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*.