Effect of Cu substitution for Ni on microstructural evolution and hydrogen storage properties of the Mg₇₇Ni_20−xCu_xLa₃ (x = 0, 5, 10 at%) alloys

Highlights: • The lattice parameters of Mg₂Ni phase increase with Cu substituting for Ni. • The Mg–Ni–Cu–La alloys keep high hydrogen storage capacities of about 4.2 wt%. • The hydride formation enthalpy of Mg₂(Ni + Cu)–H₂ is greatly reduced. - Abstract: The effect of Cu-substitution on the microstructures and hydrogen storage thermodynamic properties of Mg₇₇Ni_20−xCu_xLa₃ (x = 0, 5, 10 at%) alloys prepared by vacuum induction melting has been studied. XRD result shows that the samples consist of Mg, Mg₂Ni and LaMg₁₂ phases. With Cu substituting for Ni, the structures of all phases do not change except for a slight increase in lattice parameters of Mg₂Ni phase. The P–C-isotherms of the Mg-based alloys have two distinct hydrogen absorption/desorption plateaus, which correspond to the Mg–H₂ system and the Mg₂(Ni + Cu)–H₂ system, respectively. The hydrogen storage capacities of Mg₇₇Ni_20−xCu_xLa₃ (x = 0, 5, 10 at%) alloys are 4.18 wt%, 4.20 wt% and 4.51 wt%, respectively. In the hydrogenated samples, these alloys consist of MgH₂, monoclinic Mg₂NiH₄, orthorhombic Mg₂NiH₄, LaH₃ as well as MgCu₂. The presence of MgCu₂ indicates the reaction of Mg₂Cu with hydrogen. The hydride formation enthalpy of alloys were calculated by the van't Hoff equation. With the addition of Cu, the thermodynamic property is greatly improved, and the formation enthalpy values decrease to −51.02, −33.71 from −56.05 kJ mol⁻¹ H₂ for the Mg₂(Ni + Cu)–H₂ plateaus of Mg₇₇Ni_20−xCu_xLa₃ (x = 0, 5, 10 at%) alloys, respectively