chapter introduction and theoretical background...

Introduction And Theoretical Background

Chapter I Page|1

Introduction to Electrochemical Supercapacitor

1.1 Introduction

In the recent years, a modern technological society demands the use and

storage of energy on a major scale. As the availability of fossil fuels decreases,

the conversion of the energy from alternative and preferably renewable sources

and its efficient storage are becoming crucial to the sustainability of our

civilization. As a result we are observing an increase in renewable energy

production from sun and wind. But the sun does not shine during night, wind

does not flow on our demand, it restrict on the demand of energy. So there is

need of better energy storage devices. One such device, the supercapacitor, has

matured significantly over the last decade and emerged with the potential to

facilitate major advances in energy storage. Supercapacitors, also known as

electrochemical capacitors or ultracapacitors, have attracted much attention

because of their pulse power supply, long cycle life, simple principle and

high dynamic of charge propagation [1, 2].

1.2 Comparison Of Energy Storage Technologies

There are several energy storage systems such as conventional capacitor,

Supercapacitor and battery. The Ragone plot is plot used for performance

comparison of various energy storing devices [Fig. 1.1]. On this plot the power

density is plotted against the energy density, where the power density usually

measured in Watts per kilogram (Wkg-1) represents how fast the power system

can deliver the energy divides by how much does the power system weight. The

energy density usually measured in Watt hours per kilogram (Whkg-1) represents

the total amount of energy available divided by how much does the power

system weight.

The energy density and power density of the energy storage device can be

calculated by following equations:

CHAPTER ONE



� �1

2��

�� 1.1

� ��

�

4 �� 1.2

Where,

E - Energy density

P - Power density

C - Specific capacitance

V – Operating potential window

ESR – Equivalent series resistance

In this graph it is seen that, energy density from capacitor to fuel cell

increases dramatically whereas the power density decreases. Conventional

capacitor shows the high power density and battery and fuel cell shows the high

energy density. Supercapacitor occupies a region between conventional

capacitor and battery that is having the intermediate power density and energy

density. Supercapacitors offer a promising approach to meet the increasing

power demands of energy storage systems in the twenty first century.

Fig. 1.1 Ragone plot showing the power density against energy density for

various electrical energy storage systems


Chapter I Page|3

1.3 Historical Background

The history of supercapacitors began with Electric Double layer capacitor

(EDLC) in 1957, when a patent was granted by General Electric for an

electrolytic capacitor using porous carbon electrodes [3]. It was believed that the

energy was stored in the carbon pores and also noted that the capacitor exhibited

an “exceptionally high capacitance,” although the mechanism of charge storage

was unknown at that time. After the General electric, the modern versions of

the devices were eventually developed by researchers at Standard Oil of

Ohio in 1966 [4]. In 1969, SOHIO first attempted to market such energy storage

device using high surface area carbon materials with tetraalkylammonium salt

electrolyte. But Standard Oil failed to commercialize their invention and

licensed the technology to NEC, who finally marketed the results as

'supercapacitors' in 1978, to provide backup power for maintaining

computer memory [ 5] In 1975-1981 B. E. Conway proposed a new concept

of charge storage in which the fast and reversible redox reactions occurring

on or near the electrode surface. This type of supercapacitor referred as of

pseudocapacitor. The development of first pseudocapacitor is based on the

Ruthenium oxide (RuO2) films as an electrode [6-8]. They showed ideal

capacitive behavior with long cycling life and high reversibility. In 1990s, due

to the potential applications in the hybrid electric vehicles, the research of

supercapacitors became popular. Up to now the extensive work is done by the

researchers based on both EDLC and pseudocapacitor using different types of

metal oxides and conducting polymers as electrode materials. In the present days

several other companies like Maxwell Technologies Boost-cap U.S.A., ELTON

supercapacitor Russia, CAP-XX Australia, Nippon Chemi-Con Corporation

Japan, NessCap Republic of Korea have invested in the development of

supercapacitors.

1.4 Classification Of Electrochemical Supercapacitors

Fig. 1.2 represents the classification of electrochemical supercapacitors.

Based on the charge storage mechanism electrochemical supercapacitors mainly

categorized into two types first is EDLCs and second is pseudocapacitors. The


combination of these two types of capacitor develops the third type

Supercapacitor known as Hybrid capacitors.

Fig. 1.2 Classification of Electrochemical Supercapacitors

1.4.1 Electrical Double Layer Capacitors

EDLCs constructed with a two carbon electrodes, electrolyte solution and

a separator. Where the two carbon electrodes are electronically conducting while

the electrolyte is ionically conducting. The energy storage in EDLC is a result

of the separation of charges at the interface between the electrode and the

electrolyte, which is in the similar way as conventional capacitors. Energy

stored by non-faradaically i.e., there is no transfer of charges between electrode

and electrolyte. When the potential is applied, the charge accumulates at the

surface of electrode and the electrolyte forms the double layer. The capacitance

built at this interface is called the double layer capacitance. The thickness of

such double layer being the order of a few Angstroms. EDLC consist of two

such interfaces which are works against each other (i. e., one is charged

positively and the other negatively). The two-electrode, two- interfaces system in

a single capacitor cell is illustrated in Fig. 1.3 (a).

In the charge condition there are two interfacial drops in potential across

the capacitor cell, one across each double layer [Fig. 1.3 (a)]. Upon discharge


there is also a current dependent, ohmic IR potential drop within the solution,

and the opposite on recharge [Fig.1.3 (b)].

Because there is no transfer of charge between electrolyte and electrode,

there are no chemical or composition changes associated with non-faradic

processes. For this reason, charge storage in EDLCs is highly reversible which

allows them to achieve very high cycling stabilities up to 106 cycles. It is well

known that double layer carbon capacitors exhibit perhaps 1-5 % of their

capacitance as pseudocapacitance due to the faradaic reactivity of surface (edge)

oxygen functionalities [9].


Fig.1.3 Diagrams of electric potential profiles in an electrochemical capacitor

comprising a double layer at each of two electrodes (a) charged capacitor at

open circuit (b) capacitor passing current on discharge with IR drops [9]

In actual experiment the electrochemical behavior of each of the pair of

electrodes is to be examined, then third reference electrode is required with

respect to which the potential measurements can be independently scaled and

controlled.

1.4.1.1 Carbon Based Materials For EDLCs

Carbon materials are widely used for EDLC type supercapacitor because

of their unique combination of chemical and physical properties, namely,

high conductivity, high surface-area, good corrosion resistance, high

temperature stability, controlled pore structure, processability and

compatibility in composite materials and relatively low cost [10]. In order to

enhance the performance of the supercapacitor the different forms of carbon

materials used as electrodes for EDLCs are activated carbons, carbon

aerogels, carbon fibers and carbon nanotubes.


Chapter I Page|7

Activated Carbons

Activated carbon is the most commonly used electrode materials in

the EDLCs due to it is less expensive and possesses a higher surface area than

other carbon based materials. Activated carbon in activated process can be

tailored in such a way that the distribution of pore, pore size and pore volume

can be controlled. So it utilize a complex porous structure composed of

differently sized micropores (<20 Å wide), mesopores (20-500 Å) and

macropores (>500 Å) to achieve their high surface area. Activated carbons

achieved by treatment with KOH have been reported to exhibit high oxygen

content that influence the electrochemical characteristics. Usually, the

capacitance values of activated carbons range from 100 to 200 Fg-1 in

aqueous electrolyte and from 50 to 150 Fg-1 in organic electrolyte. Although

capacitance is directly proportional to surface area empirical evidences

suggests that for activated carbons, not all of the high surface area

contributes to the capacitance of the device. Electrolyte ions that are larger

than smaller micropores cannot diffuse into them, hence preventing some

pores from contributing to charge storage [10]. As a result the influence of

capacitance on pore size distribution of activated carbon electrodes is major

area of the research in EDLC design.

Carbon Aerogel

There are also interests in using carbon aerogels as an electrode material

for EDLCs due to their favorable characteristics like good electrical

conductivity, controllable pore structure and high useable surface area [11].

Carbon aerogels are formed from a continuous network of conductive carbon

nanoparticles with interspersed mesopores due to this continuous surface

and their ability to bond chemically to the current collector , carbon aerogels

do not require the application of an additional binding agent. As a binder less

electrode, carbon aerogels have been shown to have a lower ESR than activated

carbons. This reduced ESR which yields higher power (as per the eq. 1.2).

Hence the primary area of the interest in supercapacitor research involving

carbon aerogels.


