Ultrathin Ag Nanowires Electrode for Electrochemical Syngas

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Ultrathin Ag Nanowires Electrode for Electrochemical Syngas Production from Carbon Dioxide Wei Xi, Ruizhen Ma, Heng Wang, Zhen Gao, Weiqing Zhang, and Yunfeng Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00527 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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ACS Sustainable Chemistry & Engineering

Ultrathin Ag Nanowires Electrode for Electrochemical Syngas Production from Carbon Dioxide

Wei Xi, Ruizhen Ma, Heng Wang, Zhen Gao, Weiqing Zhang, Yunfeng Zhao*

Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, No. 391 West Binshui Road, Xiqing District, Tianjin 300384, China E-mail: [email protected]

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Abstract: Mass production of silver nanowires (Ag NWs) with ultrathin diameter (~35 nm) and a high aspect ratio (>1000) are prepared from an optimized polyol solution. The Ag NWs possessing several promising advantages, including good conductivity and excellent film formability, and have already been used to fabricate large area flexible Ag NWs-based electrode through a simple drop-casting method. The Ag NWs electrode displayed good activity to convert carbon dioxide (CO2) to carbon monoxide (CO) in high Faradic Efficiency (FE) of ~80 % under low working potential (-0.9 V vs. RHE), at the same time the fraction of CO/H2 can be tuned from 1/1 to 4/1. A little amount of HCOOH (FE: 2.6-5.6%) was also measured as by-product in liquid phase. The constant working current density and CO FE recovered super stability of Ag NWs electrode. The high resolution TEM recovered the {111} facet exposed to the surface contribuing to the effective performance. Ag NWs with larger diameter (70 nm) and Ag plate electrode were also evaluated under the same condition, where formic acid (HCOOH) was also detected besides CO as another main product. Combined with the easy-fabrication, low cost, high selectivity, and excellent stability, Ag NWs based electrode eager for further application in larger scale for syngas producing and other electrocatalysis.

KEYWORDS: Ag nanowires, flexible electrode, CO2 reduction, syngas

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Introduction Syngas is a mixture of carbon monoxide (CO) and hydrogen (H2), which can be used as intermediates in creating synthetic natural gas, gasoline, ammonia and methanol through Fischer-Tropsch process.

1-3

Traditionally, syngas is generated from water-gas shift reaction,

where coal, biomass or other carbon sources are treated by water under high temperature. The high-energy consumption and environmental concerns require the development of new technologies to produce syngas. Recently, use of electricity to extract carbon dioxide from the water and then the water gas shift to syngas is thought a promising way to give a cost effective product.

4-8

Different

products will be converted depending on metal through a certain reduction path, where the generation of the intermediate (CO2•-) is critical because it is the rate limiting step and the coordination of this intermediate determines if the 2e- reduction product will be either CO or formate.9-15 Gold (Au), Silver (Ag), and Zinc (Zn) can bind the intermediate CO2•- to varying degrees, but cannot reduce CO and typically yield CO as the major product.16 Among them, Ag is considered as the most promising catalyst because it has better stability than Zn or Ga and a much lower price than Au. However, CO2 reduction Faradaic efficiency (FE) of polycrystalline silver is normally in the range of 4% to 60% from different reports.

17-18

After

activated by plasma or ozone, the roughness of Ag plate surface would be obviously increased and therefore the CO transfer efficiency was enhanced.

19-20

Jiao et al. developed a

method to prepare a 3D porous Ag electrode through a de-alloying process, which gave the CO FE as high as 90% (-0.5 V vs. RHE).17 Amal et al. synthesized Ag foams electrode through a simple electrodeposition approach, which displayed the high CO transformation FE 3 / 26



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(94.7%).21 However, the relatively high prices of Ag limited the potential applications of high mass Ag macroscopic electrode. Nanostructured Ag nanoparticles would provide more active sites and edge/low-coordinated sites, which may exhibit higher efficiency and different catalytic behavior from that of bulk materials. Masel et al reported Ag nanoparticle gave the Faraday efficiency of ~80% by electrochemical reduction in -0.60V (vs. RHE).

