EP2059964A2

EP2059964A2 - Membrane electrode assembly and fuel cells of increased power

Info

Publication number: EP2059964A2
Application number: EP07786440A
Authority: EP
Inventors: Oemer Uensal; Thomas Schmidt; Mathias Weber
Original assignee: BASF Fuel Cell GmbH
Current assignee: BASF Fuel Cell Research GmbH
Priority date: 2006-08-02
Filing date: 2007-07-31
Publication date: 2009-05-20
Also published as: JP5698907B2; CA2659475A1; KR20090045192A; JP2009545841A; CN101523642A; JP2013152938A; WO2008014964A3; WO2008014964A2; US20090258274A1; KR101479354B1; RU2411616C2; RU2009106949A; DE102006036019A1; JP5793524B2; KR20140133884A; CN101523642B

Abstract

Membrane electrode assembly, comprising at least two electrochemically active electrodes, which are separated by at least one polymer electrolyte membrane, wherein the polymer electrolyte membrane has reinforcing elements which at least partially penetrate through the polymer electrolyte membrane. The membrane electrode assembly is preferably obtained by a process in which (i) a polymer electrolyte membrane is formed in the presence of the reinforcing elements, and (ii) the membrane and the electrodes are assembled in the desired sequence. The membrane electrode assembly is suitable in particular for use in fuel cells.

Description

description

Membrane electrode assembly and fuel cells with increased performance

The present invention relates to membrane-electrode assemblies and

Increased power fuel cells comprising at least two electrochemically active electrodes separated by a polymer electrolyte membrane.

Polymer electrolyte membrane (PEM) fuel cells are already known. In them almost exclusively sulfonic acid-modified polymers are currently used as proton-conducting membranes. Here are predominantly perfluorinated polymers application. A prominent example of this is Nafion ™ by DuPont de Nemours, Willmington USA. Proton conduction requires a relatively high water content in the membrane, typically 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 ⁰ C. Under pressure, the operating temperatures can be increased to> 120 ⁰ C. Otherwise, higher operating temperatures can be achieved without a loss of power

Fuel cell can not be realized.

For technical reasons, however, higher operating temperatures than 100 ⁰ 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, significant amounts of carbon monoxide in the reformer gas are contained in the use of so-called reformates from hydrocarbons, which usually have to be removed by a complex gas treatment or gas purification. At high operating temperatures, the tolerance of the catalysts to CO 2 increases.

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 the 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 can be made significantly more usable and thus the fuel cell system efficiency can be increased through electricity-heat coupling.

In order to reach these temperatures, membranes with new conductivity mechanisms are generally used. One approach to this is the use of Membranes that show 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). In this case, the membrane electrode assemblies and the separator plates must be pressed together at relatively high pressures in order to achieve as high as possible a high density of the system, the highest possible performance and the lowest possible volume.

In practice, however, the compression of the membrane-electrode assemblies with the Separatorplatten often causes problems, since the polymer-electrolyte membranes used 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. Most of the resulting fuel cell stacks on strongly fluctuating performance, which are caused by more or less pronounced cracks in the individual membranes and / or by different degrees of compression of the membranes.

Object of the present invention was therefore to provide membrane electrode assemblies and fuel cells with the highest possible performance, which can be produced in the simplest possible way, on a large scale, as inexpensively and as reproducibly.

The fuel cell should preferably have the following properties:

• The fuel cells should last as long as possible.

• The fuel cells should be able to be used at operating temperatures which are as high as possible, in particular above 100 ^° C.

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

• After a long period of operation, the fuel cells should have as high a quiescent voltage as possible and as little gas penetration as possible (gas cross-over). 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.) in order to increase the overall reliability as best as possible.

Furthermore, the fuel cells should have improved temperature and corrosion resistance and a comparatively low gas permeability, especially at high temperatures. A

Decrease in mechanical stability and structural integrity, especially at high temperatures, should be avoided as much as possible.

These objects are achieved by a single fuel cell with all features of claim 1.

The present invention accordingly provides a membrane-electrode assembly comprising at least two electrochemically active electrodes which are separated by at least one polymer-electrolyte membrane and wherein the above-mentioned polymer-electrolyte membrane comprises reinforcing elements which support the polymer electrolyte. Penetrate membrane at least partially.

For the purposes of the present invention suitable polymer electrolyte membranes are known per se and are not subject to any restriction. Rather, all proton-conductive materials are suitable.

Preferably, however, membranes are used which comprise acids, which acids may be covalently bound to polymers. Furthermore, a sheet material may be doped with an acid to form a suitable membrane. Furthermore, gels, in particular polymer gels, can also be used as the membrane, particularly suitable for the present purposes

Polymer membranes are described for example in DE 102 46461.

These membranes may be inter alia by swelling flat materials, for example a polymeric film, with a liquid comprising acidic compounds, or by preparing a mixture of polymers and acidic compounds and then forming a membrane by forming a sheet article and then solidifying it Membrane to be generated.

Suitable polymers include polyolefins, such as

Poly (chloroprene), polyacetylene, polyphenylene, poly (p-xylylene), polyarylmethylene, polystyrene, polymethylstyrene, polyvinyl alcohol, polyvinyl acetate, polyvinyl ether, the aromatic ring is preferably a five- or six-membered ring having one to three nitrogen atoms, which may be fused with another ring, in particular 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 repeat units.

