Ionic liquid incorporated polymer electrolytes for supercapacitor application

Transcription

1 Indian Journal of Chemistry Vol. 49A, May-June 2010, pp Ionic liquid incorporated polymer electrolytes for supercapacitor application G P Pandey, Yogesh Kumar & S A Hashmi* Department of Physics & Astrophysics, University of Delhi, Delhi , India sahashmi@physics.du.ac.in Received 27 March 2010; accepted 1 April 2010 Recent advances in the study of ionic liquids based gel polymer electrolytes have been briefly reviewed in view of their electrochemical applications, particularly, their application as electrolytes in supercapacitors. The incorporation of ionic liquids in gel polymer electrolytes, instead of organic solvents like propylene carbonate, ethylene carbonate, etc., provide added effect in terms of their thermal, electrical and electrochemical stabilities. Recent studies on poly(ethylene oxide) based polymer electrolyte plasticized with ionic liquid and ionic liquid incorporated poly(vinylidene fluorideco-hexafluoropropylene) based gel polymer electrolytes have been summarized. A special description has been given of supercapacitors (electrical double layer capacitors), studied in our laboratory, based on multiwalled carbon nanotubes, activated charcoal powder electrodes and optimized gel/polymer electrolytes. Comparative studies indicate that PVdF-HFP based gels are superior electrolytes to develop electrical double layers capacitors (in terms of higher capacitances and lower resistive values) over PEO based plasticized polymer electrolytes. Keywords: Electrolytes, Polymer electrolytes, Ionic liquids, Electrochemical capacitors, Electrical double layer capacitors, Supercapacitors, Conductivity, Cyclic voltammetry Supercapacitors, also referred to as electrochemical capacitors, are energy storage devices that combine the high energy storage capability of batteries with the high power delivery capability of the capacitors 1,2. These capacitors are very attractive for a wide range of applications for power backup in hybrid electric vehicles (HEVs), portable electronics, medical electronics and military devices. However, a major obstacle to the application of supercapacitors is their limited performance, stability, operating electrochemical potential windows and short lifetimes, which are strongly determined by the properties of the electrolytes being used. The maximum energy (E max ) and power (P max ) of a capacitor are given by the expressions: E max = (CV 2 )/2 and P max = V 2 /(4R), respectively (where C is capacitance, V is cell voltage, and R is total equivalent series resistance (ESR) of the capacitor) 1,2. Limitations of the electrodes available presently and organic electrolytes being used need to be overcome to develop high performance supercapacitors with high energy and power densities, safe operation and long life. Currently employed electrolytes in commercial supercapacitors include aqueous and organic electrolytes 3. The narrow electrochemical windows of these electrolytes lead to low values of cell voltage (aqueous: ~1.2 V, organic: ~ 2-3 V), and hence, limited energy and power densities of the capacitors. When ions of the electrolyte are transported into the double-layers at the electrodeelectrolyte interfaces, there is a decrease of salt concentration in the electrolyte (called electrolyte depletion) which limits the energy density of the capacitor 4. Also, this electrolyte depletion increases the cell resistance and thus lowers the maximum power density achievable for the capacitor. As a result, the commercially available supercapacitors, fabricated from these electrolytes and activated carbon electrodes possess a low energy density (~4-5 Wh kg -1 ) and a low power density (~1-2 kw kg -1 ) 5. Furthermore, some organic electrolytes suffer from serious safety problems as they are inherently volatile, flammable and toxic, resulting in a narrow operational temperature range 6. Next generations of portable devices will require solid-state supercapacitors with high energy and power densities, and flexibility to meet various design and power needs. In recent past, polymer electrolytes have attracted considerable attention as a substitute of liquid electrolytes for their application in batteries/supercapacitors and other electrochemical devices due to their many advantages, particularly for the production of miniaturized structures, long-term safety, and the wide operating temperature range

