Synergistic Tuning of Oxygen Vacancies and Electronic Structure Boosts Performance and Durability of Protonic Ceramic Fuel Cells

Abstract Protonic ceramic fuel cells (PCFCs) operating at intermediate temperatures require cathodes with accelerated oxygen reduction reaction kinetics and robust durability. Here, it is demonstrated that isovalent substitution of Yb ³⁺ for Y ³⁺ in BaCo _0.4 Fe _0.4 Zr _0.1 Y _0.1‐x Yb _x O _3‐δ (BCFZY (x = 0.00), BCFZYYb (x = 0.05), BCFZYb (x = 0.10)) offers a concise and effective route to co‐regulate the perovskite's electronic structure and oxygen‐defect chemistry. Yb doping induces lattice contraction, improves the Co/Fe oxidation state, and increases electron density near the Fermi level, thereby accelerating charge transfer and promoting proton‐electron co‐transport. Meanwhile, the synergistically tailored electronic structure and moderated oxygen‐vacancy population collectively weaken CO ₂ affinity and suppress BaCO ₃ nucleation, thereby delivering strong CO ₂ tolerance and rapid performance recovery. Consequently, the optimized BCFZYb cathode achieves an ultra‐low area‐specific resistance of 0.112 Ω cm ² at 550 °C and a peak power density of 0.643 W cm ⁻² at 550 °C, outperforming undoped BCFZY (0.443 W cm ⁻² ). It also shows negligible degradation over 900 h of single‐cell operation. This work establishes electronic‐structure engineering coupled with controlled vacancy tuning as a powerful strategy for designing durable, high‐activity cathodes for intermediate‐temperature PCFCs.