Multilayer electrolyte membrane

0001The present invention relates to a proton-conducting multilayer electrolyte membrane, a method for its production and a fuel cell containing such a membrane.

0002A fuel cell usually contains an electrolyte and two electrodes separated by the electrolyte. In the case of a fuel cell, one of the two electrodes is supplied with a fuel, such as hydrogen gas, and the other electrode with an oxidizing agent, such as oxygen gas, and chemical energy from the fuel oxidation is thereby converted into electrical energy.

0003The electrolyte is permeable to hydrogen ions, ie protons, but not to reactive gases such as the hydrogen gas and the oxygen gas.

0004A fuel cell generally has a plurality of individual cells, so-called MEEs (membrane electrode assemblies), each of which contains an electrolyte and two electrodes separated by the electrolyte.

0005Solids such as polymer electrolyte membranes or liquids such as phosphoric acid are used as the electrolyte for the fuel cell. Polymer electrolyte membranes have recently attracted attention as electrolytes for fuel cells. In principle, one can differentiate between two categories of polymer membranes.

0006The first category includes cation exchange membranes consisting of a polymer backbone which contains covalently bound acid groups, preferably sulfonic acid groups. The sulfonic acid group changes into an anion with the release of a hydrogen ion and therefore conducts protons. The mobility of the proton and thus the proton conductivity is directly linked to the water content. Dries the membrane, e.g. as a result of high temperature, the conductivity of the membrane and consequently the performance of the fuel cell decrease drastically. The operating temperatures of fuel cells containing such cation exchange membranes is thus limited to the boiling point of the water. For example, perfluorosulfonic acid polymers are used as materials for polymer electrolyte membranes. The perfluorosulfonic acid polymer (e.g. Nafion) generally has a perfluorocarbon backbone, such as a copolymer of tetrafluoroethylene and trifluorovinyl, and a side chain attached to it with a sulfonic acid group, such as a side chain with a sulfonic acid group attached to a perfluoroalkylene group.

0007The humidification of the fuels represents a major technical challenge for the use of polymer electrolyte membrane fuel cells (PEMBZ), which use conventional, sulfonated membranes such as Nafion.

0008As a second category, polymer electrolyte membranes with complexes of basic polymers and strong acids have been developed. So describes<patcit id="pcit0001" dnum="WO9613872A"><text>WO96 / 13872</text></patcit> and the corresponding one <patcit id="pcit0002" dnum="USPS5525436A"><text>U.S. Patent 5,525,436</text></patcit> a process for producing a proton conductive polymer electrolyte membrane in which a basic polymer such as polybenzimidazoles is treated with a strong acid such as phosphoric acid, sulfuric acid, etc.

0009Such a fuel cell, in which such a polymer electrolyte membrane is used, has the advantage that it can be operated without humidification and at temperatures of 100 ° C. or above.

0010In <nplcit id="ncit0001" npl-type="s"><text>J. Electrochem. Soc., Vol. 142, No. 7, 1995, pp. L121-L123</text></nplcit> describes the doping of a polybenzimidazole in phosphoric acid.

0011The <patcit id="pcit0003" dnum="US6197147B"><text>US Patent No. 6197147</text></patcit> discloses a process for the manufacture of membrane electrode assemblies (MEE) by means of a continuous lamination process. The raw materials required for the production of the MEE are supplied in the form of material webs and connected to one another. As ion-conductive membranes, those based on polybenzimidazole and phosphoric acid are also disclosed. The<patcit id="pcit0004" dnum="US6197147B"><text>US Patent No. 6197147</text></patcit> also refers to the revelation of <patcit id="pcit0005" dnum="US5211984A"><text>US Patent No. 5211984</text></patcit>, which discloses "solid polymer electrolyte" membranes or the spraying of a "Catalst Ink", ie a mixture consisting essentially of ionomer and catalyst, on membranes based on Nafion.

