DE102012007178A1 - Proton conducting polymer membrane based on polyoxazole, useful in membrane-electrode unit, obtainable by e.g. mixing aromatic diamino-dihydroxy compound and aromatic carboxylic acid, heating, and applying layer of mixture on carrier - Google Patents

Abstract

Proton conducting polymer membrane based on polyoxazole, obtainable by (i) mixing at least one aromatic diamino-dihydroxy compound with at least one aromatic carboxylic acid or its esters, or mixing at least one aromatic and/or heteroaromatic amino-hydroxy-carboxylic acid in polyphosphoric acid, (ii) heating the mixture obtained in step (i), (iii) applying a layer of the mixture on a carrier or on an electrode, (iv) at least partially hydrolyzing the polyphosphoric acid present in the layer, and (v) removing the obtained self-supporting membrane from the carrier, is claimed. Proton conducting polymer membrane based on polyoxazole, obtainable by (i) mixing at least one aromatic diamino-dihydroxy compound with at least one aromatic carboxylic acid or its esters containing at least two acid groups per carboxylic acid monomer, or mixing at least one aromatic and/or heteroaromatic amino-hydroxy-carboxylic acid in polyphosphoric acid to obtain a solution and/or a dispersion, (ii) heating the mixture obtained in step (i) under inert gas at 120-up to 300[deg] C to obtain polybenzoxazole polymer, (iii) applying a layer of the mixture obtained in step (ii) on a carrier or on an electrode, (iv) at least partially hydrolyzing the polyphosphoric acid present in the layer obtained in step (iii) by contacting with water and/or aqueous media, and (v) removing the obtained self-supporting membrane from the carrier, is claimed. Independent claims are also included for: (1) a membrane-electrode unit comprising at least one electrode and at least one above membrane; and (2) a fuel cell comprising at least one above membrane-electrode unit.

Description

The present invention relates to improved long life membrane electrode assemblies and fuel cells comprising two electrochemically active electrodes separated by a polymer electrolyte membrane.

In polymer electrolyte membrane (PEM) fuel cells today almost exclusively sulfonic acid-modified polymers are used as proton-conducting membranes. Here are predominantly perfluorinated polymers application.

A prominent example of this is Nafion ^™ from DuPont de Nemours, Willmington USA. Proton conduction requires a relatively high water content in the membrane, typically 4-20 molecules of water per sulfonic acid group. The necessary water content, but also the stability of the polymer in combination with acidic water and the reaction gases hydrogen and oxygen, limits the operating temperature of the PEM fuel cell stacks to 80-100 ° C. Higher operating temperatures can not be realized without a loss of power of the fuel cell. At temperatures that are above the dew point of water for a given pressure level, the membrane dries completely and the fuel cell no longer supplies electrical energy as the resistance of the membrane increases to such high levels that no appreciable current flow occurs.

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

In particular, when using so-called reformates of hydrocarbons in the reformer gas significant amounts of carbon monoxide contained, which usually must be removed by a complex gas treatment or gas cleaning. At high operating temperatures, the tolerance of the catalysts to the CO impurities increases.

Furthermore, heat is generated during the operation of fuel cells. However, cooling these systems to below 80 ° C can be very expensive. Depending on the power output, the cooling devices can be made much simpler. This means that the fuel cell systems, which are operated at temperatures above 100 ° C, the waste heat can be made much better usable and thus the fuel cell system efficiency can be increased.

In order to reach these temperatures, membranes with new conductivity mechanisms are generally used. One approach is to use membranes that exhibit ionic conductivity without the use of water. The first promising development in this direction is in the Scriptures WO 96/13872 explained. Another high-temperature fuel cell is in the document JP-A-2001-196082 disclosed.

Furthermore, it is off WO 02/088219 a second generation high temperature fuel cell based on polyazoles prepared by condensation polymerization in polyphosphoric acid (PPA) and partial hydrolysis of the same reaction mixture. These proton-conducting polymer membranes show against the WO 96/13872 known membranes improved properties. Nevertheless, these membranes for a permanent operation in a high-temperature fuel cell to be improved. In particular, at continuous use temperatures of 160-180 ° C and frequent switching on and off of the fuel cell degradation or aging of the membrane can not be excluded. Under certain circumstances, this degradation can lead to irreversible failure of the membrane-electrode assembly.

The object of the present invention is therefore to provide an improved proton-conducting polymer membrane which does not have the above-mentioned problem and thus leads to an improved MEU.

• • Die Zellen sollten bei einem Betrieb bei Temperaturen über 100°C eine lange Lebensdauer zeigen.
• • Die Einzelzellen sollten eine gleichbleibende oder verbesserte Leistung bei Temperaturen über 100°C über einen langen Zeitraum zeigen.
• • Hierbei sollten die Brennstoffzellen nach langer Betriebszeit eine hohe Ruhespannung sowie einen geringen Gasdurchtritt aufweisen (gas cross over).
• • Die Brennstoffzellen sollten insbesondere bei Betriebstemperaturen oberhalb von 100°C eingesetzt werden können und ohne zusätzliche Brenngasbefeuchtung auskommen. Insbesondere sollten die Membran-Elektroden-Einheiten permanenten oder wechselnden Druckdifferenzen zwischen Anode und Kathode widerstehen können.
• • Des Weiteren war es mithin Aufgabe der vorliegenden Erfindung eine Membran-Elektroden-Einheit zur Verfügung zu stellen, die einfach und kostengünstig hergestellt werden kann. Hierbei sollte insbesondere möglichst wenig von teuren Materialien eingesetzt werden.
• • Insbesondere sollte die Brennstoffzelle auch nach langer Zeit eine hohe Spannung aufweisen und bei niedriger Stöchiometrie betrieben werden können.
• • Insbesondere sollte die MEE robust gegen unterschiedliche Operationsbedingungen (T, p, Geometrie etc.) sein um die allgemeine Zuverlässigkeit zu erhöhen.
• • Weiterhin sollte teures Edelmetall, insbesondere Platinmetalle, sehr effektiv ausgenutzt werden.

