High-Performance Carbon Dioxide Electrocatalytic Reduction by

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High Performance Carbon Dioxide Electrocatalytic Reduction by Easily-Fabricated Large Scale Silver Nanowire Arrays Chuhao Luan, Yang Shao, Qi Lu, Shenghan Gao, Kai Huang, Hui Wu, and Ke-Fu Yao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03461 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Applied Materials & Interfaces

High Performance Carbon Dioxide Electrocatalytic Reduction by Easily-Fabricated Large Scale Silver Nanowire Arrays Chuhao Luan,a Yang Shao,*a Qi Lu,*b Shenghan Gao,a Kai Huang,a Hui Wu,*a Kefu Yaoa a

School of Materials Science and Engineering, Tsinghua University, Beijing, 100084,

China. b

Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China.

KEYWORDS: silver, nanowires, nanomoulding, electrocatalytic, carbon dioxide reduction

Abstract An efficient and selective catalyst is in urgent need for carbon dioxide electroreduction and silver is one of the promising candidates with affordable costs. Here we fabricated large scale vertically standing Ag nanowire arrays with high crystallinity and electrical conductivity as carbon dioxide electroreduction catalysts by a simple nanomoulding method that was usually considered not feasible for metallic crystalline materials. A great enhancement of current densities and selectivity for CO at moderate potentials was achieved. The current density for CO (jco ) of Ag nanowire array with 200 nm-in-diameter was more than 2500 times larger than that of Ag foil at an overpotential of 0.49 V with an 1

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efficiency over 90%. The origin of enhanced performances are attributed to greatly increased electrochemically active surface area (ECSA) and higher intrinsic activity in compare to polycrystalline Ag foil. More low-coordinated sites on the nanowires which can stabilize the CO2 intermediate better are responsible for the high intrinsic activity. In addition, the impact of surface morphology that induce limited mass transportation on reaction selectivity and efficiency of nanowire arrays with different diameter was also discussed.

Introduction

Converting carbon dioxide into energy-bearing products, can not only alleviate, if not eventually solve, the CO2 problems associated with anthropogenic activities but also supplement the diminishing fossil fuel reserves.1-5 In nature, this process is being practiced perpetually as parts of the metabolism of green plants, providing feeds for animals and balancing the CO2 concentration in atmosphere. To facilitate the carbon fixation process, the search for efficient biological and chemical pathways using renewable energy has been actively conducted for many years. Electrocatalytic reduction of CO2 in aqueous electrolyte is one of the promising strategies that receives enormous attentions in recent years.6-7 However, this process in general requires a large overpotential8 and has to compete with hydrogen evolution reaction (HER) as a side reaction.9-10 Therefore, an efficient catalyst with low overpotential and high faradaic 2


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efficiency (FE) is crucial in order to implement this technology to practical applications.11 Researchers have explored plenty of metal materials in search of active catalysts.12-18 It is found that Au and Ag are promising candidates exhibiting major reduction selection towards CO. The volcano-type plot for the CO2 reduction also shows that Au and Ag surface possessing optimal CO-binding energies for CO production,19-20 suggesting higher activities at lower energy penalties. At current stage, the state-of-the-art result for electrochemical conversion of CO2 to CO in aqueous environment was achieved on Au nanoneedles prepared by means of electrodeposition exhibiting a geometric current density of 15 mA cm-2 with a Faradaic efficiency of over 95% at a potential of -0.35 V versus the reversible hydrogen electrode (RHE).21

Although Au holds the benchmark electrochemical activity in CO2 conversion, it is not economically viable for practical implementation. As an alternative, Ag has attracted much attention due to its more affordable cost and good selectivity to CO14, 22-28 and CO could be used as an important feedstock for further application such as Fischer–Tropsch synthesis if the overall CO2 to CO conversion process could be efficiently improved. One strategy is to design nanostructured catalysts over bulk polycrystalline materials to expose more active sites. For instance, Lu et al. reported a nanoporous Ag catalyst with abundant low-coordinated surface sites produced by dealloying of an Ag-Al precursor, which showed a great improvement of current density and selectivity of CO production compared to polycrystalline Ag.23 In addition, other nanostructures such as nanowire 3



arrays,29-30 nanowires,15 nanoparticles31 ect. also showed a high Faradaic efficiency toward CO or hydrocarbons under moderate overpotentials.

