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Rational Design of Silver Sulfide Nanowires for Efficient CO2 Electroreduction in Ionic Liquid Subiao Liu, Hongbiao Tao, Qi Liu, Zhenghe Xu, Qingxia Liu, and Jing-Li Luo ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03619 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Rational Design of Silver Sulfide Nanowires for Efficient CO2 Electroreduction in Ionic Liquid Subiao Liu, Hongbiao Tao, Qi Liu, Zhenghe Xu, Qingxia Liu, Jing-Li Luo∗

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada Corresponding authors: Jing-Li Luo Telephone: +1 780 492 2232 E-mail: [email protected] Abstract

Electroreduction of CO2 holds the promise for the utilization of CO2 and the storage of intermittent renewable energy. The development of efficient catalysts for effectively converting CO2 to fuels has never been more imperative. Herein, we successfully synthesized Ag2S nanowires (NWs) dominating at the facet of (121) using a modified facile one-step method and utilized them as a catalyst for electrochemical CO2 reduction reaction (CO2RR). Ag2S NWs in IL possess a partial current density of 12.37 mA cm-2, ~14- and ~17.5-fold higher than those of Ag2S NWs and bulk Ag in KHCO3, respectively. Moreover, it shows significantly higher selectivity with a value of 92.0% at the overpotential (η) of − 0.754 V. More importantly, the CO formation begins at a low η of 54 mV. The good performance originates from not only the presence of [EMIM − CO ] complexes, but also the specific facet contribution. The partial density of states (PDOS) and work functions

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reveal that the d band center of the surface Ag atom of Ag2S(121) is closer to the Fermi energy level and has a higher d-electron density than those of Ag(111) and Ag55, which lowers transition state energy for CO2RR. Besides, density functional theory (DFT) calculations indicate that the COOH ∗ formation over Ag2S is energetically more favorable on (111) and (121) facets than that on Ag(111) and Ag55. Therefore, we conclude that the significantly enhanced performance of Ag2S NWs in IL synergistically originates from the solvent-assisted and specific facetpromoted contributions. This distinguishes Ag2S NWs in IL as an attractive and selective platform for CO2RR.

Key words: carbon dioxide reduction, sulfide, ionic liquid, nanowires, syngas, facet effect

1. Introduction Growing concerns on global warming and environmental issues arising from the anthropogenic CO2 emissions have prompted the search of environmentally friendly technologies to convert CO2 back to fuels in a sustainable manner.1,2 Electrochemical CO2 reduction reaction (CO2RR) has been considered to be an attractive strategy to produce high-value fuels (e.g. CO, CH4, etc.) and lower global carbon footprint, since it effectively incorporates the utilization of anthropogenic CO2 and the storage of intermittent renewable energy.3,4 However, CO2RR usually suffers from sluggish kinetics because of the coupled multiple proton and electron transfer processes and the inevitable hydrogen evolution reaction (HER),5,6 particularly in aqueous media. These consequently cause the poor catalytic activity (partial current density, j), high overpotential (η) and uncontrollable selectivity (Faraday efficiency, FE) towards the aimed fuels.


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In response, various materials, such as transition metal oxide and dichalcogenide (e.g., SnO27,8 and MoS29), heteroatom-doped carbon materials (e.g., N-doped carbon nanotube10 and nanofiber11) and metal organic complexes (e.g., molecular Cu6 and Re4 catalysts), have been demonstrated to be electrochemically active for CO2RR. Of all the catalysts investigated to date, metals (e.g., Au,12 Ag13,14 and Cu15), particularly the well-designed nanostructured architectures, have significantly enhanced the catalytic activity and selectivity for CO2RR. Through experimentation and computation, many studies suggest that certain structural and morphological features, such as specific facets,16 grain boundaries17 and edge-to-corner ratios,13 can selectively improve the electrochemical performance of CO2RR since the binding energies of reaction intermediates on these featured sites are relatively weaker than that of H* originated from the competitive HER. However, the high η derived from the weak binding interactions between the reaction intermediates and catalyst, and the low exchange current densities resulted from the slow electron transfer kinetics depend not only on the intrinsic electronic properties and structure of the developed material,7,18 but also on the employed solvent. Organic solvents, such as imidazoliumbased ionic liquid (IL)19,20 and acetonitrile,21,22 serving as co-catalysts have shown nontrivial effect upon the improvement of catalytic activity. Compared with aqueous media, organic solvents, particularly imidazolium-based IL, considerably promote CO2RR by lowering the energy barrier for electron transfer, increasing the CO2 solubility, stabilizing the intermediate species and hampering the competitive HER.5,9,20-22 The utilization of organic solvent for CO2RR can facilitate CO2 conversion to fuels even on some inexpensive earth-abundant metal catalysts (e.g., Bi,21 Sn and Pb22). Despite the remarkable progress in CO2RR studies, steering the reaction pathway towards desirable fuel and further lowering η still remain as a great


