CO2 Electroreduction at Low Overpotential on Oxide-Derived Cu

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CO2 electroreduction at low overpotential on oxide-derived Cu/carbons fabricated from metal organic framework Kun Zhao, Yanming Liu, Xie Quan, Shuo Chen, and Hongtao Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15402 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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ACS Applied Materials & Interfaces

CO2 Electroreduction at Low Overpotential on Oxide-Derived Cu/Carbons Fabricated from Metal Organic Framework Kun Zhao, Yanming Liu, Xie Quan*, Shuo Chen, Hongtao Yu Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Dalian, 116024, China. E-mail: [email protected]

KEYWORDS: Carbon dioxide, Electrochemical reduction, Oxide-derived Cu/carbon, Alcohol, Metal organic framework

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ABSTRACT

Electrochemical reduction of CO2 to chemical feedstocks is an attractive solution to prevent CO2 accumulation in atmosphere, but it remains a great challenge to develop the cost-effective catalysts. Herein, we synthesized oxide-derived Cu/carbon (OD Cu/C) catalysts by a facile carbonization of Cu-based MOF (HKUST-1). The resulting materials exhibited highly selective CO2 reduction to alcohol compounds with the total faradic efficiency of 45.2~71.2 % at -0.1 to 0.7 V vs RHE. High-yield methanol and ethanol has been achieved on OD Cu/C-1000 with the production rates of 5.1-12.4 and 3.7-13.4 mg L-1h-1, respectively. Notably, the onset potential for C2H5OH formation is near -0.1 V (vs. RHE), corresponding to ~190 mV of overpotential, which is among the lowest overpotentials reported to date for the reduction of CO2 to C2H5OH. The improvements in activity and selectivity of the oxide-derived Cu/carbon might be attributed to the synergistic effect between the highly dispersed copper and the matrix of porous carbon. These findings provide a new insight into design of practical catalysts for decreasing atmospheric CO2 levels and synthesizing liquid fuels.


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INTRODUCTION The accelerated depletion of fossil fuel resources leads to rising level of CO2 in the atmosphere, which results in energy crisis and global warming1-5. An attractive solution of environmental protection and energy regeneration is to develop a scalable method for transforming CO2 to energy-dense fuels and chemical feedstock6. Electrocatalysis is one of the particularly appealing ways to reduce CO2 among various approaches developed thus far, due to its moderate reaction condition, controllable selectivity and scalable application7-9. The electrochemical reduction of CO2 is a complex process with multiple proton- and electrontransfer steps, which produce various kinds of reduced products, such as carbon monoxide, formic acid, formaldehyde, methanol, methane and so forth in the liquid and gas phase9,10. Amongst these possible products, alcohols such as methanol and ethanol are desirable owing to their high energy density, availability, and safety10-12. In thermodynamics, CO2 could be reduced to CH3OH or C2H5OH at the potential of 0.02 or 0.09 V (versus reversible hydrogen electrode (RHE))13, respectively. However, these two reactions are considered to be kinetically unfavorable, as CH3OH and C2H5OH synthesized from CO2 require the transfer of 6 and 12 electrons, respectively. In addition, the activation of CO2, which serves as the first step of CO2 reduction, is difficult. Therefore, a negative potential is required for the formation of CH3OH and C2H5OH, which consume lots of energy. To overcome these limitations, a cost-effective electrocatalyst with low overpotential, high reaction kinetics and good products selectivity is highly desirable. Up to now, a variety of metal and metallic complexes electrodes have been designed to catalyze the CO2 reduction, but the major products for most electrocatalysts investigated are CO and formate14. Due to its electronic configuration, copper (Cu) is able to reduce CO2 through


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breaking the C-O bond in CO2 and allow further convert CO to more reduced species such as hydrocarbons and alcohols15. However, an overpotential of almost 1V is required for CO2 reduction on Cu electrode16, resulting in large energy losses. It is widely accepted that the overpotential for CO2 electrochemical reduction is related to the binding energies of the intermediates on the surface of electrodes, and that protonation of adsorbed CO is the most important step in dictating the overpotential17,18. The binding energy of CO on Cu surface is related to its d-electron availability, which can be easily changed by electronic effects and geometric effects16,18,19. It has been reported that the activity and selectivity for CO2 electroreduction on Cu is sensitive to its surface morphology and electronic structure. Kanan and co-workers20,21 proposed that the thermal-oxide derived electrodes can decrease the overpotential of product formation and favor the generation of hydrocarbons. Interestingly, methanol was found to form on intentionally oxidized Cu electrode with high selectivity and low overpotential22. This phenomenon possibly depends on the electronic properties of Cu1+, which could absorb atomic O into the surface of electrode and facilitate the methanol formation23. Although there are numerous studies focusing on CO2 electroreduction, the formation of C2 products, such as ethanol, is rarely reported. Converting CO2 to C2 products is in high demand because industrial synthesis of C2 products is usually more complicated and energy intensive than that of C1 products24. The C-C coupling has been proposed as key process in the formation of C2 species. It is worth noting that the CO2 reduction on Cu2O films electrode exhibits high selectively for C2 compounds at -0.99 V vs RHE, with faradic efficiencies of 34-39% for ethylene and 9-16% for ethanol25. Additionally, carbon supported metal hybrid catalysts are found to be more active toward the generation of C2 or C3 products, as they could create a virtually high CO2 pressure at the triple phase boundary25,26. Long carbon chain products, such as


