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Direct Experimental Evidence and Low Reduction Potentials for the Electrocatalytic Reduction of Carbon Dioxide on the Nanosized Fluorine Doped Tin Oxide Electrodes Shaolin Mu, Jun Wu, Qiaofang Shi, and Fengmin Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00146 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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ACS Applied Energy Materials

Electrocatalytic Reduction of Carbon Dioxide on Nanosized Fluorine Doped Tin Oxide in the Solution of Extremely Low Supporting Electrolyte Concentration: Low Reduction Potentials Shaolin Mu∗,+,

Jun Wu+,

Qiaofang Shi+, Fengmin Zhang‡

+

School of Chemistry and Chemical Engineering, Yangzhou University, Jiangsu Province, Yangzhou, 225002, P. R. China

‡

The testing center of Yangzhou University, Jiangsu Province, Yangzhou 225002, P. R. China

ABSTRACT: Fluorine doped tin oxide (FTO) has been prepared via the direct chemical reaction of tin oxide powders and hydrofluoric acid at room temperature. The image of FTO displays sphere-like structure with an average diameter of 40-100 nm. The spectra of X-ray photoelectron spectroscopy (XPS) demonstrate that fluorine is doped into SnO2. A well-defined reduction peak at ̶ 0.50 V (vs. SCE) is detected on the cyclic voltammogram (CV) of the nanosized FTO (n-FTO) electrode in CO2-saturated 3.6×10-4 µM H2SO4 solution of pH 5.5, which is strong evidence for the electrochemical reduction of CO2. This result indicates that the n-FTO electrode in such extremly low supporting electrolyte concentration exhibits good electrocatalytic ability towards CO2 reduction under lower potentials. On the basis of reduction peak current as a function of scan rate, the reduction of CO2 is first performed via adsorption of CO2 on the n-FTO electrode surface and then CO2 is reduced. The product solution obtained under a constant potential of ̶ 0.90 V (vs. Ag/AgCl with saturated KCl solution) is used for analysis of UV-vis spectra, 1H NMR, and gas chromatography, the results demonstrate the presence of formic acid and methanol in the product solution, but formic acid is a main product. Faradaic efficiency for formic acid is 82.3%. Keywords: CO2 electroreduction; Electrocatalysis; Supporting electrolyte; Low reduction potential; Formic acid

∗ Corresponding author: E-mail: [email protected] (S. L. Mu) Tel:+86 514 87975520; fax: + 86 514 87975244

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INTRODUCTION Carbon dioxide increases in the atmosphere to relate the global warming, which is caused by the combustion of fossil fuels. Therefore, reducing the production of carbon dioxide and converting it into valuable products are very significant in protecting environment and reproducing useful chemicals like formic acid and methanol, which thus has received a great deal of attention in the past decades. Various methods, such as chemical method,1,2 electrocatalytic,3-8 photocatalytic9 and photoelectrocatalytic reductions,10,11

have been used for conversion CO2 into valuable chemicals. Among

these methods, electrocatalytic reduction of CO2 is received more attention12-28 because the reduction rate can be easily controlled by applied potentials, and the product selectivity is able to select from various electrode materials.29-32 The preparation and stability of catalysts, product selectivity and reduction mechanism for CO2 electroreduction were reviewed extensively and deeply by Qiao et al,33

and

who proposed research directions of CO2 electroreduction in the future that is very significant. Generally, the electrocatalytic reduction of CO2 was carried out in the aqueous solutions with higher electrolyte concentrations, such as 0.5 M NaHCO3 solution 17, 34

and 0.520, 30, 35, 36 and 1.0 M KHCO3 solution.37

8,15,

The reason for this, higher

supporting electrolyte concentration is favorable for enhancing Faradaic efficiency and electrochemical reduction rate.38, 39 However, the pH values of these solutions are > 7, in this case, it is necessary to remove the supporting electrolyte from the product solution using an ion exchange column prior to the gas chromatographic analysis, and finally to separate the product and supporting electrolyte in the product solution. The acidic solutions were also used for CO2 electroreduction, such as 0.05 M H2SO4,40 and 0.1 M HClO4,27 which is favorable to the formation of methanol and formic acid, and the product solution can be directly used for the gas chromatographic analysis, but the applied potential would be lower than ̶ 1.1 V (SCE) because of the decrease in hydrogen overpotential due to low pH, which results in the decrease in Faradaic efficiency. 2


