Ekcrrochimico
Acra. Vol. 40, No. 6, pp. 745-753, 1995 Co~wkbt 0 1995 Elrvier Science Ltd. Printc;l &I &cat Britain. All rights rcscrved 0013%4686/95 $9.50 + 0.00
\ Pergamon
PREPARATION OF Cu-SOLID POLYMER ELECTROLYTE COMPOSITE ELECTRODES AND APPLICATION TO GAS-PHASE ELECTROCHEMICAL REDUCTION OF CO2 SEIJI KOMATSU,~ MICHIE TANAKA,~ AKIRA OKUMIJRA$ and AKIRA KUNGIt TDepartment of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770, Japan; SChemical Engineering Research Division, Shikoku Research Inst. Inc., Yashima-Nishimachi, Takamatsu 761-01, Japan; 3 Environmental Division, Yonden Consultants Inc., Yashima-Nishimachi, Takamatsu 761-01, Japan (Received 21 April 1994) Abstract-Preparation of Cu-solid polymer electrolyte (spe) composite electrodes for gas-phase electrochemical reduction of CO, was studied by electroless plating methods. Among the combinations of plating solutions and reducing solutions used, the combination of Cu-Rochelle salt and 10% NaBH,
gave the most etlicient spe electrode. With the electrodes made of cation-exchange (Nafion) and anionexchange (Selemion) membranes as spe materials, the total current efficienciesfor reduction products of CO, had maximum values of 19 and 27X, respectively. The use of the former gave C,H, as the major product, whereas, dummy exhausted reduction of CO,, Furthermore CO,
with the later, HCOOH and CO were obtained. From gas-phase electrolysis of the gas from thermal-power plants, it was found that NO has no influence on the electrobut the removal of SO, is needed in order to obtain CIH, in higher current densities. in the exhausted gas is required to be concentrated up to more than ca. 30%.
Key words: gas-phase electrochemical exchange membrane.
reduction, CO,, Cu-solid
INTRODUCTION
Electrochemical reduction of CO2 has attracted considerable attention as a possible technology for the conversion of CO2 to useful materials. It has been studied by many investigators using a variety of metallic and semiconductor electrodes in aqueous and nonaqueous solvents[ l-l 11. Copper seems to be one of the most promising electrode materials for the formation of hydrocarbons and alcohols from CO, reductionC3, 4, 81. Hori et aI.[3, 43, for example, reported the effective formation of CO, CH,, C2H,, C,H,OH, and n-C,H,OH in electrochemical reduction of COz at copper electrodes in aqueous solutions; formation of CO predominates at less negative potentials, and hydrocarbons and alcohols are favorably produced at more negative potentials. Furthermore, other studies found that the concentration and kind of electrolyte had a large influence on the product distributions and their current efliciencies in electroreduction of CO* at copper electrodes[6, 123. It was also demonstrated that the utility of a glassy carbon substrate on which Cu was in situ electrodeposited will provide practical electrocatalysis for the hydrocarbon formation from CO, at useful current densities[ 131. The solid polymer electrolyte (spe) method can be applied to electrochemical reduction of CO, in the gas phase, without solvent, because no supporting electrolyte is required. This method thus will provide the absence of the catalyst poisoning owing to
polymer electrolyte electrode, ion-
solvent or electrolyte impurities and an increased mass transfer over that in solution. Gas-phase electrochemical reduction of CO, using Cu-spe composite electrodes has been already investigated by Dewulf and Bard[14] and Cook et aI.[15]. Work by Dewulf and Bird showed that the Cu/Nafion electrode can be prepared by using a N,H, reducing solution and a Cu(II)-pyrophosphate plating solution, but it took a very long time, two weeks, to prepare the electrode[14]. Cook et al. carried out the deposition of Cu on Nafion membrane by the combination of CuSO, and NaBH, , but the technique is not clearly described in detail[15]. In the present study, we examined more sufficient fabrication techniques of the Cu-Nafion electrode by an electroless plating method in detail. Our Cuplating method, the combination of Cu-Rochelle salt as the plating reagent and 10% NaBH, as the reducing reagent gave an efficient spe electrode for COz reduction and Cu-Nafion electrodes up to a maximum diameter of 30cm was made available with good reproducibility. From the viewpoint of the influence of electrolyte solutions on the electroreduction of CO,, it appeared of interest to study further the gas-phase electrolysis of CO, using the Cu-spe electrode made of an anion-exchange (Selemion) membrane. Interestingly, the major product was altered depending on the nature of the spe material, ie C2H, for Cu-Nafion and HCOOH for Cu-Selemion. Moreover, from the standpoint of the use of the exhausted gas from thermal-power