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iso-propanol and other oxygenates, are found in carbon nanotube-based metal catalysts for CO2 electroreduction13. Baturina et al13 also showed that carbon-supported Cu nanocatalysts are more selective toward C2H4 generation versus electrodeposited Cu at the same experimental condition. Metals such as Cu, Fe and Ni doped xerogels are found to be capable of catalyzing CO2 to C1C3 products27. Recently, high selectivity of CO2 electroreduction to ethanol is achieved by Cu nanoparticle/N-doped grapheme electrode28. Unfortunately, most electrocatalysts reported previously still require a large overpotential to maintain the faradic efficiency for C2 products. Therefore, it is highly desirable to develop a novel catalyst which integrates both high current efficiency and high stability for converting CO2 to alcohols at low overpotential. It is to be noticed that the activity and selectively of these carbon-based electrocatalysts for CO2 reduction are affected by the nature of the metal, distribution of metal particles and properties of the carbon substrate. Among various carbon substrates, porous carbon materials are attractive for manufacturing carbon-based hybrid catalysts due to their large pore volume, which allows thorough distribution of the CO2 molecules over the catalysts’ surface and create a large number of active sites for CO2 conversion29. Recently, metal-organic frameworks (MOF) has been explored for CO2 reduction as their abundantly porous structure30,31. Notably, direct carbonization of metal-organic frameworks (MOFs) offers a straightforward approach to fabricate the functional porous carbon and metals/metal oxides hybrid materials32,33. The porous carbon could be directly cast from MOFs during the process of high-temperature treatment, while the metal centre in MOFs could be incorporated into the carbon matrix in situ. This structure thus offers an excellent electronic connection between the metals/metal oxides and the carbon frameworks. Moreover, the MOF-derived materials exhibit lots of distinct merits, such as


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well-dispersed metal nanoparticles, hierarchical porous structures and large surface areas, which benefit the promotion of electrocatalytic activities33-35. Herein, we synthesized oxide-derived Cu/carbon (OD Cu/C) catalysts for CO2 reduction by carbonization of Cu-based MOF (HKUST-1). The obtained materials exhibit improved selectivity for alcohol formation from CO2 at low overpotential. The effects of the metal Cu, porous carbon and oxidation state of the prepared materials on the CO2 reduction activity and products selectivity were investigated. In addition, the in situ infrared spectrum was employed to analyze the mechanism for electrocatalytic reduction of CO2 on OD Cu/C electrode. RESULTS AND DISCUSSION The schematic illustration for fabrication of oxide-derived Cu/carbon (OD Cu/C) catalysts was shown in Scheme 1. Briefly, the Cu-based MOF (HKUST-1) was used as a precursor for preparing OD Cu/C catalysts, and its crystal structure and morphology were characterized by powder X-ray diffraction (XRD, Figure S1) and scanning electron microscopy (SEM, Figure 1a), respectively. The prepared Cu based MOF showed the morphology of octahedral with uniform size of ~10µm which was consistant with the reported structure. The XRD pattern demonstrated that the Cu-based MOF was highly crystalline and was well matched the typical crystalline structure of HKUST-1.

Scheme 1. The synthesis process of oxide-derived Cu/carbon catalysts.