Chapter I Page|8

Carbon Nanotubes

Recent research trends suggest that there is an increasing interest in the

use of carbon nanotubes as a EDLCs electrode material [12, 13-17]. CNTs can

be grown directly onto the current collectors, subjected to heat treatment or

cast into colloidal suspension thin films with an open and accessible network

of mesopores. The mesopores are interconnected allowing a continuous

charge distribution and easy diffusion of ions. Thus CNTs have lower ESR

than activated carbon and they have moderate surface area hence achieve

capacitance comparable to those of activated carbons.

1.4.2 Pseudocapacitors

The construction of pseudocapacitor is similar as that of the EDLC.

Capacitance arises from reversible faradaic reactions occurring at the electrode.

There is transfer of charges between electrode and electrolyte i. e., the oxidation/

reduction takes place in the electrode material. The capacitance exhibited by

such systems is referred to as psedocapacitance since it originates in a quite

different way from that corresponding to classical electrostatic capacitance of the

type exhibited by double layer capacitors. Pseudo-capacitor device always

exhibit some electrostatic double layer capacitance component proportional to

their electrochemically accessible interfacial areas probably about 5 to 10%.

1.4.2.1 Metal Oxide Based Pseudocapacitors

Metal oxides present an attractive alternative as an electrode material for

pseudocapacitor because of its high specific capacitance at low resistance,

possibly to construct high energy, high power supercapacitors. Pseudo-

capacitances arise due to fast, reversible redox reaction at the surface of active

materials. The metal oxides like Ruthenium Oxide (Ru02), Nickel Oxide (NiO),

Cobalt Oxide (CO3O4), Iridium Oxide (IrO2) and Manganese Oxide (MnO2)

have been extensively studied as electrodes for pseudo-capacitors. The most

popular novel metal oxide for supercapacitor is RuO2 due to its high specific

capacitance and low ESR. However, RuO2 is an expensive material and this

limits their applications [18]. As an alternative, inexpensive metal oxides such as


Chapter I Page|9

MnO2 [19], NiO [20] and Fe2O3 [21] have been studied. Also, a major area of

research turn towards the development of fabrication methods and

composite materials to reduce the cost of active material, without reducing

the performance.

1.4.2.2 Conducting Polymer Based Pseudocapacitors

Conducting polymers offers the advantages of lower cost in comparison

with metal oxides and high charge density in contrast to carbon materials, they

have been paid much attention as electrode materials. They provide high specific

capacitance, as not only surface but also bulk material is involved in charge

storage mechanism and also they have high conductivity in the charged state

hence devices with low ESR are feasible. The storage in electrical conducting

polymer is due to faradaic process which takes place at the electrode materials.

When oxidation occurs, (also referred to as 'doping'), ions are transferred to the

polymer backbone. When reduction occurs ('dedoping') the ions are released

back into the solution. The detail explanation about the oxidation/ reduction of

conducting polymer is described in next point. Charging in conducting polymer

film takes place throughout the bulk volume of the film and not just on the

surface as is the case with carbon hence they achieve high levels of specific

capacitance [22]. The mechanical stress on conducting polymers during

oxidation/ reduction reactions limits the stability of these pseudocapacitors

through many charge discharge cycles. To overcome this problem the new

composite electrode materials based on the conducting polymer and carbon

materials have been studied.

Conducting polymer such as Polyaniline (PANI), Polypyrrole (Ppy) and

Polythiophene (Pth) are promising materials for super capacitor. Among these

PANI is most frequently used polymer materials due to its ease of synthesis,

reasonably high conductivity, chemical stability in the conducting state, good

environmental stability, unique doping/dedoping behavior and mechanical

flexibility [23-26].


Electrochemical Behavior Of Conducting Polymers

Conducting polymer systems offer opportunities for storing

electrochemical energy largely through their redox capacitance. Therefore, like

Li intercalation systems, they store energy largely by faradaic processes that

correspond formally to” battery type” behavior but exhibit electrical

characteristics which are those of the capacitor i. e., there is a functional relation

between the charge (q) accommodated and the potential (V) of the electrode

giving a derivative dq/dV that corresponds to pseudocapacitance.

The essential features of the electrochemical behavior of conducting

polymers that make them suitable as electrochemical capacitor materials are:

1. The continuous range of states of oxidation that arise with increasing

electrode potential

2. The reversibility of the faradaic processes corresponding to charge

withdrawal and reinjection

Fig. 1.4 Development of quasi-linear double layer at conducting polymer chain

The mechanism of electrochemical doping of a conducting polymer

film is described schematically for p-doping in Fig. l.4 (a) and for n-doping

in Fig. l.4 (b). It consists of a current collector plate i.e., on which the

conducting polymer deposited then the neutral conducting polymer film and the

electrolyte solution which contains cations and anions. When the positive


Chapter I Page|11

potential applied to the conducting polymer, the dopant anions moves in from

the solution into the conducting polymer towards delocalized charge sites on the

conducting polymer and anionic doping occurs. This anionic doping is termed as

p-doping. At this time conducting polymer gets oxidized. Abstraction of

electrons form the polymer backbone through the external circuit and

incorporation of an anion from solution into the polymer film to counter

balance the positive electronic charge.

When the negative potential is applied, a cation would move in from the

solution into the polymer backbone. This would be termed as cationic doping or

n-doping. Electrons are transported onto the polymer backbone by the

external circuit and cations enter the polymer from the solution phase in

order to maintain overall charge neutrality. Both these process are reversible.

Most conducting polymers can be p-doped and undoped and these

processes generally take place at electrode potentials that are accessible in

aqueous solutions. On the other hand, only a limited number of conducting

polymers can be reversibly n-doped, virtually all at highly reducing

electrode potentials which require cathodically stable and relatively pure

non-aqueous systems. For these reasons most workers have concentrated on

the p-doping of stable conducting polymers such as Ppy [27, 28] and PANI

[29, 30]. However, reliance on p-doping/undoping alone in a single type

of active material typically limits the overall voltage window of the device

to about 1 V. For this reason, Rudge et al. pursued other configurations of

capacitors based on conducting polymer active materials which would provide

an increased voltage window and hence an increased energy density (as per

Eq. 1.1) [22].

Types Of Supercapacitors Based On Conducting Polymers

According to Rudge et al. electrochemical capacitor systems utilizing

electroactive conducting polymers were classified into three types as follows:

1. Type I Electrochemical Capacitor: Two identical p – dopable

conducting polymer electrodes

2. Type II Electrochemical Capacitor: Two different p – dopable


Chapter I Page|12

conducting polymers electrodes

3. Type III Electrochemical Capacitor: One p – dopable and

another n –dopable conducting polymer electrodes

A schematic representation of electrochemical characteristics for the three

types of electrochemical capacitor systems as shown in Fig. 1.5 to 1.7. For each

type of system, considered a generalized schematic voltammogram is

presented for a single electrode containing the conducting polymer active

material [Figs. 1.5(a), 1.6(a) and 1.7(a)] together with a corresponding

voltage decay curve expected at constant discharge current in a capacitor

made with two such active electrodes [Figs. 1.5(b), 1.6(b) and 1.7(b)]. The

schematic forms of the voltammogram shown in these Figs. are characteristic

for conducting polymer materials which undergo conversion from an

undoped to a doped state. Typically, such an electrochemical conversion is

associated with a peaked rather than featureless voltammogram. From this it

was observed that the undoped region demonstrate the low capacitive currents

while doped region demonstrate the high capacitive currents i.e., the

differential capacitance of the material is potential dependent. During the

discharge of a capacitor fabricated with two conducting polymer electrodes,

the potential of each electrode shifts from the highest positive or highest

negative value to a final potential set by the condition of zero discharge

current and zero voltage (as shown by the arrows on the voltammograms in

Figs. 1.5-1.7 (a). The voltage decay at constant current shown in Figs. 1.5-

1.7 (b) was reconstructed in each case from the generic voltammogram. The

nonlinearity of the discharge characteristic (bold curve in Figs. 1.5-1.7 (b)) is a

result of the potential dependence of the differential capacitance.

1. Type I Electrochemical Capacitor

The Type I electrochemical capacitor consist of the two electrodes

comprise of two identical p-dopable conducting polymer films as a active

components.


Fig.1.5 (a) Schematic representation of half-cycle charge or discharge

operation of a Type I combination of polymer electrodes making an

electrochemical capacitor. (b) The decline of potential, V, with time or

charge, Q, on discharge

When the capacitor is fully charged, one of the films will be in the

undoped form, the other will be in the fully doped form and the cell

voltage is V (normally 1 V). Upon discharge, the undoped film oxidizes

(becomes doped: thick full line, Fig. 1.5(a)) while the doped film reduces

(dedopes: thick dashed line, Fig. 1.5(a)) until both are half doped and the

cell voltage is zero. Hence, the charge released on discharge, Q1, is only

one half of the full doping charge.