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challenge, which necessitates the exploration of better catalyst with higher selectivity and catalytic activity. Silver sulfide (Ag2S), as a relatively abundant material, possesses many intriguing properties, including good chemical stability and unusual solid state duel electronic and ionic conductivities at room temperature.23-25 However, there have been limited experimental and computational studies on well-designed sulfide (e.g. Ag2S) architectures for electrochemical CO2RR. In this study, nanostructured Ag2S nanowires (NWs) were synthesized using a modified facile one-step method. The as-prepared Ag2S NWs were fabricated as catalysts for CO2RR in 1-ethyl-3-methylimidazoliumtetrafluoroborate (EMIM-BF4). It was found that Ag2S NWs were highly active for CO2RR towards CO formation in IL at a low η as compared to the one in aqueous media. DFT calculations were performed to further rationalize the significantly improved catalytic activity and selectivity of Ag2S NWs towards CO2RR. 2. Results and Discussion 2.1 Characterizations of the as-synthesized Ag2S NWs. The Ag2S NWs were synthesized by simply mixing AgNO3 and mercaptoacetic acid solution, followed by shaking the mixture for ~1 min and waiting for another ~20 min at room temperature (see Supporting Information for detailed procedures).26 The scanning electron microscopy (SEM) analysis indicates formation of the abundant, highly purified and well-aligned Ag2S NWs with diameters in the range of 150~200 nm and lengths up to several micrometers (Figures 1a and b), as also confirmed by the atomic force microscope measurements (AFM, Figure 1c). The high-resolution transmission electron microscopy (HRTEM) image (Figure 1d) and the electron energy-loss

spectroscopy

(EELS)

elemental

mapping


show

homogeneous

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distributions of Ag and S in the Ag2S NWs, which further verifies the successful preparation of Ag2S NWs. Moreover, the TEM image (Figure 2a) and the corresponding fast Fourier transformation pattern (FFT, Figure 2b) demonstrate that the as-obtained Ag2S NWs are well crystallized, where the lattice fringes with an interplanar distance of ~0.2444 nm are perpendicular to the NW axis. The fringe spacing corresponds to the (121) plane of the monoclinic Ag2S crystal phase, suggesting that the growth direction of Ag2S NWs is along [121]. To further determine the structure of the Ag2S NWs, X-ray diffraction (XRD) was conducted and the Bragg reflections indicate the formation of monoclinic a-phase Ag2S (JCPDS 14-0072) of the synthesized sample (Figure S2). Furthermore, the diffraction peak indexed as (121) is comparably more intense than the others, as confirmed by the previous studies.24 This demonstrates that the as-prepared Ag2S NWs are well oriented and consistent with the analysis of HRTEM. In addition, the as-obtained Ag2S NWs possess a surface area of ~26.2 m2 g-1 and numerous mesopores with the size of ~1.6 nm (Figure 2c), both of which ensure the substantial number of active sites for CO2RR. 2.2 Electrocatalytic activity of Ag2S NWs for CO2RR. The electrochemical CO2RR activities of Ag2S NWs in both aqueous (0.1 M KHCO3) and IL media were examined on a glassy carbon electrode (GCE, 0.785 cm2) (Note: the IL medium is an electrolyte containing 50 vol% of EMIM-BF4 and 50 vol% of deionized water, which contributes to the maximum CO2RR activity9). All the electrochemical measurements were carried out in a custom-built two-compartment cell separated by Nafion 117 membrane. The polarization curves of all the samples were obtained in a sweeping potential range of −0.144 ~ −1.156 V vs. reversible hydrogen electrode (RHE, all reported potentials are based on RHE) for aqueous


