INTRODUCTION 1.1. POLYMER ELECTROLYTES P

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2 CHAPTER 1 INTRODUCTION 1.1. POLYMER ELECTROLYTES P olymer electrolytes are being remarkably emphasized as chemical science which provides the polymers with new functionalities. Consequently, a multidisciplinary research emerged to further rationalize the process and bring about innovative materials necessary in key roles like ionic conductor, mechanical separator, flexible and electronic insulator. Hence, polymer electrolytes are mainly used in electrochemical devices like batteries, electrochromic devices, solar cells and supercapacitor. Polymer electrolytes are potential materials for solving the never ending demand for high energy density in energy devices. Polymer electrolytes are defined as linear macromolecular chains bearing a large number of charged or chargeable groups when dissolved in a suitable solvent. Polymer molecules having one or a few ionic groups, in most cases terminal and anionic, are called macroions. These are primarily living polymers wherein polymer molecules present in a polymerizing reaction system grows as long as monomers (e.g., esters or nitriles of methacrylic acid) are continuously supplied. The ionic charge of the macroion gets transferred to the next monomer added, and this process continues by keeping the macroion charged for the addition of further monomers. Polymers having considerable number of ionic groups and a relatively nonpolar backbone are known as ionomers, and those polymers with many number of ionic groups and that can dissolve in water are known as polyelectrolytes. Polymers with a much higher number of ionic groups get cross linked or undergo three-dimensional polymerization which constitutes the technically important group of ion exchangers. An important unresolved area in the field of polymer electrolytes concerns the role of the anion in ionic conductivity and the degree of ion present in some systems. It can be shown that the net cation mobility, in contrast to anion, is negligible. To act as a successful polymer host, a polymer or the active part of a copolymer should generally have a minimum of three essential characteristics: 2

3 Atoms or groups of atoms with sufficient electron donating power to form coordinate bonds with cations. Low barriers to bond rotation so that segmental motion of the polymer chain can take place readily. A suitable distance between coordinating centers, because the formation of multiple intrapolymer ion bonds appears to be important. Wright s report, which talks about semi-crystalline structure complexes between PEO and salt, led to a spurt in the field of research to improve the conductivity of polymer electrolyte worldwide [1]. The correlation between amorphous phase and ion conductivity led to curious attempts to study the electrical properties of this polymer electrolyte [2]. The first proposal of Armand [3] shows the use of solid polymer electrolyte in lithium batteries. With modernization, the demand for light, highly performing, portable and cost effective energy devices also increases, which leads to an increased interest in the study of polymer electrolyte, because it plays a vital role in fulfilling these needs. Polymer electrolytes must exhibit the ionic conductivity in range 10-3 to 10-2 S cm -1 at room temperature. Electrochemical stability is an important parameter, but the instability brings about irreversible reactions leading to the gradual fading away of capacitance [4]. Even mechanical and thermal stability during charge/discharge cycles are crucial for long durability of energy devices. The growth of this polymer electrolyte field over a diverse range of innovative modifications based on applications as gone through three main stages: solid polymer electrolyte systems, blend polymer electrolyte system and gel polymer electrolyte systems. These systems progressively satisfy the requirements for use in fuel cells, supercapacitors, secondary batteries, sensors, dye sensitized solar cells and microelectronic devices. Therefore, polymer electrolyte stands as one of the keystones in electronic storage and conversion area in the current century. 3

4 SOLID POLYMER ELECTROLYTES Solid polymer electrolyte is liquid free high molecular weight polar polymer host having ionically conducting phase formed by dissolving salts. Many polymer electrolyte materials will exhibit to a greater or lesser extent the following properties. Adequate conductivity for practical purposes. Good mechanical properties. Chemical, electrochemical and photochemical stability. No possibility of leakage Ease of processing. Shape flexibility Lowering the cell weight non-volatile, all-solid-state cells do not need heavy steel casing For the success of these properties, low interfacial resistances, high ionic transport number and excellent conductivity is required. This result strong tendency of polymer to crystallize, and crystalline phases are characterized by much lower conductivities than the amorphous polymers. In general, the polyelectrolyte solutions contain a single species of polymer and one species of counter ions only. Eventually, the solution may also contain a single species of low-molar-mass electrolytes (to be called the doped salt, but it may be a strong acid or a base) having common counter ions with the polyelectrolyte unless otherwise specified and assumed not to interact chemically with the polyelectrolyte. Polymer dissolution Solutions of polyelectrolytes exhibit a behavior that may differ considerably from that of uncharged macromolecules of low-molar-mass electrolytes. The origin of this specificity lies in the combination of properties derived from those of long-chain molecules with properties that result from charge interactions. This combination is not a simple superposition, as there is a mutual influence of the characteristics of both types of properties. Thus, the presence of charges on the chain leads to intra-and inter macromolecular interactions, which may be stronger and of much longer range than in the case of uncharged macromolecules. This may have a strong influence on both the thermodynamic and dynamic properties of polyelectrolyte solutions, particularly if in the 4

