Selective reduction of CO to acetaldehyde with CuAg electrocatalysts

Edited by Richard Eisenberg, University of Rochester, Rochester, NY, and approved December 17, 2019 (received for review May 16, 2019)
January 24, 2020
117 (23) 12572-12575

Abstract

Electrochemical CO reduction can serve as a sequential step in the transformation of CO2 into multicarbon fuels and chemicals. In this study, we provide insights on how to steer selectivity for CO reduction almost exclusively toward a single multicarbon oxygenate by carefully controlling the catalyst composition and its surrounding reaction conditions. Under alkaline reaction conditions, we demonstrate that planar CuAg electrodes can reduce CO to acetaldehyde with over 50% Faradaic efficiency and over 90% selectivity on a carbon basis at a modest electrode potential of −0.536 V vs. the reversible hydrogen electrode. The Faradaic efficiency to acetaldehyde was further enhanced to 70% by increasing the roughness factor of the CuAg electrode. Density functional theory calculations indicate that Ag ad-atoms on Cu weaken the binding energy of the reduced acetaldehyde intermediate and inhibit its further reduction to ethanol, demonstrating that the improved selectivity to acetaldehyde is due to the electronic effect from Ag incorporation. These findings will aid in the design of catalysts that are able to guide complex reaction networks and achieve high selectivity for the desired product.
Electrochemical CO2 reduction (CO2R) is a promising approach to store renewable energy in the form of energy-dense fuels, or for the sustainable synthesis of feedstock chemicals (13). Copper (Cu) is currently the only electrode material that has shown significant CO2R activity toward multicarbon fuels and chemicals, albeit with a widely divergent selectivity toward over 16 different products (46). Previous studies on Cu electrodes have shown that CO reduction is the rate-limiting step in the transformation of CO2 to further reduced (>2e) products (4, 611). In addition to providing important mechanistic insights for CO2R, direct electrochemical CO reduction (COR) can also serve as a sequential route for reducing CO2 into muticarbon products when utilized in tandem with existing technologies that can efficiently generate CO from CO2 (1113). Thus, there are motivations to understand how to guide COR selectivity to the desired products.
A number of strategies have been developed to address the challenge of steering CO2R/COR selectivity, by modifying either the intrinsic or extrinsic properties of the electrocatalytic system (14). Examples of successfully controlling intrinsic properties to guide selectivity include modifying the electrocatalyst surface structure (8, 9, 1519), composition (2033), particle size (3436), and roughness factor (3739). Similarly, researchers have demonstrated that extrinsic factors such as the electrode potential (11, 39), electrolyte pH (11, 37, 38, 4043), electrolyte cation identity and concentration (4446), and molecular additives (4750) significantly influence reactivity for CO2R/COR. Although all of these studies have provided tremendous insights on how to guide selectivity to broad product groups such as C2+ oxygenates and hydrocarbons, controlling the reaction selectivity toward a specific product is still a major fundamental challenge.
In this work, we leveraged insights from the aforementioned strategies that control the intrinsic and extrinsic properties of electrocatalytic systems, with the aim of steering the selectivity of COR toward a single C2+ oxygenate. To modify the intrinsic electrocatalytic properties of Cu electrodes, we implemented an Ag galvanic exchange process as previous research indicated that CuAg bimetallic electrocatalysts enhance selectivity toward C2+ oxygenates by suppressing the competing hydrogen evolution reaction (HER) (22, 28). In addition, alkaline pH conditions were used for COR based on previous studies that show the energy efficiency and selectivity to C2+ oxygenates can be improved by governing this extrinsic property around the electrocatalyst (11, 51, 52). As hypothesized, we found that the combined approach of directing the intrinsic and extrinsic properties leads to an efficient electrocatalytic system that exhibits a Faradaic efficiency of ∼50% for COR to acetaldehyde, with hydrogen as the only other major product. Remarkably, over 90% of the CO reacted goes to the formation of acetaldehyde. Density functional theory (DFT) calculations explain the high selectivity to acetaldehyde, as the Ag ad-atoms on Cu surface lower the intrinsic activity for acetaldehyde reduction by introducing electronic effects that weaken the binding energy of its key reduction intermediate. The fundamental insights from this work provide general design strategies that can be utilized for the discovery and development of efficient electrocatalytic systems.

