EP1497882A2

EP1497882A2 - Method for producing a plasma-polymerized polymer electrolyte membrane and a polyazol membrane coated by plasma-polymerization

Info

Publication number: EP1497882A2
Application number: EP02762348A
Authority: EP
Inventors: Jörg Müller; Laurent Mex
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-07-11
Filing date: 2002-07-11
Publication date: 2005-01-19
Also published as: WO2003007411A3; WO2003007411A2; DE10133738A1; AU2002328339A1; CA2448447A1; KR20040014572A; CN1610984A; JP2005520001A

Abstract

The invention relates to a method for producing polymer-electrolyte membranes using plasma-assisted deposition in a gaseous phase. Said method simplifies the process in relation to prior art by the selection of its starting materials, carbon or fluorocarbon compounds and water. The invention also relates to a polyazol membrane coated by plasma-polymerization.

Description

description

Process for producing a plasma-polymerized polymer electrolyte membrane and a plasma-coated polyazole membrane

The invention relates to a process for the production of polymer electrolyte membranes by means of plasma-assisted deposition from the gas phase, which achieves a significant simplification compared to the prior art by the choice of its starting materials. The invention further relates to a plasma-coated polyazole membrane.

Plasma-polymerized layers generally have a high and also adjustable degree of crosslinking, which leads to a high chemical and thermal resistance (see, for example: R. Hartmann: "Plasma polymodification of plastic surfaces", Techn. Rundschau 17 (1988), pages 20-23; A. Brunold et al .: "Modification of polymers in low pressure plasma ", part 2, mo 51 (1997), pages 81-84). Through the use of monomers that lead to the incorporation of ion-conducting groups (sulfonic acid, phosphonic acid or carboxylic acid groups), ion-conducting polymer membranes can be produced with this method, which are characterized by their resistance and the high degree of crosslinking due to their barrier effect with respect to gas or Offer liquid permeation for use in fuel cells, especially direct methanol fuel cells, or electrolysis cells. In addition, the deposition technology used enables the production of thin membranes (a few 10 nm to a few 10 μm), which are deposited in particular for use in miniaturized fuel cell systems for portable applications (see, for example, DE 196 24 887 A1, DE 199 14 681 A1) or as barrier layers conventional membranes (DE 199 14 571 A1), such as phosphoric acid-doped polybenzimidazole membranes or membranes containing sulfonic acid, are of interest.

Already known plasma-polymerized ion-conducting layers are produced from various fluorocarbons in combination with trifluoromethanesulfonic acid (e.g. DE 195 13 292 C1, US 57 50 013 A), compounds with carboxyl groups (DE 196 24 887 A1) or vinylphosphonic acid (DE 199 14 681 A1). In the The use of trifluoromethanesulfonic acid also leads to fragmentation of the sulfonic acid in plasma due to the comparable binding energies between the carbon / sulfur bond and the bonds in the sulfonic acid. This results in either highly crosslinked polymers with very low ionic conductivity or polymers with sufficient ionic conductivity but a low degree of crosslinking and a high proportion of trifluoromethanesulfonic acid which is not covalently bound to the polymer backbone and thus electrolytes which are not stable in the long term (see: Ber. Bunsenges. Phys. Chem., Vol. 98 (1994), Pages 631 to 635). In all of the acid compounds mentioned there is one for plasma polymerization. Evaporation is necessary, which, in addition to the disadvantageous handling of materials hazardous to health, means an increased outlay on equipment.

The plasma polymerization according to the invention of ion-conducting layers with the use of carbon compounds, preferably alkenes and alkynes, or fluorocarbon compounds, preferably fluorinated alkenes, in combination with water offers a significant simplification in process control and a significant reduction in production costs. The fragmentation of the water in the plasma leads to the formation of OH radicals, as a result of which the carboxyl groups necessary for ionic conductivity are only formed during layer growth. The use of commercially available liquid mass flow controllers means that the evaporator required for other acid compounds is eliminated. The high vapor pressure of the water also allows separation at room temperature, while with the acid compounds mentioned, heating of the gas supply from the evaporator to the reactor and the electrodes is necessary in order to prevent condensation of the acid compounds in these areas.

For the use of these novel plasma-polymerized electrolyte membranes in fuel cells, in particular miniaturized fuel cells, the combination with catalyst layers produced in thin-layer processes (eg cathode sputtering or plasma-assisted deposition from the gas phase) and, if appropriate, porous conductive ones are suitable for their manufacture Contact layers on (DE 199 14 681 A). These depositions can be carried out in a suitable reactor, which allows both sputtering and gas phase deposition, or in interconnected separate reactors, in each of which a component of the membrane electrode assembly is deposited in a thin-film process and transported between the reactors in a vacuum, be performed. Depending on the substrates used, a stationary deposition process of the plasma-polymerized electrolytes, for example for the coating of suitably structured glass or silicon substrates, or a continuous process, in the case of large quantities or when deposition on a suitable film, can be advantageous.

According to a particular aspect of the present invention, the method described above is particularly suitable for the production of plasma-coated polyazole membranes.

Acid-doped polyazole membranes can be used in a variety of ways due to their excellent chemical, thermal and mechanical properties and are particularly suitable as polymer electrolyte membranes (PEM) in so-called PEM fuel cells.