745
746
S. KOMATSUet al.
plants, our attention was directed to the influence of CO, concentration and impurities of NO and SO2 on the gas-phase electrochemical reduction of CO2 . As far as we know, no studies have been reported on the influence of such impurities.
Table 1. Kind and composition bath
Plating solution Kind
CuSO,~SH,O KNaC,H,O,.4H,O NaOH NaCO, PH
3.5g1r’ 34.Oglrl 7.0g1-’ 3.og1-1 13.1
EDTA salt
CuSO,.SH,O
25.og1-’ 65.OgI-’ 4o.og1-’ 13.3
composite electrodes
Two kinds of ion-exchange membranes were used as the spe material: Nafion 117 (perfluoroalkanesufonate cation-exchange membrane, 0.18 mm thickness; E. I. DuPont de Nemours, Inc.) and Selemion AMV (styrene-divinylbenzene copolymer type anion-exchange membrane, 0.13 mm thickness: Asahi Glass Co. Ltd). Prior to metalization. the Nafion membrane was blasted with a # 1000 emery paper to such an extent that it turned from transparent to while in color. The adhesion of Cu deposited onto the membrane was strengthened by the above treatment. The Nafion membrane was then dipped in boiling concentrated HNO, for about 1 h, and further in boiling deionized water for about 1 h to rinse it and to increase its water content. The Selemion membrane was blasted in the same manner as the Nafion membrane and dipped in deionized water for about 24 h without the acid treatment. It is necessary to bring gas into close contract with protons on copper metal of a Cu-spe composite electrode in the case of the gas-phase reduction of CO,. Electroless plating was therefore conducted by the permeation method, which will allow a porous Cu deposit. An H-type plating cell was composed of two compartments which were separated with the membrane. In preliminary experiments, each compartment was placed in a vertical column, and then a plating solution was poured in the upper compartment and a reducing solution in the lower compartment. This, however, was proved to be a failure: gas accumulated in the lower compartment, particularly when NaBH, was used, thus leading to poor plating because of insuffcient contact of the membrane with the two solutions. Next, each compartment was horizontally placed, and plating solution and reducing solution were arranged right and left, respectively, with the membrane in between. The deposition of Cu on the membrane was performed for a maximum of 24 h at 35°C in a cell in which 2OOml of each of the solutions were poured. The apparent plating area was 12Scm’ (4cm diameter), During plating, both solutions were constantly agitated at approximately 5OOrpm with a magnetic stirrer parallel to the membrane to keep the concentration of each solution in the vicinity of the membrane uniform and to release gas bubbles trapped on the membrane surface. Electroless plating was examined using different plating and reducing solutions shown in Table 1. Namely, Rochelle salt, EDTA salt and pyrophosphate were employed as plating reagents, and formalin, NaBH, and N,H, were employed as reducing reagents. All chemicals used were of reagent grade, and the water was deion-
ized. The surface electric resistance of the Cu-Spe composite electrode was measured with multimeter
Composition
Rochelle salt
EXPERIMENTAL Preparation ofCu-spe
of Cu electroless plating
EDTA'2Na NaOH PH Pyrophosphate
CuSO,.SH,O
Na,P,O,
lOH,O 25% NH,OH PH
8.9gl-1 223.og1r1 10mll~L 11.5
Reducing solution Kind Sodium
boron hydride
Hydrazine Formalin
Composition NaBH, PH N,H, H,O PH HCHO
1 - 1.0% 9.5 - 9.8 I - 10% II.1 - 12.0 37%
probes according to the method by Liu et aI.[16], and the bonding strength between the deposited Cu and the membrane was evaluated according to JIS[17]. The amount of the copper plated was given as the weight of Cu per unit area of the membrane exposed to the solution, on the basis of the analysis of Cu ions dissolved in an acid solution by ICP. The particle size of the copper deposited on the membrane was observed by SEM, and its thickness was obtained by observing magnified sectional areas of the Cu element under EPMA. The fabricated electrodes having a surface resistance below 1 Q and well-bonded Cu were employed for the macroelectrolysis of CO,. Gas-phase electrochemical reduction of CO,
