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

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, <i>p</i>O₂, approximating the operating conditions of fuel cell cathodes at elevated temperatures, has not yet been revealed. In this work, we investigated the Pr_0.1Ce_0.9O_2−δ (PCO) surface by <i>in operando</i> 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 <i>p</i>O₂. 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 <i>p</i>O₂ compared to that in the bulk