Effect of element substitution on hydrogen storage properties of La-Mg-Ni-based alloys prepare by heat treatment

Abstract

La-Mg-Ni-based hydrogen storage alloy has become a promising material due to its high hydrogen storage capacity and easy activation. Nevertheless, the use of this alloy system is constrained by its inadequate cycle steadiness, limiting its prospects for widespread commercialisation and industrialisation. In this work, Y and Mn elements with smaller atomic radius are used to partially replace La and Ni elements with larger atomic radius in La-Mg-Ni-based hydrogen storage alloys, so as to reduce the difference in phase volume between LaMgNi₄ phase and La(NiMn)₅ phase. Annealing treatment promotes the diffusion of alloying elements, eliminates the segregation of as-cast elements, and homogenizes the spatial distribution of AB₂ and AB₅ phases in the material matrix, improving its cycle stability. The synergistic effect of the element substitution strategy combined with the annealing process on the hydrogen storage performance of La-Mg-Ni-based alloys was systematically explored, including phase composition, microstructure, kinetic performance, cycling performance, and thermodynamic performance. The research findings suggest that the primary phase structure of annealed alloys consists of LaMgNi₄ and La(NiMn)₅ phases. With the increase of Y substitution, YMgNi₄ and Ni₃Y phase structures appear. There is a small amount of YH₂ phase in the hydrogen storage alloy after dehydrogenation due to the substitution of Y, which is due to the high decomposition temperature of YH₂. The Ni₃Y phase and YH₂ introduce more phase interfaces and boundaries of grains, which enhances the hydrogen uptake and release rates. The as-cast alloy is annealed for a more uniform distribution of elements, and the structure changes from a strip-like dendrite to a cellular crystal. The interphase distribution of the cellular crystal structure provides a good diffusion channel for the migration of hydrogen atoms during the hydrogen absorption/desorption process, which significantly improves the hydrogen absorption rate of the alloy. All annealed hydrogen storage alloys can be fully activated after one hydrogen absorption/desorption cycle. The first activation of Y4 alloy at 373 K temperature only takes 15 min to reach 90 % of the maximum hydrogen absorption capacity, which is nearly 57 % shorter than that of Y0 alloy. The annealed Y4 alloy has a 1.67 wt% hydrogen absorbing ability at 313 K and a 1.26 wt% hydrogen desorption ability at 373 K. With a rise in the replacement of Y elements, the capacity retention rises from 52.21 % to 74.55 % following 50 kinetic cycles of the alloy. The substitution of Y element reduces the phenomenon of crack propagation and pulverization on the surface of the particles during the hydrogen absorption and desorption process, thus significantly improving the cycle stability of La-Mg-Ni-based hydrogen storage alloys. This work shows that the substitution of Y and Mn elements and the annealing process at 1173 K synergistically optimize the hydrogen absorption and desorption kinetics and cycle stability of La-Mg-Ni-based alloys, which provides a feasible strategy for its commercial application.