Results and Discussion

Although metallic Ag and Cu are thermodynamically immiscible at room temperature, previous work has demonstrated that CuAg surface alloys with dilute Ag content can be formed by galvanic exchange of Cu with Ag using an Ag ion precursor solution (22). It has been shown that the selectivity of CO2R to multicarbon products observed over Cu-based alloys increases concomitantly with the surface composition of Cu, suggesting that neighboring Cu atomic ensembles are required for efficient C−C coupling (53). Therefore, a low Ag surface content is preferred in order to maintain a surface rich with Cu active sites. Inspired by these previous studies, CuAg electrodes were prepared by immersing prepolished polycrystalline Cu foils into a dilute AgNO3 solution. At the surface, a relatively high Cu:Ag atomic ratio of about 7:1 was measured for these electrodes as measured by X-ray photoelectron spectroscopy (SI Appendix, Fig. S1).
The product distributions for the CuAg electrodes indicate that acetaldehyde is formed in significant quantities at all electrochemical potentials (Fig. 1A). The highest selectivity toward acetaldehyde is observed at −0.536 V vs. the reversible hydrogen electrode (RHE), with a Faradaic efficiency of ca. 50% that accounts for ∼90% of the total CO reduction partial current. Comparing the COR testing results for CuAg versus Cu (Fig. 1 and SI Appendix, Fig. S2), CuAg clearly has a higher selectivity to a single liquid product acetaldehyde, in contrast to pure Cu which forms seven different products at this potential (11), including five different liquid phase species and another two different gas phase products. While the overall selectivity for all COR products remains consistently ∼50% for CuAg within the range of electrode potentials used for testing, there are clearly potential-dependent changes in the product distribution. For example, at more negative electrode potentials, the selectivity toward acetaldehyde decreases concomitantly with an increase in selectivity toward ethylene and eventually ethanol. At −0.676 V vs. RHE, ethanol appears at the expense of acetaldehyde formation (SI Appendix, Fig. S2), supporting the suggestion that acetaldehyde is a precursor to ethanol (11, 54, 55). In contrast, selectivity to ethylene increases at more negative electrode potentials, with Tafel analysis clearly demonstrating that the electrode potential has a stronger impact on activity toward ethylene in comparison to acetaldehyde (SI Appendix, Fig. S2). This difference in the activity dependence on electrode potential indicates that acetaldehyde and ethylene may go through different pathways after CO dimerization, which is consistent with previous theoretical work focused on understanding mechanisms of C2 product formation (56). Previous studies have shown that cascading homogeneous reactions can influence the CO2R/COR product distribution (55, 57). Control 1H NMR experiments indicate that Cannizzaro-type disproportionation is not a significant homogeneous reaction pathway in 0.1 M KOH (SI Appendix, Fig. S3), suggesting that ethanol formation is likely an electrochemical process under the conditions we used for COR experiments. Instead, these 1H NMR experiments support a previous conclusion that acetaldehyde can be polymerized by homogeneous reactions in alkaline conditions (55, 58), influencing product quantification for COR. Notably, these reactions appear to be reversible as acetaldehyde can be recovered by simply neutralizing the electrolyte with 0.1 M HCl (SI Appendix, Fig. S3). The suppressed HER activity on CuAg compared to pure Cu (SI Appendix, Figs. S2B and S4) can be attributed to the presence of Ag atoms at or near the electrode surface, which likely will result in a lower *H surface coverage under reaction conditions (22, 28). The decrease in overall COR activity (SI Appendix, Fig. S2B) is likely due to Ag galvanic exchange occurring mainly on the stepped and undercoordinated Cu surface sites which are the most intrinsically active sites for COR (15, 17). To further explore the influence of the catalyst composition on reactivity, we used a previously implemented physical vapor deposition method to prepare a broad range of CuAg thin-film compositions with interphase miscibility (SI Appendix, Fig. S5) (28). Unlike previous CO2R studies that show a clear dependence of the partial activities on the CuAg composition (22, 26, 28, 29, 3133), similar activities and selectivities were observed for almost all of the bimetallic thin films (SI Appendix, Fig. S6). This result suggests that the types and proportions of surface active sites are similar for all of these catalysts under alkaline COR conditions, leading to the hypothesis that the surfaces restructure into analogous ensembles during electrolysis. Surface-sensitive operando probes could provide further evidence for surface restructuring and guide the future design of new catalysts.
Fig. 1.