The basic polyazole membranes are doped with concentrated phosphoric acid or sulfuric acid and act as proton conductors and separators in so-called polymer electrolyte membrane fuel cells (PEM fuel cells).

Due to the excellent properties of the polyazole polymer, such polymer electrolyte membranes - processed into membrane electrode assemblies (MEE) - can be used in fuel cells at continuous operating temperatures above 100 ° C, in particular above 120 ° C. This high continuous operating temperature allows the activity of the precious metal-based catalysts contained in the membrane electrode assembly (MEE) to be increased. In particular when using so-called hydrocarbon reformates, the reformer gas contains significant amounts of carbon monoxide, which usually have to be removed by complex gas processing or gas cleaning. Through the Possibility of increasing the operating temperature, significantly higher concentrations of CO impurities can be tolerated permanently.

By using polymer electrolyte membranes based on polyazole polymers, on the one hand the elaborate gas treatment or gas purification can be partially dispensed with and on the other hand the catalyst loading in the membrane electrode unit can be reduced. Both are indispensable for mass use of PEM fuel cells, since otherwise the costs for a PEM fuel cell system are too high.

Polyazoles 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)

^ -Ar ^' (II) "X n

wherein

Ar are the same or different and, for a tetra-bonded aromatic or heteroaromatic group which may be mono- or polynuclear, Ar ^{1 are the} same or different and for a divalent aromatic or heteroaromatic group which may be mono- or polynuclear, Ar ^{2 are the} same or different Ar ^{3 are the} same or different for a two or three-membered aromatic or heteroaromatic group, which may be mononuclear or polynuclear, and for a tridentic aromatic or heteroaromatic group, which may be mononuclear, Ar ^{4 are the} same or different and for a three-membered aromatic or heteroaromatic group which can be mono- or polynuclear, Ar ^{5 are the} same or different and for a tetra-aromatic or heteroaromatic group which can be mono- or polynuclear, Ar ^{6 are the} same or different and for a double-bonded aromatic or heteroaromatic group, which can be mononuclear or polynuclear, Ar ⁷ identical or ve are different and for a divalent aromatic or heteroaromatic group, which can be mono- or polynuclear, Ar ^{8 are the} same or different and for a trivalent aromatic or heteroaromatic group, which can be mono- or polynuclear, Ar ^{9 are the} same or different and for a bi- or tri- or tetra-bonded aromatic or heteroaromatic group, which may be mono- or polynuclear, Ar ^{10 are the} same or different and, for a di- or tri-bonded aromatic or heteroaromatic group, which may be mono- or polynuclear, Ar ^{11 is the} same or are different and for a divalent aromatic or heteroaromatic group, which may be mononuclear or polynuclear, X is the same or different and for oxygen, sulfur or one

Amino group which has a hydrogen atom, a group having 1-20 carbon atoms, preferably a branched or unbranched

Alkyl or alkoxy group, or an aryl group as a further radical, R carries the same or different hydrogen, an alkyl group and an aromatic group 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, benzene, pyrrole, pyrrole, pyrrole, anthracene Benzooxathiadiazol, Benzooxadiazol, benzopyridine, benzopyrazine, Benzopyrazidin, benzopyrimidine, benzopyrazine, benzotriazine, indolizine, quinolizine, pyridopyridine, imidazopyrimidine, Pyrazinopyrimidin, carbazole, Aciridin, phenazine, benzoquinoline, phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthroline and phenanthrene, which may optionally also be substituted can be from.

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 also be substituted.

Preferred alkyl groups are short-chain alkyl groups with 1 to 4 carbon atoms, such as. B. 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 can be substituted.

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

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

In principle, the polyazoles can also have different recurring units which differ, for example, in their X radical. However, it preferably has only the same X radicals in a recurring unit. Further preferred polyazole polymers are polyimidazoles, polybenzthiazoles, 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 which contains at least two units of the formulas (I) to (XXII) which differ from one another. The polymers can be present as block copolymers (diblock, triblock), statistical 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 repeating azole units.

In the context of the present invention, polymers containing recurring benzimidazole units are preferred. Some examples of the extremely useful polymers containing recurring benzimidazole units are represented by the following formulas:

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

Preferred polyazoles, but especially the polybenzimidazoles, are distinguished by a high molecular weight. Measured as intrinsic viscosity, this is at least 1.0 dl / g, preferably at least 1.2 1 dl / g.

The preparation of such polyazoles is known, in which one or more aromatic tetra-amino compounds with one or more aromatic carboxylic acids or their esters, which contain at least two acid groups per carboxylic acid monomer, are reacted in the melt to form a prepolymer. The resulting prepolymer solidifies in the reactor and is then mechanically crushed. The powdery prepolymer is usually end-polymerized in a solid-phase polymerization at temperatures of up to 400 ° C. The preferred aromatic carboxylic acids include, among others, dicarboxylic acids and tricarboxylic acids and tetracarboxylic acids or their esters or their anhydrides or their acid chlorides. The term aromatic carboxylic acids also includes heteroaromatic carboxylic acids.

