Electrochemistry of U(IV/III) in LiCl–KCl Molten Salt With Free-Standing Boron-Doped Diamond

Abstract

Molten salt reactors (MSRs) are a key technology on the pathway to zero-emission clean energy and are a part of the Generation IV advanced nuclear reactor fleet. However, large-scale development requires resolving outstanding challenges related to corrosion monitoring and nuclear material accounting. Electrochemistry provides a means to extract several fundamental aspects of an MSR system, from understanding impactful corrosion reactions to safeguarding nuclear material. This has specific importance to the nuclear fuel salt and its associated redox behavior. However, advanced electrode materials for long-term redox monitoring in molten salts still need to be developed. In this work, various electroanalytical techniques such as cyclic voltammetry (CV), chronocoulometry (CC), and differential pulse voltammetry (DPV) were used to assess the electrochemical performance of free-standing boron-doped diamond (FS-BDD) in LiCl–KCl molten salt from 723 to 923 K. The U(IV/III) system was studied as the redox couple on the FS-BDD for these measurements. Several redox properties were determined, including the formal potential (E^0′), diffusion coefficient (D), heterogeneous electron transfer rate constant (k°), formal Gibbs free energy (ΔG^0′), formal enthalpy of reaction (ΔH^0′), formal entropy of reaction (ΔS^0′), and activation energy (E_a). The calculated values were compared across the different techniques to find the most accurate and precise method for using FS-BDD in molten salt matrices. For U(IV/III), values for D were on the order of 10⁻⁵–10⁻⁶ cm²·s⁻¹ at each temperature and align with values available in the literature. Other thermodynamic data (ΔG^0′, ΔH^0′, ΔS^0′) for the redox system were also in agreement with literature. Overall, this work advances fundamental understanding by giving direct comparison of electrochemical methods of U(IV/III) in molten salts and expands the applicability of BDD as an electrode material for high-temperature electroanalytical measurements.