Enhanced de-/hydrogenation kinetics of a hyper-eutectic Mg85Ni15-Ag alloy facilitated by Ag dissolving in Mg2Ni

In order to accelerate the hydrogen absorption and desorption rates of hyper-eutectic Mg-Ni alloy, Mg ₈₅Ni _15-xAg _x ( x = 0, 0.2, 0.4, 0.6) alloys were prepared and the results show that fishbone eutectic was formed without change of phase composition. It was found that Ag atoms were dissolved in fishbone eutectic Mg ₂Ni and the average cell parameters of Mg ₂ Ni increase with the increase of Ag content. Owing to adjusting lattice parameters, the hydrogen absorption and desorption rates of Mg ₈₅Ni _14.8Ag _0.2 alloy are significantly improved. About 4.47 wt% hydrogen is absorbed at 275 ℃ only in 1 min, with the maximum capacity of 5.84 wt% in 60 min. The Mg ₈₅Ni _15-xAg _x ( x = 0.2, 0.4, 0.6) hydrides can be completely dehydrogenated within 10 min at 275 ℃. The corresponding activation energies for dehydrogenation are determined to be 80.4, 97.1 and 97.8 kJ/mol, respectively, which are lower than 117.8 kJ/mol of Mg ₈₅Ni ₁₅ . The onset dehydrogenation temperature of Mg ₈₅Ni _14.8Ag _0.2 hydride is reduced to 201 ℃, which is much lower than 220 ℃ of the Mg ₈₅Ni ₁₅ hydride. The dissolving of Ag in Mg ₂Ni leads to a lattice expansion, which facilitates the diffusion for hydrogen atoms and decrease the energy barrier required for nucleation of Mg/Mg ₂Ni. As a result, the hydrogen de-/absorption rates were obvious accelerated and the onset dehydrogenation temperature was decreased.