Optimization of Mixed Conducting Properties of Y2O3–ZrO2–TiO2 and Sc2O3–Y2O3–ZrO2–TiO2 Solid Solutions as Potential SOFC Anode Materials

The mixed conductor Y0.2Zr0.62Ti0.18O1.9 is a promising candidate as an SOFC anode material. In order to further improve the ionic conductivity, the conductivity of system Sc2O3–Y2O3–ZrO2–TiO2 was investigated when Y was substituted by Sc. Materials with single cubic fluorite structure were prepared by solid state reaction. The ionic and electronic conductivities were measured by four-terminal dc method in 5% H2 and ac impedance spectroscopy in air and 5% H2 respectively. It was found that TiO2 may dissolve to about 18 mol% in the ternary systems Y2O3–ZrO2–TiO2 and Sc2O3–ZrO2–TiO2, and 20 mol% in the quaternary system Sc2O3–Y2O3–ZrO2–TiO2. It was also observed that the ionic conductivity is related to the oxygen vacancy concentration and the size of doped ions, and electronic conductivity to the lattice parameter, the sublattice ordering, and the degree of Ti substitution. In addition, both the ionic and electronic conductivities have been improved by the introduction of scandium into the Y2O3–ZrO2–TiO2 system. The highest ionic conductivity (1.0×10-2 S/cm at 900°C) and electronic conductivity (0.14S/cm at 900°C) were observed for Sc0.2Zr0.62Ti0.18O1.9 and Sc0.1Y0.1Zr0.6Ti0.2O1.9, respectively. Considering the required levels of both ionic and electronic conductivities for ideal SOFC anode materials, Sc0.15Y0.05Zr0.62Ti0.18O1.9 is a promising candidate with ionic and electronic conductivities 7.8×10-3 and 1.4×10-1 S/cm, respectively, at 900°C.