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Even if the preparation of catalysts,41, 42 product selectivity,35,43 and the effect of electrolyte on the electrochemical reduction of CO234 have got great achievements in recent years, the electrochemical reduction of CO2 in aqueous solutions is limited by its higher overpotentials; in this case CO2 reduction is generally carried out at ≥ ̶ 1.3 V (versus SCE).7, 27,33, 43-45 Under such high negative potentials, hydrogen is formed during the CO2 electroreduction process, which results in the decrease in Faradaic efficiency and product selectivity.

Therefore, the preparation of new catalysts with

lower overpotentials and low cost is still open, and the preparation of catalysts suitable for CO2 reduction in low supporting electrolyte concentration is also a huge challenge for electrochemists; in addition, the proposed mechanism of CO2 electroreduction needs the support of more experimental evidences.44, 45 Carbon dioxide can be reduced on fluorine doped tin oxide (FTO) prepared by a chemical vapor deposition (CVD) method,44 and transparent FTO glass at the reduced state shows the interesting reduction properties in a pH region of 4-12,46 indicating that FTO is a promising material for CO2 reduction. However, not much has been reported on CO2 reduction on the FTO electrode.44 Several methods have been used to prepare FTO, such as spray pyrolysis,47, 48 chemical vapor deposition44, 49 and sol-gel methods.50, 51 Both of spray pyrolysis and CVD methods were carried out under high temperature and high vacuum, respectively. Thus the sol-gel method is usually used to prepare FTO, which shows easier composition control, better homogeneity and low cost.52 However, the sol-gel method for preparing FTO needs several steps, which thus is not convenient. In this work, a simple method for the preparation of nanosized FTO was carried out via the direct chemical reaction of SnO2 powders and hydrofluoric acid at room temperature. It was found that nanosized FTO (n-FTO) prepared by using direct chemical reaction method can catalyze CO2 electroreduction not only in 0.1 M NaHCO3 solution but also in the extremely low concentration of acid and under low reduction potentials. In this work we reported the preparation and characterization of the n-FTO, its novel electrocatalytic properties, analysis of the product solution of CO2 electroreduction, and experimental evidence for CO2 electroreduction. 3



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EXPERIMENTAL SECTION Materials. Stannic oxide was received from Aladdin Industrial Corporation; hydrofluoric acid, formic acid, and methanol were purchased from Sinopharm Chemical Reagent Company, which are analytic reagent grade. Deuterium oxide (D2O) was purchased from Qingdao Tenglong Welbo Technology Co., Ltd (China).

High

purity CO2 was used for the electrochemical reduction. Doubly distilled water was used to prepare the solutions. Transparent fluorine doped tin oxide (FTO) glasses were purchased from Huanan Xiangchen Science and Technology Company. Preparation of Nanosized FTO.

2.0 g SnO2 powders and 20 ml HF solution were

mixed and stirred for 48 h at room temperature. After reaction, the mixture was centrifuged, followed by removing the solution over the product (n-FTO). The solid product was rinsed with distilled water, and then the product with water was centrifuged again; this operation was repeated for several times until neutrality of the mixture. Finally, the product was dried at 120 oC. Preparation of n-FTO and n-FTO/RGO Composite Electrodes.

10 µl

suspension of n-FTO in water was dispersed homogeneously on a transparent FTO glass surface of 1 cm2, and then was dried at 40 oC to form n-FTO electrode. 10 µl suspension of graphene oxide (GO) in water and 10 µl suspension of n-FTO in water were dispersed on a FTO glass surface of 1 cm2 and then mixed homogeneously to form a film, followed by drying at 40 oC. This composite electrode was reduced at ̶ 0.80 V (vs.SCE) for 50 min in 0.1 M NaHCO3 solution. The purpose for this is to make GO in the film turn to reduced graphene oxide (RGO, i.e., graphene) to form an n-FTO/RGO composite electrode. Prior to electrochemical measurements, the above electrodes were cycled in 0.01 M H2SO4 solution for several cycles between ̶ 0.80 and 0.60 V (vs. SCE). Characterization of n-FTO and Electrochemical Measurement.