2. Type II Electrochemical Capacitor

The Type II electrochemical capacitor consist of the two electrodes

comprise of two different p-dopable conducting polymer films as a active

components. They have different ranges of potential for oxidation and

reduction. Their behavior in a capacitor on discharge or recharge half cycles is

illustrated in Fig. 1.6. This results in an increase of the voltage of the fully

charged capacitor to V2 (normally 1 V) and also allows a proportion of the

total doping charge, QZ, to be released on discharge. The resulting increase

in energy density over a Type I capacitor is reflected by the larger the

operating potential window and also area under the cell voltage decay curve.


Fig. 1.6 (a) Schematic representation of half-cycle charge or discharge

operation of a Type II combination of polymer electrodes making an

electrochemical capacitor. (b) The decline of potential, V, with time or charge,

Q, on discharge

3. Type III Electrochemical Capacitor

The Type III electrochemical capacitor consist of the two electrodes

comprise of one p-dopable and another n-dopable conducting polymer films

as a active components. When the capacitor is charged, one polymer film is

fully p-doped and the other is fully n-doped. As a result, the initial cell

voltage is increased further to V3 (Fig. 1.7 (b)) and the full doping charge,

Q3, is released on discharge. When the cell is fully discharged, both

polymer films are returned to their undoped state. That is the discharge half

cycle of the p-doped electrode can be worked against the discharge half cycle of

the n-doped electrode but with the substantial operating difference V0 arising

from the separate potential ranges of p and n doping.


Fig. 1.7 (a) Schematic representation of half-cycle charge or discharge

operation of a Type III combination of polymer electrodes making an

electrochemical capacitor. (b) The decline of potential, V, with time or

charge, Q, on discharge

The generic Type III electrochemical capacitor system has some

advantages over Types I and II. The instantaneous power density on

discharge should be greater, because both electrodes are in a doped

conducting state when the capacitor is fully charged. In contrast, in Types

I and II one of the polymer films is in the undoped semi-insulating state

when the capacitor is charged and this could introduce a higher resistance

into the fully charged device. This system also offers a wider range of

operating voltage (up to 3.1 V with nonaqueous solutions) and a correspondingly

increased energy density [9].

Practically, the electrochemical behavior of each of the pair of electrodes

is to be examined with three electrode system, where working and counter

electrode consist of polymer film and graphite or platinum respectively while

Saturated Calomel electrode (SCE) serve as the reference electrode.

1.4.3 Hybrid Capacitors

The combination of the two capacitor i.e., EDLC and pseudocapacitor is

called as hybrid capacitor, in which charge stored both by faradaic and non-

faradaic reactions. Hybrid capacitors attempt to exploit the relative advantages

and diminish the relative disadvantage of EDLCs and pseudocapacitors to realize

better performance characteristics. The carbon materials provide a backbone


Chapter I Page|16

with high surface area and favorable pore distribution while the pseudocapacitve

material enhances the capacitance through faradaic reactions on the surface of

the electrode [31, 32]. Based on their electrode configuration hybrid capacitors

are classified as composite, asymmetric and battery-type electrodes

Composite electrodes integrate carbon based materials with either

conducting polymer or metal oxide materials and incorporate both physical

and chemical charge storage mechanisms together in a single electrode. The

carbon based materials facilitate a capacitive double layer of charge and also

provide a high surface area backbone that increases the contact between the

deposited pseudocapacitive materials and electrolyte. The pseudocapacitive

materials are able to further increase the capacitance of the composite

electrode through faradaic reactions.

Asymmetric hybrids combine faradaic and non-faradaic processes by

coupling an EDLC electrode with a pseudocapacitor electrode. In particular,

the coupling of an activated carbon negative electrode with a conducting

polymer positive electrode has received a great deal of attention [33, 34].

Like asymmetric hybrids, battery-type hybrids couple two different

electrodes, however, battery-type hybrids are unique in coupling a

supercapacitor electrode with a battery electrode. This specialized

configuration reflects the demand for higher energy supercapacitors and

higher power batteries, combining the energy characteristics of batteries

with the power, cycle life, and recharging times of supercapacitors.

Research has focused primarily on using nickel hydroxide and lead dioxide

as one electrode and activated carbon as the other [35-37].

Theory of Conducting Polymers

1.5 Introduction

Polymers are long chain giant organic molecules assembled from many

smaller molecules called monomers. Polymers consist of many repeating

monomer units in long chains. The interlinking of many units has given the

polymer its name poly, meaning ‘many’ and mer, meaning ‘part’ (in Greek)


Chapter I Page|17

(Gowariker et al., 1987). A polymer is analogous to a necklace made from

many small beads (monomers).

The class of polymers that can conduct electricity is called conducting

polymers. The conducting polymer also sometimes called conductive polymers

or conjugated conductive polymers or organic polymeric conductors. Concept of

conjugation is presence of alternate single and double bonds between the carbon

atoms leading to creation of sigma and pi bonds. Thus delocolized electrons

become available. Thus the conducting polymer consist of an extended pi system

along the backbone, this confers the possibility of electron movement along the

chain i.e., conduction

1.6 History of Conducting Polymers

In 1958 polyacetylene was synthesized by Natta and co-workers as a

black powder possessing semiconductor properties depending on how the

polymer was processed. Pohl, Katon and their co-workers, first synthesized

and characterized semiconducting polymers in the year 1960 [38]. The

scientific curiosity was elucidated in 1967 Hideki Shirakawa of Kyoto

University synthesized a thin polyacetylene film due to a fortunate mistake.

When Shirakawa and his co-workers tried to reproduce the error they found that

they had used nearly a thousand times more Ziegler-Natta catalyst than usual

[39]. They succeeded in synthesizing polyacetylene directly in the form of a thin

silvery semiconductor film. On the other hand, in University of Pennsylvania

Alan Heeger, physicist and Alan MacDiarmid, chemist, have been working

on (SN)x having strong electronic properties and they have discovered that

bromine addition increased the conductivity tenfold. Actually, the inorganic

sulfur nitride polymer discovered in 1973 showed properties very close to

those of metal, however, its explosive nature prevented it from becoming

commercially important (Walatka, Labes and Perlste in 1973.

In1976, Alan MacDiarmid, Hideki Shirakawa and Alan Heeger along

with a group of young students found that conductivity of polyacetylene

increased by up to 6 orders of magnitude when reacted with iodine (from 10-4

Scm-1 to 102 Scm-1), this phenomenon known as doping, is as a result of charge


Chapter I Page|18

carriers. In addition, they found that the polyacetylene can be converted from

insulator to a semiconductor to a full metal by varying the level of doping [40].

The importance of this discovery was recognized in 2000 when the

“Nobel Prize in Chemistry” was awarded to the scientist who discovered the

electrically conducting polyacetelene in 1977, MacDiarmid, Shirakawa and

Heeger. Although polyacetylene is not stable in air, the fact that it could be

become conductive upon doping led to further experimentation with other

known conjugated polymers. Since 1976, a number of conducting polymers,

namely Ppy, Pth, and PANI have become the focus of much study [41-44].

1.7 Conducting Polymers And Their Structures

Since the discovery of doped polyacetylene, a number of different

polymers and their derivatives have been synthesized in order to achieve

materials with specific properties. The structures of some commonly used

conducting polymers along their monomers as shown in Fig.1.8.

Among the available conducting polymers the PANI has attracted

much attention because of its properties like ease of synthesis, controllable

electric conductivity, simplicity in doping and dedoping, chemical stability,

good environmental stability and mechanical flexibility. One of the surprising

quotation given by Prof. A.G. MacDiarmid that ”there are as many different

types of PANI as there are people who synthesize it” [45]. Therefore, the

way of synthesis decides the conductivity, band gap, chemical structure,

polymerization mechanism and ease of attachment and detachment of different

functional groups.


