Electrochemical Reduction of CO₂ Using Copper Single-Crystal Surfaces: Effects of CO* Coverage on the Selective Formation of Ethylene

Copper oxide-derived Cu catalysts are known to exhibit enhanced energetic efficiencies and selectivities towards the reduction of carbon dioxide to commercially vital C2 products such as ethylene (C2H4). However, the cause of this selectivity is not fully understood. In this work, we elucidated a fundamental reason underlying the selectivity of CO2 reduction toward C2 products by studying its reactivity on Cu(100), Cu(111), and Cu(110) single-crystal surfaces. A combination of cyclic and linear sweep voltammetries, chronoamperometry, online gas chromatography, 1H nuclear magnetic resonance spectroscopy, and density functional theory (DFT) calculations was employed for this end. A wide range of electrochemical potentials from −0.28 to −1.25 V versus the reversible hydrogen electrode was investigated. Aqueous 0.1 M KHCO3 was used as the electrolyte. We report here two general trends on Cu2O-derived Cu and Cu single-crystal surfaces: (i) the onset potential for the formation of C2H4 always starts 300–400 mV more negative than the onset potential for CO evolution, and (ii) C2H4 was formed only after a significant amount of CO gas was produced. Among the single-crystal surfaces investigated, Cu(100) required the lowest overpotential to reduce CO2 to C2H4. These observations were rationalized using DFT simulations. Of the three single-crystal surfaces modeled, the dimerization of two CO* molecules on Cu(100) exhibited the lowest energy barrier, and this barrier can be further lowered with higher CO* coverages. The application of our observed experimental trends to other previously reported Cu-based systems strongly suggests that a high surface coverage of CO* is central for the selective formation of C2H4.