Brownmillerite oxide Ca₂Fe_2−<i>x</i>Co<i>_x</i>O₅ (<i>x</i> = 0.2, 0.4, 0.6) was characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. Ca₂Fe_2−<i>x</i>Co<i>_x</i>O₅ has no reaction with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte at 1100 °C for 10 h in air. The thermal expansion coefficient (TEC) of Ca₂Fe_2−<i>x</i>Co<i>_x</i>O₅ increased with increasing Co content, and the TEC value was compatible with SDC. The electrode properties of Ca₂Fe_2−<i>x</i>Co<i>_x</i>O₅ were studied under various temperatures and oxygen partial pressures. The polarization resistance (<i>R</i>_p) of Ca₂Fe_2−<i>x</i>Co<i>_x</i>O₅ with <i>x</i> = 0.2, 0.4 and 0.6 are 0.23, 0.48 and 1.05 Ω cm² at 700 °C in air, respectively. The rate-limiting step for oxygen reduction reaction was the charge transfer process. Ca₂Fe_1.8Co_0.2O₅ cathode exhibits the lowest overpotential of about 50 mV at a current density of 70 mA cm⁻² at 700 °C in air.< Leer menos