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media and −0.136 ~ −1.164 V for IL media at a scan rate of 50 mV s-1. Figure 3a shows the linear sweep voltammetry (LSV) results of Ag2S NWs in both KHCO3 and IL media with bulk Ag in KHCO3 and IL as a representative noble-metal catalyst and GCE in KHCO3 as a reference to differentiate its own contribution to CO2RR. The Ag2S NWs in IL show roughly 4-fold higher current densities than that in aqueous media (6.9 mA cm-2, normalized by geometrical surface area), and ~2.0 times higher than bulk Ag in IL. More importantly, the CO2RR of Ag2S NWs in IL occurred at a much more positive onset potential as compared to the Ag2S NWs in KHCO3 and bulk Ag in IL (see the inset in Figure 3a). The significant increase in the current density for Ag2S NWs in IL is an indication of a remarkably solvent-assisted cathodic kinetics for CO2RR since the IL enables the lowering of the energy barrier for the electron transfer and the increase in the CO2 solubility. However, the LSV results are normally an inconclusive evidence to verify the high catalytic activity of Ag2S NWs towards CO2RR since HER and CO2RR are often simultaneously interconnected. To further distinguish the occurrence of preferred CO2RR other than HER and the highly solvent-assisted CO2RR activity of Ag2S NWs in IL, the catalyst was further investigated under various applied potentials (Figure S4). The accurate compositions of the products at each potential were analyzed by a gas chromatography (GC) and a 1H nuclear magnetic resonance (NMR). Figure 3b shows the overall FEs of the products as a function of potential. It indicates that CO, H2 and CH4 are the products with a combined FE of ~100% over the entire potential range and no other products were detected by NMR or GC. As can be seen, this system is highly selective towards CO2RR for CO formation at the potentials between −0.564 and −1.164 V, where a maximum FE of ~92% towards CO at a potential of − 0.864 V was obtained. However, at the potentials lower than − 0.564 V, a mixture of CO and H2 was


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produced due to the gradually enhanced HER competition. In comparison, Ag2S NWs, bulk Ag and GCE in KHCO3, together with bulk Ag in IL, were also tested under the same experimental conditions, and the corresponding FEs for CO formation are shown in Figure 3c. As a substrate and reference, bare GCE, despite its weak current densities, achieved a maximum FE of 6.7% for CO formation at the potential of −1.056 V (Figure 3c). Concurrently, Ag2S NWs and bulk Ag in KHCO3 show lower FEs in the applied potential range and reached their maximum values of 41.7% and 57.2% at the potentials of −0.756 and −1.056 V, respectively, as compared to Ag2S NWs in IL (92.0% at −0.864 V). More importantly, the onset potential for Ag2S NWs in IL was observed at −0.164 V (also confirmed by the LSV results in Figure 3a), indicating an η of ~54 mV over the equilibrium potential of − 0.11 V. This η corresponds to a FE of ~2.4% for Ag2S NWs in IL, whereas the Ag2S NWs in KHCO3 and bulk Ag in both media did not proceed to CO2RR at this η. To achieve an equivalent FE (~2.4%) for the formation of CO, Ag2S NWs in KHCO3 and bulk Ag in IL require a remarkably higher η of 246 mV (potential of −0.456 V) and 354 mV (potential of − 0.464 V), respectively. Figure 3d shows the CO partial current densities of Ag2S NWs and bulk Ag in both media on the basis of steady-state current densities and CO FEs at various potentials. It clearly demonstrates the exclusive catalytic activities of all samples towards CO formation during CO2RR. Ag2S NWs in IL possess a partial current density of 12.37 mA cm-2, which is ~14- and ~17.5-fold higher than those of Ag2S NWs (0.89 mA cm-2) and bulk Ag (0.71 mA cm-2) in KHCO3 at the potential of −1.156 V. The CO partial current density difference of bulk Ag in KHCO3 (0.71 mA cm-2) and IL (5.17 mA cm-2) confirms, beyond any doubt, the solvent-assisted effect for CO2RR to CO.


