Mechanism Study Of Preparing Rare Earth Alloys Used In Hydrogen Storage Alloy By Molten Salt Electrolysis

As the first choice for cathodic materials of Ni/MH batteries,rare earth hydrogen storage alloy has been massively studied on the optimization of its performance so as to improve the market competitiveness of Ni/MH batteries.Element substitution is found to be the most effective way of modification.Usually,the addition of substitute elements to the hydrogen storage alloys is mainly relying on the mixed melting process.However,for substitute element Sm with a relatively high vapor pressure and element Al with a relatively low melting point,their contents are not easy to control and the composition is uneven due to severe volatilization and segregation in the miscibility process.Using an intermediate alloy prepared by molten salt electrolysis as substitute element can well solve the problem,reducing the cost of alloy production and simplifying the process.This paper was based on the LiF-CaF2 electrolyte system and the electrochemical behavior and co-deposition behavior of La3+ and Sm3+ on Mo electrode were first studied.The results show that the reduction potential of La3+ is very close to that of Li+ in the fluoride salt system.The reduction of Sm3+ is completed in two steps and the first step is a reversible soluble/soluble process and controlled by diffusion.In LiF-CaF2 melts,only Sm2+ can be obtained through one-step reduction from Sm3+ because Sm2+ has a more negative reduction potential than Li+.When at 1123K,the diffusion coefficient of Sm3 + in the LiF-CaF2-SmF3(6wt%)system is 2.07 × 10 5cm2 · s-1 When Sm3+ and La3+ are all present in the molten salt system,no co-deposition electrochemical signals of the La3+ and Sm3+ are observed under any temperature or solubility ratios.Therefore the La-Sm alloy cannot be prepared through co-deposition after molten salt electrolysis.The electrochemical behavior of Al3+ and the electrochemical behavior of the co-deposition of Al3+ and La3+ or Al3+ and Sm3+ were studied.Also,the Al-La and Al-Sm alloys prepared through co-deposition after molten salt electrolysis are analyzed.The results show that the metal Al is formed in the deposition process of Al3+ by one-time transfer of three electrons at 1123K in LiF-CaF2-AlF3(1wt%)melts on Mo electrode.When at 1123K on the Mo electrode in the LiF-CaF2-AlF3(1wt%)-LaF3(6wt%)system,due to the under-potential deposition of the La3+ on the pre-deposited metal Al,four intermetallic compounds are formed between the metal aluminum and the metal lanthanum at a potential more positive than the deposition potential of the La3+.Therefore,Al-La alloy can be prepared through electrolytic co-deposition in molten salt.The electrolytic products attached to the electrode are obtained after adding oxide raw materials to the melts.The XRD results show that the phase composition of the electrolytic products at this time is Al and Al11 La3;and the SEM and EDS results show that Al11 La3 is dispersed in the Al matrix.When at 1123K on the Mo electrode in the LiF-CaF2-AlF3(1wt%)-SmF3(6 wt%)system,due to the under-potential deposition of the Sm3+ on the pre-deposited metal Al,four intermetallic compounds are formed.The electrolytic products attached to the electrode are obtained after adding oxide raw materials to the melts.The XRD results show that A12Sm and Al3Sm are produced;and the SEM and EDS results show that in the electrolytic process,the deposition of Al3+ ions is prior to the formation of Al-Sm alloy which is based on the deposition of Sm2+ on the metal Al.