graphene catalyst for efficient electroreduction of CO2 to CO

Journal Pre-proof Dual-atom Ag2 /graphene catalyst for efficient electroreduction of CO2 to CO Yifan Li, Chang Chen, Rui Cao, Ziwei Pan, Hua He, Kebin Zhou

PII:

S0926-3373(20)30162-4

DOI:

https://doi.org/10.1016/j.apcatb.2020.118747

Reference:

APCATB 118747

To appear in:

Applied Catalysis B: Environmental

Received Date:

8 November 2019

Revised Date:

2 February 2020

Accepted Date:

9 February 2020

Please cite this article as: Li Y, Chen C, Cao R, Pan Z, He H, Zhou K, Dual-atom Ag2 /graphene catalyst for efficient electroreduction of CO2 to CO, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118747

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Dual-atom Ag2/graphene catalyst for efficient electroreduction of CO2 to CO

Yifan Lia, Chang Chena, Rui Caob, Ziwei Pana, Hua Hea , Kebin Zhoua,*

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School of Chemical Sciences, National Engineering Laboratory for VOCs Pollution

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Control Material & Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China E-mail: [email protected]

SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource

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(SSRL), Menlo Park, California 94025, United States

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Graphical abstract

Highlights 

A dual-atom Ag2/graphene catalyst was prepared for electroreduction of CO2 to CO.

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The active site consisted of two Ag atoms and each coordinated with three N atoms.

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FECO was up to 93.4% with a current density of 11.87 mA cm-2 at -0.7 V.

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The Ag2/graphene catalyst exhibited excellent stability of more than 36 h.

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The AgN3-AgN3 sites can promote CO2 adsorption and stabilize the

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intermediates.

Abstract:

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Electrochemical reduction of CO2 into value-added carbon compounds offers a

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promising strategy to mitigate global warming, but present challenges for chemistry

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due to the poor selectivity and stability of electrocatalysts. In this work, we report a dual-atom Ag2/graphene catalyst featuring well-defined AgN3-AgN3 active site for CO2

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electrochemical reduction. This dual-atom catalyst can drive CO2 reduction reaction at a potential as high as -0.25 V, and exhibit excellent CO Faradic efficiency up to 93.4%

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with a current density of 11.87 mA cm-2 at -0.7 V and long-term stability, far surpassing the single-atom Ag1/graphene and the traditional silver nanoparticle catalysts. DFT

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calculations reveal that the dual-atom Ag site lowers the barrier for the formation of *COOH by stabilizing the *CO2 through the concomitant interactions with the C and an O atom of CO2, resulting in excellent catalytic performance.

Keywords: Dual-atom catalyst; Silver; Highly efficient; CO2 electroreduction

1. Introduction Selective electrochemical conversion of CO2 into CO is an attractive route to mitigate the excess CO2 in the atmosphere and recycle it into high valuable carbon-based compound [1-7]. It is well known that large overpotentials are needed to overcome the high activation barriers due to the extremely stable chemical bond in CO2 (C=O, 806

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kJ mol-1) [8]. Besides, hydrogen evolution reaction (HER) can easily take place as a

side reaction [9] and multi-proton coupled electron transfer commonly occurs with a

wide distribution of C1-C3 products [10]. Up to now, a range of electrocatalysts

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including carbon-based composite [11-13], metals [14-15], and organometallic

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compounds [16] have been reported to be capable of CO2 conversion. Among them, silver has been considered as one of the most selective catalysts for electrochemical

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reducing CO2 in aqueous solution, because its unique electronic structure can largely

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suppress HER and facilitate CO desorption [17-21]. However, conventional Ag-based catalysts often exhibit sluggish kinetics of CO2 electroreduction and the selectivity still

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needs to be further improved [22-23]. Atomically dispersed metal catalysts (ADMCs), i.e. single-atom catalysts, with the

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maximized metal utilization and improved catalytic reactivity, have attracted extensive attention during the past several years [24-25]. For many catalytic reactions, the activation step usually involves multiple molecules and needs multiple metal atoms to serve as adsorption sites [26-29]. In this context, one would expect the active sites of the ADMCs should consist of several atoms. With respect to CO2 electroreduction,

although CO2 is the only reactant to be activated, an catalytic site consisting of two adjacent active metal atoms is believed to be more favorable than other types of catalysts [30-31]. For instance, most recently, Li and coworkers [30] reported that a Cu atom-pair site on Pd10Te3 alloy nanowires could promote CO2 activation and reduction with the help of one Cu atom adsorbing H2O and the neighbouring Cu atom adsorbing CO2. Meanwhile, the coordination environment of the active metal centers should also

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be well defined, because the ligands have a strong impact on the electronic structure of metal atom, which directly influences the catalytic selectivity [32-34]. It was reported that when atomically dispersed Co-NX sites anchored on porous carbon was used as

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CO2 electroreduction catalysts, the coordination number of nitrogen was found to play

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a crucial role in the performance of current density and CO Faradic efficiency [33]. Therefore, it is reasonable to expect that if a pair of Ag atoms with appropriate defined

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coordination environment could be prepared, high performance of CO2 electroreduction

is a challenge.

