C4N4-Supported Single-Atom Catalysts for Nitrate Electroreduction: Electronic Structure Insights and Descriptor-Guided Screening.

In the context of carbon neutrality and sustainable development, the electrochemical nitrate reduction reaction provides a promising route for waste-to-ammonia conversion, yet remains hindered by competing hydrogen evolution and sluggish multistep proton-electron transfer kinetics. To address limitations in nitrate activation and intermediate adsorption regulation, a two-dimensional C4N4 monolayer was employed to construct 23 transition-metal-anchored TM@C4N4 single-atom catalysts. Their stability, activity, and selectivity were systematically evaluated via multistep theoretical screening, density functional theory calculations, and interpretable machine learning (SISSO). Ir@C4N4, Fe@C4N4, and Mn@C4N4 were identified as the most promising candidates, exhibiting balanced stability, activity, and suppressed side reactions. Activity shows a volcano relationship with both the d-band center and nitrate adsorption free energy. Electronic structure analyses reveal that the positively charged TM-N4 environment enhances NO3- adsorption and intermediate stabilization, while strong d-p orbital hybridization promotes interfacial charge transfer, weakens N-O bonds, and lowers reaction barriers. SISSO further identifies metal electronegativity, interfacial charge state, and coordination geometry as key descriptors governing adsorption strength. Overall, this work clarifies the atomic-scale mechanism of TM@C4N4 catalysts for NO3RR and provides design principles for high-performance electrocatalysts.