Zn oxides loaded gas diffusion electrodes

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

225

E l e c t r o r e d u c t i o n o f CO2 u s i n g C u / Z n o x i d e s l o a d e d gas d i f f u s i o n e l e c t r o d e s Shoichiro Ikeda a, Satoshi ShiozakP, Junichi Susuki a, Kaname Ito a, and Hidetomo Noda b a Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466 Japan, b Chubu Electric Power Co. Inc., Kika-sekiyama, Midori-ku, Nagoya, 459 Japan

Gas diffusion electrodes (GDEs) consist of a gas layer (mixture of hydrophobic carbon black (CBphob) and PTFE dispersion) and a reaction layer (mixture of catalyst powder, CBphob, hydrophilic CB (CBphi~), and PTFE) laminated on a Cu mesh as a current collector. As the catalyst, CuO/ZnO (3:7 by mole ratio) mixed oxides and a mixture of a Cu powder (4N, -325 mesh) and a ZnO powder were examined. Electroreduction was performed potentiostatically passing in general 200 C using a m-shaped Pyrex cell having one gas and two liquid chambers, with two lines of gas circulating systems. When using a GDE of (CuO / ZnO = 3 / 7 : CB = 6 : 5 [by weight], the reduction products were mainly C2HsOH with slight amounts of CO and HCOO-, and a comparable amount of H2. Faradaic efficiency maximum of 16.7% for C2HsOH formation with maximum selectivity of 88% was observed at -1.32 V vs. Ag-AgC1, at a partial current density of 4.23 mA/cm 2, which is about 50 times greater than that obtained on a sintered oxide electrode. In the case of the GDE of (Cu / ZnO = 3 : 7) : CB = 3 : 1 reduced by H2, the selectivity of the reduction products became poorer, like in the case of a Cu foil electrode, with lower current density, although the total faradaic efficiencies for CO2 reduction was 40.5 % with additional formation of n-C3H7OH and C2H4 at -1.30 V.

1. I N T R O D U C T I O N Gas diffusion electrodes (GDEs) having large areas of three phase boundaries are promising for use in electrochemical reactions of gaseous reactants to enhance their rates, i.e. current densities. Sammells et al. reported formation of alcohols on a perovskite loaded GDE in KOH solution [1]. We have reported massive electroreduction of CO2 using Cu-loaded GDEs [2-4], and also reported selective formation of ethanol from CO2 on a CuO/ZnO sintered electrode in KHzPO4 aqueous solution [5], however, its formation rate was extremely low as 85/zA/crff. In this paper, we will show the results on the electroreduction of CO2 using CuO/ZnO- and Cu/ZnOloaded GDEs in 0.5 M KHzPO4 aqueous solution.

226 2. EXPERIMENTAL

The active material, Le. catalyst, of CuO/ZnO (3 : 7 by mole ratio) mixed oxides was prepared from the aqueous solution (aq. soln.) of reagent grade copper nitrate and zinc nitrate by adding diluted ammonium water. The precipitate was washed and calcined at 360~ in air for 1 day. On the other hand, Cu/ZnO powder was prepared by mechanical mixing of Cu powder (Rare Metallic Co., Ltd.; 4N, -325 mesh) and ZnO (Nacalai Tesque Inc.; >99.0%) with a mole ratio of 3:7. The GDEs were prepared by almost the same manner as described in the case of Cu-GDEs [3]. GDEs consist of a gas layer (mixture of hydrophobic carbon black (CBphob)(Denki Kagaku Kogyo; AB-7) and polytetrafluoroethylene (PTFE) dispersion (Daikin Kogyo; D-l) and a reaction layer (mixture of catalyst powder, CBphob, hydrophilic CB (CBphil) (Denki Kagaku Kogyo; AB-11), and PTFE) laminated on a Cu gauze of 30 mm~ as a current collector. The electrodes were hot-pressed under 1.2 MPa in N; atmosphere. In the case of H2 reduced GDEs, they were reduced in an electric furnace at 300 ~ for 60 min under H2 flow at 50 cm3/min. The apparent working area of GDE was about 4.5 cm2. Electroreduction was performed at 25 ~ in 0.5 M KH2PO4 aq. soln. (pre-electrolyzed under N2 flow) potentiostatically, passing in general 200 C using a potentio-galvanostat (Hokuto Denko; HA-303), an electronic coulometer (Hokuto Denko; HF-201), and a LU-shaped Pyrex cell having one gas and two liquid chambers for anolyte and catholyte, separated by a cation exchange membrane, Nation NX90209. The cell had two lines of gas circulating systems for gas and catholyte. The counter and reference electrodes were Pt-Pt 60 i I ~ I I i 30 and Ag-AgC1 saturated with V: q (Total) ,,,/ 9 : /d KC1, respectively. The reduction 50 25 products were analyzed by gas chromatographs and a high % 40 20 performance liquid chromatograph as described in the previous paper [6]. g 30 15 o : .ooo

