Effects of Introducing Metal Adatoms on the Electrochemical CO2 Reduction Behavior of Copper

Abstract

The electrochemical CO2 reduction reaction (CO2RR) is considered as a promising candidate for a carbon negative technology to realize ecological and economical sustainable development. At present, copper is the only metal that can produce fuels (hydrocarbons and oxygenates) through CO2RR, but has many key scientific challenges remaining for implementation as an electrocatalyst (especially, selectivity and activity). One of the common strategies to tune electrocatalytic properties is tailoring active sites in an electrocatalyst by introducing another metal to one metal surface. In this work, we investigated the effect of introducing another metal M onto Cu(100) on the CO2RR. Physical vapor deposition was performed to obtain a 100-nm Cu(100) film epitaxially grown onto Si(100) substrate, followed by introducing 1-Å-equivalent another metal M (M: Ag, Au, In, Mo, Ni, Pd, and Sn) atoms onto the Cu(100) surface in vacuum. Atomic force microscopy and X-ray photoelectron spectroscopy guaranteed that M certainly existed on Cu and the M/Cu electrocatalyst had a smooth surface whose roughness was ±2 nm. The M/Cu electrocatalyst was supplied to a series of 15-min electrolysis in 0.1 mol dm−3 KHCO3 aqueous solution at potentials ranging from −0.6 to −1.1 V with respect to reversible hydrogen electrode at pH 6.8. Gaseous and liquid products were quantified with gas chromatography and nuclear magnetic resonance, respectively. A series of electrocatalytic evaluation pointed out the following general trends concerning the electrocatalytic properties of the M/Cu bimetallic thin films for CO2RR. For selectivity, the M atoms introduced to Cu(100) surface vary the Faradaic efficiency for each product to decrease average number of electrons transferred to a CO2 molecule. For activity, the introduction of M atoms onto Cu(100) surface suppresses the production rates for both C1 and C2+ products although it increases or decreases the total current density as seen in cyclic voltammetry, depending on M. The aforementioned experimental facts suggest the role of M that the M atoms introduce onto Cu(100) surface seem to cause some bimetallic effects on the CO2RR selectivity of Cu as well as an inhibition effect on the CO2RR activity of Cu. Electron-transfer reaction kinetics are further studied via a combined experiment and theory approach to discuss the nature of active sites in M/Cu bimetallic systems and the role of M in determining product distribution, which will be explained in the conference.