Degradation of LaNi₅ and LaNi_4.7Al_0.3 hydrogen-absorbing alloys by cycling

The changes in pressure-composition isotherms, structural and magnetic properties upon cyclic hydrogen absorption-desorption have been investigated for LaNi₅ and LaNi_4.7Al_0.3 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 LaNi_4.7Al_0.3 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 LaNi_4.7Al_0.3 sample, respectively. Hence, the degradation observed for LaNi_4.7Al_0.3 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.