3. RESULTS DISCUSSION

AND

3.1. CuO/ZnO loaded GDEs When using a GDE of (CuO / ZnO = 3 / 7 [by mole ratio]) : CB = 6 : 5 [by weight]; the standard composition, the reduction products were mainly C2HsOH (EtOH) with slight amounts of CO and HCOO-, and

o 20

X

-,:co

.

lo

9"" I

I

I

-1.7

-1.6

I

I

I

-1.5 -1.4 -1.3 V vs. Ag-AgCI

I

-1.2

Figure 1. Faradaic efficiency for reduction products of CO2 on a CuO/ZnO-GDE, (CuO / ZnO = 3 / 7 ) : CB - 6 : 5, in 0.5 M KH2PO4 aq. soln. at 25 ~

227

a comparable amount of H2 a s a byI I I I I product as shown in Figure 1. 50 I A./d Faradaic efficiency (1]) maximum of 16.7% for EtOH formation with _ - 20 .IV ........ maximum selectivity of 88% was 40 V " ---V- ............................. -V observed a t - 1 . 3 2 V vs. Ag-AgC1, v . rl (Total) E where total current density and the ~ 30 o >, < faradaic efficiency for CO2 0 E 1-) 9H2 9 (CO2) reduction, 1"1 (CO2), showed maxi.0_ .mo ma. The partial current density for ~o 20 10 ~O9 r EtOH formation was 4.23 mA/cm 2, .o_ o...--~-~ 9 J O "o which is about 50 times greater than ~ r 9 "C2HsOH ~ that obtained on a sintered (CuO / u_ 10 ZnO = 3 / 7) electrode [5]. rl(H2) o m:CO increased with the potential 0 _ ~-~-~- :-~ .................... 8 0 becoming more negative. The 9 I I I I I I selectivity for EtOH formation 0 100 200 300 400 500 600 during the CO2 reduction was more Q/C than 75% in the potential range o f Figure 2. Dependence of the faradaic efficiency for CO2 1.2 to -1.7 V as shown in Figure 1. reduction products and current density on the quantity rl(COz)M,x was, however, close of electricity passed with the standard CuO/ZnO-GDE to 19% at a maximum. The in 0.5 M KH2PO4 aq. soln. at -1.30 V. distribution of reduction products, their q, and current density at -1.3 V were maintained almost constant up to 500 C as shown in Figure 2. This fact indicates that the activity of catalyst is not changed during continuing the electrolysis up to 500 C. To improve the electrode performance, the amount of catalyst of the GDE was increased to 2.5 times as much as the standard composition, i.e. (CuO / ZnO) : CB = 3 : 1. Contrary to the Table 1 Reduction products of C02 on a GDE with (CuO/ZnO):CB = 3"1 for electrolysis 200 C passed in 0.5 M KHzPO4 aq. soln. at 25~ Potential / V vs. Ag-AgC1

