Final Report: Nanostructured Cu Electrodes for Energy-Efficient Conversion of CO 2 to Fuel

Final Report: Nanostructured Cu Electrodes for Energy-Efficient Conversion of CO 2 to Fuel Investigators Principal Investigator: Matthew W. Kanan, Assistant Professor, Department of Chemistry, Stanford University Graduate student researchers: Christina Li Allison Yau Donald Ripatti Postdoctoral researchers: Xiaofeng Feng Changhoon Li Abstract Electrochemical CO 2 conversion is a desirable technology for renewable energy storage and distributed production of fuels and chemicals. One of the greatest technical challenges for CO 2 conversion is to develop efficient, selective, and robust electroreduction catalysts that reduce CO 2 and its derivatives to desirable multi-carbon compounds. The genesis of this GCEP project was the discovery of oxide-derived Cu a thin-film metallic Cu material synthesized by reducing a Cu 2 O precursor. Compared to conventional Cu electrodes such as polycrystalline Cu foil, single crystal Cu electrodes, or commercial Cu nanoparticles, oxide-derived Cu has higher selectivity for CO 2 vs H + reduction and much higher activity for CO reduction to multi-carbon (C 2+ ) oxygenates such as ethanol and acetate. The goals of the project were to elucidate the structural origins of these improvements and apply these principles to the preparation of even more active and selective catalysts. Our research has largely achieved our overall goals and produced the following key findings: i) the suppression of H 2 evolution activity on OD-Cu is a consequence of the electrode morphology; ii) the enhancement of CO reduction activity on OD-Cu is correlated with the presence of strong CO-binding sites on the surface; iii) there is a direct correlation between grain boundary density and catalytic activity in Cu and Au nanoparticle CO/CO 2 electroreduction catalysts; iv) large increases in current density and synthesis rates are possible for Cu-catalyzed CO reduction by using gas diffusion electrodes. Future fundamental research will explore the structure of grain boundary surface terminations in detail and the effects of grain boundary geometry on activity. In addition, we have established lead catalyst structures and a first-generation cell design for the assembly of a gas diffusion CO reduction electrolyzer designed to produce multicarbon oxygenates at synthetically useful rates. 1

Introduction Electrochemical CO 2 conversion is the process of using electricity to drive the conversion of CO 2 and H 2 O into fuels and chemicals. If powered by a low-carbon intensity electricity source (solar, wind, nuclear), this process could in principle provide a source of carbon-containing fuels and commodity chemicals with much lower greenhouse gas emissions than fossil fuels. Furthermore, because the raw materials are CO 2 and H 2 O, electro-synthesis is amenable to distributed production one can imagine a network of electrolyzers that are scaled to match the local availability of renewable energy and local fuel/chemical demand. This vision is particularly intriguing for the developing world where a robust fossil fuel infrastructure has not yet been established. Arguably the greatest technical challenge for electrochemical CO 2 conversion is catalyzing the cathodic CO 2 reduction reaction. In order to be useful, electrocatalysts must operate with high rates at potentials as close as possible to the thermodynamic minima to avoid wasting energy as heat. In addition, electrocatalysts must selectively reduce CO 2 or CO instead of reducing H 2 O (H + ) to H 2, which is typically the favored reaction. While many materials are known to catalyze CO 2 electroreduction, nearly all are energetically inefficient and unselective. This GCEP project was aimed at addressing this challenge. The genesis of our project was the discovery of oxide-derived Cu a thin-film metallic Cu material produced by reducing a Cu 2 O precursor.