Microstructure and hydrogen storage properties of Mg-based Mg85Zn5Ni10 alloy powders

Mg85Zn5Ni10 ternary alloy was synthesized through vacuum induction melting for the first time. Phase compositions and microstructures of Mg85Zn5Ni10 alloy powders were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). By utilizing a Sieverts apparatus, the hydrogenation and dehydrogenation properties of Mg85Zn5Ni10 powders were measured systematically. XRD and SEM results indicated that the Mg85Zn5Ni10 alloy powders contained the major phase Mg, the eutectic Mg-Mg2Ni and Mg-MgZn2 mixtures. The possible reaction pathway can be inferred as follows: Mg+Mg2Ni+MgZn2+H-2 MgH2+Mg2NiH4+MgZn2, indicating that MgZn2 did not react with H-2. After activation, the Mg85Zn5Ni10 alloy powders could absorb 5.4wt.% hydrogen reversibly and held an excellent hydrogenation kinetics at a relatively low temperature. At 360 degrees C, the alloy powders desorbed 5.351wt.% hydrogen in 264s. However, it only had fast dehydrogenation kinetics above 300 degrees C. The existence of MgZn2 contributed to improving the kinetic properties. During the hydriding and dehydriding, the formed cracks and defects promoted the kinetics and thermodynamic properties. The activation energy for dehydrogenation was 75.514kJ/mol. The enthalpy change values of hydrogenation and dehydrogenation were calculated to be -73.064kJ/mol and 76.674kJ/mol, respectively, indicating that melting with Ni and Zn could improve the thermodynamic property of Mg slightly.