The membrane or MEU according to the invention which contains such a membrane has in particular the following properties:

• • The cells should show a long life when operated at temperatures above 100 ° C.
• • The single cells should show consistent or improved performance at temperatures over 100 ° C over a long period of time.
• • In this case, the fuel cells should have a high quiescent voltage and a low gas penetration (gas crossover) after a long period of operation.
• • The fuel cells should be able to be used in particular at operating temperatures above 100 ° C and do without additional fuel gas moistening. In particular, the membrane-electrode assemblies should be able to withstand permanent or alternating pressure differences between anode and cathode.
• • Furthermore, it was therefore an object of the present invention to provide a membrane-electrode assembly available that can be easily and inexpensively manufactured. In particular, as little as possible of expensive materials should be used.
• • In particular, the fuel cell should have a high voltage even after a long time and be operated at low stoichiometry.
• • In particular, the MEE should be robust against different operating conditions (T, p, geometry, etc.) to increase overall reliability.
• • Furthermore, expensive precious metals, especially platinum metals, should be used very effectively.

We have now found that a specific proton-conducting membrane based on polybenzoxazoles can be obtained, which fulfills the aforementioned requirement profile.

These objects are achieved by the proton-conducting membrane having all the features of claim 1, as well as the pointed out in the dependent claims preferred embodiments, and a membrane-electrode assemblies containing a proton-conducting membrane according to the invention.

• (i) Mischen von (a) einem oder mehreren aromatischen Di-Amino-Di-Hydroxy-Verbindungen mit (b) einer oder mehreren aromatischen Carbonsäuren bzw. deren Estern, die mindestens zwei Säuregruppen pro Carbonsäure-Monomer enthalten, oder Mischen von einer oder mehreren aromatischen und/oder heteroaromatischen Amino-hydroxy-carbonsäuren in Polyphosphorsäure unter Ausbildung einer Lösung und/oder Dispersion,
• (ii) Erwärmen der Mischung aus Schritt (i), vorzugsweise unter Inertgas, auf Temperaturen im Bereich von 120°C und bis zu 300°C unter Ausbildung des Polybenzoxazol-Polymeren,
• (iii) Aufbringen einer Schicht unter Verwendung der Mischung aus Schritt ii) auf einen Träger oder auf eine Elektrode,
• (iv) zumindest partielle Hydrolyse der in der Schicht aus Schritt (iii) vorhandenen Polyphosphorsäure durch in Kontaktbringen mit Wasser und/oder wasserhaltigen Medien,
• (v) Ablösen der gebildeten selbsttragenden Membran vom Träger.

The present invention is a proton-conducting polymer membrane based on polybenzoxazoles obtainable by a process comprising the steps

• (i) mixing (a) one or more aromatic di-amino-di-hydroxy compounds with (b) one or more aromatic carboxylic acids or their esters containing at least two acid groups per carboxylic acid monomer, or mixing one or more a plurality of aromatic and / or heteroaromatic amino-hydroxy-carboxylic acids in polyphosphoric acid to form a solution and / or dispersion,
• (ii) heating the mixture of step (i), preferably under inert gas, to temperatures in the range of 120 ° C and up to 300 ° C to form the polybenzoxazole polymer,
• (iii) applying a layer using the mixture from step ii) to a support or to an electrode,
• (iv) at least partial hydrolysis of the polyphosphoric acid present in the layer of step (iii) by contact with water and / or hydrous media,
• (v) detaching the formed self-supporting membrane from the support.

The aromatic di-amino-di-hydroxy compounds used according to the invention are preferably 3,3'-dihydroxy-4,4'-diaminobiphenyl, 3,3'-dihydroxy-4,4'-diamino-diphenylsulfone, 4 , 6-diamino-1,3-dihydroxybenzene (DABDO), and their salts, especially their mono- and / or di-hydrochloride derivatives.

The aromatic carboxylic acids used according to the invention are di-carboxylic acids and di-carboxylic acids in combination with tri-carboxylic acids and / or tetra-carboxylic acids. Instead of the aromatic carboxylic acids and their esters, anhydrides or acid chlorides can be used. Within the aromatic di-carboxylic acids, those in which the acid groups are in the para position on the aromatic ring are preferred.

The term aromatic carboxylic acids equally includes heteroaromatic carboxylic acids. Preferably, the aromatic dicarboxylic acids are isophthalic acid, terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid, 5-aminoisophthalic acid, 5-N, N-dimethylaminoisophthalic acid, 5-N, N-diethylaminoisophthalic acid, 2.5 Dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid. 3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalic acid, 2-fluoroterphthalic 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, diphenylether-4,4'-dicarboxylic acid, benzophenone-4,4'-dicarboxylic acid, Diphenylsulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid, 4-trifluoromethylphthalic acid, 2,2-bis (4-carboxyphenyl) hexafluoropropane, 4,4'-stilbenedicarboxylic acid, 4-carboxycinnamic acid, or their C1- C20 alkyl esters or C5 C12 aryl esters, or their acid anhydrides or their acid chlorides.

The aromatic tri-carboxylic acids or their C1-C20-alkyl esters or C5-C12-aryl esters or their acid anhydrides or their acid chlorides are preferably 1,3,5-benzenetricarboxylic acid (trimesic acid), 1 , 2,4-benzenetricarboxylic acid (trimellitic acid), (2-carboxyphenyl) iminodiacetic acid, 3,5,3'-biphenyltricarboxylic acid, 3,5,4'-biphenyltricarboxylic acid.

The aromatic tetracarboxylic acids or their C 1 -C 20 -alkyl esters or C 5 -C 12 -aryl esters or their acid anhydrides or their acid chlorides are preferably 3,5,3 ', 5'-biphenyltetracarboxylic acid, 1,2, 4,5-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- naphthalene.

The heteroaromatic carboxylic acids used according to the invention are heteroaromatic dicarboxylic acids, heteroaromatic tricarboxylic acids and heteroaromatic tetracarboxylic acids or their esters or their anhydrides. Heteroaromatic carboxylic acids are aromatic systems which contain at least one nitrogen, oxygen, sulfur or phosphorus atom in the aromatic. Preferably pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-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 / or heteroaromatic aminohydroxycarboxylic acids used according to the invention are preferably 3-amino-4-hydroxybenzoic acid and 4-amino-3-hydroxybenzoic acid

In step A) it is preferred to use mixtures of at least 2 different aromatic carboxylic acids. Particular preference is given to using mixtures which, in addition to aromatic carboxylic acids, also contain heteroaromatic carboxylic acids. The mixing ratio of aromatic carboxylic acids to heteroaromatic carboxylic acids is between 1:99 and 99: 1, preferably 1:50 to 50: 1, in particular 1:10 to 10: 1.

