WO1994001897A1

WO1994001897A1 - Fuel cell

Info

Publication number: WO1994001897A1
Application number: PCT/FR1993/000646
Authority: WO
Inventors: Guy Bronoel; Serge Besse; Noëlle TASSIN
Original assignee: Sorapec S.A.
Priority date: 1992-07-06
Filing date: 1993-06-28
Publication date: 1994-01-20
Also published as: FR2693315A1; FR2693315B1

Abstract

The fuel cell comprises at least two porous electrodes, advantageously of the triple contact type. A catalyst is dispersed at the surface of the pores, and the pores and inner surface of the electrodes are coated with a solid film of a protonic conducting polymer. The inner surface of the electrodes encloses a porous matrix filled with an acid electrolyte. Said acid electrolyte can be immobilized in a gel formed between the electrolyte and an acid-resistant gelling substance, such as silica. The catalyst surface can be partially covered by a hydrophobic polymer film, such as polytetrafluoroethylene, in the areas left uncovered by the protonic conducting polymer.

Description

FUEL CELL

The present invention relates to a gaseous fuel cell, of the hydrogen-oxygen or methanol-oxygen cell type.

In recent years, very interesting performances have been obtained with hydrogen-oxygen fuel cells in which the electrolyte consists of a polymer membrane with proton conduction. The most efficient system comprises electrodes having a so-called triple contact structure, the connection between the catalyst dispersed on the surface of the pores of the electrode and the membrane being ensured by a film of polymer which is also a proton conductor.

The Applicant has previously demonstrated that the efficiency of the cell does not depend on the use of a membrane as an electrolyte, but essentially on the mode of transport of the species and of the charges in the field where catalyst, electrolyte film coexist solid and gas.

It should also be noted that the use of membranes has various drawbacks.

Thus, for membrane thicknesses which can be easily produced industrially, the resistance induced by the membrane is greater than that of a conventional electrolyte of substantially equivalent thickness, even for the best existing materials.

In addition, maintaining a good hydration balance of the membrane proves to be a very delicate point, the transport of water molecules being more difficult within a membrane than in a liquid electrolyte. Finally, the cost of membranes is a major obstacle to the development of this type of battery.

Currently, we find in the literature of. . . current densities of up to 2 A / cm, minimizing

₂ thus the membrane surface to be used (ie 0.1 m / kW); however these values relate to electrodes having high levels of platinoids and are obtained for relatively low cell voltages

(0.5 V-0.6 V). In many applications, for example for electric vehicles, it will be necessary to operate with high yields and electrodes devoid of platinum, which will lead to current densities five to ten times lower. Under these conditions, even in large production hypotheses, the use of membranes would lead to unacceptable additional costs.

The difficulties set out above are overcome by a fuel cell without a polymer membrane, as described in the disclosure of the invention FR 2 667 728 by the Applicant, which comprises at least two porous electrodes, advantageously of the triple contact type, a catalyst being dispersed on the surface of the pores. The acid electrolyte impregnating the porous matrix is a liquid and the proton-conducting polymer film is a solid film, preferably continuous.

The capillary retention forces of the liquid electrolyte in the matrix must necessarily be greater than those created by the micro-porosity of the electrodes, in order to avoid rapid flooding of the latter. In practice, however, especially at the industrial level, the physical nature of the electrolyte located between the two electrodes soaking the porous matrix can be a crucial point, the above-mentioned characteristics being difficult to achieve and to maintain with a liquid.

The present invention provides a solution to this problem, by proposing a fuel cell of the type described. in the presentation FR 2 667 728 to which it is expressly referred here.

It first relates to a cell which comprises at least two porous electrodes, advantageously of the triple contact type, a catalyst being dispersed on the surface of the pores. The pores and the internal face of the electrodes are coated with a solid film consisting of a proton-conducting polymer. The electrodes enclose by their internal face a porous matrix filled with an acid electrolyte, which is immobilized within a gel formed between the electrolyte and a gelling substance resistant in acid medium, for example a mineral substance, such as silica .

