Combination of Alkali-Treated Soft Carbon and Activated Carbon Fiber Electrodes for Asymmetric Electric Double-Layer Capacitor

An asymmetric combination of alkali-treated soft carbon (ASC) with activated carbon fiber (ACF) electrodes has been utilized to develop a novel electric double-layer capacitor (EDLC). The capacitance of ASC electrode was significantly increased after electrochemical activation at the first high potential cycling. Electrochemical measurements were carried out by charge-discharge polarization and ac impedance methods using a 3-electrode cell with propylene carbonate (PC) dissolving 1.0 M tetrafluorobrate (BF4-) salt of tetraethylammonium (TEA(+)) or lithium (Li+). The charge-discharge performance of the cell that consists of ASC as the negative and ACF as the positive electrodes with TEABF(4) electrolyte, denoted as ASC(-)/TEABF(4)(PC)/ACF(+), showed higher specific capacitance than other systems for the cycling in the voltage range of 0.0-3.0 V. The cell using LiBF4 electrolyte, ASC(-)/LiBF4(PC)/ACF(+), gave poor capacitance behavior probably due to undesirable electrolyte decomposition at the positive electrode. The ACF(-)/ASC(+) cell using TEABF(4)/PC also showed high capacitance, high coulombic efficiency and good cycle stability. The optimized cell with TEABF(4)/PC electrolyte had over 100 Wh kg(-1) of energy density for the cycling in 0.0-3.5 V range. (C) The Electrochemical Society of Japan, All rights reserved.