Indian Journal of Pure & Applied Physics Vol. 51, May 2013, pp. 315-319 Studies on redox supercapacitor using electrochemically synthesized polypyrrole as electrode material using blend polymer gel electrolyte S K Tripathi* a, Amrita Jain a, Ashish Gupta a & Manju Kumari b a Department of Physics, Jaypee University of Engineering and Technology, AB Road, Raghogarh, Guna, India b Department of Chemistry, Viva Institute of Technology, Thane, Maharashtra, India * E-mail: sktripathi16@yahoo.com Received 10 January 2013; revised 1 February 2013; accepted 28 March 2013 The study on polymer gel electrolyte comprising of poly vinylidene fluoride co-hexafluropropylene (PVdF-HFP)- polymethyl methacrylate (PMMA)-sodium iodide (NaI) with a view to use them as electrolyte material for redox supercapacitor with electrochemically deposited polypyrrole as electrode material has been carried out. The fabricated gel electrolyte shows high ionic conductivity of the order of 1.7 10 2 S cm 1 at room temperature with excellent mechanical/dimensional stability. The redox supercapacitor has been characterized using ac impedance spectroscopy, cyclic voltammetry and galvanostatic charge discharge tests. The overall capacitance values are found to be 128 mf cm 2 ; which is equivalent to single electrode specific capacitance of 514 F g 1. It corresponds to energy density of 43.0 W h kg 1 and power density of 4.0 kw kg 1. Keywords: Redox supercapacitors, Polypyrrole, Blend polymer gel electrolytes 1 Introduction Electrochemical capacitors, also called supercapacitors or ultracapacitors, possess much higher capacitance (~10 5 times) than those achievable with conventional capacitors and can operate at substantially higher specific power than batteries 1-5. Depending upon the charge storage mechanism, there exist two main classes of supercapacitors: (a) electrochemical double layer capacitors in which the double layer capacitance arises at the electrode/ electrolyte interfaces and (b) redox capcaitors in which pseudocapacitance arises due to the Faradic reactions occurring at electrode/electrolyte interface. The electrode materials used for electrochemical capacitors, generally, possess high specific area materials such as porous carbon materials, porous metal oxides and conducting polymers. The use of carbons in the supercapacitor at present is limited due to their low specific capacitance 6,7 of ~200 F g 1. In the case of metal oxides, extensive fundamental and development work on the ruthenium oxide type of electrochemical capacitor has been carried out because of its high specific capacitance 8,9 of 700 F g 1. But due to its high cost and toxic nature, its use in practical applications is limited. On the other hand, conducting polymers like polypyrrole, polyaniline etc have shown good specific capacitance and can be electrochemically deposited at low cost 10 13. Among these, polypyrrole is,generally, used because of its high conductivity, good environmental stability and ease of synthesis. Most of the redox supercapacitors reported in the literature are based on liquid electrolyte 5,8,14. They are associated with well known disadvantages of corrosion, self-discharge, low energy density etc. Now-a-days much attention is devoted to all polymer redox capacitor based on different polymeric gel electrolyte 13 e.g. poly (methyl methacrylate) (PMMA),-EC-PC-LiClO 4 [Ref. 12], PMMA-EC-PC- NaClO 4 etc. The present paper reports the studies on redox supercapacitors constructed from electrochemically deposited polypyrrole as conducting polymer electrode with polymeric gel electrolyte based on polyvinylidene fluroride co-hexafluropropylene (PVdF-HFP)-poly (methyl methacrylate) (PMMA)- sodium iodide (NaI) as salt. The performance characteristic of the fabricated capacitor has been evaluated by using cyclic voltammetry, chargedischarge cycling and ac impedance spectroscopic techniques. 2 Experimental Details The blend polymer electrolyte has been prepared by PVdF-HFP with an average molecular weight of (MW= 400,000 Aldrich), PMMA with an average
316 INDIAN J PURE & APPL PHYS, VOL 51, MAY 2013 molecular weight of (MW = 120000, Aldrich) and inorganic salt NaI (Merck) which has been used as such without further purification. DMF (N-N dimethyl formamide) has been used as the solvent. The polymer films were developed by using the well known solution cast techniques. The mixture of polymers and doping salt in suitable amounts was stirred continuously with a magnetic stirrer for 10 h at a temperature of 60 C until homogenous solutions were obtained. The solutions were poured into Petri dishes and left to dry at room temperature to form a film for about 35-40 days. The films were kept in desiccators for further drying. The p-doped polypyrrole films were electrochemically deposited on indium tin oxide (ITO) coated conducting glasses (Blazers, sheet resistance about 80 cm 2 ). The monomer pyrrole (Aldrich) and acetonitrile solvents (Merck) were distilled before use. Electrosynthesis was carried out in one compartment of three electrode cell containing 0.1 M pyrrole and 0.2 M LiClO 4 solution in acetonitrile in the presence of nitrogen purging. The electropolymerization was carried out galvanostatically at the constant current of 2 ma for 3.0 min. The platinum foil was used as a counter electrode. The films were washed and dried before use. The bulk electrical conductivity of the polymeric gel electrolyte was measured by using ac impedance spectroscopy. It is done by sandwiching