Structure And Electrochemical Properties Of La-Mg-Ni System AB_3.8-Type La_0.75Mg_0.25Ni_3.3Co_0.5Hydrogen Storage Electrode Alloys

The La-Mg-Ni system alloy, which is a new type of hydrogen storage material appearing in recent years with higher discharge capacity than that of the rare earth system AB5type hydrogen storage alloy, has aroused wide concern among the people. However, to date, La-Mg-Ni-based alloys can’t be used for industrialization because of following reasons. Firstly, the content of Mg in La-Mg-Ni system alloys prepared by the conventional smelting methods is difficult to control. Secondly, the cyclic stability of the La-Mg-Ni system alloy needs to be improved to be used for industrialization. In view of preparation difficulty and the poor cyclic stability of the alloys, the La0.75Mg0.25Ni3.3Co0.5alloy was systemically investigated in this work. At the same time some significant results are given.Firstly, the effects of particle size of alloy, charge current and discharge current on the electrochemical properties of the La0.75Mg0.25Ni3.3Co0.5hydrogen storage alloy were investigated in this work. It was found that the alloy electrode with particle size of48μm exhibits the best electrochemical properties. And the alloy electrode with charge current of100mA/g and discharge current of60mA/g exhibits the best electrochemical properties.Secondly, in order to improve cyclic stability of the La-Mg-Ni system alloy, the structures and electrochemical properties of La0.75-xYxMgo.25Ni3.3Co0.5(x=0-0.4) alloys were investigated in detail. It was found that all of the alloys have a multiphase structure, consisting of LaNi5phase and a small amounts of (La,Mg)2Ni7phase. Lattice parameters and unit cell volumes decrease with the increase of Y content. The activation properties and the maximum discharge capacities decrease continuously with the increasing of Y content. The width of the discharge potential plateau and the cyclic stability of the alloy increase with the increasing of Y content. The peak area of cyclic voltammetric of the alloy electrodes firstly increased and then decreased. The La0.65Y0.1Mg0.25Ni3.3Co0.5alloy exhibits the largest peak area and superior electrochemical kinetic performance at x=0.1.Finally, in order to obtain suitable preparation techniques for homogeneous La-Mg-Ni system alloy with optimum and reproducible properties, the as-cast La0.7Y0.05Ni3.3Co0.5alloy was prepared by vacuum arc melting firstly. Then, La0.7Y0.05Mg0.25Ni3.3Co0.5hydrogen storage alloy was prepared by mechanical alloying subsequent annealing. And the microstructures and electrochemical properties of the preparation alloys were systematically investigated. It was found that the as-cast La0.7Y0.05Ni3.3Co0.5alloy consists of single LaNi5phase. However, the milled and annealed La0.7Y0.05Mg0.25Ni3.3Co0.5alloys all compose of (La,Mg)2Ni7and LaNi5phase. The content of (La,Mg)2Ni7phase of the annealed alloys is less than the milled alloys. All of the alloys show good activation property. The maximum discharge capacity of the as-cast La0.7Y0.05Ni3.3Co0.5alloy is larger than that of the milled La0.7Y0.05Mg0.25Ni3.3Co0.5alloy, but less than that of the annealed La0.7Y0.05Mg0.25Ni3.3Co0.5alloy. The cyclic stability of preparation alloys increases in the order of the annealed alloys> the milled alloys> the as-cast alloy.