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.

Ceramic aerogels have promising applications in extreme environments, such as aerospace, but are severely limited by the sintering-prone nature of nanounits at high temperatures. However, they typically exhibit inadequate dimensional stability when exposed to high-temperature atmospheres, which can result in the deterioration of their macroscopic characteristics, eventually restricting their applications in extreme environments. Here, a (YYbErDyGd)₂SiO₅ ceramic aerogel (ESA) was prepared at 1050 °C by introducing high entropy into a ceramic aerogel, thereby enhancing its high-temperature thermal stability via the high-entropy effect. ESA was exposed to a temperature of 1600 °C for 2 h without undergoing sintering. In addition, different fiber-reinforced ceramic aerogel composites are also explored, with a thermal conductivity of only 0.032 W/(m·K) at room temperature and 0.108 W/(m·K) at high temperature of 1000 °C. These composites demonstrate no visible damage or deformation under extreme conditions across a substantial temperature range (−196 to 1300 °C), a property that is paramount for applications in extreme environments. At 1100–1300 °C, after 2 h of calcination, the composites exhibit a shrinkage rate of only 0.25%. After 600 s of butane torch ablation at 1300 °C, the back temperature is only 110 °C. Moreover, under 60% compression deformation, its maximum compression strength is 0.386 MPa. Even after 20 high-temperature thermal cycles (1300 °C for 2 h), the sample maintains a low thermal conductivity of 0.043 W/(m·K) and a compressive strength of 0.259 MPa. This work provides a new perspective for exploring the limits of the strength and thermal properties of ceramic composites in the field of high-temperature insulation, particularly under extreme conditions.