A high-energy-density hybrid electrochemical capacitor (HEC) has been developed. The advanced HEC has graphitizable carbon activated with KOH (KOH-activated soft carbon) for the positive electrode instead of activated carbon, which is used in a conventional HEC or an electric double-layer capacitor (EDLC). The KOH-activated soft carbon had low electrostatic capacitance due to its small specific surface area in the initial state; however, the capacitance increased significantly by the first charging at potential over 4 V versus Li/Li+ and exceeded the capacitance of activated carbon. The resulting capacitance was affected by the type of anion species and organic solvent of the electrolyte. In charge-discharge tests using various kinds of electrolytes, the KOH-activated soft carbon showed the best performance in the electrolyte of LiPF6 dissolved in a binary solvent composed of ethylene carbonate (EC) and dimethyl carbonate (DMC). The advanced HEC using KOH-activated soft carbon for the positive electrode gave about 8.5 times the energy density of the conventional EDLC. (c) 2007 Elsevier B.V. All rights reserved.