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

Abstract

In order to accelerate the hydrogen absorption and desorption rates of hyper-eutectic Mg-Ni alloy, Mg 85 Ni 15- x Ag 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 2 Ni 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 85 Ni 14.8 Ag 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 85 Ni 15- x Ag 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 85 Ni 15 . The onset dehydrogenation temperature of Mg 85 Ni 14.8 Ag 0.2 hydride is reduced to 201 ℃, which is much lower than 220 ℃ of the Mg 85 Ni 15 hydride. The dissolving of Ag in Mg 2 Ni leads to a lattice expansion, which facilitates the diffusion for hydrogen atoms and decrease the energy barrier required for nucleation of Mg/Mg 2 Ni. As a result, the hydrogen de-/absorption rates were obvious accelerated and the onset dehydrogenation temperature was decreased. • The lattice size of Mg2Ni was regulated by doping Ag. • Hydrogen storage capacity of Mg85Ni14.8Ag0.2 alloy reaches 5.84 wt% at 275 ℃. • The Ea value for decomposition of Mg85Ni14.8Ag0.2 hydride decreases to 80.4 kJ/mol. • Addition of Ag reduces the initial desorption temperature of Mg85Ni15 hydrides.