COR on CuAg (A) and Cu (B) (11) electrodes under the same reaction conditions: 0.1 M KOH saturated with 1 atm CO at ambient temperature. Faradaic efficiencies (FE) and geometric current densities for COR (jCOR) are shown as a function of the electrode potential. The error bars represent one SD of the results from three independent samples.
To determine why CuAg is selective for COR to acetaldehyde, we conducted acetaldehyde reduction experiments to compare the activities of Cu and CuAg for producing ethanol (SI Appendix, Fig. S7). The results demonstrate that CuAg is significantly less active than Cu for acetaldehyde reduction, explaining its high selectivity for acetaldehyde. To rationalize the suppression in acetaldehyde reduction, the binding energy of the key reaction intermediate, CH3CH2O*, was examined using DFT. The edge sites were calculated to be the most favorable sites for Ag doping (SI Appendix, Fig. S8). Based on our previous study, Ag doping reduces the H* binding energy of Cu, corroborating the suppression in hydrogen production that is observed experimentally (SI Appendix, Fig. S2) (28). Similarly, the most stable adsorption sites of CH3CH2O on the surface are calculated to be the edge sites, as shown in the free energy diagram in Fig. 2. This diagram shows that the potential limiting step for the reduction of CH3CHO(aq) is the first proton and electron transfer step to form CH3CH2O*. As the edge Cu (site 1) is replaced by Ag, the binding energy of CH3CH2O* is weakened by 0.09 eV; when the Cu atom closest to the adsorption site (site 5) is replaced by Ag, the binding of CH3CH2O* is weakened by 0.31 eV. In both cases there is a substantially higher thermodynamic energy barrier for the conversion of CH3CHO* to CH3CH2OH(aq). Thus, Ag incorporation into Cu mitigates acetaldehyde reduction, thus greatly improving its rate of production, corroborating the experimental results. However, this improvement in product selectivity comes at the cost of decreased overall COR activity, since the Cu edge sites are the most intrinsically active for COR (15). Taken together, the results suggest that manipulating the binding energy of COR reaction intermediates by controlling the catalyst composition is a viable strategy to improve the selectivity for COR toward valuable products. To fully rationalize the high COR selectivity to acetaldehyde on CuAg it will also be important to understand how Ag influences the bifurcation between the acetaldehyde/ethanol and ethylene pathways. Although a number of DFT studies have led to hypotheses on reaction mechanisms (5962), a putative intermediate common to the bifurcation point between these pathways is yet to be determined either theoretically or experimentally. Identifying a putative intermediate will aid in the understanding of how to control CO2R/COR selectivity to a single multicarbon product, such as CuAg for COR.
Fig. 2.
Free energy diagram (Left) for CH3CHO* reduction to ethanol on the Cu(211) surface with Ag doped at different sites. The free energy of CH3CHO* on the pristine Cu(211) surface is taken as an energy reference. (Right) The adsorption structure of CH3CH2O* is shown from the top and side view. Ag_n denotes when Cu(n) on the surface is replaced by Ag. Ag_layer means the subsurface layer of Cu is replaced by Ag.
With the above knowledge in hand, we then aimed to explore the impacts of the electrode surface area on COR selectivity. It has been previously demonstrated that the electrode roughness factor can steer COR selectivity toward multicarbon oxygenates on pure Cu catalysts by suppressing the competing HER (63). To test whether this surface area effect can further enhance the selectivity to acetaldehyde for the CuAg system, a porous CuAg nanoflower electrode (CuAg-NF) was prepared and evaluated for COR similarly. Although the trends in normalized current densities indicate that there are some uncertainties in attempting to translate Ag galvanic exchange technique from the planar morphology to the NF morphology (SI Appendix, Fig. S9), we nevertheless observed substantial enhancement in both reaction rate and selectivity to acetaldehyde (Fig. 3). The Faradaic efficiency for acetaldehyde production on the CuAg-NF electrode reached 70% with an overall COR reaction rate three times greater than that of the planar system at an applied potential of only −0.33 V vs. RHE, representing a ∼0.2 V improvement in overpotential based on the geometric current density. Thus, these results further validate the concept that high electrode surface areas can improve selectivity, not just reaction rates, a principle that can likely be extended further to other catalyst materials.
Fig. 3.
Comparison between COR on planar (CuAg) and porous (CuAg-NF) electrodes under the same reaction conditions: 0.1 M KOH saturated with 1 atm CO at ambient temperature. Faradaic efficiencies (FE) and geometric current densities for COR (jCOR) are shown as a function of the electrode potential.