Current density / mA c m 2

EtOH

HCOO-

-1.20

4.1

4.2

0.9

0.3

1.3

20.5

6.7

27.2

-1.25

4.4

7.9

1.1

0.8

2.4

22.7

11.5

34.9

-1.30

5.2

12.8

1.4

1.5

3.3

25.6

19.0

44.6

-1.35

4.8

9.6

1.2

1.3

3.0

29.9

15.1

45.0

-1.40

3.9

7.2

1.1

0.6

2.8

34.5

11.9

46.4

Faradaic efficiency / % C2H4

CO

H2

T](CO2) rl(total)

228 expectations, the current density became only about 1/5 of the previous one, although CzH4 was additionally produced as listed in Table 1. This fact may indicate the decrease of gas diffusion rate in GDEs. After reduction of the GDE of (CuO / ZnO) : CB = 3 : 1 by H2 at 300~ for 60 min, the current density became 10.1 mA/cm 2 at -1.30 V and rl (CO2)Max became 34.5%, and nC3HvOH (n-PrOH) was additionally produced with rl = 9.2%. However, the selectivity for EtOH formation in the CO2 reduction became poorer than that in the case of the GDE with the standard composition as shown in Table 2. Consequently, pre-reduction of the GDE increased the 1"1(total) and rl (CO2), and did not change I"I(EtOH), but decreased the selectivity because of the produced metallic Cu, which usually leads to a variety of CO2 reduction products [6]. Table 2 Reduction products of CO2 on a H;-reduced GDE with (CuO/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KHzPO4 aq soln. at 25~ Potential / V vs. Ag-AgC1

Current density / m A c m -2 EtOH

Faradaic efficiency / % n-PrOH

HCOO-

C2H4

CO

H2

1"1(CO2) 1"1(total)

-1.20

8.2

10.1

2.9

1.0

2.1

2.3

20.0

18.4

38.4

-1.25

9.3

16.6

5.0

0.7

4.2

3.1

22.8

29.6

52.4

-1.30

10.1

11.9

9.2

1.3

7.6

4.5

25.3

34.5

59.8

-1.35

9.7

8.4

3.9

1.0

7.1

4.2

29.5

24.6

54.1

-1.40

7.8

6.8

1.3

1.1

3.8

4.0

34.8

17.0

51.8

3.2. C u / Z n O loaded GDEs To confirm the effects of Cu formed by reduction with H2 and/or in the course of the CO2 reduction in the CuO/ZnO-GDE, GDEs containing a mixture of Cu powder and ZnO have been prepared and examined. Results obtained by the as-prepared and H2-reduced GDEs are summarized in Tables 3 and 4, respectively. Table 3 Reduction products of CO; on an as-prepared GDE with (Cu/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KH2PO4 aq. soln. at 25~ Faradaic efficiency / %

Potential / V vs. Ag-AgC1

Current density / m A c m -2

EtOH

n-PrOH

HCO0-

C2H4

CO

H2

1"1(CO2)

1"1(total)

-1.20

3.9

7.2

2.4

0.8

1.5

1.8

18.2

13.7

31.9

-1.25

4.3

14.8

4.3

1.1

3.3

2.4

20.1

25.9

46.0

-1.30

5.1

11.7

7.8

0.8

5.8

3.9

24.7

30.0

54.7

-1.35

4.6

6.4

4.8

0.9

5.6

3.7

28.3

21.4

49.7

-1.40

4.2

4.3

2.9

1.1

2.7

3.1

35.1

14.1

49.2

229

n-PrOH was also produced with both GDEs. The selectivity for EtOH formation became poorer and the current density lesser than that for CuO/ZnO-GDEs of the standard composition. Faradaic efficiencies of the reduced GDE are larger than those of the as-prepared GDE. rl(EtOH)Max of 16.2% at -1.25 V, rl(COz)Max of 40.5% at -1.30 V, and rl(total)Max of 66.4% at -1.30 V were obtained with the H2-reduced GDE. These tendencies are similar to those observed for the Cu foil electrode, even though in the different electrolyte, i.e. 0.1 M KHCO3 aq. soln. [6]. Table 4 Reduction products of CO2 on a H2-reduced GDE with (Cu/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KHzPO4 aq. soln. at 25~ Potential / V vs. Ag-AgC1

