Electrocatalytic methanol oxidation at PTFE-bonded electrodes for direct methanol-air fuel cell

E/ecrrochmica Am. Vol 37, No 8, pi 1351-1353. 1992 Pnnted III Great Bntm

lWl3-4686/92 S5 00 + 0 00 PagmoDResrLtd

ELECTROCATALYTIC METHANOL OXIDATION AT PTFE-BONDED ELECTRODES FOR DIRECT METHANOL-AIR FUEL CELL G BRONOEL,*S BESET and N TAssrNt *LEI/CNRS, 1 place An&de Bnand, 92190 Meudon, France, TSORAPEC, 192 avenue Carnot, 94124 Fontenay-sous-Bols Cedex, France (Recewed

9 July 1991, VI revned form 29 October

1991)

Abstrati-Alcohol oxldatlon at PTFE-bonded electrodes has been stud& wth anodes fed w-~thgaseous methanol Polanzatlon curves show actlvltles 500 times higher than those obtamed with methanol m solution Key words fuel cell, methanol-air fuel cell, methanol oxldatlon, PTFE-bonded electrode, electrocatalyas

INTRODUCTION PTFE-bonded porous electrodes have proved to be the most efficient electrode structure for oxygen reduction and for hydrogen oxldatlon Indeed, this structure allows a fast dtffuslon of the reactive substances to the active sites, so very high current densities can be obtained at low temperatures, even with small quantities of catalyst For H2/02 (or H,/alr) fuel cells, the electrodes work with gaseous reactives On the contrary, all the results published up to now concerning direct methanol oxldatlon involve PTFE-bonded anodes dipped m an electrolyte containing the alcohol In these condlhons, platinum or platmum-ruthenmm alloys dispersed on carbon exhlblt an open circuit voltage of 0 1-O 2 V vs nhe at temperatures up to 60°C For the best catalyst, re Pt-Ru alloy dispersed on ICetJenblack[l], steady state current densities are approximately equal to 10 mA cmm2 at a potential of 0 3 V/nhe, platinum dispersed on carbon or platinum black exhibit lower performances[2-4] In the hterature, high current densities are sometimes reported but at potential higher than 0 5-O 6 V Moreover, potentlodynamlc measurements reported by Aramata and co-workers[5] have shown that methanol oxldatlon occurs at a potential of approximately 0 4 V, these results have been obtained with platinum or modified platinum bonded to a sohd polymer electrolyte An mhlbltlon of hydrogen ox+ datlon at lower potential has also been observed From these data and from our previous results[6] our work has been focused on the development of porous carbon anodes for Qrect alcohol oxldatlon The goal of the present paper 1s to descnbe the very ongmal results obtained with PTFE-bonded electrodes fed with gaseous methanol

RESULTS Two types of catalyst have been used, either platinum dispersed on carbon or nuxed tungsten-

molybdenum carbide dispersed on carbon Results obtained with gaseous methanol are compared to those correspondmg to similar electrodes dipped m an electrolyte containing methanol The catalyst loading on our PTFE-bonded anodes 1s 1 mg cm -2 for platinum and 2 mg cmm2 for nuxed tungsten-molybdenum carbide Figure 1 shows the cychc voltamograms obtained with a platmum-based electrode dipped m a 5 M CH, OH, 2 5 M sulfunc acid electrolyte mamtamed at 65”C, on these curves, methanol oxldatlon occurs at a potential of 0 4-O 5 V/nhe Figure 2 shows the steady-state polanzatlon curves obtained wth the platinum and the mlxed carbIde tested m the same conditions At a potential of 0 4 V/nhe, 0 15 mA cmm2 current densities are obtamed with platinum and 0 03 mA cme2 wth the mixed carbide It should be noted that the overpotential was limited to 0 4 V only m order to avoid the carbide oxldatlon

100 -

If

200

400

ElmV

600

800

1000

vs nhe

Fig 1 Cychc voltammograms on a Pt F+TFE-bondedelectrode m 2 5 M H,SO, at 65”C, with and wrthout CH,OH, scan rate, 5mV s-l, dotted curve, mth SM CH,OH and sohd curve unthout, CH,OH 1351

G BRONO~Let al

1352

Fig 2 Effectof catalyst on the steady state current measure-

E ImV vs nhe

ments on PTFE-bonded electrodes m 2 5 M H,SO, and 5 M CH,OH at 65”C, (x) Pt. (A) C(W-MO SO/SO)

Fig 3 Polanzauon curves for methanol oxtdatron in 2 5 M

Figure 3 grves the polarization curves obtamed on similar electrodes with gaseous methanol For these tests, the cell was maintained at 65°C in a thermostated oven The methanol feedmg was made directly from a fuel reserve heated at 65-70°C with a heatmg mantle, this system being also m the oven The distance between the alcohol reserve and the test cell entrance was lower than 1Ocm By thts method, the gaseous methanol fed directly the back side of the FIFE bonded electrode without any treatment One can see m Frg 4 a schematrc view of the expenmental mountmg The open ctrctut voltage is equal to 0 03 V and at a potential of 0 1 V/nhe a 17 mA cm-’ current density is obtained on platinum and a 2 5 mA cm-* on mixed carbide If the potential is fixed to 0 4 V/&e, the steady state current densities reach 70 mA cm-* on platmum and 14mA cm-* on mixed carbide These results show that when gaseous methanol 1s used, mstead of methanol m solution, current densities are 500 times lngher Nevertheless, the ratio between platinum and carbide activities IS constant and equal to 5 at 0 4 V/nhe

New experiments have been effected wtth anodes dipped m an electrolyte contammg very htgh concentration of methanol, higher than 15 M No increase of performances has been detected compared to the first tests These results indicate that the electrode polartzation only depends on the methanol way of feeding The work IS now contmumg m order to explam this phenomena One of our first hypotheses is the relatton between the high activity and a very fast dtssoctatrve adsorption m gaseous phase Indeed one can suppose that a catalytic drssoctatton of the methanol occurs on the dry catalytic sites of the electrode gtvmg hydrogen which will then be oxtdmed on the wetted sites

H,SO, at 65°C with gaseous CH,OH, ( x ) Pt, (A) C(WMo SO/SO)

Acknowledgement-The authors are grateful to the Agence Francaise pour la maltnse de l’enerse (AFME) for its financial support REFERENCES

I S Swathirajan and Y M M&had, J e/ec&ochem Sot 138, 5 (1991)

Teflon cell RE

CE MeOH vap t out

ErapWhp collector

W E

PTFE bonded

electr --

Methanol

-

Heating

Fig 4 Schematic view of the expenmental mounting

reserve

mantle

Electrocatalytlc methanol oxldatlon 2 J B Goodenough, A Hamnett, B J Kennedy, R Manoharan and S A Weeks, Electrochtm Acta 35,199 (1990) 3 B J Kennedy and A Hamnett, J electroanal Chem 283, 271 (1990) 4 J B Goodenough, A Hamnett, B J Kennedy and S A

1353

Weeks, Electrocham Acta 32, 1233 (1987) 5 A Aramata, T Kodera and M Masuda, J appl Electrochem 18,577 (1988) 6 G Bronoel, E Museux, G Leclenq, L Leclercq, S Besse, R Rougct, 40th ISE Mectmg, 1989, Vol I, p 305