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Coupled Metal/Oxide Catalysts with Tunable Product Selectivity for Electrocatalytic CO2 Reduction Sheng-Juan Huo, Zhe Weng, Zishan Wu, Yiren Zhong, Yueshen Wu, Jianhui Fang, and Hailiang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07707 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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ACS Applied Materials & Interfaces

Coupled

Metal/Oxide

Product

Selectivity

Catalysts for

with

Tunable

Electrocatalytic

CO2

Reduction Shengjuan Huo,*,†, ‡,||, § Zhe Weng,‡,||,§ Zishan Wu,‡,|| Yiren Zhong,‡,|| Yueshen Wu,‡,|| Jianhui Fang† and Hailiang Wang*,‡,||

†

Department of Chemistry, Science Colleges, Shanghai University, 99 Shangda Road, Shanghai

200444, China. ‡

Department of Chemistry, Yale University, New Haven, Connecticut 06520 , United States.

||

Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States.

KEYWORDS mixed metal oxides, metal/metal oxide electrocatalyst, CO2 electrocatalytic reduction reactions, selectivity, tune

ACS Paragon Plus Environment

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ABSTRACT

One major challenge to electrochemical conversion of CO2 to useful fuels and chemical products is the lack of efficient catalysts that can selectively direct the reaction to one desirable product and avoid the other possible side products. Making use of strong metal-oxide interactions has recently been demonstrated to be effective in enhancing electrocatalysis in the liquid phase. Here we report one of the first systematic study on composition-dependent influences of metal-oxide interactions on electrocatalytic CO2 reduction, utilizing Cu/SnOx heterostructured nanoparticles supported on carbon nanotubes (CNTs) as a model catalyst system. By adjusting the Cu/Sn ratio in the catalyst material structure, we can tune the products of the CO2 electrocatalytic reduction reaction from hydrocarbon-favorable to CO-selective to formic-acid-dominant. In the Cu-rich regime, SnOx dramatically alters the catalytic behavior of Cu. The Cu/SnOx-CNT catalyst containing 6.2% of SnOx converts CO2 to CO with a high Faradaic efficiency of 89% and a jCO of 11.3 mA·cm-2 at -0.99 V vs. the reversible hydrogen electrode (RHE), in stark contrast to the CuCNT catalyst on which ethylene and methane are the main products for CO2 reduction. In the Snricher regime, Cu modifies the catalytic properties of SnOx. The Cu/SnOx-CNT catalyst containing 30.2% of SnOx reduces CO2 to formic acid with a Faradaic efficiency of 77% and a jHCOOH of 4.0 mA·cm-2 at -0.99 V, outperforming the SnOx-CNT catalyst which only converts CO2 to formic acid in a Faradaic efficiency of 48%.


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1. Introduction Carbon capture and utilization technology has recently attracted much attention due to the severe greenhouse gas emissions as well as the challenging shortage of fossil fuel resources.1,2 Recycling CO2 for producing useful fuels, especially electrochemical reduction of CO2 to carbonaceous products with renewable energy sources, is regarded as an appealing way to control carbon balance.3,4 One of the key challenges for CO2 electroreduction reaction is to develop catalysts with high selectivity and activity.5 Thus far, various categories of electrocatalyst materials, including metals,6-8 metal oxides,9-11 metal chalcogenides,12-14 metal halides,15 molecular metal complexes,16,17 and novel carbon materials18,19 have been examined for CO2 reduction. Particularly, Hori’s group pioneered the study on most of the metallic or bimetallic bulk electrodes.20,21 Later work showed that nanostructuring with size, morphology and crystal faceting controls could increase the catalytic activity, selectivity and durability of metals.22,23 Among all the metals, Cu is almost the only one that is known to catalyze CO2 electroreduction to hydrocarbons with significant yield.24 Metal oxides such as RuO2,9 TiO2,25 ZnO,26 Cu2O,27 Ag2O,28 Co3O4,29 PbO30 and SnOx31 are also active for electrocatalytic CO2 reduction. Among them, SnOx is known to catalyze CO2 electroreduction to formic acid with good selectivity.32 In pursuit of better electrocatalysts for CO2 reduction, materials with multiple components or complex structures are worth exploring. Adopting metal-metal oxide heterostructures is an effective way of creating new opportunities for accomplishing better catalysis. Strong metal-oxide interactions have been widely utilized to improve kinetics for chemical catalysis in the gas phase,33-35 however similar concepts are much less investigated for electrocatalysis in the liquid phase.36-40 There have only been a few


