Capacitance Measurements in a Series of Room-Temperature Ionic Liquids at Glassy Carbon and Gold Electrode Interfaces

An extensive study has been done for the first time on the structure of the electrical double layer (EDL) at polarized glassy carbon (GC) and gold (Au) electrode interfaces in a series of room-temperature ionic liquids (RTILs) via the measurement of capacitance-potential curves. The parabolic capacitance-potential curves similar to those observed in high-temperature inorganic molten salts were obtained at GC electrode in all of the RTILs studied. Potential of zero charge (PZC) at GC electrode in imidazolium-based RTILs depends significantly on the electrochemical pretreatment of the electrode surface: Electrochemical oxidation pretreatment generates the oxide surface on GC electrode, which results in a favorable adsorption of positively charged imidazolium cations on the electrode surface and in turn shifts the PZC to the positive direction of potential, whereas at the electrochemically reduced GC electrode, on which the adsorption of the imidazolium cations is less favorable, PZC shifts to the negative direction of potential. Such an effect of electrochemical pretreatment was not observed at the highly oriented pyrolytic graphite electrode. The hump on the anodic side of the capacitance-potential curves at Au electrode in imidazolium-based RTILs results from the pi-electronic interaction of the imidazolium ring with the metal electrode, which was substantiated by using nonmetallic electrode and varying the ions of the RTILs. Such an enhanced interaction of the imidazolium ring with a gold electrode, as in the case of anion adsorption, shifts the PZC to the negative direction of potential. Such a hump as that observed at the gold electrode was not observed at the GC electrode. Similarly to the case in high-temperature inorganic molten salts, capacitances at PZC increase with increasing temperature. Different aspects of the obtained capacitance-potential curves are interpreted satisfactorily based on the hitherto proposed concept of the EDL structures.