Spin-State Modulation of FeN4 via Pre-Coordination Axial N-Ligand for Efficient Oxygen Reduction Electrocatalysis.

The high cost and scarcity of platinum-based catalysts pose a significant barrier to the widespread commercialization of fuel cells, driving the search for high-performance alternatives. Atomically dispersed FeN4 catalysts are widely regarded as promising platinum-free alternatives for the oxygen reduction reaction (ORR). To overcome the fundamental constraint, we strategically introduce axial N-coordination to the Fe center by employing phenethylamine as a Lewis-base ligand, while using π-π stacking between imidazole and a benzene ring to precisely control coordination geometry and prevent sixth-ligand binding. The introduction of an axial ligand markedly enhances the spin polarization of the Fe center and reconfigures its electronic properties by modulating the local ligand field, which promotes more favorable spin/orbital interactions with oxygen species and fine-tunes the adsorption of reaction intermediates. Through precise spin-electronic tuning, N-FeN4C with an ultralow Fe loading of 0.477% delivers outstanding ORR performance, with a half-wave potential (E1/2) of 0.92 V and a Tafel slope of 57.83 mV dec-1, which corresponds to a 151% improvement in turnover frequency (TOF). This work introduces a rational ligand-field strategy to break the symmetry of single-atom Fe sites, thereby advancing the creation of high-performance, noble-metal-free electrocatalysts.