The gas-phase electrochemical reduction of CO, was carried out by using an electrolytic cell consisting of three compartments made of Acryl, which is schematically illustrated in Fig. 1. The cathode and middle compartments were separated by the Cu-spe electrode with the metallized side facing the cathode compartment, which was filled with CO1 gas or the mixed gas. The purity of CO, was 99.995%. Dummy exhausted gas, the mixture of CO,, NO and SO,, were prepared using mass flow controller balanced by 99.999% N, gas. The gas in the system was circulated at the rate of 150 ml min- ’ at the atmospheric pressure during the electrolysis. The anode and middle compartments were separated by a nonmetallized Nafion membrane so that electrolysis products were prevented from oxidation at the anode. Ohmic contact of the Cu-spe electrode with the lead wire was achieved with a Pt ring foil inserted between the Cu-spe electrode and the cathode compartment as shown in Fig. 1. The apparent area of the Cu-spe composite electrode was 6.15cm2. The anode and middle compartments were charged with 150ml of 0.5 M K,SO, solution in total. In the case
741
Preparation of Cu-solid polymer electrolyte composite electrodes
’
k
F j
i-h
f
Fig. 1. Schematic diagram of electrolytic cell with Cu-spe composite electrode. (a) Potentiostat; (b) coulometer; (c) see; (d) manometer; (e) Pt anode; (f) Luggin capillary; (g) CO, gas bomb; (h) Cu-spe electrode; (i) Nafion; (i) Pt foil contact; (k) gas sampling.
chromatographic column, a 3 mm diameter x 2m stainless steel column packed with 30/60 mesh activated carbon, was employed under 80-160°C (20”Cmin-‘) with a He carrier gas flow of 35mlmin’. Quantification of products was performed by an integrator (Hitachi D-2500), on the basis of peak areas. The analysis of CO was conducted after conversion to CH, by a methanizer. H, was analyzed by a Hitachi 163 gas chromatograph with TCD: 3 mm diameter x 2m stainless steel column packed with 30/60 mesh activated carbon; isothermal condition at 160°C; N, carrier gas flow of 35mlmin-‘. The analysis of HCOOH was undertaken by a Yokogawa IC-7000 ion chromatograph. The column packed with Excelpak CHA-EII was employed under isothermal condition at 60°C with 25 mM Na,SO, solution flow of 1 ml min - ‘. RESULTS
AND DISCUSSION
Characterization of Cu-spe composite electrodes of the in situ Cu electrodeposition onto the CuNafion electrode, 5 x lo-4 and 5 x 10e3 M CuSO, were added to OSM K,SO, solution in the middle compartment. A platinum mesh and a saturated calomel electrode (see) were used as the anode and
reference electrodes respectively. Controlled potential electrolyses were conducted, using a Hokuto Denko Model HZ-1A potentiostat and a Hokuto Denko model HF-201 digital coulometer. The electrolytic reduction of CO, except for the long-term electrolysis was terminated when 1OOCof electricity was passed. In the long-term electrolysis, CO, gas and K$O, solution were newly changed every passage of 250-35OC. The gaseous and liquid products were analyzed as follows: gas was sampled from the cathode and middle compartments using a microsyringe and liquid from the middle compartment. The analyses of CH,, C,H,, and C2H, were performed by means of a Hitachi G-3000 gas chromatograph with FID. The
Cu-Nafon electrode. A number of Cu-Nafion electrodes were prepared using different combinations of plating solutions and reducing solutions. Their surface electric resistance and bonding strength were evaluated. The results are summarized in Table 2. Among the combination of the plating solution of Rochelle salt and five kinds of reducing solutions, only 10% NaBH, gave good deposition of Cu on the Nafion membrane. Its surface resistance was below 1 R after 1 h of the electroless plating with excellent adhesion of Cu. Others were of no Cu deposits or high surface resistance even after 24 h. When EDTA salt was used as the plating solution, the ues of 10% N,H,, 10% NaBH, and 37% formalin as reducing reagents gave a somewhat good Cu-Nafion membrane with the surface resistances below 1R. The deposited Cu, however, was observed to peel off, partly because of its extremely low bonding strength. Among the combination of pyrophosphate and five
Table 2. Characterization of Cu-Nafion electrodes Surface resist.
