Differential Capacitance of the Electrical Double Layer in Imidazolium-Based Ionic Liquids: Influence of Potential, Cation Size, and Temperature

Abstract

The interfaces formed at glassy carbon electrodes in three low-temperature ionic liquids (1-methyl-3-ethylimidazolium chloride, emimCl; 1-methyl-3-butylimidazolium chloride, bmimCl; and 1-methyl-3-hexylimidazolium chloride, hmimCl) were investigated by cyclic voltammetry and impedance spectroscopy. The potential dependence of the differential double layer capacitance was measured at several temperatures between 80 and 140 °C, and the temperature response was found to be broadly similar to that obtained with high-temperature molten salts. The differential capacitance/potential curves have a minimum and two side branches. The minimum corresponds to the point of zero charge. The differential capacitance increases in the order hmimCl < bmimCl < emimCl because the double layer is thinner when imidazolium (Rmim) cations with shorter alkyl chain lengths are used. The impedance spectra and capacitance curves indicate that cations are adsorbed at the open-circuit potential and that their surface excess concentrati...