COMPLEX BEHAVIOR OF AL DISSOLUTION IN NONAQUEOUS MEDIUM AS REVEALED BY IMPEDANCE SPECTROSCOPY

In relation to development of possible non-aqueous solution embodiments of the Al battery, the kinetics of anodic dissolution of pure Al (99.99%) in non-aqueous acetonitrile (0.33 mol dm−3 AlCl3) solution were studied by means of steady-state polarization and, especially, ac impedance methods. Two linear regions are found in the Tafel plots, one exhibiting a higher slope in the low-current region and the other a lower slope in the high-current region, which implies involvement of two parallel pathways in the dissolution process. The major features of the impedance spectra plotted in the complex plane are one capacitative loop in the first quadrant and one or two overlapping pseudo-inductive loops in the fourth quadrant, at all potentials studied. A reaction model involving three intermediate species occupying space, quasi-3-dimensionally, on the surface, with two dissolution paths, is required to represent the observed behavior. General equations for simulation of the impedance behavior of the electrode reaction with three intermediates are given and applied to the system studied. Both the experimental steady-state polarization curve and the ac impedance spectra at various potentials are fitted with reasonably good agreement by the reaction model with a single set of rate constants and other parameters. The significance of the kinetic parameters required for the simulation of the impedance behavior, as a function of both frequency and potential, is discussed, and some selected simulation results are plotted 3-dimensionally as Z′ or Z″, as f(E, ω). © 1990, The Electrochemical Society, Inc. All rights reserved.