The Importance of Ag-Cu Biphasic Boundaries for Selective

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The Importance of Ag-Cu Biphasic Boundaries for Selective Electrochemical Reduction of CO2 to Ethanol Seunghwa Lee, Gibeom Park, and Jaeyoung Lee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02822 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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The Importance of Ag-Cu Biphasic Boundaries for Selective Electrochemical Reduction of CO2 to Ethanol Seunghwa Lee1, Gibeom Park1, Jaeyoung Lee1,2,* 1

Electrochemical Reaction and Technology Laboratory, School of Earth Sciences and

Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea 2

Ertl Center for Electrochemistry and Catalysis/GRI, Chemical Energy Storage and

Transformation Center/RISE, GIST, Gwangju 500-712, South Korea Corresponding Author *E-mail: [email protected]

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ABSTRACT

In recent years, electrochemical reduction of carbon dioxide (CO2) has received a plenty of attention due to the potential that can mitigate the atmospheric CO2 concentration and produce valuable organic compounds. In particular, Cu and Cu-based catalysts have exhibited the capability to convert CO2 into multi-carbon fuels and chemicals in significant quantities. Here, we report a facile and cheap fabrication method for development of Ag-incorporated cuprous oxide (Ag-Cu2O) electrode enabling selective synthesis of ethanol via electrochemical CO2 reduction and reveal the key factor improving the ethanol (C2H5OH) selectivity. The incorporation of Ag into Cu2O leads to the suppression of hydrogen (H2) evolution and furthermore by varying the elemental arrangement (phase-separated and phase-blended) of Ag and Cu, we observe that C2H5OH selectivity can be controlled and consequently the faradaic efficiency for C2H5OH on phase-blended Ag-Cu2O (Ag-Cu2OPB) is 3 times higher than that of the Cu2O without Ag dopant. We propose that the electrochemical reaction behavior is not solely associated with a role of Ag dopant, carbon monoxide (CO) production for leading to ethanol formation pathway over ethylene, but also the doping pattern related population of Ag-Cu biphasic boundaries to relatively suppress H2 evolution reaction and encourage the reaction of mobile CO generated on Ag to a residual intermediate on Cu site.

KEYWORDS

CO2 Reduction, Bimetallic Catalyst, Ag, Cu2O, Ethanol


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INTRODUCTION

Due to the growing concerns of climate-change-related risks and the global energy crisis, recent research studies have focused on developing economical and sustainable technologies [1,2]. Among all the promising technologies, carbon dioxide (CO2) utilization is one of the most attractive, as it can reduce the concentration of CO2 in the atmosphere and also produce valuable fuels and chemicals to be a feedstock for the chemical industry [1-6]. The electrochemical reduction of CO2 has received the most attention of all the CO2 utilization routes in the past few decades because it can provide a carbon-neutral energy network by connecting with electricity production from renewable sources, such as solar, wind, and geothermal power [7-9]. Moreover, the electrochemical reduction process can be performed at ambient pressure and temperature, and it is also relatively easy to scale up for practical implementation [10-12]. The research field of electrochemical CO2 reduction has been extensively studied since the groundbreaking work by Hori et al. in the 1980s [13,14]. The reduction of CO2 to CO and HCOO- is commonly observed in the low overpotential region [7-9]. Noble metals, such as Ag and Au, are selective for the production of CO [15,16], while Sn and Pb are the best catalysts for HCOO- production [17,18]. Although recently there have been a couple of reports on metal-free catalysts that are selective for hydrocarbons [19-21], Cu and Cu-based materials are the only catalysts that exhibit notable catalytic properties for the conversion of CO2 into hydrocarbons and oxygenates, such as ethane (C2H6), ethylene (C2H4), ethanol (C2H5OH), and n-propanol (nC3H7OH) [9, 22-25]. However, there are still major barriers in the practical application of CO2 reduction to multi-carbon products using Cu-based catalysts, namely, the high overpotential to activate electrochemical CO2 conversion and the difficulty of targeting desired products [26-29].


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A superior catalyst is strongly required to overcome such barriers and subsequently obtain better catalytic activity and selectivity. A promising route for designing an efficient catalyst is to use multi-component materials. The bimetallic approach has shown remarkable results as it is able to break the linear relationship between the binding energies of CO and COOH on the electrode surface [30]. As an example, Watanabe et al. studied many Cu alloys, including Cu-Ni, Cu-Sn, Cu-Pb, Cu-Zn, Cu-Cd, and Cu-Ag, and each bimetallic catalyst exhibited catalytic behavior distinct from their respective elemental metals [31]. In contrast to the report of Watanabe et al., which only showed production of single-carbon molecules, Ishimaru and coworkers achieved highly selective conversion of CO2 to multi-carbon fuels, such as C2H4, CH3CHO, and C2H5OH on a Cu-Ag alloy electrode by a pulsed electrochemical reduction process [32]. Recently, Kim et al. investigated the synergistic effect of the electronic and geometric factors on the binding of intermediates and eventually on CO2 reduction using Cu-Au bimetallic particles with different compositions [33]. Furthermore, another report demonstrated that Cu-In derived from the in situ reduction of Cu2O in an InSO4 solution selectively catalyzed the electroreduction of CO2 to CO with extremely high stability [34]. With regards to product selectivity towards multi-carbon fuels such as ethylene, earlier experimental and computational studies have reported that local pH can play an important role in preferential production of ethylene over methane via CO dimerization, namely pH independent route [28,29,35,36]. Gupta et al. presented a mathematical model that predicts the local pH condition is significantly different from the bulk concentration and subsequently revealed that the lower buffer capacity leads to the more production of ethylene by computing a previously reported literature data based on the mathematical model [35]. Varela et al. experimentally demonstrated that on a smooth Cu electrode, a low buffer capacity or catalyst with high current


