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

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 BaCeO₃-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_0.6Fe_0.2Y_0.1Ce_0.1O_3-δ-BaCeO₃) exhibits a low resistance of 0.38 Ω cm² 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.