The aromatic dicarboxylic acids are preferably isophthalic acid, terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid, 5-aminoisophthalic acid, 5-N, N-dimethylaminoisophthalic acid, 5-N, N-diethylaminoisinoamino - Dihydroxy terephthalic 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-fluoroterphthalic acid, tetrafluorophthalic acid, tetrafluoroisophthalic acid, tetrafluoroterephthalic acid, 1, 4-naphthalenedicarboxylic acid, 1, 5-naphthalenedicarboxylic acid, 2,6-naphthalenediconic acid, dicarboxylic acid, 2,6-naphthalenediconic acid, 1, 8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl ether-4,4'-dicarboxylic acid, benzophenone-4,4'-dicarboxylic acid, diphenyl sulfone-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 ,

With 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-benzene-tricarboxylic acid (Trimesic acid), 1,2,4-benzene-tricarboxylic acid (Trimellitic acid),

(2-carboxyphenyl) iminodiacetic acid, 3,5,3'-biphenyltricarboxylic acid, 3,5,4'-

Biphenyltricarboxylic.

In the case of aromatic tetracarboxylic acids or their C1-C20-alkyl esters or C5-

C12 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, 1, 4,5,8-naphthalenetetracarboxylic acid.

The heteroaromatic carboxylic acids used are preferably heteroaromatic dicarboxylic acids and tricarboxylic acids and tetracarboxylic acids or their esters or their anhydrides. Heteroaromatic carboxylic acids are understood to mean aromatic systems which contain at least one nitrogen, oxygen, sulfur or phosphorus atom in the aromatic. It is 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-pyrimidine dicarboxylic acid, 2,5-pyrazine dicarboxylic acid, 2,4,6-pyridine tricarboxylic 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.

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

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

Mixtures of at least 2 different aromatic carboxylic acids are preferred. Mixtures which contain not only aromatic carboxylic acids but also heteroaromatic carboxylic acids are particularly preferred. The mixing ratio of aromatic carboxylic acids to heteroaromatic carboxylic acids is between 1:99 and 99: 1, preferably 1:50 to 50: 1.

This mixture is in particular a mixture of N-heteroaromatic dicarboxylic acids and aromatic dicarboxylic acids. Non-limiting examples of this are isophthalic acid, terephthalic acid, phthalic acid, 2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid, 4,6- Dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid. 3,4-dihydroxyphthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl ether-4,4 ' -dicarboxylic acid, benzophenone-4,4'-dicarboxylic acid, diphenylsulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid, 4-trifluoromethylphthalic acid, pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid, Pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazole dicarboxylic acid, 2,6-pyrimidine dicarboxylic acid, 2,5-pyrazine dicarboxylic acid.

The preferred aromatic tetraamino compounds include, among others, 3,3 ', 4,4'-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine, 1, 2,4,5-tetraaminobenzene, 3,3', 4 , 4'-tetraaminodiphenyl sulfone, 3,3 ', 4,4'-tetraaminodiphenyl ether, 3,3', 4,4'-tetraaminobenzophenone, 3,3 ', 4,4'-tetraaminodiphenylmethane and 3,3', 4,4 '-Tetraaminodiphenyldimethylmethane and their salts, especially their mono-, di-, tri- and tetrahydrochloride derivatives.

Preferred polybenzimidazoles are commercially available from Celanese AG under the trade name © Celazole.

In addition to the abovementioned polymers, a blend which contains further polymers can also be used. The blend component essentially has the task of improving the mechanical properties and reducing the material costs. A preferred blend component is polyethersulfone as described in German patent application No. 10052242.4.

To produce polymer films, the polyazole is dissolved in polar, aprotic solvents such as dimethylacetamide (DMAc) in a further step and a film is produced using conventional methods.

To remove solvent residues, the film obtained in this way can be treated with a washing liquid. This washing liquid is preferably selected from the group of alcohols, ketones, alkanes (aliphatic and cycloaliphatic), ethers (aliphatic and cycloaliphatic), esters, carboxylic acids, where the above group members can be halogenated, water, inorganic acids (such as H ₃ PO ₄ , H ₂ S0 ₄ ) and mixtures thereof.

In particular, C ₁ -C ₁₀ alcohols, C ₂ -C ₅ ketones, Ci-C-io-alkanes (aliphatic and cycloaliphatic), C ₂ -C ₆ ethers (aliphatic and cycloaliphatic), C ₂ -C ₅ esters, C1 -C ₃ carboxylic acids, dichloromethane, water, inorganic acids (such as H3PO ₄ , H ₂ S0 ₄ ) and mixtures thereof. Of these liquids, water is particularly preferred.

After washing, the film can be dried to remove the washing liquid. Drying is dependent on the partial vapor pressure of the selected treatment liquid. Drying is usually carried out at normal pressure and temperatures between 20 ° C and 200 ° C. A more gentle drying can also be done in a vacuum. Instead of drying, the membrane can also be dabbed off, thus removing excess treatment liquid. The order is not critical.

Cleaning the polyazole film from solvent residues as described above surprisingly improves the mechanical properties of the film. These properties include in particular the modulus of elasticity, the tensile strength and the fracture toughness of the film.

In addition, the polymer film can have further modifications, for example by crosslinking as in German patent application No. 10110752.8 or in WO 00/44816. In a preferred embodiment, the polymer film used contains a basic polymer and at least one blend component and additionally a crosslinking agent as described in German patent application No. 10140147.7.