22

Shape-dependent electrocatalytic reductions of CO2 to CO on triangular Ag nanoplates were investigated by Luo etc.18 Additionally, alloying with Au, Cu, Sn and Zn is another way to improve the catalysis performance.23-27Although the intrinsic property of Ag nanocatalysts have been enhanced obviously after the optimization, the discrete nanoparticles are hard to form the uniform electrode with good electrical conductivity, and various additives, including polymer binder and carbon powder, are used to enhance their conductivity and film forming ability. The addition of additives could bring a series of additional problems, such as mass transportation, blocking catalyst, and the stability. Therefore, developing a new approach to fabricate Ag nanostructure-based high quality electrode with less addictive is highly demanded for CO2 reduction. The Ag NWs with higher length/diameter ratio would be great for preparing of electrode with lower contact resistance and better economy of material utilization, which has attracted many interests as soft transparent electrode to instead indium tin oxide/fluorine doped tin oxide (ITO/FTO) glass.28-30 Ag NWs are also found good performance in energy-storage devices and water splitting.31-32 In this contribution, we combined the good catalytic performance, excellent conductivity and film forming ability of Ag NWs to prepare electrodes on plastic substrate through a facial drop-casting method to realize the 4 / 26


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electrochemical conversion of CO2 into energy molecules. The molar portion of CO/H2 also can be adjusted from 1/1 to 4/1 by changing the reduction voltage from the Ag NWs electrode with ultrathin diameter (35 nm). Specially, the largest of CO Faradaic efficiency of ~80 % was obtained at the reduction voltage of -0.9 V (vs. RHE). Considering the good stability and facial fabrication procedure, Ag NWs based electrode should have great potential for larger scale application for syngas preparation from CO2. Experimental Synthesis of Ag Ultrathin Nanowires: The large scale synthesis of silver nanowires with an average diameter of 30nm and an aspect ratio larger than 1000 were carried out by using ethylene glycol (EG) and diethylene glycol (DG) as co-mediated solvents in the presence of poly(vinyl pyrrolidine) (PVP) and nucleant through one-pot polyol method. Briefly, 0.2 mL NaCl (0.01 M) and 0.2 ml KBr (0.01M) aqueous solutions were added into the 50 mL EG/DG (4:1 in volume) with 0.66 g PVP under stirring. After that 0.54 g AgNO3 was added and stirred until completely dissolved. The reaction system then was stored in oven heated to 145 oC for 4 h, and then Ag NWs were produced by centrifugation and washing with ethanol. Prepared Ag NWs were suspended in ethanol (6 mg mL-1) for further handling. Fabrication of Ag NWs Flexible Electrode: The Ag NWs electrode was fabricated through a simple dropping coating method. Typically, a transparent polyethylene terephthalate film (PET, Shenzhen Hongmei Film Co., Ltd.) with the thickness of 1 mm was used as substrate to fabricate the flexible electrode. Firstly, PET film was cut into the size of 1 cm x 2 cm, and then was cleaned by ethanol and DI water. And then, 1 mL DI water and 60 µL Nafion® (5 wt%, Shanghai Hesen Electric Co., Ltd.) solution was added into 1 mL Ag NW ethanol suspension (6 mg mL-1) followed with 5 / 26



ultrasonic treatment for 5 min. Then, 200 µL homogenous Ag NWs solution was cast onto the cleaned PET film, and the flexible electrode was prepared after the solvent evaporated under room temperature. After drying under an infrared heating lamp, a PET substrate with the area of 2 cm2 was fully covered with only 0.6 mg Ag NWs. Characterizations: The crystallographic structures were determined by powder X-ray diffraction (XRD, Rigaku Ultima Iv). A scanning electron microscope (SEM, FEI, Verios 460L) and an aberration-corrected transmission electron microscope (TEM, FEI, Titan Cubed Themis G2 300) were utilized to characterize the morphology, structure and high-resolution structure of Ag NWs, respectively. A gas chromatography (GC9790II, Zhejiang Fuli Analyzing Instruments Co. Ltd.) equipped with a thermal conductivity (TCD) and flammable ionic detector (FID) was used to detect the product in the gas phase. The liquid product was detected in deuteroxide with DMSO as internal standard through Nuclear Magnetic Resonance (NMR, Bruker, 400M). Electrochemical experiment was tested on a CHI760e electrochemical workstation (Shanghai CHI Instruments) after degassing the solution for 30 min with either N2 (99.999 %) or CO2 (99.999 %) for the actual determination. Electrochemical Measurement: The measurement of electrochemical CO2 reduction of AgNWs electrode was performed in a homemade H-type cell (Figure S1). Two chambers were separated with a Nafion® 117 cation exchange membrane (Shanghai Hesen Electric Co., Ltd.). A conventional three-electrode system was employed for measurements. One side is equipped with a gas inlet and outlet which are able to pass the either N2 (99.999%) or CO2 (99.999%) through the solution and is airtight. The working electrode was an Ag NWs flexible electrode (1 cm x 2 cm), and only 1 cm x 1 cm was immersed into electrolyte solution (0.5 mol L−1 KHCO3 aqueous solution). An Ag/AgCl electrode (sat. KCl) was used as reference electrode, and a Pt wire was used as a counter electrode. 6 / 26


