Global climate change caused by excessive consumption of fossil fuels and increasing emissions of greenhouse gases such as CO2 is the major environmental crisis facing humanity.(1,2) One of the most urgent scientific and technological challenges of the 21st century is to effectively replace fossil fuels by renewable resources.(3,4) Electrocatalytic CO2 reduction coupled with renewable electrical sources is a sustainable pathway to produce relevant chemicals and fuels, while contributing to close the unbalanced natural carbon cycle.(5) To date, Cu is the only pure metal that can reduce CO2 further to CO and produces a wide range of hydrocarbons and oxygenates, some of them with substantial market demand.(6−9) Despite its unique potential to convert CO2 into valuable compounds, Cu catalytic performance is limited by (i) its competition with the solvent decomposition to generate hydrogen (hydrogen evolution reaction, HER).(10,11) (ii) Mixtures of different compounds are obtained, with variable product selectivity.(12−14) (iii) High applied overpotentials are necessary to increase the rate production of valuable compounds.(15) In recent years, enormous efforts have been made to develop active and selective Cu-based electrodes for the CO2RR, as reflected by few recent reviews in the field.(6,7,15−17) Notably, the faradic activities and product distribution on Cu substrates change substantially depending on surface preparation protocols, electrolyte composition, or applied potential conditions.(17−21) Well-defined single-crystalline electrodes enable the assessment of the effect of the surface structure on the preferred CO2RR mechanism pathway.(6,8,12,22,23) Nevertheless, the relations between selectivity and activity with surface structure remain under debate. This lack of understanding is likely due to various reasons: (i) the lack of knowledge of both interfacial and surface properties of Cu single-crystalline electrodes;(24−28) (ii) the unclear nature of the real surface state condition prior to the reaction onset, i.e., the surface structure modified by coadsorbed solvent;(25,26) (iii) the strong influence of the electrolyte and bulk pH on the reaction mechanism pathway;(27−30) (iv) the difficulty of pretreating Cu surface crystalline electrodes and working under extremely clean conditions.(24,28,31,32) Understanding the interfacial properties of Cu single-crystalline electrodes under reaction conditions is essential to building the bridge between theory and experiments, aiming to provide an accurate comparison between calculations and experiments carried out on real model surfaces.(30,33,34) Points i and ii are particularly important to align experiments with theory, because calculations usually assume that the reaction takes place on ideally unchanged well-ordered surfaces.(35) In this regard, several studies using in situ scanning tunnelling microscopy (STM) have demonstrated that copper surfaces undergo surface faceting and reconstruct under HER conditions.(25,26) Soriaga and co-workers,(25) for instance, observed that a Cu polycrystalline surface underwent surface reconstruction to a ⟨100⟩ orientation in alkaline media and under solvent reduction conditions. Magnussen and co-workers(26,36) reported that HER on Cu(100) induces surface reconstruction in perchloric acid media. In situ STM measurements carried out by Friebel et al.(37,38) showed that Cu(111) develops different reconstructed forms under different potential ranges, in acid and neutral media. We highlight that in situ STM measurements provide very relevant information in relation to the interfacial properties of Cu, but they are highly complex to carry out, time-consuming, and not an easy way to test the surface state condition prior to the reaction in electrocatalytic studies. To the best of our knowledge, there is little information in relation to the surface changes that Cu single-crystalline surfaces evolve during the reduction of CO2 or CO.(39,40) Electrolyte and pH effects are a focus of deep analysis while assessing relevant electrocatalytic reactions such as methanol oxidation(41) or oxygen reduction(42,43) for fuel cells as well as oxygen evolution for water electrolyzers.(44,45) In particular, both the solution pH and the electrolyte composition strongly modify the distribution of products in the CO2RR.(28,29,46) Importantly, pH effects are mostly assessed in CO-saturated solutions because studies in CO2-saturared solutions are limited to the pH range between 6 and 8.(23,47) Recent works have aimed to shed light on electrolyte and pH effects on both surface and interfacial properties of Cu, with the final purpose of rationally tuning selectivity and activity. Interestingly, Bagger et al. claimed that the onset potential for the CO2RR at different pHs is conditioned by different surface-adsorption strength of the electrolyte anion.