Here we present Ag nanowire arrays (NWA) obtained by a simple, cost-effective nanomoulding technique as an advanced catalyst for electrochemical reduction of CO2. Although the nanomoulding method has been widely used for polymers and metallic glasses, it is believed not applicable to the metallic crystalline materials due to their size effect (the smaller the stronger). Here we overcome this hurdle, successfully extend this technique on crystalline silver and obtain large scale Ag NWAs with many advantages, which endow them some superiorities on the catalytic application. Different from other nanowires fabrication techniques, nanomoulded Ag nanowires are single-crystal-like and rooted in the bottom Ag conducting film, suggesting a great electrical conductivity and facilitating the charge transfer process. The as-prepared Ag NWAs exhibited superior activity and selectivity towards CO production. For NWAs with 200 and 30 nm-in-diameter, the onset overpotential was only ~200 mV which was 300 mV smaller than that of a polycrystalline Ag foil counterpart. At moderate potential of -0.6 V vs RHE, the jco of Ag nanowire array with 200 nm-in-diameter was more than 2500 times larger than that of Ag foil at a FE of 91%. Its specific jco (jECSA ) reached a high value of 18.78 CO µA cm-2 (46-fold improvement). Both a large electrochemically active surface area (more than 50 times enhancement) as well as the intrinsically enhanced activity (reflected by jECSA ) contributed to the superior performance. The different selectivity and efficiency CO 4


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between Ag nanowire arrays (NWA) with diameter of 30 nm and 200 nm may arise from the mass transport limitation induced by different surface morphologies

Results and Discussion Figure 1a is the schematic illustration of the fabrication process based on the simple nanomoulding method. More details could be found in the experimental section. The as-obtained Ag NWA could self-assemble into four different types (I, II, III and IV) (Figure 1a) of patterns which was due to different surface tension during AAO template removal caused by different length of NWA.32 Distinguished from other nanowires fabrication techniques, nanomoulded Ag nanowires were directly rooted in the hundreds-micrometer-thick bottom Ag conducting film that could serve as an electron collector, which not only greatly facilitated the electron charge transfer process but also eliminated the necessity of using an extra support.

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Figure 1. The fabrication process and surface morphologies of as-obtained Ag NWAs. (a) A schematically illustration of the fabrication process of Ag nanowire arrays with different surface morphologies by nanomoulding at elevated temperature. (b, c) Top-down SEM images of Ag-30 nm nanowire array and (d, e) Ag-200 nm nanowire array at different magnifications. 6


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Ag NWAs with 30 nm and 200 nm wire-diameter were fabricated from two different type of AAO template. Their microstructure and crystallinity were examined using secondary electron microscope (SEM) and transmission electron microscope (TEM). Figure 1b and d showed the SEM images of the morphologies of Ag NWAs with 30 nm-in-diameter (Ag-30 nm NWA) and 200 nm-in-diameter (Ag-200 nm NWA) at low magnification, respectively. Their morphologies at high magnification were also shown in Figure 1c and e. It can be seen that the Ag-30 nm NWA self-assembled into type IVcellular pattern while Ag-200 nm NWA self-assembled into type III-bundle pattern as illustrated in Figure 1a. Both NWA structures were uniformly distributed across the sample surface up to a few tens of square millimeters (Figure S1). The Ag nanowires were sturdily connected to the bottom Ag foil which was revealed by cross-section view (45° tilted) shown in Figure S2. The length of as-fabricated Ag-30 nm and Ag-200 nm NWA were estimated to be ~40 and ~24 micrometers in average, respectively. The bright field TEM image of single 30 nm Ag nanowire and related diffraction patterns were shown in Figure S3. A series of single crystal diffraction patterns along the length of nanowire with slightly titled orientation difference confirmed the as-fabricated nanowires were of single-crystal-like, although some twin structures could be noticed. The high-resolution TEM image of the 30 nm Ag nanowire indicated the high crystallinity. As a non-localized measurement, the X-ray diffraction (XRD) analysis (Figure S4) further suggested that both Ag NWAs were well-crystallized. No additional peaks from Ag oxide,

