High Selectivity Toward C2H4 Production over Cu Particles Supported

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High selectivity towards C2H4 production over Cu particles supported by butterfly-wing-derived carbon frameworks Yajiao Huo, Xianyun Peng, Xijun Liu, Huaiyu Li, and Jun Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19423 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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ACS Applied Materials & Interfaces

High selectivity towards C2H4 production over Cu particles supported by butterfly-wing-derived carbon frameworks Yajiao Huo†, Xianyun Peng†, Xijun Liu*, Huaiyu Li, and Jun Luo*

Center for Electron Microscopy, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China

KEYWORDS: CO2 reduction, Cu particles, synergistic effect, ethylene production, butterflywing-derived carbon framework

ABSTRACT: Converting carbon dioxide to useful C2 chemicals in a selective and efficient manner remains a major challenge in renewable and sustainable energy research. Herein, we adopt butterfly wings to assist the preparation of an electrocatalyst containing monodispersed Cu particles supported by nitrogen-doped carbon frameworks for efficient reduction of CO2. Benefiting from structure advantages and the synergistic effect between nitrogen dopants and

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stepped surface-riches Cu particles, the resulting catalyst exhibited a high faradic efficiency of 63.7 ± 1.4 % for ethylene production (corresponding to an ethylene/methane products’ ratio of 57.9 ± 5.4) and an excellent durability (~100% retention after 24 h). This work presents some guidelines for the rational design and accurate modulation of metal heterocatalysts for high selectivity towards ethylene from CO2 electroreduction.

1. INTRODUCTION The accumulation of the greenhouse gas CO2 in the atmosphere as a result of anthropogenic activities, such as the fossil fuel combustion and industrial manufacturing, not only leads to global warming but also threatens sustainable human development.1, 2 The electrochemical CO2 reduction reaction (CO2RR) to valuable chemicals offers a potential solution for this widespread problem, effectively changing a societal hindrance into practical products. In reality, however, there are a variety of products generated from the CO2RR, based on a multiple-electron transfer mechanism.3–6 Generally, C2 products (especially for hydrocarbons and alcohols) are more favorable than C1 because of their higher energy density, even though the industrial synthesis of C2 is more complicated than that of C1. In addition, undesirable side reactions caused by the hydrogen evolution reaction will degrade the CO2RR performance. Therefore, there are still many challenges in the quest for suitable catalysts that has exclusive selectivity for multicarbon products. Previous works have shown that product selectivity depends on the morphological, electronic and chemical surface properties of catalysts.7–13 For instance, polycrystalline Cu had a similar activity for CH4 and C2H4, with a C2H4/CH4 products’ ratio of approximately 1 – 2.3, 5, 12, 13

More improvements were realized on single-crystal catalysts, which showed that the Cu(100)


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surface favored C2H4 production and resulted in a high C2H4/CH4 ratio value of 10.14 Unfortunately, it was still not an easy task to synthesize high-quality single-crystal catalysts and thereby limited their practical applications. Recently, stepped Cu surfaces have proven to be capable of increasing the C2H4 selectivity due to the exposure of abundant low-coordinated kink sites between adjacent crystal facets.6 Further, recent studies have shown that the nitrogen (N) site in N-doped carbon-based materials could serve as active sites for CO2RR,10, 15 especially for pyridinic-N. It was found that pyridinic-N species favored CO2 adsorption and one-electron reduction to generate *COOH intermediates, which was of paramount importance to selective formation of hydrocarbons.16–18 Inspired by these studies, we hypothesize that the selectivity for C2H4 formation can be finely tuned by the synergistic effect between pyridinic-N and Cu crystal with stepped surfaces. Herein, we report a simple synthesis of Cu particles supported by butterfly-wing-derived carbon frameworks (denoted as Cu Ps/BCF) catalyst by using butterfly wings and CuCl2 as the precursors (Figure 1). The butterfly wings can not only prevent Cu particles aggregation19 but also provide a moderate surface area for the Cu particles anchoring on the carbon frameworks surface,20, 21 which is beneficial to the electron mobility and thereby promote CO2 adsorption and reduction activity.22, 23 Moreover, the synergistic effect between Cu particles and pyridinic-N species facilitates hydrogenation and carbon-carbon coupling reactions on Cu for selective formation of C2H4.24–26 Meantime, the porous structure and the high content of pyridinic-N in Cu Ps/BCF contribute an increased CO2 capture capacity, which leads to an improved C2H4 selectivity.27 In addition, the abundant graphitic-N induces a better conductivity and thereby promote the electron transfer during the electrocatalysis.28 As a result, the Cu Ps/BCF catalyst catalyzed CO2 reduction with relatively high faradaic efficiency (FE) of 63.7 ± 1.4% for C2H4