0012In the basic polymer membranes known in the prior art, the mineral acid (usually concentrated phosphoric acid) used to achieve the required proton conductivity is either used after shaping or, alternatively, the basic polymer membrane is obtained directly from polyphosphoric acid, as in German patent application no. <patcit id="pcit0006" dnum="DE10117686"><text>10117686</text></patcit> No. <patcit id="pcit0007" dnum="DE10144815"><text>10144815</text></patcit> and no. <patcit id="pcit0008" dnum="DE10117687"><text>10117687</text></patcit> manufactured. The polymer serves as a carrier for the electrolyte consisting of the highly concentrated phosphoric acid or polyphosphoric acid. The polymer membrane fulfills further essential functions, in particular it must have high mechanical stability and serve as a separator for the two fuels mentioned at the beginning.

0013Significant advantages of such a membrane doped with phosphoric acid is the fact that this system can be operated at temperatures above 100 ° C. without the fuels otherwise having to be humidified. This is due to the property of phosphoric acid that the protons can be transported without additional water using the so-called Grotthus mechanism (<nplcit id="ncit0002" npl-type="s"><text>K.-D. Kreuer, Chem. Mater. 1996, 8, 610-641</text></nplcit>).

0014The possibility of operating at temperatures above 100 ° C results in further advantages for the fuel cell system. On the one hand, the sensitivity of the Pt catalyst to gas impurities, in particular CO, is greatly reduced. CO arises as a by-product in the reforming of the hydrogen-rich gas from carbon-containing compounds, such as natural gas, methanol or gasoline, or as an intermediate in the direct oxidation of methanol. Typically, the CO content of the fuel must be less than 100 ppm at temperatures &lt;100 ° C. At temperatures in the range of 150-200 °, however, 10,000 ppm CO or more can be tolerated (<nplcit id="ncit0003" npl-type="s"><text>NJ Bjerrum et. al. Journal of Applied Electrochemistry, 2001, 31, 773-779</text></nplcit>). This leads to significant simplifications of the upstream reforming process and thus to cost reductions for the entire fuel cell system.

0015A great advantage of fuel cells is the fact that the energy of the fuel is converted directly into electrical energy and heat during the electrochemical reaction. Water forms as a reaction product on the cathode. Heat is therefore a by-product of the electrochemical reaction. For applications in which only the electricity is used to drive electric motors, such as For automotive applications, the heat must be dissipated to avoid overheating the system. Additional, energy-consuming devices are then required for cooling, which further reduce the overall electrical efficiency of the fuel cell. For stationary applications such as the central or decentralized generation of electricity and heat, the heat can be used efficiently using existing technologies such as heat exchangers. High temperatures are aimed at to increase efficiency. If the operating temperature is above 100 ° C and the temperature difference between the ambient temperature and the operating temperature is large, then it becomes possible to cool the fuel cell system more efficiently or to use small cooling surfaces and to dispense with additional devices compared to fuel cells which, due to the membrane humidification, are below 100 ° C must be operated.

0016In addition to these advantages, such a system has two decisive disadvantages. For example, phosphoric acid is present as an electrolyte that is not permanently bound to the basic polymer due to ionic interactions and can be washed out by water. Water is formed at the cathode during the electrochemical reaction as described above. If the operating temperature is above 100 ° C, most of the water is removed as steam through the gas diffusion electrode and the loss of acid is very low. However, if the operating temperature falls below 100 ° C, e.g. when starting and stopping the cell or in partial load operation when a high current yield is desired, the water formed condenses and can lead to increased washing out of the electrolyte, highly concentrated phosphoric acid.

0017With the above-described driving style of the fuel cell, this can lead to a constant loss of conductivity and cell performance, which can reduce the service life of the fuel cell.

0018Another disadvantage of fuel cells in which the phosphoric acid acts as an electrolyte is the inhibition of the reduction reaction at the cathode, which is characterized by a high overvoltage. This leads to a low resting potential and relatively low power.

0019Furthermore, the known membranes doped with phosphoric acid cannot be used in the so-called direct methanol fuel cell (DMBZ). Such cells are of particular interest, however, because a methanol-water mixture is used as fuel. If a known membrane based on phosphoric acid is used, the fuel cell fails after a very short time.

0020The object of the present invention is therefore to provide a polymer electrolyte membrane in which the washing out of the mineral acid is reduced or prevented and which additionally has a reduced overvoltage, in particular at the cathode. In particular, it should be possible to expand the operating temperature from &lt;0 ° C to 200 ° C.

