1

CHAPTER 1

INTRODUCTION

1.1 Layered solids

Layered solids consist of stacked array of two-dimensional layers having high aspect

ratio one above the other to form three-dimensional macromolecular structures. The

class of materials in which the bonding among the atoms of the same layer is much

stronger compared to the bonding among the atoms of adjacent layers are referred as

layered solids [1-3]. The anisotropy in chemical bonding results in interesting

properties such as intercalation chemistry, ion exchange and delamination to form

colloidal dispersions of monolayers in solvents [1-5]. Variety of solids such as smectite

clays, layered double hydroxides (LDHs), metal hydroxides, hydroxysalts, zirconium

phosphates, transition metal chalcogenides, layered perovskites, layered titanates,

graphite and graphite oxide - with varied compositions and properties, come under this

class of materials [1-5]. A schematic representation of a typical layered solid is shown

in Figure 1.1.

Figure 1.1 Schematic representation of a typical layered solid

1.1.1 Classification of layered solids

Based on the charge possessed by the layers, the layered solids can be classified

as neutral, cationic and anionic layered solids [1]. Graphite consisting of one atom

thick, two dimensional neutral carbon layers stacked together is a neutral layered solid.

Cationic layered solids like cationic clays or smectite clays, layered oxides like

perovskites, birnessite and graphite oxide consist of negatively charged layers with

2

positively charged ions in the interlayer. On the other hand anionic layered solids such

as -hydroxides, hydroxysalts and LDHs contain positively charged layers with anions

in the interlayer to restore the charge neutrality. Thus anionic clays have a structure

complementary to that of the cationic clays. In all these layered solids the layers are

held together by weak van der Waal‘s forces.

1.2 Layered metal hydroxides

Most of the divalent metal hydroxides and their derivatives crystallize in layered forms

with structure derived from that of the mineral brucite, Mg(OH)2 [3,6]. Divalent metal

-hydroxides, hydroxysalts and LDHs (hydrotalcite-like compounds) belonging to the

class of anionic clays are the most studied layered hydroxides [6-10].

1.2.1 -Hydroxides of nickel and cobalt

The hydroxides of nickel and cobalt exhibit polymorphism. The -form crystallizes in a

structure similar to mineral brucite with an interlayer spacing of 4.6 Å [6]. The -form

consists of hexagonal packing of hydroxyl ions with M2+

ions occupying alternate layers

of octahedral sites [6]. This results in the stacking of charge neutral hydroxide layers of

composition M(OH)2, held together by van der Waal‘s interaction.

Figure 1.2 Structures of and -hydroxides

Hydroxides of nickel and cobalt also exist in another hydrated form known as α-

hydroxide. It is similar to brucite structure but has an enhanced basal spacing. By the

partial protonation of the hydroxyl ions positively charged layers of the hydrated

hydroxide can be generated.

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+ H+

[MII(OH)2-x(H2O)x]x+xMII(OH)2

These layers intercalate anions to yield compounds of the type [MII(OH)2-x(H2O)x](A

n-

)x/n∙mH2O [11-13]. A variety of anions ranging from Cl-, NO3

-,CO3

2- to long chain alkyl

carboxylates can be incorporated and hence the basal spacings of -hydroxides are

much larger than 4.6 Å and they depend on the size and interlayer orientation of the

intercalated anion. Figure 1.2 compares the structures of - and α-hydroxides.

1.2.2 Layered double hydroxides (LDHs)

LDHs consist of stacked array of positively charged layers of mixed metal

hydroxides with charge balancing exchangeable anions in the interlayer. The most

common mineral of this class found in nature is hydrotalcite having the composition

Mg6Al2(OH)16CO3∙4H2O [14].

Figure 1.3 Structure of a typical layered double hydroxide

In general LDHs are represented by the formula MII

1-xMIII

x(OH)2(An-

)x/n∙mH2O,

where MII

= Mg, Co, Ni, Cu, Zn, Ca; MIII

= Al, Fe, Cr, Ga; An-

is anion such as CO32-

,

NO3-, SO4

2-, etc. and x = 0.25 – 0.33. LDHs derive their structure from mineral brucite,

Mg(OH)2, with Mg2+

ions octahedrally surrounded by 6 OH- ions and the octahedra

share edges to form infinite sheets [6-10,14]. If a part, say x, of Mg2+

are

isomorphously substituted by other higher valent metal ions of similar radii [14], the

layers will acquire positive charge and anions are intercalated between the layers to

restore charge neutrality along with water molecules. These metal hydroxide sheets are

stacked one above the other forming three dimensional layered structures and the layers

are held together by weak van der Waal‘s forces [14]. The schematic structure of an

LDH is shown in Figure 1.3.

