A₂B₇-type La–Mg–Ni alloys prepared by Mg thermal diffusion for improved hydrogen storage performance

A novel approach based on thermal diffusion was used to achieve controllable Mg content in A₂B₇-type La–Mg–Ni-based alloys. The formation mechanism of the A₂B₇-type phase as a result of the thermal diffusion process and the effect of Mg content on hydrogen storage performance were investigated. X-ray diffraction (XRD) patterns and Rietveld refinement results showed that increased Mg transformed the LaNi₅ phase in the La_0.74Sm_0.03Y_0.23Ni_4.32Al_0.04 precursor alloy into a superlattice structure. Scanning electron microscopy (SEM) images showed that Mg was evenly distributed in the alloy bulk. Mg in the superlattice significantly inhibited the phase decomposition of the superlattice structure during the hydrogen absorption/desorption cycles. An A₂B₇-type La_0.57Sm_0.02Y_0.18Mg_0.23Ni_3.38Al_0.03 alloy composed of Gd₂Co₇ and Ce₂Ni₇ phases was successfully synthesized. The pressure–composition isotherm profiles showed that the alloy had a hydrogen storage capacity as high as 1.73 wt%, with good cycling stability. After 50 cycles of hydrogen absorption/desorption, the alloy retained a hydrogen storage capacity of 1.45 wt%, with a capacity retention rate of up to 84.28%. The Mg thermal diffusion process thus provides a new approach for the controlled preparation of La–Mg–Ni-based alloys.