Surface Oxide-Derived Nanoporous Gold Catalysts for

Aug 26, 2019 - Electrochemical CO2 reduction (ECR) has become a viable option as the cost of renewable energy continues to decrease. One of the major ...
0 downloads 0 Views 1MB Size
Download PDF
Letter www.acsaem.org

Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Surface Oxide-Derived Nanoporous Gold Catalysts for Electrochemical CO2‑to-CO Reduction Zhen Qi,* Juergen Biener, and Monika Biener* Materials Science Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, United States

Downloaded via 91.200.80.106 on August 30, 2019 at 07:21:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Electrochemical CO2 reduction (ECR) has become a viable option as the cost of renewable energy continues to decrease. One of the major obstacles that prevents its widespread use is the lack of efficient ECR catalysts due to our only slowly emerging understanding of catalyst design. Here, we report on a surface oxide-derived nanoporous gold catalyst prepared by one-step electrochemical dealloying that shows an extremely low overpotential (Faradaic efficiency for CO exceeds 90%) of 0.185 V (−0.3 V vs RHE) for CO2-to-CO conversion in 0.1 M KHCO3 solution. We demonstrate that surface oxide-derived nanoporous gold shows improved ECR performance with higher Faradaic efficiency compared to clean nanoporous gold which is the consequence of its smaller overpotential for CO2-to-CO reduction and simultaneous suppression of hydrogen evolution. KEYWORDS: electrochemical CO2 reduction, nanoporous, dealloying, gold, electrocatalysis

S

selectivity of ECR catalysts via changing the pore size distribution and thus facilitating the mass transport.10 Finally, oxide-derived copper and gold both have been shown to exhibit low overpotentials for ECR, presumably due to a high grain boundary density.2,7 Two groups have studied the size dependence of gold nanoparticles on CO2-to-CO reduction and found that smaller particles support higher current densities, but at the cost of decreasing Faradaic efficiency (FE) via promoting hydrogen evolution.11 Among many other ways to prepare nanostructured Au catalysts, selective dissolution of silver from Ag−Au alloys has been proven to be a particularly simple and robust process to generate nanoporous gold (npAu) foams with an open sponge structure, high surface area, and high densities of step edges and kink sites.12−14 A unique feature of this material is that the grain structure of the Ag−Au starting alloy is preserved during the dealloying process.15 The grain size of the Ag−Au starting alloy is typically tens to hundreds of microns, more than 1000 times larger than the typical ligament size of npAu. The process thus results in a single-crystal-like material with a very low density of grain boundaries. NpAu is a highly active and stable catalyst for gas phase reactions.16 The monolithic structure and bicontinuous morphology provide a continuous conductive pathway for low resistance electron transport. By contrast, metal nanoparticle based electrodes typically require the use of conductive binders and other additives which lower

ubstitutions of traditional, fossil fuel based energies with renewable energy sources such as wind, solar, and hydraulic sources have become feasible as clean energy is getting more affordable.1,2 Electrochemical CO2 reduction (ECR) over metal and alloy catalysts, such as Cu, Ag, Au, etc.,3,4 is a promising technology to address long-term, seasonal grid energy storage which remains one of the key challenges for wide-scale deployment of renewable energies. It also provides a feasible path forward to go beyond carbon neutrality by using CO2 as a plentiful and cheap resource for the synthesis of important feedstock chemicals for durable polymer products that provide permanent CO2 storage. In general, low selectivity, large negative overpotentials, and poor long-term stability still prevent large-scale industrial applications of ECR. The development of more efficient ECR catalysts is a critical component of these new technologies and requires rational catalyst design based on surface property− catalyst performance correlations. Gold is a particularly efficient electrocatalyst that has been shown to convert CO2to-CO with a high Faraday efficiency (FE) and at low overpotentials.5−7 However, the performance of ECR gold catalysts critically depends on sample preparation and properties such as morphology, particle size, grain boundaries, and surface oxides. Examples for this effect include the following: There are ultrathin Au nanowires that have a low overpotential of −0.35 V vs RHE and show long-term stability for 6 h without any noticeable change in activity which has been attributed to their high edge-to-corner sites ratio.8 The presence of grain boundaries has been shown to improve the catalytic activity of metal nanoparticles for ECR.9 Proper design of the mesoscale structure is yet another way to tune the © XXXX American Chemical Society

