EP1771904A1 - Method for the production of membrane/electrode units - Google Patents

Abstract

The invention relates to a method for production of a membrane/electrode unit, whereby a conducting polymer membrane is compressed with two electrodes until a given compression is achieved. Said method permits an increase in the resting voltage of the membrane/electrode unit in use, the amount of damage during production is reduced and a constant thickness within a production run is achieved.

Description

description

Process for the preparation of membrane-electrode assemblies

The present invention relates to a process for the production of membrane

Electrode units, especially for those with high performance, which can be used in particular in so-called polymer electrolyte membrane fuel cells, in which one or more proton-conductive polymer membranes is compressed with two or more electrodes.

A fuel cell usually contains an electrolyte and two electrodes separated by the electrolyte, in which one of the two electrodes is supplied with a fuel such as hydrogen gas or a methanol-water mixture and the other electrode is supplied with an oxidant such as oxygen gas or air. The chemical energy generated by the resulting fuel oxidation is converted directly into electrical energy.

A requirement of the electrolyte is that this be for hydrogen ions, i. Protons, but not permeable to the aforementioned fuels.

As a rule, a fuel cell has a plurality of individual cells, so-called MEEs (membrane-electrode unit), each containing an electrolyte and two electrodes separated by the electrolyte.

As an electrolyte for the fuel cell, solids such as

Polymer electrolyte membranes or liquids such as phosphoric acid for use. Recently, polymer electrolyte membranes have attracted attention as an electrolyte for fuel cells.

Polymer electrolyte membranes with complexes of basic polymers and strong ones

Acids are known, for example, from WO96 / 13872. For their preparation, a basic polymer, for example polybenzimidazole, is treated with a strong acid, such as phosphoric acid.

Furthermore, fuel cells are also known, for example from US-A-4,017,664, whose membrane comprises inorganic support materials, such as glass fiber fabric or glass fiber nonwovens, which are impregnated with phosphoric acid. In the case of the basic polymer membranes known in the prior art, the mineral acid (usually concentrated phosphoric acid) used to achieve the required proton conductivity is usually added after the molding of the polyazole film. The polymer serves as a carrier for the electrolyte consisting of the highly concentrated phosphoric acid. The polymer membrane fulfills further essential functions, in particular it must have a high mechanical stability and serve as a separator for the two fuels mentioned above.

Significant advantages of such a phosphoric acid-doped membrane is the

Fact that a fuel cell in which such

Polymer electrolyte membrane is used, can be operated at temperatures above 100 ⁰ C without otherwise necessary humidification of the fuels. This is due to the property of phosphoric acid to be able to transport the protons without additional water by means of the so-called Grotthus mechanism (K.-D. Kreuer, Chem. Mater., 1996, 8, 610-641).

The possibility of operation at temperatures above 100 ⁰ C, further benefits for the fuel cell system. On the one hand, the sensitivity of the Pt catalyst to gas contaminants, in particular CO, is greatly reduced. CO is produced as a by-product in the reforming of the hydrogen-rich gas from carbon-containing compounds, such as natural gas, methanol or gasoline or as an intermediate in the direct oxidation of methanol. Typically, the CO content of the fuel at temperatures <100 ° C must be less than 100 ppm. However, at temperatures in the range 150-200 ° C, 10000 ppm CO or more can be tolerated (NJ Bjerrum et al., Journal of Applied Electrochemistry, 2001, 31, 773-779). This leads to significant simplifications of the upstream reforming process and thus to cost reductions of the entire fuel cell system.

The performance of a membrane-electrode assembly made with such membranes is described in WO 01/18894 and Electrochimica Acta, Vol. 41, 1996, 193-197, and has a platinum loading of 0.5 mg / cm ² (anode) and 2 mg / cm ² (cathode) less than 0.2 A / cm ² at a voltage of 0.6 V. When using air instead of oxygen, this value drops to less than 0.1 A / cm ² .

A major advantage of fuel cells is the fact that in the electrochemical reaction the energy of the fuel is converted directly into electrical energy and heat. The reaction product is formed at the Cathode water. The by-product of the electrochemical reaction is heat. For applications where only electricity is used to drive electric motors, such as for automotive applications, or as multiple battery system replacement, some of the heat generated during the reaction must be dissipated to avoid overheating the system. For cooling then additional, energy-consuming devices are necessary, further reducing the overall electrical efficiency of the fuel cell. For stationary applications such as the central or decentralized generation of electricity and heat, the heat can be efficiently utilized by existing technologies such as heat exchangers. To increase the efficiency of high temperatures are sought. If the operating temperature above 100 ⁰ C and the temperature difference between the ambient temperature and the operating temperature is high, it will be possible to cool the fuel cell system more efficient or to use small cooling surfaces and to dispense with additional equipment compared to fuel cells, due to the

Membrane humidification at below 100 ⁰ C must be operated.

The preparation of membrane electrode assemblies for such fuel cells is carried out according to the prior art by pressing a sandwich-shaped construction of two flat electrodes and a flat membrane, which is arranged between the two electrodes, up to a presettable pressing pressure, preferably at elevated temperature. A circumferential seal may be disposed between the electrode and the membrane, sealing the electrode and membrane from the environment, yet not preventing contact between the electrode and the membrane. Such a sandwiched unit is then placed between two relatively movable press plates, which are then pressed together to a presettable maximum pressure to form an adhesive bond of the unit. In this case, a protective layer may be inserted between the press plates and the membrane electrode unit.

However, when using electrodes made from carbon fiber fabrics under controlled compressive pressure, the fabric structure (fiber bumps and valleys) of the electrode after pressing the membrane electrode assembly on the membrane will be distinguished. During the production of the

Membrane electrode unit may therefore happen that fiber elevations of the electrodes come to lie one above the other on both sides of the membrane. The compression pressure within this range can thereby locally increase significantly, which in turn leads to pinching or shearing, causing the membrane is damaged. This can cause shorts between the electrodes, and there may be an undesirable and harmful exchange of gas through the

Membrane take place through. This in turn leads to a reduced performance and a shortened life of the fuel cells. In addition, during installation of such pre-damaged

Membrane electrode units in a stack the risk of bruising and

Shearing further reinforced by a further compression during installation.

When using tissue or paper as the electrode material, the thickness of the MEE is more or less uncontrolled. That means that one

Product series with an acceptable dispersion of thickness in more economical sense

Way is difficult to produce.

The invention therefore an object of the invention to provide a method that does not have these disadvantages, at least partially.

This object is achieved according to the invention in the method mentioned above, that the compression is carried out up to a presettable compression.

In carrying out such a method according to the invention, it has been found, quite surprisingly, that the membrane damages are lower than in the pressure-controlled method, that the quiescent voltage (OCV) of cells produced by the method according to the invention is higher than those after the pressure-controlled Processes were prepared, and that a more uniform thickness of the membrane electrode units can be achieved within a product series. Differences in the material properties of the membrane, electrode and gasket are less pronounced during compression under controlled compression than during compression under controlled pressure. With the method according to the invention, therefore, a more uniform product can be produced than with the known method. In particular, it has been found that the electrical performance of the MEE produced according to the invention is more uniform during operation. In addition, it has been found that, when membranes containing mineral acids, in particular phosphoric acid, are used, the distribution of the acid within the membrane can be better controlled or adjusted.

The present invention therefore relates to a process for producing a membrane-electrode assembly in which one or more proton-conductive Polymer membranes are pressed with two or more electrodes, characterized in that the compression is carried out to a presettable compression.

This method can be carried out with commercially available presses known to the person skilled in the art, for example, by placing one or more spacers (shims) between the press plates. The height of the spacers corresponds to the presettable compression. For a given initial thickness D of the membrane electrode assembly before compression and a desired compression of X%, the height H of the spacers is

H = D * (100-X) / 100.

