In Situ Characterization of Ceria Oxidation States in High-Temperature Electrochemical Cells with Ambient Pressure XPS

Ambient pressure X-ray photoelectron spectroscopy (XPS) is used to measure near-surface oxidation states and local electric potentials of thin-film ceria electrodes operating in solid oxide electrochemical cells for H 2 O electrolysis and H 2 oxidation. Ceria electrodes which are 300 nm thick are deposited on YSZ electrolyte supports with porous Pt counter electrodes for single-chamber tests in H 2 /H 2 O mixtures. Between 635 and 740 °C, equilibrium (zero-bias) near-surface oxidation states between 70 and 85% Ce 3+ confirm increased surface reducibility relative to bulk ceria. Positive cell biases drive H 2 O electrolysis on ceria and further increase the percentage of Ce 3+ on the surface over 100 μm from an Au current collector, signifying broad regions of electrochemical activity due to mixed ionic-electronic conductivity of ceria. Negative biases to drive H 2 oxidation decrease the percentage of Ce 3+ from equilibrium values but with higher electrode impedances relative to H 2 O electrolysis. Additional tests indicate that increasing H 2 -to-H 2 O ratios enhances ceria activity for electrolysis.