Received: February 19, 2019 Accepted: August 26, 2019 Published: August 26, 2019 A

DOI: 10.1021/acsaem.9b00355 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

Figure 1. SEM images showing the microstructure of clean (a) and surface oxide-derived (b) npAu. The corresponding cyclic voltammetry in 0.5 M H2SO4 solution with a scan rate of 10 mV/s (c) and X-ray diffraction patterns (d).

suggests that O-npAu has a surface area of 13.3 m2/g which is higher than the surface area expected from its 16 nm ligament size indicating gold oxide is not completely reduced. The surface area of C-npAu is 3.6 m2/g, in excellent agreement with previous studies.20 As the presence of multilayer surface oxides can lead to an overestimation of the surface area based on the gold reduction peak, we independently measured the surface area of a O-npAu sample by performing additional electrochemical capacitance measurements (Figure S3) in the pure double layer region (0.5−0.6 V vs Ag/AgCl). This measurement indeed revealed a lower surface area value of 8.3 m2/g in good agreement with the expected value based on the ligament size of 16.4 ± 2.2 nm. The CVs of C-npAu are very stable. On the contrary, the shape of the gold oxide reduction peak of OnpAu changes during the first five consecutive scans (Figure S2) until it becomes stable but still different from that of CnpAu, consistent with previous studies.17,20 The distinct shape of the Au oxide reduction peak of O-npAu is a signature of Au surface oxides that formed during electrochemical dealloying.20 The presence of gold surface oxide reduces the surface mobility of gold, thus stabilizing smaller ligaments.21 X-ray diffraction (XRD) (Figure 1d) reveals peaks at 38°, 44°, 64°, 77°, and 82° that can be assigned to the (111), (200), (220), (311), and (222) crystal planes of Au, respectively. From previous studies,13,19 it is well-known that npAu maintains the crystallinity of the starting alloy; that is, ligaments within a grain of the original starting alloy form a single crystalline nanoporous network structure. XRD peaks of O-npAu are considerably broader than those of C-npAu (Figure 1d, inset), consistent with the smaller feature size of OnpAu observed in SEM (Figure 1a,b). It is important to note that XRD peak broadening in npAu samples is not caused by grain boundaries but is a consequence of long-range coherency loss within the large single crystal domains by surface stress

their electrical conductivity due to the added interparticle contact resistance. NpAu can be prepared by either free corrosion in concentrated nitric acid or by electrochemically driven dealloying.12,13,16,17 The former results in an atomically clean gold surface, while the latter can lead to the formation of a nanometer-thick gold surface oxide if high dealloying potentials are applied. The presence of the surface oxide layer reduces surface diffusion leading to a smaller feature size and a black sample color (see Supporting Information, Figure S1). In the present study, we demonstrate that gold surface oxide formation during dealloying results in npAu samples with lower overpotentials and a wider potential window for high FE CO2-to-CO conversion compared to npAu prepared without the presence of surface oxides. Our results demonstrate that the surface condition of gold nanostructures greatly affects the FE for ECR. Surface oxide-derived nanoporous gold, O-npAu, was prepared by electrochemical dealloying of Ag70Au30 alloy samples using a three-electrode setup at 0.9 V vs a pseudo-Ag/ AgCl reference electrode in 1 M HNO3. Atomically clean npAu, C-npAu, was prepared by free corrosion in concentrated HNO3. Details of the preparation procedures are described elsewhere.16,18,19 O-npAu and C-npAu share the same bicontinuous solid/void structure, but the characteristic ligament/pore size of O-npAu (Figure 1a) is considerably smaller than that of C-npAu (Figure 1b), 16.4 ± 2.2 nm vs 39.1 ± 4.8 nm, respectively. It is important to point out that the porosity of our samples (70%) is determined by the Ag content of the Ag−Au starting alloy (70 at. % Ag) which was the same for both O-npAu and C-npAu. The surface areas of both materials were determined by analyzing the gold reduction peaks in cyclic voltammograms (CVs) in 0.5 M H2SO4 at a scan rate of 10 mV/s (Figure 1c). The analysis B

DOI: 10.1021/acsaem.9b00355 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

Figure 2. Comparison between Faraday efficiency of clean and surface oxide-derived npAu for CO (a) and H2 (b) in 0.1 M KHCO3. The corresponding partial current for CO (c) and H2 (d).

