8440
J. Phys. Chem. 1995, 99, 8440-8446
Electrochemical Reduction of C02 with High Current Density in a CO2-Methanol Medium Tomonori Saeki, Kazuhito Hashimoto, and Akira Fujishima" Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1Hongo, Bunkyo-ku, Tokyo I13, Japan
Naokazu Kimura and Koji Omata Electric Power Development Company, 6-15-1 Ginza, Chuo-ku, Tokyo 104, Japan Received: September I , 1994; In Final Form: January 13, 1995@
Electrochemical reduction of COZwith high current density was studied in a CO2-methanol medium. The mole fraction of C02 in this medium varied from 0.7% to 94% with changing the pressure of the system from 1 to 60 atm. Carbon dioxide was reduced to CO, CH4, CzH4, and methyl formate at a Cu electrode. A methyl group and a formyl group of methyl formate are derived from methanol and COZ,respectively. Methyl formate production in this system corresponds to formic acid formation in aqueous systems. A Tafel plot obtained at 40 atm (the mole fraction of COZ is 33%) indicated that the reduction of C02 to CO was no longer limited by mass transfer of C02. Total current density and current efficiency of COZ reduction at -2.3 V were 436 mA cm-2 and 87%, respectively, at 40 atm. The studied pressure range, 0-60 atm, was classified into three regions with boundaries at 20 and 40 atm; 20 atm was the point above which the mass transfer of COZis sufficiently high for the reaction under the current density of 200 mA cm-*, and 40 atm was the point at which the significant change occurs in the property of CO2-methanol medium. Reduction of C02 to CO and methyl formate proceeded even at 60 atm, at which the mole fraction of COZis 94%.
Introduction Carbon dioxide fixation has been of global interest from both fundamental and practical viewpoints.' The electrochemical method is one of the promising methods to reduce C O Z . ~It is reported that, at most of metal electrodes, major reaction products are carbon monoxide and formic acid in aqueous systems? but a copper electrode effectively gives higher reduced products such as CHq and C2H4.4 In order to obtain more valuable products, addition of homogeneous catalysts to the electrolysis ~ y s t e m modification ,~ of the electrode surface by heterogeneous catalysts,6and utilization of alloys7and oxides8 as electrodes are being studied. Although the research to control the reaction products is actively done, that to increase the reaction rate has not been tried well. The rate of COz reduction (current density in electrochemistry) reported previously was several tens of mA cm-2 at most. The conversion of C02 to useful products with high current density will be one of the interesting and important topics in the C02 chemi~try.~-l~ The rate of the C02 reduction has been limited by the low mass transfer of C02. An efficient mass transfer of C02 to the electrode surface is therefore the key to obtain a high current density. The utilization of gasdiffusion electrodes is one approach to serve this purpose. The apparent current density obtained with electrodes of this type is approximately 10-100 times greater than that obtained with conventional foil electrode^.^ Another approach to achieve the efficient mass transfer of CO;? is the electrolysis of highly concentrated C02. High-pressure aqueous systems'0.' as well as systems with nonaqueous s o l v e n t ~have ~ ~ ,been ~ ~ utilized in order to obtain a higher concentration of CO2. In the study on high-pressure aqueous systems, however, the enhancement of the current density is intended only in a few recent studies;I0
'
* To whom correspondence should be addressed. E-mail: fujisima@ tansei.cc.u-tokyo.ac.jp. Abstract published in Advance ACS Abstracts, April 15, 1995. @
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50 60 70Pressure / atm Figure 1. Concentration and mole fraction of C02 in a CO2-methanol mixture at 25 O C . Data was obtained from ref 15 and plotted.
