Cyclic stability of various application-relevant metal hydrides

Six metal hydrides relevant for practical applications (four AB(5) alloys, one AB(2) alloy and Mg(Ni)) were thermally cycled at constant volume under a hydrogen gas atmosphere. Up to 64 000 absorption-desorption cycles have been completed so far (with LaNi4.7Al0.3). The respective reversible hydrogen capacities as a function of the number of cycles were determined for the selected cycling conditions. Regeneration conditions were determined when applicable. Various investigations (pressure-composition isotherms, thermal desorption spectroscopy, X-ray powder diffractometry, magnetization, laser granulometry, scanning electron microscopy-energy-dispersive X-ray analysis) were performed in order to determine the degradation and regeneration mechanisms involved. The reversible storage capacity of the AB(5) alloys decayed during cycling. This effect is stronger at higher temperatures and pressures. However, the original capacity of all four materials could be recovered by heating to about 400-500 degrees C in vacuum. Disproportionation was found to be the degradation mechanism for the AB(5) alloys. The AB(2) alloy showed no degradation after 42 400 cycles. Mg(Ni) degrades as a consequence of a reduction of the comparatively slow absorption kinetics probably caused by sintering of the original powder.