Cu And Fe On The Electrochemical Property Of Low-Co AB₅-Type Hydrogen Storage Alloys

In order to obtain low-Co AB₅ type hydrogen storage alloys with high discharge capacity, lower cost and good cycling stability, the effects of partial substituting of Cu by Fe and annealing treatment at different temperature on the phase structure and electrochemical characteristics of the low-Co AB₅ type hydrogen storage alloys were investigated in detail.The hydrogen storage alloys LaNi_3.55Mn_0.35Co_0.20Al_0.20Cu_0.85-xFe_x(x = 0.10, 0.20, 0.25, 0.40, 0.60) with low Co content were prepared by inductive melting, and LaNi_3.55Mn_0.35Co_0.20Al_0.20Cu0.75Fe0.10 with low Co content was annealing treatment at different temperature. The chemical composition of the alloys was finally tested by ICP analysis. And the phase structure of the alloys was characterized by XRD. The electrochemical kinetics test methods included linear polarization, Electrochemical Impedance Spectroscopy, anodic polarization, potentialstatic step discharge, cyclic voltammogram and galvanostatic charge-discharge etc. XRD indicates that the alloys LaNi_3.55Mn_0.35Co_0.20Al_0.20Cu_0.85-xFe_x(x = 0.10, 0.20, 0.25, 0.40, 0.60) consist of single LaNi₅ phase with hexagonal CaCu₅ structure, and bulk phase structure of the alloy is not changed by partial substitution Fe for Cu. The parameter a, the parameter c, and cell volume increase with the increase in Fe content. The electrochemical tests show that the maximum discharge capacity and the high rate dischargeability are decreased, however, the cycling stability of the alloy electrodes is significantly enhanced with increasing x. The capacity retention of the alloy electrodes at the 200th cycle (S200) increases from 77.6 % (x = 0.10) to 89.9 % (x = 0.60).LaNi_3.55Mn_0.35Co_0.20Al_0.20Cu_0.75Fe_0.10 alloy was annealed at different temperature. XRD analyses indicate that both a cast and annealed alloys consist of a CaCu5-type structure single phase, but the crystallizability and homogeneityof the annealed alloys are improved. The optical microscope analysis showed that the as-cast alloy had a crass dendrite microstructure with noticeable composition segregation, which gradually disappeared with increasing annealing temperature, and the microstructure changed to an equiaxed structure after annealing the alloy at 1233 K. The electrochemical tests indicated that the annealed alloys demonstrated much better cycling stability compared with the as-cast one. The capacity retention at the 100th cycle increased from 90.0 % (as-cast) to 94.7 % (1273 K). The annealing treatment also improved the charge capacity of the alloys. The high rate discharge ability (HRD) value of the annealed alloy slightly dropped, which was believed to be ascribed to the decreased exchange current density and the hydrogen diffusion coefficient in alloy bulk.