Mechanism-guided large-grain design of Sc–Ce co-doped BaCoO₃ dual-phase oxygen electrodes for protonic ceramic cells

Conventional oxygen electrode designs for protonic ceramic cells (PCCs) predominantly rely on the empirical notion that maximizing specific surface area inherently enhances electrochemical performance. However, this paradigm overlooks the critical mismatch between electrode microstructure and the intrinsic rate-determining step (RDS) of the proton-coupled oxygen-reduction-reaction (PC-ORR). In this study, we present a mechanism-driven microstructure engineering strategy that deliberately favors large-particle architectures over nanoscale morphologies to optimize performance when proton transport, rather than surface reaction kinetics, governs the electrode process. By employing Sc-Ce co-doping in BaCoO₃, Sc stabilizes the cubic perovskite lattice, while supersaturated Ce induces spontaneous in-situ phase reconstruction, yielding a dual-phase composite electrode composed of ~80 wt.% Co-rich cubic catalytic framework and ~20 wt.% BaCeO₃-based nanoscale proton-conducting fillers. Comprehensive electrode reaction pathway analysis reveals that Ce doping shifts the proton migration mechanism from hydration-dehydration to hydrogenation-dehydrogenation, thereby repositioning the RDS to be proton transport-dominated rather than surface-ORR-controlled. Guided by this insight, micrometer-sized particles are employed to establish continuous proton conduction channels. The optimized BaCo_0.7Sc_0.1Ce_0.2O_3-δ electrode demonstrates an ultralow polarization resistance of 0.138 Ω·cm² at 600 °C, stable operation  exceeding 120 hours, a peak power density of 2.28 W·cm^-2 in fuel cell mode, and an electrolysis current density of 2.96 A·cm^-2 at 700 °C, performance metrics that surpass most reported Co-based PCC oxygen electrodes. This work establishes a broadly applicable, mechanism-aligned microstructure design paradigm, challenging the assumption that high surface area is invariably the dominant design parameter. It provides critical guidance for the rational development of advanced protonic ceramic electrochemical devices.