Instead of the polymer films produced by means of conventional processes, the polyazole-containing polymer membranes can also be used as described in German patent applications No. 10117686.4, 10144815.5, 10117687.2. The thickness of the polyazole films can be in a wide range. The thickness of the polyazole film before doping with acid is preferably in the range from 5 μm to 2000 μm, particularly preferably 10 μm to 1000 μm, without any intention that this should impose a restriction.

To achieve proton conductivity, these foils are doped with an acid. In this context, acids include all known Lewis and Bronsted acids, preferably inorganic Lewis and Bronsted acids.

It is also possible to use polyacids, especially isopolyacids and heteropolyacids, and mixtures of different acids. For the purposes of the present invention, heteropolyacids denote inorganic polyacids with at least two different central atoms, each of which consists of weak, polybasic oxygen acids of a metal (preferably Cr, Mo, V, W) and a non-metal (preferably As, I, P, Se, Si, Te) arise as partially mixed anhydrides. They include, among others, 12-molybdate phosphoric acid and 12-tungstophosphoric acid.

The conductivity of the polyazole film can be influenced via the degree of doping. The conductivity increases with increasing dopant concentration until a maximum value is reached. According to the invention, the degree of doping is stated as mole of acid per mole of repeating unit of the polymer. In the context of the present invention, a degree of doping between 3 and 30, in particular between 5 and 18, is preferred.

Particularly preferred dopants are sulfuric acid and phosphoric acid. A very particularly preferred dopant is phosphoric acid (H ₃ P0 ₄ ). Highly concentrated acids are generally used here. According to a particular aspect of the present invention, the concentration of the phosphoric acid is at least 50% by weight, in particular at least 80% by weight, based on the weight of the dopant.

Furthermore, doped polyazole films can also be obtained by a process comprising the steps comprising the steps I) dissolving the polyazole polymer in polyphosphoric acid,

II) heating the solution obtainable according to step A) under inert gas to temperatures of up to 400 ° C.,

III) forming a membrane using the solution of the polyazole polymer according to step II) on a support and

IV) Treatment of the membrane formed in step III) until it is self-supporting.

Furthermore, doped polyazole films can be 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, which contain at least two acid groups per carboxylic acid monomer, or mixing one or more aromatic and / or heteroaromatic diaminocarboxylic acids, in polyphosphoric acid Formation of a solution and / or dispersion

B) applying a layer using the mixture according to step A) on a support or on an electrode,

C) Heating of the flat structure / layer obtainable according to step B) under inert gas to temperatures of up to 350 ° C, preferably up to 280 ° C with the formation of the polyazole polymer.

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

The aromatic or heteroaromatic carboxylic acid and tetraamino compounds to be used in step A) have been described above.

The polyphosphoric acid used in step A) is commercially available polyphosphoric acids such as are available, for example, from Riedel-de Haen. The polyphosphoric acids H _{n + 2} P _n θ _{3n +} ι (n> 1) usually have a content calculated as P ₂ 0 ₅ (acidimetric) of at least 83%. Instead of a solution of the monomers, a dispersion / suspension can also be produced. The mixture produced in step A) has a weight ratio of polyphosphoric acid to the sum of all monomers of 1: 10000 to 10000: 1, preferably 1: 1000 to 1000: 1, in particular 1: 100 to 100: 1. The layer formation in step B) takes place by means of measures known per se (casting, spraying, knife coating) which are known from the prior art for polymer film production. All carriers which are inert under the conditions are suitable as carriers. To adjust the viscosity, phosphoric acid (concentrated phosphoric acid, 85%) can optionally be added to the solution. As a result, the viscosity can be adjusted to the desired value and the formation of the membrane can be facilitated.

The layer produced according to step B) has a thickness between 20 and 4000 μm, preferably between 30 and 3500 μm, in particular between 50 and 3000 μm.

If the mixture according to step A) also contains tricarboxylic acids or tetracarboxylic acids, this results in branching / crosslinking of the polymer formed. This contributes to the improvement of the mechanical property. Treatment of the polymer layer produced according to step C) in the presence of moisture at temperatures and for a period sufficient until the layer has sufficient strength for use in fuel cells. The treatment can be carried out so far that the membrane is self-supporting so that it can be detached from the support without damage.

The inert gases to be used in step C) are known in the art. These include in particular nitrogen and noble gases such as neon, argon, helium.

In a variant of the process, the formation of oligomers and / or polymers can already be brought about by heating the mixture from step A) to temperatures of up to 350 ° C., preferably up to 280 ° C. Depending on the selected temperature and duration, the heating in step C) can then be partially or completely dispensed with. This variant is also the subject of the present invention.

The treatment of the membrane in step D) takes place at temperatures above 0 ° C. and below 150 ° C., preferably at temperatures between 10 ° C. and 120 ° C., in particular between room temperature (20 ° C.) and 90 ° C., in the presence of Moisture or water and / or water vapor or and / or water-containing phosphoric acid of up to 85%. The treatment is preferably carried out under normal pressure, but can also be carried out under the action of pressure. It is essential that the treatment takes place in the presence of sufficient moisture, as a result of which the polyphosphoric acid present contributes to the solidification of the membrane by partial hydrolysis with the formation of low molecular weight polyphosphoric acid and / or phosphoric acid.