[1, 2] Compared to conventional Cu electrodes such as polycrystalline Cu foil, single crystal Cu electrodes, or commercial Cu nanoparticles, oxide-derived Cu (OD-Cu) has higher selectivity for CO 2 vs H + reduction and much higher activity for CO reduction to multi-carbon (C 2+ ) oxygenates such as ethanol and acetate. The goals of the project were to elucidate the structural origins of these improvements and apply these principles to the preparation of even more active and selective catalysts. As described in the Results section below, these goals were largely achieved and we are now at the point of evaluating catalysts in prototype electrolysis cells designed to attain synthetically useful rates. Our vision for electrochemical CO 2 conversion has evolved over the course of the project. We originally targeted a catalyst that would electrochemically reduce CO 2 all the Figure 1: Two-step conversion of CO 2 into C 2+ oxygenates using Cu-catalyzed electrochemical CO reduction. 2

way to a C 2+ oxygenate. Cu can catalyze this reaction, but it is inefficient and unselective. While OD-Cu and related catalysts show improvement, their performance is still far from what would be necessary for a practical device. We now favor a two-step process to produce liquid fuels and commodities (Figure 1): i) CO 2 conversion to CO; ii) CO conversion to C 2+ oxygenates. The advantage of the two step approach is that the catalysts/conditions for CO 2 and CO reduction can be optimized independently. CO 2 to CO conversion proceeds at high rates and very high efficiency in high-temperature solid oxide electrolysis cells.[3] Leveraging this technology, the remaining challenge is CO conversion. Using OD-Cu or other appropriately structured Cu electrocatalysts (see below), it is possible to achieve much higher selectivity and activity for CO conversion to C 2+ oxygenates than when starting from CO 2. Our efforts have therefore focused on optimizing this step and, in particular, elucidating experimentally validated design principles for catalyst development. Background Electrochemical CO 2 and CO reduction Electrochemical CO 2 reduction was first systematically studied beginning in the 1980s by Hori and co-workers.[4] In the past several years, there has been a resurgence of interest in this area, spurred by increased concern about the rising atmospheric CO 2 concentration. The key metrics for a catalyst are activity, selectivity, and durability. For electrocatalysts, activity is measured as the current density (current per catalyst area) vs the applied potential. The goal is to develop catalysts that have high current densities at potentials close to the thermodynamic minimum (the equilibrium potential). The excess potential (overpotential) that must be applied to attain a desired rate diminishes the energetic efficiency of the electrolysis. The equilibrium potentials for CO 2 and CO reduction to various products are shown in Figure 2. The selectivity of an electrocatalyst is expressed as Faradaic efficiency, which is the percentage of the current that corresponds to the formation of a particular product. A major selectivity challenge is to favor CO 2 or CO reduction over H + reduction to H 2 (i.e. the H 2 evolution reaction), which is typically more facile on an electrode surface. Finally, practical electrochemical devices require catalysts that maintain their activity and selectivity over thousands of hours of operation. Copper is the most heavily studied material for CO 2 and CO reduction catalysis. Polycrystalline and numerous single-crystal Cu electrodes have been evaluated in CO 2 - saturated aqueous bicarbonate (HCO 3 ) solutions and CO-saturated neutral or alkaline solutions at ambient temperature.[4-10] These studies have revealed a complex, potentialdependent product distribution. At low overpotential, H 2 evolution is the major reaction on conventional Cu electrodes. At moderate to high overpotentials, CO 2 reduction to CO and HCO 2 become significant products. At very high overpotentials, methane and ethylene are the major products, followed by smaller amounts of oxygenates including ethanol and acetate. In CO-saturated solutions without CO 2, methane, ethylene and oxygenates are also formed but only at similarly negative potentials.[9-11] Sweep voltammetry experiments have observed transient activity for CO reduction to ethylene with an onset potential of 0.3 V for single crystal Cu(100) electrodes, but sustained activity in bulk electrolyses has not been demonstrated for these surfaces in this potential range.[11, 12] The requirement for large overpotentials compromises the energetic 3

