Electrochemical Reduction of Carbon Dioxide to Formate on TinâLead

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Electrochemical Reduction of Carbon Dioxide to Formate on Tin-Lead Alloys Song Yi Choi, Soon Kwan Jeong, Hak Joo Kim, Il-Hyun Baek, and Ki Tae Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01336 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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ACS Sustainable Chemistry & Engineering

Electrochemical Reduction of Carbon Dioxide to Formate on Tin-Lead Alloys Song Yi Choi, Soon Kwan Jeong, Hak Joo Kim, Il-Hyun Baek, Ki Tae Park* Green Energy Process Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon, 305-343 Republic of Korea

* Corresponding author. Tel.: +82-42-860-3257. E-mail address: [email protected]

ABSTRACT Electrochemical reduction of carbon dioxide (CO2) to formate (HCOO-) in aqueous solution is studied using tin-lead (Sn-Pb) alloys as new electrocatalysts. In electrochemical impedance spectroscopy (EIS) measurements, lower charge-transfer resistance is observed for the alloy electrodes when compared to the single metal electrodes such as Sn and Pb. The results of X-ray photoelectron spectroscopy (XPS) and cyclic voltammetric (CV) analysis show that the Sn in the Sn-Pb alloys facilitates the formation of oxidized tin (SnOx) and metallic lead (Pb0) on the alloy surface by inhibiting the formation of low-conductive lead oxide (PbO) film. The CV analysis confirms that the Sn-Pb alloys exhibit higher reduction current than the single metal electrodes under CO2 atmosphere. The Faradaic efficiency (FE) and the partial current density (PCD) of HCOO- production on the alloy electrodes are investigated by electroreduction experiments at -2.0 V (vs. Ag/AgCl) in an H-type cell. As results, respectively more than 16 % and 25 % higher FE and PCD of HCOO- are obtained from the Sn-Pb alloys compared to the single metal electrodes. A Sn-Pb alloy including surface composition of Sn56.3Pb43.7 exhibits the highest FE of 79.8 % with the

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highest PCD of 45.7 mA cm-2. Keywords: electrochemical reduction; carbon dioxide; formate; electrocatalyst; alloy

INTRODUCTION The conversion of CO2 into useful chemicals and fuels has been the target of recent research in order to provide a means to control carbon balance by recycling CO2.1-5 Electrochemical conversion of CO2 provides the storage of renewable electricity in high energy-density form as reduction products, determined by electrode material, solvent, local pH, electrolyte and CO2 pressure.7-10 Electrochemical reduction of CO2 at heterogeneous metal surface in aqueous solution is a promising technique, because the system is simple and the product can be selectively controlled by changing the electroreduction conditions, such as electrode and electrolyte.3 In particular, electroreduction of CO2 to HCOO- appears to have the best chance for the practical development of technically and economically viable processes11, because HCOO- can be obtained with high selectivity in aqueous electrolyte.6,10 In recent years, demand for formic acid keeps increasing in pharmaceutical synthesis and in paper and pulp production, including its traditional uses such as textile finishing and additive in animal feeds.12 In addition, formic acid has been proposed as a fuel for fuel cells and hydrogen (H2) storage.13-16 Accordingly, many researchers have been reported results of the electroreduction of CO2 into HCOO- on various metal catalysts such as Sn, Pb, and In. 12,16-27

Recent research interest has been focused on tin oxides (SnOx) as electrocatalysts

as they are shown to exhibit a high activity for CO2 reduction to HCOO-. 28-30 In addition, it has been found that the electrocaltalytic activities for the electro-oxidation of organic compounds are changed dramatically by the presence of sub-monolayer ad-


