Electrochemical reduction of carbon dioxide to ethylene: Mechanistic approach

Journal of CO2 Utilization 1 (2013) 43–49

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Review

Electrochemical reduction of carbon dioxide to ethylene: Mechanistic approach Kotaro Ogura * Department of Applied Chemistry, Yamaguchi University, Ube, Japan

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 January 2013 Received in revised form 9 February 2013 Accepted 15 March 2013 Available online 17 April 2013

It has been appraised that the world energy spending will more than double by 2050. The global energy depends mostly on fossil fuels at present, while the estimated amount of fossil-fuel deposits goes on decreasing. The greater part of fossil fuels may be exhausted within next hundred years. In view of this situation, the electrochemical and selective conversion process of CO2 to ethylene that can be driven by the electricity derived from renewable energy is attractive, since CO2 can be utilized as an energy carrier regardless of fossil fuel. The developed CO2 conversion process takes place under rather specific conditions involving three-phase (gas/solution/solid) interface, concentrated solution of potassium halide, low pH, and copper or Cu(I) halide-confined metal electrode. Herein, the bases for leading to the augmentation of the efficiency and selectivity in the electrochemical reduction of CO2 to ethylene are discussed in association with the reduction pathway. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: Carbon dioxide Electrochemical reduction Three-phase interface Heterogeneous inner-sphere reaction Energy carrier

Contents 1. 2. 3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . Selective formation of ethylene from Adsorption of CO2 onto electrode . . . Electrochemical reduction of CO2 . . . Conclusions . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .

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1. Introduction The global population is presently seven billion, and keeps on increasing with big strides. It is estimated that the world will reach a population of nine billion by mid-century, and the total energy consumption of the world will more than double by 2050. The bulk of the global energy at present depends on fossil fuels. Nevertheless, fossil-fuel deposits are on the decrease, and a great portion of these fuels may be exhausted within next hundred years. In the light of these circumstances, it is attractive to recycle CO2 via conversion into a high energy-content fuel that is fit for use in the existing infrastructure. The electrochemical conversion of CO2 to organic substances is an interesting process for producing fuels. However, this process is thermodynamically uphill, requiring intense input energy, and it is serviceable only when renewable energy sources such as sunlight, wind or terrestrial heat are applicable to the purpose. Generation of electricity by renewable source is subjected

* Corresponding author. Tel.: +81 836332858; fax: +81 836332858. E-mail address: [email protected] 2212-9820/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcou.2013.03.003

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43 44 46 47 49 49

to the temporal mismatch of supply and demand, because renewable energy is intermittent and unstable. Efficient storage of electricity is essential to steady utilization of renewable energy. Nevertheless, electricity is not very storable: e.g., the energy density of Li-ion battery, 0.63 MJ/kg; lead battery, 0.16 MJ/kg; ultra capacitor, 0.02 MJ/kg; (cf. CH4, 55.5 MJ/kg; C2H4, 50.4 MJ/kg), indicating that the use of renewable energy as the energy source for long-distance transport vehicle and aircraft is very difficult unless the renewable energy can be transformed to the chemical energy preserved in chemical compounds such as hydrocarbons. To settle this problem, the application of the electrochemical conversion of CO2 to energy-rich substances that is driven by the electricity originated in renewable energy is useful. In this process, the renewable energy is stored in chemical bonds, and the reaction products can be later used to generate energy in another form. This scheme is quite similar to natural photosynthesis that green plants convert CO2 and water into glucose and molecular oxygen under sunlight, and the CO2 electroreduction process run by renewable energy can be regarded as artificial photosynthesis. We have developed the electrochemical process that CO2 is selectively converted to ethylene at cathode with accompanying

K. Ogura / Journal of CO2 Utilization 1 (2013) 43–49

44

the evolution of oxygen gas at anode in aqueous solution [1–5]. The energy source for this process is the electric power supplied by a solar panel. The overall reaction is represented by reaction (1)

(1)