5 solution, the electrostatic potential arising from the charges fixed on the macromolecular chains is not sufficiently screened. In the dissolved state, dissolved macromolecules tend to coil up, but the macroions of polyelectrolytes stretch out, owing to repulsive forces between their charged groups, and violent mechanical action may actually break their chains. The accumulation of like charges closely spaced along the macromolecular chain leads to a certain rigidity of the macroion, owing to the repulsive forces. This causes the solution to have a higher viscosity than expected for a soluble uncharged macromolecule and gives rise to some other specific rheological features (shear flow involving chain stretching). The strength of the electrostatic interactions may be moderated by increasing the concentration of added salt, which results in a screening effect by the small ions, but high concentrations of salt may also affect the solvent quality. These interactions strongly affect not only the average dimensions of the polyelectrolyte chain, but also the intra macromolecular dynamics of the chains. The screening effects are primarily the result of the interactions between the bound and mobile charges (i.e., between the charged chains and the small ions in solution). These interactions are not completely comparable to the charge-charge interactions occurring in solutions of low-molar-mass electrolytes. This occurs from the divergent behavior of the bound and mobile charges, even in dilute polyelectrolyte solutions. The former are considerably restricted in their motions by the chain on which they are fixed. Local charge fluctuations arising from charges bound to same chains will therefore be highly correlated and hence leading to a fundamental asymmetry between the bound and mobile charges. On the molecular level, charges bound to a same chain will have to cluster to some extent around each other in all possible configurations, in contrasts to the mobile charges, which, in principle, can move independently through the entire volume of the solution. Around the chainbound charge clusters the electrostatic potential will be much higher than elsewhere in the system, but this potential may fluctuate in accordance with the conformational fluctuations of the macromolecular chain. Movements of ions in SPE After dissolution of salt in the polymer solution, the polymer must solvate the ions by overcoming the lattice energy of the ionic salt and thereby forms a complex [5]. The three main criteria in formation of complex are 5

6 (a) Electron pair donicity (DN) (b) Acceptor number (AN) and (c) Entropy term. The efficiency of polymer to solvate the ions are given by the term DN which follows Lewis acid base concept. Therefore, the polymer to act as a host polymer electrolyte it should have donor sites such as oxygen, sulfur or nitrogen either in the backbone or in a group attached in the form of a side chain to the polymer. Correspondingly, the solvation of the anion is described the AN term which is Lewis base. The DN number must be greater than AN. Since PEO has higher DN than cations, it can effectively solvate cations possessing counter anions that are bulky delocalized anions such as I, ClO 4, BF 4 or CF 3 SO 3. Entropy term describes the spatial movement of polymer in the solvating unit. In PEO containing ethylene oxy (CH 2 CH 2 O) have the most favorable spatial orientation of the solvating units. Smaller ions such as Li + are easily solvated and form polymer salt complexes. Whilst, in order to get solvated by poly(ethylene phthalte) (PEP) by larger cations such as Na +, K + etc., there is necessity of bulky counter anions such as I, SCN, or CF 3 SO 3. Along with the above factors the polymer must lack extensive intermolecular hydrogen bonding because it will affect the solvation ability of the polymer. The other factor is that it should withstand high torsion by keeping itself flexible. This can be achieved by using polymer having low glass transition temperature T g. Low T g favors large segmental motion of the polymer chain and thus increase the ionic conductivity. Ion transport relies on local relaxation processes in the polymer chains which may provide liquid like degrees of freedom, giving the polymer properties similar to those of a molecular liquid. The macroscopic properties that are similar to those of a solid are the result of chain entanglements and possibly crosslinking. Ion transport in polymer electrolytes is considered to take place by a combination of ion motions between ion coordinating sites. Ratner et al [6] have developed a dynamic percolation model for description of ion transport in polymer electrolytes. This macroscopic model that characterizers the ionic motion in terms of jumps between neighboring positions. For anions, which are not strongly solvated by polymer host and for cations, the local coordination environment evolves slowly, as a single M + --B (M + = metal ion, B = Lewis base site on polymer) linkage is changed at a time. The jump of 6

7 a cation then corresponds to a completed exchange of one ligand. This process is sketched in Figure 1. Figure 1a shows the motion of ions coupled to that of the polymer chain; lateral displacement brought about by 180 o bond rotation at C-O bond. Figure 1b presents the first step in the transfer of a cation between chains. Anions may also be involved as part of either as ion pair or an ion triplet. Figure 1: Cation transport mechanism in a polyethylene oxide based polymer electrolyte. As the salt concentration is increased, the competing process becomes relatively faster than the segmental motion/site percolation mechanism. A consistent mechanism for cation transport at high concentration might involve transition between ion clusters [7] such as M X M X M Activated cation jumps of this nature would not only be confined to such small associated clusters as triples and ion pairs and would also involve cation-polymer bond exchange (a) M X M (b) X M Proton Conduction Mechanism Proton transfer phenomena in polymer electrolyte follow two principal mechanisms one is chemical mechanism (Vehicle type) and another is Grotthus mechanism [8]. In media which supports strong hydrogen bonding, the Grotthus mechanism is preferred, the vehicle mechanism is characteristic of species with weaker bonding. Consequently, Grotthuss type mechanisms are progressively dominated by vehicle-type mechanisms with increasing temperature. In proton doped polymer electrolyte, the proton remains shielded by electron density along its entire diffusion path, so that in this effect the momentary existence of a free proton is not seen. The most trivial case of proton migration requires the translational dynamics of bigger species, this is the chemical mechanism. In this mechanism the proton diffuses through the medium together with a vehicle (for example, with H 2 O as H 3 O + ). The counter diffusion of unprotonated vehicles (H 2 O) allows the net transport 7