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would be achieved. However, the obtaining of such a single-site heterogeneous catalyst

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Herein, we report a dual-atom Ag2/graphene catalyst (namely, Ag2-G) for CO2 electrochemical reduction. The active site consists of two adjacent silver atoms and

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each coordinate with three nitrogen atoms and the AgN3-AgN3 sites are strongly anchored on the graphene matrix by formation of Ag-C bonds. Delightfully, the Ag2-G could catalyze the CO2 reduction at a high potential of -0.25 V, showing superior CO Faradic efficiency up to 93.4% with a current density of 11.87 mA cm-2 at -0.7 V and maintaining the FECO about 90% for electrolyzing over 36 h. DFT calculations reveal

that the dual-atom Ag site lowers the barrier for the formation of *COOH intermediate by stabilizing the CO2 adsorption intermediate through the concomitant interactions of the C and an O atom of CO2 with two Ag atoms, respectively.

2. Experimental 2.1 Materials

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Graphite flake (>325 mesh, 99.95%, Aladdin), Potassium permanganate (KMnO4, AR, Beijing Chemical Co.), Sulfuric acid (H2SO4, 98%, Beijing Chemical Co.), Hydrogen peroxide (H2O2, 30%, Beijing Chemical Co.), Hydrochloric acid (HCl, 38%,

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Beijing Chemical Co.), 1,10-phenanthroline monohydrate (99%, Sinopharm Chemical

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Reagent Co. Ltd), Silver nitrate (AgNO3, 99.8%, Sinopharm Chemical Reagent Co. Ltd), Ethanol (99.8%, Sinopharm Chemical Reagent Co. Ltd), Acetonitrile (99.8%,

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Aladdin), Methanol (99.5%, Beijing Chemical Co.), Phthalazine (98.0%, Aladdin),

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Dimethyl sulfoxide (DMSO, 99.8%, Aladdin), Sodium citrate (99.0%, Beijing Chemical Co.), Sodium borohydride (NaBH4, 98%, Shanghai Macklin Biochemical Co.

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Ltd), Potassium bicarbonate (KHCO3, 99.5%, Shanghai Macklin Biochemical Co. Ltd ) were used as received. Nafion 117 membrane was purchased from Dupont. The 18.2

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MΩ cm ultrapure water was obtained from the Mili-Q System. 2.2. Synthesis of catalysts 2.2.1. Graphene oxide Graphene Oxide (GO) was synthesized according to the Hummers’ method with a minor modification [35]. In brief, 5 g KMnO4 was added to a mixture of 1 g powdered

flake graphite and 40 mL H2SO4. The rate of addition was controlled to prevent the temperature of the suspension from exceeding 20 °C. Then the temperature of the suspension was brought to 35 °C and maintained for 2.5 h. Next, 160 mL water was slowly stirred into the paste, causing violent effervescence. After the reaction completed, 7 mL H2O2 was added to the reaction jar. Then, filtered and washed the

oxide was obtained by centrifugation and lyophilization. 2.2.2. Mononuclear Ag complex

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products with 8 mL HCl and 2000 mL ultrapure water. Finally, the dry form of graphene

The mononuclear Ag complex ([Ag(phen)2]NO3) was prepared by the method

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reported previously [36], mixed 0.396 g 1,10-phenanthroline monohydrate dissolved in

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15 mL methanol to a solution of 0.17 g AgNO3 in 15 mL acetonitrile. The light yellow precipitate formed immediately. After stirring for 1.5 h in darkness, the yellow

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precipitate was filtered off and washed with methanol.

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2.2.3. Binuclear Ag complex

The binuclear Ag complex ({[Ag(NO3-O)(phtz-N)]2(μ-phtz-N,N′)2}) was prepared

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by the method reported previously [37]. The solution of AgNO3 (169.9 mg) in 5 mL of ethanol was slowly added to the solution containing phthalazine (130.2 mg) in 20 mL

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of warm ethanol under stirring. The mixture was stirred for 3 h in the dark at room temperature. Then the solid product was filtered off and dissolved in 20 mL of acetonitrile. The complex was crystallized after stand in the refrigerator for four days. The colorless crystals were filtered off and dried in the dark. 2.2.4. Single-atom Ag1-G catalyst

In a typical synthesis of single silver atom catalyst (Ag1-G), 100 mg GO was dispersed in 100 mL DMSO mixed with 100 mL DMSO solution contained 3.5 mg [Ag(phen)2]NO3. The mixture was sonicated for 1 h and stirred at room temperature for 24 h in darkness, then separated by centrifugation at 8500 rpm for 20 min and washed with DMSO for three times. Afterward, the above product was lyophilized to form a GO foam with uniformly distributed [Ag(phen)2]NO3 (called [Ag(phen)2]NO3-GO).

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Finally, the [Ag(phen)2]NO3-GO foam was annealed at 550 °C for 2 h under a gas flow of 80 sccm N2. 2.2.5. Dual-atom Ag2-G catalyst

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In a typical synthesis of dual silver atom catalyst (Ag2-G), 100 mg GO was dispersed

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in 100 mL DMSO mixed with 100 mL DMSO solution contained 2.4 mg {[Ag(NO3O)(phtz-N)]2(μ-phtz-N,N′)2}. The mixture was sonicated for 1 h and stirred at room

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temperature for 24 h in darkness, then separated by centrifugation at 8500 rpm for 20

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min and washed with DMSO for three times. Afterwards, the above product was lyophilized to form a GO foam with uniformly distributed {[Ag(NO3-O)(phtz-N)]2(μ(called

{[Ag(NO3-O)(phtz-N)]2(μ-phtz-N,N′)2}-GO).

