Diffusion behaviors of HF in molten LiF-BeF2 and LiF-NaF-KF eutectics studied by FPMD simulations and electrochemical techniques

Abstract

The hydrofluorination reaction has proven to be a very effective approach to remove impurities in the preparation and usage of molten LiF-BeF 2 (Flibe) and LiF-NaF-KF (Flinak) eutectic salts, however, the diffusion behaviors of HF in two molten fluorides are incompletely understood. Here we design two systems, HF+Flibe and HF+Flinak, to reveal the diffusion properties and local structures as well as their correlations by first principles molecular dynamics (FPMD) simulations and electrochemical methods . It is shown that the strong interaction between Be 2+ and F – in molten Flibe reduces their diffusivities while providing more space for Li + migration. The diffusion coefficient of H + in molten Flinak is higher than that in molten Flibe determined by the fast exchange of H + between two F anions, and the larger diffusivity of F – in molten Flinak is related to the irregular movement of free F anions. Besides, the quasi-gaseous transport behavior of HF in two molten fluorides is proposed in this study according to the stable H-F bonds. Moreover, the experimental diffusion coefficient of HF in molten Flinak estimated by the method of convolution linear sweep voltammetry is consistent with our simulation value, and the larger diffusivity is well explained by the higher stretching vibration frequency between H + and F – in molten Flinak. Finally, the ionic diffusion coefficients are descripted by the first sharp diffraction peaks of structure factors, the nearest neighbor peaks of radial distribution functions , and the major bond angels of angular distribution functions, and it is speculated from these FPMD simulations that the peak position has a higher weight on diffusion coefficient. Overall, in-depth understandings of the structure-property relationships of HF+Flibe and HF+Flinak systems provide insights into the molten salt purification technology and the daily operation of molten salt reactors .