These mixtures are in particular mixtures of N-heteroaromatic di-carboxylic acids and aromatic dicarboxylic acids. Non-limiting examples thereof are isophthalic acid, terephthalic acid, phthalic acid, 2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid. 3,4-dihydroxyphthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl ether-4,4 ' dicarboxylic acid, benzophenone-4,4'-dicarboxylic acid, diphenylsulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid, 4-trifluoromethylphthalic acid, pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid, Pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoldicarboxylic acid, 2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid.

The polyphosphoric acid used in step A) are commercially available polyphosphoric acids, such as are obtainable, for example, from Riedel-de Haen. The polyphosphoric acids H _{n + 2} P _n O _{3n + 1} (n> 1) usually have a content calculated as P ₂ O ₅ (acidimetric) of at least 79.8% which corresponds to a concentration of min. 110% H ₃ PO ₄ corresponds. Instead of a solution of the monomers, it is also possible to produce a dispersion / suspension.

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

As the polyoxazole polymers are understood which have at least one oxygen heteroatom and at least one nitrogen heteroatom in the aromatic system. The aromatic system may be mononuclear or polynuclear and also includes fused aromatic ring systems. Particularly preferred are aromatic systems in which an aromatic ring has at least one oxygen heteroatom and at least one nitrogen heteroatom.

The aromatic ring is preferably a five- or six-membered ring having a nitrogen atom and an oxygen atom which may be fused to another ring, in particular another aromatic ring.

High-temperature stability in the context of the present invention is a polymer which can be operated as a polymer electrolyte in a fuel cell at temperatures above 120 ° C permanently. Permanently means that a membrane according to the invention can be operated for at least 100 hours, preferably for at least 500 hours at at least 80 ° C., preferably at least 120 ° C., more preferably at least 160 ° C., without the power corresponding to that given in WO 01/18894 A2 method can be measured by more than 50%, based on the initial power decreases.

worin
Ar gleich oder verschieden sind und für eine vierbindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar¹ gleich oder verschieden sind und für eine zweibindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar² gleich oder verschieden sind und für eine zwei oder dreibindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar³ gleich oder verschieden sind und für eine dreibindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar⁴ gleich oder verschieden sind und für eine dreibindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar⁵ gleich oder verschieden sind und für eine vierbindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar⁶ gleich oder verschieden sind und für eine zweibindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar⁷ gleich oder verschieden sind und für eine zweibindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar⁸ gleich oder verschieden sind und für eine zweibindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
Ar⁹ gleich oder verschieden sind und für eine zweibindige aromatische oder heteroaromatische Gruppe stehen, die ein- oder mehrkernig sein kann,
X gleich oder verschieden ist und für Sauerstoff steht,
n eine ganze Zahl größer gleich 10, bevorzugt größer gleich 100 ist,
umfassen.A particularly preferred group of polyoxazole polymers are those which contain repeating oxazole units of the general formula (I) and / or (II) and / or (III) and / or (IV) and / or (V) and / or (VI) and / or (VII)

wherein
Ar are the same or different and represent a four-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{1 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{2 are the} same or different and represent a two or trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{3 are the} same or different and represent a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{4 are the} same or different and represent a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{5 are the} same or different and represent a four-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{6 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{7 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{8 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,
Ar ^{9 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear,
X is the same or different and represents oxygen,
n is an integer greater than or equal to 10, preferably greater than or equal to 100,
include.

Preferred aromatic or heteroaromatic groups are derived from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulfone, quinoline, pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine, tetrazine, pyrol, pyrazole, anthracene, benzopyrrole, benzotriazole, Benzooxathiadiazole, benzooxadiazole, benzopyrdine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzopyrazine, benzotriazine, indolizine, quinolizine, pyridopyridine, imidazopyrimidine, pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline, phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthroline and phenanthrene, which may also be substituted can be off.

In this case, the substitution pattern of Ar ¹ , Ar ⁴ , Ar ⁶ , Ar ⁷ , Ar ⁸ , Ar ^{9 is} arbitrary, in the case of phenylene, for example, Ar ¹ , Ar ⁴ , Ar ⁶ , Ar ⁷ , Ar ⁸ , Ar ⁹ 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 polyoxazoles having repeating units of the formula (I) in which the radicals X within a repeating unit are identical.

The polyoxazoles can in principle 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 polyoxazole polymers are polybenzoxazoles.

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 (VII) 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 repeating oxazole units is a polyoxazole containing only units of formula (I) and / or (II).

The number of repeating oxazole units in the polymer is preferably an integer greater than or equal to 10. Particularly preferred polymers contain at least 100 repeating oxazole units.

wobei n jeweils eine ganze Zahl größer gleich 10, vorzugsweise größer gleich 100 ist.In the context of the present invention, polymers containing recurring units of the following formulas are preferred.

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

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

The preparation of polyoxazole is known in principle, wherein the underlying monomers are reacted in the melt to form a prepolymer. The resulting prepolymer solidifies in the reactor and is then mechanically comminuted. The powdery prepolymer is customarily polymerized in a solid state polymerization at temperatures of up to 400 ° C.

In addition, it is known, in particular in the production of laboratory quantities, to condense the basic monomers in polyphosphoric acid and then to precipitate them by introducing them into water and to wash them neutrally.

The aforementioned polyoxazole polymers can be used individually or as a mixture (misery). Particular preference is given here to blends which contain polyazoles, as in WO 02/088219 . WO 02/081547 . WO 03/022412 described, and / or polysulfones. The preferred blend components are polyethersulfone, polyetherketone and polymers modified with sulfonic acid groups as in the patent application EP-A-1337319 and US 2004/075172 described. The use of blends improves mechanical properties and reduces material costs.

Insofar as the polyoxazoles according to the invention, in particular the polybenzoxazoles, are to be used as a miso, they are mixed with the polymers described below, but in particular with polysulfones and / or polyethersulfone or already added in the preparation of the polyoxazole, for example in step i) (reactor blend ).

