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.

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 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.

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The electrocatalytic reduction of carbon dioxide to hydrocarbons and oxygenates on Cu electrodes proceeds through surface-adsorbed CO. The adsorption and desorption of this intermediate play a key role in determining the product selectivity of this electrocatalytic process. It is therefore critical to understand the molecular factors that determine the adsorption of CO on Cu electrodes. In prior studies, it was suggested that specifically adsorbing anions of the supporting electrolyte compete with CO for surface sites at low overpotentials. However, prior infrared (IR) spectroscopy of CO adsorption on Cu electrodes did not compare the relative CO coverages in the presence of different anions and was restricted to a narrow range of electrolyte concentrations (0.1 to 0.2 M). Therefore, the impact of anions on the adsorption of CO on Cu is not fully understood to date. Herein we systematically explored the adsorption and desorption of CO on polycrystalline Cu electrodes in the presence of specifically and nonspecifically adsorbing anions (Cl–, SO4 2–, ClO4 –) at two different concentrations (10 mM and 1 M) of the corresponding sodium salts. With surface-enhanced IR absorption spectroscopy (SEIRAS), we monitored the CO stretch band of atop-bound CO (COatop) and an infrared band of the hydration shells of interfacial anions as a function of electrode potential. We found that at an electrolyte concentration of 10 mM, the adsorption and desorption of COatop are virtually independent of the identity of the anions. In contrast, at an electrolyte concentration of 1 M, the COatop coverage is significantly impacted by the electrolyte anions. The saturation coverages of COatop are lower in the 1 M electrolytes compared with those in the 10 mM electrolytes. The magnitude and mechanism of the modulation depend on the identity of the anions. Weakly and nonspecifically adsorbing anions (SO4 2–, ClO4 –) limit the COatop saturation coverage by blocking a fraction of CO adsorption sites. However, their site-blocking ability depends on the CO coverage, as evidenced by the hysteresis in the CO adsorption/desorption profiles, which likely originates from a reversible CO-induced surface reconstruction. Chloride ions, which can specifically adsorb on Cu electrodes, lower the CO coverage by modulating the CO adsorption energy. This modulation manifests itself in (1) a distinctly higher CO stretch frequency in the presence of this anion relative to that measured in the other electrolytes and (2) the absence of the hysteresis in the adsorption/desorption profiles. Our study highlights the intricate interplay between anions and surface-adsorbed CO at the Cu electrode/electrolyte interface.

Copper is the only pure metal electrocatalyst capable of converting carbon dioxide to hydrocarbons at significant reaction rates. However, the poor product selectivity of this process on copper remains a critical challenge. Modification of the aqueous electrolyte/copper interface with organic thin films has emerged as a promising means for tuning the selectivity toward valuable C≥2 hydrocarbons. Recently, it was demonstrated that the addition of N-substituted arylpyridinium derivatives to the electrolyte substantially alters the reaction selectivity (Han et al. ACS Cent. Sci. 2017, 3, 853–859). The changes in selectivity were shown to sensitively depend on the chemical structure of the added N-substituted arylpyridinium. For example, 1-(4-tolyl)­pyridinium (T-Pyr) increases the Faradaic efficiency of C≥2 products to ≈80% (compared to ≈25% observed for the unmodified Cu/electrolyte interface), whereas 1-(4-pyridyl)­pyridinium (P-Pyr) blocks the pathways to C≥2 products. It has been demonstrated that the reduction of N-substituted arylpyridinium derivatives leads to the formation of organic thin films on Cu electrodes. However, the mechanisms by which these thin films regulate the reaction selectivity are poorly understood. Herein, using surface-enhanced infrared absorption spectroscopy, we elucidate how films formed from T-Pyr and P-Pyr give rise to distinct interfacial properties that result in the observed differences in catalytic selectivity. We find that the two films alter the reactivity of the surface in two distinct ways: (1) in the presence of T-Pyr, the interfacial pH increases at moderate current densities (<5 mA cm–2), as evidenced by the appearance of a carbonate absorption band in the infrared spectrum of the interface. By contrast, the band of solution carbonate is comparatively small or absent in the spectra of the unmodified Cu/electrolyte contact and the corresponding interface modified by P-Pyr. Scanning electron microscopy of the modified Cu surfaces reveals that the T-Pyr- and P-Pyr-derived films exhibit distinct morphologies. Taken together, these observations suggest that the film formed by T-Pyr leads to an increase in the interfacial pH by limiting the mass transport to/from the interface. Therefore, the favorable selectivity for C≥2 products is a result of the limited proton availability at that interface. (2) In the presence of P-Pyr, the lineshape of the CO stretch band of atop-bound CO (COatop) is markedly different from that of COatop on unmodified and T-Pyr-modified Cu electrodes. Using pyridine, we show that N-heterocycles with a lone pair on the nitrogen atom (such as P-Pyr) compete with CO for undercoordinated Cu sites. On the basis of these results, we conclude that the selective poisoning of these undercoordinated Cu sites impedes the reduction of the CO intermediate to higher order reduction products in the presence of P-Pyr. Taken together, our results explain the observed reactivity trends in terms of the altered interfacial properties in the presence of the films. Our study ...

Understanding the effects of solution pH on the rates and mechanisms of multiproton/electron transfer reactions at aqueous electrolyte/electrode interfaces has been an active area of research for many decades. Recent interest in this topic has been driven by observations that the reaction selectivity and rates of electrocatalytic processes for energy storage and renewable fuel synthesis can profoundly change with electrolyte pH. Further, a subset of these reactions, such as CO and CO2 reduction, are often carried out under near-neutral bulk pH conditions. Such conditions, in combination with insufficient mass transport and limited pH buffer capacity, can lead to substantial deviations of the near-electrode pH from the bulk pH of the electrolyte. Such pH gradients, together with electrodes whose surface chemistry is highly dependent on pH, can give rise to complex feedback between the reactions that are catalyzed on the surface and the local pH conditions and surface speciation. In this Perspective, we discuss representative studies that characterize and quantify the effects of (local) pH on electrocatalysis with innovative experimental and theoretical methods. We further highlight possible future directions of investigation.