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.

Protonic ceramic electrolysis cells (PCECs) have attracted significant interest because of their efficiency and environmental sustainability in energy conversion. However, their commercial application is hindered by the absence of effective and robust electrodes capable of operating in harsh environments, such as those characterized by high vapor or CO₂ concentrations. In this study, we developed a stable steam electrode composed of PrBaMn₂O_5+δ (PBM) and the durable proton conductor BaZr_0.85Y_0.15O_3−δ (BZY), which was enhanced with the deposition of PrO_x nano-catalysts. The composite electrode exhibited a low polarization resistance (~0.34 Ω·cm² at 600 °C), comparable to that of conventional cobalt-based electrodes. Additionally, extensive testing over hundreds of hours under severe conditions revealed exceptional durability, with no significant degradation observed. Notably, the electrode composited with cube-shaped BZY microcrystals and PBM showed a higher proton conductivity of 2.15×10⁻⁵ S·cm⁻¹ at 500 °C, representing an entire order of magnitude greater than that of the electrode composited with irregular nanosized BZY. In addition, the single cell achieved a superior electrolysis current of 2.0 A·cm⁻² at 700 °C and 1.3 V. These findings demonstrate the superiority of constructing an innovative interface between the mixed ionic‒electronic conductor (MIEC) and the proton conductor. Our work presents a promising strategy for designing durable steam electrodes for PCECs through a rational compositing approach.

BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY) is a new type of triple-conductive cathode material with proton-oxygen ion-electron conduction for proton ceramic fuel cells (PCFC), with excellent electrocatalytic activity in low and intermediate temperature range (500–700 ℃). However, the thermal expansion coefficient of BCFZY is much larger than that of electrolytes, which seriously hindered the electrode performance in PCFC. In this study, BCFZY was introduced into BaZr_0.85Y_0.15O_3−δ porous backbone using a precursor solution infiltration method to prepare composite cathode, with thermal expansion mismatch was effectively alleviated, so that the electrochemical performance was obviously improved. The optimum infiltrating concentration is ~46.7 wt.% and the polarization resistance of the optimized electrode is 0.20 Ω·cm² at 600 ℃, which is about 63% lower than that of the BCFZY electrode prepared by using screen-printing method. The relaxation time (DRT) distribution method was used to analyze electrochemical processes of the electrode, suggesting that the improved performance of the BCFZY electrode is mainly related to the acceleration of the charge transfer, the ion formation and the gas diffusion processes.