Cobalt-free SrFeO_3−δ is an attractive mixed ionic-electronic conducting (MIEC) oxygen-electrode platform for intermediate-temperature solid oxide fuel cells (IT-SOFCs), but its electrochemical response is strongly coupled to Fe valence, oxygen non-stoichiometry, and structural phase stability. Here, B-site Ce-substituted SrFe_1−xCe_xO_3−δ (x = 0, 0.05, 0.10, 0.15, and 0.20) was prepared by a solid-state route to establish a composition-structure-defect-transport relationship. X-ray diffraction and Rietveld refinement show an initial expansion of the perovskite lattice after Ce introduction, followed by a lattice-parameter plateau and weak CeO₂/Sr₃Fe₂O_7−δ segregation at higher Ce contents. Electron microscopy confirms an interconnected granular morphology, while HRTEM/SAED of SrFe_0.85Ce_0.15O_3−δ verifies well-crystallized perovskite domains. XPS reveals a Ce-dependent redistribution of the Fe³⁺/Fe⁴⁺ and surface oxygen environments, with the x = 0.15 composition displaying the most favorable defect-associated oxygen response. Electrical conductivity follows a non-monotonic composition dependence and is maximized at x = 0.15, indicating an optimum balance between defect formation and continuity of the Fe-O-Fe transport network. A single cell employing SrFe_0.85Ce_0.15O_3−δ as the oxygen electrode delivers a peak power density of 0.53 W cm⁻² at 800 °C. The corresponding polarization resistance decreases from 0.47 Ω at 600 °C to 0.36 Ω at 800 °C, confirming strongly activated oxygen-electrode kinetics. These results identify controlled B-site Ce substitution as an effective route for tuning the electronic/defect chemistry of cobalt-free SrFeO_3−δ cathodes.