) Transmittance

-OH -1 Transmittance (a.u) Transmittance 1055 cm (Epoxide group)

1425.3 cm-1 (C-O-H) PPBP PPBP EBP EBP 1600 1400 1200 1000 800 600 400 Wavenubmer (cm-1)

4000 3500 3000 2500 2000 1500 1000 500 Wavenubmer (cm-1)

(b)

Transmittance(%) PPBP-P(m-ANA) EBP-P(m-ANA) PPBP-P(m-ANA)-NiO Transmittance (%) P(m-ANA) EBP-P(m-ANA)-NiO

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

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Table 5.1 Assignment of FT-IR peaks for Pure P(m-ANA), PBP-P(m-ANA) and EBP-P(m- ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO.

Unit: cm-1 C-H (Out of Plane) C-O-C C=C Stretching C=O O-H Pure P(m-ANA) 826 - 1574 1695 3227 PPBP-P(m-ANA) 876 - 1560 1699 3442 EBP-P(m-ANA) 874 1066 1562 1699 3442 PPBP-P(m-ANA)-NiO 874 - 1558 1700 3443 EBP-P(m-ANA)-NiO 876 1061 1568 1707 3430

5.4.2 Electrochemical Performance Evaluation as Supercapacitor Electrodes

In this section, the results of the electrochemical performance of synthesized composites were covered to estimate their potential application for supercapacitor electrodes. In addition, effect of intrinsic chemical interactions in the composites on their electrochemical performance was also investigated. For this work, both cyclic voltammetry (CV) and galvanostatic charge- discharge (GCD) were analyzed with a three-electrode system in PVA/H3PO4 electrolyte.

Results of the CV testing are compared in Figure 5.4 (a) where it is seen that all the samples behaved in different ways. The shape change of the CV curves obtained from each electrode were related to materials that made up the composite electrodes. PPBP, which consisted only of carbon, exhibited electrochemical double-layer capacitance behavior, showing rectangular shape in its CV curve [182]. When carbon-based electrodes also exhibit pseudo-capacitance behavior, the shapes of their CV curves will deviate from the rectangular shape, exhibiting redox peaks. In addition, the electrodes containing EBP or P(m-ANA) showed oblate forms with specific redox peaks in their CV curves. Such deviation and redox peaks occur due to the following two reasons; (1) additional pseudo-capacitance behavior can be achieved by redox reaction of oxygen containing functional groups in CNT [183]; (2) change of the charge storage mechanism can be occurred depending on reduction and oxidation mode of conductive polymer [243]. On the other hand, the different CV curve areas of each sample was attributed from presence or non-presence

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of chemical interactions in the composite. This is because specific capacitance is proportional to

CV curve area. In this regard, in accordance with the result, the sequence of their CV curve areas was as follow: EBP-P(m-ANA)-NiO > EBP-P(m-ANA) > PPBP-P(m-ANA)-NiO > PPBP-P(m-

ANA ) > EBP > PPBP. From this comparison, it is clearly observed that EBP based composites, which have additional chemical interactions, typically showed wider area than PPBP based composites.

In GCD curves, carbonaceous electrode exhibited typical behavior of EDLC. For example, both

PPBP and EBP showed symmetrical and linear discharge slope [244]. However, GCD curves of composites containing P(m-ANA) or NiO, such as PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-

P(m-ANA)-NiO and EBP-P(m-ANA)-NiO, deviated from linearity with showing potential plateaus. This is due to the contribution of pseudo-capacitance behavior [245]. In addition, time length of prepared electrodes was also compared to clarify their charge store ability. According to the result, the longest discharge length was achieved from EBP-P(m-ANA)-NiO, followed by

PPBP-P(m-ANA)-NiO, EBP-P(m-ANA), PPBP-P(m-ANA), EBP and PPBP. These values corresponded to the results obtained from CV curves. Furthermore, from GCD curves, smaller

IR-drop value was also found in EBP-P(m-ANA)-NiO (0.0674 V) when compared to PPBP-

P(m-ANA)-NiO (0.1004 V). Similar to this, EBP-P(m-ANA) (0.1129 V) showed smaller IR- drop than PPBP-P(m-ANA) (0.1498 V). Based on these values, their ESR (RESR) values were also calculated by using equation (3). The result showed that EBP-P(m-ANA)-NiO had the lowest ESR value of 0.1348 Ω, indicating the highest ionic conductivity compared to the other samples as shown in Table 5.2 [156,157]. Hence, all these results achieved from GCD curves can be another evidence to support the positive effect of chemical interactions on electrochemical performance.

