Production and optimisation of molten salt compatible micro- and nanoelectrodes for enhanced electroanalysis in LiCl-KCl Eutectic

The unique properties of molten salts have made their utilisation in a variety of industrial processes indispensable. Many of these benefits make them an attractive option for the reprocessing of spent nuclear fuel. In the UK, electrochemical pyroprocessing is being developed using a molten salt of LiCl-KCl eutectic. Fully understanding the electrochemical processes which would occur and developing a robust sensing system for on-line monitoring is essential to advance understanding and maintain both control and safety over the process. Performing measurements within a molten salt is challenging due to the inherently high temperatures (usually around 450°C) and corrosive species that can be present in the molten salt. For the purpose of electrochemical sensing, microelectrodes and nanoelectrodes have been shown to have enhanced properties, including lower detection limits and insensitivity to convection, because of the increased efficiency of their mass transport and the more rapid establishment of their steadystate diffusion profile. These properties make them ideal for both studying the fundamental electrochemical processes and for on-line monitoring of a pyroprocessing system. Traditional methods of manufacture of these miniaturised devices do not allow precise control over size, which limits reproducibility and requires extensive calibration of the exact dimensions. Microfabrication enables reliable production of both single microelectrodes and microelectrode arrays of exact dimension and offers a route to nanoelectrodes for quantitation in aqueous systems. However, attempts to use such microelectrodes in molten salt have also been hampered by materials degradation particularly at the seals between different materials. Whilst the traditional methods often use materials that are not compatible with molten salts, microfabrication has another advantage of being able to choose certain materials and tune their properties. This work aimed to bring the greatly needed, enhanced sensing properties of micro and ring nanoelectrodes to the field of molten salts, and hence pyroprocessing, by leveraging the greater control over device manufacture offered by microfabrication techniques. It presents the systematic development, production, characterisation and iterative optimisation of the single micro- and nanoelectrode designs which can withstand the harsh environment of the molten salt. For nanoelectrodes there was evidence of contribution to the measured signal from the conductive adhesion layer. Eliminating this requirement of an adhesion layer enabled the fabrication of molten-salt-compatible nanoelectrodes for the first time. These optimised micro- and nanoelectrodes were then used to gain insight into charge transfer kinetics and mass transfer processes during molten salt electrochemistry. This work demonstrates the fundamentals of utilising a single nanoelectrode for measurement, avoiding the array overlap which can complicate the electroanalysis. Europium was studied within this work due to its nuclear relevance and soluble-soluble one electron transfer process. Quantitative analysis has enabled the relative contributions of mass transport, alongside charge transfer kinetics and thermodynamics, when using these micro- and nanoelectrode systems to be obtained, compared and contrasted. Together this demonstrates the potential for these electrodes to be utilised to study other nuclear relevant species and to perform on-line monitoring in molten salt systems