0021In addition, it was therefore an object of the present invention to provide a membrane which, even during operation, has a low permeability to a wide variety of fuels, such as, for example, hydrogen or methanol, and this membrane should also have a low oxygen permeability.

0022The object of the invention is achieved by a multilayer membrane system made of a polymer electrolyte membrane doped with mineral acid and which is coated on at least one side with a barrier layer for the mineral acid. In this configuration, the membrane doped with mineral acid fulfills the essential functions of separating the fuels and ensuring mechanical stability. The barrier layer should prevent the loss of mineral acid and reduce the overvoltage at the cathode.

0023A polymer electrolyte membrane according to the invention has a very low methanol permeability and is particularly suitable for use in a DMBZ. This enables permanent operation of a fuel cell with a variety of fuels such as hydrogen, natural gas, gasoline, methanol or biomass. The membranes enable a particularly high activity of these fuels. Due to the high temperatures, methanooxidation can take place with high activity.

0024The present invention therefore relates to a multilayer electrolyte membrane containing at least<ul id="ul0001" list-style="none"><li>A. a sheet material doped with one or more mineral acids,</li><li>B. at least one barrier layer which covers at least one of the two surfaces of the material mentioned under A. and has a barrier effect against mineral acids.</li></ul>

0025In the sheet materials according to A., both basic polymers, mixtures of basic polymers with other polymers, chemically inert supports, preferably ceramic materials, in particular silicon carbides (SiC) as described in <patcit id="pcit0009" dnum="US4017664A"><text>US-A-4017664</text></patcit> and <patcit id="pcit0010" dnum="US4695518A"><text>US-A-4695518</text></patcit> are used. These materials are capable of transporting protons according to the Grotthus mechanism.

0026A temperature-stable and chemically inert carrier which is filled with phosphoric acid in order to achieve proton conductivity can be used as the flat material. For example, ceramic materials such as silicon carbide SiC (<patcit id="pcit0011" dnum="US4017664A"><text>US-A-4017664</text></patcit> and <patcit id="pcit0012" dnum="US4695518A"><text>US-A-4695518</text></patcit>) or inorganic glasses in question. This carrier can represent, for example, a fabric or a fleece. Furthermore, the carrier can also be made of porous materials.

0027Porous organic polymers with an open pore structure can also be used as the chemically inert carrier. The open pore volume is more than 30%, preferably more than 50% and very particularly preferably more than 70%. The glass transition temperature of the organic base polymer of such membranes is higher than the operating temperature of the fuel cell and is preferably at least 150 ° C., preferably at least 160 ° C. and very particularly preferably at least 180 ° C. Such membranes are used as separation membranes for ultrafiltration, gas separation, pervaporation, nanofiltration, microfiltration or hemodialysis.

0028Methods for producing such membranes are in HP Hentze, M. Antonietti

0029"Porous polymers and resins" in <nplcit id="ncit0004" npl-type="b"><text>F. Schüth "Handbook of Porous Solids" p. 1964-2013</text></nplcit> described.

0030Organic foams can also be produced as chemically inert carriers. These foams can be produced by using gases such as CO in the synthesis of the organic polymer₂ are released or volatile liquids are used. Processes for the production of organic foams are in<nplcit id="ncit0005" npl-type="b"><text>D. Plumber, KC Frisch "Handbook of Polymeric Foams and Foam Technology</text></nplcit>" and <nplcit id="ncit0006" npl-type="s"><text>FA Shutov Advances in Polymer Science Volume 73/74, 1985, pages 63-123</text></nplcit> described. Supercritical CO₂ be used.

0031A particularly useful carrier is a phase separation membrane made of polybenzimidazole as in <patcit id="pcit0013" dnum="US4693824A"><text>US 4693824</text></patcit> or <patcit id="pcit0014" dnum="US4666996A"><text>US 4666996</text></patcit> or <patcit id="pcit0015" dnum="US5091087A"><text>US 5091087</text></patcit> described can be produced. By networking using the in<patcit id="pcit0016" dnum="US4634530A"><text>US 4634530</text></patcit> described method, the chemical stability of these membranes against phosphoric acid or polyphosphoric acid can be further improved.

0032Expanded polymer films such as expanded Teflon can also be used as carrier materials. Methods for producing proton-conducting membranes by filling such an expanded perfluorinated membrane are in<patcit id="pcit0017" dnum="US5547551A"><text>US 5547551</text></patcit> described.

