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

The anodic polarization process in metal-hydride electrodes occurs via the following consecutive steps: (i) diffusion of absorbed hydrogen from the bu lk to the surface of the electrode, (ii) hydrogen transfer from absorbed st ate to adsorbed state at the electrode surface, and (iii) electrochemical o xidation of the adsorbed hydrogen on the electrode surface. A theoretical t reatment is presented to account for the dependencies on these three consec utive 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 su m of the charge-transfer, hydrogen-transfer, and hydrogen-diffusion resista nces, which is in contradiction with the generally accepted idea that the r esistance measured from linear micropolarization is only the charge-transfe r resistance. For a metal-hydride electrode with a flat pressure plateau at a low state of discharge (SOD), the resistance measured from linear microp olarization 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 measu red from linear micropolarization is higher than total resistances measured from AC impedance. (C) 2000 Elsevier Science S.A. All rights reserved.