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 μgNH3 h–1 cm–2 yield rate at −0.68 V vs RHE, remarkably surpassing that of Fe-NC (<80%, 4330.9 μgNH3 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-Hx, 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.
The development of oxygen reduction reaction (ORR) catalysts with high activity, stability, and economic applicability plays a decisive role in reducing expenses and enhancing the discharge performance of seawater-based zinc-air batteries (SWZABs). Co- and Fe-based single-atom catalysts (M-N4-C) with metal-N4 structure offer advantages of well-defined active structure and high active site utilizations. However, the oxygen electrocatalytic performance of M-N4-C remains a formidable challenge due to the highly stable centrosymmetric electronic structure. To overcome the dilemma, we develop a Co-N4Cl-C with axial coordination of Cl atoms. The axial coordination drags the Co atoms out of the Co-N4 centrosymmetric configuration. This alters the electronic configuration of Co single-atom sites, resulting in a valence state change from +1.83 to +0.67 and forming a localized negative charge environment. These alternations enhance the electronic orbital overlap between Co single-atom sites and oxygen species, promote the rapid evolution of *OOH intermediates, and inhibit the adsorption of toxic Cl- ions, ensuring the ORR kinetics and stability. Co-N4Cl-C exhibits a high oxygen reduction onset potential of 1.05 mV and a half-wave potential of 0.88 mV vs. the reversible hydrogen electrode. The SWZAB, featuring a Co-N4Cl-C catalyst cathode, Zn anode, and NaCl electrolyte supplemented with KOH, reaches a discharge voltage platform of 1.27 V and a peak power density of 179 mW·cm-2, even at a current density of 10 mA·cm-2. This study sheds important light on advancing single-atom catalysts with superior ORR performance and economic viability.
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