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densities result in a relatively higher selective production of ethylene since higher local pH under such conditions makes the products such as hydrogen and methane less favorable [36]. Another promising approach is the use of oxidized-metal composites for electrode development [37-39]. The concept of oxide-derived materials has considerably raised many interesting issues. According to the reports of Kanan et al., oxide-derived metals exhibit enhanced catalytic properties due to their abundant grain-boundaries on the catalyst surfaces [40,41]. The reports of the Kanan group contributed to a boom of use of oxidized-metal composites for electrochemical CO2 reduction. More recently, we comprehensively reviewed the trend of electrode build-up of oxidized-metal composites for the electrochemical reduction of CO2 [42]. Additionally, we developed a biphasic Cu2O-Cu electrode for the selective production of a variety of multi-carbon fuels [24]. It is rational to develop a catalyst that incorporates a different metal into an oxidized-metal electrode considering both techniques exhibit improved activity and selectivity for CO2 reduction. Two different reports that combined these two techniques showed remarkable results [43,44]. Ren et al. developed a series of oxide-derived CuxZn catalysts for the enhanced electrochemical reduction of CO2 to ethanol [43]. They found that the selectivity toward ethanol could be tuned by varying the amount of Zn in the CuxZn catalysts, and they were able to achieve a maximum faradaic efficiency of 29.1%. Furthermore, it was demonstrated that the COproducing sites for selective ethanol production are important by comparing Cu-Ni and Cu-Ag bimetallic catalysts. Larrazabal and co-workers prepared Cu2O catalysts by a solvothermal method for the electrochemical CO2 reduction and investigated the catalytic effect of introducing p-block elements, such as Sn, In, Ga, and Al, into the Cu2O material [44]. The addition of Sn and In into Cu2O was observed to have a promotional effect on the selective CO production. In


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contrast to Sn and In, which showed selective CO production, Al drove the reaction selectivity toward ethylene despite only low doping amounts relative to Sn and In. They suggested that the selectivity of the Al-doped Cu2O catalyst might be linked to a stabilizing effect of Al on the Cu1+ species, and thus, more abundant Cu1+ species could play a key role in ethylene production compared to the Sn- and In-modified catalysts. In this regard, we developed a facile and cheap fabrication method, which enables easy control of the elemental compositions and structure of the catalysts. As it is known to be very selective for the production of CO, Ag was employed and introduced into Cu2O by electrochemical codeposition. We prepared the multi-component catalysts with two different elemental arrangements (phase-separated and phase-blended) by varying the chemical solvent in the deposition process and evaluated their performances at various overpotentials in 0.2 M KCl as well compared their performances with those of an unmodified Cu2O catalyst.

EXPERIMENTAL METHODS

All electrochemical experiments were carried out using a three-electrode assembly with a commercial Ag/AgCl reference electrode and Pt mesh (4 cm2) counter electrode. The recorded potentials were converted with respect to the reversible hydrogen electrode (RHE). The two types of working electrodes of Ag-incorporated Cu2O were fabricated by electrochemical deposition in different solvents, namely, ammonia solution (NH3 30wt%, Junsei Chemical) and potassium cyanide (KCN >98.0%, Sigma-Aldrich). The experimental details for the electrode fabrication were as follows: (1) phase-separated Ag-Cu2O was made in an NH3-based electrolyte solution containing 0.4 M CuSO4 (> 99%, Sigma-Aldrich), 0.02 M AgNO3 (> 99%, SigmaAldrich), lactic acid (>92%, Kanto chemical), and 5.0 M NaOH (>97%, Daejung). As shown in


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Figure S1, the Ag precursor was first dissolved and stabilized in NH3 solution for 5 min, and then, the other chemicals were added gradually with continuous stirring. Ag and Cu2O were simultaneously structured onto the gas diffusion layer (GDL) (TGPH-060, Toray) using the resulting solution by applying 0.39 V (vs. RHE) up to a total charge of -10 C. (2) For the phaseblended Ag-Cu2O, a KCN solution was used instead of NH3, but the procedure for the solution preparation was the same. The possible potential window for co-deposition was kinetically controlled and different in the two solvents that were employed. Hence, two types of Ag-Cu2O electrodes were produced. Controlling the potential region for co-deposition is explained in detail in the following Results and Discussion section (see Figure 1). The electrochemical CO2 reduction tests were performed in a customized H-type electrolytic cell. A cation-exchange membrane (Nafion N115, DuPont) separated the working electrode compartment from the anode compartment. The catholyte was prepared from 300 mL of deionized water (18 MΩ) and potassium chloride (> 99.5%, Kanto Chemical Co.), and the anolyte was made from 80 mL of 0.1 M potassium carbonate (> 99.7%, Sigma-Aldrich). For CO2 electrochemical reduction tests, high-purity CO2 gas (99.999%) was purged into the electrolyte at a flow rate of 40 mL min-1 until the electrolyte was saturated. After CO2 purging, linear sweep voltammograms were repeatedly conducted at a scan rate of 20 mV to reduce and stabilize the electrodes. Chronoamperometry was performed at fixed potentials between -0.9 V and -1.4 V (vs. RHE) for 180 min to determine the optimal conditions for CO2 reduction. Liquid-phase products generated from the electrochemical conversion of CO2 were analyzed using UV spectroscopy (UV-1800, Shimadzu) and gas chromatography-mass spectrometry (GCMS, Agilent Technologies). Moreover, gas-phase products were quantified by gas chromatography (GC, Agilent 7890A, Agilent Technologies) with a thermal conductivity


