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Role of the oxide layer on Sn electrode in electrochemical reduction of CO2 to formate
Applied Surface Science 356 (2015) 24–29
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Role of the oxide layer on Sn electrode in electrochemical reduction of CO2 to formate Rui Zhang a , Weixin Lv b , Lixu Lei a,∗ a b
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, China
a r t i c l e
i n f o
Article history: Received 5 July 2015 Received in revised form 27 July 2015 Accepted 4 August 2015 Available online 5 August 2015 Keywords: Carbon dioxide Electrochemical reduction Formate Oxide layer Tin electrode
a b s t r a c t The importance of oxide layer on Sn electrode to the efficiency of CO2 reduction has been evaluated by comparing the activities of Sn electrodes treated via etching and annealing. In KHCO3 aqueous solution saturated with CO2 , Sn electrode with a native oxide layer exhibits high catalytic activity for CO2 reduction, in contrast, Sn electrode which is etched in HCl solution to remove the oxide layer or treated by annealing exhibits lower activity for CO2 reduction. If the etched Sn electrode is exposed in the air over 24 h, its catalytic performance for CO2 reduction will restore, meanwhile, the electrolysis current density for this electrode is around 1.5-fold than that for a untreated Sn electrode because the etching treatment makes the electrode rough and behave a larger superficial area. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Excessive consumption of fossil fuels has gradually led to the depletion of these finite natural resources while increasing the concentration of atmospheric carbon dioxide (CO2 ) [1]. The transformation of CO2 into value-added chemicals and fuels has attracted much attention in recent years. Up to now, many chemical, photochemical, electrochemical and photoelectrochemical methods have been investigated for the purpose [2–4]. Among these routes, electrochemical reduction of CO2 represents a potentially “clean” technique only at the expense of a sustainable supply of electric energy [5]. To date, formic acid [6,7], CO [8,9], methanol [10], and oxalic acid [11] have been prepared by electrochemical reduction of CO2 . Among them, formic acid or formate (depending on pH) is a fundamental chemical which is widely used in the textile, tanning, rubber processing and pharmaceutical industries [12]. Formic acid could also function as an energy vector for storing excess of electricity, as it can be used in direct formic acid fuel cell to produce electricity [13]. Therefore, it is more convenient to produce formic acid than gaseous product such as CO from electrochemical reduction of CO2 . Electrochemical reduction of CO2 to formate has been attracted much attention in recent years. Metal complex [14], pure metal
∗ Corresponding author. Tel.: +86 25 52090620/6421; fax: +86 25 52090618. E-mail address: [email protected] (L. Lei). http://dx.doi.org/10.1016/j.apsusc.2015.08.006 0169-4332/© 2015 Elsevier B.V. All rights reserved.
[15], polymer [6] and alloy [16] have been used for electrochemical reduction of CO2 to formate. Among them, metal electrodes have been studied extensively due to their excellent performances for electrochemical reduction of CO2 . However, different metal electrodes showed difference catalytic properties for electrochemical reduction of CO2 to formate. Noda et al. studied the electrochemical reduction of CO2 at 27 kinds of metal electrodes in 0.1 mol L−1 KHCO3 aqueous solution, and found that In, Sn, Hg and Pb electrodes showed excellent catalytic activities for reduction of CO2 to formate and had higher faradaic efficiencies (> 50%) than other metal electrodes [17]. Among all the metal electrodes used for electrochemical reduction of CO2 to formate, Sn electrode is the most commonly used electrode because of its low cost and relatively low toxicity [18]. Most researchers obtained highly faradaic efficiencies for producing formate using Sn electrode. Wu et al. reported that the obtained highest faradaic efficiency was 95% on a Sn electrode in KHCO3 aqueous solution [19]. In our previous work, the highest faradaic efficiency of 91% was obtained on a Sn electrode in KHCO3 aqueous solution [20]. Zhang et al. reported that the highest faradaic efficiency of 28% was obtained on a Sn electrode which was pretreated in 1 mol L−1 HNO3 to remove surface oxide [21]. The catalytic performance for producing formate is greatly reduced when the Sn electrode is removed its surface oxide according to the above reported results. Sn electrodes are known to possess a native oxide layer, and the oxide layer may have some influences for electrochemical reduction of CO2 . A lot of researchers used the Sn electrode
R. Zhang et al. / Applied Surface Science 356 (2015) 24–29
