Studies of electrochemical properties of TiNi alloy used as an MH electrode. II. Discharge kinetics

Abstract

Abstract The electrochemical polarization and concentration polarization during the discharge of TiNi alloy electrodes are analyzed in terms of a model presented in part I. The ratio of electrochemical polarization to concentration polarization during discharge ( η e / η c ) p increases with the number of charge/discharge cycles due to the faster decline in electrocatalytic performance ( I 0 ) than in the apparent diffusion coefficient D α A , but the ratio decreases with increasing discharge current density and state of discharge (SOD). However, the ratio of electrochemical polarization to concentration polarization at the end of discharge ( η e / η c ) e increases with discharge current density. In contradiction to the generally accepted idea, the discharge capacity is controlled by the rate controlling step at the end of discharge, the discharge capacity has no relationship with the rate controlling step in discharge process. At low discharge current density ( I I dc ), for which the electrochemical polarization is lower than 0.11 V, the diffusion of hydrogen from the bulk through the oxide film to the surface of electrode is the step controlling of the discharge capacity. However when the discharge current density is higher than I ec , for which the electrochemical polarization is higher than 0.199 V, but lower than the electrochemically limited current density I Le [= I 0 exp(0.332 βF / RT )], the discharge capacity is determined by the charge-transfer reaction on the electrode surface. When discharge current density is between I dc and I ec , both electrochemical reaction and hydrogen diffusion limit the discharge capacity.