Manipulation of New Married Edge-Adjacent Fe₂N₅ Catalysts and Identification of Active Species for Oxygen Reduction in Wide pH Range

Transition metal-nitrogen-carbon-based catalysts (M-N-C) serve as promising alternatives for oxygen reduction reaction (ORR). However, their synthesis generally involves complex pyrolysis reactions, resulting in their high structure heterogeneity and consequently making it difficult to distinguish the catalytic active sites. Herein, atomically dispersed Fe-2 on the hollow carbon spheres are synthesized as the model for insight into the active sites at the atomic level. By virtue of the systematic SCN- poisoning experiments and theoretical calculations, the authors find that both edge-adjacent Fe2N5 and graphitic N sites exhibit the pH-dependent poisoning kinetics, beyond a simple and traditional "SCN- poisoning M-N-x sites" notion, helping us to discriminate the edge-adjacent Fe2N5 structure and graphitic N species as the bi-active ORR sites in a wide pH range. Moreover, this is the first work to synthesize the new married edge-adjacent Fe2N5 structure in an experimental aspect. The original work offers an important insight to pinpoint the active species in different pH media, which can broaden the fundamental understanding to design M-N-C and metal-free-carbon-based catalysts for ORR.