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atoms at optimum surface compositions, and that synergistic action between surface atoms is obtained.31 Based on these previous studies, similar enhancement effects can also be expected for CO2 reduction on the electrode surfaces modified by ad-atoms or by alloying, which may make possible reversibility as well as selective reduction. The introduction of low H2-overpotential metal sites on a high H2-overpotential metal surface, or vice versa, allows catalytic interaction between both H2 and CO2 adsorbed on the neighboring sites, resulting in a reduction of the overpotential and an improvement of selectivity for the CO2 reduction.32 Several results of alloy catalysts for CO2 reduction have been reported.33-37 Watanabe et al. have investigated the catalytic properties of various Cu-based alloy electrodes.33 Kyriacou and Anagnostopoulos used Au-modified Cu electrodes and observed that the production of CH4 falls when the surface contains more gold.34 Christophe et al. used Cu-Au alloys with various Au contents (1 - 50 %) and reported that Au50Cu50 alloy appears to be the most efficient substrate for the conversion of CO2 into CO.35

Jia et al.

also developed nanostructured Cu-Au alloys for reduction of CO2 to alcohols (methanol of ethanol) by template-assisted electrodeposition.36 Faradaic efficiency of methanol on Cu63.9Au36.1/NCF (nano-porous Cu film) was analyzed to be 15.9 %, which was about 19 times that on pure copper. Recently, Kortlever et al. investigated the electrochemical CO2 reduction to formic acid on carbon-supported bimetallic Pd-Pt Nanoparticles.37 They found that PdxPt(100-x)/C have a very low onset potential for the reduction of CO2 to formic acid and obtained a FE of 88 % toward formic acid after 1 h electrolysis at 0.4 V vs. RHE with an average current density of ~5 mA cm-2 from the Pd70Pt30/C catalyst. In the present work, electrochemical reduction of CO2 to HCOO- in aqueous solution



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was studied using Sn-Pb alloys as electrocatalyst for the first time. The Sn-Pb is a commonly used material in soldering, and the alloy can be obtained easily by a typical co-deposition system without any additive agent.38 The alloys were obtained by electrochemical deposition on a carbon paper with different compositions and characterized by morphology, structure, and composition. The electrochemical behavior of the alloys was measured by EIS and CV analysis. In addition, FE, PCD and concentration of HCOO- on the alloys were investigated and compared with those on Sn and Pb electrodes.

EXPERIMENTAL SECTION Preparation of Sn-Pb alloys Sn-Pb alloys were prepared by electrodeposition on a carbon paper (Toray, TGPH-060) as a substrate using conventional three electrode cell with a volume of 100 ml at 298 K under atmospheric pressure. Before experiment, the carbon paper was treated in ethanol with an ultrasonic bath for 10 min in order to remove impurities on the surface, after which the electrode was dried at 323 K for 30 min. One side of carbon paper was exposed to electrodeposition bath with 1 cm2 area by masking the other side with an epoxy resin (Sigma Aldrich). A Pt coil (BAS Inc., A-002234) with about 3.6 cm2 surface area and a Ag/AgCl (Radiometer analytical, XR300) filled with saturated KCl were used for the counter electrode and the reference electrode, respectively. The electrodeposition bath was prepared by addition of SnO (Alfa, 99.9 %) and/or PbO (Sigma Aldrich, 99.999 %) with different compositions into aqueous solution of 1 M HBF4, prepared from 48 % (w/w) HBF4 (Sigma Aldrich). Five electrodes with different compositions (Sn, Sn-Pb alloys, and Pb) were obtained by varying PbO concentration


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(0.0, 2.0, 3.3, 5.0, and 10.0 mM) in the electrodeposition bath. The total concentration of metal cations (Sn2+ and Pb2+) in electrolyte was 10.0 mM for all experiments. Eight samples per each composition were prepared for characterizations and electrochemical measurements. The electrodeposition bath was bubbled with purified N2 gas (99.99 % (v/v)) at least for 30 min before experiment, and this atmosphere was kept during the experiment. The electrodeposition was carried out using potentiostat/galvanostat (Princeton Applied Research, VersaSTAT 3) by applying constant current of -16 mA with 3.6 C total consumed charge. The prepared electrodes were rinsed with deionized water and dried at room temperature for 24 h.

