Surface Defect Chemistry and Electronic Structure of Pr_0.1Ce_0.9O_2−δ Revealed in Operando

Understanding the surface defect chemistry of oxides under functional operating conditions is important for providing guidelines for improving the kinetics of electrochemical reactions. Ceria-based oxides have applications in solid oxide fuel/electrolysis cells, thermo-chemical water splitting, catalytic convertors, and red-ox active memristive devices. The surface defect chemistry of doped ceria in the regime of high oxygen pressure, pO2, approximating the operating conditions of fuel cell cathodes at elevated temperatures, has not yet been revealed. In this work, we investigated the Pr0.1Ce0.9O2−δ (PCO) surface by in operando X-ray photoelectron and absorption spectroscopic methods. We quantified the concentration of reduced Pr3+, at the near-surface region of PCO as a function of electrochemical potential, corresponding to a wide range of effective pO2. We found that the Pr3+ concentration at the surface was significantly higher than the values predicted from bulk defect chemistry. This finding indicates a lower effective defect formation energy at the surface region compared with that in the bulk. In addition, the Pr3+ concentration has a weaker dependence on pO2 compared to that in the bulk.