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production at –1.0 V versus the reversible hydrogen electrode (RHE). The value for C2H4/CH4 products’ ratio was 57.9 ± 5.4, which is among the best data for Cu-based catalysts reported thus far. Also, it does not show any obvious decay in the current densities, while its selectivity for producing C2H4 is always greater than 60% during the long test period of 24 h, indicative of their very favorable stability. Our study will open up new opportunities for design and development of Cu-based catalysts for the selective formation of hydrocarbons.

Figure 1. Scheme for the synthesis and the electrocatalytic reduction of CO2 into ethylene over butterfly-wing-derived carbon formwork supported Cu particles.

2. EXPERIMENTAL 2.1 Synthesis. The butterfly wings (Papilio maackii) are specimens purchased from Beijing Jiaying Yishu Insectarium. The wings were degreased in an ethanol ultrasonic bath for


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30 min and washed several times with DI water to obtain hydrophilic surface. After that, a certain amount of the pre-treated butterfly wings were soaked in 20 ml 3 mmol copper chloride (CuCl2·2H2O) solution. In order to guarantee the homogeneity, a 30 min sonication treatment was conducted. Finally, the Cu Ps/BCF was obtained via heating treat the precursor after freezedrying at 1000 °C for 1 h under the NH3 atmosphere. Meantime, the control samples were synthesized by adjusting the amounts of CuCl2·2H2O and butterfly wings. The pure BCF and Cu particles (Cu Ps) were synthesized only without adding CuCl2·2H2O solution or butterfly wings under the same experimental conditions. In addition, the BCF sample was further treated in 20 vol% HNO3 solution for 5 h, and the resulting suspension was centrifuged, in which powders were collected. After that, the powders were rinsed several times with distilled water and subsequently were dried in vacuum at 70 °C for 12 h. Finally, the acid-treated BCF sample was obtained (denoted as BCF-acid). 2.2 Characterization. The crystalline phases were identified by Rigaku D/max 2500 Xray diffractometer (XRD) with Cu Kα radiation (λ = 0.154598 nm) at 40 kV voltage, 40 mA current, 10° min–1 scan rate, and 2θ range of 10° – 100°. The size and morphology of the prepared samples were characterized by field-emission scanning electron microscopy (FESEM, Quanta FEG 250) and transmission electron microscopy (TEM, Talos F200X) equipped with energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific) was used for the elemental mapping using monochromatic Al Kα radiation source. The binding energies calibration was performed by referencing the C 1s main peak at 284.8 eV. N2 adsorption–desorption and CO2 uptake isotherms were measured on a Tristar II 3020 Micromeritics analyser.


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2.3 Preparation of Electrodes. For the preparation of the working electrode, the catalyst inks were firstly prepared by dispersing 10 mg catalyst powers, 1 ml 0.05 wt.% Nafion solution, 0.6 ml DI water, and 0.6 ml isopropyl alcohol. A following high-power sonication (500 W) was employed to achieve a fine dispersion. The resulting suspension was uniformly coated onto the carbon paper. After drying overnight in a vacuum oven, the electrode was obtained with an approximate of 5 mg cm–1 catalyst loading. 2.4 CO2 Electrochemical Tests. All electrochemical measurements were carried out at room temperature (25 °C) in a typical H-type cell with a 0.1 M KHCO3 solution on an electrochemical workstation (CHI 660D). Platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. Linear sweep voltammetry (LSV) was carried out to examine the electrochemical activity of the samples with a scan rate of 10 mV s–1 in the voltage range between –0.6 and 0 V vs RHE. The electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 100 kHz to 10 mHz with a perturbation amplitude of 5 mV. All of the measured potentials were referenced to the RHE according to the Nernst equation potential: ERHE(V) = ESCE(V) + 0.0592 × pH + 0.241 V. 2.5 Product Analysis. Before testing, CO2 was bubbled into the 0.1 M KHCO3 for 30 min to reach saturation. The potentiostatic reduction of CO2 was then carried out undersaturated CO2 levels (2 mL min-1). The resulting gas products were qualitatively analyzed at regular time intervals by gas chromatography (Agilent GC-7890) equipped with a Poraplot Q column and a molecular sieve column. The hydrocarbons and permanent gases were analyzed by flame ionization detector (FID) and thermal conductivity detector (TCD) with N2 as a carrier gas, respectively. The liquid products were analyzed by nuclear magnetic resonance (NMR) (Bruker AVANCE AV III 400) spectroscopy. To quantify the liquid products, the electrolyte (0.5 mL)