The partial hydrolysis of the polyphosphoric acid in step D) leads to a solidification of the membrane and to a decrease in the layer thickness and formation of a membrane with a thickness between 15 and 3000 μm, preferably between 20 and 2000 μm, in particular between 20 and 1500 μm, which is self-supporting is. The intra- and intermolecular structures (interpenetrating networks IPN) present in the polyphosphoric acid layer according to step B) lead in step C) to an orderly membrane formation, which is responsible for the special properties of the membrane formed.

The upper temperature limit of the treatment in step D) is generally 150 ° C. With extremely short exposure to moisture, for example superheated steam, this steam can also be hotter than 150 ° C. The duration of the treatment is essential for the upper temperature limit.

The partial hydrolysis (step D) can also take place in climatic chambers in which the hydrolysis can be specifically controlled under the influence of moisture. The humidity 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 treatment time depends on the thickness of the membrane.

As a rule, the duration of treatment is between a few seconds to minutes, for example under the action of superheated steam, or until to whole 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 and a relative humidity of 40-80%, the treatment time is between 1 and 200 hours.

The membrane obtained in step D) can be self-supporting, i.e. it can be detached from the carrier without damage and then processed directly if necessary.

About the degree of hydrolysis, i.e. the duration, temperature and ambient humidity, the concentration of phosphoric acid and thus the conductivity of the polymer membrane can be adjusted. The concentration of phosphoric acid is reported as moles of acid per mole of repeat unit of the polymer. The process comprising steps A) to D) enables membranes with a particularly high phosphoric acid concentration to be obtained. A concentration (mol of phosphoric acid based on a repeating unit of the formula (I), for example polybenzimidazole) of between 10 and 50, in particular between 12 and 40, is preferred. Such high levels of doping (concentrations) are only very high by doping polyazoles with commercially available orthophosphoric acid difficult or not accessible at all.

Following the treatment in step D), the membrane can still 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.

The crosslinking can also be effected by the action of IR or NIR (IR = InfraRot, ie light with a wavelength of more than 700 nm; NIR = Near IR, ie light with a wavelength in the range from approx. 700 to 2000 nm or an energy in the range of approx. 0.6 to 1.75 eV). Another method is radiation with β-rays. The radiation dose is between 5 and 200 kGy. According to a modification of the method described in which doped polyazole foils are produced by using polyphosphoric acid, these foils can also be produced by a method comprising the steps

1) Reaction of one or more aromatic tetra-amino compounds with one or more aromatic carboxylic acids or their esters, 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 up to 350 ° C, preferably up to 300 ° 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 to temperatures of up to 300 ° C, preferably up to 280 ° C to form the dissolved polyazole polymer.

4) Forming a membrane using the solution of the polyazole polymer according to step 3) on a support and

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

The method steps shown under items 1) to 5) were previously explained in more detail for steps A) to D), reference being made to this, in particular with regard to preferred embodiments.

The polyazole film can be provided with a plasma-polymerized ion-conducting layer before or after the doping with acid. However, the plasma polymerization is preferably carried out after the doping.

The polyazole film can be provided with a layer according to the invention, which is a plasma-polymerized ion-conducting electrolyte membrane. This layer prevents washing out of acid, so that this layer can also be referred to as a barrier layer. It has been shown that it is advantageous if the barrier layer is on the cathode side of the polymer electrolyte membrane, since the overvoltage is significantly reduced.

Furthermore, both sides of the polyazole film can also be provided with a layer according to the invention. This results in a sandwich-like structure, the polyazole film, which may optionally be doped with acid, forming the middle layer, while the layers obtainable by the plasma process according to the invention are on the outside.

The implementation of the plasma polymerization is known to the person skilled in the art and is described, for example, in the aforementioned documents. Further information on the implementation of plasma polymerization can be found in Ullmann's Encyclopedia of Industrial Chemistry, on CDROM, 5 ^th Ed., Keyword Plastics, Processing, Coating Processes and in Boing, H., Plasma Science and Technology, Carl Hanser Verlag, Munich 1982.

In the context of the present invention, the term plasma describes a partially ionized gas. A plasma can be generated by exciting a gas with electromagnetic radiation. The irradiation can take place both continuously and pulsed. Furthermore, DC or AC voltage sources can be used to generate the plasma. Devices for generating plasma can be obtained commercially, for example, from GaLa Gabler Labor Instrumente GmbH.

Depending on the process, the plasma polymerization can be carried out at a pressure of 0.001 to 1000 Pa, preferably 0.1 to 100 Pa and particularly preferably 1 to 50 Pa. The temperature during the plasma coating is preferably in the range from 0 ° to 300 °, preferably 5 to 250 ° C, without this being intended to impose any restriction.

