The performance of a double-layer capacitor ͑DLC͒ composed of activated carbon electrodes and 1-ethyl-3-methylimidazolium fluoride ͑EMIF͒•2.3HF, which has extremely high conductivity with low viscosity, was examined and compared with those using the popular ionic liquid EMIBF 4 , conventional aqueous electrolyte 35 wt % H 2 SO 4 , and nonaqueous electrolyte 1 M Et 3 MeNBF 4 /propylene carbonate. The DLC using EMIF•2.3HF showed an intermediate capacitance and internal resistance between the aqueous and nonaqueous electrolyte systems due to its intermediate double-layer capacitance and electrolytic conductivity. EMIF•2.3HF afforded much higher capacitance than EMIBF 4 even at low temperatures, however, it had a lower decomposition voltage ͑ϳ2 V͒, resulting in lower energy density. The capacitance of EMIF•2.3HF was extremely dependent on the applied voltage.
The limiting reduction and oxidation potentials and electrolytic conductivities of new quaternary ammonium salts were examined for electrochemical capacitor applications, whose anions have already been tested as lithium salts for lithium battery applications. The anodic stability was in the following order
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