Figure 2: Equilibrium potentials for selected CO 2 and CO reduction reactions vs the reversible hydrogen electrode (RHE). efficiency of electrosynthesis and promotes electrode deactivation. Computational studies have provided insight into possible reduction pathways on Cu and identified possible potential-determining steps.[13-15] In addition to bulk Cu surfaces, a recent study observed that Cu nanoparticles ranging in size from 2 nm to 15 nm have comparable or worse selectivity for CO 2 reduction vs H 2 evolution than polycrystalline Cu.[16] Very small (<2 nm) Cu nanoparticles showed a greater propensity for H 2 O reduction, suggesting that surface atoms with low coordination numbers promote H 2 formation. Oxide-derived Cu We reported OD-Cu and its CO 2 reduction activity in 2012 and its activity for CO reduction to liquid fuels in 2014. OD-Cu is prepared by reducing a 1 µm thick Cu 2 O precursor, either via electroreduction or treatment with H 2 at 130 C. Compared to Cu foil and commercial Cu nanoparticles, OD-Cu has lower specific activity (current per Cu area) for H 2 evolution and higher specific activity for CO reduction to multi-carbon oxygenates. Quantitatively, OD-Cu electrodes exhibit up to 54-fold suppression of H 2 evolution and up to 45-fold enhancement of CO reduction compared to conventional Cu electrodes. As a result, OD-Cu attains higher Faradaic efficiency for CO 2 reduction to CO and HCO 2 at low overpotentials and higher Faradaic efficiency for CO reduction to ethanol and acetate at moderate overpotential. Transmission electron microscopy (TEM) and grazing incidence X-ray diffraction studies have revealed that OD-Cu materials are nanocrystalline, meaning that they are comprised of continuous networks of nanocrystals linked by grain boundaries (Figure 3).[2] Based on this characterization, we hypothesized that the grain boundaries are responsible for the unusual catalytic properties of OD-Cu. Grain boundary surface terminations may contain active sites that are not otherwise stable[17] and the grain boundaries impose irregular shapes on the nanocrystals that may affect the step density on the particle surfaces that are distal from the grain boundaries. Testing this model explicitly with OD-Cu is challenging because it is difficult to modulate and quantify the grain boundary density in this material. It is also unclear if the factors that determine the catalytic properties of a nanocrystalline film will be applicable to discrete nanoparticles that are not part of a continuous material. Developing design principles for nanoparticles 4

Figure 3: Schematic depiction of the synthesis of oxide-derived Cu and its nanocrystallinity. is essential for electrochemical devices because nanoparticles have much higher (orders of magnitude) surface area-to-mass ratios than thin-film catalysts. These considerations inspired us to broaden our investigation of grain boundaries beyond oxide-derived materials to study of their effects on nanoparticles. Solution vs gas-diffusion electrolysis cells Solution-phase electrolyses are useful for comparing the intrinsic activities of different catalysts but impractical for electrosynthesis on a preparative scale. The rate of CO 2 reduction is limited by the relatively low solubility of CO 2 in aqueous solutions, which is ~30 mm at 1 atm saturation. Depending on the cell design, stir rate, and electrode geometry, the maximum rate of CO 2 reduction that can be achieved in CO 2 - saturated aqueous solution is ~5 10 ma per cm 2 of geometric electrode area (i.e. the macroscopic electrode area without accounting for roughness factor). As a reference point, a geometric current density of 10 ma cm 2 for CO 2 reduction to ethanol corresponds to a synthesis rate of ~1.4 mg cm 2 h 1. The mass transport of CO 2 to an electrocatalyst is greatly improved with the use of a gas diffusion electrode (GDE).