Figure 3. Partial current density for CO (a) and H2 (b) of surface oxide-derived npAu in 0.1 and 0.5 M KHCO3 solution at the low overpotential region. The current densities are normalized to the electrode’s geometric surface area.

negative value, −0.5 V, for O-npAu, respectively. The higher current density in CO2-saturated 0.1 M KHCO3 (with respect to the Ar purged electrolyte) below −0.2 V can be assigned to ECR, and the higher ECR related current density of O-npAu (compared to C-npAu) observed in this region suggests that this material is the more active ECR catalyst. Potentiostatic ECR experiments confirm the better performance of the O-npAu catalyst indicated by the CV experiments. O-npAu shows a higher Faradaic efficiency for CO (FECO), especially in the low overpotential region between −0.5 to −0.3 V where FECO exceeds 90% (Figure 2a). By contrast, the FECO of C-npAu only approaches 90% at −0.4 V. For the entire high overpotential region below −0.5 V, the FECO of OnpAu remains 20% higher than that of C-npAu. The opposite

induced microstrain.22 Energy dispersive spectroscopy (EDS) results (Figure S4) indicate a residual Ag content of 1.4 at. % for C-npAu and 4.4 at. % for O-npAu, respectively. To quickly assess their ECR performance, we first measured the CVs of C-npAu and O-npAu in Ar- and CO2-saturated 0.1 M KHCO3 solution (Figure S5a,b). All potentials are referred to RHE unless otherwise specified and corrected for the measured pH difference of CO2 (6.8) and Ar (8.2) purged 0.1 M KHCO3 electrolytes. Between −0.2 and 0.4 V, both electrodes show pure electric double layer (EDL) charging currents reflecting the high surface area of both materials, with a higher EDL current density for the higher surface area OnpAu. The onset of hydrogen evolution in Ar-saturated 0.1 M KHCO3 occurs at −0.4 V for C-npAu and at a slightly more C

DOI: 10.1021/acsaem.9b00355 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

cleaning the electrolyte solution with Cheleax 100. Furthermore, to exclude Pt leaching and deposition on the npAu working electrode, we exchanged the Pt counter electrode against a glassy carbon counter electrode. The results discussed above demonstrate that O-npAu has a lower overpotential and wider potential window for CO2-toCO reduction than that of C-npAu. The geometric surface area of O-npAu is 2−3 times larger than that of C-npAu because of its smaller feature size (Figure 1a,b), but its current density is only about 30% higher in the low overpotential region between −0.6 and −0.3 (Table S1). Therefore, the current does not scale with the available surface area, again indicating a diffusion limitation. Besides the smaller ligament/pore size and the potential presence of some remaining subsurface oxide, OnpAu and C-npAu share the same pore−ligament morphology and single crystalline character of its Au ligaments. The improved CO2-to-CO reduction performance of oxide-derived Au has previously been explained by a high density of grain boundaries formed during the Au oxide reduction process.7 A high density of grain boundaries also seems to improve the selectivity of Au nanoparticles for ECR.9 However, both OnpAu and C-npAu have a very low grain boundary density because the dealloying process preserves the original grain structure of the Ag−Au starting alloy which is in the range of hundreds of microns, 3−4 orders of magnitude larger than the ligament size (tens of nanometers). The improved performance of O-npAu may thus be attributed to the presence of a low concentration of chemically inert surface/subsurface Au oxide species formed during electrochemical dealloying. Interestingly, the formation of Au surface oxide layers by ozone treatment of C-npAu has been demonstrated to be a critical step toward activation of npAu for gas phase partial oxidation of alcohols.16 Like the electrochemical dealloying process used to prepare O-npAu, ozone treatment of C-npAu leads to the formation of a nanometer-thick Au oxide surface layer. While most of this oxygen can be removed by either electrochemical oxidation−reduction cycles (this work) or by CO exposure,24 the remaining oxygen seems to be very stable and is involved in modifying reactivity and selectivity.16 Alternatively, the improved performance of O-npAu may be attributed to dislocations formed during gold oxide reduction.25 Finally, the improved performance may also be structure related as the smaller pore size of O-npAu facilitates the depletion of the proton source HCO3− resulting in a higher selectivity for CO2 reduction. SEM micrographs collected from C-npAu and O-npAu electrodes after 12 h of ECR reveal that the ligament size increased from 39.1 ± 4.8 to 80.6 ± 10.9 nm for C-npAu and from 16.4 ± 2.2 to 29.4 ± 4.0 nm for O-npAu, respectively (Figure S5). Electrochemical surface area measurements (Figure S8) performed after 12 h of CO2 electrolysis reaction show that the surface areas of C-npAu and O-npAu decrease from 3.6 to 2.4 m2/g and from 8.3 to 5 m2/g, respectively, in good agreement with SEM observations (Figure S7). However, it is important to note that FECO for O-npAu stays relatively constant at about 90% (Figure 4) even after running ECR for 12 h, thus suggesting that the feature size alone cannot explain the better performance of O-npAu. However, if it turns out that ligament coarsening needs to be prevented, one strategy could be the use of npAu prepared from AgAu leaf with a thickness of 100 nm. The structural confinement of npAu leaf in the third-dimension limits coarsening of the structure to 10 nm at which point the 3D structure collapses to a 2D structure.26 Indeed, two recent