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 ⁰ C permanently. Permanently means that a membrane according to the invention can be operated for at least 100 hours, preferably at least 500 hours, at at least 80 ^° C., preferably at least 120 ^° C., more preferably at least 160 ^° C., without the performance according to the method described in WO 01 / 18894 A2 method can be measured 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). Here are in particular

Blends preferred which contain polyazoles and / or polysulfones. The preferred blend components are polyether sulfone, polyether ketone and polymers modified with sulfonic acid groups, as described in German patent application DE 100 522 42 and DE 102 464 61.

Furthermore, polymer blends which comprise at least one basic polymer and at least one acidic polymer, preferably in a weight ratio of from 1:99 to 99: 1 (so-called acid-base polymer blends), have proven especially useful for the purposes of the present invention. 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)

(XV)

Ar ^{4 are the} same or different and are a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{5 are the} same or different and represent a ^four- membered aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ⁶ are the same or different and are a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{7 are the} same or different and are a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{8 are the} same or different and represent a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{9 are the} same or different and represent a di- or tri- or tetravalent 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 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 represents a hydrogen atom, a 1-20 carbon atoms group, preferably a branched or unbranched

Alkyl or alkoxy group, or an aryl group as another radical R is identical or different hydrogen, an alkyl group or an aromatic group and in formula (XX) is an alkylene group or an aromatic group, with the proviso that R in formula (XX ) is not 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, benzopyrazine, 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 are} ortho, meta and para-phenylene. Particularly preferred groups are derived from

Benzene and biphenylene, which may optionally be substituted, from. 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.

In principle, the polyazoles can also have different recurring units which differ, for example, in their radical 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:

10

where n and m are integers greater than or equal to 10, preferably greater than or equal to 100.

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 commercially available under the trade name Celazole®.

Preferred polymers include polysulfones, especially polysulfone having aromatic and / or heteroaromatic groups in the backbone. 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 ISO 1133. Here, polysulfones having a Vicat softening temperature VST / A / 50 of 18O ⁰ C to 230 ⁰ C are preferred. In still a preferred embodiment of with n <o

The polysulfones described above may under the trade names ^® Victrex 200 P ₁ ^® Victrex 720 P, ^® Ultrason E, ^® Ultrason S, ^® Mindel, ^® Radel A ₁ ^® Radel R, ^® Victrex HTA, ^® Astrel and ^® Udel be obtained commercially.

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.

For the production of polymer films, a polymer, preferably a polyazole, can be dissolved in a further step in polar, aprotic solvents, such as, for example, dimethylacetamide (DMAc), and a film produced by means of classical processes. In this case, the reinforcing elements are preferably introduced into the film during film production. To remove residual solvent, the film thus obtained can be treated with a washing liquid as described in German patent application DE 101 098 29. The cleaning of the polyazole film from solvent residues described in the German patent application surprisingly improves the mechanical properties of the film. These properties include in particular the modulus of elasticity, the tear strength and the fracture toughness of the film.

In addition, the polymer film may have further modifications, for example by crosslinking, as described in German patent application DE 101 107 52 or in WO 00/44816. In a preferred embodiment, the polymer film used contains a basic polymer and at least one Embedded membrane and protrude, if at all, only sporadically out of her. The membranes reinforced according to the invention can no longer be delaminated without destruction

A distinction should be made between laminar structures in which the polymer

Electrolyte membrane and the reinforcing elements each form separate layers, which are indeed connected to each other, but do not penetrate each other. Such laminar structures are not included in the present invention, the present invention includes only those reinforced polymer electrolyte membranes in which the reinforcing elements are at least partially connected to the membrane. As a partial composite, a composite of reinforcing element and membrane is considered in which the reinforcing elements expediently absorb a force such that in the force-elongation diagram at 20 ^° C. the reference force of the polymer electrolyte membrane with reinforcing elements, compared with the polymer Electrolyte membrane without

Reinforcing elements, in the range between 0 and 1% elongation at least one location by at least 10%, preferably by at least 20% and most preferably by at least 30%, different.

According to the invention, the polymer electrolyte membrane is preferably fiber-reinforced and the reinforcing elements preferably comprise monofilaments, multifilaments, long and / or short fibers, hybrid yarns and / or bi-component fibers. In addition to a reinforcing element made of concrete fibers, the reinforcing element can also form a textile surface. Suitable textile surfaces are nonwovens, woven fabrics, knitted fabrics, knitted fabrics, felts, scrims and / or meshes, particularly preferably scrims,

Fabrics and / or nonwovens. Nonlimiting examples of the above-mentioned fabrics are those of poly (acrylic), poly (ethylene terephthalate), poly (propylene), poly (tetrafluoroethylene), poly (ethylene-co-tetrafluoroethylene) (ETFE), 1: 1 alternating copolymer of ethylene and chlorotrifluoroethylene (E-CTFE), polyvinylidene fluoride (PVDF), poly (acrylonitrile) and polyphenylene sulfide (PPS).

Fabrics designate products of predominantly crossed threads of monofilaments and / or multifilament threads. The mesh size of the textile surface may usually be 20 to 2000 microns, for the purposes of the present invention, textile surfaces, in particular tissue, scrim and grid, have proven particularly useful with a mesh size in the range of 30 to 300 microns. The mesh size can be determined, for example, by electronic image analysis of an optical or TEM image.

