Improving Nitric Oxide Reduction Reaction Activity of TMN4–C Model Catalysts by Axial Atom Coordination

In comparison with the conventional four-nitrogen coordinated transition metal (TMN4), we clarify that the electrochemical nitric oxide reduction reaction (NORR) activity can be significantly improved by axially coordinating nonmetal atoms (O, F, Cl) over the metal sites. In light of an electron-withdrawing effect, the axial fifth ligand disrupts the electron distribution symmetry and regulates the local electronic structure of the metal active center. It subsequently moderates the TM–NO interaction and thus enhances the activity. In particular, MnN4O–C, FeN4O–C, CoN4O–C, and CoN4F–C are identified as promising NORR catalysts with ultralow limiting potential (UL) of −0.07, −0.07, −0.07, and −0.05 V, respectively. In addition, the axial atom can also passivate the competing hydrogen evolution reaction (HER), increasing the selectivity toward NH3 formation. It therefore can be concluded that the present work affirms a novel strategy for the rational design of advanced electrocatalysts, highlighting the significance of optimal metal–ligand match and the coordination microenvironment tuning of the active centers.