Mechanistic Insights into Water-Mediated CO2 Electrochemical Reduction Reactions on Cu@C2N Catalysts: A Theoretical Study

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.

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