[18, 19] A GDE is composed of a porous, hydrophobic electrode material (typically a carbon fiber paper) that is coated on one side with catalyst particles and an ionomer. The catalyst side is placed in contact with an electrolyte solution or an ion exchange membrane while the hydrophobic side is exposed to CO 2. During operation, CO 2 diffuses through the hydrophobic pores to the catalyst particles that are in contact with electrolyte. This direct gas diffusion to the catalyst surface removes the limitation imposed by the solubility of CO 2 in solution and enables much higher current densities. To our knowledge, CO reduction has not been studied with a gas diffusion electrode. CO reduction can be performed in an alkaline electrolyte without quenching the HO by HCO 3 formation. Results As discussed above, the salient features of electroreduction catalysis with OD-Cu electrodes that distinguish it from ordinary Cu electrodes are i) suppressed specific activity (activity per Cu surface area) for H 2 O (H + ) reduction to H 2 (i.e. H 2 evolution) and ii) enhanced specific activity for CO reduction to multi-carbon oxygenates. The combination of these two kinetic features results in improved selectivity for CO 2 reduction and CO reduction over competitive, undesired H 2 evolution. Our research on OD-Cu and related materials over the course of this project has produced the following key findings to advance the science of electrochemical fuel synthesis: 5

1) The suppression of H 2 evolution on OD-Cu is a consequence of the electrode morphology. By evaluating the specific H 2 evolution activity of OD-Cu electrodes prepared with different morphologies, we showed that the extent of suppression of this reaction is correlated to the electrode roughness factor. This result suggests that the effect arises from mass transport phenomena that affect adsorbate coverage on the electrode surface and/or the availability of H + donors such as HCO 3. 2) The enhanced specific activity for CO reduction on OD-Cu is correlated with the presence of strong CO-binding sites on the surface. 3) There is a direct correlation between grain boundary density and catalytic activity in Cu and Au nanoparticle CO/CO 2 electroreduction catalysts. We have provided a new design principle for fuel-producing electroreduction catalysts by establishing quantitative grain boundary activity relationships. We have shown that there is a direct correlation between specific activity and grain boundary density for electrocatalytic CO reduction to multi-carbon oxygenates on Cu nanoparticles and CO 2 reduction to CO on Au nanoparticles. 4) Large increases in current density and synthesis rates are possible for Cu-catalyzed CO reduction by using gas diffusion electrodes. We have taken the first steps to translate our catalysts to prototype devices. By improving CO mass transport, >30-fold enhancements in CO reduction current density are possible. CO binding properties of OD-Cu surfaces The enhanced specific activity for CO reduction on OD-Cu suggests that the material has different surface sites for this reaction compared to conventional Cu materials. Temperature programmed desorption of CO (CO-TPD) provides information on the strength of CO binding to surface sites, which directly affects the potential energy surface for CO electroreduction.[20] Using CO-TPD, we have found that OD-Cu has CO binding sites with significantly higher affinity (by 7 kj/mol) than Cu terraces or highly stepped Cu surfaces. The presence of these high-affinity sites is qualitatively correlated with specific activity for CO electroreduction, suggesting that at least a portion of these surfaces contain highly active sites for these reactions (Figure 6). The details of this study are described in a paper published in JACS in 2015.