CYCLIC STABILITY OF VARIOUS APPLICATION-RELEVANT METAL-HYDRIDES

Six metal hydrides relevant for practical applications (four AB(5) all oys, one AB(2) alloy and Mg(Ni)) were thermally cycled at constant vol ume under a hydrogen gas atmosphere. Up to 64 000 absorption-desorptio n cycles have been completed so far (with LaNi4.7Al0.3). The respectiv e reversible hydrogen capacities as a function of the number of cycles were determined for the selected cycling conditions. Regeneration con ditions were determined when applicable. Various investigations (press ure-composition isotherms, thermal desorption spectroscopy, X-ray powd er diffractometry, magnetization, laser granulometry, scanning electro n microscopy-energy-dispersive X-ray analysis) were performed in order to determine the degradation and regeneration mechanisms involved. Th e reversible storage capacity of the AB(5) alloys decayed during cycli ng. This effect is stronger at higher temperatures and pressures. Howe ver, 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 kineti cs probably caused by sintering of the original powder.