is true for the Faradaic efficiency for hydrogen evolution (FEH2) (Figure 2b) which is lower for O-npAu compared to that of C-npAu in the whole potential region, especially in the low overpotential region between −0.5 and −0.3 V. The partial current density for CO2-to-CO reduction (ICO) is higher for O-npAu over the entire potential region (Figure 2c). The partial current density for H2 evolution (IH2), however, is the same for C-npAu and O-npAu (Figure 2d). The higher FECO thus is a consequence of the selective enhancement of the CO2-to-CO reduction by O-npAu. Additional potentiostatic ECR experiments were performed in higher concentration (0.5 M) KHCO3 electrolytes (Figure 3) to explore if the higher FECO of O-npAu (compared to CnpAu) in 0.1 M KHCO3 (Figure 2) is caused by proton depletion. For O-npAu, both ICO and IH2 increase with increasing KHCO3 electrolyte concentration (Figure 3). Below −0.3 V, the partial current densities of CO in 0.1 and 0.5 M KHCO3 solutions are roughly the same; for more negative potentials, ICO in 0.5 M KHCO3 is about 40% higher than the corresponding value in 0.1 M KHCO3 solution. The increase of IH2 in 0.5 M KHCO3 is even higher, thus effectively reducing FECO and increasing FEH2 in 0.5 M KHCO3 (Figure S6). For example, IH2 in 0.5 M KHCO3 solution at −0.6 V is 52 mA/cm2, about 13 times higher than the 4 mA/cm2 measured in 0.1 M KHCO3 solution. The FECO for O-npAu in 0.5 M KHCO3 exhibits a maximum of ∼60% at −0.34 V, considerably less than that in 0.1 M KHCO3, which is consistently above 90% in the potential range between −0.5 and −0.3 V (Figure S6). The selective increase of IH2 in 0.5 M KHCO3 can be explained by the higher concentration of bicarbonate anions which are known to be a more effective source of protons than H2O, thus facilitating hydrogen evolution.7 These results provide evidence that proton depletion limits the hydrogen evolution in the case of the lower concentration (0.1 M KHCO3) electrolyte. One of the main obstacles preventing the applications of ECR is the fast loss of catalyst reactivity due to the changes of surface morphology and/or composition caused by the high surface mobility of metal atoms under reaction conditions. Long-term stability tests of C-nAu and O-npAu at −0.34 V in 0.1 M KHCO3 for 12 h (Figure 4) show that the FE for H2 and CO of both samples is stable over 12 h, and that FECO is consistently higher for O-npAu (∼90%) than for C-npAu (∼60%). We noticed that the stability in these long-term tests strongly depends on the testing conditions,23 and that the high stability reported here required the use of ultrapure K2CO3 and

Figure 4. Stability test of clean (hollow) and oxide-derived (solid) npAu at −0.35 V RHE vs in 0.1 M KHCO3 solution for 12 h. D

DOI: 10.1021/acsaem.9b00355 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