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All potential values are in reference to Ag/AgCl electrode unless mentioned otherwise. All experiments performed under room temperature (20 ± 3 ◦C) and ambient pressure. The electrolyte was saturated with CO2 before each electrolysis process, and CO2 gas was continuously aerated at a flow rate of 2 mL min−1 during the electrolysis process. The electrolysis experiments were terminated when the total charge passed reached 50 C. The average current density was given in the total current divided by the geometric surface area of the electrode for all cells. The current efficiency (Faradaic efficiency) of CO production and H2 was obtained from itotal, which is a measured current by the potentiostat, and the partial current (iH2 or CO) from the GC chromatogram peak areas (equations 1 and 2): [1]

[2] where VH2 or CO is the volume concentration of H2 or CO, based on calibration of the GC; F is the Faradaic constant; p0 is the pressure; T is the ambient temperature; and R is the ideal gas constant (R = 8.314 J mol K−1). Solution resistance was obtained by measuring the electrochemical impedance spectroscopy (EIS) at various potential to correct iR loss. The measured potentials for CO2 reduction reaction were compensated for iR loss and were reported versus the reversible hydrogen electrode (RHE) by using the following equation [3]: [3] The potential of the reference electrode for Ag/AgCl (saturated KCl) is 0.197 V. HCOOH Detection Method: The KHCO3 solution after electrolysis was collected and analyzed on a 1H NMR spectrometer to quantify liquid products. Standard curve was made by using sodium 7 / 26



formate (HCOONa·2H2O, Sinopharm Chemical Reagent Co. Ltd.) and the internal standard (DMSO, dimethyl sulfoxide), as shown in Figure S2a. A 0.4 mL sample of the KHCO3 solution after electrolysis was mixed with the addition of 0.1 mL D2O and 0.1 mL DMSO (100 ppm) solution as an internal standard. The 1H spectrum was measured with water suppression by a pre-saturation method, as shown in Figure S2b. The area ratio of the formate peak to the DMSO peak was compared to the standard curve to quantify the concentration of formate. Results and Discussion Preparation of Ag NWs with ultra narrow diameter and long length has attracted more and more attention. Polyol method was commonly used to prepare Ag NWs with higher L/D ratio. 33-37 Firstly, we optimized the synthesis conditions by employing the mixture of ethylene glycol (EG) and diethylene glycol (DG) as solvents. By using this modified polyol method, Ag NWs can be synthesized in large quantities (Figure S3). By modifying the ratio of EG/DG in the synthesis system, the reduction ability and viscosity would be adjusted correspondingly. As illustrated in Figure S4, a variety of Ag nanostructures were prepared by modulation the EG/DG ratio in volume from 1/0 to 0/1. Specially, high quality Ag NWs will be fabricated with the EG/DG ratio from 1/0 to 3/2. Ag NWs with the diameter of 100 nm are synthesized in pure EG condition. The adding of DG can effectively decrease the diameter to ~35 nm Ag NWs with the EG/DG of 4/1 to 3/2. The mixture of Ag nanoparticles and Ag NWs will be obtained if the portion of DG is further increased, and no Ag NWs are found from pure DG system. The Ag NWs synthesized from EG/DG (4/1) was further characterized by SEM and TEM. The SEM image in large-scale view recovered the ultra-long Ag NWs were quite uniform (Figure 1a and 1b). The TEM image in Figure 1c displayed the Ag NWs occupied a smooth surface with the 8 / 26


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average diameter of ~35 nm. The average length of Ag NWs could over tens of microns, so the L/D ratio could as high as 1000. The surface of Ag NWs, indicated by high resolution image (Figure 1d), may be the {111} facet. The lattice d-spacing 0.236 nm of {111} crystal plane is identical to that of Ag (insight picture of Figure 1d), which is further verified by the powder X-ray diffraction (XRD) pattern of metallic Ag with face centered cubic structure (FCC, JCPDS NO. 4-783) as showed in Figure S5.

