Biomimetic Dual-Iron-Site Single-Atom Nanozymes for N₂-Selective Electrocatalytic Denitrification

Electrocatalytic denitrification (ECDN) for the reduction of NO₃^- to N₂ offers an effective and environmentally benign method for removing nitrogen from wastewater, but challenges remain for poor N₂ selectivity. To address this issue, this study reports a dual-iron-site single-atom nanozyme (SAN, FePc@FeNOC) electrocatalyst, resembling the natural cytochrome c-dependent nitric oxide reductase (cNOR). The FePc@FeNOC electrocatalyst exhibits a NO₃^- removal efficiency as high as 96.1%, accounting for N₂ selectivity of 93.3% and Faradaic efficiency of 82.8% at a reaction time of 10 h. The theoretical results reveal that the potential-determining step of ECDN to N₂ is more thermodynamically favorable than that to NH₃ by FePc@FeNOC, as indicated by the lower free energy barrier for *NO to *N₂O₂ (0.82 eV) compared with that for *NO to *NOH (0.87 eV). The *N₂O₂ intermediate demonstrates enhanced charge separation compared with *NOH. The charge redistribution strengthens the electrostatic coupling between FePc@FeNOC and *N₂O₂, which not only stabilizes the intermediate structure but also creates a thermodynamic driving force for N₂ formation. We further demonstrate that the superior N₂-selectivity (90%) of FePc@FeNOC can offer a promising electrocatalyst for removing nitrogen from realistic photovoltaic wastewater with a low energy consumption of 9.8 kWh kgN₂^-1. This work provides a proof-in-concept demonstration of mimicking cNOR toward the sustainable treatment of nitrate-contaminated wastewater.