The open screen surface a _{0 of} the textile surface, in particular of the fabric, scrim and grid, can usually be in the range from 0.1 to 98%, preferably in the range from 20 to 80%. She can talk about the relationship Adhesively consolidated nonwovens are preferably obtained by bonding the fibers with liquid binders, in particular with acrylate polymers, SBR / NBR, polyvinyl ester or polyurethane dispersions, or by melting or dissolving so-called binder fibers which have been added to the nonwoven during production.

In cohesive consolidation, the fiber surfaces are conveniently dissolved by suitable chemicals and pressure bonded or heat sealed at elevated temperature.

In a particularly preferred embodiment of the present invention, the nonwoven fabrics are further reinforced by additional threads, woven or knitted fabric.

The weight per unit area of the nonwoven fabrics is advantageously 30 g / m ² to 500 g / m ² , in particular 30 g / m ² to 150 g / m ² .

Nonlimiting examples of particularly preferred nonwovens are SEFAR PETEX ©, SEFAR FLUORTEX ©, SEFRA PEEKTEX ©.

The composition of the reinforcing elements can in principle be chosen freely and adapted to the specific application. However, the reinforcing elements expediently contain glass fibers, mineral fibers, natural fibers, carbon fibers, boron fibers, synthetic fibers, polymer fibers and / or ceramic fibers, in particular SEFAR CARBOTEX®, SEFAR PETEX®, SEFAR FLUORTEX®,

SEFRA PEEKTEX ©, SEFAR TETEX MONO ©, SEFAR TETEX DLW, SEFAR TETEX Multi from SEFAR, but also DUOFIL ©, EMMITEX Yarn ©. Also possible are those made from acids, corrosion resistant materials, e.g. Hastelloy o. Ä., Are manufactured, as well as square mesh, Tressen-, Körpertertressen or multiplex fabric of the company GDK.

Basically, all types and materials are suitable, insofar as they are largely innert under the prevailing conditions when operating in a fuel cell and meet the mechanical requirements for reinforcement.

The reinforcing elements, which may be part of a woven, knitted, knitted or nonwoven fabric, may have a virtually round cross-section or may have other shapes, such as dumbbell, kidney-shaped, triangular or multilobal cross-sections. Also bicomponent fibers are possible.

The reinforcing elements preferably have a maximum diameter in the range of 10 μm to 500 μm, preferably in the range of 20 μm to 300 μm, 22

In this case, the proton-conductive membranes are preferably obtained by a process comprising the steps

I) dissolving polymers, in particular polyazoles in polyphosphoric acid,

II) heating the solution obtainable according to step I) under inert gas to 5 temperatures of up to 400 ⁰ C,

III) arranging reinforcing elements on a support,

IV) forming a membrane using the solution of the polymer according to step II), optionally after intermediate cooling, on the support of step III) in such a way that the reinforcing elements at least the solution

10 partially penetrate and

V) treatment of the membrane formed in step III) until it is self-supporting.

Such a procedure, but without the incorporation of reinforcing elements, is described, for example, in DE 102 464 61, from which the person skilled in the art can derive further valuable information regarding steps I), III), IV) and V). The corresponding membranes without reinforcing elements are available, for example under the trade name Celtec ^®.

20 Within the scope of a further particularly preferred variant of the present invention

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

A) mixing one or more aromatic tetra-amino compounds with one or more aromatic carboxylic acids or their esters, the

25 contain at least two acid groups per carboxylic acid monomer, or

Mixing one or more aromatic and / or heteroaromatic diaminocarboxylic acids in polyphosphoric acid to form a solution and / or dispersion,

B) arranging reinforcing elements on a support,

30 C) applying a layer using the mixture according to step A) on the carrier from step B) in such a way that the reinforcing elements at least partially penetrate the mixture,

D) heating of the sheet / layer obtainable according to step C) under inert gas to temperatures of up to 350 ⁰ C, preferably up to 280 ⁰ C below

35 Formation of the polyazole polymer.

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

This variant requires the use of reinforcing elements whose melting point is above the temperatures mentioned in step D). 40

Insofar as the use of reinforcing elements is to take place, the melting point of which is below the temperatures mentioned in step D), step D) (heating 23

the mixture from step A) also directly after step A). After subsequent cooling, step C) can take place.

Furthermore, it is also possible to omit step B) and to perform the feeding of the 5 reinforcing elements before or during step D). Depending on

Texture of the materials, the reinforcing elements via a calender, which is possibly heated, take place. Here, the reinforcement is pressed into the still ductile base material.

10 Such a procedure, but without the installation of

Reinforcement elements, for example, described in DE 102 464 59, from which the skilled person further valuable information regarding the steps A), C), D) and E) can be found. The corresponding membranes without reinforcing elements, for example, under the trade name Celtec ^®

15 available.

The aromatic or heteroaromatic carboxylic acid compounds to be used in step A) preferably comprise di-carboxylic acids and tricarboxylic acids and tetracarboxylic acids or their esters or their anhydrides or their acid chlorides. The term aromatic carboxylic acids equally includes heteroaromatic carboxylic acids.

Preferably, the aromatic dicarboxylic acids are isophthalic acid, terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid,

25 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-

30 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

35,4,4'-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.