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1.3 Layered metal oxides

Layered metal oxides [15] like Cs0.7Ti1.825O4, KCa2Nb3O10, K0.45MnO2, K4Nb6O17,

RbTaO3, KTiNbO5and Cs6 + xW11O36 are all made of stacked negatively charged slabs

built from corner- and/or edge-shared MO6 (M = Ti, Nb, Mn, Ta, W) octahedral units,

and alkali metal cations (K +

, Rb+, Cs

+) occupying the interlayer space. A common

feature of these layered oxides is their cation-exchange property involving interlayer

alkali metal ions. Ion-exchange and intercalation properties facilitate the process for

chemically modifying the composition of the interlayer space at ambient temperature,

while retaining the host slab units. While many of these layered oxides are employed in

photocatalytic water splitting, some are capable of reversible redox reactions and hence

find application in energy storage.

1.3.1 Layered manganese oxide - Birnessite

Birnessite, a layered manganese oxide, has been studied as a potential supercapacitor

electrode material. [16] The heterovalent manganese cations (i.e. Mn3+

and Mn4+

)

renders the layers, composed of edge shared MnO6 octahedra, negative (Figure 1.4).

Exchangeable solvated cations accommodate the interlayer gallery thus neutralizing the

negatively charged layers. Birnessite is chemically expressed as AxMnO2·yH2O, where

A is H+ or a metal cation such as Na

+, K

+ or Ca

2+. Like the other layered solids,

birnessite can also be exfoliated into monolayers [17]. The exfoliated monolayers can

be assembled into functional composites [18-24].

Figure 1.4 Structure of birnessite

Following the pioneering work by Lee and Goodenough [25, 26] on the

pseudocapacitative behavior of manganese dioxide, there has been a lot of interest in

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fabricating MnO2 based composites as electrodes for supercapacitor applications.

Redox reaction between the III and IV oxidation state of Mn ions involved in the

pseudocapacitative (Faradic) reactions occurring on the surface and in the bulk of the

electrode are the major charge storage mechanisms for manganese oxides. Due to poor

conductivity and lack of structural stability, hydrated manganese oxide exhibits a low

specific capacitance of 100 – 200 Fg-1

.

1.4 Graphite oxide

Graphite oxide (GO) [27], a derivative of graphite [28], is a layered material consisting

of hydrophilic oxygenated graphene oxide sheets (Figure 1.5) bearing covalently

attached oxygen containing functional groups on their basal planes and edges [29]. It

was first synthesized in 1859 [27]. There are different methods available for the

synthesis of GO such as methods due to Brodie [27]; Staudenmaier [30]; Clauss,

Boehm and Hofmann [31]; and Hummers and Offeman [32]. Depending on the extent

of oxidation and the interlamellar water content the interlayer distance increases from

3.35 Å of the starting graphite to 7-10 Å. According to Lerf–Klinowski model the basal

planes in GO are decorated mostly with epoxide and hydroxyl groups whereas carbonyl

and carboxyl groups are located presumably at the edges [33-35].

Figure 1.5 Comparison of structures of graphite and graphite oxide

These oxygen functionalities render the GO layers hydrophilic (lyophilic). Polar

molecules can readily intercalate into the interlayer galleries solvating the GO layers

resulting in complete delamination [29]. GO is an excellent host matrix for the

interlayer accommodation of long chain aliphatic compounds (like amines, ammonium

ions) [29,36], transition metal ions [37] and metal oxide nanoparticles [38,39],

hydrophilic molecules and polymers [40,41] to form intercalated GO nanocomposites.

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Delamination of graphite oxide occurs in dilute alkaline aqueous solution and the

interlayer modified GO can be delaminated in organic solvents [42, 43]. The two

dimensional sheets of GO obtained through delamination can be used as precursors for

the synthesis of new range of materials for various applications [51-56]. GO is also the

precursor to graphene sheets and graphene based nanocomposites which find

applications as high performance battery electrodes [54] sensors, catalysts, potential

hydrogen storage matrices, transistors [55] and resonators [56] due to the fundamental

electronic properties and large surface-to-volume ratio through de-oxidation either

thermally or by chemical reduction.

1.5 Properties of layered solids

Layered solids are employed in a variety of applications such as catalysis [8],

electrochemical energy storage devices [54], ion exchangers and sorbents [57, 58] due

to their interesting physical and chemical properties such as ion diffusion/mobility,

surface acidity or basicity, intercalation and delamination [1-5]. Some of the properties

of layered solids are discussed below.

1.5.1 Intercalation reactions in layered solids

The interlamellar space between the layers can accommodate ions/molecules [59-63].

The process of intercalation has been studied to be reversible and topotactic, that is,

during insertion or de-insertion of the guest species the structure of the layers remains

intact. The layers that are held by weak Van der Waal‘s forces can be separated

depending on the size of the ions/molecules. The process of intercalation/de-

intercalation can be carried out chemically or electrochemically. The electrochemical

process has been exploited in energy storage devices. For example, in rechargeable

lithium ion batteries, graphite or layered metal oxides have been used as electrodes

which undergo insertion/de-insertion of lithium ions thus contributing to the capacity

delivered by the cell [28, 54]. The inserted ions can be exchanged in the presence of

excess of other ions; this process is referred to as ion-exchange reactions.

