Mechanistic Insights into Water-Mediated CO<sub>2</sub> Electrochemical Reduction Reactions on Cu@C<sub>2</sub>N Catalysts: A Theoretical Study

Abstract: CO2 molecules can be converted into various fuels and industrial chemicals through electrochemical reduction, effectively addressing the problems of global warming, desertification, ocean acidification, and other adverse environmental changes and energy supply issues such as excessive utilization of nonrenewable fossil fuels. Generally, the pathway of the CO2 reduction reaction (CO2RR) involves multiple proton–electron pairs transferred to the reactants, resulting in the production of multiple reduction products. Here, protons are derived from water molecules under aqueous solvent conditions. Therefore, exploring the effect of water molecules on the proton–electron pair transfer process in CO2RRs is essential. In this study, we developed a water-mediated hydrogen shuttle model (H-shuttling) as a hydrogenation model to investigate the effect of water molecules on the proton–electron pair transfer process in CO2RRs and compared it with the widely used water-free direct hydrogenation model (H-transfer), wherein the hydrogen atom is used as a proton. Because copper is a metal electrode material capable of producing hydrocarbons from CO2 electroreduction with a high faraday efficiency, and nitrogen-doped graphene (C2N) exhibits excellent catalytic CO2 activation, we selected a single copper atom-embedded C2N (Cu@C2N) as the catalyst. Furthermore, to study the effect of graphene on the CO2RR activity of Cu@C2N/G, we selected a graphene-loaded Cu@C2N composite (Cu@C2N/G) as the catalyst because graphene was utilized as a substrate to boost the conductivity of the catalyst. In the two hydrogenation models, we investigated the mechanisms of CO2RRs on Cu@C2N and Cu@C2N/G catalysts through density functional theory calculations. Notably, in the H-shuttling model, the H atom combines with the water molecule to form H3O, which transfers one of its own H atoms to a reactant on the catalyst surface, yielding a reaction intermediate. The H-shuttling model enhances the interaction between the catalyst and intermediate. Graphene, as a substrate, transfers electrons to the Cu@C2N surface of the Cu@C2N/G catalyst, which is demonstrated by calculations of the Bader charge transferred between the reaction intermediate and catalyst, as well as the Gibbs free energy of the CO2 reduction elementary reaction. This effectively lowers the Gibbs free energy of the potential-determining step and enhances the CO2RR catalytic activity of Cu@C2N/G. Moreover the limiting potentials of the CO2RR and hydrogen evolution reaction are determined to obtain the activity and selectivity of the Cu@C2N and Cu@C2N/G catalysts. The results indicate that CO2 molecules on the Cu@C2N and Cu@C2N/G catalysts generate HCOOH at low applied potentials, and are able to produce CO, CH3OH, CH4, and H2 as the applied potentials increases.