Award Date

May 2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

First Committee Member

Kenneth Czerwinski

Second Committee Member

Artem Gelis

Third Committee Member

Daniel Koury

Fourth Committee Member

Marisa Monreal

Fifth Committee Member

Cory Rusinek

Sixth Committee Member

Matthew Sheridan

Number of Pages

260

Abstract

Molten salt chemistry has a range of applications within nuclear technology, including for the Molten Salt Reactors (MSRs) and pyroprocessing to recover valuable actinides for energy and national security needs. However, the high-temperature, corrosive nature of molten salts makes them particularly challenging to deploy on an industrial scale and study in benchtop measurements. Material accountability and corrosion monitoring of MSR fuels are essential components to the successful deployment of MSRs, and electroanalytical techniques like cyclic voltammetry (CV) and spectroelectrochemistry (SEC) can provide a wealth of information to describe salt systems in situ. To perform such measurements, it is imperative to have materials that can withstand the harsh environment of molten chloride and fluoride salts. It is hypothesized that boron-doped diamond (BDD), a well-studied, ‘designer’ electrode material, can serve as a resilient working electrode in molten salt systems given its previous applications to other harsh environments.To better understand the use of BDD in molten salts, it was first studied in aqueous systems using CV to determine the impact of crystal structure, morphology, and carbon hybridization (e.g., sp2 or sp3) on the electrochemical response. Potassium ferricyanide, hexaammineruthenium(III) chloride, europium trichloride, and uranyl nitrate were all evaluated using CV on the two distinct sides of free-standing BDD to find that there is, in fact, a difference between large and small-grain structures of polycrystalline diamond. Values for formal potential (Eo), peak separation (DEp), diffusion coefficients (D), and heterogeneous electron transfer rate constants (k) were compared for each side of the BDD to quantify the quality of each response. Then, a novel optically transparent electrode (OTE) design from free-standing BDD was applied to SEC measurements of aqueous potassium ferricyanide to determine Eo and D. Following successful measurements, this OTE could be a resilient design applicable for actinide species in molten salt systems to help characterize MSR fuels. Then, BDD was exposed to chloride (NaCl-KC) and fluoride (FLiNaK) molten salts under various conditions for extended periods to determine if it would survive under the most extreme environments (i.e., exposed to air, in inert atmosphere, using non-purified salts, using thermally dried salts, etc.). The material was then evaluated using scanning electron microscopy (SEM), profilometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) to determine changes to the surface structure and chemistry after exposure. There was virtually no change to the material after exposure except in the case of chloride salt that was not dried and was exposed to air where etching was seen in some regions of the crystal structures. However, when aqueous CV was performed on the samples to determine changes to the electrochemical performance, there was very little change, including for the corroded sample. Finally, BDD was used to perform CV on Eu3+/2+ and U4+/3+ redox couples in chloride salts using various BDD electrode geometries. Successful measurements of Eu were performed using free-standing BDD and thin-film BDD in LiCl-KCl, and U was evaluated with thin-film BDD in LiCl-KCl and MgCl2-NaCl electrolytes. The impact of BDD crystal structure with the differing electrode geometries for Eu and changing electrolyte cation side for U were significant and produced an interesting set of results that opened doors for a wide range of future studies. Ultimately, BDD successfully performed measurements of f-block species relevant to MSR fuels and should be further explored under various conditions and optimized electrode designs.

Keywords

Boron-doped diamond; Electrochemistry; f-Block chemistry; Molten salts; Semiconductors

Disciplines

Chemistry | Engineering Science and Materials | Materials Science and Engineering | Oil, Gas, and Energy

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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