The images of

the SnO2 powders and n-FTO powders dispersed on a FTO glass surface were observed by a field emission scanning electron microscopy (SEM) S4800 II FE-SEM. The XPS spectra of n-FTO were collected on a Thermo ESCALAB 250 spectrometer 4


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with an Al Kα X-ray source (1350 eV). The conductivity of n-FTO powders was measured on a pressed pellet using a four-probe technique. The pH of the solution was determined by using a PXD-12 pH meter. A conventional three-electrode cell, containing an n-FTO or n-FTO/RGO composite working electrode, a platinum foil counter electrode and a saturated calomel reference electrode (SCE), was used for the electrochemical experiments that were performed on a CHI 407 workstation and an Autolab Nova 1.8 instrument, respectively. High purity CO2 was used for its electrochemical reduction. The flow rate of CO2 was controlled at 1 L min-1. The tested solution was deaerated by bubbling CO2 for 20 min prior to electrochemical measurements and then a continuous flow of CO2 was maintained over the tested solution during the experiment process. In order to use

1

H NMR technique to analyze the product solution, the

electoreduction of CO2 was performed in a D2O solution of 3.6×10-4 µM H2SO4 of pH 5.5. A home-made Ag/AgCl electrode (with saturated KCl solution) was used for the reference electrode that shows a little water leakage compared to the SCE reference electrode, as a result it decreases water content in the D2O solution. Thus, the home-made Ag/AgCl electrode plays an important role in decreasing the effect of water on the 1H NMR measurement that is favorable to the detection of the product signal. Analysis of the Product Solution.

After the electoreduction of CO2 at a constant

potential, the product solution was analyzed using UV-vis spectroscopy, NMR technique, and gas chromatography. The UV-vis spectra of the samples were measured in a quartz cuvette with a path length of 1 cm using a Shimadzu 2550 double-beam spectrophotometer. 1H NMR spectrum of the product solution was recorded on a 600 MHz Bruker spectrometer. Gas chromatographic analysis of the product solution was carried out on 9790II gas chromatograph with a flame ionization detector (FID). RESULTS AND DISCUSSION Characterization.

The SEM technique was used to observe the surface images 5



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and sizes of SnO2 powders and n-FTO particles, which are shown in Figure 1A and 1B, respectively. Figure 1A indicates that SnO2 powders with diameter of approximately 60-120 nm are aggregated together to form larger blocky particles. The image of n-FTO in Figure 1B displays sphere-like structure with an average diameter of 40-100 nm, and they are homogeneously and closely dispersed on the surface of a transparent FTO glass. Figure 1 demonstrates that after the chemical reaction of SnO2 with HF, the size of n-FTO particles is smaller than that of SnO2 powders. As mentioned in the introduction section, there are several methods to prepare FTO; however, here the n-FTO was first prepared using the direct chemical reaction between SnO2 powders and HF. Thus a key problem is that fluorine is doped into SnO2 or not using this method. In that case the products were determined using XPS technique. Figure 2A shows XPS spectra of SnO2 and the products prepared from different reaction times, respectively. Curve 1 indicates that signals of Sn and O2 are detected, but no F trace is detected in SnO2. This result is expected. However, a weak signal at 684.4 eV is detected in curves 2 and 3, respectively, indicating that fluorine is doped into SnO2 to form FTO and the XPS signal intensity of fluorine in the product increases with reaction time. It was found that the content of F in the n-FTO is 8% after the chemical reaction of 144 h. Curves 1, 2 and 3 in Figure 2B are the signals of fluorine corresponding to above three samples, which are amplified. Also, no fluorine signal is detected in curve 1 of Figure 2B, but the marked F signal is observed in curves 2 and 3. In addition, the positions of Sn3d and O1s in curves 2 and 3 of Figure 2A shift slightly towards the higher binding energies compared to that on curve 1. This is due to the fact that the electro-negativity of fluorine is stronger than that of tin and oxygen, which results in the shift of binding energy of Sn3d and O1s to the high binding energy direction, indicating the formation of Sn-F and O-F bonds. Electric Properties of n-FTO.