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detector (TCD) and gas chromatography-mass spectrometry (GC-MS, Agilent Technologies). After the quantification, the faradaic efficiencies (FE) toward each product were calculated as follows:

FE (%) =

× ×

× 100

Where n is number of moles of electrons to participate in the faradaic reaction, F is the Faraday constant (96485 C mol-1), and C is the amount of charge passed through the working electrode. The morphologies of the electrodes were observed using field-emission scanning electron microscopy (FE-SEM, S-4700, Hitachi) and high-resolution transmission electron microscopy (HR-TEM, Technai-F20, Ultra twin). Energy dispersive X-ray microanalysis (EDXMA) mapping was simultaneously performed during both analyses to analyze further the elemental compositions. X-ray diffraction (XRD, MiniFlex II, Rigaku) and X-ray photon spectroscopy (XPS; Thermo Fisher Scientific) were employed to examine the crystallinity changes and surface compositions before and after electrochemical CO2 reduction, respectively. In addition, more detailed surface-sensitive analysis was carried out using angle-resolved XPS (Theta Prove ARXPS system MXR1 Gun-400 mm, 15 kV spectrometer). In situ X-ray absorption near edge structure (XANES) measurements were performed in the range of the Cu K-edge at beamline 7D of the Pohang Accelerator Laboratory (PAL) to confirm the electronic and geometric local structures of sample electrodes. Real time data from the XANES measurements were recorded in fluorescence mode at room temperature during the CO2 reduction reaction. The data were subsequently analyzed using IFEFFIT software.


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RESULTS AND DISCUSSION

The two types of Ag-incorporated Cu2O electrodes were prepared by electrochemical codeposition in different solutions containing Cu and Ag precursors. Figure 1 shows the linear sweep voltammograms obtained in the solutions. In principle, electrochemical deposition of metal ions can be kinetically controlled using different solvents [45]. Since Cu and Cu2O are deposited in a much larger variety of solutions, the choice of media for Ag-Cu2O deposition is mainly limited to those in which Ag may be deposited [46]. In general, M(NH3)2+ complex ion can be reduced at smaller reduction potential than M+ ion and hence ammonia (NH3) is often added to the solution for controlling the onset potential and potential window of electrodeposition [45-47]. Also, as reported in earlier work by Ibrahim et al., the alloy polarization curve lies between the curves for each metal ion deposition and the limiting current range (potential window for co-deposition) increased with increasing the NH3 concentration [48]. In this regards, first, as seen in Figure 1, Ag deposition started at a more positive potential value compared to Cu deposition in both solutions. In the KCN solution, each onset potential for the deposition of Ag and Cu2O was located at a less positive potential rather than in NH3 solution, and besides, the potential window for co-deposition of Ag and Cu2O was also relatively narrower. This is in accordance with previous studies [45,47,48]. The LSV curve for Ag-Cu2O deposition in NH3 solution exhibits two limiting current plateaus in contrast to that in KCN solution. Under the limiting current region between 0.9 V and 0.6 V, Ag deposition proceeds via mass transport control and in the following limiting condition beyond onset for Cu2O deposition (see Figure 1a), Ag-Cu2O co-deposition would be expected. As shown in Figure 1, the different ranges of codeposition resulting from the different solutions could allow us to design two different Ag-Cu2O electrodes with different elemental patterns by controlling the deposition rate of each Ag and Cu


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ion. Note that we tried to make the compositions of Cu and Ag (in the range of 6:4 to 7:3) of the two electrodes equal and after making the electrodes, we analyzed their crystalline and elemental compositions.

Figure 1. Linear sweep voltammograms for the preparation of Ag-incorporated Cu2O in NH3 (black line) and KCN (red line) solutions. The possible potential window for the co-deposition of Ag and Cu2O is wider in (a) NH3 solution than in (b) KCN solution. Figure 2 exhibits the XRD patterns for Ag-Cu2O samples fabricated in NH3 solution and KCN solution. In NH3 solution, co-depositions of Ag and Cu2O were performed at 0.05 V intervals from 0.49 V based on the LSV result (see Figure 1a). As shown in Figure 2a, Ag(111) peak exhibited a relatively higher intensity than that of Cu2O(111) at 0.49 V and then the Cu2O(111)


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peak gradually increased up to 0.39 V. At less positive potential region than 0.39 V, it seems that almost no Ag deposition occurs and instead metallic Cu(111) at 43.2° can be rapidly deposited rather than Cu2O. Therefore, we selected the Ag-Cu2O at 0.39 V where the peak at 36.7° matches pure Cu2O(111) and the other peak at 38.2° corresponds to pure Ag(111), which indicates the Ag-Cu2O is formed in the bimetallic catalyst maintaining their own crystalline phase of Cu2O and Ag respectively. On the other hand, the prepared sample in KCN (Figure 2b) shows a specific peak between the 2 theta degree positions (36.7° and 38.2°) together with disappearance of pure Cu2O(111) and peak broadening of Ag(111) at an applied potential of 0.24 V. Theoretically, introducing other atoms into a structure of host material can lead to peak shifts because of the difference in size of the atoms and subsequent changes in lattice parameters depending on whether the introduced atom is larger or smaller than the size of host atom [49-51]. Given that in this context the peak of Ag-Cu2O (37.6°) prepared in KCN, it is rational to assume that the peak is originated from the shift of Cu2O(111) peak due to partial substitution of Ag for host Cu2O and/or formation of a solid Ag-Cu2O solution at grain boundary [49-51]. In any cases, it is clear that Ag and Cu2O can be structured in the form of phase-blended in KCN in contrast to phase-separated composition in NH3 solution [51]. As shown in Figure 2b, at 0.34 V, Ag was electrochemically deposited earlier in KCN and from 0.29 V the Ag-Cu2O related peak (37.6°) started to increase. Overall in the experimental range of 0.34 V to 0.14 V, Ag is consistently deposited to some extent and once co-deposition begins (from 0.29 V), the Ag-Cu2O is solely formed as a blended composite without any pure Cu2O form. In terms of XRD pattern for the Ag-Cu2O in NH3, we further analyzed how much Ag and Cu2O are present in the form of phase-separated or phase-blended in detail because the Ag-Cu2O peak is slightly shifted to higher angle position. As shown in Figure S2, we approximately calculated


