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An electrical double-layer capacitor showing a sleight increase of resistance when used under continuous application of high voltage and maintaining high energy residual ratio after standing for a long time includes an electrode element including a pair of electrodes disposed opposite to each other

An electrical double-layer capacitor showing a sleight increase of resistance when used under continuous application of high voltage and maintaining high energy residual ratio after standing for a long time includes an electrode element including a pair of electrodes disposed opposite to each other with a separator interposed therebetween, and is impregnated with a nonaqueous electrolyte solution prepared by dissolving quaternary ammonium salts into cyclic carbonates and containing impurities of 30 ppm or less of glycols, 30 ppm or less of primary alcohols and less than 20 ppm of tertiary amines. The water content may be 50 ppm or less. The quaternary ammonium salt may be triethylmethylammonium tetrafluoroborate. The cyclic carbonate may be propylene carbonate. The nonaqueous electrolyte solution may have a concentration of 0.1 to 2.5 mol/liter. The electrode may be a polarizable electrode composed of activated carbon.

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1. An electrical double-layer capacitor comprising:an electrode element consisting of a separator and a pair of electrodes disposed opposite to each other with said separator interposed therebetween, said electrode element being impregnated with a nonaqueous electrolyte solution, wherein said nonaqu

1. An electrical double-layer capacitor comprising:an electrode element consisting of a separator and a pair of electrodes disposed opposite to each other with said separator interposed therebetween, said electrode element being impregnated with a nonaqueous electrolyte solution, wherein said nonaqueous electrolyte solution is prepared by dissolving a quaternary ammonium salt in a cyclic carbonate, and the impurities contained in said nonaqueous electrolyte solution impregnated into said electrode element comprise 30 ppm or less of glycols, 30 ppm or less of primary alcohols and less than 20 ppm of tertiary amines. 2. The electrical double-layer capacitor according to claim 1, wherein a water content in said nonaqueous electrolyte solution impregnated into said electrode element is 50 ppm or less.3. The electrical double-layer capacitor according to claim 1, wherein said quaternary ammonium salt is triethylmethylammonium tetrafluoroborate.4. The electrical double-layer capacitor according to claim 1, wherein said cyclic carbonate is propylene carbonate.5. The electrical double-layer capacitor according to claim 4, wherein said nonaqueous electrolyte solution is prepared by dissolving triethylmethylammonium tetrafluoroborate in propylene carbonate in a concentration ranging from 0.1 to 2.5 mol/liter.6. The electrical double-layer capacitor according to claim 1, wherein said electrode is a polarizable electrode composed of an activated carbon.