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was mixed with 0.1 mL D2O and 0.03 µL dimethyl sulfoxide (DMSO, 99.99%) as an internal standard. The 1H spectrum was measured with water suppression using the pre-saturation method. The Faradaic efficiencies were calculated from the amount of the charge required to produce each product divided by the total charge injected over a specific time or during the overall run (for liquid products).

3. RESULTS AND DISCUSSION

Figure 2. Morphological characterizations of Cu Ps/BCF: (a) SEM image of BCF; (b, c) SEM images of Cu Ps/BCF (particle size distribution chart in inset); and (d) EDS elemental mapping images of N, Cu and C element for Cu Ps/BCF.

As shown in Figure S1a, the raw butterfly wings were composed of tiny overlapped and elongated rectangular scales of two or more layers, forming a roof-tile-like arrangement. High


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magnification SEM image in Figure S1b revealed that quasi-periodic triangular roof-type ridges run the length of the scale. In addition, double-row staggered quasi-periodic lattice works were present between every two ridges, forming a honeycomb-like structure. Besides, the EDS result (Figure S1c) suggested that these wings contain C, H, O, N, P and Mg, beacuse they are biological tissues that mainly consist of proteins and polysaccharides.29 After the NH3 heat treatment, the unique architecture of the butterfly wings is still retained (Figures 2a and S2). Meanwhile, the corresponding EDS in Figure 2a demonstrated that BCF contains the same component as the raw butterfly wings (Figure S3). An extremely good replication of the fine detail of the original scale structures is also clearly seen in the Cu Ps/BCF composite. As displayed in Figure 2b and Figure S4, the monodispersed Cu particles within an approximately 1 – 5 µm in diameter and an average size of 2.29 µm were distributed on the BCF. Interestingly, the Cu particles were not regular and smooth spherical structure but a sphere or ellipsoid with many stepped surfaces (Figure 2c). The unique surface microstructure can make the Cu particles expose a great amount of terraces, edges, facets, corners, which can primly serve as the active sites during the electrolysis.4,

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The pore size distribution obtained by the Barrett-Joyner-

Halenda (BJH) method indicated the main pore size concentrated around 2.97 nm in Cu Ps/BCF, which is shown in Figure S5. As previous reported,27 the presence of micropores was considered an important factor in promoting the CO2 adsorption. However, when the butterfly wings were not used, the synthesized Cu Ps (Figure S6) present a cobblestone-like morphology with a big size (up to tens of micrometers). These results indicated that the butterfly wings played a critical role in the preparation procedures, which can prevent the aggregation of Cu particles and inhibit them enlarge in size.30, 31 Furthermore, the TEM image (Figure S7) and the corresponding EDS


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element mapping images (Figure 2d) confirmed that the particle belongs to Cu and displayed that a highly homogeneous N element distributed on the carbon frameworks.

Figure 3. Structural characterizations of Cu Ps/BCF: (a) XRD pattern; (b) Cu 2p XPS spectra; (c) C 1s XPS spectra; and (d) N 1s XPS spectra.

To determine the possible phase and composition of Cu Ps/BCF, XRD and XPS analyses were further carried out and the results were shown in Figure 3. The XRD pattern for BCF could be indexed to amorphous carbon (Figure 3a), further verified by its High-resolution TEM image and the corresponding Fourier transform (FFT) pattern in Figure S8. The sharp and symmetric diffraction peaks revealed for Cu Ps/BCF catalyst can be indexed as (111), (200), (220), (311), and (222) phases of Cu (JCPDS, No. 04-0836), which was strong confirm the presence of metallic Cu and its high crystallinity. Furthermore, the high-resolution XPS spectrum offered more information about the surface composition. In detail, the Cu 2p spectrum has been deconvoluted (Figure 3b), which peaks at 932.7 and 952.6 eV, corresponding to the presence of