40 For the aromatic tri-, tetra-carboxylic acids or their C1 -C20-alkyl esters or

C5-C12-aryl-esters or their acid anhydrides or their acid chlorides are preferably 1,3,5-benzenetricarboxylic acid (trimesic acid), 1,4-benzene-tricarboxylic acid. tricarboxylic acid (trimellitic acid), (2-carboxyphenyl) iminodiacetic acid, 3,5,3'-biphenyl tricarboxylic acid or 3,5,4'-biphenyl tricarboxylic 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-benzenetetracarboxylic acid, benzophenonetetracarboxylic acid, 3,3 ', 4,4'-biphenyltetracarboxylic acid, 2, 2', 3,3'-biphenyltetracarboxylic acid, 1, 2,5,6-naphthalenetetracarboxylic acid or 1, 4,5,8- naphthalene.

10

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 have at least one

15 contain 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-pyridinedicarboxylic acid, 3,5 Pyrazole dicarboxylic acid, 2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid, 2,4,6-pyridinetricarboxylic acid or benzimidazole-5,6-

20 dicarboxylic acid or its C 1 -C 20 -alkyl esters or C 5 -C 12 -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%, 25 in particular 0.5 and 10 mol%.

The aromatic and heteroaromatic diaminocarboxylic acids used are preferably diaminobenzoic acid or its mono- and dihydrochloride derivatives.

30

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

35 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. Not limiting examples 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- 25

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, diphenylether-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, S.S' ^^ '- Tetraaminodiphenylsulfon, 3,3', 4.4 '-Tetraaminodiphenylether, 3,3', 4,4'-tetraaminobenzophenone, 3,3 ', 4,4'-tetraaminodiphenylmethane and 3,3', 4,4'-tetraaminodiphenyldimethylmethane and salts thereof, in particular their mono-,

15 di-, tri- and tetrahydrochloride derivatives.

The polyphosphoric acid used in step A) are commercially available polyphosphoric acids, as are obtainable, for example, from Riedel-de Haen. The polyphosphoric acids H _n - P _n O _{3n +} I (n> 1) usually have a content calculated as P ₂ O ₅ (acidimetric) of at least 83%. Instead of a

Solution of the monomers can also be produced 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 25: 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 innert to 30 designating 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 according to step C) has a thickness between 20 and 4000 μm, preferably between 30 and 3500 μm, in particular between 50 and 3000 μm.

Insofar as the mixture according to step A) also contains tricarboxylic acids or tetracarboxylic acid, this results in a branching / crosslinking of the polymer formed. This contributes to the improvement of the mechanical

Feature at. Treating the polymer layer produced according to step D) in the presence of moisture at temperatures and for a time sufficient for the layer to have 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 flat structure obtained in step C) is heated to a temperature of up to 350 ⁰ C, preferably up to 280 ⁰ C and particularly preferably in the range of 200 ⁰ C to 25O ⁰ C. The inert gases 10 to be used in step D) are known in the art. These include in particular nitrogen as well

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 polymers can already be effected. 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) is carried out at temperatures above 0 ⁰ C and below 15O ⁰ C, preferably at temperatures between 10 ⁰ C and 120 ⁰ C, in particular between room temperature (20 ⁰ C) and 90 ⁰ 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

25 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 contributes to the solidification of the membrane by partial hydrolysis to form low molecular weight polyphosphoric acid and / or phosphoric acid.

30

The partial hydrolysis of the polyphosphoric acid in step E) leads to solidification of the membrane and to a decrease in the layer thickness and formation of a membrane having a thickness between 15 and 3000 .mu.m, preferably between 20 and 2000 .mu.m, in particular between 20 and 1500 .mu.m, the self-supporting is.

35

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.

40

The upper temperature limit of the treatment according to step E) is usually 150 ^° C. In the case of extremely short exposure to moisture, for example from 27

superheated steam, this steam can also be hotter than 150 ⁰ 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. In this case, the moisture can be adjusted in a targeted manner by the temperature or saturation of the contacting environment, for example gases, such as air, nitrogen, carbon dioxide or other suitable gases, or water vapor. The duration of treatment depends on the parameters selected above. 10

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

In general, the duration of treatment is between a few seconds to minutes, for example under the action of superheated steam, or up to 15 to full days, for example in the 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 carrier without damage and then 25 optionally 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

30 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. Preferably, a concentration (mol of phosphoric acid based on a repeat unit of the formula (I), for example polybenzimidazole) is between 10 and 50, in particular between

35 12 and 40. Such high levels of doping (concentrations) are very difficult or even not accessible by doping of polyazoles with commercially available ortho-phosphoric acid.

An advantageous modification of the method described above, in which doped polyazole films are produced by the use of polyphosphoric acid, comprises the steps 28

1) reacting 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

5 heteroaromatic diaminocarboxylic acids in the melt at temperatures of up to 350 ⁰ C, preferably up to 300 ⁰ C,

2) dissolving the solid prepolymer obtained in step 1) in polyphosphoric acid,

3) heating the solution obtainable according to step 2) under inert gas

10 temperatures of up to 300 ⁰ C, preferably up to 280 ⁰ C, to form the dissolved polyazole polymer,

4) placing reinforcing elements on a support,

5) forming a membrane using the solution of the polyazole polymer according to step 3) on the support from step 4) in such a way that the

15 reinforcing elements penetrate the solution at least partially, and

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

The method steps described under points 1) to 6) were previously explained in more detail for steps A) to E), reference being made to this, in particular with regard to preferred embodiments.