1.5.2 Delamination/Exfoliation

Delamination is one of the most important processes that have been studied in

case of layered solids [5]. Delamination is a process where the layers are separated such

that each layer behaves as an independent particle. All classes of layered solids can be

delaminated to monolayers or few layers. Delamination can be carried out thermally or

7

chemically. Graphite consisting of neutral hydrophobic layers is delaminated thermally

by heating at high temperatures in inert atmosphere [28]. Chemically the layers can be

separated by modifying the interlayer region with hydrophilic or hydrophobic guest

species and soaking the modified layered solid in a suitable solvent of similar nature

[5,64,65]. Vigorous stirring or sonication is usually used to ascertain complete

delamination of layers to form a colloidal dispersion of monolayers. As the layers are

held by weak Van der Waal‘s forces the solvent molecules penetrate into the interlayer

gallery and cause separation of layers (Figure 1.6). The process of delamination is

controlled by the solvation of the interlayer region, the lattice energy, and the layer

charge density [5]. Higher charge density requires additional interlayer modification to

obtain stable dispersions.

Figure 1.6 Delamination of a layered solid

The stability of the colloid can be enhanced by proper choice of

intercalant/solvent combination. The colloidal dispersion of the delaminated layered

solid is thermodynamically unstable and layers may re-assemble to the parent layered

solid [5]. This re-assembly process has been explored to make nanocomposites. The re-

assembly process can be carried out in the presence of guest species like polymers,

biomolecules, nanoparticles, layers of another layered solid etc which results in layered

solid–polymer/nanoparticle composites. These nanocomposites have been studied to

exhibit properties different from that of the precursor layers or guest species. The

nanocomposites have been employed in a variety of applications such as fire

retardation, energy storage in batteries and supercapacitors, UV absorption, catalysis,

photocatalysis, sorption and water splitting reactions [15, 67-68].

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1.6 Energy Storage Devices based on layered solids

As explained in the previous sections, layered solids can be subjected to intercalation of

ions/molecules into the interlayer gallery or the layers themselves can undergo redox

reactions involving electron transfer. Due to these features, layered solids have been

employed as electrode materials in energy storage devices. 3D Graphite and lately 2D

graphene are used in Li ion battery, fuel cells and supercapacitors. Similarly layered

metal hydroxides are used in alkaline storage batteries and supercapacitors. Principles

and mechanism of energy storage and delivery in the case of alkaline storage batteries

and supercapacitors are briefly described in the following sections.

1.6.1 Alkaline secondary batteries

Nickel based alkaline secondary cells such as nickel–cadmium, nickel–iron, nickel–

hydrogen, nickel–metal hydride storage batteries with the positive nickel hydroxide

electrode has replaced the toxic lead acid battery in the fields of electronics, aerospace

and defense. The - and -polymorphs of nickel hydroxide exhibit interesting redox

intercalation chemistry. These hydroxides undergo reversible oxidative deintercalation

of protons to yield oxide-hydroxides, MO(OH) [69,70]. This redox behavior of nickel

hydroxides is exploited in their application as positive electrode materials in alkaline

secondary batteries.

The electrochemistry of nickel hydroxide suggests the existence of two redox

couples during electrochemical cycling, namely, the / and /γ couples. The Bode‘s

diagram [69, 70] given below shows the electrochemical reactions involved in these

couples.

The redox chemistry of -nickel hydroxide can be better understood using the reaction

scheme involving a ~ 2e- exchange as proposed by Kamath and co-workers [71].

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-nickel hydroxide comes closest to the ideal electrode material as it exhibits complete

reversibility of steps (a) and (b) with a close to 2e- exchange during electrochemical

cycling. When the reversibility is restricted to step (a) alone, the / couple results,

while the complete reversibility of both steps (a) to (b) is characteristic of the /γ

couple. The salient features of each of these couples are listed below.

/ couple: This couple involves the cycling of the active material between the divalent

-nickel hydroxide and trivalent -nickel oxide hydroxide according to the equation

This couple involves a 1e- exchange and the theoretical reversible discharge capacity

delivered by the / couple corresponds to 456 mAh/g of Ni [72].

/γ couple: The /γ couple is superior to / couple for the reason that the oxidation

state of Ni in the γ-phase is 3.7. The redox reaction involving the /γ couple can be

represented as

This couple involves a 1.7e- exchange and the theoretical reversible discharge capacity

delivered by the /γ couple corresponds to 775 mAh/g of Ni. γ-phases from pure -

nickel hydroxide with nickel oxidation state of + 3.7 have been realized in thin films

[73].

Therefore it is ideal to use -nickel hydroxide as electrode material [74].

However, the α-form is a metastable phase and it rapidly ages to the β-form in the

alkaline medium of the battery [69]. There have been a lot of reports on the

stabilization of the α-γ couple to improve charge storage capacity in cells. Substitution

of a part of Ni in -nickel hydroxide with other metal ions like Al3+

, Zn2+

, Co2+

, Fe3+

and Mn3+

has been demonstrated as an effective means to stabilize the -phase [74-86].