This reaction is thermodynamically uphill, and solar energy is stored in the chemical bonds of ethylene. The energy preserved by 1 kg of CO2 is 16.0 MJ, and ethylene itself has the energy density of 50.4 MJ/kg. The decomposition voltage for reaction (1) is 1.15 V. The electric charge required for the fixation of one mole of CO2 to C2H4 is 161 Ah, and the needed electric power is 161 Ah  1.15 V = 185 Wh/mol-CO2. It is adduced that CO2 is very stable and a large input energy is necessary to chemically activate it. The hydrogenation of CO2 is actually carried out with a heterogeneous catalyst at high temperature. The efficient photochemical reduction takes place only in non-aqueous solvents with a sophisticated homogeneous catalyst, and the usual electrochemical reduction needs a large overpotential. On the other hand, our studies indicate that CO2 is electrochemically active if the electrode process is properly tailored [1,5]. The main feature of our process is that the electrolysis is performed under rather specific conditions involving three-phase (gas/solution/solid) interface, concentrated solution of potassium halide, low pH, and copper or Cu(I) halide-confined metal electrode. Electrochemical and selective formation of ethylene from CO2 has been also reported by Zhai et al. [6]. They performed the electrolysis at a three-phase interface in a 2 M KCl solution with a porous copper working electrode against a glass frit, where CO2 gas was continuously poured from the glass frit side to the electrolyte side of the electrode during the electrolysis. It is interesting that nitrogen oxide contained in the initial gas as an impurity has no effect on the selective formation of ethylene in such an electrolysis manner [6,7]. Wang et al. have described that a three-phase interface is essential to the formation of hydrocarbons in the photolysis from CO2, water and sunlight on cobalt nanostructures [8]. Goncalves et al. have accomplished the electrochemical reduction of CO2 with a copper foil electrode in an aqueous solution containing CuSO4 with slow agitation during the electrolysis, and reported the products including methane and ethylene [9]. Afterward, they employed a Cu mesh electrode with electrodeposits of copper as the cathode, and found that ethylene is selectively generated in the detriment of methane [10]. In the electroreduction of CO2, Yano et al. have selectively obtained ethylene by applying a periodic anodic polarization in a potential pulse mode [11]. They attributed the improved selectivity to the catalytic activity of a copper oxide generated during the anodic scan. On the other hand, Le et al. have pointed out that the

anodically deposited copper oxide is apt to be reduced to copper metal itself in the cathodic run according to their Auger spectroscopic analysis [12], and it is presumably newly formed copper metal that is involved in the selective generation of ethylene. Thus, it is obvious that the presence of copper deposits and Cu(I) halide as well as a three-phase interface on copper leads to the increase in selective reduction of CO2 to ethylene. However, the effect of electrodeposits and phase interface at cathode upon the CO2 reduction is not thoroughly understood, which was here considered carefully in relation to the reduction scheme. 2. Selective formation of ethylene from CO2 The conventional electrochemical reduction of CO2 has been carried out at a two-phase (solution/solid) interface with a metalfoil electrode. In such a process, CO2 is necessary to be first dissolved in solution to undergo the electron-transfer at the solution/electrode interface. As the reduction advances, CO2 is exhausted in the vicinity of the electrode surface, and CO2 needs to diffuse from the bulk solution to the electrode for further reduction. The rate of this movement regulates the whole rate of reaction, and the conversion efficiency becomes extremely suppressed. Hence, the enrichment of CO2 in solution is the key to higher efficiency. On the other hand, the presence of abundant protons is indispensable for efficient electrochemical reduction of CO2. However, the preparation of concentrated solution of CO2 with aqueous acidic medium is difficult owing to the solubility limit. This situation is antinomic, and the choice of catholyte seems to be compromised by using weakly acidic or alkaline solution in the conventional electrolysis of CO2. For example, Hori et al. have reported the results of the CO2 electroreduction in weakly acidic solutions as shown in Table 1 [13]. The yield of C2H4 was highest except in K2HPO4 solution, but the formation was not very selective (cf. our results described later). Needless to say, the CO2 electroreduction should be most efficient provided that the concentrations of both CO2 and protons are high enough on the electrode. In order to circumvent such an antinomic problem, we have developed the electrolysis process that CO2 is reduced at a gas/solution/solid interface [14,15]. In this method, a metal-mesh electrode is partially immersed in solution, and the reduction reaction takes place most efficiently at the threephase. This idea originates in the previous findings that the electrochemical reduction of CO is expedited on a metal-foil electrode partially submerged in solution [16]. The enhanced rate of reaction is attributed to the generation of effective reaction zone. In this type of electrolysis, high concentrations of CO2 are possible, since it is directly supplied to the electrode without passing through solution. The CO2 concentration at the reaction zone is independent of the solubility, permitting us to use acidic solution as the catholyte. Thus, the concentrations of both CO2 and protons can be maintained high on the electrode during the electrolysis irrespective of the transport problem. Another advantage of applying such an electrolysis manner is that the deactivation of

Table 1 Current efficiencies of products from the electroreduction of CO2 at a Cu electrode at 5 mA/cm2 in various solutions at 19 8C. pHa

Electrolyte (M)

KHCO3 KCl KClO4 K2SO4 K2HPO4 a

0.1 0.1 0.5 0.1 0.1 0.1 0.5

6.8 5.9 5.9 5.8 6.5 7.0

Potential vs NHE

1.41 1.44 1.39 1.40 1.40 1.23 1.17

pH values were measured for bulk solutions after electrolyses.

