Preserving a Kinetically‐Metastable Nanophase by Limited Calcination for High‐Performance Protonic Ceramic Cells

ABSTRACT Sluggish oxygen reduction/evolution reactions (ORR/OER) at the air electrode critically limit the efficiency of reversible protonic ceramic cells (r‐PCCs), yet conventional high‐temperature calcination of the air electrode leads to undesired surface passivation, while low‐temperature calcination leads to insufficient crystallization, both of which severely impair electrocatalytic activity. Here we employ a thermally‐limited calcination process to kinetically retain a nanophase in the air electrode that only forms at a selected calcination temperature (termed “metastable”), thereby forming rich heterointerfaces for active ORR/OER. Specifically, controlled calcination temperature induces selective Ce incorporation into the host lattice of Ba(Co,Fe,Y)O 3‐δ while preserving the metastable BaCeO 3 ‐related nanophase. This nanostructure enriches oxygen‐vacancy‐related defects, accelerates surface exchange and bulk diffusion, promotes proton incorporation, and improves thermomechanical compatibility with the electrolyte. The as‐developed electrode (BaCo 0.6 Fe 0.2 Y 0.1 Ce 0.1 O 3‐δ ‐BaCeO 3 ) exhibits a low resistance of 0.38 Ω cm 2 at 550°C. Single cells with this electrode deliver a high peak power density of 1.44 W cm −2 at 650°C and an electrolysis current density of −2.47 A cm −2 at 1.3 V. These findings establish a promising strategy of thermally‐limited calcination for designing high‐performance reversible protonic ceramic cells.