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Subsequently, the oxide derived Cu nanoparticles embedded in a porous carbon matrix were produced by carbonization of HKUST-1 under Ar atmosphere. The thermogravimetric analysis (TGA, Figure S2) curve of HKUST-1 showed three serious weight loss during the pyrolysis, which were attributed to the loss of volatile solvent, removal of trapped water molecules from MOF and decomposition of organic linkers, respectively36. There was no significant weight loss when the temperature was heated up to around 400 ℃ and the residual materials could be the carbon, copper oxides and metallic copper compounds37. The carbonization of Cu-based MOF was conducted at 900℃, 1000℃ and 1100℃ for 6h under Ar atmosphere, and the obtained catalysts were denoted as OD Cu/C-900, OD Cu/C-1000 and OD Cu/C-1100, respectively. The Powder X-ray diffraction of the obtained OD Cu/C samples (Figure S3) presented the distinctive peaks at 2θ = 43.5°, 50.7°, 74.7°, and 89.9°, which could be assigned to Cu (JCPDS card No.040836). The diffraction peaks at 2θ = 36.5° and 61.4° belonged to the Cu2O residue. The diffraction peak of carbon (002) for these three materials was low because of the high intensity of Cu pattern. High resolution SEM image (Figure 1b-d and Figure 1f-h) showed that the morphologies of the prepared porous OD Cu/C composite catalysts were octahedral, which presented a rough surface and retained the size of the MOF template. Moreover, the surface of products was decorated with a number of Cu particles and holes, indicating the formation of copper and carbon porous hybrid materials during the carbonization process. The sizes of Cu nanoparticles broaden with increasing carbonization temperature, and the average particle sizes were approximately 53.8 nm, 59.6 nm and 220 nm for OD Cu/C-900, OD Cu/C-1000 and OD Cu/C-1100 catalysts, respectively (Figure S4). As shown in transmission electron microscopy (TEM) images (Figure 1e), the oxide derived Cu nanoparticles embedded in a porous carbon


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matrix were produced by carbonization of HKUST-1 at different temperature under Ar atmosphere.

Figure 1. a) SEM images of HKUST-1; b), f) SEM images of the OD Cu/C-900; c), g) SEM images of the OD Cu/C-1000; d), h) SEM images of the OD Cu/C-1100; e) TEM images of the OD Cu/C-1000. To evaluate the electrochemical behavior of the porous OD Cu/C catalysts for CO2 reduction, linear sweep voltammetry (LSV) test in Ar or CO2 saturated 0.1 M KHCO3 solution (pH 8.8 and pH 6.8, respectively) were performed. As shown in Figure 2a, the monotonically increased cathodic current in Ar-saturated electrolyte was possibly caused by the hydrogen evolution reaction (HER), while the higher current density in CO2-saturated solution could be ascribed to the CO2 electroreduction reaction that occurred in parallel with the hydrogen evolution reaction. The obvious peak was observed in CO2-saturated electrolyte, whereas no reduction peak appeared at their linear sweep voltammograms under Ar, demonstrating that CO2 can be electrochemically reduced on these three catalysts. Similar curve with a slight shoulder near 0.77 V (vs. RHE) in CO2-saturated 0.1 M KHCO3 aqueous was reported by Hori’s group, and they suggested that the reduction peak might be attributed to the adsorbed CO during the


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reduction reaction1. Additionally, the onset potential and reduction peak potential of OD Cu/C for CO2 reduction was about -0.1 V and -0.5 V, respectively, which were positive than those of most of Cu electrocatalysts reported recently1,13,18. Among these materials, the current density of OD Cu/C-1000 catalyst (1.0 mA cm-2 at -0.5 V (vs. RHE)) was higher than that of OD Cu/C-900 (0.37 mA cm-2) and OD Cu/C-1100 (0.73 mA cm-2). To avoid the effect of local pH, LSV curves of CO2 reduction were also performed in a phosphate buffered solution, the onset potential for CO2 reduction was similar with that in KHCO3 solution (Figure S5). Furthermore, the electrochemical impedance spectroscopy (EIS) of OD Cu/C catalysts were tested in CO2 saturated 0.1 KHCO3 solution. As shown in Figure 2b, the OD Cu/C-1000 catalysts exhibited relatively lower charge-transfer resistance, which was beneficial for enhanced electronic interaction during the electroreduction CO2. These results suggested that OD Cu/C-1000 catalyst was highly active for CO2 reduction.

Figure 2. Linear sweep voltammetry curves of OD Cu/C materials in Ar- or CO2-saturated solution with a scan rate of 50 mV s-1 (a), EIS data of OD Cu/C catalysts in CO2-saturated


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solution with an AC amplitude of 5 mV and a frequency range between 400 kHz and 0.1 Hz (b), Methanol and ethanol production rates for CO2 electrochemical reduction: c) OD Cu/C-900, d) OD Cu/C-1000, e) OD Cu/C-1100 (applied potential from -0.1 to -0.7 V vs.RHE and electrolyte concentration 0.1 M KHCO3).