Faraday efficiency (%) CH4

C2H4

EtOH

n-PrOH

CO

HCOO

H2

Total

29.4 11.5 14.5 10.2 12.3 17.0 6.6

30.1 47.8 38.2 48.1 46.0 1.8 1.0

6.9 21.9 – 15.5 18.2 0.7 0.6

3.0 3.6 – 4.2 4.0 tr 0.0

2.0 2.5 3.0 2.4 2.1 1.3 1.0

9.7 6.6 17.9 8.9 8.1 5.3 4.2

10.9 5.9 12.5 6.7 8.7 72.4 83.3

92.0 99.8 96.0 99.4 98.5 96.7

K. Ogura / Journal of CO2 Utilization 1 (2013) 43–49

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Fig. 1. CO2 attraction by electric double layer and formation of formate radical by inner-sphere mechanism.

copper electrode, which has been often pointed out in a prolonged electrolysis of CO2 [17–21], does not break out. This favorable result is associated with the electrochemically reducible nature of the poisonous species at low pH that cause the deactivation [14]. In the CO2 electroreduction in acidic solution, however, there is a different problem to be overcome: the reduction of protons to molecular hydrogen is accelerated at low pH, competing intensely with the target reaction. It is important for the efficient reduction of CO2 how to suppress the hydrogen evolution in acidic solution. We have succeeded in restraining the competitive reaction by facilitating the specific adsorption of halide anions onto copper electrode [1,22]. It is known that a double layer is built up at the metal/solution interface [23]. Metallic ions with strongly bound water molecules interact with the metal electrode by electrostatic forces, while halide anions with weakly bound water molecules can remove a part of water and form a direct chemical bond with the metal electrode. In the latter case, a surface concentration of anions is in excess of that provided with pure electrostatic forces. This consequence is called specific adsorption in general. The specific adsorption is ascribed not to the electrostatic attraction but to a strong chemical affinity of the anion for meal. The halide anions and solvated cations are located on the inner and outer Helmholtz planes, respectively, as shown in the left side of Fig. 1. Specifically adsorbed anions can block the adsorption of protons [24], and the overpotential for the hydrogen evolution is enhanced. Thus, the proton electroreduction competitive to the reduction of CO2 can be suppressed. On the other hand, the reduction of CO2 is expedited by the existence of specifically adsorbed anions as described later. In our electrolysis, highly concentrated solution of potassium halide (>3 M) was normally used, and the inner Helmholtz plane with specifically adsorbed halide anions must

be extremely stabilized. For this reason, the hydrogen evolution may be effectively controlled even in acidic solution. In a typical electroreduction of CO2 at a three-phase interface on copper in a KBr solution (Table 2), the major product was ethylene, and small quantities of methane, CO and formic acid were obtained as byproducts [3]. The formation of ethylene was promoted by increasing the concentration of KBr. The current efficiency reached to 63% at the KBr concentration of 3 M. In contrast to this, the current efficiency for the hydrogen evolution was decreased with an increase in concentration of KBr. Hydrogen evolution was predominant with the current efficiency of 97% at the KBr concentration of 0.1 M, but it sharply dropped to 9% at 3 M. Therefore, it is obvious that the formation of ethylene is enhanced but the evolution of hydrogen gas is restrained by increasing the concentration of KBr. On the other hand, the current efficiencies of the byproducts under the same conditions were lower: CH4, 16.8%; CO, 6.2%; HCOOH, 2.6%. Hori et al. have claimed that the deactivation of Cu electrode in the CO2 reduction is not caused by the adsorption of products or intermediates onto the electrode but by the deposits of heavy metals included as impurities in the electrolyte [25]. However, this view is questionable because of the following reason. As noted above, the ethylene formation was enhanced by increasing the concentration of KBr. In such a highly concentrated solution, the concentrations of impurities included in electrolyte should be concurrently increased. However, the yield of ethylene continued to grow even in the solution of 3 M KBr, suggesting that the effect of impurities such as heavy metals on the CO2 electroreduction is not major at least in acidic solution. Although the increase in concentration of halide anions was valid for the selective production of ethylene, the coating of copper electrode with a Cu(I) halide film led to more advantageous results