These stabilized phases referred to as layered double hydroxides (LDHs) have a

structure similar to -hydroxides. Addition of cobalt oxide/hydroxide has also been

shown to improve the performance as it enhances the conductivity and prevents

conversion of the phase to phase [87-89].

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1.6.2 Supercapacitors

Supercapacitors or ultracapacitors or electrochemical supercapacitors (ES) are energy

storage devices which can be reversibly charged/discharged rapidly. They exhibit

higher specific energy compared to conventional capacitors and high specific power

compared to batteries makes them function in-between capacitors and batteries [90].

Two electrodes, electrolyte and a separator which electrically isolates the two

electrodes, constitute an ES device (Figure 1.7). The energy is stored in the

electrochemical double layer that is formed at the electrode/electrolyte interface. Based

on the mode of charge storage in ES they are classified into two types: electrochemical

double layer capacitors and redox capacitors or pseudocapacitors.

Figure 1.7 Schematic representation of electrical double layer capacitor in its charged

state [90].

1.6.2.1 Electrochemical double layer capacitors (EDLCs)

Nanoscopic charge separation at the electrochemical interface, between an electrode

and an electrolyte contributes to the energy stored or released in EDLCs [91, 92-94].

The energy storage is inherently rapid as it simply involves movement of ions to and

11

from the electrode surface compared to additional heterogeneous charge-transfer and

chemical phase changes in batteries that introduces relatively slow steps into the

process of energy storage and delivery. As a result, EDLCs exhibit a very high degree

of reversibility in repetitive charge–discharge cycling with demonstrated cycle life in

excess of 500,000 cycles [95].

Carbon in its various forms – activated carbon, carbon blacks, aerogels, fibers,

glassy carbon, carbon nanotubes and graphene, is currently the most extensively

studied and widely employed electrode material in EDLCs [96, 97]. Development in

EDLCs focuses on achieving high surface area, low resistive electrode materials.

Compared to conventional dielectric capacitors, EDLCs have high energy density as the

energy stored is inversely proportional to the thickness of the double layer. The double

layer capacitance, Cdl, at each electrode interface [90] is given by

Cdl= εA/4πt

ε= dielectric constant of the electrical double-layer region,

A= surface-area of the electrode

t= is the thickness of the electrical double layer

Hence a combination of factors like the high surface-area (typically >1500 m2g

−1) with

extremely small charge separation (few angstroms) accounts for the high capacitance

[98]. The energy (E) and power (Pmax) of supercapacitors [90] are calculated according

to

E = ½ CV2

Pmax= V2/4R

C = dc capacitance in Farads

V = nominal voltage

R = equivalent series resistance (ESR) in ohms

The characteristics of the electrode material – surface-area and the pore-size

distribution, largely define the capacity. Due to low density of carbons coupled with

high porosity, it is usually the volumetric capacitance of each electrode that determines

the energy density.

EDLCs are capable of storing large amount of charge that can be delivered at

much higher power ratings than rechargeable batteries. High power capability, long

life, a wide thermal operating range, low weight, flexible packaging, and low

12

maintenance are some of the advantages of EDLCs over more traditional energy

storage devices. EDLCs are used either by themselves as the primary power sources or

in combination with batteries or fuel cells in a wide range of energy capture and storage

applications like hybrid electric vehicles and short term power sources for mobile

electronic devices. Though the energy density of EDLCs is very high compared to

conventional dielectric capacitors, it is still significantly lower than batteries or fuel

cells. Coupling with batteries (or any another power source) is still required for

supplying energy for longer periods of time [90].

Carbon based electrical double layer capacitors (EDLCs) store energy due to

charge separation at the electrode/electrolyte interface. Due to low double layer

capacitance ( 40µFcm-2

) and incomplete utilization of high surface area, carbon

materials generally deliver lower capacitance compared to pseudocapacitors [90-94].

1.6.2.2 Pseudocapacitors

In case of pseudocapacitors, the application of voltage to the electrode material results

in faradaic current passing through the cell. The electrode undergoes fast and reversible

redox reactions (faradic reactions) and the passage of charges across the double layer,

constitutes the current [99]. This process is similar to charging/discharging process

involved in the case of batteries. Conducting polymers, metal oxides (RuO2, MnO2,

Co3O4, NiO) [91, 100-102] and layered hydroxides (Ni(OH)2, Co(OH)2 and Co-Al

layered double hydroxide) [103] are the most studied pseudocapacitor materials. Three

types of faradic process have been studied to contribute to the faradic current – (1)

reversible adsorption, for example, adsorption of hydrogen on the surface of platinum

or gold; (2) redox reactions of transition metal oxides and (3) reversible

electrochemical doping–dedoping in conductive polymer based electrodes.

