Atomically Dispersed Fe–Nx Sites in a Triazine–Bis(imino)pyridine Covalent Organic Framework Enabling Bifunctional Nitrate-to-Ammonia Conversion and Oxygen Evolution

Abstract

Electrochemical nitrate reduction to ammonia (NO3RR) provides a sustainable strategy for simultaneously removing nitrate pollutants and producing carbon-neutral ammonia; however, achieving efficient and selective nitrate conversion under neutral conditions while integrating the anodic oxygen evolution reaction (OER) remains challenging. Herein, we report a coordination-engineered triazine–bis(imino)pyridine covalent organic framework hosting atomically dispersed Fe–Nx active sites (Fe–Tta–Dfp) as an intrinsically bifunctional electrocatalyst for coupled NO3RR and OER. The crystalline π-conjugated framework combines electron-deficient triazine units that promote nitrate enrichment with bis(imino)pyridine coordination pockets that stabilize isolated Fe centers, enabling precise modulation of the electronic structure and catalytic microenvironment. Comprehensive structural characterization confirms long-range crystallinity, hierarchical porosity, preserved framework integrity, and homogeneous atomic Fe dispersion without metallic or oxide aggregates. Benefiting from the synergistic interaction between triazine-assisted nitrate adsorption and coordination-defined Fe–Nx sites, Fe–Tta–Dfp exhibits outstanding NO3RR activity in neutral 0.1 M KHCO3 containing 0.1 M KNO3, achieving a Faradaic efficiency of 94.93% at −0.5 V vs RHE with an NH3 yield rate of 553.57 μg h–1 cm–2 at −0.6 V vs RHE, while effectively suppressing nitrite formation and eliminating detectable hydrazine. The catalyst also displays excellent operational stability with negligible Fe leaching and sustained activity during prolonged electrolysis. Moreover, the same Fe–Nx centers efficiently catalyze oxygen evolution, demonstrating genuine bifunctionality within a single COF platform. Comparative studies using structurally related control COFs reveal that the cooperative integration of electron-deficient triazine nodes and bis(imino)pyridine-coordinated Fe sites is essential for maximizing catalytic performance. Density functional theory calculations further show that Fe incorporation optimizes adsorption energetics, enhances orbital coupling, and lowers the rate-determining free-energy barriers for both NO3RR and OER, in excellent agreement with experimental observations. This work establishes coordination-engineered COFs with atomically dispersed metal sites as a versatile platform for multifunctional electrocatalysis, offering a general strategy for integrated nitrogen valorization and sustainable energy conversion.