2 744 INDIAN J CHEM, SEC A, MAY-JUNE 2010 of the devices. Poly(ethylene oxide) (PEO) based electrolytes are one of the most promising materials due to their excellent thermal and electrochemical properties. However, PEO-based polymer electrolytes are hindered by a low room-temperature ionic conductivity due to their predominant semi-crystalline nature. Gel polymer electrolytes, an important type of polymer based electrolytes, have recently found global interest due to their specific electrochemical properties, which make them suitable for various technological applications These materials are formed by immobilizing the salt solutions (for example LiClO 4 in propylene carbonate) in the host polymers such as poly(vinylidene fluorideco-hexafluoropropylene) (PVdF-HFP), polymethylmethacrylate (PMMA), polyacrlonitrile (PAN), etc The gel electrolytes have some important advantages over liquid electrolytes; the risk of leakage is reduced and formation of proper electrode-electrolyte contact. However, the main drawbacks associated with these materials are their poor dimensional stability, reduction in the electrical conductivity with time, interfacial instability towards electrode materials, which are deleterious in terms of poor reliability and safety of the devices. Since their first description in , ionic liquids (ILs) have been used in a wide range of applications Certain unique properties of these ILs, including high ionic conductivity (~ S cm -1 ), large liquid phase range ( C), wide electrochemical window (~4-6 V), non-volatility, non-flammability, and non-toxicity, have made them excellent environmental friendly electrolytes for various electrochemical systems 19. Since the maximum energy and power of an electrochemical capacitor is directly proportional to the square of its cell voltage, the use of ILs with large electrochemical window as electrolytes would significantly enhance their performance characteristics. The very high ionic concentration of ILs would be able to eliminate the electrolyte depletion problem also as encountered with conventional electrolytes, and therefore, the performance characteristics of the capacitors would be improved significantly. Further, the superior chemical and environmental stability of ILs ensures safe operation and a long life for the supercapacitors. Fuller and coworkers 20,21 have developed ionic liquid incorporated gel polymer electrolytes (ILGPEs) based on hydrophilic and hydrophobic imidazolium salts immobilized in host polymer PVdF-HFP, thereby opening up a new ILGPE field for their electrochemical applications. ILGPEs have the combined properties of both ILs and gel polymer electrolytes, possessing high ionic conductivity (~10-3 S cm -1 ) and excellent mechanical/dimensional stability. This paper gives a brief overview of the developments in the area of IL based gel polymer electrolytes (ILGPEs) recorded during the last one decade. Some experimental results on a few newly synthesized ionic liquid incorporated polymer electrolytes for their application in solid-state supercapacitors, recently investigated in our laboratory have been presented. EDLC cells based on gel polymer electrolytes have been characterized using different experimental techniques such as ac impedance spectroscopy and cyclic voltammetry. Ionic Liquid Polymer Electrolytes: An Overview Although the room temperature conductivity values of the gel polymer electrolytes are significantly high due to the use of organic liquid components (e.g. EC, PC, DEC, etc.), they always suffer from significant problems of flammability, volatility, and electrochemical and thermal instability 8. Moreover, it is believed that the relatively narrow electrochemical windows of these organic solvents are responsible for the electrochemical instability of the gel polymer electrolytes 8. Therefore, it is of the prime importance to develop gel polymer electrolytes that consist of non-volatile solvents with a wider potential window and better thermal and electrochemical properties. In recent years, room temperature ionic liquids (RTILs) have been proposed as nonvolatile polar media in a variety of electrochemical systems 17,18. The RTILs are room temperature molten salts typically consisting of bulky, asymmetric organic cations and inorganic anions and possess many attractive properties 17,18. More recently, ionic-liquidbased gel polymer electrolytes (replacing molecular plasticizer, e.g., EC, PC, etc., by ionic liquids) have been examined for their application as promising electrolytes in batteries 17,22, supercapacitors 17, and fuel cells 17,26. However, contribution of target ion to the overall ion transport is quite low in the ionic liquid-based gel polymer electrolytes, which limits the use of these materials, particularly in battery applications. However, in the case of supercapacitors it does not affect the performance of the devices.