[21] The origin and structure of the high-affinity CO binding sites in OD-Cu are presently unknown. Since OD-Cu has a relatively high density of GBs, we proposed that the high affinity sites are found on GB surface terminations. This hypothesis motivated our efforts to quantify the relationship between GB density and electrocatalytic activity. Grain boundary activity relationships in nanoparticle catalysts The combination of electrocatalysis, electron microscopy, and TPD studies of OD-Cu led us to propose that the GBs are responsible for creating highly active sites for CO reduction. Testing this model explicitly with OD-Cu is very challenging because it is difficult to quantify and control its GB density. In addition, it is unclear if GB effects in OD-Cu are applicable to nanoparticle catalysts, which are more desirable for use in practical electrosynthesis devices. In light of these issues, we sought to investigate the relationship between GBs and catalytic activity with metal nanoparticles whose GB densities could be varied and quantified directly by TEM without laborious sample 6

Figure 6: Overview of CO temperature programmed desorption (TPD) study of OD-Cu. OD- Cu is a nanocrystalline material with a high density of grain boundaries (a) and strong CO binding sites (b). OD-Cu loses its and high selectivity (Faradaic efficiency) for CO reduction upon annealing at 350 C (c, d). The loss of activity is accompanied by a change in the surface binding properties. preparation. Using metal vapor deposition and thermal annealing, we have prepared electrodes composed of Cu nanoparticles on carbon nanotubes (Cu/CNT) with different GB densities that can be quantified by TEM. Using these electrodes, we have shown that there is a direct correlation (linear relationship) between the fraction of the nanoparticle surfaces comprised of GB surface terminations (hereafter referred to as the grain boundary surface density) and the specific activity for CO reduction to ethanol and acetate (with Cu/CNT). The most GB-rich Cu nanoparticles that we have investigated are ~3 more active than our most active OD-Cu material and reach Faradaic efficiencies for ethanol and acetate as high as 70%. Key results are summarized in Figure 7. More details may be found in our recent publication.[22] To see if GBs are important for other catalytic materials, we performed a similar study with vapor-deposited Au nanoparticles on CNTs (Au/CNT). Au is a well known catalyst for CO 2 reduction to CO, although there is very little understanding of the surface structures that are responsible for this activity. Using Au/CNT samples spanning a range of GB densities, we found that there is a direct correlation between the GB surface density and the specific activity for CO 2 reduction to CO. This study was published in 2015.[23] To our knowledge, our results for CO reduction on Cu nanoparticles and CO 2 reduction on Au nanoparticles are the first quantitative GB activity correlations in electrocatalysis. 7

Figure 7: Grain boundary-dependent CO electroreduction activity. a) TEM images of Cu/CNT catalysts asdeposited and after annealing at temperatures ranging from 200 C to 500 C. Top row shows lowresolution images and bottom row shows high-resolution image of one NP from each sample. b) Faradaic efficiency (product selectivity) for CO reduction on Cu/CNT catalysts as a function of the annealing pretreatment temperature. The three data sets correspond to three different potentials (vs RHE). The remaining Faradaic efficiency (up to 100%) is accounted for by H2O reduction to H 2. c) specific current density for CO reduction (current corresponding to CO reduction per Cu surface area) vs the grain boundary (GB) surface density quantified by TEM. The three plots correspond to three different potentials. All electrolyses for the data in (b) and (c) were performed in 0.1 M KOH saturated with 1 atm CO. Figure adapted from ref. 27. Conclusions We have identified the structural parameters of OD-Cu that are correlated to its salient electrocatalytic properties and developed a design principle for nanoparticle CO2/CO electroreduction catalysts. For OD-Cu, the suppression of H2 evolution activity is correlated to the RF, and the enhancement of CO reduction activity is correlated to the presence of strong CO binding sites. The strong CO binding sites are qualitatively correlated to the density of grain boundaries in OD-Cu. These observations inspired us to 8

