Electrochemical behavior of aluminum in 1-ethyl-3-methylimidazolium chloroaluminate ionic liquids

Abstract

The mechanism of electrochemical oxidation and reduction of aluminum in 1-ethyl-3-methylimidazolium-based (EMImCl) chloroaluminate ionic liquids is studied across the concentration range of N = 1.1–2.0 (N the molar ratio of AlCl3 to EMImCl) at temperatures between 303 K and 418 К. Anodic stationary polarization curves (SPCs) indicate that the process of anodic dissolution of aluminum is complicated by passivation. The current density in the passive region and the passivation potential decrease with a higher concentration of Al2Cl7– ion in the melt. To elucidate the processes taking place at the aluminum electrode, the relationship between the concentration of the ions in the vicinity of the electrode and the potential applied in the cathodic and anodic region was analyzed by in-situ Raman spectroscopy for the concentrations of N = 1.1 and 2.0. The resulting dependences of relative peak intensity vs. potential correlate with the cathodic and anodic polarization curves. In the anodic region the content of Al2Cl7– grows with the potential, while the concentration of AlCl4– ion decreases. In the cathodic region the opposite process is observed: the concentration of Al2Cl7– decreases and that of AlCl4–increases with growing potential. It is shown that a precipitate of AlCl3 forms on the electrode surface in the region of passive aluminum dissolution, which is responsible for the electrode passivation. Based on the results obtained, we propose a model for the process of aluminum oxidation/reduction preceded and followed by a chemical reaction of aluminum chloride formation for the cathodic and anodic process, respectively. Temperature dependences of diffusion coefficients are studied by chronopotentiometry. The results indicate that diffusion coefficients are not affected by the concentration across the temperature range studied. The activation energy for diffusion was found to be 13.65 kJ∙mol−1. Impedance spectroscopy was used to investigate the concentration and temperature dependences of exchange current density. The exchange currents increase with increasing aluminum chloride content in the melt. The transfer coefficient and electrochemical reaction rate constant were found to be α = 0.17 ± 0.04, ks = (1.7 ± 0.4)∙10−6 cm∙s−1, respectively. The activation energy for the electrochemical reaction rate constant does not depend on the concentration and was found to be 33.5 kJ∙mol−1.