Pseudocapacitive behaviour of RuO2 in a proton exchange ionic liquid

We studied the electrochemical behaviour of a thermally-prepared RuO2 electrode in a protic ionic liquid made from 2-methylpyridine and trifluoroacetic acid to determine if the current obtained on such electrode originates from double-layer charging or from pseudocapacitance. A thorough comparison of the electrochemical behavior in the ionic liquid and in aqueous electrolyte confirmed that pseudocapacitance is observed. The shape of the cyclic voltammograms obtained for the RuO2 electrode in the protic ionic liquid is similar to that obtained in H2SO4 and shows distinct redox peaks attributed to Faradaic reactions across the electrolyte∣electrode interface, which give rise to pseudocapacitance. The specific capacitance of the electrode in the protic ionic liquid (83 F g−1) is in the same order of magnitude to that obtained in water-based electrolyte but is ten times higher than that in an aprotic ionic liquid composed of 1-ethyl-3-methylimidazolium tetrafluoroborate. Our observations lead to the conclusion that pseudocapacitance can occur on a ruthenium dioxide electrode in an ionic liquid. Our results could be of interest to develop new metal oxide electrochemical capacitors based on non-aqueous electrolytes.