0
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20 30 40
most of the previous studies have been on the improvement of the current efficiency of C02 reduction.",12 Electrolysis in nonaqueous solvents is advantageous in the C02 reduction because carbon dioxide is more soluble in several nonaqueous solvents than in water.I3 There is an additional advantage; a side reaction of H2 evolution proceeds less efficiently in nonaqueous systems.I2 For all the promising features, a reduction of C02 with both high current density and high current efficiency has not been reported yet. Research on COz reduction with high current density is still on the initial stage. We have been studying the electrochemicalreduction of liquid C02. The reaction of C02 at its highest concentration is of great interest. Furthermore, the highest concentration of COz will be effective in various kinds of electrochemical carboxylation of organic materials with CO2.I4 In such reactions involving C02, however, a polar material needs to be added to liquid C02 to obtain an ionic conduction, since nonpolar CO;? alone does not provide an ionically conductive electrolyte solution. We chose methanol as the polar material that is to be added to liquid C02 in this study. Methanol and COz are completely miscible with each other as shown in Figure l . I 5 The concentration of C02 amounts to about 8 mol dm-3 (33%)
0022-365419512099-840$09.00/0 0 1995 American Chemical Society
Electrochemical Reduction of C02
J. Phys. Chem., Vol. 99, No. 20, 1995 8441
Figure 2. Illustration of the electrode assembly. (a) Cu working electrode, (b) Ag quasi-reference electrode, (c) Pt counter electrode, (d) Torr Seal cement; (e) 1/4" (6.35 mm) stainless steel tube, (f) Swagelok tube fitting.
at 40 atm and about 17 mol dm-3 (94%) at 60 atm, respectively, in a CO2-methanol mixture at 25 "C. Methanol, a protic solvent, is expected to serve as a hydrogen atom source for the formations of H2 andor hydrocarbons in this system. We have succeeded in the electrochemical reduction of C02 in a C02-methanol medium with extremely high current density and current efficiency.I6 In this paper, we will describe the details of this reaction, particularly the product determination, the evidence of the sufficient mass transfer, and the effect of the pressure.
Experimental Section Electrolysis was performed using a three-electrode system in a one-compartment cell. The high-pressure apparatus was assembled from an SUS-316 stainless steel tube and Swagelok tube fittings. A glass inner test tube was used to avoid contact of the electrolyte with the metal apparatus. The volumes of gas and liquid phases were 13.2 and 1.0 mL, respectively. Electrodes used were a Cu working electrode (99.999% in purity and 1.0 or 0.33 cm2 in geometric area, donated by Sumitomo Electric Industry Co.), a Pt counter electrode (99.99%, Nilaco), and a Ag quasi-reference electrode (99.99%, Nilaco). These electrodes were sealed in a stainless steel tube I/4 in. (6.35 mm) in diameter with Torr Seal resin, as is shown in Figure 2. Stability of the reference electrode was tested before every electrolysis. Its variation was within 20 mV and was satisfactory for the present study. The working electrode was electrochemically etched in a concentrated H3P04 before use. This procedure was important to obtain reproducible results. Cyclic voltammograms indicated that copper oxide did not exist on the electrode surface. Commercially available methanol of reagent grade (Nacalai Tesque) was used as a supporting solvent without further purification. Tetrabutylammoniumtetrafluoroborate (TBABF4, Aldrich) was used as a supporting electrolyte as purchased. No precipitation of the supporting salt was observed even at 60 atm, at which the mole fraction of C02 is 94%. The electrolyte solution was deaerated by the introduction of C02 to about 40 atm followed by the flushing of the gas to atmospheric pressure. This procedure was repeated three times. Carbon dioxide gas of desired pressure was then introduced to the solution, and the system was left for 1 h to reach equilibrium. Temperature was controlled by using a water bath during the experiment. , All electrolyses were conducted galvanostatically using a potentiostat-galvanostat (Hokuto, HA-501) until about 10 C were passed. After the electrolysis, high-pressure gas was released into a larger volume vessel to reduce the system pressure. Gas and liquid phase products were then analyzed by a gas chromatograph (GC) with a flame ionization detector (FID)(Ohkura, GC-202 with Porapak R column packing; a catalytic methanizer was used for detection of carbon monoxide) and by a gas chromatographwith a thermal conductivity detector
0
5
10 Charge / C
15
Figure 3. Electrochemical formation of CO (a), HCOOCHj (b), H2 (c), CHq (d), and C2H4 (e). The electrolysis was performed at 20 OC, under 40 atm, with a current density of 200 mA cm-* (-2.0 V).
(TCD) (Hitachi, GC-163 with 12X molecular sieves). Gas chromatography-mass spectrometry (GC-MS,Shimadzu, GCMS-QP-11OOEX) studies of electrolyses in I3C02- 12CH30H( I2C4H9)4NBF4and 12C02-13CH30H-(I2C4H9)4NBF4systems were carried out to identify whether each product was formed from C02, methanol, or the supporting salt. Ohmic loss in potential was compensated by calculating from values of measured resistance and current. As will be discussed later, methyl formate (HCOOCH3) was found as a product of C02 reduction. It should be also considered that methyl formate may be under the dissociation equilibrium of methanol and formic acid. The solutions of various initial concentrations of methyl formate containing 0. I mol dm-3 of (C4H9)4NBF4 were prepared to calibrate the results of GC analysis, so that the same dissociation conditions as in the electrolysis may be provided.