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examples in the area of electrocatalytic CO2 reduction. Xie et al. have achieved selective and stable CO2 reduction to formate with a 2D Co/Co3O4 atomic layer catalyst.41 Pérez-Ramírez et al. have reported Cu/In(OH)342 and Ag/In(OH)343 catalysts for electrochemical CO2 reduction to CO with moderate Faradaic efficiency (FE). Jiao et al. have recently developed Ag-Sn bimetallic nanoparticles with a naturally formed ultrathin SnOx shell which can reduce CO2 to formate in a FE of ca. 80% at -0.8 V vs. reversible hydrogen Electrode (RHE).44 More recently, Sun et al. have found that a 0.8 nm SnOx shell on Cu nanoparticles could dramatically change the catalytic product selectivity from hydrocarbon-favorable to highly CO selective (FE ca. 90%).45 These findings demonstrate that metal/(hydro)oxide interfaces, if properly designed, can positively influence the electrocatalytic kinetics of CO2 reduction. Despite the progress, systematic studies on metal/oxide heterostructured catalyst systems, e.g., the dependence of catalytic properties, especially product selectivity, on material composition, are still lacking. Here we report on Cu/SnOx nanoparticles anchored on carbon nanotubes (CNTs) as CO2 reduction electrocatalysts whose catalytic selectivity can be widely tuned by taking advantage of metal-oxide interactions in different composition regimes. In the Cu-rich regime, as little as 0.01% (Sn/(Cu+Sn) atomic ratio) of SnOx is enough to significantly reduce the selectivity of Cu for hydrocarbons. As the amount of SnOx increases to 6.2%, the Cu/SnOx-CNT catalyst becomes remarkably CO selective. A FE of 89% together with a partial current density of 11.3 mA cm-2 is obtained for CO2 conversion to CO at -0.99 V (vs. Reversible hydrogen Electrode (RHE)). In the relatively Sn-rich regime, the intrinsic catalytic property of SnOx becomes dominant and Cu can serve to further improve the product selectivity for formic acid. The Cu/SnOx-CNT catalyst containing 30.2% of SnOx can reduce CO2 to formic acid in a FE of 77% and a partial current density of 4.0 mA cm-2 at -0.99 V, outperforming the SnOx-CNT material.


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2. Experimental Section 2.1. Material Synthesis and Characterization 2.1.1. Material synthesis To synthesize CuOy/SnOx-CNT materials with various Cu/Sn ratios, various amounts of Cu(OAc)2·2H2O and SnCl2 ethanol solutions (Table S1) were first mixed with 8 mg of mildlyoxidized multiwall CNTs46 dispersed in 2 ml of ethanol and bath-sonicated for 1 h to give a uniform dispersion. Different amounts of 0.5 M KOH ethanol solution (Table S1) were then slowly added under vigorous magnetic stirring to ensure an appropriate pH value for the reaction system. The system was allowed to react for 6 h in an oil bath at 80 oC. After reaction, the solid products were centrifuged at 11000 rpm, successively washed for several times by ethanol and water, and then freeze-dried for use. 2.1.2. Structural characterizations X-ray diffraction (XRD) patterns were collected on a Rigaku SmartLab X-ray Diffractometer equipped with a Cu-target X-ray tube (λ=0.154 nm) and operated at 40 mA and 44 kV. X-ray photoelectron spectroscopy (XPS) was performed using a PHI VersaProbe II X-ray Photoelectron Spectrometer with a monochromatic 1486.7 eV Al Kα X-ray source. The energy scale was calibrated using the Cu 2p3/2 peak (932.67 eV) of a clean Cu foil and the Au 4f7/2 peak (84.00 eV) of a clean Au foil. The binding energies (B. E.) were adjusted by shifting the C 1s peak to 284.6 eV. X-ray fluorescence (XRF) measurements (Rigaku ZSX Primus II) were performed for the average elemental composition. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were conducted with a Hitachi SU8230 SEM


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microscope. 1HNMR spectra were recorded on a V600 Varian VNMRS 600 MHz NMR spectrometer.