Characterizations The surface compositions of prepared electrodes were analyzed through X-ray photoelectron spectroscopy (XPS) on a spectrometer (Thermo, iLab 2000). An inductively coupled plasma-atomic emission spectrometer (ICP-AES; Thermo Scientific iCAP 6500 duo) was used for analyzing the bulk compositions of the electrodeposits. The morphologies of deposits of Sn, Pb, and Sn-Pb alloys on a carbon paper were observed using a field emission scanning electron microscopy (FESEM; HITACHI, S4800) at 5 kV. X-ray diffraction (XRD) patterns of the electrodeposited Sn, Pb, and SnPb alloys were obtained from a diffraction-meter (Rigaku, Dmax-2500pc) in a 2Θ range from 30 to 80 degree.

Electrochemical Measurements Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were carried in the three-electrode cell, which has same construction with



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the electrodeposition experiment except for electrolyte. The electrolyte used for electrochemical measurements was an aqueous solution of 0.5 M KHCO3 (Sigma Aldrich, 99.7 %) prepared from the solid salt. EIS measurements were carried out using a potentiostat/galvanostat (BioLogic, SP-240) at 298 K. The impedance spectra were recorded in a frequency range from 1 MHz to 100 mHz at two different electrode potentials of 0 V and -2.0 V (vs. Ag/AgCl) with a amplitude of 30 mV. The electrolyte was saturated with CO2 gas (99.999 %) prior to measurements. CV measurements were performed after degassing the electrolyte at least for 30 min with N2 gas, used as a baseline, and the actual determinations were carried out using CO2 saturated electrolyte. Cyclic voltammogram was recorded from -2.2 V to 0.2 V (vs. Ag/AgCl) with scan rate of 50 mV∙s-1 at 298 K under ambient pressure to initially test the feasibility of CO2 reduction with fabricated electrodes.

Electrocatalytic Reduction of CO2 Electroreduction experiments were performed at -2.0 V (vs. Ag/AgCl) for 2 h under ambient temperature and pressure in an H-type cell, which has cathode and anode chambers separated by an ion exchange membrane (Nafion® 115). Six samples of each composition, prepared at same condition, were investigated to obtain a statistical distribution of the results. The schematic diagram of the electroreduction system is shown in Figure 1. The volume of the both electrolyte chambers was about 65 ml. An Ag/AgCl electrode (Radiometer analytical, XR300) was used as a reference electrode along with a Pt coil (BAS Inc., A-002234) as a counter electrode. An aqueous solution 0.5 M KHCO3 was used as catholyte, while 0.5 M KOH (Sigma Aldrich, 90 %) was used as anolyte. The catholyte was saturated with CO2 by bubbling for 5 h before


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electrolysis. CO2 gas at a flow rate of 30 ml∙min−1 was supplied continuously to the catholyte using glass sparger throughout the experiment. Quantitative analysis of the gas and liquid products were carried out by 20 min intervals to investigate the FE, PCD and concentration of HCOO-. The outlet gas was analyzed by gas chromatograph (GC; YL Instrument, 6500GC system), equipped with both thermal conductivity detector (TCD) and flame ionization detector (FID). Liquid products were analyzed using a high performance liquid chromatograph (HPLC; Themo Scientific, Ultimate 3000) to determine the amount of produced HCOO-.

Figure 1. Schematic diagram of the electroreduction of CO2 in an H-type cell reaction system.



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RESULTS AND DISCUSSION Characterization of Sn-Pb alloys The morphology of the electrodes was investigated by SEM, and a series of SEM images at the same magnification is described in Figure 2. This displays that Sn, Pb and Sn-Pb alloys were deposited successfully on the surface of carbon fiber. However, the deposited shapes of the Sn-Pb alloys are different from those of single metals. In the cases of single metals, crystals with tetragonal shape were observed, while deposits with rounded shape were observed for the alloys. This result coincides well with previous work in the literature. Petersson et al. reported that the microstructure of the electrodeposited Sn changes drastically in the presence of Pb, because Pb inhibits the deposition of Sn on Sn and prevent the formation of inherent Sn crystal.39 Therefore, the rounded shape of the alloys indicates that the Sn-Pb alloys were fabricated by codeposition of Sn and Pb. In addition, the deposits were grown in particles of a variety of sizes, generally consisted in particle sizes of 2 - 3 μm in diameter. In particular, the alloys exhibited more agglomerated forms of particles compared to single metals. Thus, it is difficult to define the exact active area of the prepared electrodes, especially in the case of using a substrate having a complicated structure such as a carbon paper. Therefore, a simple geometric electrode area, the 2-dimensional area of the carbon paper (1×1 cm2), was used in this study.