Results and Discussion Reduction Products In the electrochemical reduction of C02 in a CO2-methanol medium, under both atmospheric and high pressure, CO, Ch,C2&, and H2 were detected as products in the gas phase. In the liquid phase, methyl formate, HCOOCH3, and dimethoxymethane, CH3OCH20CH3, were detected. In the control experiment, the system being kept for several hours without passing current, no such products presented above were detected except for the impurities in the C02 used. The amounts of impurities were subtracted from those of the experimental results. The amounts of all products increased monotonically with increasing charge passed, as shown in Figure 3. It is confirmed that these products were formed electrochemically. GC-MS chromatograms of CO produced in the I3CO212CH30H-( 12C4H9)4NBF4experiment are shown in Figure 4. Gases come out in the sequence 02, N2, and CO when MS13X is used as a column-packing material. The peak at 5.0 min is assigned to CO. The ion chromatogram of m/z = 29 has a large peak at 5.0 min where CO appears, while that of
8442 J. Phys. Chem., Vol. 99, No. 20, 1995
Saeki et al.
.€ fn t
Q)
-c c
I
71
5 10 Retention time / min Figure4. Ion chromatogramsof the GC-MS analysis in 13C02-12CHaOH-( I2C4H9)4NBF4 experiment: (a) total ion chromatogram, (b) ion chromatogram of d z = 29 (c) ion chromatogram of d z = 28. The electrolysis was performed at 20 OC, under 40 atm, with a current density of 200 mA cm-' (-2.0 V). Molecular sieve 13X was used as 0
a column packing.
100 -
31
not take part in the formation of dimethoxymethane. It was formed by the oxidation of methanol at the Pt anode, since l2CH30I2CH2Ol2CH3was formed even in the I3C02-I2CH3OH-( 12C4H9)4NBF4system. Shown in Figure 5 is a spectral comparison of electrolytically formed HCOOCH3 with the standard one. Panel a in Figure 5 depicts the mass spectrum of the standard reagent, HI2C00I2CH3. The mass spectra of HCOOCH3 formed in the I3CO212CH30H-( I2C4H9)4NBF4experiment and I2C02- 13CH30H(12C4H9)4NBF4experiment are presented in panels b and c of Figure 5 , respectively. The peak at d z = 60 in panel a of Figure 5 is assigned to the molecular ion, Hi2C00I2CH3+.In panel b, the peak appears at d z = 61, indicating that either one of the two carbon atoms in the HCOOCH3 molecule is derived from I3CO2. Peaks at d z = 15,31, and 32 in panel a of Figure 5 are attributed to 12CH3+,I2CH30+,and 12CH30H+ fragments, respectively. These fragments have a methyl group. All of these peaks appear at the same positions in panel b of Figure 5 (I3CO2experiment), but, in Figure 5, panel c, (13CH3OH experiment), the peaks have shifted in the larger mass number direction by 1. These results clearly demonstrate that the methyl group is derived from methanol and that the formyl group is from C02. The mass peak of the formyl group at d z = 29, in the form of an HCO+ fragment, should appear at d z = 30 in the 13C02-12CH30H-( I2C4H9)4NBF4 experiment. Actually a large peak appears at d z = 30 in panel b, while only a weak peak appears in the mass spectrum of the standard material (Figure 5 , panel a). On the basis of these results, the formation of methyl formate can be explained as follows;
HCOOH
100, 30
\
0
10
20
32
40
30
50
60
70
m/Z
Mass spectra of HCOOCH3: (a) standard chemical, H1'C00'2CH~, (b) electrolytically formed HCOOCH3 in the I3CO2I2CH30H-( I2C4H9)4NBF4 experiment; (c) electrolytically formed Hexperiment. The COOCHj in the 12C02-13CH30H-(12C4H9)4NBF4 electrolysis was performed at 20 OC, under 40 atm, with a current density of 200 mA cm-2 (-2.0 V).
