1023-1935/01/3710- $25.00 © 2001

åÄIä “Nauka

/Interperiodica”1085

Russian Journal of Electrochemistry, Vol. 37, No. 10, 2001, pp. 1085–1088. Translated from Elektrokhimiya, Vol. 37, No. 10, 2001, pp. 1250–1254.Original Russian Text Copyright © 2001 by Tyurin, Bogatyreva, Zhutaeva, Korovin, Marinich, Radyushkina, Tarasevich, Yuskov.

INTRODUCTION

To date, an impressive success has been achieved inthe development of low-temperature air–hydrogen fuelcells with a polymer electrolyte and a platinum catalyston a carbon support [1]. Considerable effort was aimedat decreasing the specific content of platinum andincreasing the degree of its utilization, by decreasingthe size of platinum crystallites deposited on a carbonsupport, among other measures. This led to such adecrease in the content of platinum in the active layerof electrodes that its contribution to the overall cost ofa fuel cell was relatively low. However, during massproduction of power installations based on fuel cells theplatinum consumption is still appreciable. Besides, inthe case of a direct methanol fuel cell, due to the pene-tration of organic fuel through the membrane, of urgentneed is the development of a catalyst that would beinactive towards methanol. In connection with this ofgreat interest are non-platinum catalysts of the oxygenreduction process, especially macrocyclic complexesof metals subjected to a thermal treatment (so-calledpyropolymers) adsorbed on carbon supports [2, 3].These catalysts have sufficient enough activity, stabil-ity, and selectivity. During the last few years, a group ofCanadian and Belgian researchers has published aseries of works [4–6], where macrocyclic complexesadsorbed on carbon black Vulcan XC-72 are used ascatalysts in fuel cells with polymer electrolyte Nafion-117. This carbon material has a low resistivity of 0.2–1.0 ohm cm [7], average size of particles 30 nm [8], andspecific area 220 m

2

g

–1

[9]. A French group used finelydivided carbon Norit SX Ultra with a specific surfacearea of 1200 m

2

g

–1

as the support [10, 11]. Either group

of researchers used double-layered gas diffusion oxy-gen electrodes. The gas supplying layer in these elec-trodes is made of special porous paper resembling feltmanufactured by Toray Carbon Paper Industries(United States). The paper is filled with a mixture ofpowdered carbon and polytetrafluoroethylene (PTFE)at a 70 : 30 ratio. The sources for the production ofpyropolymers in these works were cobalt phthalocya-nine, cobalt tetracarboxyphthalocyanine [4–6], andcobalt dibenzotetraazaannulene [10, 11]. The charac-teristics of fuel cells assembled by both groups areclose, specifically, at a temperature of

23°C

and an oxy-gen electrode potential of 0.5 V (relative to a reversiblehydrogen electrode), the current density amounts to100 mA cm

–2

; at a temperature of

80°C

and an oxygenelectrode potential of 0.7 V, the current density alsoamounts to 100 mA cm

–2

[4–6, 10, 11].

In [12, 13], we proposed methods for the formationof composite polymer materials consisting of cobaltporphyrin (CoPP) pyrolyzed on acetylene black (AD)or ultradisperse dynamic diamond (UDD) and a proton-conducting polymer electrolyte. Electrocatalytic activ-ity of these materials was examined by methods basedon use of a floating gas diffusion electrode and a rotat-ing disk electrode in solutions of sulfuric acid. In mod-eling conditions, electroreduction of oxygen on thecomposition AD + CoPP occurs with the participationof four electrons, and that on the composition UDD +CoPP, with the participation of 2.5–3 electrons.

The aim of this work was to examine the activity ofthese compositions as catalysts of an oxygen electrodein a system with a polymer electrolyte.

