(338f) Computing Chemical Reaction Equilibria in Molten Salts from Molecular Simulations: Composition of Molten Carbonate Electrolytes in Contact with Gaseous Species

Several common molecules including water, oxygen, and carbon dioxide have significant solubility in molten salt phases. Upon absorption, these molecules chemically react with the molten salt to form a diversity of species. For example, dissolved oxygen in molten alkali-metal carbonates is believed to exist almost entirely as a combination of peroxide and superoxide species. The precise ratio of these “activated oxygen species” (OASs) depends on several factors including the metal-cation composition and melt acidity. The solvation of CO₂ and H₂O leads to similar complex reaction products. The interplay between dissolved gas composition and factors such as temperature and melt cation composition is only poorly understood. Molecular simulations can provide insight into structure-function relationships, which can then guide rational design of improved electrolytes. However, determining chemical reaction equilibria (CRE) in condensed phases, especially for complex molecular fluids like molten salts, is computationally challenging. In this talk we describe our work developing methods for calculating CRE for salts and solutes relevant for Molten Carbonate Fuel Cells. Our methods utilize stepwise ion insertions to calculate the chemical potentials of all species combined with quantum chemical methods to determine gas-phase thermochemical data when needed. We show our first calculations of equilibrium concentrations of CO₂, O₂, and H₂O, and their activated products, in Li/K and Li/Na molten carbonate electrolytes. Finally, we discuss how changes in CRE can be related back to physical properties such as entropy and polarizability of alkali-metal cations, and transport of electrochemical species.