Synthesis And Properties Of Mg₂Ni Based Hydrogen Storage Alloys

The Mg-based hydrogen storage alloys are considered as one of the most promising hydrogen storage alloys, offering the combination of great hydrogen storage capacity, low density , abundant resource and low cost. However, their difficult hydrogen-release conditions, overstable hydrides, bad kinetics properties in hydrogen absorbing process, poor corrosion resistance in alkali liquor and short cycle life are the blocks in the road for Mg-based hydrogen storage alloys to become a strong competitor in this field. In this paper, four kinds of Mg-based hydrogen storage alloys within two series, Mg₂N and Mg₂Ni_0.9Zr_0.1 respectively, were prepared by the two methods. One is induction melting, and the other is the synergistic effect of induction melting and diffusion annealing. And twenty kinds of Mg-based hydrogen storage alloys within six series, Mg₂Ni, Mg-Ni-Fe, Mg-Ni-Al, Mg-Ni-Cr, Mg-Ni-Ti and Mg-Ni-Al-Cr respectively, were synthesized by two-step method. All the prepared alloys above were subjected to detailed examinations, including XRD, metallographic examination, SEM, chemical constitution analysis, TEM, granularity analysis, electrochemical properties analysis (including discharge capacity, cycle stability, D value). The results were as follow: (1) Two alloys (Mg₂Ni and Mg₂Ni_0.9Zr_0.1) can be produced by induction melting at a rather low temperature. XRD results indicated that both the two alloys share the same main phase of Mg₂Ni. The electrochemistry analysis showed that the fist discharge capacity of Mg₂Ni and Mg₂Ni_0.9Zr_0.1 at room temperature were 45.6mA·h·g^-1 and 56.7mA·h·g^-1, respectively, and the discharge capacity were 97% and 87% after 20 charge and discharge cycles. In particular, alloys'electrochemistry properties were significantly improved by diffusion annealing. (2) 20 kinds of alloys with six series (Mg₂Ni, Mg-Ni-Fe series, Mg-Ni-Al series, Mg-Ni-Cr series, Mg-Ni-Ti series and Mg-Ni-Al-Cr series) can be synthesized by two-step method (mechanical activation and sinter in solid). XRD results showed that the main phase of all the alloys was Mg2Ni, and another hydrogen-storage phase (Mg₃AlNi₂) was found both in Mg-Ni-Al and Mg-Ni-Al-Cr series alloys. The electrochemistry analysis indicated that the Mg-Ni-Al-Cr series,whose average discharge capacity at room temperature was as high as 110mA·h·g^-1 and average attenuation rate of discharge capacity was about 50% after 20 cycles, held the best electrochemical properties in the six tested series. In addition, the effects of preparation technology, substitute elements, hydrogen-storage phase Mg3AlNi2, D value and the "white grain"in SEM photos on the electrochemistry-properties improvement of the alloys were also discussed in this paper.