Degradation Mechanism against Hydrogenation Cycles in Mg2–xPrxNi4 (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 Mg2–xPrxNi4 (x = 0.6 and 1.0). Mg1.0Pr1.0Ni4 shows significant degradation and loss of capacity after only a few cycles. In stark contrast, Mg1.4Pr0.6Ni4 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 Mg1.0Pr1.0Ni4. These changes were not observed in Mg1.4Pr0.6Ni4. In pair distribution function (PDF) patterns, r-dependent peak broadening was observed and it became significant with an increasing number of cycles in Mg1.0Pr1.0Ni4, suggesting an increase of the dislocation density. Mg1.4Pr0.6Ni4 showed higher hardness and more pulverization upon hydrogenation than Mg1.0Pr1.0Ni4. These results suggest that the dominant mechanism, the stress induced by hydr...