as the cathode for electrochemical reduction of CO2 , but only few of them mentioned the oxide layer on the surface of the Sn electrode. Recently, Wu et al. reported that the Sn nanoparticles (100 nm) with native oxide layer showed better catalytic performance for electrochemical reduction of CO2 than the Sn nanoparticles after heat treatment [22]. Chen et al. reported that Sn/SnOx layer on Ti substrate is highly selective towards CO and HCOOH compared to pure Sn, and they obtained the highest faradaic efficiency of 95% for CO2 reduction (40% for producing formate and 55% for producing CO) at −0.7 V vs. RHE [23]. Baruch et al. studied the electrochemical reduction of CO2 on Sn cathode and reported that a metastable oxide layer presented on the surface of the Sn cathode under reducing potentials [24]. In this paper, the Sn electrode was etched to remove its oxide layer, and the activity of the etched Sn electrode was compared to that of untreated Sn electrode. After that, the etched Sn electrode was exposed in the air to form an oxide layer, and its catalytic activity for CO2 reduction was evaluated. 2. Experimental 2.1. Preparation of the electrodes The Sn plates (99.99%, 0.5 mm thick, 1 cm2 ) were treated with different methods. Firstly, Sn plates were annealed in air for 3 h at 120, 140, 160 and 180 ◦ C, respectively. Secondly, Sn plate was etched by cathodizing in 2 mol L−1 HCl solution at −3.0 V for 5 min, and the sample was denoted as E-Sn. Then, the E-Sn was exposed in the air over 24 h to form an oxide layer on its surface, and the sample was denoted as OE-Sn. Caution! During the process of etching the Sn plate, the poisonous gas of chlorine is produced, which may be collected by a NaOH solution. 2.2. Electrochemical experiments The cell used here is an airtight and undivided glass cell equipped with a gas inlet and outlet which is able to pass the either N2 (99.99%) or CO2 (99.99%) through the solution. A conventional three-electrode system was used during the measurements. A Sn plate (99.99%, 0.5 mm thick, 1 cm2 ) with a native oxide layer was used as the Sn electrode. The working electrode was the Sn electrode (untreated), the E-Sn, or the OE-Sn. A Pt plate (2 cm2 ) and an Ag/AgCl electrode (sat. KCl) were used as counter and reference electrodes, respectively. All potential values are in reference to Ag/AgCl electrode unless mentioned otherwise. The electrolyte used was 40 mL of 0.1 mol L−1 KHCO3 aqueous solution. All experiments were performed under room temperature (20 ± 3 ◦ C) and ambient pressure. Cyclic voltammetry (CV) experiments were carried out using a CS350 electrochemical workstation (Wuhan CorrTest Instrument Co., Ltd., China) after degassing the solution for 30 min with either N2 or CO2 for the actual determinations. The current density is determined on the geometrical area of the electrode. Electrochemical impedance spectroscopy (EIS) was performed in 0.1 mol L−1 KHCO3 aqueous solution after being bubbled with CO2 for 30 min, which was recorded at the frequency range from 5 × 104 to 0.1 Hz with amplitude of 0.01 V. Controlled potential electrolysis was carried out using a LAND CT2001C cell performance-testing instrument (Wuhan Electronics Co., Ltd., China) in the same three-electrode electrochemical cell. The electrolyte was saturated with CO2 before each electrolysis process, and CO2 gas was continuously aerated at a flow rate of 10 mL min−1 during the electrolysis process. The electrolysis experiments were terminated when the total charge passed reached 50 C. The average current density is expressed as the total current divided
25
Fig. 1. Faradaic efficiencies for producing formate on the Sn electrodes annealed at different temperatures.
by the geometric surface area of the electrode for all cells. Only the liquid product was tested with emphasis on the formation of formate. 2.3. Analysis and calculations Scanning electron microscope (SEM) images were taken with a Hitachi S-4800 microscope at an acceleration voltage of 15 kV. The valence state of Sn on the electrode surface was quantified by X-ray photoelectron spectroscopy (XPS, VGESCALAB 250). The products in the electrolyte after electrolysis without any further treatment were analyzed by ion chromatography (ICS-900 Dionex). The column was an IonPac AS11-HC anionic column using 0.02 mol L−1 KOH as the mobile phase at the rate of 1 mL min−1 . 10 L of the electrolyte were used for each time. The faradaic efficiency for producing formate (f) is determined by means of: f = 2nF/Q
(1)
where n is the moles of the formate produced; F is Faraday’s constant (96485 C mol−1 of electrons); and Q is the total charge in Coulomb passed across the electrode during the electrolysis (Q = 50 C here). 3. Results and discussion It had been reported that the oxide layer on the surface of the Sn electrode is in favor of the CO2 reduction. In this paper, we evaluated the catalytic performance of the Sn electrodes annealed at different temperatures for electrochemical reduction of CO2 . As we known, the melting point of Sn is 231.9 ◦ C, and the heat treatment below it may form thick oxide layer. In our experiments, the Sn plates were annealed in air for 3 h at 120, 140, 160 and 180 ◦ C, respectively, and the faradaic efficiencies for producing formate obtained on these Sn plates are shown in Fig. 1. The faradaic efficiencies on these electrodes are lower than that on the Sn plate without heat treatment. The higher temperature the Sn plate is annealed at, the lower the faradaic efficiency can be obtained. It indicates that the Sn electrode with the native oxide layer formed in air has better catalytic performance for electrochemical reduction of CO2 to formate than the Sn electrode after heat treatment. According to the analysis results of Fig. 1, the Sn electrode after heat treating is not benefit for electrochemical reduction of CO2 to formate, in the following experiments, we studied the electrocatalytic activities of the Sn electrodes after etching in HCl solution. Fig. 2 depicts the CV curves of the Sn, E-Sn and OE-Sn electrodes in
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R. Zhang et al. / Applied Surface Science 356 (2015) 24–29
Fig. 2. CV curves obtained on (a) Sn, (b) E-Sn and (c) OE-Sn electrodes in 0.1 mol L−1 KHCO3 solutions after being bubbled with N2 or CO2 for 30 min; (d) CV curves obtained on Sn, E-Sn and OE-Sn electrodes in 0.1 mol L−1 KHCO3 solutions after being bubbled with N2 for 30 min.