Furthermore, such a procedure, but without the incorporation of reinforcing elements, for example described in DE 102 464 59, from which the skilled person further valuable information regarding the steps 1) -3) and 25 5) and 6) can refer. The corresponding membranes without

Reinforcing elements are for example available under the trade name Celtec ^®.

In a further preferred embodiment of the present invention, 30 monomers comprising monomers containing phosphonic acid groups and / or monomers comprising sulfonic acid groups are used for the preparation of the polymer electrolyte membranes. Particularly expedient embodiments of this variant include the steps

A) preparing a mixture comprising phosphonic acid group-comprising monomers and at least one polymer,

B) arranging reinforcing elements on a support,

C) applying a layer using the mixture according to step A) on the carrier from step B) in such a way that the reinforcing elements at least partially penetrate the mixture,

40 D) polymerization of the in the planar structure obtainable according to step C) existing phosphonic acid monomers comprising monomers. 29

Within the scope of still another particularly preferred variant of the present invention, doped polyazole films are obtained by a process comprising the steps

A) dissolving the polyazole polymer in organic phosphonic anhydrides to form a solution and / or dispersion,

B) heating the solution from step A) under inert gas to temperatures of up to 400 ⁰ C, preferably up to 350 ⁰ C, in particular of up to 300 ⁰ C,

C) placing reinforcing elements on a support,

D) forming a membrane using the solution of the polyazole polymer 10 from step B) on the support from step C) and

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

Such a procedure, however, without the incorporation of reinforcing elements, is described, for example, in WO 2005/063851, 15 from which the skilled person further valuable information regarding steps A), B),

D) and E). The corresponding membranes without reinforcing elements are available, for example under the trade name Celtec ^®.

The organic phosphonic anhydrides used in step A) are cyclic compounds of the formula

O

R-P-O

/ \ / R O P ^

R - P -O ° O

25 or linear compounds of the formula

9 9 9 - O-PtO-P] - _n OPO -

^RRR n> 0

or anhydrides of the multiple organic phosphonic acids, such as the formula of anhydrides of diphosphonic acid 30

O O

-tO-P-R'-P-O-R R n> 1

in which the radical R and R 'are identical or different and represent a C 1 -C ₂₀ -carbon-containing group. 5

C ₂₀ carbon-containing group is preferably the radicals -C ₂ -alkyl, particularly preferably methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, - in the context of the present invention, a C t-butyl, n-pentyl, s-pentyl, cyclopentyl, n-hexyl, cyclohexyl, n-octyl or cyclooctyl, C 1 -C ₂₀ -alkenyl,

Ethenyl, propenyl, butenyl, pentenyl, cyclopentenyl, hexenyl,

Cyclohexenyl, octenyl or cyclooctenyl, C ₁ -C ₂₀ -alkynyl, particularly preferably ethynyl, propynyl, butynyl, pentynyl, hexynyl or octynyl, C ₆ -C ₂₀ -An / I, particularly preferably phenyl, biphenyl, naphthyl or anthracenyl, C ₁ -C 4 -alkyl ₂ o - fluoroalkyl, particularly preferably trifluoromethyl, pentafluoroethyl or 2,2,2-trifluoroethyl, C ₆ -C _2O -

15 aryl, particularly preferably phenyl, biphenyl, naphthyl, anthracenyl, triphenylenyl,

[1, 1 ', 3', 1 "] terphenyl-2'-yl, binaphthyl or phenanthrenyl, C ₆ -C ₂₀ fluoroaryl, particularly preferably tetrafluorophenyl or heptafluoronaphthyl, C ₁ -C _2O- alkoxy, more preferably methoxy, ethoxy , n-propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy or t-butoxy, C ₆ -C ₂ o-aryloxy, more preferably phenoxy, naphthoxy,

20 biphenyloxy, anthracenyloxy, phenanthrenyloxy, C ₇ -C ₂ o-arylalkyl, particularly preferably 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, pt-butylphenyl, C ₇ -C ₂ o-alkylaryl, more preferably benzyl, ethylphenyl, propylphenyl, diphenylmethyl, triphenylmethyl or naphthalenylmethyl, C ₇ - C ₂ o-aryloxyalkyl,

25 particularly preferred o-methoxyphenyl, m-phenoxymethyl, p-phenoxymethyl, Ci ₂ -

C ₂₀ -Aryloxyaryl, particularly preferably p-phenoxyphenyl, Cs-C ^ heteroaryl, particularly preferably 2-pyhdyl, 3-pyridyl, 4-pyridyl, quinolinyl, isoquinolinyl, acridinyl, benzoquinolinyl or benzoisoquinolinyl, C ₄ -C ₂ o-heterocycloalkyl , furyl, benzofuryl, 2-pyrolidinyl, 2-indolyl, 3-indolyl, 2,3-

30 dihydroindolyl, C ₈ -C ₂₀ arylalkenyl, particularly preferably o-vinylphenyl, m-

Vinylphenyl, p-vinylphenyl, C ₈ -C ₂₀ -arylalkinyl, particularly preferably o-ethynylphenyl, m-ethynylphenyl or p-ethynylphenyl, C ₂ - C ₂₀ - heteroatom-containing group, particularly preferably carbonyl, benzoyl, oxybenzoyl, benzoyloxy, acetyl, acetoxy or nitrile, wherein one or more dC ₂ o-carbon-containing groups

35 can form a cyclic system. 31

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 abovementioned C ₁ - C ₂ o-carbon-containing groups which have aromatic systems, one or more non-adjacent CH groups may be replaced by -O-, -S-, -NR ¹ - or -CONR ² and one or more several H atoms can 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.

