Axial Chlorine-Induced Symmetry-Breaking Iron Single-Atom Catalyst for Electrochemical Ammonia Synthesis

Electrochemical nitrate reduction reaction (NO₃^–RR) presents a sustainable method for ammonia synthesis. Single-atom catalysts possessing the symmetric planar four-ligand structure (M-N₄) serve as advantageous catalytic active sites for NO₃^–RR. However, the inherent extreme symmetry of the standard M-N₄ structure limits the reaction kinetics. Herein, we introduce a symmetry-breaking iron single-atom catalyst coordinated with axial chlorine on nitrogen-doped carbon (Cl-Fe-NC) for NO₃^–RR. Cl-Fe-NC exhibits a 99.4% ammonia Faradaic efficiency (FE) at −0.28 V vs reversible hydrogen electrode (RHE) with a 9396.7 μg_NH3 h^–1 cm^–2 yield rate at −0.68 V vs RHE, remarkably surpassing that of Fe-NC (<80%, 4330.9 μg_NH3 h^–1 cm^–2 at the same potential). Operando synchrotron radiation Fourier transform infrared (SR-FTIR) spectroscopy confirms that key intermediates, such as *NO, *NO-H_x, and σ(N–H), are formed. Density functional theory (DFT) calculations attribute the optimized free energy of NO₃^–RR intermediates to the axial chlorine design, reducing the potential determination step barrier energy by up to 0.66 eV. The presence of axial Cl atoms modulates the symmetry of the single Fe atom, enhancing the adsorption of nitrate ions and the enrichment of critical intermediates during NO₃^–RR while inhibiting the hydrogen evolution reaction (HER). This discovery opens avenues for boosting electrochemical ammonia synthesis through the precise modulation of atomic structures by doping heteroatoms for symmetry breaking.