0.1 mol L−1 KHCO3 solutions saturated with N2 or CO2 . For the E-Sn electrode, the Sn electrode after etching was quickly transferred to the electrolysis cell for less than 40 s (including 10 s of ultrasonic cleaning) for preventing oxidation; and for the OE-Sn electrode, the etched Sn electrode was exposed to air over 24 h to rebuild an oxide layer at room temperature. The anodic peaks between −0.3 and −0.9 V and the cathodic peak between −0.8 and −1.1 V in CV curves can be attributed to the formation and the reduction of tin oxides respectively [25]. On the cathodic end of the CV curves, sharp increases of the current densities can be observed under both N2 and CO2 . Under N2 , this increase is due to the evolution of H2 only; under CO2 , the enhanced current must be caused by both the reduction of the CO2 and the evolution of H2 . For the Sn electrode (Fig. 2a) and the OE-Sn electrode (Fig. 2c), the current densities under CO2 are higher than those under N2 . However, for the E-Sn electrode, the current density under CO2 is close to that under N2 (Fig. 2b). The CV curves of the Sn, E-Sn and OE-Sn electrodes under N2 are plotted together and shown in Fig. 2d. It can be seen that the current density of the E-Sn electrode is obviously higher than those of the Sn and OE-Sn electrodes. It reveals that the hydrogen evolution reaction is more likely to happen on the E-Sn electrode, therefore, the E-Sn electrode is unfavorable to the CO2 reduction. Fig. 3 shows the SEM images of the Sn electrode and the OE-Sn electrode. The morphologies of the two electrodes are obviously different. The OE-Sn electrode has a rough surface and presents a larger superficial area than the Sn electrode. As we know, the total current passed through an electrode is related to the electrode
area for electrochemical reaction; the larger the cathode area is, the faster the reaction rate is. Fig. 4 shows the Sn 3d5/2 XPS spectra of the Sn electrode and the OE-Sn electrode. The peaks appear at about 484.0 and 486.3 eV can be attributed to Sn0 and SnOx . Here, the OE-Sn electrode was exposed to air over 5 days before the XPS analyze. For the OE-Sn electrode, the peak area percentages of Sn0 and SnOx are 9.0% and 91.0%, respectively; and for the Sn electrode, the peak area percentages of Sn0 and SnOx are 1.1% and 98.9%, respectively. Chen et al. studied the oxide content on the surface of the Sn electrode by utilizing XPS, and found that the oxide (SnOx ) of the etched Sn electrode exhibited a 18% peak, and the etched electrodes after about 1 day of exposure to air exhibited a 60–70% SnOx peak [23]. The method of etching used in this paper is the same with their report. It indicates that the etching process can remove the oxide layer of the Sn electrode effectively, and the surface of the etched Sn electrode can be oxidized in the air. Following our previous work [20], the electrochemical reduction of CO2 was carried out at the found optimal electrolysis conditions, that is, the electrolysis potential was −1.8 V vs. Ag/AgCl and the concentration of the KHCO3 aqueous electrolyte was 0.1 mol L−1 . It can be seen from Fig. 5 that the E-Sn and OE-Sn electrodes have higher electrolysis current densities than the Sn electrode. We can see from Fig. 3 that the E-Sn electrode with the rough surface presents a larger superficial area than the Sn electrode. The current density would increase as the electrode area increases. The reaction time to obtain definite amount of the product can be reduced by using
R. Zhang et al. / Applied Surface Science 356 (2015) 24–29
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Fig. 3. SEM images of (a) the Sn electrode and (b) the OE-Sn electrode.
larger electrode. At the same passed charge (50 C here), the OE-Sn electrode has faster reaction rate than the Sn electrode. It can be seen from Fig. 6 that the faradaic efficiencies for producing formate on the Sn, E-Sn and OE-Sn electrodes are 84%, 43% and 85%, respectively. The faradaic efficiency obtained on the E-Sn electrode is obviously lower than those obtained on the Sn and OE-Sn electrodes. These results are consistent with the CV results. As we known, hydrogen evolution reaction easily takes place in aqueous electrolytes by cathodic polarization, usually competing with CO2
reduction reaction. Through the experiments of CV and controlled potential electrolysis, it clearly shows that the oxide layer on the surface of Sn electrode is essential for electrochemical reduction of CO2 to formate, because the oxide layer can effectively retard the hydrogen evolution reaction. Electrolysis experiments were performed in CO2 saturated 0.1 mol L−1 KHCO3 solutions applying a constant potential in the range from −1.6 to −2.0 V (vs. Ag/AgCl) at 0.1 V intervals, and the results are shown in Fig. 7. It can be seen that the faradaic efficiency for producing formate increases at first until −1.8 V, where it reaches the maximum. As the potential decreases further, the faradaic efficiency begins to decrease. It can be seen that the difference of the faradaic efficiencies between the Sn electrode and the OE-Sn electrode is small, whereas the formate production rate of the OE–Sn electrode is faster than that of the Sn electrode. Fig. 8 shows the Nyquist plots for the Sn electrode and the OE-Sn electrode in 0.1 mol L−1 KHCO3 solutions after being bubbled with CO2 for 30 min. Since the onset potential for electrochemical reduction of CO2 to formate was −1.25 V (see CV results), the applied potential was set at −1.25 V. It can be seen that the Nyquist plots show two semicircles with two different time constants. Table 1 lists the parameters for the EIS analysis by Zview2 software. The
Fig. 4. XPS spectra of the Sn electrode and the OE-Sn electrode.