Conclusion

In summary, CuAg bimetallic electrodes have been investigated for CO reduction under alkaline conditions. Unprecedented selectivity to acetaldehyde was obtained at low overpotentials on planar CuAg electrodes. The selectivity and activity to acetaldehyde was further improved by increasing the CuAg electrode surface area. DFT calculations demonstrate that the Ag ad-atoms can tune the surface binding energy of reduced aldehyde intermediates, resulting in a suppression of acetaldehyde reduction to ethanol. This study emphasizes how deliberate modification to the catalyst surface and reaction conditions can greatly improve product selectivity, providing design principles that can be extended to other catalysts and related electrocatalytic reactions.

Data Availability

Electrochemical data and other physical characterization data used to produce the results in this paper are provided in SI Appendix, Figs. S1–S9 and Table S1.

Acknowledgments

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a Department of Energy (DOE) Energy Innovation Hub, supported through the Office of Science of the US DOE under Award DE-SC0004993. Part of this work was performed at the Stanford Nano Shared Facilities, supported by the National Science Foundation, under Award ECCS-1542152. We thank the Stanford NMR Facility. L.W. and Y.J. thank the Knut Alice Wallenberg Foundation for financial support. C.G.M.-G. gratefully acknowledges support from the Swiss National Science Foundation (Grant P400P2_180767).

Supporting Information

Appendix (PDF)

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Information & Authors

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Published in

Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 117 | No. 23
June 9, 2020
PubMed: 31980521

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Submission history

Published online: January 24, 2020
Published in issue: June 9, 2020

Keywords

  1. electrocatalysis
  2. CO2 reduction
  3. CO reduction
  4. bimetallics
  5. heterogeneous catalysis

Acknowledgments

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a Department of Energy (DOE) Energy Innovation Hub, supported through the Office of Science of the US DOE under Award DE-SC0004993. Part of this work was performed at the Stanford Nano Shared Facilities, supported by the National Science Foundation, under Award ECCS-1542152. We thank the Stanford NMR Facility. L.W. and Y.J. thank the Knut Alice Wallenberg Foundation for financial support. C.G.M.-G. gratefully acknowledges support from the Swiss National Science Foundation (Grant P400P2_180767).

Notes

This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, “Status and Challenges in Decarbonizing our Energy Landscape,” held October 10–12, 2018, at the Arnold and Mabel Beckman Center of the National Academies of Sciences and Engineering in Irvine, CA. NAS colloquia began in 1991 and have been published in PNAS since 1995. From February 2001 through May 2019 colloquia were supported by a generous gift from The Dame Jillian and Dr. Arthur M. Sackler Foundation for the Arts, Sciences, & Humanities, in memory of Dame Sackler's husband, Arthur M. Sackler. The complete program and video recordings of most presentations are available on the NAS website at http://www.nasonline.org/decarbonizing.
This article is a PNAS Direct Submission.

Authors

Affiliations

SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, CA 94305;
Drew C. Higgins
SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, CA 94305;
SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
Department of Chemical Engineering, McMaster University, Hamilton, ON L8S 4L8, Canada;
Yongfei Ji
SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, CA 94305;
SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, CA 94305;
SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
CatTheory Center, Department of Physics, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
Christopher Hahn1 [email protected]
SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
Thomas F. Jaramillo1 [email protected]
SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, CA 94305;
SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;

Notes

1
To whom correspondence may be addressed. Email: [email protected] or [email protected].
Author contributions: L.W., D.C.H., K.C., C.H., and T.F.J. designed research; L.W., D.C.H., Y.J., and C.G.M.-G. performed research; L.W., D.C.H., Y.J., and C.G.M.-G. analyzed data; and L.W., K.C., C.H., and T.F.J. wrote the paper.

Competing Interests

The authors declare no competing interest.

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    Selective reduction of CO to acetaldehyde with CuAg electrocatalysts
    Proceedings of the National Academy of Sciences
    • Vol. 117
    • No. 23
    • pp. 12501-13176

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