Regulating d-band center and d-p orbital alignment to enhance K₂NiF₄-type cathode performance for protonic ceramic fuel cells

Regulating the transition-metal d-band center and metal-oxygen orbital alignment represents a potentially useful approach to improve cathodic oxygen reduction reaction (ORR) kinetics in protonic ceramic fuel cells (PCFCs). Herein, K₂NiF₄-type La_0.5Sr_1.5MnO_4+δ (LSMO) and Fe-based La_0.5Sr_1.5FeO_4+δ (LSFO) cathodes were investigated to clarify how electronic structure regulation affects oxygen defect chemistry, oxygen/proton transport, and proton-involved ORR activity. Density functional theory (DFT) calculations reveal that the work-function difference between LSFO and LSMO is negligible, whereas LSFO possesses a more favorable Fe 3d–O 2p energy alignment. This electronic configuration lowers the energetic cost of oxygen vacancy formation, improves hydration-related proton incorporation, reduces the proton hopping barrier, and decreases the thermodynamic free energy requirement for key ORR steps. Experimentally, LSFO exhibits enriched oxygen vacancy-related surface oxygen species, much higher electronic conductivity, and accelerated oxygen/proton surface exchange and diffusion. $P_{{\text{O}}_2}/P_{{\text{H}}_2\text{O}}$-dependent electrochemical impedance spectra further confirm that LSFO promotes multiple p-ORR subprocesses, including interfacial charge/proton transfer, oxygen reduction, and surface oxygen transport. Consequently, the optimized LSFO cathode achieves a peak power density of 2.2 W·cm⁻² and a polarization resistance of 0.04 Ω·cm² at 700 °C, outperforming LSMO and many reported Ruddlesden‒Popper (R–P) cathodes. Moreover, the LSFO cell exhibits stable operation over 400 h. These results demonstrate that coordinated regulation of the d-band center and d-p orbital alignment is a viable way to design high-performance K₂NiF₄-type PCFC cathodes.