Application of Low-Viscosity Ionic Liquid to the Electrolyte of Double-Layer Capacitors

The performance of a double-layer capacitor (DLC) composed of activated carbon electrodes and 1-ethyl-3-methylimidazolium fluoride (EMIF)⋅2.3HF, which has extremely high conductivity with low viscosity, was examined and compared with those using the popular ionic liquid conventional aqueous electrolyte 35 wt % and nonaqueous electrolyte 1 M /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 even at low temperatures, however, it had a lower decomposition voltage (∼2 V), resulting in lower energy density. The capacitance of EMIF⋅2.3HF was extremely dependent on the applied voltage.