Fig. 6. The faradaic efficiencies and the average current densities for electrochemical reduction of CO2 to formate on the Sn, E-Sn and OE-Sn electrodes. Table 1 Results of EIS fitting according to the equivalent circuit shown in Fig.8.
Fig. 5. Variations of the current density with electrolysis time during the electrochemical reduction of CO2 on Sn, E-Sn and OE-Sn electrodes at −1.8 V vs. Ag/AgCl.
Electrode
Sn
OE-Sn
Rs () R1 () R2 ()
31.35 214.9 167.7
30.30 126.8 132.4
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R. Zhang et al. / Applied Surface Science 356 (2015) 24–29
Fig. 7. Variations in the faradaic efficiency (a) and the production rate (b) for producing formate on the Sn electrode and the OE-Sn electrode with the electrolysis potential.
CO2 reduction but hydrogen evolution reaction accelerates. These research results indicate that the surface oxide of the Sn electrode is essential for CO2 reduction. 4. Conclusions
Fig. 8. Nyquist plots for the EIS analysis on the Sn electrode and the OE-Sn electrode in 0.1 mol L−1 KHCO3 solutions after being bubbled with CO2 for 30 min. The inset is the equivalent circuit. The applied potential was −1.25 V vs. Ag/AgCl.
The oxide layer on the Sn electrode has been removed by cathodizing in HCl solution. The faradaic efficiencies for electrochemical reduction of CO2 to formate on the Sn, E-Sn and OE-Sn electrodes are 84%, 43% and 85%, respectively. The faradaic efficiency on the E-Sn electrode is obviously lower than those on the Sn and OESn electrodes. The CV results illustrate that the seriously hydrogen evolution reaction causes the catalytic activity of the E-Sn electrode for CO2 reduction decrease. The etching treatment makes the Sn electrode have a rough surface and behave a larger superficial area. The electrolysis current density for the CO2 reduction on the OE-Sn electrode is around 1.5fold than that on the untreated Sn electrode which can accelerate the reaction rate. Large current density is beneficial to the industrial demand. Acknowledgements
solution resistance (Rs ) values for the Sn electrode and the OE-Sn electrode were similar. Obvious differences are found in the resistances of R1 and R2 . Both the values of R1 and R2 for OE-Sn electrode are smaller than that for the Sn electrode, mainly resulting from the larger superficial area of the OE–Sn electrode which has more catalytic activity point. Although many studies on electrochemical reduction of CO2 have been carried out, the mechanism of this reaction is still ambiguously, it hinders the further research in this field. For the mechanism of CO2 reduction on the metal electrode, it is generally assumed that the bare metal surface is catalytically active for CO2 reduction. However, oxide layer forms easily on the surface of Sn in the air. The oxide layer may have some influences on the performance of CO2 reduction. This problem has been neglected for the study of CO2 reduction until Chen et al. reported their experiments [23]. Baruch et al. studied the electrochemical reduction of CO2 on Sn cathode by utilizing in situ attenuated total reflectance infrared spectroscopy, and found that the peaks attributed to a surface-bound monodentate tin carbonate species (intermediate of CO2 reduction) only appeared at potentials where CO2 reduction was observed, and disappeared when the tin surface was chemically etched to remove surface oxide [24]. In this work, the CV and the electrolysis results illustrate that Sn electrode possessing oxide layer shows excellent catalytic activity for CO2 reduction, and the Sn electrode removed its oxide layer shows poor catalytic activity for
The authors would like to thank the Department of Education for the Fundamental Research Funds for the Central Universities, Jiangsu Province for the College graduate research and innovation projects (CXLX13 086) for the financial support. References [1] X.Y. Zhao, B.B. Luo, R. Long, C.M. Wang, Y.J. Xiong, Composition-dependent activity of Cu-Pt alloy nanocubes for electrocatalytic CO2 reduction, J. Mater. Chem. A 3 (2015) 4134–4138. [2] Y. Oh, X.L. Hu, Organic molecules as mediators and catalysts for photocatalytic and electrocatalytic CO2 reduction, Chem. Soc. Rev. 42 (2013) 2253–2261. [3] Q. Shen, Z.F. Chen, X.F. Huang, M.C. Liu, G.H. Zhao, High-yield and selective photoelectrocatalytic reduction of CO2 to formate by metallic copper decorated Co3 O4 nanotube arrays, Environ. Sci. Technol. 49 (2015) 5828–5835. [4] Q.J. Feng, S.Q. Liu, X.Y. Wang, G.H. Jin, Nanoporous copper incorporated platinum composites for electrocatalytic reduction of CO2 in ionic liquid BMIMBF4 , Appl. Surf. Sci. 258 (2012) 5005–5009. [5] C.W. Li, M.W. Kanan, CO2 reduction at low overpotential on cu electrodes resulting from the reduction of thick Cu2 O films, J. Am. Chem. Soc. 134 (2012) 7231–7234. [6] S. Zhang, P. Kang, S. Ubnoske, M.K. Brennaman, N. Song, R.L. House, J.T. Glass, T.J. Meyer, Polyethylenimine-enhanced electrocatalytic reduction of CO2 to formate at nitrogen-doped carbon nanomaterials, J. Am. Chem. Soc. 136 (2014) 7845–7848. [7] X.Q. Min, M.W. Kanan, Pd-Catalyzed electrohydrogenation of carbon dioxide to formate: high mass activity at low overpotential and identification of the deactivation pathway, J. Am. Chem. Soc. 137 (2015) 4701–4708.