Alternatively, the pressing process can also be carried out path-controlled. In this case, the press plates with the help of an electronic control unit and a hydraulic or mechanical traversing moved so far toward each other that they last a distance of H, as calculated above, having each other. However, such methods are known to the person skilled in the art and require no further explanation here.

Particular embodiments of the method according to the invention are disclosed in the following description and in the dependent claims. It is also possible for one or more of the features disclosed in these sections to be independent inventions, either alone or in combination with the features of the main claim, and the features can also be combined as desired.

In a first particular embodiment of the method according to the invention, the pressing is carried out at elevated temperature. For the purposes of this invention, the term "elevated temperature" is to be understood as meaning the temperature range from room temperature to about 200 ^° C. Preferred are

Pressing temperatures in the range of 40 to 22O ⁰ C, in particular 60 to 200 ⁰ C, particularly preferably 80 to 18O ⁰ C, depending on the materials used. The elevated temperatures can be adjusted for example via heated press plates.

A further particular embodiment is characterized in that one or more seals, preferably in the structural edge area, are arranged between the membrane and the or each electrode. Another particular embodiment is characterized in that the presettable value of the compression is selected from the interval of incl. 1 to 40%. A preferred interval includes values of 15 to 40%, especially 15 to 25%. This preferred interval is particularly favorable for mineral acid-doped membranes. The compression is calculated as the quotient of the final thickness of the resulting composite and the sum of the initial thicknesses of the individual elements multiplied by 100 in order to arrive at the unit "%".

Another particular embodiment is characterized in that the compression up to the presettable compression within a period of

1 to 100 seconds. Preferably, the adjustable compression is achieved within a period of 20 seconds, more preferably 10 seconds. The beginning of the time interval defines the time from which pressure is exerted on the MEU by the pressure plates. The press plates can then be kept in this state for a further preselectable period of time, in order then to be driven apart again and to release the obtained pressure. This further pre-selectable period can take values of 10 to 100 seconds, preferably 15 to 50 seconds, more preferably about 30 seconds. These values depend on the hardness of the selected membrane.

Another particular embodiment is characterized in that the compression at a compression speed in the range of 0.2 to

2% / s is performed, but not more than 20% / s is performed. Measured at absolute speeds of the press plates, a preferred maximum "compression rate" is 0.2 millimeters per second, with typical MEU thicknesses of about 1 mm. At faster speeds there would be a risk that the membrane would be damaged by bruising and shearing. Preferred absolute compression speeds are in the range of 1 to 200 μm / s. At typical thicknesses of the MEEs in the range of approx.

1 mm, 20 to 200 μm / s are particularly advantageous. ,

Another particular embodiment is characterized in that the compression is carried out in a ramp.

Another particular embodiment is characterized in that the polymer membrane has proton conductivity. Another particular embodiment is characterized in that a

Membrane electrode unit is compressed, comprising at least one doped with at least one mineral acid polymer membrane containing at least one polymer having at least one basic heteroatom selected from the group O, S and / or N, at least two electrodes, wherein at least one electrode has a Catalyst comprising at least one noble metal of the platinum group, in particular Pt, Pd, Ir, Rh, Os, Ru, and / or at least one noble metal Au and / or Ag.

The membrane-electrode unit produced by this method according to the invention (hereinafter also referred to as MEU according to the invention), in addition to the increased quiescent voltage, still supplies high current intensities without the voltage dropping too much. Furthermore, the membrane

Electrode units have good durability at high currents.

The polymer membrane contained in the membrane-electrode assembly is preferably formed by polymers comprising at least one nitrogen atom. These polymers are also referred to as basic polymers. The basicity of

Polymers can also be defined by the molar ratio of nitrogen atoms to carbon atoms. In the context of the present invention, particular preference is given to polymers whose molar ratio of nitrogen atoms to carbon atoms is in the range from 1: 1 to 1: 100, preferably in the range from 1: 2 to 1:20. This ratio can be determined by elemental analysis.

Basic polymers, especially polymers having at least one nitrogen atom, are known in the art. In general, according to the invention, polymers having a nitrogen atom in the main chain and / or in the side chain can be used.

The polymers having a nitrogen atom in the main chain include, for example, polyphosphazenes, polyimines, polyisocyanides, polyetherimine, polyaniline, polyamides, polyhydrazides, polyurethanes, polyimides, polyazoles and / or polyazines.

Preferably, the polymer membranes comprise polymers having at least one nitrogen atom used in a repeat unit. This can also be done Copolymers can be used which comprise repeating units with a nitrogen atom and repeating units without a nitrogen atom.

According to a particular aspect of the present invention, basic polymers having at least one nitrogen atom are used. The term basic is known in the art, in particular Lewis and Bronstedtbasizität to understand this.

The repeating unit in the basic polymer preferably 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.

Polymers based on polyazole generally contain 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 (XVI) and / or (XVII) and / or (XVIII) and / or (XIX) and / or (XX) and / or (XXI) and / or (XXII)

(Xviii;

(XXII)

wherein

Ar are the same or different and, for a four-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{1 are the} same or different and are a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{2 are the} same or different and, for a two- or three-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{3 are the} same or different and for a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ⁴ are the same or different and for a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{5 are the} same or different and for a vierbindige aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{6 are the} same or different and a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,

Ar ^{7 are the} same or different and 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 is the} same or are different and for a di- or tri- or tetra-aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{10 are the} same or different and are a di- or trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear , Ar ^{11 are the} same or different and 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 group, preferably a branched or unbranched alkyl or alkoxy group, or an aryl group as a further radical

R is the same or different hydrogen, an alkyl group and an aromatic

Group is, with the proviso that R in the 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 according to the invention are derived from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulfone, thiophene, furan, pyrrole, thiazole, oxazole, imidazole, isothiazole, isoxazole, pyrazole, 1,3,4-oxadiazole , 2,5- Diphenyl-1, 3,4-oxadiazole, 1, 3,4-thiadiazole, 1, 3,4-triazole, 2,5-diphenyl-1, 3,4-triazole, 1, 2,5-triphenyl-1, 3,4-triazole, 1, 2,4-oxadiazole, 1, 2,4-thiadiazole, 1, 2,4-triazole, 1, 2,3-triazole, 1, 2,3,4-tetrazole, benzo [ b] thiophene, benzo [b] furan, indole, benzo [c] thiophene, benzo [c] furan, isoindole, benzoxazole, benzothiazole, benzimidazole, benzisoxazole, benzisothiazole, benzopyrazole, benzothiadiazole, benzotriazole, dibenzofuran,

Dibenzothiophene, carbazole, pyridine, bipyridine, pyrazine, pyrazole, pyrimidine, pyridazine, 1, 3,5-triazine, 1, 2,4-triazine, 1, 2,4,5-triazine, tetrazine, quinoline, isoquinoline, quinoxaline, Quinazoline, cinnoline, 1,8-naphthyridine, 1,5-naphthyridine, 1,6-naphthyridine, 1,7-naphthyridine, phthalazine, pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine, diphenyl ether, anthracene, benzopyrrole, benzooxathiadiazole,

Benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazine, indolizine, pyridopyridine, imidazopyrimidine, pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline, phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthroline and phenanthrene, which may optionally be substituted.

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 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. As 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 are polymers containing recurring

Benzimidazole units preferred. Some examples of the most useful polymers containing recurring benzimidazole units are represented by the following formulas:

10

H

10

10

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

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

Preferred polyazoles are characterized by a high molecular weight. This is especially true for the polybenzimidazoles. Measured as intrinsic viscosity, this is in the range of 0.3 to 10 dl / g, preferably 1 to 5 dl / g.

