Hydrogen surface concentration and overpotential for galvanostatic discharge of hydride electrodes; 1: Development of the model

The huge economic potential of rechargeable batteries using hydrogen storage materials as negative electrodes has triggered new interest in the study of hydrogen absorption-desorption by metals or alloys. The equations for hydrogen surface concentration and anodic overpotentials have been obtained for the galvanostatic discharge of hydride electrodes. The anodic process occurs in three consecutive steps: (1) diffusion of absorbed hydrogen from the bulk to the surface of the electrode; (2) transfer of hydrogen from absorbed state to adsorbed state at the electrode surface; and (3) electrochemical oxidation of the adsorbed hydrogen at the electrode surface. With step (2) treated as a heterogeneous reaction, the time dependence of the anodic overpotentials has been derived as a function of the electrode size, diffusion constant of hydrogen in the bulk electrode, interface hydrogen transfer characteristics, and discharge current. The rate capacities of electrodes also are obtained from expressions for the overpotential and hydrogen surface concentration. The overpotential and rate capacities have been obtained for three different electrode shapes, i.e., plate, cylinder, and sphere, to facilitate the discussion of experimental data on the ribbon, fiber, and powder electrodes. The time scales for discharge are discussed in terms of electrode size and shape, hydrogen diffusion constant, and discharge current.