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[17] H. Noda, S. Ikeda, Y. Oda, K. Imai, M. Maeda, K. Ito, Electrochemical reduction of carbon-dioxide at varous metal-electrodes in aqueous potassium hydrogen carbonate solution, Bull. Chem. Soc. Jan. 63 (1990) 2459–2462. [18] G.K.S. Prakash, F.A. Viva, G.A. Olah, Electrochemical reduction of CO2 over Sn-Nafion coated electrode for a fuel-cell-like device, J. Power Sources 223 (2013) 68–73. [19] J.J. Wu, F.G. Risalvato, F.S. Ke, P.J. Pellechia, X.D. Zhou, Electrochemical reduction of carbon dioxide I. effects of the electrolyte on the selectivity and activity with Sn electrode, J. Electrochem. Soc. 159 (2012) F353–F359. [20] W.X. Lv, R. Zhang, P.R. Gao, L.X. Lei, Studies on the faradaic efficiency for electrochemical reduction of carbon dioxide to formate on tin electrode, J. Power Sources 253 (2014) 276–281. [21] S. Zhang, P. Kang, T.J. Meyer, Nanostructured tin catalysts for selective electrochemical reduction of carbon dioxide to formate, J. Am. Chem. Soc. 136 (2014) 1734–1737. [22] J.J. Wu, F.G. Risalvato, S. Ma, X.D. Zhou, Electrochemical reduction of carbon dioxide III. The role of oxide layer thickness on the performance of Sn electrode in a full electrochemical cell, J. Mater. Chem. A 2 (2014) 1647–1651. [23] Y.H. Chen, M.W. Kanan, Tin oxide dependence of the CO2 reduction efficiency on tin electrodes and enhanced activity for tin/tin oxide thin-film catalysts, J. Am. Chem. Soc. 134 (2012) 1986–1989. [24] M.F. Baruch, J.E. Pander, J.L. White, A.B. Bocarsly, Mechanistic Insights into the reduction of CO2 on tin electrodes using in situ ATR-IR spectroscopy, ACS Catal. 5 (2015) 3148–3156. [25] S.D. Kapusta, N. Hackerman, Anodic passivation of tin in slightly alkaline solutions, Electrochim. Acta 25 (1980) 1625–1639.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Role of the oxide layer on Sn electrode in electrochemical reduction of CO2 to formate Rui Zhang a , Weixin Lv b , Lixu Lei a,∗ a b
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China School of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 224051, China
a r t i c l e
i n f o
Article history: Received 5 July 2015 Received in revised form 27 July 2015 Accepted 4 August 2015 Available online 5 August 2015 Keywords: Carbon dioxide Electrochemical reduction Formate Oxide layer Tin electrode
a b s t r a c t The importance of oxide layer on Sn electrode to the efficiency of CO2 reduction has been evaluated by comparing the activities of Sn electrodes treated via etching and annealing. In KHCO3 aqueous solution saturated with CO2 , Sn electrode with a native oxide layer exhibits high catalytic activity for CO2 reduction, in contrast, Sn electrode which is etched in HCl solution to remove the oxide layer or treated by annealing exhibits lower activity for CO2 reduction. If the etched Sn electrode is exposed in the air over 24 h, its catalytic performance for CO2 reduction will restore, meanwhile, the electrolysis current density for this electrode is around 1.5-fold than that for a untreated Sn electrode because the etching treatment makes the electrode rough and behave a larger superficial area. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Excessive consumption of fossil fuels has gradually led to the depletion of these finite natural resources while increasing the concentration of atmospheric carbon dioxide (CO2 ) [1]. The transformation of CO2 into value-added chemicals and fuels has attracted much attention in recent years. Up to now, many chemical, photochemical, electrochemical and photoelectrochemical methods have been investigated for the purpose [2–4]. Among these routes, electrochemical reduction of CO2 represents a potentially “clean” technique only at the expense of a sustainable supply of electric energy [5]. To date, formic acid [6,7], CO [8,9], methanol [10], and oxalic acid [11] have been prepared by electrochemical reduction of CO2 . Among them, formic acid or formate (depending on pH) is a fundamental chemical which is widely used in the textile, tanning, rubber processing and pharmaceutical industries [12]. Formic acid could also function as an energy vector for storing excess of electricity, as it can be used in direct formic acid fuel cell to produce electricity [13]. Therefore, it is more convenient to produce formic acid than gaseous product such as CO from electrochemical reduction of CO2 . Electrochemical reduction of CO2 to formate has been attracted much attention in recent years. Metal complex [14], pure metal
∗ Corresponding author. Tel.: +86 25 52090620/6421; fax: +86 25 52090618. E-mail address: [email protected] (L. Lei). http://dx.doi.org/10.1016/j.apsusc.2015.08.006 0169-4332/© 2015 Elsevier B.V. All rights reserved.
