Integrating preparation of borides and separation of alkaline- and rare-earth ions through an electrochemical alloying approach in molten salts

Abstract

Electrifying synthesis has become important because of the trend of using renewable electricity to drive materials synthesis. Herein, a series of alkaline-earth and rare-earth borides are synthesized by an electrochemical boridization reaction in molten salt. During electrolysis, a specific cation from the molten salt is reduced and simultaneously alloyed with boron powder to generate borides. The selective reduction and an electrochemical alloying of alkaline-earth and rare-earth metals with boron stem from the semiconductive properties of boron that has a high electronegativity and can form stable metal-boron (M-B) bonds. Meanwhile, boron, as a separation active agent, effectively separates and recovers the alkaline- and rare-earth elements from the fission products in the electrolyte solution and purifies the electrolyte through synthesizing corresponding borides. Both thermodynamic calculations and electrochemical measurements show that the electrochemical boridization reaction happens at a potential sequence from the positive to negative follows SrB6 → CeB6 → BaB6 → SmB6 → LaB6 → CaB6, which is different from the sequence of the deposition of pure metals. In addition, a higher temperature and cell voltage can speed up the boridization reaction. To further improve the energy efficiency, a CaSi2||B paired electrolysis cell can produce porous Si at the anode and CaB6 at the cathode, promising a zero-emission approach for materials synthesis. Therefore, the utilization of the electrochemical alloying approach (EAA) in molten salts not only provides an efficient way to prepare materials but also is able to purify molten salts.