Electrochemical characteristics of activated carbon/Ppy electrode combined with P(VdF-co-HFP)/PVP for EDLC

Electrodes were fabricated by activated carbon powders such as BP-20 and YP-17, conducting agents such as polypyrrole (Ppy), Super P, vapor grown carbon fiber (VGCF) and acetylene black (AB), and binders such as poly-(vinylidenefluoride-hexafluoropropylene) [P(VdF-co-HFP)] and polyvinyl-pyrrolidone (PVP). In the electrode fabrication of unit cells, it was noted that electrochemical characteristics, mechanical strength and flexibility were greatly increased with 90 wt.% of BP-20, 5 wt.% of Ppy and 5 wt.% of P(VdF-co-HFP)/PVP mixed binder. The electrochemical characteristics of unit cell with Ppy were as follows: 37.6 F/g of specific capacitance, 0.98 Omega of AC-ESR, 2.92 Wh/kg and 6.05 Wh/L of energy density, and 754 W/kg and 1562 W/L of power density. According to the impedance measurement of the electrode with conducting agent, we found that it was possible to charge rapidly by the fast steady-state current convergence due to low equivalent series resistance (AC-ESR), fast charge-transfer rate at interface between electrode and electrolyte and low RC time constant. It was also concluded that the specific capacitance originated from the complex phenomena of the faradaic capacitance by oxidation-reduction of conducting agent and the non-faradaic capacitance by adsorption-desorption of activated carbon, as compared with the plateau portion of cyclic voltammogram (CV) curve and scan rate. The self-discharge of unit cell showed that diffusion process was controlled by the ion concentration difference of initial electrolyte due to the characteristics of EDLC charged by ion adsorption in the beginning, but current leakage through the double-layer at the electrode/electrolyte interface had a minor effect and voltages of curves remained constant except Ppy. We found that the high capacitance EDLC could be applied to industrial usage showing the constant DC-ESR by IR drop regardless of discharge current, although the specific capacitance was decreased when compared with that of unit cell. The electrochemical characteristics of 2.3 V/3000 F grade EDLC were as follows: 0.35 mOmega of DC-ESR (100 A discharge), 0.14 mOmega of AC-ESR (ac amplitude 100 mV), 2.80 Wh/kg (3.73 Wh/L) of energy density and 4.64 kW/kg (6.19 kW/L) of power density. Power output was compatible with electric vehicle applications, uninterrupted power supply and engine starter, in due consideration of Ragone relations. (C) 2004 Elsevier Ltd. All rights reserved.