In addition to water, the precursors to be used for plasma coating comprise a matrix-forming component. The matrix-forming component in particular comprises unsaturated organic compounds. These include under other alkenes, especially ethylene, propylene, 1-hexene, 1-heptene, vinylcyclohexane, 3,3-dimethyl-1-propene, 3-methyl-1-diisobutylene, 4-methylpentene-1; Alkynes, especially ethyne, propyne, butyne, hexyne-1; Vinyl compounds comprising an acidic group, especially vinylphosphonic acid, vinylsulfonic acid, acrylic acid and methacrylic acid; Vinyl compounds which comprise a basic group, in particular vinylpyridine, 3-vinylpyridine, 2-methyl-5-vinylpyridine, 3-ethyl-4-vinylpyridine, 2,3-dimethyl-5-vinylpyridine, N-vinylpyrrolidone, 2-vinylpyrrolidone, N -Vinylpyrrolidine and 3-vinylpyrrolidine; fluorinated alkenes, especially monofluoroethylene, difluoroethylene, trifluoroethylene, tetrafluoroethylene, hexafluoropropylene, pentafluoropropylene, trifluoropropylene, hexafluoroisobutylene, trifluorovinylsulfonic acid, trifluorovinylphoshonic acid and perfluorovinylmethyl ether.

The aforementioned compounds can be used individually or as a mixture.

The proportion of matrix-forming component is generally 1 to 99% by weight, preferably 50 to 99% by weight, particularly preferably 60 to 99% by weight, based on the gas mixture used for the plasma coating.

The proportion of water is generally 1 to 99% by weight, preferably 1 to 50% by weight, particularly preferably 1 to 40% by weight, based on the gas mixture used for the plasma coating.

Furthermore, the gas mixture can comprise an inert carrier gas. These include, for example, noble gases such as helium and neon.

The components of the gas mixture used for plasma polymerization can be mixed before being introduced into the coating chamber. Furthermore, the different connections can also be introduced separately into the chamber. The treatment time can be in a wide range. The polyazole film, which can optionally be doped, is preferably coated for 10 seconds to 10 hours, preferably 1 minute to 1 hour, under plasma conditions.

The flow velocities of the gases in the vacuum chamber, the energy used to generate the plasma and further process parameters can be in a wide range, and the parameters customary for the method used can be selected. This information can generally be found in the operating instructions for the respective systems.

The preferred methods for producing a coating according to the invention include, in addition to plasma polymerization with continuous power feed, also the plasma pulse chemical vapor deposition method (PICVD).

The PICVD method is described, for example, in Journal of the ceramic society of Japan, 99 (10), 894-902 (1991), the coating of curved surfaces also being disclosed (cf. WO 95/26427).

In the PICVD processes, the electromagnetic radiation that excites the plasma is usually supplied in a pulsed manner with a continuous flow of the coating gases, with a thin layer (typically about 1 nm, monolayer range) being deposited on the substrate with each pulse. After each power pulse there is a pulse pause, so that high coating speeds are possible without significant temperature stress on the substrate. The height and duration of the power pulses and the duration of the pulse pause are particularly important for the production of a layer. The pulse height is a performance measure in a PICVD process. It corresponds to the pulse power, i.e. H. the product of generator voltage and generator current during the pulse duration. The proportion of the power that is actually injected into the plasma depends on a number of parameters, e.g. B. from the dimensioning of the pulse-emitting component and the reactor.

Depending on the pulse height a) from a threshold value that is characteristic of each gas, different excitations and reactions in the plasma are generated, b) different thicknesses of the plasma zone are set.

When using the PICVD process, elementary layers (monolayers) of different compositions can be deposited in layers from pulse to pulse by selecting the appropriate pulse height. This is done in particular by a suitable choice of the pulse pause, so that the same gas composition is always present with each pulse, for example by a clean separation of the exhaust gas from the fresh gas. The same is not possible with the usual PCVD method.

The following ranges of the performance parameters are particularly preferred: pulse duration: 0.01 to 10 milliseconds, in particular 0.1 and 2 milliseconds; Pulse pause: 1 to 1000 milliseconds, in particular 5 to 500 milliseconds; and pulse height: 10 to 100,000 watts.

The PICVD process is carried out with alternating voltage pulses with a frequency between 50 kHz and 300 gigahertz, the frequencies of 13.56 MHz and 2.45 GHz being particularly preferred.

The flow rate of the gas in the PICVD process is generally chosen so that the gas can be regarded as stationary during the pulse. Accordingly, the mass flows are generally in the range from 1 to 200 standard cm ³ / minute, preferably in the range from 5 to 100 standard cm ³ / minute.

The intrinsic conductivity of the plasma-polymerized ion-conducting layer is between 0.001 S / cm and 0.3 S / cm at 80 ° C., depending on the mixing ratio of the matrix-forming component and the water in the plasma, without any intention that this should impose a restriction. These values are determined in a simple manner by means of impedance spectroscopy, the plasma-polymerized layers being deposited on a dielectric support on which two or four electrodes, preferably platinum or gold electrodes, are deposited beforehand, depending on the measurement technique, using thin-film technology. A Temperature-dependent measurement of the conductivity is carried out by heating the sample, for example by means of a heating plate with temperature regulation via a temperature sensor, which is positioned in the immediate vicinity of the layer to be measured, or by heating the sample in a suitable measuring cell in an oven.

Due to the manufacturing process and the resulting high degree of cross-linking, the plasma-polymerized ion-conducting layers are characterized by high stability. Aging and stability studies can e.g. by an annealing in the temperature range between 100 ° C and 500 ° C, with a structural examination of the structure of the plasma-polymerized layers, e.g. Using infrared spectroscopy, statements about the structural changes occurring as a result of the tempering and thus about the stability of these layers are permitted.

