Performance of Electric Double Layer Capacitors with Polymer Gel Electrolytes
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Chemicals, Battery Grade), was used after drying under a vacuum at 70-80 °C for 24 h. High purity PC (Mitsubishi Chemical, Battery Grade) was used as a plasticizer without further purification. PVdF gel systems were prepared by mixing PVdF and TEABF 4 in PC at 80 °C, followed by evaporation of the PC solvent in a vacuum oven at 150 C as shown in Scheme 1. The resulting composites were transparent and rubbery films. The films are represented by composition of the precursor solutions as follows; PVdF(1.792 g)/TEABF 4 (x g)/PC(10 ml), where 1.792 g and x g are the mass of each component in the precursor solutions containing 10-2 dm3 of PC. The actual composition of the resulting composites was determined from a weight loss (corresponding to the mass of vaporized PC) of the samples during heating. -
(CH2CF2)n
-
(PVdF)
dissolving PVdF (1.792 g) and TEABF4 (x g)
in PC (10-2 dm 3) at 80 "C
evaporating PC ina vacuum oven at 150 "C
PVdF(1.792 g)/TEABF 4 (x g)/PC (10 m
Scheme 1 Preparation of PVdF gel electrolytes. The electrolytic conductivity was measured by an ac impedance meter (10 kHz) in the temperature range of -20 °C to 60 *C. The thickness of the gel films was about 2 mm. A model cell for EDLCs was constructed with the gel electrolyte film and two electrodes made of an activated carbon fiber cloth (mass: 51.6 mg, geometric surface area: 2.3 cm 2 ) as illustrated in Fig. 1. The activated carbon fiber cloth (Toyobo, BW552) was made from a phenolic resin and has a high surface area; the specific surface area is about 1.3 x 103 m2 g-1 (thickness: ca. 0.62 mm). The two electrodes were fixed face to face by small Teflon props. 6 ,7 The distance between the electrodes was 2.0 mm and the diameter of the electrodes was 17 mm. A pair of the fixed electrodes was soaked in the precursor solution, followed by evaporation of the PC solvent in the vacuum oven under the above-mentioned condition. The resulting gel electrolytes were in good contact with the electrodes. The actual compositions of the composites between the electrodes were determined as follows. When x = 0.44 g, PVdF: 179.8 mg, TEABF 4 :44.2 mg, PC: 415.7 mg; when x = 0.88 g, PVdF: 161.1 mg, TEABF 4 : 79.1 mg, PC: 412.4 mg; when x = 1.32 g, PVdF: 150.2 mg, TEABF 4 : 110.6 mg, PC: 393.6 mg; when x = 1.76 g, PVdF: 153.4 mg, TEABF 4 :150.7 mg, PC: 432.9 mg. For comparison with the gel-type capacitors, model capacitors with a liquid electrolyte were constructed as shown in Fig. 2, and their performance was tested, the liquid-type capacitors were constructed with PC containing TEABF 4 (0.8 M) and two electrodes (mass: 51.6 mg, geometric surface area: 2.3 cm 2 ) which were identical to those in the gel-type capacitors. A polypropylene non-woven cloth was used as a separator, which was impregnated with the electrolyte solution. The capacitor performance was investigated by charge-discharge cycling. The capacitors were charged to 3.0 or 2.0 V, then discharged to 1.0 V at a constant cycling current (1.0 mA: 0.44 mA
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Fig. I Schematic diagram for the
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