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 EMIBF4, conventional aqueous electrolyte 35 wt % H2SO4, and nonaqueous electrolyte 1 M Et3MeNBF4/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 EMIBF4 even at low temperatures, however, it had a lower decomposition voltage (similar to2 V), resulting in lower energy density. The capacitance of EMIF . 2.3HF was extremely dependent on the applied voltage. (C) 2003 The Electrochemical Society.