Insight into Electrochemical CO2 Reduction on Surface-Molecule

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Insight into Electrochemical CO2 Reduction on Surface Molecule-Mediated Ag Nanoparticles Cheonghee Kim, Taedaehyeong Eom, Michael Shincheon Jee, Hyejin Jung, Hyungjun Kim, Byoung Koun Min, and Yun Jeong Hwang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01862 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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Insight into Electrochemical CO2 Reduction on Surface Molecule-Mediated Ag Nanoparticles Cheonghee Kim,a†‡ Taedaehyeong Eom,b‡ Michael Shincheon Jee,a,c Hyejin Jung,a,d Hyungjun Kim,b* Byoung Koun Min,a,d,e* and Yun Jeong Hwanga,d* a

Clean Energy Research Center, Korea Institute of Science and Technology (KIST), Seoul

02792, Republic of Korea b

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced

Institute of Science and Technology (KAIST), Daejeon, 34141, Republic of Korea c

Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic

of Korea d

Korea University of Science and Technology, Daejeon 34113, Republic of Korea

e

Green School, Korea University, Seoul 02841, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT

Electrochemical CO2 reduction reaction to valued hydrocarbon molecules is an attractive process because it can be coupled with renewable energy resources for carbon recycling. For an efficient CO2 conversion, designing a catalyst with high activity and selectivity is crucial because CO2 reduction reaction in aqueous media competes with hydrogen evolution reaction (HER) intensely. We developed a strategy to tune CO2 reduction activity by modulating the binding energies of the intermediates on the electrocatalyst surfaces with the assistance of functional group containing molecules. We discovered that the amine functional group on Ag nanoparticle is highly effective to improve selective CO production (Faradaic efficiency to 94.2%) by selectively suppressing HER, while the thiol group rather increases HER activity. A density functional theory calculation supports that attaching amine molecules to Ag nanoparticles destabilizes H binding which effectively suppresses HER selectively, while an opposite tendency is found with thiol molecules. In addition, the product selectivity changes depending on the functional group are also observed when the organic molecules are added after nanoparticle synthesis or nanoparticles are immobilized with an amine (or thiol) containing anchoring agent. CO Faradaic efficiencies were consistently improved when Ag nanoparticle was modified with amine groups compared with that of thiol counterpart.

KEYWORDS: Silver, Nanoparticle, Electrocatalyst, CO2 Reduction Reaction, Selectivity


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INTRODUCTION The current world energy production habits still heavily depend on traditional fossil fuels which aggravate climate effects as increasing atmospheric concentration of carbon dioxide.1, 2 Fossil fuels are also used as important chemical feedstocks in the petrochemical industry to make plastics, lubricants, cosmetics, medicines, etc. To mitigate global warming and climate changes caused by these fossil fuels dependency, various renewable energy technologies have been developed to produce electricity but insufficient attention has been paid to produce chemicals in a sustainable manner.3,

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Therefore, recently, many researchers have attempted to produce

valuable carbon compounds via CO2 conversion. Among various CO2 utilization techniques, electrochemical CO2 reduction in an aqueous solution can complete a clean and sustainable carbon recycling by combining with renewable energy resources such as solar energy (i.e. solar chemical technique) as a power source.5 However, developing an efficient CO2 reduction catalysts faces challenges in resolving the large overpotential for CO2 reduction and inevitable competition with hydrogen evolution reaction (HER) in aqueous environment.6 Recently, a number of heterogeneous catalysts have been developed for electrocatalytic CO2 reduction; and catalytic activity has improved by modifying the bulk surface to nanostructures.713

Most studies suggest that the structural and morphological factors such as the prevalence of

edge sites, high index facets, nano-sized pores, or defects on the nanostructured surface contribute to selective CO2 reduction. However, not all nanostructured surfaces are reported to be efficient for CO2 reduction reaction because newly exposed surfaces may become better active sites for HER instead of CO2 reduction. For example, Lu et al. reported that nanoporous Ag was highly active for CO2 to CO production, while Ag nanoparticle or Ag nanowire showed negligible activity.8 The ambivalent characteristic of nanostructures often results in the particular


