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, ΔH_f,ox, and the ionic conductivity, σ_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-°C. Solid solutions of Ce _1-xNd_xO_2-0.5x, Ce_1-xSm _xO_2-0.5x, and Ce_1-xSm_0.5xNd _0.5xO_2-0.5x are studied. The ΔH_f,ox at 702-°C is determined by considering the excess heat content between 25 and 702-°C combined with the value of ΔH_f,ox at 25-°C. Both σ_i and ΔH_f,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 ΔH_f,ox and σ_i towards higher concentrations. This shift results from a gradual increase in dissociation of the defect associates. A unique correlation: The correlation between the energetics and the ionic conductivity, σ_i, of ceria that was singly or doubly doped with neodymia and samaria has now also been confirmed for high temperatures. The maximum formation enthalpy of the oxide solid solution, ΔH_f,ox, coincides with the maximum σ_i at/near dopant fractions of 0.10, 0.15, and 0.20 in Ce_1-xNd_xO _2-0.5x, Ce_1-xSm_xO_2-0.5x, and Ce _1-xSm_0.5xNd_0.5xO_2-0.5x, respectively.