Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls

http://pubs.acs.org/journal/aelccp

Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls Using Copper Silver Thin Films with Interphase Miscibility Drew Higgins,†,‡ Alan T. Landers,‡,§ Yongfei Ji,†,‡ Stephanie Nitopi,†,‡ Carlos G. Morales-Guio,†,‡ Lei Wang,†,‡ Karen Chan,†,‡ Christopher Hahn,*,†,‡ and Thomas F. Jaramillo*,†,‡ Downloaded via UNIV STRASBOURG on November 20, 2018 at 12:59:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Department of Chemical Engineering, Stanford University, 443 Via Ortega Way, Stanford, California 94305, United States SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States § Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, California 94305, United States ‡

S Supporting Information *

ABSTRACT: Steering the selectivity of Cu-based electrochemical CO2 reduction (CO2R) catalysts toward targeted products will serve to improve the technoeconomic outlook of technologies based on this process. Using physical vapor deposition as a tool to overcome thermodynamic miscibility limitations, CuAg thin films with nonequilibrium Cu/Ag alloying were prepared for CO2R performance evaluation. In comparison to pure Cu, the CuAg thin films showed significantly higher activity and selectivity toward liquid carbonyl products, including acetaldehyde and acetate. Suppressed activity and selectivity toward hydrocarbons and the competing hydrogen evolution were also demonstrated on CuAg thin films, with a greater degree of suppression observed at increasing nominal Ag compositions. Compositional-dependent CO2R trends coupled with physical characterization and density functional theory suggest that significant miscibility of Ag into the Cu-rich phase of the catalyst underpinned the observed CO2R trends through tuning of adsorbate and reaction intermediate binding energies on the surface.

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catalysts has been shown as an effective technique to steer product selectivity,10,11 which is a system that we explore herein to gain a mechanistic understanding of the underlying factors governing CO2R on these materials. This insight will help guide the development and further understanding of reliable techniques to improve the activity of Cu while simultaneously steering selectivity toward more desirable products, significantly perpetuating the advancement of CO2R technologies. From the wide array of CO2R products, oxygenated species such as alcohols (i.e., methanol, ethanol, isopropanol) and carbonyls (i.e., formaldehyde, acetaldehyde, propionaldehyde) are attractive as they are generally more valuable than their hydrocarbon counterparts12 and are in liquid form under ambient conditions, simplifying processing, storage, and distribution. Previous work has provided insight into factors that control the selectivity of Cu toward hydrocarbon or oxygenated products, including surface structure,13,14 electrolyte (ions, surface pH) effects,15−18 and alloying.19−21 Several

lectrochemical CO2 reduction (CO2R) is an attractive approach to store renewable energy (wind, solar, hydro) in the form of chemicals and energy-dense fuels.1−4 While routinely demonstrated on a laboratory scale, electrochemical CO2R remains challenging from a technoeconomic standpoint due to the poor catalyst efficiency and selectivity. Copper (Cu) is the only known metal that can selectively catalyze the formation of products requiring more than two electron transfers, referred to hereafter as “further reduced products” that include C−C coupled hydrocarbons, alcohols, and carbonyls.3,5 However, the activity of Cu toward these useful products is intrinsically low under typical electrochemical testing conditions employing CO2-sparged 0.1 M KHCO3 electrolyte, requiring more than 800 mV of overpotential to achieve geometric CO2R current densities on the order of 1 mA/cm2 on polycrystalline copper foils.6,7 Furthermore, the reaction pathways and resulting selectivity of Cu are quite divergent.8,9 Our previous work has elucidated the CO2R activity and selectivity of a planar Cu foil as a function of electrode potential, with at least 16 different products observed in varying quantities.7 Separating these gas- and liquid-phase products would likely require costly and energyintensive separation processes. The addition of Ag to Cu-based © XXXX American Chemical Society

Received: September 14, 2018 Accepted: November 12, 2018

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DOI: 10.1021/acsenergylett.8b01736 ACS Energy Lett. 2018, 3, 2947−2955

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Cite This: ACS Energy Lett. 2018, 3, 2947−2955

