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

The kinetics of H(D) absorption into a b-phase PdHx (PdDx) electrode are discussed numerically, based on the Volmer–Tafel route of the hydrogen (deuterium) evolution reaction and thermodynamic and kinetic data of H(D) in the b-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 l8dDj/(D( Dx) where d is the dimension of the electrode (thickness for plate, radii for cylinder or sphere); Dj is the current density step; D( is the average diffusion coefficient; Dx is the loading ratio step of H(D) caused by the current step. If l 1 (large scale of dimension, high current density and/or low temperature), the absorption rate is controlled by diffusion; in contrast, if l 1, the rate-determining step is the interface process and the charging efficiency approaches 100%; otherwise, the kinetics are under mixed control if l 1. © 1999 Elsevier Science S.A. All rights reserved.