On the Electrochemical Thermodynamics of Minor Components in Molten Salt Mixtures

Molten salt mixtures have important applications in, for example, industrial metallurgical processes, energy storage for solar plants, and the development of advanced nuclear reactor systems. Several unanswered questions in these systems involve the thermodynamics of minor components in the molten salt solution. Consequently, we examine molten salt systems using electrochemical thermodynamics, published experimental data, and atomistic simulations to accurately characterize interactions between a minor component and the base salt for multiple salts. The utility of an infinite dilution reference state is demonstrated and used to characterize the range over which solute-solvent interactions dominate over solute-solute interactions for minor components. Under such conditions, the activity is readily defined and cell potential can be easily determined as a function of concentration with use of an appropriate standard potential. Experimental data show that reactions can proceed at very different potentials in different salt melts, and molecular dynamics simulations are used to quantify differences between salts. Simulations show that the chemical potential of an anion varies between melts as influenced by the different cations present in each melt. Hence, attempts to use a common reference reaction for different salt mixtures are at best an approximation. Simulations also demonstrate that solute-solute interactions become significant at lower concentrations if the minor component includes an anion that is different from that of the base salt. This work helps to enhance our understanding of the behavior of minor components in molten salts, which is important for the development of future energy technologies.