Fig. 1.8 Examples of Conducting Polymers

1.8 Polyaniline (PANI)

PANI is one of the oldest conducting polymers known. The first PANI

also called aniline black was prepared by Letheby in 1862 by anodic oxidation

of aniline in sulfuric acid [46]. PANI exists in a variety of forms that differ

in chemical and physical properties [47-50]. PANI will become conductive by

doping in which the polymer is partially oxidized or reduced. Depending upon

the method of preparation of PANI exists in four main oxidation states as

follows:

1. Leucoemeraldine base

2. Emeraldine base

3. Emeraldine salt

4. Pernigraniline base


1.8.1 Chemical Structure Of PANI

The general structure of PANI is as shown in Fig. 1.9 [51, 52]. It consist

of alternating reduced benzenoid (-B-N=Q-N=) units and oxidized quinoid (B-

NH-B-NH-) repeats units. Where “B” denotes benzoic and “Q” denotes quinoid

rings.

Fig. 1.9 General structure of PANI

In the structure n represents the degree of polymerization and (1-y)

represents the oxidation states which can be varied from 0.0 to 1.0. Thus the

changing ratio of amine to imine yields various structures, such as

leucoemaraldine, a reduced form of emeraldine base [53]. Only one form,

called the emeraldine salt, is electrically conducting. The various structures of

PANI in several oxidation states, ranging from the completely reduced

leucoemeraldine base state to completely oxidized pernigraniline base state as

shown in Fig. 1.10.

Fig. 1.10 Various stages of PANI.


Chapter I Page|21

For (1-y) we have completely reduced colorless state of PANI, denoted as

leucoemerildine as shown in Fig. 1.10 (a). With (1-y) = 0.5, we have the half

oxidized blue state of PANI, denoted as emeraldine base in Fig. 1.10 (c). And

finally, with (1-y) =1.0, we have the completely oxidized purple state of PANI,

denoted as pernigraniline. All states except leucoemeraldine can be protonated.

The states with no protonated are denoted as base form. Upon protonation, the

polymer denoted as a salt. For example protonation of the emeraldine base (Fig.

1.10 (c)) converted into emeraldine (Fig.1.10 (b)) salt form.

The insulating emeraldine base can be converted into the emeraldine salt

form by non-redox doping process with protonic acids (HA). The conducting

emeraldine salt can also be obtained through a redox doping process in acidic

conditions from its corresponding reduced leucoemeraldine base form or

oxidized pernigraniline base form by either a chemical or an electrochemical

step. However, the non-redox doping process is different from the redox doping

in that it does not involve the addition or removal of electrons from the polymer

backbone. Instead, the imine nitrogen atoms of the polymer are protonated to

give a polaronic form where both spin and charge are delocalized along the

entire polymer backbone. Both the redox doping process and the non-redox

doping process are reversible, the conductive emeraldine salt form can be

converted back to its corresponding insulating base forms if the conditions

change, either physically (for non-redox doping) or (electro-) chemically (for

redox doping).

1.8.2 Synthesis Of PANI

The most common synthesis of PANI involves oxidative polymerization,

in which the polymerization and doping occurs simultaneously. The synthesis of

PANI can be classified into two major categories:

1. Chemical Polymerization

2. Electrochemical Polymerization


Chapter I Page|22

1. Chemical Polymerization

Chemical polymerization (oxidative coupling) is one of the most useful

techniques to prepare large amounts of conducting polymers. Chemical

oxidative polymerization of PANI involves the use of either hydrochloric or

sulfuric acid in the presence of ammonium peroxo-di-sulfate as the oxidizing

agent in the aqueous medium. Where the oxidant is used to withdraw a proton

from an aniline molecule without forming a strong co-ordination bond either

with the substrate / intermediate or with the final product. It has been suggested

that the use of such strong oxidizing agents may result in defects such as cross-

links. However smaller quantity of oxidant is used to avoid oxidative

degradation of the polymer formed.

In the typical chemical oxidative polymerization of PANI, a aniline is

polymerized using ammonium persulphate (APS) as an oxidant. For carrying the

polymerization reaction, chilled monomer of definite molarity is added to pre-

cooled acidic solution. To achieve better yield and better quality of polyaniline

and to avoid the formation of oligomers, the reaction is carried out in low

temperature range (0-5 0C), by placing the beaker in ice bath. APS taken in 0.1

M is added to the above solution slowly (~1ml/min) and the resulting solution is

stirred in order to ensure the completion of the reaction. The slower addition of

APS will avoid the formation of oligomers. The formation of green color

indicates the formation of PANI. The polymerization was completed within 30

minute.

2. Electrochemical Polymerization

Electrochemical polymerization is less frequently employed for bulk

production and more frequently for preparation of thin films. It consist of a three

electrode system such as working electrode, counter electrode and reference

electrode, which contains a solution of monomer and supporting

electrolyte. The conducting substrate serves as a working electrode in the

electrochemical polymerization process on which the polymer film was

deposited. In the three electrode cell, counter electrode is generally a graphite or

platinum plate while reference electrode is a SCE or Ag/AgCl electrode.


Chapter I Page|23

Electrochemical polymerization can be performed either potentiometrically to

obtain thin films or galvanostatically to obtain thick films. Polymer films

thickness or geometry can be controlled via monitoring charge passed.

In typical procedure for electropolymerization of PANI, aniline monomer is

dissolved in an aqueous sulfuric acid solution used as electrolyte. The applied

potential is either held at a constant value within the range 0·7-1·2 V or is cycled

between -0·2 V and 0·7 to 1·2 V. The PANI is usually formed as a thin film on

the anode.

1.8.3 Polymerization mechanism

The various complex stages of oxidative polymerization of aniline is as

shown in following Fig.1.11. It is generally accepted that the polymerization

mechanism is sufficiently analogous for the chemical and the electrochemical

syntheses that observations from both cases can be combined to determine the

reaction pathway. The polymerization begins with the formation of anilinium

radical cation and then proceeds via successive additions of the radical cation to

the end of the (oxidized) growing chain.

Oxidative polymerization of aniline consists of following steps:

1. Oxidation of monomer to radical action.

2. Dimerization of radical cation followed by proton loss to form a neutral

dimer.

3. Oxidation of neutral dimer to radical action.

4. Oxidation of dimer radical action with another cation to form dication.

In this way the reaction proceeds and consequently trimer, tetramer and

finally polymer are formed.


Fig. 1.11 Schematic structure for polymerization of Aniline

1.8.4 Conduction Mechanism In PANI

The mechanism of conduction and behavior of charge carriers in the

conducting polymers have been explained using the concept of polarons and

bipolarons. Low doping levels give rise to polarons, whereas higher doping

levels produce bipolarons. Both polarons and bipolarons are mobile and can

move along the polymer chain by the rearrangement of double and single bonds

in the conjugated system that occurs in an electric field.


Fig. 1.12 Polaron and bipolaron lattice. (a) Emeraldine salt in bipolar form.

(b)Dissociation of the bipolarons into two polarons. (c) Rearrangement of the

charges into‘polaron lattice’ [54, 55]

The conductivities of PANI can be transformed from insulating to

conducting through doping. Both n-type (electron donating, such as Na, K, Li,

Ca) and p-type (electron accepting, such as I2, BF4, Cl) dopants have been used

to make an insulator to conductor transition in electronic polymers. The common

dopants for PANI are hydrochloric acid, sulfuric acids and sulfonic acids. For

the degenerate state polymers, the charges added to the polymer backbone at low

doping levels are stored in charged soliton and polaron states created for

degenerate polymers and as charged polarons or bipolarons created for

nondegenerate systems. Such a situation is also encountered in PANI, which do

not have two degenerate ground states. That is, the ground state is non-

degenerate due to the non-availability of two energetically equal Kekule

structures.

Therefore there cannot be a link to connect them. In the doping process,

the heteroatoms nitrogen will be protonated and become a bipolar form (Fig.

1.12). The conventional distortion of molecular lattice can create a localized

electronic state, thereby lattice distortion is self consistently stabilized (Fig.

1.12). Thus, the charge coupled to the surrounding (induced) lattice distortion to

lower the total electronic energy is known as polaron (i. e., an ordinary radical

ion). A bipolaron consist of two coupled polarons.


Chapter I Page|26

It is well known that PANI with conjugated π-electron backbones can be

oxidized or reduced more easily and more reversibly than conventional

polymers. Charge transfer agents (dopants) effect this oxidation or reduction and

in doing so convert an insulating polymer to a conducting polymer with near

metallic conductivity in many cases. Further, we have to look at the basis of

doping effects on the band structure of PANI in order to wholly understand the

conduction mechanism in PANI.