8 of protons. The observed conductivity, therefore, is directly dependant on the rate of vehicle diffusion. In another principal mechanism, the vehicles show pronounced local dynamics, but reside on their sites. The protons are transferred from one vehicle to the other by hydrogen bonds (protons hopping). Simultaneous reorganization of the proton environment leads in the formation of an uninterrupted path for proton migration. This mechanism is known as the Grotthus mechanism. This reorganization usually involves the reorientation of solvent dipoles (for example H 2 O), which is an inherent part of establishing the proton diffusion pathway. The rates of proton transfer Г trans and reorganization of its environment Г rep affect directly this mechanism. All rates directly connected to the diffusion of protons (Г D, Г trans, Г rec ). Figure 2: Proton transportation by Grotthus- type mechanism. These two principal mechanisms essentially reflect the difference in nature of the hydrogen bonds formed between the protonated species and their environment. Dependence of cation mobility on the relative molar mass of the polymer host Cations which have a low solvent exchange rate will not exhibit significant long range motion in high molar mass polymer based electrolytes, and will be unable contribute to direct current in practical cells. For lower molecular mass systems, mechanisms involving polymer chain diffusion become important. For these dissociative steps involving ion-solvent bond scission is not required. Simple linear polymers of relatively low molar mass have chain diffusion coefficients which are inversely proportional to their molar masses. The Rouse-Zimm model, originally proposed to describe the viscoelastic properties of dilute solutions of coiling polymers, is also found to describe successfully the motion of such molecules in the pure liquid state, correctly predicting the molecular weight dependence of the self-diffusion coefficient [7]. Above a critical value of the molar mass, chain entanglement becomes significant and a new model of polymer dynamics must be considered. At short times, a high molecular weight amorph- 8

9 ous linear polymer behaves as a rubber since the knots formed by two polymer strands do not have time to become untied. At longer times Brownian motion results in local disentanglement allowing the chains to slide past one another. A literature overview on SPE Solid polymer electrolyte has been constantly studied in the field of energy device with different dopants and mode of preparations. Rodriguez et al. [9] have prepared SPE films consisting of mixtures of poly(vinypyrrolidone) and LiClO 4 with various mass ratios using dip-coating method. A conductivity of S cm -1 at 60 C was obtained and reported that residual amount of solvent is important in preserving the ionic conductivity. They demonstrated that these films are used as transparent SPE in supercapacitors. Ramya et al. [10] focused the study on the proton-conducting polymer electrolytes; poly(n-vinyl pyrrolidone) ammoniumthiocyanate and poly(n-vinyl pyrrolidone) ammonium acetate prepared using solution casting technique. The room temperature ionic conductivity was found to be high as S cm 1 for 80 mol% PVP 20 mol% NH 4 SCN and S cm 1 for 75 mol% PVP 25 mol% CH 3 COONH 4. The Raman analysis confirms the interaction of the ammonium proton of NH 4 SCN and CH 3 COONH 4 with the carbonyl group of the PVP. The presence of broad band also indicated amorphous nature of SPE. Shukla et al. [11] reported for the first time, optimization of salt (LiClO 4 ) concentration on structural, morphological, electrical, and ion polymer interaction in PMMA-based solid polymer films. They suggested that the active coordination site for the cation (Li + ) was C=O, out of the two possible electron donating functional groups (i.e. C=O and O CH 3 ) in PMMA. An optimum ionic conductivity of ~ S cm 1 has been recorded at 100 C (~PMMA glass transition). The temperature dependence of conductivity follows typical Vogel Tamman Fulcher behaviour. Ramesh et al. [12] reported a series of different composition of polymer electrolytes based on poly(vinyl chloride) (PVC) as host polymer, lithium tetraborate (Li 2 B 4 O 7 ) as dopant salt, and dibutyl phthalate (DBP) as plasticizer. The highest ionic conductivity of S cm 1 was achieved upon addition of 30 wt% of DBP and obeyed Arrhenius behaviour. Frequency dependence-conductivity in the study revealed that conductivity increased with frequency and temperature. 9

10 Wee et al. [13] emphasized on the effect of the ionic conductivity in the PSS:H film containing supercapacitors at different levels of relative humidity (RH) using impedance spectroscopy, cyclic voltammetry and galvanostatic charge-discharge techniques. High capacitance values (85 F g 1 at 80% RH) are obtained for these supercapacitors due to the extremely high effective electrode area of the CNTs and the enhanced ionic conductivity of the PSS:H film at increasing RH level. They concluded that the combined advantages of the high ionic conductivity of electrolytes and the mechanical properties of polymers render polyelectrolytes an attractive candidate for printable solid-state supercapacitors. Dissnayake et al. [14] successfully synthesized nano-sized silica from local rice husk ash and prepared the nanocomposite solid polymer electrolyte, PEO 9 LiTf:SiO 2. They suggested with help of other literature that the O 2 and OH surface groups in the filler surface interact with the Li + ions and provide hopping sites for migrating Li + ions through transient hydrogen bonding, creating additional high-conducting pathways. This would contribute to a substantial conductivity enhancement through increased ionic mobility. Silva et al. [15] have studied the elastomers constituted of the homopolymer of epichlorohydrin, its copolymer with ethylene oxide and its terpolymer with ethylene oxide and allyl-glycidyl-ether as polymeric electrolytes in polymer/liclo 4 systems. Chitapalli et al. [16] have reported an infrared spectroscopic study on the effect of plasticizers such as ethylene carbonate (EC) and propylene carbonate (PC) on the PEO- LiCF 3 SO 3 system POLYMER BLEND ELECTROLYTES Polymer blend electrolytes are physical mixture of two or more polymer chains to form a homogeneous (liquid) solvent-free system though sometimes the various phases are chemically bonded together, wherein ionically conducting phase is formed by dissolving inorganic salts. Hydrogen bonding, charge transfer interactions, dipoledipole forces forms the basis for miscibility of polymer blends. Manifestation of properties of polymer blends depends upon the miscibility of the components and structure. Blended mixtures may offer distinct properties, one set of properties related to one member of the blend, and another set of properties related to the second member of the blend. The property mixing of polymeric blends is dependent on a number of fac- 10

