Modification of a Carbon Electrode Surface by Cold Plasma Treatment for Electric Double Layer Capacitors
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high-frequency power supply (13.56 MHz) generating a pulsed electric power (10 kW). After the ACFC was introduced into a reaction chamber filled with Ar, this chamber was degassed until 0.1 Torr. The ACFC was then exposed to plasma generated by the pulsed power supply for 30 min.; the pulse on-time and off-time were 0.15 and 34.85 ins, respectively. The supplying gas for the pulsed plasma generation was Ar-02 (2 vol.%), Ar-0 2 (6 vol.%), Ar-0 2 (10 vol.%), or Ar-02 (20 vol.%); vol.% stands for volumetric percentage of 02 in the supplying Ar-0 2 gas. Throughout this treatment the Ar-02 mixed gas was supplied intermittently into the chamber; the flow rate was 500 cm 3 min.-1, the on-time and off-time of the gas supply were 7 ms. X-ray photoelectron spectroscopy (XPS) was performed to analyze the distribution of atoms and functional groups at the surface of the treated and untreated ACFC. BET method was applied to determine the specific surface area and the pore size at the ACFC surface. High purity propylene carbonate (PC; Mitsubishi Chemical, Battery Grade) was used as a solvent without further purification. An electrolytic salt, tetraethylammonium tetrafluoroborate (TEABF 4 ), was used after drying under a vacuum at 80 °C for 24 h. Two types of model EDL capacitor were fabricated with Teflon cell cases as shown in Fig. 2a and b; one has two ACFC electrodes with an identical area (diameter: 10 mnm) and the other contains two ACFC electrodes with different areas (diameter: 6 mm for a working electrode, 35 mm for a counter electrode). The former, "a symmetric capacitor", as a conventional configuration was used for the total capacitance measurement. On the other hand, the latter, "an asymmetric capacitor", was applied to the determination of single electrode capacitance, i.e., negative or positive electrode capacitance, and to ac impedance analysis as described below. The current collector of the electrode was nickel mesh. A polypropylene non-woven cloth was used as a separator, which was impregnated with the electrolyte solution. The charge-discharge characteristics of the model capacitors were measured under a constant current cycling. The cutoff voltages of the capacitors were 2 V for charging and 1 V for discharging. The ac impedance at a ACFC working electrode interface in the asymmetric model capacitor shown in Fig. 2b was measured by a frequency analyzer (Solartron, 1250) and a potentiostat (Solartron, 1186) at polarization of 1 or -1 V vs. the counter electrode. The ac potential was 10 mVp-p, and the frequency was scanned from 65 kHz to 10 mHz. Capacitor assembly and all capacitor tests were carried out under a dry Ar atmosphere at room temperature (20-25 'C). High-Frequency Power Supply
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b
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(PulsedPower)
Sample • •
Gas Source ~~~(Ar, 02)
Vacuum Pump
Fig. 1 Apparatus for treatment of ACFCs with pulsed cold plasma.
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Fig.2 Configurations of model EDL capacitors. a: symmetric capacitor; 1: ACFC electrodes, 2: electrolyte + separator, 3: Teflon spacer. b: asymmetric capacitor; 1: working ACFC elec
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