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metallic Cu in Cu Ps/BCF composite.32, 33 This result was consistent with that of pure Cu Ps (Figure S9). In Figure 3c and Figure S10a, the C 1s spectrum of Cu Ps/BCF and BCF can be deconvoluted into three subpeaks at 284.8, 286.3, and 290.5 eV which resemble C=C, C-N, and π-π, respectively (Figure 3c).34, 35 Figure 3d and Figure S10b showed that both samples contain pyridinic-N and graphitic-N.36–38 The changes in the relative amounts of the two N species from BCF to Cu Ps/BCF are 38.0 → 49.7% (pyridinic-N), and 62.0 → 50.3% (graphitic-N), of which the most remarkable is the increase in the pyridinic-N content and the decrease in the graphitic-N content in Cu Ps/BCF. As previously reported, both pyridinic and graphitic nitrogen atoms show high activities for CO2RR, of which graphitic-N improve the electrical conductivity28 and pyridinic-N facilitate CO2 adsorption and subsequent proton-electron transfer;16, 26 whereby the integration of Cu particles and nitrogen-doped carbon frameworks may exhibit enhanced selectivity and activity of CO2RR. The electrocatalytic performance of Cu Ps/BCF towards CO2 reduction was evaluated in an H-cell consisting of a working and a counter electrode, separated by a piece of Nafion-115 proton exchange membrane to prevent the oxidation of CO2 reduction products. The electrolysis was performed in a CO2-saturated electrolyte of 0.1 M KHCO3 solution. Figure 4a showed the LSV curves of CO2 reduction on BCF, Cu Ps, and Cu Ps/BCF acquired by sweeping the potential in the range of 0 to –0.6 VRHE at a sweep rate of 10 mV s–1. Clearly, the Cu Ps/BCF exhibited much higher current density than the BCF or Cu Ps samples alone.


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Figure 4. Catalytic performance in the electrochemical CO2 reduction: (a) LSV curves in CO2saturated solutions at a scan of 10 mV s–1; (b) FEs for C2H4, CH4, CO, formate and H2 at various applied potentials; (c) Comparison of FEC2H4, FECH4, and the corresponding ratio of FEC2H4 to FECH4 with previous reported catalysts: Cu nanoparticle,6 Cu(711),14 Cu mesocrystals,39 Cu/OLC,40 CuCl-confined Cu-mesh,41 (10% Cu-95% Si),42 CuBr,43 and Electrodeposited Cu2O.44 Reproduced with permission.6 Copyright 2011, The Royal Society of Chemistry. Reproduced with permission.14 Reproduced with permission.39 Copyright 2014, The Royal Society of Chemistry. Reproduced with permission.40 Copyright 2017, Elsevier. Reproduced with permission.41 Copyright 2003, The Electrochemical Society. Reproduced with permission.42


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Copyright 2017, Elsevier. Reproduced with permission.43 Copyright 2004, Elsevier. Reproduced with permission.44 Copyright 2014, The Royal Society of Chemistry; (d) Nyquist plots; (e) Tafel plots of partial current dentist of CO2RR for the catalysts; and (f) Chronoamperometry results at –1.0 VRHE and the corresponding ratio of FEC2H4 to FECH4.

The possible products of CO2RR were analyzed using an on-line chromatography (GC) and a nuclear magnetic resonance (NMR) spectrometer. As displayed in Figure 4b, the FEs for the carbonaceous products (C2H4, CH4, CO, and formate) on Cu Ps/BCF at various applied potentials were presented. The product distribution trend exhibited by Cu Ps/BCF towards CO2 reduction is consistent with previous reports.6, 14, 39–44 It can be found that the largest FE of the total carbonaceous product is obtained at the applied potential of –1.0 VRHE. Obviously, the Cu Ps/BCF exhibits better selectively for CO2 reduction to the C2H4 formation. It increases as a more negative potential is applied, from 11.3 ± 2.2 % at –0.4 VRHE to a maximum of 63.7 ± 1.4 % at –1.0 VRHE, subsequently, it declines to 39.8 ± 4.5% at –1.2 VRHE (Figure 4b). Additionally, the FE of the remainder products is 1.1 ± 0.1% (CH4), 2.9 ± 0.3% (CO), and 8.7 ± 1.5% (formate) at –1.0 VRHE, respectively. As a comparison, the FEs of the products on the BCF and Cu Ps is clearly below that of on Cu Ps/BCF (Figure S11), which strongly demonstrates that the presence of the synergistic effect between Cu Ps and N-doped carbon frameworks brings about a higher CO2 reduction performance. Moreover, at –1.0 VRHE, the ratio value of FEs towards C2H4/CH4 (FEC2H4/FECH4) is 57.9 ± 5.4, much higher than that of BCF (8.4 ± 2.5) and Cu Ps (0.7 ± 0.1) (Figure S12). When adjusting the concentration of CuCl2·2H2O precursor, an optimal Cu Ps/BCF mass ratio was archived at 9:16 (Figure S13). In addition, the FEC2H4, FECH4, and the corresponding ratio of FEC2H4 to FECH4 of the Cu Ps/BCF and other electrodes reported in the