Table 2 Current efficiencies for the products obtained in the electrochemical reduction of CO2 on a Cu-mesh electrode in KBr solutions of various concentrations.a [KBr] (M)

0.1 0.3 0.5 1.0 1.5 2.0 2.5 3.0

Current efficiencies (%) Ethylene

Methane

CO

Ethane

Formic acid

Acetic

Lactic

H2

0.1 1.2 14.5 32.4 40.8 40.5 51.4 63.0

2.4 4.9 10.5 16.4 25.8 28.8 17.9 16.8

3.9 17.3 33.2 28.0 16.7 14.9 14.0 6.2

0.4 1.9 2.0 1.6 1.4 1.1 1.1 1.1

1.6 7.4 5.4 6.5 2.3 4.4 3.5 2.6

0.0 1.1 0.0 0.0 0.6 1.2 0.9 1.2

0.9 2.2 1.9 1.9 1.3 1.3 1.0 0.9

96.8 68.2 34.6 14.3 11.0 9.4 12.4 9.2

Conversion (%)b

Selectivity (%)c

h (%)d

0.2 1.5 3.0 5.0 5.1 6.3 8.0 8.0

0.4 1.4 10.1 21.1 32.0 32.2 41.5 57.7

106 104 102 102 103 103 105 104

a Initial pH, 3.0; the surface area of a Cu-mesh electrode, 10.2 cm2; electrolysis potential, 1.8 V vs Ag/AgCl; electrolysis time, 30 min; initial volume of CO2, 480 cm3; catholyte, 205 cm3. b Conversion percentage of CO2 in the 30 min-electrolysis. c Selectivity for the formation of ethylene on the basis of C content. d Total current efficiency in the cathodic reduction.

K. Ogura / Journal of CO2 Utilization 1 (2013) 43–49

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Table 3 Current efficiencies for the products obtained in the electrochemical reduction of CO2 on a copper(I) halide-confined Cu-mesh electrode.a Electrode substrate

Cuc CuCld CuBre CuIf a b c d e f

Current efficiency (%) Ethylene

Methane

CO

Ethane

Ethanol

Formic acid

Acetic

Lactic

H2

40.0 60.5 79.5 71.8

5.1 6.6 5.8 4.3

2.0 1.8 2.4 2.8

0.3 2.8 1.2 0.8

2.6 1.9 1.6 1.6

0.0 0.1 0.7 0.0

0.0 0.2 0.2 0.0

0.0 0.4 0.3 0.1

53.2 18.8 9.3 11.0

Conversion (%)

Selectivity (%)b

h (%)

14.4 17.1 24.3 21.9

61.9 79.2 82.4 86.9

104 93.1 101 92.4

The surface area of copper-mesh substrate, 10.2 cm2; electrolysis potential, 2.4 V vs Ag/AgCl; electrolysis time, 30 min. Selectivity for the formation of ethylene on the basis of C content. Pure Cu-mesh used in a 3 M KCl solution of pH 3. CuCl (9.5 mm)-confined Cu-mesh used in a 3 M KCl solution of pH 3. CuBr (7.9 mm)-confined Cu-mesh used in a 3 M KBr solution of pH 3. CuI (7.5 mm)-confined Cu-mesh used in a 3 M KI solution of pH 3.

[3]. The covering was achieved by the electrochemical oxidation of a copper-mesh electrode in acidic solutions of potassium halides in which Cu(I) halide was anodically deposited. In a substrate other than copper, the coating was made by the pulse-electrolysis with potential pulses between 0.1 V vs Ag/AgCl and 0 V in potassium halide solutions containing cupric ions [5]. In this case, the cuprous halide film was deposited by the cathodic reduction of cupric ion. Cu2þ þ X þ e ! CuX

(2)

