Cathodic Decomposition Reference Electrodes As a Generalized Approach for Thermodynamic Calculations in Complex Molten Salts

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The rise of molten salt use in green energy applications has led to increased efforts to model and understand the underlying thermochemical properties of active species within molten salts. Molten salts can become increasingly complex during the operational lifetime through corrosion or generation of new species, resulting in changes to the thermophysical and thermodynamic properties. For example, next-generation molten salt nuclear reactors start with a binary or ternary fuel salt, but generation of fission products during operation can significantly change the behavior of active species in the salt and interactions with container materials. While processes in established molten salt electrolytes such as LiCl-KCl eutectic can use well-characterized reference electrodes such as Ag/AgCl to quantify thermochemical properties1, emerging salt systems do not have well-established reference electrode systems. Using standard reference electrodes based upon anodic electrolyte decomposition (e.g., chlorine or fluorine gas-based electrodes) introduces additional engineering and safety challenges during construction and measurement of thermochemical properties. These experimental barriers have previously limited investigation of the thermodynamics of novel molten salt systems.Here, we use cathodic decomposition electrodes (CDEs) to measure the thermochemical properties of molten salt systems. The CDE defines 0 V as the cathodic decomposition of the pure solvent electrolyte at all temperatures. A CDE was used to make activity measurements in chloride and fluoride salts. Both steady-state emf measurements in concentration cells and transient measurements were made in an electrolyte containing active species such as GdCl3 or NiF2. The universal nature of a CDE reference can improve accuracy and repeatability of activity measurements in molten salt systems and fill the knowledge gaps that prevent prediction of thermochemical and thermophysical properties in molten salt systems. L. YANG and R. G. HUDSON, “Some Investigations of the Ag∕AgCl in LiCl-KCl Eutectic Reference Electrode,” J. Electrochem. Soc. 106 11, 986 (1959); https://doi.org/10.1149/1.2427195.