The CO2 electroreduction activities of the OD Cu/C catalysts obtained at different carbonization temperature were further measured in a series of constant-potential electrolysis in 0.1M KHCO3 saturated with CO2 at ambient temperature. The yield of methanol and ethanol within 3 h on OD Cu/C-900, OD Cu/C-1000 and OD Cu/C-1100 catalysts were presented in Figure 2. Interestingly, these oxide-derived Cu catalysts exhibited the same onset potential for methanol and ethanol formation of -0.1 V vs RHE, which correspond to 120 mV and 190 mV of overpotential for generation of methanol and ethanol from CO2, respectively. The low overpotential of alcohol formation might be caused by the Cu2O in OD Cu/C, which was identified as an important contributor to the generation of methanol at low overpotential on oxide-derived Cu electrode21,23. Notably, the OD Cu/C composites catalysts obtained at different carbonization temperatures showed different activity for products formation. The highest production rates for methanol occurred at -0.3 V on OD Cu/C-1000 with the value of 12.4 mg L−1h−1. The decrease of methanol production rate at high applied potentials on OD Cu/C-1000 catalysts might be associated with the limited mass transport of CO2 in the system when ethanol formation rate was high. In addition, the maximum average production rates for ethanol reached 13.4 mg L−1h−1 at -0.7 V, which was 4.3 and 2.5 times as great as that of OD Cu/C-900 and OD Cu/C-1100 under the same potential, respecctively. The production rate of ethanol was still lower when compared with that of formic acid as previously reported, because the ethanol


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synthesized from CO2 required 6 times more electrons (12 electrons) than that for formic acid formation. H2 was only gas phase product detected during the CO2 reduction at applied potential. The faradaic efficiency of products formed on OD Cu/C-1000 catalysts at applied potential was shown in Table S1. Importantly, the faradaic efficiency for C2H5OH formation on OD Cu/C1000 catalysts was comparable to or even higher than those of Cu Nanowire arrays38, Cu-Au alloy39, Cu2O ﬁlms25 and CuO nanoparticles40 at a relatively lower potential (Table S2), which are among the best electrocatalysts reported recently for the generation of C2H5OH from CO2 at low overpotential. To verify methanol and ethanol derived from CO2 reduction, electrolytic experiment was performed on OD Cu/C-1000 catalysts in Ar-saturated KHCO3 solution (without CO2) at -0.7 V vs. RHE (Figure S6). The results showed that the methanol and ethanol were undetectable after electrolysis for 3 h, confirming that methanol and ethanol were synthesized from CO2 reduction.


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Figure 3. Nitrogen adsorption-desorption isotherms and porous size distribution of OD Cu/C catalysts (a), Cu 2p XPS spectra of OD Cu/C-900 (b), OD Cu/C-1000 (c), OD Cu/C-1100 (d). The pore structure, specific surface area, particle diameters and metal content of these MOF derived hybrid materials can be controlled by pyrolysis temperature33. To gain more insight into the different selectivity and activity for CO2 reduction, pore structure and composition of the OD Cu/C catalysts were analyzed by Brunauer-Emmett-Teller (BET), inductively coupled plasma atomic emission spectrometer (ICP-AES) and X-ray photoelectron spectroscopy (XPS). The results of BET and porous distribution (Figure 3a and Table S3) demonstrated that all OD Cu/C materials were hierarchically porous with type IV N2 isotherm. The 3D porous structure could minimize the diffusion resistance for mass transport, which favored the efficient transfer of CO2


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and fast emission of products. Nonetheless, the Cu nanoparticles were agglomerated at high carbonization temperature, resulting in the formation of large sized Cu nanoparticles (Figure 1h and Figure S4) and the destruction of porous structure (Figure 3a). The low surface area, low porosity and large sized Cu nanoparticles were responsible for the low activity of OD Cu/C-1100 toward CO2 reduction. It was notable that the OD Cu/C-1000 had lower BET value than that of OD Cu/C-900, but displayed higher electrocatalytic activity. This might result from the large charge-transfer resistance, which was observed from EIS image (Figure 2b). The OD Cu/C-900 exhibited the smallest mass-transfer resistance among these three OD Cu/C catalysts, due to its high surface area and well configured porous structure. But the charge-transfer resistance of OD Cu/C-900 was larger than that of OD Cu/C-1000, indicating that the OD Cu/C-900 has relatively higher charge transfer resistance for CO2 reduction. These result indicated that the surface area was not the only crucial role in determining the electrochemical activity for CO2 reduction. The high-resolution Cu 2p spectrum (Figure 3b-d) was fitted to two peaks at 932.4 and 932.8 eV that associated with Cu0 and Cu1+, respectively. The Cu content of catalysts surface (Table S4) increased from 3.93 at% to 5.42 at% with the increasing carbonization temperature from 900 to 1100 ℃. The total content of Cu was confirmed by the inductively coupled plasma atomic emission spectrometer (ICP-AES, Table S5), and the same trend was observed. According to the XPS results, the ratios of corrected peak areas for Cu2O and Cu were decreased. Consequently, the Cu2O content for OD Cu/C-1000 (2.46 at%) was higher than that of Cu/C-900 (2.07 at%) and OD Cu/C-1100 (2.27 at%). It was noteworthy that the proportions of Cu2O decreased with broadening the size of Cu nanoparticle. These results were consistent with the previous studies that the smaller metal nanoparticles on the surface were easier oxidized than large one6. It has been reported that the selectivity and activity of Cu0 produced from copper oxide reduction


