The effects of hydrogen concentration, particle size distribution and shape variations on rate of hydrogen absorption and desorption of MmNi₅-based hydrogen-absorbing alloys

A model for hydriding-dehydriding reactions was constructed by considering the hydrogen concentration dependency of diffusion coefficient, the particle size distribution, and the shape variation of MmNi₅-based hydrogen-absorbing alloys was constructed. The effects of the diffusion coefficient and particle conditions on the rate of hydrogen absorption or desorption for MmNi₅-based hydrogen-absorbing alloys were numerically investigated by the finite element method by comparing calculated results with the available experimental ones for pure hydrogen. When the experimentally determined diffusion coefficient was used and assumed to be constant regardless of concentration, the calculated results were in a good agreement with experimental ones. In the desorption processes, however, the results calculated with a diffusion coefficient that was dependent on hydrogen concentration showed better agreement with the experimental ones than those calculated with a constant diffusion coefficient. Thus, like that of LaNi5-based alloys, the diffusion coefficient of MmNi₅-based alloys seems to depend on the hydrogen concentration. When particle size distribution was considered, the termination time of hydriding-dehydriding reaction was longer than when average particle size was used and it was shown that the larger particles determined the termination time. From these results, it is important to consider the particle size distributions to estimate the hydrogen diffusion coefficient in the hydrogen absorption-desorption measurements. When shape variations were considered, the reaction rate increased with an increase in the oblateness for the constant projected area. Furthermore, the reaction rate of cubic particles was larger than those of spherical and ellipsoidal particles. Thus, the diffusion coefficient calculated on the assumption of spherical particles is expected to be larger than the actual one.