The Phase Structures And Electrochemical Properties Of La-Mg-Ni-Co Based AB₃-type Hydrogen Storage Alloys

In this thesis, previous research works on non-AB₅ type RE-based hydrogen storage alloys have been extensively reviewed. On this basis, the La-Mg-Ni-Co based AB₃-type hydrogen storage electrode alloys were selected as the subject of this study. By means of XRD analysis and the electrochemical test methods such as galvanostatic charge-discharge, linear polarization, potentialstatic discharge and EIS etc., the phase structure, electrochemical properties of La_0.7Mg_0.3(Ni_0.85Co(0.15))_x (x=3.1-3.6) alloys were studied systematically, and from which the La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.4 alloy with the highest discharge capacity and a better overall electrode properties was selected for further study. Then the effect of annealing treatment on the structure and electrochemical properties of La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.4 hydrogen storage alloy electrode and the effect of Cu substitution for Ni on the phase structure and electrochemical properties of the La_0.7Mg_0.3(Ni_0.85-xCo_0.15Cu_x)_3.4 (x=0-0.03) hydrogen storage alloy electrode were investigated in order to improve the overall electrochemical properties of the alloys.The XRD analyse of the La_0.7Mg_0.3(Ni_0.85Co_0.15x (x=3.1-3.6) alloys shows that all the alloys mainly consist of the (La,Mg)Ni3 phase with the rhombohedral PuNi3-type structure a few impurity phases (LaNi₅ and LaNi₂), leads to an increase in both crystallizability and composition homogeneity, and leads to a decrease in cell volume expansion rate (ΔV/V) on hydriding(24.7% as x=3.1 to 23.2% as x=3.4). Electrochemical tests indicate that the maximum discharge capacity , high-rate dischargeability and cycling stability of the alloys increases a little with increasing of x and pass though a maximum value at x=3.4, and then decreases with the further increase of x. Among the alloys studied, the alloy annealing at 1123K shows a relatively good overall properties with C_max=403.2mAh/g, HRD₁₂₀₀=84.6 % and S₁00=36.8%.Based on these results, The influences of annealing treatment (1073K— 1223K, 9h ) on the phase structure and electrochemical properties of La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.4 hydrogen storage alloy were investigated. XRD analyse indicate that both the as-cast and annealed alloys are consisted of a main phase with hexagonal PuNi₃-type structure and a few impurity phases (LaNi₅ and LaNi₂), but the annealed alloys show a improved crystallizability and composition homogeneity, and a noticeable decrease in cell volume expansion rate on hydriding. Electrochemical tests indicate that the annealed alloys show a higher discharge capacity and a great improvement in cyclic stability, but a lower high-rate dischargeability compared withthe as-cast alloy. It is found that the increase in discharge capacity of the annealed alloys is closely related to the increase of abundance of main phase in the alloys, and the improvement of cyclic stability for the annealed alloys is mainly ascribed to their lower volume expansion on hydriding and more uniform compositon. while, the decrease in both the electrocatalytic activity and diffusion rate of hydrogen in alloy bulk is the main reason for their relatively lower high-rate dischargeability. Among the alloys studied, the alloy annealing at 1123K shows a relatively good overall properties with COTa;c=414.4mAh/g, HRDi2oo=75.4%% and S/oo=59.2%.To further improve the overall properties of the alloys, the effect of Cu substitution for Ni on the phase structure and electrochemical properties of the Lao.7Mgo.3(Nio.85.xCoo.i5Cux)3.4 (x=0-0.03) alloys was studied. XRD analyse indicate that both the as-cast and annealed alloys are consisted of a main phase with hexagonal PuNi3-type structure and a few impurity phases (LaNis and LaNi2) .The lattice parameters and cell volume of the main phase increase with increasing Cu content. It is found that the substitution leads to a cell volume expansion of the alloys, but a further decrease in cell volume expansion rate on hydriding (AF/F=18.9% as x=0.03), with increasing of x the discharge capacity of the alloys decreases a little (Cmax=371.4mAh/g as x=0.03) but the high-rate dischargeability decreased sharply (HRD1200 decreases from 85.8% as x=0 to 68.7% as x=0.03). Among the alloys studied, the Lao.7Mgo.3(Nio.83Coo. 15(^110.02)3.4 alloy shows a relatively good overall properties with Cmax=388.6mAh/g, HRDi2oo=75.2% and 5/00=79.8%. However, the high-rate dischargeability and cycling stability of the alloys is needed to be further improved for practical application.