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the given XRD spectrum (Ag-Cu2O at 0.39 V in NH3) with a linear combination of reference spectra (pure Cu2O and Ag-Cu2O at 0.24 V in KCN) [52,53]. Consequently, the approximate quantitative analysis shows that in respect of Cu2O related species in the structure of the AgCu2O sample, mostly phase-separated Cu2O consists of 85% with slightly phase-blended Cu2O (~15%).

Figure 2. XRD patterns of (a) the fabricated samples in NH3 solution and (b) in KCN solution as a function of applied cathodic potential. Black dotted line and gray dotted line indicate Cu2O(111) and Ag(111) position respectively.

Through repeated LSV scans (see Figure S3), both Ag-incorporated Cu2O samples were partially reduced, stabilized, and consequently changed into the two types of Ag-incorporated biphasic Cu2O-Cu electrodes (see Figure 3). We abbreviate the phase-separated and phaseblended electrodes as Ag-Cu2OPS and Ag-Cu2OPB, respectively.


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Figure 3. XRD patterns of Ag-Cu2OPS (a) before and (b) after stabilization as well as Ag-Cu2OPB (c) before and (d) after stabilization.

The compositions of all the samples were investigated by SEM-EDX and TEM-EDX. The results of both analyses are summarized in Table 1. As shown, all the electrode samples have a high atomic percent of Cu (~ 66%) and low atomic percent of Ag (~ 34%), which indicates that there is no considerable structural collapse and nanoparticle loss at the electrode during the LSV scans. Moreover, as evident from SEM images in Figure S4, the bulk structures of Ag-Cu2OPS and Ag-Cu2OPB did not change considerably during the LSV scans.


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Table 1. Summary of the atomic percentages of all the Ag-incorporated Cu2O samples before an d after stabilization through LSV scans. Atomic percent (%) Catalyst SEM

TEM

Cu

68.48

Cu

65.93

Ag

31.52

Ag

34.07

Cu

64.36

Cu

66.53

Ag

35.64

Ag

33.47

Cu

64.57

Cu

66.84

Ag

35.43

Ag

33.16

Cu

66.27

Cu

65.49

Ag

33.73

Ag

34.51

As-prepared Ag-Cu2OPS After stabilization

As-prepared Ag-Cu2OPB After stabilization


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Figure 4. (a, f) High-resolution and (b, g) low-resolution TEM images and (c, h) STEM images of Ag-Cu2OPS and Ag-Cu2OPB after stabilization as well as EDX mapping analysis identifying (d, i) Cu and (e, j) Ag with the resultant atomic percentages of each area in (c, h) the STEM images.


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HR-TEM images (see Figures 4 and S5) show that the four Ag-incorporated Cu2O electrodes (before and after stabilization) have different morphological and phase arrangements; however, they have similar bulk compositions of Cu and Ag. Furthermore, combined with the EDX mapping results of identical area, the chemical microstructures of the electrodes were clearly examined. As shown in Figures 4 and S5, Ag-Cu2OPS before and after stabilization consists of interconnected crystalline features with a particle size of ~50 nm. The EDX mapping of the same area notably exhibits separate phases of Cu (red) and Ag (green). In other words, Ag-Cu2OPS shows the surface composition where a biphasic boundary split distinctively into two phases formed by each element. Contrary to the Ag-Cu2OPS samples, the Ag-Cu2OPB samples appear to be finely dispersed in both fresh and stabilized catalysts, as shown in Figures 4 and S5. The EDX mapping of the fresh Ag-Cu2OPB shows that Cu (red) and Ag (green) elements are homogeneously distributed unlike the Ag-Cu2OPS samples, but after the electrochemical reduction by the repeated LSV scanning, it seems that Ag-Cu2OPB also experienced partial phase separation. This finding was also confirmed by the analysis of the identical area where a slight change of the atomic compositions of Ag-Cu2OPB before and after stabilization was observed (see Figures 4h and S5h). According to the segregation and surface mixing energies for the transition metals, Cu must be thermodynamically and at least partially segregated from Ag [5456]. Therefore, one can assume that, although as-prepared Ag-Cu2OPB was stable and homogeneously distributed, during repeated LSV scanning for stabilization, Cu2O was partially reduced to a metallic Cu phase, and thus, the partial segregation between metallic Cu and Ag was expected to occur. Nonetheless, considering the distribution of elements and atomic compositions, Ag-Cu2OPB reveals more homogeneously distributed Cu and Ag elements compared to that of Ag-Cu2OPS.