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depends on the properties of the oxide layer14. Similarly, the Cu1+ was considered to allow the valance band electrons to participate in the CO2 or CO adsorption, which might lead to a stronger adsorption of atomic O onto Cu1+ centers16,17, and thus facilitate the alcohol generation at low overpotentials. In addition, the particle size might also be a contributor to the different product selectivity during CO2 reduction. Therefore, the high selectivity and activity for CO2 reduction to alcohol on OD Cu/C-1000 catalysts was probably ascribed to the high Cu2O content, correct Cu particles size and well-dispersed 3D porous structure that was formed during the pyrolysis process.

Figure 4. XRD pattern (a) and XPS spectra (b) of OD Cu/C-1000 and Cu/C-1000 catalysts, TEM images and SAED pattern of OD Cu/C-1000 (c) and Cu/C-1000 (d).


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To clarify the correlation between Cu2O and alcohol generation from CO2 reduction on OD Cu/C-1000 catalyst, HKUST-1 was also annealed at 1000℃ under H2 flow and the resulting product was denoted as Cu/C-1000. The XRD (Fig. 4a) and XPS (Fig. 4b) analysis demonstrated that the Cu/C-1000 materials were composed of copper and carbon, and there was no Cu2O existing in Cu/C-1000. The morphology of structure was perfectly retained (Figure S7), and the particle size of Cu/C-1000 was similar to that of OD Cu/C-1000 material. Moreover, the BET surface area of Cu/C-1000 catalysts was calculated to be 98.1 m2 g-1. According to the transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns (Figure 4c, d), both the OD Cu/C-1000 and Cu/C-1000 catalysts were composed of copper nanoparticles covered by the porous carbon, but their crystalline structure were different. The Cu (111), Cu (220) and Cu2O (111) crystal plane were observed on OD Cu/C-1000 (Figure 4c), whereas the crystal plane of Cu2O was absent on Cu/C-1000 (Figure 4d). These observations were consistent with the XRD results, indicating that OD Cu/C-1000 and Cu/C-1000 had similar morphology but different crystal structure.

Figure 5. Faradaic efficiency for CO2 electrochemical reduction:a) OD Cu/C-1000, b) Cu/C1000 (applied potential from -0.1 to -0.7 V vs.RHE and electrolyte concentration 0.1 M KHCO3).


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The electrochemical performance of CO2 reduction on Cu/C-1000 catalysts was investigated by linear sweep voltammetry (LSV) test in Ar or CO2 saturated 0.1 M KHCO3 solution. As shown in Figure S8, the onset potential of Cu/C-1000 for CO2 reduction was about -0.25 V , which was more negative than that of OD Cu/C-1000 (-0.1 V). It suggested that the OD Cu/C1000 catalyst was more active for CO2 electroreduction than Cu/C-1000 catalysts. The faradaic efficiency (FE) of carboxylic acid and alcohol products were measured on OD Cu/C-1000 and Cu/C-1000 catalysts at the applied potential between -0.1 and -0.7 V (vs. RHE). As shown in Fig 5a, the OD Cu/C-1000 catalyst presented a high selectivity toward alcohol formation, producing methanol with FE of 13.8~43.2% and ethanol with FE of 24~34.8% at potential between -0.1 and -0.7 V. In addition, the faradaic efficiency of 24% for ethanol production was obtained at -0.1 V vs.RHE, corresponding to 190 mV of overpotential for this products. It is worth noting that the overpotential for C2H5OH formation on OD Cu/C-1000 was the lowest among the electrocatalysts reported to date for production of C2H5OH product from CO225,41,42. For comparison, basically no C2H5OH generation was observed for Cu/C-1000 at -0.1 V vs. RHE, and the maximum faradaic efficiency for C2H5OH on Cu/C-1000 was only 11.8% (-0.7 V voltages). Peterson and Nørskov17 demonstrated that the overpotential required for CO2 reduction was related to the binding energies of adsorbed intermediates produced during the reduction reaction, which can be tuned by changing the electronic properties of catalyst materials. The valance band electrons of Cu2O were thought to participate in the adsorption of CO221,23, which might cause a stronger binding strength of CO2 and CO on electrode surface and reduce the overpotential required for CO2 electrochemical reduction. Therefore, the remarkably low overpotential for alcohol formation from CO2 was possibly caused by Cu2O formed in OD Cu/C-1000 composite materials.


