Abstract:

The interfacial structure and capacitance of a Pt foil electrode were investigated by electrochemical impedance spectroscopy in various ionic liquids including BMIMPF6, BMIMBF4, BMIMClO4, BMIMTf2N, BMIMCl, BMIMBr, C3OHMIMBF4, C3OHMIMClO4 and BMMIMPF6 (BMIM: 1-butyl-3-methylimidazolium, C3OHMIM: 1-(3-hydroxypropyl)-3-methylimidazolium, BMMIM: 1-butyl-2-methyl-3-methylimidazolium, Tf2N: bis(trifluoromethylsulfonyl)amide). The results indicate that when the anion and cation of the ionic liquid are of comparable size and do not adsorb specifically on the electrode, the“capacitance-potential”curve near the potential of zero charge exhibits either one peak or two peaks. The potential of zero charge corresponds to either the peak potential or the valley potential on the capacitance-potential curve with one peak or two peaks, respectively. When the potential is positive or negative to the potential of zero charge, the structure of the“electrode/ionic liquid”interface could be explained by the compact layer theories. In the presence of specific ion adsorption, the capacitance peak disappears and while the electrode potential shifts against the potential of zero charge, the interfacial capacitance rapidly increases. The effects of small Li+ ions on the electrode/ionic liquid interfacial structure and capacitance were also studied. When LiTf2N was added to BMIMTf2N, the small Li + ion changed the ionic composition and hence the structure of the electrode/ionic liquid interface, and also decreased the interfacial capacitance. Based on these findings, the arrangement and conformation of the anions and cations of the ionic liquid at the electrode/ionic liquid interface are elaborated under different conditions with particular attention to the specific adsorption of both the cation and the anion.

Key words: Electrochemical capacitance, Ionic liquid, Electrode/electrolyte interface, Interfacial ion collocation, Electrochemical impedance spectroscopy