Synthesis and hydrogen sorption properties of Mg2FeH6–MgH2 nanocomposite prepared by reactive milling

The complex hydride Mg2FeH6 is an interesting material for hydrogen storage due to its high gravimetric hydrogen capacity as well as for having the highest known volumetric hydrogen density – 150 kg m−3. Several papers have recently reported its synthesis from the stoichiometric precursors 2Mg–Fe or 2MgH2–Fe through sintering process and ball milling under argon or hydrogen atmosphere. However, regardless of processing conditions, a remaining iron always was identified by X-rays diffraction, which resulted in a lower hydrogen storage capacity. In the present paper, the Mg2FeH6–MgH2 nanocomposite was successfully synthesized through high-energy ball milling from 3Mg–Fe mixture under hydrogen atmosphere at room temperature. After the ball milling, X-rays diffraction patterns showed that the iron was kept to a minimum, which was also confirmed by simultaneous thermal analysis of differential scanning calorimetry and thermogravimetry. The gravimetric density of the ball milled 3Mg–Fe is more than 5 wt.% of hydrogen. In the case of 2Mg–Fe, processed in the same condition, the measured hydrogen capacity was 3.5 wt.%. The hydrogen sorption kinetics analyses were performed in a Sievert's apparatus in temperatures ranging from 250 °C to 350 °C. Enhanced hydrogen sorption kinetics was observed for these 3Mg–Fe milled powders in comparison with the 2Mg–Fe ones.

► Mg2FeH6–MgH2 nanocomposite was synthesized by HEBM from 3Mg–Fe mixture. ► The gravimetric density of the ball milled 3Mg–Fe is more than 5 wt.% of hydrogen. ► The 2Mg–Fe mixture processed in the same condition has only 3.5 wt.% of hydrogen. ► The 3Mg–Fe powder presents better hydrogen sorption properties than the 2Mg–Fe one.