The conductivity of n-FTO is 2.41×10-5 S cm-1, so

n-FTO is a semiconductor. Generally, the electrochemical reduction of CO2 was performed in NaHCO3 solution. In this case, The n-FTO electrode was first tested in this solution, its cyclic voltammogram (CV) was recorded in curve 1 of Figure 3A, in which a reduction peak occurs at ̶ 0.51 V, and its peak current density is 0.140 mA 6


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cm-2. This reduction peak is attributed to SnO2 reduction.

The n-FTO/RGO

composite electrode was also tested in this solution, its CV is shown in curve 2 of Figure 3A, in which a reduction peak still occurs at ̶ 0.51 V, but its peak current density is 0.257 mA cm-2, indicating that both electrodes have the same redox properties, but the reduction peak current density of the n-FTO /RGO composite electrode is higher than that of n-FTO electrode. The impedance plots of both n-FTO and n-FTO/RGO composite electrodes are recorded in curve 1 and curve 2 of Figure 3B, respectively, which were measured at ̶ 0.45 V, in 0.1 M NaHCO3 solution. A semicircle with different charge transfer resistance (Rct) values displays in both curves of Figure 3B, in which Rct of n-FTO/RGO composite electrode is much smaller than that of n-FTO. This is caused by RGO in the composite electrode because of high conductivity and large specific surface area for RGO. The result of impedance measurement gives the good explanation why the reduction peak current density of the composite electrode is larger than that of the n-FTO electrode. The above results demonstrate that the electric properties of the composite electrode are better than those of the n-FTO electrode. The n-FTO/RGO composite electrode was also tested in CO2-saturated 0.1 M NaHCO3 solution of pH 8.7, its CV is shown in S-1. The CV in S-1 displays a reduction peak at ̶ 0.41 V (vs. SCE). It is clear that its peak potential is more positive than that in the solution free of CO2 in curves 1 and 2 of Figure 3A, which is caused by CO2 reduction on the n-FTO/RGO composite electrodes. In this work, we focus our attention on the electrochemical reduction of CO2 in the solution of extremely low supporting electrolyte concentration, which will be discussed more detail as follows. Evidence of CO2 Electrochemical Reduction.

The electrochemical reduction of

CO2 on the n-FTO and n-FTO/RGO composite electrodes prepared from the chemical reaction of 48 h was examined by using cyclic voltammetry; the results are shown in Figure 4. Curve 1 of Figure 4A is the CV of the n-FTO/RGO composite electrode in 3.6×10-4 µM H2SO4 solution of pH 5.5; there are no redox peaks to be detected in the potential region of

̶ 0.80 and 0.60 V. However, a pair of redox peaks occurs on the 7



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curve 2 of Figure 4A, which is obtained from the same electrode and same solution but in the presence of CO2. This CV was recorded when the electrode reached to a steady state in CO2-saturated solution, in which an oxidation peak and a reduction peak occur at 0.20 V and ̶ 0.50 V, respectively. It is clear that this pair of redox peaks is caused by CO2 reduction compared to that of curve 1 of Figure 4A. An oxidation peak at 0.20 V would be caused by the oxidation of the product formed during the CO2 reduction process and the reduction peak at ̶ 0.50 V is the direct experiment evidence for CO2 electroreduction. This result is similar to that of CO2 electroreduction on Sn electrode in the solution consisting of 0.45 M KHCO3 and 0.5 M KCl, in which an oxidation peak and a reduction peak occurred at ̶ 0.75 and ̶ 0.91 V (vs. Ag/AgCl), respectively.7 It is clear that the reduction peak potential of CO2 on the n-FTO/RGO electrode is more positive than that of the Sn electrode, indicating that the catalytic performance of the n-FTO/RGO electrode is better than that of the Sn electrode. The current-voltage curve of linear sweeping voltammetry for the fluorine doped tin oxide (SnO2:F) prepared using CVD method indicated that a reduction peak at ̶ 0.85 V (vs. SCE) was detected in CO2-saturated 0.1 M Na2SO4 solution of pH 4.0.44 Comparison the reduction peak potential of CO2 on SnO2:F electrode shows that the reduction peak potential of CO2 on the n-FTO/RGO composite electrode is 0.35 V more positive than that of the SnO2:F electrode. A reduction peak of CO2 on the Pd shells on Au nanocores electrode was observed at ̶ 0.95 V (vs. Ag/AgCl) in 0.1 M Na2SO4 solution.4 Therefore, n-FTO/RGO composite electrode has low reduction peak potential of CO2 compared with those of FTO:F electrode prepared with CVD method and Pd shells on Au electrode. In addition, no oxidation peak on the CVs was observed on the electrodes used in references,4, 36, 44 this is quite different from n-FTO/RGO composite electrode shown in curve 2 of Figure 4A and n-FTO electrode shown in curve 1 of Figure 4B. Curves 1 and 2 of Figure 4B present the CVs of the n-FTO electrode and n-FTO/RGO composite electrode in the potential region of ̶ 1.30 and 0.60 V, in the CO2-saturated H2SO4 solution of pH 5.5, respectively, in which curve 1 shows a reduction peak at ̶ 0.49 V on the n-FTO electrode that is close to the reduction peak potential of the n-FTO/RGO composite 8