2.2. Electrochemical measurements Electrochemical measurements were carried out with a Bio-Logic VMP3 electrochemistry workstation. A home-made gas-tight H-type electrochemical cell separated by a cation exchange membrane (Nafion N115, Du Pont) was used. The electrolyte was 0.1 M KHCO3 aqueous solution which had been electrochemically purified for 48 h before use.16 When saturated with CO2, the pH of the electrolyte was 6.8. The working electrode was carbon fiber paper (Tory, 30% PTFE) with an active area of 0.5 cm2 covered by drop-drying 50 µL of a catalyst ink (prepared by dispersing 4 mg of catalyst material in 350 µL of ethanol and 50 µL of 5 wt% Nafion solution with bath sonication for more than 30 min). The catalyst mass loading was 1 mg cm-2. A graphite rod and a Ag|AgCl electrode were used as the counter electrode and the reference electrode, respectively. The reference electrode was periodically calibrated and the potential applied on the working electrode was corrected using the iR compensation function of the electrochemistry workstation. The working electrode compartment of the cell was continuously purged with CO2 at a flow rate of 10 sccm in the reaction process. Gaseous products from the cell were injected via an automatic sampling loop into a gas chromatograph and analyzed to determine their concentrations. The gas chromatograph (Multiple Gas Analyzer, MG#5, SRI Instruments) was equipped with molecular sieve and HayeSep D columns with N2 as the carrier gas. Hydrogen was analyzed by a thermal conductivity detector, and carbon monoxide, methane, and ethylene were determined using a flame ionization detector. The liquid products such as formic acid were


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quantified by 1HNMR using a solvent presaturation technique to suppress the water peak. Faradaic efficiency was determined from the amount of charge passed to produce each product divided by the total amount of charge passed at a specific time or during the overall run.

3. Results and Discussion CuOy and SnOx nanoparticles were simultaneously grown on CNTs using a solution-phase synthesis (Scheme 1, Table S1). The CNTs not just serve as a support to anchor and disperse the oxide nanoparticles but can provide electron conduction paths during electrocatalysis. The elemental compositions of the materials were determined by XRF spectroscopy. The measured Sn/(Cu+Sn) atomic ratios matched reasonably well with the designed ratios (Table S2). Morphology and phase information of the series of as-prepared materials with varied Sn% values were characterized by SEM (Figure S1). Figure 1a-e and Figure 2a, 2b show the SEM images and XRD patterns of 5 materials with representative Sn% values. It can be recognized from the images that CuOy and SnOx nanoparticles were successfully deposited on CNTs. Figure 1 Figure 2

XRD reveals that the pure CuOy nanoparticles grown on CNTs (CuOy-CNT-#1) are crystalline CuO. The diffraction pattern remains unchanged with 0.01% SnOx incorporated (CuOy/SnOxCNT-#2). As the Sn% increases to 6.2% (CuOy/SnOx-CNT-#7), a mixed diffraction pattern of CuO and Cu2O is observed. The appearance of Cu2O is attributed to reduction of Cu2+ by Sn2+. With a further increase of Sn% to 30.2% (CuOy/SnOx-CNT-#12), Cu2O becomes the dominant phase. No diffraction peaks of any Sn compounds are detected for the CuOy/SnOx-CNT materials, indicating that the Sn in these materials likely exists as amorphous SnOx. This is consistent with


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the observation that pure SnOx nanoparticles grown on CNTs (SnOx-CNT-#13) are also amorphous. We also note that the XRD peaks of CuO and Cu2O do not shift as the Sn/Cu ratio changes, which can exclude the possibility of forming single-phase tin-doped copper oxides. Figure S2 shows transmission electron microscopy (TEM) images of CuOy/SnOx-CNT-#7. Nanoparticles in the size range of 5-10 nm are anchored on CNTs, with the lattice fringes of CuO clearly visible. XPS spectra were used to characterize the surface composition and chemical environment of the CuOy/SnOx-CNT materials. The Cu 2p core level spectrum of CuOy-CNT-#1 exhibits clear CuO features: Cu 2p1/2 and 2p3/2 peaks at 953.8 and 933.9 eV with their corresponding satellite peaks (Figure 2c). As the Sn% increases, Cu2O features start to appear in the spectra. The CuOy/SnOx-CNT-#12 samples exhibit B.E. peaks at 952.7 and 932.8 eV without obvious satellites, characteristics of cuprous oxide species.47 The spectral evolution dependent on the compositional changes of the materials is consistent with the XRD results. The corresponding Sn 3d core level spectra are plotted in Figure 2d. The 3d3/2 and 3d5/2 components appear at 495.4 and 486.9 eV, respectively, which can be tentatively assigned to Sn(IV).47 The surface Sn/(Sn+Cu) atomic ratios derived from the XPS spectra are 5.0% and 27% for CuOy/SnOx-CNT-#7 and CuOy/SnOx-CNT-#12, respectively. These values are comparable to the corresponding bulk atomic ratios obtained from XRF, suggesting that the CuOy and SnOx nanoparticles are evenly distributed between on the surface and in the bulk, without forming core-shell like structures. The O 1s core level spectra of the materials are shown in Figure S3. The spectral changes following the compositional changes of the materials agree well with those observed in the Cu 2p spectra. Figure 3


