Facet Dependence of CO 2 Reduction Paths on Cu Electrodes

Experimental results have shown that CO 2 electroreduction is sensitive to the surface morphology of Cu electrodes. We used density functional theory (DFT) to evaluate the thermodynamics and kinetics of CO 2 reduction pathways on Cu(100) and Cu(111) with the aim of understanding the experimentally reported differences in CO 2 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 CH 2 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 C 2 intermediates, under relatively lower overpotential (−0.4 to −0.6 V vs RHE). Further reduction of these C 2 intermediates, however, require larger potentials (∼−1.0 V vs RHE) and conflicts with the experimentally observed low potential pathway to C 2 products on Cu(100). Calculations show that the presence of (111) step sites on the flat (100) terrace can reduce the overpotential for C 2 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*.