Pyropolymer-based Oxygen Electrode for Solid Polymer Electrolyte Systems

V. S. Tyurin, G. P. Bogatyreva*, G. V. Zhutaeva, N. V. Korovin**, M. A. Marinich*,K. A. Radyushkina, M. R. Tarasevich, and A. Yu. Yuskov**

Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 117071 Russia* Bakul’ Institute for Superhard Materials, National Academy of Sciences of Ukraine, Avtozavodskaya ul. 2, Kiev, 254153

Ukraine** Moscow Power Engineering Institute, ul. Krasnokazarmennaya 17, Moscow, 111250 Russia

Received December 8, 2000

Abstract

—The activity of composite materials (acetylene black or ultrafinely divided dynamic diamond + Co-pyropolymer + Nafion solution) in the oxygen electroreduction in a 0.5 M H

2

SO

4

solution and when using aNafion-117 film 200

µ

m thick as a proton-conducting electrolyte is compared. It is established that the additionof Nafion in the active mass leads to a decrease in the electrocatalytic activity of the latter. The same compositecatalyst (at an insignificant thickness of the active layer) in contact with a solid polymer electrode makes it pos-sible to obtain current densities ten times those in a sulfuric acid solution. Possible reasons for these effects arediscussed.

1086

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 37

No. 10

2001

TYURIN

et al

.

EXPERIMENTAL

The source for the production of the pyropolymerwas cobalt tetra(

p

-methoxyphenyl)porphyrin (CoT-MPP) with the empirical equation

C

48

H

40

N

4

O

4

Co

, con-taining 7.4 wt % Co. For the carbon support we usedacetylene black, so-called technical carbon, particlesize 20 nm, specific area

S

BET

= 85 m

2

g

–1

, and UDDmanufactured by Ukrainian firm ALIT, in the form ofmicropowder with

S

BET

= 170 m

2

g

–1

and the size ofsubgrains 4 nm [13]. To obtain the pyropolymer, a CoT-MPP solution in chloroform (concentration 1 mM) wasmixed with the carbon support. The mixture was driedin air and then heated in an argon atmosphere for30 min at

800°C

. The amount of CoTMPP added toacetylene black was 16.5 wt % [12] and to UDD, 5.5 wt% [13].

The activity of the catalysts was examined on a gasdiffusion electrode with a working surface area of0.3 cm

2

; the electrode was shaped as a disk 1 cm indiameter and 0.3–0.5 mm thick; the disk was preparedby rolling 40–50 mg of hydrophobized acetylene blackon a caprone gauze [12]. A paste prepared from the cat-alyst under study on a support and a 5-% Nafion solu-tion (Aldrich) : water in a 1 : 2 volume ratio (5–10

µ

l)was applied to this disk on the side facing the proton-conducting membrane of Nafion-117 (DuPont). Theactivity of electrodes thus prepared was estimated frompolarization curves (PC) recorded with the aid of aPI-50-1.1 potentiostat (at potential scan rate

v

= 2 mV s

–1

)and an N307

x–y

recorder in a device speciallydesigned for this purpose (Fig. 1), where a Nafion-117film 200

µ

m thick was used as the proton-conductingelectrolyte.

The device comprised two symmetrical half-casings

5

and

9

(of Teflon), between which a Nafion-117 film

1

was clamped by means of tightening plates

10

and

11

(of polymethyl methacrylate) fixed by tightening pins

12

. Preliminarily, the Nafion-117 film underwent clean-ing and swelling in 0.5 M

H

2

SO

4

. The electrode undertest

2

and the anode

2

'

were pressed to either side of thisfilm with the aid of bolts

13

, guiding cylinders

4

and

8

,and gas-distributing metallic meshes

3

and

7

. One ofthe electrodes served as an electrode under study, onwhich the catalyst activity was determined. The otherelectrode, of a similar design, served as an auxiliaryelectrode, on which the oxygen evolution occurred.Oxygen was supplied to electrode

2

with a layer of cat-alyst

6

on it through gas-distributing mesh

3

andremoved through a groove cut along an element ofmetallic cylinder

4

and a channel in housing

5

. To main-tain the membrane humidity constant, the oxygen priorto entering the cell was passed through a layer of waterand after exiting housing

5

was sent onto gas-distribut-ing mesh

7

of the auxiliary electrode, after which it lefthousing

9

for a flask with water. By the rate of bub-bling, it was possible to judge on the fact of the oxygensupply to the cell as well as on the oxygen consumptionand pressure (expressed in millimeter of water columnthrough which the gas bubbled). The potential of anelectrode under test was determined relative to a silver–silver chloride reference electrode. The capillary of thelatter was pressed to projection

14

of the Nafion-117film. The measurements were taken at

20°C

.

