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.
