Chapter 23
Electrochemical Reduction of CO with Gas-Diffusion Electrodes Fabricated Using Metal and Polymer-Confined Nets 2
K. Ogura, H. Yano, andM.Nakayama Department of Applied Chemistry, Yamaguchi University, Tokiwadai 2-16-1, Ube 755-8611, Japan
Electrochemical reduction of CO has been performed with a Cu/gas-diffusion electrode, a Cu plate electrode, and a modified Pt/gas-diflusion electrode in KCl (pH 3) and KHCO (pH 7.2) solutions. Among these electrolysis systems, the most efficient reduction of C O was obtained in a KCl solution of pH 3 with a Cu/gas-diffusion electrode on which CO, C H and CH were mainly formed with some quantities of solution products such as lactic acid and ethanol. The hydrogen evolution was sufficiently retarded even at highly negative potential, and the total current efficiency was almost 100 %. On the other hand, the electrochemical reduction of CO was carried out at a Cu plate electrode in a K H C O solution of pH 7.2. This solution has been exclusively used as an electrolyte in the reduction of CO at a Cu plate electrode. Major products were CH and CH, and the total current efficiency was about 43 %. The cause for such a low current efficiency was attributed to the formation of graphitic carbon and/or 2
3
2
2
4
4
2
3
2
4
2
344
4
© 2002 American Chemical Society
345 copper oxide. At a Prussian blue (PB)/polyaniline (PAn)/metal complex-confined Pt/gas-diflusion electrode in a KCl solution of pH 3, lactic acid was mainly produced, and alcohols and acetic acid were minor products. In this electrolysis system, C O is bifunctionally captured and activated by ΡAn and a metal complex: the electrophilic caibon atom of CO2 is bound to the amino group of ΡAn, and the basic oxygen atom coordinates to the central metal of the complex. 2
Chemical fixation of carbon dioxide is of importance in connection with the mitigation of the concentration of green-house gas in the atmosphere. Many processes have been proposed for the chemical conversion of C 0 including the hydrogénation over heterogeneous and homogeneous catalysts at high temperature and electrochemical reduction (J). One of the most essential matters in C 0 fixation is to achieve it under an input energy as low as possible to avoid a secondary generation of C 0 . Because of this, an electrochemical reduction process taking place at room temperature seems to be promising. In the application of this method, however, there are still difficult problems to be settled, e. g., the deactivation of electrode in a prolonged electrolysis. There are many works on the electrochemical reduction of C 0 with Cu electrode in a hydrogencarbonate solution (2-9). This electrolysis system is known to produce mainly methane and ethylene. In a prolonged electrolysis of C 0 , however, the rate of formation of hydrocarbons is found to go down to very small value. It is pointed that the high faradaic efficiencies of CH4 and C H4 reported in most of literatures are not the steady-state values but rather maximum efficiencies observed in a short period of electrolysis (8). This is attributed to the formation of a poisoning species. Therefore, it is very difficult to use the electrochemical process with Cu electrode for the reduction of C 0 over long periods. The poisoning of this reaction has been attempted to be alleviated by applying a periodic anodic polarization, and the reactivation of the electrode been confirmed (8). Although a neutral hydrogencarbonate solution is the unique electrolyte in which hydrocarbons are produced on a Cu plate electrode in the reduction of C 0 , this solution becomes alkaline with the progress of the reduction, resulting in less efficiency of C 0 conversion. On the other hand, a Cu plate electrode does not lead to the reduction of C 0 in acidic solution but only to the hydrogen evolution. 2
2
2
2
2
2
2
2
2
2