Cu plating solution Rochelle
salt
EDTA salt
Phyrophosphate
Reducing solution 1% 10% 1% 10% 37%
A side
B sidet
Adhesion
Remarks
-
No. Cu deposited even after 24 h. > 100 kR after 24 h. Although partly < 1 Q reproducibility was poor. Well-bonded and < 1 R electrode was prepared at 1 h. No. Cu deposited even after 24 h.
N,H, N,H, NaBH, NaBH, HCHO
x x x x x
1%) N,H, 10% N,H, 1% NaBH, 10% NaBH, 37”% HCHO
x x x x x
x x
lo/;, N,H, lo”/;, NZH, 1% NaBH, 10% NaBH, 37”/1, HCHO
0
0
x x x
x
x
0
x
-
No. Cu deposited even after 24 h. < 1 R electrode was prepared, but adhesion was poor. > 100 kR after 24 h despite uniform deposition. < 1 R electrode was prepared, but adhesion was poor. < I R electrode was prepared, but adhesion was poor. Well-bonded and < 1 R electrode was prepared at 24 h. > 100 kR after 24 h despite uniform deposition. Although partly < IOR, reproducibility was poor. Cu scarcelv deoosited after 5 h. No. Cu deposiied even after 24 h.
Symbols: surface resistance (0) < 1 R, (A) 1 _ lOR, ( x) > 10R. Adhesion: (0) peeling at cut part, (-) no test conducted due to high surface resistance. t A and B sides: the plating solution and the reducing solution side, respectively.
no peeling, (A) partial
peeling,
( x) larger
S. KOMATSUet al.
148
kinds of reducing solutions, only 1% N,H, gave good deposition of Cu. Its surface resistance went up to below 1 n only after 24 h with excellent adhesion of Cu. Others were of no Cu deposit or high surface resistance even after 24 h. The morphology of the Cu deposited in and on the Nafion membrane was greatly dependent on how plating solutions and reducing solutions were combined: Cu was deposited on the reducing solution side when the pyrophosphate was combined with N,H,, while on the plating solution side in cases of other combinations. It will be understood that the relative permeation rate of plating reagent and reducing reagent across an ion-exchange membrane determines on which side the metal is deposited. The Pt-Nafion electrode prepared using H,PtCl, and N,H, or NaBH, allows Pt to be deposited on the plating solution side. This may be interpreted in terms of the electrical repulsion between the Nafion cation-exchange membrane and an anion of the Pt complex[l& 193, ie the reducing reagent permeates across the membrane in preference to the plating reagent. According to this idea, all the deposition of Cu in the present work will take place on the reducing solution side since all the Cu-complexes used are cationic. As described above, however, Cu was deposited on the plating solution side except for the combination of pyrophosphate and N,H,. The reason may be interpreted as follows : even if the Cucomplex is cationic, the permeation rate across the Nafion membrane will be influenced by its ion size. This is supported by the result that when a macromolecular organic cation such as a surfactant is subjected to electrodialysis through a cation-exchange membrane, electric voltage becomes significantly higher[20]. Therefore, the permeation of the reducing reagent is preferred rather than those of Cucomplexes of EDTA and Rochelle salt with larger ion size, resulting in the deposition of Cu on the plating solution side. On the other hand, the combination of pyrophosphate and NaBH, gave Cu-deposits on the plating solution side, although the combination of pyrophosphate and N,H, resulted in Cu deposits on the reducing solution side. This may be attributed to the accelerated permeation rate of NaBH, by the evolution of H, gas caused by self-decomposition. Figure 2 shows SEM of two kinds of Cu-Nafion electrodes which were prepared by the combinations of the Rochelle salt and 10% NaBH, for 1 h (CuNafion electrode-I) and the pyrophosphate and 1% N,H, for 24h (Cu-Nafion electrode-II): their Cu
Table 3. Current
efficiencies
of products
