WO2012073085A1

WO2012073085A1 - Membrane-electrode unit and fuel cells with improved lifetime

Info

Publication number: WO2012073085A1
Application number: PCT/IB2011/002781
Authority: WO
Inventors: Oliver Gronwald; Detlef Ott; Seonghan Yu; Sigmar BRÄUNINGER; Thomas Justus Schmidt
Original assignee: Basf Se; Basf (China) Company Limited
Priority date: 2010-12-02
Filing date: 2011-11-21
Publication date: 2012-06-07
Also published as: DE112011104001A5

Abstract

The present invention relates to a membrane-electrode unit comprising at least two electrochemically active electrodes which are separated by at least one polymer-electrolyte membrane, the aforementioned polymer-electrolyte membrane having at least one reinforcement, characterized in that the reinforcement comprises at least one film that has holes through which the polymer-electrolyte membrane is in contact with both electrochemically active electrodes. The membrane-electrode unit is suitable for applications in fuel cells, in particular in high-temperature polymer-electrolyte fuel cells.

Description

description

Membrane electrode assembly and fuel cells with improved lifetime

The present invention relates to membrane-electrode assemblies and

Fuel cells with improved life, the least two

electrochemically active electrodes separated by a polymer electrolyte membrane.

Polymer electrolyte membrane (PEM) fuel cells are already known. In them are currently almost exclusively sulfonic acid-modified polymers as

used proton-conducting membranes. Here are predominantly perfluorinated polymers application. Prominent example is Nation ™ by DuPont de Nemours, Willmington USA. For the proton conduction is a relatively high

Required water content in the membrane, which is typically at 4 - 20 molecules of water per sulfonic acid group. The necessary water content, but also the stability of the polymer in combination with acidic water and the reaction gases hydrogen and oxygen, usually limits the operating temperature of the PEM fuel cell stacks to 80-100 ° C. Under pressure, the

Operating temperatures can be increased to> 120 ° C. Otherwise, higher

Operating temperatures can not be realized without a loss of power of the fuel cell.

For technical reasons, however, higher operating temperatures than 100 ° C in the fuel cell are desirable. The activity of the noble metal-based catalysts contained in the membrane-electrode assembly (MEU) is high

Operating temperatures much better. In particular, when using so-called reformates of hydrocarbons significant amounts of

Carbon monoxide contained in the reformer gas, which usually must be removed by a complex gas treatment or gas cleaning. At high

Operating temperatures increases the tolerance of the catalysts to the CO impurities.

Furthermore, heat is generated during the operation of fuel cells. However, cooling these systems to below 80 ° C can be very expensive. Depending on

Power output, the cooling devices can be made much simpler. This means that in fuel cell systems that are operated at temperatures above 100 ° C, the waste heat made much better usable and thus the fuel cell system efficiency can be increased by electricity-heat coupling.

In order to reach these temperatures, membranes with new conductivity mechanisms are generally used. One approach for this is the use of membranes, which show an electrical conductivity without the use of water. The first promising development in this direction is set out in document WO 96/13872.

Since the tapped voltage of a single fuel cell is relatively low, several membrane-electrode units are generally connected in series and connected to each other via planar separator plates (bipolar plates). The membrane-electrode assemblies and separator plates must be included

relatively high pressures are pressed together to achieve the highest possible density of the system, the highest possible performance and the lowest possible volume.

However, in practice, the compression of the membrane-electrode assemblies with the Separatorplatten often causes problems, since the polymer electrolyte membrane used have a comparatively low mechanical strength and stability and therefore can be easily damaged during pressing.

Furthermore, it is difficult to achieve reproducible results due to the required high compression of the polymer electrolyte membrane on the one hand and their low mechanical stability on the other hand. In most cases, the resulting fuel cell stacks have greatly fluctuating performances due to more or less pronounced cracks in the individual

Membranes and / or caused by different degrees of compression of the membranes. In addition, a creep of the electrolyte membrane is observed in long-term operation that is at least partially due to the low mechanical stability.

Object of the present invention was therefore to provide membrane electrode assemblies and fuel cells with the highest possible performance that can be produced in the simplest possible way, on a large scale, as inexpensively and reproducibly as possible and also have an improved life. The fuel cells should preferably have the following properties:

• The fuel cells should last as long as possible.

• The fuel cells should be operated at the highest possible operating temperatures,

especially above 100 ° C, can be used.

• Single cells should show consistent or improved performance over the longest possible period of operation.

• The fuel cells should be as high as possible after a long period of operation

Quiescent voltage and the lowest possible gas passage have (gas crossover). Furthermore, they should be able to be operated at the lowest possible stoichiometry.

• The fuel cells should, if possible, do without additional fuel gas humidification.

• The fuel cells should be able to withstand permanent or changing pressure differences between anode and cathode in the best possible way.

• In particular, the fuel cells should be robust against different ones

Operating conditions (T, P, geometry, etc.) to be general

To increase reliability as best as possible.

• Furthermore, the fuel cells should have an improved temperature and

Corrosion resistance and a comparatively low

Gas permeability, especially at high temperatures have. A decrease in mechanical stability and structural integrity, especially at high temperatures, should be avoided as much as possible.

These tasks are solved by a single fuel cell with all

Features of claim 1.

The subject matter of the present invention is accordingly a membrane-electrode unit which has at least two electrochemically active electrodes, which are separated by at least one polymer electrolyte membrane, comprises the abovementioned polymer electrolyte membrane has at least one reinforcement, characterized in that the reinforcement comprises at least one foil having holes through which the polymer electrolyte membrane is in contact with both electrochemically active electrodes.

The polymer electrolyte membranes or polymer electrolyte materials suitable for the purposes of the present invention are known per se. Preferably For example, membranes are used which comprise acids, which acids may also be covalently bound to the polymers.

Basic polymers preferably form the polymer in the polymer electrolyte membrane. These membranes comprise acids or are doped with acids, so that an acid-base complex forms between the basic polymer and the acid. As such basic polymer membranes are almost all known polymer membranes into consideration, in which the protons can be transported. In this case, acids are preferred which release protons without additional water, e.g. by means of the so-called Grotthus mechanism.

As the basic polymer in the context of the present invention is preferably a basic polymer having at least one nitrogen, oxygen or

Sulfur atom, preferably at least one nitrogen atom, in one

Repeat unit used. Furthermore, basic polymers comprising at least one heteroaryl group are preferred.

The repeating unit in the basic polymer according to a preferred embodiment contains an aromatic ring having at least one nitrogen atom. The aromatic ring is preferably a five- or

six-membered ring having one to three nitrogen atoms which may be fused to another ring, especially another aromatic ring.

According to a particular aspect of the present invention

high-temperature-stable polymers are used which contain at least one nitrogen,

Oxygen and / or sulfur atom in one or in different

Repeating units included.

High temperature stability in the context of the present invention is a polymer which can be operated as a polymeric electrolyte in a fuel cell at temperatures above 120 ° C permanently. Permanently means that a membrane according to the invention at least 100 hours, preferably at least 500 hours, at least 80 ° C, preferably at least 120 ° C, more preferably

at least 160 ° C, without the performance, which can be measured according to the method described in WO 01/18894 A2, by more than 50%, based on the initial power decreases.

In the context of the present invention, all the abovementioned polymers can be used individually or as a mixture (blend), also with other polymers. Blends which contain polyazoles and / or polysulfones are particularly preferred. The preferred blend components are polyethersulfone,

Polyether ketone and sulfonic acid-modified polymers, as described in International Publication WO 02/36249 and WO 2004/034500.