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optimal range of size or morphology, which adds to the difficulty in developing efficient catalysts. Therefore, it is required to discover a novel strategy that controls CO2 reduction activity by other factors such as chemical properties beyond the structural properties. On the gold and silver electrode promising CO2 electrocatalyst metals to produce CO,14 ionic liquids such as 1-ethyl-3methylimidazolium tetrafluoroborate (EMIM-BF4) have been used to reduce the required onset potential15 because of its CO2 capture ability through complex formation.16 Similarly, polyethylenimine was reported to be effective in stabilizing the *CO2˖-, a critical intermediate species.17, 18 With the presence of pyridinium ions in an electrolyte, copper also has exhibited improved selectivity in electrochemical CO2 reduction.19 These studies demonstrated that various organic molecules can act as cocatalysts in the electrolyte for CO2 reduction.7, 8, 12, 13 In addition, Schmitt et al. reported that 3,5-diamino-1,2,4-triazole (DAT) exposed-Ag electrode can increase the CO production selectivity by weakening the CO bonding strength compared to the untreated Ag electrode which was confirmed via in situ surface-enhanced Raman spectroscopy (SERS).20 We also recently reported specific interaction between Ag nanoparticles and the anchoring agents (Ag-S) affected the stability of the intermediate COOH resulting enhance catalytic activity.21 These studies indicate that functional groups of the organic molecules can significantly reduce overpotential or improve CO selectivity for the electrochemical CO2 reduction. However, the direct correlation between the functional groups and CO2 reduction activity of the metal nanoparticle has not yet been proven systematically. In this study, we demonstrate that small organic molecules on the metal nanoparticle surfaces can modify the chemical property and consequentially modify the catalytic activity for CO2 reduction reaction. As a model system, we investigated the effect of various functional groups on


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Ag nanoparticles for the electrochemical CO2 reduction reaction. The strategy to control the catalytic activity of nanoparticles with organic functional groups has been supported by using experimental and density functional theory (DFT) calculation results. We prepared metal nanoparticles in a wet chemistry method by using different surface capping agents to modify chemical properties of the metal nanoparticle surfaces.22, 23 Three types of Ag nanoparticles were prepared with different surface capping agents;24 oleylamine having an amine functional group, oleic acid having a carboxyl functional group, and dodecanethiol having a thiol functional group; and their activity for electrochemical CO2 reduction reaction were evaluated. The oleylaminecapped Ag nanoparticles showed the highest Faradaic efficiency (94.2 ± 1.5%) for electrochemical conversion of CO2 to CO due to the exceptional suppression of HER. On the other hand, the thiol-capped Ag nanoparticles exhibited indiscriminate increase for both HER and CO2 reduction reaction and yielded a poorer CO production selectivity. Similar trend was observed when comparing the electrochemical CO2 reduction reaction of previously reported cysteamine anchored Ag/C treated with hexylamine and hexanethiol. Furthermore, we synthesized ethylendiamine (a molecule with two amine end groups) anchored Ag/C to provide more evidence for the generic effect that functional groups on the Ag surface can tune the catalytic activities for CO2 reduction reaction and HER.


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EXPERIMENTAL SECTION Synthesis and characterization of Ag Nanoparticles on carbon support (surface capping agents: OLA, OA, DDT) Synthesis procedures for oleylamine-, oleic acid-, and dodecanethiol-capped Ag nanoparticles were modified from previously reported methods.23 For oleylamine-capped Ag nanoparticles, 170 mg of silver nitrate (AgNO3; Aldrich, 99.9999%) was dissolved in 20 mL of oleylamine (OLA; Aldrich, 70%) and 0.1 mL of oleic acid (OA; Aldrich, 90%) with vigorous stirring, and this precursor solution was heated to 180 °C for 2 hr under the nitrogen atmosphere. The oleic acid-capped Ag nanoparticles were synthesized by using 170 mg of silver nitrate, 10 mL of oleic acid, and 10 mL of ethylene glycol (EG; Aldrich, 99.8%). This dissolved solution was heated to 160 °C for 1 hr under the nitrogen atmosphere with vigorous stirring. The resulting products were cooled down at room temperature, washed with acetone, and then re-dispersed in n-hexane. The exact amount of silver in the solution was measured by inductively coupled plasma optical emission spectrometry (ICP-OES; Thermo Scientific iCAP 6300 Duo). Carbon black (Ketjen black) was dispersed in n-hexane by ultra-sonication, then 10 wt % of the synthesized Ag nanoparticles were added with stirring. The mixture was filtered and washed with additional isopropanol, and then it was dried at 50 °C in the vacuum oven. The dodecanethiol-capped Ag nanoparticles were prepared by surface ligand exchange. The OLA-capped Ag nanoparticles were dispersed in n-hexane by stirring. Then, 11.7 µL of 1-dodecanethiol (DDT; Aldrich, ≥ 98%) was added dropwise in the dispersed OLA-capped Ag nanoparticle solution followed by stirring for 1hr. Total Ag mass and dodecanethiol molar concentration ratio was 1 mg : 1 µM. Carbon black was dispersed in n-hexane by ultra-sonicating and stirring for 1 hr. The 10 wt % of DDT-capped Ag nanoparticles solution was added in the prepared carbon solution with stirring


