Electrical Double-Layer Structure in Ionic Liquids: A Corroboration of the Theoretical Model by Experimental Results

We experimentally observed for the first time a bell-shaped (convex parabolic) differential capacitance versus potential (C-dl-E) curve, which is expected according to the theory of Kornyshev given for the electrical double layer (EDL) of metal electrode/ionic liquid (IL) interface, at platinum and gold electrodes in four different [quaternary ammonium, imidazolium, and pyrrolidinium cations and bis(trifluoromethanesulfonyl)imide anion-based] ILs with cations and anions of similar sizes. The C-dl-E curves measured at a glassy carbon (nonmetallic) electrode in the same set of ILs were found to be U-shaped, in contrast to those obtained at platinum and gold electrodes. The present study corroborates the so-called Kornyshev's model of the EDL at metal electrode/IL interfaces and at the same time demands a theoretical model for the nonmetallic electrode/IL interface. The EDL formation in ILs is discussed.