Furthermore, for the purposes of the present invention have also

Polymer blends are particularly useful, which comprise at least one basic polymer and at least one acidic polymer, preferably in a weight ratio of 1: 99 to 99: 1 (so-called acid-base polymer blends). Particularly suitable acidic polymers in this context include polymers having sulfonic acid and / or phosphonic acid groups. Very particularly suitable acid-base polymer blends according to the invention are described in detail, for example, in the publication EP1073690 A1.

A particularly preferred group of basic polymers are polyazoles. A basic polymer based on polyazole contains recurring azole units of the general formula (I) and / or (II) and / or (III) and / or (IV) and / or (V ) and / or (VI) and / or (VII) and / or (VIII) and / or (IX) and / or (X) and / or (XI) and / or (XII) and / or (XIII) and / or (XIV) and / or (XV) and / or (XVI) and / or (XVII) and / or (XVIII) and / or (XIX) and / or (XX) and / or (XXI) and / or (XXII)





wherein

Ar are the same or different and represent a four-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{1 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{2 is the} same or are different and represent a two- or three-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{3 are the} same or different and are a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{4 are the} same or different and represent a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{5 are the} same or different and are a four-membered aromatic or

heteroaromatic group, which may be mononuclear or polynuclear,

Ar ^{6 are the} same or different and are for a divalent aromatic or

heteroaromatic group, which may be mononuclear or polynuclear,

Ar ^{7 are the} same or different and are for a divalent aromatic or

heteroaromatic group, which may be mononuclear or polynuclear,

Ar ^{8 are the} same or different and for a trivalent aromatic or

heteroaromatic group, which may be mononuclear or polynuclear,

Ar ^{9 are the} same or different and are a two- or three- or vierbindige

aromatic or heteroaromatic group, which may be mononuclear or polynuclear,

Ar ^{10 are the} same or different and represent a divalent or trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{11 are the} same or different and are for a divalent aromatic or

heteroaromatic group, which may be mononuclear or polynuclear,

X is the same or different and is oxygen, sulfur or a

Amino group which carries a hydrogen atom, a 1-20 carbon atoms having group, preferably a branched or unbranched alkyl or alkoxy group, or an aryl group as a further radical

R is the same or different than hydrogen, an alkyl group or a

is an aromatic group and in formula (XX) is an alkylene group or an aromatic group, with the proviso that R in formula (XX) is other than hydrogen, and

n, m is an integer greater than or equal to 10, preferably greater than or equal to 100.

Preferred aromatic or heteroaromatic groups are derived from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulfone, quinoline, pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine, tetrazine, pyrol, pyrazole, anthracene, benzopyrrole, benzotriazole,

Benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazine, indolizine, quinolizine, pyridopyridine, imidazopyrimidine, pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline, phenoxazine,

Phenothiazine, acridizine, benzopteridine, phenanthroline and phenanthrene, which may optionally be substituted from.

Here, the substitution pattern of Ar ¹ , Ar ⁴ , Ar ⁶ , Ar ⁷ , Ar ⁸ , Ar ⁹ , Ar ¹⁰ , Ar ^{11 is} arbitrary, in the case of phenylene, for example, Ar ¹ , Ar ⁴ , Ar ⁶ , Ar ⁷ , Ar ⁸ , Ar ⁹ , Ar ¹⁰ , Ar ¹¹ Ortho, meta- and para-phenylene. Particularly preferred groups are derived from benzene and biphenylene, which may optionally also be substituted.

Preferred alkyl groups are short chain alkyl groups of 1 to 4

Carbon atoms, such as. For example, methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. The

Alkyl groups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms such as. For example, fluorine, amino groups, hydroxy groups or short-chain alkyl groups, such as. For example, methyl or ethyl groups.

Preference is given to polyazoles having repeating units of the formula (I) in which the radicals X within a repeating unit are identical.

The polyazoles can in principle also have different recurring

Have units that differ, for example, in their remainder X.

Preferably, however, it has only the same X radicals in a repeating unit.

Other preferred polyazole polymers are polyimidazoles, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (pyridines), poly (pyrimidines) and poly (tetrazapyrenes).

In a further embodiment of the present invention, the polymer containing recurring azole units is a copolymer or a blend containing at least two units of the formulas (I) to (XXII) which differ from each other. The polymers can be present as block copolymers (diblock, triblock), random copolymers, periodic copolymers and / or alternating polymers.

In a particularly preferred embodiment of the present invention, the polymer containing recurring azole units is a polyazole which contains only units of the formula (I) and / or (II).

The number of repeating azole units in the polymer is preferably an integer greater than or equal to 10. Particularly preferred polymers contain at least 100 recurring azole units. In the context of the present invention, polymers containing recurring benzimidazole units are preferred. Some examples of the most useful polymers containing benzimidazole recurring units are represented by the following formulas:

where n and m are integers greater than or equal to 10, preferably greater than or equal to 100. In the case of the abovementioned benzimidazoles, it is also possible for one or more nitrogen heteroatoms to be replaced by other heteroatoms; preferably these other heteroatoms are oxygen and / or sulfur atoms. Such compounds are also to be subsumed under the name benzimidazole.

The polyazoles used, but especially the polybenzimidazoles are characterized by a high molecular weight. Measured as intrinsic viscosity this is at least 0.2 dl / g, preferably 0.8 to 10 dl / g, in particular 1 to 10 dl / g.

Preferred polybenzimidazoles are under the trade name Celazole®

commercially available.

The preferred polysulfones include in particular polysulfones having aromatic and / or heteroaromatic groups in the main chain. According to a particular aspect of the present invention, preferred

Polysulfones and polyethersulfones a melt volume rate MVR 300/21, 6 is less than or equal to 40 cm ^3/10 min, especially less than or equal to 30 cm ^3/10 min and particularly preferably less than or equal to 20 cm ^3/10 min measured according to IS01133. Here, polysulfones having a Vicat softening temperature VST / A / 50 of 180 ° C to 230 ° C are preferred. In still a preferred embodiment of the present invention, the number average molecular weight of the

Polysulfones greater than 30,000 g / mol.

The polymers based on polysulfone include, in particular, polymers which contain repeating units having linking sulfone groups corresponding to the general formulas A, B, C, D, E, F and / or G:

in which the radicals R independently of one another are identical or different

represent aromatic or heteroaromatic groups, these radicals have been previously explained in more detail. These include in particular 1, 2-phenylene, 1, 3-phenylene, 1, 4-phenylene, 4,4'-biphenyl, pyridine, quinoline, naphthalene, phenanthrene. Preferred polysulfones for the purposes of the present invention include homopolymers and copolymers, for example random copolymers. Particularly preferred polysulfones comprise recurring units of the formulas H to N:

The above-described polysulfones can 720 P, ^® Ultrason E, ^® Ultrason S, ^® Mindel, ^® Radel A, ^® Radel R, ^® Victrex HTA, ^® Astrel and ^® Udel be obtained commercially under the trade names Victrex 200 P, ^® Victrex.

In addition, polyether ketones, polyether ketone ketones, polyether ether ketones, polyether ether ketone ketones and polyaryl ketones are particularly preferred. These

High performance polymers are known per se and can be obtained commercially under the trade names Victrex® PEEK ™, ^® Hostatec, ^® Kadel. According to the present invention, the polymer electrolyte membrane has at least one film as a reinforcement, wherein the film has holes through which the polymer electrolyte membrane with both electrochemically active

Electrodes in contact.

