Effect Of Elemental Substitution And Annealing On Electrochemical Characteristics Of La-Mg-Ni Based Hydrogen Storage Alloys

In order to obtain a new La–Mg–Ni based AB3 type hydrogen storage alloy with high discharge capacity, lower cost and good cycling stability, the effect of substituting Co by Mn, Si, Cr, Cu, Fe and annealing treatment on the phase structure and electrochemical characteristics of La–Mg–Ni based hydrogen storage alloys was studied in this paper.The prepared La0.80Mg0.20Ni2.85Al0.11M0.53 (M = Ni, Co, Cu), La0.80Mg0.20- Ni2.85Al0.11Mn0.20M0.33(M = Co, Cu) and La0.80Mg0.20Ni2.85Al0.11M0.53 (M = Ni, Si, Cr, Cu, Fe) hydrogen storage alloys were consisted of LaNi3 phase and LaNi5 phase.For the La0.80Mg0.20Ni2.85Al0.11Co0.20M0.33 (M = Co, Mn, Cu, Fe) alloys, the capacity retention at the 200th cycle of Mn substitution alloy increased from 82.2% to 86.7% comparing with Co substitution. The Cu substituted alloy electrodes improved the kinetics properties of the alloys, and the HRD1100 of Cu substitution alloy increased from 64.6% to 69.2% comparing with Co substitution. The discharge capacity of Mn, Cu and Fe substituted low-cobalt alloy was reduced.For the cobalt-free La0.80Mg0.20Ni2.85Al0.11M0.53 (M = Ni, Si, Cr, Cu, Fe) alloys, Si-, Cu- and Fe-adding increased the cycling stability of La–Mg–Ni based alloys. The maximum discharge capacity decreased from 362 to 215 mAh/g in the order of Ni>Fe>Cu>Cr>Si. In addition, the electrochemical kinetics of the alloy electrodes was also researched by measuring the high rate discharge ability (HRD), the hydrogen diffusion coefficient (D) and the limiting current density (IL).La0.80Mg0.20Ni2.85Al0.11Fe0.53 alloy was annealed at 1123 K, 1223 K and 1273 K. The absorption/desorption plateau became lower after annealing treatment. In addition, the high rate discharge ability (HRD), the exchange current densities (I0) and the hydrogen diffusion coefficient (D) of the annealed alloys increased then decreased with the increasing annealing temperature. At 1123 K, the maximum discharge capacity could reach 337 mAh/g. The charge retention and cycle life at the 200th cycle was up to 93.0% and 81.6%, respectively.