The faradic electrochemical processes widens the working voltage thus

enhances the specific capacitance of the supercapacitors. The faradic electrochemical

processes, occurring both on the surface and in the bulk near the surface of the solid

electrode results in larger capacitance and energy density (10–100 times higher)

compared to EDLCs [99, 104]. As the faradaic processes are normally slower than

nonfaradaic processes, pseudocapacitors usually suffer from low power density

compared to EDLCs and lack stability during cycling similar to batteries.

13

Electrolytes used in supercapacitors: The specific energy of supercapacitors is

proportional to the square of the operating voltage. The specific energy and the power

of supercapacitors is also determined by the operating cell voltage which in turn is

dependent on the electrolyte stability. Aqueous electrolytes such as acids (e.g., H2SO4)

and alkalis (e.g., KOH) have the inherent disadvantage of a restricted voltage range

with a relatively low decomposition voltage of ∼1.23V though they have the advantage

of high ionic conductivity (up to ∼1 S cm−1

), low cost and wide acceptance [105]. Due

to the higher dielectric constant that pertains to aqueous systems, the observed specific

capacitance of high surface-area carbons in aqueous electrolytes tends to be

significantly higher than that of the same electrode in non-aqueous solutions that allow

the use of cell operating voltages above 2.5V [105-107]. Many commercial

supercapacitors for higher energy applications employ non-aqueous electrolyte

mixtures such as propylene carbonate or acetonitrile, containing dissolved quaternary

alkyl ammonium salts. However it should be noted that the electrical resistivity of non-

aqueous electrolytes is at least an order of magnitude higher than that of aqueous

electrolytes and therefore the capacitors employing non-aqueous electrolytes will have

a higher internal resistance.

Scope of present work

The recent trend in energy storage applications is designing hybrid materials or

composites of different materials as electrodes, in which the two different components

can work in synergy to deliver high capacity, stability and performance

[54,99,103,108]. For example, in supercapacitors, graphene based hybrids such as

graphene-Co3O4 [54] delivers much higher performance compared to only graphene or

Co3O4. The flexibility, electrical conductivity and high surface area of graphene not

only improve the electrical conductivity of Co3O4 but also anchor Co3O4 to graphene

layers which minimizes loss of electrochemical active surface area and thus both

contribute to the capacity. Similar principles are involved in designing hybrids for

various energy storage applications.

This thesis deals with synthesis of nickel hydroxide based layered composites

and graphene based composites as electrode material for energy storage devices.

Delamination of layered solids such as layered metal hydroxides, graphite oxide or

layered birnessite in water or in organic solvent, results in colloidal dispersion of the

layers. The layers from the colloidal dispersions are restacked in the presence of other

14

layers of interest to make composites. The resulting composites consist of structurally

similar or dissimilar layers stacked randomly together. These composites, an intimate

mixture of the two different layers usual exhibit properties different from the parent

individual solids. Hence these composites which possess advantages of two different

layers are studied as electrode material for rechargeable alkaline storage batteries and

supercapacitors.

First we established a protocol to delaminate nickel and cobalt based anionic

clays in water through intercalation of zwitterionic p-aminobenzoate. Using the

colloidal dispersions thus obtained we prepared interstratified composites of -

hydroxides of nickel and cobalt and studied their potential for pseudocapacitor

application. Then we went on to make binary and ternary composites comprising nickel

based LDHs and -hydroxides of nickel and cobalt for positive electrode of alkaline

batteries. Graphene based hybrid capacitor electrode materials were developed starting

from graphite oxide.

Plan of thesis

This thesis contains five chapters. Chapter 1 gives a brief introduction to the general

structure and classification of layered solids; and structures and properties of anionic

clays, layered birnessite and graphite oxide. The chapter also briefly describes the

principles and the mechanism involved in supercapacitors and rechargeable alkaline

storage batteries.

Chapter 2 consisting of two sections explains the exfoliation of zwitterion

intercalated -hydroxide of nickel and cobalt in water in section A and the use of the

exfoliated layers for making interstratified composites for supercapacitor application in

section B.

In chapter 3, we discuss the synthesis of binary and ternary composites of Ni-

Al/Ni-Zn layered double hydroxides with -hydroxides of nickel and cobalt through

exfoliation and costacking. The composites thus obtained are studied as positive

electrodes of rechargeable alkaline batteries.

Chapter 4 deals with graphene based materials as electrodes for supercapacitor

applications. It has two sections. Section A explains the thermal reduction of graphite

oxide at various temperatures to form chemically modified graphene and its

electrochemical behavior as a supercpacitor electrode material. In Section B we discuss

the delamination of alkylamine intercalated layered birnessite (MnOx) in 1-butanol and

15

subsequent costacking of the MnOx layers from the colloidal dispersion with colloidal

dispersions of alkylamine intercalated graphite oxide to obtain graphite oxide-birnessite

composite. Thermal decomposition of the composite, results in graphene-birnessite

layered hybrid which is studied as supercpacitor electrode material.

Chapter 5 summarizes the major conclusions that we have arrived at in this

work. We have also discussed the areas of failure and future prospects of this work.

