朱慶宮 孫曉甫 康欣晨 馬 珺 錢慶利 韓布興

（中國(guó)科學(xué)院化學(xué)研究所，中國(guó)科學(xué)院膠體界面與化學(xué)熱力學(xué)重點(diǎn)實(shí)驗(yàn)室，北京分子科學(xué)國(guó)家實(shí)驗(yàn)室，北京 100190）

泡沫銅負(fù)載硫化亞銅電極高效電催化還原二氧化碳制備甲酸

朱慶宮 孫曉甫 康欣晨 馬 珺 錢慶利 韓布興*

（中國(guó)科學(xué)院化學(xué)研究所，中國(guó)科學(xué)院膠體界面與化學(xué)熱力學(xué)重點(diǎn)實(shí)驗(yàn)室，北京分子科學(xué)國(guó)家實(shí)驗(yàn)室，北京 100190）

電催化還原二氧化碳制備甲酸是備受關(guān)注的熱點(diǎn)問(wèn)題。而電極材料是決定還原效率的重要因素。本文通過(guò)電沉積方法在泡沫銅上直接制備納米結(jié)構(gòu)硫化亞銅薄膜，并采用掃描電鏡（SEM）、X射線衍射（XRD）對(duì)其結(jié)構(gòu)性能進(jìn)行了系統(tǒng)研究。以硫化亞銅作為陰極電催化材料、0.5 mol·L–11-丁基-3-甲基咪唑四氟硼酸鹽的乙腈溶液為電解液，在該體系中可高效催化轉(zhuǎn)化二氧化碳為甲酸。 結(jié)果表明，這一電解體系可有效實(shí)現(xiàn)電化學(xué)反應(yīng)，甲酸的法拉第效率（FEHCOOH）可以達(dá)到85%，同時(shí)甲酸還原電流密度可達(dá)到5.3 mA·cm–2。

硫化亞銅；泡沫銅；甲酸；電化學(xué)；二氧化碳

1 Introduction

CO2is an abundant and inexpensive C1resource. Conversion of CO2into value-added chemicals is an interesting topic1–4. Among the chemical ways to convert CO2into useful products, the electrochemical method is one of the most efficient routes5–8.

HCOOH is a very useful chemical. Electrochemical reduction of CO2to HCOOH has been studied extensively9–17. For example, tin and tin oxide electrodes have been reported to produce formate with a wide range of faradaic efficiencies. Tin particles found faradaic efficiency of 80%9, while a value of 58% was reported on tin oxide electrode10. This value was significantly increased to 93% when tin oxide nanocrystals were employed as the cathodic material11. It was also found that the faradaic efficiency for formate on carbon nanotube/Ir pincer dihydride complex/polyethylene composite was 96%. However, the current density was only 1.0 mA·cm–2in this reported sys-tem12. In and Pb metals are also well known catalysts for HCOOH formation13. The faradaic efficiencies were observed to be 90% and 45% on In and Pb metal electrodes, respectively. Unfortunately, the current density was limited to 2.0 mA·cm–2for either In or Pb electrode. Recent progress of Cu based material has found that, the annealed Cu2O electrode exhibits strong reduction activities towards CO2, and this electrode produced CO with 40% and HCOOH with 33% faradaic efficiencies14. These results indicate that it is very difficult to achieve satisfactory conversion rate of CO2and high selectivity to HCOOH simultaneously. Therefore, obviously development of efficient electrodes for electrochemical reduction of the CO2to HCOOH is highly desirable although some elegant work has been conducted on this interesting topic.

As an environment-friendly and abundant material, copper sulfides have attracted considerable attention due to their various potential applications, such as dye-sensitized solar cell18, optical filters, and nanoscale switches19–21. The availability of Cu2S nanostructures with well-defined morphologies and dimensions may enable new types of applications. Due to their unique optical and electrical properties, they are also widely applied in thin films and composite materials22–24. The synthesis of new porous metal sulfide materials has great potential for exploring new electrode materials.

Herein we report the first work on electrochemical reduction of CO2to produce HCOOH using Cu2S modified Cu foam electrode. Cu foam with three-dimensional open network structure was used as the substrate of Cu2S formation, on which the nanostructure Cu2S was formed directly. The morphology and thickness of Cu2S layer can be easily controlled by varying the deposition potential and time. It was found that the Cu2S/Cufoam electrode was very efficient for the CO2reduction to HCOOH. As far as we know, this is the first work for the electrocatalytic reduction of CO2to HCOOH using Cu2S cathode.