1.8.5 Effect of doping on band structure

Before the details of band structure of PANI are discussed, let’s see few

details about the doping. The chemical oxidation of the conducting polymer by

anions or its reduction by cations is called as doping. The extent of enhancement

of electric conductivity of a polymer primarily depends on the chemical

reactivity of the dopant with the polymer. There are two types of doping as

follows:

Polymer +Dopant [Polymer+ – Dopant

-]

(Anions) Oxidation, p- doping

Polymer +Dopant [Polymer- – Dopant

+]

(Cations) Reduction, n- doping

In the organic semiconductor band structure consist of valence band and

conduction band. The difference between the valence band and conduction band

is called as band gap. Similar picture observed in case of the polymer only the

valence band is called highest occupied molecular orbital (HOMO) and

conduction band is lowest unoccupied molecular orbital (LUMO). The band

energy spacing between the HOMO and LUMO is the band gap. The energy

band diagram in polymers as a result of doping are indicated in Fig. 1.13 [56].

In a polymer just as in a crystal the interaction of a polymer unit cell with

all its neighbours leads to the formation of electron band. At a zero doping level

the polymer is neutral and its band structure is that of a standard semiconductor

with a band gap [Fig.1.13 (a)]. Removal of one electron from the conducting


polymer chain produces a polaron (i. e an ordinary radical ion) as shown in

Fig.1.13 (b). A local distortion of the lattice takes place around the charge

created. The localized electronic states in the gap due to upward shift ∆ε of the

HOMO and the downward shift of the LUMO appear. Upon electron removal

(Oxidation) the ionization energy is lowered by an amount ∆ε. Polaron, a radical

ion associated with a lattice distortion is created and the presence of localized

electronic states in the gap referred to a polaron states. Removal of second

electron forms a bipolaron. A bipolaron is defined as a pair of like charges

associated with a strong lattice distortion, the electronic band structure

corresponding to the presence of one bipolaron is depicted in Fig.1.13 (c). The

electronic states appearing in the gap of a bipolaron is further away from the

band edges than for a polaron. Further doping results in overlapping between the

bipolaron states and formation of bipolaron bands [Fig.1.13 (d)]. The band gap is

widened due to the fact that the bipolaron states coming in the gap are taken

from the valence band and conduction band edge [57].

Fig.1.13 Band structures of polymer chain (a) Undoped polymer (b) Formation

of polaron (c) Formation of bipolaron (d) Formation of bipolaron band

In this way the conducting polymer varies from insulating state to the

conducting metallic state.


Chapter I Page|28

1.9 Literature Survey

PANI is one of the most promising materials because of its environmental

stability, controllable electrical conductivity and easy processability [58]. PANI

is so far the most extensively studied material. The detailed survey on

chemically polymerized PANI as electrode material for electrochemical

supercapacitor is given below. The aim of this literature survey is to understand

relevant experiences, promising concepts and limitations. This will be helpful to

prepare PANI as electrode material for electrochemical supercapacitor with

better performance.

In order to overcome the shortcomings of PANI electrode, lots of work is

carried out by the number of researchers as PANI composite with carbon based

material or metal oxides as electrode material for supercapacitor. Therefore, the

literature survey on chemically synthesized PANI based electrode for

supercapacitor divided into three main groups viz. 1] Pristine PANI 2] PANI

composite with carbon based material 2] PANI composite with metal oxides.

1] Pristine PANI

This category involves the different PANI nanostructures, doped

PANI and PANI coated with different electrode.

Chemical oxidative polymerization of aniline with acetic acid instead of

HCl (conventional synthesis) dramatically changes the morphology of the doped

PANI powder from nanofibrillar to almost extensively nanofibers of the reduced

leucoemeraldine state studied by Mallikarjuna et al.. Also they observed

variation in the diameter of nanofiber ranges from 20 nm to 50 nm with

increasing concentration of acetic acid [59]. PANI nanofibers with high surface

area and their electrochemical performances as electrode materials in an aqueous

redox supercapacitor is studied by Shivakkumar et al. using interfacial

polymerization. Preparation of electrodes carried out onto the Ti foil by means

of a doctor blade technique. Also they shown both the specific capacitance and

the cycleability for the redox supercapacitor are found to depend on the charging

potential [60]. Guan et al. fabricated PANI nanofibers by interfacial

polymerization in the presence of para-phenylenediamine (PPD). They found


Chapter I Page|29

that PANI nanofibers prepared in the presence of PPD were longer and less

entangled than those in the absence of PPD due to a much faster polymerization

rate in initial stage. A higher SC value of 548 Fg-1 and a SE of 36 Whkg-1 were

obtained in PANI-PPD nanofibers compared to those of PANI-N at a constant

discharge current density of 0.18 Ag-1 [61].

Aqueous conducting PANI nanofibers were prepared by Zhang et al.

using acidic phosphate ester containing hydrophilic ethylene glycol

segment to dope nanofibrous dedoped PANI, in which the fibers was

synthesized by the ferric nitrate as oxidant through pseudo-high dilution

method. Supercapacitors electrode made from thin nanofibrous PANI

(diameter of 17–26 nm) gave a high specific capacitance of 160 Fg-1 at

a discharge rate of 0.4 Ag-1 within the potential range of −1 to 1 V

versus saturated calomel reference electrode in organic non-protonic

electrolyte solution, which was higher than that of thick nanofibrous or

spherical polyaniline. Since large BET surface area of 70 m2g-1 was obtained

for the thin nanofibrous PANI which leads to higher utilization of the

active materials. [62]

Liu et al. prepared a porous PANI material by using sodium

dodecylsulfate as a soft template and ammonium persulfate as an oxidant. This

porous materials posses high specific surface area than bulk materials this leads

to the high utilization of the active materials which enhance the capacitive

performance [63].

A porous and mat-like PANI /sodium alginate (PANI/SA) composite was

polymerized by Li et al. in an aqueous solution with sodium sulfate as a

template. They demonstrates nanostructured PANI/SA electrode shows a good

reversible stability and fast response to oxidation/reduction on high current

changes and an excellent electrochemical discharge capacitance as high as 2093

Fg-1. This outstanding electrochemical characteristic is attributed to the

nanostructured electrode materials, which generate a high electrode/electrolyte

contact area and short path lengths for electronic transport and electrolyte ions

[64].


Chapter I Page|30

One of the challenging issues in development of high performance

supercapacitor is to improve its electronic conductivity of the PANI electrode

because it is less than metal oxide. Extensive research work has been focused on

enhancing electronic conduction of the electrodes by using metal doping. Jie Li

et al. have demonstrated the capacitive behavior of H+ and Zn2+ doped with

PANI. They reported the specific capacitance values 415 and 427 Fg-1 at 30

mVSec-1 respectively [65].

The numerous researches have been performed with doping of protonic

acid into PANI for increasing conductivity [66, 67]. Although PANI doped with

HCl has high conductivity and is stable in air atmosphere, it has disadvantage

such as insolubility due to that PANI has strong intermolecular interaction

(hydrogen bonding) induced by short distance between counter ions. In addition,

many research has been carried out to increase conductivity and processability of

this polymer [68] .To improve its solubility, non-protonic acid doping agents

such as sodium dodecylsulfate are used [69]. The preparation of PANI doped

with dimethylsulfate (PANI- DMS) as a nucleophilic dopant and electrochemical

properties of PANI-DMS as electrode for both lithium secondary battery and

redox are discussed by Ryu et al. [70]. Also they reports on PANI doped with

DMS as electrode material for supercapacitor synthesized by chemical

polymerization in which, first emeraldine base was prepared by chemical

polymerization then emeraldine base powder was mixed with 100 ml of a 1M

DMS aqueous solution at 40-60 0C for 16 h. Electrode depositions were carried

out by doctor blade technique [71].

The electrochemical characteristics of Li/PANI doped with lithium salt

were also examined to study the electrochemical reaction mechanism during

repeated charge discharge process reported by Ryu et al. [72]. From this result it

was observed that PANI doped with lithium salt is easily oxidized and reduced

by electrochemical potential in electrolyte solution. Further they demonstrate the

symmetric redox supercapacitor based on PANI-HCl (PANI doped with HCl)

and PANI-LiPF6 (PANI doped with LiPF6) and investigate their electrochemical

properties in two kinds of electrolytes such as Et4NCF3SO3 and Et4NBF4. As a


Chapter I Page|31

result, the PANI-LiPF6 system with Et4NBF4 electrolyte offers better capability

than PANI-HCl system due to lower internal resistance [73]. Moreover they

fabricate two types of supercapacitors: redox type (symmetric type) based on

two LiPF6-doped PANI (PANI-LiPF6) electrodes and hybrid type (asymmetric

type), based on PANI-LiPF6 and active carbon electrodes. The hybrid type of

supercapacitor was shown to have better electrochemical performance than that

of redox type in both 0–1 and 0–3 V ranges. The specific capacitance of the

hybrid type was also larger than that of the redox type in both ranges. The active

carbon electrode function to increase the voltage of the supercapacitor and the

polymer electrode maintains the redox reaction for a relatively long time [74].