Figure 1. SEM images (a and b), TEM image (c) and high-resolution image (d) and atomic structure (insight) of Ag NWs. The Ag NWs were well connected with each other and many aggregation pores in the range of hundreds of nanometers were found (Figure 1b), which were ensured the conductivity and beneficial for mass transfer. For the evaluation of CO2 reduction, half of electrode (1 cm2) was immersed into 9 / 26



the electrolyte (0.5 M KHCO3), the up side of the substrate was cut off for clarity. The CO2 reduction process was undergoing in a gas-tight H-Cell equipped with three electrodes. A PET substrate coated with Ag NWs was invoked as working electrode, which was simply connected with alligator clip. An Ag/AgCl electrode (saturated with KCl) was positioned in the same chamber as the reference electrode. The CO2 was bubbled into the 0.5 M KHCO3 with the gas flow speed of 2 cm3 min-1 controlled by a mass control flowmeter (MCF), and the outlet gas was connected to a on-line gas chromatography (GC) for production detection. A Pt foil was used as a counter electrode and was placed in another chamber, where a Nafion ion-exchange membrane separated two chambers. The electrochemical performance of Ag NWs electrode with the diameter of 35 nm was firstly evaluated by a cyclic voltammetry (CV) process, where the electrode potential was controlled using a CHI760E electrochemical workstation. Figure 2 depicts the CV curves of Ag NWs electrode in 0.5 M KHCO3 solution saturated with N2 or CO2 at room temperature (25 oC), respectively. In N2 saturated solutions, the onset potential was observed at -1.3 V (vs. Ag/AgCl) with the max current density less than 1 mA cm-2. Comparatively, the onset potential was only -1.1 V (vs. Ag/AgCl) in CO2 saturated KHCO3 solution and the max current density was almost five fold to that of N2 condition. The monotonously increased current in N2 saturated KHCO3 electrolyte was possibly caused by the hydrogen evolution reaction (HER), while the higher current density and lower onset potential in CO2-saturated solution could be attributed to the CO2 electroreduction reaction that occurred in parallel with the HER. In addition, two peaks at the voltages near -0.7 V (vs Ag/AgCl) in N2 and CO2 LSV curves were found and would derive from the adsorption and desorption of HCO3on the Ag catalyst since the voltammograms curves were tested in 0.5 M KHCO3 solution saturated with N2 or CO2.

38

To verify the explanation, we also checked CV curves of Ag NWs (35nm) 10 / 26


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electrode in 0.5M potassium chloride (KCl). As showed in Figure S6a, no additional peak was found in N2 purged solution in 0.5M KCl solution. If CO2 was purged into KCl solution, two peaks were always well detected even using KCl as electrolyte (Figure S6b). As we know CO2 and H2O can form HCO3-, so we can confirm that the two peaks around -0.7 V are original from the adsorption and desorption of HCO3- on the Ag catalyst.

0 -2

Current density (mA cm )

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CO2

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-3

-4 -1.6

-1.4

-1.2

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Figure 2. Cyclic voltammetry obtained with Ag NWs (35 nm) electrode in N2 and CO2 saturated 0.5 M KHCO3 solution with scan rate of 50 mV s-1. The current density of Ag NWs (35 nm) electrode was further recorded with time under different voltage (Figure 3). With the working potential increase, the current would increase synchronously. At the initial stage, c.a. before 3000 s, the j was not so stable. The product in the gas phase was analysized by a GC equipped with a TCD and FID detector. CO was found in a major product from CO2, and H2 was also detected originated from the electrolyting of H2O, as well as small amount of formic acid (HCOOH) was found in solution by 1H NMR measurement. Faradaic efficiency (FE) is the key index to evaluate the percentage of electrons consummation during the conversion of CO2 to CO and other hydrocarbons. The FE of CO was is tending towards to the stability after the reaction 11 / 26



time over about 5000 s for the working potential from -0.8 V to -1.1 V (vs. RHE). The CO FE would keep in a high value during the whole reaction period. The FE of HCOOH would keep in the low range of 2.59 % (-1.3 V vs. RHE) to 5.6% (-0.9 V vs. RHE). It was noteworthy that the total weight of Ag catalyst was 0.6 mg only for Ag NWs (35nm) electrode, thus the generation speed of CO was low under low working potential (Figure 3a to 3d). Coupling with a certain of CO2 feeding speed (2 mL min-1) and low CO generating speed under low working potential, a certain time (~ 6000 s) was needed to full-fill and reached a constant concentration in the reaction cell and pipeline.