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ACS Energy Letters

Figure 1. (a) Symmetric XRD patterns of as-prepared and CO2R tested Cu50Ag50 thin films. (b) High-resolution Ag 3d, Cu 2p, O 1s, and C 1s XPS scans of as-prepared and CO2R tested Cu50Ag50 thin films. SEM and AFM images of Cu50Ag50 (c) as-prepared and (d) after 1 h of CO2R electrolysis at −0.98 V vs RHE.

impact on the extent of metastable Cu/Ag alloying and resulting catalytic properties.10,11,22 Therefore, utilizing alternative CuAg preparation methods to achieve nonequilibrium miscibility between Cu and Ag and more prevalent Cu−Ag interactions allows for investigations of how the properties of this promising bimetallic catalytic system govern CO2R activity and selectivity, which can be used to guide future catalyst designs. Different synthetic approaches have been utilized previously to achieve alloying in thermodynamically immiscible Ag-based bimetallics. AgCo nanoparticles were prepared by synthesizing an insoluble salt-phase (Ag3[Co(CN)6)] precursor with intermetallic mixing.35 This salt was then rapidly heated to 500 °C in a reductive atmosphere for 10 min and then quenchcooled to produce nanoparticles with a AgCo surface alloy that provided notable electrochemical oxygen reduction activity enhancements. CuAg nanoparticles have been prepared by magnetron sputtering, employing kinetically rapid and nonequilibrium gas-phase condensation of Cu and Ag plasmas to achieve metastable configurations in the resulting particles.36 Our group has previously adopted electron-beam physical vapor deposition (PVD) to prepare CuAg thin films with metastable interphase alloying.37 The “quench-cooling” nature of PVD38 enabled preparation of CuAg thin films consisting of a Cu-rich and a Ag-rich phase with significant nonequilibrium miscibility (i.e., >5 atom %) of the solute metals indicated by

studies have focused on catalysts comprised of copper and silver,10,11,22−25 two metals that are thermodynamically immiscible under ambient conditions.26 The CuAg bimetallic system couples one metal catalyst that has high selectivity for CO2R to CO (Ag)27−30 and another with the capability to further reduce CO (Cu).31,32 In a recent study, CuAg metal foils consisting of segregated Cu and Ag phases were prepared by arc melting, quenching, and cold rolling for electrochemical CO2R testing.10 These CuAg foils provided a significant increase in oxygenate selectivity without a concomitant change in overall CO2R activity. Suppression of hydrogen evolution reaction (HER) activity also led to an improved overall selectivity for CO2R. The CO2R activity and selectivity changes were speculated to result from the formation of a Cu−Ag surface alloy under electrochemical reaction conditions that induced compressive strain on the Cu atoms, thereby favorably tuning their binding energies toward reactive species.10 This hypothesis was supported by a previous study using scanning tunneling microscopy to demonstrate the formation of a Ag−Cu surface alloy on a Cu(100) single crystal when Ag coverage was less than 0.13 of a monolayer Ag.33 Despite the thermodynamic immiscibility of Ag−Cu in the bulk, the formation of a surface alloy was predicted by theory to be stabilized due to repulsive Ag−Ag interactions in the surface layer.34 While promising, past work on CuAg has demonstrated that the catalyst preparation method has a large 2948