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The CuOy/SnOx-CNT materials with varied compositions were assessed as electrocatalysts for CO2 reduction reactions in 0.1 M aqueous KHCO3. At -0.99 V vs. RHE, the catalysts manifest interesting composition-dependent product selectivity (Figure 3). The CuOy-CNT-#1 material catalyzes CO2 reduction to CH4 and C2H4 in FEs of 13.5% and 34.3%, respectively, with CO and formic acid being minor products. The product distribution is similar to the reported Cu electrocatalysts under similar conditions.48,49 This indicates that the CuO nanoparticles are reduced to metallic Cu at this potential.50 Incorporation of a small amount of SnOx into the catalyst material structure can alter the catalytic performance. With 0.01% of SnOx, the CuOy/SnOx-CNT-#2 catalyst generates more CO and less hydrocarbons than the CuOy-CNT-#1. As the SnOx content increases, the catalyst becomes more and more selective for CO production. With 6.2% of SnOx, the CuOy/SnOx-CNT-#7 catalyst selectively converts CO2 to CO in a high FE of 89%. Further increase of SnOx content lowers the selectivity for CO and increases that for formic acid. On the other end of the compositional spectrum, the SnOx-CNT-#13 material mainly reduce CO2 to formic acid in a FE of 48%, which is characteristic behavior of SnOx. Adding a copper component to the catalyst material structure can influence the catalytic performance. With a SnOx content of 30.2%, CuOy/SnOx-CNT-#12 catalyst reduces CO2 to formic acid in a FE of 77% with a jHCOOH of 4.0 mA·cm-2, much higher than that on the SnOx-CNT-#13 catalyst. The linear sweep voltammograms of the 5 representative catalysts are shown in Figure S4. It is noted that catalysts with higher SnOx contents deliver lower current densities at the same potential, indicating that the overall catalytic activity decreases with the SnOx content in the material structure. This trend is in good agreement with the steady-state current densities obtained for the CuOy/SnOx-CNT catalysts in the chronoamperometry mode at -0.99 V (Figure 3).


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Figure 4 For the 5 representative catalysts, we further measured their electrocatalytic kinetics for at different applied potentials. The results are plotted in Figure 4. For CuOy-CNT-#1 (Figure 4a) and CuOy/SnOx-CNT-#2 (Figure 4b), more CH4 and less CO are produced at more negative potentials; the FE for C2H4 reaches a maximum at -0.99 V. Both the product distribution and the potential-dependent hydrocarbon FE resemble those reported for Cu electrocatalysts.48,49 For the CO-selective catalyst CuOy/SnOx-CNT-#7 (Figure 4c), a maximum FE for CO of ca. 89% is achieved at -0.99 V with a stable partial current density of 11.3 mA cm-2. Both the high CO FE and current density can be maintained for at least 5 h of continuous electrolysis (Figure S(5a)). For the formic acid-selective catalyst CuOy/SnOx-CNT-#12 (Figure 4d), a maximum FE for formic acid of ca. 79% is achieved at -1.09 V with a stable partial current density of 6.2 mA cm-2. Both the high formic acid FE and current density can be maintained for at least 5 h of continuous electrolysis (Figure S(5b)). SnOx-CNT-#13 (Figure 4e) exhibits similar potential-dependent product distribution as CuOy/SnOx-CNT-#12, except for a lower FE for formic acid and a higher FE for CO. In addition, it is worth noting that the Cu-containing CuOy/SnOx-CNT-#12 catalyst manifests better catalytic durability than the SnOx-CNT-#13 (Figure S6). Figure 5 The 5 representative catalysts after 1.5 h of continuous electrolysis at -1.0 V were characterized with XRD, XPS, SEM and EDX. The diffraction peaks in the XRD patterns of the used CuOy/SnOx-CNT catalyst materials all belong to metallic Cu (Figure 5a). The used SnOxCNT-#13 remains amorphous. These results suggest that the CuOy nanoparticles are reduced to Cu while the SnOx nanoparticles remain oxidized under the electrocatalytic conditions, which is consistent with previous studies. Furthermore, no shifts are observed for the Cu diffraction peaks,


