An extensive study of the Mg-Fe-H material obtained by reactive ball milling of MgH2 and Fe in a molar ratio 3:1

A mixture of MgH2 and Mg2FeH6 was synthesized by reactive ball milling of magnesium hydride and iron in hydrogen atmosphere. The material is highly nanocrystalline, with typical dimensions of the order of 10 nm; after hydrogen cycling at ~400 °C, well defined XRD peaks of Mg2FeH6 can be observed. Volumetric measurements of hydrogenation/dehydrogenation provide clear evidence of the presence of both hydrides even at lower temperatures. The relative content of magnesium-iron hydride increases on increasing H2 cycling temperature, passing from ~44% at 335 °C to ~54% at 390 °C. Already at 250 °C the composite releases ~3wt% H2 in ~1000 s, while above 340 °C, more than 4wt% H2 can be discharged in less than 100s, following the Johnson-Mehl-Avrami-Kolmogorov equation, with an exponent n = 1, compatible with a reaction controlled transformation. Finally, also the electrochemical performances in a lithium cell have been investigated: the material is able to undergo a conversion reaction and gives on the first discharge more than 1400 mAhg-1. The overpotentials decrease after materials activation by H2 sorption treatments. Moreover, for the first time, the partial reversibility of the conversion reaction for materials containing magnesium iron hydride is here reported. © 2017 Hydrogen Energy Publications LLC