Determining Optical Basicity of Molten Chloride Salts for Solar Thermal Applications
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Authors
Sharpless, Laurel
Issue Date
2024
Type
Thesis
Language
en_US
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Abstract
As demand for sustainable energy sources increases in response to calls for lower carbon emissions, research and development to improve relevant materials is imperative. Concentrated solar power (CSP) and thermal energy storage (TES) are valuable technologies that, when coupled, allow for intermittent solar power to be stored as thermal energy in molten salt. Thermal storage allows for energy to be accessed when sunlight is not available. This improves wide-scale grid stability, which is a common issue with other renewable energy sources. Molten salt is commonly used in CSP/TES, as its high operating temperature, high energy density, and thermal conductivity contribute to high operating efficiencies. However, molten salts are highly corrosive in atmospheres containing water and oxygen that are present in the CSP/TES. The salt can corrode the storage materials, typically nickel-based alloys, causing ruptures and subsequent plant shutdowns. In aqueous systems, pH can be a predictor of corrosion, but the non-aqueous, harsh environment of molten chlorides makes pH measurements irrelevant. Alternative basicity metrics, such as optical basicity (OB), are needed to predict corrosion. OB allows for measurement of the Lewis basicity, or the extent of electron donation from ligands in the molten salt, via the nephelauxetic effect of an added probe ion. In this work the OB of several alkali and alkaline earth chloride salts was measured by doping them with d10s 2 probe ions, Pb2+ and Bi3+, and quantifying the probe ion's redshift relative to that observed in the reference salt (LiCl-KCl eutectic). The probe ion absorbance frequency was observed in-situ using UV-vis spectroscopy for salts melted in a custom furnace. This work found Pb2+ and Bi3+ to be valid probes for molten chloride salts, with alkali chlorides being the most basic, alkali/alkaline earth chlorides containing Ca2+ and Sr2+ being slightly less basic, and AlCl3-NaCl being the least basic. Increasing temperatures also reflected a higher basicity for LiCl-KCl and AlCl3-NaCl. While Bi3+ OB data was consistent with Pb2+ data, there was significant volatilization of the probe that made detection impractical. The addition of transition metals such as Fe (III) and Cr (II) introduced charge transfer spectra that overshadowed the probe ion absorption peaks in the UV region, making OB determination impossible. Finally, UV-vis absorption data was collected for molten Li-Na-KCO3 eutectic. This eutectic was observed to rapidly corrode silica, so sapphire was used as an alternative material. The carbonate eutectic was also observed to readily precipitate the Pb2+ probe ion as lead oxide, so the OB could not be determined by UV-vis in this case.
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CC BY