Time-scales of electrochemical desorption and sorption of H in relation to dimensions and geometries of host metal hydride electrodes

Abstract

The time-scales of electrochemical H sorption and desorption processes at H-storing metals are of current interest in the development of rechargeable metal hydride anode materials for new secondary battery systems and in relation to the use of Pd and other metals as hosts for D in the supposed cold fusion process. The sorption process is related to the cathodic H 2 evolution mechanism at the surface of the host metal through the H coverage θ H prevailing at a given overpotential at that surface. θ H is determined by the electrosorption isotherm for H and the sorption rate is determined by the gradient of the chemical potential of H in the near-surface region. The efficiency of sorption of H is examined in terms of the kinetics of H 2 evolution in relation to θ H and the sorption rate. It is shown, by means of quantitative numerical calculations, how the time-scales for H desorption (or sorption) from (into) H-sorbing host metals or alloys depend on (a) their physical dimensions and (b) their geometries. An important aspect of the present work is the quantitative comparison of H desorption rates from thin plates and spherulites of comparable volumes and surface areas, as well as from cylinders, and normalization parameters for this purpose are derived. The time-scales for H desorption are shown to be sensitive to electrode dimensions and geometries, and are evaluated quantitatively. Corresponding effects arise in the diffusion-controlled current densities that can be realized in H oxidation at metal hydride anodes and which determine power densities attainable in metal hydride batteries.