The gel may contain, if necessary, mineral or organic stabilizing agents, stable in an acid medium. It can also be introduced into the retention matrix with a fibrous alveolar structure or of the sieve type.

The idea of gelling an acid electrolyte like H ₂ S0 ₄ has already been applied to Pb / Pb0 ₂ lead-acid batteries. However the operation of the accumulator, where the ions S0 ₄ ^{2 ~} involved in the electrode reaction, is different from a stack of H _2/0 _2, where only H + ions participate in the reaction.

Another aspect of the invention starts from the observation, unexpected and independently made by the Applicant, that a catalyst such as platinum, covered on certain parts of its surface by a film of polymer with proton conduction and on the free parts by a hydrophobic polymer such as polytetrafluoroethylene exhibited an electrocatalytic reactivity with respect to hydrogen (oxidation of H ₂ to H ⁺ ) much greater than that obtained if the surface was covered only with the proton-conducting polymer. Knowing that polytetrafluoroethylene is not a proton conductor, it has been necessary to deduce that the surfaces covered by the hydrophobic polymer seem to constitute a "source" supply of hydrogen to the catalytic sites in contact with the proton polymer.

The invention therefore also relates to a fuel cell, which comprises at least two porous electrodes, advantageously of the triple contact type, a catalyst being dispersed on the surface of the pores. The pores and the internal face of the electrodes are coated with a solid film consisting of a proton-conducting polymer, and the electrodes enclose by their internal face a porous matrix filled with an acid, liquid or gel electrolyte. The catalyst surface is partially covered, at the locations left free by the proton-conducting polymer, with a film of hydrophobic polymer. The hydrophobic polymer is for example polytetrafluoroethylene (PTFE) or fluoroethylene-propylene (FEP).

It goes without saying that both the proton-conducting polymer and the hydrophobic polymer must be stable in the acidic environment prevailing in the fuel cell.

The matrix is a porous structure which can be made of polyethylene or polypropylene mixed with a fine silica powder, as described in the description FR 2 667 728 already cited.

As is well known, the catalyst is advantageously platinum or else a mixture of platinoids for the hydrogen electrode and an organometallic compound for the oxygen electrode; the proton-conducting polymer is advantageously a fluorinated polymer or

- _ perfluorinated carrier of SO3 groups.

Advantageously, the relative proportions of polymers are such that 40 to 90% of the total surface of the catalyst is covered with proton-conducting polymer and 60 to 10% with the hydrophobic polymer.

The porous structure has the dual function of supplying gas (gas diffusion layer) and charge collection. This porous structure is then filled using a mixture consisting of the hydrophobic polymer forming a binder and carbon powder, carbon black for example. The quantity of hydrophobic polymer is chosen, in the proportions mentioned above, in order to obtain the best compromise between hydrophobicity and electronic conductivity.

On one side of this porous structure, an active layer is then produced in the form of a very thin film, where the electrocatalytic reaction will take place. The layer comprises the catalyst support carbon, the hydrophobic polymer and the proton conductor.

Power electrodes are thus easily produced.

The invention will be better understood in relation to the examples which follow, given by way of nonlimiting examples.

Examples 1 to 6

A triple-contact electrode is manufactured by a rolling-compacting technique or by mechanical coating on a substrate of a mixture comprising a carbon black coated with a dispersion of platinum at a rate of 10% by mass relative to the carbon, and a binder.

The substrate is a metallic gold foam, or a porous carbon matrix of the brand Stackpole PC 206 from the company Stackpole Corporation, or of the brand Panex P B 3 from the company Zoltek, in accordance with the table below.

The platinum carbon black, of the brand Vulcan XC 72 of the firm Cabot or of the brand Printex XE 2 of the firm of Degussa, is suspended in water, then the binder is introduced, polytetrafluoroethylene (PTFE 30 N ) by Dupont De Nemours, in the form of a suspension. The respective proportions are such that the ratio of the mass of binder on the mass of carbon and platinum is 0.3, it being understood that this ratio can range from 0.1 to 0.6.