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A facile route for preparing Ag-incorporated biphasic Cu2O-Cu catalysts with different structural arrangements was developed based on the agreement of STEM-EDX, HR-TEM, and XRD results. After identifying and stabilizing the electrode catalysts, we evaluated their catalytic activities and selectivities for electrochemical reduction of CO2. KCl was chosen as a catholyte for the detailed studies because we previously confirmed that KCl resulted in not only higher local pH condition for favorable production of multi-carbon fuels but also formation of biphasic Cu2O-Cu structure which can remarkably enhance the selectivity toward multi-carbon fuels even including C3-C4 compounds [24]. However, KHCO3 substituted for KCl in anode compartment due to the formation of chlorine gas (Cl2), a toxic and pulmonary irritant. For the reason, all subsequent CO2 reduction related-experiments were carried out in KCl solutions. As shown in Figure 5, LSV curves were obtained at a cathodic sweeping rate of 20 mV sec-1 in both N2saturated and CO2-saturated 0.2 M KCl solutions on Ag-Cu2OPS, Ag-Cu2OPB and Cu2O without Ag dopants. All three samples exhibit similar overpotential for the activation of CO2 reduction, but there is a striking difference in the hydrogen evolution reaction activity. On the Cu2O electrode with continuous N2 purging, H2 formation began faster than those on the Ag-Cu2O samples and consequently we could see the vigorous increase in current density for the hydrogen (H2) formation with respect to negative potential direction. In the sufficient CO2 condition, because electrode surface is covered by adsorbed CO molecules generated from CO2 reduction, the decrease in the reduction current measured should be considered as the indication of CO2 reduction and suppression of H2 formation [9,23]. Therefore, the notably extensive change in current density was observed on the Cu2O compared with the Ag-Cu2O samples. Clearly, it reveals that the Ag-Cu mixed surface is intrinsically less active for H2 evolution reaction relative to a polycrystalline Cu surface. The potential range for CO2 reduction was determined from the


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obtained LSV curves (Figure 5) and the potential dependent product distributions were examined subsequently between -0.9 V and -1.4 V. All CO2 reduction tests were carried out for 3h at each potential respectively. Figure S6 presents the current density profiles for CO2 reduction at each electrolytic condition. As shown in Tables S1-S3, a variety of multi-carbon compounds were produced, and impressive faradaic efficiencies of 20.1% and 34.15% on Ag-Cu2OPS and AgCu2OPB at -1.2 V (vs. RHE) were obtained for ethanol, respectively. We can see that, in the case of Cu2O without Ag in 0.2 M KCl (Table S3), the maximum faradaic efficiency for ethanol was low, 10.5% and H2 formation was relatively higher than the two Ag-incorporated electrodes, as expected from the LSV results in Figure 5. Based on the LSV measurements and previous literature survey [32,33,57], it can be expected that the specific property to suppress H2 evolution reaction is linked to the favorable ethanol formation over ethylene because the suppression property of intermediate adsorption for H2 formation can also help to suppress the hydrogenation of CO-related intermediates toward C2H4 during the CO2 reduction [32,33,58].

Figure 5. LSV curves obtained on (a) Ag-Cu2OPS, (b) Ag-Cu2OPB and (c) Cu2O in N2-purged (short-dotted line) and CO2-purged (red-solid line) 0.2 M KCl solutions. The downward dotted arrow indicates the cathodic potential for the start of hydrogen evolution. Earlier studies have suggested that the major C2 product on a Cu-based electrode is ethylene, instead of ethanol, over a wide range of potentials because CH2CHO*, which is a selectivitydetermining intermediate toward C2H4 or C2H5OH in the late stage during the CO2 reduction


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process, prefers energetically to form C2H4 [28,29,58]. Hence, despite considerable endeavors, it is still a challenge to achieve a high faradaic efficiency for ethanol. On the basis of the faradaic efficiency, we successfully developed a new catalyst that is more selective for ethanol compared to biphasic Cu2O-Cu without Ag dopant.

Figure 6. Comparison of the faradaic efficiencies for (a) CO, (b) C2H5OH, (c) HCOO- and (d) C2H4 on the (▲) Ag-Cu2OPS, (●) Ag-Cu2OPB and (■) Cu2O electrodes. The calculated faradaic efficiencies of the three electrodes for the major reaction products provide an interesting viewpoint related to the selectivity for ethanol and ethylene (see Figure 6). In the low cathodic potential region, CO production remarkably increased on both types of Agincorporated electrodes, while the faradaic efficiencies for HCOO- were similar in all the


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electrode samples. In addition, the production of ethanol was favored over ethylene on AgCu2OPS and Ag-Cu2OPB, whereas ethylene was produced selectively over ethanol on Cu2O. The partial current densities for the main products were also analyzed to compare actual production rate between the three electrodes as shown in Figure S6. Overall propensity for each product seems almost similar to that of the faradaic efficiency (Figure 6) and thus when considering both the selectivity and productivity, the Ag-Cu2OPB was best for selective electrochemical reduction of CO2 to ethanol. We assumed that the enhanced CO production could contribute to improving the product selectivity toward ethanol over ethylene. The observed trends of the Faradaic efficiencies for CO and ethanol (Figure 6a and b) agree well with the aforementioned study by Ren et al., suggesting the increase in ethanol selectivity via CO-insertion route from the increase in CO production [43]. However, we need to investigate further the surface structural characteristics of Ag-Cu2OPB and Ag-Cu2OPS considering that the atomic arrangements of Cu and Ag were different in both electrodes. The different arrangements might be the factor for the difference in the ethanol selectivity of the two electrodes. Hence, we investigated the atomic compositions and characteristics of Ag-Cu2OPS and Ag-Cu2OPB to find the origin of their different ethanol selectivity despite similar bulk compositions of Ag and Cu elements. As shown in Table 2, the surface characterizations of the catalysts through XPS show that, in both Ag-incorporated samples, the surface proportion of Cu was relatively increased after stabilization and that the Ag-Cu2OPS surface has the highest atomic ratio of Cu to Ag compared to its bulk composition among all cases, including before and after stabilization. The different surface atomic composition between the two different Ag-incorporated Cu2O samples might explain the reason why the hydrogen evolution reaction was relatively more suppressed on AgCu2OPB than on Ag-Cu2OPS, as seen in Figure 5, Tables S1 and S2. The higher C2H5OH


