<p>The Fe–N–C single-atom catalyst mainly generates CO (g) rather than high-value-added hydrocarbons in electrocatalytic CO₂ reduction reaction (CO₂RR), and its mechanism bottleneck is still unclear. This study revealed the thermodynamic and kinetic limitations of the key hydrogenation step from *CO → *CHO through density functional theory calculations: the adjacent co adsorption energy barrier between *CO and *H on the Fe–N–C surface is as high as 1.4&#xa0;eV, which is due to the electron redistribution effect induced by *CO on the surface, resulting in weak hydrogen adsorption at the N site (energy barrier increased from 1.05 to 1.4&#xa0;eV). After introducing K⁺ into the electrolyte, its local electric field adjusts the Fe charge population (+ 1.43 e →  + 1.14 e) and differentially regulates the adsorption of intermediates: The binding energy of *CO decreases, while the adsorption of *H increases. Orbital analysis shows that K⁺ promotes hybridization between the 5σ electrons of CO and the 3d orbitals of Fe (overlap integral enhancement), while weakening the C–O bond (bond length 1.168&#xa0;Å → 1.285&#xa0;Å) by feeding back electrons from Fe to the *CO 2π* orbitals (energy level difference reduced from 9.72 to 7.85&#xa0;eV). K⁺ induced hydration shell reduces the hydrogen bond density between *H and water molecules, enhancing their stability. This work elucidated the structure–activity relationship of Fe–N–C catalysts through multiscale theoretical simulations, deepening the understanding of their reaction bottlenecks at the atomic scale and providing theoretical guidance for designing efficient catalysts for hydrocarbon generation.</p> Graphical abstract <p></p>