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6 (a)

PPBP -2 EBP Current Density (A/g) Density Current PPBP-P(m-ANA) EBP-P(m-ANA) -4 PPBP-P(m-ANA)-NiO EBP-P(m-ANA)-NiO -6 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V)

(b) PPBP 0.8 EBP PPBP-P(m-ANA) EBP-P(m-ANA) 0.6 PPBP-P(m-ANA)-NiO EBP-P(m-ANA)-NiO

0.4

0.2 Current Density (A/g) Density Current

0.0

-0.2 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Time (S)

Figure 5.4 CV (a) and GCD (b) Curves of PPBP, EBP, PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO.

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Table 5.2 Specific capacitance, IR-drop and ESR values of PPBP, EBP, PPBP-P(m-ANA), EBP- P(m-ANA), PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO.

Specific Capacitance IR-drop ESR CV GCD PPBP 23.33 F/g 15.94 F/g 0.089 V 0.178 Ω EBP 50.86 F/g 41.12 F/g 0.0716 V 0.1432 Ω PPBP-P(m-ANA) 340.04 F/g 454.55 F/g 0.1498 V 0.2996 Ω EBP_P(m-ANA) 486.38 F/g 714.29 F/g 0.1129 V 0.2258 Ω PPBP-P(m-ANA)-NiO 374.88 F/g 555.56 F/g 0.1004 V 0.2008 Ω EBP-P(m-ANA)-NiO 576.04 F/g 1000 F/g 0.0674 V 0.1348 Ω

As shown in Table. 5.2, their specific capacitances were also calculated by using equation

(1) and (2). Both set of tests revealed that the specific capacitance achieved from EBP based composites were higher than PPBP based composites. This can be explained by the presence of chemical interactions in the EBP based composite since hydrogen bonds can be formed in between the two components due to the presence of epoxide functional groups on the buckypaper. Due to this interaction, both higher electrical conductivity and lower charge transfer resistance can be achieved. Therefore, electrochemical behavior can be improved [172-174]. In addition, when NiO nanoparticle were finally coated, specific capacitances of both PPBP-P(m-

ANA)-NiO and EBP-P(m-ANA)-NiO were relatively higher than those of NiO nontreated electrodes. This result indicated that positive effect of NiO on enhancement of specific capacitance. This effect was also ascribed to presence of hydrogen bonding interaction between

COOH in P(m-ANA) and NiO. This is because such chemical interaction enhanced charge transfer efficiency by reducing resistance in the prepared electrodes [252,253]. In particularly, when compared between PPBP-P(m-ANA)-NiO and EBP-P(m-ANA)-NiO, the latter electrode showed higher specific capacitance than former electrode. This result implies that full-chemical

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interactions between individual components in the electrode is also important factor to achieve improved specific capacitance.

To further clarify the practical electrochemical performance of the electrodes, their CV and GCD analysis were carried out at different scan rates and current densities,. In figure 5.5 (a) - (f), it was found that increasing the scan rate made a change in CV curve shapes of all electrodes. In particularly, electrodes, containing pseudo capacitance materials, exhibited that their CV curves became closer to oblate form with redox peaks disappearing as scan rate was increased. This behavior was a result of having slower migration and diffusion rate of counter ions when compared with transfer rate of electrons in the electrodes [186].

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(a) 2.0 10mV/s (b) 4 10mV/s 20mV/s 20mV/s 1.5 30mV/s 30mV/s 40mV/s 40mV/s 50mV/s 2 50mV/s 1.0 100mV/s 100mV/s

0.5

0 0.0 Current Density Current (A/g) Density CurrentDensity (A/g)

-0.5 -2

-1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Voltage (V)

0 0

-5 Current Density CurrentDensity (A/g)

Current Density (A/g) CurrentDensity -5

-10

-10 -15

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Voltage (V) 20 (e) 10mV/s (f) 10mV/s 20mV/s 10 20mV/s 15 30mV/s 30mV/s 40mV/s 40mV/s 10 50mV/s 50mV/s 5 100mV/s 5 100mV/s

0 0 -5 Current Density (A/g) Current Density Current (A/g) -5 -10

-15

-10 -20 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Voltage (V)

Figure 5.5 CV curves of PPBP (a), EBP (b), PPBP-P(m-ANA) (c), E BP-P(m-ANA) (d), PPBP- P(m-ANA)-NiO (e) and EBP-P(m-ANA)-NiO (f) under different scan rates.