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To understand the correlation between catalytic activities for CO2 electrochemical reduction and the structural components of carbon and metal copper in the OD Cu/C composite catalysts, the well-defined octahedral porous Cu and porous carbon (Figure S9, S10 and Table S3) were synthesized. The porous Cu was prepared by heating OD Cu/C-1000 at 380 ℃ in air and subsequently calcinated at 350 ℃ in H2 atmosphere. The porous carbon was obtained by treating OD Cu/C-1000 with 10 wt% HF for several times. The porous Cu hollow architectures could produce alcohol products during the process of CO2 reduction, but the faradaic efficiency for methanol (5.9-6.2%) and ethanol (1.6-1.9%) were low at the applied potential between -0.5 and 0.7 V vs. RHE (Fig. S11). There was no alcohol detected on the porous carbon catalysts. In comparison, the OD Cu/C-1000 exhibited the faradaic efficiency of 13.8-8.3% and 31.4-34.8% for methanol and ethanol at -0.5 to -0.7 V, respectively. Subsequently, the electrochemical impedance spectroscopy (EIS) of porous Cu and porous carbon were investigated. As shown in Figure S12, the OD Cu/C-1000 catalysts exhibited a semicircle diameter smaller than those of porous Cu and porous carbon, indicating that the OD Cu/C-1000 have a lower charge transfer and mass transfer resistance during CO2 reduction. The enhanced charge-transfer properties of OD Cu/C-1000 might due to the interaction between porous carbon and metal copper. The high alcohol production performance of the oxide-derived Cu/carbon might be attributed to the synergistic effect between a highly dispersed copper and a matrix of porous carbon. Briefly, the Cu nanoparticles embedded in the carbon substrate created catalytic sites for CO formation by dissociating CO2, and for protonating adsorbed CO to generate alcohol products. However, further reduction of CO was unfavorable, because it was difficult to form active hydrogen species and favorable to form H2 on the metal surface. As the nanostructure carbon based electrode was possible to use confinement effect inside the nanopore to increase the local CO2


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concentration13,26,43, the metal and carbon hybrid structure created a high CO2 pressure at the electrode/solution interface, leading to a fully covered surface by CO or CO2 species, which inhibited the reaction between protons and electron to form H2 and benefited C-C coupling6. On the other hand, the localized electrons on the carbon surface were able to transfer to protons, and formed available hydrogen for further reaction with adsorbed CO6,41. Additionally, the porous carbon structure minimized the diffusion resistance for mass transfer, contributing to the efficient transfer of CO2 and diffusion of the alcohol. To investigate the stability of the OD Cu/C-1000 catalysts, five successive batches (each bath lasted 3h) for CO2 electroreduction were performed at an applied potential of -0.7 V versus RHE. As shown in Fig. S13a, the production rates remained around 13 mg L-1 h-1 for ethanol and 7.6 mg L-1 h-1 for methanol during 5 successive batches, indicating that no observable deactivation occurred on OD Cu/C-1000 catalysts. Furthermore, the catalysts were characterized by SEM and XPS spectra (Figure S14) after CO2 reduction. The octahedral morphology was intact, but the surface roughness increased. The XPS spectrum showed a similar structure with the OD Cu/C1000 catalysts before electrolysis, indicating that the catalysts surface was composed of Cu2O and Cu after CO2 reduction. In contrast, the porous Cu exhibited a poor stability during 5 consecutive cycles (Figure S13b). These results suggested that the prepared OD Cu/C composite catalysts possessed good stability. This might be caused by the carbon matrix, which protected the Cu from deactivation during electroreduction of CO2. On the basis of the aforementioned results, we hypothesized that the improvements in activity and selectivity of the oxide-derived Cu/carbon may be attributed to the synergistic effect between the highly dispersed copper and the matrix of porous carbon. To summarize, the superior performance of oxide-derived Cu/C catalysts could be attributed to the factors as


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following: 1) the Cu2O formation in the OD Cu/C-1000 catalyst system modified the surface structure of the polycrystalline copper, causing a strong adsorption of atomic O into the Cu, which facilitated the alcohol generation; 2) the carbon supported metal hybrid structure had a significantly different triple phase boundary at electrode/solution interface, resulting in a higher CO2 pressures at the surface of OD Cu/C catalyst, favoring C-C bond formation; 3) strong electronic interactions between porous carbon substrate and Cu nanoparticles provided more electrons to form active hydrogen species, which benefit to further convert the adsorbed CO to alcohol; 4) high surface area and 3D hierarchical porous structure provided a large electroactive area for CO2 conversion and minimize the diffusion resistance for mass transfer and electrons mobility, favoring the efficient transfer of CO2 and fast diffusion of the alcohol; 5) the carbon matrix was able to protect the incorporated Cu from deactivation during electroreduction of CO2, thus prolonging the life of catalysts.