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Figure 2. SEM images of (a) pristine carbon paper and deposits of electrodes obtained by electrodeposition with different compositions: (b) Sn, (c) Sn77.3Pb22.7, (d) Sn56.3Pb43.7, (e) Sn35.1Pb64.9, and (f) Pb.

The surface and bulk compositions with metal loading amount of the prepared electrode were described in Table 1. The surface compositions were almost same with those of bulk compositions as shown in Table 1, which show that the deposits of Sn-Pb alloys were grown up with uniform compositions. The metal loading amount was increased



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from 1.4 mg cm-2 for Sn to 2.7 mg cm-2 for Pb with increasing Pb ratio, because of the higher molecular weight of Pb compared to that of Sn. When considering metal loading amount in terms of mmol cm-2, all the prepared electrodes exhibit similar loading amount of 12.7 mmol cm-2 within 5.1 % deviation.

Table 1. Compositions and metal loading amounts of the prepared electrodes. Surface composition (at. %) Electrode Sn

Sn

Bulk composition (at. %)

Pb

Sn

Pb

Loading amount (mg cm-2) 1.4

100.0

-

100.0

-

Sn77.3Pb22.7

77.3

22.7

78.1

21.9

1.7

Sn56.3Pb43.7

56.3

43.7

55.4

44.6

2.1

Sn35.1Pb64.9

35.1

64.9

35.6

64.4

2.3

-

100.0

-

100.0

2.7

Pb

Figure 3 shows the XRD patterns of prepared electrodes with Sn, Pb, and Sn-Pb alloys. The Sn electrode has 11 reflections in the 2Θ range investigated (Figure 3 (a)), showing a random distribution of crystals on the surface with preferential orientation in (200) and (101) directions. For Pb electrode, 6 reflections with preferential orientation in the (111) direction were observed as shown in Figure 3 (e). A reflection at 54.5 degree corresponds to (008) surface of the carbon paper. For the Sn-Pb alloys, reflections for both Sn and Pb were observed in the diffractogram (Figure 3 (b)-(d)), indicating that Sn and Pb were electrodeposited side by side on the surface of carbon paper. In addition, the peaks intensity of Pb was increased with Pb ratio, while that of Sn was decreased. Moreover, all peak positions of Sn and Pb have little excursion to directions of approaching to neighboring peaks for Sn-Pb alloys. In particular, (112) peak of Sn and (311) peak of Pb were closed each other and merged like one peak. This result shows that the crystal grain is not pure metal phase, but solid solution phase of Sn-Pb alloy.40


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• ••

•

(312)

••

(420) (411)

(112) (400) (321)

• •

(301)

(220) (211)

(200) (101)

Lead (Pb) • Tin (Sn) * Carbon (C)

Intensity / a.u.

•

••

(a) Sn (b) Sn77.3Pb22.7

30

(400)

(222)

(311)

(200)

(220)

(c) Sn56.3Pb43.7

(111)

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*

40

50

(d) Sn35.1Pb64.9 (e) Pb

60

70

80

2θ / degree Figure 3. XRD patterns of the prepared electrodes with different compositions: (a) Sn, (b) Sn77.3Pb22.7, (c) Sn56.3Pb43.7, (d) Sn35.1Pb64.9, and (e) Pb.

XPS signals of Sn and Pb were obtained and the peak binding energies of Sn and Pb signals from the prepared electrodes are given in Table 2. The binding energy was calculated from reference C 1s core level at 284.6 eV. In the high resolution Sn 3d spectrum of the electrodes including Sn and Sn-Pb alloys (Figure 4), metallic tin (Sn0) was not observed, and there was only oxidized tin (SnOx) on their surface. On the other hand, the Pb 4f spectra of Sn-Pb alloys show both oxidized lead (PbOx) and metallic lead (Pb0) as shown in Figure 5. However, the Pb electrode exhibits only divalent lead (Pb2+) peaks of 4f5/2 and Pb 4f7/2 at 145.1 and 140.2 eV, respectively (Figure 5 (a)).