RESULTS AND DISCUSSION

Figure 2 presents polarization curves for the cathodein the semilogarithmic coordinates. The curves wereobtained at various amounts of the active mass on thecathode. The ordinate is the logarithm of the specific

12345 6 7 8 910 11

1213

14

O

2

13

2

'

6

+

Fig. 1.

Schematic of device used for studying the activity ofan oxygen cathode; see text for explanations.

1 2

3

7

546

4

3

2

1

0.7 0.5 0.3

E

, V

log

I

[mA g

–1

]

Fig. 2.

PC for oxygen reduction on electrodes with (

1

) 2,(

2

) 2.5, (

3

) 4.5, and (

4

) 9 mg of AD + CoPP + Nafion solu-tion; and (

5

) 2.5 mg of AD + CoPP, (

6

) 2.3 mg of UDD +CoPP + Nafion solution, and (

7

) AD + CoPP + Nafion solu-tion in free electrolyte [12].

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 37

No. 10

2001

PYROPOLYMER-BASED OXYGEN ELECTRODE 1087

activity (mA per gram of an active mass consisting of acarbon support, pyropolymer, and a Nafion solution).Attention is drawn to the presence of dependence of thespecific activity on the catalyst amount (curves

1

–

4

).This is especially clearly seen from the data presentedin Fig. 3 for all studied amounts of the catalyst in theactive layer (from 0.9 to 6 mg) for a potential of 0.5 V.For the composition AD + CoPP + Nafion solution, themaximum activity was obtained on electrodes contain-ing 2.0 mg of active mass, i.e. 7 mg per cm

2

of theworking surface area of the electrode (Fig. 3, curve

1

). Atthe same catalyst amount in the active layer, the magni-tude of the current sharply increases following the addi-tion of Nafion to the active mass (Fig. 2; curves

2

,

5

).These data differ from those we obtained earlier in [12]when investigating the electrocatalytic activity of com-positions AD + CoPP and AD + CoPP + Nafion solutionin a free sulfuric acid electrolyte (Fig. 2, curve

7

). Aswas established in [12], in 0.5 M

H

2

SO

4

, the oxygenelectroreduction rate on the AD + CoPP compositionapplied to the surface of a floating electrode free of abinder and Nafion was higher than that on the composi-tion AD + CoPP + Nafion solution by 20–30%. More-over, the specific activity was independent of the cata-lyst amount in the active layer. These experimental datawere explained on the basis of the assumption that thedistribution of agglomerates of the catalyst and Nafionin the case of the AD + CoPP + Nafion solution compo-sition was not optimum. However, the fact that valuesof the current in the present work differ by two ordersof magnitude (Fig. 2; curves

2

,

5

) cannot be explainedonly in this manner.

One more difference in the operation of electrodeswith a membrane, as compared with their operation ina free electrolyte, is a substantial increase in the specific

activity of the former (by more than an order of magni-tude) for catalyst weights of 2–4 mg (Fig. 3; curves

1

,

3

). On a qualitative level, we explain this difference bya decrease in the contact resistance between particles ofthe catalyst itself and between particles of the catalyst,Nafion, and the solid membrane: in the first place, thecatalyst layer exists under these conditions in a com-pressed state (see description of device for electrodetesting) and, secondly, there is no disjoining action onthe part of the free electrolyte.

