Symmetry Breaking of FeN₄ Moiety via Edge Defects for Acidic Oxygen Reduction Reaction

Fe−N−C catalysts, with a planar D 4h symmetric FeN 4 structure, show promising as noble metal‐free oxygen reduction reaction catalysts. Nonetheless, the highly symmetric structure restricts the effective manipulation of its geometric and electronic structures, impeding further enhancements in oxygen reduction reaction performance. Here, a high proportion of asymmetric edge‐carbon was successfully introduced into Fe−N−C catalysts through morphology engineering, enabling the precise modulation of the FeN 4 active site. Electrochemical experimental results demonstrate that FeN 4 @porous carbon (FeN 4 @PC), featuring enriched asymmetric edge‐FeN 4 active sites, exhibits higher acidic oxygen reduction reaction catalytic activity compared to FeN 4 @flaky carbon (FeN 4 @FC), where symmetric FeN 4 is primarily distributed within the basal‐plane. Synchrotron X‐ray absorption spectra, X‐ray emission spectra, and theoretical calculations indicate that the enhanced oxygen reduction reaction catalytic activity of FeN 4 @PC is attributed to the higher oxidation state of Fe species in the edge structure of FeN 4 @PC. This finding paves the way for controlling the local geometric and electronic structures of single‐atom active sites, leading to the development of novel and efficient Fe−N−C catalysts.