Fe Loading Triggers High-Spin State: Expediting H₂O/O₂ Swap in Fuel Cell Cathodes.

Fe-N-C catalysts have long suffered from kinetically sluggish oxygen reduction reaction (ORR) because of the overly strong binding to oxygen-related reaction species. Modulating the spin state of active centers emerges as a promising strategy to break the kinetic bottleneck. Herein, we show that high-spin (HS) Fe²⁺ (t2g⁴eg²) centers, with the emergence of unpaired 3d electrons in the dz² orbital, offer a substantially reduced reaction energy barrier through synergistic regulation of H₂O-O₂ adsorption. Facial low-spin (LS) Fe³⁺(t2g⁵eg⁰) to HS Fe²⁺ transition was achieved via densifying the Fe site density, stemming from the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. Synchrotron-based Fe Kβ X-ray emission spectroscopy (XES) and L-edge soft X-ray absorption spectroscopy (sXAS) directly reveal t2g-eg redistribution. These HS sites promote antibonding orbital occupancy compared to LS states, corroborating a correlation between spin density, reduced activation energy, and turnover frequency. Consequently, the single cell test achieves a peak power density of approximately 1.31 W cm⁻² and delivers a current density of 65.1 mA cm⁻² at 0.9 ViR-free, surpassing the DOE 2025 target and most of the previously reported Fe-N-C catalysts.