Figure 5.
d z = 28 has a peak about 1/100 smaller than that of the former. The latter is attributed to I2CO formed from I2C02 contained as an impurity in the I3COused. Carbon monoxide is therefore concluded to be produced from C02, not from methanol or the supporting electrolyte. In the same manner, C& and C2& were determined to be produced from C02. Consequently,these are the reduction products from C02. On the other hand, C02 did
+ CH,OH -HCOOCH, + H20
(3)
Therefore, the methyl formate formation in the present system corresponds to the formic acid (HCOOH) production in the aqueous C02 reduction system. It is noted that the spectrum of HCOOCH3 electrolytically formed in the 1T02-13CH30H-(1qfi9)4NBF4 experiment has two molecular ion peaks, at d z = 61 and 62 (Figure 5, panel c). The latter originates in Hi3C00I3CH3. The peak intensity at d z = 62 is approximately a half of that at d z = 61, indicating that about one-third of HCOOCH3 was produced from solvent molecules. Two possible pathways are considered for the formation of this product; one is the re-reduction of C02 produced by the anodic oxidation of methanol, and the other is the reaction of methanol with HCOOH formed by the oxidation of methanol. On the basis of the following three reasons, we concluded that the amount of HCOOCH, produced by the former process (reduction process) is much larger than that produced by the latter process (oxidation process). (1) The sum of the current efficiencies of all the products is almost 100% in all experiments, without remarkable excess or loss, when it is calculated under the assumption that all HCOOCH3 is formed by the cathodic reaction. (2) A strong bubble evolution at the anode was observed during the electrolysis. Since no anodic production of CO was found in GC-MS analysis in the 13C02'TH3OH experiment (Figure 4, see above), the bubble was most likely C02. Anodically produced C02 from methanol could easily reach the cathode and react in a one-compartment electrolysis cell. (3) The efficiency of HCOOCH3 formation in the absence of C02 is only about 5% (estimated as a reduction product) at maximum, while that in the presence of C02 is greater than 20%.
Electrochemical Reduction of C02
J. Phys. Chem., Vol. 99, No. 20, 1995 8443
I6o
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-2.0
-1 .o
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-0.5
Potentialvs. Ag QRE / V
Figure 6. Cyclic and linear sweep voltammograms of methanol and C02-methanol mixture. Scan rate was 50 mV s-'. Dashed line: under 1 atm of Nz. Solid line: under 1 atm (a), 20 atm (b), and 40 atm (c) of c02.
-2.5
-2
-1.5
-1
Potential I V Figure 8. Effect of the electrode potential on the current efficiencies of products under 40 atm (25 "C): A, current efficiencies for C 0 2 reduction (a) and HZevolution (b); B, current efficiencies of reduction products from C02; CO (c), HCOOCH3 (d), C h (e), and C2H4 (0.
6 S
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Figure 7. Effect of the electrode potential on the current efficiencies of products under 1 atm (25 "C): A, current efficiencies for C02 reduction (a) and H2 evolution (b); B, Current efficiencies of reduction products from C02; CO (c), HCOOCH3 (d), CHq (e), and C2H4 (0.
Electrochemical Reduction of C02 with High Current Density and Efficiency. Cyclic voltammograms at various pressures are presented in Figure 6. The cathodic current was observed with the onset potential of - 1.O V under 1 atm of N2. In the scan in the negative direction, a shoulder wave was observed around -1.4 V, while it was not observed in the reverse scan. When N2 was replaced with 1 atm of C02, a larger shoulder wave was observed with the same onset potential. The cathodic current under C02 in the more negative potential region approached the curve under N2. Thus, excess current under C02, in comparison with the voltammogram under N2, was observed only at this shoulder-wave region, around -1.6 V, under 1 atm. This observation indicates that C02 reduction is favorable only in this narrow potential range. The magnitude of this shoulder wave increased with increasing CO;?pressure. The excess current in the voltammogram under C02 around the shoulder wave is therefore attributable to the C02 reduction. The shoulder was no longer observed above 20 atm. Efficiencies of the product formation at the various electrode potentials under 1 and 40 atm of C02 are presented in Figures 7 and 8, respectively. Under 1 atm, H2 formation proceeded