Figure 6. In situ infrared spectrum of CO2 reduction on OD Cu/C-1000 at -0.7 V vs.RHE with different electrolysis time (a), In situ infrared spectrum of CO2 reduction on OD Cu/C-1000 at the potential range from -0.1 to -0.7 V vs.RHE (b).


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The in situ FTIR spectroelectrochemical techniques was employed for better understanding the mechanism of the CO2 electrochemical reduction on OD Cu/C-1000 catalysts by detecting the adsorbed species formed on the electrode’s surface. The in-situ infrared reflectance (IR) spectroscopy analysis was characterized according to the previous work24. Figure 6 showed the electrolysis time-dependent infrared spectrum recorded at the region between 750 and 3600 cm-1 at -0.7 V vs. RHE, where the signal of OD Cu/C-1000 electrode in 0.1 M KHCO3 solution was used as the background. During electrolysis, the electrolyte was saturated by injecting purity CO2 into the IR cell. The peak observed at 2341 cm-1 corresponds to the antisymmetric stretching mode of dissolved CO2 in aqueous solutions44, and its intensity maintained at the same level during the test. The absorption bands at 1345 and 1589 cm-1 were caused by the asymmetric and symmetric stretching of formate ions (HCOO-)44 and the band around 3300 cm-1 was associated with the O-H stretching of alcohol. The intensity of these peaks increased with accumulation time, suggesting the formation of carboxylic acids and alcohol products. The small bands of bridge-bonded CO (COB) and linear-bonded CO (COL) at approximately 1880 and 2060 cm-144,45 were observed, and its intensity almost remained unchanged in the experiment. This result implies that CO is the intermediate during CO2 reduction. The bands appeared at 2844 and 2916 cm-1 can be assigned to C-H stretching vibration of -CH2-. While a band at 1447 cm-1 was attributed to deformation vibration of -CH3. Moreover, the bonds related to C-O, C=C, and C=O were observed at about 1043, 1665 and 1722 cm-1, respectively46,47. These results indicated the formation of formic acid, methanol and ethanol. The In situ FTIR spectrums of OD Cu/C-1000 at the potential from -0.1 to -0.7 V vs RHE were measured (Figure 6a). Notably, the intensity of peaks appeared at 1345 cm-1 (HCOO-), 1665 cm-1 (C=C), 2844 and 2916 cm-1 (-CH2-), and 3300 cm-1 (O-H) increased with the applied potential negative shifted. This phenomenon indicted that


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more products such as carboxylic acids and alcohol products formed at -0.7 V. Another interesting finding was that the O-H band at -0.3V have a similar intensity with that spectrum observed at -0.5 V, but the peak intensity associated with C=C and -CH2- bands were different. These results were in good consistent with the different CO2 reduction selectively at the potential of -0.3 V and -0.5 V. At -0.3 V, methanol was the major products of CO2 electroreduction whereas generation of ethanol was dominant at the applied potential of -0.5 V. Additionally, the FTIR spectrums of OD Cu/C-900 and OD Cu/C-1100 were collected under CO2 atmosphere at 0.7 V vs RHE (Figure S15). Similar spectra features have been observed after 20 min electrolysis. As shown in Figure S11b, the intense peaks at 3300 cm-1 was obtained on the surface of OD Cu/C-900 catalysts while the peaks related to the C=C and -CH2- were weak. These results suggested that methanol might be the major products of OD Cu/C-900. Interestingly, the intensity of asymmetric (1345 cm-1) and symmetric stretching (1589 cm-1) of formate ions (HCOO-) were increased on OD Cu/C-1100 catalysts. An additional peak attributed to the C-H was also observed at 1116 cm-1 on OD Cu/C-1100 material. These results indicated HCOOH might be the major product for OD Cu/C-1100 during CO2 reduction. Therefore, the variety of adsorbed species formed on the catalysts might provide an explanation for different selectivity for CO2 reduction on OD Cu/C catalysts.


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Scheme 2. Proposed reaction paths for CO2 electroreduction on OD Cu/C-1000, producing formate acid (HCOOH), methanol (CH3OH), and ethanol (C2H5OH). Based on previous experimental studies and the density function theory (DFT)48,49, possible reaction path for CO2 electroreduction on OD Cu/C-1000 are proposed in Scheme 2. In this process, one-electron transfers to CO2 molecule and forms the intermediate CO2*- anion radical. Subsequently, the obtained CO2*- reacts with proton-electron pair and forms the carboxyl (*COOH) intermediate, which further reacts with the second proton-electron pair to obtain the the product of formate acid. Alternatively, the adsorbed *COOH could be reduced to *CO by breaking the C-O bond and removing water molecule. The *CO is considered to be key intermediate during CO2 electrochemical reduction as it can be further reduced to hydrocarbon or oxygenates. Following the formation of *CO radical, the adsorbed *CO reacts with another proton-electron pair to form hydroxymethylidyne (*COH) or formyl (*CHO) radical. The *COH and *CHO are then further reduced to the product of methanol. The plausible reaction pathway for ethanol formation requires the creation of a C-C bond, which could be significantly


