Comparative Impedance Study of Cd(0001) Electrode in EMImBF₄and KI Aqueous Solution at Different Temperatures

The electrochemical impedance spectroscopy was applied in order to characterize and compare the dependence of differential capacitance on temperature at the Cd(0001) | 1-ethyl-3-methylimidazolium tetrafluoroborate and Cd(0001) | 0.1 x M KI + 0.033(1− x ) M K 2 SO 4 interfaces. For both systems, the impedance spectra were fitted using the modified Frumkin–Melik-Gaikazyan model, including the high-frequency "true" double layer capacitance, equilibrium adsorption capacitance, Warburg-like diffusion impedance and parallel charge transfer resistance. The modified Frumkin–Melik-Gaikazyan model gives better fitting of calculated results to experimental data compared to alternative equivalent circuits. This indicates that the adsorption from aqueous solution and room temperature ionic liquid follows similar general kinetic steps. However, in contrast to the aqueous electrolyte solutions, the high-frequency capacitance for Cd(0001) in room temperature ionic liquid increases with increasing temperature. The presented theoretical explanations lead to the conclusion that the experimental series differential capacitance is determined by the multilayered structure of the electrical double layer, which gradually "dissolves" with increasing temperature.