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AFM imaging (Figure 1c, inset) that depicts a roughness factor (RF) of 1.002 that is consistent with metallic and bimetallic thin films that we have prepared previously.40,41 After electrochemical CO2R testing, moderate surface coarsening is observed (Figure 1d), resulting in nonuniform grains and the formation of dispersed larger particles with sizes on the order of several hundred nm that appear to be a coalescence of smaller crystallites. Despite this surface coarsening, significant topological features are not observed, indicating that the Cu50Ag50 thin film is still planar. AFM confirms this, demonstrating a RF of 1.104 (Figure 1d, inset). Therefore, these changes are not expected to play a significant role when interpreting electrochemical activity results on a geometric electrode area basis. All electrochemical CO2R testing was done in 0.1 M KHCO3 electrolyte using a custom-built electrochemical cell that enables high product detection sensitivity,7 with detailed experimental methods provided in the Supporting Information. A 1 h chronoamperometry test at −0.98 V vs the reversible hydrogen electrode (RHE) was used as Cu has a high selectivity to further reduced products at this potential. Furthermore, our previous work using the same electrochemical cell has shown that planar Cu catalysts can be tested in the absence of mass transport limitations at this potential,7,42 which is an important criterion for intrinsic activity evaluation.43 Gas-phase and liquid-phase products were detected by online gas chromatography (GC) and postelectrochemistry nuclear magnetic resonance (NMR) spectroscopy, respectively. Faradaic efficiencies (selectivity) and geometric area-based partial current densities (activity) as a function of nominal CuAg thin film composition are shown in Figure 2. For different CuAg compositions, Figure 2a shows the selectivity and activity toward the competing HER, along with the 2e− CO2R products, CO, and HCOO−. Figure 2b shows the selectivity and activity toward further reduced (i.e., >2e−) products that are known to be derived from the reduction of adsorbed *CO intermediates.32,44 To more specifically probe product distributions, Faradaic efficiencies toward products formed with a selectivity of >0.5% are shown in Figure 2c for pure Cu and Cu50Ag50 as a representative case, with the remaining CuAg compositions along with pure Ag provided in Figures S5−S8. Analysis of the electrochemical results shows that the overall activity toward >2e− CO2R products is suppressed for all CuAg compositions in comparison to Cu (Figure 3a). To account for CO2R products requiring a different number of electron transfers (and therefore varying current densities per CO2 molecule reduced), the turnover rates of CO2 molecules on an electrode geometric area basis are shown in Figure 3b. While total CO2 turnover rates are only marginally affected by the addition of Ag to Cu, the rates toward further reduced (>2e−) products are much lower for the CuAg thin films in comparison to Cu. Cu is the only transition metal that has been shown to be capable of selectively producing >2e− CO2R products.3 Therefore, considering that Cu atoms are the most likely active sites in the CuAg thin films for >2e− CO2R, current densities normalized to the nominal Cu composition of the thin films were plotted (Figure S9) and indicate that there is a decrease in the intrinsic activity of Cu toward further reduced products with the addition of Ag. This observation differs from results reported recently for CuAg foils,10 which showed no changes in the intrinsic activity of Cu toward further reduced products upon the addition of varying

