| Summary: | The oxygen vacancy formation behavior and electrochemical and thermal properties of Ba<sub>0.5</sub>Sr<sub>0.5</sub>Fe<sub>1−x</sub>Mn<sub>x</sub>O<sub>3−δ</sub> (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 O<sub>ads</sub>/O<sub>lat</sub> ratio decreased, and the binding energy of the O<sub>lat</sub> 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<sup>−1</sup> (x = 0.15) at 450 °C. The area-specific resistance of BSFMn0.15 was 0.14 Ω cm<sup>2</sup> at 700 °C and the thermal expansion coefficient (TEC) gradually decreased to 12.7 × 10<sup>−6</sup> K<sup>−1</sup>, which is similar to that of Ce<sub>0.8</sub>Sm<sub>0.2</sub>O<sub>2</sub> (SDC) (12.2 × 10<sup>−6</sup> K<sup>−1</sup>). 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.
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