Electrochemical Separation of Barium into Liquid Bismuth by Controlling Deposition Potentials

Electrochemical separation of barium from multi-component molten salt electrolyte (BaCl₂-LiCl-CaCl₂-NaCl) at 500–700 °C is demonstrated using a liquid bismuth electrode which possesses strong chemical interactions with barium. While the standard emf analysis suggests Na to be the first species to deposit in this electrolyte followed by Ca, Li, and finally Ba, barium was found to be the first species to be reduced into the bismuth electrode followed by Ca. The exceptional deposition behavior of barium was ascribed to the activity of the constituent alkali/alkaline-earth metals in the bismuth metal. The activity of barium in bismuth was extremely low (as low as 10⁻¹⁵), shifting the redox potential of barium to the most positive potentials and enabling the separation of barium into liquid bismuth. By exploiting the differential interactions of constituent ions with the liquid bismuth, it was possible to separate conventionally non-separable barium species from the electrolyte solution. In addition, high coulombic efficiencies of the liquid bismuth electrode (>99%) suggest that electrode processes are chemically reversible for co-deposition of barium and calcium. The analyses of electrode potentials at various current densities and electrochemical impedance spectra indicate charge transfer as the most significant overpotential mechanism during electrolysis.