The preferred polysulfones include in particular polysulfone having aromatic and / or heteroaromatic groups in the main chain. According to a particular aspect of the present invention, preferred polysulfones and polyether sulfones have a melt volume rate MVR 300 / 21.6 smaller 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 / Measured after 10 minutes ISO 1133 on. Here, polysulfones having a Vicat softening temperature VST / A / 50 of 180 ° C to 230 ° C are preferred. In still a preferred embodiment of the present invention, the number average molecular weight of the polysulfones is greater than 30,000 g / mol.

worin die Reste R unabhängig voneinander gleich oder verschieden eine aromatische oder heteroaromatische Gruppen darstellen, wobei diese Reste zuvor näher erläutert wurden. Hierzu gehören insbesondere 1,2-Phenylen, 1,3-Phenylen, 1,4-Phenylen, 4,4'-Biphenyl, Pyridin, Chinolin, Naphthalin, Phenanthren.The polymers based on polysulfone include, in particular, polymers which have recurring units with linking sulfone groups corresponding to the general formulas A, B, C, D, E, F and / or G:

wherein the radicals R, independently of one another or different, represent an aromatic or heteroaromatic group, these radicals having been explained in more detail above. These include in particular 1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 4,4'-biphenyl, pyridine, quinoline, naphthalene, phenanthrene.

Preferred polysulfones for the purposes of the present invention include homopolymers and copolymers, for example random copolymers. Particularly preferred polysulfones comprise recurring units of the formulas H to N:

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

In addition, polyether ketones, polyether ketone ketones, polyether ether ketones, polyether ether ketone ketones and polyaryl ketones are particularly preferred. These high-performance polymers are known per se and can be obtained commercially under the trade names Victrex ^® PEEK ^{™, ®} Hostatec, ^® Kadel.

Other suitable glare materials are the in WO 02/088219 . WO 02/081547 . WO 03/022412 described polyazoles. These can be produced by adding the corresponding monomers in the mixture according to step i) in polyphosphoric acid. For this purpose, the monomers are used for the blend material polyazole in amounts of up to 100 mol% based on the monomers for the polyoxazole, so that a maximum of 50 mol% polyazole are present in the blend and the remaining 50 mol% polyoxazole. The lower limit is not limited, but 5 to 10 mol% of monomers of the polyazole based on the monomers are preferred for the polyoxazole. Below these levels, the contributions of the blend polyazole are very limited. By adding polyazoles, the excellent application properties of polyoxazoles can be improved.

The heating of the mixture in step ii) takes place in the temperature range from 120 to 300 ° C. In this case, it is advantageous to increase the temperature gradually, preferably at intervals of 20-30 ° C. The duration of the heating is usually between 2 and 100 hours, preferably between 5 and 80 hours

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

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

In a variant of the method can be carried out by heating according to step ii) even after the formation of the layer according to step iii). This Dünschichtpolymerisation takes place basically in the temperature range of 120 to 300 ° C, but can also act for a short time higher temperatures. Instead of a purely thermal heating and a high-energy electromagnetic radiation, z. As IR and / or NIR radiation, microwaves and the like, are used. In addition, only a warming, z. B. in the lower temperature range and then further heating in the form of said layer.

In a variant of the present invention, the membrane formation can also take place directly on the electrode instead of on a support. The at least partial hydrolysis in step iv) can be shortened accordingly, since the membrane no longer has to be self-supporting. Such a membrane is also the subject of the present invention.

The at least partial hydrolysis of the remaining polyphosphoric acid in step iv) takes place in such a way that the membrane located on the support is brought into contact with water and / or an aqueous medium.

The hydrolysis is preferably carried out at temperatures above 0 ° C and below 200 ° C, preferably at temperatures between 10 ° C and 120 ° C, in particular. between room temperature (20 ° C) and 90 ° C. Water-containing medium is water and / or water vapor and / or water-containing phosphoric acid of up to 85%. The hydrolysis 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 is at least partially hydrolyzed. This form u. a. low molecular weight polyphosphoric acid and / or phosphoric acid, which contribute to the solidification of the membrane. Insofar as the ambient air has sufficient humidity, z. Min. 30% relative humidity, the hydrolysis can also be carried out by the ambient air.

In addition to the partial hydrolysis, a complete hydrolysis of the existing polyphosphoric acid is possible.

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

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

The at least partial hydrolysis in step iv) causes a sol-gel transition and results in a rubbery membrane in which the polybenzoxazole acts as a superabsorbent for the polyphosphoric / phosphoric acid. The membranes of the invention have a high content of phosphoric acid and are not comparable with subsequently doped membranes.

The at least 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 due to the temperature or saturation of the contacting environment, for example, gases such as air, nitrogen, Carbon dioxide or other suitable gases, or steam can be adjusted specifically. The duration of treatment depends on the parameters selected above.

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

In general, the treatment time is between a few seconds to minutes, for example under the action of superheated steam, or up to full 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 at least partial hydrolysis is carried out at room temperature (20 ° C.) with ambient air having a relative atmospheric humidity of 40-80%, the treatment duration is between 1 and 200 hours.

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

About the degree of hydrolysis, d. H. the duration, temperature and ambient humidity, the concentration of phosphoric acid and thus the conductivity of the polymer membrane according to the invention is adjustable.

The at least partial hydrolysis can also be carried out in a water-containing liquid, which liquid may also contain suspended and / or dispersed constituents.

The viscosity of the hydrolysis liquid can be within wide ranges, wherein for adjusting the viscosity, an addition of solvents or a temperature increase can take place. Preferably, the dynamic viscosity is in the range of 0.1 to 10,000 mPa · s, in particular 0.2 to 2000 mPa · s, these values being, for example, according to DIN 53015 can be measured.

The at least partial hydrolysis according to step iv) can be carried out by any known method. For example, the membrane obtained in step iii) can be immersed in a liquid bath. Furthermore, the hydrolysis liquid can be sprayed on the membrane. Furthermore, the hydrolysis liquid can be poured over the membrane. The latter methods have the advantage that the concentration of acid in the hydrolysis fluid remains constant during the hydrolysis. However, the first method is often less expensive to implement.

The hydrolysis liquid comprises water-containing mixtures of oxygen acids of phosphorus and / or sulfur, in particular phosphinic acid, phosphonic acid, phosphoric acid, hypodiphosphonic acid hypodiphosphoric acid, oligophosphoric acids, sulfurous acid, disulfurous acid and / or sulfuric acid. These acids can be used individually or as a mixture.

The hydrolysis liquid comprises water, the concentration of the water generally not being particularly critical. According to a particular aspect of the present invention, the hydrolysis liquid comprises 5 to 80 wt .-%, preferably 8 to 70 wt .-% and particularly preferably 10 to 50 wt .-% water. The amount of water that is formally contained in the oxygen acids is not taken into account in the water content of the hydrolysis liquid.