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excluding the possibility of forming Cu-Sn alloys. Therefore, we believe the active electrocatalysts are metallic Cu and amorphous tin oxide nanoparticles anchored on CNTs, denoted as Cu/SnOx-CNT. This is also confirmed by XPS studies. The Cu 2p peaks of the used CuOy/SnOx-CNT catalysts are at B. E. of 932.8 and 952.7 eV (Figure 5b), suggesting that the materials have metallic Cu exposed on surface, the same as the used CuOy-CNT-#1 catalyst. The Sn 3d peaks of the used CuOy/SnOx-CNT catalysts are at B.E. of 486.8 and 495.4 eV (Figure 5c), suggesting that the materials still maintain an oxidized Sn surface, the same as the used SnOxCNT-#13 catalyst. Based on the XPS spectra, the surface Sn/(Sn+Cu) atomic ratios are 9.4% and 32.6% for Cu/SnOx-CNT-#7 and Cu/SnOx-CNT-#12, respectively. The Sn% values are slightly higher compared to those of the as-synthesized CuOy/SnOx-CNT materials, which can be explained by morphological changes during CuOy reduction to Cu. Morphology and elemental distribution of the 5 used electrocatalysts were further characterized by SEM coupled with EDX. It is evident from the SEM images that the electro-generated Cu nanoparticles are larger in size (10-30 nm) than the original CuOy nanoparticles (Figure 6a, c, e, g, i), likely due to partial aggregation during the electroreduction process, which is also verified by the TEM imaging results (Figure S7). The corresponding elemental maps clearly verify the trend of the Cu/SnOx ratio in the catalyst material structures (Figure 6b, d, f, h, j). Figure 6 Figure 7 This work represents one of the first studies that systematically probe composition-dependent influences of metal-oxide interactions on electrocatalytic CO2 reduction. Composition, namely Sn/Cu ratio, of our catalyst materials is controlled in the co-precipitation step where CuOy and SnOx nanoparticles are grown on CNTs. The Cu/SnOx metal/oxide interfaces are formed in-situ


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under CO2 electroreduction conditions. The Cu-SnOx interactions render new catalytic properties strikingly different from either Cu or SnOx based catalysts (Figure 7). On one side of the compositional spectrum, SnOx dramatically alter the catalytic behavior of Cu. The Cu/SnOx-CNT-#7 catalyst converts CO2 to CO with a high selectivity (89% FE), which is in sharp contrast to the Cu-CNT-#1 catalyst that mainly reduces CO2 to hydrocarbons in a moderate FE. The selectivity also distinguishes from the SnOx-CNT-#13 catalyst which mainly generates formic acid. Thus, the Cu-SnOx interaction generates distinct catalytic sites on the catalyst surface. This can be explained by electronic and geometric effects. Theoretical analysis has shown that a good catalyst for electrochemical CO2 reduction to CO should bind *COOH strongly enough to lower the energy requirement for the reaction and also bind *CO weakly enough to remove the CO product without triggering further protonation and reduction.51 At the Cu/SnOx interface, electron transfer from the oxide semiconductor to the metal nanoparticles is favorable due to the electronic structure alignment at the junction.52 Similar electron transfer phenomena have been reported for Ag/In(OH)3 and Cu/In(OH)3 interfaces.42,43 Increased electron density on the Cu site may strengthen the binding of *COOH (CO2 is a Lewis acid) but weaken the binding of *CO (CO is a Lewis base), and thus favors CO2 reduction to CO. In terms of geometric effect, SnOx is oxygen-affinitive and may help stabilizing the *COOH adsorbed on an adjacent Cu site by binding with one of the O atoms. Such a dual site mechanism may also contribute to the observed catalytic selectivity for CO. We believe that the Cu sites, while influenced and assisted by the adjacent SnOx sites, are responsible for catalyzing CO2 reduction to CO. This differs from a recent report where ultrathin SnO2 coating on Cu nanoparticles can selectively reduce CO2 to CO.45