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for 1 hr. The mixture was filtered and washed with additional isopropanol, and then it was dried at 50 °C in the vacuum oven. The prepared catalysts were analyzed by X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha), X-ray diffraction (XRD; D8 ADVANCE LynxEye), transmission electron microscopy (TEM; TECNAI F20 G2 at 200 kV), and ICP-OES.

Preparation of amine and thiol treatment with cysteamine anchored Ag/C Cysteamine anchored Ag/C (CA Ag/C, 5 nm sized Ag nanoparticle) was synthesized as reported previously.21 For amine treatment of CA Ag/C (amine treated CA Ag/C), prepared CA Ag/C was dispersed in n-hexane by ultra-sonication then hexylamine (Aldrich, 99%) was added with stirring for 1 hr. The mixture was filtered and dried. The procedure of thiol treatment of CA Ag/C (thiol treated CA Ag/C) was the same as the method above. Hexanethiol (Aldrich, 95%) was added in the dispersed CA Ag/C solution instead of hexylamine. The mixture was filtered and dried. Total Ag mass and molar concentration ratio of functional group (-NH2 and -SH) was 1 mg : 0.3 mM.

Synthesis of ethylenediamine anchored Ag/C Ethylemediamine anchored Ag/C (EDA Ag/C; Fluka, ≥ 99.5%) was synthesized by modifying previously reported method.21 Ethylenediamine was used instead of cysteamine with the same molar concentration (0.648 mM) following the same procedure. The solution was kept at 160 °C for 5 hr. The product solution was cooled down to room temperature, washed with isopropanol, filtered and dried.


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Preparation of working electrode The catalysts were dispersed into a mixture of isopropanol and Nafion solution (5 wt %, Aldrich) to prepare a catalyst ink by sonication for 30 min. The actual Ag metal amount in the solution was measured by inductively coupled plasma (ICP) elemental analysis. Glassy carbon plate (Alfa Aesar) was used as the electrode substrate after mechanically polishing and complete cleaning with deionized water. Using spray gun (Gunpiece GP-2, Japan), the catalyst ink solution was sprayed onto the glassy carbon plate which was weighted on the balance before and after spraying to measure the amounts of the Ag/C catalyst. From the ICP results, the loaded Ag metal amounts were obtained as 0.06, 0.07 and 0.07 mgAg/cm2 for OLA Ag/C, OA Ag/C DDT Ag/C, respectively. For amine treated CA Ag/C, thiol treated CA Ag/C and EDA anchored Ag/C catalysts, the mass of sprayed silver was 0.08 mgAg/cm2. The active area of the electrode was 0.5 cm2.

Electrochemical measurements Platinum and Ag/AgCl (3 M NaCl) were used as a counter electrode and a reference electrode, respectively. Electrochemical measurements were performed by using CHI 760E potentiostat in a two-compartment electrochemical cell, and Nafion 117, a proton exchange membrane, was used to separate the catholyte and the anolyte. The electrolyte solution (0.5 M KHCO3; SigmaAldrich, ≥ 99.95%) was purged with high purity CO2 (99.999%) gas for at least 1 hr, and the pH of the electrolyte was 7.3 after saturation. To obtain stable electrochemical signals, chronoamperometry was performed at -2.0 V (vs Ag/AgCl) for 10 min prior to CO2 reduction reaction

measurements.

