Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy

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 degrees C in 1 mbar reactant gases H-2 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 H-2 electro-oxidation and H2O electrolysis. The active regions extend similar to 150 mu m 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 similar to 150 mu m active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.