Air-stable magnesium nickel hydride with autocatalytic and self-protective effect for reversible hydrogen storage

Among the factors which restrict the large-scale utilization of magnesium-based hydride as a hydrogen storage medium, the high operating temperature, slow kinetics, and air stability in particular are key obstacles. In this work, a novel method, namely hydriding combustion synthesis plus short-term mechanical milling followed by air exposure, was proposed to synthesize air stable and autocatalytic magnesium nickel hydride (Mg2NiH4), which shows excellent hydrogen absorption/desorption kinetics, capacity retention and oxidation resistance. The short-term-milled Mg2NiH4 can desorb 2.97 wt.% hydrogen within 500 s at 230 °C. Even after exposure under air atmosphere for 67 days, it can still desorb 2.88 wt.% hydrogen within 500 s at 230 °C. The experimental and theoretical results both indicated that the surface of as-milled Mg2NiH4 was easy to be oxidized under air atmosphere. However, the in-situ formed Ni during air exposure of Mg2NiH4 improved the hydrogen desorption kinetics, and the formed surface passivation layer maintained the hydrogen storage capacity and avoided further poisoning, which we called autocatalytic and self-protective effect. Such a novel dual effect modified the reaction activity and oxidation resistance of the air-exposed Mg2NiH4. Our findings provide useful insights into the design and preparation of air stable metal-based hydride for large-scale utilization and long-term storage.