The

CO2

reduction

reaction

were

measured

by

using

chronoamperometry at each fixed potential and gaseous products (i.e. H2 and CO) were


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quantified by a gas chromatography (GC; Younglin 6500 GC) equipped with a pulsed discharge detector (PDD). Ultra-high purity helium (He; 99.9999%) was used as the carrier gas. CO2 average flow rate (Q) was 120 cc/min measured by a universal flow meter (Agilent technologies ADM 2000) at the exit of the electrochemical cell. Following the previous reports,21, 25, 26 current efficiency (Faradaic efficiency; F.E.H₂or CO) of H2 or CO production was obtained from itotal, a measured current by the potentiostat, and the partial current (iH₂or CO) from GC chromatogram peak areas where VH₂or CO is volume concentration of H2 or CO based on calibration of the GC, F is Faradaic constant, p0 is pressure, T is room temperature, and R is ideal gas constant, 8.314 J·mol·K-1. 2 × × . . (%) = × 100 = × 100 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. The potential of the reference electrode for Ag/AgCl (3 M NaCl) is 0.21 V. E (vs RHE) = E (vs Ag/AgCl) + 0.21 V + 0.0591 V × pH To analyze formate liquid product, formate ion concentration in the electrolyte was measured by an ion chromatography (IC, DIONEX IC25A) with a conductivity detector.

Density functional theory calculations


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Density functional theory (DFT) calculations were performed using Spanish Initiative for Electronic Simulation with Thousands of Atoms (SIESTA) program,27, 28 and Perdew-BurkeErnzerhof (PBE) exchange correlation functional was used coupled with double-ζ level of basis sets.29 Only the gamma point was used for the reciprocal space sampling, because the simulation system is non-periodic particle system, and mesh cut off energy and force tolerance were set as 250 Ry and 0.05 eV Å-1, respectively, for the geometry optimization.

RESULTS AND DISCUSSION The electrochemical CO2 reduction reaction was performed using three different types of silver nanoparticles with specific surface capping agents; namely oleylamine, oleic acid, and dodecanethiol-capped Ag nanoparticles. These were synthesized24 and deposited on the carbon support, denoted as OLA Ag/C, OA Ag/C, and DDT Ag/C, respectively. Each surface capping agent represents a different functional group featuring a long carbon chain that contains a single amine, a carboxyl, or a thiol end group. Alkyl chains with amine or thiol end group such as oleylamine and dodecanethiol are well known as surface capping agents in a wet chemistry synthesis of nanoparticles.23, 30-33 The average sizes of Ag nanoparticles were controlled to be 4.88 ± 0.8 nm, 4.99 ± 1.0 nm and 4.91 ± 0.8 nm for OLA Ag/C, OA Ag/C, and DDT Ag/C, respectively (Figure S1, supporting information). Distinctive CO Faradaic efficiency dependence was shown for different types of the surface capping agents of Ag nanoparticles. Figure 1 shows the iR-corrected potential dependent CO Faradaic efficiency profiles and partial current densities of H2 and CO production measured at the steady-state current density. During the CO2 reduction reaction, the current was stable at


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constant potential conditions (Figure S2). Similar to previous reports, our silver electrocatalysts had H2 and CO as the two major gaseous products, and their summation was near 100% regardless of capping agents. The formate Faradaic efficiency as liquid products was confirmed to be less than 0.5 % for all catalysts (Table S1). In addition, our previous study confirmed that carbon support was incapable of CO2 reduction reaction and rather active for HER catalyst under high biased potential regions.21 The OLA Ag/C had the highest CO Faradaic efficiency compared to both OA Ag/C and DDT Ag/C over the entire applied potential region (Figure 1a). The maximum CO Faradaic efficiency was 94.2 ± 1.5%, 89.1 ± 1.1%, and 65.5 ± 6.7% for OLA Ag/C, OA Ag/C, and DDT Ag/C, respectively (Figure 1a). When CO Faradaic efficiencies of all samples were compared at a fixed potential of -0.75 V vs RHE (Figure 1b), Faradic efficiency of OLA Ag/C (F.E.CO = 92.6 ± 3.8%) showed the highest selectivity compared to those of OA Ag/C and DDT Ag/C (F.E.CO = 87.8 ± 1.2% and 65.2 ± 5.8%, respectively). The discrepancy in the Faradaic efficiencies widened when the applied potential was -1.0 V vs RHE. Faradaic efficiencies of OLA Ag/C (F.E.CO = 88.9 ± 1.3%) and OA Ag/C (F.E.CO = 80.9 ± 3.0%) relatively maintained while that of DDT Ag/C dropped significantly to F.E.CO = 22.7 ± 4.9%. The Faradaic efficiency of DDT Ag/C turned to be more sensitive to the applied potentials, but OLA Ag/C showed over 80% of Faradaic efficiency in the much wider ranges of applied potentials which is notably different even compared with the polycrystalline bulk Ag electrode.26


