Degradation mechanism against hydrogenation cycles in Mg _2-xPr_xNi₄ (x = 0.6 and 1.0)

A long cycle life of metal hydrides is paramount for applications. We present an investigation of the degradation mechanism against hydrogenation cycles in Mg_2-xPr_xNi₄ (x = 0.6 and 1.0). Mg_1.0Pr_1.0Ni₄ shows significant degradation and loss of capacity after only a few cycles. In stark contrast, Mg _1.4Pr_0.6Ni₄ did not show any reduction of hydrogen storage capacity until 100 cycles at 313 K. The peak broadening of X-ray diffraction (XRD) patterns and accumulation of lattice strain were observed concomitantly with an increase of hydrogenation cycles only in Mg _1.0Pr_1.0Ni₄. These changes were not observed in Mg_1.4Pr_0.6Ni₄. In pair distribution function (PDF) patterns, r-dependent peak broadening was observed and it became significant with an increasing number of cycles in Mg_1.0Pr _1.0Ni₄, suggesting an increase of the dislocation density. Mg_1.4Pr_0.6Ni₄ showed higher hardness and more pulverization upon hydrogenation than Mg_1.0Pr_1.0Ni ₄. These results suggest that the dominant mechanism, the stress induced by hydrogen occupation release, must change from the formation of dislocations to pulverization by increasing the Mg content of the alloy. We discuss how this leads to better cycling properties.