Effect of carbonate impurity on thermophysical properties and structure of chloride molten salt

Abstract

This study focuses on the effects of carbonate ion impurities on the thermophysical properties of NaCl-KCl mixed molten salts, the intrinsic mechanisms via structural evolution. Results show that carbonate impurities have an insignificant effect on density, but increase the specific heat capacity 4. 35 % at 4 mol % CO₃²⁻. Due to new clusters formation of carbonate impurity and metal cations, the movement of ions is restricted, and the self-diffusion coefficient of Na⁺ decrease 21.62 %, 6. 80 % for Cl⁻ and 6.12 % for K⁺, which further led to an increase in viscosity. Additionally, carbonate impurities introduce diverse short-range interactions that disrupt the initial molten salt structure and limit effective collision and migration between ions, leading to a decrease in thermal conductivity. Density, viscosity and thermal conductivity all show a negative temperature dependence, primarily due to elevated temperatures increasing ion spacing, loosening the system, and weakening ionic interactions. Finally, correlations between temperature, impurities and thermophysical properties were established. This study provides a valuable theoretical basis for the establishment and optimization of standards for impurity content of molten salts and offers insights for the development of cost-effective high-temperature molten salts energy storage.