0033Highly porous thermosets which have been produced by chemically induced phase separation can also be used as carrier materials. In this process, a slightly volatile solvent is added to a mixture of several monomers capable of crosslinking. When crosslinked, this solvent becomes insoluble and a heterogeneous polymer is formed. Evaporation of the solvent creates a chemically inert, porous thermoset, which can then be impregnated with phosphoric acid or polyphosphoric acid.

0034A particularly suitable carrier can be made from inorganic materials, for example from glass or materials, which have at least one compound made of a metal, a semimetal or a mixed metal or phosphorus with at least one element from the 3rd to 7th main group. The material particularly preferably has at least one oxide of the elements Zr, Ti, Al or Si. The carrier can be made of an electrically insulating material, such as. B. Minerals, glasses, plastics, ceramics or natural materials exist. The carrier preferably has special fabrics, nonwovens or porous materials made of quartz or glass resistant to high temperatures and acids. The glass preferably contains at least one compound from the group SiO₂, Al₂O₃ or MgO. In a further variant, the carrier comprises woven, non-woven or porous materials made of Al₂O₃-, ZrO₂-, TiO₂-, Si₃N₄, or SiC ceramics. In order to keep the total resistance of the electrolyte membrane low, this carrier preferably has a very large porosity but also a small thickness of less than 1000 μm, preferably less than 500 μm and very particularly preferably less than 200 μm. Carriers are preferably used which have interwoven fibers made of glass or quartz, the fabrics preferably consisting of 11-Tex yarns with 5-50 warp or weft threads and preferably 20-28 warp and 28-36 weft threads. 5.5-Tex games with 10-50 warp or weft threads and preferably 20-28 warp and 28 -36 weft threads are very preferably used.

0035As previously stated, supports made of woven, non-woven or porous materials can be used. Porous materials can be known in particular based on organic or inorganic foams.

0036Preferred carriers are permeable to mineral acids without a barrier layer. This property can be demonstrated by the barrier effect test set out in the examples. According to a particular aspect of the present invention, at least 5% of a mineral acid in the sheet is released within 1 hour if the sheet material is exposed to a large excess of water (at least 100 times the amount by weight of the film), which one Has temperature of 80 ° C.

0037Depending on the field of application, the flat structure according to step A) can be stable at high temperatures. High temperature stable means that the support is stable at a temperature of at least 150 ° C., preferably at least 200 ° C. and particularly preferably at least 250 ° C. Stable means that the essential properties of the carrier are retained. There is no change in the mechanical properties or the chemical composition when the sheet material is exposed for at least 1 hour.

0038The carrier is generally chemically inert. Chemically inert means that a flat material doped with a mineral acid is stable. Resistant means that the material is not decomposed by the acid. After 100 hours, the material shows at least 95% of the mechanical properties that the material had at the start of the measurement. This applies, for example, to the modulus of elasticity and microhardness.

0039Almost all known polymer membranes in which the protons are transported without additional water, for example by means of the so-called Grotthus mechanism, can be considered as the basic polymer membrane doped with mineral acid.

0040A basic polymer with at least one nitrogen atom in a repeating unit is used as the basic polymer in the sense of the present invention.

0041The repeating unit in the basic polymer preferably contains an aromatic ring with at least one nitrogen atom. The aromatic ring is preferably a five- or six-membered ring with one to three nitrogen atoms, which can be fused to another ring, in particular another aromatic ring.

0042Polymers based on polyazole generally contain repeating azole units of the general formula (I) and / or (II) and / or (III) and / or (IV) and / or (V) and / or (VI) and / or ( VII) and / or (VIII) and / or (IX) and / or (X) and / or (XI) and / or (XII) and / or (XIII) and / or (XIV) and / or (XV) and / or (XVI) and / or (XVII) and / or (XVIII) and / or (XIX) and / or (XX) and / or (XXI) and / or (XXII)<chemistry id="chem0001" num="0001"><img file="EP1518282B1_D0001.tif" /></chemistry><chemistry id="chem0002" num="0002"><img file="EP1518282B1_D0002.tif" /></chemistry><chemistry id="chem0003" num="0003"><img file="EP1518282B1_D0003.tif" /></chemistry><chemistry id="chem0004" num="0004"><img file="EP1518282B1_D0004.tif" /></chemistry><chemistry id="chem0005" num="0005"><img file="EP1518282B1_D0005.tif" /></chemistry><chemistry id="chem0006" num="0006"><img file="EP1518282B1_D0006.tif" /></chemistry><chemistry id="chem0007" num="0007"><img file="EP1518282B1_D0007.tif" /></chemistry><chemistry id="chem0008" num="0008"><img file="EP1518282B1_D0008.tif" /></chemistry><chemistry id="chem0009" num="0009"><img file="EP1518282B1_D0009.tif" /></chemistry><chemistry id="chem0010" num="0010"><img file="EP1518282B1_D0010.tif" /></chemistry><chemistry id="chem0011" num="0011"><img file="EP1518282B1_D0011.tif" /></chemistry><chemistry id="chem0012" num="0012"><img file="EP1518282B1_D0012.tif" /></chemistry><chemistry id="chem0013" num="0013"><img file="EP1518282B1_D0013.tif" /></chemistry><chemistry id="chem0014" num="0014"><img file="EP1518282B1_D0014.tif" /></chemistry><chemistry id="chem0015" num="0015"><img file="EP1518282B1_D0015.tif" /></chemistry><chemistry id="chem0016" num="0016"><img file="EP1518282B1_D0016.tif" /></chemistry><chemistry id="chem0017" num="0017"><img file="EP1518282B1_D0017.tif" /></chemistry><chemistry id="chem0018" num="0018"><img file="EP1518282B1_D0018.tif" /></chemistry><chemistry id="chem0019" num="0019"><img file="EP1518282B1_D0019.tif" /></chemistry><chemistry id="chem0020" num="0020"><img file="EP1518282B1_D0020.tif" /></chemistry><chemistry id="chem0021" num="0021"><img file="EP1518282B1_D0021.tif" /></chemistry><chemistry id="chem0022" num="0022"><img file="EP1518282B1_D0022.tif" /></chemistry>wherein<dl id="dl0001" compact="compact"><dt>Ar</dt><dd>are identical or different and are for a four-membered aromatic or heteroaromatic group which can be mononuclear or polynuclear,</dd><dt>Ar¹</dt><dd>are the same or different and represent a divalent aromatic or heteroaromatic group which can be mononuclear or polynuclear,</dd><dt>Ar²</dt><dd>are identical or different and are for a two or three-membered aromatic or heteroaromatic group which can be mononuclear or polynuclear,</dd><dt>Ar³</dt><dd>are the same or different and for a three-membered aromatic or heteroaromatic group, which can be mononuclear or polynuclear,</dd><dt>Ar⁴</dt><dd>are the same or different and for a three-membered aromatic or heteroaromatic group, which can be mononuclear or polynuclear,</dd><dt>Ar⁵</dt><dd>are identical or different and are for a four-membered aromatic or heteroaromatic group which can be mononuclear or polynuclear,</dd><dt>Ar⁶</dt><dd>are the same or different and represent a divalent aromatic or heteroaromatic group which can be mononuclear or polynuclear,</dd><dt>Ar⁷</dt><dd>are the same or different and represent a divalent aromatic or heteroaromatic group which can be mononuclear or polynuclear,</dd><dt>Ar⁸</dt><dd>are the same or different and for a three-membered aromatic or heteroaromatic group, which can be mononuclear or polynuclear,</dd><dt>Ar⁹</dt><dd>are identical or different and are for a two- or three- or four-membered aromatic or heteroaromatic group, which may be mono- or polynuclear,</dd><dt>Ar¹⁰</dt><dd>are identical or different and are for a di- or tri-bonded aromatic or heteroaromatic group which may be mononuclear or polynuclear,</dd><dt>Ar¹¹</dt><dd>are the same or different and represent a divalent aromatic or heteroaromatic group which can be mononuclear or polynuclear,</dd><dt>X</dt><dd>is identical 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</dd><dt>R</dt><dd>is identical or different to hydrogen, an alkyl group and an aromatic group and</dd><dt>n, m</dt><dd>is an integer greater than or equal to 10, preferably greater than or equal to 100.</dd></dl>