2 Experimental

2.1 Materials

1-Butyl-3-methylimidazolium tetrafluoroborate （BmimBF4, purity > 99%） was purchased from the Centre of Green Chemistry and Catalysis, LICP, CAS, P. R. China. Acetonitrile （A. R. grade） and acetone （A. R. grade）, were provided by Sinopharm Chemical Reagent Co., Ltd., P. R. China. Nafion N-117 membranes （0.180 mm thick, ≥ 0.90 meq·g–1exchange capacity）were purchased from Alfa Aesar China Co., Ltd. Cu-foam was purchased from Kunshan Jiasheng Electronics Co. Ltd., P. R. China. Before used, the ionic liquids （ILs） were dried in vacuum oven at 80 °C for 48 h and the water content was less than 1.0 g·L–1as determined by Karl-Fischer method25.

2.2 Preparation and characterization of Cu2S electrodes

The Cu-foam was pretreated by degreasing in acetone, etching in 4.0 mol·L–1HCl for 15 min, and rinsing thoroughly with ultrapure water prior to use. Arrays of Cu2S on copper foam were prepared via anodization of copper foam followed by thermal treatments. The electrodeposition procedure was performed on DC power supply LW6020KD （Longwei Instrument Co. Ltd, HK）, which was similar to that reported in the literature26,27. The anodization was carried out in an electrochemical cell with a copper foam anode, a platinum foil cathode. The electrolyte was 0.145 mol·L–1Na2S aqueous solution. The anodization was performed by applying a constant voltage of 0.6 V to the copper foam for 300 s at 55 °C during which a faintblue film was formed on the copper foam surface. The anodized copper foam was then washed with distilled water and heat-treatment. The deposition underwent mainly following electrochemical processes.

X-ray diffraction （XRD） analysis of the samples was performed on the X-ray diffractometer （Model D/MAX2500, Rigaka Denki Co., Ltd., Japan） with Cu-Kαradiation, and the scan speed was 5 （°）·min–1. The morphologies of Cu2S/Cufoam electrodes were characterized by a HITACHI S-4800 scanning electron microscope （Hitachi High-Technologies CO., Japan）.

2.3 CV study

An electrochemical workstation （CHI 6081E, Shanghai CH Instruments Co., China） was used for the experiments. Cyclic voltammetry （CV） measurements were carried out in a single compartment cell with three-electrode configuration, which was similar to that reported previously28. The cell consisted of Cu2S/Cu-foam or Cu-foam electrode, a platinum gauze auxiliary electrode, and Ag/Ag+（0.01 mol·L–1AgNO3in 0.1 mol·L–1TBAP-MeCN） reference electrode. Prior to experiment, the electrolyte was bubbled with CO2（or N2） for 30 min until CO2-saturated solution （or N2-saturated solution） was formed. CV measurements in gas-saturated electrolyte were taken between–0.8 and –2.2 V （vs Ag/Ag+） at a sweep rate of 20 mV·s–1. For better mixing, slight magnetic stirring was applied in the process.

2.4 CO2reduction electrolysis and product analysis

The appratus and procedures were similar to those reported previously for electrochemical reduction of CO2to CH428. Briefly, the electrolysis was performed under room temperature （25 °C） in an H-type cell （Fig.1）. The cathode and anode compartments were separated by a proton exchange membrane（Nafion 117）. MeCN containing 0.5 mol·L–1BmimBF4and 0.5mol·L–1H2SO4aqueous solution were used as cathodic and anodic electrolytes, respectively. Before measurements, CO2was bubbled through the catholyte for 30 min with stirring and electrolysis was conducted under a steady flow of CO2（5.0 cm3·min–1）, the gaseous product was collected in a gas sampling bag. After a desired electrolysis time, the gaseous product was analyzed by gas chromatography （GC, HP 4890D；Agilent Technologies, Inc., Wilmington, United States）, which was equipped with thermal conductivity detector （TCD） andflame ionization detector （FID） using helium as the internal standard. Products soluble in the catholyte were analyzed by proton nuclear magnetic resonance （1H-NMR） method, which recorded on a Bruker Avance III 400 HD spectrometer （Bruker, Karlsruhe, Germany） in DMSO-d6with tetramethylsilane（TMS） as an internal standard. The total current density and faradaic efficiency of the products were calculated on the basis of GC and NMR analyses. The faradaic efficiency of the products was calculated from GC analysis data28,29. The experiments were run at different potentials.

