Characterization of microstructure, hydrogen storage kinetics and thermodynamics of a melt-spun Mg86Y10Ni4 alloy

Abstract

Ternary Mg86Y10Ni4 alloy was successfully prepared by vacuum induction melting and subsequent melt-spinning technique. The phase composition and microstructure of the melt-spun and hydrogenated samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The melt-spun alloy had an amorphous structure, and it transformed into nanocrystalline during the first hydrogenation process. The hydrogenated sample was composed of MgH2, Mg2NiH4, YH2, and a small amount of YH3. The hydrogen absorption/desorption kinetics and thermodynamics were measured by Sievert's apparatus at various temperatures. It was found that the melt-spun Mg86Y10Ni4 alloy could be fully activated after five hydrogenation and dehydrogenation cycles at 380 °C, and it exhibited a reversible gravimetric hydrogen storage capacity of about 5.3 wt%. The enhanced hydrogen sorption kinetics during the first few cycles can be attributed to the increased specific surface caused by the pulverization and cracking of the alloy particles. The activation energy for dehydrogenation reaction was determined to be 67 kJ/mol and 71 kJ/mol by using Arrhenius equation and Kissinger equation respectively. The thermodynamics of the sample was also evaluated by pressure–composition–isotherms, and the results shown that the enthalpy and entropy changes of Mg/MgH2 transformation in the Mg86Y10Ni4 alloy were slightly higher than that of pure Mg/MgH2.