The film used according to the invention can in this case be arranged in the membrane, preferably the film is arranged centrally and is completely through the

Polymer electrolyte of the membrane covered or the membrane is

sandwiched by two films according to the invention. In the sandwich-type arrangement, it must be ensured that the polymer electrolyte of the membrane is still in contact with the electrochemically active electrodes.

The membranes reinforced according to the invention can no longer be delaminated without destruction

The reinforcing films used according to the invention can in turn

be fiber reinforced. Suitable reinforcing fibers preferably comprise

Monofilaments, multifilaments, long and / or short fibers, hybrid yarns and / or bicomponent fibers. The reinforcing fibers of the film according to the invention may also form a textile surface, preferably a scrim and / or mesh.

The reinforcing foils used according to the invention have holes, so that the polymer electrolyte membrane is in contact with both electrochemically active electrodes. The shape of the holes and their arrangement are not limited. The holes may have symmetrical or asymmetrical shapes, the same applies to the arrangement of the holes. To be favoured

recurrent, geometric holes, in particular round, oval, elliptical, dumbbell and / or kidney-shaped holes and angular, in particular triangular, quadrangular, pentagonal and / or hexagonal holes, rhombic, diamond-shaped, square and / or diamond-shaped holes. The distribution of the holes may be uniform or uneven.

The holes can be produced by punching and / or cutting from the film web.

For diamond-shaped holes, the long intervals of the diamond holes (LWD) are in the range of 0.5 mm to 13 mm, the short intervals of the diamond holes (SWD) in the range of 0.25 mm to 8 mm, and the thickness of the lands (Strand Width) in the range of 0.15 mm to 2 mm. Such reinforcing films are particularly well suited.

The thickness of the reinforcing films used according to the invention is

preferably 5 m to 80 pm, more preferably 10 to 70 m, in particular 15 to 60μητι. The thickness can be determined by electronic image analysis of an optical or TEM image.

The reinforcing films used according to the invention preferably have a tensile strength of at least 5 MPa, particularly preferably at least 10 MPa, in particular at least 14 MPa.

The reinforcing films used according to the invention preferably have an elongation at break in the range from 0.5% to 100%, preferably in the range from 1% to 60%.

The reference force is measured according to EN 29073, part 3 on 5 cm wide specimens with a measuring length of 100 mm. The numerical value of the prestressing force, indicated in

Centinewton [cN] is the numerical value of the basis weight of the sample, expressed in grams per square meter.

The reinforcing films used according to the invention have sufficient stability with respect to the polymer electrolyte material. Due to the

Operating temperature of the membrane electrode units of the invention of min. 120 ° C are suitable as materials for the reinforcing films used in the invention such materials whose continuous service temperature min. 120 ° C, preferably min. 160 ° C, especially min. 175 ° C, is. The open area of the film formed by the holes is preferably 10 to 98%, more preferably 15 to 95%, in particular 20 to 90%, of the total surface area of the film.

Suitable polymers for the reinforcing films used according to the invention are in particular polytetrafluoroethylene (PTFE), polyfluoroethylene propylene (PFEP), polychlorotrifluoroethylene (PCTFE), polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene, with perfluoropropyl vinyl ether, with trifluoronitrosomethane, with carbalkoxy perfluoroalkoxy vinyl ether, and polysulfones, especially those described above polysulfones.

Other suitable polymers for the invention used

Reinforcing foils are in particular polyphenylene, poly (p-xylylene),

Polyarylmethylene, poly (N-vinylacetamide), polyvinylimidazole, polyvinylcarbazole, polyvinylpyrrolidone, polyvinylpyridine, polyvinyl chloride, polyvinylidene chloride,

Polyvinylidene difluoride, polyvinyl fluoride, polyvinylidene fluoride;

Polymers with C-O bonds in the main chain, for example polyphenylene oxide and / or polyether ketone;

Polymers having C-S bonds in the main chain, for example, polysulfide ethers, polyphenylene sulfide, polysulfones, polyethersulfone;

Polymers with C-N bonds in the main chain, for example polyimines,

Polyisocyanides, polyetherimine, polyetherimides, polyaniline, polyaramides, polyamides, polyhydrazides, polyurethanes, polyimides, polyazoles, polyazole ether ketone, polyazines, polybenzoxazole, polybenzoxazines; and / or inorganic polymers, for example polysilicates and / or

Polyphosphazenes.

In a preferred embodiment of the invention, the reinforcing sheet also includes those having functional chemical groups that form a covalent chemical bond between the reinforcing sheet polymer and the polymer of the polymer electrolyte membrane.

By chemical functionalization is meant the introduction of a functional group capable of forming a covalent chemical bond between the fibers and the polymer of the polymer electrolyte membrane. suitable Functional groups are preferably amines, carboxylic acids, carboxylic acid esters, carboxylic anhydrides, carboxylic acid halides, carboxylic acid amides, acetals, alcohols, ethers, sulfonic acid halides and halides.

Preferred polymers for the functionalized reinforcing films are those based on polyether ketone (PEK), polyether ether ketone (PEEK), polysulfone,

Polyethersulfone, polyazole, polyacrylonitrile (PAN), polyphosphazene,

Polyphenylene oxide, polyetherimide and polyaramid.

The abovementioned polymers are functionalized, preferably in the form of their film. For this purpose, the polymer is first nitrated and then reduced to the amine, or the polymer is first carboxylated then saponified in the corresponding carboxylic acid ester. Suitable procedures are described, for example, in WO 01/64322 and WO 01/64773.

The reinforcing foils thus have functional groups which are capable of reacting with the polymer of the polymer electrolyte membrane or, in a further structure of the polymers, react with these and form a covalent bond between the polymer of the foil-like reinforcement and the polymer of the polymer

Form polymer electrolyte membrane. Thus, it is advantageous if the

Polymer electrolyte membrane still monomers and / or oligomers constituents (each at least 1 wt .-%).

The polymer electrolyte membranes can be prepared in a manner known per se, wherein they are expediently provided directly with the reinforcing foils used according to the invention during their production, preferably by forming the polymer electrolyte membrane in the presence of the reinforcing foils used according to the invention.

In a preferred variant of the present invention, doped polyazole films are obtained by a process comprising the steps

A) mixing (i) one or more aromatic tetra-amino compounds with (ii) one or more aromatic carboxylic acids or their esters containing at least two acid groups per carboxylic acid monomer, or (iii) mixing one or more aromatic and / or (iv) heteroaromatic diaminocarboxylic acids, in polyphosphoric acid to form a solution and / or dispersion,

B) arranging the film comprising holes on a support, C) applying a layer using the mixture according to step A) on the support from step B) in such a way that the mixture penetrates the holes of the film and covers both sides of the film, preferably completely,

D) heating of the sheet / layer obtainable according to step C) under inert gas to temperatures of up to 350 ° C, preferably up to 280 ° C, in particular up to 230 ° C, to form the polyazole polymer, wherein the polyazole Polymer fills the existing holes so that it covers both sides of the film,

E) Treatment of the membrane formed in step D) (until it is self-supporting).

This variant requires the use of film materials whose melting point is above the temperature range mentioned in step D) and which have sufficient stability with respect to polyphosphoric acid or phosphoric acid.

Instead of the carrier in step B), the film according to the invention may also be used.

