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 Ni-based 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.