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2.3 Kinetics analysis of Ag2S NWs for CO2RR. To gain better kinetic insights into Ag2S NWs for CO2RR in both solvents, the CO partial current densities at various overpotentials were calculated and an ECSAcorrected Tafel plot is shown in Figure 4a. It has been identified that the singleelectron transfer (i.e. CO + e → CO∙ ) determines the reaction rate for CO2RR since it strides over a higher energy barrier than other elementary steps.27 As compared with the Tafel slopes of 150 mV dec-1 for Ag2S NWs in KHCO3 and 137 mV dec-1 for bulk Ag in IL, a much lower Tafel slope of 115 mV dec-1 was obtained on Ag2S NWs in IL, slightly less than the theoretical value of 118 mV dec-1 expected for the ratedetermining single-electron transfer at the electrode,28 implying a faster kinetics for the formation of the adsorbed CO∙ intermediate. The exchange current density ( ) derived from Tafel plot (Figure 4b), a reflection of intrinsic rate of electron transfer as well as a measure of the energy required for CO2RR to CO, is 2.2 × 10 mA cm-2 for Ag2S NWs in IL, which is 7- and 3.1-fold higher than that for bulk Ag (3.1 × 10 mA cm-2) and Ag2S NWs ( 6.9 × 10 mA cm-2) in KHCO3, respectively. Additionally, as an evaluation parameter of catalyst in renewable energy storage, the maximum values of energy efficiency (EE) of Ag2S NWs in IL, Ag2S NWs and bulk Ag in KHCO3 were calculated and the results are shown in Figure 4b. The high FE for the desired product (i.e., CO) and low η of Ag2S NWs in IL jointly contribute to an EE of ~58.5%, considerably higher than Ag2S NWs in KHCO3 (~27.9%), bulk Ag in KHCO3 (~33.2%) and IL (~50.1%). These evidences make Ag2S NWs in IL stand out among the state-of-the-art catalysts and support the claim that it is a promising system towards CO2RR for CO formation. Moreover, Ag2S NWs in IL exhibit significantly higher CO mass activities [e.g., 2.37 to 19.28 A g-1 in Figure 4c, than that of Ag2S NWs (0.09 to 1.65 A g-1) in KHCO3 and bulk Ag (0.17 to 0.91 A g-1)] in IL at the


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selected potential range of −0.56 to −0.96 V. To evaluate the stability of Ag2S NW, potentiostatic measurements at fixed potentials over bulk Ag and Ag2S NW were carried out over an extended period of 19 h in both IL and KHCO3 media (Figure 4d). The Ag2S NWs in KHCO3 showed negligible decay in the steady-state current density during the testing period, suggesting its comparable stability relative to Ag, the noble metal representative. In contrast, the current density of Ag2S NWs in IL fluctuated slightly throughout the durability test, which might be caused by the small disturbance of the stirring electrolyte on the large current response. However, the Ag2S NWs in IL showed no morphological change after the stability test (Figures 1a and S5), suggesting that the Ag2S NWs in IL are stable for CO2RR. The X-ray photoelectron spectroscopy (XPS) results (Table S2, Figures S6a and b), XRD patterns (Figure S6c) and TEM analysis (Figure S6d) of this catalyst before and after electrolysis further verified its good long-term stability. 2.4 The origins for the enhanced CO2RR performance. Understanding the reasons credited for the enhanced CO2RR performance of Ag2S NWs in IL (e.g., low onset potential and high CO selectivity at a lowered η) is of prime importance in discovering the mechanism and designing desirable materials. It has been demonstrated, as mentioned earlier, that the presence of [EMIM] cations in CO2-saturated media can form the [EMIM − CO ] complexes29 which could physically adsorb on the negatively charged catalyst and subsequently increase the solubility of CO2 and the probability of substantial close collision of CO2 with catalyst.20,29 Furthermore, the [EMIM − CO ] complexes significantly lower the energy barrier of electron-transfer process, which consequently reduce the η towards CO2RR.30 However, we conjecture that the significantly enhanced CO2RR performance originates not only from the solvent-assisted contribution, but also the