3 PANDEY et al.: IONIC LIQUID BASED POLYMER ELECTROLYTES FOR SUPERCAPACITOR APPLICATION 745 As mentioned earlier, the very high ionic concentration of ILs would be able to eliminate the electrolyte depletion problem encountered with conventional electrolytes and therefore enhance the capacitor performances. Table 1 lists some important ionic liquid based polymer electrolytes and their conductivity values. As discussed earlier, PEO based electrolytes are one of the most promising materials due to their good thermal and electrochemical properties. However, these electrolytes are hindered by a low room temperature ionic conductivity. The addition of organic molecular solvents (e.g. EC, PC, etc.) leads to the increase in the ionic conductivity but the reactivity of such solvents in these plasticized electrolytes results in a poor interfacial stability. Additionally, the volatile nature of these solvents may cause a problem in the device, if short circuits create localized heating. In contrast, the addition of room temperature ILs to PEO based electrolytes may improve the ionic conductivity of the polymer electrolytes without the detrimental effects noted for molecular solvents. The selection of an adequate RTIL, however, is required for its use in the electrochemical devices. For example, RTILs based on the 1-ethyl-3- methylimidazolium (EMI) cation have many desirable properties such as a low viscosity, wider electrochemical stability window, and high ionic conductivity 45-47, but they have poor stability toward Li-metal 48,49. So EMI + cation based ILs are not adequate for Li metal battery application, although these can be useful for supercapacitor applications. Many IL-PEO based electrolytes were investigated by various research groups, as listed in Table 1. Watanabe and coworkers 50,51 investigated many IL-polymer electrolyte system based on certain kinds of onium salts, such as pyridinium and imidazolium salts. They found that compatible binary systems, where ILs are solidified (gelled) by polymers, can be used as ion-conducting polymer electrolytes (ion gels). The ion transport in such ion gels is found to be decoupled from the segmental motion of the polymers, leading to relatively high ionic conductivities even at their glass transition temperatures 51. The flexibility of the molecular design of ILs allows the preparation of many new ILs having electrochemical functionalities such as proton and lithium ion transport. The new ILs and their ion gels appear to be promising candidates as new materials for electrochemical applications. PEO or PVdF-HFP, and the ionic liquids 1-n-propyl-2,3-dimethylimidazolium tetrafluoroborate (MMPIBF 4 ) and 1-n-propyl-2,3-dimethylimidazolium hexafluorophosphate (MMPIPF 6 ), with and without Li salt, were used to prepare gel polymer electrolytes by Sutto 36. On the basis of various electrical and electrochemical studies, he concluded that both in terms of basic ionic conductivity as well as electrochemical behavior, the PVdF-HFP gels perform better than the PEO gels. PEO, because of its ability to interact with charged species via the Table 1 Some ionic liquid based polymer electrolytes IL based gel polymer electrolyte Conductivity (S cm -1 ) Temp ( C) Ref. PVdF-HFP EMITf/ EMIBF 4 ~ RT 20 PVdF-HFP EMIBETI LiBETI ~10-3 RT 27 PVdF-HFP DMOImTf LiCF 3 SO 3 ~10-4 RT 28 PVdF-HFP DMPI-Im-TFA ~ RT 29 PVdF-HFP-EMITf-Zn(Tf) 2 ~ PVdF-HFP-BMMITFSI-LiTFSI ~ RT 31 PVdF-HFP-EImTFSI-HTFSI ~ PVdF-HFP-PYR 13 TFSI-LiTFSI ~ 10-3 RT 33 PVdF-HFP-P 13 TFSI LiTFSI ~ RT 34 PVdF-HFP-PMP/DMP HTFSI PEO/PVdF-HFP-MMPIBF 4 / MMPIPF 6 LiBF 4 / LiPF PVdF-HFP-EImTFSI/MImTFSI/ MPyTFSI PVdF-HFP- PY 24 TFSI-LiTFSI- EC-PC ~ PEO- BMITFSI-LiTFSI ~ PYR 13 TFSI-P(EO) 20 LiTFSI ~ PEO-LiTFSI-PYR 1A TFSI > P(EO) 20 LiTFSI-BMPyTFSI ~ PVdF-HFP/EMITf-Mg(Tf) 2 ~ PVdF-HFP/EMITf-NaTf ~

