STUDIES OF ELECTROCHEMICAL PROPERTIES OF TINI ALLOY USED AS AN MH ELECTRODE - II - DISCHARGE KINETICS

The electrochemical polarization and concentration polarization during the discharge of TiNi alloy electrodes are analyzed in terms of a mod el presented in part I. The ratio of electrochemical polarization to c oncentration polarization during discharge (eta(e)/eta(c))(p) increase s with the number of charge/discharge cycles due to the faster decline in electrocatalytic performance (I-0) than in the apparent diffusion coefficient D-alpha(A), but the ratio decreases with increasing discha rge current density and state of discharge (SOD). However, the ratio o f electrochemical polarization to concentration polarization at the en d of discharge (eta(e)/eta(c))(e) increases with discharge current den sity. In contradiction to the generally accepted idea, the discharge c apacity is controlled by the rate controlling step at the end of disch arge, the discharge capacity has no relationship with the rate control ling 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 him to the surface of electrode is the step controlling of the discharge capacit y. However when the discharge current density is higher than I-ec, for which the electrochemical polarization is higher than 0.199 V, but lo wer than the electrochemically limited current density I-Le [= I-0 exp (0.332 beta F/RT)], the discharge capacity is determined by the charge -transfer reaction on the electrode surface. When discharge current de nsity is between I-dc and I-ec, both electrochemical reaction and hydr ogen diffusion limit the discharge capacity. (C) 1998 Elsevier Science Ltd. All rights reserved.