The results acquired in the CO2 electroreduction with Cu(I) halide-confined electrodes are exhibited in Table 3 [3]. The current efficiency of ethylene was largely enhanced by covering with Cu(I) halides: 40% on bare Cu, 61% with CuCl film, 80% with CuBr film, 72% with CuI film. The selectivity for the ethylene formation was also increased: 62% on bare Cu, 79% with CuCl film, 82% with CuBr film, 87% with CuI film. In contrast with this, the current efficiency for the hydrogen evolution was lowered by the modification: 53% on bare Cu, 19% with CuCl film, 9% with CuBr, 11% with CuI film. On the other hand, the confinement of Cu(I) halide had little effect on the formation of byproducts. Thus, the modification of copper electrode as well as the application of concentrated solution of potassium halide to the catholyte can produce the electrochemical environment that permit to selectively convert CO2 to ethylene and to suppress the hydrogen evolution. In Table 4, the current efficiencies for the products acquired with a CuCl-confined Ag-mesh electrode are listed as a function of thickness of CuCl film [5]. Pure silver is known as the peculiar metal that allows the selective reduction of CO2 to CO [26,27]. In fact, the current efficiency of CO observed here with a pure silver electrode was about 80%, and ethylene was hardly formed. However, such a unique property of silver was greatly lost by covering with the CuCl film. It is seen from Table 4 that the current efficiency of ethylene is enhanced from 1.3% to 64% by increasing the film thickness until 10.2 mm, while the reduction of CO2 to CO

is suppressed from 79.6% to 9.3% by thickening the coated film. This result indicates that CO is not a precursor for the generation of ethylene and the C–C bond formation is accelerated by the coated Cu(I) halide film. 3. Adsorption of CO2 onto electrode Carbon dioxide is a linear triatomic molecule with two equivalent C–O bonds, and the outermost shells of three atoms have sixteen electrons in all. The MO energy level of CO2 is as follows: (K)(K)(K)(3sg)2(2su)2(4sg)2(3su)2(1pu)4(1pg)4(2pu)0. The impact of low energy electron upon CO2 results in the formation of a negative anion (CO2) by introducing an electron into the lowest unoccupied anti-bonding orbital (2pu). The generated anion radical is more stabilized by a change in geometry from the linear structure to a bent form, and the energy level of the lowest unoccupied orbital (3.8 eV) corresponds to the electron affinity (EA) of CO2. This value is large relatively to EA of other common substances: 0.45 eV (O2); 2.38 eV (Cl2); 2.55 eV (Br2); 2.6 eV (I2); 3.0 eV (F2) [28,29]. These data indicate that CO2 is possessed of a high electron affinity associated with the central carbon atom. Hence, the unoccupied orbital is ready to accept electron pairs of other ion or molecule. In the present situation, the lone-paired electrons of specifically adsorbed anions may be admitted to the unoccupied orbital of CO2. In other words, CO2 is attracted to the electrode on account of the electron flow from the adsorbed anion to the vacant orbital of CO2. This attraction may cause the structural transformation of CO2 from the linear form to the bend, as schematically shown in the middle section of Fig. 1. The occurrence of the Xads–C bond results in the weakening of the C–O bond and the chemical activation of CO2 occurs. The nucleophilic oxygen of CO2 may interact with potassium cations located on the outer Helmholtz plane. The role of counter cation is discussed later. Although the attraction of CO2 to the electrode takes place at open-circuit potential, this step is very important for

Table 4 Effects of thickness of CuCl film on the current efficiencies for the products obtained in the electrochemical reduction of CO2 on a CuCl-confined Ag-mesh electrode.a Thickness (mm)b

0 3.4 7.9 8.9 10.2 12.9 15.3

Current efficiency (%) Ethylene

Methane

CO

Ethane

EtOH

Formic acid

Acetic

Lactic

H2

1.3 16.5 28.8 42.8 64.0 60.4 60.2

4.9 1.8 1.8 4.0 3.8 2.7 4.5

79.6 59.7 48.9 31.8 9.3 14.4 15.8

0.7 1.1 0.7 1.0 1.8 0.9 0.8

0.4 2.6 2.7 2.3 2.0 0.4 1.8

2.4 1.3 1.3 1.0 2.0 2.0 2.6

0.3 1.4 1.8 2.0 1.0 0.5 0.6

1.4 0.5 0.8 1.5 1.1 0.4 0.3

7.4 9.2 12.7 14.5 13.0 13.5 9.2

Conversion (%)

Selectivity (%)c

Q (C)

h (%)

11.6 13.0 11.7 9.3 10.4 10.7 10.1

0.5 0.6 15.3 28.0 59.7 52.9 49.0

153 215 212 206 323 307 278

98.4 94.1 99.5 101 98.0 95.2 95.8

a Electrolyte, 3 M KCl; initial pH, 3.0; the surface area of a silver-mesh substrate, 10.2 cm2; electrolysis potential, 1.8 V vs Ag/AgCl; electrolysis time, 30 min; initial concentration of CO2, 100%; volume of CO2, 480 cm3. b Thickness of coated CuCl. c Selectivity for the formation of ethylene on the basis of C content.