4 746 INDIAN J CHEM, SEC A, MAY-JUNE 2010 oxygen atoms in the polymer backbone, limits the ion mobility in the gel. However, the nonpolar nature of the PVdF-HFP gels provides structural integrity, while at the same time allowing for the formation of high ionically conductive channels in which the lithium-containing ionic liquid is separated. According to these studies, it seems clear that the use of polar polymer systems is desired for the formation of solid polymer electrolytes. The formation of gel polymer electrolytes should utilize polymers, which limits the interaction of the Li + ion (or other charged species) with the polymer backbone and a hydrophobic type of plasticizer. Kim et al. 52,53 investigated ionic liquid based polymer gel electrolytes composed of morpholinium salt and PVdF-HFP copolymer. They have chosen morpholinium salt as a component for forming the gel electrolyte after some preliminary considerations, such as (i) simple synthesis and purification, and short reaction time, (ii) economical cost, (iii) wide electrochemical window, (iv) non-detectable vapor pressure, and, (v) high thermal stability up to ~400 C. In general, ionic conductivities of the gels increase with increasing temperature and weight ratio of IL. Also, the addition of PC into gel electrolyte led to an improvement in ionic conductivity. Although the gel electrolyte containing PC displayed the ionic conductivity of about 10-2 S cm -1 at 60 C, the vapor pressure of the solvents such as PC can lead to hazardous conditions. The key advantage of the gel electrolytes composed of only IL and polymer is their non-volatility. Ye et al. 34 reported that the ionic liquid, 1-methyl-3-propylpyrrolidinium bis(trifluoromethane-sulfonyl) imide (P 13 TFSI), is a promising solvent for the electrolyte of Li batteries because it possesses a wide electrochemical stability window and relatively low viscosity and is stable with respect to Li reduction reaction. Dimensionally stable, flexible, nonvolatile polymer gel electrolyte was synthesized by mixing LiTFSI/P 13 TFSI solution with PVDF-HFP copolymer and characterized for electrochemical device applications. PVdF-HFP/ 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [EMIM][Tf 2 N] based composite ILGPEs were investigated and used to fabricate supercapacitors with improved performance and cycle life by Lu et al. 54 A few PVdF-HFP/1-ethyl-3- methylimidazolium trifluoromethanesulfonate (EMITf) based gel polymer electrolyte and PEO-IL plasticized electrolytes have been investigated by us for their application in the solid state supercapacitors 43,44,55. Materials and Methods Preparation of electrodes Multiwalled carbon nanotube (MWCNT) powder having a specification of O.D. I.D. L = nm 2-6 nm µm and average wall thickness of 5 15 graphene layers was procured from Aldrich. The MWCNTs were kept in vacuum oven overnight at ~100 C before use. To prepare electrodes for supercapacitor (electric double layer capacitor, EDLC) application, the composition of MWCNTs, acetylene black, and poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) as a binder with a ratio of 70:10:20 (w/w) was mixed in mortar and pestle to form a slurry in acetone. The slurry was coated on the flexible high density graphite sheets (25 µm thick, FMI Composite Ltd.). The prepared electrodes were vacuum-dried overnight at ~100 C before fabricating EDLC cell assemblies. Activated charcoal (AC) electrodes for EDLC application were also prepared following the same method, as mentioned above. Preparation of IL based gel/plasticized polymer electrolytes The copolymer PVDF-HFP, ionic liquid EMITf, Mg-salt magnesium trifluoromethanesulfonate [Mg(Tf) 2 ], PC, and EC were obtained from Sigma Aldrich. The EMITf and Mg(Tf) 2 were vacuum dried at ~80 C overnight before use. A solution cast method was used to prepare the gel polymer electrolyte films. First, the