A hybrid capacitor (HC) with an aqueous electrolyte solution was assembled using a Zn negative electrode and an activated carbon (AC) positive electrode. The electrochemical properties of the HC were compared with an electric double-layer capacitor (EDLC) having two AC electrodes. The HC cell successfully functioned as an electrochemical capacitor, with a maximum operating voltage of 1.4 V, which was 1.5 times higher than that of the EDLC cell, and a specific capacitance of ca. 3.7 times higher than that of the EDLC cell. In addition, the HC cell displayed high-rate dischargeability comparable to that of the EDLC cell. Due to these advantages, the HC cell was found to be superior to the EDLC cell in terms of energy and power density. The HC could be a promising candidate for water-based electrochemical capacitors with high energy and power densities. (c) 2007 The Electrochemical Society.