Carbon–ionic liquid double-layer capacitors Original Research Article

A series of electrochemical capacitors, based on activated carbon powders (ACP, specific surface area 870 and 2600 m2/g) and ionic liquids as electrolytes, were prepared and tested. The ionic liquids consisted of 1-ethyl-3-methyl imidazolium (EMIm+), 1-butyl-3-methyl imidazolium (BMIm+) and 1-methyl-1-propyl pyrrolidinium (BMPy+) cations, as well as of tetrafluoroborate, hexafluorophosphate and bis((trifluoromethyl)sulfonyl) imide anions. A typical capacitor consisted of two electrodes each with a mass of ca. 15–30 mg, and showed a capacity of ca. 0.35–1.5 F; this leads to a specific capacity of the carbon electrode material within the range of 45 (ACP 870 m2/g)–180 F/g (ACP 2600 m2/g). The specific capacity expressed versus total surface of carbon material was within the range of 5.2–6.9 μF/cm2. The electrochemical stability window of ionic liquids determined at the glassy carbon electrode is within the range of ca. 3.0–4.2 V. The energy stored in a capacitor based on activated carbons and ionic liquids may be high, due to a broad practical electrochemical stability window of ca. 3 V. Ionic liquids are characterised by negligible vapour pressure; such a capacitor emits no volatile organic compounds and may be regarded as environmentally friendly.