Abstract
Earlier studies have shown a strong correlation between the enthalpy of formation, ΔHf,ox, and the ionic conductivity, σi, 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 Ce1−xNdxO2−0.5x, Ce1−xSmxO2−0.5x, and Ce1−xSm0.5xNd0.5xO2−0.5x are studied. The ΔHf,ox at 702 °C is determined by considering the excess heat content between 25 and 702 °C combined with the value of ΔHf,ox at 25 °C. Both σi and ΔHf,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 ΔHf,ox and σi towards higher concentrations. This shift results from a gradual increase in dissociation of the defect associates.
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