Degradation of LaNi5 and LaNi4.7Al0.3 hydrogen-absorbing alloys by cycling

Abstract

The changes in pressure-composition isotherms, structural and magnetic properties upon cyclic hydrogen absorption-desorption have been investigated for LaNi5 and LaNi4.7Al0.3 in order to identify the origin of their performance degradation. The hydrogen storage capacity of LaNi5 was observed to degrade by 35% after cycling for 2000 times whereas the degradation for LaNi4.7Al0.3 under the same cycling condition was only 13%. Magnetic measurements showed the presence of superparamagnetic Ni precipitates in these degraded samples. Assuming that the precipitation of Ni is the result of disproportionation, our magnetic measurements yield a degradation of 13% for the LaNi5 sample and 9% for the LaNi4.7Al0.3 sample, respectively. Hence, the degradation observed for LaNi4.7Al0.3 can be mostly explained by taking into account the disproportionatlon reaction during cycling. On the contrary, the calculated degradation of LaNi5 explains only approximately 1/3 of the experiment. Our structural analysis suggests that the excess portion of degradation in LaNi5 could be due to the evolution of stable protume sites in LaNi5.