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Effect of scan rate on the changes of specific capacitances for all electrode materials was also determined as depicted in Figure 5.6. According to the result, variation in the values were clearly observed from all prepared electrodes in this study. Moreover, the specific capacitance was diminished as much as scan rate increased. This behavior was ascribed to low ion diffusion and migration into the inner surface of electrodes since enough time was not allowed from high scan rate [187,246].

650 600 PPBP EBP 550 PPBP-P(m-ANA) 500 EBP-P(m-ANA) 450 PPBP-P(m-ANA)-NiO 400 EBP-P(m-ANA)-NiO 350 300 250 200

Specific Capacitance (F/g) Capacitance Specific 150 100 50 0 10 20 30 40 50 60 70 80 90 100 Scan Rate (mV/s)

Figure 5.6. Specific capacitance values of PPBP, EBP, PPBP-P(m-ANA)-NiO and EBP-P (m- ANA)-NiO under different scan rates.

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Figure 5.7 (a) ~ (f) show the GCD curves of all prepared electrodes. The result was revealed that lower current density resulted in longer charge-discharge time span and smaller IR-drop values.

These results were contributed from slow electrochemical process. This is because it can provide enough time for enhancement of possibilities that electrolyte ions can approach more deep pores in electrodes [189]

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(a) (b) 0.8 0.25A/g 0.8 0.25A/g 0.5A/g 0.5A/g 0.75A/g 0.75A/g 0.6 1.0A/g 0.6 1.0A/g 2.0A/g 2.0A/g

0.4 0.4 Voltage Voltage (V) 0.2 Voltage (V) 0.2

0.0 0.0

-0.2 0 20 40 60 -0.2 0 30 60 90 120 150 Time (S) Time (S) (C) (d) 0.25A/g 0.25A/g 0.8 0.8 0.5A/g 0.5A/g 0.75A/g 0.75A/g 0.6 1.0A/g 0.6 1.0A/g 2.0A/g 2.0A/g

0.4 0.4

0.2 0.2 CurrentDensity(A/g) Current Density Current (A/g) Density

0.0 0.0

-0.2 -0.2 0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 200 400 600 800 1000 1200 Voltage (V) Time (S) (e) (f) 0.25A/g 0.25A/g 0.8 0.8 0.5A/g 0.5A/g 0.75A/g 0.75A/g 0.6 1.0A/g 0.6 1.0A/g 2.0A/g 2.0A/g

0.4 0.4

0.2 0.2 Current(A/g)Density Current Density (A/g) Current Density

0.0 0.0

-0.2 -0.2 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Time (S) Time (S)

Figure 5.7 GCD curves of PPBP (a), EBP (b), PPBP-P(m-A NA) (c), EBP-P(m-ANA) (d), PPBP-P(m-ANA)-NiO (e) and EBP-P(m-ANA)-NiO (f) under different current densities.

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Cyclic stability is also important parameter to assess potential as supercapacitor electrode in real field. According to this, cyclic stabilities of fabricated electrodes were measured as depicted in Figure 5.8. All the measured electrodes showed different capacitance retentions after

25,000 cycles at the scan rate of 200mV. Interestingly, when compared with PPBP based composites, EBP based composites always showed more stable behavior while steadily maintaining capacitance retention over 80% of their initial specific capacitance values. Based on this result, it was revealed that cycling stability of EBP based composites can be acquired due to presence of chemical interactions, such as hydrogen bonding between individual components in the electrodes.

300 PPBP 280 EBP 260 PPBP-P(m-ANA) EBP-P(m-ANA) 240 PPBP-P(m-ANA)-NiO 220 EBP-P(m-ANA)-NiO 200 180 160 140 120 100 80

Specific Capacitance (F/g) Capacitance Specific 60 40 20 0 0 500 1000 1500 2000 2500 Number of Cycles

Figure 5.8 Cyclic stability of PPBP, EBP, PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-P (m- ANA)-NiO and EBP-P (m-ANA)-NiO during 2,500 cycles in 200mV/s.