Finally,

the

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phtz-N,N′)2}

{[Ag(NO3-O)(phtz-N)]2(μ-phtz-N,N′)2}-GO was annealed at 550 °C for 2 h under a gas

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flow of 80 sccm N2. 2.2.6. AgNO3-G

In a typical synthesis of AgNO3-G, 1 mg AgNO3 and 100 mg GO were dispersed in 200 mL ultrapure water under stirring for 24 h in darkness. Then separated by centrifugation at 8500 rpm for 20 min and washed with ultrapure water for three times.

Finally, the product was lyophilized and annealed at 550 °C for 2 h under a gas flow of 80 sccm N2. 2.2.7. Reduction-graphene oxide (G) In a typical synthesis of G, 100 mg GO was dispersed in 200 mL DMSO under stirring for 24 h in darkness. Then separated by centrifugation at 8500 rpm for 20 min. Finally, the product was lyophilized and annealed at 550 °C for 2 h under a gas flow of

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80 sccm N2. 2.2.8. Ag nanoparticles-G (Ag NPs-G)

In a typical synthesis of Ag NPs-G, 50 mg G and 100 mg sodium citrate were

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sonicated in the 241.25 mL ultrapure water for 30 min. Then, 0.15 mL AgNO3 solution

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(0.02 M) was injected into the above mixture solution and stirred for 30 min. Afterward, 6.25 mL NaBH4 solution (8 g/L) was added into the mixture and stirred at 800 rpm for

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30 min. Finally the product was separated by centrifugation at 8500 rpm for 20 min and

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washed with ultrapure water for three times. The Ag NPs-G sheets can be obtained after the lyophilized progress.

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2.3. Materials Characterization

Transmission electron microscopy (TEM) and EDS mapping images were carried out

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on a JEM-1200EX. Scanning electron microscope (SEM) images were operated with Hitachi S4800 FE SEM. Atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization and the corresponding element mappings were acquired on a JEM-ARM200F (acceleration voltage, 120 kV). Specific surface areas were obtained on Brunauer-Emmett-Teller

(BET) nitrogen adsorption-desorption on an ASAP 2460 analyzer (Micromeritics, USA) at 77 K. X-Ray diffraction (XRD) patterns were conducted on Rigaku Smartlab powder diffractometer with monochromatized Cu Kα radiation (λ=0.15418 nm, 9 kW). X-ray photoelectron spectroscopy (XPS) was measured by Thermo ESCALAB 250 Xi. Quantitative analysis of the Ag loading was carried using an Agilent ICP-OES 730. The Raman spectra were attained on a Renishaw Raman microscope using 532 nm laser

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excitation with a laser power of 5 mW. X-ray Absorption Spectroscopy (XAS). The Ag

K-edge X-ray absorption spectra were measured at the Stanford Synchrotron Radiation

Lightsource (SSRL) on the unfocussed 20-pole 2 T wiggler side-station beam line 7-3

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under standard ring conditions of 3 GeV and ~500 mA. A Si(220) double crystal

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monochromator was used for energy selection. The monochromator was detuned by 30% to diminish components from higher harmonics. The XAFS and FT-XAFS were

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fitted using the Artemis software. The structural parameters varied during the fitting

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process were the bond distance (R) and the bond variance σ2, which is related to the Debye-Waller factor resulting from thermal motion, and static disorder of the absorbing

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and scattering atoms.

2.4. Electrochemical measurements

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2.4.1. Electrochemical characterization Linear sweep voltammograms and potentiostatic electrolysis were performed using

a CHI 600E electrochemical Station (Shanghai Chenhua Instruments Company). Gasphase products were analyzed by Gas Chromatograph SP-2100 (FID) and Gas Chromatograph GC-7920 (TCD). Liquid-phase products were determined by nuclear

magnetic resonance (1H-NMR) spectra recorded on an Ascend 400 spectrometer (500 MHz, Bruker, Germany). 2.4.2. Electrochemical Measurements All electrochemical measurements were measured in a three-electrode system at ambient temperature. The carbon fiber paper (1 cm2) with the catalyst layer was acting as the working electrode. The Pt foil (1 cm2) and Ag/AgCl (saturated KCl) served as

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the counter electrode and reference electrode, respectively. A gastight H-type glass cell was used as the electrolyzer for CO2 reduction. A Nafion 117 membrane was inserted

between the anodic and cathodic chambers. The reference and working electrodes were

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placed in the cathode chamber, and the counter electrode was placed in the anode

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chamber. Each part contained 100 mL 0.5 M KHCO3 electrolyte. The solution of cathode was purged with high purity CO2 (99.999%) for 1 h with a flow rate of 25 mL

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min-1 before electrolysis (CO2 saturated electrolyte pH = 7.2), and the flow of CO2 was

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maintained throughout the electrochemical measurement. Linear sweep voltammetry (LSV) with a scan rate of 10 mV s-1 was carried out in CO2-staurated electrolyte. For

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comparison, LSV measurements were also performed in an N2 (99.999%)-saturated electrolyte (pH = 8.8). The durability tests were analyzed by keeping electrolysis at -

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0.7 V (versus RHE) for 36 h. The potentials were converted to the reversible hydrogen electrode (RHE) by Nernst equation: E(versus RHE) = E(versus Ag/AgCl) + 0.199 V + 0.059 × pH