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On the other side of the compositional spectrum, the catalytic behavior is generally dominated by the SnOx component and formic acid is the major product. However, Cu-SnOx interaction can significantly increase the FE for formic acid. The Cu/SnOx-CNT-#12 catalyst reduces CO2 to formic acid in a FE of 77%, much higher than that for the SnOx-CNT-#13 catalyst. The improved formic acid production can also be explained by possible electronic and geometric effects at the Cu/SnOx interface. Electron relocation from the oxide to the metal may strengthen the binding of SnOx to *OCHO which is a key intermediate for CO2 electroreduction to formic acid. In fact, enhanced catalytic selectivity for formic acid has recently been observed for SnOx coated on AgSn or Cu nanoparticles.44 The Cu site may also adsorb *H and spill it over to the adjacent SnOx site to facilitate CO2 hydrogenation.8

4. Conclusions In summary, we have systematically studied Cu/SnOx heterostructured nanoparticles supported on CNTs for catalyzing electrochemical CO2 reduction reactions. By adjusting the Cu/SnOx ratio in the material structure, the major product for CO2 electroreduction can be successfully tailored from hydrocarbons to CO to formic acid. Electronic interactions and geometric synergistic effects at the Cu/SnOx interface are possibly responsible for the tunable catalytic selectivity. The results add the composition-tunable Cu/SnOx-CNT materials to the list of product-selective electrocatalysts for CO2 reduction. The structural insights from this study may guide future development of better catalyst materials for energy conversion.

ASSOCIATED CONTENT


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Supporting Information. This Supporting Information is available free of charge on the ACS Publications websites via the Internet at http://pubs.acs.org. Tabulated material synthesis recipes and additional characterization results. (PDF)

AUTHOR INFORMATION Corresponding Author *Hailiang Wang: E-mail: [email protected] *Shengjuan Huo: E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §

Shengjuan Huo and Zhe Weng contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was sponsored by the Doctoral New Investigator grant from the ACS Petroleum Research Fund and the Natural Science Foundation of China (Grant No. 21103105). Shengjuan Huo gratefully acknowledges the financial support from China Scholarship Council for her visiting research at Yale University.

REFERENCES


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Scheme 1. Schematic illustration of CuOy/SnOx nanoparticle growth on CNTs.


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Figure 1. SEM images of (a) CuOy-CNT-#1, (b) CuOy/SnOx-CNT-#2, (c) CuOy/SnOx-CNT-#7, (d) CuOy/SnOx-CNT-#12 and (e) SnOx-CNT-#13.


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Figure 2. (a) YSn (Sn/(Sn+Cu)) atomic ratio, (b) XRD patterns and (c) Cu 2p and (d) Sn 3d core level XPS spectra for CuOy-CNT-#1, CuOy/SnOx-CNT-#2, CuOy/SnOx-CNT-#7, CuOy/SnOxCNT-#12, and SnOx-CNT-#13.

Figure 3. Composition-dependent product selectivity and total current density for the CuOy/SnOx-CNT materials as electrocatalysts for CO2 reduction at -0.99 V vs. RHE in 0.1 M aqueous KHCO3 (pH 6.8) saturated with CO2. The square symbols represent steady-state current densities.


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Figure 4. FEs and partial current densities for major products of CO2 electroreduction catalyzed by (a) CuOy-CNT-#1, (b) CuOy/SnOx-CNT-#2, (c) CuOy/SnOx-CNT-#7, (d) CuOy/SnOx-CNT#12 and (e) SnOx-CNT-#13 at various applied potentials.


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Figure 5. (a) XRD patterns and (b) Cu 2p and (c) Sn 3d core level XPS spectra for CuOy-CNT#1, CuOy/SnOx-CNT-#2, CuOy/SnOx-CNT-#7, CuOy/SnOx-CNT-#12, and SnOx-CNT-#13 after CO2 electroreduction catalysis.


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Figure 6. SEM images of (a) CuOy-CNT-#1, (c) CuOy/SnOx-CNT-#2, (e) CuOy/SnOx-CNT-#7, (g) CuOy/SnOx-CNT-#12, and (i) SnOx-CNT-#13 after CO2 electroreduction catalysis. The corresponding overlaid EDX elemental maps are shown in (b), (d), (f), (g), (h), and (j).


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Figure 7. Correlations between material structures and product distributions for compositiontunable Cu/SnOx-CNT materials as electrocatalysts for CO2 reduction.


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