Investigation On Alloying Elements And Stoichiometric Ratio Composition Affect The Electrochemical Properties Of La-Mg-Ni System Hydrogen Storage Alloys

In this thesis, previous research works on La-Mg-Ni system alloys have been extensively reviewed. On this basis, the La-Mg-Ni based ABn(n=3.0,3.3,3.5,3.7)type hydrogen storage electrode alloys was selected as the subject of this study.By means of XRD, EPMA, SEM analysis and the electrochemical test methods,the lower Mg hydrogen storage alloys,which were substituted by different stoichiometric ratio composition and alloying elements, electrochemical properties were investigated systemically.Hydrogen storage alloysLa_0.63Gd_0.2Mg_0.17Ni_3-xCo_0.3Al_x(x= 0,0.1,0.2,0.3,0.4) were obtained by induction melting followed by annealing and the effect of Al substitution for Ni on the microstructure and electrochemical properties was investigated. X-ray diffraction (XRD) analysis and EPMA show La0.63Gd0.2Mg0.17Ni3-xCo0.3Alx alloys that mainly consist of CaCu₅-type phase and Ce₂Ni₇-type and PuNi₃-type and Pr₅Co₁9-type as the as matrix phase. The addition of elements Al has no significant influence on the microstructures of the alloys. Howeve,CaCu₅-type phase increase obviously with the addition of Al, the Ce₂Ni₇-type(Gd₂Co₇-type) phase decreases, Meanwhile, appear other phase. Electrochemical test and analysis results show that the activation performance has the minor influence but the discharge capacities have been developed more or less by the addition of Al element. All the alloys reached the highest capacity by one two circulating. the highest capacity is 371.1 mAh/g. At the same time, replaced by Al the cyclic stability of the alloys has been improved at certain extent, the maximum discharge capacity reach 92 percent. In addition, with the elements of Ni replaced by Al alloy electrode the high rate discharge performance reduced.On this basis, AB₃.5 alloy was selected as the subject of study. Focuses on Y, Mn elements to replace the La and Ni respectively with low Mg content alloy La0.63-xYxGd0.2Mg0.17Ni3.1-2XMn2xCo0.3Al0.1(x=0.1,0.2,0.3,0.4,0.5), which structure and electrochemical properties were investigated. Experiment results showed that, La0.63-xYxGd0.2Mg0.17Ni3.1-2XMn2xCo0.3Al0.1alloy is composed mainly of CaCu₅-type phase and Ce₂Ni₇-type and PuNi₃-type and Pr₅Co₁9-type as the as matrix phase.Among them the content of CaCu₅-types is low. This series of alloy Pr₅Co₁9 and PuNi₃ type phases are moe than La_0.63Gd_0.2Mg_0.17Ni_3.1Co_0.3Al_0.1alloy. Meanwhile, it shows general overall electrochemical properties, the highest capacity of the alloy is 392.7mAh/g. and S₁00 reachs to 84.76 percent. Electrochemical tests show: The alloy electrode discharge capacity and S₁00 increase at first, then decreased, Compared with the La0.63Gd0.2Mg0.17Ni3.2Co0.3 alloy, the Pr₅Co₁9 and PuNi₃ alloys of La0.5Gd0.2Mg0.3Ni3.1Co0.3Al0.1 are increased obviously. In addition, The high-rate dischargeability (HRD) of La0.63Gd0.2Mg0.17Ni3.2Co0.3 has good performance, HRD900 reaches 82.6%.Finally,The AB_x (x=3.0, 3.5 and 3.7) type alloys was chosen as research object, Study on the influence of different annealing time on the electrochemical properties of those alloys.The results indicate that the heat treatment has a great influence on the electrochemical properties.Among those alloys, the maximum discharge capacity of the 3.5 and 3.7 type alloys increase with the time of heat treating and the maximum capacity reached to 397.8mAh/g.The cycle of stability, has improved by the heat treatment. Especially,AB₃.7 type alloys are the best.The circulation of 100 capacity to maintain high rates S₁00 reach to 94.86%.