Of the aforementioned acids, phosphoric acid and / or sulfuric acid are particularly preferred, these acids in particular comprising 5 to 70 wt .-%, preferably 10 to 60 wt .-% and particularly preferably 15 to 50 wt .-% water.

According to the invention, the concentration of phosphoric acid is given as mol of acid per mole of repeat unit of the polymer. In the context of the present invention, a concentration (moles of phosphoric acid relative to a repeat unit of the formula C ₁₈ H ₁₀ N ₂ O ₂ , ie polybenzoxazole) of between 10 and 50, preferably between 13 and 40, in particular between 15 and 35 moles is suitable. Such high doping levels (concentrations) are not accessible by subsequent doping of polymer films.

The membranes of the invention preferably contain between 2 and 15% by weight of polyoxazoles and between 40 and 70% by weight of phosphoric acid, the remainder being water. Particular preference is given to polyoxazole contents of from 5 to 10% by weight and contents of phosphoric acid of from 50 to 60% by weight, the remainder being water.

Following the hydrolysis according to step iv) or after the detachment according to step v), the membrane can still be treated 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.

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

The polymer membrane according to the invention has improved material properties compared to the previously known polymer membranes.

The membranes according to the invention have good proton conductivity. This is at temperatures of 160 ° C at least 0.1 S / cm, preferably at least 0.105 S / cm. The proton conductivity is determined without additional humidification of the required gases.

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 made either in step i) or after the polymerization.

Non-limiting examples of proton-conductive fillers are Sulfates like: 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 like 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 , Like polyacid 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 PMo ₁₂ 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 like (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 , Oxides like Al ₂ O ₃ , Sb ₂ O ₅ , ThO ₂ , SnO ₂ , ZrO ₂ , MoO ₃ Silicates like Zeolites, zeolites (NH ₄ +), phyllosilicates, framework silicates, H-natrolites, H-mordenites, NH ₄ -alcines, NH ₄ -sodalites, NH ₄ -gallates, H-montmorillonites Acids like HClO ₄ , SbF ₅ Fillers like Carbides, in particular SiC, Si ₃ N ₄ , fibers, in particular glass fibers, glass powders and / or polymer fibers, preferably based on polyazoles.

In addition, this membrane may also contain perfluorinated sulfonic acid additives (0.1-20 wt%, preferably 0.2-15 wt%, very preferably 0.2-10 wt%). These additives improve performance near the cathode to increase oxygen solubility and oxygen diffusion and reduce the adsorption of phosphoric acid and phosphate to platinum. ( Electrolyte additives for phosphoric acid fuel cells. Gang, Xiao; Hjuler, HA; Olsen, C .; Mountain, RW; Bjerrum, NJ Chem. Dep. A, tech. Univ. Denmark, Lyngby, Den. J. Electrochem. Soc. (1993), 140 (4), 896-902 and Perfluorosulfonimide as an additive in phosphoric acid fuel cell. Razaq, M .; Razaq, A .; Yeager, E .; DesMarteau, Darryl D .; Singh, S. Case Cent. Electrochem. Sci., Case West. Reserve Univ., Cleveland, OH, USA. J. Electrochem. Soc. (1989), 136 (2), 385-90 .).

Non-limiting examples of perfluorinated sulfonated additives are:
Trifluoromethanesulfonic acid, potassium trifluoromethanesulfonate,
Sodium trifluoromethanesulfonate, lithium trifluoromethanesulfonate,
Ammonium trifluoromethanesulfonate, potassium perfluorohexanesulfonate,
Sodium perfluorohexanesulfonate, lithium perfluorohexanesulfonate,
Ammonium perfluorohexanesulfonate, perfluorohexanesulfonic acid,
Potassium nonafluorobutanesulfonate, sodium nonafluorobutanesulfonate,
Lithium nonafluorobutanesulfonate, ammonium nonafluorobutanesulfonate,
Cesium nonafluorobutanesulfonate, triethylammonium perfluorohexasulfonate,
Perflurosulfoimides and Nafion.

Further, the membrane may also contain additives which trap (primary antioxidants) or destroy (secondary antioxidants) the peroxide radicals produced during oxygen reduction operation, and thereby as in US Pat JP 2001118591 A2 described life and stability of the membrane and membrane electrode unit improve. The functionality and molecular structures of such additives are in F. Gugumus in Plastics Additives, Hanser Verlag, 1990 ; NS Allen, M. Edge Fundamentals of Polymer Degradation and Stability, Elsevier, 1992 ; or H. Zweifel, Stabilization of Polymeric Materials, Springer, 1998 described.

Non-limiting examples of such additives are:
Bis (trifluoromethyl) nitroxide, 2,2-diphenyl-1-picrinylhydrazole, phenols, alkylphenols, hindered alkylphenols such as Irganox, aromatic amines, sterically hindered amines such as Chimassorb; sterically hindered hydroxylamines, sterically hindered alkylamines, sterically hindered hydroxylamines, sterically hindered hydroxylamine ethers, phosphites such as irgafos, nitrosobenzene, methyl-2-nitroso-propane, benzophenone, benzaldehyde-tert-butylnitrone, cysteamine, melanines, lead oxides, manganese oxides, nickel oxides , Cobalt oxides.

The polymer membrane of the invention have additional improved material properties compared to the previously known polymer membranes based on polyazoles. Thus, the membranes of the invention based on polyoxazoles show improved compression resistance. This improved compression resistance results in improved, long-term stability with constant or nearly constant electrochemical performance.

The polyazole membranes according to the invention have a compression resistance which is improved by a factor of 2 compared with polybenzimidazole membranes at operating temperatures of the membrane-electrode units of more than 100 ° C., preferably 180-200 ° C. This is determined in a known to the expert external test cell by measuring the decrease in thickness of a membrane sample within a predetermined time window under the action of a profiled test specimen at the relevant operating temperatures.