346 In the present study, we have developed a gas-diffusion electrode consisting of a metal mesh electrode and a glass filter. As described later, the electrolysis of C 0 at the gas-diffusion electrode with Cu mesh electrode led chiefly to the formation of CO and C2H4 with high yields in acidic solution, and the poisoning problem of the electrode was not involved. In the gas-diffusion electrode, C 0 can be always supplied in large quantities to the electrode surface and the hydrogen evolution was effectively suppressed even in acidic solution. In addition to the experiments with Cu plate and mesh electrodes, a platinum mesh substrate on which an inorganic conductor and a conducting polymer were immobilized was also used as an electrode for the C 0 reduction. 2
2
2
Experimental The electrolysis cell with a gas-diffusion electrode used (Figure 1) was essentially the same as that reported elsewhere (JO). The working electrodes were a copper mesh, a copper plate and a platinum mesh, and their effective surface areas coming into contact with electrolyte were 10.2 cm , 10.2 cm and 19.3 cm , respectively. Copper mesh and copper plate were made from pure copper (99.9 % purity, Nilaco Co.). The platinum mesh was modified with Prussian blue (PB, KFe^fFe^CNQe]) (inner layer), conducting polymer (PAn) (outer layer), and a metal complex, following the procedure described previously (11). Anions incorporated in the prepared Pt mesh/PB/PAn electrode were released by immersing it in a phosphate buffer of pH 7, and bis(l,8-dihydroxynaphthalene-3,6disulfonato) ferrate(II) (Fe L") complex was electrodeposited from its aqueous solution. As shown in Figure 1, these mesh electrodes were put on a glassfilterthrough an O-ring, and both were bound to a Teflon cylinder tightened with a Teflon screw cup. This was connected to the cathode compartment through an O-ring which was separated from the anode compartment by a cation-exchange membrane. The average pore size of glassfilterwas 20 ju m. Purified C 0 gas was forced up through the glass filter, and the liquid meniscuses spread out on the mesh electrode to form a thin layer electrolyte. The electrolysis with a Cu plate electrode was performed using a Η-type cell. The electrolytes used were 0.5 M KC1 solutions of various pH's and 0.5 M KHCO3 solution of pH 7.2. These solutions were prepared from reagent grade KC1, KHCO3, H Q and K O H with doubly distilled water. The counter electrode was a platinum plate, and the reference electrode was a Ag/AgCl/saturated KC1 electrode, The quantitative analyses of the reaction products were done with a gas 2
2
2
n
2
c
Figure 1. Schematic diagram of the electrolysis cell A, gas inlet; B, gas outlet; C, counter electrode; D, electrolyte; E, working electrode; F, reference electrode; G , glassfilter;H, O-ring; I Teflon screw cap; J , cation-exchange membrane.
Β
F
348 chromatograph (Shimadzu GC-8A), a steam chromatograph (Ohkura SSC1) and an organic acid analyzer (Shimadzu LC-LoAD).
Results and Discussion Reduction of C0 Electrodes
with Copper/Gas-Diffusion and Copper Plate
2
The electrochemical reduction of CO2 was performed with Cu/gasdiffusion and Cu plate electrodes in a 0.5 M KC1 solution of pH 3 and a 0.5 M KHCO3 of pH 7.2, respectively, and the results are shown versus potential in Figure 2. These solutions were selected because the yields of CO2 reduction were maximum in each electrolysis system. As seen from this figure, the reduction of C 0 occurs at more negative potential than -1.2 V vs Ag/AgCl on the Cu plate electrode, but on the Cu/gas-diffiision electrode the CO2 reduction is observed even around -0.6 V although the full-scale reduction begins from -1.2 V. The total concentration of the products obtained with the gas-difiusion electrodes was about 3.5 times as large as that with the plate electrode at -2.4 V. Hence, the gas-diffusion electrode is found to be much more effective for the C 0 reduction than the plate electrode. Faradaic efficiencies for the products obtained in the electrochemical reduction of C 0 with a Cu plate electrode and a Cu/gas-diflusion electrode are shown in