Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived Cu_<i>x</i>Zn Catalysts

Abstract

The electrochemical reduction of carbon dioxide (CO2) to ethanol (C2H5OH) and ethylene (C2H4) using renewable electricity is a viable method for the production of these commercially vital chemicals. Copper (Cu) and its oxides are by far the most effective electrocatalysts for this purpose. However, the formation of ethanol using these catalysts is generally less favored in comparison to that of ethylene. In this work, we demonstrate that the selectivity of CO2 reduction toward ethanol could be tuned by introducing a cocatalyst to generate an in situ source of mobile CO reactant. Cu-based oxides with different amounts of Zn dopants (Cu, Cu10Zn, Cu4Zn, and Cu2Zn) were prepared and used as catalysts under ambient pressure in aqueous 0.1 M KHCO3 electrolyte. By varying the amount of Zn in the bimetallic catalysts, we found that the selectivity of ethanol versus ethylene production, defined by the ratio of their Faradaic efficiencies (FEethanol/FEethylene), could be tuned by a factor of up to ∼12.5. Ethanol formation was maximized on Cu4Zn at −1.05 V vs RHE, with a remarkable Faradaic efficiency and current density of 29.1% and −8.2 mA/cm2, respectively. The Cu4Zn catalyst was also catalytically stable for the production of ethanol for at least 5 h. The importance of Zn as a CO-producing site was demonstrated by performing CO2 reduction on Cu–Ni and Cu–Ag bimetallic catalysts. Operando Raman spectroscopy revealed that the as-deposited Cu-based oxide films were reduced to the metallic state during CO2 reduction, after which only signals belonging to CO adsorbed on Cu sites were recorded. This showed that the reduction of CO2 probably occurred on metallic sites rather than on metal oxides. A two-site mechanism to rationalize the selective reduction of CO2 to ethanol is proposed and discussed.