Preparation and performance study of SrCo<sub>0.8</sub>Fe<sub>0.125</sub>Zr<sub>0.075</sub>O<sub>3−<italic>δ</i

Solid oxide fuel cells (SOFCs) can convert the chemical energy stored in fossil fuels to electricity with high efficiency and low emissions. Methane has high calorific value, and it is the main component of natural gas, coal-bed gas and biogas. Therefore, it is a potential fuel for SOFCs. One of major requirement in intermediate temperature SOFCs (IT-SOFCs) is that the cathode material has high ionic and electronic conductivity. The cubic perovskite SrCo_0.8Fe_0.2O_{3−<italic>δ</italic>} is known to be excellent mixed ionic and electronic conductivity. However, as a cathode in SOFCs, SrCo_0.8Fe_0.2O_{3−<italic>δ</italic>} is impeded by its low structural stability, especially in the presence of CO₂. This disadvantage can drastically decrease the cell performance. Partial substitution the B-site ions of perovskite with a fixed oxidation state metal can enhance its structural stability. Zirconium has stable oxidation state, in this study, 7.5% zirconium was doped into SrCo_0.8Fe_0.2O_{3−<italic>δ</italic>} to form a structurally stable cubic perovskite SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>}. SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>} showed metallic electrical conductivity above 500 ℃, the maximum conductivity achieves 321 S/cm at the temperature region of 200–850 ℃ in air. SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>} as the cathode was operated in a BZCYYb electrolyte-based IT-SOFCs. X-ray diffraction indicated that SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>} was cubic perovskite structure, and there was no new phase for the calcined SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>}–BZCYYb mixture powder, which indicates that SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>} had good chemical compatibility with electrolyte at the operating temperature. CO₂-temperature programmed desorption experiments indicated that SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>} possessed adequate structural stability and CO₂ tolerance. The electrochemical performances of SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>} as a cathode were surveyed for SOFCs with a structure of Ni-BZCYYb|BZCYYb|BZCYYb-SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>} using 97% CH₄-3% H₂O fuel. The cell shows a maximum power density of 526 mW/cm² at 700 ℃, and a best stability of 50 h under a current density of 350 mA/cm² at 650 ℃. These results indicate that the SrCo_0.8Fe_0.125Zr_0.075O_{3−<italic>δ</italic>} has significant CO₂-tolerance, and it is a good candidate as the cathode material for IT-SOFCs directly operating on methane fuel.