liquid electrolyte was prepared by dissolving 0.3 M Mg(Tf) 2 in IL. The polymer PVDF-HFP was separately dissolved in acetone. The liquid electrolyte was then mixed with the PVDF-HFP/acetone solution and stirred magnetically for 4-5 h. The weight ratio of the liquid electrolyte:pvdf-hfp was kept at 80:20. The gel polymer electrolyte films with a composition of 0.3 M Mg(Tf) 2 in EMITf/PVDF-HFP/(EC:PC mixture) with a w/w ratio of 65:20:15 were also prepared. All the compositions were casted over glass petri dishes, and acetone was allowed to evaporate slowly. Finally, semi-transparent freestanding gel polymer electrolyte films (thickness ~ µm) were obtained. These films were stored in a desiccator for the characterization and device fabrication. The IL plasticized PEO based polymer electrolytes were also prepared by solution cast technique. PEO (~m. wt. ~ ) and lithium trifluoromethanesulfonate (LiTf) were obtained from Sigma-Aldrich, and used without any further purification. The

5 PANDEY et al.: IONIC LIQUID BASED POLYMER ELECTROLYTES FOR SUPERCAPACITOR APPLICATION 747 PEO was dissolved in a common solvent, acetonitrile, by stirring magnetically at ~25 C for about 24 h. Then an appropriate amount of LiTf was mixed in PEO to maintain the EO/Li + ratio ~25. The mixture was then stirred thoroughly for ~20 h and different amounts of IL, EMITf, were added during the stirring. Finally, the mixture was poured into teflon petri dishes and allowed to evaporate the common solvent slowly to obtain solid-like, free-standing, polymer electrolyte films of thickness ~ µm. Fabrication and characterization of EDLC cells Symmetrical cells of EDLCs were fabricated by sandwiching gel/plasticized polymer electrolyte films between two MWCNT or AC composite electrodes (of area ~1.5 cm 2 ) in specifically designed sample holders. The cells were laminated before their characterization. The performance characteristics of the EDLC cells were evaluated using ac impedance spectroscopy and cyclic voltammetry. The details of these techniques have been discussed by us elsewhere 2,25. Results and Discussion Characteristics of gel/plasticized polymer electrolytes The room temperature electrical conductivity (σ) of PEO 25.LiTf has been obtained to be ~ S cm -1, which is quite low from device application point of view. A sudden initial increase of almost one order of conductivity has been observed when ~5 wt.% of IL is added to the PEO-complex. Thereafter, a gradual but slow increase in conductivity has been obtained on further addition of IL up to a maximum conductivity of S cm -1 at room temperature for its 40 wt. %. Beyond this, the material is not dimensionally stable. This composition of PEO 25. LiTf + 40 wt. % IL has been chosen for the EDLC fabrication. Another set of EDLCs has been fabricated using the gel polymer electrolytes of the following compositions: ILGPE-1: 0.3 M Mg(Tf) 2 in EMITf/PVDF-HFP with a ratio of 80:20, w/w, ILGPE-2: 0.3 M Mg(Tf) 2 in EMITf/PVDF- HFP/(EC:PC mixture) with a ratio of 65:20:15, w/w. The detailed studies on the preparation and the physical/electrochemical characterization of the gel polymer films have been reported elsewhere 25,43. These films, which are obtained in the form of freestanding thick films, offer ionic conductivity of the order of 10-3 S cm -1 at room temperature with the excellent mechanical, thermal, and electrochemical stabilities 43. We have adopted another approach to improve upon the performance of gel electrolytes by substituting some proportion of the ionic liquid with the EC:PC mixture to form the electrolyte ILGPE-2. Such a modification has been reported earlier by a few researchers 34,38. Ye and coworkers 34 reported that adding small amounts of EC to the polymer gel electrolytes comprising lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in 