weights per unit and thicknesses were 2 mgcn-‘, and 2-4 pm, 8-15 pm, respectively. lOmgcm_’ Observation of the surfaces of two electrodes enlarged under SEM revealed that Cu-Nafion electrode-II consists of porous deposits with angular particles, about 3 pm diameter, assembled, while CuNafion electrode-1 consists of slightly round particles, about 1 pm diameter, assembled, and is larger in surface area and less porous than Cu-Nafion electrode-II. Cu-Selemion electrode. Two combinations of the Rochelle salt/lo% NaBH, and the pyrophosphate/ 1% N,H, were applied to preparation of the CuSelemion electrode. As a result, the former gave a good electrode of which the surface resistance was below 1 R after 1 h of the electroless plating with excellent adhesion of Cu. Cu was deposited on the plating solution side. The latter, however, could not give a good electrode even after 24 h of the plating. Gas-phase electrochemical reduction ofC0, Cu-NaJion electrode. Controlled potential electrolyses of gaseous CO, were undertaken at - 1.5 V vs. see using two kinds of Cu-Nafion electrodes. The electrolysis products were present not only in the cathode compartment but also in the middle compartment, meaning that part of the products passed through the Nafion membrane during electrolysis. The ratio of the sum of current eficiencies for the formation of products in each compartment was approximately 1O:l. The sum of the current efliciency for each reduction product from cathode and middle compartments are summarized in Table 3. H, accounted for ca. 87 and 95% current efficiencies for Cu-Nafion electrode-1 and -11, respectively. The reduction products of CO, included C,H,, a major product accounting for 3.5-8.8%, CO, 2.0-2.6%, and small quantities of C,H, and CH,, together with HCOOH, 3.2-5.9%, depending on the electrodes used. Comparison of Cu-Nafion electrode-I and -II, revealed that the former gave about two times higher current efficiency for the reduction of CO, than the latter. Possibly this will be due to the smaller Cu particle size of Cu-Nafion electrode-I. The potential dependence of current efficiency and partial current density for the reduction of CO2 (ic) were examined using Cu-Nafion electrode-I. These results are shown in Fig. 3. In the potential region of - 1.3 to - 1.8 V vs. see, the current eficiency for H, evolution was SO-90%, but it went up to nearly 100% at the more negative potential of - 1.9 V vs.
in gas-phase CO, ekctroreduction at - 1.5 V vs. scet Current
Electrode Cu-Nafion electrode-I Cu-Nafion electrode-II
at Cu-Nafion
electrodes
efficiencies/%
C,H,
CH,
CO
C,H,
HCOOH
Totalf
H,
Sum total
8.8
2.6
0.1
5.9
17.4
86.8
104.2
3.5
2.0
0.1
3.2
8.8
95.3
104.1
7 Counter solution = 0.5 M K,SO,: quantity of electricity passed = 1OOC; T = room temperature. $ Total current efficiency for reduction products from CO,.
Preparation of Cu-solid polymer electrolyte composite electrodes
Fig. 2. Top view of SEM micrograph of Cu-Nafion electrode surface prepared from (a) Rochelle salt/ 10% NaBH, for 1 h and (b) pyrophosphate/l% N,H, for 24 h. see. On the other hand, total current efficiency for reduction products of CO, had a maximum value of 19% at - 1.5 V vs. see as shown by curve c in Fig. 3.
The reduction products such as CO, HCOOH, C2H, and C2H, were produced at around 5% current ticiencies in the potential region of - 1.3 to - 1.5 V vs. see. The increase in overpotential brought about the increase in the current efficiency for the C2H, formation and the decreases in these for the formation of other products. The current efficiency for the C$H, formation went up to 14% at - 1.8 V vs. see and a maximum partial current density at this potential was ca. 5mAcm-* which was nearly equal to the value obtained by Hori et aZ.[2], who have reported ca. 4.5 mA cm - * at - 1.47 V vs. she in the CO2 saturated 0.05 M K,SO, solution at a Cu metal electrode. Such a potential dependence is almost consistent with the electrolytic reduction of CO,saturated solution at the Cu metal electrode by Noda et aI.[22], except for the formation of CH, and alcohols.
