Modeling of the hydrogen sorption kinetics in an AB₂ laves type metal hydride alloy

Hydrides of the AB₂ Laves type alloys (A=Zr, Ti; B = transition metal – Fe, Co, Ni, Mn, Cr, V) have been extensively studied as materials for the storage of gaseous hydrogen. They contain up to 4 H atoms/formula unit AB₂, thus achieving reversible H storage capacities in the range between 1.5 and 2.0 wt% H and offering high rates of hydrogen charge and discharge, thus making them suitable for designing efficient hydrogen stores operating at ambient conditions. In the present study, we performed an experimental study and modeling of the thermodynamics and the kinetics of interaction in the AB₂-hydrogen system. The experimental data was collected by studying a model alloy with a composition Ti_0.15Zr_0.85La_0.03Ni_1.126Mn_0.657V_0.113Fe_0.113. Hydrogen absorption and desorption were studied in a volumetric Sieverts type apparatus at isothermal conditions using a single-step change/discharge and stepwise methods. The results obtained from the model simulation show that the reaction follows the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, with the value of exponent n = 1–1.25 for absorption and 1 for desorption. This indicates that the rate-limiting hydrogen absorption and desorption steps are jointly governed by hydrogen diffusion and grain boundary nucleation of alpha-solid solution and beta-hydride. The activation energies for both hydrogen absorption and desorption decrease along with increasing hydrogen content in the hydride.