Influence of elemental composition on crystal structure and hydrogen storage performance for Mg-containing alloys

The influence of elemental composition on the crystal structure, hydrogen storage and electrochemical properties for Mg-containing alloys was investigated. As La/Mg ratio decreased, the slight change of Ni content was detected. XRD results indicated that these alloys were composed of LaNi₅ and Mg-containing phases. The lattice parameters of Mg-containing phases decreased. Meanwhile, the mass fraction of Mg-containing phases varied with the change of La/Mg and Ni. The hydrogen storage capacity reached ～1.6 wt.% for La/Mg ≧ 3:1 and decreased to ～0.71 wt.% for La/Mg=1. Two hydrogen absorption processes were observed because of the existence of the multiphases for La/Mg ≧ 3:1. With decreasing La/Mg ratio, the equilibrium pressure rose due to the shrinkage of the lattice parameter. Meanwhile, one hydrogen absorption process was obviously present. The discharge capacity of these as-cast alloys was higher, but the cyclic stability was poor for La/Mg ≧ 3:1 due to the partial amorphisation. It was better for La/Mg≤2 although the discharge capacity was lower. The polarization resistance increased, contrarily the exchange current density decreased with decreasing Mg content.