15

The organic phosphonic anhydrides used in step A) can also be used in combination with polyphosphoric acid and / or with P ₂ O ₅ . The polyphosphoric acid is commercially available polyphosphoric acids such as those available from Riedel-de Haen, for example. The polyphosphoric acids

20 H _{n + 2} P _n O _{3n +} I (n> 1) usually have a content calculated as P ₂ O ₅

(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 used in step A) 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

R -PO ₃ H ₂

30 H ₂ 2O ₃ 3P - R - PO ₃ 3H ₂ 2

R tPO ₃ H ₂ L

wherein the radical R is the same or different and is a C ₁ - C ₂ o- carbon-containing group and n> 2. Particularly preferred radicals R have already been described above. 35

The organic phosphonic acids used in step A) are commercially available, for example the products of the company Clariant or Aldrich. The organic phosphonic acids used in step A) do not comprise any vinyl-containing phosphonic acids as described in German Patent Application No. 10213540.1. 5

The mixture produced in step A) has a weight ratio of organic phosphonic anhydrides to the sum of all polymers of 1: 10,000 to 10,000: 1, preferably 1: 1000 to 1,000: 1, in particular 1: 100 to 100: 1. Inasmuch as these phosphonic anhydrides in admixture with polyphosphoric acid or simple and

10 or more organic phosphonic acids are used, these are to be considered in the phosphonic anhydrides. Furthermore, it is possible to add further organophosphonic acids, preferably perfluorinated organic phosphonic acids, to the mixture produced in step A). This addition may take place before and / or during step B) or before step C).

15 done. This allows the viscosity to be controlled.

The method steps described under points B) to E) have already been explained in more detail above, with reference being made thereto, in particular with regard to preferred embodiments.

20

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 additionally improves the properties of the membrane. For this purpose, the membrane on a

25 temperature of at least 150 ⁰ C, preferably at least 200 ⁰ C and particularly preferably at least 250 ⁰ C are heated. The oxygen concentration in this process step is usually in the range of 5 to 50% by volume, preferably 10 to 40% by volume, without this being intended to limit it.

30

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

35 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 limitation. 33

The preparation of the reinforced polymer electrolyte membranes can be carried out in a manner known per se. Particularly preferred is the introduction of the reinforcing elements into a flowable or at least still ductile polymer composition and / or monomer or oligomer composition, preferably a polymer melt, solution, dispersion or suspension, and the subsequent solidification of the polymer composition, for example by cooling or removal of volatiles (solvents) and / or chemical reaction (eg, crosslinking or polymerization).

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 can be obtained by coating the electrodes with platinum and / or ruthenium. The term "electrode" means that the material is electrically conductive, and the electrode may optionally comprise 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.

20

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

As gas diffusion layers usually planar, electrically conductive and 25 acid-resistant structures are used. These include, for example, graphite fiber

Papers, carbon fiber papers, graphite cloth 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 is coated with at least one electrically conductive material, for. As carbon (for example carbon black) is impregnated. For this purpose, particularly suitable support materials include fibers, for example in the form of nonwovens, papers or fabrics, in particular carbon fibers, glass fibers

35 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 9720358.

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%.

5 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 the 10 gas diffusion layers may 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. 15

This property generally includes gas diffusion layers of 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 used in the form of alloys with one another. Furthermore, these substances can also be used in alloys with base metals, such as, for example, Cr, Zr, Ni,

25 Co and / or Ti are used. In addition, it is also possible to use the oxides of the abovementioned noble metals and / or base metals. Usually, the above-mentioned metals are used by known methods on a carrier material, usually carbon with a high specific surface, in the form of nanoparticles.

30

According to a particular aspect of the present invention, the catalytically active compounds, i. H. the catalysts, used in the form of particles, which preferably have a size in the range of 1 to 1000 nm, in particular 5 to 200 nm and preferably 10 to 100 nm.

35

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.

40

According to a particular embodiment of the present invention, the catalyst layer has a thickness in the range of 1 to 1000 .mu.m, in particular from 5 to 35

500 microns, preferably from 10 to 300 microns, 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). 5

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.

10

The catalyst layer is generally not self-supporting but is usually applied to the gas diffusion layer and / or the membrane. In this case, a part of the catalyst layer can diffuse, for example, into the gas diffusion layer and / or the membrane, whereby

Form 15 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 such that the first electrode contacts the front side of the polymer electrolyte membrane.

20 electrolyte membrane and the second electrode the back of the polymer electrolyte

Membrane 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),

25 preferably the cathode, in the direction of the second electrode (back), 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 abovementioned references regarding 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

35 description. 36

The preparation of the membrane-electrode assembly according to the invention will be apparent to those skilled in the art. In general, the various components of the membrane-electrode assembly ϋ are superimposed and interconnected by pressure and temperature, usually at a temperature in the

Range of 10 to 300 ⁰ C _1, in particular 20 ⁰ 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 low for many applications, it is preferred that several be used in the present invention

Combined fuel cells via separator plates to a fuel cell (fuel cell stack). In this case, the separator plates, if appropriate in conjunction 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 15. 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 are characterized by a significantly improved mechanical stability and strength and therefore can be used for the production of fuel cell stacks with very high performance. The usual performance fluctuations of the resulting fuel cell stacks are not

30 more observed and it is achieved a previously unknown quality, reliability and reproducibility.

