Formation of AlNi phase and its influence on hydrogen absorption kinetics of Mg77Ni23-xAlx alloys at intermediate temperatures

Abstract

In order to reduce the obstacle influence of coarse Mg2Ni phase on hydrogen absorption kinetics in Mg–Ni alloys, aluminum was doped and Mg77Ni23-xAlx (x = 0, 3, 6, 9) alloys were prepared. The results show that AlNi phase was formed when Al was added, the size of primary Mg2Ni phase decreases with increasing Al content till 6 at.%, while primary Mg2Ni phase was diminished and primary Mg phase was formed when Al content increased to 9 at.%. The initial hydrogenation rates of Mg77Ni23-xAlx alloys were increased, which is resulted from the refined primary Mg2Ni and the catalytic AlNi phase. More importantly, the hydrogenation rates and capacities were significantly improved at 150 °C, especially for the Mg77Ni17Al6 alloy. The apparent activation energy of the Mg77Ni17Al6 alloy for hydrogenation was reduced to 73.68 kJ/mol from 102.27 kJ/mol of the Mg77Ni23 alloy. Its enthalpy changes for hydrogenation at low and high platforms are 72.3 kJ/mol and 53.9 kJ/mol, respectively. The multiple channels and short distance for hydrogen atoms diffusion provided by refined primary Mg2Ni phase, the solid dissolution of Al in Mg2Ni lattice, and catalytic effect of AlNi on hydrogenation, leading to the improvement of the hydrogen storage properties.