First real-time tracking of oxidation states during fast redox of UO$_2$ using a microfluidic electrochemical cell and HR-XANES

Real-time tracking of the oxidation states of a UO2 electrode during electrochemical oxidation and reduction was achieved using operando high-resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy at the ACT station of the CAT-ACT beamline at the KIT Light Source, Karlsruhe, Germany. This was made possible by utilizing a particle-attached microfluidic electrochemical cell (PAMEC) developed at PNNL, and employing KIT’s advanced actinide M-edge HR-XANES technique.1-2 The PAMEC is a three-electrode system consisting of a working electrode (WE) made of the materials of interest a platinum (Pt) reference electrode, and a Pt counter electrode (Figure 1A).3 The electrochemical analyzer connected to the PAMEC device controlled the redox process, e.g., applying constant potential on the UO2 WE to reduce (-1.1 V vs Pt) or oxidize it (0.5 V vs Pt), while HR-XANES simultaneously scanned its surface chemistry. The U M4-edge HR-XANES spectra revealed the evolution of U from U(IV) to U(V) and finally to U(VI) during the oxidation process. We were able to demonstrate the reversibility of this process by reducing the same electrode back to pure U(IV), as confirmed by HR-XANES (Figures 1B-1C). ... mehrTo our knowledge, this study reports the first in-situ and operando measurement of real-time oxidation state changes of UO2. The spectra obtained also provided insight into the electronic structure of U(VI) in the UO2 alteration process. This successful international scientific collaboration showcases the potential of PAMEC for in-situ and operando experiments with UO2 and highlights its promising broad application for characterization of spent nuclear fuel systems.
(1) Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; Gonzalez-Robles, E.; Rothe, J.; Altmaier, M.; Denecke, M. A.; Geckeis, H., The role of the 5f valence orbitals of early actinides in chemical bonding. Nature Communications 2017, 8, 16053. 10.1038/ncomms16053
(2) Schacherl, B.; Prüssmann, T.; Dardenne, K.; Hardock, K.; Krepper, V.; Rothe, J.; Vitova, T.; Geckeis, H., Implementation of cryogenic tender X-ray HR-XANES spectroscopy at the ACT station of the CAT-ACT beamline at the KIT Light Source. Journal of synchrotron radiation 2022, 29, 80-88. 10.1107/s1600577521012650
(3) Yao, J.; Lahiri, N.; Tripathi, S.; Riechers, S. L.; Ilton, E. S.; Chatterjee, S.; Buck, E. C., A microfluidic electrochemical cell for studying the corrosion of uranium dioxide (UO2). RSC Advances 2022, 12, 19350-19358. 10.1039/D2RA02501A