Integration of Morphology and Electronic Structure Modulation on Atomic Iron-Nitrogen-Carbon Catalysts for Highly Efficient Oxygen Reduction

Atomic transition-metal-nitrogen-carbon catalysts (M-N-Cs) hold great promise as Pt-group-metal-free candidates for electrochemical reactions, yet their rational design and controllable synthesis remain fundamental challenges. Here, the molten-salts mediated pyrolysis is demonstrated to be an effective and facile strategy for simultaneous morphology and electronic structure modulation of prototypical Fe-N-C materials, which functions as efficient oxygen reduction electrocatalysts. Taking advantage of the strong polarity and salt templating effects, the as-obtained Fe-N/C-single atom catalyst (SAC) possesses hierarchical porous nanosheet morphology with an impressive specific surface area of 2237 m(2) g(-1) and unique FeN4Cl moieties as isolated active centers. The Fe-N/C-SAC delivers remarkable alkaline oxygen reduction reaction (ORR) activity with a half-wave potential of 0.91 V and record kinetic current density up to 55 mA cm(-2), outperforming the benchmark Pt/C. By virtue of dechlorination treatment, it is experimentally identified that the enhanced ORR activities are essentially governed by the axially bound Cl. Theoretical calculations rationalize this finding and demonstrate that the well-defined fivefold-coordinated configuration accelerates 4e(-) pathway kinetics through near-optimal adsorption of the *OH intermediates and tunes the potential determining step from *OH reduction to *OOH formation. This study provides fundamental insights into the coordination-engineered strategy in single-atom catalysis.