Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH2) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH2 via in situ formed additives from the conversion of an MgNi2 alloy upon de/hydrogenation. The MgH2–16.7 wt%MgNi2 composite was synthesized by ball milling of Mg powder and MgNi2 alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH2, and the others reacted with the MgNi2 generating Mg2NiH4, which produced in situ Mg2Ni during dehydrogenation. Results showed that the Mg2Ni and Mg2NiH4 could induce hydrogen absorption and desorption of the MgH2, that it absorbed 2.5 wt% H2 at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg2NiH4 decreased greatly by 100 K, whereas it is 623 K for MgH2. Furthermore, benefiting from the catalyst effect of Mg2NiH4 during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol−1 H2 from 129.5 kJ mol−1 H2.