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To determine prepared electrodes as potential supercapacitor electrode, a Ragone plot is included in Figure 5.9. Energy and power densities were computed by using equation (4) and (5), respectively. A close look at the plot showed that value of energy density was in inversely proportion to the that of power density. In addition, when compared with other control groups, it was also noticed that EBP-P(m-ANA)-NiO exhibited the highest energy density of 120.82

Wh/Kg at a power density of 381.48 W/Kg of power density and maintained an energy density of 35.58 Wh/Kg even at the highest power density of 784.72 W/Kg. According to our survey of the related literature, Table 5.3 shows that the maximum energy density achieved from EBP-

P(m-ANA)-NiO was higher than other electrodes consisting of similar materials reported recently. Furthermore, higher energy and power densities were achieved from EBP based electrodes in comparison with PPBP based electrodes. Therefore, it was demonstrated that chemical interactions in the electrode effect its electrochemical performance positively.

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150 PPBP EBP PPBP-P(m-ANA) EBP-P(m-ANA) PPBP-P(m-ANA)-NiO 100 EBP-P(m-ANA)-NiO

50 Energy Density (Wh/Kg) Density Energy

200 400 600 800 1000 1200 Power Density (W/kg)

Figure 5.9 Ragone plot of PPBP, EBP, PPBP-P(m-ANA), EBP-P(m-ANA), PPBP-P(m-ANA)- NiO and EBP-P(m-ANA)-NiO.

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Table 5.3 A comparison of the energy density of reported electrode materials.

Electrode Material Experimental Conditions Energy Ref. Density Description (Wh/Kg) Electrolyte Reference Potential Range (V)

EBP- P(m-ANA)-NiO PVA/H3PO4 SCE -0.2~0.8 120.82 This work

PPBP- P(m-ANA)-NiO PVA/H3PO4 SCE -0.2~0.8 62.5 This work NiO/PAni-MWCNT KCl/ACN Ag/AgCl 0.0~0.6 55.9 [247]

CoHCF/CNF/PPY Na2SO4 Ag/AgCl -0.1~1.1 102 [248]

Ce-MOF/GO KOH/K3(Fe(CN)6 Ag/AgCl 0~0.5 111.05 [249]

rGO/Fe3O4/PANI PVA/H3PO4 SCE -0.2~0.9 47.7 [250]

EBP- P(m-ANA) PVA/H3PO4 SCE -0.2~0.8 78.09 This work

PPBP-P(m-ANA) PVA/H3PO4 SCE -0.2~0.8 45.68 This work

PmAP/CNG-10 H2SO4 Ag/AgCl -0.1~0.9 45.2 [251]

CNT/PANI H2SO4/PVA - 0~0.8 7.1 [220]

PANI/rGO H2SO4/PVA SCE -0.2-0.8 29.3 [221]

EBP PVA/H3PO4 SCE -0.2 ~0.8 4.92 This work

PPBP PVA/H3PO4 SCE -0.2 ~0.8 1.84 This work

5.4.3 Conclusions

In this work, hybrid supercapacitor electrodes were designed and synthesized by combining EDLC (functionalized buckypaper) and Pseudo-capacitance (poly(meta-anthranilic acid and nickel oxide nanoparticles) materials. For electrode fabrication, simple methods, such as epoxide treatment, in-situ polymerization and dip-coating methods, were utilized. Since both

EDLC and pseudo- capacitance materials were combined in one electrode, synergistic effects were induced and significantly improved electrochemical performance was achieved. Moreover, chemical interactions, such as hydrogen bonding, were intentionally introduced to maximize the synergistic effect. EBP based electrodes showed a higher level of performance when compared with PPBP based electrodes. The specific capacitance of EBP-P(m-ANA)-NiO reached 1,000

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F/g and maintained over 80% capacity retention after 2,500cycles. The best samples showed an energy density of 120.82 Wh/kg. The chemical interactions engineered into the structure were critical for reaching these very high values. The EBP-P(m-ANA)-NiO has a high potential for additional development into commercial high performance supercapacitor electrodes.

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CHAPTER 6

Overall Conclusion and Future Work

6.1 Overall Conclusions

CNT buckypaper based composites have been applied to various fields due to their excellent properties such as high both mechanical strength and electrical conductivity.