K. Ogura / Journal of CO2 Utilization 1 (2013) 43–49

the subsequent electrochemical reduction. If such a spontaneous electron flow to CO2 does not occur, the chemical activation would be very difficult. For example, the reaction of CO2 with H2 needs high temperature and the presence of a catalyst in which the activation step involves the dissociative adsorption of reaction gases onto the catalyst and the recombination of adsorbed species, requiring a large input energy. Halide anions have the highest ability of specific adsorption among the common anions: I > Br > Cl > NO3 > CO32 > SO42 [30]. Gibbs energies of adsorption of halide anions increase with an increase in degree of the specific adsorption, and the interaction energies of these anions with solvent molecules decrease in the opposite way: 56.8 kJ/mol(Cl) > 51.0 kJ/mol(Br) > 44.7 kJ/ mol(I) [31]. Adsorbed halide anions are bound to the electrode with a covalent character (I > Br > Cl), which would make electrons easily transfer via the Xads–C bond in the subsequent electrochemical step. The stronger the adsorption of the halide anion to the electrode, the more strongly CO2 is restrained, resulting in higher reduction currents of CO2 [32]. Such a movement of electrons corresponds to a heterogeneous inner-sphere reaction in which electrons move via a bridge ion (Xads) [32,33]. In this case, the overpotential necessary for the CO2 electroreduction is largely mitigated [5]. Generally, the overpotential required for the uncatalyzed reduction of CO2 is more than 1 V [34] in which no bridge ion exists and the electron-transfer occurs by tunneling across the solution/electrode interface with a large activation energy. The standard potential for forming ethylene is 0.079 V vs NHE, while the onset of CO2 reduction took place at 0.28 V vs NHE in our study [32]. The importance of the role of adsorbed anions in the CO2 electroreduction was emphasized above, whereas counter cations were found to act an essential part in the electrolysis as well. Alkaline and alkaline-earth metallic ions might be considered as a candidate for the counter cation of halide anion, but the actual reduction of CO2 was very inefficient in solutions containing these cations except for alkaline metals. Even with alkaline cations, the conversion efficiency of CO2 to ethylene decreased in the order: K+  Na+ > Li+. The role of cation is to compensate the charge of Xads–CO2 occurring on the inner Helmholtz plane. Solvated cation cannot approach to the electrode beyond the outer Helmholtz plane of closest approach, and the size of solvated cation would influence the charge compensation. The radii of K+, Na+ and Li+ ions are 1.33 A˚, 0.96 A˚ and 0.60 A˚, respectively, and the degree of hydration of these ions should increase in the order: K+ < Na+ < Li+, implying that water molecules around K+ are more labile and detachable than those around Na+ and Li+. Hence, the direct interaction of K+ with the nucleophilic oxygen atom of CO2 becomes possible as shown in the middle section of Fig. 1. The CO2 thus stabilized in the double layer is ready to accept the electrons from the cathode in the electrochemical step. Therefore, potassium cations are vital to the efficient electroreduction of CO2.

47

formed by reducing CO2 on a Pt disk electrode in the hydrogen adsorption region by means of in situ Fourier transform infrared (FTIR) spectroscopy [35]. They found two bands at 1253 cm1 and 1360 cm1 which are attributable to the CO stretching and OH deformation of COOH radical, respectively. According to Sexton [36], IR absorption bands from COOH radical should be in range of 1300–1700 cm1, because the asymmetric OCO stretching would be expected near 1640 cm1 and the symmetric OCO stretching near 1340 cm1. In situ FTIR spectrum obtained by Ogura et al. showed the absorption band at 1400 cm1 due to CO stretching of COOH radical [5]. Their spectrum was acquired during the CO2 electroreduction on a Cu electrode in a concentrated solution of KBr. These results support the generation of COOH radical in the initial stage of electrochemical reduction of CO2. In the three-phase current-potential curve for 3 M solution of KBr with CO2 [5], the onset of cathodic current was at 0.5 V vs Ag/ AgCl, and the abrupt rise in current was seen at 0.7 V. The current leveled off at 1.1 V and ascended again at 1.7 V. On the other hand, this tendency of the current-potential curve was not observed in the solution with N2 except for current increasing in the potential region more negative than 1.5 V. Such a difference between the polarization curves with CO2 and N2 verifies the presence of electron-transfer from the electrode to CO2 at the three-phase interface. In general, there is no big difference between current–potential curves measured with a usual copperfoil electrode in aqueous solutions with CO2 and N2 as shown in Fig. 2 [13], because the reduction of CO2 takes place in concurrence with the vigorous evolution of hydrogen at the two-phase interface. So, it is difficult to infer the existence of electrontransfer at the electrode only from the polarization curve. The Tafel plots of the excess current (Di) acquired at the two-phase and three-phase interfaces are shown in Fig. 3, where Di is the current observed with the CO2 minus that with only N2 present in a 3 M KBr solution [32]. The Tafel slopes of the three-phase and twophase experiments were 113 and 78 mV/decade, respectively. In the three-phase experiment, the Tafel slope was taken between 0.86 and 0.93 V. Similarly, for the two-phase experiment, the Tafel slope was taken between potentials of 0.67 and 0.73 V. The former slope was close to the value of 140 mV/decade that was

