Effect of graphene addition on activation and kinetic properties of La–Mg–Ni-based hydrogen storage alloys

The La_1.7Pr_0.3Mg₁₆Ni hydrogen storage alloy was prepared by medium-frequency induction melting, and then the composite hydrogen storage alloy powder of La_1.7Pr_0.3Mg₁₆Ni + x wt.% (x = 0, 2, 4, and 6) graphene was prepared by ball milling for 10 h. The effect of the addition of graphene on the activation and hydrogen de/absorption properties of La_1.7Pr_0.3Mg₁₆Ni alloy was studied. The result demonstrated that these composite alloys were composed of La₂Mg₁₇, La₂Ni₃, and Mg₂Ni phases. After saturated hydrogen absorption, it was composed of LaH₃, Mg₂NiH₄, and MgH₂ phases, while during the dehydrogenation process, it was composed of LaH₃, Mg, and Mg₂Ni phases. The addition of graphene can help get a more homogeneous granule after ball milling and accelerate the first activation of dehydrogenation/hydrogen absorption. The hydrogen release activation energy of the alloys first decreases and then increases as the graphene content increases from x = 0 wt.% to x = 6 wt.%. The minimum activation energy of the composite hydrogen storage alloy is 51.22 kJ mol⁻¹ when x  = 4 wt.%.