HYDROGEN SURFACE CONCENTRATION AND OVERPOTENTIAL FOR THE GALVANOSTATIC DISCHARGE OF HYDRIDE ELECTRODES .2. QUANTITATIVE NUMERICAL-CALCULATIONS

The electrochemical hydrogen desorption process from metal hydride electrodes is discussed in terms of the model presented in Part I. Based on the previously obtained equations, quantitative numerical calculations are presented for the near-surface concentration of hydrogen during electrochemical desorption from H-storing materials. The time dependence of H concentration is discussed in terms of the size and shape of the host (plates, cylinders, or spheres), to analyze the results obtained for ribbon, fiber, and powder electrodes. Experimental data are presented concerning the anodic overpotential decay during galvanostatic discharge of several hydride electrodes in ribbon and powder form. The data are fit by the theoretical model and the values of the parameters characterizing the discharge process are determined. These include the interface hydrogen transfer coefficient, K(t), the reaction transition time, tau(r), as well as the symmetry coefficient for the electrochemical oxidation, beta. K(t) is positive in most of the investigated cases, showing that hydrogen is transferred more easily from the absorbed to the adsorbed state than in the opposite direction. For a crystalline host showing a phase transformation, the model may be applied only in the single-phase regions, i.e., at the beginning and at the end of the discharge process.