Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N‑C Acidic ORR Catalyst and Its in Situ Reduced Fe‑N₄ Active Site Identification Revealed by X‑ray Absorption Spectroscopy

Developing highly efficient, low-cost oxygen reduction catalysts, especially in acidic medium, is of significance toward fuel cell commercialization. Although pyrolyzed Fe-N-C catalysts have been regarded as alternatives to platinum-based catalytic materials, further improvement requires precise control of the Fe-N_<i>x</i> structure at the molecular level and a comprehensive understanding of catalytic site structure and the ORR mechanism on these materials. In this report, we present a microporous metal–organic-framework-confined strategy toward the preferable formation of single-atom dispersed catalysts. The onset potential for Fe-N-C is 0.92 V, comparable to that of Pt/C and outperforming most noble-metal-free catalysts ever reported. A high-spin Fe³⁺-N₄ configuration is revealed by the ⁵⁷Fe Mössbauer spectrum and X-ray absorption spectroscopy for Fe L-edge, which will convert to Fe²⁺-N₄ at low potential. The in situ reduced Fe²⁺-N₄ moiety from high-spin O_<i>x</i>-Fe³⁺-N₄ contributes to most of the ORR activity due to its high turnover frequency (TOF) of ca. 1.71 e s^–1 sites<sup>–1