Effects of Ni precursors on the formation of Mg-Fe-Ni intermetallic hydrides, kinetics, and reversibility.

This work focuses on the effects of Ni precursors (metallic Ni or Mg 2 NiH 4 ) on the formation of Mg-Fe-Ni intermetallic hydrides as well as their de/rehydrogenation kinetics and reversibility. After ball milling and sintering, the formation of Mg 2 FeH 6 and Mg 2 NiH 4 are found in both samples, while MgH 2 is observed only in the sample with metallic Ni. Both samples show comparable hydrogen capacities of 3.2-3.3 wt% H 2 during the 1 st dehydrogenation, but the sample with metallic Ni decomposes at a lower temperature (Δ T = 12 °C) and shows faster kinetics. Although phase compositions after dehydrogenation of both samples are comparable, their rehydrogenation mechanisms are different. This affects the kinetic properties upon cycling and reversibility. Reversible capacities of the samples with metallic Ni and Mg 2 NiH 4 during the 2 nd dehydrogenation are 3.2 and 2.8 wt% H 2 , respectively, while those during the 3 rd -7 th cycles reduce to ∼2.8 and 2.6 wt% H 2 , respectively. Chemical and microstructural characterizations are carried out to explain de/rehydrogenation pathways.