Formation of Mg₂Ni/Cu phase and de-/hydrogenation behavior of Mg₉₁Ni_9-xCu_x alloy at moderate temperatures

The addition of Cu is proved effective to improve the hydrogen storage property of hypoeutectic Mg–Ni alloy. In this study, Mg₉₁Ni_9-xCu_x (x = 3, 4.5, and 6) alloys are prepared by melting, and their isothermal de-/hydrogenation properties are characterized, especially at moderate temperatures. The results show that a much thicker eutectic structure is formed in Mg₉₁Ni₃Cu₆ alloy, and the hydrogen storage properties deteriorate gradually with the Cu addition. Cu atoms can increase the lattice constants of Mg₂Ni and Mg_6.33Ni by replacing Ni atoms. After repeated absorption and desorption cycles, Mg₂Ni/Cu crystals are preserved by forming high-density Mg₂Ni/Cu nanophases, and the diffusion of H atoms and nucleation of hydrides are facilitated. Also, the nucleation of hydrides in Mg₉₁Ni₆Cu₃ alloy are facilitated by the “synergistic effect”. Comparing with the Mg₉₁Ni_4.5Cu_4.5 and Mg₉₁Ni₃Cu₆ alloys, the Mg₉₁Ni₆Cu₃ alloy shows a higher hydrogen capacity of 5.60 wt % with faster kinetic property at 175 °C under 1 MPa hydrogen. The Mg₉₁Ni₆Cu₃ hydride shows the faster desorption rate at 225 °C. In Mg₉₁Ni_4.5Cu_4.5 and Mg₉₁Ni₃Cu₆ alloys, the Mg₂Ni/Cu phases gradually transform into Mg₂Cu-like phases, and the independent hydrogenation processes lead to their reduced hydrogen storage capacities.