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previous literature were compared and summarized in Figure 4c and Table S1. Obviously, the Cu Ps/BCF exhibits a high competitive superiority as compared with the previously reported Cubased electrocatalysts. Exceptionally, it reported that a Cu2O film with 8.8 µm thickness electrode can offer a FEC2H4/FECH4 ratio of 143.67.8 However, the overall performances have been significantly compromised by the low FE (21.55%) for C2H4 production (Table S1). It can be seen from Figure 4d that the radius of the half semicircle of the Nyquist plots for Cu Ps/BCF is smaller than those of Cu Ps and BCF, indicating the improved electron transfer ability by adding BCF in Cu Ps/BCF composites, which is crucial for the enhanced electrochemical CO2 reduction. Moreover, the reaction kinetics for the C2H4 formation was further analyzed using Tafel slope. As shown in Figure 4e, the resulting Tafel slope of the Cu Ps/BCF was 104 mV dec–1, much smaller than that of BCF (163 mV dec–1) and Cu Ps (128 mV dec–1), respectively. This result suggests the kinetics of *COOH formation is fast over the Cu Ps/BCF.45 Another crucial metric for the design of ideal CO2RR electrocatalyst is stability. As shown in Figure 4f, it can be found that the current density exhibit a no obvious decay during the 24 h continuous electrolysis. Intriguingly, the ratio of FEC2H4 to FECH4 still presents a good retention, suggesting the excellent electrocatalytic stability of the Cu Ps/BCF. Further, the SEM characterization of Cu Ps/BCF after the electrolysis have been performed, and the obtained SEM images were shown in Figure S14. Obviously, after the electrolysis, no aggregation of Cu particles was observed (Figure S14a). Meanwhile, the size distribution of these Cu particles before and after the electrolysis was close to each other. These results manifested that the carbon framework can effectively prevent agglomeration of Cu nanoparticles. However, the highmagnification SEM image (Figure S14b) clearly showed that a structural transformation occurred on the surface of Cu particles during the electrolysis. In detail, there are numerous


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cube-like nanoparticles appeared after the electrolysis. This finding is consistent with the previous work.46 In addition, according to the above results, there are traces of Mg and P elements in the BCF sample. To determine whether these trace elements will have an impact on CO2 reduction performances, an acid leaching was performed (see details in Experimental section). After that, the EDS and XPS spectra (Figure S15) indicated that Mg and P do not exist in the acid-treated BCF (namely BCF-acid) sample. Further, the comparison of the CO2RR properties between BCF (Figure S11a) and BCF-acid (Figure S16) clearly confirmed that the trace elements (Mg and P) play a trivial role in the CO2 reduction process.

Figure 5. Scheme for the electroreduction of CO2 into C2H4 over Cu Ps/BCF catalyst.

On the basis of above results and discussion, the clearly improved selectively towards the electroreduction of CO2 to C2H4 of Cu Ps/BCF may be attributed to the synergistic effect between Cu particles and BCF (Figure 5). It has been confirmed that the pyridinic-N in N-doped carbon materials can facilitate the formation of *COOH intermediates by lowering the free


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energy barrier.17, 26 When Cu-based catalysts supported by such an N-doped carbon material (i.e., BCF), the *COOH formed around pyridinic-N may subject to next step protonation and dehydration to yield *CO on Cu,6, 11, 47–50 which would undergo further protonated to *CHxO and finally reduced to C2H4 via a C-C coupling reaction.26,

