The molten carbonate fuel cell (MCFC) is a promising technology option for recovering carbon dioxide from flue gases for carbon capture, for example in natural-gas fired power plants. MCFCs use the electrochemical driving force of hydrogen reaction with oxygen to concentrate CO<sub>2</sub> across an electrolyte layer while simultaneously generating electricity. In order to improve MCFC performance, a deeper understanding of the complex chemical reactions in molten carbonate electrolytes is needed. This includes the solvation of reactant gas molecules, chemical reaction in the molten phase, and diffusion to the electrode surface. The dissolved gases determine the reactants in the oxygen reduction reaction (ORR) and influence ionic conductivity and other properties. The MCFC operating conditions (~600°C) pose a challenge for experimental methods, and simulations can provide valuable insight. The three solutes most relevant for the MCFC are carbon dioxide, oxygen, and water vapor. These can react with the molten salt to form a potentially large number of molecules. A main challenge for computations is to determine force-field parameters defining all chemical interactions in a system; the number of parameters grows rapidly with increasing number of species. In this talk, we discuss our work developing new force-fields for molten salts and dissolved species. Our methods utilize density functional theory molecular dynamics (DFT-MD). Because there is no reliance on available experimental data, this makes our approach broadly applicable to other molten salts. We show that DFT-MD can used to accurately compute bulk properties of MCFC electrolytes including densities and ionic conductivities. The MD trajectories comprise a large data set of information about liquid structure and atomic forces. We show that common machine-learning methods can be used to translate between MD and force-field parameter sets in a robust and automated fashion. Our new force fields significantly expand the compositional space of MCFC electrolytes that can currently be studied using atomistic methods. Finally, we show how these potentials help pave the way for computing chemical reaction equilibria using free-energy methods.<br/>

Developing new force fields for molten salts: Toward understanding electrolyte chemistry in the molten carbonate fuel cell

2019 Fall National Meeting

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