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specific facet promotion. To confirm this, construction of the Gibbs free energy diagram through DFT calculations was performed based on the computational hydrogen electrode model (Figure S11).31 Figures 5a and S7 show the calculated Gibbs free energy (∆G) diagram for CO2RR and HER on the facets of (111) and (121) for Ag2S, versus (111) and 55-cluster for Ag, respectively (see Table S3 for the adsorption energies of H*, COOH*, CO* on all surfaces). On Ag(111) and Ag55 cluster, the formation of COOH* is considerably endergonic and therefore determines the reaction rate. It should be noted that the Ag55 cluster requires less energy than Ag(111) to form COOH* due to the presence of undercoordinated Ag atoms, as confirmed by other studies.9,32 This explains the higher η of bulk Ag as compared to that of Ag nanoparticles. However, the COOH* formation over metallic edges of Ag2S is energetically more favorable because of the strong binding on the facets of (111) and (121), suggesting a higher catalytic activity for CO2RR after the doping of S, as confirmed by the group of Georgios that modifying a transition metal surface with Se or S affects the binding of the reaction intermediates to the catalyst surface.33 Importantly, the TEM image (Figure 2a) and the corresponding FFT pattern (Figure 2b) have already demonstrated that the as-prepared Ag2S NWs predominantly grew at the direction of (121), where a lowest activation energy is required for the COOH* formation. It is also found that the incorporation of S not only considerably reduces the quantity of silver and the overall cost of catalyst, but also can achieve even higher catalytic activity for CO2RR as compared to the noble metal representative Ag alone. Moreover, different from pure Ag, it exclusively enjoys any perfect cleavage planes after the incorporation of S.34 The dominating planes of (121) and (111) show threedimensionally complicated surface structures, which consequently renders these planes more active sites for intermediate adsorption than those of transition metals


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since intermediates bind to both the metallic site (Ag) and the covalent S. The combined effect of the two binding sites might result in a deviation from the transition-metal scaling relation.35 Moreover, different from CO* in our models, both C and O (the O atom not connected with H) of COOH* (–OC–OH*) bind with surface silver atoms and the underlying surfaces undergo certain reconstructions. Therefore, the enhanced catalytic activity for CO2RR beyond these existing transition metal surfaces requires breaking the linear scaling relation between the binding energies of CO* and COOH* during CO2RR. As confirmed by the group of Nørskov,36 they proposed to break the scaling relationship through constructing three dimensional active sites over nanostructures, where certain intermediate can be selectively formed relative to others via binding larger intermediates on multiple sites. All these factors synergistically cause a stronger mutual interaction between COOH* and the silver sulfide slab as compared to the CO* (see Supporting Information for more details). The energetic structure ordering suggests that the CO2RR towards CO formation is kinetically more favorable on the Ag2S NWs than on Ag. In addition, mechanistic insight into the two-electron CO2RR to CO on Ag2S NWs was explored, as shown in Scheme 1. Kinetic analysis supports a mechanism that involves an initial rate-determining one e transfer to CO2 to form adsorbed CO∙ after the formation of [EMIM − CO ] complex, followed by a H transfer with HCO serving as the H donor. Subsequent relative fast steps involving additional e and H transfers complete the CO2RR to CO. Concurrently, transition metalterminated edges of dichalcogenide are demonstrated to have a higher d-electron density and a similarly metallic property since d-band center of the surface metal atoms is closer to the Fermi energy level relative to those of bulk transition metal.29 More importantly, the energy of the d states (the d-band center) relative to the Fermi


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level is a good indicator of the bond strength, and the d-band model has been proven particularly useful in understanding bond formation and trends in reactivity.37,38 The higher the d states are in energy (relative to the Fermi energy level), the higher in energy the antibonding states are, and as a result, the stronger the bond (lower binding energy).38 Therefore, we calculated the partial density of states (PDOS) of the surface Ag atoms of Ag and Ag2S, as shown in Table S4, Figures 5b and c. The results verify that d-band center of the surface Ag atom of Ag2S(121) is closer to the Fermi energy level and shows a higher d-electron density than those of Ag(111) and Ag55. This lowers transition state energy for CO2RR over Ag2S NWs and subsequently enhances the electrocatalytic activity. In addition, the work functions of bulk Ag and Ag2S NWs were experimentally measured by ultraviolet photoelectron spectroscopy (UPS) (Figure S8). The experimental results show almost the same work functions of bulk Ag and Ag2S NWs, which in turn confirms the similarly metallic property of Ag2S NWs. Since the Ag2S NWs grow along the facet of [121], DFT simulations were also computationally conducted to distinguish its work function (Figure 5d). It has been demonstrated that the work function correlates with the experimental activity as measured by current density.9 In this case, Ag2S(121) has the lowest work function, suggesting its fast electron transfer properties and subsequently enhanced CO2RR catalytic activity. Both the UPS measurements and DFT simulations as well as the confirm the good electronic properties of Ag2S NWs in comparison to bulk Ag, because Ag2S NWs contribute to a faster electron transfer and consequently a higher electrocatalytic activity. Therefore, we conclude that the exceptional performance of Ag2S NWs in IL is attributed to a synergistic effect of solvent-assisted and specific facet-promoted contributions. 3. Conclusions