The current efficiency for the formation of hydrocarbon products was two times higher than that by Cook et aI.[15], who have reported 7% current elliciency in the gas-phase electrolysis of CO,, but our value was somewhat lower than the value (17%) obtained by Dewulf and Bard[14]. Our method for the fabrication of the Cu-Nafion electrode, however, would be favorable at the point of the electroless plating time. In addition, Cu-Nafion electrodes up to a maximum of 30cm diameter (about 7OOcm*) can now be made available with good reproducibility. The time dependence of current density and current efficiency for the gas-phase electrolysis of CO2 were examined using Cu-Nafion electrode-I at - 1.7V vs. see at room temperature. These results are illustrated in Fig. 4. The mean value of the electrolytic current density decreased gradually as shown by curve a in Fig. 4. As shown by curves c, d, e, f, g and h, the current efficiencies for each reduction product of CO2 were nearly constant, even
S. KOMATWet al.
750
N
pj&yy-y -1.3
-1.5 E/V
k
j
-1.7 vs SCE
-1.9
i$j “---
-2.1
1
3
5
Time/h
E 5 8
2 5
70:
:
zo-
C
E
3it
I
,d
lo-
20
E g
e
elf?OO
3
1
3
h
5
Time/h
10
0
l
-1.3
-1.5 E/V
-1.7
-1.9
Fig. 4. Time dependence of mean current density and current efftciency at Cu-Nafion electrode-I at - 1.7V vs. see. (a) Current density; (b) current efficiency for H, evolution; (c) total current elliciency for the reduction products from CO,; (d)-(h) current elkiencies for C,H,, HCOOH, CO, CH, and C,H, , respectively.
-2.1
vs SCE
Fig. 3. Potential dependence of current efficiency and partial current density (ic) for the reduction of CO, at CuNafion electrode-I. (a) Partial current density for the reduction of CO,; (b) current effkiency for H, evolution; (c) total current efliciency for the reduction products from CO,; (d)-(h) current efficienciesfor C,H,, CO, HCOOH, C,H, and CH,, respectively.
after the electrolysis of 5 h. The current elliciency for the formation of H, was also almost constant. It has been reported that the electrochemical reduction of CO, at Cu electrodes affords hydrocarbons in high current efficiencies at short electrolysis times, but the current efficiency decreases at longer times and finally the electrodes become almost inactiveC7, 11-J. As shown above, Cu-Nafion electrode-I was demonstrated to be very stable for the electrochemical reduction of CO2 . In situ electrodeposited copper Cu-Najion electrode. Controlled potential electrolyses of gaseous CO, were undertaken at - 1.7 V vs. see using in situ electrodeposited copper Cu-Nafion electrode-I. Bright copper was newly electrodeposited on the anode compartment side of Cu-Nafion electrode,
Table 4. Gas-phase electroreduction
but partial blistering of Cu-Nafion was observed. Table 4, presents current efficiencies for reduction products and the mean current density in the reduction of CO, in the presence of 5 x 10m4 and 5 x 10e3 M CuSO, passing 1OOC of electricity. Current efficiencies and current density decreased compared with the absence of CuSO,, particularly, the addition of higher concentration of CuSO, resulting in a significant decrease in current efficiency for C2H4 formation. This indicates the situation contrary to the results by Cook et aI.[13], who showed the effectiveness of in situ electrodeposited copper layers on glassy carbon electrode for the electroreduction of CO, in aqueous solutions. The reason is unknown at present. In any event, in situ electrodeposited copper Cu-Nafion electrode will not be suitable for the gas-phase electrochemical reduction of co,. Cu-Selemion electrode. It is known that pH of the electrolyte solution and kind of supporting electrolyte have an influence on the product distributions
of CO, at in situ electrodeposited
copper Cu-Nafion electrode-I at - 1.7 V vs.
scet Current efficiencies/% CuSO, added non 5 x lo-“M 5 x lo-“M
C,H, 11.8 9.0 5.9
CH,
CO
HCOOH
Total1
H,
Sum total
Mean current density (mAcm-s)
0.2 0.1 -
1.2 0.5 0.4
1.5 8:;
14.7 10.5 7.0
89.0 93.9 96.6
103.7 104.4 103.6
26.4 24.7 21.5
t Counter solution = 0.5 M K,SO,: quantity of electricity passed = 1OOC; T = room temperature. $ Total current eficiency for reduction products from CO,.
