Determination of reaction resistances for metal-hydride electrodes during anodic polarization

Abstract

The anodic polarization process in metal-hydride electrodes occurs via the following consecutive steps: (i) diffusion of absorbed hydrogen from the bulk to the surface of the electrode, (ii) hydrogen transfer from absorbed state to adsorbed state at the electrode surface, and (iii) electrochemical oxidation of the adsorbed hydrogen on the electrode surface. A theoretical treatment is presented to account for the dependencies on these three consecutive steps, of the reaction resistances, anodic limiting current, cathodic limiting current, and exchange current. The theoretical analysis predicts that the total resistances measured from linear micropolarization is the sum of the charge-transfer, hydrogen-transfer, and hydrogen-diffusion resistances, which is in contradiction with the generally accepted idea that the resistance measured from linear micropolarization is only the charge-transfer resistance. For a metal-hydride electrode with a flat pressure plateau at a low state of discharge (SOD), the resistance measured from linear micropolarization is approximately equal to the sum of three resistances measured from AC impedance, namely, charge-transfer, hydrogen-transfer and hydrogen-diffusion resistances; however, when the SOD is high, the resistance measured from linear micropolarization is higher than total resistances measured from AC impedance.