Moisture impurities in molten halide salts: Analytical challenges to mitigate corrosion in next-generation energy systems

This review focuses on the critical issue of water and related impurities in halide salts, particularly relevant in the context of high-temperature energy storage and conversion systems like molten salt nuclear reactors, concentrating solar power, pyroprocessing of nuclear fuel, and thermal energy storage. The corrosive nature of molten halide salts is exacerbated by hydroxides, oxides, oxyhalides, and hydrogen halide acids that form when halide salts are heated and fused with water present. Precise quantification of water and related species is vital for advancing fundamental research necessary for the development of these systems and for safe and economical operation of commercial systems in the future. The literature regarding (i) the structure and speciation of water and water-related impurities, (ii) drying and purification methods to reduce water in halide salts, and (iii) methods for detecting and quantifying water and related reaction products have been reviewed and compiled here. This review highlights the complex behavior of water in halide salts and the challenges involved in rigorous quantification. Pathways towards better understanding fundamental molten salt chemistry and developing quantification methods with improved accuracy are also discussed. Development of complementary analytical techniques will not only be crucial for high-fidelity molten salt experimental research but also essential for developing and optimizing the safety, performance, and longevity of molten halide salt-based energy systems.