Transition State Characterization and Electrode Potential Effects on Thermally Driven *CO–*CO Coupling in the Free Energy Landscape

Among the proposed mechanisms of the electrocatalytic CO 2 reduction reaction (eCO 2 RR) to convert CO 2 into valuable C 2 chemicals and fuels, *CO–*CO coupling is a key C–C bond formation step. However, static density functional theory (DFT) calculations often struggle to accurately describe this process at the liquid–solid interface. In this work, we employed ab initio molecular dynamics with umbrella sampling to calculate the free energy changes of *CO–*CO coupling on Cu(111) in the presence of water. Through detailed and rigorous analysis, the bridge/hollow–bridge/hollow (*OC–CO) configuration was identified as the most favorable transition state structure at the interface, offering valuable insight into ongoing debates over the true nature of reaction sites. This preference is supported by a larger orbital-overlap integral between *C 2p and Cu 3d orbitals and enhanced charge transfer from *OC–CO to the surface. Importantly, we demonstrate that the *CO–*CO coupling barrier, despite being a thermally driven (non-electrochemical) step, is sensitive to electrode potential. A barrier reduction of ∼0.11 eV per 1.0 V negative shift was observed, potentially enhancing reaction rates by an order of magnitude. These findings underscore the dynamic nature of interfacial environments and highlight the need to reconsider the interplay between electrochemical and thermally driven steps in the eCO 2 RR under varying electrode potentials.