Bonding State and Thermal Expansion Coefficient of Mn-Doped Ba_0.5Sr_0.5FeO_3-δ Perovskite Oxides for IT-SOFCs

The oxygen vacancy formation behavior and electrochemical and thermal properties of Ba_0.5Sr_0.5Fe_1-xMn_xO_3-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 O_ads/O_lat ratio decreased, and the binding energy of the O_lat 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 Ce_0.8Sm_0.2O₂ (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.