Fig.2 （a） XRD patterns of Cu2S； （b） photographs of Cu-foam electrodes before and after deposition of Cu2S； （c） SEM images of Cu-foam；（d） SEM images of Cu2S/Cu-foam

3 Results and discussion

3.1 Material characterization

Growing oxide or sulfide films on top of a metallic surface by electrodeposition is a widely used method of producing structured electrode surfaces on different types of substrates29–31. We choose the copper foam as substrate electrode, because it possesses a three-dimensional （3D） open network structure, provided a high volumetric specific surface area. In this study, the Cu2S film was obtained by anodizing a Cu foam in Na2S aqueous solution. The crystal structure of the Cu2S powders scratched from Cu2S/Cu-foam was characterized by XRD. The XRD pattern is shown in Fig.2a, which is consistent with that of monoclinic Cu2S22,24. There was no characteristic peak for impurities. So the XRD results indicated that pure phase Cu2S was formed on Cu foam. Fig.2b shows photographs of the Cu-foam and Cu2S/Cu-foam. A uniform faint-blue film was formed on the copper foam surface in the Cu2S/Cu-foam, and the coating basically reproduced the surface architecture of the Cu foam. The morphology of the Cu2S/Cu-foam was examined by scan-ning electron microscopy （SEM）. Figs.2c and 2d show some typical SEM images of Cu-foam and Cu2S/Cu-foam at different magnifications, respectively. The surface morphology of the microspheres could be clearly observed in Fig.2d, which had two-dimensional （2D） nanosheet-assembled flowerlike pattern. These nanosheets were aligned perpendicular to the surface of Cu-foam, and covered the Cu-foam surface completely and compactly. Since the deposited Cu2S packed densely on the entire copper foam skeleton, the Cu2S/Cu-foam has good mass transport property for electrolyte diffusion.

3.2 Electrocatalytic study

The Cu2S/Cu-foam was then used as the electrode for selective reduction of CO2to HCOOH in MeCN with 0.5 mol·L–1BmimBF4as the electrolyte, which is commonly used in electrochemical reduction of CO232,33. The CO2reduction efficiency of Cu2S/Cu-foam electrode was examined by cyclic voltammetry （CV） with applied voltage from –0.8 to –2.2 V （vs Ag/Ag+） （Fig.3a）. The onset of irreversible cathodic wave with higher current densities than in N2-saturated system is indicative of CO2reduction process. For comparison, the CV curves using Cu-foam electrode is determined and the results are presented in Fig.3b. The Cu2S/Cu-foam electrode yielded much higher current density than the Cu-foam electrode in the CO2-saturated system, as can be known by comparing Figs.3a and 3b.

Fig.3 CV scans at 20 mV·s-1under N2-saturated and CO2-saturated MeCN containing 0.5 mol·L-1BmimBF4of（a） Cu2S/Cu-foam electrode and （b） Cu-foam electrode

3.3 Long-term electrolysis study

To explore the electrocatalytic response shown in Fig.3, the controlled potential electrolysis （CPE） experiments were carried out in an electrolysis device shown in Fig.1. After initiating the CPE at different voltages （from –1.5 to –2.2 V, vs Ag/Ag+） for 5 h, the gas in the headspace was collected and analyzed by GC, and the liquid mixture was analyzed by1HNMR. It was found that H2was the only product in gas, HCOOH and CH3OH were observed in liquid mixtures, respectively. HCOOH was found to be the major product through all the applied potential. The faradaic efficiency for HCOOH（FEHCOOH） as a function of applied potential using the Cu2S/Cufoam electrode is shown in Fig.4a. It can be concluded that current densities increased with applied potentials, while the FEHCOOHreached a maximum （85%） at –2.0 V （vs Ag/Ag+）. HCOOH was produced slowly as the applied potentials were more positive than –2.0 V （vs Ag/Ag+）, while the FEHCOOHdecreased when the applied potential was more negative than –2.0 V （vs Ag/Ag+）. The Cu-foam electrode was also used in the electrolysis and the results are also given in Fig.4a. The FEHCOOHof the Cu-foam electrode was much lower （maximum 38.9% at–1.8 V （vs Ag/Ag+） than that of the Cu2S/Cu-foam electrode, i.e., the selectivity to HCOOH over the Cu2S/Cu-foam electrode was higher than that over Cu-foam electrode.