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CO Production

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Figure 1. CO Faradaic efficiency (a) depending on applied potential and (b) at fixed potentials of -0.75 and -1.0 V (vs RHE). Mass activity for (c) H2 and (d) CO production depending on applied potential. CO2 electrochemical reduction was performed in CO2-saturated 0.5 M KHCO3, the mass activity was calculated using the following equation: current density (mA/cm2) / loaded Ag amount (mg/cm2) on the electrode.

In order to discern the variance of selectivity for CO production, we compared mass activities for total current (Figure S3), HER (Figure 1c), and CO production by CO2 reduction reaction (Figure 1d), respectively, of OLA Ag/C, OA Ag/C, and DDT Ag/C electrodes. The H2 partial mass activity (Figure 1c) clearly shows that the OLA and OA Ag/C effectively suppressed the competitive HER. The HER suppression became more appreciable under more negative bias


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potentials (< -0.9 V vs RHE), where DDT Ag/C showed a sharp increase in H2 partial mass activity accompanied by decrease of CO Faradaic efficiency. Conversely, DDT Ag/C noticeably had the highest CO partial mass activity compared to the OLA Ag/C and the OA Ag/C in the lower cathodic potential region (-0.4 to -0.9 V vs RHE). These results correspond with our previous study that specific interaction between Ag nanoparticles and the thiol (Ag-S) influences the stabilization of the COOH intermediate resulting in a decrease in CO2 reduction overpotential.21 However, the CO partial mass activity significantly decreased under more cathodic potential region with DDT Ag/C as shown in Figure 1d. The concave shape of the potential dependent CO mass activity exhibited by DDT Ag/C implies that the eventual poor activity for CO2 reduction reaction is due to the CO production active sites turning to be more active for hydrogen production under largely cathodic potentials beyond the mass transport limitation of CO2. On the other hand, OLA Ag/C maintains a high CO production mass activity even under higher negative bias potentials. The amine group has been suggested to have an ability to weaken the CO bond on the Ag surface resulting in improvement of CO production activity because weaker binding of CO would more readily release the product from the catalyst surface.17, 20 The outstanding CO selectivity of OLA Ag/C from CO2 conversion is due to the suppression of HER and suitable activity of CO2 reduction reaction. In addition, the CO2 reduction reaction activities of the previously reported silver-based electrocatalysts are summarized in Table S2. Amine modified Ag/C showed concurrently high CO Faradaic efficiency and relatively high mass activity. The Ag nanoparticles with specific surface capping agents were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD patterns confirmed the face-centered cubic Ag crystal structure for all of the three different types of Ag nanoparticles