4. Electrochemical reduction of CO2 CO2 is first attracted to the electrode at open-circuit potential by specifically adsorbed anions, and electrons are poured into CO2 in the subsequent electrochemical reduction. The electron injection is performed via the Xads–C bond, which is consistent with the inner-sphere mechanism. The oxygen atom of CO2 rich in electrons attracts a proton, and a formate radical is generated, which is schematically shown in the right side of Fig. 1. The formate radical should be directly adsorbed on the copper substrate, because it has a stronger affinity for Cu than halide anion. The formation of COOH radical in the initial stage of CO2 electroreduction has been confirmed by the spectroscopic analysis. Iwashita et al. have studied the structure of the adsorbed species

Fig. 2. Voltammograms obtained with N2 and CO2 saturated solutions. (A) N2 saturated phosphate buffer solution (0.1 M KH2PO4 + 0.1 M K2HPO4, pH 6.7). (B) CO2 saturated phosphate buffer solution (initial composition: 0.05 M KH2PO4 + 0.15 M K2HPO4, pH 6.7 after equilibration with saturated CO2). (C) 0.1 M KHCO3 saturated with CO2 (pH 6.8).

48

K. Ogura / Journal of CO2 Utilization 1 (2013) 43–49

Fig. 3. Tafel plots of the excess current density (Di) of CO2 electrochemical reduction in a 3 M KBr solution of pH 3.

previously observed at a three-phase interface of a different shape [5]. The CO2 reduction with two Tafel slopes may follow the mechanism with different and consecutive rate-determining steps (rds) [37]. CO2 þ Hþ þ e ! COOHads

(3)

COOHads þ e ! COOH

(4)

The Tafel slopes of 113 and 58 mV/decade correspond to the rds of reactions (3) and (4), respectively. Hence, the electrochemical reduction of CO2 at the three-phase interface occurs following reaction (3) with the rds of one electron. On the other hand, at twophase interface, the CO2 reduction takes place with the rds of two electrons (reactions (3) and (4)), and the total reaction is first order with respect to CO2 concentration, involving two electrons. The Tafel slope at the three-phase interface was not affected by the immobilization of Cu(I) halide, indicating the same rds occurs on the modified electrode [5]. Reaction pathway for the CO2 electroreduction was studied in more detail by identifying reaction intermediates by means of in situ FTIR spectroscopy. The measurements were performed at bare and CuBr-confined Cu electrode during the electrochemical reduction of CO2 in a concentrated solution of KBr [5]. The