(10) Hall, A. S.; Yoon, Y.; Wuttig, A.; Surendranath, Y. Mesostructure-Induced Selectivity in CO2 Reduction Catalysis. J. Am. Chem. Soc. 2015, 137, 14834−14837. (11) Mistry, H.; Reske, R.; Zeng, Z. H.; Zhao, Z. J.; Greeley, J.; Strasser, P.; Cuenya, B. R. Exceptional Size-Dependent Activity Enhancement in the Electroreduction of CO2 over Au Nanoparticles. J. Am. Chem. Soc. 2014, 136, 16473−16476. (12) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Evolution of Nanoporosity in Dealloying. Nature 2001, 410, 450− 453. (13) Qi, Z.; Vainio, U.; Kornowski, A.; Ritter, M.; Weller, H.; Jin, H. J.; Weissmuller, J. Porous Gold with a Nested-Network Architecture and Ultrafine Structure. Adv. Funct. Mater. 2015, 25, 2530−2536. (14) Biener, J.; Biener, M. M.; Madix, R. J.; Friend, C. M. Nanoporous Gold: Understanding the Origin of the Reactivity of a 21st Century Catalyst Made by Pre-Columbian Technology. ACS Catal. 2015, 5, 6263−6270. (15) Van Petegem, S.; Brandstetter, S.; Maass, R.; Hodge, A. M.; ElDasher, B. S.; Biener, J.; Schmitt, B.; Borca, C.; Van Swygenhoven, H. On the Microstructure of Nanoporous Gold: An X-Ray Diffraction Study. Nano Lett. 2009, 9, 1158−1163. (16) Zugic, B.; Wang, L. C.; Heine, C.; Zakharov, D. N.; Lechner, B. A. J.; Stach, E. A.; Biener, J.; Salmeron, M.; Madix, R. J.; Friend, C. M. Dynamic Restructuring Drives Catalytic Activity on Nanoporous Gold-Silver Alloy Catalysts. Nat. Mater. 2017, 16, 558−564. (17) Qi, Z.; Weissmuller, J. Hierarchical Nested-Network Nanostructure by Dealloying. ACS Nano 2013, 7, 5948−5954. (18) Zhu, C.; Qi, Z.; Beck, V. A.; Luneau, M.; Lattimer, J.; Chen, W.; Worsley, M. A.; Ye, J. C.; Duoss, E. B.; Spadaccini, C. M.; Friend, C. M.; Biener, J. Toward Digitally Controlled Catalyst Architectures: Hierarchical Nanoporous Gold Via 3D Printing. Sci. Adv. 2018, 4, eaas9459. (19) Parida, S.; Kramer, D.; Volkert, C. A.; Rosner, H.; Erlebacher, J.; Weissmuller, J. Volume Change During the Formation of Nanoporous Gold by Dealloying. Phys. Rev. Lett. 2006, 97, No. 035504. (20) Jin, H. J.; Parida, S.; Kramer, D.; Weissmuller, J. Sign-Inverted Surface Stress-Charge Response in Nanoporous Gold. Surf. Sci. 2008, 602, 3588−3594. (21) Biener, J.; Wittstock, A.; Biener, M. M.; Nowitzki, T.; Hamza, A. V.; Baeumer, M. Effect of Surface Chemistry on the Stability of Gold Nanostructures. Langmuir 2010, 26, 13736−13740. (22) Graf, M.; Ngô, B.-N. D.; Weissmüller, J.; Markmann, J. X-Ray Studies of Nanoporous Gold: Powder Diffraction by Large Crystals with Small Holes. Physical Review Materials 2017, 1, No. 076003. (23) Clark, E. L.; Resasco, J.; Landers, A.; Lin, J.; Chung, L. T.; Walton, A.; Hahn, C.; Jaramillo, T. F.; Bell, A. T. Standards and Protocols for Data Acquisition and Reporting for Studies of the Electrochemical Reduction of Carbon Dioxide. ACS Catal. 2018, 8, 6560−6570. (24) Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Baumer, M. Nanoporous Gold Catalysts for Selective Gas-Phase Oxidative Coupling of Methanol at Low Temperature. Science 2010, 327, 319− 322. (25) Min, X.; Chen, Y.; Kanan, M. W. Alkaline O2 Reduction on Oxide-Derived Au: High Activity and 4e- Selectivity without (100) Facets. Phys. Chem. Chem. Phys. 2014, 16, 13601−4. (26) Ding, Y.; Kim, Y. J.; Erlebacher, J. Nanoporous Gold Leaf: ″Ancient Technology″.Advanced Material. Adv. Mater. 2004, 16, 1897−1900. (27) Zhang, W. Q.; He, J.; Liu, S. Y.; Niu, W. X.; Liu, P.; Zhao, Y.; Pang, F. J.; Xi, W.; Chen, M. W.; Zhang, W.; Pang, S. S.; Ding, Y. Atomic Origins of High Electrochemical CO2 Reduction Efficiency on Nanoporous Gold. Nanoscale 2018, 10, 8372−8376. (28) Wen, X. S.; Chang, L.; Gao, Y.; Han, J. Y.; Bai, Z. M.; Huan, Y. H.; Li, M. H.; Tang, Z. Y.; Yan, X. Q. A Reassembled Nanoporous Gold Leaf Electrocatalyst for Efficient CO2 Reduction Towards CO. Inorg. Chem. Front. 2018, 5, 1207−1212.