To prepare the catalyst ink, 1 mg of catalyst and 20 μL of 5 wt% Nafion solution were introduced into 250 μL of water-ethanol solution (V water : V ethanol = 4:1) and

sonicated for 3 h. Then, 54 μL of catalyst ink was transferred onto a carbon paper (1 cm2) and dried in air, given a catalyst loading of 0.2 mg cm-2. The gas-phase products were detected at each potential every 30 min by using an online gas chromatograph. In detail, CO concentration was quantified by flame ionization detector (FID), while H2 was determined using a thermal conductivity detector (TCD). During the CO2 electrolysis, the electrolyte of cathode was purged with

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purity CO2 with a constant flow rate of 25 cm3 min-1 which monitored by an LZB-3WB Scientific mass flow controller. The cathodic compartment of the H-type electrolyser

was connected the outlet into the gas sampling loop of SP-2100 gas chromatograph

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(FID) and Gas Chromatograph GC-7920 (TCD) equipped with a combination of

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HayeSep Q, HayeSep T, HayeSep N columns and a 5A molecular sieves. The detectors were calibrated by different concentrations of standard gases.

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The liquid products were analyzed by nuclear magnetic resonance (NMR) after

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continuous electrolysis for 2 h, 0.5 mL electrolyte was collected and mixed with 100 μL D2O in an NMR tube, but no liquid products were detected after 2 h continuous

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electrolysis.

Faradaic Efficiency (FE) of CO and H2 were calculated by using the concentration

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detected by GC based on the equation: FE =

Z × P × F ×  × νi 𝑅×𝑇×𝐼

Where Z is the number of electrons transferred per mole CO2 to gas product, which is 2 for CO and H2 production. P is atmospheric pressure (1.01 × 105 Pa), F is Faraday’s constant (96485 C mol-1),  is the gas flow rate at the exit of the cell measured by a

flow meter (m3s-1), ʋi is the volume concentration of gas product in the exhaust gas from the cell determined by online GC, T is the reaction temperature (298.15 K), R is the idea gas constant (8.314 J mol-1K-1), and I is the steady-state cell current at each potential. The partial current density of CO was determined by total current density and FE of

JCO = Jtotal × FECO

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CO, as following:

The Tafel slope was calculated based on the Tafel equation as follows: JCO η = blog( ) J0

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CO, and J0 is the exchange current density.

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Where Ƞ is the overpotential, b is the Tafel slope, JCO is the partial current density of

on the following formula:

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The turnover frequency (TOF, h-1) of the CO2 reduction reaction was computed based

Iproduct × MAg × 3600 Z × F × ω × mcatalyst

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TOF =

Where Iproduct is the partial current for CO (A); MAg =107.8682 g mol-1 is the atomic

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mass of Ag; ω is the content of Ag loading in the catalyst (wt%) which is determined

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by ICP analysis; mcatalyst is the catalyst loading mass on the carbon fiber paper (g). 2.5. Computational details The geometry optimization and calculation of electrochemical reaction pathways

were carried out by density functional theory calculations on the Vienna ab initio simulation package (VASP) code with consideration of spin-polarization, and the projector augmented wave was used to pseudopotential for the core electrons, the cutoff

energy of the valence electrons is 400 eV, and used the generalized gradient approximation in the form of Perdew Burke Ernzerhof for the exchange correlation potentials. The convergence of energy and forces were 1×10-4 eV and 0.05 eV Å-1, respectively. The Gamma point was employed as k-point. Grimme’s D3 correction was used to take account of dispersion interaction. The single Ag atom and dual Ag atom catalysts were simulated using a 19.7*17.0*20.0 angstrom3 slab graphene sheet model

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with AgN4 and AgN3-AgN3 sites strongly anchored on it with periodical boundary

conditions was used. Only adsorbates (CO2, CO2H, etc) and Ag atoms were allowed to relax at the stage of vibrational analysis [38, 39].

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As the reaction steps containing proton-electron pairs transfer, the free energy of

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each intermediate state was calculated based on the computational hydrogen electrode (CHE) method. The main idea of the CHE is the Gibbs free energy of proton-electron

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pairs related in the PECT progress [40] can be referenced by the free energy of gas-

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phase H2, whereas the chemical potential of proton-electron pairs is equated with a half that of H2 at 0 V versus standard hydrogen electrode: μ (H++e-) = ½ μ (H2).

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The adsorption energy of intermediates is defined as: Eads = EG/M – EG - EM

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Where EG/M, EG and EM denote the total energies in this adsorbed system, graphene

substrate, and an adsorbate at a free state, respectively, all of them can be obtained from the DFT calculations.

It is known that the CO2 electrochemical reduction to CO is following the progresses: (* is the clean adsorption site, *COOH is refers to the COOH adsorbed on the activity site, and *CO is representing the activity site with CO adsorbed on it) CO2 + H + + 𝑒 − + ∗ → COOH∗ COOH∗ + H + + e− → CO∗ + H2 O CO∗ → CO + ∗

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The free energy change ΔG1 for the first proton-coupled electron-transfer (PCET) step under the zero overpotential can be calculated by the following equation: ΔG1 = ΔE + ΔZPE + Δ0−298K H − TΔS = ΔEtotal + ΔG298K

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= [𝐸𝑡𝑜𝑡𝑎𝑙 (𝐶𝑂𝑂𝐻 ∗ )– 𝐸𝑡𝑜𝑡𝑎𝑙 (𝐶𝑂2 + 12𝐻2 + ∗)]

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+ [𝐺298𝐾 (𝐶𝑂𝑂𝐻 ∗ ) − 𝐺298𝐾 (𝐶𝑂2 + 12𝐻2 +∗)] The Etotal and G298K is the calculated total energy and free energy correction at 298K

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that includes the zero-point energies (ZPE), the entropy of the intermediates only take

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the vibrational entropy into account.