spectro-electrochemical cell used was constructed of Pyrex and glass blown to admit a Teflon cylinder [38]. An infrared transparent calcium fluoride window was glued to the Pyrex cell. The angle of incidence on the CaF2 window was 708. The working electrode was a pure copper disk, and a copper disk was further modified by coating with a CuBr film in a separate experiment. In situ reflection spectra measured with a CuBr-modified Cu electrode are illustrated in Fig. 4. The electrode was scanned from the rest potential to 1.8 V vs Ag/AgCl and polarized at this potential for 30 min. Upward peaks in DR/R spectra, which represents the decrease in concentration of species existing in the thin film layer extended at the solution/electrode interface, are evidently seen at 2350 cm1 and 1650 cm1. The former peak is attributed to CO2 [39]. This peak appears from 0.6 V, and the height is growing larger upon the potential shift to more negative side, and leveling off at 1.2 V. It is therefore suggested that the electrochemical reduction of CO2 to intermediates starts at 0.6 V and the reduction attains to a steady state at 1.2 V. The appearance of the latter peak was accompanied by an upward broad band observed at 3000–3600 cm1, and hence the upward peak at 1650 cm1 can be assigned to water molecule [39]. The downward peak seen at 1400 cm1, which is indicative of an increase in concentration of species generated at the electrode/solution interface, is ascribable to the CO stretching vibration of adsorbed formate, and another downward peak at 1485 cm1 is attributed to the CH2 deformation vibration of –CH2C(O)– group [39]. A shoulder with slight intensity was observed downward around 1420 cm1, which is assignable to CH25 5CH– group. Furthermore, a downward peak is seen at 1100 cm1 that is attributable to –CH5 5CO group [39]. There are many mechanistic studies on the electrochemical reduction of CO2 at copper electrode [17,18,40]. The basic pathway is as follows: the formation of adsorbed CO is first assumed as the key intermediate, this species is protonated to form surface carbene (Cu5 5CH2), further protonation of the carbene yields methane, and ethylene is formed by the reaction of two species of surface carbine, which is represented in Scheme 1 [40]. Recently, however, Batista and Temperini have investigated the CO2 electroreduction on a copper electrode by means of surface enhanced Raman scattering (SERS) in aqueous solution [41], and interposed an objection for the reaction scheme involving

Scheme 1.

Fig. 4. In situ FTIR spectra of a CuBr-coated Cu electrode in a 3 M KBr solution of pH 3 saturated with CO2. Potential is shown at intervals of 0.1 V from the rest to 1.8 V.

K. Ogura / Journal of CO2 Utilization 1 (2013) 43–49

49

Scheme 2.

adsorbed CO and surface carbene. They have evidenced the generation of ethylene at a low overpotential in acidic solution, and pointed out that hydrocarbons are formed by a pathway not via adsorbed CO, because there was no any band assignable to CO in their SERS spectrum. A surface carbenoid species which is assumed as a possible intermediate in the traditional mechanism has never been originally detected either by the trapping [17] or in situ FTIR experiment [5]. The conversion scheme of CO2 to ethylene proposed by Batista et al. is consistent with our results. A CuCl film coated on Ag electrode was found to hinder the generation of CO but conversely to facilitate the formation of ethylene (Table 4), suggesting that the formation of ethylene is not via CO. Therefore, the reaction pathway through CO and surface carbene is not proper for the electrochemical reduction of CO2 to ethylene at least in acidic solution. As noted above, the rate-determining step in the initial electrochemical reduction of CO2 is the generation of formate radical. The radicals are further subjected to the reductive coupling, and ethylene is finally produced through the intermediates such as –COOH, –CH2C(O)–, –CH5 5CO, and CH25 5CH–. The above scheme is consistent with these results (Scheme 2). In relation to this scheme, it is indicated that the efficiency for the formation of ethylene from CO2 should be dependent on the concentration of formate radicals. The increase in concentration of the adsorbed radicals heightens the probability of the C–C combination. The role of the coated Cu(I) halide is to create the interfacial environment suitable to the adsorption of formate radicals. The cuprous ions are reduced to copper metal during the cathodic electrolysis, but the halide anions should remain on the electrode surface. Hence, the specific adsorption of halide anions on the inner Helmholtz plane may be intensified, and CO2 is favorably attracted by the adsorbed anions. The newly formed copper may serve as additional active sites that adsorb the formate radicals. Such a situation would be facilitated at a three-phase interface, because the concentration of CO2 is always high enough at the reaction zone and the effect of solvent molecules is limited. A creation of active sites on copper would be also brought about by in situ cathodic reduction of electrodeposited copper particles or copper oxides. In this case, however, the catalytic activity of fresh copper will be vanished soon because of the adsorption of solvent molecules and reaction intermediates, and it is necessary to periodically oxidize and refresh the substrate for maintaining the activity for the CO2 reduction. Consequently, CO2 is first attracted to the electrode by a specifically adsorbed halide anion, and the lone-paired electrons of the adsorbed anion flow to the vacant orbital of CO2. In the subsequent electrochemical stage, electrons are transferred to CO2 via the Xads–C bond, and CO2 undergoes the electronation and protonation to form the adsorbed formate radical. The reductive coupling of two COOH radicals results in the generation of various intermediates, and ethylene is finally formed. This product is then expelled from the three-phase interface by new CO2 molecules, and thus the catalytic cycling is repeated. 5. Conclusions The electrochemical conversion of CO2 to ethylene is thermodynamically uphill, and the driving of this process by the electricity

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