as the main reaction. In a very narrow potential range around -1.6 V, the C02 reduction proceeded predominantly. This phenomenon is consistent with the prediction by the CV. Carbon monoxide, CH4, C2H4, and HCOOCH3 were formed as reduction products, as was described previously. Carbon monoxide and HCOOCH3 formations were favorable at relatively positive potentials. Methane formation was favorable in the potential region more negative than -1.6 V. Such a dependence of the product distribution on the electrode potential is similar to that of the electroreduction of aqueous C02 at a Cu electrode. In the aqueous systems, the main product changes in the sequence of HCOOH, CO, C2%, and CHq with the electrode potential shifting in the negative dire~ti0n.I~ These observations indicate that similar reactions take place at the electrode surface in both aqueous and methanol systems under 1 atm of C02, and the formation of HCOOCH3 in the present system corresponds to the HCOOH formation in aqueous systems. It is also indicated that methanol works as a protic solvent. Recently, production of CO and CHq was reported in the electrochemicalreduction of C02 in methanol under 1 atm,'* but the analysis of liquid phase products was missing and the total current efficiency was far below 100%. The rest is most likely attributed to the methyl formate formation. Under 40 atm, on the other hand, the drastic increase in the efficiency of C02 reduction onset at around -1.0 V (Figure 8A), at which the onset of the large cathodic current occurred in the CV (Figure 6). Carbon monoxide and HCOOCH3 were formed as main products. Formation of CO increased with increasing negative potential, while that of HCOOCH3 took a maximum at - 1.8 V. Furthermore, in contrast to the observation under 1 atm or in aqueous systems, the current efficiency of C02 reduction did not decrease with increasing potential in the negative direction. This strongly demonstrates that a sufficient supply of C02 to the electrode surface was maintained so that C02 at the electrode surface was hardly exhausted even under high current density at high applied bias. Current efficiency of hydrocarbon formation was lower under 40 atm than under 1 atm over the entire potential range. Figure 9 shows the Tafel plot obtained under 40 atm, at which the mole fraction of CO;! is 33% and its concentration is 8 M.I6
Saeki et al.
8444 J. Phys. Chem., Vol. 99, No. 20, 1995 A
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Potential I V Figure 9. Tafel plots of H2 evolution (a), CO formation (b), and HCOOCH3 formation (c) under 40 atm (25 "C).
Hydrogen evolution and HCOOCH3 formation become diffusion-controlled at - 1.4 and - 1.8 V, respectively. On the other hand, the current density of CO production keeps increasing even at -2.3 V. The result strongly demonstrates that the production of CO is not limited by the diffusion of C02 in this potential range. A maximum current density as high as 500 mA cm-2 was recorded at -2.3 V. At this potential, the partial current densities for the production of CO and HCOOCH3 were 234 and 173 mA cm-2, respectively,and the total current density for C02 reduction was found to be 436 mA cm-2. It is emphasized that these current density values are as high as those found in the industrial electrolyses, such as in the chloralkali industry (200 mA cm-2 at the anode) and in the electrolytic refinement of aluminum (500 mA cm-2 at the cathode). Although the ohmic loss in the electrode potential is compensated, it seems still existent in values shown in Figure 9, judging from the high Tafel slope (300 mV).I9 Therefore, the actual electrode potential may be more positive than -2.3 V for 500 mA cm-2. In this system, a sufficiently high supply of C02 to the electrode is achieved from the matrix solvent to the electrode at high C02 pressure, not from the dissolved species in the bulk solution. Therefore, it would be appropriate to call the reduction of highly concentrated C02 in a C02-methanol medium at high C02 pressure the electrochemical reduction of "liquid C02." Effect of Pressure. The electrochemical reduction of C02 under various C02 pressures was studied galvanostatically at 200 mA cm-2. The current efficiencies of H2 evolution and C02 reduction are shown in Figure 10A. The efficiency of CO:! reduction increased from 23% at 1 atm to 92% at 20 atm. Hydrogen evolution decreased from 95% to less than 20% in this pressure range. Above 20 atm, the selectivities for C02 reduction and H2 evolution were almost constant around 95% and 5%, respectively. Such changes in the selectivities around 20 atm are interpreted as follows. The mass transfer of C02 is too low to achieve the current density of 200 mA cm-2 below 20 atm but is sufficiently high above 20 atm. The efficiency of the formation of each product also varied with the pressure (Figure 10B). Below 20 atm, the efficiencies of both CO and HCOOCH3 formations increased with increasing pressure. The efficiency of CH4 formation changed in a different way; the maximum efficiency of CHq formation was observed at 10 atm. These data suggest that the balance between the COZand the hydrogen source (Hf, H20, or methanol) that are supplied to the electrode surface is important. Above 20 atm, the current efficiency of CO production was.almost constant
50
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0
4 1
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20 30 40 Pressure I atm
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Figure 10. A, effect of pressure on the current efficiencies of C02 reduction (a), H2 evolution (b), and the total efficiency (c). The electrolyses were performed at 20 OC, with a current density of 200 mA cm-2 (potential varied between -2.0 and -2.3 V). B, effect of pressure on the current efficiencies of CO (a), HCOOCH3 (b), CH4 (c), and C2H4 (d). The electrolyses were performed at 20 "C, with a current density of 200 mA cm-2 (potential varied between -2.0 and -2.3 V).