1-methyl-3- propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (P 13 TFSI) ionic liquid immobilized with PVDF- HFP dramatically improves their ionic conductivity and net Li ion transport kinetics. Recently, an improvement in the ionic conductivity and in the stabilization of the interface with the lithium electrode has been reported due to the addition of a discrete amount of an EC:PC solvent mixture to the gel membranes comprising N-n-butyl- N-ethylpyrrolidinium N,N-bis (trifluoromethane) sulfonamide-lithium N,N-bis(trifluoromethanesulfonimide) (Py 24 TFSI-LiTFSI), ionic liquid solutions entrapped in a PVDF-HFP matrix 38. These studies motivated us to use ILGPE-2 as an electrolyte in the fabrication of EDLCs. The room temperature electrical conductivity of ILGPE-1 has been observed to be ~ S cm -1, while conductivity of ~ S cm -1 has been observed for ILGPE-2. This order of conductivity of the gels is acceptable from the EDLC application point of view. The addition of the EC:PC co-solvent in the ionic liquid solution may lead to a decrease in the viscosity of the liquid electrolyte, which is a possible reason for conductivity enhancement in ILGPE-2. The ILGPE-2 film has also been observed to be relatively flexible, which would help in getting the proper electrode and electrolyte contacts. The electrochemical stability (i.e. working voltage range) of electrolytes is an important parameter to be evaluated from their application point of view in electrochemical devices, such as supercapacitors, etc. The working voltage range for all polymer electrolytes was tested by CV using stainless steel electrodes. Figure 1 shows a typical cyclic voltammograms of IL based gel/polymer electrolytes. The electrochemical stability has been found to be in the range of -2.5 to 2.5 V, which is an acceptable working voltage range from the EDLC application point of view. It may be noted that the potential window (working voltage range) does not reduce on introducing the EC:PC mixture in the gel electrolyte system.

6 748 INDIAN J CHEM, SEC A, MAY-JUNE 2010 Fig. 2 Typical impedance plot of MWCNT electrode based capacitor cell-a recorded at room temperature in the frequency range from 100 khz to 10 mhz. Fig. 1 Cyclic voltammograms of (a) PEO 25 LiCF 3 SO wt. % IL, and, (b) IL based gel polymer electrolytes, ILGPE-1 and ILGPE-2, with stainless steel electrodes recorded at room temperature (20 C) at the scan rate of 10 mv s -1. Fabrication and performance studies of EDLCs The EDLC cells with the following configurations were constructed using different IL-based polymer electrolytes and MWCNT or AC composite electrodes: Cell A: MWCNT PEO 25 LiCF 3 SO wt. % IL MWCNT Cell B: MWCNT ILGPE-1 MWCNT Cell C: MWCNT ILGPE-2 MWCNT Cell D: AC ILGPE-1 AC Cell E: AC ILGPE-2 AC These capacitor cells have been characterized using impedance spectroscopy and cyclic voltammetric technique. Figures 2-4 show the typical impedance spectra of different capacitor cells (A-E). The MWCNT and AC-composite electrode materials show the capacitive behavior with the IL based gel/plasticized polymer electrolytes, reflected by the steep rising behavior of the impedance plots in the lower frequency range, which are close to the ideal capacitors. The ideal impedance behavior of a pure capacitor is a straight line parallel to the imaginary axis (Z ), whereas in practical capacitors, the steep rising capacitive impedance response is observed in the low frequency region accompanied with high frequency semicircular features, owing to the Fig. 3 Typical impedance plot of MWCNT electrode based capacitor cells recorded at room temperature in the frequency range from 100 khz to 10 mhz. [, cell B;, cell C]. bulk and interfacial properties of the capacitor cells. The impedance spectroscopic technique enables the evaluation of various parameters associated with bulk properties of electrolytes and electrode-electrolyte interfaces including charge-transfer resistances, potential-dependent faradaic resistances, low frequency capacitance values, etc., to be evaluated separately in different frequency ranges. The expanded part of the impedance response at higher frequencies (Figs 2-4 inset) reflects the bulk properties of the electrolytes and the charge transfer process at the electrode-electrolyte interfaces. The values of bulk

