Modulating coordination chemistry of Fe active sites enables near-unity CO selectivity in CO2 electrolysis

The development of high-performance metal-nitrogen-carbon (M-N-C) catalysts for electrochemical CO2 reduction (CO2RR) requires precise control over atomic dispersion and coordination environments. Here, we report a mechanochemical ball-milling strategy to synthesize an iron-nitrogen-carbon catalyst (Fe-NC-BM) featuring uniformly dispersed Fe species within a nitrogen-doped carbon matrix. Ball-milling promotes homogeneous Fe site distribution, introduces abundant defects, and modulates the electronic structure. This catalyst achieves a CO Faradaic efficiency exceeding 99% across −0.5 to −1.2 V vs RHE, with a current density of 41.7 mA cm–2 at −1.1 V, more than twice that of the non-ball-milled counterpart (17.0 mA cm–2). Aberration-corrected STEM and XPS analyses confirm that ball-milling enhances Fe dispersion, prevents aggregation during pyrolysis, and fosters Fe–N4 site formation. The mechanical forces also induce an interconnected nanostructure, increasing active site exposure and enabling efficient charge and mass transport. Defect engineering further tunes the electronic structure and lowers the reaction energy barrier, as supported by DFT calculations. This work demonstrates that ball-milling is a solvent-free and effective pretreatment strategy for simultaneously enhancing the density and intrinsic activity of active sites, providing a promising pathway for the rational design and large-scale production of next-generation CO2RR electrocatalysts.