Ryu and Kim et al. suggest a new concept of a hybrid power source

having one shared electrode with the redox reaction occurring. It consist of three

electrodes (a lithium metal, a PANI-LiPF6 shared electrode and another PANI-

LiPF6 electrode), which concurrently play the roles of a lithium secondary

battery and a redox supercapacitor. In the case of using the powdered lump as

electrode and the porous separator, the hybrid cell had a longer discharge time

and more reduced voltage drop than a lithium secondary battery on a pulsed

discharging mode [75].

Park et al. studied the performance of a hybrid type EC capacitor which

has electronically conducting p-dopable PANI as the positive electrode material

and activated carbon (AC) with high specific surface area as the negative

electrode material instead of n-dopable conducting polymers. The idea is based

on a novel design of electrodes for improving the performance of

electrochemical capacitor capacitor [76]. These components have already been

shown to have good cycle characteristics due to the electrostatic charge–

discharge mechanism (AC) and very reversible doping dedoping process (PANI)

respectively [77].

For supercapacitors the Solid electrolytes are advantageous over liquid

electrolytes in respect of easy handling and reliability without electrolyte

leakage. It is expected that sulfonated poly(ether ether ketone) SPEEK can also

act as potential material for an all solid capacitor. In the preliminary Shivaraman


Chapter I Page|32

et al. studied an all solid capacitor with PANI as the electrode material and

SPEEK as the solid electrolyte has been prepared. The study has demonstrated

that a SPEEK-based electrolyte may be a promising material for supercapacitors.

[78]. Some of the well-known room temperature proton conductors with high

ionic conductivity are reported such as phosphotungstic acid (H3PW12O40.nH2

O:PTA) and phosphomolybdic acid (H3PMo12O40.nH2O:PMA) [79] . However,

the conductors tend to dehydrate under low humidity or pelletization pressure. In

order to overcome this problem, composites of heteropolyacid hydrates have

been prepared by dispersing A12O3 or salt hydrates like Al2 (SO4)3.16H2O and

ammonium paratungstate ((NH4) 10W12O41.2H2O:APT). Since, heterogeneous

doping or the formation of dispersed phase composites has been found to be an

efficient method for enhancing the ionic conductivity with good mechanical

properties. Wang et al. first time used the proton conducting composite

phosphotungstic acid PTA/Al2 (SO4)3.18H2O as the electrolyte of symmetric

supercapacitor based on PANI. The effect of the weight ratio of PTA/Al2

(SO4) 3.18H2O on capacitance performance was also discussed [80].

2] PANI-Carbon Based Materials Composite

The long term stability during cycling is a major demand for an industrial

application of electrically conducting polymers (ECPs). Swelling and shrinkage

of ECPs is well known and may lead to degradation of the electrode during

cycling. It occurs because the doping of polymers requires the

insertion/deinsertion of counter ions, which cause a volume change. Thus, the

mechanical stress in the polymer film relates directly with the cycle life of

polymer based capacitors. This has been overcome to some extent by the use of

composite structures, for example, a combination of ECP and insulating

polymers with good mechanical properties such as poly-N(vinyl alcohol) and

polystyrene [81,82]. However, in each case, the conductivity of the composite

materials is lower than in the pristine ECP. Hence, the most interesting solution

is to use carbon materials to improve the mechanical properties of the electrodes.

Moreover, the presence of carbon in the bulk of ECPs allows to ensure a good

electronic conduction in the electrode when the polymer is in its insulating state.


Chapter I Page|33

However, electro-conducting carbon additives generally provide a low specific

capacity in comparison to ECPs. Therefore, it is necessary to minimize the

carbon content in the composite in order to obtain a high value of specific

capacitance and energy per total mass of supercapacitor electrode. For these

reasons, it can be profitable to use a moderate amount of nanotubular materials

as surface area enhancing component and as electronic conductor in ECP based

electrodes. Thin layers of ECP can be deposited on the nanotubes, while still

keeping the advantage of their open mesoporous network that allows an easy

diffusion of ions. Hence, electrodes containing an important mass of ECP can be

prepared from such nanocomposites and used in supercapacitors. Different kinds

of nanotubes and their composites have been proposed by many researchers as

electrode materials for supercapacitors.

Khomenko et al. fabricated ECP/ Multiwalled Carbon Nanotubes

(MWCNTs) composites by chemical oxidative polymerization of a monomer on

the surface of MWCNTs in order to overcome the problems encountered with

the electrochemical method. They demonstrate that pellet electrodes obtained by

simply pressing the nanocomposite materials can be employed for

electrochemical capacitors, especially when an asymmetric configuration (Type

II) is realized. The specific capacitance of a Type II capacitor with

PPy/MWCNT composite as negative and PANI/MWCNT as positive electrode

achieved a value up to 320 Fg-1 [83]. Frackowiak et al. stated that application of

CNTs enables to extract fully the energy from the ECP, however, only if the

operating voltage is optimally selected for the positive and negative electrode.

They developed three types of electrically conducting polymers (ECPs), i.e.,

PANI, Ppy and poly-(3, 4-ethylenedioxythiophene) (PEDOT) have been tested

as supercapacitor electrode materials in the form of composites with MWCNTs

and highlighted the effect of potential range during cycling [84].

The PANI/CNT composite was prepared by Shivakkumar et al. adopting

the in situ chemical synthesis method. The nanoporous PANI/CNT composite

provides a large surface area that allows excellent electrolyte access as well as

providing low internal resistance which enhances the electrochemical


Chapter I Page|34

performance. PANI/CNT composite based device shows larger capacitance than

that of PANI nanofiber with improved electrochemical stability [85].

Li et al. fabricated carbon nanotubes PANI nanocomposites with

core/shell structures by ultrasonic assisting in situ polymerization. They

observed that the improvements on electrical and electrochemical properties for

such nanocomposites with core/shell structures [86].

The composites of protonic acid doped PANI with MWCNTs were

synthesized by Dong et al. via an in situ chemical oxidative polymerization. The

capacitor behaviors of the composites in neutral system (NaNO3) were tested in

detail and the improvement mechanisms of capacitor value of the composites

were discussed by impedance measurements. The results indicated that

MWCNTs have an obvious improvement effect, which make the composites

more active sites for faradaic reaction and larger specific capacitance than pure

PANI. So the PANI/MWCNTs composites proposed here will be effectively

used as electrodes for the supercapacitors [87].

Mi et al. reported a novel microwave assisted technique for the rapid

fabrication of PANI /MWCNTs composites. Electrochemical tests indicated that

this composite had a high specific capacitance (322 Fg-1) and good rate

capability [88].

The novel hierarchical PANI/sMWCNT nanocomposites were

synthesized by Sun et al. through the interfacial polymerization method in the

presence of the sulfonated multiwalled carbon nanotubes (sMWCNT). They

found that oxidant content had a significant influence on the PANI content, the

degree of oxidation and the microstructure of composites, which markedly

affected the electrochemical performance [89].

The electrochemical properties of the composites with core/shell

structures consisting of PANI and MWCNT as the supercapacitor electrode

materials were evaluated by Zhou et al. They achieved specific capacitance 560

Fg-1 by using a composite with 66 wt% PANI content as the supercapacitor

electrode [90].


Chapter I Page|35

Yoon et al. report on the synthesis and electrochemical properties of

nanocomposites of leucoemeraldine base (LB), emeraldine salt (ES) and

pernigraniline base (PB) with MWCNTs are prepared through the chemical

polymerization by control of its oxidation state by using oxidizing and

reducing agents. They observed that ES/MWCNT shows the higher specific

capacitance of 328 Fg-1 than the LB/MCWNT and PB/MWCNT

nanocomposites. Since the ES/MWCNT composite has smaller Rs and Rct

than the LB/MWCNT and PB/MWCNT composites because of the high

electrical conductivity of the ES form of PANI in the composite [91].

There are many reports on CNT/PANI composite materials obtained by

the chemical methods in the powder form. Generally, some binding materials are

needed to be added to make electrode coatings or tablets. These binding

materials may decline the electrical and electrochemical properties of the

electrodes and also it is a complicated process. The pellet electrodes free of

binding substance have been reported by Khomenko et al. and Frackowiak et al.

but their brittle mechanical nature is still a shortcoming for practical

applications. Meng et al. prepare the interesting paper like CNT/PANI

composites by using the CNT network as the template. These paper like

CNT/PANI composites not only are free of binding materials but also show ideal

mechanical property [92].