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(Figure S4). X-ray photoelectron spectroscopy (XPS) of the carbon support, OLA Ag/C, OA Ag/C and DDT Ag/C show notable differences depending on the functional group of the capping agent (Figure S5). Figure S5a shows the presence of a N 1s peak for the OLA Ag/C associated with the amine group of OLA. However, N peak did not appear clearly because limited N atoms are coordinated with Ag atoms which act stabilizers to prevent aggregation of nanoparticles where the relative difference in the quantities for Ag and N atoms yielded a low N peak.32 The DDT Ag/C also show a slight N 1s peak possibly due to the residual OLA since DDT Ag/C was derived by ligand exchange of OLA Ag. Meanwhile, OA Ag/C and carbon black do not have any N 1s spectra further confirming that the characteristic N 1s peak originates from OLA. In addition, only DDT Ag/C shows two distinct S 2p peaks (Figure S5b) where one results from the interaction between the dodecanethiol and the Ag surface forming a Ag-S bond yielding a binding energy signal at 163 eV.24 The other S 2p signal at 169 eV is attributed to the S in Nafion, a component of electrode preparation, and is also present for carbon black, OLA Ag/C, and OA Ag/C. Additionally, XPS was performed on the Ag nanoparticles after CO2 reduction reaction (Figure S6) which showed that the Ag surfaces still retained their functional groups even after CO2 reduction reaction. In addition, the concentrations of Ag ion in the electrolytes were analyzed by ICP pre- and post-CO2 reduction for 5 hr. No Ag ions were detected (Table S3) in the electrolyte indicating Ag catalysts were not leached out during CO2 reduction reaction. To investigate the effect of amine and thiol functional groups on the CO2 reduction activity further, we prepared cysteamine anchored Ag nanoparticle/C (denoted as CA Ag/C), a previously reported catalysts having a high efficiency for electrochemical CO2 reduction reaction,12 that has undergone a post-treatment of amine (denoted as amine treated CA Ag/C) and thiol (denoted as thiol treated CA Ag/C) containing molecules. For the treatment, we used


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hexylamine and hexanethiol, respectively, to have the same length of carbon chain so that the effect of functional group would be more obvious. Figure 2a shows the applied potential dependent CO Faradaic efficiency for CA Ag/C, amine treated CA Ag/C, and thiol treated CA Ag/C. The amine treated CA Ag/C had an improved CO Faradaic efficiency, where the maximum CO Faradaic efficiency of 87.1 ± 0.6% was at the smallest overpotential (540 mV) in comparison to that of the untreated CA Ag/C (maximum FEco = 78.2 ± 6.2% at 620 mV) and thiol treated CA Ag/C (maximum FEco = 76.8 ± 4.3% at 620 mV). The total, H2, and CO mass activity are shown in Figure S7 for CA Ag/C, amine treated CA Ag/C, and thiol treated CA Ag/C. Both the amine treated CA Ag/C and thiol treated CA Ag/C electrodes showed slightly improved total mass activities compared to CA Ag/C. Especially, improvement in CO production current was appreciable for amine or thiol treated CA Ag/C. However, an increase of HER activity was only observed with the thiol treated CA Ag/C while the difference was negligible with the amine treated CA Ag/C. Although cysteamine already has both thiol and amine functional groups at each carbon atom, the additional treatment of amine molecules still showed improvement in selective CO production. Meanwhile, the additional treatment with thiol molecules showed undistinguishing improvement for both of HER and CO production reaction, adversely affecting the CO Faradaic efficiency despite of decrease in overpotential for CO production. This experiment confirmed that amine is an effective functional group to achieve high selectivity for CO2 reduction reaction over HER.


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100

a 80 60 40 CA Ag/C Amine treated CA Ag/C Thiol treated CA Ag/C

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c

-1.0

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Potential (E, V vs RHE) -0.75 V vs RHE -1.0 V vs RHE

80 60 40 20 0 CA Ag/C Amine treated CA Ag/C EDA Ag/C Thiol treated CA Ag/C

Figure 2. (a) CO Faradaic efficiency depending on applied potential for CA Ag/C, amine treated CA Ag/C, thiol treated CA Ag/C, (b) CO Faradaic efficiency depending on applied potential for CA Ag/C and EDA Ag/C, and (c) CO Faradaic efficiency at fixed potential of -0.70 V and -1.0 V (vs RHE).