evaluate the effect of grain boundaries on the electroreduction activity of Cu nanoparticle catalysts. We found that the specific activity for CO reduction on dispersed Cu nanoparticle catalysts was linearly related to their grain boundary density, which was quantified by TEM. Similarly, the specific activity for CO 2 reduction on Au nanoparticles is also correlated to grain boundary density. Our studies inspire further efforts to ascertain the structure of grain boundary surface terminations and explore the effects of grain boundary geometry on activity. Furthermore, we have shown that CO reduction rates are greatly improved by using a gas diffusion electrode, paving the way toward the fabrication of prototype CO reduction electrolyzers. Publications and Presentations Publications 1. Lee, C. H.; Kanan, M. W. Controlling H + vs CO 2 Reduction Selectivity on Pb Electrodes. ACS Catal. 5:1, 465 469 (2015). 2. Feng, X.; Jiang, K.; Fan, S.; Kanan, M. W. Grain-Boundary-Dependent CO 2 Electroreduction Activity. J. Am. Chem. Soc. 137:14, 4606 4609 (2015). 3. Verdaguer-Casadevall, A.; Li, C. W.; Johansson, T. P.; Scott, S. B.; McKeown, J. T.; Stephens, I. E. L.; Kanan, M. W.; Chorkendorff, I. Probing the active surface sites for CO reduction on oxide-derived copper electrocatalysts. J. Am. Chem. Soc. 137: 9808 9811 (2015). 4. Feng, X.; Jiang, K.; Fan, S.; Kanan, M. W. A direct grain-boundary activity correlation for CO electroreduction on Cu. ACS Cent. Sci. 2: 169 174 (2016). Presentations (selected invited talks) 1. Kanan, M. W. Recycling CO 2 Using Nanocrystalline Electrocatalysts. NASA Weekend Workshop on CO 2 -Based Manufacturing. Mountain View, CA. June, 2014. 2. Kanan, M. W. Recycling CO 2. International Conference on Photochemical Conversion and Storage of Solar Energy. Berlin, Germany. July, 2014. 3. Kanan, M. W. Electroreduction Catalysis on Nanocrystalline Metal Surfaces. ACS National Meeting, San Francisco, CA. August, 2014. 4. Kanan, M. W. Electrocatalytic CO 2 Recycling. Pacific Coast Catalysis Meeting. Stanford, CA. September, 2014. 5. Kanan, M. W. Heterogeneous Electrocatalysts that Convert CO 2 into Liquid Fuels. GCEP Symposium, Stanford, CA. October, 2014. 6. Li, C. W. CO 2 and CO Reduction on Oxide-Derived Cu Electrodes. GCEP Symposium, Stanford, CA. October, 2014. 7. Kanan, M. W. Electrocatalysis at the Nanoscale: Defect-Rich Nanoparticles for CO 2 and CO Reduction. Manufacturing of Green Fuels from Renewable Energy Workshop. Technical University of Denmark. April, 2015. 8. Kanan, M. W. Electrocatalytic CO 2 and CO Conversion at Metal Grain Boundaries. ExxonMobil Chemical Company. Baytown, TX. May, 2015. 9. Kanan, M. W. New Electrochemical and Chemical Methods for Recycling Carbon Dioxide. Solar Fuels Network International Discussion Meeting. London, UK. July 2015. 10. Kanan, M. W. Grain Boundary Dependent CO 2 and CO Electroreduction Catalysis. ACS National Meeting. Boston, MA. August, 2015. 9

