Facet Dependence of CO₂ Reduction Paths on Cu Electrodes

Experimental results have shown that CO₂ electroreduction is sensitive to the surface morphology of Cu electrodes. We used density functional theory (DFT) to evaluate the thermodynamics and kinetics of CO₂ reduction pathways on Cu(100) and Cu(111) with the aim of understanding the experimentally reported differences in CO₂ reduction products. Results suggest that the hydrogenation of CO∗ to hydroxymethylidyne (COH∗) or formyl (CHO∗) is a key selective step. Cu(111) favors COH∗ formation, through which methane and ethylene are produced via a common CH₂ species under high overpotential (←0.8 V vs RHE). On Cu(100), formation of CHO∗ is preferred and ethylene formation goes through C-C coupling of two CHO∗ species followed by a series of reduction steps of the C₂ intermediates, under relatively lower overpotential (-0.4 to -0.6 V vs RHE). Further reduction of these C₂ intermediates, however, require larger potentials (∼-1.0 V vs RHE) and conflicts with the experimentally observed low potential pathway to C₂ products on Cu(100). Calculations show that the presence of (111) step sites on the flat (100) terrace can reduce the overpotential for C₂ production on the Cu electrode, which may be present on Cu(100) due to reconstruction. On Cu(100), a change in CO∗ coverage from low to high with increasing negative applied potential can trigger a switch from ethylene/ethanol to methane/ethylene as the reduction products by affecting the relative stability of CHO∗ and COH∗.