Celazole is particularly preferred from Celanese. The properties of the polymer film and polymer membrane can be improved by screening the starting polymer as described in German Patent Application No. 10129458.1.

Very particular preference is given to using para-polybenzimidazoles in the preparation of the polymer electrolyte membranes. In this case, the polybenzimidazoles in particular include six-membered aromatic groups which are linked in 1, 4 position. Particular preference is given to using poly [2,2 '- (p-phenylene) -5,5'-bisbenzimidazole].

The polymer film used for doping based on basic polymers may have further additives to fillers and / or auxiliaries. In addition, the polymer film may have further modifications, for example by crosslinking, as in German Patent Application No. 10110752.8 or in WO 00/44816. In In a preferred embodiment, the polymer film used for doping, comprising a basic polymer and at least one blend component, additionally contains a crosslinker as described in German Patent Application No. 10140147.7. An essential advantage of such a system is the fact that higher doping levels and thus higher conductivity can be achieved with sufficient mechanical membrane stability.

In addition to the abovementioned basic polymers, it is also possible to use a blend of one or more basic polymers with another polymer. The Blendkomponeηte has essentially the task to improve the mechanical properties and reduce the cost of materials. A preferred blend component is polyethersulfone as described in German Patent Application No. 10052242.4.

Among the preferred polymers which can be used as Blenkomponente include, inter alia, polyolefins, such as poly (cloroprene), polyacetylene, polyphenylene, poly (p-xylylene), polyarylmethylene, polyarmethylene, polystyrene, polymethylstyrene, polyvinyl alcohol, polyvinyl acetate, polyvinyl ether, polyvinylamine, Poly (N-vinylacetamide), polyvinylimidazole, polyvinylcarbazole, polyvinylpyrrolidone, polyvinylpyridine, polyvinyl chloride, polyvinylidene chloride, polytetrafluoroethylene,

Polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene, with perfluoropropyl vinyl ether, with trifluoronitrosomethane, with sulfonyl fluoride vinyl ether, with carbalkoxy perfluoroalkoxy vinyl ether, polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, polyacrolein, polyacrylamide, polyacrylonitrile, polycyanoacrylates, polymethacrylimide, cycloolefinic copolymers, in particular of norbornene;

Polymers having C-O bonds in the main chain, for example polyacetal, polyoxymethylene, polyether, polypropylene oxide, polyepichlorohydrin, polytetrahydrofuran, polyphenylene oxide, polyether ketone, polyesters, in particular polyhydroxyacetic acid, polyethylene terephthalate, polybutylene terephthalate, polyhydroxybenzoate, polyhydroxypropionic acid, polypivalolactone, polycaprolactone,

Polymalonic acid, polycarbonate;

Polymer C-S bonds in the main chain, for example, polysulfide ether, polyphenylene sulfide, polyethersulfone; Polymers C-N bonds in the main chain, for example polyimines, polyisocyanides, polyetherimine, polyaniline, polyamides, polyhydrazides,

Polyurethanes, polyimides, polyazoles, polyazines; Liquid crystalline polymers, especially Vectra and

Inorganic polymers, for example polysilanes, polycarbosilanes, polysiloxanes, polysilicic acid, polysilicates, silicones, polyphosphazenes and polythiazyl. For use in fuel cells having a continuous use temperature above 100 ^° C., preference is given to those blend polymers which have a glass transition temperature or Vicat softening temperature VST / A / 50 of at least 100 ^° C., preferably at least 150 ^° C. and very particularly preferably at least 180 ^° C. , These are polysulfones with a Vicat

Softening temperature VST / A / 50 of 180 ⁰ C to 230 ° C is preferred.

Preferred polymers include polysulfones, especially polysulfone having aromatics in the backbone. Have According to a particular aspect of the present invention, preferred polysulfones and polyether sulfones have 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 to ISO 1133.

In a particular aspect, the polymer membrane may be at least one

Polymer having aromatic sulfonic acid groups. Aromatic sulfonic acid groups are groups in which the sulfonic acid group (-SO ₃ H) is covalently bonded to an aromatic or heteroaromatic group. The aromatic group may be part of the backbone of the polymer or part of a side group, with polymers having aromatic groups in the

Main chain are preferred. The sulfonic acid groups can also be used in many cases in the form of the salts. Furthermore, it is also possible to use derivatives, for example esters, in particular methyl or ethyl esters, or halides of the sulfonic acids, which are converted into the sulfonic acid during operation of the membrane.

Preferred aromatic or heteroaromatic groups are derived from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulfone, thiophene, furan, pyrrole, thiazole, oxazole, imidazole, isothiazole, isoxazole, pyrazole, 1, 3,4-oxadiazole, 2,5-diphenyl-1, 3,4-oxadiazole, 1, 3,4-

Thiadiazole, 1, 3,4-triazole, 2,5-diphenyl-1, 3,4-triazole, i, δ-triphenyl-I, S-triazole, 1, 2,4-oxadiazole, 1, 2, 4-thiadiazole, 1, 2,4-triazole, 1, 2,3-triazole, 1, 2,3,4-tetrazole, benzo [b] thiophene, benzo [b] furan, indole, benzo [c] thiophene, Benzo [c] furan, isoindole, benzoxazole, benzothiazole, benzimidazole, benzisoxazole, benzisothiazole, benzopyrazole, benzothiadiazole, benzotriazole, dibenzofuran, dibenzothiophene,

Carbazole, pyridine, bipyridine, pyrazine, pyrazole, pyrimidine, pyridazine, 1, 3,5-triazine, 1, 2,4-triazine, 1, 2,4,5-triazine, tetrazine, quinoline, isoquinoline, quinoxaline, quinazoline, Cinnoline, 1,8-naphthyridine, 1,5-naphthyridine, 1,6-naphthyridine, 1,7-naphthyridine, phthalazine, pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine, Diphenyl ethers, anthracene, benzopyrrole, benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazine, indolizine, pyridopyridine, imidazopyrimidine, pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline, phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthroline and phenanthrene optionally also be substituted. preferred

Substituents are halogen atoms such as. As fluorine, amino groups, hydroxy groups or alkyl groups.

The substitution pattern is arbitrary, in the case of phenylene, for example, ortho, meta and para-phenylene can be. 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 aikyl groups and the aromatic groups may be substituted.

The polymers modified with sulfonic acid groups preferably have a content of sulfonic acid groups in the range from 0.5 to 3 meq / g, preferably 0.5 to

2 meq / g. This value is determined by the so-called ion exchange capacity (IEC).

To measure the IEC, the sulfonic acid groups are converted into the free acid. For this purpose, the polymer is treated in a known manner with acid, wherein excess acid is removed by washing. Thus, the sulfonated polymer is first treated for 2 hours in boiling water. Excess water is then swabbed and the sample for 15 hours at 160 ⁰ C in a vacuum drying cabinet at p <1 mbar. Then the dry weight of the membrane is determined. The thus dried polymer is then dissolved in DMSO at 80 ⁰ C for 1 h. The solution is then titrated with 0.1 M NaOH. From the

Consumption of the acid up to the equivalent point and the dry weight is then calculated the ion exchange capacity (IEC).

Polymers having sulfonic acid groups covalently bonded to aromatic groups are known in the art. So can polymer with aromatic

Sulfonic acid groups are prepared for example by sulfonation of polymers. Methods for sulfonating polymers are described in F. Kucera et. al. Polymer Engineering and Science 1988, Vol. 38, No. 5, 783-792. Here you can the sulfonation conditions are chosen so that a low degree of sulfonation is formed (DE-A-19959289).