Due to their dimensional stability, the membrane-electrode assemblies according to the invention can be used with fluctuating ambient temperatures and atmospheric humidity

35 easily stored or shipped. Even after prolonged storage or after shipment to places with significantly different climatic conditions, the dimensions of the membrane-electrode units 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 results in manufacturing

40 of the fuel cell simplifies and saves time and money. 37

An advantage of preferred membrane-electrode assemblies is that they allow the operation of the fuel cell at temperatures above 120 ⁰ C. This applies to gaseous and liquid fuels, such as hydrogen-containing gases, which are prepared for example in an upstream reforming step of hydrocarbons. 5 For example, oxygen or air can be used as the oxidant.

Another advantage of preferred membrane-electrode assemblies is that they have a high tolerance to carbon monoxide in operation above 120 ⁰ C even with pure platinum catalysts, ie without a further 10 alloy constituent. At temperatures of 160 ^° C., for example, more than 1% of CO can be contained in the fuel gas, without this leading to a noticeable reduction in the power of the fuel cell.

Preferred membrane-electrode units can be operated in fuel cells without the need to humidify the fuel gases and the 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, as the

20 guiding the water cycle is simplified. Next, this is also the

Behavior at temperatures below 0 ⁰ C of the fuel cell system improved.

Surprisingly, preferred membrane-electrode assemblies allow the fuel cell to be cooled to room temperature and below without any problem

25 and can be put back into operation without losing performance. On the other hand, conventional phosphoric acid-based fuel cells sometimes have to be maintained at a temperature above 40 ^° C., even when the fuel cell system is switched off, in order to avoid irreversible damage.

30

Furthermore, the preferred membrane-electrode assemblies of the present invention exhibit 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 ⁰ C with dry reaction gases

35 can be operated continuously without a noticeable

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

In this case, even after a long time, 40 for example more than 5000 hours, the fuel cells according to the invention show a high rest voltage, which after this

Time is preferably at least 900 mV. To measure the quiescent voltage, a fuel cell with a hydrogen flow on the anode and an air 38

Flow operated on the cathode without power. The measurement is made 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 5 of the quiescent voltage are valid for a temperature of 160 ⁰ C. In addition, the

Fuel cell after this time preferably a low gas passage (gas cross-over). To measure the crossover, the anode side of the fuel cell is operated with hydrogen (5 Uh), the cathode with nitrogen (5 L / h). The anode serves as a reference and counter electrode. The cathode as a working electrode. The cathode

10 is set to a potential of 0.5 V and the diffusing through the membrane

Hydrogen oxidized 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 the H ₂ cross over apply for a temperature of 160 ⁰ C.

15

Furthermore, the membrane-electrode assemblies according to the invention are distinguished by improved temperature and corrosion resistance and a comparatively low gas permeability, in particular at high temperatures. A decrease in mechanical stability and structural

Integrity, especially at high temperatures, is optimally avoided according to the invention.

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

25

For more information about membrane-electrode assemblies, reference is made to the literature, and more particularly to US-A-4,191,618, US-A-4,212,714 and US-A-4,333,805. The disclosure contained in the above references [US-A-4,191,618, US-A-4,212,714 and US-A-4,333,805]

30 regarding the design and manufacture of membrane-electrode assemblies, as well as the electrodes to be selected, gas diffusion layers and catalysts is also part of the description.

35 example

Membrane electrode unit A (reference)

Anode: The anode catalyst is Pt supported on carbon. 40 Cathode: The cathode catalyst is a Pt alloy supported on carbon. 39

Membrane A: The membrane is a phosphoric acid-doped polymer membrane whose polymer consists of para-polybenzimidazole.

Membrane Electrode Assembly B: 5 Anode: The anode catalyst is Pt supported on carbon.

Cathode: The cathode catalyst is a Pt alloy supported on carbon. Membrane A: The membrane is a phosphoric acid-doped polymer membrane whose polymer consists of para-polybenzimidazole. The membrane was applied on both sides to a polyether ether ketone nonwoven (Sefar Peektex® 50 .mu.m thick.

10

Experiment:

The two membrane electrode assemblies were continuously operated in fuel cells having an active area of 50 cm ² at 200 ⁰ C for 350h (anode gas: hydrogen at 1.2 stoichiometry; cathode gas with air stoichiometry 2) and

15 during which current-voltage characteristics recorded. The electricity

Voltage characteristics are a measure of the performance of the fuel cell. During operation, the cell resistance (I kHz impedance measurement) was measured. The change in cell resistance is a measure of the change in the electrical contact between the membrane-electrode unit and the flow field used.

20 plates. If the membrane loses its thickness during operation, it will grow

Cell resistance.

Figure 1 shows the current-voltage characteristics after 350h at 200 ⁰ C.

25 Table 1 shows the change in cell resistance during the operation of

Membrane electrode Einhait A.

Table 2 shows the change in cell resistance during operation of membrane electrode bone B.

30

The current-voltage characteristic after 350 h of membrane electrode assembly A is significantly below the characteristic of membrane electrode assembly B. For example, at a current of 0.5A / cm ^2, only the cell voltage of membrane electrode assembly A is 26mV below the cell voltage of membrane-electrode unit B.