Furthermore, it is also possible to omit step B) and to perform the feeding of the reinforcing sheet before or during step D).

Furthermore, it is also possible to attach the hole-containing reinforcing film by pressure on both sides of the membrane, for example by means of pressure and / or temperature by suitable calenders, rollers, rollers or presses, so that a sandwich-like structure results. In this variant of the invention, it is advantageous if the polymer electrolyte material is still ductile, so that the holes are filled with polymer electrolyte material as described above.

The preparation of the abovementioned membrane, but without the incorporation of reinforcing foils, is described, for example, in WO 2004/033079, from which the skilled person can obtain further valuable information regarding steps A), C), D) and E). The corresponding membranes without reinforcing films are for example available under the trade name Celtec ^®.

The in step A) to be used aromatic or heteroaromatic

Carboxylic acid compounds preferably include di-carboxylic acids and tri-carboxylic acids and tetra-carboxylic acids or their esters or their anhydrides or their acid chlorides. The term aromatic carboxylic acids includes

equally heteroaromatic carboxylic acids. Preferably, the aromatic dicarboxylic acids are isophthalic acid, terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid, 5-aminoisophthalic acid, 5-N, N-dimethylaminoisophthalic acid, 5-N, N-diethylaminoisophthalic acid, 2.5 Dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalic acid, 2-fluoroterephthalic acid, tetrafluorophthalic acid, tetrafluoroisophthalic acid,

Tetrafluoroterephthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl ether-4,4'-dicarboxylic acid, benzophenone 4,4'-dicarboxylic acid, diphenylsulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid, 4-trifluoromethylphthalic acid, 2,2-bis (4-carboxyphenyl) -hexafluoropropane, 4,4'-stilbenedicarboxylic acid , 4-carboxycinnamic acid, or their C1-C20-alkyl esters or C5-C12-aryl esters, or their acid anhydrides or their acid chlorides.

The aromatic tri-, tetra-carboxylic acids or their C 1 -C 20 -alkyl esters or C 5 -C 12 -aryl esters or their acid anhydrides or their acid chlorides are preferably 1,3,5-benzenetricarboxylic acid (trimesic acid ), 1, 2,4-benzenetricarboxylic acid (trimellitic acid), (2-carboxyphenyl) -iminodiacetic acid, 3,5,3'-biphenyltricarboxylic acid or 3,5,4'-biphenyltricarboxylic acid.

The aromatic tetracarboxylic acids or their C 1 -C 20 -alkyl esters or C 5 -C 12 -aryl esters or their acid anhydrides or their acid chlorides are preferably 3,5,3 ', 5'-biphenyltetracarboxylic acid, 1, 2,4 , 5-Benzoltetracarboxylic acid, benzophenonetetracarboxylic acid, 3,3 ', 4,4'-biphenyltetracarboxylic acid, 2,2', 3,3'-biphenyltetracarboxylic acid, 1, 2,5,6-naphthalenetetracarboxylic acid or 1,5,5,8-naphthalenetetracarboxylic acid ,

The heteroaromatic carboxylic acids used are preferably heteroaromatic dicarboxylic acids or tricarboxylic acids or tetracarboxylic acids or their esters or their anhydrides. Heteroaromatic carboxylic acids are aromatic systems which contain at least one nitrogen, oxygen, sulfur or phosphorus atom in the aromatic.

Preferably pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-dicarboxylic acid, pyridinedicarboxylic acid, 3,5-pyrazoldicarboxylic acid, 2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid, 2,4,6-pyridinetricarboxylic acid or benzimidazole-5,6-dicarboxylic acid or their C1-C20-alkyl-ester or C5-C12-aryl Esters, or their acid anhydrides or their acid chlorides.

The content of tricarboxylic acid or tetracarboxylic acids (based on the dicarboxylic acid used) is between 0 and 30 mol%, preferably 0.1 and 20 mol%, in particular 0.5 and 10 mol%.

It is preferable that the aromatic and

heteroaromatic diaminocarboxylic acids to diaminobenzoic acid or their mono- and dihydrochloride derivatives.

Preference is given to using mixtures of at least 2 different aromatic carboxylic acids. Particular preference is given to using mixtures which, in addition to aromatic carboxylic acids, also contain heteroaromatic carboxylic acids. The mixing ratio of aromatic carboxylic acids to

heteroaromatic carboxylic acids is between 1:99 and 99: 1, preferably 1:50 to 50: 1.

These mixtures are, in particular, mixtures of N-heteroaromatic dicarboxylic acids and aromatic dicarboxylic acids. Non-limiting examples thereof are isophthalic acid, terephthalic acid, phthalic acid, 2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid. 3,4-dihydroxyphthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl ether-4,4 ' dicarboxylic acid,

Benzophenone-4,4'-dicarboxylic acid, diphenylsulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid, 4-trifluoromethylphthalic acid, pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid, pyridine-2 , 6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoldicarboxylic acid, 2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid.

The tetra-amino compounds to be used in step A) preferably comprise 3,3 ', 4,4'-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine, 1, 2,4,5-tetraaminobenzene, 3,3', 4,4'-tetraaminodiphenylsulfone, 3,3 ', 4,4'-tetraaminodiphenyl ether, 3,3', 4,4'-tetraaminobenzophenone, 3,3 ', 4,4'-tetraaminodiphenylmethane and 3,3', 4, 4 ' Tetraaminodiphenyldimethylmethane and salts thereof, in particular their mono-, di-, tri- and tetrahydrochloride derivatives.

The polyphosphoric acid used in step A) is

commercially available polyphosphoric acids, such as those available from Riedel-de Haen, for example. The polyphosphoric acids Η _{η +} 2Ρηθ3η ₊ ι (n> 1) usually have a content calculated as P2O5 (acidimetric) of at least 83%. Instead of a solution of the monomers, it is also possible to produce a dispersion / suspension.

The mixture produced in step A) has a weight ratio of polyphosphoric acid to sum of all monomers of 1: 10,000 to 10,000: 1, preferably 1: 1000 to 1000: 1, in particular 1: 100 to 100: 1, on.

The layer formation according to step C) takes place by means of measures known per se (casting, spraying, doctoring) which are known from the prior art for polymer film production. As the carrier, all under the conditions as internal to be designated carrier are suitable. To adjust the viscosity, the solution may optionally be treated with phosphoric acid (concentrated phosphoric acid, 85%). This allows the viscosity to be adjusted to the desired value and the formation of the membrane can be facilitated.

The layer produced in step C) has a thickness μιη 20-4000 ^μ η τι, preferably between 30 and 3500, particularly between 50 and μητι 3000th

Insofar as the mixture according to step A) also tricarboxylic acids or

Containing tetracarboxylic acid, thereby a branching / crosslinking of the polymer formed is achieved. This contributes to the improvement of the mechanical property.

The treatment of the polymer layer produced according to step D) is carried out in

Presence of moisture at temperatures and sufficient for a period of time until the layer has sufficient strength for use in fuel cells. The treatment can be carried out so far that the membrane is self-supporting, so that it can be detached from the carrier without damage.

According to step D), the planar structure obtained in step C) is set to a

Temperature of up to 350 ° C, preferably heated up to 280 ° C and more preferably in the range of 200 ° C to 250 ° C. The inert gases to be used in step D) are known in the art. These include in particular nitrogen and noble gases, such as neon, argon, helium.

