Microstructure characteristics, hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

Abstract

In this paper, the Mg95-X-Nix-Y5 (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH2, Mg2NiH4, and YH3 phases. And, the in-situ formed YH3 phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg2Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni5, Ni10, and Ni15 samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H2. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg80Ni15Y5 alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H2. Besides, the dehydrogenated activation energy of the Mg80Ni15Y5 alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni5 alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.