(29,30) Another relevant example is that halide-rich solutions induce morphological changes on polycrystalline Cu, which highly affects selectivity for the CO2RR.(18,20) To the best of our knowledge, there are only a few reports focusing on the pH and electrolyte effects on the interfacial properties of Cu single-crystalline electrodes for CO2RR or CORR.(19,23,28,48) Among other methods, cyclic voltammetry is the most widely employed electrochemical technique to characterize single crystalline surfaces.(49) Base cyclic voltammograms (CVs) show singular and sharp structure-sensitive features, which contain relevant information in relation to the distribution of the surface charge, an important interfacial property. These features depend on the pH of the bulk solution, electrolyte, and applied potential limits, thus providing the electrochemical fingerprint of single-crystalline electrodes in contact with different electrolytes. It has been traditionally used as a main control test of the quality and cleanness of both employed surfaces and solutions(49) while studying an electrocatalytic reaction. The reliability of the reported voltammetric profiles was later confirmed by in situ spectroscopic and/or microscopic techniques.(50) We consider that reporting base voltammograms of Cu single-crystalline electrodes is necessary when assessing the structure-sensitivity of the CO2RR and reproducing results. However, base voltammetric analysis is often overlooked in the literature, as recently discussed by Engstfeld et al.(31) and Tiwari et al.(32) To highlight the vital importance of recording base CVs while assessing either the CO2RR or the CORR, we have carried out a systematic voltammetric study of the Cu(111) single-crystalline electrode in contact with classical phosphate and bicarbonate aqueous solutions. The aim of this study is to provide the voltammetric fingerprint of Cu(111) to be used as a parameter control of the surface state and experimental conditions in studies with Cu. At the same time, we aim to show that base CVs of Cu single-crystalline electrodes actually contain relevant information on the interfacial properties to be considered in electrocatalytic studies. Here, we analyze different aspects that are essential for the investigation of the interfacial properties for CORR or CO2RR on Cu(111): (i) stability of the base CVs while cycling successively; (ii) evolution of the voltammogram features with different applied potential limits, which serves to evaluate the condition of the surface state at any applied potential; (iii) effect of the pH bulk solution; and (iv) influence of the specific nature of the electrolyte on the voltammogram response. Furthermore, we analyze the behavior of Cu(111) with and without the presence of CO in solution, aiming to evaluate to what degree the presence of CO in the electrified interface modifies the properties of Cu(111). Full experimental details and setup are provided in the Supporting Information. Below we summarize the scheme of experiments that we have carried out to assess the above-mentioned aspects:(a)Assessment of the blank cyclic voltammogram of Cu(111) in contact with classic 0.1 M phosphate solution (pH 5) at different potential regions.(b)Base voltammogram of Cu(111) in contact with a CO-saturated 0.1 M phosphate solution (pH 5) and at different potential regions.(c)Base voltammogram of Cu(111)|0.1 M phosphate solution interface in the neutral and mildly alkaline region, with and without CO in solution.(d)Base voltammogram of Cu(111)|0.1 M bicarbonate solution interface, with and without CO in solution to elucidate the effect of the electrolyte. Assessment of the blank cyclic voltammogram of Cu(111) in contact with classic 0.1 M phosphate solution (pH 5) at different potential regions. Base voltammogram of Cu(111) in contact with a CO-saturated 0.1 M phosphate solution (pH 5) and at different potential regions. Base voltammogram of Cu(111)|0.1 M phosphate solution interface in the neutral and mildly alkaline region, with and without CO in solution. Base voltammogram of Cu(111)|0.1 M bicarbonate solution interface, with and without CO in solution to elucidate the effect of the electrolyte. Figure 1 shows the base CVs of Cu(111) in contact with a 0.1 M phosphate buffer solution at pH 5 under different potential limits. The small potential window CV scan in Figure 1A (bottom panel) displays a sharp nonsymmetric feature centered at −0.55 V vs SHE. This feature was assigned to the phosphate species adsorption on the surface in previous studies.