Activation mechanism and dehydrogenation behavior in bulk hypo/hyper-eutectic Mg-Ni alloy

To investigate the effect of microstructure on the better de-/hydrogenation property of Mg-based alloy, hypo-eutectic Mg-8Ni (at. %) alloy and hyper-eutectic Mg-15Ni alloy are prepared by metallurgy method. The phase constitutions and microstructures are characterized by XRD and SEM/EDS. Mg-8/15Ni alloy is composed of primary Mg/Mg₂Ni and eutectic Mg-Mg₂Ni. In isothermal sorption test, Mg-15Ni alloy shows preferable activation performance and faster de-/hydrogenation rates than Mg-8Ni alloy. The respective hydrogen uptake capacity in 165min is 5.62 wt% and 5.76 wt% H₂ at 300 °C 3 MPa. Intersections of Mg-Mg₂Ni eutectic phase boundaries with particle surface provide excellent sites and paths for the dissociation and permeation of hydrogen. The de-/hydrogenation enthalpy and entropy values are determined by PCI measurement. Based on the DSC curves at different heating rates, the desorption behavior of Mg-8/15Ni hydride is revealed and the respective activation energy is calculated to be 134.67 kJ mol⁻¹ and 88.34 kJ mol⁻¹ H₂ by Kissinger method. Synergic dehydrogenation occurs in eutectic MgH₂-Mg₂NiH₄, which facilitates the primary MgH₂ in Mg-8Ni hydride to decompose at a lower temperature. The rapid H diffusion and synergic effect in eutectic MgH₂-Mg₂NiH₄ collectively contribute to the lower dehydrogenation energy barrier of Mg-15Ni hydride.