11 tors, one of the major being the miscibility of the polymers in one another. This miscibility is in turn dependent on the nature of the polymers composing the blend and the amount of each component in the blend. Here, polymer blends will be divided into miscible and immiscible polymer blends [17]. Preparation of polymer blends Solution mixing laboratory, paint industry but the problem is removing of a solvent. Interpenetrating networks crosslinked materials. Melt mixing (physical, reactive) most important method in industry. When two different polymers are dissolved effectively in a common solvent, there is fast formation of thermodynamic equilibrium among the polymers in the molecular level [18]. The difficulty with this procedure is due to the fact that many polymers become incompatible above a certain concentration when their solutions in a common solvent are combined. This means that the originally homogeneous solutions of polymers A and B separate into two phases when being combined, whereby each of the phases contain different quantitative proportions A:B [e.g., polystyrene and poly(vinyl acetate) in toluene]. But, even when two polymers have been dissolved in a common solvent, composition and phase morphology of the solid polymer blends obtained by precipitation or evaporation of the solvent depend strongly on the work-up method. Evaporation, which means slow increase of polymer concentration up to the solid state, can lead to inhomogeneities because the macromolecules have time to separate according to their molecular size and chemical composition. During fast precipitating processes this is far less the case, for example, by addition of a precipitating agent during spray precipitation or by rapid evaporation. Miscibility of polymers In the preparation of blend polymer the most important characteristic must be known is the phase behaviour in order to interpret the mechanism of ions. The miscible blend polymer electrolytes could provide a homogeneous pathway for the conduction of ion rather than specified to particular polymer in immiscible blend system. Molecular weight is one of the lead factors in bringing about miscibility. Mostly in low molecular weight polymers the combinatorial entropy contribution is higher than high molecular 11

12 weight polymers. This criterion makes solvent-solvent mixtures provide broader range of miscibility than polymer-solvent combinations. Furthermore, in polymer-polymer mixtures the range of miscible combinations is even much smaller. Viscosity method is useful to study the miscibility of the polymer blends. Intrinsic viscosity of polymer solutions depends on the nature of polymer, solvent and temperature [19]. It also depends on the polymer-solvent interaction. A literature overview on polymer blend electrolyte Wieczorek and Stevens [20] studied blends consisting of a polyether, PMMA, and LiCF 3 SO 3. The electrolytes showed a maximum room temperature conductivity of S cm -1. They also prepared blends PVC and PMMA polymers using PC as the plasticizer and LiCF 3 SO 3 as the salt. Due to insolubility of PVC in the solvent PC, phase separation was observed. The inclusion of PVC into PMMA helped to increase the mechanical stability of the gel, but unfortunately decreased the lithium ion conductivity. The ions were found to preferentially move towards the plasticizer- rich phase or the PMMA-rich phase. Dimensionally stable blends consisting of PEG-PAN-PC-EC-LiClO 4 were prepared by Munichandraiah et al. [21]. Compared with gels PEO-PC-LiClO 4 and PAN- PC-EC- LiClO 4, the PEG containing gels showed lower room temperature conductivities, but higher mechanical stabilities. In the blend mixture consisting of poly(pphenylene terephthalamide) (PPTA), polyethylene glycol (PEG), polycarbonate in PC- EC, and a lithium salt, conductivities as high as S cm -1 at room temperature were observed at 0.8 M LiBF 4 salt per mole of PPTA content. Above 1 M of the salt content, the conductivity was found to fall rapidly suggesting the ion conductivity to be due to LiBF 4 interaction at the amide bond sites of the PPTA. Rajendran and Ramesh [22] prepared blend polymer electrolytes comprising EC, PC, diethylene carbonate (DEC) and g-butyrolactone (GBL) complexed with PVC(5) PEMA(20) LiClO 4 (8) wt% systems and the maximum conductivity value S cm -1 at 28 C was observed. This conductivity obtained depends on the specific nature of the plasticizer, including viscosity, dielectric constant, polymer plasticizer interaction and ion plasticizer interaction. 12

13 Sengwa et al. [23] prepared blend polymer electrolyte films consisting of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) with lithium triflate (LiCF 3 SO 3 ) as a dopant ionic salt and poly(ethylene glycol) (PEG) as plasticizer. Their results reveals that besides the amorphicity, the ionic conductivity of these electrolytes is also governed by the relaxation time, The dielectric strength, and the transport of ions is due to hopping mechanism which is coupled with segmental motion of polymers chain. Room temperature ionic conductivity values of the PEO PMMA blend based electrolytes are found about one to two orders of magnitude higher than that of the PEO and PMMA based electrolytes. Blend of poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) as the polymer host with LiCF 3 SO 3 as a Li + cation salt and TiO 2 nanoparticle which acts as a filler were prepared by Ali et al. [24]. Optimum ambient temperature conductivity achieved was Scm 1. They suggested that complex formation has taken place in the amorphous phase and TiO 2 nanoparticle filler increases the ionic conductivity of blend composite by impeding the reorganization of crystalline phase of PEO/PEG chains. Shukur et al. [25] prepared a proton conducting ethylene carbonate plasticized chitosan poly(ethyleneoxide) (PEO) doped with ammonium nitrate (NH 4 NO 3 ) electrolyte films. The conductivity was S cm -1. The capacitance and capacity values for the EDLC at 140 th cycle are 134 mf g -1 and ma hg -1, respectively. Arof et al. [26] based on natural polymer prepared chitosan/iota (i)-carrageenan blended film doped with ortho-phosphoric acid (H 3 PO 4 ) as ionic dopant and PEG as plasticizer. The ionic conductivity value was S cm 1. The conductivity temperature relationship was Arrhenious, and the activation energy for the highest conducting sample was 0.09 ev. FTIR results indicate that H + is the conducting species as 2 peaks due to HPO 4 and H 2 PO 4. Buraidah and Arof [27] suggested that the existence of strong hydrogen bonding between the hydroxyl groups in chitosan and the hydroxyl groups in PVA as a result of blending provides good mechanical properties. The highest conducting sample 55 wt.% (chitosan PVA) 45 wt.% NH 4 I exhibited conductivity of S cm 1. The chitosan:pva ratio is 1:1. This is higher than the conductivity for the unblended electrolyte 55 wt.% chitosan 45 wt.% NH 4 I which was S cm 1. Hence, the blending in 13

