Bonding State and Thermal Expansion Coefficient of Mn-Doped Ba0.5Sr0.5FeO3−δ Perovskite Oxides for IT-SOFCs

The oxygen vacancy formation behavior and electrochemical and thermal properties of Ba0.5Sr0.5Fe1−xMnxO3−δ (BSFMnx, x = 0–0.15) cathode materials were investigated. For thermogravimetric analysis, the weight decreased from 1.98% (x = 0) to 1.81% (x = 0.15) in the 400–950 °C range, which was due to oxygen loss from the lattice. The average oxidation state of the B-site increased, the Oads/Olat ratio decreased, and the binding energy of the Olat peak increased with Mn doping. These results indicate that Mn doping increases the strength of the metal–oxygen bond and decreases the amount of oxygen vacancies in the lattice. The electrical conductivity of BSFMnx increased with the temperature due to the thermally activated small-polaron hopping mechanism showing a maximum value of 10.4 S cm−1 (x = 0.15) at 450 °C. The area-specific resistance of BSFMn0.15 was 0.14 Ω cm2 at 700 °C and the thermal expansion coefficient (TEC) gradually decreased to 12.7 × 10−6 K−1, which is similar to that of Ce0.8Sm0.2O2 (SDC) (12.2 × 10−6 K−1). Mn doping increased the metal–oxygen bonding energy, which reduced the oxygen reduction reaction activity but improved the electrical conductivity and thermal stability with SDC.