In a variant of the process, by heating the mixture from step A) to temperatures of up to 350 ° C., preferably up to 280 ° C., the formation of oligomers and / or low molecular weight polymers can already be effected. In

Depending on the selected temperature and duration, then the heating in step D) can be omitted partially or completely. This variant is also the subject of the present invention.

The treatment of the membrane in step E) takes place at temperatures above 0 ° C and below 150 ° C, preferably at temperatures between 10 ° C and 120 ° C, in particular between room temperature (20 ° C) and 90 ° C, in the presence of moisture or water and / or water vapor and / or water-containing phosphoric acid of up to 85%. The treatment is preferably carried out under

Normal pressure, but can also be done under the action of pressure. It is essential that the treatment is carried out in the presence of sufficient moisture, whereby the present polyphosphoric acid by partial hydrolysis

Training low molecular weight polyphosphoric acid and / or phosphoric acid contributes to the solidification of the membrane.

The partial hydrolysis of the polyphosphoric acid in step E) leads to a

Solidification of the membrane and a decrease in the layer thickness and formation of a membrane having a thickness between 15 and 3000 μητι, preferably between 20 and 2000 pm, in particular between 20 and 1500 μιτι, which is self-supporting.

The intra- and intermolecular structures (interpenetrating networks IPN) present in the polyphosphoric acid layer according to step C) lead to an ordered membrane formation in step C), which is responsible for the particular properties of the membrane formed.

The upper temperature limit of the treatment according to step E) is generally 150 ° C. With extremely short exposure to moisture, for example from superheated steam, this steam may also be hotter than 150 ° C. Essential for the upper temperature limit is the duration of the treatment.

The partial hydrolysis (step E) can also be carried out in climatic chambers in which the hydrolysis can be controlled in a controlled manner under defined action of moisture. Here, the moisture by the temperature or saturation of the contacting environment such as gases, such as air, nitrogen, carbon dioxide or other suitable gases, or steam can be adjusted specifically. The duration of treatment depends on the parameters selected above.

Furthermore, the duration of treatment depends on the thickness of the membrane.

As a rule, the treatment duration is between a few seconds to

Minutes, for example under the action of superheated steam, or up to whole days, for example in air at room temperature and low relative humidity. The treatment time is preferably between 10 seconds and 300 hours, in particular 1 minute to 200 hours.

If the partial hydrolysis is carried out at room temperature (20 ° C.) with ambient air having a relative humidity of 40-80%, the treatment duration is between 1 and 200 hours.

The membrane obtained according to step E) can be made self-supporting, i. it can be detached from the wearer without damage and subsequently

if necessary be further processed directly.

About the degree of hydrolysis, i. the duration, temperature and

Ambient humidity, the concentration of phosphoric acid and thus the conductivity of the polymer membrane is adjustable. The concentration of phosphoric acid is reported as moles of acid per mole of repeat unit of the polymer. By the method comprising steps A) to E), membranes having a particularly high phosphoric acid concentration can be obtained. A concentration (mol of phosphoric acid relative to a repeating unit of the formula (I), for example polybenzimidazole) is preferably between 10 and 50, in particular between 12 and 40, particularly preferably between 15 and 25. Such a high

Doping levels (concentrations) are very difficult or impossible to access by doping polyazoles with commercially available ortho-phosphoric acid.

An advantageous modification of the previously described method, in which doped polyazole films are produced by the use of polyphosphoric acid, comprises the steps

1) reaction of one or more aromatic tetra-amino

Compounds with one or more aromatic carboxylic acids or their esters, the at least two acid groups per carboxylic acid monomer or one or more aromatic and / or heteroaromatic diaminocarboxylic acids in the melt at temperatures of up to 350 ° C, preferably up to 300 ° C,

2) dissolving the obtained according to step 1) solid prepolymers in

polyphosphoric

3) placing the film comprising holes on a support,

4) impregnating the holes comprising film with a solution according to step 2) and heating under inert gas to temperatures of up to 300 ° C, preferably up to 280 ° C, in particular up to 230 ° C, to form the polyazole polymer, wherein the polyazole polymer fills the existing holes so that it covers both sides of the film,

5) treatment of the membrane formed in step 4) until it is self-supporting.

The method steps described under points 1) to 5) were previously explained in more detail for the steps A) to E), reference being made to this, in particular with regard to preferred embodiments.

Furthermore, such a procedure, but without the installation of

Reinforcing foils, for example described in WO 2004/033079, from which the skilled person can take further valuable information regarding the steps 1) -5). The corresponding membranes without reinforcing elements are available, for example under the trade name Celtec ^®.

Within the scope of yet another particularly preferred variant of the present invention, doped polyazole films are obtained by a process in which the polyphosphoric acid is wholly or partly replaced by organic phosphonic anhydrides. Such an approach, but without the incorporation of reinforcing films, is described for example in WO 2005/063851.

The organic phosphonic anhydrides mentioned above are cyclic compounds of the formula

or linear compounds of the formula

or anhydrides of the multiple organic phosphonic acids, such as the formula of anhydrides of diphosphonic acid

in which the radical R and R 'are identical or different and represent a C 1 -C ₂₀ -carbon-containing group.

In the context of the present invention, the radicals C ₁ -C ₂₀ -alkyl, particularly preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, n-butyl, Butyl, t-butyl, n-pentyl, s-pentyl,

Cyclopentyl, n-hexyl, cyclohexyl, n-octyl or cyclooctyl, C ₁ -C 20 -alkenyl, particularly preferably ethenyl, propenyl, butenyl, pentenyl, cyclopentenyl, hexenyl, cyclohexenyl, octenyl or cyclooctenyl, C 1 -C ₂₀ -alkynyl, especially preferably ethynyl, propynyl, butynyl, pentynyl, hexynyl or octynyl, C ₆ -C ₂₀ aryl, more preferably phenyl, biphenyl, naphthyl or anthracenyl, Ci - C ₂ o - fluoroalkyl more preferably trifluoromethyl, pentafluoroethyl or 2,2,2 Trifluoroethyl, C ₆ -C ₂ o-aryl, more preferably phenyl, biphenyl, naphthyl, anthracenyl, triphenylenyl, [1, 1 ', 3', 1 "], terphenyl-2'-yl, binaphthyl or phenanthrenyl, C ₆ - C ₂₀ -Fluoraryl, more preferably tetrafluorophenyl or heptafluoronaphthyl, CrC ₂₀ alkoxy, more preferably methoxy, ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy or t-butoxy, C ₆ -C ₂ o-aryloxy, particularly preferred phenoxy, naphthoxy, biphenyloxy, anthracenyloxy, phenanthrenyloxy, C ₇ -C ₂ o-arylalkyl, particularly Favor t o -tolyl, m-tolyl, p-tolyl, 2,6-dimethylphenyl, 2,6-diethylphenyl, 2,6-di-i-propylphenyl, 2,6-di-t-butylphenyl, ot-butylphenyl, mt Butylphenyl, p-butylphenyl, C ₇ -C ₂ o-alkylaryl, more preferably benzyl, ethylphenyl, propylphenyl,

Diphenylmethyl, triphenylmethyl or naphthalinylmethyl, C -C ₂ o-aryloxyalkyl, particularly preferably o-methoxyphenyl, m-phenoxymethyl, p-phenoxymethyl, C ₂ - C ₂₀ -Aryloxyaryl, particularly preferably p-phenoxyphenyl, C ₅ -C ₂₀ -heteroaryl, particularly preferably 2-pyridyl, 3-pyridyl, 4-pyridyl, quinolinyl, isoquinolinyl, Acridinyl, or Benzochinolinyl Benzoisochinolinyl, C ₄ -C2o-heterocycloalkyl, more preferably furyl, benzofuryl, 2-pyrrolidinyl, 2-indolyl, 3-indolyl, 2,3-dihydroindolyl, C8 C20 ^_ "arylalkenyl, particularly preferably o-vinylphenyl, m - vinylphenyl, p-vinylphenyl, C ₈ -C ₂ o-arylalkynyl, particularly preferably o-ethynylphenyl, m-ethynylphenyl or p-ethynylphenyl, C ₂ - C ₂ o - heteroatom-containing group, particularly preferably carbonyl, benzoyl, oxybenzoyl, benzoyloxy, Acetyl, acetoxy or nitrile understood, wherein one or more d-C2o-carbon-containing groups can form a cyclic system.