Table I and Π, respectively. The major products except hydrogen are CH4 and C2H4 on the Cu plate electrode, and the total current efficiency averages about 43 % (Table I). This value is very low compared to that obtained with the Cu/gas-diffiision electrode as looted below. The cause for such a low current efficiency may be attributed to the formation of poisoning species on the Cu plate electrode. The identification of this species is under investigation, but graphitic carbon is one of candidates (5). On the other hand, the major products obtained with the Cu/gas-diffiision electrode are CO, C2H4 and CH4 as shown in Table Π. The total current efficiency was almost 100 %, which is a contrast to the value of about 43 % on the Cu plate electrode. It is therefore indicated that the electrolysis system with the Cu/gas-diffiision electrode is not subject to poisoning. This is probably because the poisoning process is considerably suppressed in acidic solution. Major compounds generated in the two electrolysis systems are shown versus potential in Figures 3 and 4. The formation of CH4 and C2H4 was observed on the Cu plate electrode at more negative potential than -1.4 V, 2
2
2
349
-2.6
-2.2
-1.8
-1.4
-1
-0.6
-0.2
Potential/VvsAg/AgCI Figure 2. Relationship between the total concentration ( Σ of the products based on the C content and the electrode potential on Cu plate (O) and Cu/gas-diffusion ( φ) electrodes in 0.5 M KHCO3 and 0.5 MKCl solutions, respectively. Σ C = [CH OH] + [HCOOH] + [CO] + [CH ] + 2([CHsCHO] + [C H OH] + [CH3COOH] + [C2H4} + [C H
C H4 + 40H"
(6)
2
2
2
2
2
The standard potentials for reaction (4) and (5) are known to be 0.206 V vs SHE and 0.132 V vs SHE, respectively. As another possible scheme for the formation of graphite instead of reaction (4), the involvement of copper oxide is considered as following. C 0 + Cu + H 0 + 2e" 2
2
-> C + CuO + 20H"
(7)
In fact, copper oxide has been detected on the Cu plate electrode used for the C 0 reduction in hydrogencarbonate solution (9). The formation of carton on the catalyst surface has been often referral in the reduction of CO over a heterogeneous catalyst at high temperature, and hydrocarbons are produced by hydrogénation of surface carbon (12). The formation of surface carbon has been identified also in the methanation of C 0 (13). However, a striking difference between electrochemical and high temperature reduction of C 0 is that the former reduction on Cu plate electrode is not accompanied by the generation of CO. The cause for such a difference should to clarified by further investigation. 2
2
2
Reduction of C 0 with a Modified Pt/Gas-Diffusion Electrode 2
The electrochemical reduction of C 0 was performed with a modified Pt/gas-diffiision electrode. The modification of a Pt substrate was achieved with an inorganic conductor and a conducting polymer, and besides a metal complex was immobilized to the conducting polymer (10). The results obtained are shown in Table III, where the flow rate of C 0 was changedfrom2.0 to 99.0 ml min . The major product was lactic acid, and methanol, ethanol, acetone, 2-propanol, and formic and acetic acids were formed as minor products. The current efficiency for the reduction of C 0 2
2
1
2
2
2.5
5.0 12.7
26.7
20.6
15.1
63.7
57.0
32.0
142
135
134
137
141
1.9
2.2
2.2
3.3
2.3
25.6
44.3
36.6
32.8
16.9
e
2
5
2
Faradaic efficiency for the reduction of C0 .
t
3
X C = [CHjOH] + [HCOOH] + 2([C H OH] + [ C ^ C O O H ] ) + S t f C ^ C O C H J + [CH Œ(OH)COOH] + [CH,CH(OH)CH,])
3
2
87.6
105
85.5
54.5
36.8
Total concentration of the products on the basis of C content, μ mol dm" .
11.7
5.6
3.0
4.9
28.3
Electric charge passed during the electrolysis.
4.3
0.0
3.3
2.0
0.0
d
17.0
1.1
0.0
7.8
4.1
(%)
Volume of the catholyte after electrolysis.
21.4
99.0
1.0
2.7
2.8
(μΜ)
e
c
2.8
27.0
5.1
0.0
d
sc,
b
0.7
16.0
1.0
c
(Q
Q
Electrolysis Potential, -0.8 V vs A g / A g C l ; initial pH, 3.0; electrolysis time, 3 h; net area of the substrate, 19.3 cm .
