Constant Electrode Potential Quantum Mechanical Study of CO2 Electrochemical Reduction Catalyzed by N-Doped Graphene

In this study, quantum mechanics combined with a constant electrode potential model were employed to study CO2 electrochemical reduction (CO2ER) on N-doped graphenes under U = −1.0 VSHE. Our goal was to investigate whether metal-free N-doped graphene itself can reduce CO2 and identify the reaction centers. We considered both the thermodynamics and kinetics of the process. Among the 26 reaction sites that were screened, only 2 of these sites could reduce CO2 to CO(g) with a kinetic barrier (ΔG‡⧧) of ∼0.55 eV for the rate-determining step and downhill thermodynamics for each elementary step. Both sites are composed of carbon atoms on the edge of graphene and adjacent to graphitic nitrogen atoms. Two other motifs (composed of either pyridinic or pyrrolic N) were also able to reduce CO2 to surface-bound CO with ΔG⧧ values less than 0.70 eV. However, despite favorable thermodynamics, the reduction of the bound CO to CHO suffered from larger ΔG⧧ values (>0.97 eV), rendering the reaction inaccessible. Therefore, N-doped graphene is able to reduce CO2 to CO(g) or surface-bound CO. However, further reactions beyond two-proton–two-electron reduction are unlikely. In addition, the evaluation of the performance of a site for the CO2ER must consider both the thermodynamics and kinetics of the process.