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In summary, Ag2S NWs were successfully synthesized using a modified facile onestep method and utilized to investigate both solvent- and specific facet-promoted electroreduction of CO2 to CO in aqueous and IL media. Ag2S NWs in IL possess a partial current density of 12.37 mA cm-2, ~14- and ~17.5-fold higher than those of Ag2S NWs (0.89 mA cm-2) and bulk Ag (0.71 mA cm-2) in KHCO3, respectively. Moreover, it shows significantly higher selectivity with a value of 92.0% at an η of −0.754 V as compared to bulk Ag in IL (82.7%), Ag2S NWs (41.7%) and bulk Ag (57.2%) in KHCO3 at overpotentials of −0.964 V, − 0.646 V and − 0.946 V, respectively. More importantly, the formation of CO began at a low η of 54 mV, confirming the enhanced catalytic activity of Ag2S NWs in IL. In addition, only slight degradation was observed over 19 h of test, further verifying the excellence of Ag2S NWs as a catalyst for CO2RR in IL. The good electrochemical performance originates from the presence of [EMIM − CO ] complexes which not only physically adsorb on the negatively charged catalyst and subsequently increase the solubility of CO2 and the probability of substantial close collision of CO2 with catalyst, but also significantly lower the energy barrier of electron-transfer process, which consequently reduce the η towards CO2RR. Furthermore, DFT calculations indicate that the COOH* formation over Ag2S NWs is energetically more favorable on the facets of (111) and (121) than that on Ag(111) and Ag55, suggesting a higher catalytic activity for CO2RR, as confirmed by the results of UPS and PDOS. Computational analysis confirmed that the exceptional performance of Ag2S NWs in IL is attributed to a synergistic effect of solvent-assisted and specific facet-promoted contributions. These findings can serve as stepping stone in advancing our understanding on CO2RR mechanism and exploring new catalysts for efficient CO2RR. Supporting Information


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Synthesis and characterizations of materials; procedures of electrochemical measurements and product analysis; Faraday efficiency and energy efficiency calculations; DFT calculations; determinations of ECSA and work function; supporting figures and tables; explaination of the COOH* which breaks the apparent scaling relation over Ag2S; the investigation of shape effects over Ag2S NWs and NPs; the adsorption energies of H*, COOH*, CO* on all surfaces; data for d band center; the demonstration of the surface models and other related content. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada. References (1) Faunce, T.; Styring, S.; Wasielewski, M. R.; Brudvig, G. W.; Rutherford, A. W.; Messinger, J.; Lee, A. F.; Hill, C. L.; Fontecave, M.; MacFarlane, D. R. Energy Environ. Sci. 2013, 6, 1074-1076. (2) Wang, Z.-L.; Li, C.; Yamauchi, Y. Nano Today 2016, 11, 373-391. (3) Costentin, C.; Robert, M.; Savéant, J.-M. Acc. Chem. Res. 2015, 48, 2996-3006. (4) Oh, S.; Gallagher, J. R.; Miller, J. T.; Surendranath, Y. J. Am. Chem. Soc. 2016, 138, 1820-1823. (5) Zhang, Z.; Chi, M.; Veith, G. M.; Zhang, P.; Lutterman, D. A.; Rosenthal, J.; Overbury, S. H.; Dai, S.; Zhu, H. ACS Catal. 2016, 6, 6255-6264. (6) Weng, Z.; Jiang, J.; Wu, Y.; Wu, Z.; Guo, X.; Materna, K. L.; Liu, W.; Batista, V. S.; Brudvig, G. W.; Wang, H. J. Am. Chem. Soc. 2016, 138, 8076-8079. (7) Li, F.; Chen, L.; Knowles, G. P.; MacFarlane, D. R.; Zhang, J. Angew. Chem., Int. Ed. 2017, 56, 505-509.