Current density / mA cm 2

-1.20 -1.25

Faradaic efficiency / % H2

I"I(COz)

3.2

21.3

20.8

42.1

4.8

23.4

34.8

58.2

7.1

25.9

40.5

66.4

8.6

6.9

30.2

31.4

61.6

5.5

6.6

36.7

22.8

59.5

EtOH

n-PrOH

HCOO-

C2H4

7.3

9.8

3.6

1.1

3.1

8.6

16.2

6.1

1.5

6.2

-1.30

7.9

12.8

10.2

1.3

9.1

-1.35

6.8

8.3

6.5

1.1

-1.40

6.5

5.6

3.8

1.3

CO

rl (total)

Table 5 Summary of C02 electroreduction results for 200 C passed at Cu/Zn oxides loaded gas diffusion electrodes in 0.5 M KHzPO4 aq. soln. at 25~ CuO/ZnO-GDE Weight ratio of catalyst : CB

6:5

IdMax/mA cm -2

25.3

Pot. of rl (total)Ma,,, / V

-1.70

-1.40

1"1(total)Max / %

48.9

Pot. of rl (CO2)Max / V

3 :1 5.2

Cu/ZnO-GDE

3 : 1(red.) 10.1

3 :1

3 : 1(red.)

5.1

8.6

-1.30

-1.30

-1.30

46.4

59.8

54.7

66.4

-1.32

-1.30

-1.30

-1.30

-1.30

rl (CO2)Max / %

19.0

19.0

34.5

30.0

40.5

Pot. of rl (EtOH)Ma,,, / V

-1.32

-1.30

-1.25

-1.25

-1.25

rl (EtOH)Ma~, / %

16.7

12.8

16.6

14.8

16.2

Selectivity of EtOH / %

87.9

68.7

56.1

57.1

47.1

Pot. of rl (n-PrOH)Max / V

-

-

-1.30

-1.30

-1.30

rl (n-PrOH)Max /

-

-

9.2

7.8

%

10.2

(red.): Hz-reduced, CB: Total amount of carbon black, IdMax"Maximum of current density, Pot.: Potential vs. Ag-AgC1, rl (X)Max: Maximum faradaic efficiency for formation of X.

230 From these results, it is found that there are zinc oxide and copper oxide, lather than the metallic copper, which control the selective formation of ethanol from CO 2.

4. CONCLUSION Using ZnO/CuO-loaded gas diffusion electrodes, ethanol has been selectively produced with ca. 17% of faradaic efficiency by electroreduction of CO2 the same as with the sintered ZnO/CuO electrode in aqueous KH2PO4 solution but with about 50 times higher current density than the latter. H2-reduced GDEs or Cu/ZnO-loaded GDEs produced in addition ethylene and n-propanol, but with lower current density and selectivity. Optimum conditions for electroreduction of CO 2 on each GDE are summarized in Table 5.

REFERENCES

1. R.L. Cook, R.C. McDuff, and F. Sammells, Proc. Intern. Symp. on Chem. Fixation of Carbon Dioxide (ISCF-CO2-91 Nagoya), Dec. 2-4, Nagoya, Japan, (1991) 39. 2. T. Ito, S. Ikeda, M. Maeda, H. Yoshida, and K. Ito, Proc. Intern. Symp. on Chem. Fixation of Carbon Dioxide (ISCF-CO2-91 Nagoya), Dec. 2-4, Nagoya, Japan, (1991) 313. 3. S. Ikeda, T. Ito, K. Azuma, K. Ito, and H. Noda, Denki Kagaku, 63 (1996) 303. 4. S. Ikeda, T. Ito, K. Azuma, N. Nishi, K. Ito, and H. Noda, Denki Kagaku, 64 (1996) 69. 5. S. Ikeda, Y. Tomita, A. Hattori, K. Ito, H. Noda, and M. Sakai, Denki Kagaku, 61 (1993) 807. 6. H. Noda, S. Ikeda, Y. Oda, and K. Ito, Chem. Lett., 289 (1989).