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selectivity resulting from improved CO production might also be associated with the abundant Ag sites of Ag-Cu2OPB (22.1%), which are increased compared to that of Ag-Cu2OPS (12.5%) (see Table 2). According to previous studies by theoretical calculations [59,60], metallic Cu is expected to be segregated preferentially from Ag; this was confirmed by the HR-TEM images and EDX analyses of the Ag-Cu2OPB and Ag-Cu2OPS samples after stabilization, as shown in Figure 4. However, the lower surface energy of Ag relative to Cu suggests that preferential segregation between Cu and Ag metals is not solely responsible for the different surface atomic percentages from the bulk. In general, a core-shell arrangement is formed when one metal has a lower surface energy than the other metal, following the rule of the lower surface energy material coating the higher surface energy material [61,62]. Hence, a Ag-abundant surface rather than a Cu-abundant surface is contradictory to the results of Ag-Cu2OPS and Ag-Cu2OPB. Therefore, another viewpoint to understand the surface composition in the Cu-Ag multi-component systems is required.


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Table 2. Summary of surface atomic percentages of all the Ag-incorporated Cu2O samples befor e and after stabilization through LSV scanning. Atomic percent (%) Catalyst XPS Cu

76.21

Ag

23.79

Cu

87.5

Ag

12.5

Cu

67.17

Ag

32.83

Cu

77.9

Ag

22.1

As-prepared Ag-Cu2OPS After stabilization

As-prepared Ag-Cu2OPB After stabilization


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Figure 7. XRD patterns of Ag-Cu2OPS (a) before and (b) after stabilization as well as a AgCu2OPS sample composed of a higher amount of Ag (c) before and (d) after stabilization. Several theoretical and experimental studies have suggested that the presence of oxidizing species leads to Cu segregation to the surface and that, consequently, a thin layer of copper oxide on top of the silver surface is formed in the surface region [62,63]. Considering the surface exposure to air and to the aqueous experimental conditions, we can expect that, after stabilization, the Cu2O component is reduced partially to metallic Cu and Cu is subsequently segregated from Ag and is then segregated to the surface. To trace the segregation of Cu to the surface, we analyzed the catalyst surfaces using angle-resolved XPS (AR-XPS), which is a surface-sensitive


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technique. As shown in Figure S7, the AR-XPS measurements provide more accurate surface profiling of chemical compositions as a function of the angle to the surface normal. As the angle increased, the outermost surface is analyzed intensively. The gradual increase in the relative ratio of Cu to Ag indicates that Cu can segregate and diffuse from the bulk to the surface in both the Ag-Cu2O electrodes. Based on the observations, we confirmed the Ag-incorporated Cu2O samples should have Cu-rich surface regions relative to the bulk composition irrespective of the phase arrangement. One thing that should not be missed is why Ag-Cu2OPB has a favorable surface composition with more Ag content for ethanol production than that of Ag-Cu2OPS. It seems that more Ag is present on the surface of Ag-Cu2OPB than Ag-Cu2OPS, and therefore, this increase leads to a decrease in hydrogen affinity and a subsequent increase in selectivity toward ethanol formation via a CO-insertion route due to more CO production. A different Ag-Cu2OPS electrode with a higher atomic percentage of Ag was prepared to confirm the abovementioned hypothesis (see Figure 7 and Table S4). It is clear that the two AgCu2OPS samples have different atomic compositions based on the intensities of the main peaks, which correspond to metallic Ag (38.2°) and Cu (43.5°). The increased amount of Ag in the new Ag-Cu2OPS sample was confirmed clearly by SEM-EDX analysis (Table S4). The ratio of Ag to Cu in the new Ag-Cu2OPS sample was almost 1:1. In contrast to the analytical results of the bulk composition by XRD and SEM-EDX, the surface analysis by XPS (see Table S4) suggests that the composition of Ag on the surface is not increased considerably despite the increase in the amount of Ag in Cu2O.


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Figure 8. Faradaic efficiency of ethanol (blue column) and surface atomic composition of Ag (red column) of the electrode samples: (a) Ag-Cu2OPS, (b) Ag-increased Ag-Cu2OPS and (c) AgCu2OPB. Based on the analytical results, we estimated that the degree of phase segregation of Cu to the surface could be dependent on initial structure and atomic arrangement, and nevertheless, the increased Ag in Ag-Cu2OPS could improve the selectivity for ethanol production. Figures 8 and S8 reflect the difference of the product selectivity affected by the different surface atomic composition of Ag. The Ag-increased Ag-Cu2OPS exhibited a slightly enhanced selectivity toward CO (2.1%) and for the subsequent CO-insertion-derived ethanol production (21.9%) in comparison with those of the original Ag-Cu2OPS, as seen in Figure S8. However, if the surface composition of Ag plays a critical role in the selective production of ethanol via the CO-insertion