X-ray diffraction analysis (XRD; see Figure 1 and Table S1 in ref 37). Furthermore, PVD-prepared thin films have a planar morphology that is more ideal for analysis of the intrinsic electrocatalytic activity and selectivity in comparison to high surface area catalyst systems where convoluting mass transport and nanostructuring effects must be considered. In this work, we leverage the advantages of PVD-prepared CuAg thin films to investigate the effects of composition and interphase miscibility on CO2R activity and selectivity. After synthesis, XRD analysis indicated that all as-prepared CuAg thin films have significant nonequilibrium bulk-phase miscibility, indicated by the shift in the Ag(111) and Cu(111) diffraction peaks to higher and lower 2θ angles, respectively (Figure 1a for Cu50Ag50; Figure S1 for all other compositions). All compositions indicated that bulk miscibility was retained following CO2R testing, with the only changes to the XRD pattern being the emergence of a small diffraction peak at ca. 36.5° for the Cu-rich (Cu95Ag5, Cu70Ag30) and pure Cu thin films. This peak is attributed to Cu-oxide species that likely form due to inevitable air exposure prior to XRD measurements. High-resolution X-ray photoelectron spectroscopy (XPS) spectra for Cu50Ag50 (Figure 1b) shows the presence of Ag, Cu, O, and C. The near-surface metal composition of asprepared Cu50Ag50 was 49 atom % Cu and 51 atom % Ag, in close agreement with the nominal composition. The surface of Cu50Ag50 became relatively Cu-enriched as a result of electrochemical CO2R testing, with XPS indicating a nearsurface composition of 64 atom % Cu and 36 atom % Ag. This Cu enrichment is consistent with previous work.10,22 On the basis of reaction selectivity changes, we expect that this surface enrichment of Cu occurs over the first several minutes of electrolysis (Figure S2), which is likely a result of adsorbateinduced surface segregation owing to the stronger binding energy of reactive CO2R intermediates on Cu in comparison to Ag.39 Underneath of this Cu-enriched layer, XPS sputter depth profiling (Figure S3) indicates that there is a subsurface region enriched with Ag (i.e., depleted Cu), with these composition variant regions confined to the top ca. 8 nm of the thin film. The compositional rearrangement at the surface of the catalyst motivated us to evaluate whether miscibility between Cu and Ag atoms was retained in the near-surface region following CO2R electrochemistry. XRD measurements were conducted at SSRL beamline 2-1 in a grazing incidence configuration to achieve enhanced scattering contributions from the surface versus the bulk. At an X-ray energy of 11.5 keV and incidence angle of 0.32° (ca. 20 nm probing depth), grazing incidence XRD patterns indicated that the Cu50Ag50 thin film maintained a significant degree of near-surface Cu/Ag miscibility in both the Cu-rich and Ag-rich phases post-CO2R testing (Figure S4), consistent with the results of bulk XRD measurements shown in Figures 1A and S1. While the characterization in this work is done ex situ on as-prepared and tested thin films, the development of surface-sensitive in situ or operando methods would enable probing the atomic and electronic structures of the catalyst surface under reaction conditions and would represent a significant contribution to the field of CO2R electrocatalysis. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images are shown in Figure 1c,d for asprepared and CO2R tested Cu50Ag50 thin films, respectively. The as-prepared thin film consists of relatively uniform sized (ca. 20 nm) pseudospherical grains and a smooth surface absent of any topological features. This is corroborated by 2949


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compositions of Ag. Importantly, the as-prepared foils from this previous work had minimal (2e− products, this hydrocarbon suppression is clearly observed when activity measurements are normalized to the nominal Cu composition (Figure S10a). The largest activity decrease is observed for methane (Figures S11 and S12a), showing an almost order of magnitude lower activity going from Cu to Cu95Ag5, along with further activity decreases with higher Ag contents. A similar

Figure 2. Composition dependence of CuAg thin films. (a) Selectivity and activity toward the 2e− products H2, CO, and HCOO−. (b) Activity and selectivity toward further reduced (>2e−) hydrocarbon, alcohol, and carbonyl CO2R products. (c) Selectivity of Cu50Ag50 in comparison to pure Cu toward different CO2R products. For additional details on the values and standard deviations, please see Tables S1−S4 in the Supporting Information.

Figure 3. As a function of nominal CuAg composition: (a) Total current density (gray squares) and the amount of current density going toward further reduced (>2e−) C1, C2, and C3 carbon chain length CO2R products. (b) Total molecular rate of CO2R and to further reduced (>2e−) products. 2950


