An Investigation On The Phase Struture And Electrochemical Properties Of The La₂MgNi_7.5Co_1.5 Hydrogen Storage Electrode Alloy

In this thesis, based on the review of research and development of the AB_3.0-3.8 type rare earth-based hydrogen storage alloys, to improve the electrochemical properties of the alloy was selected as the starting point of this study. By means of XRD EPMA , master-sizer analysis and the electrochemical test methods including the galvanostatic charge-discharge, linear polarization and potentialstatic discharge etc., the relationships among compositions, phase structure and electrochemical properties were studied systematically for the alloys. Firstly, the effect of partial substitution of Al for Mg on the phase structure and electrochemical properties of La₂MgNi_7.5Co_1.5 alloy was studied. On the basis of it, the capacity degradation mechanism of La₂Mg_1-xAl_xNi_7.5Co_1.5(x=0.0, 0.1) alloy electrodes were studied to clarify the intrinsic reason of the better cyclic stability of La₂Mg_0.9Al_0.1Ni_7.5Co_1.5 alloy when compared with La₂MgNi_7.5Co_1.5 alloy. Based on these results, Ni was partly substituted by Mn, and the phase structure and electrochemical properties of the La₂Mg_0.9Al_0.1Ni_7.5-xCo_1.5Mn_x(x=0.0 - 0.9) alloys were investigated. Besides, the effect of melt spun on the phase structure and electrochemical properties of La₂Mg_0.9Al_0.1Ni_7.5Co_1.5 alloy was studied preliminarily.For the La₂MgNi_7.5Co_1.5 alloys, the effect of partial substitution of Al for Mg on the phase structure and electrochemical properties was investigated systematically. The results indicate that the alloy consist of two phases such as LaNi₃ phase (PuNi₃-type) and αLa₂Ni₇ phase (Ce₂Ni_7-type). When the aluminum join in the alloys(x=0.1), the LaNi₃ phase decreases and the αLa₂Ni₇ phase becomes the main phase. While the more Al substitution, the LaNi₃ phase disappears and the abundance of αLa₂Ni₇ phase decreases, the LaNi₅ phase appears and becomes the main phase in La₂Mg_1-xAl_xNi_7.5Co_1.5 alloys. With the increasing of Al content, the H/M activation properties and the maximum discharge capacity of the alloys descend gradually. But the cyclic stability of the alloys have been improved markedly with the increasing of Al content, the capacity retention rate of the alloys increase from 45.8%( x=0.0) to 85.4%( x=0.5) after 100 charge/discharge cycles (S₁₀₀). The high rate dischargeability (HRD) of the alloy electrodes increase with the increasing of Al content. When x≤0.3, the electrode reaction kinetics of the alloys is controlled by charge-transfer velocity on the surface of electrode alloys, but when x=0.5, the electrode reaction kinetics is controlled by both the charge-transfer velocity on the surface of electrode alloys and the hydrogen diffusion velocity in the bulk of the alloys.The study on the capacity degradation mechanism of La₂Mg_1-xAl_xNi_7.5Co_1.5(x=0.0, 0.1) alloys show that the lattice parameter a, c and the unit-cell volume v of LaNi₃ phase and αLa₂Ni₇ phase increase, and the c/a also increases after the hydrogen enters into the alloys. What's more, the unit-cell volume expansion of the two phases is anisotropic and the rate of LaNi₃ phase is bigger than αLa₂Ni₇ phase. With the aluminum join in the alloys, three new phases such as La(OH)₃ Mg(OH)₂ and Ni appear. It shows that the element La and Mg in the alloys are oxidized in the discharge cycles process. Master-sizer analysis shows that the average grain of La₂MgNi_7.5Co_1.5 alloy is smaller than La₂Mg_0.9Al_0.1Ni_7.5Co_1.5 alloy remarkably; the oxide in the former is more than the latter too. The cyclic stability of La₂Mg_0.9Al_0.1Ni_7.5Co_1.5 alloy is superior to La₂MgNi_7.5Co_1.5 alloy, the most important reason of it is the main phase of the alloys become αLa₂Ni₇ phase from LaNi₃ phase, the unit cell expansion rate of LaNi₃ phase is bigger than αLa₂Ni₇ phase and the unit cell expansion rate is decrease both LaNi₃ phase and αLa₂Ni₇ phase when the aluminum join in the alloy. Therefore, the anti-pulverization characteristic and then the corrosion degree decrease, that is the main reason for the improvement of electrochemical cyclic stability of the alloys.The phase structure and electrochemical properties of the La₂Mg_0.9Al_0.1Ni_7.5-xCo_1.5Mn_x(x=0.0-0.9) alloys were investigated systematically. The results show that the LaNi₃ phase disappears and the abundance of αLa₂Ni₇ phase decreases, the La₅Ni₁₉ phase and LaMgNi₄ phase appear and increase with the increasing of Mn content, and the lattice parameter a, c and the unit-cell volumes v of αLa₂Ni₇ phase, LaMgNi4 phase, La₅Ni₁₉ phase increase. With the increasing of Mn content, the plateau pressure H/M, activation properties and the maximum discharge capacity of the alloys descend gradually. When x=0.3, the alloy shows the best cycle stability. While the more Mn substitution, the capacity retention rate of the alloys decrease. In addition, the high rate discharge ability (HRD) of the alloy electrodes decreases gradually with increasing of x.The effect of melt spun on the phase structure, microstructure and electrochemical properties of La₂Mg_0.9Ni_7.5Co_1.5Al_0.1 hydrogen storage alloys were investigated systematically. The analysis shows that the annealed alloy is consisted of αLa₂Ni₇ main phase with Ce₂Ni₇ type structure and a little LaNi₃ second phases with PuNi₃ type structure. With the increasing of quenching rate, the LaNi₅ phase, βLa₂Ni₇ phase and LaMgNi4 phase appear in the alloys, the content of the new phase increases but the content of αLa₂Ni₇ phase and LaNi₃ phase decrease. EPMA analysis indicates that the alloys prepared by melt spun are columnar crystal structure, and columnar crystal structure become finer with the increasing of quenching rate. The electrochemical measurement shows that the maximum discharge capacity and the high rate discharge ability decrease with the increase of quenching rate. The alloy electrode demonstrates better cyclic stability with the further increase of quenching rate as 20m/s, after 100 charge/discharge cycles (S₁₀₀), the capacity retention rate of the alloy is 88.1%.