With regard to polymers having aromatic sulfonic acid groups whose aromatic radicals are part of the side group, reference may be made in particular to polystyrene derivatives.

Thus, the document US-A-6110616 describes copolymers of butadiene and styrene and their subsequent sulfonation for use in fuel cells.

Furthermore, such polymers can also be obtained by polyreactions of monomers comprising acid groups. So can perfluorinated

Polymers as described in US-A-5422411 are prepared by copolymerization of trifluorostyrene and sulfonyl-modified trifuorostyrene.

According to a particular aspect of the present invention, high temperature stable thermoplastics are used which have sulfonic acid groups attached to aromatic groups. In general, such polymers have aromatic groups in the main chain. Thus, sulfonated polyether ketones (DE-A-4219077, WO96 / 01177), sulfonated polysulfones (J. Membr. Sei. 83 (1993) p.211) or sulfonated polyphenylene sulfide (DE-A-19527435) are preferred.

The polymers with sulfonic acid groups bound to aromatics described above can be used individually or as a mixture, particular preference being given to mixtures having polymers with aromatics in the main chain.

The molecular weight of the polymers having sulfonic acid groups bonded to aromatics can be within wide ranges, depending on the nature of the polymer and its processability. The weight-average molecular weight M _w is preferably in the range from 5000 to 10 000 000, in particular 10 000 to 1000 000, particularly preferably 15000 to 50 000. According to a particular

Aspects of the present invention are polymers having aromatic-bonded sulfonic acid groups which have a low polydispersity index M _w / M _n . The polydispersity index is preferably in the range 1 to 5, in particular 1 to 4.

To further improve the performance properties, the sheet material fillers, in particular proton-conductive fillers, have. Non-limiting examples of proton-conductive fillers are

Sulfates such as: CsHSO ₄ , Fe (SO ₄ ) ₂ , (NH ₄ ) ₃ H (SO ₄ ) 2, LiHSO ₄ , NaHSO ₄ , KHSO ₄ ,

RbSO ₄ , LiN ₂ H ₅ SO ₄ , NH ₄ HSO ₄ ,

Phosphates such as Zr ₃ (PO ₄ ) ₄ , Zr (HPO ₄ ) ₂ , HZr ₂ (PO ₄ ) ₃ , UO ₂ PO ₄ .3H ₂ O, H ₈ UO ₂ PO ₄ , Ce (HPO ₄ ) ₂ , Ti ( HPO ₄ ) ₂ , KH ₂ PO ₄ , NaH ₂ PO ₄ , LiH ₂ PO ₄ , NH ₄ H ₂ PO ₄ ,

CsH ₂ PO ₄ , CaHPO ₄ , MgHPO ₄ , HSbP ₂ O ₈ , HSb ₃ P ₂ O ₁₄ , H ₅ Sb ₅ P ₂ O ₂₀ , polyacid such as H ₃ PW ₁₂ O ₄₀ .nH ₂ O (n = 21-29 ), H ₃ SiW ₁₂ O ₄₀ .nH ₂ O (n = 21-29), H _x WO ₃ , HSbWO ₆ , H ₃ PMOi ₂ O ₄₀ , H ₂ Sb ₄ O ₁₁ , HTaWO ₆ , HNbO ₃ , HTiNbO ₅ , HTiTaO ₅ , HSbTeO ₆ , H ₅ Ti ₄ O ₉ , HSbO ₃ , H ₂ MoO ₄ selenites and arsenides such as (NH ₄ ) ₃ H (SeO ₄ ) ₂ , UO ₂ AsO ₄ , (NH ₄ ) ₃ H (SeO ₄ ) ₂ , KH ₂ AsO ₄ ,

Cs ₃ H (SeO ₄ ) ₂ , Rb ₃ H (SeO ₄ ) ₂ , phosphides such as ZrP, TiP, HfP

Oxides such as Al ₂ O ₃ , Sb ₂ O ₅ , ThO ₂ , SnO ₂ , ZrO ₂ , MoO ₃

Silicates such as zeolites, zeolites (NH ₄ +), phyllosilicates, framework silicates, H-natrolites, H-mordenites, NH ₄ -alcines, NH ₄ -sodalites, NH ₄ -gallates, H-

Montmorillonite acids such as HCIO ₄ , SbF ₅ fillers such as carbides, in particular SiC, Si ₃ N ₄ , fibers, in particular glass fibers,

Glass powders and / or polymer fibers, preferably based on polyazoles.

These additives can be present in the proton-conducting polymer membrane in conventional amounts, but the positive properties such as high conductivity, long life and high mechanical stability of the membrane should not be overly impaired by the addition of too large amounts of additives. In general, the membrane comprises at most 80% by weight, preferably at most 50% by weight and particularly preferably at most 20% by weight of additives.

To prepare the polymer film, the polymer components are first dissolved or suspended as described in the applications cited above, for example DE No. 10110752.8 or WO 00/44816, and then used to prepare the polymer films. Furthermore, the polymer films according to DE No. 10052237.8 can be produced continuously.

Alternatively, the film formation may be carried out according to the method described in Japanese Application No. Hei 10-125560. In this case, the solution is poured into a cylinder with a cylindrical inner surface, and then the cylinder is rotated. At the same time, the solvent is allowed to evaporate by the centrifugal force caused by the rotation; wherein a cylindrical polymer film of substantially uniform thickness forms on the inner surface of the cylinder.

With this method, the basic polymer can be formed with a uniform matrix. This method described in Japanese Patent Application Hei 10-125560 is also part of the present specification.

Subsequently, the solvent is removed. This can be done by means of methods known to the person skilled in the art, for example by drying. Subsequently, the film of basic polymer or polymer blend is impregnated or doped with a strong acid, preferably a mineral acid, wherein the film can be treated beforehand as described in German Patent Application No. 10109829.4. This variant is advantageous in order to exclude interactions of the residual solvent with the barrier layer.

For this purpose, the film comprising at least one polymer having at least one nitrogen atom is immersed in a strong acid, so that the film is impregnated with the strong acid and becomes the proton-conducting membrane. For this purpose, the preferably basic polymer is usually immersed in a highly concentrated strong acid having a temperature of at least 35 ⁰ C over a period of several minutes to several hours.

The strong acid used is mineral acid, in particular phosphoric acid and / or sulfuric acid.

In the context of the present description, "phosphoric acid" means polyphosphoric acid (H _n + 2PnO _3n + i (n> 1) usually has a content calculated as P ₂ O ₅ (acidimetric) of at least 83%), phosphonic acid (H ₃ PO ₃ ), Orthophosphoric acid (H ₃ PO ₄ ), pyrophosphoric acid (H ₄ P ₂ O ₇ ), triphosphoric acid (H ₅ P ₃ O ₁₀ ) and metaphosphoric acid. The phosphoric acid, in particular orthophosphoric acid, preferably has a concentration of at least 80 weight percent, more preferably a concentration of at least 85

Weight percent, more preferably a concentration of at least 87 weight percent, and most preferably a concentration of at least 89 weight percent. The reason for this is that it is preferable basic polymer can be impregnated with a larger number of molecules of strong acid at increasing concentration of the strong acid.

The obtained polymer electrolyte membrane is proton conductive. After doping, the degree of doping expressed as moles of acid per repeating unit should be greater than

6, preferably greater than 8 and most preferably greater than 9.

Instead of the polymer membranes produced by classical methods

The basis of preferably basic polymers may also be the polyazole-containing polymer membranes as described in German Patent Applications No. 10117686.4, US Pat.

10144815.5, 10117687.2 described, can be used. Such, provided with at least one barrier layer polymer electrolyte membranes are also the subject of the present invention.