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Furthermore, to test the tendency of amine molecule’s ability to improve CO selectivity, we developed a synthesis method of Ag nanoparticle on the carbon support immobilized by ethylenediamine as an anchoring agent (denoted as EDA Ag/C, Figure S8a) instead of cysteamine. Ethylenediamine is nearly identical to cysteamine except the thiol is replaced by another amine to have two amine end groups. EDA Ag/C had effective HER suppression (Figure S8b and Figure S8c) and improved CO mass activity above -0.87 V vs RHE. (Figure S8d) compared to the CA Ag/C. While the Faradaic efficiency of CA Ag/C and EDA Ag/C was similar in the low cathodic potential region (-0.3 to -0.65 V vs RHE), EDA Ag/C showed an improved selectivity for CO production in higher biased potential region as a result (Figure 2b). Figure 2c shows that CO Faradaic efficiencies of all samples were compared at fixed potentials of -0.75 V and -1.0 V vs RHE. Especially, even under -1.0 V vs RHE, EDA Ag/C samples maintained high Faradaic efficiency (FEco = 85.9 ± 3.1%) unlike the CA anchored Ag/C nanoparticles (FEco = 31.2 ± 11.1%). All the experimental results conclude that the amine functional group (represented by the amine treated CA Ag/C and EDA Ag/C) generally has higher selectivity of CO production than the thiol functional group. Using density functional theory (DFT) calculations, we added further insight on the distinct effect of amine and thiol functional groups on the electrochemical catalytic activity of Ag nanoparticles. We modelled the Ag nanoparticle using a cuboctahedral shape of Ag147. The corner sites of Ag147 nanoparticle are the most undercoordinated sites having 5 nearest neighbor atoms. This leads the binding energy of Ag to the corner site to be 0.3-0.5 eV smaller than that to the other sites such as an edge and a facet site (Table S4). We thus conceived that the substitution of a corner site with a ligand molecule requires the smallest energetic cost.


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Figure 3. (a) Atomistic structure of Ag nanoparticle (Ag NP) model used in DFT calculations. Binding energies were calculated at different active sites (labeled from 1 to 6) covering the corner, edge, and facet sites. (b) Volcano plot of HER, shown as a function of H binding free energy (∆GB,H). HER activities of nanoparticles are calculated at the most active site of 2, 2, and 3, for HER of Ag nanoparticle (Ag NP), thiol capped Ag NP, and amine capped Ag NP, respectively. (c) Volcano plot of CO2 reduction reaction, shown as a function of COOH binding free energy (∆GB,COOH). CO2 reduction reaction activities of nanoparticles are calculated at the most active site of 2, 1, and 5, for CO2 reduction reaction of Ag NP, thiol capped Ag NP, and amine capped Ag NP, respectively. Dashed lines show the activities of each elementary step. (Gray, dark-gray, and white spheres in (a) represent for Ag, C, and H, respectively, and yellow spheres in (a) represent for S or NH)

To investigate the effect of capping agents, we substituted four out of twelve corner Ag atoms of our model of Ag nanoparticle with either propylamine or propanethiol as singly deprotonated species, i.e. yielding Ag143(C3H7NH)4 or Ag143(C3H7S)4, respectively. We then calculated the binding energies of H, COOH, and CO on various catalytic active sites with different coordination number (CN); one site with CN=5 on the corner (1 of Figure 3a), two sites with CN=7 on the edge (2 and 3 of Figure 3a), two sites with CN=8 on the (100) facet (4 and 5 of Figure 3a), and one site with CN=9 on the (111) facet (6 of Figure 3a). Site specific H, COOH and CO binding energies (∆EB) on these sites (Table S5 and Table S6) were then converted into


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the free energy quantities (∆GB) using pre-established linear relationship between ∆EB and ∆GB, which were used to calculate the reaction free energy (∆G°(R)) of each elementary step of HER or CO2 reduction reaction and determine the calculated activity (see supporting information for details).34, 35 Figure 3b and 3c show the volcano plots for HER and CO2 reduction plotted as a function of ∆GB,H and ∆GB,COOH, respectively. Since the activity of CO2 reduction is dependent both on ∆GB,COOH and ∆GB,CO, we used the scaling relation between ∆GB,COOH and ∆GB,CO (Figure S12) to represent two-dimensional volcano plot into one-dimensionally. The plot shows that the ∆GB,H of Ag nanoparticle is enhanced compared with ∆GB,H of the Ag (111) slab at the “weak binding side (right slope)” (labelled as Ag on Figure 3b), which is effective in improving the HER activity of Ag nanoparticle. Such tendency of increasing ∆GB,H becomes more pronounced when the Ag nanoparticle is capped with thiols, further improving the HER activity by climbing up the volcano at the right slope. When the Ag nanoparticle is capped with amines, however, ∆GB,H is surprisingly weakened, and becomes even comparable with ∆GB,H of the Ag (111) slab by climbing down the volcano at the right slope, as bulk silver is a well-known for having a suppressed HER activity. For the case of CO2 reduction, similar to HER, nanosizing effect substantially increases ∆GB,COOH, which is even further enhanced by capping the Ag nanoparticle with thiol groups, and thereby improves CO production (climbing up the volcano at the right slope). However, unlike HER, capping the Ag nanoparticle with amine groups marginally deteriorates the COOH binding affinity, leading to a similar CO2 reduction activity to the uncapped. This shows the selective improvement of CO Faradaic efficiency without elevating HER activity for the amine capped Ag nanoparticles, which is in stark contrast to the cases of Ag nanoparticle or thiol capped Ag