around 45%, while that of HCOOCH3 formation remained constant and then decreased above 40 atm. Thus, another changing point appears at 40 atm, in addition to the point at 20 atm. The efficiency of C h production decreased slightly above 20 atm and then increased above 40 atm. Although the formation of C2& was rather low, the changes in its current efficiency around 20 and 40 atm were observed; the efficiency increased with increasing pressure below 20 atm, remained constant above 20 atm and then decreased above 40 atm. Since the supply of C02 is sufficient for the reaction with the current density of 200 mA cm-2 above 20 atm and does not change around 40 atm, as was discussed above, such changes in the current efficiencies of the products around 40 atm are not due to the change in the supply of COz. Figure 11 depicts the cell resistance in this pressure range. The resistance represents the property of the solution such as the solvation structure. There is a knee in the curve at 40 atm, indicating that the property of the CO2-methanol solution changes above this pressure. Since the changes in the product distribution and the solution property occurred at the same pressure, the variation of the product distribution above 40 atm is attributed to the change of the solution property.
Electrochemical Reduction of C02
J. Phys. Chem., Vol. 99, No. 20, 1995 8445
TABLE 1: ElectrochemicalReduction of COZin C02-Methanol Media gas N2
co2 c02 co2 co2 c02 co2 a
current efficiency (%a)
press. (atm)
xco*a
currentb (mA.cm-2)
potentialc (V)
H2
co
CHq
C2H4
HCOOCH3
total
1 1 40 40 40 52 60
0
2.0 2.0 2.0 5.0 500 2.0 6.0
-1.6 -1.6 -1.2 -1.6 -2.3 -1.5 -1.5
88.8 91.7 77.4 53.7 4.0 46.1 51.3
n.d.d 1.8 3.0 11.7 46.6 11.7 16.6
n.d. 0.2
n.d. tr. 0.8 n.d. 2.6 n.d. n.d.
4.6 23.3 17.2 21.7 34.6 27.0 29.2
93.4 117.0 98.8 90.7 91.2 84.8 97.1
(a)
0.7 33 33 33 60 94
0.4 3.6 3.4 n.d. n.d.
Calculated mole fraction from the data in ref 15. Suuuorting salt free basis. Total current density. Potential vs Ag quasi-reference electrode.
-
0 0
10
20 30 40 50 60 70
Pressure / atm Figure 11. Cell resistance measured at 10 kHz between the Cu working electrode and the Pt counter electrode.
Eventually, the pressure range is classified into three consecutive regions with two boundaries at 20 and 40 atm. Above 20 atm the mass transfer of C02 is sufficiently high for the reaction under a current density of 200 mA cm-2. At 40 atm significant change in the properties of the CO2-methanol mixture starts to occur. The electrolyses under high pressure up to 60 atm are summarized in Table 1. The reaction above 40 atm is of great interest since the property of the CO2-methanol solution is dramatically changed from that under lower pressure. The reduction of C02 to CO and HCOOCH3 proceeded remarkably even under 60 atm. Hydrocarbon formations, on the contrary, were inefficient. At a Cu electrode, hydrocarbons such as CHq and C2H4 are mainly formed at the current density of 2-5 mA cm-2 in aqueous systems.4a Although methanol is also a protic solvent and actually H2 was formed under 60 atm, hydrocarbon formation was rather small in this system. At 60 atm, the mole fraction and the concentration of C02 are calculated to be 94% and 17 M, respe~tive1y.l~Because of the high resistance, large ohmic loss was an obstacle for a higher current density at this pressure.