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facilitated by constructing nanoporous carbon supported catalysts6. In the process of the ethanol formation, the C-C is coupled between surface-bound C1 oxygenates, followed by the formation of enol-like surface species. The enol-like intermediates are then converted to ethanol by hydrogenation and dehydroxylation. Further studies are required to elucidate the detailed mechanism of the ethanol formation via CO2 electrochemical reduction on the OD Cu/C materials. CONCLUSIONS In summary, we have successfully synthesized the oxide-derived Cu/carbon catalysts by a facile carbonization of metal-organic framework (HKUST-1). The oxide-derived Cu/carbon catalyst exhibited high activity, good selectivity and stability for electrochemical reduction of CO2 to alcohol products. A highly selective CO2 reduction to ethanol was achieved on OD Cu/C1000 at overpotentials as low as 190 mV. Moreover, the production rates for methanol and ethanol were achieved to be 5.1-12.4 and 3.7-13.4 mg L-1h-1 at potentials between -0.1 and -0.7 V (vs.RHE). The improvements in activity and selectivity of the oxide-derived Cu/carbon might be attributed to the synergistic effect between the highly dispersed copper and the matrix of porous carbon. We anticipate the strategy for constructing the oxide-derived Cu/carbon catalyst in this work could provide new insights into development of the electrocatalysts with superior activity and high selectivity for converting CO2 to alcohol products.


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ASSOCIATED CONTENT Supporting Information. Experimental section, XRD pattern (Figure S1) and TGA (Figure S2) curve of HKUST-1, XRD pattern of the OD Cu/C catalysts (Figure S3), Cu nanoparticle size of OD Cu/C catalysts (Figure S4), the LSV curves of OD Cu/C catalysts in phosphate buffer solution (Figure S5), 1H NMR analysis of products (Figure S6), SEM images and Cu particle size of Cu/C-1000 (Figure S7), the LSV curves of OD Cu/C-1000 and Cu/C-1000 (Figure S8), XRD spectrum (Figure S9), SEM images (Figure S10) and Faradaic efficiency (Figure S11) of porous Cu and porous carbon, the EIS data of different catalysts (Figure S12), the stability of the OD Cu/C-1000 and porous Cu catalysts(Figure S13), the SEM and XPS of OD Cu/C-1000 after electrolysis (Figure S14), the in situ FTIR of OD Cu/C-900 and OD Cu/C-1100 catalysts (Figure S15), the faradaic efficiency of OD Cu/C-1000 catalysts (Table S1), the comparison of faradaic efficiency (Table S2), the BET analysis (Table S3) and content of Cu and Cu2O (Table S4) of catalysts, the total Cu content of catalysts (Table S5). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding author: Xie Quan E-mail: [email protected]；Tel.: +86-411-84706140; Fax: +86-411-84706263.


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (NO.21590813), the Programme of Introducing Talents of Discipline to Universities (B13012) and the Fundamental Research Funds for the Central Universities (DUT16TD02). REFERENCES (1) Gattrell, M.; Gupta, N.; Co, A. A Review of the Aqueous Electrochemical Reduction of CO2 to Hydrocarbons at Copper. J. Electroanal. Chem. 2006, 594, 1-19. (2) Liu, Y.; Yang, Y.; Sun, Q.; Wang, Z.; Huang, B.; Dai, Y.; Qin, X.; Zhang, X. Chemical Adsorption Enhanced CO2 Capture and Photoreduction over a Copper Porphyrin based Metal Organic Framework. ACS Appl. Mater. Interfaces 2013, 5, 7654-7658. (3) Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G.; Jiao, F. A Selective and Efficient Electrocatalyst for Carbon Dioxide Reduction. Nat. Commun. 2014, 5, 3242-3248. (4) Centi, G.; Quadrelli, E. A.; Perathoner, S. Catalysis for CO2 Conversion: a Key Technology for Rapid Introduction of Renewable Energy in the Value Chain of Chemical Industries. Energy Environ. Sci. 2013, 6, 1711-1731. (5) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and Homogeneous Approaches to Conversion of CO2 to Liquid Fuels. Chem. Soc. Rev. 2009, 38, 8999. (6) Genovese, C.; Ampelli, C.; Perathoner, S.; Centi, G. Electrocatalytic Conversion of CO2 on Carbon Nanotube-Based Electrodes for Producing Solar Fuels. J. Catal. 2013, 308, 237-249.


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Table of Contents Graphic


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CO2 Electroreduction at Low Overpotential on Oxide-Derived Cu

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