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

Abstract

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 CO 2 solubility as well as product selectivity in CO 2 reduction reactions (CO 2 RR). In our work, we have studied the activity for CO 2 RR 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 AuCu 3 intermetallics in contact with 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide [BMIM][NTf 2 ] electrolytes with 1.5 M H 2 O. 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 CO 2 reduction and does not require stabilization of a CO 2 − 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 ∼3.2 for AuCu 3 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.

Deciphering the role of aromatic cations in electrochemical CO2 reduction: interfacial ion assembly governs reaction pathways

“…To date, imidazolium-based ionic liquids have been the most widely used ionic liquid cation in electrochemical CO 2 reduction. 34–38 Electrolytes based on dialkylimidazolium ionic liquids, such as 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF 4 ]), have been repeatedly demonstrated to significantly lower the overpotential of electrochemical CO 2 reduction. 38–41 Many prior studies attributed the catalytic activity of dialkylimidazolium to the stabilization of adsorbed CO 2 by protons at C4/5 positions via hydrogen bonds.…”