From Table 1 it can be seen that with membrane electrode assembly A, the resistance increases from 2.30 to 3.30 mOhm during the operating time, since membrane A loses thickness due to the influence of pressure and temperature, while membrane electrode assembly B loses thickness the resistance remains constant over the same period as the reinforced membrane B maintains its thickness. 40

Table 1 :

Membrane electrode unit A:

Operating time [h] cell resistance

5 60h 2.30 mOhm

200h 2.90 mOhm

350h 3.30 mOhm

Table 2:

10 membrane electrode unit B:

Operating time [h] cell resistance

60h 2.05 mOhm

200h 2.05 mOhm

350h 2.10 mOhm

15

Claims

41Patentansprüche:

A membrane-electrode assembly comprising at least two electrochemically active electrodes separated by at least one polymer-electrolyte membrane 5, characterized in that the polymer-electrolyte membrane has reinforcing elements at least partially covering the polymer-electrolyte membrane penetrate.

10 2. membrane-electrode assembly according to claim 1, characterized in that the polymer-electrolyte membrane is fiber-reinforced.

3. membrane electrode assembly according to claim 2, characterized in that the reinforcing elements monofilaments, multifilaments, short and / or

15 Long fibers, nonwovens, fabrics, knits and / or knitted fabrics include.

4. membrane electrode assembly according to claim 2 or 3, characterized in that the reinforcing elements glass fibers, mineral fibers, natural fibers, carbon fibers, boron fibers, synthetic fibers, polymer fibers and / or

20 ceramic fibers include.

5. membrane electrode assembly according to at least one of the preceding claims, characterized in that the reinforcing elements have a maximum diameter in the range of 10 microns to 500 microns.

25

6. membrane electrode assembly according to at least one of the preceding claims, characterized in that the reinforcing elements have a Young's modulus of at least 5 GPa.

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

Claims, characterized in that the reinforcing elements have an elongation at break of 0.5 to 100%.

8. membrane electrode assembly according to at least one of the preceding 35 claims, characterized in that the volume fraction of

Reinforcing elements, based on the total volume of the polymer electrolyte membrane, in the range of 5 vol .-% to 95 vol .-%.

9. membrane electrode assembly according to at least one of the preceding 40 claims, characterized in that the reinforcing elements absorb such a force that in the force-elongation diagram at 20 ⁰ C, the reference force of the polymer electrolyte membrane with reinforcing elements, 42

compared with the polymer electrolyte membrane without reinforcing elements, differs in the range between 0 and 1% elongation at least one point by at least 10%.

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

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

11. membrane electrode assembly according to claim 10, characterized in that the polymer electrolyte membrane 10 having phosphoric acid or derived from phosphoric acid derivatives.

12. membrane electrode assembly according to claim 11, characterized in that the content of acid between 3 and 50 moles per repeating unit of

15 polymers.

13. A process for producing a membrane-electrode assembly according to at least one of the preceding claims, characterized in that one

(i) forming a polymer electrolyte membrane 20 in the presence of the reinforcing elements,

(ii) assembling the membrane and electrodes in the desired order.

14. The method according to claim 13, characterized in that one forms the polymer electrolyte membrane 25 by a method comprising the steps

I) dissolving polymers, in particular polyazoles in polyphosphoric acid,

II) heating the solution obtainable according to step I) under inert gas to temperatures of up to 400 ⁰ C,

IM) placing reinforcing elements on a support,

30 IV) Forming a membrane using the solution of the polymer according to

Step II) on the support of step IM) in such a way that the reinforcing elements at least partially penetrate the solution and V) treatment of the membrane formed in step III) until it is self-supporting. 35

15. The method according to claim 13, characterized in that one forms the polymer electrolyte membrane by a method comprising the steps

A) mixing one or more aromatic tetra-amino

Compounds with one or more aromatic carboxylic acids or 40 esters thereof containing at least two acid groups per carboxylic acid

Containing monomer, or mixing of one or more aromatic and / or heteroaromatic Diaminocarbonsäuren, in polyphosphoric acid 43

forming a solution and / or dispersion,

B) arranging reinforcing elements 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

5 reinforcing elements at least partially penetrate the mixture,

D) heating of the sheet / layer obtainable according to step C) under inert gas to temperatures of up to 350 ⁰ C, preferably up to 280 ⁰ C to form the polyazole polymer.

E) treatment of the membrane formed in step D) (until it is self-supporting 10).

16. The method according to claim 13, characterized in that one forms the polymer electrolyte membrane by a method comprising the steps

1) reaction of one or more aromatic tetra-amino

15 compounds containing 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 at temperatures of up to 350 ⁰ C, preferably up to 300 ⁰ C,

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

polyphosphoric

3) heating the solution obtainable according to step 2) under inert gas to temperatures of up to 300 ⁰ C, preferably up to 280 ⁰ C, to form the dissolved polyazole polymer,

25 4) placing reinforcing elements on a support,

5) forming a membrane using the solution of the polyazole polymer according to step 3) on the carrier of step 4) in such a way that the reinforcing elements at least partially penetrate the solution, and

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

17. The method according to claim 13, characterized in that one forms the polymer electrolyte membrane by a method comprising the steps

B) arranging reinforcing elements on a support,

40 D) polymerization of the in the planar structure obtainable according to step C) existing phosphonic acid monomers comprising monomers. 44

18. Fuel cell, comprising at least one membrane-electrode unit according to one or more of claims 1 to 12.