References

[1] G. Alberti, U. Costantino, in Comprehensive Supramolecular Chemistry, Layered

Solids and their Intercalation Chemistry, ed. G. Alberti and T. Bein, Elsevier,

Oxford, Vol. 7, (1996) (and references therein)

[2] R. W. McCabe (1992) Inorganic materials (eds) D. W. Bruce and D. O‘Hare (New

York: John Wiley and Sons) p. 295.

[3] H. R. Ostwald, R. Asper, in ‗Physics and chemistry of materials with Layered

Structures‘, ed. R. M. A. Leith, D. Reidel, Dordrecht, Vol 1 (1977) 73.

[4] Jean Rouxel, in Comprehensive Supramolecular Chemistry, Layered metal

chalcogenides and their intercalation Compounds, ed. G. Alberti and T. Bein,

Elsevier, Oxford, Vol. 7 (1996).

[5] A. B. Jacobson, in ‗comprehensive Supramolecular Chemistry: Colloidal

dispersion of compounds with layer and chain structures‘; Alberti, G., Bein, T.,

Eds.; Elsevier: Oxford.Vol. 7 (1996) 315.

[6] J. G. Hooley, in Preparation and Crystal Growth of Materials with Layered

Structure, ed. R. M. A. Leith, D. Reidel, Dordrecht (1977).

[7] W. T. Reichle, Chemtech 16 (1986) 58.

[8] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173.

[9] F. Trifiro, A. Vaccari, in Comprehensive Supramolecular Chemistry, ed. J. L.

Atwood, D.D. MacNicol, J. E. D. Davis and F. Vogtle, Pergamon press, Oxford,

Vol. 7 (1996) Chapter 10.

[10] A. Vaccari, Chem. Ind. (Milan) 74 (1992) 174.

[11] P. V. Kamath, G. H. A. Therese, J. Gopalakrishnan, J. Solid State Chem. 128

16

(1997) 38.

[12] J. Ismail, M. F. Ahmed, P. V. Kamath, G. N. Subbanna, S. Uma and J.

Gopalakrishnan, J. Solid State Chem. 114 (1995) 550.

[13] M. Rajamathi, G. S. Thomas, P. V. Kamath, Proc. Indian Acad. Sci.,Chem.

Sci.,113 (2001) 671.

[14] F. Trifiro, A. Vaccari, in: G. Alberti, T. Bein (Eds.), Comprehensive

Supramolecular Chemistry: Hydrotalcite-Like Anionic Clays (Layered Double

Hydroxides), Elsevier, Oxford (1996) Vol. 7, p.251.

[15] R. Ma, T. Sasaki, Adv. Mater. 22 (2010) 5082.

[16] W. Wei, X. Cui, W. Chena, D. G. Ivey Chem. Soc. Rev. 40 (2011) 1697.

[17] J. E. Post, D. R. Veblen, Am. Mineral. 75 (1990) 477. (references therein)

[18] Z.–H. Liu, K. Ooi, H. Kanoh, W.-P. Tang, T. Tomida Langmuir 16 (2000) 4154.

[19] Y. Omomo, T. Sasaki, L. Z. Wang, M. Watanabe J. Am. Chem. Soc. 125 (2003)

3568.

[20] X. Yang, Y. Makita, Z.–H. Liu, K. Sakane, K. Ooi, Chem. Mater.16 (2004) 5581.

[21] N.Sakai, Y. Ebina, K. Takada, T. Sasaki J. Phys. Chem. B 109 (2005) 9651.

[22] Y. Cui, Z. –H. Liu, M. Wang, K. Ooi Chem. Lett. 35 (2006) 740.

[23] S. Suzuki, S. Takahashi, K. Sato, M. Miyayama Key Eng. Mater. 320 (2006) 223.

[24] T. Sasaki J. Ceram. Soc. Jpn. 115 (2007) 9.

[25] H. Y. Lee, J. B. Goodenough J. Solid State Chem. 144 (1999) 220.

[26] H. Y. Lee, V. Manivannan, J. B. Goodenough, C. R. Acad. Sci., Ser. IIc: Chim. 2

(1999) 565.

[27] Brodie, Philos. Trans., 149 (1859) 249.

[28] D. D. L. Chung, J. Mater. Sci. 37 (2002) 1475.

[29] R. C. Croft, Quartz. Rev. 14.1 (1960) 1.

[30] L. Staudenmaier, Ber. Dtsch. Chem. Ges. 31 (1898) 1481.

[31] A. Clauss, Boehm, Hofmann, Z. anorg. Chem., 291 (1957) 205.

[32] Hummers and Offeman, J. Am. Chem. Soc. 80 (1958) 1339.

[33] H. He, T. Riedl, A. Lerf, J. Klinowski, J. Phys. Chem. 100 (1996) 19954.

[34] A. Lerf, H. He, T. Riedl, M. Foster, J. Klinowski, J. Solid State Ionics, 101-103

(1997) 857.

