Effect of annealing temperature on microstructures and electrochemical performances of La0.75Mg0.25Ni3.05Co0.2Al0.1Mo0.15 hydrogen storage alloy

•Annealing facilitates transformation of LaNi5 into (La, Mg)2Ni7 and (La, Mg)5Ni19 phase in a proper temperature.•Annealing enhancesCmax and cyclic stability, but impairs the HRD and activation ability.•The improved mechanism of comprehensive electrochemical performance is studied with respect to phase analysis.

The effect of annealing temperature on microstructures and electrochemical performances of La0.75Mg0.25Ni3.05Co0.2Al0.1Mo0.15 hydrogen storage alloy is investigated. The alloys was prepared via vacuum induction melting followed by annealing treatment for 8 h at the temperatures of 1073, 1123, 1173, and 1223 K, respectively. The major phases in the as-cast and annealed alloys are consisted of (La, Mg)2Ni7, (La, Mg)5Ni19 and LaNi5 phase while the residual phase MoNi4 existing in the as-cast alloy decomposes into Mo phase after heat treatment. Annealing treatment facilitates LaNi5 phase transforms into (La, Mg)2Ni7 and (La, Mg)5Ni19 phase, but the tendency is suppressed as annealing temperature increases. Furthermore, the annealed alloys have more homogeneous phase distribution and better crystallization. Electrochemical experiments manifest that annealing treatment is beneficial for the increase of the maximum discharge capacity while impairs the activation ability of the alloy electrodes. The cycle life is prolonged in consequence of the homogeneous phase distribution created by annealing treatment at 1073–1173 K. Nonetheless, annealing treatment distinctly deteriorates the kinetic property of the alloys electrodes which is mainly ascribed to the disappearance of MoNi4 phase with high-efficiency electrocatalytic activity. The comprehensive electrochemical properties of the annealed alloy at 1173 K presents a good balance between superior discharge capacity, kinetic property and remarkably improved cyclic stability.