7 PANDEY et al.: IONIC LIQUID BASED POLYMER ELECTROLYTES FOR SUPERCAPACITOR APPLICATION 749 Fig. 4 Typical impedance plot of AC electrode based capacitor cells recorded at room temperature in the frequency range from 100 khz to 10 mhz. [, cell D;, cell E]. Table 2 Electrical parameters of EDLC cells from the impedance analysis calculated at 10 mhz EDLC cell R b (Ω cm 2 ) R ct (Ω cm 2 ) C (mf cm -2 ) C (F g -1 ) Cell-A Cell-B Cell-C Cell-D Cell-E resistance R b and interfacial charge-transfer resistance R ct of the different capacitor cells can be easily evaluated from the intercepts on the real axis of the impedance response of EDLC cells. These resistances along with the capacitance values evaluated at frequency 10 mhz are listed in Table 2. Although all the polymer based electrolytes used in the present studies offer capacitive behavior with MWCNT/AC electrodes, the comparative values of different parameters (R b, R ct and C) are quite different. Substantially lower value of specific capacitance and relatively higher value of resistive parameters are obtained for EDLC (cell-a) with PEO based electrolyte. The substantial improvement in these parameters has been observed for cells B-E, when gel polymer electrolytes ILGPE-1 and ILGPE-2 are used. As discussed above, it has been observed that inclusion of EC-PC leads to a slight increase in the conductivity of the IL based gel electrolyte. Simultaneously, the gel polymer films become highly flexible due to the presence of EC-PC mixture, which ensures excellent electrode-electrolyte contacts. As a result, a significant enhancement in the capacitance value and Fig. 5 Cyclic voltammograms of capacitor cell-a at different scan rates. Fig. 6 Cyclic voltammograms of capacitor cell-c at different scans rates. improvement in the other electrical properties of the capacitor cell values have been observed using the electrolyte like ILGPE-2 (Table 2). Cyclic voltammetry (CV) is another useful technique, generally employed in a three-electrode configuration, to characterize the liquid electrolyte based supercapacitors. The information about the nature of charge storage at the individual interfaces in the anodic and cathodic regions can be gathered by this technique. CV can also be performed in a two electrode configuration, particularly, in solidstate-like supercapacitors from which the overall behavior of the capacitor cells can be characterized. The CV, in the present case, can be performed in a two electrode configuration, particularly in solid-state

8 750 INDIAN J CHEM, SEC A, MAY-JUNE 2010 supercapacitors, from which the overall behavior of the capacitor cells can be characterized. Figures 5 and 6 show the typical cyclic voltammograms of capacitor cell-a and C respectively, in the potential range of -0.8 to 0.8 V at different scan rates. EDLC cells show an almost square shape of the CV having a near mirror image symmetry of the current response about the zero current line for different scan rates. This indicates the capacitive behavior of the cell-a and C with a double-layer formation at the interfaces. The rectangular shape of voltammograms persists even up to higher scan rates ( mv s -1 ). This indicates the fast switching behavior of the ions at the MWCNT electrode/polymer electrolyte interfacial contacts. Conclusions A brief overview of the IL based gel/polymer electrolytes has been presented. Some recent