[35] A. Lerf, H. He, M. Foster, J. Klinowski, J. Phys. Chem. B. 102 (1998) 4477.

17

[36] (a) I. Dekany, R. Kruger-Grasser, A. Weiss, Colloid Polymer Sci. 276 (1998) 570.

(b) A. B. Bourlinos, D. Gournis, D. Petridis, T. Szabo, A. Szeri, I. Dekany,

Langmuir 19 (2003) 6050.

[37] N. I. Kovtyukhova, G. A. Karpenko, A. A. Chuiko, J. Inorg. Chem. 37 (1992)

566.

[38] T .Cassagueau, J. H. Fendler, J. Phys. Chem. B 103 (1999) 1789.

[39] K. Morishige, T. Hamada, Langmuir 21 (2005) 277.

[40] P. Liu, K. Gong, P. Xiao, Carbon 37 (1999) 2073.

[41] O. C. Compton, SonBinh T. Nguyen, Small 6 (2010) 711.

[42] X. Yang, Y. Makita, Zh. Liu, K. Ooi, Chem. Mater. 15 (2003) 1228.

[43] C. Nethravathi, M. Rajamathi, Carbon, 44 (2006) 2635.

[44] S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J.

Zimmey, E. A. Stach, R. D. Piner, S. T. Nguyen, R. S. Ruoff, Nature, 442

(2006) 282.

[45] N. A. Kotov, I. Dekany, J. H. Fendler, Adv. Mater. 8 (1998) 637.

[46] T. Cassagneau, J. H. Fendler, Adv. Mater. 10 (1998) 877.

[47] N. I. Kovtyukhova, P. J. Olliver, B. J. Martin, T. E. Mallouk, S. A. Chizhik,

E. V. Buzaneva, A. D. Gorchinskiy, Chem. Mater. 11 (1999) 771.

[48] T. Szabo, A. Szeri, I. Dekany, Carbon, 43 (2005) 87.

[49] S. Stankovich, R. D. Piner, X. Chen, N. Wu, S. T. Nguyen, R. S. Ruoff, J.

Mater. Chem. 16 (2006) 155.

[50] N. A. Kotov, I. Dekany, J. H. Fendler, Adv. Mater. 8 (1998) 637.

[51] X. S. Du, M. Xiao, Y. Z. Meng, A. S. Hay, Synth. Met. 143 (2004) 129.

[52] P. Liu, K. Gong, P. Xiao, Carbon 37 (1999) 2073.

[53] C. Nethravathi. M. Rajamathi, Carbon 46 (2008) 1994.

[54] M. Liang and L. Zhi, J. Mater. Chem., 19 (2009) 5871.

[55] M. J. Allen, V. C. Tung, R. B. Kaner, Chem. Rev. 110 (2010) 132.

[56] D.M. Tanenbaum, J. M. Parpia, H. G. Craighead, P. L. McEuen, Science 315

(2007) 490.

[57] A. I. Khan, D. O‘ Hare, J. Mater. Chem. 12 (2002) 3191.

[58] R.L. Goswamee, P. Sengupta, K.G. Bhattacharyya, D.K. Dutta, Appl. Clay

Sci.,13 (1998) 21.

[59] D.W. Bruce, D. O‘Harer, ―Inorganic Materials,‖ Wiley, Chichester, 1997.

[60] A.M. Fogg, V.M. Green, H.G. Harvey, D. O'Hare, Adv. Mater., 11 (1999)1466.

18

[61] C. Schauffaul, J. Prakt. Chem., 21 (1841) 129.

[62] A.de Roy, C. Forano, E. El Malki, J. P. Besse, in: M. L. Ocelli, E. R. Robson

(Eds.), Anionic clays: Trends in pillaring Chemistry, Synthesis of microporous

materials. VanNostrand Rienhold, New York, (1992) Vol. 2, p.108.

[63] M. Figlarz, B. Gerand, A. Delahaye-Vidal, B. Dumont, F. Harb, A. Coucou,

F. Fievet, Solid State Ionics 43 (1990) 170.

[64] P. H. Nadeau, Applied Clay Science, 2 (1987) 83.

[65] M. M. J. Treacy, S. B. Rice, A. J. Jacobson, J. T. Lewandowski, Chem. Mater.

2 (1990) 279.

[66] T. E. Mallouk, J. A. Gavin, Acc. Chem. Rev., 31 (1998) 209.

[67] D. M. Kaschak, J. T. Lean, C. C. Waraksa, G. B. Saupe, H. Usami, T. E.

Mallouk, J. Am. Chem. Soc., 121 (1999) 3435.

[68] M. Osada, Y. Ebina, K. Takada, T. Sasaki, Adv. Mater., 18 (2006) 295.

[69] H. Bode, K. Dehmelt, J. Witte, Electrochim. Acta, 11 (1966) 1079.

[70] J. Chevanas, J. C. Joubert, J. J. Capponi, M. Marezio, J. Solid State Chem. 6

(1973) 1.