The CPE was also performed to investigate the effect of applied potentials on the partial current density of HCOOH. Fig.4b displays the plots of the partial current density versus the applied potential for the production of HCOOH as a function of catalyst. We observed that the current density increased continuously with the applied potential. The current density on the Cu2S/Cu-foam was much higher than that on the Cu-foam electrodes, indicating that the Cu2S/Cu-foam electrode was much more efficient than Cu-foam electrode. We also studied the dependence of the current density on electrolysis time on the electrodes up to 6 h （Fig.4c）. The current density was not changed with time, indicating that the electrodes were stable in the electrolysis process.

We propose that the high selectivity of the catalyst toward HCOOH could be attributed to the surface morphology of the Cu2S film. At the end of the reaction, the surface of the electrode was again examined by XRD and the result is shown in Fig.4d. Indeed, only Cu0reflexions were observed in the XRD patterns of the reduced film, which indicates that the bulk of the Cu2S films has reduced to metallic Cu during the course of the CO2reduction. We assume that the sulfide state of a Cu2S film has been proposed to be partially conserved during the initial phase of the CO2reduction process. Cu+ions were thus suggested to be catalytic active species for reducing CO2initially. However, the surface of a Cu2S film reduces and remains as metallic Cu particles during electrochemical CO2reduction. We thus believe that Cu0particles are the catalytic active species forreducing CO229,30. This finding is in agreement with many reports presented earlier14,34,35. When the steady state currents were compared, the electrodes deposited with Cu2S films exhibited higher current density compared with Cu foam （Fig.4c）. This can be attributed to the former's larger surface roughness and, hence, electrochemically active surface areas. The remarkable electrocatalytic conversion behavior between Cu2S and Cu foams also suggests that the active sites on the surfaces of sulfide derived Cu nanoparticles enable better stabilization forintermediate than the sites on polycrystalline Cu.

Fig.4 （a） Applied electrolysis potential dependence of FEHCOOHfor Cu2S/Cu-foam and Cu-foam electrodes； （b） reduced current density of HCOOH vs potential for Cu2S/Cu-foam and Cu-foam electrodes； （c） total current density profiles for Cu2S/Cu-foam and Cu-foam electrodes （potential applied: -2.0 V （vs Ag/Ag+）， electrolyte: MeCN containing 0.5 mol·L-1BmimBF4）； （d） XRD pattern of the Cu2S/Cu-foam after electrolysis

4 Conclusions

In summary, we have fabricated Cu2S/Cu-foam electrode by an electrodeposition method, which is utilized as cathodic electrode in the electrochemical reduction of CO2to HCOOH. The as-synthesized Cu2S/Cu-foam electrode exhibited very high activity, selectivity, and stability for electrochemical reaction in MeCN with 0.5 mol·L–1BmimBF4. The current density and selectivity depend strongly on the reduction potential. At optimized condition, the selectivity of HCOOH can be as high as 85% with a reduced current density of higher than 5.3 mA·cm–2. Control experiments show that the Cu2S/Cu-foam electrode is much more efficient than the Cu-foam electrode.

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Cu2S on Cu Foam as Highly Efficient Electrocatalyst for Reduction of CO2to Formic Acid

ZHU Qing-Gong SUN Xiao-Fu KANG Xin-Chen MA Jun QIAN Qing-Li HAN Bu-Xing*
（Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China）

The electrocatalytic reduction of CO2to HCOOH is an interesting topic and the efficiency usually depends strongly on the materials of the electrodes. Herein, nanostructured Cu2S on Cu-foam was prepared by electro-deposition method and characterized by means of scanning electron microscope（SEM） and X-ray diffraction （XRD）. The Cu2S/Cu-foam electrode was used for the first time in the electrocatalytic reduction of CO2to HCOOH, and acetonitrile （MeCN） with 0.5 mol·L–11-butyl-3-methylimidazolium tetrafluoroborate （BmimBF4） was used as the electrolyte. It was demonstrated that the electrolysis system was νery efficient for the electrochemical reaction, and faradaic efficiency of HCOOH（FEHCOOH） and reduction current density could reach 85% and 5.3 mA·cm–2, respectiνely.

Copper（I） sulfide; Copper foam; Formic acid; Electrochemistry; CO2reduction

O646

10.3866/PKU.WHXB201512101

Received: October 24, 2015； Revised: December 10, 2015； Published on Web: December 10, 2015.

*Corresponding author. Email: hanbx@iccas.ac.cn； Tel: +86-10-62562821.

The project was supported by the National Natural Science Foundation of China （21403253, 21533011, 21321063）.

國(guó)家自然科學(xué)基金（21403253, 21533011, 21321063）資助項(xiàng)目?Editorial office of Acta Physico-Chimica Sinica