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route, it is expected that the faradaic efficiency of ethanol should be at least 25% in proportion to the amount of Ag in the surface region. However, CO2 reduction using Ag-increased Ag-Cu2OPS recorded a selectivity of only 21.9% for ethanol, as shown in Figure 8b. Given that the increased Ag content and CO selectivity (2.1%) were on a similar level for Ag-Cu2OPB (2.3%), as seen in Figure S8, the production of C2H5OH was not improved as much as expected. Instead, as shown in Figure 8, while Ag composition in the surface region of Ag-Cu2OPB rose by a similar amount of increment in Ag-increased Ag-Cu2OPS relative to Ag-Cu2OPS, ethanol selectivity on the AgCu2OPB was remarkably improved irrespective of the trend of surface atomic composition of Ag. Therefore, we can assume that Ag-Cu2OPB can specifically catalyze the electrochemical reduction of CO2 to ethanol because of its unique properties as well as the higher amount of Ag on its surface compared to that of Ag-Cu2OPS. In a recent report [43], the authors suggested that Ag is a less efficient dopant in comparison to Zn, and they did not observe a dramatic enhancement of selectivity for ethanol on Cu-based catalysts irrespective of the amount of Ag dopant. On the basis of their experimental results, the surface amount of Ag might not play a critical role for the selective production of ethanol from electrochemical CO2 conversion. This is in good agreement with our hypothesis that the higher selective production of ethanol on Ag-Cu2OPB cannot be attributed simply to the higher surface amount of Ag. It seems that another advantage derived from different structural characteristics could contribute to promoting the selective reduction of CO2 to ethanol. In this regard, the phase pattern related biphasic boundaries, which are due to neighboring sites being occupied by different elements, namely, Cu and Ag, might be a key factor for the observed better selectivity for ethanol for Ag-Cu2OPB. As described in this study (see Figures 3 and 4), the structural arrangement of the Ag-incorporated Cu2O samples after stabilization is determined following the


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structure of the parent samples although Ag-Cu2OPB is partially segregated not completely stable during the electrochemical reduction process. As a result, Ag-Cu2OPS has less Ag-Cu biphasic boundaries than that of Ag-Cu2OPB.

Figure 9. (a) Comparison of the in situ measured Cu K-edge XANES spectra for (i) Ag-Cu2OPB, (ii) Ag-Cu2OPS and (iii) Cu2O before applying the cathodic potential and (iv) Ag-Cu2OPB, (v) Ag-Cu2OPS and (vi) Cu at an applied cathodic potential of -1.2 V. Figure 9b and 9c show the comparison between (iv), (v) and (vi) in detail. In situ X-ray absorption near edge structure (XANES) measurements were carried out at the Cu K-edge region to investigate further the local structures of the electrodes during electrochemical reduction of CO2. All XANES data were recorded using a customized in situ X-ray absorption spectroscopy (XAS) cell in fluorescence mode. CO2 reduction was performed at room temperature in CO2-saturated 0.2 M KCl solution under an applied cathodic potential of -1.2 V.


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The XANES spectrum is sensitive to the geometric and electronic environment of the absorbing atoms, and thus, the analytical method can be utilized to confirm the atomic arrangement of the alloy. As shown in Figure 9, the XANES spectra at the Cu K-edge under the electrolytic conditions of cathodic potential show very little difference. Despite the introduction of Ag into Cu2O, as depicted in Figure 9b, the Ag-Cu2OPS sample has similar features as metallic Cu, indicating that the Cu character was relatively maintained more than that of Ag-Cu2OPB due to its atomic arrangement, phase separation. Based on the observed white line of the electrode samples as seen in Figure 9c, one can assume that the different selectivity for ethanol between two types of Ag-Cu2O was not critically correlated with electronic properties. This is in line with the experimental results showing similar onset potentials for CO2 reduction on both the Ag-Cu2O samples as seen in Figure 5. In other words, the Ag-dopant-derived changes in the electronic properties might be related to the reduced H2 selectivity and the enhanced selectivity for ethanol relative to pure Cu2O to a certain extent, but it is not enough to explain why ethanol was produced more selectively on Ag-Cu2OPB compared to on Ag-Cu2OPS. We can see that the peaks of Ag-Cu2OPB were shifted slightly to a low energy level. This can be attributed to the increase of the bond length of Cu-Cu in Ag-Cu2OPB relative to those in Ag-Cu2OPS and Cu2O [64,65]. This is a small difference, but it is in accordance with the observations that Ag and Cu in AgCu2OPB are relatively well mixed, as already demonstrated in the XRD and TEM analyses. Therefore, there are more Ag-Cu biphasic boundaries present in Ag-Cu2OPB compared to AgCu2OPS. In this respect, we postulate that the Ag-Cu biphasic boundary is a key factor to explain the higher selectivity for ethanol production on Ag-Cu2OPB than on Ag-Cu2OPS. In a couple of previous studies, a so-called “CO-insertion mechanism” has been proposed for the selective


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production of ethanol in CO2 reduction [43,67-69]. At the late stage of the reaction pathway, where the intermediate can be converted to either C2H4 or C2H5OH, if CO inserts into the bond between CH2* and the Cu surface, the conversion reaction proceeds toward CH2CO* formation and eventually to the production of C2H5OH via CH3CHO [58,70]. To further prove the hypothesis of CO-insertion mechanism we carried out direct CO electroreduction tests on AgCu2OPS, Ag-Cu2OPB and Cu2O electrodes at -1.2 V for 3 h in 0.2 M KCl solution. Although absolute faradaic efficiencies for CO reduction were much lower than those of CO2 reduction because CO is sparingly soluble in water, we confirmed that in terms of faradaic efficiencies the relative ratio of ethanol to ethylene was increased from 0.55 to 2.88 using Cu2O sample while the values for both Ag-Cu2OPS and Ag-Cu2OPB did not critically change. They are summarized in Table S5. It indicates that on the Cu2O, favorable product was tuned by an intentional CO insertion from ethylene to ethanol [43,67] and expectedly the relative selectivity between ethylene and ethanol was not affected on the series of Ag-Cu2O which seem to favorably catalyze the production of ethanol via CO-insertion route. In the context of this CO-insertion mechanism, the elemental distribution of Cu for an active site and Ag for a CO-producing site might affect the ethanol selectivity. The formation of Ag-Cu interconnected sites, i.e., biphasic boundaries, should be abundant and not too far from each other to highly encourage CO insertion into the bond between CH2* and Cu during electrochemical CO2 reduction. Comparing the phase arrangements of Ag-Cu2OPS and Ag-Cu2OPB, it is clear that the appropriate Ag-Cu biphasic boundaries for CO insertion into the CH2 intermediate are more present on the surface of AgCu2OPB than on Ag-Cu2OPS, and consequently, higher product selectivity for ethanol was achieved using Ag-Cu2OPB relative to Ag-Cu2OPS.


