Synergistic effect of TiH2 and air exposure on enhancing hydrogen storage performance of Mg2NiH4

Air stability of magnesium-based hydrogen storage materials is one of the key challenges for its large-scale application, as the hydrides could be oxidized easily during air exposure. In this work, TiH2 and graphite were added into magnesium nickel hydride (Mg2NiH4) by short-time (0.5 h) mechanical milling to clarify the effect of air exposure on hydrogen storage behavior of the composites. Among the various Mg2NiH4-additive composites, the Mg2NiH4-TiH2 composite showed the highest hydrogen desorption and absorption capacity retention of 76.9 % and 75.6 % after air exposure for 30 days, respectively. It is worth noting that both the hydrogen absorption and desorption kinetics of the composites were enhanced after air exposure, especially for the Mg2NiH4 co-added with TiH2 and graphite composite. The excellent air stability and enhanced hydrogen reaction kinetics of Mg2NiH4-TiH2 composite should be attributed to the ternary-synergistic effect, including the protective effect of passivation layer, catalytic and sacrificial effect of TiH2, and catalysis of in-situ formed Nibased products. They prevented further oxidation of the active matrix and provided more active sites for hydrogen absorption and desorption reactions. Our findings might guide the design and development of reversible metal hydrides with excellent reaction activity and air stability.