11. Feng, X.; Kanan, M. W. Electroreduction Catalysis with Defect-Rich Nanoparticles. American Vacuum Society Meeting, San Jose, CA. October, 2015. References 1. C. W. Li, M. W. Kanan, CO 2 reduction at low overpotential on Cu electrodes resulting from the reduction of thick Cu 2 O films. J. Am. Chem. Soc. 134, 7231-7234 (2012). 2. C. W. Li, J. Ciston, M. W. Kanan, Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper. Nature 508, 504-507 (2014). 3. Y. M. Xie, J. Xiao, D. D. Liu, J. Liu, C. H. Yang, Electrolysis of Carbon Dioxide in a Solid Oxide Electrolyzer with Silver-Gadolinium-Doped Ceria Cathode. J. Electrochem. Soc. 162, F397-F402 (2015). 4. Y. Hori, in Modern Aspects of Electrochemistry, C. G. Vayenas, R. E. White, M. E. Gamboa-Aldeco, Eds. (Sringer, New York, 2008), vol. 42, pp. 89 189. 5. K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energ. Environ. Sci. 5, 7050-7059 (2012). 6. Y. Hori, K. Kikuchi, S. Suzuki, Production of CO and CH 4 in Electrochemical Reduction of CO 2 at Metal-Electrodes in Aqueous Hydrogencarbonate Solution. Chem. Lett. 1695-1698 (1985). 7. Y. Hori, A. Murata, R. Takahashi, Formation of Hydrocarbons in the Electrochemical Reduction of Carbon-Dioxide at a Copper Electrode in Aqueous-Solution. J. Chem. Soc. Farad. Trans. I 85, 2309-2326 (1989). 8. Y. Hori, I. Takahashi, O. Koga, N. Hoshi, Selective formation of C2 compounds from electrochemical reduction of CO 2 at a series of copper single crystal electrodes. J. Phys. Chem. B 106, 15-17 (2002). 9. Y. Hori, A. Murata, R. Takahashi, S. Suzuki, Electroreduction of CO to CH 4 and C 2 H 4 at a Copper Electrode in Aqueous-Solutions at Ambient-Temperature and Pressure. J. Am. Chem. Soc. 109, 5022-5023 (1987). 10. Y. Hori, R. Takahashi, Y. Yoshinami, A. Murata, Electrochemical reduction of CO at a copper electrode. J. Phys. Chem. B 101, 7075-7081 (1997). 11. K. J. P. Schouten, Z. S. Qin, E. P. Gallent, M. T. M. Koper, Two Pathways for the Formation of Ethylene in CO Reduction on Single-Crystal Copper Electrodes. J. Am. Chem. Soc. 134, 9864-9867 (2012). 12. K. J. P. Schouten, E. P. Gallent, M. T. M. Koper, Structure Sensitivity of the Electrochemical Reduction of Carbon Monoxide on Copper Single Crystals. ACS Catal. 3, 1292-1295 (2013). 13. A. A. Peterson, J. K. Norskov, Activity Descriptors for CO 2 Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 3, 251-258 (2012). 14. J. H. Montoya, A. A. Peterson, J. K. Norskov, Insights into CC Coupling in CO 2 Electroreduction on Copper Electrodes. Chemcatchem 5, 737-742 (2013). 15. J. H. Montoya, C. Shi, K. Chan, J. K. Norskov, Theoretical Insights into a CO Dimerization Mechanism in CO 2 Electroreduction. J. Phys. Chem. Lett. 6, 2032 2037 (2015). 10

16. R. Reske, H. Mistry, F. Behafarid, B. R. Cuenya, P. Strasser, Particle Size Effects in the Catalytic Electroreduction of CO 2 on Cu Nanoparticles. J. Am. Chem. Soc. 136, 6978 6986 (2014). 17. T. Radetic, F. Lancon, U. Dahmen, Chevron defect at the intersection of grain boundaries with free surfaces in Au. Phys. Rev. Lett. 89, 085502 (2002). 18. M. L. Perry, J. Newman, E. J. Cairns, Mass Transport in Gas Diffusion Electrodes: A Diagnostic Tool for Fuel Cell Cathodes. J. Electrochem. Soc. 145, 5-15 (1998). 19. T. Yamamoto, D. A. Tryk, A. Fujishima, H. Ohata, Production of syngas plus oxygen from CO 2 in a gas-diffusion electrode-based electrolytic cell. Electrochim. Acta 47, 3327-3334 (2002). 20. S. S. Fu, G. A. Somorjai, Interactions of O 2, CO, CO 2, and D 2 with the Stepped Cu(311) Crystal-Face - Comparison to Cu(110). Surface Sci. 262, 68-76 (1992). 21. A. Verdaguer-Casadevall, C. W. Li, T. P. Johansson, S. B. Scott, J. T. McKeown, M. Kumar, I. E. Stephens, M. W. Kanan, I. Chorkendorff, Probing the Active Surface Sites for CO Reduction on Oxide-Derived Copper Electrocatalysts. J. Am. Chem. Soc. 137, 9808-9811 (2015). 22. X. Feng, K. Jiang, S. Fan, M. W. Kanan, A direct grain boundary activity correlation for CO reduction on Cu nanoparticles. ACS Cent. Sci. 2, 169 174 (2016). 23. X. Feng, K. Jiang, S. Fan, M. W. Kanan, Grain-boundary-dependent CO 2 electroreduction activity. J. Am. Chem. Soc. 137, 4606-4609 (2015). 24. E. Bertheussen, A. Verdaguer Casadevall, D. Ravasio, J. H. Montoya, D. B. Trimarco, C. Roy, S. Meier, J. Wendland, J. K. Nørskov, I. E. Stephens, Acetaldehyde as an Intermediate in the Electroreduction of Carbon Monoxide to Ethanol on Oxide Derived Copper. Angew. Chem. Int. Ed. 55, 1450 1454 (2016). Contacts Matthew Kanan: mkanan@stanford.edu Xiaofeng Feng: xffeng@stanford.edu Christina Li: cwli1@stanford.edu Allison Yau: alliyau@stanford.edu Changhoon Lee: chl1@stanford.edu 11