Reaction mechanisms of CO₂ electrochemical reduction on Cu(1 1 1) determined with density functional theory

Density functional theory (DFT) was used to determine the potential-dependent reaction free energies and activation barriers for several reaction paths of carbon dioxide (CO₂) electrochemical reduction on the Cu(1 1 1) surface. The role of water solvation on CO₂ reduction paths was explored by evaluating water-assisted surface hydrogenation and proton (H) shuttling with various solvation models. Electrochemical OH bond formation reactions occur through water-assisted H-shuttling, whereas CH bond formation occurs with negligible H₂O involvement via direct reaction with adsorbed H* on the Cu(1 1 1) surface. The DFT-computed kinetic path shows that the experimentally observed production of methane and ethylene on Cu(1 1 1) catalysts occurs through the reduction of carbon monoxide (CO*) to a hydroxymethylidyne (COH*) intermediate. Methane is produced from the reduction of the COH* to C* and then sequential hydrogenation. Ethylene production shares the COH* path with methane production, where the methane to ethylene selectivity depends on CH2â̂ - and H* coverages. The reported potential-dependent activation barriers provide kinetics consistent with observed experimental reduction overpotentials and selectivity to methane and ethylene over methanol for the electroreduction of CO₂ on Cu catalysts.