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(8) Kumar, B.; Atla, V.; Brian, J. P.; Kumari, S.; Nguyen, T. Q.; Sunkara, M.; Spurgeon, J. M. Angew. Chem., Int. Ed. 2017, 56, 3645-3649. (9) Asadi, M.; Kim, K.; Liu, C.; Addepalli, A. V.; Abbasi, P.; Yasaei, P.; Phillips, P.; Behranginia, A.; Cerrato, J. M.; Haasch, R. Science 2016, 353, 467-470. (10) Wu, J.; Yadav, R. M.; Liu, M.; Sharma, P. P.; Tiwary, C. S.; Ma, L.; Zou, X.; Zhou, X.-D.; Yakobson, B. I.; Lou, J. ACS Nano 2015, 9, 5364-5371. (11) Kumar, B.; Asadi, M.; Pisasale, D.; Sinha-Ray, S.; Rosen, B. A.; Haasch, R.; Abiade, J.; Yarin, A. L.; Salehi-Khojin, A. Nat. Commun. 2013, 4, 2819. (12) Zhu, W.; Michalsky, R.; Metin, O.; Lv, H.; Guo, S.; Wright, C. J.; Sun, X.; Peterson, A. A.; Sun, S. J. Am. Chem. Soc. 2013, 135, 16833-16836. (13) Liu, S.; Tao, H.; Zeng, L.; Liu, Q.; Xu, Z.; Liu, Q.; Luo, J. L. J. Am. Chem. Soc. 2017, 139, 2160-2163. (14) Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G.; Jiao, F. Nat. Commun. 2014, 5, 3242. (15) Ma, M.; Djanashvili, K.; Smith, W. A. Angew. Chem., Int. Ed. 2016, 55, 66806684. (16) Back, S.; Yeom, M. S.; Jung, Y. ACS Catal. 2015, 5, 5089-5096. (17) Feng, X.; Jiang, K.; Fan, S.; Kanan, M. W. J. Am. Chem. Soc. 2015, 137, 46064609. (18) Li, Y.; Cui, F.; Ross, M. B.; Kim, D.; Sun, Y.; Yang, P. Nano Lett. 2017, 17, 1312-1317. (19) Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C.; de Arquer, F. P.; Safaei, T. S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.; Kelley, S. O.; Sargent, E. H. Nature 2016, 537, 382-386.


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Figure 1. (a, b) High-resolution SEM images of as-prepared Ag2S NWs; (c) AFM image and the corresponding height profile of Ag2S NWs; (d) high-resolution TEM image and the corresponding electron energy-loss spectroscopy (EELS) mapping showing elemental distributions of Ag and S.


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Figure 2. (a) TEM image and (b) the corresponding fast Fourier transformation; (c) N2 adsorption-desorption isotherm of Ag2S NWs, the inset shows the pore size distribution.


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Figure 3. (a) Cathodic LSV results scanning at 50 mV s-1; (b) overall FEs (i.e., CO, CH4 and H2) for Ag2S NWs in IL; (c) CO FEs of different catalysts at various potentials and (d) the corresponding CO partial current densities.


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Figure 4. (a) Tafel plots and (b) exchange current densities and maximum EEs of all catalysts; (c) CO mass activity as a function of potential; (d) long-term stabilities of Ag2S NWs and bulk Ag in both KHCO3 and IL.


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Figure 5. (a) Free energy diagrams for CO2RR on different facets of Ag [i.e., (111) and 55-cluster] and Ag2S [i.e., (111) and (121)]; calculated PDOS of the surface Ag atom of (b) bulk Ag [i.e., Ag55 and Ag(111)] and (c) Ag2S NWs [i.e., Ag2S(111) and Ag2S(121)]; (d) work functions of Ag and Ag2S NWs based on DFT and experimental calculations.


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Scheme 1. Proposed mechanism for CO2RR to CO on Ag2S(111) and Ag2S(121) in IL.


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