In the above-mentioned Ci - groups by -O-, -S-, -NR ¹ - - or -CONR ² - C ₂ o-carbon containing groups, one or more non-adjacent CH ₂ may be replaced and one or more H Atoms can be replaced by F.

In the aforementioned Ci - C ₂ o-carbon-containing groups the

aromatic systems, one or more non-adjacent CH groups may be replaced by -O-, -S-, -NR ¹ - or -CONR ² , and one or more H atoms may be replaced by F.

The radicals R ¹ and R ² are identical or different at each occurrence H or an aliphatic or aromatic hydrocarbon radical having 1 to 20 C-atoms.

Particularly preferred are organic phosphonic anhydrides which are partially or perfluorinated.

The organic phosphonic anhydrides used can also be used in

Combination with polyphosphoric acid and / or with P ₂ O ₅ are used. The polyphosphoric acid is commercially available polyphosphoric acids such as those available from Riedel-de Haen, for example. The polyphosphoric acids Hn ₊ 2PnO ₃ n ₊ i (n> 1) usually have a content calculated as P2O5

(acidimetric) of at least 83%. Instead of a solution of the monomers, it is also possible to produce a dispersion / suspension.

The organic phosphonic anhydrides can also be used in combination with simple and / or multiple organic phosphonic acids.

The simple and / or multiple organic phosphonic acids are compounds of the formula

in which the radical R is identical or different and represents a C 1 -C ₂₀ -carbon-containing group and n> 2. Particularly preferred radicals R have already been described above.

The organic phosphonic acids are commercially available, for example the products of the company Clariant or Aldrich.

The organic phosphonic acids do not include vinyl-containing phosphonic acids as described in International Publication WO 03/075389.

The produced mixture has a weight ratio of organic

Phosphonic anhydrides to sum of all polymers from 1: 10,000 to 10,000: 1, preferably 1: 1000 to 1000: 1, in particular 1: 100 to 100: 1, on. Insofar as these phosphonic anhydrides are used in admixture with polyphosphoric acid or simple and / or multiple organic phosphonic acids, these have to be considered in the case of the phosphonic anhydrides.

Furthermore, further organophosphonic acids, preferably perfluorinated organic phosphonic acids, can be added to the mixture produced.

The membrane, in particular the membrane based on polyazoles, can be crosslinked by the action of heat in the presence of atmospheric oxygen on the surface. This hardening of the membrane surface improves the

Properties of the membrane in addition. For this purpose, the membrane on a

Temperature of at least 150 ° C, preferably at least 200 ° C and particularly preferably at least 250 ° C are heated. The oxygen concentration in this process step is usually in the range from 5 to 50% by volume.

preferably 10 to 40 vol .-%, without this being a limitation.

The crosslinking can also be effected by the action of IR or NIR (IR = infra red, ie light with a wavelength of more than 700 nm; NIR = near IR, ie light with a wavelength in the range of about 700 to 2000 nm or an energy in the range of about 0.6 to 1.75 eV). Another method is the irradiation with ß-rays. The radiation dose is between 5 and 200 kGy.

Depending on the desired degree of crosslinking, the duration of the crosslinking reaction can be in a wide range. In general, this reaction time is in the range of 1 second to 10 hours, preferably 1 minute to 1 hour, without this being a restriction.

According to the invention, the membrane-electrode assembly comprises at least two electrochemically active electrodes (anode and cathode) which are separated by the polymer electrolyte membrane. The term "electrochemically active" indicates that the electrodes are capable of catalyzing the oxidation of hydrogen and / or at least one reformate and the reduction of oxygen, this property being obtained by coating the electrodes with platinum and / or ruthenium The term "electrode" means that the material is electrically conductive. The electrode may optionally have a noble metal layer. Such electrodes are known and are described, for example, in US 4,191,618, US 4,212,714 and US 4,333,805.

The electrodes preferably include gas diffusion layers in contact with a catalyst layer.

As gas diffusion layers usually planar, electrically conductive and acid-resistant structures are used. These include, for example, graphite fiber papers, carbon fiber papers, graphite fabrics and / or papers rendered conductive by the addition of carbon black. Through these layers, a fine distribution of the gas and / or liquid streams is achieved.

Furthermore, gas diffusion layers can be used which contain a mechanically stable support material, which with at least one electrically conductive material, for. As carbon (for example carbon black) is impregnated. Support materials particularly suitable for these purposes comprise fibers, for example in the form of nonwovens, papers or fabrics, in particular carbon fibers, glass fibers or fibers containing organic polymers, for example polypropylene,

Polyester (polyethylene terephthalate), polyphenylene sulfide or polyether ketones.

Further details of such diffusion layers can be found, for example, in WO 97/20358. The gas diffusion layers preferably have a thickness in the range from 80 μm to 2000 μm, in particular in the range from 100 μm to 1000 μm and particularly preferably in the range from 150 μm to 500 μm.

Furthermore, the gas diffusion layers favorably have a high porosity. This is preferably in the range of 20% to 80%.

The gas diffusion layers may contain conventional additives. These include, but are not limited to, fluoropolymers, e.g. Polytetrafluoroethylene (PTFE) and

surface-active substances.

According to a particular embodiment, at least one of

Gas diffusion layers consist of a compressible material. In the context of the present invention, a compressible material is characterized by the property that the gas diffusion layer can be pressed without loss of its integrity by pressure on half, in particular to one third of its original thickness.

This property generally comprises gas diffusion layers

Graphite fabric and / or paper rendered conductive by the addition of carbon black.

The catalytically active layer contains a catalytically active substance. These include precious metals, in particular platinum, palladium, rhodium, iridium and / or ruthenium. These substances can also be in the form of

Alloys are used with each other. Furthermore, these can

Substances may also be used in alloys with base metals such as Cr, Zr, Ni, Co and / or Ti. In addition, it is also possible to use the oxides of the abovementioned noble metals and / or base metals. Usually, the above metals are by known methods on a

Support material, usually carbon with a high specific surface area, used in the form of nanoparticles.

According to a particular aspect of the present invention, the

catalytically active compounds, d. H. the catalysts, used in the form of particles, preferably having a size in the range from 1 to 1000 nm,

in particular 5 to 200 nm and preferably 10 to 100 nm. According to a particular embodiment of the present invention, the weight ratio of fluoropolymer to catalyst material comprising at least one noble metal and optionally one or more support materials is greater than 0.05, said ratio preferably being in the range of 0.1 to 0.6.