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AAO or other metal residue could be identified on the XRD spectra. The contamination-free surfaces of both Ag NWAs were confirmed by X-ray photoelectron spectroscopy (XPS) analysis (Figure S5).

Figure 2. The electrochemical performances of Ag-200 nm NWA, Ag-30 nm NWA and polycrystalline Ag foil towards CO2 reduction. (a) A comparison of current density versus potential with a scan rate of 5 mV s-1, (b) CO Faradaic efficiencies (solid black line) and H2 Faradaic efficiencies (dashed red line) for Ag-200 nm NWA (circle), Ag-30 nm NWA 8


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(square) and polycrystalline Ag foil (triangle), respectively. (c-e) The stability test of current densities (f-h) and CO Faradaic efficiencies at different potentials for Ag-200 nm NWA, Ag-30 nm NWA and polycrystalline Ag foil, respectively.

The electrochemical performances of both Ag NWAs as well as polycrystalline Ag foil counterpart were evaluated in CO2-saturated 0.5 M KHCO3 (99.95%) electrolytes (pH 7.2) in a gas-tight two-compartment cell, with a magnetic stirrer bar stirring at a rate of 700 rpm. Figure 2 presents the results of CO2 electroreduction. Except for the linear sweep voltammetry curve, current densities and FEs were collected after 30 min in the stability test (Figure 2c-e) to ensure that they could reflect the steady state of the catalytic activity. Ag-200 nm NWA and Ag-30 nm NWA demonstrated a geometric current density (jtotal ) of ~4.92 mA cm-2 and ~6.81 mA cm-2 respectively at a moderate potential of -0.6 V (referenced to RHE unless stated otherwise), corresponding to an overpotential of 0.49 V with the CO2/CO equilibrium potential of -0.11 V. Although the jtotal of Ag-30 nm NWA was higher than that of Ag-200 nm NWA, its FE was less than that of Ag-200 nm NWA. As shown in Figure 2b, Ag-200 nm NWA exhibited a FE as high as 91% at -0.6 V, but for Ag-30 nm NWA, the FE is only 59% at the same potential. As the potential dropped to -0.5 V, both jtotal and FEs decreased (1.42 mA cm-2 with 82% FE for Ag-200 nm NWA, and 2.35 mA cm-2 with 29% FE for Ag-30 nm NWA). At a more negative potential of -0.7 V, the jtotal increased but the FEs decreased for both Ag-200 nm NWA and Ag-30 nm 9



NWA (12.22 mA cm-2 with 84% FE for Ag-200 nm NWA, and 16.43 mA cm-2 with 52% FE for Ag-30 nm NWA), which were most likely due to the promotion of competing H2 production. As comparison, the polycrystalline Ag foil exhibited a significantly smaller jtotal (0.32 mA cm-2, 0.20 mA cm-2 and 0.11 mA cm-2 at -0.7 V, -0.6 V and -0.5 V, respectively) with poor selectivity (less than 20%) towards CO.