The polyazole membranes provided with a layer obtainable by plasma polymerization show a surprisingly high conductivity over a wide temperature range. Thus, the membranes obtainable according to the invention show a surprisingly high conductivity both at low temperatures in the range from 0 ° C. to 50 ° C. and at high temperatures above 120 ° C.

According to a particular aspect of the present invention, the polyazole membranes provided with a layer obtainable by plasma polymerization and doped with an acid have a high conductivity of at least 0.005, in particular at least 0.01 S / cm, particularly preferably at least 0.02S / cm at 120 ° C. without any limitation. These values are determined using impedance spectroscopy.

The specific conductivity can be measured by means of impedance spectroscopy in a 4-pole arrangement in potentiostatic mode and using platinum electrodes (wire, 0.25 mm diameter). The distance between the current-consuming electrodes is 2 cm. The spectrum obtained is with a simple model consisting of a parallel arrangement view of an ^ohmic resistance and a capacitor evaluated. The sample cross section of the phosphoric acid-doped membrane is immediately before the sample assembly measured. To measure the temperature dependence, the measuring cell is brought to the desired temperature in an oven and controlled via a Pt-100 thermocouple positioned in the immediate vicinity of the sample. After reaching the temperature, the sample is kept at this temperature for 10 minutes before starting the measurement.

Furthermore, the acid present in the polyazole film is surprisingly retained in the film by the coating according to the invention, without the acid being washed out during operation at low temperatures.

To measure the barrier effect of a layer obtained according to the invention by plasma polymerization using the example of phosphoric acid-doped membranes, the procedure can be as follows:

The barrier effect is measured in a simple manner by changing the pH of water as a function of time. For this purpose, a measuring cell is used, which consists of two chambers, which are separated by a plasma-polymerized layer according to the invention. The water to be measured and a pH electrode are located in one chamber, while in the other a solution, preferably a phosphoric acid solution of known concentration or a phosphoric acid-doped polyazole membrane is placed in direct contact with the plasma-polymerized layer.

To separate the two chambers, the plasma-polymerized layer according to the invention is advantageously placed on a porous support, e.g. a porous film or porous ceramic. This coated carrier is inserted into a suitable holder, which separates the two chambers of the measuring cell and leaves a defined surface area of the plasma-polymerized layer on the carrier accessible on both sides.

In addition, a polyazole membrane coated according to the invention and doped with an acid shows a very low overvoltage. This property is maintained over a long period of operation and many start-up cycles. Furthermore, the polyazole membranes provided with a coating according to the invention have a surprisingly high durability, which is evident both in operation at low and at high temperatures.

The present invention also relates to a membrane-electrode unit which has at least one polymer membrane according to the invention based on polyazoles.

For further information on membrane electrode assemblies, reference is made to the technical literature, in particular to the patents US-A-4,191, 618, US-A-4,212,714 and US-A-4,333,805. The disclosure contained in the abovementioned references [US-A-4,191, 618, US-A-4,212,714 and US-A-4,333,805] with regard to the construction and manufacture of membrane-electrode assemblies, as well as the electrodes, gas diffusion layers and Catalysts are also part of the description.

In a further variant, a catalytically active layer can be applied to the membrane according to the invention and this can be connected to a gas diffusion layer.

The present invention also relates to a membrane-electrode unit which contains at least one polymer membrane according to the invention, optionally in combination with a further polymer membrane based on polyazoles or a polymer blend membrane.

An advantage of MEAs that include polyazole membranes is that they allow the fuel cell to operate at temperatures above 120 ° C. This applies to gaseous and liquid fuels, e.g. Gases containing hydrogen, e.g. be produced from hydrocarbons in an upstream reforming step. As an oxidant, e.g. Oxygen or air can be used.

Another advantage of the MEAs comprising polyazole membranes is that, when operated above 120 ° C., they also work with pure platinum catalysts, ie without one another alloy component, have a high tolerance to carbon monoxide. At temperatures of 160 ° C, for example, more than 1% CO can be contained in the fuel gas without this leading to a noticeable reduction in the performance of the fuel cell.

The MEAs with doped polyazole films can be operated in fuel cells without the fuel gases and oxidants not having to be humidified despite the possible high operating temperatures. The fuel cell still works stably and the membrane does not lose its conductivity. This simplifies the entire fuel cell system and brings additional cost savings, since the management of the water cycle is simplified. This also improves the behavior of the fuel cell system at temperatures below 0 ° C.

The MEAs, which have a doped polyazole film, surprisingly allow the fuel cell to be cooled to room temperature and below without any problems and then to be put back into operation without losing performance. In contrast, conventional fuel cells based on phosphoric acid must always be kept at a temperature above 80 ° C. even when the fuel cell system is switched off in order to avoid irreversible damage.

Furthermore, the MEAs with a polyazole membrane show very high long-term stability. It has been found that a fuel cell according to the invention may e.g. more than 1000 hours, preferably more than 2000 hours and particularly preferably more than 5000 hours at temperatures of more than 120 ° C. can be operated continuously with dry reaction gases without a noticeable degradation in performance being ascertainable. The power densities that can be achieved are very high even after such a long time.

