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 waterassisted surface hydrogenation and proton (H) shuttling with various solvation models. Electrochemical O-H bond formation reactions occur through water-assisted H-shuttling, whereas C H 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 CH2*, 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. (c) 2014 Elsevier Inc. All rights reserved.