The stability test of the electrocatalytic behaviors of the Ag NWA samples was also carried out. The potentials for electrolysis were chosen to be -0.5 V, -0.6 V and -0.7 V, respectively. The current densities and FEs were recorded every 30 minutes during 2-hour test period (Figure 2c-h). Both of Ag NWA samples exhibited stable geometric current densities and faradaic efficiencies. Notably, there were a sharp decrease of total current densities (jtotal ) at the beginning, which were most likely a result of the reduction of oxide layer.23 After the initial sharp decrease, the current density still decreased slowly for 5~15 min before it was stable. We believe a mass transportation balance process is responsible for the current density slow down process which will be discussed in detail later. In addition, 24-hour stability tests were also performed (Figure S6) and no obvious difference in current densities and FEs was found throughout the whole process, suggesting excellent long-term stability of Ag NWAs at the moderate potentials. The SEM images (Figure S7) of Ag NWAs after the 24-hour stability test also revealed their highly stable nature as CO2 electroreduction catalysts. The XRD (Figure S4) and XPS (Figure S5) analysis were performed on the samples after 24-hour stability test, 10


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confirming no noticeable change in crystal structure and surface condition.

To investigate the different reaction selectivity among Ag-200 nm NWA, Ag-30 nm NWA and polycrystalline Ag foil counterpart, the FEs of CO2 reduction as a function of potential from -0.3 V to -0.8 V were plotted in Figure 2b. For polycrystalline Ag foil, the production of CO was only detectable at a potential more negative than -0.6 V. Its FE increased gradually as the overpotential became larger which was accompanied by a decline of competing H2 production. In comparison, the performance of Ag-200 nm NWA and Ag-30 nm NWA were greatly improved. The onset potential for CO2 reduction reduced to ~-300 mV (responding to an overpotential of 190 mV) for both Ag NWAs, which was 300 mV better than that of polycrystalline Ag foil counterpart. The maximum FE was 91% for Ag-200 nm NWA and 59% for Ag-30 nm NWA, both of which were achieved at -0.6 V. Their FEs also decreased as the potential became more negative which was believed due to the more severe mass transfer limitation of CO2 at higher reaction rate.

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Figure 3. The comparison of the partial current density and specific current density for Ag-200 nm NWA, Ag-30 nm NWA and polycrystalline Ag foil towards CO2 elecoreduction. (a) The partial current densities for CO at different potentials. (b) The specific current densities for CO at different potentials.

To gain further insights into Ag NWAs as CO2 electroreduction catalysts, jco at different potentials were calculated and plotted in Figure 3a. At -0.6 V where maximum FE was achieved, the jco of Ag-200 nm NWA and Ag-30 nm NWA reached 4.47 mA cm-2 and 4.01 mA cm-2, respectively, which was about 2500 and 2250 times higher than that of polycrystalline Ag foil. As the potential became more negative, the jco of Ag-200 nm NWA surpassed that of Ag-30 nm NWA with an over 50% higher value at -0.8 V.

Because jco was determined by both total number of active sites and intrinsic activity,33 individual discussion on the effect of ECSA and jECSA was crucial to better CO understand the behavior of jco at different Ag catalysts. The ECSAs were calculated based on the double layer capacitance (Figure S8).22,

34

For 1 cm2 of the geometric

electrode area, the estimated ECSA were 238 cm2, 396 cm2 and 4.36 cm2 for Ag-200 nm NWA, Ag-30 nm NWA, and polycrystalline Ag foil, respectively. After normalizing the jco with the ECSA, the jECSA which represented the intrinsic activities were calculated. CO As shown in Figure 3b, the jECSA for Ag-200 nm NWA and Ag-30 nm NWA reached a CO 12


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value of 18.78 µA cm-2 and 10.14 µA cm-2 at -0.6 V where their FEs were maximum, respectively, greatly exceeding that of polycrystalline Ag foil (~0.41 µA cm-2). To confirm the enhanced performance, we also compare the Ag NWAs with other Ag catalysts in Table S1.17, 22-23, 35

