Kinetic mechanisms of hydriding and dehydriding reactions in La–Mg–Ni alloys investigated by the modified Chou model

Abstract

The phenomenon of volume change is very important in the hydriding and dehydriding reactions to clarify their kinetic mechanisms. However, few of the theoretical models take this factor into account in the study of kinetic mechanisms. In the present work, the modified Chou model with considering the volume change by Pilling-Bedworth Ratio (PBR) β is applied to interpret the kinetic mechanisms of hydriding and dehydriding reactions in La–Mg–Ni ternary hydrogen storage alloys. The calculation results agree well with the experimental data, indicating that the modified Chou model can well describe the kinetic behaviors of La–Mg–Ni alloys. Diffusion is determined to be the rate-controlling step of hydriding and dehydriding for the examples in this work. The activation energies are calculated to be 39.4 kJ/mol for hydriding reaction (β = 1.29) and 93.1 kJ/mol for dehydriding reaction (β = 0.78) of Mg–10.6La–3.5Ni nanoparticles at 453–623 K. For the hydrogen absorption kinetics of La2Mg16Ni alloy under 0.5–3 MPa at 598 K, β is 1.25 and the equilibrium hydrogen pressure is calculated to be 0.332 MPa. For the hydrogen desorption kinetics of Mgx(LaNi3)100−x alloys at 623 K, the characteristic times increase in the order of tc(x = 70,β= 0.79) < tc(x= 60, β= 0.79) < tc(x= 40, β= 0.97) < tc(x= 50, β = 0.85). The value of β is 0.77 and the hydrogen desorption rate of Mg–5La–10Ni alloy prepared by the ultrahigh pressure (UHP) method is faster than that of the alloy prepared by the as-cast method, whose characteristic times are 7428 s and 4651 s, respectively. Furthermore, the calculation accuracy can also be improved by the modified Chou model compared with the original Chou model after taking PBR into consideration.