14 this system helped to increase the conductivity although there exist an interaction between the blended polymers GEL POLYMER ELECTROLYTES Gel polymer electrolyte is often known as plasticized polymer electrolyte which is neither liquid nor solid, or conversely both liquid and solid. Gel contains a solid skeleton of polymers or long chain molecules cross-linked intra-molecularly or intermolecularly entrapping a uninterrupted liquid phase. The chemical composition and other factors such as hydrogen bonding vary the chemistry of gels from viscous fluid to moderately rigid solids. Nevertheless, they are very soft and stretchy or jelly like. Due to large number of liquids are filled in microspores most gel materials exhibit liquid like characteristics microscopically and macroscopically have solid character. The presence of the ultraporous structure in the gel system is likely to provide channels for ion migration. The cross-linking occurs either physically or chemically depending upon the conditions and functional groups present on the polymer chains. As the polymer networks are solvated by a large amount of the trapped solvent so gels generally possess high ionic mobility. HC C O O C O - Ca 2+ HC O - C H 2 CH N Cu 2+ N CH CH 2 CH C O - Ca 2+ O O - C CH O C H 2 CH N Cu 2+ N CH 2 CH Coulomb s force Hydrogen bond Coordination bond 14

15 Formation of helix Hydrophobic bond Covalent bond Figure 3. Different types of crosslinkage in gel polymer Conductivity The gel polymer electrolytes are prepared by heating a mixture containing the appropriate amounts of the polymer, solvents, and a lithium salt to about C. This range of temperature is above the glass transition temperature of the polymer in order to form viscous clear liquids. Gel films are made in hot condition by solution casting and allowed the solution to cool slowly under pressure of electrodes. Commonly used plasticizers are less-evaporating solvents for gel polymer electrolytes, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl formamide (DMF), diethyl phthalate (DEP), di-ethyl carbonate (DEC), methylethyl carbonate (MEC), dimethyl carbonate (DMC), g-butyrolactone (GBL), glycolsulfite (GS) and alkyl phthalates. The solvents have been used separately or as mixtures. Gels are able to retain up to 80% of solvent trapped in the polymer matrix. The use of high permittivity solvents allows a greater dissociation of the lithium salt and increases the mobility of the cation. In polymer gel electrolytes the salt generally provides free/mobile ions which take part in the conduction process and the solvent helps in solvating the salt and acts as a conducting medium. Whilst, the polymer is reported to induce mechanical stability by increasing the viscosity of electrolyte. Dissociation of Li salt and migration of Li + in gel polymer matrix takes place by means of complex formation between polar group in a polymer chain and Li +. A plasticized electrolyte, which is essentially a gel electrolyte but is unusually associated with the addition of small amounts of a high dielectric constant solvent to a conducting polymer electrolyte to enhance its conductivity. This kind of property gives rise to the possibility of synthesizing good ion conducting gel mate- 15

16 rials. The donor number of the polymer repeat units vs. that of solvent determines the interaction between solvent-cation or polymer-cation. Figure 4a, shows the interaction of high DN solvent like propylene carbonate with ions rather than lesser DN polymer like PVDF, herein, solvent helps the ions move through the polymer chain. In Figure 4b, the higher DN of polymer like PEO causes direct interaction with the ions. (a) DN solvent > DN polymer (b) DN polymer > DN solvent Figure 4: (a) Direct interaction between solvent-cation (b) direct interaction between polymer-cation A literature overview on GPE Meng et al. [28] demonstrated a novel kind of ultrathin all-solid-state supercapacitor configuration with an extremely simple process using two slightly separated polyaniline-based electrodes well solidified in the H 2 SO 4 -poly(vinyl alcohol) gel electrolyte. The thickness of the entire device is much comparable to that of a piece of commercial standard A4 print paper. Under its highly flexible (twisting) state, the integrate device showed a high specific capacitance of 350 F g -1 for the electrode materials, well cycle stability after 1000 cycles and a leakage current of as small as 17.2 μa. Furthermore, due to its polymer-based component structure, it showed a specific capacitance as high as 31 F g -1 for device, which is more than 6 times that of current highlevel commercial. Choudhury et al. [29] wrote a comprehensive review on hydrogel electrolyte based EDLCs which is promising and environmentally friendly. The specific capaci- 16

