Hydrogen desorption kinetics of a LaNi4.7Al0.3 metal hydride electrode using potentiostatic measurements

The reaction kinetics of an activated LaNi4.7Al0.3 metal hydride (MH) electrode during the discharge process were studied using charge/discharge and potentiostatic measurements. The electrochemical measurements were conducted at varying applied potentials and temperatures. The potentiostatic measurements show that the reacted fraction alpha(t) (alpha(t) is the ratio of the discharge capacity at time, t, to the maximum discharge capacity) of the MH electrode during the discharge process depends on both the applied potential and temperature. Analysis of the kinetic data indicates that the primary rate controlling step for the discharge process of the LaNi4.7Al0.3 electrode is the beta --> alpha phase transformation process at temperatures in the range 298-328 K and at applied potentials in the range from - 0.8 to - 0.6 V vs. Hg/HgO. The dependence of reacted fraction alpha(t) of the MH electrode reaction on time t obeys a nucleation and growth (ng) equation: -ln(1 - alpha(t)) = kt(m) (k and m are material parameters of the MH electrode during the discharge process; the reaction rate constant K-ng = k(1/m); t is the discharge time (min)). The increase in applied potential leads to a decrease in the hydrogen activity. The nucleation and growth of the alpha-hydride phase, resulting from the decrease in the hydrogen activity, leads to an increase in the reacted fraction. The reaction rate constant (K-ng) increases with increasing temperature which is in agreement with an Arrhenius-type temperature behavior. The calculated apparent activation energy (E-a) from the Arrhenius equation for the discharge reaction of the MH electrode was 41.1 kJ (mol H)(-1) for temperatures ranging from 298 to 328 K. (C) 2000 Elsevier Science B.V. All rights reserved.