Examples Substrate Platinized carbon black 1 metallic gold foam Vulcan XC 72 2 metallic gold foam Printex XE 2 3 Stackpole PC 206 Vulcan XC 72 4 Stackpole PC 206 Printex XE 2 5 Panex PWB 3 Vulcan XC 72 6 Panex PWB 3 Printex XE 2

After complete drying, the above mixture is taken up in an organic solvent, for example isopropanol, and the electrode is prepared by rolling and / or pasting, then heat treated at 300 ° C. for 2 hours.

This electrode is impregnated with a mixture consisting of a 5% solution of Nafion 117 and a suspension of PTFE 30 N from Dupont De Nemours, in such a way

~ that the mass of Nafion 117 deposited is 1 mg / cm, and that of PTFE 0.5 mg / cm ² , it being understood that these ratios can

~ - vary between 0.5 and 10 g / crrr and 0.1 and 2 π-g / cπr respectively.

Nafion (trademark of DuPont de Nemours) is a perfluorinated polymer with proton conduction, carrying SO ² groups.

After drying, two electrodes thus formed are placed in an assembly which plates them on a porous polyethylene matrix having a thickness of 0.2 mm and soaked with a 2N solution of H ₂ S0 ₄ -

By supplying one of the electrodes and the other with H _{2 at 0} with an overpressure of about 50 g / cm, it is found that at 23 ° C. the system delivers under a voltage

0.7 V a current of the order of 300 mA / cm ² . This value is at compare to that of electrodes impregnated with a solution of Nafion 117 alone and which is 200 mA / cm ² .

Examples 7 to 12

Two electrodes manufactured and impregnated as described above in Examples 1 to 6 are placed in an assembly which plates them on a porous polyethylene matrix having a thickness of 0.2 mm. The difference from these examples is that the electrolyte is a gel made up of a mixture of H ₂ S0 ₄ 2N and Si0 ₂ , the content by weight of Si0 ₂ being 7% in the electrolyte.

This system, tested under the same conditions as above, has comparable characteristics, while avoiding the problems which a liquid electrolyte can pose.

It is further observed that the properties of the gel can be further improved by the addition of a stabilizing compound, for example polyacryla ide.

Claims

1. Fuel cell, comprising at least two porous electrodes, advantageously of the triple contact type, a catalyst being dispersed on the surface of the pores, the pores and the internal face of the electrodes being coated with a solid film made of a polymer with proton conduction, and the electrodes enclosing by their internal face a porous matrix filled with an acid electrolyte, characterized in that the acid electrolyte is immobilized within a gel formed between the electrolyte and a gelling substance resistant in medium acid.

2. Fuel cell according to claim 1, characterized in that the gelling substance is a mineral substance.

3. Fuel cell according to claim 2, characterized in that the mineral substance is silica.

4. Fuel cell according to claim 1 to 3, characterized in that the gel contains an inorganic or organic stabilizing agent.

5. Fuel cell comprising at least two porous electrodes, advantageously of the triple contact type, a catalyst being dispersed on the surface of the pores, the pores and the internal face of the electrodes being coated with a solid film consisting of a polymer with proton conduction, and the electrodes enclosing by their internal face a porous matrix filled with an acid electrolyte, characterized in that the surface of the catalyst is partially covered, at the locations left free by the proton-conducting polymer, with a film of hydrophobic polymer.

6. Fuel cell according to claim 5, characterized in that the electrolyte is a liquid electrolyte.

7. Fuel cell according to claim 5, characterized in that the electrolyte is an electrolyte immobilized in a gel according to claim 1.

8. Fuel cell according to one of claims 5 to 7, characterized in that the hydrophobic polymer is polytetrafluoroethylene.

9. Fuel cell according to one of claims 5 to 8, characterized in that the relative proportions of polymers are such that 40 to 90% of the total surface of the catalyst is covered with proton-conducting polymer and 60 to 10% by the hydrophobic polymer.