Modification Of Rare-metal-based Alloys And Electrodeposition Of Mg-Ni Alloys For Hydrogen Storage

An AB₅-typed hydrogen storage alloy, La_0.65Ce_0.28Pr_0.02Nd_0.05Ni_3.8Co_0.7Mn_0.3Al_0.2(MIB_5.0), was prepared by twin-roller rapid quenching technology. It was thenmodified with transitional metal oxides of Cr₂O₃, V₂O₅, ZnO, NiO and MnO₂ by ballmilling, respectively. The physical and chemical performances of modified alloyswere studied by X-ray diffraction(XRD), scanning electron microscopy (SEM),electrochemical impedance spectroscopy(EIS), galvanostatic cyclic(GC), and cyclicvoltammetry(CV). It is found that the modification by V₂O₅ and Cr₂O₃ can enhancethe discharge capacity of the alloy, and the modification by V₂O₅, Cr₂O₃, Bi₂O₃ canimprove the rate discharge property of the alloy. V₂O₅ can promote the thinner of thealloy grains, increase the surface area of the alloy and enhance the discharge capacity.The alloy modified with 10% (wt) V₂O₅ has the highest capacity, 332.6mAh/g. V₂O₅can also enhance the kinetics of hydrogen absorption and desorption and improve therate discharge property of the alloy.Purified multiwalled carbon nanotubes (MWCNTs) with different diameters andlengths were used as hydrogen storage materials. It is found that MWCNTs can hardlystore any hydrogen by electrochemical method. MWCNTs were mixed withAB₅-typed alloy and the electrochemical properties of the mixtures were studied. It isfound that the addition of MWCNTs in the alloy can enhance the discharge capacityof the alloy, especially when it is mixed with MWCNTs with small diamenter andlong length. However, the rate discharge properties were not affected by the mixing.Mg-Ni alloy was prepared by electrodeposition on copper electrode in aqueoussolution containing 0.04 mol/L Ni²⁺ and 0.4 mol/L Mg²⁺, with EDTA as chelatingagent. It is found that the concentration of EDTA (C_EDTA) and the pH of solution arethe key factors for the codeposition of Mg-Ni. The higher the C_EDTA is, the moredifficultly the Mg-Ni codeposite. However, Mg can not be deposited in the solutionwithout EDTA. The appropriate C_EDTA was 0.01 mol/L. High pH makes Mg-Nicodeposition easier, but if pH is higher than 7.8, deposition in the solution will takeplace. The appropriate pH is in the range of 4.0-7.0. Mg-Ni alloys with 34.94% ofMg can be obtained when C_EDTA is 0.01mol/L and pH is 3.98. Electrochemical experiments show that this codeposited alloy can store hydrogen but just in a lowdensity. Further study should be done to improve the hydrogen storing ability of thecodeposited alloy.