Vacancy–Redox Coupling at Interface‐Engineered Heterostructures Enhances Reversible Energy Conversion in Protonic Ceramic Cells

ABSTRACT Achieving efficient and durable oxygen electrocatalysis in protonic ceramic cells (PCCs) demands precise control of defect chemistry and cation redox under steam. Here, we design a hierarchically engineered oxygen electrode comprising a three‐dimensional, mesh‐like PrNi 0 . 7 Co 0 . 3 O 3−δ (PNC) scaffold conformally integrated with a vacancy‐rich PrO x nanophase. This architecture extends the reactive zone while the PrO x – PNC interphase enables vacancy‐mediated redox coupling between Pr and Co, buffering local oxygen chemical potential and stabilizing the defect landscape during reversible operation. The enhanced activity is attributed to vacancy‐assisted steam activation and defect‐mediated oxygen surface exchange and is consistent with interfacial modulation of metal–oxygen covalency within an O 2p band center framework. The electrode delivers 1.75 W cm − 2 in fuel‐cell mode and 2.77 A cm − 2 at 1.3 V in electrolysis at 600°C, maintains >92% Faradaic efficiency, and shows minimal degradation over 200 h. Our results establish a general strategy for coupling hierarchical transport with chemically active, redox‐buffered interphases to achieve both high kinetics and durability in protonic electrochemical systems.