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ACS Energy Letters activity loss trend exists for ethylene (Figure S11), albeit to a much smaller extent. Previous experimental work20 has shown a positive correlation between the selectivity of Cu-based catalysts toward hydrocarbons and the competing HER. This is postulated to arise due to increased coverage of *H on the catalyst surface, which potentially plays a role in hydrogenation of CO2R intermediates toward hydrocarbons.20,46 Our current results align with this as the activity toward hydrogen (Figure 2a) and hydrocarbons (Figure 2b) for the CuAg thin films shows very similar trends. We hypothesize that this could be due to the significant intermiscibility between Ag and Cu in the thin films. As Ag has a significantly lower hydrogen binding energy compared to Cu,47 the presence of Ag atoms at or near the catalyst likely results in a lower *H surface coverage under reaction conditions and therefore suppressed rates of HER and hydrogenation of oxygenated intermediate species. This hypothesis was evaluated by density functional theory (DFT) investigations using the computational hydrogen electrode48 to model the free energy of adsorbate species, as shown in previous works.49−51 A (3 × 3 × 5) supercell of Cu(211) was used with the bottom two layers fixed (Figure 4a), with computational details provided in the Supporting Information. The Cu(211) was used as a model of a prototypical step defect, which should dominate the HER activity because Cu lies on the weak-binding leg of the HER volcano.52 The energies of the Ag dopants at different sites, indicated by letters in Figure 4a, were evaluated. The energies relative to the edge site a, the most favorable site for Ag doping, are shown in Table 1. We found the most favorable site for *H adsorption to be the hollow site by the edge, as shown in Figure 4b. When the Cu atom nearest to the adsorption site (site 5 in Figure 4b) is replaced by Ag, the free energy of *H adsorption is increased by 0.13 eV in comparison to the undoped Cu(211) surface (Figure 4c). As site 5 was determined by the free energy analysis shown in Figure 4a and Table 1 to be the most energetically favorable site for Ag dopants, it is most likely that Ag atoms reside at this edge site. The *H adsorption energy calculations suggest that Cu(211) with an edge atom replaced by Ag will have decreased HER activity and decreased H* coverage, with the latter potentially contributing to the lower activity toward hydrocarbons. The free energy of *H adsorption with the Ag dopant at different sites is provided in Table S5. The other sites (1−4) have a negligible effect on the adsorption energy of *H. For example, when Ag is doped at site 1, the binding energy of *H is weakened by only 0.01 eV in comparison to undoped Cu(211). These calculations suggest that the effect of Ag doping on *H binding energies is very local. Therefore, increasing the concentration of Ag dopants in Cu will result in decreased *H surface coverages due to the increased likelihood of having a Ag-dopant in close proximity to *H adsorption sites. The steadily decreasing activity toward HER and hydrocarbons observed experimentally for the CuAg thin films with increasing Ag compositions aligns with these calculations. Furthermore, this notion is supported by comparing our experimental results with the previously discussed work on CuAg foils with minimal bulk miscibility between Cu and Ag.10 This previous study demonstrated activity suppression toward HER and hydrocarbons for a 20:80 nominal Ag:Cu atomic ratio in comparison to pure Cu, albeit no further hydrocarbon/HER activity losses were observed at increasing Ag compositions. This directly contrasts the steady

Figure 4. (a) Side view of the Cu(211) slab used to investigate Ag doping at the different sites indicated by the letters, with the formation energy values shown in Table 1. (b) Top view and side view of a Cu(211) structure showing the most favorable *H binding site and letters indicating the various sites investigated computationally to be replaced by a Ag dopant. (c) Free energy diagram for HER on the Cu(211) surface with Ag doped at site 5 versus undoped Cu(211). * stands for an adsorption site.

Table 1. Formation Energies for Ag Doping at the Sites Indicated by Letters in Figure 4a Relative to the Formation Energy for Ag Doping at Site a Ag dopant site

formation energy/eV

a b c d e f

0.00 0.18 0.27 0.67 0.59 0.58

decrease in hydrocarbon/HER activity demonstrated in the present work (Figure 2a,b) at higher Ag compositions. The key difference is that the CuAg thin films presented herein have increasing Ag miscibility in the Cu-rich phase as a function of nominal Ag composition. Taken together, the results suggest that the increased likelihood of CO2R-active Cu atoms having neighboring Ag atoms in our CuAg thin films with significant nonequilibrium miscibility can serve to suppress HER and hydrocarbon formation beyond what is observed in phasesegregated CuAg catalysts. 2951


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Figure 5. (a) Percentage of overall CO2R current going toward C1, C2, and C3 products. Partial current densities toward C2 products, including (b) acetaldehyde, (c) ethanol, and (d) ethylene plotted as a function of nominal Cu composition within the thin films. The dashed lines are what would be expected if the thin films behaved as a linear combination of Cu and Ag. Note that the nominal composition refers to bulk thin film composition, whereby the actual surface composition under electrochemical CO2R conditions is likely Cu-enriched.