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Meantime, since Cu Ps showed

abundant low-coordinated sites on the stepped surfaces, the strong bonding with *COOH may also further promote the protonation of *COOH and hence boost the electrolysis.6, 11, 47 Note that, such synergistic effect mainly occurred at or around the Cu Ps/BCF interface.51, 52 Accordingly, for the region far away from the interface, this effect would be weakened and even disappeared. On the other hand, previous studies have shown that increasing the CO2 pressure can optimize the activity and selectivity of CO2 reduction.53–55 In our work, Cu Ps/BCF can adsorb high amounts of CO2 molecule due to its abundant micropores (Figure S5) and the high content of pyridinic-N (Figure 3d),56, 57 as evidenced by the CO2 adsorption isotherms in Figure S17, which indicated that Cu Ps/BCF has a higher CO2 capture capacity compared with that of Cu Ps. It is assumed that the largely increased CO2 adsorption could maintain a local microenvironment with high CO2 concentration around Cu Ps, and hence endows them with a high coverage of *CO moieties.27 This may facilitate subsequent proton–electron transfer reactions to produce C2H4.27 Additionally, graphitic N atoms are known to maintain an sp2-hybridized graphitic structure and thus can improve the electrical conductivity of Cu Ps/BCF by providing delocalized electrons.27 Then, the good electrical conductivity can promote the electron transfer between catalyst surfaces and reaction intermediates, leading to an activity enhancement. The synergistic effect observed above was further supported by the reduction catalysis of the Cu Ps/BCF samples with different mass ratios. The mass ratio of Cu Ps : BCF can be easily tuned by controlling the input amount of CuCl2•2H2O in the synthetic process. The SEM images


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clearly demonstrated that Cu particles in these Cu Ps/BCF samples possess similar morphology and size distribution (Figure S18). Their corresponding XPS analysis further showed that the presence of two forms of nitrogen, namely graphitic-N and pyridinic-N (Figure S19). Of note, the pyridinic-N content of these Cu Ps/BCF catalysts is close to each other (Figure S20). According to these results, it is reasonable to infer that if the adding amount of CuCl2•2H2O is too low, the synergistic effect should be very slight. Thus, increasing the CuCl2•2H2O content can make this effect more obvious. But, if the CuCl2•2H2O content is too large, the intrinsic activity of Cu would be highlighted, worsening the effect. The above speculation is consistent with the experimental result in Figure S13, which indicates that when the CuCl2•2H2O content increased from 0 to 4 mmol, the activity towards C2H4 production of Cu Ps/BCF initially increased, reached the optimal value at 3 mmol (corresponds to a mass ratio of 9 : 16) and then decreased. Moreover, the reduction catalysis of the Cu Ps/BCF sample synthesized by adding 4 mmol CuCl2•2H2O demonstrated a very broad product distribution as a function of potential (Figure S21), similar to the result measured by Hori et al3 with a Cu electrode. The CH4 and C2H4 are the dominant products at sufficiently negative potentials. At less negative potentials, H2, formate and CO are instead dominant.

4. CONCLUSIONS In summary, Cu Ps/BCF has been successfully synthesized by a one-step pyrolysis using butterfly wings and copper salt as the precursors. The Cu particles with unique morphological properties present a massive of low-coordinated active sites at the stepped surfaces (e.g., terraces, edges, facets, and corners). Meantime, the nitrogen dopants result in an enhanced conductivity and help to concentrate more proton/hydrogen around Cu, thus facilitating the


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conversion of *COOH to *CO, *CHxO, and C–C coupling to C2H4. Therefore, the Cu Ps/BCF catalyzes CO2 reduction to C2H4 at a moderate applied potential with high selectivity and excellent durability in contrast to pure BCF or Cu particles. This study may open a new horizon toward achieving highly selective and robust CO2 conversion properties based on Cu/N-doped carbon materials.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxx. SEM images, TEM image, XPS spectra, EDS spectrum, and Faradic efficiencies for raw butterfly wing, N2 adsorption–desorption isotherms, BCF, Cu Ps and BCF-acid sample, the ratio of FEC2H4 to FECH4, CO2 uptake isotherms, the particle size distribution, the pyridinic-N content, and a comparison Table (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions Y.H. and X.P. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21601136), National Program for Thousand Young Talents of China, the Tianjin Municipal Education Commission, the Tianjin Municipal Science and Technology Commission (15JCYBJC52600), and the Fundamental Research Funds of Tianjin University of Technology. REFERENCES (1)

Duan, X.; Xu, J.; Wei, Z.; Ma, J.; Guo, S.; Wang, S.; Liu, H.; Dou, S., Metal-free Carbon

Materials for CO2 Electrochemical Reduction. Adv. Energy Mater. 2017, 29, 1700759. (2)

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Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution. J. Chem. Soc. Faraday Trans. 1989, 85, 2309–2326. (4)

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High Selectivity Toward C2H4 Production over Cu Particles Supported

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