The second proton-coupled electron-transfer step energy change can be calculated

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as:

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ΔG2 = [𝐸𝑡𝑜𝑡𝑎𝑙 (𝐶𝑂∗ + 𝐻2 𝑂)– 𝐸𝑡𝑜𝑡𝑎𝑙 (𝐶𝑂𝑂𝐻 ∗ + 12𝐻2 )] + [𝐺298𝐾 (𝐶𝑂∗ + 𝐻2 𝑂) − 𝐺298𝐾 (𝐶𝑂𝑂𝐻 ∗ + 12𝐻2 )]

The finally step of CO desorption can be calculated as： ΔG3 = [𝐸𝑡𝑜𝑡𝑎𝑙 (𝐶𝑂 +∗)– 𝐸𝑡𝑜𝑡𝑎𝑙 (𝐶𝑂∗ )] + [𝐺298𝐾 (𝐶𝑂 +∗) − 𝐺298𝐾 (𝐶𝑂∗ )]

We apply the ZPE and TΔS values reported by Peterson et. al [41]. 3. Results and Discussion

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Figure 1. Schematic illustration of the synthesis process of Ag2-G catalyst. Figure 1 describes our strategy to prepare the dual-atom Ag2-G catalyst. It was

synthesized by anchoring binuclear silver-containing aromatic molecules on graphene

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via π-π interaction, followed by a temperature-programmed pyrolysis process (see

Supporting Note S1). The binuclear Ag complex ({[Ag(NO3-O)(phtz-N)]2(μ-phtz-

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N,N′)2}) was used as the metal precursor, in which, each Ag atom coordinates with

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three N atoms of phthalazines and the two Ag atoms form a weak interaction, as indicated by a Ag···Ag distance of 3.434 Å. The large planar aromatic structures of the

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precursor can be strongly adsorbed on the graphene surface via π-π interaction [42]. Thermal gravimetric analysis (TGA) and Raman spectra measurements reveal that

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graphitization of aromatic groups could take place while a part of the ligands can be

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reserved during pyrolysis of the Ag complex at appropriate temperature (Figure S2). In order to protect the coordinating N atoms adjacent to silver and prevent Ag aggregation during pyrolysis, the annealing conditions were carefully optimized. Finally, the Ag2G catalyst was successfully prepared. The content of Ag is 0.10 wt% as determined by ICP. For comparison, using the same strategy, single-atom catalyst (Ag1-G) was also

prepared with the mononuclear Ag complex ([Ag(phen)2]NO3) as the metal precursor.

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The loading of Ag in Ag1-G was determined to be 0.27 wt% (Table S1).

Figure 2. Characterization of Ag1-G and Ag2-G catalysts. (a) TEM image of Ag1-G. (b) Aberration-corrected HAADF-STEM image of Ag1-G. (c) Enlarged HAADFSTEM image of Ag1-G. (d) Corresponding element maps showing the distribution of C (red), N (blue) and Ag (green), respectively. (e) TEM image of Ag2-G. (f) Aberrationcorrected HAADF-STEM image of Ag2-G. (g) Enlarged HAADF-STEM image of Ag2-

G. (h) Corresponding element maps showing the distribution of C (red), N (blue) and Ag (green), respectively. TEM and SEM analyses were performed to examine the morphology of the two catalysts (Figure 2a, e and Figure S4, S5). Both the catalysts have thin and smooth surfaces without any obvious Ag nanoparticles (NPs). In Raman spectra, the typical graphite D band and G band can be identified clearly at 1350 and 1580 cm-1,

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respectively (Figure S15) [43]. The XRD patterns only display a broad peak at 2θ = 26° and a tiny peak at 2θ = 44°, corresponding to the (002) and (004) plane of graphene. No diffraction peaks of Ag can be observed, indicating that Ag atoms did not

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agglomerate during the pyrolysis process (Figure S4b and Figure S5b) [44]. The

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dispersion of Ag atoms on the graphene was further confirmed by aberration-corrected HAADF-STEM in sub-angstrom resolution. As shown in Figure 2b, c, f, and g, the

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uniformly dispersed bright dots corresponding to the Ag atoms on graphene surface of

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Ag1-G and Ag2-G can be clearly observed because of the sensitive Z-contrast of heavy elements [45]. The corresponding element mapping images reveal the homogeneous

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distributions of C, N, Ag over the graphene (Figure 2d, h). Interestingly, Ag1-G presents a typical single-atom feature for Ag atoms, while in the case of Ag2-G, isolated metallic