• 1) Umsetzung einer Mischung von (a) einem oder mehreren aromatischen Di-Amino-Di-Hydroxy-Verbindungen und (b) einer oder mehreren aromatischen Carbonsäuren bzw. deren Estern, die mindestens zwei Säuregruppen pro Carbonsäure-Monomer enthalten, oder einer Mischung von einer oder mehreren aromatischen und/oder heteroaromatischen Amino-hydroxy-carbonsäuren in der Schmelze bei Temperaturen von bis zu 350°C, vorzugsweise bis zu 300°C,
• 2) Lösen des gemäß Schritt 1) erhaltenen festen Prä-Polymeren in Polyphosphorsäure,
• 3) Erwärmen der Lösung erhältlich gemäß Schritt 2), vorzugsweise unter Inertgas, auf Temperaturen im Bereich von 120°C und bis zu 300°C unter Ausbildung des Polybenzoxazol-Polymeren,
• 4) Bilden einer Membran unter Verwendung der Lösung des Polybenzoxazol – Polymeren gemäß Schritt 3) auf einem Träger,
• 5) zumindest partielle Hydrolyse der in der Schicht aus Schritt (iii) vorhandenen Polyphosphorsäure durch in Kontaktbringen mit Wasser und/oder wasserhaltigen Medien,
• 6) Ablösen der gebildeten selbsttragenden Membran vom Träger.

According to a modification, the preparation of the membrane based on polyoxazoles according to the invention can also be carried out by a process comprising the steps

• 1) reacting a mixture of (a) one or more aromatic di-amino-di-hydroxy compounds and (b) one or more aromatic carboxylic acids or their esters containing at least two acid groups per carboxylic acid monomer, or a mixture of one or more aromatic and / or heteroaromatic amino-hydroxy-carboxylic acids in the melt at temperatures of 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), preferably under inert gas, to temperatures in the range of 120 ° C. and up to 300 ° C. to form the polybenzoxazole polymer,
• 4) forming a membrane using the solution of the polybenzoxazole polymer according to step 3) on a support,
• 5) at least partial hydrolysis of the polyphosphoric acid present in the layer of step (iii) by contact with water and / or hydrous media,
• 6) detachment of the formed self-supporting membrane from the carrier.

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

The possible fields of use of the doped polymer membranes according to the invention include, inter alia, the use in fuel cells, in electrolysis, in capacitors and in battery systems. Because of their property profile, the doped polymer membranes are preferably used in fuel cells.

The present invention also relates to a membrane-electrode assembly comprising at least one polymer membrane according to the invention. For more information about membrane-electrode assemblies, refer to the specialized literature, in particular to the patents US Patent No. 4,191,618 . US Pat No. 4,212,714 and US-A-4,333,805 directed. The references in the abovementioned references US Patent No. 4,191,618 . US Pat No. 4,212,714 and US-A-4,333,805 ] contained disclosure regarding the construction and manufacture of membrane-electrode assemblies, as well as the electrodes to be selected, gas diffusion layers and catalysts is also part of the description.

The membrane-electrode assembly according to the invention has two gas diffusion layers separated by the polymer electrolyte membrane. Usually flat, electrically conductive and acid-resistant structures are used for this purpose. These include, for example, graphite fiber papers, carbon fiber papers, graphite fabrics and / or papers made conductive by the addition of carbon black. Through these layers, a fine distribution of the gas and / or liquid streams is achieved.

This layer generally has a thickness in the range from 80 μm to 2000 μm, in particular from 100 μm to 1000 μm and particularly preferably from 150 μm to 500 μm.

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

This property generally comprises a gas diffusion layer of graphite fabric and / or paper rendered conductive by the addition of carbon black.

In addition to the two gas diffusion layers which are separated by the polymer electrolyte membrane, the membrane electrode assembly according to the invention still has a catalyst layer on each side of the membrane. The catalyst layer may be on both sides of the membrane or on the interface of the gas diffusion layer to the membrane.

The catalyst layer or catalyst layers contain or contain catalytically active substances. These include precious metals of the platinum group, d. H. Pt, Pd, Ir, Rh, Os, Ru, or the precious metals Au and Ag. Furthermore, it is also possible to use alloys of all the aforementioned metals. Further, at least one catalyst layer may contain alloys of the platinum group elements with base metals such as Fe, Co, Ni, Cr, Mn, Zr, Ti, Ga, V, etc. In addition, the oxides of the aforementioned noble metals and / or non-noble metals can also be used.

The catalytically active particles which comprise the abovementioned substances can be used as metal powder, so-called black noble 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, electrically conductive metal oxides, such as SnO _x , TiO _x , or phosphates, such as. B. FePO _x , NbPO _x , Zr (PO _x ) _z can be used _as a 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 assume 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 limitation. The particle size of the carrier, in particular the size of the carbon particles, is preferably in the range of 20 to 1000 nm, in particular 30 to 100 nm. The size of the metal particles present thereon is preferably in the range of 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 catalytically active particles set forth above can generally be obtained commercially.

Furthermore, the catalytically active layer may contain conventional additives. These include, inter alia, fluoropolymers such. As polytetrafluoroethylene (PTFE), proton-conducting ionomers and surfactants.

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, greater than 0.1, wherein this ratio is preferably in the range of 0.2 to 0.6.

According to a particular embodiment of the present invention, the catalyst layer has a thickness in the range from 1 to 1000 .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 noble metal content of the catalyst layer is 0.1 to 10.0 mg / cm ² , preferably 0.3 to 6.0 mg / cm ² and more preferably 0.3 to 3.0 mg / cm ² , These values can be determined by elemental analysis of a flat sample.

The preparation of membrane electrode assemblies according to the invention will be apparent to those skilled in the art. Generally, the various components of the membrane-electrode assembly are superimposed and bonded together by pressure and temperature. In general, at a temperature in the range of 10 ° to 300 ° C, in particular 20 ° to 200 ° C and at a pressure in the range of 1 to 1000 bar, in particular from 3 to 300 bar laminated.

The finished membrane-electrode unit (MEU) is ready for operation after cooling and can be used in a fuel cell.

The membrane electrode assembly (MEU) according to the invention is suitable for operation at temperatures above 160 ° C and allows gaseous and / or liquid fuels, such. B. hydrogen-containing gases, the z. B. be prepared from hydrocarbons in an upstream reforming step. As an oxide can be z. As oxygen or air can be used.

Another advantage of the membrane-electrode assembly according to the invention is that when operating above 120 ° C with pure platinum catalysts, d. H. without a further alloying component, have a high tolerance to carbon monoxide. At temperatures of 160 ° C z. B. be more than 1% CO contained in the fuel gas, without resulting in a significant reduction in the performance of the fuel cell.

