An extensive study of the Mg Fe H material obtained by reactive ball milling of MgH 2 and Fe in a molar ratio 3:1

Abstract A mixture of MgH 2 and Mg 2 FeH 6 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 Mg 2 FeH 6 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 H 2 cycling temperature, passing from ∼44% at 335 °C to ∼54% at 390 °C. Already at 250 °C the composite releases ∼3wt% H 2 in ∼1000 s, while above 340 °C, more than 4wt% H 2 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 H 2 sorption treatments. Moreover, for the first time, the partial reversibility of the conversion reaction for materials containing magnesium iron hydride is here reported.