Impedance of a reaction involving two adsorbed intermediates:: aluminum dissolution in non-aqueous lithium imide solutions

The model presented considers the dissolution of a trivalent metal in three consecutive steps involving two adsorbed intermediates. If mass transport effects are negligible, it is possible to construct equivalent circuits in which adsorption-related elements are doubled compared to the case of a single adsorbate. In the case where mass transport affects the dissolution, the Faradaic admittance can be evaluated as a fraction of two power series and no simple equivalent circuit can be constructed from conventional circuit elements. Depending on the mechanism assumed, the low-frequency behavior can be either similar to a Warburg impedance or different fundamentally. The impedance of aluminum dissolution is discussed in the case of insignificant mass transport. The Langmuir isotherm is supposed to hold for intermediate adsorption, and only anodic partial reactions are accounted for. It has been concluded that the second step is rate-determined and that solvent takes part in the desorption of the product only. An empirical correlation was found between the dipole moment of the solvent used and the ratio of the rate constants of non-rate determining steps. (C) 2000 Elsevier Science S.A. All rights reserved.