findings of the present laboratory on the ionic liquid incorporated polymer electrolytes based solid state electrical double layer capacitors (supercapacitors) are discussed. From the comparative studies, it has been concluded that PVdF-HFP based gel electrolytes perform better than the PEO based electrolytes. The nonpolar nature of the PVdF-HFP gels provides structural integrity, while at the same time allowing for the formation of high ionically conductive channels, offering their suitability as electrolytes in supercapacitors. Acknowledgement The authors acknowledge the financial support received from the Department of Science & Technology, New Delhi (SR/S2/CMP-45/2005) and University of Delhi, Delhi (11-17 research grant). GPP and YK are grateful to CSIR, New Delhi, for the award of research associateship and senior research fellowship, respectively. References 1 Conway B E, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, (Kluwer Academic/Plenum Publishers, New York) Hashmi S A, Natl Acad Sci Lett, 27 (2004) Duong T Q, Annual Progress Report for Energy Storage Research and Development, 2003, (FreedomCAR & Vehicle Technologies Program, Energy Storage Research and Development, US Department of Energy, USA), May Zheng J P, Huang J & Jow T R, J Electrochem Soc, 144 (1997) Simon P & Gogotsi Y, Nature Mat, 7 (2008) Xu K, Chem Rev, 104 (2004) Song J Y, Wang Y Y & Wan C C, J Power Sources, 77 (1999) Agrawal R C & Pandey G P, J Phys D: Appl Phys, 41 (2008) Ciuffa F, Croce F, D Epifanio A, Panero S & Scrosati B, J Power Sources, 127 (2004) Stephan A M, Eur Polym J, 42 (2006) Kim H S, Periasamy P & Moon S I, J Power Sources, 141 (2005) Vondrak J, Reiter J, Velicka J, Klapste B, Sedlarıkova M & Dvorak J, J Power Sources, 146 (2005) Akashi H, Shibuya M, Orui K, Shibamoto G & Sekai K, J Power Sources, 112 (2002) Quartarone E, Tomasi C, Mustarelli P, Appetecchi G B & Croce F, Electrochem Acta, 43 (1998) Walden P, Bull Acad Imper Sci (St Petersburg), (1914) Rogers R D & Seddon K R, Science, 302 (2003) Ohno H, Electrochemical Aspects of Ionic Liquids, (Wiley Interscience, New Jersey) Armand M, Endres F, MacFarlane D R, Ohno H & Scrosati B, Nature Mat, 8 (2009) Buzzeo M C, Evans R G & Compton R G, Chem Phys Chem, 5 (2004) Fuller J, Breda A C & Carlin R T, J Electrochem Soc, 144 (1997) L Fuller J, Breda A C & Carlin R T, J Electroanal Chem, 459 (1998) Egashira M, Todo H, Yoshimoto N & Morita M, J Power Sources, 178 (2008) Lewandowski A & Swiderska A, Solid State Ionics, 161 (2003) Wei D, Wakeham S J, Ng T W, Thwaites M J, Brown H & Beecher P, Electrochem Commun, 11 (2009) Pandey G P, Hashmi S A & Kumar Y, J Electrochem Soc, 157 (2010) A Sekhon S S, Lalia, B S, Park J S, Kim C S & Yamada K, J Mater Chem, 16 (2006) Bansal D, Cassel F, Croce F, Hendrickson M, Plichta E & Salomon M, J Phys Chem B, 109 (2005) Singh B & Sekhon SS, J Phys Chem B, 109 (2005) Navarra M A, Panero S & Scrosati B, Electrochem Solid- State Lett, 8 (2005) A Xu J J, Ye H & Huang J, Electrochem Commun, 7 (2005) Sutto T E, Ollinger M, Kim H, Craig B, Arnold C B & Piqué A, Electrochem Solid-State Lett, 9 (2006) A Fernicola A, Panero S, Scrosati B, Tamada M & Ohno H, Chem Phys Chem, 8 (2007) Tizzani C, Appetecchi G B, Carewska M, Kim G T & Passerini S, Aust J Chem, 60 (2007) Ye H, Huang J, Xu J J, Khalfan A & Greenbaum S G, J Electrochem Soc, 154 (2007) A Martinelli A, Matic A, Jacobsson P, Börjesson L, Navarra M A, Panero S & Scrosati B, J Electrochem Soc, 154 (2007) G Sutto T E, J Electrochem Soc, 154 (2007) P Fernicola A, Panero S & Scrosati B, J Power Sources, 178 (2008) Sirisopanaporn C, Fernicola A & Scrosati B, J Power Sources, 186 (2009) Choi J W, Cheruvally G, Kim Y H, Kim J K, Manuel J, Raghavan P, Ahn J H, Kim K W, Ahn H J, Choi D S & Song C E, Solid State Ionics, 178 (2007) 1235.

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