Heterojunction Architecture Resolves the Stability‐Performance Paradox in Reversible Protonic Ceramic Cells

The practical deployment of protonic ceramic cells (PCCs) is currently impeded by an intrinsic trade‐off between chemical stability and electrochemical performance. Here, we resolve this paradox through a heterojunction architecture that synergistically integrates a high‐conductivity, Zn‐modified chemically stable electrolyte with a self‐assembled, multiphasic oxygen electrode, PrNi 0.5 Co 0.3 Zr 0.1 Y 0.1 O 3‐δ (PNCZY). This design establishes a functionally graded interface with engineered band alignment, creating a Schottky‐type barrier that selectively suppresses electronic leakage while facilitating rapid proton transport. The resulting device exhibits exceptional bidirectional performance, achieving a peak power density of 956 mW cm −2 at 600°C in fuel cell mode and an electrolysis current density of −2300 mA cm −2 at 1.3 V. Crucially, the cell maintains > 96% Faradaic efficiency under high steam loads, confirming the efficacy of the carrier‐selective interface. This work provides a generalizable blueprint for decoupling material properties through interfacial engineering, paving the way for durable, high‐efficiency energy conversion technologies.