Studies on hydriding kinetics of some La-based metal hydride alloys

In this paper, the hydriding kinetics of LaNi5, LaNi4.7Al0.3 and LmNi(4.91)Sn(0.15) is presented. Experiments were carried out by maintaining the pressure ratio (supply pressure to equilibrium pressure at the mid-point of the pressure-concentration-isotherm) equal to 2 and by maintaining nearly isothermal reaction conditions. Two widely used reaction kinetics models, namely Johnson-Mehl-Avrami (JMA) model and Jander diffusion model (JDM) are considered for the analysis. Two JMA models are considered; in the first model, the order of the reaction is assumed as unit and in the second model, the rate constant is calculated by estimating the order by fitting the reaction kinetics data with a reaction kinetics equation. The activation energy and pre-exponential constants of the above-mentioned alloys are estimated by constructing the Arrhenius plot. Activation energies estimated from the different models are compared and the accurate values of activation energy for the different alloys are determined by comparing the reaction kinetics data obtained from the models with the experimental data. The rate-controlling step of the hydriding reaction is obtained for all the alloys investigated. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.