Copper Interface for Relay Electroreduction of Carbon Dioxide

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Silver/Copper Interface for Relay Electroreduction of Carbon Dioxide to Ethylene Jiaqi Wang, Zhe Li, Cunku Dong,* Yi Feng, Jing Yang, Hui Liu,* and Xiwen Du* Institute of New Energy Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China

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S Supporting Information *

ABSTRACT: CO2 electroreduction provides an effective solution on CO2 emission and greenhouse effect. However, it is a big challenge to produce hydrocarbon fuels with high energy density via the electroreduction of CO2. Here we report the efficient production of ethylene by constructing Ag−Cu bimetallic catalyst with sharp interface; the Faradaic efficiency for ethylene formation is enhanced to 42%, more than 2 times that of pure Cu catalyst. The high yield of ethylene can be rationalized by the relay catalysis of Ag and Cu component around the Ag/Cu interface.

KEYWORDS: CO2 electroreduction, catalyst, interface, ethylene, Ag/Cu, Faradaic efficiency

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produced under the catalysis of Ag component, then captured by adjacent Cu atoms, and further transform into hydrocarbon under the catalysis of Cu component. Although several groups developed AgCu bimetallic catalysts for CO2 electroreduction, they focused on the homogeneous AgCu alloy or core−shell structure.24−27 Hitherto, AgCu heterostructure with exposed Ag/Cu interface has not been investigated for its potential on the selective electroreduction of CO2 to hydrocarbons with high energy density. In this work, we test the above hypothesis by constructing Ag/ Cu composite with tiny Ag nanoparticles distributed evenly on the surface of large Cu nanoparticles. As expected, the heterojunction shows excellent catalytic activity on CO2 electroreduction, the FE for ethylene reaches 42% at the potential of −1.1 V vs. RHE, which was much higher than that of pure Ag catalyst (0%), Cu catalyst (19.5%), and homogeneous AgCu alloy (26.8%). In addition, the high FE kept for 30 h without noticeable deterioration, indicating an excellent longterm durability. Ag/Cu composite was produced by sequential precipitation (SP) of Ag/Cu2O heterostructure and then electroreduction. For comparison, homogeneous AgCu alloy was prepared by homochronous precipitation (HP) of AgCuOx nanoparticles and subsequent electroreduction (see details in the Supporting Information). According to the X-ray diffraction (XRD) profiles shown in Figure 1a, the as-prepared Ag/Cu2O sample contains Cu2O (PDF No. 05−0667) and Ag (PDF No. 04−0783), the

O2 electroreduction has attracted considerable attention among various strategies for converting CO2 into fuels.1,2 The most concern for this technology is exploring highly effective catalysts to enhance Faradaic efficiency (FE) and reduce energy consumption.3,4 Nowadays, common catalysts for CO2 electroreduction are based on metallic materials.5,6 For instance, the noble metals, such as Au,7 Ag,8 and Pd,9 exhibit high FE for CO formation at moderate overpotentials, whereas nonprecious metals, such as Sn,10 In11 and Bi,12 are favorable to the production of formate. Particularly, Cu is the only catalyst to reduce CO2 into hydrocarbon fuels, including methanol, methane, and ethylene,13 among which ethylene is known as an important industrial gas and owns high energy density.14,15 So far, it is still a big challenge to improve the selectivity of ethylene.16 Interface engineering has emerged as a powerful tool for enhancing catalytic activity in many fields, such as CO oxidation,17 water−gas shift reaction18 and methanol synthesis,19 because the interface can create unique catalytic sites.20 Yet, this technique has shown great potential on promoting CO2 electroreduction. For instance, Gao et al. engineered Au/CeOx interface to enhance the yield and selectivity of CO.21 Back et al. employed Au/Cu interface to obtain a comparable activity with Au but significantly reduced the consumption of Au.22 Ma et al. reported that the phase-separated Cu/Pd catalyst achieved good selectivity for ethylene.23 Therefore, heterojunction catalysts with numerous interfaces are efficient for CO2 electroreduction. Regarding the components of heterojunction catalysts, previous works indicate that Ag catalyst is conducive to produce CO gas,8 and Cu catalyst facilitates the transformation of adsorbed CO into hydrocarbon.13 These findings inspire us to propose a Ag/Cu heterojunction, where CO molecules are first © XXXX American Chemical Society

Received: November 24, 2018 Accepted: January 8, 2019 Published: January 8, 2019 A

DOI: 10.1021/acsami.8b20545 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. Morphology and structure of Ag/Cu2O and AgCuOx. (a) XRD patterns, (b) XPS spectra of Ag 3d region, (c) XPS spectra of Cu 2p region, (d) SEM image of Ag/Cu2O, and (e) SEM image of AgCuO.