Table 2. Peak binding energies of Sn 3d and Pb 4f signals from prepared electrodes. Sn 3d

Pb 4f

Electrode Sn 3d3/2

Sn 3d5/2

Pb 4f5/2

Pb 4f7/2

Sn

495.9

487.5

-

-

Sn77.3Pb22.7

495.6

487.2

143.8

139.0

Sn56.3Pb43.7

495.7

487.3

143.9

139.1

Sn35.1Pb64.9

495.8

487.4

144.1

139.2

-

-

145.1

140.2

Pb

Sn3d5/2 Sn3d3/2

Intensity / a.u.

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(a) Sn (b) Sn77.3Pb22.7 (c) Sn56.3Pb43.7 (d) Sn35.1Pb64.9

500

498

496

494

492

490

488

486

Binding energy / eV Figure 4. XPS spectra of Sn 3d for Sn and Sn-Pb alloys electrodes.


484

482

480

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Pb4f7/2 Pb4f5/2

Intensity / a.u.

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(a) Pb (b) Sn35.1Pb64.9 Pb2+

(c) Sn56.3Pb43.7

Pb2+

150

148

Pb0

Pb0

(d) Sn77.3Pb22.7

146

144

142

140

138

136

134

Binding energy / eV Figure 5. XPS spectra of Pb 4f for Sn-Pb alloys and Pb electrodes.

Table 3 shows the atomic ratio of Pb0/Pb2+ and the Pb0 composition at the surface of SnPb alloys and Pb electrodes. The atomic ratio of Pb0/Pb2+ was evaluated by measuring the area under each peak after deconvolution of the Pb 4f spectra based on the total Pb content in the catalysts, and the composition of Pb0 at the surface was calculated by considering the total Sn and Pb amount of catalyst. As a result, Pb0 was not detected from the Pb electrode and the ratio of Pb0/Pb2+ was increased with Sn ratio in the alloys. This result shows that the oxidation of Pb0 into PbO was inhibited by Sn in the Sn-Pb alloys. In addition, among the alloy electrodes, the Sn56.3Pb43.7 shows the highest Pb0 composition of 2.8 at.% at the surface, because the total Pb content increases with Pb ratio in the alloys, although the ratio of Pb0/Pb2+ is increased with increasing Sn.



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Accordingly, Sn in the alloys forms SnOx and Pb0 on their surface and inhibits the formation of PbO film.

Table 3. The ratio of Pb0/Pb2+ and at Pb0 composition at the surface of Sn-Pb alloys and Pb electrodes. Electrode

Atomic ratio of Pb0/Pb2+ (%)

Pb0 at the surface (at. %)

Sn77.3Pb22.7

10.2 / 89.8

2.3

Sn56.3Pb43.7

6.5 / 93.5

2.8

Sn35.1Pb64.9

3.3 / 96.7

2.1

Pb

0.0 / 100.0

0.0

From the high resolution Sn 3d spectrum of Sn electrode (Figure 4 (a)), Sn 3d3/2 and Sn 3d5/2 binding energies appear at 495.9 and 487.5 eV, respectively. However, the Sn 3d signals of Sn-Pb alloys (Figure 4 (b)-(d)) were shifted to lower binding energy compared to Sn electrode, and the signal undergoes a greater shift as the Sn ratio increases in the alloys. Similar to the result of Sn 3d spectra, the peaks of Pb 4f for the Sn-Pb alloys also shifted to lower binding energy compared to the Pb electrode, and the greater shift was observed for the Sn-Pb alloy with higher Sn ratio. This tendency of peak shift for the alloy composition was observed only in the Sn-Pb alloys. The shifting of binding energy to lower levels is because of the increase in the electrical conductivity of the electrode surface.41,42 The non-neutralized positive charge on the surface of insulator (or on the surface which has very low conductivity) will increase the binding energy of electrons in the surface and XPS peaks will shift to higher energy levels. Therefore, the lower shift in binding energy indicates that the surfaces of alloys have higher electrical conductivity compared to single metal electrodes, and the conductivity