[15], polymer [6] and alloy [16] have been used for electrochemical reduction of CO2 to formate. Among them, metal electrodes have been studied extensively due to their excellent performances for electrochemical reduction of CO2 . However, different metal electrodes showed difference catalytic properties for electrochemical reduction of CO2 to formate. Noda et al. studied the electrochemical reduction of CO2 at 27 kinds of metal electrodes in 0.1 mol L−1 KHCO3 aqueous solution, and found that In, Sn, Hg and Pb electrodes showed excellent catalytic activities for reduction of CO2 to formate and had higher faradaic efficiencies (> 50%) than other metal electrodes [17]. Among all the metal electrodes used for electrochemical reduction of CO2 to formate, Sn electrode is the most commonly used electrode because of its low cost and relatively low toxicity [18]. Most researchers obtained highly faradaic efficiencies for producing formate using Sn electrode. Wu et al. reported that the obtained highest faradaic efficiency was 95% on a Sn electrode in KHCO3 aqueous solution [19]. In our previous work, the highest faradaic efficiency of 91% was obtained on a Sn electrode in KHCO3 aqueous solution [20]. Zhang et al. reported that the highest faradaic efficiency of 28% was obtained on a Sn electrode which was pretreated in 1 mol L−1 HNO3 to remove surface oxide [21]. The catalytic performance for producing formate is greatly reduced when the Sn electrode is removed its surface oxide according to the above reported results. Sn electrodes are known to possess a native oxide layer, and the oxide layer may have some influences for electrochemical reduction of CO2 . A lot of researchers used the Sn electrode
R. Zhang et al. / Applied Surface Science 356 (2015) 24–29
as the cathode for electrochemical reduction of CO2 , but only few of them mentioned the oxide layer on the surface of the Sn electrode. Recently, Wu et al. reported that the Sn nanoparticles (100 nm) with native oxide layer showed better catalytic performance for electrochemical reduction of CO2 than the Sn nanoparticles after heat treatment [22]. Chen et al. reported that Sn/SnOx layer on Ti substrate is highly selective towards CO and HCOOH compared to pure Sn, and they obtained the highest faradaic efficiency of 95% for CO2 reduction (40% for producing formate and 55% for producing CO) at −0.7 V vs. RHE [23]. Baruch et al. studied the electrochemical reduction of CO2 on Sn cathode and reported that a metastable oxide layer presented on the surface of the Sn cathode under reducing potentials [24]. In this paper, the Sn electrode was etched to remove its oxide layer, and the activity of the etched Sn electrode was compared to that of untreated Sn electrode. After that, the etched Sn electrode was exposed in the air to form an oxide layer, and its catalytic activity for CO2 reduction was evaluated. 2. Experimental 2.1. Preparation of the electrodes The Sn plates (99.99%, 0.5 mm thick, 1 cm2 ) were treated with different methods. Firstly, Sn plates were annealed in air for 3 h at 120, 140, 160 and 180 ◦ C, respectively. Secondly, Sn plate was etched by cathodizing in 2 mol L−1 HCl solution at −3.0 V for 5 min, and the sample was denoted as E-Sn. Then, the E-Sn was exposed in the air over 24 h to form an oxide layer on its surface, and the sample was denoted as OE-Sn. Caution! During the process of etching the Sn plate, the poisonous gas of chlorine is produced, which may be collected by a NaOH solution. 2.2. Electrochemical experiments The cell used here is an airtight and undivided glass cell equipped with a gas inlet and outlet which is able to pass the either N2 (99.99%) or CO2 (99.99%) through the solution. A conventional three-electrode system was used during the measurements. A Sn plate (99.99%, 0.5 mm thick, 1 cm2 ) with a native oxide layer was used as the Sn electrode. The working electrode was the Sn electrode (untreated), the E-Sn, or the OE-Sn. A Pt plate (2 cm2 ) and an Ag/AgCl electrode (sat. KCl) were used as counter and reference electrodes, respectively. All potential values are in reference to Ag/AgCl electrode unless mentioned otherwise. The electrolyte used was 40 mL of 0.1 mol L−1 KHCO3 aqueous solution. All experiments were performed under room temperature (20 ± 3 ◦ C) and ambient pressure. Cyclic voltammetry (CV) experiments were carried out using a CS350 electrochemical workstation (Wuhan CorrTest Instrument Co., Ltd., China) after degassing the solution for 30 min with either N2 or CO2 for the actual determinations. The current density is determined on the geometrical area of the electrode. Electrochemical impedance spectroscopy (EIS) was performed in 0.1 mol L−1 KHCO3 aqueous solution after being bubbled with CO2 for 30 min, which was recorded at the frequency range from 5 × 104 to 0.1 Hz with amplitude of 0.01 V. Controlled potential electrolysis was carried out using a LAND CT2001C cell performance-testing instrument (Wuhan Electronics Co., Ltd., China) in the same three-electrode electrochemical cell. The electrolyte was saturated with CO2 before each electrolysis process, and CO2 gas was continuously aerated at a flow rate of 10 mL min−1 during the electrolysis process. The electrolysis experiments were terminated when the total charge passed reached 50 C. The average current density is expressed as the total current divided
25
Fig. 1. Faradaic efficiencies for producing formate on the Sn electrodes annealed at different temperatures.
