Surface chemistry of carbon electrodes in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid - an in situ infrared study

Three different carbon materials were studied in very dry 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) ionic liquid using infrared and electrochemical spectroscopy methods. Advantages of in situ infrared measurements with magnetron sputtered thin carbon films are demonstrated over thicker porous carbide-derived carbon and pyrolytic graphite electrodes. Differences in the double layer structure exist for the three carbon materials, confirmed by the different shape of capacitance-potential curves and infrared spectra. It was established that imidazolium cation dimerizes at potential E < -2V (vs. Ag vertical bar AgCl wire in the same ionic liquid), whereas anion as well as cation decomposition takes place at E > +1.6 V. An explanation is given why EMImBF4 with a potential window of 4.1 V for the carbon film electrode gives symmetric supercapacitor voltage of only 2.8V. (C) 2014 Elsevier Ltd. All rights reserved.