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increases with Sn ratio in the alloys. This is coincided well to the result reported by Simon et al., who found that the Pb-Sn films formed on Sn-rich alloys are more electrically conductive through electrochemical impedance study.43

Electrochemical Measurements EIS measurements were carried out to investigate the electrical conductivity of the electrodes and the kinetics of electron-transfer processes on the prepared electrodes. Figure 6 (a) and (b) show the Nyquist impedance plots obtained for the electrodes with electrode potentials of 0 V and -2.0 V (vs. Ag/AgCl), respectively. It can be seen in Figure 6 (a) that the Sn-Pb alloy electrodes exhibit smaller impedance arcs than those of Sn and Pb when there are almost no electrochemical reactions, which indicates the alloy electrodes have lower charge-transfer resistance. In addition, the diameter of arcs decreased with higher Sn ratio for the alloys. This EIS responses obtained at 0 V are consistent well with the result of the lower shift in XPS peaks for the alloy electrodes. Figure 6 (b) shows the kinetics of electron-transfer processes on the electrodes when CO2 reduction reaction occurs at -2.0 V on the electrodes. The ohmic resistance values mainly caused by the electrolyte solution were almost constant to about 4.8 Ω; however, the diameter of the arcs varies depending on the electrode composition. The alloy electrodes show smaller impedance arcs than Sn and Pb electrodes similar to the result at 0 V, and the Sn56.3Pb43.7 exhibits the smallest diameter of the arc. Therefore, it is expected that faster electrochemical reduction occurs on the alloy electrodes, especially on the Sn56.3Pb43.7.



800 Sn Sn77.3Pb22.7 Sn56.3Pb43.7 Sn35.1Pb64.9 Pb

600

-ZIm / Ω

(b)

400

200

0

0

200

400

600

800

1000

ZRe / Ω 5 Sn Sn77.3Pb22.7 Sn56.3Pb43.7 Sn35.1Pb64.9 Pb

4

-ZIm / Ω

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3

(b)

2

1

0

4

5

6

7

8

9

10

11

ZRe / Ω Figure 6. Nyquist impedance plots of the prepared electrodes in a 0.5 M KHCO3 solution in a frequency range from 1 MHz to 100 mHz with 30 mV amplitude at (a) 0 V and (b) -2.0 V (vs. Ag/AgCl).


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Figure 7 shows the cyclic voltammogram of prepared electrodes in aqueous 0.5 M KHCO3 solution, obtained under N2 (dashed lines) and CO2 (solid lines) atmosphere. The voltammogram can be divided by three main peaks denoted as A, B, and C. The anodic peaks of A and B are related to oxidation of Sn and Pb; respectively, and the cathodic peak C indicates reduction of SnOx and PbOx. For Sn electrode, the anodic peak A between -0.5 and -0.8 V and the cathodic peak C at -0.9 V in voltammogram (Figure 7 (a)) can be attributed to the formation and the reduction of SnOx in basic media, respectively.44 Thus, the peak A was not observed for the Pb electrode. Similarly, the peak B was not appeared in the voltammogram of Sn electrode. A large cathodic peak C of the Pb electrode in Figure 7 (e) indicates the reduction of PbOx. However, all the three peaks including the two anodic peaks (A and B) and the cathodic peak (C) can be observed in the voltammogram of Sn-Pb alloys as shown in Figure 7 (b) - (d). The intensity of peak B and C, attributed to the formation and reduction of PbO, were decreased with decreasing Pb ratio in the alloys at the both atmosphere of N2 and CO2. However, especially under CO2 atmosphere, the peak intensities of B and C of the SnPb alloys were more significantly decreased than those of Pb as shown in Figure 7 (b) – (e). This indicates that Pb0 in the Sn-Pb alloys was not oxidized easily into PbO under CO2 atmosphere. This behavior along with the XPS results, which showing the increased Pb0/Pb2+ ratio by Sn content in the alloy, supports the role of Sn in the inhibition of the formation of PbO. For all the tested electrodes, H2 evolution starts at about -1.7 V and increases drastically at more negative potential than -2.0 V in the N2-bubbled solution. However, the increase of cathodic current begins at -1.5 V in the CO2 bubbled solution, indicating that participation of CO2 reduction contributes to the enhancement of cathodic current. In



