The Phase Structure And Electrochemical Properties Of La-Mg-Ni Based AB₃-type Hydrogen Storage Electrode 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 based AB3-type hydrogen storage electrode alloys were selected as the subject of this study. By means of XRD, SEM, XPS and AES analysis and the electrochemical test methods such as galvanostatic charge-discharge, linear polarization, potentialstatic discharge and EIS etc., the phase structure, electrochemical properties and cycling capacity degradation mechanism of LaxMg3.xNi9 (x=l.0-2.3) alloys were studied systematically, and from which the La2MgNi9 alloy with the highest discharge capacity and a better overall electrode properties was selected for further study. Then the part substitution of 6 different elements for Ni in La2MgNi9 alloy was conducted for examining the effect of element substitution on the phase structure and electrochemical properties of La2Mg(Nio.95Mo.o5)9 (M=A1, Co, Cu, Fe, Mn, Sn) alloys. Based on these works, Co and Al were selected as the substitution elements for Ni in La2MgNi9 alloy and the effect of the amount of substitutions of Co or/and Al on the phase structure and electrochemical properties of the La2Mg(Ni1.xCox)9 (x=0.1-0.5) and La2Mg(Nii.xAlx)9 (x=0.01-0.03) quaternary alloys and the La2Mg(Nio.g-xCoo.2Alx)9 (x=0.01-0.03) quinary alloys were studied in order to improve the overall electrochemical properties of the alloys.For the LaxMg3_xNi9 (x= 1.0-2.3) ternary alloys, the effects of La content (x) on the phase structure and electrochemical properties were systemically investigated. It is found that the main phase of the LaxMg3.xNi9 (x=l.0-2.3) alloys has the hexagonal PuNis type structure, in which the Pul atom position (3a) is occupied by La atoms only and the Pu2 atom position (6c) is occupied by the rest La atoms and Mg atoms. Both the lattice parameters and the unit cell volume of the alloys increase with the increase of x. The hydride of the alloys preserves the PuNis type structure but shows a large unit cell expansion rate for the alloys with x=1.7-2.2, the AF/F value reaches 23.6-25.9%. The hydrogen storage capacity (H/M) and the discharge capacity of the alloys increases with increasing of x and passed though a maximum (H/M=1.03, Cmax=397.5mAh/g) at x=2.0, and then decreases with the further increase of x. The high-rate dischargeability (HRD) of the LaxMg3.xNi9 (x=l.7-2.2) alloy electrodes decreases with increasing of x (HRDiaoo decreases from 62.3% as x=1.7 to 26.5% as x=2.2), which is attributed to the decreasing of electrocatalytic activity and the hydrogen diffusion rate in the alloys. The cycling capacity retention rate of the alloys with r=1.7-2.2 after 100 cycles (Sioo) reaches 55.7-62.9%, which needs to be further improved. Based on the study of the cycling capacity degradation mechanism of the alloys, it is found that the structure stability of the alloys and their hydrides is good during cycling and has not effect on the cycling capacity degradation of the alloys. The main reason of the fast cycling capacity decay of the alloys is the corrosion of La and Mg on the alloy surface and the serious pulverzation of the alloys during cycling due to their high cell expansion on hydriding, leading to a higher rate of corrosion.For improving the cycling stability of the alloys, the Ni in La2MgNi9 alloy was partly substituted by 6 alloy elements (M), and the La2Mg(Nio.9sMo.o5)9 (M=A1, Co, Cu, Fe, Mn, Sn) quaternary alloys were studied to examine the effect of element substitution on the phase structure and electrochemical properties of the alloys. It is found that the alloys and their hydrides still preserve the PuNis type structure and show a noticeable decrease in unit cell expansion rate (AF/F) on hydriding. The substitution leads to some decrease in the hydrogen storage capacity, discharge capacity and in the high-rate dischargeability (exceptfor Sn substitutied one), but leads to a noticeable improvement in cycling stability for most of the substituted alloys. Among the M elements studied, Co substitution result...