(24,28,48) Enlarging the potential limits of the CV toward values where the solvent reacts (top panel, black line) modifies the whole voltammogram shape. New structure-sensitive features have appeared, marked with arrows in Figure 1A, for the sake of clarity. We observe a broad peak located before the HER onset reaction in the cathodic region (scanning in the negative direction from 0.13 V to −0.87 V vs SHE) and new features in the anodic region (scanning in the positive direction from −0.87 to 0.13 V vs SHE) approaching to the onset surface oxidation. Figure 1. Cyclic voltammograms of the interface of Cu(111) in contact with 0.1 M buffer phosphate solution at pH 5 without CO (A) and in a CO-saturated solution (B). Scan rate: 50 mV/s. Bottom panels show small potential window CVs, and top panels show wide potential window CVs. Red dashed line shows the passivation of the surface in the blank solution and in the CO-saturated solution. Illustrations on the top part of the panels represent the Cu(111) in contact with the working solutions in the hanging meniscus configuration. Adding CO in the solution and maintaining a CO atmosphere (Figure 1B) affect the characteristic structure features, evidencing that CO poisons the surface and modifies the interfacial properties of Cu(111). Interestingly, both nonsaturated and CO-saturated solutions present the same group of features (marked by arrows in Figure 1A,B), which are clearly influenced by the CO, and show the same progress with applied potential limit conditions: First, enlarging progressively the potential limit in the negative scan modifies the features in the anodic region, while the CVs overlap in the cathodic region (Figure S1). This result clearly indicates that the surface state changes with the selected potential range and can be tuned by modulating the applied potential limits. Stable CVs are obtained, evidencing the high cleanliness of the experimental conditions (Figure S1). Second, the surface passivates by reaction with the solvent or electrolyte, even under the presence of CO (Figure 1A,B, red-dashed line). These results agree with in situ STM measurements carried out by Schlaup et al.,(51) who showed that the Cu(111) surface remains unstable in a neutral phosphate buffer solution and at different potential ranges. Moreover, our voltammetric results show that this surface instability remains under the presence of CO in solution and that both surface state condition and interfacial properties can be tuned and monitored through the applied potential limits. Another relevant aspect while assessing the surface structure–selectivity–activity relationships is the effect of the pH bulk solution on the cyclic voltammetry fingerprint of Cu(111). In Figure 2, we investigate the effect of the pH in the base CVs by assessing the solutions of 0.1 M phosphate buffer at pH 7 and 11. The top panels show the blank CVs, while the bottom panels show the CVs measured under the presence of CO. At neutral and mildly alkaline media, the phosphate peak (red lines) is even sharper. It is worth mentioning that sharp peaks on a CV are usually a sign of high surface ordering because sharp features mostly evolve ordered adlayers of the adsorbed electrolyte in long terrace domains.(49,52) To give support to this assumption, we have also provided, in the Supporting Information, base CVs of Cu(100) and polycrystalline Cu in contact with phosphate solution at pH 7 and have compared them with those obtained on Cu(111). Figures S2 and S3 clearly evidence the strong structure sensitivity of the voltammogram features. At pH 7 and 11, the saturation of the solution with CO modifies the voltammogram features similar to what we report at pH 5. The phosphate adsorption–desorption features modified with CO (Figure 2B, red line, bottom panels) are two quasi-reversible and symmetric peaks. The broad peak located close to the HER onset (top panel) and the CORR onset (bottom panel) decreases in intensity at pH 7 and is almost negligible at pH 11 (marked with arrows), showing its dependence with the pH. Features in the positive scan shift to higher potential values while enlarging the potential scan in the negative direction (blue and black dashed lines in all the panels). These results indicate that the Cu(111) orientation intrinsically undergoes surface changes with the applied potential and those changes seem to be independent of the pH. Figure 2. Cyclic voltammograms of the interface Cu(111) in contact with 0.1 M buffer phosphate solution at (A) pH 7.4 and (B) pH 11. Top panels are blank cyclic voltammograms, while bottom panels show the CVs of CO-saturated solutions. Red, blue, and black dashed lines in any panel correspond to consecutive cycles scanned at different lower potential limits. Scan rate: 50 mV/s. Aiming to elucidate the role of the electrolyte and differentiate it from the effect of the intrinsic nature of Cu(111) surface