Real-Time Tracking of Oxidation States of Nano Particulate UO₂ Electrode Using HR-XANES at Uranium M₄ Edge and a Microfluidic Technique

Studying redox reactions in actinide chemistry presents a significant challenge, which can be addressed by designing and utilizing innovative in-situ cells that combine electrochemistry with spectroscopy. This approach is particularly advantageous when advanced high-resolution X-ray emission and absorption spectroscopic techniques, which are highly sensitive to actinide oxidation states and binding environments, are employed.¹<br/>We will present the first synchrotron application of such an in-situ cell. The experiments were conducted at the ACT station of the CAT-ACT beamline at the KIT Light Source in Germany. High-resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy at the uranium (U) M₄-edge was utilized to monitor the oxidation states of a uranium dioxide (UO₂) working electrode within a microfluidic electrochemical cell. The study observed that U(IV) oxidized to U(V) and subsequently to U(VI) with increasing redox potentials, and the process reversed back to U(IV) with decreasing redox potentials.<br/>Coupling a microfluidic electrochemical cell with HR-XANES opens new avenues for investigating the electronic structure of redox-sensitive materials. This methodology requires only micrograms of sample, making it possible to conduct research that would be otherwise inaccessible with highly radioactive bulk materials such as spent nuclear fuel.<br/><br/>Acknowledgement:<br/>We gratefully acknowledge funding from the European Research Council (ERC) Consolidator Grant 2020 under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 101003292). This research was funded by the U.S. Department of Energy, Nuclear Energy (NE-8), Spent Fuel Waste form Science and Technology (SFWST) program. PNNL is operated by Battelle for the United States Department of Energy under Contract DE-AC05-76RL01830.<br/><br/>References:<br/>1. Pidchenko, I.; Kvashnina, K. O.; Yokosawa, T.; Finck, N.; Bahl, S.; Schild, D.; Polly, R.; Bohnert, E.; Rossberg, A.; Göttlicher, J.; Dardenne, K.; Rothe, J.; Schäfer, T.; Geckeis, H.; Vitova, T., Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles. <i>Environmental Science & Technology </i><b>2017,</b> <i>51</i> (4), 2217-2225.