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

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 Yb³⁺ for Y³⁺ in BaCo_0.4Fe_0.4Zr_0.1Y_0.1-xYb_xO_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 CO₂ affinity and suppress BaCO₃ nucleation, thereby delivering strong CO₂ tolerance and rapid performance recovery. Consequently, the optimized BCFZYb cathode achieves an ultra-low area-specific resistance of 0.112 Ω cm² 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.