Summary The present work is summarized as follows: (1) A high concentration of C02 is electrochemically reduced at a Cu electrode in a COz-methanol medium, producing CO, HCOOCH3, CH4, and C2H4. Methanol serves as a protic solvent in this system. (2) Dependence of the product distribution on the electrode potential under 1 atm is similar to those in aqueous systems. Hydrogen production is the principal reaction at the high cathodic potential. On the contrary, under 40 atm, C02 reduction to CO proceeds mainly even at a highly negative potential. At this pressure, the mole fraction of COz is 33%. The C02 reduction efficiency hardly decreases even at -2.3 V under 40 atm. (3) A high concentration of CO:! is beneficial to its mass transfer. Sufficiently high mass transfer of COz was confirmed
by the Tafel plot and the electrolyses under various pressures ranging from 1 to 50 atm. The current density for C02 reduction under 40 atm amounted to 436 mA cm-2, which is a value comparable to or even larger than that used in industrial electrolyses. (4)The pressure range is classified into three regions with two boundaries. The first boundary at 20 atm is the level above which the mass transfer of C02 is sufficiently high to achieve the current density of 200 mA cm-2, and the second boundary at 40 atm is the point where the change in the properties of the COz-methanol mixture starts to occur. Reduction of C02 proceeded under a pressure as high as 60 atm, at which the mole fraction of COz is 94%. However, the large resistance becomes an obstacle for a higher current density. In the electrochemical system to treat C02, the anodic and the cathodic reactions should be ideally the C02 reduction and the oxygen evolution, respectively. On the basis of the present result, such a system as follows is proposed; cathode ICO,-methanol mixture (this system)((aqueous solutionlanode The separation of the two electrolyte solutions will be achieved by using an ion exchange membrane. This research is now in progress and will be reported elsewhere. Finally, it should be also noted that the other nonaqueous solvents such as acetonitrile,dimethylformamide and propylene carbonate, which are also miscible with C02, were also preliminarily tested, but decomposed at relatively high potential and gave unexpected products. Methanol is therefore found to be the best supporting solvent to compose a polar solution with highly concentrated C02.
Acknowledgment. The present work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. The work was also partially supported by the Intemational Joint Research Program of New Energy and Industrial Technology Development Organization (NEDO). The authors are grateful to Dr. L. A. Nagahara and Mr. B. Ohtsuka for their careful reading of the manuscript. References and Notes (1) (a) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277,637-638. (b) Jessop, P. G.; Ikariya, T.; Noyori, R. Nature 1994,368, 231-233. (c) Thampi, K. R.; Kiwi, J.; Gratzel, M. Nature 1987,327,506508. ( 2 ) (a) Taniguchi, I. Modern aspects of electrochemistry; Bockris, J. O'M., White, R. E., Conway, B. E., Eds.; Plenum Publishing Corporation: New York, 1989; Vol. 20, pp 327-400. (b) Electrochemical and electrocatalytic reaction of carbon dioxide; Sullivan, B. P., Krist, K., Guard, H. E., Eds.; Elesevier: Amsterdam, 1993. (3) (a) Azuma, M.; Hashimoto, K.; Hiramoto, M.; Watanabe, M.; Sakata, T. J. Electrochem. Soc. 1990,137, 1772-1778. (b) Noda, H.; Ikeda, S.; Oda, Y .; Imai, K.; Maeda, M.; Ito, K. Bull. Chem. SOC.Jpn. 1990, 63, 2459-2462.