17 tance value was 150 F g -1 for crosslinked potassium poly(acryate) (PAAK) KOH H 2 O based EDLC with activated carbon fibre cloth electrodes, This value was the highest amongst alkali salt used. A pseudocapacitor having neutral hydrogel (PAAK-KCl-H 2 O) and MnO 2. nh 2 O as electrode showed higher specific capacitance values of 168 F g -1. Furthermore, acidic pristine PVA hydrogel and RuO x. xh 2 O/C electrode containing pseudocapacitor exhibited specific capacitance value as high as 1000 F g -1. They suggested that the water retention property is better in natural-based hydrogel electrolytes such as cross-linked gelatin hydrogel electrolytes which are highly hydrophilicin nature than synthetic hydrogel electrolytes containing PVA. Senthilkumar et al. [30] synthesised a hydroquinone mediated PVA H 2 SO 4 gel electrolyte phenyl hydroquinne (PHHQ) and activated carbon from bio-waste were prepared for supercapacitor fabrication. PHHQ delivered a higher capacitance (941 F g -1 at 1 ma cm -2 ) and energy density (20 Wh kg -1 at 0.33 W g -1 ) than the PVA H 2 SO 4 gel electrolyte (425 F g -1 at1 ma cm -2, 9Wh kg -1 at 0.33 W g -1 ). Incorporation of hydroquinone brought about redox reaction in the system, thereby causing to increase in capacitance. Kumar et al. [31] showed that the addition of polymethylmethacrylate to liquid electrolytes containing trifluoromethanesulfonic acid (HCF 3 SO 3 ) in PC result in an increase in conductivity of gel electrolytes. The maximum ionic conductivity was S cm -1 at 25 C. The gel electrolytes were found to be stable over a temperature range of 50 to 100 C. Ma et al. [32] utilized a novel PVA KOH P-phenylenediamine and demonstrated excellent electrochemical performance with large potential window of 2.0 V. They exhibited high ionic conductivity of 25 ms cm 1, high electrode specific capacitance of 611 F g 1 at a current density of 0.5 A g 1, reaching an energy density as high as W h kg 1 at a power density of kw kg 1 and good cycling stability. Hashmi et al. [33] reported the effect of the dispersion of zinc oxide (ZnO) nanoparticles in the zinc ion conducting host polymer poly(vinylidine fluoride-cohexafluoropropylene) PVdF-HFP gel polymer electrolyte. The nanocomposites gave ionic conductivity values greater than 10 3 S cm 1 with good thermal and electrochemical stabilities. The variation of ionic conductivity with temperature followed the Vogel Tamman Fulcher behavior. ZnO:Zn 2+ species form space charge regions which induce 17

18 local electric field. This local field would be responsible for enhancement in Zn 2+ ion conduction in this GPE. Various polymers such as poly(propylene oxide), poly(ethylene imine), thioalkane, polystyrene, poly(vinyl alcohol), poly(vinylidenefluoride), poly(vinylidene carbonate), poly(acrylonitrile), poly(vinyl chloride), poly(vinyl sulfone), poly(p-phenylene tereththalamide), and poly(vinylpyrolidone), have been reported to form GPEs with conductivities ranging between 10-4 and 10-3 S cm -1 under ambient conditions [34-41] ELECTROCHEMICAL CAPACITOR (SUPERCAPACITOR) Electrical energy can be stored in two fundamentally different ways: (i) indirectly, in batteries as potentially available chemical energy requiring faradaic oxidation and reduction of the electro-active reagents to release charges that can perform electrical work when they flow between two electrodes having different electrode potentials, and (ii) directly, in an electrostatic way as negative and positive electric charges on the plates of a capacitor by a process termed as non-faradic electrical energy storage. Electrochemical cells are devices that convert chemical energy into electrical energy and vice versa. Due to the redox reaction in the electrode-electrolyte interface this electrical energy can be obtained. The electrochemical cells mainly consist an negative electrode, an positive electrode having active mass and an ion-conducting electrolyte. In response to the changing global landscape, energy has become a primary focus of the major world powers and scientific community. There has been great interest in developing and refining more efficient energy storage devices. One such device, the capacitor, has matured significantly over the last three decades and emerged with the potential to facilitate major advances in energy storage. Figure 5 represents the energy density and power density distribution curves of electrochemical capacitor with comparison to other energy devices. This plot is called as Ragone plot. 18

19 Figure 5. Ragone plot showing performance of different energy devices (Source: Nature publishing group) [42] Brief evolution of capacitor to supercapacitor Capacitor stores energy at the electrode/electrolyte interface due to formation electrochemical double layer and are currently called by several names under trade name and capability of capacitors. The names such as supercapacitor, ultracapacitor or electrochemical double layer capacitor are often used; since there is an additional contribution to the capacitance other than double layer affects it is called electrochemical capacitor. In this thesis the technology is referred to supercapacitor but based on the mechanism of charge storage sometimes the term electrochemical double layer capacitor (EDLC) is used. First generation capacitors from condensers Early capacitors were also known as condensers, a term that is still occasionally used today. The term was first used for this purpose by Alessandro Volta in 1782, with reference to the device ability to store a higher density of electric charge than a normal isolated conductor. 19

