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

Abstract

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.