toward liquid-based oxygenate species. As the overall activity of CuAg toward oxygenated species is not increased versus Cu (Figure S14), the selectivity increase arises due to the suppression in activity toward methane and, to a lesser extent, ethylene (Figure S11). The activity and selectivity toward oxygenates can be further broken down to evaluate trends toward alcohol or carbonyl species (Figure 2b). In general, there is a decrease in selectivity and activity toward alcohols for CuAg thin films versus Cu. On the other hand, CuAg thin films notably provide a significant increase in selectivity and activity toward carbonyl products at all nominal CuAg compositions. Cu70Ag30 in particular shows a more than 4-fold Faradaic efficiency increase and an almost 3-fold partial current density increase toward carbonyls relative to Cu (Figure 2b). It is important to note that the observed hydrocarbon, alcohol, and carbonyl production trends for CuAg continue to hold when normalizing for nominal Cu composition (Figure S10), based on the assumption that Cu surface atoms are most likely the active sites for the >2e− products. Figure 2c provide a more specific breakdown of product distributions for Cu50Ag50, with additional compositions provided in Figures S5−S8. For Cu50Ag50, the selectivity toward ethanol and n-propanol was lower in comparison to that for Cu, while the selectivity toward allyl alcohol showed a modest increase. While the trend of decreasing ethanol and npropanol selectivity with increasing Ag contents held across the composition space (Figures S5−S8), selectivity toward allyl alcohol showed very little compositional dependence. The plot in Figure 2c also highlights that the increased carbonyl selectivity for CuAg is due to improved selectivity toward

The effect of Ag in Cu on C−C coupling selectivity is demonstrated by plotting total current densities along with partial current densities toward further reduced C1, C2, and C3 products (Figure 3a). This data is replotted in Figure 5a to show the proportion of the total current going toward >2e− CO2R products with varying carbon chain lengths. All CuAg compositions are found to have an increased proportion of >2e− CO2R current density going toward C−C coupled products in comparison to pure Cu. In the case of Cu50Ag50, 91% of the total CO2R current to >2e− products goes toward C2+ product formation, a significant improvement over the 75% C2+ selectivity for pure Cu. Partial current densities toward C1, C2 and C3 products plotted as a function of nominal Cu composition (Figure S12a−c) clearly depict that, while there is increased selectivity for C−C coupling in CuAg thin films, lower partial current densities and decreased CO2 turnover rates (Figure S12d) toward these products are observed in comparison to pure Cu. The increased proportion of >2e− CO2R current density going toward C−C coupling on the CuAg thin films (Figure 5a) is therefore largely attributed to the significant suppression in activity toward methane (discussed previously), the only >2e− C1 product observed in our study. A plot of the partial current density ratios of oxygenate (alcohols, carbonyls) to hydrocarbon products for different CuAg compositions is shown in Figure S13. Pure Cu shows an oxygenate:hydrocarbon partial current density ratio of 0.41, whereas CuAg shows ratios ranging from 0.87 to 1.22. This demonstrates that the addition of even small amounts of Ag to Cu provides a clear advantage in steering CO2R selectivity 2952


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was demonstrated versus Cu, leading to an increased selectivity for C−C coupled >2e− products. DFT simulations suggest that Ag doping in Cu weakens the binding energy of *H species, an effect that is localized to *H adsorption sites in proximity to the Ag dopant. This effect was used to explain the steadily decreasing activity and selectivity toward hydrocarbons and the HER as a function of increased nominal Ag compositions. Owing to the significant miscibility between Ag and Cu in the thin films, higher nominal Ag compositions lead to an increased proportion of the surface with weakened *H binding energies. This more complete understanding of the underlying mechanisms of electrochemical CO2R on CuAg will enable the development of strategies to further steer selectivity toward liquid carbonyl products. Translating the selectivity benefits of CuAg to nanoscale morphologies can potentially enable avoiding the formation of Ag-rich domains that are ubiquitous in bulk systems and result in increased selectivity toward carbon monoxide products. Furthermore, nanostructured CuAg catalysts are a more suitable morphology for integration into high surface area vapor-fed devices, and their development should be accompanied by long-term durability evaluation.