The membrane-electrode assembly according to the invention can be operated in fuel cells, without the fuel gases and the oxidants must not be moistened despite the possible high operating temperatures. The fuel cell is still stable and the membrane does not lose its conductivity. This simplifies the entire fuel cell system and brings additional cost savings, since the management of the water cycle (cooling) is simplified.

The membrane-electrode assembly according to the invention can be easily cooled to room temperature and below and then put back into operation without losing power.

In addition, the MEUs according to the invention can be produced inexpensively and easily.

The proton-conducting polymer membrane based on polyoxazoles according to the invention is distinguished by a considerably improved compression resistance. The membranes of the invention show a markedly small decrease in thickness at 200.degree. Thus, a membrane based on polyoxazoles in the described test method after 10 minutes at 200 ° C still a residual thickness of min. 50%, while a comparable membrane based on polyazoles (polybenzimidazole) only has a residual thickness of about 40%.

Preferably, a membrane according to the invention based on polyoxazole in the described test method after 20 minutes at 200 ° C, a residual thickness of min. 40%, while a comparable membrane based on polyazoles (polybenzimidazole) has only a residual thickness of less than 30%.

Particularly preferably, a membrane according to the invention based on polyoxazole in the described test method after 60 minutes at 200 ° C, a residual thickness of min. 35%, while a comparable membrane based on polyazoles (polybenzimidazole) only has a residual thickness of less than 25%.

General measuring methods:

Measuring method for IEC

The conductivity of the membrane depends strongly on the content of acid groups expressed by the so-called ion exchange capacity (IEC). To measure the ion exchange capacity, a sample with a diameter of 3 cm is punched out and placed in a beaker filled with 100 ml of water. The liberated acid is titrated with 0.1 M NaOH. The sample is then removed, excess water blotted and the sample dried at 160 ° C for 4 h. Then the dry weight, m ₀ , is determined gravimetrically with an accuracy of 0.1 mg. The ion exchange capacity is then calculated from the consumption of the 0.1 M NaOH to the first titration end point, V ₁ in ml, and the dry weight, m ₀ in mg, according to the following formula: IEC = V ₁ · 300 / m ₀

Measuring method for specific conductivity

The specific conductivity is measured by means of impedance spectroscopy in a 4-pole arrangement in the potentiostatic mode and using platinum electrodes (wire, 0.25 mm diameter). 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

Measuring method for compression resistance

A membrane sample of 4.91 cm ² diameter (d = 2.5 cm) is punched out and placed on a hot plate on a piece of Kapton foil. A metal die with three recesses (depth = 1 mm, width 2 mm, length 1.5 cm, spacing of 5 mm each) is placed on the membrane sample by means of guide rails and the thickness reduction at 200 ° C (± 10 ° C) by means of a Mitutoyo DC III measured for a duration of 120 minutes. The thickness decrease in [%] is calculated by [Thickness _after / Thickness _Start ] × 100.

Hereinafter, the present invention will be explained in more detail by an example and a comparative example, without this being intended to be limiting.

example 1

A solution of 4% by weight with equimolar amounts of 19.76 g of 3,3'-dihydroxy-4,4'-diaminobiphenyl and 15.25 g of terephthalic acid in polyphosphoric acid (116%) is heated to 240 ° C within 40 h. The resulting polybenzoxazole-polyphosphoric acid solution is cooled to a temperature of 100 ° C and applied by means of a hand doctor on a support in a 450 micron thick layer and hydrolyzed after cooling in 50 wt -% - phosphoric acid overnight and a self-supporting polybenzoxazole-phosphoric acid Membrane obtained. The membrane assemblies are listed in Table 1.

Comparative Example:

A solution of 2% by weight with equimolar amounts of 3,3 ', 4,4'-tetraaminobiphenyl and terephthalic acid in polyphosphoric acid (112%) is heated to 280 ° C within 100 h. The resulting polybenzimidazole-polyphosphoric acid solution is cooled to a temperature of 100 ° C and applied by means of a hand doctor on a support in a 450 micron thick layer and hydrolyzed after cooling in 50 wt% phosphoric acid overnight and a self-supporting polybenzimidazole phosphoric acid Membrane obtained. The membrane properties are listed in Table 1. Table 1: Comparison of the properties of reinforced and standard membrane Comparative example example thickness 390 μm 455 μm Proportion of phosphoric acid 54.1% 52.4% by weight Proportion of solid 5.6% by weight 6.5% by weight acid concentration 57.3% by weight 56.0% by weight Inherent viscosity 5.7 dL / g 1.9 dL / g proton conductivity 100 mS / cm at 160 ° C 105 mS / cm at 160 ° C

The compression resistance of the membrane of the invention and the comparative membrane are in the 1 shown, wherein the measurement was carried out as set forth above.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 96/13872 [0007, 0008]
• JP 2001-196082A [0007]
• WO 02/088219 [0008, 0045, 0052]
• WO 01/18894 A2 [0028]
• WO 02/081547 [0045, 0052]
• WO 03/022412 [0045, 0052]
• EP 1337319 A [0045]
• US 2004/075172 [0045]
• JP 2001118591 A2 [0086]
• US 4191618 A [0093, 0093]
• US 4212714 A [0093, 0093]
• US Pat. No. 4,333,805 A [0093, 0093]

Cited non-patent literature

• ISO 1133 [0047]
• DIN 53015 [0071]
• Electrolyte additives for phosphoric acid fuel cells. Gang, Xiao; Hjuler, HA; Olsen, C .; Mountain, RW; Bjerrum, NJ Chem. Dep. A, tech. Univ. Denmark, Lyngby, Den. J. Electrochem. Soc. (1993), 140 (4), 896-902 [0084]
• Perfluorosulfonimide as an additive in phosphoric acid fuel cell. Razaq, M .; Razaq, A .; Yeager, E .; DesMarteau, Darryl D .; Singh, S. Case Cent. Electrochem. Sci., Case West. Reserve Univ., Cleveland, OH, USA. J. Electrochem. Soc. (1989), 136 (2), 385-90 [0084]
• F. Gugumus in Plastics Additives, Hanser Verlag, 1990 [0086]
• NS Allen, M. Edge Fundamentals of Polymer Degradation and Stability, Elsevier, 1992 [0086]
• H. Zweifel, Stabilization of Polymeric Materials, Springer, 1998 [0086]

Claims (21)