Figure 3. Current density and CO Faradaic Efficiency as a function of electrolyzing time on Ag NWs (35 nm) electrode at the potential from -0.8 to -1.3 V vs. RHE. To further investigate the effect of the diameter to the CO2 conversion, a PET electrode coated with Ag NWs with larger diameter (70 nm) was prepared (Figure S7). As given in Figure 4, the Ag NWs (70nm) electrode presented the products of CO, H2 and HCOOH, producing CO with FE of 22.4-59.3% (-0.9 V vs. RHE) and H2 with FE of 19.7-49.1% as well as HCOOH with FE of 3.8%-7.3 % (-1.1 to -1.3 V vs. RHE) at working potential between -0.8 and -1.1 V (vs. RHE). With the working of potential further increase, no CO was detected any more, and the H2 is the main product. 12 / 26


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Figure 4. H2, CO, and HCOOH FE as a function of electrolyzing time on Ag NWs (70 nm) electrode at the potential from -0.8 to -1.3 V vs. RHE. The catalyst performance of Ag plate (0.2 mm, 99.99 %, Beijing Zhongnuo New Materials Co. Ltd.) electrode was investigated as the reference to demonstrate the advantages of Ag NWs-based electrode (Figure 5). The CO was detected from Ag plate electrode with the working potential from -0.9 V to -1.3 V (vs. RHE), and the maximum CO FE around 60% was recorded under -0.9 V to -1.1 V (vs. RHE). Besides CO, formic acid (HCOOH) was was further in the liquid through 1H NMR. The FE of HCOOH enhanced from 6.7 % (-0.8 V vs. RHE) to 62.8 % (-1.3 V vs. RHE) with the increase of working voltage. Therefore, the Ag NWs electrode can convert CO2 to CO in high selectivity, and therefore the performance of producing syngas of Ag NWs based electrode was very promising. Moreover, the mass of the Ag plate is 217.3 mg, while the mass loading of Ag NWs electrode is only 0.6 mg, therefore the Ag NWs based electrode greatly improved the utilization of materials.

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Figure 5. H2, CO, and HCOOH FE as a function of electrolyzing time on Ag plate electrode at the potential from -0.8 to -1.3 V vs. RHE. We further analyze the performance of conversation CO2 into syngas (CO and H2) and HCOOH. As illustrated in Figure 6a, the CO rate reaches its peak under -0.9 V (vs. RHE), and will decrease with the increase of working potential for Ag catalysts, and the CO FE will higher than that of other Ag catalysis electrodes. Specially, the CO FE would reach at a high level around 78 % and the H2 FE is around of 20 % of Ag NWs (35 nm). Therefore, the syngas from this condition are the component of CO and H2 with the molar fraction of 4:1. The stability of Ag NWs under -0.9 V (vs. RHE) was further evaluated as indicated in Figure 6b. The CV curves of Ag NWs electrode gave no markable change after 1000 cycles testing in CO2 saturated 0.5 M KHCO3 solution. The current density and CO FE would keep constant as long as 12 hours.

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0

b

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E H R . s v V 9 . 0 -

O C

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120

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60 % E F O 40C

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20

80 2

m c A m j

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FE (%)

60 40

(

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20

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Time (h)

Figure 6. (a) H2, HCOOH, and CO FE vs. working potential of Ag NWs (35nm, full filled), Ag NWs (70nm, slash-line filled) and Ag plate (cross-line filled) electrodes; and (b) the stability measurement of Ag NWs (35nm) electrode. The CO2 reduction performance of Ag NWs electrode is comparable with that of reported Ag nanoparticles-based electrode, eg. Ag/C (CO FE of 79.2%, -0.75 V vs RHE) 39, SS-Ag-NPs (CO FE of 65.4%, 0.846 V vs. RHE) 18, and nanoparticle with O bonded on surface, but still lower than that of well-designed Ag nanoparticle catalysts, such as triangular silver nanoplates (CO FE of 96.8%, -0.8 V vs. RHE)18 and nanoparticles with O bonded on surface40. The experimental and theoretical work reveals that the abandonment of edge-to-corner ratio is favourable for the CO2 converting to CO of Ag nanostructures, the facet of {111} of Ag represents the most stable low-index facet among the planed for Ag structure. 41-42 As shown in Figure 1d, the atomic structure of Ag NWs surface is typically {111} planes. Additionally, the Ag NWs synthesized with PVP as capping molecules would comprise with fivefold twins riched in edges and corners according to the previous report.

