Dissertation Seminar: Chen Wang will present his dissertation research “Anodic Metal Dissolution in Haloaluminate Molten Salts/Ionic Liquids” to the department.
The anodization of aluminum was investigated in the low-melting, mixed halide molten salt system, LiAlBr4-NaAlCl4-KAlCl4 (30-50-20 m/o), and in the Lewis acidic chloroaluminate ionic liquid, aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl3-EtMeImCl) using rotating disk electrode voltammetry. In both cases, at modest overpotentials, the anodization reaction proceeds under mixed kinetic/mass-transport control. However, at larger positive overpotentials and correspondingly higher anodization rates, the reaction transitions to a mass transport-limited process governed by the dissolution of a passive layer of AlCl3(s) and/or AlBr3(s) on the electrode surface. In AlCl3-EtMeImCl, the passive current density scales linearly with the concentration of AlCl4– in the ionic liquid. The heterogeneous rate constant, referenced to the equilibrium potential, and the transfer coefficient of the Al anodization reaction were measured in the absence of passivation in both ionic solvents. The exchange current densities were independent of the composition of the AlCl3-EtMeImCl ionic liquid, and the anodic transfer coefficients were close to zero in both cases. Surprisingly, the kinetic results were independent of the Al grain size.
The anodic dissolution of copper was also investigated in the AlCl3-EtMeImCl ionic liquid. A kinetic analysis of the anodic dissolution of copper in the Lewis acidic and basic compositions of the AlCl3-EtMeImCl ionic liquid was completed. In the Lewis acidic ionic liquid, the current density is potential-dependent and time-independent under all experimental conditions. That is, the anodization of copper proceeds under mixed kinetic/mass-transport control without complication from surface species, e.g., salt precipitates. However, in the Lewis basic ionic liquids, when the anodic dissolution rate is increased above a certain critical value, the current density becomes potential-independent. This behavior is due to the formation of a CuCl surface film, and the passive current density is governed by the dissolution of this species. At lower anodization rates, when the Cu reaction exhibits mixed control, modeling indicated that CuCl2– is the diffusion-limited species. The anodic transfer coefficient was close to 0.5.