Defect chemistry of singly and doubly doped ceria: correlation between ion transport and energetics.

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-x Ndx O2-0.5x , Ce1-x Smx O2-0.5x , and Ce1-x Sm0.5x Nd0.5x O2-0.5x 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.