Polymer membranes may preferably be obtained by a process comprising the steps of i) preparing a mixture comprising polyphosphoric acid, at least one polyazole and / or at least one or more compounds which are suitable for forming polyazoles under the action of heat in step ii), ii) heating the mixture obtainable according to step i) under inert gas

Temperatures of up to 400 ^° C., iii) applying a layer using the mixture according to step i) and / or ii) on a support, iv) treating the membrane formed in step iii).

For this purpose, one or more compounds which are suitable for the formation of polyazoles under the action of heat in step ii) may be added to the mixture according to step i).

For this purpose, mixtures are suitable which comprise one or more aromatic and / or heteroaromatic tetra-amino compounds and one or more aromatic and / or heteroaromatic carboxylic acids or derivatives thereof which comprise at least two acid groups per carboxylic acid monomer. Furthermore, one or more aromatic and / or heteroaromatic diaminocarboxylic acids can be used for the preparation of polyazoles. The aromatic and heteroaromatic tetra-amino compounds include inter alia 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 S.S' ^^ ' -Tetraaminodiphenyldimethylmethane and salts thereof, in particular their mono-, di-, tri- and tetrahydrochloride derivatives. Of these, 3,3 ', 4,4'-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine and 1,2,4,5-tetraaminobenzene are particularly preferred.

Furthermore, the mixture i) aromatic and / or heteroaromatic

Carboxylic acids include. These are dicarboxylic acids and tricarboxylic acids and tetracarboxylic acids or their esters or their anhydrides or their acid halides, in particular their acid halides and / or acid bromides. Preferably, the aromatic dicarboxylic acids are isophthalic acid, terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid,

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-Dihydroxyphthalsäure. 3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalic acid, 2-

Fluoroteraphthalic 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, A-carboxycinnamic acid, or their C1-C20- Alkyl esters or C5-C12 aryl esters, or their acid anhydrides or their acid chlorides.

Very particular preference is given to using mixtures which comprise dicarboxylic acids whose acid residues are in the para position, for example terephthalic acid.

The heteroaromatic carboxylic acids are heteroaromatic

Dicarboxylic acids and tricarboxylic acids and 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 it is 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, 2,4,6-pyridinetricarboxylic acid, benzimidazole-5,6-dicarboxylic acid. As well as their C1-C20 alkyl esters or C5-C12 aryl esters, or their acid anhydrides or their acid chlorides.

Furthermore, the mixture i) may also contain aromatic and heteroaromatic diaminocarboxylic acids. These include, inter alia, diaminobenzoic acid, 4-phenoxycarbonyl-3, '4'-diaminodiphenylether and their mono- and dihydrochloride derivatives.

The mixture prepared in step i) preferably comprises at least 0.5% by weight, in particular 1 to 30% by weight and particularly preferably 2 to 15% by weight of monomers for the preparation of polyazoles.

According to a further aspect of the present invention, the mixture prepared in step i) comprises compounds which under the action of heat in step ii) are suitable for the formation of polyazoles, which compounds are prepared by reacting one or more aromatic and / or heteroaromatic tetra-amino Compounds with one or more aromatic and / or heteroaromatic carboxylic acids or their derivatives which contain 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 400 ^° C., in particular up to 350 ⁰ C, preferably up to 28O ⁰ C are available. To be used for the preparation of these prepolymers

Compounds have been previously set forth.

In addition, monomers containing covalently bonded acid groups can be used to prepare polyazoles. These include, inter alia, aromatic and heteroaromatic dicarboxylic acids or their derivatives which have at least one phosphonic acid group, for example 2,5-dicarboxyphenylphosphonic acid, 2,3-dicarboxyphenylphosphonic acid 3,4-dicarboxyphenylphosphonic acid and 3,5-dicarboxyphenylphosphonic acid; aromatic and heteroaromatic dicarboxylic acids or their derivatives which have at least one sulfonic acid group, in particular 2.5-

Dicarboxyphenylsulfonic acid, 2,3-dicarboxyphenylsulfonic acid 3,4-dicarboxyphenylsulfonic acid and 3,5-dicarboxyphenylsulfonic acid; aromatic and heteroaromatic diaminocarboxylic acids which comprise at least one phosphonic acid group, for example 2,3-diamino-5- carboxyphenylphosphonic acid, 2,3-diamino-6-carboxyphenylphosphonic acid and 3,4-diamino-6-carboxyphenylphosphonic acid; aromatic and heteroaromatic diaminocarboxylic acids comprising at least one sulfonic acid group, for example, 2,3-diamino-5-carboxyphenylsulfonic acid, 2,3-diamino-6-carboxyphenylsulfonic acid and 3,4-diamino-6-carboxyphenylsulfonic acid.

A polyazole membrane prepared according to the method set forth above may contain the optional components set forth above. These include in particular blend polymers and fillers. Blend polymers may be present dissolved, dispersed or suspended in the mixture obtained according to step i) and / or step ii), inter alia. In this case, the weight ratio of polyazole to polymer (B) is preferably in the range from 0.1 to 50, preferably from 0.2 to 20, particularly preferably from 1 to 10, without this being a restriction. If the polyazole is formed only in step ii), the weight ratio can be calculated from the weight of the monomers to form the polyazole, taking into account the compounds liberated in the condensation, for example water.

To further improve the performance properties of the membrane may also be added fillers, in particular proton-conductive fillers, and additional acids. The addition can be carried out, for example, in step i), step ii) and / or step iii). Furthermore, if these are present in liquid form, these additives can also be added after the polymerization according to step iv). These additives have been previously described.

The polyphosphoric acid used in step i) are commercially available polyphosphoric acids, such as are obtainable, for example, from Riedel-de Haen. The polyphosphoric acids H _{n + 2} P _n Oa _{n +} 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 mixture obtained in step i) is heated according to step ii) to a temperature of up to 400 ⁰ C, in particular 350 ° C, preferably up to 280 ⁰ C, in particular 100 ⁰ C to 25O ⁰ C and particularly preferably in the range of 200 ° C heated to 250 ⁰ C. In this case, an inert gas, for example nitrogen or a noble gas, such as neon, argon, is used.

The mixture prepared in step i) and / or step ii) may additionally contain organic solvents. These can make the workability positive influence. For example, the rheology of the solution can be improved so that it can be more easily extruded or laced.

The formation of the planar structure according to step iii) takes place by means of measures known per se (casting, spraying, knife coating, extrusion) which are known from the prior art for polymer film production. Suitable carriers are all suitable carriers under the conditions as inert. These supports include, in particular, films of polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene, polyimides, polyphenylene sulfides (PPS) and polypropylene (PP).

Furthermore, the formation of the membrane can also take place directly on the electrode provided with a barrier layer.

The thickness of the planar structure according to step iii) is preferably between 10 and 4000 .mu.m, preferably between 15 and 3500 .mu.m, in particular between

20 and 3000 microns, more preferably between 30 and 1500μm and most preferably between 50 and 1200 microns.

The treatment of the membrane in step iv) is carried out in particular at temperatures in the range of ^{0 0} C and 150 ⁰ C, preferably at temperatures between 1 ^{0 0} C and

120 ⁰ C, in particular between room temperature (2O ⁰ C) and 9O ⁰ C, in the presence of moisture or water and / or water vapor. The treatment is preferably carried out under normal pressure, but can also be effected 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 contributes to solidification of the membrane to form low molecular weight polyphosphoric acid and / or phosphoric acid.

The partial hydrolysis of the polyphosphoric acid in step iv) leads to a solidification of the membrane and to a decrease in the layer thickness and formation of a membrane. The solidified membrane generally has a thickness between 15 and 3000 .mu.m, preferably 20 and 2000 .mu.m, in particular between 20 and 1500 microns.