751
Preparation of Cu-solid polymer electrolyte composite electrodes
g
1
&(--y-y -1.3
-1.5 E/V
-1.7 vs SCE
-1.9
C,H,,
vs SCE
Fig. 5. Potential dependence of current efficiency and partial current density (ic) for the reduction of CO, at CuSelemion electrode. (a) Partial current density for the reduction of CO,; (b) current efficiency for H, evolution; (c) total current efficiency for the reduction products from CO,; (d)-(g) current efficiencies for HCOOH, CO, C,H, and C,H,, respectively.
and their current efficiencies in electroreduction of CO, at copper electrodesC6, 12-J.In this connection, it would be interesting to study the gas-phase electroreduction of CO2 using the Cu-spe composite electrode made of an anion-exchange membrane (Selemion) in place of a cation-exchange membrane (Nafion). Figure 5 shows a typical potential dependence of current efficiency and partial current density for the reduction of CO,. There are remarkable differences between Cu-Selemion and CuNafion electrodes. HCOOH and CO were the major products of the reaction and their current effciencies at - 1.5V vs. see were 15 and lo%, respectively. That is, the utility of the Selemion in place of the Nafion membrane as the spe material leads to the change of the major product from C,H, to HCOOH. Furthermore, the total current efficiency for the reduction products of CO2 had a maximum value of 27% at - 1SV vs. see (cf. 19% for CuNafion electrode-I). A maximum partial current density was 5mAcmm2 at - 1.9V vs. see, which was nearly equal to that at Cu-Nafion electrode-I. The current efficiency for H, generation was 75-90% as curve b in Fig. 5. This difference in the major product is explainable in terms of the ease of proton transport within the porous ion-exchange membrane. As a matter of course, Nafion has an advantage over Selemion in respect to a supply of protons. The number of
as follows: CO, + 2H’ + 2e + HCOOH
-2.1
1 E/V
protons involved in the electrochemical reduction of CO, depends on the kind of products. HCOOH and
CO2 + 6H+ + 6e + fC,H,
+ 2H,O.
That is, the formation of C,H, requires more protons than that of HCOOH. Therefore, the electrochemical reduction of CO, at the Cu-Nafion electrode gave C,H, as the major product, whereas HCOOH was produced with the Cu-Selemion electrode. An analogous result was also observed by Hori et aI.[23], ie the electroreduction of CO, at Cu metal electrodes KHCO, aqueous solutions gave preferentially C2H, at lower pH, whereas HCOOH and CH, at higher pH. Figure 6 shows the time dependence of the mean current density and current efficiencies for reduction products in the gas-phase electrolysis of CO, at CuSelemion electrode at - 1.7V vs. see at room temThe electrolytic current decreased perature. gradually as shown by curve a in Fig. 6. As shown by curves c, d, e, f and g, each current efficiency was almost independent of the electrolysis time. That is, Cu-Selemion electrode was also found to be very stable for the electroreduction of CO, as well as CuNafion electrode-I. InJIuence of concentration of CO, and impurity of NO and SO, on gas-phase electrochemical reduction of co2
Concentrations of CO,, NO, and SO, exhausted from a typical thermal-power plant (electric power; 250MW, fuel, coal) are about 15%, 90ppm
!/:;;I 1
3
5
Time / h
Time / h Fig. 6. Time dependence of mean current density and current efftciency at Cu-Selemion electrode at - 1.7 V vs. see. (a) current density; (b) current efficiency for H, evolution; (c) total current efficiency for the reduction products from CO,; (d)-(g) current efficiencies for HCOOH, CO, C2H4 and C,H, respecttvely.