Figure 2. Morphology and structure of Ag/Cu and AgCu catalysts. (a) EDS spectra. (b) XRD patterns. HAADF-STEM images and combined EDS mappings of (c) Ag/Cu and (d) AgCu showing elemental distribution of Cu (red) and Ag (green).

consists of big gray particles with sizes about 300 nm loaded with fine bright nanoparticles with sizes around 90 nm (Figures 1d and Figure S2). TEM images and EDS mapping indicate that the big and tiny particles comprise Cu2O, and Ag, respectively. In contrast, the AgCuOx sample is composed of nanoparticles with consistent contrast, and EDS mapping illustrates that Cu, Ag, and O elements distribute evenly in the nanoparticles (Figure S3).

same phase constitution is found for the AgCuOx sample. The energy-dispersive spectroscopy (EDS) spectra in Figure S1 reveal that the atomic ratio of Ag:Cu:O is 1:23.4:12.8 for Ag/ Cu2O sample, and 1:24.9:13.2 for AgCuOx. In both samples, Ag and Cu element presents in Ag0 and Cu+ states, respectively, according to X-ray photoelectron spectrometer (XPS) results shown in Figures 1b, c. We also observed the two samples by scanning electron microscope (SEM). The Ag/Cu2O sample B


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Figure 3. Electrocatalytic properties of Ag/Cu, AgCu, and Cu catalysts. (a) Faradaic efficiencies of CO (gray), formate (blue), ethanol (violet), and ethylene (orange) for Ag/Cu catalyst in the potential range from −0.8 to −1.2 V (vs. RHE). (b) Ethylene FEs and (c) partial current density of Ag/Cu catalyst (orange), AgCu catalyst (violet) and Cu catalyst (blue) in the potential range from −0.8 to −1.2 V (vs. RHE). (d) Long-term stability of Ag/ Cu catalyst at a potential load of −1.1 V (vs. RHE) and the corresponding FEs of ethylene.

hydrogen evolution reaction (HER), which is a competing process of CO2 reduction. By comparison, a more dramatic current increase is observed in CO2-saturated 0.1 M KHCO3 solution, indicating that the electroreduction of CO2 is catalytically favorable. Gas chromatograph (GC) technique and nuclear magnetic resonance (NMR) spectrometer were employed to analyze the constitution of the products under the catalysis of four catalysts. The results indicate that Ag/Cu catalyst facilitates the formation of CO, HCOOH, C2H4, C2H5OH (Figures S11 and S12, Table S1). Particularly, this catalyst shows excellent selectivity to C2H4 (Figure 3a). By contrast, the AgCu and Cu catalysts induce much less C2H4 (Figure S13) and pure Ag catalyst does not produce hydrocarbons at all, and the product is mainly CO (more than 80% at −0.8 V vs RHE) (Figure S14). Figure 3b summarizes the FEs of C2H4 formation under the catalysis of Ag/Cu, AgCu and Cu, respectively. Ag/Cu catalyst exhibits the highest ethylene FE (42% at −1.1 V vs. RHE) compared to AgCu (26.8%), pure Cu (19.5%). Meanwhile, Ag/ Cu catalyst presents much higher total current density and C2H4 partial current density (2.31 mA at −1.1 V vs. RHE) than AgCu (1.17 mA) and Cu (0.63 mA) (Figure 3c and Figure S15). We also varied the Ag content in Ag/Cu catalyst from 2 wt % to 10 wt % (Figure S16) and tested their electroreduction properties (Figure S17). As shown in Figure S18, the Ag/Cu catalyst with 6 wt % Ag content achieves the highest FE for ethylene. Further increase in the Ag content causes serious aggregation of Ag nanoparticles, leading to a drop in FE of ethylene (Figure S19).

The Ag/Cu heterostructure and AgCu alloy were obtained via electroreduction of Ag/Cu2O and AgCuOx samples, respectively. The EDS spectra in Figure 2a show that the Ag contents of Ag/Cu and AgCu catalysts are 5.94 and 5.68 wt %, respectively. According to the Cu−Ag binary diagram, the largest solid solubility of Ag in Cu is 2 wt % at room temperature, thus part of Ag atoms may precipitate from Cu matrix in both Ag/Cu and AgCu catalysts. Indeed, XRD patterns in Figure 2b illustrate that the both catalysts contain Ag (PDF No. 04−0783) and Cu (PDF No. 04−0836) phases. EDS mapping demonstrates that, in Ag/Cu catalyst, tiny Ag nanoparticles with the size of several ten nanometres anchored on the surface of large Cu particles (Figure 2c); while in AgCu catalyst, Cu and Ag elements distribute evenly across the whole nanoparticles (Figure 2d), indicating that the Ag nanoparticles are very small and embed evenly in Cu particles. In addition, pure Cu and Ag nanoparticles were prepared as control samples. Cu catalyst was obtained by electroreduction of Cu2O nanoparticles (see details in the Supporting Information), while Ag catalyst was fabricated by chemical reduction of AgNO3. Both Cu and Ag catalysts show similar sizes with their counterparts in Ag/Cu catalyst (Figures S4−S7). Besides, the as-prepared Ag/Cu, AgCu and Cu catalysts have a similar specific surface area (Figure S8). We then conducted electrochemical reduction of CO2 by using the above four catalysts (Figure S9). The linear sweep voltammetry (LSV) profiles are presented in Figure S10. In N2saturated solution, the current increase should arise from C


