Microstructures and electrical properties of calcium substituted LaFeO3 as SOFC cathode

Mixed ionic and electronic conductors of La1−xCaxFeO3−δ (LCF, x =0.0–0.5) have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometry, and four-probe electrical conductivity measurements. Ca substitution on La site reduces the LaFeO3 sinterability and the cell volume of this orthorhombic crystal. Dense samples for property studies can be sintered at 1320 °C. Nevertheless, the sintering temperature is near the decomposition temperature of LCF for those solid solutions of x ≥ 0.3. The LCF decomposition is evident when a La-poor secondary phase, not detected in XRD, was revealed in SEM micrographs of 1270 °C thermally etched samples of x ≥ 0.3. Dilatometric studies demonstrate linear increments in thermal expansion with increasing temperature in samples of x ≤ 0.2, while show strange bendings in thermal expansion curves of x = 0.4 and 0.5. The bending in thermal expansion indicates influences of the secondary phase. The TEC value of compositions of x ≤ 0.2 is between 10.8 and 11.7 × 10−6 °C−1. The LCF electrical conductivity increases with the Ca content and its temperature dependence can be described by the small polaron hopping mechanism. The composition around x = 0.15 promises to be a superior cathode for SOFC since it thermally matches with 8 mol% YSZ and 10 mol% Dy, Er substituted LAMOX electrolytes and possesses electrical conductivity near 90 S cm−1 at 800 °C.