Claims

claims

1. A method for producing a plasma-polymerized ion-conducting electrolyte membrane, characterized in that it is produced by means of a plasma-assisted co-polymerization with a matrix-forming component, preferably carbon or fluorine-carbon compounds, and water.

2. A method for producing a plasma-polymerized ion-conducting electrolyte membrane according to claim 1, characterized in that fluorinated alkenes, preferably tetrafluoroethylene, are used as precursors for the matrix-forming component.

3. A process for producing a plasma-polymerized ion-conducting electrolyte membrane according to claim 1, characterized in that alkenes, preferably ethylene, are used as precursors for the matrix-forming component.

4. A method for producing a plasma-polymerized ion-conducting electrolyte membrane according to claim 1, characterized in that alkynes, preferably acetylene, are used as precursors for the matrix-forming component.

5. A method for producing a plasma-polymerized ion-conducting electrolyte membrane according to one or more of claims 1 to 4, characterized in that the layers are deposited in a parallel-plate plasma reactor.

6. A method for producing a plasma-polymerized ion-conducting electrolyte membrane according to claim 1 to 5, characterized in that the coating is carried out stationary.

7. The method for producing a plasma-polymerized ion-conducting electrolyte membrane according to claim 1 to 5, characterized in that the coating is carried out in a continuous process.

8. Use of the plasma-polymerized ion-conducting electrolyte membranes produced according to one or more of claims 1 to 7 in a fuel cell.

9. Use of the plasma-polymerized ion-conducting electrolyte membranes produced according to one or more of claims 1 to 7 as a thin barrier layer with respect to gas or liquid permeation on a polymer electrolyte membrane not produced by means of plasma polymerization.

10. Use of the plasma-polymerized ion-conducting electrolyte membranes produced according to one or more of claims 1 to 7 in an electrolysis cell.

11. Plasma-coated polyazole membrane, characterized in that a polyazole film is coated with a plasma-polymerized ion-conducting layer obtainable by a process according to claims 1 to 7.

12. Polyazole membrane according to claim 11, characterized in that the polyazole film is doped with an acid.

13. Polyazole membrane according to claim 12, characterized in that the degree of doping is between 3 and 15.

14. Polyazole membrane according to one of claims 11 to 13, characterized in that the plasma-polymerized ion-conducting layer has a thickness in the range from 10 nm to 20 microns.

15. polyazole membrane according to any one of claims 11 to 14, characterized in that the polyazole film polymers containing 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)

^ N AΓM- (I) X n

NN

- Ar ⁶ - ^ -Ar ⁶ -τ- (V)

X n

wherein

Ar are the same or different and, for a tetra-bonded aromatic or heteroaromatic group which may be mono- or polynuclear, Ar ^{1 are the} same or different and for a divalent aromatic or heteroaromatic group which may be mono- or polynuclear, Ar ^{2 are the} same or different Ar ^{3 are the} same or different for a two or three-membered aromatic or heteroaromatic group, which may be mononuclear or polynuclear, and for a tridentic aromatic or heteroaromatic group, which may be mononuclear, Ar ^{4 are the} same or different and for a three-membered aromatic or heteroaromatic group which can be mono- or polynuclear, Ar ^{5 are the} same or different and for a tetra-aromatic or heteroaromatic group which can be mono- or polynuclear, Ar ^{6 are the} same or different and for a double-bonded aromatic or heteroaromatic group, which can be mononuclear or polynuclear, Ar ⁷ identical or ve are different and for a divalent aromatic or heteroaromatic group, which can be mono- or polynuclear, Ar ^{8 are the} same or different and for a trivalent aromatic or heteroaromatic group, which can be mono- or polynuclear, Ar ^{9 are the} same or different and for a two- or three- or four-membered aromatic or heteroaromatic group which can be mono- or polynuclear, Ar ^{10 are} identical or different and for a di- or tri-aromatic or heteroaromatic group which can be mono- or polynuclear,

Ar ^{11 are the} same or different and are for a double-bonded aromatic or heteroaromatic group which can be mononuclear or polynuclear,

X is the same or different and for oxygen, sulfur or an amino group which carries a hydrogen atom, a group having 1-20 carbon atoms, preferably a branched or unbranched alkyl or alkoxy group, or an aryl group as a further radical

R is identical or different to hydrogen, an alkyl group and an aromatic group and n, m is an integer greater than or equal to 10, preferably greater than or equal to 100, includes.

16. Polyazole membrane according to one of claims 11 to 15, characterized in that the polyazole film polymers selected from the group polybenzimidazole, poly (pyridines), poly (pyrimidines), polyimidazoles, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles and poly ( tetrazapyrene).

17. Polyazole membrane according to one of claims 12 to 16, characterized in that the polyazole film is obtainable by a process comprising the steps

A) dissolving the polyazole polymer in polyphosphoric acid,

B) heating the solution obtainable according to step A) under inert gas to temperatures of up to 400 ° C.,

C) Forming a membrane using the solution of the polyazole polymer according to step B) on a support and

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

18. membrane electrode assembly containing at least one plasma-coated polyazole membrane according to claims 11 to 16.