Electrochemical Methods for Analysis of Hydroxide and Oxide Impurities in Li, Mg/Na, and Ca Based Molten Chloride Salts

Abstract

With applications that include thermal energy storage in Gen3 concentrated solar power (CSP), molten salt batteries in grid scale energy storage, fuel and coolant in molten salt reactors (MSR), and oxide reduction/electrorefining of spent nuclear fuel; molten salts are potentially the most important all around medium for advanced energy systems. In particular, molten halide salts are known to be extremely hygroscopic, typically resisting complete thermal dehydration. Incomplete removal of water leads to hydrolysis; promoting formation of volatile HCl and insoluble oxide/hydroxide impurities in the molten salt. Depending on the application, this can accelerate corrosion, lower product yield through formation of insoluble oxides and oxychlorides, and/or reduce cell efficiency in electrolytic processes. Consequently, the generation, speciation, and electrochemical response of oxide and hydroxide impurities in various molten chloride salts have been widely studied. In this presentation, previously published work with LiCl-Li2O, MgCl2-KCl-NaCl, CaCl2, and CaCl2-CaO-CeCl3 molten chloride salts will be reviewed and discussed to reveal underlying commonality and lessons learned. In LiCl-Li2O and MgCl2-KCl-NaCl, LiOH and NaOH, respectively, were found in the salt after melting water-contaminated starting material. Electrode response signals were ascribed to both LiOH and NaOH contamination, and a nondestructive, in-situ method for determining their concentrations using cyclic voltammetry (CV) was developed. In CaCl2, Ca(OH)2 was found to be unstable, reverting to CaO at operating temperatures. Electrode signals were ascribed to the CaO, and an in situ measurement technique using CV to measure the CaO concentration was developed. The effect of CaO contamination on an electrorefining process was further studied in CaCl2-CaO-CeCl3, where CaO presence was shown to promote the formation of both insoluble Ce2O3 and CeClO. Electrochemical methods in all cases were further substantiated through support analyses including titration, thermogravimetric analysis, quadrupole mass spectrometry, and inductively coupled plasma mass spectrometry. These quantitative electroanalytical methods are shown to be both effective and versatile, showing promise as the development of real-time, in-line sensors for many molten salt processes continues to be of great consequence. Moreover, their versatility in different molten chloride salt systems is encouraging, as they can become an integral step in the evaluation of new use-cases for other molten halide salts and/or their suitability for new applications.