20 Figure 6. Schematic representation of energy stored in electrostatic capacitor In a capacitor, energy is stored via electrostatic field generated due to the removal of charge carriers, typically electrons, from one metal plate and depositing them on another. Hence, it is called as electrostatic capacitor. Potential is developed between two plates because of this charge separation and hence energy can be harnessed from the capacitor. This energy stored will be proportional to both the number of charges stored and the potential between the plates. Furthermore, these number of charges stored is essentially a function of size and the material properties of the plates, while the potential between the plates depends upon dielectric breakdown. Different materials sandwiched between the plates to separate them result in different voltages to be stored. Optimizing the material leads to higher energy densities for any given size of capacitor. Electrostatic capacitors, shown schematically in Figure 6, consist of a pair of conductors separated with a dielectric such as air, mica, polymer film, ceramic, etc. These capacitors operate at gigahertz (GHz) frequencies with charge times of ~10 9 s. Second generation electrolytic capacitors Electrolytic capacitors are made up of tantalum, ceramic and aluminium either containing solid or liquid electrolyte separated by a separator. The construction of the cell is similar to that of batteries. Third generation electrochemical double layer capacitors Capacitor evolved as electric/electrochemical double layer capacitor (EDLC) in the third generation. The main focus was on electrical charge at electrode-electrolyte interface in the order reach ~10 6 F. In this generation, organic and aqueous electrolytes 20

21 were used in the construction of the cell and activated carbon was majorly exploited as electrode material. The power density and low energy density output from these EDLCs were complementary to batteries. Being a modified capacitor EDLC has longer cycle life and power density than batteries. Even possess higher energy density as compared to conventional capacitors. Thus, due to this hybrid concept between battery and conventional capacitor EDLC made its use in engine start or acceleration for hybrid vehicles, backup power sources for electronic devices, load-levelling, engine start and electricity storage generated from solar or wind energy. The practical use of EDLCs began in 1957 by General Electric and patented for an electrolytic capacitor consisting of four porous carbon electrodes [43]. Later, many companies successfully commercialized supercapacitors having capacity from 7 to 4000 F and voltage from 2 to 6 V. Various flexible supercapacitors too have been prepared with suitable energy density. Flexible supercapacitors have great potential for applications in wearable, miniaturized, portable, large-scale transparent and flexible consumer electronics. Flexible electrodes are primarily based on carbon material since carbon networks such as carbon fabric, carbon film, carbon paper, carbon textile can be used for electrochemical performances. The electrolyte component of flexible supercapacitor is either liquid electrolyte or polymer electrolyte. Liquid electrolyte varies from aqueous to organic solutions. Mixture of organic solution and salts are widely used. Polymer electrolyte generally mixture gelling agent, solute and solvent has great potential in future transparent, flexible energy storage devices due to its safety and with no need of a separator [44-46]. Principle and applications. The principle of EDLC works on development of double-layer capacitance at the electrode/electrolyte interface. Specific capacitance can be defined by various ways from double layer capacitors, such as normalization to unit surface area, volume or mass of the device. However, since the focus of this perspective is materials, capacitance is normalized to unit mass of active material, which simplifies comparisons between materials. It is important to emphasize that since supercapacitor voltage decreases linearly with state of charge, not all of the stored energy can generally be used. The energy density and power density depends upon the equivalent series resistance (ESR) 21

22 which is comprised of the electrode resistance, electrolyte resistance and resistance due to the diffusion of ions in the electrode porosity. A simple EDLC is shown in the Figure 7 in which two carbons rods are dipped in the salt water. The dissociated ions are dispersed well in the solution when there is no current applied to the cell. When the current is applied between the rod current starts flowing and charge accumulates on the electrode/electrolyte interface. An excess or a deficit of electric charges is accumulated on the electrode surfaces and equal counter balancing of opposite charge accumulates on the electrolyte end. When switch is opened the voltage remains due to stored energy. The energy is from the double layer capacitor developed in series. These two parallel regions of charge form the source of the term inner Helmholtz double layer. The thickness of the double layer depends on the concentration of the electrolyte and on the size of the ion which varies from few angstroms and hence the surface area is measured in thousands of square meters per gram (m 2 g -1 ) of electrode material. Figure 7. Electrochemical double layer capacitor showing persisting of double layer at the interface after switch is opened. As described by Helmholtz from this inner double layer the double layer capacitance C (F cm -2 ) is calculated using equation: C S o r (1) D 22

23 where ε 0 is the electric constant ( F m -1 ), ε r the relative dielectric constant of the interface (whether liquid or solid), S is the specific surface area of the electrodes (m 2 g -1 ), and D(m) is the separation of the electrode plates. Furthermore, this capacitance model was later refined by Gouy and Chapman, and Stern and Geary, who suggested the presence of a diffuse layer in the electrolyte due to the accumulation of ions close to the electrode surface as described in the Figure 8. The Stern model showing the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP). The IHP refers to specifically adsorbed ions and OHP refers to nonspecifically adsorbed ions. The diffuse layer begins from OHP layers towards the bulk of the EDLC. Figure. 8. Stern model showing inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP), the potential drop at the electrode/electrolyte interface and diffuse layer. Using a series of EDLCs high capacitances with desired voltage can be achieved. This is due to use of high surface area porous electrodes makes the EDLCs slower than conventional capacitors. Figure 9 shows electrical double-layer capacitance coming from electrode material particles, such as at the interface between the carbon particles and electrolyte. 23

24 Figure 9. Principles of a single-cell double-layer capacitor The effectiveness of increased capacitance due to nano-porous carbon is represented in Figure10. In this high surface area represented in cylinder the electrolyte fills and formation of electric double layer on the interior wall surface of the pore occurs which gives to capacitor connected in series with each other and hence increasing the capacitance of the cell [47]. Figure 10. Cylindrical representation of nanopore in a carbon electrode of an electrochemical capacitor 24