According to a particular embodiment of the present invention, the catalyst layer has a thickness in the range from 1 to 1000 μιτι, in particular from 5 to 500 μιη, preferably from 10 to 300 μηι on. This value represents an average value that can be determined by measuring the layer thickness in the cross-section of images that can be obtained with a scanning electron microscope (SEM).

According to a particular embodiment of the present invention, the noble metal content of the catalyst layer is 0.1 to 10.0 mg / cm ² , preferably 0.2 to 6.0 mg / cm ² and more preferably 0.2 to 3.0 mg / cm ² , These values can be determined by elemental analysis of a flat sample.

The catalyst layer is generally not self-supporting but is usually applied to the gas diffusion layer and / or the membrane.

Here, a part of the catalyst layer, for example, in the

Gas diffusion layer and / or the membrane diffuse, resulting in

Form transition layers. This can also lead to the fact that the catalyst layer can be regarded as part of the gas diffusion layer.

According to the invention, the surfaces of the polymer electrolyte membrane are in contact with the electrodes in such a way that the first electrode is the front side of the polymer electrolyte membrane and the second electrode the rear side of the polymer electrolyte membrane is partially or completely, preferably only partially, covered. Here, the front and the back of the polymer electrolyte membrane denote the side facing away from the viewer or the polymer electrolyte membrane, wherein a viewing from the first electrode (front), preferably the cathode, in the direction of the second electrode (behind), preferably the anode takes place.

For more information about inventively suitable polymer electrolyte membranes and electrodes is on the literature, especially on the

Patent Applications WO 01/18894 A2, DE 195 09 748, DE 195 09 749, WO

00/26982, WO 92/15121 and DE 197 57 492. The disclosure contained in the above references with regard to the structure and the Production of membrane-electrode assemblies, as well as the electrodes to be selected, gas diffusion layers and catalysts is also part of the

Description.

The preparation of the membrane electrode assembly according to the invention is the

Professional obviously. In general, the various components of the membrane-electrode assembly are superimposed and pressure and

Temperature is connected together, wherein usually at a temperature in the range of 10 to 300 ° C, in particular 20 ° C to 200 ° and at a pressure in the range of 1 to 1000 bar, in particular from 3 to 300 bar, is laminated.

Since the performance of a single fuel cell is often too low for many applications, in the context of the present invention preferably a plurality of individual fuel cells via separator plates to a fuel cell

(Fuel cell stack) combined. If necessary, the separator plates should be in

Interaction with other sealing materials seal the gas spaces of the cathode and the anode to the outside and between the gas spaces of the cathode and the anode. For this purpose, the separator plates are preferably applied sealingly to the membrane-electrode assembly. The sealing effect can be further increased by pressing the composite of Separatorplatten and membrane electrode unit.

The separator plates preferably each have at least one gas channel for reaction gases, which are conveniently arranged on the sides facing the electrodes. The gas channels are to allow the distribution of reactant fluids.

It has been found, particularly surprisingly, that the membrane electrode units according to the invention are distinguished by a markedly improved mechanical stability and strength and can therefore be used to produce fuel cell stacks with particularly high power. The hitherto customary power fluctuations of the resulting fuel cell stacks are no longer observed and it is achieved a hitherto unknown quality, reliability and reproducibility.

The membrane-electrode assemblies of the invention can be easily stored or shipped due to their dimensional stability at fluctuating ambient temperatures and humidity. Even after prolonged storage or after shipment to places with significantly different climatic conditions The dimensions of the membrane-electrode assemblies are perfectly suited for installation in fuel cell stacks. The membrane-electrode assembly then no longer needs to be conditioned on-site for external installation, which simplifies fuel cell fabrication and saves time and cost.

An advantage of preferred membrane-electrode assemblies is that they allow operation of the fuel cell at temperatures above 120 ° C. This is true for gaseous and liquid fuels, e.g. Hydrogen-containing gases, e.g. be prepared from hydrocarbons in an upstream reforming step. As the oxidant, e.g. Oxygen or air can be used.

Another advantage of preferred membrane-electrode assemblies is that, when operating above 120 ° C, they can also be reacted with pure platinum catalysts, i. without a further alloying component, have a high tolerance to carbon monoxide. At temperatures of 160 ° C, e.g. more than 1% CO contained in the fuel gas, without resulting in a significant reduction in the performance of the fuel cell.

Preferred membrane-electrode assemblies can be operated in fuel cells without the need to humidify the fuel gases and oxidants despite the possible high operating temperatures. The fuel cell is still stable and the membrane does not lose its conductivity. This simplifies the entire fuel cell system and brings additional cost savings, since the management of the water cycle is simplified. Furthermore, this also improves the behavior at temperatures below 0 ° C. of the fuel cell system.

Surprisingly, preferred membrane-electrode assemblies allow the

Fuel cell can be easily cooled to room temperature and below and then put back into operation without losing power. On the other hand, conventional phosphoric acid-based fuel cells sometimes have to be kept at a temperature above 40 ° C when switching off the fuel cell system to avoid irreversible damage.

Furthermore, the preferred membrane-electrode assemblies of the

present invention, a very high long-term stability. It has been found that a fuel cell according to the invention over long periods, for example more than 5000 hours, at temperatures of more than 120 ° C with dry reaction gases can be operated continuously without a noticeable

Performance degradation is detected. The achievable power densities are very high even after such a long time.

The hole-containing film used according to the invention gives the polymer electrolyte membrane a high dimensional stability, which is also advantageous for the membrane-electrode assembly. Creep of the polymer electrolyte membrane is prevented or minimized, so that a high quiescent voltage is obtained even after a long time, for example more than 5000 hours. The quiescent voltage reflects the quality of the membrane-electrode assembly. The resulting by the formation of a constant membrane thickness qualities of the membrane-electrode unit cause a very good reproducibility in the production.

The fuel cells according to the invention show, even after a long time,

For example, more than 5000 hours, a high rest voltage, which is preferably at least 900 mV after this time. To measure the quiescent voltage, a fuel cell with a hydrogen flow on the anode and an air flow on the cathode is de-energized. The measurement is done by the

Fuel cell is switched from a current of 0.2 A / cm ² to the de-energized state and then recorded there for 5 minutes, the quiescent voltage. The value after 5 minutes is the corresponding resting potential. The measured values of the quiescent voltage apply for a temperature of 160 ° C. In addition, the fuel cell after this time preferably shows a low gas passage (gas cross-over). To measure the crossover, the anode side of the fuel cell is operated with hydrogen (5 lJh) and the cathode with nitrogen (5 l / h). The anode serves as a reference and counter electrode. The cathode as a working electrode. The cathode is set to a potential of 0.5 V and oxidized through the membrane diffusing hydrogen at the cathode mass transport-limited. The resulting current is a measure of the hydrogen permeation rate. The current is <3 mA / cm ² , preferably <2 mA / cm ² , more preferably <1 mA / cm ² in a 50 cm ² cell. The measured values of H ₂ crossover are valid for a temperature of 160 ° C.

Furthermore, the membrane-electrode assemblies of the invention are characterized by improved temperature and corrosion resistance and a

comparatively low gas permeability, especially at high

Temperatures, off. A decrease in mechanical stability and structural integrity, especially at high temperatures, is achieved according to the invention

best avoided. In addition, the membrane-electrode assemblies according to the invention can be produced inexpensively and easily.

