The utilization of identical anode and cathode materials in symmetrical solid oxide fuel cells (SSOFCs) can efficiently reduce their fabrication costs and prolong their operational life. Herein, Nb-doped double perovskite Sr2CoFeO6-δ of a traditional cathode was employed as the electrode material for SSOFCs. Notably, the electrochemical performance and reversibility were improved due to Nb doping. The Nb-doped sample Sr2CoFe0.7Nb0.3O6−δ.5 (SCFN3) was synthesized in air, which decomposed into Co–Fe alloy, SrCoO2.25, and Sr3FeNbO6−δ in H2. Following calcination in air, the sample reverted to the original perovskite structure. The SCFN3 sample exhibited good chemical compatibility with SOFC electrolytes such as La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM), Sm0.2Ce0.8O2−δ, and BaZr0.1Ce0.7Y0.1Yb0.1O3−δ. A single cell consisting of 300-μm LSGM electrolyte exhibited maximum power densities of 951, 510, 563, and 397 mW/cm2 at 850℃ when using H2, C3H8, liquid C2H5OH, and solid C as fuels, respectively. These findings adequately demonstrate the fuel-flexible characteristics of SOFCs.