Defect Chemistry of Singly and Doubly Doped Ceria: Correlation between Ion Transport and Energetics

Earlier studies have shown a strong correlation between the enthalpy of formation, Delta H-f,H-ox, and the ionic conductivity, sigma(i), near room temperature in doped ceria systems, which are promising solid electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The present work demonstrates that this correlation holds at the operating temperature of IT-SOFCs, 600-700 degrees C. Solid solutions of Ce1-xNdxO2-0.5x, Ce1-xSmxO2-0.5x, and Ce1-xSm0.5xNd0.5xO2-0.5x are studied. The Delta H-f,H-ox at 702 degrees C is determined by considering the excess heat content between 25 and 702 degrees C combined with the value of Delta H-f,H-ox at 25 degrees C. Both sigma(i) and Delta H-f,H-ox show maxima at x=0.15 and 0.20 for the singly and doubly doped ceria, respectively, suggesting that the number of mobile oxygen vacancies in these solid solutions reaches a maximum near those compositions. An increase in temperature results in a shift of the maximum in both Delta H-f,H-ox and si towards higher concentrations. This shift results from a gradual increase in dissociation of the defect associates.