Differential capacitance of the electrical double layer in imidazolium-based ionic liquids: Influence of potential, cation size, and temperature

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 degrees 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 concentration increases with negative polarization. Adsorption of the cation becomes stronger as the length of the alkyl chain decreases. Adsorption of chloride anions occurs at positive potentials and is weakest with bmimCl. The increase in the differential capacitance with temperature is most probably due to ion association within the double layer, which diminishes as temperature increases. The electrochemical window narrows as the temperature increases but is almost unaffected by the length of the alkyl chain of the Rmim cation.