Structure and electrochemical properties of cobalt-free perovskite cathode materials for intermediate-temperature solid oxide fuel cells

Highlights: • Phase structure of BaFeO_3-δ is very sensitive to Ti⁴⁺, Zr⁴⁺ and Ce⁴⁺ doped in Fe-site. • Co-free BaCe_0.05Fe_0.95O_3-δ cathode with a cubic structure has the best performance. • The BaCe_0.05Fe_0.95O_3-δ cathode on SDC exhibits R_pol 0.046 Ω cm² at 700 °C. Cobalt-free perovskite oxides, BaFeO_3-δ and BaM_0.05Fe_0.95O_3-δ (M = Ti, Zr and Ce), were prepared as the cathode of intermediate-temperature solid oxide fuel cells (IT-SOFCs). High temperature X-ray diffraction (HT-XRD) results show that BaFeO_3-δ and BaTi_0.05Fe_0.95O_3-δ (BTF) have a complex phase composition, and the phase composition changes with temperature. BaZr_0.05Fe_0.95O_3-δ (BZF) has a cubic perovskite structure above 400 °C, while BaCe_0.05Fe_0.95O_3-δ (BCF) has a cubic perovskite structure from room temperature (25 °C) to 1000 °C. Thermogravimetric analysis (TG) and oxygen temperature-programmed desorption (O₂-TPD) analysis reveal that the phase transition affects the oxygen-release properties of these materials. In contrast to BaFeO_3-δ and BTF showing phase-transition-dependent electrical conductivity, BZF and BCF show typical curves similar to other mixed ionic-electronic perovskite oxides. On an electrolyte of Sm_0.2Ce_0.8O_1.9 (SDC), a single cell with a BCF cathode exhibits polarization resistances of 0.027, 0.046, 0.087 and 0.186 Ω cm² at 750, 700, 650 and 600 °C, respectively. The peak power densities of the single cell are 482, 415 and 315 mW cm^-2 at 750, 700 and 650 °C, respectively. This indicates that the Co-free BCF has a good electrocatalytic activity toward the oxygen reduction reaction and is a promising cathode for IT-SOFCs.