Revealing the Predominant Surface Facets of Rough Cu Electrodes under Electrochemical Conditions

Abstract

Metal electrodes with rough surfaces are often found to convert CO or CO2 to hydrocarbons and oxygenates with high selectivity and at high reaction rates in comparison with their smooth counterparts. The atomic-level morphology of a rough electrode is likely one key factor responsible for its comparatively high catalytic selectivity and activity. However, few methods are capable of probing the atomic-level structure of rough metal electrodes under electrocatalytic conditions. As a result, the nuances in the atomic-level surface morphology that control the catalytic characteristics of these electrodes have remained largely unexplored. Because the CO stretching frequency of atop-bound CO (COatop) depends on the coordination of the underlying metal atom, the IR spectrum of this reaction intermediate on the copper electrode could, in principle, provide structural information about the catalytic surface during electrolysis. However, other effects, such as dynamic dipole coupling, easily obscure the dependence of the frequency on the surface morphology. Further, in the limit of low COatop coverage, where coupling effects are small, the CO stretching frequencies of COatop on Cu(111) and Cu(100) facets are virtually identical. Therefore, on the basis of the CO stretching frequency, it is not straightforward to distinguish between these two ubiquitous surface facets, which exhibit vastly different CO reduction activities. Herein, we show that key features of the atomic-level surface morphology of rough copper electrodes can be inferred from the potential dependence of the line shape of the CO stretching band of COatop. Specifically, we compared two types of rough copper thin-film electrodes that are routinely employed in the context of surface-enhanced infrared absorption spectroscopy (SEIRAS). We found that copper films that are electrochemically deposited on Si-supported Au films (CuAu–Si) are poor catalysts for the reduction of CO to ethylene in comparison to copper films (Cu–Si) that are electrolessly deposited onto Si crystals. As quantified by differential electrochemical mass spectrometry (DEMS), the onset potential for ethylene is ∼200 ± 65 mV more cathodic for CuAu–Si than that for Cu–Si. To reveal the origin of the disparate catalytic properties of Cu–Si and CuAu–Si, we probed the surfaces of the electrodes with cyclic voltammetry (CV) and SEIRAS. The CV characterization suggests that the (111) surface facet predominates on CuAu–Si, whereas the (100) facet is more common on Cu–Si. SEIRAS reveals that the line shape of the CO stretching of COatop is composed of two bands that are attributable to COatop on terrace and defect sites. The different surface structures manifest themselves in the form of starkly different potential dependences of the line shape of the CO stretching mode of COatop on the two types of electrodes. With a simple Boltzmann model that considers the different adsorption energies of COatop on terrace and defect sites, and the resulting COatop populations on terrace and defect sites, we de...

Revealing the CO Coverage-Driven C–C Coupling Mechanism for Electrochemical CO2 Reduction on Cu2O Nanocubes via Operando Raman Spectroscopy

“… 36 − 38 In some reports, the broad C–O stretching band was attributed to different CO adsorption configurations or sites, including bridge-bonded and atop-bonded CO or CO adsorbed on terrace and defect sites. 4 , 53 , 39 Moreover, the P1 and P2 bands, which reflect the interaction between reaction intermediates and the Cu electrode surface, change regularly with the potential. P2 displays a blue shift in the peak frequencies as the electrode potential decreases ( Figure 2 b).…”

Sub‐Second Time‐Resolved Surface‐Enhanced Raman Spectroscopy Reveals Dynamic CO Intermediates during Electrochemical CO2 Reduction on Copper

“…By fitting the Raman spectra, we deconvolute the spectra of anodized Cu‐MP in the C=O stretching range into three peaks, based on the previous work by Gunathunge et al. [ 6a , 6b , 8a , 20 ] (Figure 2 d ): (1) bridged CO at ≈2030 cm −1 , (2) low‐frequency band linear CO (LFB‐CO) at ≈2060 cm −1 , and (3) high‐frequency band linear CO (HFB‐CO) at ≈2095 cm −1 . The first species to be observed in this region are two wide peaks centered at 2058 cm −1 (LFB‐CO), which appear within 2 s after the onset of cathodic bias.…”

Dynamic Evolution of Active Sites in Electrocatalytic CO2 Reduction Reaction: Fundamental Understanding and Recent Progress

“…discovered that (100) facet was predominant on copper films electrochemically deposited on Si substrate, rather than (111) facet for Cu deposited on Si‐supported Au films ( Figure A). [ 132 ] Kim et al. investigated the surface evolution of polycrystalline Cu electrode via operando electrochemical STM.…”

“…A) Raman features difference of CO species adsorbed on specific (100) facet formed on copper films (Cu-Si) and (111) facet formed on copper films electrodeposited on Si-supported Au films (CuAu-Si). Reproduced with permission [132]. Copyright 2020, American Chemical Society.…”