The functionalized groups help to disperse CNTs homogeneously in the

reaction medium, the monomers can be adsorbed on the surface of f-MWCNTs

based on the strong electron and hydrogen bonding interactions between

functionalized groups and the amino groups of monomer. In addition the

covalent functionalized group on the surface of CNTs can dope into the PANI

chain, which would avoid potential microscopic phase separation in the

nanocomposite and ensure the high electrochemical properties. So Gao et al.

reported nanocomposite of benzenesulfonic functionalized MWCNTs doped

Polyaniline (PANI /f-MWCNTs) was synthesized via a low temperature in situ

polymerization method. The electrochemical results show that PANI /f-

MWCNTs have better capacity and cycleability. It could be attributed to the f-


Chapter I Page|36

MWCNTs, which makes more PANI contact with the electrolyte to participate in

faradaic redox reactions and dopes with the PANI polymer chain through the

benzenesulfonic acid groups to form stable polyemeraldine salts [93]. Similarly,

Zhu et al. prepared a sulfonated multi-walled carbon nanotubes (sMWCNTs) by

a diazotization. PANI/sMWCNTs were synthesized successfully by in situ

oxidative polymerization method in the HClO4 solution with specific

capacitance 515 Fg-1. The cycling stability of PANI/sMWCNT composites

possessed higher (below 10% capacity loss after 1000 cycles) compared to pure

PANI nanorods (29.4% capacity loss) [94].

Three Ordered Mesoporous Carbon (OMC)/PANI composites were

synthesized by Li et al. using different compounding processes, namely the in

situ polymerization of aniline in the presence of OMC or its precursor

(carbon/silica composite) and the direct physical mixing of PANI and OMC.

They observed that whatever starting material was used, either OMC or

carbon/silica composite, the interactions between OMC and PANI exist in the

chemically synthetic OMC/PANI composites, which would be beneficial to the

improvement of both electric conductivity and further specific capacitance.

Because of the high degree of dispersion of PANI molecules and double fixing

effects of the surface and mesopore of OMC on PANI, this made more PANI

molecules available for faradaic reaction and OMC a stronger support for the

maintenance of electrical conductivity and mechanical strength of PANI [95].

A new group of porous carbon like carbide derived carbons (CDCs)

materials for supercapacitor which possess high specific surface area with pore

sizes that can be fine tuned by controlling the chlorination temperature and by

the choice of starting carbide. Zheng et al. prepared (CCDC)/ (PANI) composite

materials by in situ chemical oxidation polymerization of an aniline solution

containing well dispersed CCDC. The CCDC/PANI composite in the application

of supercapacitor showed excellent electrochemical performances. Since the

capacitance of the CCDC/PANI is combination of double layer capacitance

(CCDC) and faradaic pseudocapacitance (PANI). The specific capacitance of

CCDC/PANI electrode is as high as 713.4 Fg-1 at 1 mVSec-1 compared with


Chapter I Page|37

154.0 Fg-1 of CCDC electrode. As well, the capacitance retention of the coin

supercapacitor using CCDC/PANI composite as electrode active material was up

to 80.1% after 1000 cycles [96].

Lei et al. showed that the chemical oxidative polymerization of aniline in

the presence of the HCS yielded composite materials with a layer of PANI

deposited on the external surface of the HCS. They varied the content of PANI

in the HCS-PANI composite from 11% to 74%. From that it was observed that

the supercapacitive performance of the HCS-PANI composite is strongly

dependent on the amount of PANI on the HCS. The maximum value of 525 Fg-1

was achieved for the HCS-optimized composite, at current density of 0.1 Ag-1

[97].

Graphene, a two-dimensional monolayer of sp2-bonded carbon atoms, has

attracted increasing attention in recent years, mainly due to its extraordinarily

high electrical and thermal conductivities [98, 99], great mechanical strength

[100], large specific surface area and potentially low manufacturing cost [101].

Graphene and chemically modified graphene sheets possess high conductivity

[102], high surface area and good mechanical properties [103] comparable with

or even better than CNTs. In addition, graphene based materials can be easily

obtained by simple chemical processing of graphite [104]. Therefore, the

potential of using grapheme based materials for supercapacitor has attracted

much attention very recently.

Wang et al. report a simple process to synthesis the nanocomposite of

graphene oxide (GO) doped PANI via in situ polymerization in the presence of

GO and monomer with the aim of improving the electrochemical capacitance

performance of PANI. The fibrillar morphology self assembled with PANI

nanofibres is induced and influenced greatly by the addition of graphene oxide

sheets. The electrical conductivity and specific capacitance of the nanocomposite

are remarkably enhanced compared with individual PANI [105].

The preparation of free standing GO-PANI and graphene-PANI hybrid

papers via rapid mixture polymerization of aniline on the surfaces of GO and

graphene papers respectively reported by Yan et al.. The remarkable


Chapter I Page|38

combination of advantages coming from graphene (and GO) papers and

nanostructural PANI includes flexibility, electrochemical activity and

capacitance and biocompatibility making such paper like hybrid papers

promising materials for electrochemical capacitor [106].

For obtaining highly conducting systems suitable to supercapacitor

applications Gomez et al. synthesized graphene–PANI nanocomposites varying

the monomer to graphene ratio. The high specific capacitance and good cyclic

stability have been achieved using 1:2 aniline to graphene ratio by weight of

graphene–PANI polymer [107].

Graphene nanosheets/ PANI nanofibers (GNS/PANI) composites are

synthesized by Li et al. via in situ polymerization of aniline monomer in

HClO4 solution. They optimize the GNS to aniline ratio to obtain the good

electrode in terms of their electrochemical performance. The introduction of

GNS into the composites provides a relatively large surface area for

dispersing PANI nanofibers, which can effectively enhance the kinetic

for both charge transfer and ion transport throughout the electrode. The

maximum specific capacitance is 1130 Fg-1 at a scan rate of 5 mVSec-1 in

1.0 M H2SO4 solution [108].

Aligned PANI nanowires are precisely synthesized by Zu et al. on two-

dimensional GO nanosheets by dilute polymerization. The electrochemical

studies proved that PANI-GO nanocomposites possessed a synergistic effect of

PANI and GO, which showed excellent electrochemical capacitance and better

stability than each individual component [109]. Also Wang et al. studied the

effect of GO on the properties of its composite with PANI. They observed higher

specific capacitance of 746 Fg-1 which is higher than the value obtained by the

electrochemical method [110].

Wu et al. prepared chemically converted graphene (CCG) and PANI

nanofibers (PANI-NFs) composite films by vacuum filtration the mixed

dispersions of both components. The composite film has a layered structure and

PANI-NFs are sandwiched between CCG layers. The conductivity of graphene-


Chapter I Page|39

PNF film was measured to be as high as 5.5102 Sm-1, which is one order higher

than that of pure PANI-NF film [111].

Zhang et al. studied electrochemical properties of series of homogeneous

composites of chemically modified graphene and PANI prepared by in situ

polymerization. They found that the highest specific capacitance of 480 Fg-1 at a

0.1 Ag-1 current density is achieved for the optimized composite with good

cycling stability [112].

Yan et al. synthesized the composite of GNS/CNT/PANI via in situ

polymerization. GNS is used as support materials for deposition of PANI

particles and CNTs as conductive wires interconnected among GNS/PANI

particles. CNTs can provide highly conductive path resulting in the improvement

of conductivity of composite and also maintain the mechanical strength.

GNS/CNT/PANI shows the maximum specific capacitance 1035 Fg-1 at a scan

rate of 1 mVSec-1 in 6 M KOH and demonstrating that excellent cycle stability

than GNS/PANI and CNT/PANI composites [113].

Lu et al. report on a novel design and synthesis of a hierarchical film

with coaxial PANI/CNT nanocables uniformly sandwiched between GN sheets.

In the composite, GN sheets serve as outer current collector to improve

the electrical conductivity and elastic buffering to accommodate the

volumetric change of the PANI chains, whereas the CNTs function as a

support material to effectively enhance the utilization of PANI, inner

current collector to form conducting 3D nano-network and rigid core to

improve the mechanical stability of PANI. PANI provides faradaic

capacitance for overall capacitance and favors the wettability of GN

sheets. This composite film shows the superior electrochemical properties,

including high electrochemical capacitance (569 Fg-1 and 188 Fcm-3 for

gravimetric and volumetric capacitances), good rate capability (60%

capacity retention at 10 Ag-1) and excellent electrochemical stability

(4% capacity loss after 5000 cycles) [114].