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diatoms as shown in the region tagged by circles in Figure 2g, demonstrate the formation of dual-atom active sites in the latter. Because the HAADF-STEM images describe a two-dimensional projection of the samples along the incident beam direction, the detailed features of dual-atom Ag are somewhat different from each other in three dimensions [46]. For example, the bright double dots groups marked by red circles

corresponding to the parallel Ag2 structure and the largest distance (~0.35nm) is consistent with the Ag ‧ ‧ ‧ Ag distance in {[Ag(NO3-O)(phtz-N)]2(μ-phtz-N,N′)2}, whereas the bright dots tagged by green circles describe the dual Ag atom align with the projection [47]. To further reveal the origin of Ag1 and Ag2 site, the Ag loading amounts of the two catalysts were reduced. The HAADF-STEM images show that the Ag atoms still present as single-atom and dual-atom in Ag1-G and Ag2-G, respectively

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(Figure S6). These results confirm that the dual Ag atoms in Ag2-G catalyst and single

Ag atom in Ag1-G catalyst are derived from the starting binuclear and mononuclear Ag

complexes, respectively, rather than from aggregation or separation during preparation

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process.

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XPS analyses were conducted to detect the chemical composition and elemental state of the Ag1-G and Ag2-G catalysts. Figure S7 and S8 demonstrate the presences of Ag,

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C and N elements. The binding energies of the Ag 3d3/2 and Ag 3d5/2 peaks locate at

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374.68 eV and 368.4 eV, respectively, suggesting that the valence state of Ag is between 0 and +1 in both catalysts [48]. The C 1s spectra of the two catalysts are also

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similar and can be deconvoluted into three species: the peak at 284.75 eV for the C=C in aromatic rings; that at 285.6 eV for the sp2 hybridized C bonded with N; and the

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tertiary peak at 288.2 eV for the π-π* characteristics of the sp2 carbon network [49]. The N 1s spectrum of Ag2-G can be deconvoluted into three peaks: pyrrolic-N (400.3 eV), pyridinic-N (398.5 eV) and graphitic-N (401.5 eV) [50]. But only graphitic-N and pyridinic-N are present in Ag1-G catalyst. The absence of pyrrolic-N in Ag1-G could be ascribed to the native molecular structure of the mononuclear Ag complex precursors

which lack this type N species. These XPS results indicate that the composition and

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structure of the Ag complexes are partially inherited in the Ag1-G and Ag2-G catalysts.

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Figure 3. Electronic states of Ag atom in the Ag1-G catalyst and Ag2-G catalyst. (a) Ag K-edge XANES spectra. (inset is the expanded view of the rising edge) (b) Fourier

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transformed (FT) k2-weighted EXAFS spectra of Ag1-G, Ag2-G and Ag foil. (c) Wavelet transformed k3-weighted EXAFS (WT-EXAFS) of Ag1-G, Ag2-G and Ag foil. White dotted line indicating the position of the Ag-Ag scattering observed in Ag2-G and Ag foil. (d,e) Corresponding EXAFS R-space fitting curves for Ag1-G and Ag2-G,

respectively. The inset of (d) and (e) is the schematic model of Ag1-G and Ag2-G, respectively (Ag in green, N in blue, C in gray). To further reveal the electronic structure and coordination environment of Ag1-G and Ag2-G, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were performed. As shown in Figure 3a, the absorption-edge positions of Ag1-G and Ag2-G shift to higher energy side with respect

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to that of Ag foil, indicating that the Ag atoms in Ag1-G and Ag2-G are positively charged as the electron donors are anchored on the graphene surface. Comparing the

Ag2-G with Ag1-G, the rising-edge position of Ag2-G shifts to lower energy side (closer

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to the Ag foil) which can be attributed to the existence of Ag-Ag bonds in Ag2-G, vide

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infra. The Fourier-transformed (FT) k2-weighted EXAFS spectra (Figure 3b) show Ag K-edge for Ag1-G and Ag2-G, as well as the reference spectrum of Ag foil. The main

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peak at about 2.67 Å observed in the FT curve of Ag foil can be attributed to the Ag-

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Ag bond. Ag1-G spectrum displays two peaks at 1.75 and 2.55 Å which correspond to the Ag-N and Ag-C coordination, respectively. No peak corresponding to the Ag-Ag

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coordination can be detected. This result indicates that the Ag atoms in Ag1-G are atomically dispersed without forming Ag clusters or nanoparticles. The Ag2-G

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spectrum displays a tiny peak at 2.67 Å, which corresponds to the Ag-Ag coordination compared with Ag foil. An obvious peak exhibits at about 1.85 Å, could be attributed to the Ag-C and Ag-N coordinations. To clearly justify the atomic dispersion of Ag on graphene, the wavelet transform (WT) was performed because of the high resolution in R-space and k-space (Figure 3c)

[51]. The WT-EXAFS of Ag foil shows three prominent features centered at k~4.5, 8.5 and 10.5 Å -1, the feature at k~10.5 Å -1 is present in Ag2-G (white dotted line) while none of the features were observed in Ag1-G. This provides further evidence for the presence of Ag-Ag bonds in Ag2-G but no in Ag1-G. The quantitative coordination configuration of Ag atom in Ag1-G and Ag2-G can be obtained by EXAFS fitting in Figure 3d, 3e and the fitting parameters are listed in Table S3 and Table S4. The Ag-N

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coordination number is 4 in Ag1-G and 3 in Ag2-G, which are consistent with the

structure of the starting mononuclear and binuclear complexes, respectively. Ag-C coordination number is 2 in Ag1-G and 1 in Ag2-G, indicating the presence of strong