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Figure 10. A hypothetic CO-insertion mechanism scheme indicating the transfer of CO from one metal site that weakly binds CO (Ag) to another site that binds residual C1 intermediate species (Cu) in the case of (a) Ag-Cu2OPS and (b) Ag-Cu2OPB.

Figure 10 describes a hypothetical CO-insertion mechanism scheme considering the different ratio of Ag to Cu and proportion of Ag-Cu biphasic boundaries on the surfaces of Ag-Cu2OPS and Ag-Cu2OPB. In this mechanism, both factors are crucial for controlling product selectivity toward ethanol rather than ethylene because Ag plays a role in mobile CO production, and the Ag-Cu neighboring distance must be favorable for the further CO-insertion process to occur efficiently. In summary, we have demonstrated that the incorporation of Ag into Cu2O results in


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the suppression of H2 evolution and subsequent enhancement of the ethanol selectivity with more CO production relative to the original Cu2O electrode. In addition, the elemental arrangement of Ag-Cu2OPB has two advantages for the production of ethanol in comparison with that of AgCu2OPS in terms of (1) maintaining surface atomic composition of Ag to properly produce CO that can further react with residual intermediate on Cu and (2) population of Ag-Cu biphasic boundaries for favorable CO-insertion reaction to proceed toward ethanol formation.

CONCLUSION

In this work, we prepared Ag-incorporated biphasic Cu2O-Cu catalysts with different elemental mixing patterns through a facile and widely useful synthetic method, electrochemical co-deposition. We found that the introduction of Ag into Cu2O has a catalytic effect on tuning the selectivity toward ethanol over ethylene in the electrochemical reduction of CO2 due to the suppression of H2 production and the increase of CO population. Compared with the faradaic efficiency for ethanol of 10.5% on biphasic Cu2O-Cu without Ag dopant, the faradaic efficiencies for ethanol of 20.1% and 34.15% were recorded on Ag-Cu2OPS and Ag-Cu2OPB, respectively. In this context, it seems that the phase-blended mixing pattern of Ag-Cu2OPB has a couple of critical advantages for the selective production of ethanol. Through several catalyst surface characterization methods, it was demonstrated that the ratio of Cu to Ag could be maintained to a certain extent after electrode reduction on the surface of Ag-Cu2OPB, but Cu was predominantly dispersed in the surface of Ag-Cu2OPS relative to the bulk region. Although the more abundant Ag on surface of Ag-Cu2OPB can increase CO production for subsequent ethanol production, it was not enough to explain the different selectivity for ethanol between two different Ag-Cu2O samples. At last, we postulated that considering their elemental arrangements,


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the phase-blended pattern of Ag-Cu2OPB, even despite partial separation after oxide reduction, seems to provide a specific advantage because Ag and Cu are more neighboring each other within an appropriate distance for efficient CO insertion than that in the phase-separated pattern of Ag-Cu2OPS. In conclusion, the remarkable effect derived from the geometric characteristics of Ag-Cu2OPB leads to a dramatic enhancement of the product selectivity toward ethanol, and this can provide more insights into designing multi-component catalysts for electrochemical reduction of CO2.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail for Jaeyoung Lee: [email protected] Funding Sources Korea Institute of Energy Technology Evaluation and Planning (KETEP) Gwangju Institute of Science and Technology (GIST)

Notes The authors declare no competing financial interest. Supporting Information. Scheme of electrode preparation, XRD, LSV curves, SEM, TEM-EDX, Chronoamperograms, Tables of faradaic efficiencies of all the products, AR-XPS analysis


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ACKNOWLEDGMENT This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030031720). This work was supported by the “Climate Technology Development and Application” research project (K07723) through a grant provided by GIST in 2017.

ABBREVIATIONS Ag-Cu2OPB, Phase-blended Ag-incorporated Cu2O; Ag-Cu2OPS, Phase-separated Agincorporated Cu2O. REFERENCES [1] Sharkun, J. D.; Clark, P. U.; He, F.; Marcott, S. A.; Mix, A. C.; Liu, Z.; Bliesner, B. O.; Schmittner, A.; Bard, E. Nature 2012, 484, 49-54. [2] Wei, T.; Yang, S.; Moore, J. C.; Shi, P.; Cui, X.; Duan, Q.; Xu, B.; Dai, Y.; Yuan, W.; Wei, X.; Yang, Z.; Wen, T.; Teng, F.; Gao, Y.; Chou, J.; Yan, X.; Wei, Z.; Guo, Y.; Jiang, Y.; Gao, X.; Wang, K.; Zheng, X.; Ren, F.; Lv, S.; Yu, Y.; Liu, B.; Luo, Y; Li, W.; Ji, D.; Feng, J.; Wu, Q.; Cheng, H.; He, J.; Fu, C.; Ye, D.; Xu, G.; Dong, W. Proc. Natl. Acad. Sci. USA 2012, 109, 12911-12915. [3] Najafabadi, A. T. Int. J. Energy Res. 2013, 37, 485-499. [4] Oelkers, E. H.; Gislason, S. R.; Matter, J. ELEMENTS 2008, 4, 333-337


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The Importance of Ag-Cu Biphasic Boundaries for Selective

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