by the geometric surface area of the electrode for all cells. Only the liquid product was tested with emphasis on the formation of formate. 2.3. Analysis and calculations Scanning electron microscope (SEM) images were taken with a Hitachi S-4800 microscope at an acceleration voltage of 15 kV. The valence state of Sn on the electrode surface was quantified by X-ray photoelectron spectroscopy (XPS, VGESCALAB 250). The products in the electrolyte after electrolysis without any further treatment were analyzed by ion chromatography (ICS-900 Dionex). The column was an IonPac AS11-HC anionic column using 0.02 mol L−1 KOH as the mobile phase at the rate of 1 mL min−1 . 10 L of the electrolyte were used for each time. The faradaic efficiency for producing formate (f) is determined by means of: f = 2nF/Q
(1)
where n is the moles of the formate produced; F is Faraday’s constant (96485 C mol−1 of electrons); and Q is the total charge in Coulomb passed across the electrode during the electrolysis (Q = 50 C here). 3. Results and discussion It had been reported that the oxide layer on the surface of the Sn electrode is in favor of the CO2 reduction. In this paper, we evaluated the catalytic performance of the Sn electrodes annealed at different temperatures for electrochemical reduction of CO2 . As we known, the melting point of Sn is 231.9 ◦ C, and the heat treatment below it may form thick oxide layer. In our experiments, the Sn plates were annealed in air for 3 h at 120, 140, 160 and 180 ◦ C, respectively, and the faradaic efficiencies for producing formate obtained on these Sn plates are shown in Fig. 1. The faradaic efficiencies on these electrodes are lower than that on the Sn plate without heat treatment. The higher temperature the Sn plate is annealed at, the lower the faradaic efficiency can be obtained. It indicates that the Sn electrode with the native oxide layer formed in air has better catalytic performance for electrochemical reduction of CO2 to formate than the Sn electrode after heat treatment. According to the analysis results of Fig. 1, the Sn electrode after heat treating is not benefit for electrochemical reduction of CO2 to formate, in the following experiments, we studied the electrocatalytic activities of the Sn electrodes after etching in HCl solution. Fig. 2 depicts the CV curves of the Sn, E-Sn and OE-Sn electrodes in
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Fig. 2. CV curves obtained on (a) Sn, (b) E-Sn and (c) OE-Sn electrodes in 0.1 mol L−1 KHCO3 solutions after being bubbled with N2 or CO2 for 30 min; (d) CV curves obtained on Sn, E-Sn and OE-Sn electrodes in 0.1 mol L−1 KHCO3 solutions after being bubbled with N2 for 30 min.
0.1 mol L−1 KHCO3 solutions saturated with N2 or CO2 . For the E-Sn electrode, the Sn electrode after etching was quickly transferred to the electrolysis cell for less than 40 s (including 10 s of ultrasonic cleaning) for preventing oxidation; and for the OE-Sn electrode, the etched Sn electrode was exposed to air over 24 h to rebuild an oxide layer at room temperature. The anodic peaks between −0.3 and −0.9 V and the cathodic peak between −0.8 and −1.1 V in CV curves can be attributed to the formation and the reduction of tin oxides respectively [25]. On the cathodic end of the CV curves, sharp increases of the current densities can be observed under both N2 and CO2 . Under N2 , this increase is due to the evolution of H2 only; under CO2 , the enhanced current must be caused by both the reduction of the CO2 and the evolution of H2 . For the Sn electrode (Fig. 2a) and the OE-Sn electrode (Fig. 2c), the current densities under CO2 are higher than those under N2 . However, for the E-Sn electrode, the current density under CO2 is close to that under N2 (Fig. 2b). The CV curves of the Sn, E-Sn and OE-Sn electrodes under N2 are plotted together and shown in Fig. 2d. It can be seen that the current density of the E-Sn electrode is obviously higher than those of the Sn and OE-Sn electrodes. It reveals that the hydrogen evolution reaction is more likely to happen on the E-Sn electrode, therefore, the E-Sn electrode is unfavorable to the CO2 reduction. Fig. 3 shows the SEM images of the Sn electrode and the OE-Sn electrode. The morphologies of the two electrodes are obviously different. The OE-Sn electrode has a rough surface and presents a larger superficial area than the Sn electrode. As we know, the total current passed through an electrode is related to the electrode
area for electrochemical reaction; the larger the cathode area is, the faster the reaction rate is. Fig. 4 shows the Sn 3d5/2 XPS spectra of the Sn electrode and the OE-Sn electrode. The peaks appear at about 484.0 and 486.3 eV can be attributed to Sn0 and SnOx . Here, the OE-Sn electrode was exposed to air over 5 days before the XPS analyze. For the OE-Sn electrode, the peak area percentages of Sn0 and SnOx are 9.0% and 91.0%, respectively; and for the Sn electrode, the peak area percentages of Sn0 and SnOx are 1.1% and 98.9%, respectively. Chen et al. studied the oxide content on the surface of the Sn electrode by utilizing XPS, and found that the oxide (SnOx ) of the etched Sn electrode exhibited a 18% peak, and the etched electrodes after about 1 day of exposure to air exhibited a 60–70% SnOx peak [23]. The method of etching used in this paper is the same with their report. It indicates that the etching process can remove the oxide layer of the Sn electrode effectively, and the surface of the etched Sn electrode can be oxidized in the air. Following our previous work [20], the electrochemical reduction of CO2 was carried out at the found optimal electrolysis conditions, that is, the electrolysis potential was −1.8 V vs. Ag/AgCl and the concentration of the KHCO3 aqueous electrolyte was 0.1 mol L−1 . It can be seen from Fig. 5 that the E-Sn and OE-Sn electrodes have higher electrolysis current densities than the Sn electrode. We can see from Fig. 3 that the E-Sn electrode with the rough surface presents a larger superficial area than the Sn electrode. The current density would increase as the electrode area increases. The reaction time to obtain definite amount of the product can be reduced by using
R. Zhang et al. / Applied Surface Science 356 (2015) 24–29
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Fig. 3. SEM images of (a) the Sn electrode and (b) the OE-Sn electrode.