the electrolyte between two stainless steel blocking electrodes. The prototype supercapacitors were assembled by sandwiching the polymeric gel electrolyte films between two symmetrical polypyrrole electrodes. The performance characteristics of capacitor cell were determined by using impedance measurements, cyclic voltammetry and galvanostatic charge-discharge tests. The impedance measurements were carried by using computer controlled LCR HI TESTER (Model 3522-50, Hioki, Japan) in the frequency range 1 mhz- 100 khz. The capacitance values were calculated by using the Eq. (1): The linear sweep cyclic voltammetry were performed at different scan rate with the help of computer controlled CHI 608C, CH Instruments, USA. The capacitance values from this technique were evaluated by using the relation: C = i/s (2) where i represents the current and s is the scan rate. The discharge test of the supercapacitor cell was carried out galvanaostatically at different constant currents for various cycles. The discharge capacitance C was calculated from the linear part of the discharge curve using the relation: C = i t / V (3) where i is the constant current and t is the time interval for the change of voltage V. 3 Results and Discussion The composition of polymeric gel electrolyte PVdF(HFP)-PMMA-NaI was initially optimized in order to get both highly conducting and mechanically stable and flexible film for their application as electrolyte in solid-state redox supercapacitors. Fig. 1 shows the variation of ionic conductivity of PVdF(HFP)-PMMA-NaI blend polymer electrolytes system. It is observed that [PVdF(HFP) (80 wt%)- PMMA (20 wt%)](20 wt%)-[1m NaI] 80 wt% of polymer exhibits maximum conductivity of about 1.67 10 2 S cm 1 at room temperature. The maximum C = 1/ Z (1) where (=2 f) is the angular frequency and Z is the imaginary part of the total complex impedance. The single electrode specific capacitance values were evaluated by multiplying the overall capacitance by a factor of two and dividing the mass of single active electrode. Fig. 1 Variation of ionic conductivity for blend polymer gel electrolyte [PVdF(HFP) (80wt%)-PMMA (20wt%)]-[1M NaI] 80 wt% as a function of polymer blend concentration in different weight percentage
TRIPATHI et al.: REDOX SUPERCAPACITOR USING ELECTROCHEMICALLY SYNTHESIZED POLYPYRROLE 317 conductivity is due to the proper PVdF(HFP)-PMMA and salt interaction. Higher conductivity in blend polymer electrolytes system could be attributed to the higher amorphicity and due to steric hindrance. Higher amorphicity provides mobile Na + ion more free volume giving rise to higher conductivity of electrolytes system. This order of conductivity of polymer electrolyte is quite acceptable for their use in redox supercapacitor as they offer low resistance when used in the thin film form (~250 µ cm 2 ). The symmetrical redox supercapacitor has been constructed using electrochemically deposited p-doped polypyrrole (ppy) electrode material on ITO with optimized polymeric gel electrolyte. Cell A: ppy [PVdF(HFP) (80 wt%)-pmma (20 wt%)]-[1m NaI] 80 wt% ppy 3.1 Cyclic voltammetric tests Figure 2 shows the linear sweep voltammogram of cell A at different scan rate. At constant scan rate the profile of current response is ideally a rectangle when capacitance is constant (i.e. it is potential independent). On the other hand, peak structured voltammogram is observed in case of potential dependent capacitance which indicates the differential profile. In the present studies, cyclic voltammograms for capacitor cell are almost close to an ideal shape of rectangle at lower scan rate up to 20 mv s 1 as shown in Fig. 2. This is characteristic of capacitive behaviour and indicative of fast switching rate of ions at the site of electrode-electrolyte interfaces. 3.2 Impedance analysis The complex impedance plot of cell A is shown in Fig. 3. The polypyrrole shows the capacitive behaviour (i.e. steep rising response of Z asymptotically for approximately constant value of Z, i.e. it is almost parallel to Z axis) in the lower frequency range up to 1 mhz with optimized gel electrolyte which is used to fabricate redox supercapacitor in the present investigations. It can be noted that instead of distinct semicircular spur, almost linear impedance (Z versus Z ) response has been observed in the lower frequency region as shown in Fig. 3. This indicates the interfacial characteristics having specific porous structure of the electrodes in contact with highly flexible gel electrolytes. The values of bulk resistance R b and interfacial charge transfer resistance R ct have been evaluated from the intercepts of the impedance response on real (Z ) axis as shown in Fig. 2. The capacitance values at frequencies 1 and 10 mhz have been calculated using Eq. (1). The values of R b, R ct, overall resistance R and capacitance C for capacitor cell has been summarized in Table 1. The values of capacitance have been observed to be in the range of 128 mf cm 2 which is equivalent to single electrode specific capacitance of 514 F g 1 mass of polypyrrole. Fig. 2 Cyclic voltammograms of redox capacitor cell A Fig. 3 Typical impedance plot of redox capacitor cell A Cell R ct (Ω cm 2 ) Table 1 Electrical parameters of redox capacitor cell from impedance analysis R b 10 mhz 1 mhz (Ω cm 2 ) R C R C (Ω cm 2 ) (mf cm 2 ) a (F g 1 ) b (Ω cm 2 ) (mf cm 2 ) a (F g 1 ) b A 1297 56 1725 35 137 2476 128 514 a Overall capacitance of the cells. b Single electrodes specific capacitance of the cells.