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electrode.

The reduction currents on both curves increase from ̶ 1.0 to ̶ 1.3 V,

indicating the formation of hydrogen accompanied with CO2 reduction, but the increasing rate is quite lower than those of metal and metal oxide electrodes4, 23, 33, 52, 53

based on the slope of the straight line between ̶ 1.0 and ̶ 1.3 V in Figure 4B. This

indicates that both n-FTO and n-FTO/RGO composite electrodes possess higher overpotential for hydrogen evolution like the Sn electrode.7 High overpotentials for hydrogen evolution are favorable to the electrochemical reduction of CO2 in aqueous solutions. In addition, the current density of reduction peak on the n-FTO/RGO composite electrode is higher than that of n-FTO electrode in Figure 4B, which is caused by the high conductivity and large specific surface area of RGO. Figure 4 (A) indicates that the reduction peak current density of the n-FTO/RGO composite electrode prepared from the chemical reaction of 48 h is ̶ 0.227 mA cm-2 , however the reduction peak current density of the n-FTO/RGO composite electrode prepared from the chemical reaction time of 144 h is 0.095 mA cm-2 (S-2) . It is clear that the electrocatalytic activity of n-FTO catalyst prepared from the chemical reaction of 48 h is higher than that of the chemical reaction of 144 h, i.e., the electrocatalytic activity of the catalyst with lower doped content of fluorine is better than that with higher doped content of fluorine. Thus, the n-FTO catalyst prepared from the chemical reaction of 48 h was used in this work for CO2 electroreduction. Consider a well-defined oxidation peak and reduction peak on curve 2 of Figure 4A, thus the n-FTO/RGO composite electrode was used to study the effect of the scan rate on the electrochemical reduction of CO2, the result is shown in Figure 5. The result in Figure 5 demonstrates that the cathodic peak current increases with increasing the scan rate from 5 to 100 mV s-1; while the oxidation peak current also increases with increasing scan rate. Such close relationship between cathodic and anodic peak currents and scan rate gives us a broad hint that the oxidation peak is caused by the oxidation of the product produced by CO2 electroreduction; thus their peak currents increase simultaneously with increasing scan rate.

However, the anodic peak

potential and the cathodic peak potential shift towards positive potential and negative potential directions with increasing scan rate, respectively, which results in the 9



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increase in the difference of anodic and cathodic peak potentials, ∆Ep, from 0.217 V at scan rate of 5 mV s-1 to 0.621 V at scan rate of 100 mV s-1 (S-3). The increase of ∆Ep is mainly caused by the voltage drop of the solution due to very low conductivity of the H2SO4 solution of pH 5.5. Despite ∆Ep is dependent on the scan rate, there is still a pair of redox peaks on CVs in the scan rate region of 5 to 100 mV s-1, indicating that the charge transfer in the reduction process of CO2 is not controlled by kinetic effects in this scan rate region, even in the solution of extremely low supporting electrolyte concentration. On the basis of data on Figure 5, the reduction peak current and oxidation peak current as a function of scan rate are shown in Figure 6A and 6B, respectively. A straight line with correlation coefficient of 0.99621 is shown in Figure 6A for the cathodic reaction; also a straight line with correlation coefficient of 0.99585 is obtained in Figure 6B for the anodic reaction, indicating that the rate-determining step for both electrode reactions is controlled by adsorption of reactants on the electrode surface. This result provides strong evidence that the electrochemical reduction mechanism of CO2 is performed via its adsorption. Constant Potential Reduction of Carbon Dioxide.