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particular, higher reduction currents were obtained from the Sn-Pb alloys than those from Sn and Pb electrodes under CO2 atmosphere. The Sn56.3Pb43.7 exhibits the highest reduction current among the tested electrodes in a potential range from -1.5 V to -2.2 V in the presence of CO2. When compare the reduction current density obtained from the CV result at -2.0 V under CO2 atmosphere, the Sn56.3Pb43.7 shows -20.2 mA cm-2, whereas Sn and Pb display -17.1 and -13.9 mA cm-2, respectively. The increased reduction current comes from the reduction of CO2, which indicate that the Sn-Pb alloys facilitate selective reduction of CO2. However, product analysis from the CO2 reduction experiment is needed to investigate the FE and PCD of HCOO-, because both the CO2 reduction and H2 evolution reactions occur in this potential range.


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20 A

0

C

-20

(a) Sn

-40 20 A

B

0 C

-20

(b) Sn77.3Pb22.7

-40 20 A

0

B

C

(c) Sn56.3Pb43.7

-20 -40 40 20 0 -20 -40 -60

A

B

C

(d) Sn35.1Pb64.9

40 20

B

0 C

-20

(e) Pb

-40 -60 -2.4

-2.0

-1.6

-1.2

-0.8

-0.4

0.0

0.4

Potential / V vs. Ag/AgCl Figure 7. Cyclic voltammogram of (a) Sn, (b) Sn77.3Pb22.7, (c) Sn56.3Pb43.7, (d) Sn35.1Pb64.9, and (e) Pb in a 0.5 M KHCO3 solution under N2 (dash line) and CO2 (solid line) atmosphere in a potential range from -2.2 V to 0.2 V (vs. Ag/AgCl) with scan rate of 50 mV s-1.



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Electrocatalytic Reduction of CO2 Electroreduction experiments were carried out to investigate the FE and the PCD of HCOO- using Sn, Pb and Sn-Pb alloys electrodes in H-type cell. The FEs of the liquid and gas products such as HCOO-and H2 were evaluated respectively by HPLC and GC, and the PCD was calculated by assuming the electrode areas are similar each other. Figure 8 shows the comparison results of FEs of HCOO- and H2, and PCD of HCOOfor the tested electrodes at -2.0 V (vs. Ag/AgCl) over a period of 2 h. Each composition was investigated by 6 times to obtain a statistical distribution of the results, and the results of FE and PCD of HCOO- were obtained within standard deviations of 2.6 % and 1.8 mA cm-2, respectively. The FEs of HCOO- and H2 exhibit the opposite dependence on the composition of alloy electrodes as shown in Figure 8 (a). The FE of HCOO- was increased with Sn ratio from 0 at.% (Pb electrode) to 56.3 at.% (Sn56.3Pb43.7 electrode) and then started to decrease for the electrodes including the higher Sn ratio than that. Generally, the Sn-Pb alloys exhibit higher FE of HCOO- than those of Sn and Pb electrodes, and the Sn56.3Pb43.7 shows the highest FE of HCOO- about 79.8 % with the lowest FE of H2 about 18.7 %. On the other hand, the lowest FE of HCOO- (43.8 %) and the highest FE of H2 (55.9 %) were obtained from the Pb electrode. The low FE of HCOO- of the Pb electrode used in this study is because of the low electrical conductivity of the electrode surface, composed of PbO as given in Table 3. The PCD of HCOO- shows a similar dependency with the FE of HCOO- on the composition of alloy electrodes. The Sn56.3Pb43.7 shows the best PCD of HCOO- about 45.7 mA cm-2, higher than those of Sn (36.5 mA cm-2) and Pb (31.3 mA cm-2) electrodes as shown in Figure 8 (b).


