Anodic Dissolution of Aluminum in the Aluminum Chloride-1-Ethyl-3-methylimidazolium Chloride Ionic Liquid

The anodic dissolution of aluminum metal was investigated in the Lewis acidic chloroaluminate ionic liquid, aluminum chloride-1-ethyl-3-methylimidazolium chloride. The investigation was conducted on aluminum rotating disk electrodes as a function of potential, ionic liquid composition, and temperature. Two different dissolution mechanisms were realized. At modest overpotentials, dissolution takes place under mixed kinetic-mass transport control. However, as the overpotential is increased to induce higher dissolution rates and/or the ionic liquid is made more acidic, the dissolution reaction transitions to a potential-independent passivation-like process ascribed to the formation of a porous solid layer of AlCl₃(s). At a fixed temperature and composition, the limiting passivation current density displays Levich behavior and also scales linearly with the concentration of AlCl₄ ^- in the ionic liquid. The heterogeneous kinetics of the Al dissolution reaction were measured in the active dissolution potential regime. The exchange current densities were independent of the composition of the ionic liquid, and the anodic transfer coefficients were close to zero and seemed to be independent of the Al grain size.