43-45

Besides, the cross of Ag NWs formed a large amount of pores, which is helpful to the transformation of CO2 and electrolyte, and further the reduction capability is enhanced. These structural features of Ag NWs would be beneficial to the CO2 conversion to CO and further syngas. Some factors other 15 / 26



than the crystalline structure were also involved in the CO production efﬁciency. To further identify the other factors causing the conversion of CO2 into CO and HCOOH, X-ray photoelectron spectroscopy (XPS) analysis of Ag NWs and plate was performed, and the resulting shifts are presented at Figure 7. The energy shift of Ag NWs (35 nm) moving in a negative direction comparing with Ag NWs (70 nm) and Ag plate. It has been demonstrated that the shift in core-level binding energy toward negative direction might affect the binding strength of the most important reaction intermediate (COOH*) on the Ag surface. The negative shift in core-level binding energy leads to the stabilization of intermediate as well, thus increasing CO efﬁciency. 41, 46-47

Figure 7. Representative results of XPS Ag 3d spectra for Ag catalysts. Moreover, the main product of Ag-based catalyst should be CO from the classic CO2 electrochemical mechanism on metallic catalysts.

9

However, HCOOH was also detected as by

product to Ag NWs and main product of Ag plate in this study. This may because the reduction path for CO2 takes into account many factors including facet, morphology, and even impurities. 18, 42 For the Ag catalyst in this study, the carbon, oxygen and nitrogen species were also observed from Ag catalysis electrodes (Figure S8). The Ag NWs synthesized from polyol method where employing polyvinyl pyrrolidone (PVP) as capping agent, therefore the Ag NWs is actually a composite of Ag 16 / 26


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NWs coated with PVP on surface.34, 48-49 For Ag plate, the C/O species should derive from the partial oxide and carbonate or surface adsorbed impurities. These complex surface composition may response to the converting CO2 into different products. 50 Conclusion In this paper, we have optimized the synthesis condition to obtain the Ag NWs with ultrathin diameter and high length to diameter ratio. The optimized Ag NWs with the diameter of 35 nm and 70 nm are used to fabricate electrocatalysis electrodes on PET substrates. The capability of generating syngas from CO2 and H2O is investigated in a three electrode system on Ag NWs electrodes. After surveying the working potentials from -0.8 V to -1.3 V (vs. RHE), the CO/H2 ratio of syngas is regulated from 1:1 to 4:1 on Ag NWs (35 nm) electrode, and maximum faradic efficiency of CO2 conversion is found to be almost 80% under -0.9 V (vs. RHE). Stability and recycle performance is further investigated in detail. The overall performance of Ag NWs based electrode is comparable to the nanoparticle-based electrode and much better than the Ag plate electrode. The high catalytic activity of Ag NWs is attributed to the abundant of corners and edges as actives and improved stabilization ability to intermediate. Considering the less mass loading, facial fabrication process, Ag NWs based electrodes have great potential for large scale applications in the field of electrocatalysis.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: The images of reaction cell; 17 / 26



Representative HCOOH 1HNMR spectra; The SEM images of Ag nanostructures; The XRD patterns of electrodes. AUTHOR INFORMATION Corresponding Author *Tel.: +86-22-6021-0415. E-mail: [email protected].

ORCID Yunfeng Zhao: 0000-0002-1442-992X Author Contributions W.X. and R.Z.M contributed equally to this work. Notes The authors declare no competing financial interest. Acknowledgement: This work was supported by the National Natural Science Foundation of China (NSFC: 21402136, NSFC-FDCT: 5171101212), Natural Science Foundation of Tianjin City (16JCYBJC17000, 16ZXCLGX00120). Y. Z. acknowledges support from the “Talent Program” of Tianjin University of Technology, “Youth Thousand Talents Program” and “131 Project” of Tianjin City. Authors thank for Prof. Yi Ding for fruitful discussion. References: (1) Khodakov, A. Y.; Chu, W.; Fongarland, P. Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels. Chem. Rev. 2007, 107 (5), 1692-1744, DOI: 10.1021/cr050972v. 18 / 26


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TOC（For Table of Contents Use Only） Ag NWs with ultrathin diameter displayed high efficiency for converting CO2 into syngas under low working potential.

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Ultrathin Ag Nanowires Electrode for Electrochemical Syngas

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