152
S. KOMATSUet al.
Table 5. Influence of SO, and NO on current efficiencies for products and current density in gas-phase CO, electroreduction at Cu-Nafion electrode-1 at - 1.7 V vs. scet Current efficiencies/%
Mixed gas$ CO,
NO
60%
-
60% 60%
200ppm -
SO,
C,H, 14.9 15.0 9.3
-
170ppm
CH,
CO
0.1 0.1 -
0.5 0.4 1.2
HCOOH 1.2 1.1 6.1
Totals
H,
Sum total
Mean current density (mA cme2)
16.7 16.6 16.6
85.9 87.8 86.5
102.6 104.4 103.1
21.8 21.5 18.5
t Counter solution = 0.5 M K,SO,: quantity of electricity passed = 1oOC; T = room temperature. 1 Balanced by 99.999% N, gas. § Total current efficiency for reduction products from CO,.
(NO/NO,:9/1) and 30ppm (nearly 100% SO,) at the outlet of de-NO, and de-SO, equipment, respectively. It would be interesting to apply directly the exhausted gas to the gas-phase electrochemical reduction of CO,. Figure 7 shows a typical influence of CO1 concentration on current efficiencies for reduction products and the mean current density in the electroreduction of CO, at Cu-Nafion electrode at - 1.7 V vs. see. The current density increased with an increase in CO2 concentration as shown curve a. The total current efficiency for CO2 reduction products increased exceedingly to have 14-30% CO, concentration and then above this concentration, increased gradually to have 17% as shown by curve c. The current efficiency for H, evolution decreased
looIL 80
t
A
.
e f,g
80
50
100
co2 /%
Fig. 7. CO, concentration dependence of mean current density and current elliciency at Cu-Nafion electrode-I at - 1.7V vs. see. (a) current density; (b) current efficiency for H, evolution; (c) total current elliciency for the reduction products from CO,; (d)-(g) current etllciencies for C,H,, HCOOH, CO and C,H, , respectively.
from ca. 100% to 85% with increasing CO, concentration. C*H, began to produce at around 5% CO, and an increase in CO, concentration resulted in the increase of the current efficiency for C2H, up to 50% CO*. Above 50% CO1 it was almost constant, as shown by curve d. Other reduction products such as HCOOH, CO, and C,H, began gradually to produce with increasing CO, concentration, although their current efficiencies were very low. Similar concentration dependence of CO2 was observed in the electrolytic reduction of CO1 saturated 0.05M L&CO, aqueous solution at indium electrode by Ito et aI.[24], at the point of exceeding increase of current efficiency at lower CO, pressure and gradual increase at higher CO, pressure. Influence of impurities of NO and SO2 on the gasphase electrochemical reduction of CO, was examined using Cu-Nafion electrode-I at - 1.7 V vs. see at room temperature. The following mixed gases were used: 60% COz gas balanced by high purity N, gas containing 200ppm NO and 170ppm SO,, which were about two and six times higher than these in the exhausted gas from a typical coal thermal-power plant, respectively. These results are listed in Table 5. NO had no influence on current efficiency for each reduction product. However, the addition of SO, caused C2H, to decrease in current efficiency by the increment of HCOOH. The reason is unknown at present, but it would be pointed out that a copper corrosion of Cu-Nafion electrode was observed after the electrolysis of the SO,-containing CO1 gas. Consequently, when the exhausted gas from thermal-power plants was used in the gas-phase electrolysis at Cu-spe composite electrode, COz in the exhausted gas should be concentrated more than two times and the removal of SO, is recommended for the purpose of obtaining higher current efficiency of C,H, formation and more stable Cu-spe composite electrode. Acknowledgement-The authors are grateful to Dr H. Takenaka, Government Industrial Institute of Osaka, Japan for his valuable suggestions on the method of pretreatment of the Nafion membrane in the preparation of Cu-spe electrodes by an electroless plating method. One author (A. K.) also gratefully acknowledges financial support from a Grant-on-Aid for Scientific Research on Priority Areas (no. 05235235) from the Japanese Ministry of Education, Science and Culture. This paper has been completed with the support of Shikoku Electric Power Company, Inc.
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