The enhanced ECSA and intrinsic activity of Ag NWAs are responsible for the enhanced current densities and FEs compared to that of Ag foil during electrolysis. However, for Ag-200 nm NWA and Ag-30 nm NWA, the different selectivity and efficiency are believed to come from the mass transfer effect induced by different surface morphology. In Figure 2c-e, the current density slow down process is corresponding to a continuous decrease of local CO2 partial pressure on the catalyst surface until it reaches a balance between CO2 consumption rate (determined by jco ) and compensation rate via mass transportation. When the current density became stable, the local CO2 partial pressure also dropped to a balanced lower value, resulting in a decreased intrinsic activity and therefore a decreased CO2 partial current densities.36-37 According to a continuum model for mass transfer combined with microkinetic model in previous literature,37 the limitation of mass transport may affect the current density of Ag NWAs not only at high overpotential but also at a relatively low overpotential. The cellular pattern of Ag-30 nm NWA with longer and denser nanowires are confronted with a more severe problem of mass transportation compared with Ag-200 nm NWA, leading to a lower CO2 partial pressure and therefore a lower intrinsic activity. Because of the aqueous reaction 13



environment, a lower surface utilization by CO2 conversion typically accompanied by higher H2 production due to the unlimited H2O molecules, further resulting in a low FEs of Ag-30 nm NWA. For Ag-200 nm NWA, the ECSA was quite consistent to the actual surface area (242 cm2) estimated from the geometry of nanowire array (Figure S2a). However, for Ag-30 nm NWA, only ~44% of the estimated surface area (893 cm2) was electrochemically active due to the severe surface agglomeration, which suggested a low utilization of surface area. In addition, the gas production would gather on the surface of the samples during the experiment, resulting in a decrease of effective ECSA. Ag-30 nm NWA is favor of the bubbles’ aggregation compared with Ag-200 nm NWA due to the smaller separation distance between the nanowires, and thus sacrifices more area of ECSA, which ா஼ௌ஺ may also partly account for the lower ݆஼ை of Ag-30 nm NWA.

14


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Figure 4. CO partial current density Tafel analysis for Ag-200 nm NWA, Ag-30 nm NWA and polycrystalline Ag foil.

To probe the intrinsically high activity of Ag NWAs over Ag foil catalysts, Tafel analysis was performed as shown in Figure 4. Despite various mechanisms for CO2 electroreduction over Ag have been discussed in the literature,37 the most classic interpretation proposed by Hori et al. involved an initial electron transfer to CO2 forming a surface adsorbed radical anion (CO2-) followed by a chemical proton donation process forming COOH adsorbate and another electron transfer process forming CO adsorbate which eventually left surface as free CO molecule:9

e−

* + CO 2 → * CO

− 2

e−

H 2O

→ − * COOH

− OH

→ − * CO → * + CO

− OH

(1)

It was believed the first step forming CO2- intermediate proceeds at a much more negative potential than the following steps, and was therefore the rate determining step for the whole process. In this scenario, Tafel slope was calculated to be about 120 mV dec-1 according to kinetics under the assumption of negligible intermediate coverage.23, 38 This was consistent with the Tafel slope observed on polycrystalline Ag foil (Figure 4). However, the Tafel slope of Ag-200 nm NWA and Ag-30 nm NWA was significantly smaller than 120 mV dec-1 with fitting results of 56 mV dec-1 and 80 mV dec-1,

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respectively. This was typically regarded as the indication of a fast initial electron transfer before the second chemical step, leading to a theoretical Tafel slope value of about 60 mV dec-1 under the assumption of negligible intermediate coverage.23,

38

The Tafel study

suggested that Ag nanowire surfaces were able to stabilize the CO2- intermediate better than a flat surface which might be the fundamental cause for their much improved intrinsic activity. The exposure of denser low-coordinated surface sites due to the curved nano-sized surface could be responsible for the stronger binding of reaction intermediates.36

