Hydrogen sorption kinetics of La–Ni–Sn storage alloys

In this work we identify the limiting mechanism in the hydrogen absorption reaction of LaNi5−xSnx alloys. For this purpose the absorption reaction was modeled as a process involving several partial reactions occurring sequentially. The analysis was performed by comparing the experimental data with the descriptions derived from four different kinetic expressions, each considering that there is only one mechanism governing the hydrogen absorption reaction while the other steps occur near equilibrium conditions. Kinetic curves corresponding to the binary alloy LaNi5 and to the substituted alloys LaNi4.73Sn0.27 and LaNi4.55Sn0.45 were measured using the Sieverts volumetric technique. Experiments were carried out over a temperature range from 300 K to 348 K and under different hydrogen pressures. In all cases, the best fit to the experimental data was achieved by applying an expression that assumes the limiting step of the overall reaction is the diffusion of hydrogen atoms through the hydride layer.