Numerical simulation of hydrogen (deuterium) absorption into beta-phase hydride (deuteride) palladium electrodes under galvanostatic conditions

The kinetics of H(D) absorption into a beta-phase PdHx (PdDx) electrode are discussed numerically, based on the Volmer-Tafel route of the hydrogen (de uterium) evolution reaction and thermodynamic and kinetic data of H(D) in t he beta-phase PdHx (PdDx). It is found that the asymptotic loading ratio of H(D) is determined only by the Tafel step under galvanostatic conditions. The kinetics of H(D) absorption can be characterised by a parameter lambda proportional to d Delta j/((D) over bar Delta x) where d is the dimension o f the electrode (thickness for plate, radii for cylinder or sphere); Delta j is the current density step; (D) over bar is the average diffusion coeffi cient; Delta x is the loading ratio step of H(D) caused by the current step . If lambda much greater than 1 (large scale of dimension, high current den sity and/or low temperature), the absorption rate is controlled by diffusio n; in contrast, if lambda much less than 1, the rate-determining step is th e interface process and the charging efficiency approaches 100%; otherwise, the kinetics are under mixed control if lambda similar to 1. (C) 1999 Else vier Science S.A. All rights reserved.