Engineering Selectivity in Electrochemical CO2 Reduction via Transition-Metal Anchored Aza-COF Monolayers

Abstract Developing efficient and selective electrocatalysts for electrochemical CO2 reduction reaction (CO2RR) remains a major challenge for sustainable fuel production from CO2 for mitigating its excessive concentrations and bringing climate neutrality. Herein, density functional theory (DFT) calculations were employed to systematically explore the catalytic potential of transition-metal-anchored aza-covalent organic framework monolayers (TM–aza-COFs) as single-atom catalysts for CO2RR. A total of 27 transition metals from the 3d, 4d, and 5d series were investigated as isolated active sites on the aza-COF support. Binding energy calculations identified seventeen thermodynamically stable TM–aza-COF systems. Electronic structure analyses based on projected density of states and Bader charge calculations reveal that the stability of these systems stems from strong hybridization between TM d orbitals and the N/C p orbitals of the framework, promoting efficient charge transfer and stabilizing the dispersed metal centers. CO2 adsorption studies demonstrate substantial activation, characterized by pronounced O–C–O bending and elongation of the C–O bonds with adsorption energies ranging from –1.18 to –2.12 eV for 3d metals and –0.56 to –2.11 eV for 4d/5d metals. Moreover, free energy calculations indicate that CO2RR is thermodynamically favored over the competing hydrogen evolution reaction across all screened catalysts. Based on limiting potentials together with CO and HCOOH desorption energetics, eight promising candidates—Cr, Ti, Fe, Ni, Zr, Mo, Tc, and Pd anchored on the aza-COFs—were selected for detailed evaluation toward deeper reduction products. Among them, Cr-aza-COF and Fe-aza-COF exhibit the highest catalytic activity for CH3OH and CH4 formation in a selective manner, delivering low limiting potentials of –0.38 and –0.45 V, respectively. Further, the superior performance of these catalysts was found to originate from optimal adsorption of CO2RR intermediates and reduced energy barrier for the potential-determining steps as evidenced by crystal orbital Hamilton population (COHP) plots. The present findings provide important mechanistic insights for the rational development of efficient and stable COF-supported single-atom catalysts for CO2RR.