25 The conductivity in carbon is greater than electrolyte, so the electrical signal passes through the inner side of the cylinder faster than through the pores in the electrolyte. The charge stored on the pore mouth is accessible with small electrolyte resistance while charge stored within the pore takes longer time and path with high series resistance. This limits the EDLC up to 1 to 3 V compared to conventional capacitor The main feature of EDLC is that no charge transfer takes places across the electrode and electrolyte interface. This process is called as non-faradaic process. This implies that the concentration of the electrolyte remains constant during charge/discharge cycle. Mechanism Let the two electrode surfaces expressed as E S1 and E S2, an anion as A -, a cation as C +, and the electrode/electrolyte interface as //, the electrochemical processes for charging and discharging can be expressed as equation(2) (5). [48] On one electrode (positive): E 1 Charging S1 A ES // A e (2) Discharging ES1 // A e ES1 A (3) On the other electrode (negative): Charging ES2 C e ES2 // C (4) Discharging ES2 // C ES2 C e (5) And the overall charging and discharging process can be expressed as eqn (6) and (7): Charging ES1 ES2 C A ES1 // A ES2 //C e (6) Discharging ES1 // A ES2 // C ES1 ES2 C A (7) Activated carbon (AC) as electrode material Carbon is one of the most abundantly available and structurally diverse materials, and most present day EDLCs employ porous carbons as the active electrode material. Abundantly available organic materials such as coconut shells, charcoal, nut shell, wood, including food waste [49], are a particularly attractive natural resource for the 25

26 production of porous carbon materials commercially. Porosity of activated carbon plays a major role to increase capacitance with increased surface area. Nonetheless, the carbon structure including pore shape, surface functional groups, and ionic conductivity must be considered. Activation treatment is required for improving capacitance by increasing the surface area by opening pores that are closed, clogged, or obstructed [50]. The activation of porous carbons can be achieved by two ways; physical activation involves treatment at high temperatures from 600 to 1000 C using steam, or CO 2 or air. Chemical activation is carried out between 400 to 600 C with activating reagents like KOH, NaOH, H 3 PO 4 and ZnCl 2 [51]. ACs produced by activation processes have broad pore size distribution which are divided as micropores (<2 nm), mesopores (2-50 nm) and macropores (>50 nm). The theoretical EDL capacitance range from μf cm -2, but experimentally small EDL capacitance <10 μf cm -2 was obtained with a high surface area up to 3000 m 2 g -1 [52]. This shows that not all pores are effective in charge accumulation [53]. Graphene as electrode material Graphite is becoming an important raw material in many applications like energy devices, micro-sensors, super-adsorbents, semiconductors etc. For conversion of graphite to reduced graphene oxide (rgo) many methods have been used [54,55]. Few green methods have also been implemented like the one by Chen [56], without the use of polymer or surfactant. But, the use of ammonia in this method is a concern when considered in large amounts. Microwave treatment has also been used for exfoliation of graphite oxide to rgo [57]. The green method wherein ionic liquid assisted microwave reduction of GO gave a specific capacitance of 135 Fg -1 and was reported to be rapid and facile [58]. Using Gum Arabic and ultrasonification graphite was exfoliated, but again the concern is that to bring about 100% pure graphene, 100 hrs of ultrasonification and acid treatment was required [59]. Similarly, many attempts have been made to develop eco-friendly methods to prepare rgo [60-62], which is usually associated with complex processes for removal of reducing agents. Using sodium carbonate, rgo was efficiently reduced from GO, but it took 4hr for reduction [63]. Graphene has been used as electrode material in supercapacitors [64-66]. The graphene honeycomb lattice is composed of two equivalent sub-lattices of carbon atoms bonded together with σ bonds. A single atomic layer of graphene having high surface 26

27 area is idealistic electrode material, but removal of heteroatoms and functional groups completely will affect the capacitance [67]. Graphene is known to have a theoretical surface area of 2630 m 2 g -1 and Hantel et al. [68] claimed to have achieved 2687 m 2 g -1 from partially reduced graphene oxide. Fewer layers of graphene showed 1400 m 2 g - 1 surface area [67] and porous as prepared using microwave [69]. Highly porous 3D graphene produced from biomass showed good conductivity and specific capacitance of 231 Fg -1 [70].Graphene and rgo are slight different in structure, since rgo has more lattice defects and trace amount of functional groups when compared to graphene. The rgo/polymer binder having mesoporous rgo was reported for good ionic liquid accessibility and hence showed specific capacitance of 250 F g -1 [71]. Even macroporus low BET surface area significantly adsorbed oil better than micro or mesoporous graphene sheets. Nonetheless, compared to surface area of carbon nanofibers and activated carbon, rgo has a lower surface area. Redox based electrochemical double layer capacitor Redox based electrochemical double layer capacitor (EDLC) as also called as pseudocapacitor. In pseudocapacitor similar to batteries when a potential is applied a fast reversible reaction occur and transfers the charge across the electrode/electrolyte interface by overcoming the double layer barrier. This process is called Faradaic process. The materials such as conducting polymers and several metal oxides, including RuO 2, MnO 2, and Co 3 O 4 undergo redox reactions [48]. Depending upon the type of material used mainly three types of Faradaic processes occur; (a) reversible adsorption wherein is occurs due to adsorption of hydrogen on the surface of platinum or gold, (b) redox reactions of transition metal oxides (e.g. RuO 2 ), and (c) reversible electrochemical doping dedoping in conducting polymer based electrodes. The capacitances of pseudocapacitor are usually higher than EDLC since the electrochemical redox processes occur both on the surface and in the bulk near the surface of the solid electrode. Conway et al. [52] reported that the capacitance of a pseudocapacitor can be times higher than the electrostatic capacitance of an EDLC. Nevertheless the pseudocapacitor has relatively less power density because redox reactions occur at the electrode and this further increase the instability during cycling. 27