Conclusions We developed an easy nanomoulding method to fabricate single-crystal-like Ag nanowire arrays with a diameter of 200 nm and 30 nm for efficient electroreduction of CO2. Due to the significantly enhanced intrinsic activity and ECSA of Ag nanowire array, a large current density of ~5 mA cm-2 and faradaic efficiency of 91% was achieved on Ag-200 nm NWA at an overpotential of less than -0.5 V. Superior long-term stability of Ag nanowire array was demonstrated with 24-hour electrolysis. Tafel analysis revealed the enhanced intrinsic activity of Ag NWAs is originated from the denser low-coordinated surface sites due to the curved nano-sized surface. As the discussion between different activity of Ag-200 nm NWA and Ag-30 nm NWA, an optimized surface morphology of Ag NWA should make a balance between a larger effective ECSA and a faster mass 16


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transportation.

Experimental Section Materials KHCO3 (99.95% trace metals basis), KOH (99.99% trace metals basis) and KF (99.0% ACS reagent) were purchased from Sigma Aldrich. The chemicals were used without purification. All electrolyte solutions were prepared using reagent grade water (Millipore Type 1,18MΩ resistivity). Anodic aluminum oxide was purchased from Shangmu Tech. co. Ltd. Ag foil (99.999%) was purchased from Alfa Aesar. CO (99.999%), CO2 (99.999%), H2 (99.999%) and N2 (99.999%) gases were purchased from Beijing huanyu jinghui city gas technology co. Ltd.

Fabrication of Ag nanowire arrays Firstly, an anodic aluminum oxide (AAO) template was placed on Ag foil between two press heads and heated to 400 °C. Secondly, a pressure of 500 MPa was applied on the press heads which extruded Ag into the nanopores of the AAO template. Two types of AAO templates were used to obtain Ag-200 nm and Ag-30 NWAs in the work: (1) the diameters of nanopores were about 200 nm and the separation distance of nanopores was about 250 nm; (2) the diameters of nanopores were about 30 nm and the separation distance of nanopores was about 65 nm. The length of Ag NWA was extended by holding the pressure at elevated temperature for 30 minutes. After the sample was cooled to room 17



temperature, the AAO template was removed using 1 M KOH solution at 75 °C to expose the nanowire array. For the filamentary material, the surface morphologies are controlled by the material’s Young’s modulus, surface tension, length of nanowire, diameter of nanowires as well as the separation distance between nanowires, which has been well described in the literature.32 For our Ag NWAs, the surface morphology is only the function of length of nanowires by using a certain type of AAO template. Simply increasing the temperature, pressure or holding time, we could increase the length of nanowires and thus obtain different type of surface morphologies. With provided experimental parameters, the surface morphologies of Ag NWAs used in our work could be well reproduced. Characterizations Surface morphologies and cross-sections of the Ag NWAs were characterized with scanning electron microscope (SEM, ZEISS Merlin) with an acceleration voltage of 30 kV. The microstructure of Ag-30 nm nanowire was examined on transmission electron microscope (TEM, FEI Tecnai G20) at 200 kV. XRD spectra were obtained using Rigaku D/max-RB with Cu Kα radiation. XPS spectra was collected using a Thermo Scientific ESCALAB 250Xi instrument with Al-Kα radiation.

Electrochemical measurements and product analysis All of experiments were conducted using a CH Instruments 760D. A two-compartment cell was used for electrolysis. A Nafion-115 proton exchange membrane was used to 18


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separate cathode and anode to exclude the oxidation of CO2 reduction products and the electro-deposition of Pt ion from the counter electrode in long-term experiment. The gas products (CO and H2 in major) were collected from the hermetic space above the cathode and measured by the gas chromatography that has already been calibrated before with standard gas. The counter electrode was a piece platinum gauze (99.9%) which was purchased from Sigma Aldrich. The reference electrode is Ag/AgCl (3.0 M KCl) which was purchased from BASi. Potential vs Ag/AgCl was converted to Potential vs RHE using the relationship of E (RHE) = E (Ag/AgCl) + 0.210 + 0.059 × pH. The products in cathodic

compartment

were

determined

by

gas

chromatography

(GC-7920,

CEAULIGHT). N2 (99.999%) was used as carrier gas.