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nanoparticle showing substantially improved HER activity and diminishing CO Faradaic efficiency. This clearly supports our experimental observation that OLA Ag/C had higher CO faradaic efficiency than DDT Ag/C about 30%, which is originated by the successfully suppressed H2 partial mass activity for OLA Ag/C than that for DDT Ag/C in the all applied potential range (Figure 1c) although DDT Ag/C shows higher CO partial mass activity than OLA Ag/C in the low applied potential region (< -0.9 V vs RHE; Figure 1d).

CONCLUSIONS In summary, the electrochemical CO2 reduction activity and CO product selectivity can be modulated on the Ag nanoparticle electrocatalysts by introducing functional groups containing molecules since the binding energies of the reaction intermediate species can be influenced by the type of the functional groups (i.e. amine, carboxyl, and thiol). The amine-capped Ag/C showed superior selectivity as 94.2% of CO Faradaic efficiency due to effective suppression of HER and suitable activity of CO2 reduction reaction. DFT calculations consistently suggest that the amine-capped Ag nanoparticles stabilize COOH intermediate while destabilizing H to be an effective electrocatalyst for selective CO2 reduction. On the other hand, the thiol-capped Ag nanoparticles exhibited increases in both HER and CO2 reduction reaction rate, unfavorably disposed towards HER catalyst compared with the amine or carboxyl-capped Ag nanoparticles by indiscriminately increasing both of ∆GB,H and ∆GB,COOH. The trends of the functional group effect were further verified by testing electrochemical CO2 reduction activity with Ag nanoparticles after treating amine or thiol molecules. In addition, ethylenediamine showed the higher selectivity for CO production compared with cysteamine when it was used as an


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anchoring agent to immobilize Ag nanoparticle on the carbon support. These series of experiments strengthen the strategy to develop efficient CO2 electrocatalysts by modifying the catalyst surface with organic functional groups, specifically amine-containing molecules. In short, we demonstrate that catalytic activity and selectivity for electrochemical CO2 reduction can be modulated by judicious choice of the molecules containing functional groups.

ASSOCIATED CONTENT Supporting Information. XRD data, XPS spectra, further electrocatalytic CO2 reduction data, and additional computational details. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * H. Kim: [email protected] * B. K. Min: [email protected] * Y. J. Hwang: [email protected] Present Addresses † The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division, Technical University Berlin, Berlin 10623, Germany Author Contributions ‡ C.K. and T.E. contributed equally to this work.


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the program of the Korea Institute of Science and Technology (KIST) and University-Institute cooperation program of the National Research Foundation of Korea, funded by the Korean Government (MSIP). REFERENCES (1) Turner, J. A. Science 1999, 285, 687-689. (2) Goeppert, A.; Czaun, M.; May, R. B.; Prakash, G. K. S.; Olah, G. A.; Narayanan, S. R. J. Am. Chem. Soc. 2011, 133, 20164-20167. (3) Centi, G.; Quadrelli, E. A.; Perathoner, S. Energy Environ. Sci. 2013, 6, 1711-1731. (4) Graves, C.; Ebbesen, S. D.; Mogensen, M.; Lackner, K. S. Renew. Sustain. Energy Rev. 2011, 15, 1-23. (5) Mikkelsen, M.; Jorgensen, M.; Krebs, F. C. Energy Environ. Sci. 2010, 3, 43-81. (6) Whipple, D. T.; Kenis, P. J. A. J. Phys. Chem. Lett. 2010, 1, 3451-3458. (7) Chen, Y.; Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 134, 19969-19972. (8) Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G.; Jiao, F. Nat. Commun. 2014, 5, 3242-3247.


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TOC


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Insight into Electrochemical CO2 Reduction on Surface-Molecule

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