318 INDIAN J PURE & APPL PHYS, VOL 51, MAY 2013 Table 2 Typical charge-discharge characteristics of redox capacitor cell at a current density of 5.0 ma cm 2 Cell R i Discharge capacitance, C d Working voltage (Ω cm 2 ) (mf cm 2 ) a (F g 1 ) b (V) Energy density (Wh kg 1 ) Power density (kw kg 1 ) A 418 79 313 1.0 V 44.0 4.0 a Overall capacitance of the cells. b Single electrodes specific capacitance of the cells. Fig. 4 Charge-discharge curve of redox capacitor cell A 3.3 Charge-discharge tests The capacitor cell has also been tested with the constant charge-discharge methods. The typical charge-discharge characteristics of the capacitor cell are shown in Fig. 4. The cell was charged up to 1.0 V. The linear portion of the discharge characteristics confirms the capacitive behaviour of cell. The initial sudden change in voltage response with respect to time while charging and discharging has been found due to ohmic loss across the internal resistance (R i ) of the cell. The internal resistance of the cell has been estimated from this voltage loss. The values of discharge capacitance have been evaluated from the linear part of the discharge characteristics using Eq. (3). The values of discharge capacitance C d and internal resistance R i are listed in Table 2. The energy density of capacitor cell has been estimated from the corresponding value of capacitance. The power density values have also been evaluated by dividing the energy density values by the discharge time of the cell. The energy and power density values are also listed in Table 2. 4 Conclusions Combining all the above experimental studies, the following important conclusion can be drawn: (1) The [(PVdF-HFP) (80 wt%)-(pmma) (20 wt%)] (20 wt%) [NaI (1.0 M)] (80 wt%) based polymeric gel electrolyte, having room temperature conductivity of about 1.7 10 2 S cm 1 is suitable electrolyte for the fabrication of electrochemical redox supercapacitor with polypyrrole deposited on ITO as conducting polymer electrode. (2) The overall capacitance of about 128 mf cm 2 which is equivalent to single electrode specific capacitance of 514 F g 1 mass of polypyrrole is obtained. This value corresponds to energy density of 43.4 Wh kg 1 and power density of 4.0 kw kg 1. (3) The internal and overall resistance of cell been found to be in the order of few hundred ohm cm 2, which is comparatively lower than that of the solid-state cell based on conventional solid polymer electrolyte. (4) The Columbic efficiency of the gel electrolyte is nearly 100% which is equivalent to liquid electrolyte based cell which assures the liquid like behaviour of the gel electrolyte. Acknowledgement The authors are grateful to the Department of Science & Technology, Government of India, for providing financial support through Grant-in-Aid for Scientific Research (D.O. Nos.: SR/FTP/PS-02/2007; Dated 30.07.2008). References 1 Conway B E, Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications, Kluwer/Plenum Publishers, New York, 1997. 2 Conway B E, J. Electrochem Soc, 138 (1991) 1539. 3 Arbizzani C, Mastragostino M, Scosati B, in: Nalwa H S (Ed.), Handbook of Organic Conductive Molecules and Polymers, vol 4, Wiley, Chichester, UK, 1997, p. 595. 4 Burke A F, Murfy T C, in: Goughtly D H, Vyas B, Takamura T, Huff J R (Eds), Materials for Energy Storage and Conversion: Batteries, Capacitors and Fuel Cells, Materials Research Society, Pittusburg, 1995, p. 375. 5 Sarangapani S, Tilak B V & Chen C P, J Electrochem Soc, 143, 3791 (1996). 6 Barbieri O, Hahn M, Herzog A & Kotz R, Carbon, 43 (2005) 1303.
TRIPATHI et al.: REDOX SUPERCAPACITOR USING ELECTROCHEMICALLY SYNTHESIZED POLYPYRROLE 319 7 Gryglewicz G, Machnikowski J, Grabowska E L, Lota G & Frackowiak E, Electochem Acta, 50 (2005) 1197. 8 Zheng J P, Cygon P J & Jow T R, J Electrochem Soc, 142 (1995) 2699 9 Takasu Y & Murakami Y, Electrochim Acta, 45 (2005) 4135. 10 Nam K W, Yoon W S & Kim K B, Electrochim Acta, 47 (2002) 3201. 11 Hu C C & Wang C C, J Electrochem Soc, 150 (2003) 179. 12 Tripathi S K, Kumar Ashok & Hashmi S A, Solid State Ionics, 177 (2006) 2979. 13 Hashmi S A, Kumar Ashok & Tripathi S K, Euro Polym J, 41(2005) 1373. 14 Carlberg J C & Inganas O, J Electrochem Soc, 144 (1997) L61.