[71] J. Ismail, M. F. Ahmed, P. V. Kamath, J. Power Sources, 36 (1991) 507.

[72] M. S. Kim and K. B. Kim, J. Electrochem. Soc., 145 (1998) 507.

[73] D. A. Corrigan, S. L. Knight, J. Electrochem. Soc., 136 (1989) 613.

[74] C. Faure, C. Delmas, M. Fouassier, J. Power Sources, 35 (1991) 279.

[75] M. Dixit, R. S. Jayashree, P. V. Kamath, A. K. Shukla, V. G. Kumar, N.

Munichandraiah, J. Electrochem. Solid State Lett., 2 (1996) 239.

[76] P. V. Kamath, M. Dixit, L. India et al. J. Electrochem. Soc. 141 (1994) 2956.

[77] L. Indira, M. Dixit, P.V. Kamath, J. Power Sources 52 (1994) 93.

[78] P. V. Kamath, M. Dixit, L. Indira, et al, J. Electrochem. Soc. 141 (1994) 2956.

[79] K. T. Ehlsissen, A. Delahaye-Vidal, P. Genm, M. Figlarz and P. Willmann, J.

Mater. Chem, 3 (1993) 883.

[80] L. Demaurges-Guerlou, J. J. Braconmer and C. Delmas, J. Solid State Chem.

104 (1993) 359.

[81] P. Oliva, J. Leonardi , J. F. Laurent , C. Delmas, J.J .Braconnier, M. Figlarz,

F. Fievet, J. Power Sources, 8 (1982) 229.

[82] S. Le Bihan, J. Guenot, M. Figlarz, C. R. Acad. Sci., C 270 (1970) 2131.

[83] J. J. Braconnier, C. Delmas, C. Fouassier, M. Figlarz, Beaudouin, P.

Hagenmuller, Rev. Chim. Miner. 21 (1984) 496.

19

[84] L. Demaurges-Guerlou, J. J. Braconnier, C. Delmas, J. Solid State Chem., 104

(1993) 359.

[85] K. T. Ehisissen, A. Delhaye-Vidal, P. Genin, M. Figlarz, P.Willman, J. Mater.

Chem. 3 (1993) 883.

[86] X. M. Liu, Y. H. Zhang, X. G. Zhang, S. H. Fu, Electrochim. Acta 49 (2004)

3137.

[87] M. Oshitani, H. Yufu, K. Takashima, S. Tsuji, Y. Matsumaru, J.

Electrochem. Soc., 136 (1989) 1590.

[88] A. Yuan, S. Cheng, J. Zhang, C. Cao, J. Power Sources, 77 (1999) 178.

[89] C. Sigala, A. Verbaere, J. L. Mansot, D. Guyomard, Y. Piffard, and M.

Tournoux, J. Solid State Chem., 132, (1997) 372.

[90] A.G. Pandolfo, A.F. Hollenkamp, J. Power Sources 157 (2006) 11.

[91] B.E. Conway, Electrochemical Supercapacitors—Scientific Fundamentals and

Technological Applications, Kluwer, New York, (1999).

[92] S. Sarangapani, B.V. Tilak, C.-P. Chen, J. Electrochem. Soc. 143 (1996)

3791.

[93] A. Burke, J. Power Sources 91 (2000) 37.

[94] R. K¨otz, M. Carlen, Electrochim. Acta 45 (2000) 2483.

[95] X. Andrieu, Energy Storage Syst. Electron.: New Trends Electrochem.

Technol., 1 (2000) 521.

[96] A.S. Fialkov, Russ. J. Electrochem. 36 (2000) 389.

[97] M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Nano Lett. 8 (2008) 3498.

[98] A.K. Shukla, S. Sampath, K. Vijayamohanan, Curr. Sci. 79 (2000) 1656.

[99] G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797.

[100] M. S. Wu, P. C. Chiang, Electrochem. Solid-State Lett., 7 (2004) A123.

[101] W. Sugimoto, H. Iwata, Y. Murakami, Y. Takasu, J. Electrochem. Soc., 151

(2004) A1181.

[102] X. Dong, W. Shen, J. Gu, L. Xiong, Y. Zhu, H. Li, J. Shi, J. Phys. Chem. B,

110 (2006) 6015.

[103] Q. Wang, D. O‘Hare, Chem. Rev., 112 (2012) 4124. (and references therein)

[104] B. E. Conway, V. Birss, J. Wojtowicz, J. Power Sources, 66 (1997) 1.

[105] O. Haas, E.J. Cairns, Annu. Rep. Prog. Chem., Sect. C 95 (1999) 163.

[106] B.E. Conway, J. Solid State Electrochem. 7 (2003) 637.

[107] M.F. Rose, C. Johnson, T. Owens, B. Stephens, J. Power Sources 47 (1994)

20

303.

[108] L. Cao, F. Xu, Y-Y. Liang, Hu-Lin Li, Adv. Mater. 16 (2004) 1853.