Especially, in energy storage application, they have been actively studied as supercapacitor electrodes due to their outstanding electrochemical performance. In this regard, literature review in this dissertation revealed that ternary composite consisting of CNT buckypaper, conductive polymer and metal (oxide) can be the solid foundation to develop high performance supercapacitor electrodes due to their synergistic effect between these different components in the composites. According to this, ternary composites were fabricated to develop high performance supercapacitor electrode in this study. In particularly, with conventional synergistic effect appeared in the composite, three different strategies were reflected to improve electrochemical performance. The followings are contents of applied strategies and their positive effect on electrochemical performance for the prepared composite-based supercapacitor electrodes in this dissertation.

[1] The functionalized buckypaper was introduced to develop supercapacitor electrodes with

high performance. Unlike other most of reported papers that used carboxylic acid as

functional group on the buckypaper, epoxide functional groups were attached on the

buckypaper since these groups have been reported that they can make buckypaper to have

large pores, Therefore, polymer matrix can be formed in this epoxide functionalized

buckypaper (EBP), completely. In addition, research progressed in this dissertation also

revealed that EBP exhibited wider surface area, higher surface energy and enhanced

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electrical conductivity when compared with purified pristine buckypaper (PPBP). These

factors played important roles to decide properties of supercapacitor electrodes with high

performance. Therefore, specific capacitances of EBP were calculated from Galvanostatic

Charge/Discharge (GCD) curve, resulting in 41.12 F/g. This value was about 3time higher

than that of PPBP (15.94 F/g).

[2] The prepared composites in this dissertation were designed and fabricated to have internal

chemical interactions, among all three different components, to improve their

electrochemical performance. In this regard, hydrogen bond and metal binding formations

were intentionally formed. In case of hydrogen bond, it was formed between oxygen in EBP

and amine in conductive polymers. In addition, metal binding formation was also generated

by using high affinity property of nitrogen in Polypyrrole and Polyaniline toward silver

nanoparticles. On the other hand, nickel oxide nanoparticles (NiO) were chemically

interacted with carboxylic acid in Poly(meta-anthranilic acid). Through these interactions,

electrochemical performances of prepared composite-based supercapacitor electrodes were

significantly improved when compared with control groups with no chemical interactions.

For example, in case of NiO/P(m-ANA)/EBP composite based supercapacitor electrode, its

increase efficiency was about 200%. In this regard, chemical interactions can be important

role to enhance electrochemical performance of supercapacitor electrode by increasing

charge-transfer efficiency and electrical conductivity.

[3] In prepared composite containing both EBP and polyaniline in this dissertation, crystalline

polyaniline was formed and therefore, more organized and better packed structure was

generated, resulting in enhancement of electrical conductivity for the composite. Finally,

electrochemical performance was improved. This phenomenon can be induced by epoxide

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functional groups in EBP. This is because, as oxidants, they may participate in polyaniline

polymerization process, providing proper nature for epitaxial growth of crystalline

polyaniline.

Above all, with conventional synergistic effect between components, three different applied strategies in this dissertation can positively affect to the development of high- performance supercapacitor electrodes. In addition, prepared composites can be promising materials for high performance supercapacitor electrodes.

6.2 Future Works

Through this research, it was observed that applied three strategies (use of buckypaper functionalization, introduction of intrinsic chemical interactions and crystalline conductive polymer) played an effective role to improve electrochemical performance of supercapacitor electrode. For example, higher energy density was achieved when compared with supercapacitor electrodes reported in other papers. However, these obtained values in this dissertation were still lower than current lithium ion batteries. Therefore, in the future, future research is required to improve such electrochemical performance.

First method to improve electrochemical performance is the study of more diverse functional groups. In current reported methods including my research, functional groups, frequently applied in the buckypaper, are limited to carboxylic acid and epoxide. This is because buckypaper can be easily damaged by specific condition required for chemical reaction. For instance, buckypaper structure can be destructed by sonicating. Hence, if proper experimental methods to attach new functional groups in buckyaper can be found through further research, different type of functionalized buckypaper can be developed, resulting in introduction various chemical interaction and improvement of electrochemical performance.

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Second method is to clarify the proper deposition amount of conductive polymers in the prepared composite. In this dissertation, it was clearly found that they positively affect to improve electrochemical performance by inducing additional pseudo-capacitance property.

However, according to reported papers, too small or much quantity of conductive polymers have a reverse effect on electrochemical performance. For example, too much conductive polymer deposition reduces specific capacitance since thickness of composite can be increased.

Therefore, if detailed research is carried out to find optimal polymerization time for proper quantity of conductive polymer nanoparticles in the composite, maximum electrochemical performance that composite exhibits can be obtained.

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