Characteristics of A2B7-type La[sbnd]Y[sbnd]Ni-based hydrogen storage alloys modified by partially substituting Ni with Mn

Abstract

LaY2Ni10.5−xMnx (x = 0.0, 0.5, 1.0, 2.0) alloys are prepared by a vacuum induction-quenching process followed by annealing. The structure, as well as the hydriding/dehydriding and charging/discharging characteristics, of the alloys are investigated via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), pressure-composition isotherms (PCI), and electrochemical measurement. The alloys have multiphase structures mainly composed of Gd2Co7-type (3R) and Ce2Ni7-type (2H) phases. Partial substitution of Ni by Mn clearly increases the hydrogen storage capacity of the alloys. The x = 0.5 alloy exhibits a maximum hydrogen storage capacity of 1.40 wt % and a discharge capacity of 392.9 mAh g−1, which are approximately 1.5 and 1.9 times greater than those of the x = 0.0 alloy, respectively. The high-rate dischargeability (HRD) of the x = 0.5 alloy is higher than that of the other alloys because of its large hydrogen diffusion coefficient D, which is a controlling factor in the electrochemical kinetic performance of alloy electrodes at high discharge current densities. Although the cyclic stability of the x = 0.5 alloy is not as high as that of the other alloys, its capacity retention ratio is as high as 56.3% after the 400th cycle. The thermodynamic characteristics of the x = 0.5 alloy satisfy the requirements of the hydride electrode of metal hydride–nickel (MH–Ni) batteries.