Speciation of Cu Surfaces During the Electrochemical CO Reduction Reaction

Abstract

Cu-catalyzed selective electrocatalytic upgrading of carbon dioxide/monoxide to valuable multicarbon oxygenates and hydrocarbons is an attractive strategy for combating climate change. Despite recent research on Cu-based catalysts for the CO2 and CO reduction reactions, surface speciation of the various types of Cu surfaces under reaction conditions remains a topic of discussion. Herein, in situ surface-enhanced Raman spectroscopy (SERS) is employed to investigate the speciation of four commonly used Cu surfaces, i.e., Cu foil, Cu micro/nanoparticles, electrochemically deposited Cu film, and oxide-derived Cu, at potentials relevant to the CO reduction reaction in an alkaline electrolyte. Multiple oxide and hydroxide species exist on all Cu surfaces at negative potentials, however, the speciation on the Cu foil is distinct from that on micro/nanostructured Cu. The surface speciation is demonstrated to correlate with the initial degree of oxidation of the Cu surface prior to the exposure to negative potentials. Combining reactivity and spectroscopic results on these four types of Cu surfaces, we conclude that the oxygen containing surface species identified by Raman spectroscopy are unlikely to be active in facilitating the formation of C2+ oxygenates in the CO reduction reaction.

Understanding the electric and nonelectric field components of the cation effect on the electrochemical CO reduction reaction

“…The adsorbed CO bands observed in this work correspond well with those reported on metallic Cu surfaces under the ultrahigh vacuum conditions ( 31 – 33 ), and thus, the possibility of CO adsorbed on these oxygen-containing species is not considered. Furthermore, the product distribution in the CORR on the chemically deposited Cu films and Cu microparticles is similar at −0.6 V ( 30 ). Thus, the spectral features observed in the SEIRAS experiments should be representative of the surface of Cu microparticles, which are frequently used in the CO 2 RR and CORR in both batch and flow configurations ( 29 , 34 , 35 ).…”

“…This discrepancy could be attributed to the presence of a different counter ion (hydroxide versus bicarbonate) in the electrolyte. Hydroxide-derived oxygen-containing species are known to be present on the copper surface at the CORR conditions ( 30 , 49 ). Adsorbed hydroxide, along with cations at the outer Helmholtz (OHP), has previously been proposed to affect the water dissociation process needed to release protons for the HER ( 50 ).…”

“…Scanning electron microscopy images before and after potential steps in hydroxide show no difference in particle size ( 29 ). X-ray diffraction patterns of the as-deposited Cu film show the presence of the (111) and (200) Cu facets ( 30 ), but polycrystalline Cu is known to undergo reconstruction in alkaline conditions at reducing potentials and oxidation when exposed to air, making its thorough characterization challenging ( 63 ).…”

“…Scanning electron microscopy images before and after potential steps in hydroxide show no difference in particle size (29). X-ray diffraction patterns of the as-deposited Cu film show the presence of the (111) and (200) Cu facets (30), but polycrystalline Cu is known to undergo reconstruction in alkaline conditions at reducing potentials and oxidation when exposed to air, making its thorough characterization challenging (63). Electrolyte A 0.1 M alkali metal/organic cation hydroxide solution (Sigma-Aldrich, ≥99.9%), preelectrolyzed for 24 hours using a constant reducing current of −5 mA cm −2 to deposit most of the metal impurities onto a Cu foil (Sigma-Aldrich, 99.998%), was used as the electrolyte for the spectroscopic tests.…”

In situ study of surface species and structure over Oxide-Derived Copper Catalysts for Electrochemical CO2 Reduction

“…According to the previous report, [39] the band can be assingned CuO x or CuO x OH y species. When conducting in D 2 O instead of H 2 O, the band at 524 cm -1 displayed a negligible shift (Supporting Information, Figure S12), indicating that the band was attributed to CuO x species rather that the CuO x OH y .…”