The upper temperature limit of the treatment according to step iv) is usually

150 ⁰ C. With extremely short exposure to moisture, for example superheated steam, this steam may also be hotter than 15O ⁰ C. Essential for the upper temperature limit is the duration of the treatment. The partial hydrolysis (step iv) 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 specifically adjusted 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.

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 (2O ⁰ C) with ambient air at a relative humidity of 40-80%, the treatment time is between 1 and 200 hours.

The membrane obtained according to step iv) can be made self-supporting, i. It can be detached from the carrier without damage and then optionally further processed directly.

The treatment according to step iv) leads to a hardening of the coating. If the membrane is formed directly on the electrode, the treatment is carried out according to

Step iv) until the coating has a sufficient hardness to be pressed to a membrane-electrode assembly can. A sufficient hardness is given if a correspondingly treated membrane is self-supporting. In many cases, however, a lower hardness is sufficient. The hardness determined in accordance with DIN 50539 (microhardness measurement) is generally at least 1 mN / mm ² , preferably at least 5 mN / mm ² and very particularly preferably at least 50 mN / mm ² , without this being intended to limit it.

About the degree of hydrolysis, ie the duration, temperature and ambient humidity, the concentration and the amount of phosphoric acid and thus the conductivity of the polymer membrane are adjustable. The concentration of phosphoric acid is reported as moles of acid per mole of repeat unit of the polymer. In the context of the present invention, a concentration (mol of phosphoric acid relative to a repeating unit of the formula (III), ie Polybenzimidazole) between 13 and 80, especially between 15 and 80, is preferred. Such high doping levels (concentrations) are very difficult or even impossible to access by the addition of polyazoles with commercially available ortho-phosphoric acid.

Preferred polymer membranes have a high proton conductivity. This is at temperatures of 120 ⁰ C at least 0.1 S / cm, preferably at least 0.11 S / cm, in particular at least 0.12 S / cm. If the membranes comprise polymers with sulfonic acid groups, the membranes show a high conductivity even at a temperature of 7O ⁰ C. The

Among other things, conductivity depends on the sulfonic acid group content of the membrane. The higher this proportion, the better the conductivity at low temperatures. In this case, a membrane according to the invention can be moistened at low temperatures. For this purpose, for example, the compound used as an energy source, for example hydrogen, be provided with a proportion of water. In many cases, however, the water formed by the reaction is sufficient to achieve humidification.

The specific conductivity is determined by means of impedance spectroscopy in a 4-PoI arrangement in the potentiostatic mode and using platinum electrodes

(Wire, 0.25 mm diameter) measured. The distance between the current-collecting electrodes is 2 cm. The obtained spectrum is evaluated with a simple model consisting of a parallel arrangement of an ohmic resistor and a capacitor. The sample cross-section of the phosphoric acid-doped membrane is measured immediately prior to sample assembly. To measure the temperature dependence, the measuring cell is brought to the desired temperature in an oven and controlled by a Pt-100 thermocouple placed in the immediate vicinity of the sample. After reaching the temperature, the sample is held at this temperature for 10 minutes before starting the measurement.

A membrane-electrode unit according to the invention comprises in addition to the polymer membrane at least two electrodes, which are in contact with the membrane in each case.

In general, the electrode comprises a gas diffusion layer. The gas diffusion layer generally exhibits electron conductivity. Usually flat, electrically conductive and säu reresistente structures are used for this purpose. These include, for example, carbon fiber papers, graphitized carbon fiber papers, Carbon fiber fabrics, graphitized carbon fiber fabrics and / or sheets rendered conductive by the addition of carbon black.

Furthermore, the electrode contains at least one catalyst layer which has at least one noble metal of the platinum group, in particular Pt, Pd, Ir, Rh ₁ Os, Ru, and / or at least one noble metal Au and / or Ag.

Furthermore, it is advantageous if the electrode has at least one catalyst layer which i. at least one noble metal of the platinum group, in particular Pt, Pd, Ir, Rh, Os,

Ru, and / or at least one precious metal contains Au and / or Ag, ii. at least one according to the electrochemical series of noble metal as the metal mentioned under (i), in particular selected from the group Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga ₁ V ₁ contains.

Preferably, the catalyst is formed in the form of an alloy of the metals (i) and (ii). In addition to the alloy, further catalytically active substances, in particular noble metals of the platinum group, ie Pt, Pd, Ir, Rh, Os, Ru ₁ or also the noble metals Au and Ag, can be used. Furthermore, the

Oxides of the aforementioned noble metals and / or non-noble metals are used.

The catalytically active particles comprising the aforementioned substances can be used as metal powder, so-called black precious metal, in particular platinum and / or platinum alloys. Such particles generally have a size in the range of 5 nm to 200 nm, preferably in the range of 7 nm to 100 nm.

In addition, the metals can also be used on a carrier material. Preferably, this support comprises carbon, which can be used in particular in the form of carbon black, graphite or graphitized carbon black. Furthermore, it is also possible to use electrically conductive metal oxides, such as, for example, SnO _x , TiO _x , or phosphates, such as, for example, FePO _x , NbPO _x , Zr _y (PO _x ) _z as carrier material. Here, the indices x, y and z denote the oxygen or metal content of the individual compounds, which may be in a known range, since the

Transition metals can take different oxidation states.

The content of these supported metal particles, based on the total weight of the metal-carrier compound, is generally in the range of 1 to 80 wt .-%, preferably 5 to 60 wt .-% and particularly preferably 10 to 50 wt .-%, without this being a restriction. The particle size of the carrier, in particular the size of the carbon particles, is preferably in the range from 20 to 100 nm, in particular from 30 to 60 nm. The size of the metal particles present thereon is preferably in the range from 1 to 20 nm, in particular 1 to

10 nm and more preferably 2 to 6 nm.

The sizes of the different particles represent mean values and can be determined by transmission electron microscopy or powder X-ray diffractometry.

The catalyst layer has a thickness in the range of 0.1 to 50 microns.

The membrane-electrode unit according to the invention has a loading of catalyst between 0.1 and 10 g / m ² , based on the surface of the

Polymer membrane.

The weight ratio of the noble metals of the platinum group or of Au and / or Ag to the less noble according to the electrochemical series of metals is between 1: 100 to 100: 1.

The catalytically active particles set forth above can generally be obtained commercially.

The preparation of the catalysts is described, for example, in US392512,

JP06007679A940118, or EP450849A911009

The catalyst can be applied inter alia to the gas diffusion system. Subsequently, the gas diffusion system provided with a catalyst layer can be connected to a polymer membrane in order to arrive at a membrane-electrode unit according to the invention.

This membrane can also be connected to a gas diffusion plant. In this case, it is also possible to use a gas diffusion system provided with a catalyst layer. In general, however, a gas diffusion system which does not comprise a catalyst is sufficient.

For applying at least one catalyst layer to a polymer membrane, various methods can be used. For example, in step iii) a support provided with a coating containing a catalyst to provide the layer formed in step iii) or iv) with a catalyst layer.

Here, the membrane can be provided on one side or on both sides with a catalyst layer. If the membrane is provided with only one side of a catalyst layer, the opposite side of the membrane must be pressed with an electrode having a catalyst layer. If both sides of the membrane are to be provided with a catalyst layer, the following methods can also be used in combination to achieve optimum results

Result.