8446 J. Phys. Chem., Vol. 99, No. 20, 1995 (4) (a) Hori, Y.; Murata, A.; Takahashi, R. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2309-2326. (b) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki,S. J. Am. Chem. Soc. 1987,109,5022-5023. (c) Hori, Y.; Murata, A.; Takahashi. R.; Suzuki, S. J. Chem. Soc., Chem. Commun. 1988, 1719. (d) Hori, Y.; Murata, A.; Yoshinami, Y. J. Chem. Soc., Faraday Trans. 1 1991, 87, 125-128. ( 5 ) (a) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Chem. Soc., Chem. Commun. 1984, 1315-1316. (b) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Am. Chem. Soc. 1986, 108,7461-7467. (c) Bhungun, I.; Lexa, D.; Saveant, J.-M. J. Am. Chem. SOC.1994,116,50155016. (d) Ishida, H.; Tanaka, K.; Tanaka. T. J. Chem. Soc., Chem. Commun. 1987, 131-132. (e) Ishida, H.; Tanaka, K.; Tanaka, T. Organometallics 1987, 6, 181- 186. (6) (a) Christensen, P. A,; Hamnett, A,; Muir, A. V. G. J. Electroanal. Chem. 1988,241, 361-371. (b) Meshitsuka, S.; Ichikawa, M.; Tamaru, K. J. Chem. Soc., Chem. Commun. 1974, 158-159. (c) Lieber, C. M.; Lewis, N. S. J. Am. Chem. Soc. 1984, 106, 5033-5034. (7) (a) Watanabe, M.; Shibata, M.; Kato, A.; Sakata, T.; Azuma, M. J. Electroanal. Chem. 1991, 305, 319-328. (b) Watanabe, M.; Shibata, M.; Kato, A.; Azuma, M.; Sakata, T. J. Electrochem. SOC. 1991,138,33823389. (8) (a) Bandi, A. J. Electroanal. Chem. 1990, 137, 2157-2160. (b) Frese, K. W. J. Electrochem. SOC. 1991, 138, 3338-3344. (9) (a) Schwartz, M.; Cook, R. L.; Kehoe, V. M.; MacDuff, R. C.; Patel, J.; Sammels, A. F. J. Electrochem. SOC. 1993, 140, 614-618. (b) Cook, R. L.; MacDuff, R. C.; Sammels, A. F. J. Electrochem. Soc. 1990, 137,607-608. (c) Fumya, N.; Matsui, K. J. Electroanal. Chem. 1989,271, 181-191. (d) Furuya, N.; Koide, S. Electrochim. Acta 1991, 36, 13091313. (10) Hara, K.; Tsuneto, A.; Kudo, A.; Sakata, T.; J. Electrochem. Soc. 1994, 141, 2097-2103. (1 1) (a) Nakagawa, S.; Kudo, A.; Azuma, M.; Sakata, T. J. Electroanal. Chem. 1991, 308, 339-343. (b) Kudo, A,; Nakagawa, S.; Tsuneto, A,;
Saeki et al. Sakata, T. J. Electrochem. SOC. 1993, 140, 1541-1545. (c) Ito, K.; Ikeda, S.; Okabe, M. Denki Kagaku 1980, 48, 247-252. (12) (a) Ikeda, S.; Takagi, T.; Ito, K. Bull. Chem. Soc. Jpn. 1987, 60, 2517-2522. (b) Ikeda, S.; Saito, Y.; Yoshida, M.; Noda, H.; Maeda, M.; Ito, K. J. Electroanal. Chem. 1989, 260, 335-345. (c) Ito, K.; Ikeda, S.; Yamauchi, N.; Iida, T.; Takagi, T. Bull. Chem. Soc. Jpn. 1985, 58, 30273028. (13) Gennaro, A.; Isse, A. A.; Vianello, E. J. Electroanal. Chem. 1990, 289, 203-215. (14) (a) Tyssee, D. A.; Baizer, M. M. J. Org. Chem. 1974, 39, 28192823. (b) Tyssee, D. A.; Baizer, M. M. J. Org. Chem. 1974, 39, 28232828. (c) Yeh, L . 3 . R.; Bard, A. J. J. Electrochem. Soc. 1977, 124, 355360. (d) Mizen, M. B.; Wrighton, M. S. J. Electrochem. SOC. 1989, 136, 941-946. (e) Lund, H.; Simonet, J. J. Electroanal. Chem. 1975, 65, 205218. (0 Bulhoes, L. 0. S.; Zara, A. J. J. Electroanal. Chem. 1988, 248, 159-165. (g) Ticianelli, E. A.; Avaca, L. A. J. Electroanal. Chem. 1989, 258, 369-377. (h) Nagaoka, T.; Nishi, N.; Fujii, K.; Ogura, K. J. Electroanal. Chem. 1992, 322, 383-389. (15) Brunner, E.; Hiiltenschmidt, W.; Schlichthirle, G. 1. Chem. Thermodyn. 1987, 19, 273-291. (16) Saeki, T.; Hashimoto, K.; Kimura, N.; Omata, K.; Fujishima, A. J. Electrochem. Soc. 1994, 141, L130-132. (17) Noda, H.; Ikeda, S.; Oda, Y.; Ito, K. Chem. Lett. 1989, 289-292. (18) Naitoh, A.; Ohta, K.; Mizuno, T.; Yoshida, H.; Sakai, M.; Noda, H. Electrochim. Acta 1993, 38, 2177-2179. (19) Typical reported values of Tafel slope for C02 reduction to CO are 124 mV in aqueous systems and 230 mV in non-aqueous systems; see: Ohlawa, K.; Noguchi, Y.; Nakayama, S.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1994, 369, 247-250 and references therein.
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