larger electrode. At the same passed charge (50 C here), the OE-Sn electrode has faster reaction rate than the Sn electrode. It can be seen from Fig. 6 that the faradaic efficiencies for producing formate on the Sn, E-Sn and OE-Sn electrodes are 84%, 43% and 85%, respectively. The faradaic efficiency obtained on the E-Sn electrode is obviously lower than those obtained on the Sn and OE-Sn electrodes. These results are consistent with the CV results. As we known, hydrogen evolution reaction easily takes place in aqueous electrolytes by cathodic polarization, usually competing with CO2
reduction reaction. Through the experiments of CV and controlled potential electrolysis, it clearly shows that the oxide layer on the surface of Sn electrode is essential for electrochemical reduction of CO2 to formate, because the oxide layer can effectively retard the hydrogen evolution reaction. Electrolysis experiments were performed in CO2 saturated 0.1 mol L−1 KHCO3 solutions applying a constant potential in the range from −1.6 to −2.0 V (vs. Ag/AgCl) at 0.1 V intervals, and the results are shown in Fig. 7. It can be seen that the faradaic efficiency for producing formate increases at first until −1.8 V, where it reaches the maximum. As the potential decreases further, the faradaic efficiency begins to decrease. It can be seen that the difference of the faradaic efficiencies between the Sn electrode and the OE-Sn electrode is small, whereas the formate production rate of the OE–Sn electrode is faster than that of the Sn electrode. Fig. 8 shows the Nyquist plots for the Sn electrode and the OE-Sn electrode in 0.1 mol L−1 KHCO3 solutions after being bubbled with CO2 for 30 min. Since the onset potential for electrochemical reduction of CO2 to formate was −1.25 V (see CV results), the applied potential was set at −1.25 V. It can be seen that the Nyquist plots show two semicircles with two different time constants. Table 1 lists the parameters for the EIS analysis by Zview2 software. The
Fig. 4. XPS spectra of the Sn electrode and the OE-Sn electrode.
Fig. 6. The faradaic efficiencies and the average current densities for electrochemical reduction of CO2 to formate on the Sn, E-Sn and OE-Sn electrodes. Table 1 Results of EIS fitting according to the equivalent circuit shown in Fig.8.
Fig. 5. Variations of the current density with electrolysis time during the electrochemical reduction of CO2 on Sn, E-Sn and OE-Sn electrodes at −1.8 V vs. Ag/AgCl.
Electrode
Sn
OE-Sn
Rs () R1 () R2 ()
31.35 214.9 167.7
30.30 126.8 132.4
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R. Zhang et al. / Applied Surface Science 356 (2015) 24–29
Fig. 7. Variations in the faradaic efficiency (a) and the production rate (b) for producing formate on the Sn electrode and the OE-Sn electrode with the electrolysis potential.
CO2 reduction but hydrogen evolution reaction accelerates. These research results indicate that the surface oxide of the Sn electrode is essential for CO2 reduction. 4. Conclusions
Fig. 8. Nyquist plots for the EIS analysis on the Sn electrode and the OE-Sn electrode in 0.1 mol L−1 KHCO3 solutions after being bubbled with CO2 for 30 min. The inset is the equivalent circuit. The applied potential was −1.25 V vs. Ag/AgCl.
The oxide layer on the Sn electrode has been removed by cathodizing in HCl solution. The faradaic efficiencies for electrochemical reduction of CO2 to formate on the Sn, E-Sn and OE-Sn electrodes are 84%, 43% and 85%, respectively. The faradaic efficiency on the E-Sn electrode is obviously lower than those on the Sn and OESn electrodes. The CV results illustrate that the seriously hydrogen evolution reaction causes the catalytic activity of the E-Sn electrode for CO2 reduction decrease. The etching treatment makes the Sn electrode have a rough surface and behave a larger superficial area. The electrolysis current density for the CO2 reduction on the OE-Sn electrode is around 1.5fold than that on the untreated Sn electrode which can accelerate the reaction rate. Large current density is beneficial to the industrial demand. Acknowledgements
solution resistance (Rs ) values for the Sn electrode and the OE-Sn electrode were similar. Obvious differences are found in the resistances of R1 and R2 . Both the values of R1 and R2 for OE-Sn electrode are smaller than that for the Sn electrode, mainly resulting from the larger superficial area of the OE–Sn electrode which has more catalytic activity point. Although many studies on electrochemical reduction of CO2 have been carried out, the mechanism of this reaction is still ambiguously, it hinders the further research in this field. For the mechanism of CO2 reduction on the metal electrode, it is generally assumed that the bare metal surface is catalytically active for CO2 reduction. However, oxide layer forms easily on the surface of Sn in the air. The oxide layer may have some influences on the performance of CO2 reduction. This problem has been neglected for the study of CO2 reduction until Chen et al. reported their experiments [23]. Baruch et al. studied the electrochemical reduction of CO2 on Sn cathode by utilizing in situ attenuated total reflectance infrared spectroscopy, and found that the peaks attributed to a surface-bound monodentate tin carbonate species (intermediate of CO2 reduction) only appeared at potentials where CO2 reduction was observed, and disappeared when the tin surface was chemically etched to remove surface oxide [24]. In this work, the CV and the electrolysis results illustrate that Sn electrode possessing oxide layer shows excellent catalytic activity for CO2 reduction, and the Sn electrode removed its oxide layer shows poor catalytic activity for
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