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interaction between Ag atoms and graphene matrix. In Ag2-G, the Ag-Ag coordination

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number is 1, suggesting that Ag atoms exist as dual-atom in Ag2-G. In addition, the coordination number in Ag foil is 12 [52], further confirmed that the atomic dispersion

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of Ag throughout the whole Ag2-G catalyst without any Ag nanoparticles. The results

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are consistent with other characterization (TEM, HADDF-STEM, XRD, XPS et. al). Therefore, the dominant reactive sites for Ag1-G and Ag2-G can be postulated as AgN4

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and AgN3-AgN3, respectively. Supportively, the density functional theory (DFT) calculations indicate that the ortho-C and para-C in graphene serve as anchors for

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stabilizing the single Ag atom and dual Ag atom through the strong coordination interaction (Figure S9). The corresponding atomic structure of the models are illustrated in the inset of Figure 3d and Figure 3e. All the above results demonstrate that the dual-atom Ag2-G catalyst has been successfully obtained. To confirm the importance of using aromatic complexes as the

metal precursors, we used silver nitrate to replace the silver complexes (denoted as AgNO3-G). It was found that the silver nitrate decomposed in annealing process and Ag atoms easily agglomerated to form Ag nanoparticles. (Figure S10-S12). This further indicates that the aromatic ligands of the silver complexes can act as steric hindrance around the Ag atoms and prevent metal atoms from agglomerating. To demonstrate the unique performance of Ag2-G on the CO2 reduction reaction, we also prepared the Ag

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NPs-G (Figure S10 and S13-14) and pure graphene for comparison.

Figure 4. Catalytic performances of the catalysts in CO2 reduction reaction. (a) LSV curves acquired in CO2-saturated electrolyte at the scan rate of 10 mV s-1. (b) LSV curves of the Ag2-G catalyst in N2- or CO2-saturated electrolyte. (c) FECO (top) and FEH2 (bottom) of the as-synthesized catalysts at various applied potentials. (d) Partial current densities of CO on the catalysts under different potentials. (e) Catalytic stability test with Ag2-G catalyst at -0.7 V in the CO2-saturated electrolyte for 36 h.

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The CO2 electroreduction performances of the catalysts were evaluated in CO2saturated 0.5 M KHCO3 electrolyte. For all the Ag-containing catalysts, only CO and H2 were detected in the gas-phase products and no liquid product could be verified by H nuclear magnetic resonance (Figure S17). The current density and onset potential

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1

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were measured from linear sweep voltammetry (LSV) to investigate the electrocatalytic activity. As illustrated in Figure 4a, the pure graphene (G) exhibits a poor activity with

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the total current density of -5.67 mA cm-2 at -1.0 V (V versus RHE) and really low

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onset potential (-0.77 V). The Ag NPs-G shows a better activity with the onset potential of -0.58 V. Remarkably, when atomically dispersed Ag atoms are present, the total

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current density and the onset potential enhance dramatically. Especially, the Ag2-G catalyst exhibits the highest total current density of -44.3 mA cm-2 at -1.0 V and the

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highest onset potential of -0.25 V, surpassing Ag1-G and other catalysts significantly (Table S5).

Beside the current density and onset potential, the selectivity is also important to assess a catalyst. In general, HER is a competing reaction with CO2 electroreduction reaction, particular in aqueous electrolytes [53]. To probe the features of

electrocatalytic selectivity, LSV in N2-saturated 0.5 M KHCO3 electrolyte was carried out. As shown in Figure S18, G and Ag NPs-G catalysts exhibit the higher reduction current density in N2-saturated electrolyte than that in CO2-saturated electrolyte, indicating the poor CO2 reduction activity compared with HER. On the contrary, both Ag1-G and Ag2-G were found favorable for CO2 reduction. As shown in Figure 4b, when tested in N2-saturated electrolyte, the onset potential of Ag2-G was decreased to

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-0.58 V and the total current density was decreased to -19.61 mA cm-2 at -1.0 V,

indicating that Ag2-G is more effective toward electrocatalytic CO2 reduction than HER. In order to quantify the selectivity of these catalysts in CO2 reduction reaction, the

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electrochemical reduction was operated in potentiostatic electrolysis at different

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potentials and the corresponding Faradic efficiency (FE) values were obtained. As shown in Figure 4c, Ag2-G possesses high selectivity for converting CO2 to CO over a

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wide potential range (-0.5 V to -0.9 V) and the FECO reaches its maximum value of

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93.4% at -0.7 V and the FEH2 is lower than 7%, indicating that the HER is suppressed. On the other hand, Ag1-G shows modest FECO (79% at -0.7 V), which is lower than

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Ag2-G but significant higher than other catalysts. The CO production turnover frequencies per Ag site were calculated to be 30 s-1 for Ag2-G and 5.1 s-1 for Ag1-G at

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the potential of -0.7 V, indicating the high intrinsic CO2 reduction activity of the former. While under the identical testing conditions, G and Ag NPs-G have no significant activity toward the CO2 reduction, thus the contribution of the G substrate and Ag nanoparticles can be ignored.