To know the products of

CO2 electroreduction on the n-FTO electrode, CO2 reduction was carried out at a constant potential of ̶ 0.90 V (vs. Ag/AgCl with saturated KCl solution); at this potential, no hydrogen is formed based on the CVs shown in Figure 4B. Curve 1 of Figure 7 shows the change in the current with time, which was recorded in the solution of nitrogen-saturated D2O solution containing H2SO4 with pH 5.5. Its reduction current decreases with time. Finally, its current is 3 µA, and the quantity of electricity consumed during reduction of 8.5 h is 0.1033 C, that is caused by the reduction of the n-FTO itself. Curve 2 of Figure 7 obtained from the same electrode, but in CO2-saturated solution, its reduction current decreases slowly with time in long-term CO2 reduction process. This result indicates that n-FTO electrode shows a little electrochemical polarization and good stability because the solution was not stirred during the long-term reduction process. After stopping experiment, the final current is 43.6 µA and the quantity of electricity consumed during reduction of 8.5 h 10


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are 1.404 C. Comparison of results obtained from above different experimental conditions demonstrates that the reduction current and coulombs consumed in CO2-saturated solution are much higher than those in nitrogen-saturated solution. This difference is caused by CO2 reduction. On the basis of coulombs passed in nitrogen-saturated solution and CO2- saturated solution, 7.4% of the total coulombs consumed in the CO2 reduction process are lost by the reduction of the n-FTO electrode itself. To obtain the sufficient amount of the products for the following analysis, the electrochemical reduction of CO2 at ̶ 0.90 V was carried out for ca 70 h at the same electrode, indicating that the electrode has the better stability. In fact, the n-FTO and n-FTO/RGO electrodes can be repeatedly used for CO2 electroreduction in our laboratory. Analysis of the Product Solution.

Products for the electroreduction of CO2 are

dependent on the electrode material, applied potential, and pH of the solution. To determine the products in the product solution, UV-vis spectroscopy, 1H NMR, and gas chromatography were used for analysis of the product solution. Curves 1 and 2 of Figure 8 show the UV-vis spectra of formic acid and the product solution, respectively, in which an absorption band at 204 nm occurs on curve 1 for formic acid solution, and a band at 198 nm is detected on curve 2 for the product solution, the latter band is close to that of formic acid. This result indicates that formic acid is present in the product solution; however no any other signals were detected in the region of 190-300 nm. Figure 9 displays the 1H NMR spectrum of the product solution, which was obtained from CO2 reduction in the D2O solution with 3.6×10-4 µM H2SO4 as mentioned above. The chemical shift of 8.36 and 3.27 in Figure 9 would be attributed to CH proton of formic acid and methyl protons of methanol, respectively, since the chemical shift of CH proton in formic acid is 8.06 and the chemical shift of methyl protons in methanol is 3.39,55 which are put in Figure 9. A strong signal at the chemical shift of 4.8 in Figure 9 is caused by H2O that comes from H2SO4 solution in the product solution and the reference electrode of Ag/AgCl (with saturated KCl solution). Other weak signals in Figure 9 are not clear to us because products of CO2 11



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1

electroreduction are more complicated. The result from

H NMR spectrum

demonstrates the presence of formic acid and methanol in the product solution, and the amount of formic acid is predominant over methanol.

A absorption band of

methanol in the UV-vis spectrum occurs at 183 nm, which is beyond the measuring region of the UV-vis spectrometer used in our experiment, thus methanol signal in Figure 8 was not observed. The result of gas chromatography for the product solution is shown in Figure 10, in which the retention times of 2.994 and 3.054 minutes are attributed to methanol and formic acid, respectively, based on the retention times of formic acid and methanol (S-4). The results on Figure 10 indicate that the amount of formic acid in the product solution predominates over that of methanol. Faradaic efficiency for formic acid is 82.3% at ̶ 0.90 V. On the basis of the results from the analysis of the product solution using above spectroscopic techniques, formic acid is main product for CO2 electroredution on the n-FTO electrode in the acidic solution. The oxidation peak at 0.2 V on curve 2 of Figure 4 is caused by the oxidation of the product for CO2 reduction as discussed above, thus this oxidation peak is caused by the oxidation of formic acid. In addition, the electrochemical reduction of CO2 is performed via its adsorption as mentioned above. Even if only a pair of redox peaks occurs on the CVs of Figure 5 in a scan rate region of 5 to 100 mV s-1; it seems that no intermediates were formed.