In the membrane-electrode assembly of the present invention, the catalysts contained in the electrode and the catalyst layer adjacent to the gas diffusion layer are different at the cathode and anode sides, respectively. In a particularly preferred embodiment of the invention, at least the cathode side comprises a catalyst containing i. at least one noble metal of the platinum group, in particular Pt, Pd, Ir, Rh, Os,

Ru, and / or at least one precious metal Au and / or Ag ii. at least one less noble according to the electrochemical series

Metal as the metal mentioned under (i), in particular selected from

Group Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga, V on.

According to the invention, the catalyst layer can be applied by a method in which a catalyst suspension is used. In addition, powders comprising the catalyst may also be used.

The catalyst suspension contains a catalytically active substance. These have been previously described.

Furthermore, the catalyst suspension may contain conventional additives. These include, but are not limited to, fluoropolymers, e.g. Polytetrafluoroethylene (PTFE), thickening agents, in particular water-soluble polymers, e.g. Cellulose derivatives, polyvinyl alcohol, polyethylene glycol, and surface-active substances.

The surface-active substances include in particular ionic surfactants, for example fatty acid salts, in particular sodium laurate, potassium oleate; and alkylsulfonic acids, alkylsulfonic acid salts, in particular Natriumperfluorohexansulfonat, Lithiumperfluorohexansulfonat, Ammoniumperfluorohexansulfonat, Perfluorohexansulfonsäure, Kaliumnonafluorbutansulfonat, and nonionic surfactants, in particular ethoxylated fatty alcohols and polyethylene glycols.

Furthermore, the catalyst suspension may comprise liquid components at room temperature. These include organic solvents, which may be polar or nonpolar, phosphoric acid, polyphosphoric acid and / or water. The catalyst suspension preferably contains from 1 to 99% by weight, in particular from 10 to 80% by weight, of liquid constituents.

The polar, organic solvents include, in particular, alcohols, such as ethanol, propanol and / or butanol.

The organic, nonpolar solvent includes, among others, known ones

Thin film thinner, such as DuPont 8470 thin film thinner containing turpentine oils.

Particularly preferred additives are fluoropolymers, in particular tetrafluoroethylene polymers. 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 0.1, this ratio preferably being in the range of 0 , 2 to 0.6.

The catalyst suspension can be applied to the membrane by conventional methods. Depending on the viscosity of the suspension, which may also be in paste form, various methods are known with which the suspension can be applied. Suitable methods for coating films, fabrics, textiles and / or papers, in particular spraying and

Printing processes, such as stencil and screen printing processes, inkjet processes, roller application, in particular anilox rollers, slot nozzle application and doctoring. The particular method and the viscosity of the catalyst suspension are dependent on the hardness of the membrane.

The viscosity can be influenced by the solids content, in particular the proportion of catalytically active particles, and the proportion of additives. The viscosity to be adjusted depends on the application method of the catalyst suspension, the optimum values and their determination being familiar to the person skilled in the art. Depending on the hardness of the membrane, an improvement in the binding of catalyst and membrane can be achieved by heating and / or pressing. In addition, the bond between membrane and catalyst increases by a treatment according to step iv).

Furthermore, the application of a catalyst layer can take place simultaneously with the treatment of the membrane until it is self-supporting according to step iv). This can be done, for example, by applying a water-containing catalyst suspension to the planar structure according to step iii).

In this case, the suspension in the form of fine droplets can be sprayed onto the planar structure according to step iii). In addition to water, the suspension may also contain other solvents and / or diluents. Depending on the water content, the curing of the membrane takes place according to step iv). Accordingly, the water content can be in wide ranges. Preferably, the

Water content in the range of 0.1 to 99, in particular 1 to 95 wt .-%, based on the catalyst suspension.

According to a particular aspect of the present invention, the catalyst layer is applied by a powder method. This is a

Catalyst powder used, which may contain additional additives, which have been exemplified above.

For application of the catalyst powder, spraying methods and screening methods can be used, inter alia. In the spraying process, the powder mixture is sprayed onto the membrane with a nozzle, for example a slot nozzle. In general, subsequently, the membrane provided with a catalyst layer is heated in order to improve the connection between the catalyst and the membrane. The heating can be done for example via a hot roll. Such methods and devices for applying the powder are, inter alia, in

DE 195 09748, DE 195 09 749 and DE 197 57492 described.

In the sieving process, the catalyst powder is applied to the membrane with a shaking sieve. An apparatus for applying a catalyst powder to a membrane is described in WO 00/26982. After applying the

Catalyst powder, the binding of catalyst and membrane by heating and / or step iv) can be improved. Here, the membrane provided with at least one catalyst layer can be heated to a temperature in the range of 50 to 200 ⁰ C, in particular 100 to 18O ⁰ C. In addition, the catalyst layer may be applied by a method in which a coating containing a catalyst is applied to a support, and then the coating on the support containing a catalyst is transferred to a polymer membrane. An example is such

Method described in WO 92/15121.

The carrier provided with a catalyst coating can be prepared, for example, by preparing a catalyst suspension described above. This catalyst suspension is then applied to a carrier film, for example made of polytetrafluoroethylene. After applying the suspension, the volatiles are removed.

The transfer of the coating containing a catalyst can take place, inter alia, by hot pressing. For this purpose, the composite is comprising a

Catalyst layer and a membrane and a carrier film heated to a temperature in the range of 5O ⁰ C to 200 ⁰ C and pressed at a pressure of 0.1 to 5 MPa. In general, a few seconds are enough to bond the catalyst layer to the membrane. This time is preferably in the range from 1 second to 5 minutes, in particular 5 seconds to 1 minute.

According to a particular embodiment of the present invention, the catalyst layer has a thickness in the range from 1 to 1000 .mu.m, in particular from 5 to 500, preferably from 10 to 300 .mu.m. 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 membrane provided with at least one catalyst layer comprises 0.1 to 10.0 mg / cm ² , preferably 0.3 to 6.0 mg / cm ² and particularly preferably 0.3 to 3.0 mg / cm ² . These values can be determined by elemental analysis of a flat sample.

Furthermore, the membrane, which may also be provided with a catalyst layer, can still be crosslinked by the action of heat in the presence of oxygen. This hardening of the membrane additionally improves the properties of the membrane. For this purpose, the membrane can be heated to a temperature of at least 15O ⁰ C, preferably at least 200 ⁰ C and particularly preferably at least 25O ⁰ C. 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. The crosslinking can also be effected by the action of IR or NIR (IR = infrared, 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 to irradiate with

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.

A membrane electrode assembly according to the invention shows a surprisingly high power density. According to a particular embodiment, preferred membrane-electrode units have a current density of at least 0.3 A / cm ² , preferably 0.4 A / cm ² , particularly preferably 0.5 A / cm ² . This current density is in operation with pure hydrogen at the anode and air (about 20 vol .-% oxygen, about 80 vol .-% nitrogen) at the cathode at atmospheric pressure (absolute 1013 mbar, with open cell output) and 0.6V Cell voltage measured. In this case, particularly high temperatures in the range of 150-200 ° C, preferably 160-180 ⁰ C, in particular of 170 ⁰ C are used.

The aforementioned power densities can be achieved even at low stoichiometry of the fuel gases on both sides. According to a particular aspect of the present invention, the stoichiometry is less than or equal to 2, preferably less than or equal to 1.5, most preferably less than or equal to 1.2.

According to a particular embodiment of the present invention, the catalyst layer has a low precious metal content. The noble metal content of a preferred catalyst layer which is comprised by a membrane according to the invention is preferably at most 2 mg / cm ² , in particular at most 1 mg / cm ² , very particularly preferably at most 0.5 mg / cm ² . According to a particular aspect of the present invention, one side of a membrane has a higher metal content than the opposite side of the membrane. Preferably, the metal content of one side is at least twice as high as the metal content of the opposite side. The method according to the invention will be explained in more detail below with reference to FIGS. 1 to 3. A limitation of the invention in any way is not intended thereby.