acetaldehyde, propionaldehyde, and acetate. The latter is considered a carbonyl species due to its carboxylic acid group, but its formation likely follows a different mechanistic pathway than acetaldehyde53 and could include contributions from Cannizzaro-type disproportionation reactions of two acetaldehyde molecules.54 To further analyze the selectivity changes for CuAg, a comparison of Cu composition (nominal) normalized current densities toward related C2 products acetaldehyde, ethanol, and ethylene is provided in Figure 5b−d. The modest reduction in ethanol formation rates (Figure 5c) can seemingly be correlated with the increased activity toward acetaldehyde (Figure 5b). Acetaldehyde is postulated as a likely reaction intermediate for ethanol formation as the further electrochemical reduction of carbonyls to alcohol has been previously demonstrated,32 resulting in acetaldehyde being postulated as a likely reaction intermediate for ethanol formation in the CO2R pathway. This increased selectivity toward lesser reduced oxygenated species (i.e., carbonyls) was previously observed on CuAg foils and attributed to weakening of the adsorption energy of reactive oxygen-containing intermediate species.10 Our previous work demonstrated that a portion of Cu atoms in PVD-prepared CuAg thin films could undergo Cu-oxide reduction at potentials more anodic than those on pure Cu.37 This provided evidence for weakened binding energies toward oxygen-containing species, attributed to the significant metastable Cu/Ag miscibility that modulated the electronic structure of the surface Cu atoms. We hypothesize that a similar effect is at play in the present work, whereby increased surface Cu atom nobility leads to decreased binding energies with oxygenated reaction intermediates. This can result in desorption of oxygenated CO2R species from the catalyst surface before further hydrogenation, explaining the increased Faradaic efficiencies toward acetaldehyde at the expense of ethanol. A similar rationale could be used to explain the increased activity toward acetaldehyde (Figure 5b) at the expense of ethylene (Figure 5d); however, recent theoretical work53 has suggested that the CO2R pathway bifurcates after *CO (or *CHO) dimerization into an acetaldehyde/ethanolforming pathway and an ethylene-forming pathway. Acetaldehyde is therefore an unlikely intermediate for ethylene, a notion consistent with previous experiments on acetaldehyde reduction.32,55 This is also consistent with results of this work, whereby the magnitude of activity enhancement toward acetaldehyde (Figure 5b) and other C−C coupled carbonyls (Figure S10c) for CuAg versus Cu pales in comparison to the activity decrease observed for ethylene (Figure 5d). Therefore, additional mechanisms underlying the suppression of hydrocarbon pathways are likely at play, including the reduced surface coverage of *H species discussed previously. In summary, PVD was demonstrated as a versatile synthesis platform for preparing CuAg thin films with controllable compositions that consist of Cu-rich and Ag-rich phases with significant nonequilibrium interphase miscibility. Our results show that Ag miscibility into Cu is a reliable method for increasing the electrocatalytic CO2R selectivity and activity toward liquid carbonyl products, likely due to decreased surface binding energies of oxygen-containing intermediate species. While overall intrinsic CO2R activity toward further reduced products was decreased for CuAg versus Cu, significant suppression was observed in the activity toward hydrocarbons and the competing HER. For methane, almost an order of magnitude lower activity for all CuAg compositions

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b01736. Experimental details, supplementary figures for data interpretation, and tables showing Faradaic efficiencies and current densities (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.H.). *E-mail: [email protected] (T.F.J.). ORCID

Drew Higgins: 0000-0002-0585-2670 Carlos G. Morales-Guio: 0000-0002-5840-5591 Karen Chan: 0000-0002-6897-1108 Christopher Hahn: 0000-0002-2772-6341 Thomas F. Jaramillo: 0000-0001-9900-0622 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under Award ECCS-1542152. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Additional thanks go to the Stanford NMR Facility. D.H. gratefully acknowledges support from the Banting Postdoctoral Fellowships program, administered by the Government of Canada. C.G.M-G. gratefully acknowledges support from the Swiss National Science Foundation (Grant No. P2ELP2_168600). L.W. 2953


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ACS Energy Letters thanks the Knut & Alice Wallenberg Foundation for financial support. Dr. Ezra Clark is gratefully acknowledged for numerous insightful discussions and feedback provided throughout the course of this project.

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