Shrinking Core Model for the Discharge of a Metal Hydride Electrode

Metal hydride particles are used to make negative electrodes 1-3 in nickel/metal hydride batteries. The performance of these electrodes is affected by both the kinetics of the processes occurring at the metalelectrolyte interface and the hydrogen diffusion within the bulk of the metal alloy particle. Two different phases exist in the metal hydride particles. The hydriding or charging process of metal hydride particles was discussed in detail in Ref. 4 where equations governing the diffusion of hydrogen in the particle during charging (hydriding) were derived from the fundamental laws of mass and momentum transfer. Zhang et al. 4 were the first to develop rigorous boundary conditions based on jump balances. They provided a closed-form solution for the charging of metal hydride electrodes assuming a known (constant) concentration at the surface. They derived expressions describing the motions of the ab interface and the weight fraction of hydrogen entering the electrode particle from the electrolyte. They predicted that for particles of smaller radius and smaller diffusion coefficients, the pseudosteady-state (PSS) solution 5-12 does not provide an accurate solution of the governing equations. Unfortunately, their model cannot be used to predict the effect of applied current density on the concentration profiles and charge/discharge curves for the metal hydride electrodes. The discharge process of a metal hydride particle includes a phase change as shown schematically in Fig. 1. In the fully charged state (Fig. 1a) the metal hydride particle is in the b phase. The discharge process begins when the adsorbed hydrogen (Hads) at the surface of the particle reacts electrochemically with hydroxide ions as follows discharge