An aqueous capacitor has been developed mainly for the purpose of recovering the regenerating energy for automobiles. A rapid charging within few seconds is required as well as the safety and the considerably large energy density. We have made two types of capacitors using a saturated sodium perchlorate aqueous solution (SSPAS) as an electrolyte, i.e., one consisted of a mixture of graphite and activated carbon, and the other mixtures of graphite and metal oxides. The former was an electric double layer type its energy density being 23 Wh/ kg under the 70% of activated carbon and the applied voltage of 2.5 V. The generation of gases was not detected until the cutoff voltage 2.7 V using a SS444 foil as a current collector. Experiments using a titanium foil as a current collector revealed that the charge and discharge capacities were independent of temperatures from 0 to 60°C. In the applied electric current density of 20 mA/cm 2 , the capacity was determined to be 0.03 mAh/cm 2 with the coulombic efficiency of 100% leading to the charging time of 5.4 s. The present result confirms a feasibility of recovering the regenerating energy for automobiles.

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