Nanoparticle-Mediated Synthesis of High-Density Single-Atom Catalysts for Acidic Oxygen Reduction Reaction

Iron-nitrogen-carbon materials are promising nonprecious-metal catalysts for oxygen reduction reaction, yet their active-site density is inherently limited by conventional high-temperature synthesis. Herein, we introduce a precursor-mediated synthesis strategy that overcomes this bottleneck by using ferrous oxalate as a highly dispersed iron source. Controlled low-temperature decomposition generates ultrafine FeO_x nanoparticles, which are subsequently converted into atomically dispersed Fe-N₄ sites during pyrolysis. The concomitant release of CO₂ concurrently etches the carbon matrix, creating additional defects and microporosity. This approach achieves a high site density of 1.18 × 10²⁰ sites g^-1 (3.4 ± 0.6 atoms nm^-2). The resulting catalyst exhibits outstanding ORR performance in acidic medium, with a half-wave potential of 0.825 V vs RHE, and achieves a peak power density of 1.43 W cm^-2 in a practical H₂-O₂ fuel cell. Operando X-ray absorption spectroscopy further reveals the reversible structural dynamics of the Fe-N₄ sites, which switch between tetra- and penta-coordinated configurations during the reaction, directly evidencing that such coordination flexibility optimizes intermediate adsorption/desorption and enhances catalytic activity. This work provides a practical route to break the site-density ceiling in Fe-N-C catalysts and delivers insights into coordination-environment evolution of single-atom centers under working conditions.