Electrochemical engineering considerations for gas evolution in molten sulfide electrolytes

Abstract

The current interrupt and galvanostatic electrochemical impedance spectroscopy techniques were utilized to characterize the ohmic, charge transfer, and mass transfer over-potential behavior of gas evolving electrodes in aqueous, molten chloride, and molten sulfide electrolyte solutions under steady-state natural convective flow conditions as a means to gain access to thermodynamic, physicochemical, and hydrodynamic properties of these systems. Previous efforts purposely chose operating conditions under which one or more sources of overpotential were negligible to facilitate analysis of the total overpotential observed at the expense of maintaining operating conditions of industrial relevance. This work represents a preliminary effort to understand the fundamental material properties of a molten sulfide electrolyte, by application of materials-blind electrochemical techniques that were validated on previously well characterized systems-oxygen evolution in aqueous KOH and chlorine evolution in eutectic LiCl-KCl-CsCl. For the first time, values are reported for the saturation concentration of dissolved sulfur gas, an approximate range of Schmidt number for dissolved sulfur, and natural convection limiting current densities in a molten sulfide electrolyte consisting of Cu₂S-BaS-La₂S₃ at 1300°C.