Conducting PANI -grafted reduced graphene oxide (PANI -g-rGO)

composite with highly enhanced properties is reported by Kumar et al. In the


Chapter I Page|40

typical experiment they prepare PANI -g-rGO, amine-protected 4-aminophenol

was initially grafted to graphite oxide (GO) via acyl chemistry where a

concomitant partial reduction of GO occurred. Simultaneous reduction of GO

sheets and subsequent deprotection of the amine groups followed by an

oxidative polymerization of aniline yielded a highly conducting PANI- g -rGO

composite. Initial changes in surface functionalities confirmed that PANI was

covalently grafted to the reduced GO sheets, thus forming highly conductive

networks. Electrical conductivity of these hybrid assemblies showed a value as

high as 8.66 Scm-1 [115].

For the application of asymmetric supercapacitors Hung et al synthesized

PANI nanofibers via a chemical method of rapid mixing. They demonstrate the

asymmetric supercapacitor, consisting of a PANI nanofiber cathode and a

graphene anode, with proper complementary potential windows, which shows

the device energy and power densities of 4.86 Whkg-1 and 8.75 kWkg-1

respectively [116].

3] PANI-Metal Oxide Composites

The incorporation of PANI into layered MnO2 is of interest because the

resulting nanocomposites could possess synergic properties from both

components, such as enhancement in electrochemical cycling stability or

electronic conductivity. In particular, both these components are expected to be

electrochemical active, which may improve the electrochemical storage ability.

Zhang et al. present new strategy for constructing birnessite type layered

MnO2material intercalated with conducting polymer. The process consists of

incorporation of PANI into layered manganese oxide nanocomposite via

exchange reaction of PANI with n -octadecyltrimethyl-ammonium intercalated

layered MnO2 in N -methyl-2-pyrrolidone (NMP) solvent. They suggest that the

benzene rings of PANI are arranged in a zigzag conformation and located

perpendicular to manganese oxide layers. The PANI -intercalated layered MnO2

nanocomposite shows not only the enhancement of specific capacitance but also

the improvement of electrochemical cyclic stability compared with those of

pristine PANI and manganese oxide [117].


Chapter I Page|41

To improve the cycleability of PANI, by means of stabilizing the radical

cations those are formed during the charging process. Song et al. made PANI

into a composite with Nafion. On the other hand, hydrous RuO2 has been

recognized as one of the most promising electrodes due to its high specific

capacitance, highly reversible redox reactions, wide potential window and very

good cycleability but due to its high cost and poor abundance of RuO2 posses

problems for its commercial use. Further, it has been reported that only a very

thin layer of RuO2 participates in the charge storage process and the rest of

material under this thin layer remains inactive and cause low gravimetric. So in

the present study, the PANI /Nafion composite is used as a matrix for the

deposition of a thin layer of hydrous RuO2. The resultant ternary composite

electrode of PANI /Nafion/hydrous RuO2 displays good cycleability in aqueous

1.0 M H2SO4 electrolyte and delivers an initial specific capacitance value of 325

Fg-1 and 260 Fg-1 after 104 cycles (80% capacitance retention) for a loading of 50

wt. % hydrous RuO2 [118].

The conductance of conducting polymers was very low at dedoped state,

which results in high ohmic polarization of supercapacitor, which would reduce

the reversibility and stability of supercapacitor. In order to solve the problem, Xu

et al. prepared a PANI/neutral red/TiO2 composite electrode material by

chemical oxidation polymerization. The electrochemical tests like cyclic

voltammetry, galvanostatic charge–discharge, impedance and cycle life

measurements demonstrate that the PANI/PNR/TiO2 composite is a promising

material in the application of supercapacitors [119].

Wang et al reported Mesoporous MnO2 / PANI composite with unique

morphology of hierarchical hollow submicron spheres was synthesized

successfully by interfacial synthesis. This composite illustrate the enhanced

electrochemical properties due to its unique hollow microstructure with well

defined mesoporosity and the coexistence of conducting PANI [120].

(PANI/ MnWO4) nanocomposite was successfully synthesized by

Saranya et al. by in situ polymerization method under ultrasonication and the

MnWO4 was prepared by surfactant assisted ultrasonication method. The


Chapter I Page|42

in situ polymerized PANI / MnWO4 composite shows higher specific

capacitance (475 Fg-1) than the physical mixture (346 Fg-1) of PANI and

MnWO4 at a constant discharge current of 1 mAcm-2. Because of the

interaction of PANI and MnWO4 shows the low resistance than the

physical mixture due to the low resistance [121].

Recently ternary composites of carbon materials, conducting polymers

and MnO2 have been investigated to utilize their advantages and overcome their

disadvantages in supercapacitors. Ternary composites fabricated from poly (4-

styrenesulfonic acid) (PSS) dispersed MWCNT, conducting polymers and MnO2

with MnO2 embedding in conducting polymer matrices have shown interesting

electrochemical properties and stability for supercapacitor applications. Li et al.

fabricated Multi-walled carbon nanotube (MWCNT)/ (PANI)/MnO2 (MPM)

ternary coaxial structures are as supercapacitor electrodes via a simple wet

chemical method. The electrostatic interaction between negative poly (4-

styrenesulfonic acid) (PSS) molecules and positive Mn2+ ions facilitates the

formation of MnO2 nanostructures on MWCNTs. They also studied the effect of

PANI thickness on the subsequent MnO2 nanoflakes attachment onto MWCNTs

and the MPM structures suggests that the interaction between PSS and Mn2+ ions

is necessary to build MPM ternary coaxial nanostructures. They suggest that

MWCNT/PANI/MnO2 ternary coaxial composites provide large interaction area

between the MnO2 nanoflakes and electrolyte to improve the electrochemical

utilization of the hydrous MnO2 as well as decrease the contact resistance

between MnO2 and PANI layer coated MWCNTs, leading to interesting

electrochemical properties for the applications in supercapacitors [122].

1.10 Purpose Of Dissertation

Among the family of conducting polymers, the PANI has

attracted much attention because of its ease of synthesis, reasonably high

conductivity, chemical stability in the conducting state, good environmental

stability, unique doping/dedoping behavior and mechanical flexibility. These

encouraging characteristics of PANI provides a wide range of applications and


Chapter I Page|43

have attracted great interest in energy storage devices, chemical sensor devices,

light emitting diodes, electrochromic devices, anticorrosion coatings etc.

PANI is one of the most promising material which is frequently

used as electrode material for supercapacitor due to its four oxidation states

(Leucoemeraldine, Emeraldine base, Emeraldine salt and Pernigraniline)

which contribute to its high specific capacitance. The excellent electrode

required for the supercapacitor consists of good electronic conductivity,

electrochemical stability and high surface area. The electronic conductivity of

green protonated emeraldine form of PANI is lower than that of metals.

So, one of the challenging issues in development of high performance

supercapacitor is to improve its electronic conductivity of the PANI

electrode which is reversibly controlled both by the charge transfer doping

and by protonation. Extensive research work has been focused on enhancing

electronic conduction of the electrodes by using metal doping. On the other

hand, the main drawback of using PANI as supercapacitor electrode is mainly

connected with their poor stability during cycling. To overcome the stability

problem of PANI upon extended cycling their composite with carbon materials

like CNTs, activated carbons etc. have been widely reported which also

enhances the surface area of the electrodes.

In the present investigation, the main endeavor is to synthesize PANI

nanostructured films and to improve its electrical conductivity and surface area,

which enhances its electrochemical performance. In this direction the efforts will

be made to deposit metal ions (Mn, Ag) doped PANI films and activated carbon/

PANI films by using simple chemical route.

1.11 Plan of work

The aim of this work is to enhance the electronic conductivity and

surface area of PANI electrodes and hence the specific capacitance. One of the

major factors affecting the specific capacitance is the film thickness. Hence,

initially the PANI electrodes with different thickness have been synthesized by

using chemical polymerization on to the stainless steel substrate and optimized

thickness in terms of specific capacitance has been determined. This optimized


Chapter I Page|44

thickness was used for fabricating further electrodes. In order to develop with

improved performance electrodes the Mn doped PANI, Ag doped PANI,

Activated carbon/PANI (AC/PANI) and Ag doped AC/PANI (Ag-AC/PANI)

electrodes were synthesized. In addition, the doping concentration of Mn and Ag

were varied to determine their effects on the magnitude of specific capacitance

of PANI electrode.


Chapter I Page|45

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