Engineering Synergistic Oxygen-Proton Properties for High-Performance Reversible Protonic Ceramic Cell Air Electrodes.

Reversible protonic ceramic cells (RePCCs) promise integration with renewable energy, supporting sustainable energy systems. RePCC performance hinges on the air electrode activity, where optimal proton, oxygen, and electron transport are essential. However, in air electrodes, oxygen exchange requires vacancies, while hydration consumes them, creating a fundamental trade-off. Conventional material design strategies overemphasize hydration, overlooking their impact on oxygen transport. Here, using a simple Nb-doped Sr₃Fe₂O_7-δ (SF) perovskite system, this study demonstrates that balanced oxygen-proton transport properties are essential for high-performance air electrodes. Specifically, SF exhibits abundant oxygen vacancies, yet excessive hydration occupies these vacancies, thereby limiting oxygen-ion transport and impairing oxygen electrocatalytic activity. Optimal Nb doping maintains the oxygen vacancy concentration while effectively suppressing excessive hydration due to the enhanced electrostatic repulsion between lattice cations and protons resulting from Nb doping. The resulting Sr₃Fe_1.9Nb_0.1O_7-δ (SFNb0.1) electrode achieves a balance between oxygen and proton transport. Furthermore, Nb doping stabilizes the material's crystal structure. As a result, the electrode shows enhanced activity and stability. This work underscores balanced oxygen-proton transport as a key design principle for high-performance RePCC air electrodes.