Electrode properties of a spinel family, AFe₂O₄ (A = Co, Ni, Cu), as new cathode for solid oxide fuel cells

In this work, the spinel-type oxides of AFe₂O₄ (A = Co, Ni, Cu) prepared via a glycine–nitrate process were investigated as possible cathode materials for solid oxide fuel cells. The as prepared sample, CoFe₂O₄ and NiFe₂O₄ are cubic spinel structure, while the CuFe₂O₄ is tetragonal spinel structure. The XRD results show that AFe₂O₄ (A = Co, Ni, Cu) is chemically compatible with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) at fuel cell operation temperatures. At a given temperature, the order of the electrical conductivity of the ceramic samples was CuFe₂O₄ > CoFe₂O₄ > NiFe₂O₄. The electrical conductivity of CuFe₂O₄ reaches a maximum value of 2.7 S cm⁻¹ at 850 °C in air. The order of average thermal expansion coefficient was CuFe₂O₄ < NiFe₂O₄ < CoFe₂O₄ in the temperature range of 30–1000 °C in air. The thermal expansion coefficients of the AFe₂O₄ (A = Co, Ni, Cu) samples are very close to that of typical electrolyte materials. CuFe₂O₄ exhibits the smallest area specific resistance among the three samples, i.e., 0.37 Ω cm² at 800 °C in air. Peak power density of single cells with CuFe₂O₄ as cathode on a 300 µm-thick LSGM electrolyte reaches 326 mW cm⁻² at 800 °C. In this series, CuFe₂O₄ exhibits a favorable oxygen reduction reaction activity, thus it may be a promising candidate in SOFCs.