properties, we investigated the effect of the nature of the electrolyte by recording new CVs in a 0.1 M potassium bicarbonate solution. Figure 3 displays the CVs of Cu(111) in contact with 0.1 M KHCO3 before (top panel) and after (bottom panel) saturating the solution with CO (bottom panel). We essentially observe the same group of features that were recorded in the prepared fresh phosphate buffer solutions (Figure 3, blue solid and black dashed lines). Notably, we also observe that the bicarbonate or anion pair of peaks (at −0.617 V vs SHE, Figure 3, red line) locates at potential values that are slightly more positive in bicarbonate solutions (Figure S4). This is clearly an electrolyte effect likely related to a difference between the adsorption energy of bicarbonate anion and phosphate species on Cu(111).(29,30) The remaining features present in the CVs that we report can be assumed to be related with the intrinsic nature of Cu(111) orientation and less influenced by the identity of the anion or the pH in the selected solutions. Figure 3. Cyclic voltammograms of the interface Cu(111) in contact with 0.1 M KHCO3, pH 9.3: (A) without CO and (B) CO-saturated solution. Red, blue, and black dashed lines in any panel correspond to consecutive cycles scanned at different lower potential limits. Scan rate: 50 mV/s. Our experimental work highlights the importance of recording and showing the base CVs when measuring the CO2RR and CORR on well-ordered Cu electrode surfaces. We also show that well-defined base CV profiles contain key information related to the influence of the selected potential range, bulk solution pH, and employed electrolyte in the interfacial properties of Cu(111) in contact with CO-saturated solutions. In particular, we note the strong influence the selected potential range has on the voltammogram shapes, which could be considered as a control parameter of the real surface state condition at the onset of CO2 or CO reduction. In addition, our voltammetric analysis clearly evidences that the surface state conditions can be modulated by tuning the applied potential limits. Understanding the complex surface and interfacial properties of well-ordered Cu surfaces is, from our point of view, necessary to close the gap between computational models of the CO2RR or CORR on different Cu facets and experimental results on single-crystalline electrodes.(12,53) This will allow the aligning of theory with experiments, which is necessary to get rational explanation of the reaction mechanisms as well as the structure–reactivity relations for CO2RR and CORR on Cu single-crystalline electrodes. We encourage future in situ and potential-control analysis to shed light on the origin of the different structural features that we have reported and explain their dependence with the applied potential limits. The nature of the interaction between both solvent (water) and coadsorbed electrolyte species deserves deep consideration. As we showed, the surface remains passivated prior to the reaction, even under the presence of CO, which may affect the electrocatalytic performance of Cu. We conclude that well-defined studies, under potential control, that monitor the interfacial properties of Cu single-crystalline electrodes will be essential to understand the structure–selectivity–activity relations and, ultimately, to rationally design highly efficient CO2RR and CORR electrocatalysts. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.9b02456.Results from additional experiments carried out on Cu(111) and Cu(100) and a Cu polycrystalline disk (PDF) Results from additional experiments carried out on Cu(111) and Cu(100) and a Cu polycrystalline disk (PDF) Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS The authors declare no competing financial interest. Electronic Supporting Information files are available without a subscription to ACS Web Editions. The American Chemical Society holds a copyright ownership interest in any copyrightable Supporting Information. Files available from the ACS website may be downloaded for personal use only. Users are not otherwise permitted to reproduce, republish, redistribute, or sell any Supporting Information from the ACS website, either in whole or in part, in either machine-readable form or any other form without permission from the American Chemical Society. For permission to reproduce, republish and redistribute this material, requesters must process their own requests via the RightsLink permission system. Information about how to use the RightsLink permission system can be found at http://pubs.acs.org/page/copyright/permissions.html. We gratefully acknowledge the Villum Foundation for the award of a Villum Young Investigator Grant (Project Number: 19142). This article references 53 other publications.