For more information about membrane-electrode assemblies, refer to the

Special literature, in particular to the patents US-A-4,191, 618, US-A-4,212,714 and US-A-4,333,805 referenced. The disclosure contained in the above references [US-A-4,191,618, US-A-4,212,714 and US-A-4,333,805]

with regard to the construction and production of membrane-electrode assemblies, as well as the electrodes to be selected, gas diffusion layers and catalysts is also part of the description.

embodiments

Example 1: Preparation of a reinforced membrane

The stretched plastic (DEXMET 2PTFE5-50H, thickness 80 pm, open area 80%, 28 cm x 28 cm, see Figure 1) is fixed on a support and by means of a hand doctor a 450 μιτι thick layer consisting of heated to 100 ° C.

Polybenzimidazole in Polyphorsphorsäure (solids content 5% by weight) applied. The polybenzimidazole solution is then hydrolyzed after cooling in 50% strength by weight phosphoric acid overnight and a reinforced polybenzimidazole-phosphoric acid membrane is obtained. The properties of the reinforced membrane are listed in Table 1.

Example 2:

The membrane of the invention shows improved mechanical strength under the typical operating conditions of a high temperature polymer electrolyte fuel cell (160 ° C). Under load of 100 g / cm ² as a function of time, the comparative example has a relative decrease in thickness of over 80%, while in the inventive membrane of Example 1 under the same compression conditions, the relative reduction in thickness is less than 70%, cf. FIG. 2. The membrane according to the invention accordingly has a greatly improved compression resistance compared to unreinforced membranes

consistent material properties such as proton conductivity (Table 1)

Comparative Example:

A solution of 2% by weight with equimolar amounts of 3,3 \ 4,4-tetraaminobiphenyl and terephthalic acid in polyphosphoric acid (112%) is heated to 280 ° C over 100 h. The resulting polybenzimidazole-polyphosphoric acid solution is cooled to a temperature of 100 ° C and by means of a hand doctor on a Carrier applied in a 450 μιτι thick layer. and hydrolyzed after cooling in 50 wt% phosphoric acid overnight to obtain a self-supporting polybenzimidazole-phosphoric acid membrane. The membrane properties are listed in Table 1.

Claims

claims:

1. Membrane electrode assembly containing at least two electrochemically active

Electrodes separated by at least one polymer electrolyte membrane, the above-mentioned polymer electrolyte membrane having at least one reinforcement, characterized in that the reinforcement comprises at least one foil having holes through which the polymer electrolyte membrane is in contact with both electrochemically active electrodes.

2. membrane electrode assembly according to claim 1, characterized in that the polymer electrolyte membrane has at least one centrally disposed film and is preferably completely covered by the polymer electrolyte of the membrane.

3. Membrane-electrode assembly according to claim 1, characterized in that the polymer-electrolyte membrane is sandwiched by two foils.

4. membrane electrode assembly according to claim 2, characterized in that the film is fiber-reinforced, preferably by monofilaments, multifilaments, long and / or short fibers, hybrid yarns and / or bi-component fibers, which also includes a scrim and / or grid can form.

5. membrane electrode assembly according to claim 1, characterized in that the holes of the film are arranged symmetrically or asymmetrically to each other.

6. membrane electrode assembly according to claim 1, characterized in that the holes of the film round, oval, elliptical, dumbbell and / or kidney-shaped and / or angular, in particular triangular, quadrangular, pentagonal, hexagonal, rhombic, diamond-shaped, square and / or diamond shaped.

7. membrane electrode assembly according to claim 1, characterized in that the holes of the film for the long distances sizes in the range of 0.5 mm to 13 mm, for the short distances sizes in the range of 0.25 mm to 8 mm and thicknesses of the webs ranging from 0.15 mm to 2 mm.

8. membrane electrode assembly according to claim 1, characterized in that the thickness of the film 5μηη to 80μιη, preferably 10 to 70 m, more preferably 15 to 60μιτι, is.

9. membrane electrode assembly according to claim 1, characterized in that the tensile strength of the film is at least 5 MPa, more preferably at least 10 MPa, in particular at least 14 MPa.

10. membrane electrode assembly according to claim 1, characterized in that the breaking elongation of the film in the range of 0.5% to 100%, preferably in the range of 1% to 60%.

11. membrane electrode assembly according to claim 1, characterized in that the holes of the film an open area of 10 to 98%, preferably 15 to 95%, particularly preferably 20 to 90%, based on the total surface of the film form ,

12. membrane electrode assembly according to claim 1, characterized in that the film polytetrafluoroethylene (PTFE), Polyfluorethylenpropoylen (PFEP), polychlorotrifluoroethylene (PCTFE), polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene, with perfluoropropyl vinyl ether, with trifluoronitrosomethane, with carbalkoxy perfluoroalkoxyvinylether , and polysulfone includes.

13. Membrane electrode unit according to at least one of the preceding

Claims, characterized in that the polymer electrolyte membrane comprises polyazoles, in particular polybenzimidazoles.

14. membrane electrode assembly according to claim 13, characterized in that the polymer electrolyte membrane having phosphoric acid or derived from phosphoric acid derivatives.

15. membrane electrode assembly according to claim 14, characterized in that the content of acid between 3 and 50 moles per repeating unit of

Polymer is.

16. membrane electrode assembly according to at least one of the preceding

Claims, characterized in that the reinforcing sheet has functional, chemical groups which have a covalent, chemical bond between form the polymer of the reinforcing sheet and the polymer of the polymer electrolyte membrane.

17. A process for producing a membrane-electrode assembly according to at least one of the preceding claims, comprising the steps:

A) mixing (i) one or more aromatic tetra-amino compounds with (ii) one or more aromatic carboxylic acids or their esters containing at least two acid groups per carboxylic acid monomer, or (iii) mixing one or more

aromatic and / or (iv) heteroaromatic diaminocarboxylic acids, in polyphosphoric acid to form a solution and / or dispersion,

B) arranging the film comprising holes on a support,

C) applying a layer using the mixture according to step A) on the support from step B) in such a way that the mixture penetrates the holes of the film and covers both sides of the film, preferably completely,

E) Treatment of the membrane formed in step D) (until it is self-supporting).

18. A process for producing a membrane-electrode assembly according to at least one of the preceding claims, comprising the steps:

1) reaction of one or more aromatic tetra-amino compounds with one or more aromatic carboxylic acids or their esters containing at least two acid groups per carboxylic acid monomer, or of one or more aromatic and / or heteroaromatic diaminocarboxylic acids in the melt

Temperatures of up to 350 ° C, preferably up to 300 ° C,

2) dissolving the obtained according to step 1) solid prepolymers in

polyphosphoric

3) placing the film comprising holes on a support,

4) impregnating the holes comprising film with a solution according to step 2) and heating under inert gas to temperatures of up to 300 ° C, preferably up to 280 ° C, in particular up to 230 ° C, below Forming the polyazole polymer, wherein the polyazole polymer fills the existing holes so that it covers both sides of the film, 5) treatment of the membrane formed in step 4) until it is self-supporting.

19. The method according to claim 17 or 18, characterized in that the

Mixture from step A) or step 1) to a temperature of up to 350 ° C, preferably up to 280 ° C, heated to form low molecular weight polyazole polymers or oligomers and the heating in step D) or in Step 4) is completely or partially omitted.

20. The method according to at least one of claims 17 to 19, characterized

in that the reinforcing film has functional chemical groups which form a covalent chemical bond between the polymer of the reinforcing film and the polymer of the polymer electrolyte membrane

21. Fuel cell, comprising at least one membrane-electrode unit

according to one or more of claims 1 to 16.