Electrode properties of Co-doped Ca2Fe2O5 as new cathode materials for intermediate-temperature SOFCs

Brownmillerite oxide Ca2Fe2−xCoxO5 (x = 0.2, 0.4, 0.6) was characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. Ca2Fe2−xCoxO5 has no reaction with Sm0.2Ce0.8O1.9 (SDC) electrolyte at 1100 °C for 10 h in air. The thermal expansion coefficient (TEC) of Ca2Fe2−xCoxO5 increased with increasing Co content, and the TEC value was compatible with SDC. The electrode properties of Ca2Fe2−xCoxO5 were studied under various temperatures and oxygen partial pressures. The polarization resistance (Rp) of Ca2Fe2−xCoxO5 with x = 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. Ca2Fe1.8Co0.2O5 cathode exhibits the lowest overpotential of about 50 mV at a current density of 70 mA cm−2 at 700 °C in air.