From the total current density and FECO, the partial current density of CO production could be obtained. As shown in Figure 4d, the Ag1-G and Ag2-G curves are close to their LSV curves, but the partial current density of other catalysts are close to 0, further demonstrating that the Ag nanoparticles on graphene play little contribution to CO2 reduction but hydrogen evolution at the test potentials. In addition, the Tafel slopes (Figure S19) were used to analyze the kinetics of CO2 reduction reaction with different

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catalysts. Ag2-G exhibits the lowest Tafel slope, indicating its favorable kinetics for the

conversion of CO2 to CO. The CO2 reduction stability of Ag2-G was tested at -0.7 V for 36 h (Figure 4e). The current density of about 11.8 mA cm-2 was obtained

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throughout the test without the notable decrease and the FECO could be maintained at

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about 90% after 36 h, revealing its remarkable long term stability. The used Ag2-G catalyst was characterized (Figure S20), and the results confirmed the dual-atom

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structure remained unchanged. To the best of our knowledge, such high CO2

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electroreduction performance has seldom been reported for Ag-based catalysts up to

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date (Table S6).

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Figure 5. Free energy profiles of CO2 reduction with Ag1-G and Ag2-G.

To gain insight into the activity difference of Ag1-G and Ag2-G catalysts toward CO2

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reduction, DFT calculations [54-55] were carried out to obtain the reaction pathways

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of both catalysts [56-57]. As illustrated in Figure 5, both reactions take place via similar sequential four stages: CO2 adsorption (CO2 + * → *COO), proton-coupled electron

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transfer (PCET) (*COO + H++ e- → *COOH), further PCET (*COOH + H+ + e- → *CO + H2O), and CO release and catalyst recovery (*CO → CO + *). Nevertheless,

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the energetics of the two sites are different. Comparing the optimized structures of the

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*COO, *COOH and *CO intermediates, the carbon and an oxygen of CO2 simultaneously interact with the two Ag atoms of AgN3-AgN3 sites in Ag2-G, while only the carbon of CO2 interacts with the Ag atom of AgN4 sites in Ag1-G. Consequently, the intermediates (*COO, *COOH, and *CO) of Ag2-G are much more stable than their counterparts of Ag1-G. Overall, the rate-determining barrier (0.7 eV) of Ag2-G is lower than that of Ag1-G (0.97 eV), explaining our experimental

observation that Ag2-G is a more effective catalyst than Ag1-G. Interestingly, the stronger adsorption of Ag2-G does not significantly affect the energetics of the two elementary PCET stages. The two PCET stages of Ag2-G are endergonic/exergonic by 0.70/0.74eV, which are close to those (0.74/0.71eV) of Ag1-G. Thus, the reduced ratedetermining barrier of Ag2-G mainly originates from its stronger adsorption of *COO than that of Ag1-G. Essentially, AgN3-AgN3 site improves its catalytic activity by

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converting a high barrier (0.97eV) of Ag1-G to two relatively low barriers (i.e. 0.70 and 0.66eV, respectively). Thus, the dual-atom AgN3-AgN3 site in Ag2-G is a more effective active site for CO2 reduction reaction than the single-atom AgN4 in Ag1-G. In

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order to explain the high selectivity of CO, another DFT calculations of binding energy

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towards the *COH intermediate was performed. As shown in Figure S21, *COH is an important intermediate for forming hydrocarbon according to the mechanistic pathways

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of the CO2 conversion on the Ag catalyst [58]. *COH is protonated form of *CO, and

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the energy barrier of *CO protonation directly determines the hydrocarbon production from CO2 [59]. The limiting potential of forming *COH step (2.61 eV) is much larger

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than CO release (0.16 eV) from *CO. This result indicates that hydrocarbon production

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is less favored than CO on the dual-atom Ag catalyst.

4. Conclusions In summary, we have developed a dual-atom catalyst features isolated active site comprises AgN3-AgN3 on graphene. The catalyst exhibits more excellent electrocatalytic performance than single-atom Ag catalyst and other conventional

electrocatalysts toward CO2 reduction. Combining the experimental data and the DFT results, the formation of AgN3-AgN3 sites in Ag2-G was found to promote CO2 adsorption and stabilize the intermediates, thus reducing the energy barrier for the formation of *COOH. We believe this synthesis strategy may provide a new approach to prepare effective catalysts for CO2 electroreduction reaction and also pave a new

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path for design of atomically dispersed metal atoms catalysts.

Conflicts of Interest The authors declare no conflict of interest.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Credit Author Statement

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K. B. Zhou and Y. F. Li conceived the project. Y. F. Li and C. Chen carried out this experiment and wrote this manuscript. R. Cao. carried out the XAFS and provided the analyses. Z. W. Pan and H. He participated in preparing the manuscript. K.B. Zhou was responsible for the overall direction of the project. Y. F. Li, C. Chen and R. Cao contributed equally to this work.

Acknowledgements

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This work was supported by the National Natural Science Foundation of China

(21473199), and Chinese Academy of Sciences. We thank Stanford Synchrotron Radiation Lightsource (SSRL) BL7-3 for providing the beam time. We thank Prof. ZhiXiang Wang for helping us to analyze the DFT results. R.Cao acknowledges support from DOE funded LDRD program and SSRL. K. B. Zhou and Y. F. Li conceived the

project. Y. F. Li and C. Chen carried out this experiment and wrote this manuscript. R. Cao. carried out the XAFS and provided the analyses. Z. W. Pan and H. He participated in preparing the manuscript.

K. B. Zhou was responsible for the overall direction of

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the project. Y. F. Li, C. Chen and R. Cao contributed equally to this work.

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