Electrodeposition of aluminium from the chloroaluminate ionic liquid 1-ethyl-3-methylimidazolium chloride

Abstract

The mechanism of aluminium reduction in 1-ethyl-3-methylimidazolium chloride/chloroaluminate ionic liquids has been investigated. Stationary polarisation curves for aluminium electrode in 1-ethyl-3-methylimidazolium chloride chloroaluminate ionic liquid are S-shaped and contain limiting current plateau sections, which indicates heterogeneous kinetics of aluminium electrochemical reduction. The shift in the half-wave potential on the voltammograms may be interpreted using the model of the homogeneous chemical reaction preceding the electrochemical reduction of aluminium. Limiting current values have been obtained for the first time. The limiting currents grow with the increase in the concentration of Al2Cl7– anion and are determined by the diffusion of Al2Cl7– particle. The limiting current density in the neutral liquid (4.39 μA·cm−2) is by 3–4 orders of magnitude lower than the maximum current densities in acidic liquids (1.81 – 25.37 mA·cm−2 at 1.1 ≤ N ≤ 2.0). The possible preceding homogeneous chemical reaction has a much higher rate constant compared to the diffusion rate constant. Aluminium is reduced from Al2Cl7– anion at overvoltages below 1.5 V. At overvoltages above 1.5 V aluminium can be reduced from AlCl4– anion, and at the overvoltage of 2.2 V the reduction of [EMIm]+ becomes possible. The values for the diffusion coefficient of Al2Cl7– anion in the IL at different concentrations of Al2Cl7– have been determined to evaluate the contribution of migration flow to the total flow of ions at 30 °C. The diffusion coefficient of Al2Cl7– anions was found to be 9.3·10−7 cm2·s−1. It was shown that the thickness of the diffusion layer in the investigated electrochemical system decreases with the increase of Al2Cl7– concentration.