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binding energy of Cu (EB[CO] = −1.52 eV) is three times that of Ag (EB[CO] = −0.48 eV), as depicted by Figure S21. Therefore, once the CO molecules form on Ag atoms, they may escape from Ag atoms, pass through the Ag/Cu interface, and are captured by the Cu atoms (Figure 4d). In the following steps toward ethylene, a rate-determining step is the formation C−C bond via CO dimerization.3 A prerequisite for CO dimerization is the existence of closely contacted CO molecules, otherwise, the CO will evolve into C1 products.3,31 The Ag/Cu interface favors the generation dense CO molecules along the Ag/Cu boundaries, because the Ag side can produce CO molecules at high turnover frequency, and the Cu atoms can capture the CO molecules efficiently. As a result, a large amount of CO molecules are accumulated at the Cu side of the Ag/Cu interface. Next, the *CO−*CO dimerization is mediated by electron transfer, rendering a *C2O2− intermediate,3 and continuous coupling proton−electron transfer into the intermediates leads to protonation and dehydration, eventually giving rise to the ethylene molecules (Figure 4e).32−35 The low productivity of AgCu alloy catalyst can be rationalized as below: the starting material for preparing AgCu alloy is single-phase AgCuOx where Cu and Ag elements distribute homogeneously. After reduction, Ag element precipitates as pure Ag phase (Figure 2b) because its concentration (5.68 wt %) is higher than its solubility in Cu at room temperature (2%). However, Cu and Ag are mutually soluble according to Ag−Cu phase diagram, hence, there must be a transition AgCu layer between Ag and Cu area. This transition layer weakens the direct transfer of CO molecules from Ag to Cu, significantly reduces the density of CO molecules along the Cu boundary, depresses the CO dimerization, and eventually lowers the FE of ethylene. In conclusion, we build up Ag/Cu interface and investigated its catalytic activity on CO2 electroreduction. We found that the Ag/Cu interface is crucial for the production of ethylene, the FE of ethylene can reach 42% at −1.1 V (vs RHE) and remain stable over 30 h. The high FE of ethylene arises from the relay catalysis of Ag and Cu sites around the interface. Our work provides a new idea on engineering bimetallic catalysts, and paves a new way toward high productivity of ethylene by CO2 electroreduction. The relay catalysis at interface can be extended to develop various catalysts for efficiently producing hydrocarbon fuels.

Meanwhile, the stability of Ag/Cu was evaluated at a constant potential load of −1.1 V for 30 h (Figure 3d). The current density maintained steady at −5.9 mA cm−2 without any degradation, and the FE of ethylene formation kept stable with slight fluctuation around 42%. In addition, the Ag/Cu catalyst after the stability test was characterized by XRD, SEM and TEM. As shown in Figure S20, the morphology and phase of the Ag/ Cu catalyst with interface remained unchanged. Ag/Cu catalyst exhibits overwhelming superiority on producing ethylene to AgCu catalyst. Since the Ag/Cu interface is the most noticeable structural difference between the two catalysts the Ag/Cu interface should play a crucial role on the selectivity for ethylene. Next, we discuss the possible mechanism related to the Ag/Cu interface, which was schematically shown in Figure 4.

Figure 4. Proposed mechanism for the electroreduction of CO2 to ethylene at Ag/Cu interface. (a) CO2 molecules adsorbed on Ag nanoparticle. (b) Formation of *COOH intermediate on Ag nanoparticle by transferring a proton and an electron to a CO2 molecule. (c) Formation of *CO intermediate on Ag nanoparticle by transferring an electron and a proton to *COOH. (d) Transfer of CO intermediate from Ag nanoparticle to Cu side. (e) Formation of ethylene by coupling two CO molecules. The orange arrows indicate proton and electron transfer. The yellow arrow indicates the migration of CO molecule from Ag side to Cu side. The pink arrows indicate the coupling two CO molecules.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b20545. Materials, detailed synthetic procedures, characterization methods, calculation, and additional experimental results (PDF)

It was widely accepted that CO is a key intermediate for producing hydrocarbons by the electroreduction of CO2.13,28 The formation of CO intermediate usually experiences two steps, first, the CO2 molecules are captured by the Ag atoms, along with the transfer of one proton and one electron (Figure 4a, b)29 CO2 + * + (H+ + e−) → *COOH

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.D). *E-mail: [email protected] (C.K.D.) *E-mail: [email protected] (H.L).

(1)

Second, the *COOH intermediates are reduced to *CO by the following equation (Figure 4c) *COOH + (H+ + e−) → *CO + H 2O

ASSOCIATED CONTENT

S Supporting Information *

ORCID

(2)

Cunku Dong: 0000-0001-8277-6707 Xiwen Du: 0000-0002-2811-147X

Feaster et al. demonstrated that Ag atoms possess higher activity than Cu atoms on producing CO molecules.30 Meanwhile, our density functional theory (DFT) calculations show that the CO

Notes

The authors declare no competing financial interest. D


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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (51871160, 51671141, and 51471115)

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Copper Interface for Relay Electroreduction of Carbon Dioxide

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