However the

scan rate in above scan rate region is too slow, thus the radical intermediate, such as the radical anion of carbon dioxide, cannot be detected in the aqueous solution because of the short lifetime of free radicals. Fortunately this free radical has been detected recently in the electrochemical reduction of carbon dioxide in N, N–dimethylformamide by scanning electrochemical microscopy,56 which provides significant evidence for the mechanism of CO2 electroreduction. Several mechanisms for the electroreduction of CO2 to formic acid/formate have been suggested such as the electroreduction of CO2 on the FTO electrode prepared by a CVD method44, on a tin electrode13, on metal-free boron-doped graphene electrode45, on metal electrodes43, on nano-copper electrode20, and on the tin-lead alloys 12


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electrode57. Clearly, the mechanism for the electroreduction of CO2 to formic acid is more complicated. One of problems is the detection of the intermediates formed in the electroreduction process of CO2. Based on a wealth of experimental evidence reported in this work and the significant detection of carbon dioxide radicals in the CO2 electroreduction process56, the mechanism of CO2 electroreduction in the weak acidic solution is suggested as follows: CO2(ads)

CO2(aq) CO2(ads)

.CO2(ads)

+

.CO2(ads)

1 e-

+

2 H+

+

2 e-

HCOOH

The oxidation peak on the CV is caused by the oxidation of formic acid: HCOOH

CO2

+ 2 H+ + 2 e

The anodic oxidation of formic acid on the n-FTO electrodes is similar to that on the Sn electrode in Na2SO4 supporting electrolyte,7 which is favorable for CO2 electroreduction.

CONCLUSIONS Nanosized fluorine doped tin oxide (n-FTO) was prepared via the direct chemical reaction between tin oxide powders and hydrofluoric acid at room temperature. This method provides a simple and convenient way to prepare FTO compared to other methods. The cost of n-FTO catalyst is low. Fluorine doped in SnO2 makes n-FTO become an n-type semiconductor under the cathodic polarization. As a result, a reduction peak occurs in the reduction process of n-FTO, which plays an important role in mediating the charge transfer for the electroreduction of CO2.

A well-defined

reduction peak at ̶ 0.50 V (vs. SCE) is detected on the CV of the n-FTO electrode in CO2-saturated 3.6×10-4 µM H2SO4 solution of pH 5.5. To the best of our knowledge, this is first report on the CO2 electroreduction in such low supporting electrolyte concentration and a well-defined reduction peak on the CVs is strong evidence for the electrochemical reduction of CO2. The results from the relationship between reduction peak current and scan rate give a broad hint that CO2 electroreduction is performed 13



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via adsorption of CO2 on the electrode surface. The n-FTO electrode shows high overpotential for hydrogen evolution and good stability. The mechanism of CO2 electroreduction in the weak acidic solution is suggested. The results from measurements of UV-vis spectroscopy,

1

H NMR, and gas chromatography

demonstrate that the products of CO2 electroreduction on the n-FTO electrode are formic acid and methanol, but the main product is formic acid. In addition, the n-FTO catalyst possesses higher Faradaic efficiency and better selectivity for CO2 electroreduction. ACKNOWLEDGMENTS We thank the testing center of Yangzhou University for technical assistance. S Supporting Information ○

Cyclic voltammogram (CV) of n-FTO/RGO electrode in CO2-saturated NaHCO3 solution, the CV of the n-FTO electrode prepared from the chemical reaction time of 144 h, the relationship between (∆Ep) and scan rate for CO2 electroreduction, and gas chromatography of methanol and formic acid.

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Page 21 of 26

Figure 1. SEM images: (A) SnO2 powders, (B) n-FTO.

Sn3d5/2 Sn3d3/2 O1s

F1s

B F1s

3

F1s

2

Intensity/a.u.