A proton-conducting polymer membrane based on polyoxazoles obtainable by a process comprising the steps (i) mixing (a) one or more aromatic di-amino-di-hydroxy compounds with (b) one or more aromatic carboxylic acids or their esters containing at least two acid groups per carboxylic acid monomer, or Mixing one or more aromatic and / or heteroaromatic amino-hydroxy-carboxylic acids in polyphosphoric acid to form a solution and / or dispersion, (ii) heating the mixture of step (i), preferably under inert gas, to temperatures in the range of 120 ° C and up to 300 ° C to form the polybenzoxazole polymer, (iii) applying a layer using the mixture from step ii) to a support or to an electrode, (iv) at least partial hydrolysis of the polyphosphoric acid present in the layer of step (iii) by contact with water and / or hydrous media, (v) detaching the formed self-supporting membrane from the support. Membrane according to Claim 1, characterized in that 3,3'-dihydroxy-4,4'-diaminobiphenyl, 3,3'-dihydroxy-4,4'-diamino-diphenylsulfone, as aromatic di-amino-di-hydroxy compounds, 4,6-diamino-1,3-dihydroxybenzene (DABDO), and salts thereof, in particular their mono- and / or dihydrochloride derivatives. be used. Membrane according to Claim 1, characterized in that as aromatic dicarboxylic acids isophthalic acid, terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid, 5-aminoisophthalic acid, 5-N, N-dimethylaminoisophthalic acid, 5-N, N-diethylaminoisophthalic acid, · 2,5-dihydroxyterephthalic acid, 2,5-dihydroxyisophthalic acid, 2,3-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-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl ether-4,4'-dicarboxylic acid, benzophenone-4,4'-dicarboxylic acid, diphenylsulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid, 4- Trifluoromethylphthalic acid, 2,2-bis (4-carboxyphenyl) hexafluoropropane, 4,4'-stilbenedicarboxylic acid, 4-carboxycinnamic acid, or their C1-C20-alkyl esters or C5-C12-aryl esters, or their acid anhydrides or their acid chlorides be used. Membrane according to Claim 1, characterized in that the aromatic carboxylic acid used is tri-carboxylic acids, tetracarboxylic acids or their C1-C20-alkyl esters or C5-C12-aryl esters or their acid anhydrides or their acid chlorides, 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 acid and / or 2,4,6-pyridinetricarboxylic acid are used. Membrane according to Claim 1, characterized in that the aromatic carboxylic acid tetracarboxylic acids are their C1-C20-alkyl esters or C5-C12-aryl esters or their acid anhydrides or their acid chlorides, preferably benzene, 1,2,4,5-tetracarboxylic acids; 1,4,5,8-naphthalenetetracarboxylic acids 3,5,3 ', 5'-biphenyltetracarboxylic 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. Membrane according to Claim 4, characterized in that 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% , is. Membrane according to Claim 1, characterized in that the heteroaromatic carboxylic acids used are heteroaromatic dicarboxylic acids and tricarboxylic acids and tetracarboxylic acids which contain at least one nitrogen, oxygen, sulfur or phosphorus atom in the aromatic, preferably pyridine-2,5-dicarboxylic acid, pyridine 3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoldicarboxylic acid, 2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid, 2,4,6-pyridine tricarboxylic acid, benzimidazole-5,6-dicarboxylic acid, and their C1-C20 alkyl esters or C5-C12 aryl esters, or their acid anhydrides or their acid chlorides. Membrane according to Claim 1, characterized in that 3-amino-4-hydroxybenzoic acid and 4-amino-3-hydroxybenzoic acid are used as the aromatic and / or heteroaromatic aminohydroxycarboxylic acids A membrane according to claim 1, characterized in that in step A) a polyphosphoric acid having a content calculated as P ₂ O ₅ (acidimetric) of at least 79.8% which corresponds to a concentration of min. 110% H ₃ PO ₄ is used. Membrane according to Claim 1, characterized in that the polyoxazole polymers contain repeating oxazole units of the general formula (I) and / or (II) and / or (III) and / or (IV) and / or (V) and / or (VI ) and / or (VII)

wherein Ar is the same or different and represents a tetravalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{1 may be the} same or different and represents a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ² are the same or different and are a two or three-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{3 are the} same or different and are a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ⁴ are the same or different and are a trivalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{5 are the} same or different and represent a four-membered aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{6 is the} same or are different and represent a divalent aromatic or heteroaromatic group, which may be mononuclear or polynuclear, Ar ^{7 are the} same or different and represent a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, Ar ^{8 are the} same or different and represent a divalent aromatic or heteroaromatic group, the - or polynuclear, Ar ^{9 are the} same or different and are a divalent aromatic or heteroaromatic group which may be mononuclear or polynuclear, X is the same or different and is oxygen, n is an integer greater than or equal to 10, preferably greater is equal to 100. Membrane according to Claim 1, characterized in that the polyoxazole is polybenzoxazoles. Membrane according to claim 1, characterized in that in step iii) a polymer containing recurring units of the formula

where n is an integer greater than or equal to 10, preferably greater than or equal to 100, is formed. Membrane according to claim 1, characterized in that the hydrolysis according to step iv) at temperatures between 0 ° C and 200 ° 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 takes place. Membrane according to claim 1, characterized in that the duration of the hydrolysis according to step iv) is between 10 seconds and 300 hours, preferably 1 minute to 200 hours. Membrane according to claim 1, characterized in that in step iii) a layer having a thickness between 20 and 4000 μm, preferably between 30 and 3500 μm, in particular between 50 and 3000 μm, is produced. Membrane according to Claim 1, characterized in that the membrane present after step iv) has a thickness between 15 and 3000 μm, preferably between 20 and 2000 μm, in particular between 20 and 1500 μm. Membrane according to one or more of Claims 1 to 16, characterized in that the content of polyoxazoles is between 2 and 15% by weight and the content of phosphoric acid is 40 and 70% by weight and the remainder is water. Membrane according to one or more of claims 1 to 17, characterized in that the membrane comprises at least one further polymer from the group polyazole, polysulfone, polyethersulfone, polyetherketone, wherein the polymers may also have sulphonic acid groups. Membrane according to one or more of claims 1 to 18, characterized in that the membrane has a proton conductivity, measured at 160 ° C and without additional humidification of the required gases, of at least 0.1 S / cm, preferably at least 0.105 S / cm , Membrane electrode unit comprising at least one electrode and at least one membrane according to one or more of claims 1 to 19. A fuel cell containing one or more membrane-electrode assemblies according to claim 20.

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