Abstract
Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO2−x/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H2 and H2O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO2−x electrodes undergo Ce3+/Ce4+ oxidation–reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H2 electro-oxidation and H2O electrolysis. The active regions extend ∼150 μm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce3+/Ce4+ shifts in the ∼150 μm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.
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Acknowledgements
This work was financially supported by the ONR through Contract number N000140510711. We thank the University of Maryland Nanocenter and the University of Maryland Energy Research Center (UMERC) for support. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231. Work at Sandia National Laboratories is supported by the Laboratory Directed Research and Development programme under contract DE-AC04-94AL85000 of the United States Department of Energy.
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All co-authors contributed to the conception and design of experiments. The ALS team collected and analysed the XPS data. The Sandia team collected the electrochemical data. The Maryland team fabricated and characterized cells and collated data analysis. Z.H. and M.A.L. initiated and partially financially supported the collaboration.
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Zhang, C., Grass, M., McDaniel, A. et al. Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy. Nature Mater 9, 944–949 (2010). https://doi.org/10.1038/nmat2851
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DOI: https://doi.org/10.1038/nmat2851
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