Fig. 1 shows a schematic representation of the structure of a device for

Compression of a membrane electrode assembly in cross section.

Fig. 2 shows a table in which results from compression of membrane electrode units of three different conditions are included.

3 shows a graphical representation of the results shown in FIG.

Between two relatively movable press plates 5 of a press, which is not shown here, there is a membrane electrode unit MEE for pressing according to the inventive method. The MEU comprises a flat membrane 1, which is arranged between two likewise flat electrodes 2. Between the membrane 1 and the electrodes 2, a likewise flat seal can be arranged in each case, which has a recess, so that an electrode space is formed, which is sealed against the environment. Between the press plates 5 and the electrodes 2 can each incompressible

Protective layer 4 may be arranged to prevent damage to the press plates 5 by any leaking mineral acid. According to the invention, one or more spacers 3 of height H are arranged between the press plates 5, which ensure that the press plates 5 can be moved toward one another at most up to a distance H. This then gives the maximum

Compression of the MEE. The height H is dimensioned such that the known compression of the membrane 1, the electrodes 2, possibly the seals and the protective layer 4, the desired compression when the press plates 5 are pressed together to the distance H at known thicknesses. Since the individual components of the MEU are manufactured separately, their thicknesses can be precisely measured in advance.

The time profile of the distance between the press plates 5 to each other can have the shape of a ramp, the slope is adjustable. For pressing at elevated temperature, the press plates 5 can be heated.

During pressing of the MEU, the following processes take place: a pressure is produced on the MEU, which causes the membrane 1 to be squeezed to a certain extent into the tissue of the electrodes 2 in the case of a soft membrane 1. Insofar as the membrane still has shares of oligomeric compounds, these can enriched the interface. This results in a compression and an intimate bond, which ensures the cohesion of the MEA due to the surface adhesion. This has the advantage that the interfaces between the polymer membrane 1 and the electrodes 2 are increased. Suitable soft membranes are preferably polymer membranes which are doped with or contain one or more mineral acids. In particular, in phosphoric acid-doped membranes containing up to 90% phosphoric acid. When MEUs are pressed with such membranes, part of the mineral or phosphoric acid, possibly with oligomeric proportions, enters the electrodes, which improves the performance of the MEUs. Here, the membrane 1 can be compressed by up to 75%.

The compression of the MEU can also be done in two stages, for example by a post-compression when installed in a fuel cell stack. Therefore, another particular embodiment of the method according to the invention is characterized in that one or more already pressed membrane electrode

Units are pressed again in a further pressing process. The total compression can take values from the range of 20 to 40% based on the original thickness of the individual components anode-anode-cathode.

The experiments described below serve to further explain the invention.

MEAs were made with the same membrane and electrode thickness at different compression rates. These were then installed in an electrochemical measuring cell, re-compressed therein, and recorded current-voltage characteristics. The table in Fig. 2 shows the results, Fig. 3 is a graphical representation thereof. In the first part of Fig., The measured open circuit voltage (OCV) is plotted as a function of the compression in the production (MEA-Compr.) And the compression in the cell (Cell-Compr.). With registered is a comparison test, in which a MEE according to the known method with a set pressure of 1

N / mm ^{2 was} pressed. The other two subfigures show the measured terminal voltage UkI at two different current densities each as a function of the same parameters. As already stated, these results show that the quiescent voltage in the MEE compressed according to the invention is higher than in the case of pressure-controlled production. In addition, it can be seen that as recompression increases during cell installation, the quiescent voltage drops. An optimum is achieved with a manufacturing compression of 20% and a cell compression of likewise 20%. The measurement results also show a decrease in the terminal voltage at higher current densities with decreasing built-in Compression. The reason for this is presumably a slightly higher contact resistance between the electrodes and the bipolar plate, which serves to reduce the current.

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 aforementioned references [US-A-4,191,618, US-A-4,212,714 and US-A-4,333,805] regarding the construction and manufacture of membrane-electrode assemblies, as well as the electrodes to be selected, gas diffusion layers and Catalysts is too

Part of the description.

Claims

claims:

1. A process for producing a membrane-electrode assembly in which one or more proton-conductive polymer membranes (1) with two or more electrodes (2) are pressed, characterized in that the

Compression is performed up to a presettable compression.

2. The method of claim 1, wherein the pressing is carried out at elevated temperature.

3. The method according to one or more of claims 1 to 3, wherein the presettable value of the compression of the interval of incl. 1 to 40% is selected.

4. The method according to one or more of claims 1 to 3, wherein the

Compression is performed up to the presettable compression within a period of 1 to 100 seconds.

5. The method according to one or more of claims 1 to 4, wherein the compression with a compression speed in the range of 0.2

^'is carried out to 20% / s.

6. The method according to one or more of claims 1 to 5, wherein the compression is performed ramped.

7. The method according to one or more of claims 1 to 7, wherein a stacked, alternating sequence of a plurality of electrodes and a plurality of membranes is pressed.

8. The method according to one or more of claims 1 to 8, wherein one or more already pressed membrane electrode assemblies are pressed again in a further pressing operation.

9. The method according to one or more of claims 1 to 8, wherein a membrane-electrode assembly is pressed, comprising

A) at least one doped with at least one mineral acid

Polymer membrane containing at least one polymer having at least one basic heteroatom selected from the group O, S and / or N,

RPSTATiGUNGSKOPIE B) at least two electrodes, wherein at least one electrode comprises a catalyst comprising at least one noble metal of the platinum group, in particular Pt, Pd, Ir, Rh, Os ₁ Ru, and / or at least one noble metal Au and / or Ag.

10. The method according to claim 9, wherein as polymers having at least one nitrogen atom, a basic polymer having at least one nitrogen atom in a repeating unit is used.

11. The method according to claim 9 or 10, characterized in that the basic polymer contains at least one aromatic ring having at least one nitrogen atom.

12. The method according to claim 9, characterized in that the basic

Polymer is a polyimidazole, a polybenzimidazole, a polybenzothiazole, a polybenzoxazole, a polytriazole, a polyoxadiazole, a polythiadiazole, a polypyrazole, a polyquinoxaline, a poly (pyridine), a poly (pyrimidines) or a poly (tetrazapyrene).

13. The method according to any one of claims 9 to 12, characterized in that a mixture of one or more basic polymers is used with a further polymer.

14. The method according to claim 9, characterized in that the membrane

Phosphoric acid and / or sulfuric acid as a mineral acid.

15. The method according to one or more of Ansprüqhe 9 to 14, characterized in that the polymer membrane comprises para-polybenzimidazoles.

16. The method according to any one of claims 9 to 15, wherein the catalyst is applied to the polymer membrane.

17. The method according to one or more of claims 9 to 16, wherein the catalyst layer has a thickness in the range of 0.1 to 50 microns.

18. The method according to one or more of claims 9 to 17, wherein the catalyst comprises catalytically active particles having a size in the range of 5 to 200 nm.

19. The method according to one or more of claims 9 to 18, wherein the catalyst comprises supported catalytically active particles, wherein the size of the catalyst particles in the range of 1 to 20 nm.

20. The method according to one or more of claims 9 to 19, wherein the membrane-electrode unit comprises 0.1 to 10 g / m ^{2 of} a catalytically active substance.

21. The method according to one or more of claims 9 to 20, wherein the

Weight ratio of the noble metals of the platinum group or of Au and / or Ag (i) to the less noble according to the electrochemical series of metals (ii) between 1: 100 to 100: 1.

22. Use of presses for carrying out a method according to one or more of claims 1 to 21.