Effects of annealing temperature on the structure and properties of the LaY2Ni10Mn0.5 hydrogen storage alloy

The effects of annealing at 1123, 1148, 1173 and 1198 K for 16 h on the structure and properties of the LaY2Ni10Mn0.5 hydrogen storage alloy as the active material of the negative electrode in nickel-metal hydride (Ni-MH) batteries were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy linked with an energy dispersive X-ray spectrometer (SEM-EDS), pressure-composition isotherms (PCI) and electrochemical measurements. The quenched and annealed LaY2Ni10Mn0.5 alloys primarily consist of Ce2Ni7- (2H) and Gd2Co7-type (3R) phases. The homogeneity of the composition and plateau characteristics of the annealed alloys are significantly improved, and the lattice strain is effectively reduced. The alloys annealed at 1148 K and 1173 K have distinctly greater hydrogen storage amounts, 1.49 wt% (corresponding to 399 mAh g(-1) in equivalent electrochemical units) and 1.48 wt%, respectively, than the quenched alloy (1.25 wt%, corresponding to 335 mAh g(-1) in equivalent electrochemical units). The alloys annealed at 1148 K and 1173 K have relatively good activation capabilities. The annealing treatment slightly decreases the discharge potentials of the alloy electrodes 'but markedly increases their discharge capacity. The maximum discharge capacities of the annealed alloy electrodes (372-391 mAh cl) are greater than the extreme capacity of the LaNi5-type alloy (370 mAh g(-1)). The cycling stability of the annealed alloy electrodes was improved. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.