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 (BF
4−) 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 LiBF
4 electrolyte, ASC(−)/LiBF
4(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.
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