0.0
9.8
3
(cm )
b
V
electrode
a
0.6
2.0
Acetic Lactic Acetone Formic
2-Pr
MeOH EtOH
( ml min )
1
Faradaic efficiency (%)
Flow rate
KCl
a
with a polymer composite/gas-diffusion
as a function of gas flow rate i n a solution of 0 . 5 M
reduction of C 0
Table III. Faradaic efficiencies for the products obtained i n the electrochemical
359 increased with an increase of the flow rate of C 0 , but became lower at the flow rate of 99.0 ml min' . At this flow rate, the electric charge passed during the electrolysis was also smaller. These results are probably attributed to the limited area of the liquid meniscus which was found to be important for the C 0 reduction on the mesh metal/gas-difi&ision electrode, because a part of the mesh electrode could not keep contact with the electrolyte under such a highflowrate. In the mediated reduction of C 0 , PB and ΡAn are in the reduced states as represented by reaction (8) and (9), respectively. 2
1
2
2
ffl
n
+
n
n
m [ F e ( C N ) ] + K + e- - » K Fe {Fe (CN) ] (PB) (Everitt's salt, ES) 6
+
n
2
+
6
n
PAn /Fe L" + K + e-
-> PAn/Fe L7K
+
(8)
(9)
The reduction scheme of C 0 on the polymer-modified electrode (11) is schematically shown in Figure 7. In this scheme, C 0 is bifimctionally captured by ΡAn and the metal complex: the electrophilic carbon atom of C 0 is bound to the amino group of ΡAn, and the basic oxygen atom coordinates to the central metal of the complex. Electrons from the Pt substrate reach the PAn/PB interface where Yt is catalytically reduced to H on the zeolitic lattice of ES. The bifimctionally activated C 0 is hydrogenated by H . The reaction intermediates are stabilized in the nonaqueous atmosphere of conducting polymer, and various products including lactic acid and methanol are generated by further advanced hydrogénation. 2
2
2
a d s
2
ads
References 1. Halmann, M. Chemical Fixation of Carbon Dioxide; CRC Press: Boca Raton, FL, 1993. 2. Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 1695-1698. 3. Hori, Y.; Kikuchi, K.; Murata, Α.; Suzuki, S. Chem. Lett. 1986, 897898. 4. Cook, R. L.; MacDuff, R. C.; Sammells, A. F. J. Electrochem. Soc. 1987, 134, 1873-1874. 5. DeWulf, D. W.; Bard, A. J. Catal. Lett.. 1988, 1, 73-79. 6. DeWulf, D. W.; Jin, T.; Bard, A. J. J. Electrochem. Soc. 1989, 136, 1689-1691.
Figure 2
ofC0
7. Schematic
representation n
electrode.
of the electrochemical
on a Pt/PB/PAn-Fe L
reduction
361 7. Hori, Y; Wakebe, H.; Tsukamoto, T.; Koga, Ο. Electrochim. Acta 1994, 39, 1833-1839. 8. Jermann, B.; Augustynski, J. Electrochim. Acta 1994, 39, 1891-1896. 9. Smith, B. D.; Irish, D. E . ; Kedzierzawski, P.; Augustynski, J. J. Electrochem. Soc. 1997, 144, 4288-4296. 10. Ogura,K.;Endo, N. J. Electrochem. Soc. 1999, 146, 3736-3740. 11. Ogura, K.; Endo, N.; Nakayama, M . J. Electrochem. Soc. 1998, 145, 3801-3809. 12. Reymond, J. P.; Mériaudeau, P.; Teichner, S. J. J. Catal. 1982, 75, 3948. 13. Sulymosi, F.; Erdohelyi, Α.; Kocsis, M . J. Chem. Soc., Faraday Trans. 1 1981, 77, 1003-1012.
