Acid-base equilibria in molten fluoride salts and their impact on molten salt reactors

Molten halide salts find use in a variety of applications, including fission reactors, fusion reactors, energy storage, and electrochemical processes. The breadth of applications of molten salts is made possible by the wide range of thermo-physical properties that can be obtained by tuning the salt composition. This wide range of variability can in part be attributed to acid-base equilibria, which can affect both solvent and solute behavior. While acid-base equilibria have been largely reviewed for chloride salts, owing to their wide use in electrochemical applications, only limited and sparse literature is present on acid-base equilibria in fluoride salts (i.e., fluoroacidity). Through this paper, we aim to clarify the inherent connection of fluoroacidity and molten salt chemical structure, illustrating how knowledge of the fluoroacidity of a given salt can be used to predict the physicochemical properties of such salt. In our analysis, we observe that fluoroacidity influences solvent (viscosity, self-diffusivities, electrical conductivity, thermal conductivity) and solute properties (solubility, diffusion coefficients, vapor pressures). We complement this analysis with a review of experimental and computational methods to evaluate the fluoroacidity of the salt, describing a wide range of techniques that can be used to indirectly probe fluoroacidity. The effects of radiation on fluoroacidity and the relationship between fluoroacidity and corrosion emerge as key knowledge gaps. Additionally, the development of direct methods for fluoroacidity quantification emerges as an important engineering objective.