Phase structure and electrochemical hydrogen storage performance of La4MgNi18M (M = Ni, Al, Cu and Co) alloys

Abstract

The substitution of transition metals such as Ni is one of the effective methods to improve the electrochemical performance of La-Mg-Ni alloys because this considerably affects the chemical composition and phase structure of the alloys. In this study, A5B19-type hydrogen storage alloys with the elemental composition of La4MgNi18M (M = Ni, Al, Cu, and Co) were prepared by vacuum induction melting followed by annealing. In addition, the effect of the incorporation of M on the phase transformation and electrochemical performance was investigated. The X-ray diffraction profiles revealed the presence of AB5, A2B7 (Ce2Ni7 and Gd2Co7), and A5B19 (Pr5Co19 and Ce5Co19) phases, and no A2B7 (Ce2Ni7 and Gd2Co7) phase, in the La4MgNi18Al alloys. The substitution of Al clearly led to the increased abundance of the A5B19 phase and the disappearance of the A2B7 phase. Cu and Co exhibited similar roles in the phase composition and contributed to the formation of the A2B7 phase. The substitution of Ni by Al, Cu, or Co led to the increase in the maximum discharge capacity Cmax, which is mainly related to the increase in the cell volume and the abundances of the A2B7 and A5B19 phases. With increasing annealing temperature, the Cmax of the La4MgNi18M alloy electrodes first increased and then decreased, with the highest value obtained at an annealing temperature of 1223 K. The change trend in the cycling stability of the La4MgNi18M alloy electrodes was inversely proportional to that of the abundance of the LaNi5 phase. The substitution of Ni by Al, Cu, or Co led to the enhancement in the cycling stability of the alloy electrodes, which was related to the improvement in the corrosion resistance and anti-pulverization ability.