studies reported that the ligament size in npAu made from AgAu leaf samples did not coarsen during ECR.27,28 In summary, we have shown that surface oxide-derived npAu shows enhanced reactivity and selectivity for electrochemical CO2-to-CO reduction. Surface oxide-derived npAu supports higher CO2-to-CO reduction currents with a FECO of 90%. Our results suggest that the development of new strategies for creating surface oxide-derived catalysts is a critical component toward preparing stable, reactive, and selective catalysts for electrochemical CO2-to-CO reduction.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00355.

■

Experimental details and additional results (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Monika Biener: 0000-0001-7289-5905 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for the support of this research under the auspices of the U.S. Department of Energy under Contract DE-AC52-07NA27344, through LDRD award 17-LW-013. We thank Dr. Jianchao Ye for performing XRD.

■

REFERENCES

(1) Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New Insights into the Electrochemical Reduction of Carbon Dioxide on Metallic Copper Surfaces. Energy Environ. Sci. 2012, 5, 7050. (2) Li, C. W.; Ciston, J.; Kanan, M. W. Electroreduction of Carbon Monoxide to Liquid Fuel on Oxide-Derived Nanocrystalline Copper. Nature 2014, 508, 504−507. (3) Bliem, R.; van der Hoeven, J.; Zavodny, A.; Gamba, O.; Pavelec, J.; de Jongh, P. E.; Schmid, M.; Diebold, U.; Parkinson, G. S. An Atomic-Scale View of CO and H2 Oxidation on a Pt/Fe3o4 Model Catalyst. Angew. Chem., Int. Ed. 2015, 54, 13999−14002. (4) Hicken, A.; White, A. J. P.; Crimmin, M. R. Selective Reduction of CO2 to a Formate Equivalent with Heterobimetallic Gold-Copper Hydride Complexes. Angew. Chem., Int. Ed. 2017, 56, 15127−15130. (5) Manthiram, K.; Surendranath, Y.; Alivisatos, A. P. Dendritic Assembly of Gold Nanoparticles During Fuel-Forming Electrocatalysis. J. Am. Chem. Soc. 2014, 136, 7237−7240. (6) Zhu, W. L.; Michalsky, R.; Metin, O.; Lv, H. F.; Guo, S. J.; Wright, C. J.; Sun, X. L.; Peterson, A. A.; Sun, S. H. Monodisperse Au Nanoparticles for Selective Electrocatalytic Reduction of CO2 to CO. J. Am. Chem. Soc. 2013, 135, 16833−16836. (7) Chen, Y. H.; Li, C. W.; Kanan, M. W. Aqueous CO2 Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles. J. Am. Chem. Soc. 2012, 134, 19969−19972. (8) Zhu, W. L.; Zhang, Y. J.; Zhang, H. Y.; Lv, H. F.; Li, Q.; Michalsky, R.; Peterson, A. A.; Sun, S. H. Active and Selective Conversion of CO2 to CO on Ultrathin Au Nanowires. J. Am. Chem. Soc. 2014, 136, 16132−16135. (9) Feng, X.; Jiang, K.; Fan, S.; Kanan, M. W. Grain-BoundaryDependent CO2 Electroreduction Activity. J. Am. Chem. Soc. 2015, 137, 4606−4609. E

DOI: 10.1021/acsaem.9b00355 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX