Surface Reconstruction and Catalytic Performance Enhancement of the Perovskite Air Electrode for Reversible Protonic Ceramic Fuel Cells via Nonstoichiometry Modulation

Reversible protonic ceramic fuel cells (R-PCCs) offer efficient energy storage and conversion. Therefore, PCCs show significant potential to revolutionize the large-scale adoption of intermittent renewable energy sources. However, their performance is significantly hindered by the reduced oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activities at air electrodes as the operating temperature decreases. Here, we synergistically developed a perovskite air electrode, (Ba_0.85K_0.15)_<i>x</i>Co_0.7Fe_0.2Nb_0.1O_3-δ ((BK)_<i>x</i>CFN, <i>x</i> = 0.90, 0.95, 1.00, 1.05, 1.10), through nonstoichiometry design and a steam-induced surface reconstruction strategy. The introduction of nonstoichiometric ratios notably promotes surface reconstruction of the (BK)_<i>x</i>CFN air electrode, a process further amplified under humid air conditions. Compared to stoichiometric BKCFN, the nonstoichiometric (BK)_<i>x</i>CFN forms in situ exsolution BaCoO₃ nanoparticles on the surface under water vapor, increasing electrocatalytically active sites. When employed as the air electrode in anode-supported R-PCCs, the (BK)_1.05CFN electrode, with 5% A-site excess, exhibited superior bifunctional catalytic activity. It achieved a polarization resistance as low as 0.057 Ω·cm² at 700 °C in humid air with 3% H₂O, representing a 19.3% improvement over BKCFN (0.068 Ω·cm²). Additionally, the single cell with the configuration of (BK)_1.05CFN|BZCYYb|Ni-BZCYYb reached a peak power density of 828 mW·cm^-2 and an electrolytic current density of 1.80 A·cm^-2 at 1.3 V under humid air at 700 °C. The single cell demonstrated excellent operational stability over 100 h in a humid air environment under step-potential test conditions. This study provides a promising strategy for designing high-performance air electrodes for R-PCCs, leveraging nonstoichiometric composition design and surface reconstruction, which has extended applications to other energy material fields.