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

Fe−N−C catalysts, with a planar D_4h symmetric FeN₄ 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 FeN₄ active site. Electrochemical experimental results demonstrate that FeN₄@porous carbon (FeN₄@PC), featuring enriched asymmetric edge-FeN₄ active sites, exhibits higher acidic oxygen reduction reaction catalytic activity compared to FeN₄@flaky carbon (FeN₄@FC), where symmetric FeN₄ 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 FeN₄@PC is attributed to the higher oxidation state of Fe species in the edge structure of FeN₄@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.