Unveiling Electron Dynamics in the Electrochemical Reduction of CO₂ to Methane on Copper

Electrochemical reduction of CO₂ (CO₂ER) into fuels is a crucial strategy for mitigating climate change and meeting sustainable energy demands. Among catalytic materials, copper stands out due to its ability to convert CO₂ into a diverse range of hydrocarbons and oxygenates with significant current density. Quantum mechanical studies have greatly advanced the understanding of CO₂ER on copper surfaces; however, most have focused on thermodynamics and/or kinetics to elucidate reaction mechanisms or explain experimental trends, leaving orbital-level insights largely unexplored. In this study, density functional theory calculations combined with intrinsic bond orbital analysis to track orbital evolution across 13 protonation steps involved in CO₂ER to methane are employed. Based on these results, an arrow-pushing diagram is constructed to illustrate the electron flow for each step. This methodology allows to identify the key orbital used by each CO₂ER intermediate to accommodate the transferred proton. Furthermore, this approach also reveals that the copper electrode actively participates in six protonation steps by exchanging pairs of electrons with CO₂ER intermediates that are either selectivity-determining or rate-determining steps. These insights deepen the understanding of CO₂ER mechanisms and provide a foundation for developing strategies to enhance its efficiency and selectivity.