The ability of copper to catalyze the electrochemical reduction of CO 2 has been shown to greatly depend on its nanoscale surface morphology. While previous studies found evidence of irreversible changes of copper nanoparticle and thin film electrodes following electrolysis, we present here the first observation of the reversible reconstruction of electrocatalytic copper surfaces induced by the adsorbed CO intermediate. Using attenuated total internal reflection infrared and surface-enhanced Raman spectroscopies, the reversible formation of nanoscale metal clusters on the electrode is revealed by the appearance of a new CO absorption band characteristic of CO bound to undercoordinated copper atoms and by the strong enhancement of the surface-enhanced Raman effect. Our study shows that the morphology of the catalytic copper surface is not static but dynamically adapts with changing reaction conditions.

Electrocatalysis is central to the production of renewable fuels and high-value commodity chemicals. The electrolyte and the electrode together determine the catalytic properties of the liquid/solid interface. In particular, the cations of the electrolyte can greatly change the rates and reaction selectivity of many electrocatalytic processes. For this reason, the careful choice of the cation is an essential step in the design of catalytic interfaces with high selectivity for desired high-value products. To make such a judicious choice, it is critical to understand where in the electric double layer the cations reside and the various distinct mechanistic impacts they can have on the electrocatalytic process of interest. In this perspective, we review recent advances in the understanding of the electric double layer with a particular focus on the interfacial distribution of cations and the cations’ hydration states in the vicinity of the electrode under various experimental conditions. Furthermore, we summarize the different ways in which cations can alter the rates and selectivity of chemical processes at electrified interfaces and identify possible future areas of research in this field.

Surface-adsorbed CO is generally considered a reactive on-pathway intermediate in the aqueous electrochemical reduction of CO2 on Cu electrodes. Though CO can bind to a variety of adsorption sites (e.g., atop or bridge), spectroscopic studies of the Cu/electrolyte contact have mostly been concerned with atop-bound CO. Using surface-selective infrared (IR) spectroscopy, we have investigated the reactivities and coverages of atop- and bridge-bound CO on a polycrystalline Cu electrode in contact with alkaline electrolytes. We show here that (1) a fraction of atop-bound CO converts to bridge-bonded CO when the total CO coverage drops below the saturation coverage and (2) unlike atop-bound CO, bridge-bonded CO is an unreactive species that is not reduced at a potential of −1.75 V vs SHE. Our results suggest that bridge-bonded CO is not an on-pathway intermediate in CO reduction. Using density functional theory (DFT) calculations, we further reveal that the activation barrier for the hydrogenation of bridge-bonded CO to surface-adsorbed formyl on Cu(100) is higher than that of the reduction of atop-bound CO, in qualitative agreement with our experimental findings. The possible modulation of the catalytic properties of the interface by the electrochemically inert bridge-bonded CO population should be considered in future studies involving CO2 or CO reduction on Cu under alkaline conditions.

The product selectivity of many heterogeneous electrocatalytic processes is profoundly affected by the liquid side of the electrocatalytic interface. The electrocatalytic reduction of CO to hydrocarbons on Cu electrodes is a prototypical example of such a process. However, probing the interactions of surface-bound intermediates with their liquid reaction environment poses a formidable experimental challenge. As a result, the molecular origins of the dependence of the product selectivity on the characteristics of the electrolyte are still poorly understood. Herein, we examined the chemical and electrostatic interactions of surfaceadsorbed CO with its liquid reaction environment. Using a series of quaternary alkyl ammonium cations (methyl 4 N + , ethyl 4 N + , propyl 4 N + , and butyl 4 N + ), we systematically tuned the properties of this environment. With differential electrochemical mass spectrometry (DEMS), we show that ethylene is produced in the presence of methyl 4 N + and ethyl 4 N + cations, whereas this product is not synthesized in propyl 4 N + -and butyl 4 N + -containing electrolytes. Surface-enhanced infrared absorption spectroscopy (SEIRAS) reveals that the cations do not block CO adsorption sites and that the cation-dependent interfacial electric field is too small to account for the observed changes in selectivity. However, SEIRAS shows that an intermolecular interaction between surface-adsorbed CO and interfacial water is disrupted in the presence of the two larger cations. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. Our study provides a critical molecular-level insight into how interactions of surface species with the liquid reaction environment control the selectivity of this complex electrocatalytic process.hydrogen bonding | cation effects | electrocatalysis | carbon dioxide reduction | catalytic selectivity T he reaction environment profoundly impacts the kinetics of many chemical processes. Examples include the influence of the solvating environment on the rates of electron transfer (1), isomerization (2), peptide folding (3), and organic reactions (4), as well as the sensitivity of enzymatic catalysis to changes in the molecular structure of the active site (5). For a chemical process that can lead to multiple reaction products, solvent effects can impact the relative rates of product formation and therefore the product selectivity (6, 7). These effects, which can have complex energetic and/or dynamical origins (1,8,9), are fundamentally rooted in intermolecular interactions between the reactants and their environment. In the context of heterogeneous electrocatalysis, the reaction environment is asymmetric; i.e., reactants at the electrochemical interface are interacting with the solid electrode and the liquid electrolyte. Understanding the interactions of intermediates with their interfacial environment is essential for controlling the reaction paths of electrocatalytic processes that exhibit poor product selectivity.The reduc...

The catalytic selectivity and reactivity of an electrocatalytic interface can profoundly depend on the identity of the supporting electrolyte's cation. In the case of CO reduction on copper electrodes, these cation effects have been utilized to suppress undesired hydrogen evolution and to promote the formation of C reduction products. However, to more effectively steer the catalytic selectivity of the electrolyte/copper interface by cations, it is crucial to reveal the various physical mechanisms by which cations impact the catalytic properties of this prototypical interface for CO reduction. Herein, we employ surface-sensitive infrared spectroscopy to probe how alkali cations (Li, K, and Cs) control the coverage of CO, a key intermediate in CO reduction, on a polycrystalline copper electrode. We find that surface-adsorbed CO experiences an increasingly larger interfacial electric field with increasing cation size. The reduction of CO is further promoted by the two larger cations, leading to a significant drop of the CO coverage at high cathodic potential around -1 V vs. RHE. Our results demonstrate for the first time that the coverage of CO on the electrode is very sensitive to the identity of the cation. Since the relative coverage of CO and hydrogen on the copper surface affects the catalytic rates of CO reduction and hydrogen evolution, our results represent an essential step towards a better understanding of how cation effects control the product distribution.

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...

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is key to the development of processes that can convert CO and CO 2 to hydrocarbons, and nitrate to ammonia. The hydrogen evolution reaction (HER) often competes with these processes. Few studies studied this reaction on Cu under alkaline conditions. Herein, we examined the HER on Cu electrodes under alkaline conditions in Na + -and Cs + -containing electrolytes. We found that in 0.1 M solutions of NaOH and CsOH of the highest commercially available purity grades, trace impurities of iron deposit on the Cu electrode during electrolysis. As a result, the rate of the HER is enhanced by up to a factor of ≈5 over the course of eleven cyclic voltammograms (CV) from 0.15 to −0.65 V vs the reversible hydrogen electrode. After removal of the iron impurities, the CVs are stable as a function of cycle number. Comparison of the CVs in pre-electrolyzed 0.1 M NaOH and CsOH reveals that changing the cation from Na + to Cs + has no measurable effect on the HER. With density functional theory (DFT), we further rationalized our experimental findings. We discuss the implications of our results for electrocatalytic processes on Cu electrodes.