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Redox and complexation chemistry in chloride ionic liquids and molten salts
Unger, Aaron J.
Unger, Aaron J.
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2023
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2024-10-18
Abstract
Molten salts are a unique solvent class composed entirely of ions that have emerged as an important technology in both advanced nuclear technologies and other green energy applications. In particular, molten salts based on the chloride anion have garnered prevalence among innovative molten salt reactors and pyrochemical reprocessing where there is a trove of novel lanthanide and actinide chemistry that must be well characterized to successfully engineer and deploy these systems. Molten chloride salts also offer a direct route to produce high purity lithium metal which is a critical resource for a variety of green energy applications. Despite the demand to describe the complicated chemical environment in molten salts, research on molten salt chemistry is hindered by the difficulty of performing laboratory scale research using conventional experimental approaches to describe system chemistry. This work aims to facilitate the study of molten chloride salt chemistry through application of their organic, lower-temperature relatives known as room-temperature ionic liquids (RTILs). Further, the work seeks to address important open questions about the chemistry of lithium reduction in molten salts related to both its industrial production and influence in pyrochemical processing of used nuclear fuel.
RTILs are a subset of molten salts that employ an organic cation together with a variety of anions to reduce the lattice energy of the salt such that is achieves the liquid state below 100 °C. As a form of molten salts, they offer a simplified route to predict the properties of solvated metals in their high-temperature inorganic analogues by using conventional experimental approaches, such as spectrophotometry, that are not simple to achieve in molten salts without specialized experimental setups. RTILs retain the ionic character of the high-temperature systems but may offer a streamlined route to predicting critical experiments to target in molten salts. However, there is a decisive gap in literature describing the behavior of f-elements in RTILs containing a chloride anion that must be characterized to successfully compare the two systems. A portion of the work presented herein offers perspective into the fundamental chemistry of solvated f-element complexes in a variety of chloride RTILs in order to determine the comparability to inorganic molten salt systems. The work assessed the influence of RTIL cation properties on the structure and stability of solvated f-element complexes. For both lanthanides and actinides, RTIL cations were found to have substantial second-sphere coordination interactions with the anionic metal complexes by interacting over M-Cl-Cation bonding networks. Further, more polarizing RTIL cations stabilized lower oxidation states of redox active lanthanides and actinides by inductively removing electron density across the bridged chlorides.
The final section of this work targets molten chloride salt chemistry by investigating the thermodynamics and kinetics of lithium reduction from molten chloride salts. Current industrial methods of producing lithium metal operate under low current efficiencies, making the process economically inefficient and producing a variety of side reactions that compete with lithium. The kinetics of lithium reduction were studied under a variety of molten salt compositions and working electrode materials to characterize the thermodynamics and kinetics of lithium reduction in these systems and optimize the process efficiency. The results demonstrate the sensitivity of the reduction to slight changes in experimental conditions and illustrates ways to improve the industrial technique. Overall, the work demonstrates a variety of chloride salt-based chemistry to improve molten salt techniques for nuclear and green energy applications. These studies set precedence for describing novel f-element chemistry in chloride RTILs and offer preliminary optimizations for producing lithium metal from molten chloride salts.
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