Cu/Au(111) Surfaces and AuCu Intermetallics for Electrocatalytic Reduction of CO₂ in Ionic Liquid Electrolytes

Room-temperature ionic liquids (RTIL) are important alternatives to aqueous electrolytes in electrocatalytic reactions, batteries, and fuel cells. They are known to reduce existing high overpotentials and increase CO2 solubility as well as product selectivity in CO2 reduction reactions (CO2RR). In our work, we have studied the activity for CO2RR of Au(111), Cu(111), and Cu-modified Au(111) electrodes with 1/3, 2/3, and 3/3 Cu monolayers, as well as of AuCu and AuCu3 intermetallics in contact with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTf2] electrolytes with 1.5 M H2O. Using offline gas chromatography (GC), we demonstrate the formation of H-2 and mainly CO as the only reduction products at Au(111), while exclusively H-2 is formed at Cu(111). Synergistic electronic and geometric effects lead to higher levels of CO formation at Cu-modified Au(111) electrodes in comparison to neat Au(111). Operando IR absorption spectroscopy (IRAS) of the bulk electrolyte shows the formation of a 2-imidazolium carboxylic acid intermediate that can lower the overpotential for CO2 reduction and does not require stabilization of a CO2- radical anion as an alternative intermediate at the interface. Systematic variation of the copper content at the catalysts' surfaces enables us to adjust the H-2/CO syngas ratio to a maximum of 1.8 for Cu-modified Au(111) electrodes and similar to 3.2 for AuCu3 catalysts at electrolysis times of 20 min, demonstrating a large tunability of the syngas ratio with electrode potential. The observed range of H-2/CO ratios includes the ideal ratio of 2 for the Fischer-Tropsch process to produce hydrocarbons and the ratio of 3 needed for methanation.