A

Intensity/a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2 1

1 400

450

500

550

600

650

Bi n di n g e ne r g y / e V

700

680

685

690

695

700

Bi n d i n g e n e r g y / e V

Figure 2. XPS spectra, (A) curves (1) SnO2, (2) and (3) n-FTO obtained from the chemical reaction time of 48 and 144 h, respectively; (B) F1s in n-FTO. 21



16000

0.10 0.05

A

2

14000

B

12000

1

10000

-0.05

Z'' / ohm

I / m A c m-2

0.00

-0.10 -0.15

8000 6000

-0.20

4000

-0.25

2000

1 2

0 0 2000 4000 6000 8000 10000120001400016000

-0.30 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Z' / o h m

Ev s . S CE/ V

Figure 3. (A) the CVs, at a scan rate of 60 mV s-1, and (B) impedance plots, at ̶ 0.45 V (vs.SCE), curves: (1) n-FTO electrode and (2) n-FTO/RGO composite electrode, in 0.1 M NaHCO3 solution.

0.3

0.3

A 0.2

2

0.1

1

0.2

0.0 -0.1 -0.2 -0.3 -0.8

B 2

0.1

I / m A C m-2

I / m A c m-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

1

0.0 -0.1 -0.2 -0.3

-0.6

-0.4

-0.2

0.0

0.2

E v s . S CE/V

0.4

0.6

-0.4 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

E v s . S CE / V

Figure 4. (A) CVs of n-FTO/RGO composite electrode, (1) in 3.6×10-4 µM H2SO4 solution of pH 5.5, (2) in CO2–saturated H2SO4 solution of pH 5.5; (B) CVs of n-FTO electrode (1) and n-FTO/RGO composite electrode (2), in CO2–saturated H2SO4 solution of pH 5.5; at a scan rate of 60 mVs-1. 22


Page 23 of 26

300 6

200

5

I / µ Α c m-2

4

100

3 2 1

0 -100 -200 -300 -0.8 -0.6 -0.4 -0.2

0.0

0.2

0.4

0.6

E v s . S CE / V

Figure.5. Effect of scan rate on CVs of n-FTO/RGO composite electrode in CO2–saturated H2SO4 solution of pH 5.5, curves: (1) 5, (2) 10, (3) 20, (4) 40, (5) 60 and (6) 100 mV s-1.

250

A

250

200 150 100 50 0 0

B

200

I / µ A c m-2

-I / µ A c m-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150 100 50

20

40

60

80

υ / mVs-1

100

0 0

20

40

60

80

100

υ / mVs-1

Figure 6. Based on the data of Figure 5, (A) the relationship between the cathodic peak current and scan rate, (B) the relationship between the anodic peak current and scan rate.

23



0.00 1

-0.02

I/mA

-0.04 2

-0.06 -0.08 -0.10 -0.12 0

1

2

3

4

t/h

5

6

7

8

9

Figure 7. Reduction of carbon dioxide on n-FTO electrode at a constant potential of ̶ 0.90 V (vs. Ag/AgCl with saturated KCl solution). The area of the electrode is 2 cm2. Curves: (1) in the nitrogen-saturated D2O solution containing H2SO4 with pH 5.5, (2) in the CO2-saturated D2O solution containing H2SO4 with pH 5.5.

1.6 1.4

2

1.2

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

1.0 0.8 0.6 0.4

1

0.2 0.0 -0.2

180

200

220

240

260

280

300

Wa v e l e n g t h / n m

Figure 8. UV-vis spectra: (1) formic acid solution, (2) the product solution.

24


Page 25 of 26

Figure 9. 1H NMR spectrum of the product solution.

6.1 6.0 Formic acid

5.9

Signal intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.8 5.7 5.6 5.5

Methanol

5.4 5.3 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

t / mi n

Figure 10. Gas chromatography of the product solution.

25



Table of contents graphic:

0 .3 2

0 .2

-2

C O

I/mAcm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

2

-s a tu ra te d

s o lu tio n

0 .1

1

0 .0 -0 .1

F re e

of

C O

2

-0 .2 -0 .3 - 0 .8

- 0 .6

-0 .4

- 0 .2

0 .0

0 .2

0 .4

0 .6

E / v s . S C E /V

26


Electrocatalytic Reduction of Carbon Dioxide on ... - ACS Publications

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