Doping-Enabled Symmetry Control in BaCoO₃ for Enhanced Oxygen Reduction Reaction

Stabilizing crystal structure is a powerful route to unlock new functionalities in perovskite oxides. BaCoO₃ (BCO) derivatives are a versatile platform for designing mixed ionic-electronic conductors for solid oxide fuel cell (SOFC) cathodes, yet their stable hexagonal structure limits performance unless transformed into the cubic perovskite phase. Here, seven compositions are systematically investigated -undoped BCO and Sc-, Y-, Zr-, Hf-, Nb-, and Ta-doped variants-and show that the hexagonal-to-cubic transition is the decisive factor governing oxygen reduction kinetics. Among the dopants, Ta proves most effective, achieving a polarization resistance as low as ≈0.004 Ω cm² at 650 °C, by promoting cubic symmetry, which balances oxygen ion transport, oxygen vacancy formation, and surface oxygen adsorption. These findings highlight chemical doping as an effective means to stabilize cubic BCO and offer general design principles for tailoring crystal symmetry and functionality in perovskite oxides for electrochemical energy conversion.