Associated Content Supporting information Figure S1. Optical images and low magnification SEM images of Ag nanowire arrays. Figure S2. Cross-section view of SEM images of Ag nanowire arrays. Figure S3. TEM images of Ag-30 nanowires. Figure S4. XRD characterization of Ag nanowire arrays and Ag foil. Figure S5. XPS characterization of Ag nanowire arrays before and after electrocatalytic. Figure S6. Long-term stability test of Ag nanowire arrays and Ag foil under different potential. Figure S7. Top-down view of SEM images of Ag nanowire arrays after electrocatalytic. 19



Figure S8. Measurement of double-layer capacitance. Table S1 Summary of performances for carbon dioxide electroreduction including Ag NWAs in this work and other Ag catalysts.

Author information Corresponding authors Yang Shao: [email protected]; Qi Lu: [email protected]； Hui Wu: [email protected]. Author Contributions Y.S. and H.W. conceived of the idea. C.H.L., Y.S. and K.F.Y. performed the Ag nanowire arrays fabrication. Y.S. and C.H.L helped with SEM, TEM, XRD and XPS measurement. C.H.L., Y.S., Q.L., W.X. and H.W. conducted the measurement of electrochemical performance and product analysis. C.H.L., Q.L. Y.S. and H.W. co-wrote the manuscript. All authors discussed the results and commented on the manuscript. Notes The author declare that they have no competing interests.

Acknowledgments 20


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We acknowledge the support from the National Natural Science Foundation of China (Grant No. 51771096 and 51571127) and National Key Basic Research and Development Programme (Grant No. 2016 YFB0300502). This work made use of the resources of the Beijing National Center for Electron Microscopy at Tsinghua University.

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31. Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I., Ionic Liquid-Mediated Selective Conversion of CO2 to CO at Low Overpotentials. Science 2011, 334 (6056), 643-644. 32. Tawfick, S. H.; Bico, J.; Barcelo, S., Three-dimensional lithography by elasto-capillary engineering of filamentary materials. MRS Bulletin 2016, 41 (02), 108-114. 33. Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T. F., Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355 (6321), eaad4998. 34. Motheo, A. J.; Machado, S. A. S.; Kampen, M. H. V.; Jr, J. R. S., Electrochemical Determination of Roughness of Silver Electrode Surface. Journal of the Brazilian Chemical Society 1993, 4 (3), 122-127. 35. Liu, S.; Tao, H.; Zeng, L.; Liu, Q.; Xu, Z.; Liu, Q.; Luo, J. L., Shape-Dependent Electrocatalytic Reduction of CO2 to CO on Triangular Silver Nanoplates. J Am Chem Soc 2017, 139 (6), 2160-2163. 36. Rosen, J.; Hutchings, G. S.; Lu, Q.; Rivera, S.; Zhou, Y.; Vlachos, D. G.; Jiao, F., Mechanistic Insights into the Electrochemical Reduction of CO2 to CO on Nanostructured Ag Surfaces. ACS Catalysis 2015, 5 (7), 4293-4299. 37. Singh, M. R.; Goodpaster, J. D.; Weber, A. Z.; Headgordon, M.; Bell, A. T., Mechanistic insights into electrochemical reduction of CO2 over Ag using density functional theory and transport models. Proceedings of the National Academy of Sciences of the United States of America 2017, 114 (42), E8812. 38. Chen, Y.; Li, C. W.; Kanan, M. W., Aqueous CO2 reduction at very low overpotential on oxide-derived Au nanoparticles. J Am Chem Soc 2012, 134 (49), 19969-19972.

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