Effect of Ni content on microstructural evolution and hydrogen storage properties of Mg–xNi–3La (x = 5, 10, 15, 20 at.%) alloys

Highlights: • The Mg–(5, 10, 15, 20 at.%)Ni–3La alloys keep high hydrogen storage capacity of 4.51–5.50 wt%. • The dehydriding rate first increases, and then decreases with an increase of Ni content. • The Mg–(5, 10, 15, 20 at.%)Ni–3La alloys show low apparent activation energy of 80.36–95.43 kJ/mol. • The Mg–15Ni–3La alloy shows an optimum hydrogen desorption kinetics. - Abstract: The effect of Ni content on microstructural evolution and hydrogen storage properties of Mg–xNi–3La (x = 5, 10, 15, 20 at.%) ternary alloys prepared by vacuum induction melting has been investigated. The as-cast Mg–xNi–3La (x = 5, 10, 15, 20 at.%) alloys consist of Mg, Mg₂Ni, LaMg₁₂ and La₂Mg₁₇ phases. An increase of Ni content results in an amount increase of the Mg₂Ni phase and coarsening of the Mg₂Ni microstructure in the alloys. The P–C-isotherms of the Mg-based alloys show that the hydrogen storage capacity decreases in the order "Mg–5Ni–3La (5.50 wt%) > Mg–10Ni–3La (5.16 wt%) > Mg–15Ni–3La (4.60 wt%) > Mg–20Ni–3La (4.51 wt%)". But the change of Ni content does not alter the thermodynamic properties of Mg–H₂ system and Mg₂Ni–H₂ system in Mg–xNi–3La (x = 5, 10, 15, 20 at.%) alloys. All of the four alloys show excellent desorption kinetics. The desorption kinetics of the alloys first increases, and then decreases with the increase of Ni content. The Mg–15Ni–3La alloy shows the highest dehydriding rate and the lowest apparent activation energy value of 80.36 kJ/mol, which is the optimum kinetics of hydrogen desorption among the alloys. This is attributed to the most appropriate amount of the Mg₂Ni phase and eutectic microstructures in the alloy