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Faradaic efficiency / %

100 HCOOH2

(a)

80

60

40

20

0

Sn

Sn77.3Pb22.7 Sn56.3Pb43.7 Sn35.1Pb64.9

Pb

50

PCD of HCOO- / mA cm-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


(b) 40

30

20

10

0

Sn

Sn77.3Pb22.7 Sn56.3Pb43.7 Sn35.1Pb64.9

Pb

Figure 8. The comparison results of (a) FEs of HCOO- and H2, and (b) PCD of HCOOfor the Sn-Pb alloys with Sn and Pb after 2 h of electrolysis at -2.0 V (vs. Ag/AgCl).



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Figure 9 shows the FE, PCD and cumulative concentration of HCOO- for the Sn56.3Pb43.7 compared to Sn and Pb at -2.0 V (vs. Ag/AgCl) over a period of 2 h. It can be seen in Figure 9 (a) and (b) that the electrodes exhibit stable FE and PCD of HCOOduring 2 h. In addition, the Sn56.3Pb43.7 exhibits 16 % and 36 % higher FE of HCOOthan those of Sn and Pb electrodes, and shows 25 % and 46 % enhancement in PCD of HCOO- compared to Sn and Pb electrodes, respectively. As a result, the largest amount of HCOO- was produced on the Sn56.3Pb43.7 electrode during 2 h, and the cumulative concentration of HCOO- reached 26.4 mmol L-1 as shown in Figure 9 (c). Additionally, the reduction of the oxide (SnOx and/or PbO) film on the electrode surface into metals such as Sn and/or Pb must be considered in the long-term electrolysis at the reduction condition.


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100

FE of HCOO- / %

(a) 80 60 40 20

PCD of HCOO- / mA cm-2

0 50

Concentration of HCOO- / mmol L-

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


30

(b)

45 40 35 30 25

(c)

Sn56.3Pb43.7 Sn Pb

25 20 15 10 5 0

0

20

40

60

80

100

120

140

Time / min Figure 9. The FE, PCD and cumulative concentration of HCOO- of the Sn56.3Pb43.7 compared to Sn and Pb electrodes at -2.0 V (vs. Ag/AgCl) over a period of 2 h. 23 ACS Paragon Plus Environment


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CONCLUSIONS The SEM images and the X-ray diffractogram of Sn-Pb alloys show that the alloys are deposited successfully on the carbon paper. The lower shift in XPS binding energy and smaller impedance arc for the alloy electrodes in EIS measurements indicates that the surfaces of alloys have higher electrical conductivity and stronger electron donating ability compared to the single metal electrodes. The electrochemical behavior and the electrocatalytic performance were dependent on the Sn-Pb alloy composition. Both the FE and PCD of HCOO- were increased as the Sn ratio increases until 56.3 at.% and then started to decrease for the electrodes including the higher Sn ratio than that. Therefore, the Sn56.3Pb43.7 showed the best performances, more than 16 % and 25 % higher in FE and PCD of HCOO- respectively than the single metal electrodes. Based on these results, it is supposed that the SnOx and Pb0 on the surface improve the electrical conductivity and catalytic activity of the alloys, and facilitate selective reduction of CO2 to HCOOon the Sn-Pb alloys.

ACKNOWLEDGEMENTS This work was supported by the Energy & Resource Recycling Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No.20155020200720)


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For Table of Contents Use Only

Title: Electrochemical Reduction of Carbon Dioxide to Formate on Tin-Lead Alloys

Authors: Song Yi Choi, Soon Kwan Jeong, Hak Joo Kim, Il-Hyun Baek, Ki Tae Park*

Synopsis: Sn-Pb alloy, a new electrocatalyst for CO2 reduction into HCOO-, exhibits higher performance in selectivity and activity than the single metals.


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Electrochemical Reduction of Carbon Dioxide to Formate on TinâLead

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