Enhanced dehydrogenation properties of Mg-Ni-Cu hydride after air exposure

It is difficult to strike the balance between air-stability and high reactivity of magnesium-based hydrogen storage materials. The high reactivity requires more fresh surface without oxidation, but this always comes with severe surface passivation since the fresh metal surface can react with the O₂/H₂O/CO₂ in the air easily. In this work, a new strategy was investigated to improve the reactivity and air-stability by substituting part of nickel (Ni) in Mg-Ni hydride with copper (Cu). The hydrides with the composition Mg₈₀Ni_20-yCu_y (y = 1, 2, 3, 4) were synthesized and analyzed, and the results demonstrated that Cu substitution enhanced the hydrogen storage properties of the hydrides. Notably, the Mg₈₀Ni₁₉CuH_x hydride showed the best performance, including superior air stability, the highest hydrogen storage capacity, and the greatest improvement after air exposure. The onset dehydrogenation temperature of Mg₈₀Ni₁₉CuH_x decreased from 310.0 °C to 237.5 °C after 3 months of air exposure. At 245 °C, the air-exposed Mg₈₀Ni₁₉CuH_x released 4.15 wt% H₂ within 1000 seconds, retaining more than 97 % of its initial capacity before air exposure. After 3 months of air exposure, the observation revealed that an Mg(OH)₂ passivation layer formed and nickel particles were generated in situ on the surface of Mg₈₀Ni₁₉CuH_x, where the passivation layer preserved the hydride's air stability, and the Ni particles enhanced the dehydrogenation kinetics. This work offers a promising approach for the large-scale production of highly reactive yet air-stable hydrides.