As is seen in Figs. 2 (curve

6

) and 4, use of UDD asthe support leads to a decrease in the cathode activity bytwo orders of magnitude in the Tafel region of thepolarization curve (d

E

/

d = 0.1 V) and by 1.5orders of magnitude during a polarization of more than0.2 V relative to a steady-state potential of an oxygencathode, which is equal to 0.65 V. Such a big differencein the activity may be connected with a non-optimum,for this particular support, composition of the compos-ite active mass with regard to both the amount of CoPPand the amount of the Nafion solution (such an optimi-zation was beyond the scope of this paper). The charac-ter of the dependence of the specific activity on the cat-alyst amount in the active layer of the electrode is thesame as in the case of pyropolymer on acetylene black,although the maximum activity at

E

= 0.5 V (Fig. 3,curve

2

) is less pronounced.

Thus, the addition of Nafion to the active layer con-sisting of acetylene black and CoPP ensures substantialexpansion of the reaction zone at the electrode/mem-brane interface, which in the region of the extremum(Fig. 3, curve

1

) probably corresponds to the operatingarea of the catalytic-layer surface. On the other hand,however, the results we obtained suggest that the

Ilog

4

3

2

0 2 4 6

log

I

[mA g

–1

]

g

, mg

1

2

3

3

2

1

0.6 0.4 0.2

E

, V

log

I[mA g–1]

Fig. 3. Dependence of the logarithm of specific activity inoxygen electroreduction at 0.5 V on the amount of activemass at the electrode: (1) AD + CoPP + Nafion solution,(2) UDD + CoPP + Nafion solution, and (3) AD + CoPP [12].

Fig. 4. PC for oxygen reduction on electrode with (1) 1,(2) 2.3, (3) 4.0, and (4) 6.2 mg of UDD + CoPP + Nafionsolution.

32

41

1088

RUSSIAN JOURNAL OF ELECTROCHEMISTRY Vol. 37 No. 10 2001

TYURIN et al.

desired expansion of the gas/catalyst/membrane inter-face at the expense of use of a Nafion solution in theactive layer at its amount of >2.0 mg (which corre-sponds to the thickness of this layer of about 30 µm)has not materialized. We could make an attempt to findan explanation for this effect in an analysis of theNafion properties.

In the literature, when analyzing the microstructureof a Nafion membrane, attention is most frequentlyfocused on the model of a micelle-like ion cluster thathas three regions, specifically, a water cage, an interme-diate region, and a perfluorinated polymer foundation.A study by the small-angle X-ray scattering methoddemonstrated the clusters to have a diameter of 4 nm[14]. A study of by the method of IR spectroscopy ofabsorption of H2O and HDO led to the conclusion [15]that Nafion contains two types of water. One type formshydrogen bonds, and the other does not. The energy ofthe hydrogen bond of a water molecule in the mem-brane is 62% of that in the water bulk. This fact, cou-pled with the results of studying the diffusion of ions inNafion [16], point to an amorphous structure of ionclusters. The structure is formed by a fluorocarbonfoundation surrounded by a region with a high watercontent and sulfonate ion-exchange groups. The inter-mediate region comprises a mixture of nonbonded sidechains, small groups of water molecules that form noclusters of sulfonate ion-exchange groups, and counte-rions. It was also assumed [17] that the ion clusters in aNafion membrane have a layered structure.

