Electrolytic Synthesis Of Rare Earth-based AB₅Alloys For Hydrogen Storage In The Molten Salt

Rare earth-based AB5type alloy can be widely used for hydrogen storage, such as the electrode materials of Ni/MH batteries. Up to now, in order to improve their electrochemical properties and reduce raw material costs, plenty of study is related to substitution of rare metals and nickel in AB5type alloy by the cheaper metals. However, the traditional preparation techniques of these alloy preparation usually consist of complex processes with large energy consumption and high cost, which limit their wide application. Therefore, an easier, sustainable and more environmentally friendly process for production of the hydrogen storage alloy is highly desired.In this study, LaNi5, CeNi5, LaNi5-xCux, CeNi5-xCux, LaCu6and CeCu6alloys powders were prepared by electro-deoxidation from oxide mixtures in molten salt. The study focused on some important factors, which can influence on the electrolytic process, such as the method to prepare oxide cathode, electrolysis voltage and temperature. The reduction pathway from the mixture oxides were studied by examination of partially reduced samples using XRD, SEM with EDX analysis, which were obtained by interrupting the reduction process after different times. AB5alloy powders obtained by electro-deoxidation were heat-treated in situ in the molten salt. The main results are summarized as follows.The mixing oxide pellets can be preparing by being ball-milled with anhydrous, pressed in to a cylindrical pellet under10MPa, sintered at1200℃in air for4h. The sintered temperature for pellets of La2O3/CeO2-NiO-CuO was900℃, and of La2O3/CeO2-CuO was just850℃. The prepared pellets can severe as a cathode for electrochemical preparation of the alloy.The direct electrochemical reduction of La2O3/CeO2-NiO to LaNi5/CeNi5was achieved in molten CaCl2-NaCl at850℃and800℃, respectively, at a controlled potential, which is applied of3.1V for the first hour, thereafter in the constant manner for the rest period of time. The total electrolysis time was about10h. Successful direct electrochemical reduction of La2O3/CeO2-NiO-CuO to LaNi5-xCux/CeNi5-xCux, was demonstrated in molten KCl-LiCl at650℃under voltage of3.5V for10h. The reduction mechanism was investigated by analyzing chemical compositions and phase constitutions of intermediate products of electrolysis. The results revealed that the reduction of La2O3/CeO2-NiO in molten CaCl2-NaCl may take place in two steps:first, Ni-oxide was reduced to Ni, La2O3/CeO2reacted with Cl" to LaOCl/CeOCl, then reduced and alloyed with Ni to LaNis/CeNis in situ. The reduction of the oxide mixture in molten KCl-LiCl, the first stage of the reaction also involved the rapid formation of Ni-Cu alloy and Ni, thereafter La2O3/CeO2was electrochemically reduced and alloyed with Ni-Cu alloy or Ni. Thermodynamic and dynamic analysis showed that it is difficult for formation of LaOCl/CeOCl in KCl-LiCl molten salt, and there is no LaOCl/CeOCl detected from the XRD analysis of intermediate products.Alloy powders obtained by electro-deoxidation was heat-treated in situ under cathodic polarization conditions, which number of crystal defects should be reduced and lattice strain relieved. After heat treatment, the surface of alloy particles became smoother and the morphology changed with the varied temperatures of heat-treating temperature.