To our knowledge, no microscopic studies of physi-cochemical properties of a composite consisting of apyropolymer-based catalyst and a polymer electrolytehave ever been undertaken. In [8], studies of this kindwere performed for a catalyst for an oxygen electrode,which consisted of platinum (20 wt %) on carbon blackVulcan XC-72 (average size platinum and carbon blackcrystallites 2.3 and 30 nm, respectively). The interac-tion between the catalyst and the ionomer was investi-gated by methods of scanning and transmission micros-copy and X-ray diffractometry. The researchers discov-ered considerable differences in the size of carbonblack particles and Nafion’s micelles, and a tendencytowards agglomeration of micelles with the formationof an inverted micellar structure. The average size ofthe Nafion micelles was found to substantially dependon the degree of dilution of the Nafion solution. For a5-% Nafion solution diluted with water in a 1 : 1 ratio,the average size of micelles or agglomerates was foundto be 200 nm. The authors of [8] analyzed the obtaineddata and found no appreciable changes in the structuralproperties of Nafion and catalyst as compared withproperties of the source components of the composite;nor did they discover any chemical interaction betweenNafion and platinum. They noted two major problems.One was the absence of an intimate interaction betweenNafion and the catalyst, which was attributed to a sub-stantial difference in their size (200 and 30 nm, respec-

tively). The other involved the absence of any percepti-ble mutual penetration of particles of Nafion and thecatalyst, which led to some limitations on the ion trans-port in the composite electrode. Apparently, there is noground to assume that the system Co-pyropolymer +carbon black will much differ from the system platinum +carbon black.

The ratio between the size of agglomerates ofNafion and particles of carbon black with a pyropoly-mer is close to that for Nafion and platinum on carbonblack. On this basis we feel it justified to explain thedecrease we observed in the specific activity of the cat-alyst following an increase in its weight (layer thick-ness) precisely by a limitation on the ion transport intothe reaction zone. Besides, an increase in the active-layer thickness gives rise to limitations on the oxygensupply as well. All these hindrances may, probably, becircumvented by optimizing the ratio between the num-ber and size of constituents of the active layer.

REFERENCES

1. Srinivasan, S., in Electrochemistry in Transition, Mur-phy, O.J., Srinivasan, S., and Conway, B.E., Eds., NewYork: Plenum, 1992, p. 577.

2. Tarasevich, M.R. and Radyushkina, K.A., Elek-trokhimiya, 1986, vol. 22, p. 1155.

3. Tarasevich, M.R., Radyushkina, K.A., andBogdanovskaya, V.A., Elektrokhimiya porfirinov (TheElectrochemistry of Porphyrins), Moscow: Nauka, 1991.

4. Ladouceur, M., Lalande, G., Guay, D., et al., J. Electro-chem. Soc., 1993, vol. 140, p. 1974.

5. Tamizhmani, G., Dodelet, J.P., Guay, D., and Lalande, G.,J. Electrochem. Soc., 1994, vol. 141, p. 41.

6. Lalande, G., Cote, R., Tamizhmani, G., et al., Electro-chim. Acta, 1995, vol. 40, p. 2635.

7. Mosdale, R. and Stevens, P., Solid State Ionics, 1993,vol. 61, p. 251.

8. Arico, A.S., Creti, P., Antonucci, P.L., et al., Electro-chim. Acta, 1998, vol. 43, p. 3719.

9. Bett, J.S., Kunz, H.R., Aldykiewicz, A.J., et al., Electro-chim. Acta, 1998, vol. 43, p. 3645.

10. Gouerac, P., Biloul, A., Contamin, O., et al., J. Electroa-nal. Chem., 1997, vol. 422, p. 61.

11. Gouerac, P. and Savy, M., Electrochim. Acta, 1999,vol. 44, p. 2653.

12. Tyurin, V.S., Radyushkina, K.A., Levina, O.A., andTarasevich, M.R., Elektrokhimiya, 2001, vol. 37, p. 981.

13. Zhutaeva, G.V., Radyushkina, K.A., Marinich, M.A.,et al., Elektrokhimiya, 2001, vol. 37, p. 1224.

14. Yeo, S.C. and Eisenberg, A., J. Appl. Polym., 1977,vol. 21, p. 875.

15. Falk, M., Can. J. Chem., 1980, vol. 58, p. 1495.16. Yeager, H.L. and Steck, A., J. Electrochem. Soc., 1981,

vol. 128, p. 1880.17. Litt, M.A., Polym. Prepr. (Am. Chem. Soc., Div. Polym.

Chem.), 1997, vol. 38, p. 80.