0043Aromatic or heteroaromatic groups preferred according to the invention are derived from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulfone, thiophene, furan, pyrrole, thiazole, oxazole, imidazole, isothiazole, isoxazole-pyrazole, pyrazole , 2,5-diphenyl-1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,3,4-triazole, 2,5-diphenyl-1,3,4-triazole, 1,2,5 Triphenyl-1,3,4-triazole, 1,2,4-oxadiazole, 1,2,4-thiadiazole, 1,2,4-triazole, 1,2,3-triazole, 1,2,3,4-tetrazole, benzo [b] thiophene, benzo [b] furan, indole, benzo [c] thiophene, benzo [c] furan, isoindole, benzoxazole, benzothiazole, benzimidazole, benzisoxazole, benzisothiazole, benzopyrazole, Benzothiadiazole, benzotriazole, dibenzofuran, dibenzothiophene, carbazole, pyridine, bipyridine, pyrazine, pyrazole, pyrimidine, pyridazine, 1,3,5-triazine, 1,2,4-triazine, 1,2,4,5-triazine, tetrazine, Quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline, 1,8-naphthyridine, 1,5-naphthyridine, 1,6-naphthyridine, 1,7-naphthyridine, phthalazine, pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine, anthracene, benzopyrrole, benzooxathiadiazole, benzopyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazidopyridine, Aciridine, phenazine, benzoquinoline, phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthroline and phenanthrene, which can optionally also be substituted.

0044Here is the substitution pattern of Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ any, in the case of phenylene, for example, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ be ortho-, meta- and para-phenylene. Particularly preferred groups are derived from benzene and biphenyls, which may also be substituted.

0045Preferred 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.

0046Preferred aromatic groups are phenyl or naphthyl groups. The alkyl groups and the aromatic groups can be substituted.

0047Preferred substituents are halogen atoms such as. B. fluorine, amino groups, hydroxyl groups or short-chain alkyl groups such as. B. methyl or ethyl groups.

0048Preference is given to polyazoles having repeating units of the formula (I) in which the radicals X are the same within a repeating unit.

0049In principle, the polyazoles can also have different recurring units which differ, for example, in their radical X. However, it preferably has only the same X radicals in a recurring unit.

0050Further preferred polyazole polymers are polyimidazoles, polybenzthiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles poly (pyridines), poly (pyrimidines), and poly (tetrazapyrenes).

0051In 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.

0052In a particularly preferred embodiment of the present invention, the polymer containing recurring azole units is a polyazole which only contains units of the formula (I) and / or (II).

0053The 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.

0054For the purposes 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:<chemistry id="chem0023" num="0023"><img file="EP1518282B1_D0023.tif" /></chemistry><chemistry id="chem0024" num="0024"><img file="EP1518282B1_D0024.tif" /></chemistry><chemistry id="chem0025" num="0025"><img file="EP1518282B1_D0025.tif" /></chemistry><chemistry id="chem0026" num="0026"><img file="EP1518282B1_D0026.tif" /></chemistry><chemistry id="chem0027" num="0027"><img file="EP1518282B1_D0027.tif" /></chemistry><chemistry id="chem0028" num="0028"><img file="EP1518282B1_D0028.tif" /></chemistry><chemistry id="chem0029" num="0029"><img file="EP1518282B1_D0029.tif" /></chemistry><chemistry id="chem0030" num="0030"><img file="EP1518282B1_D0030.tif" /></chemistry><chemistry id="chem0031" num="0031"><img file="EP1518282B1_D0031.tif" /></chemistry><chemistry id="chem0032" num="0032"><img file="EP1518282B1_D0032.tif" /></chemistry><chemistry id="chem0033" num="0033"><img file="EP1518282B1_D0033.tif" /></chemistry><chemistry id="chem0034" num="0034"><img file="EP1518282B1_D0034.tif" /></chemistry><chemistry id="chem0035" num="0035"><img file="EP1518282B1_D0035.tif" /></chemistry><chemistry id="chem0036" num="0036"><img file="EP1518282B1_D0036.tif" /></chemistry><chemistry id="chem0037" num="0037"><img file="EP1518282B1_D0037.tif" /></chemistry><chemistry id="chem0038" num="0038"><img file="EP1518282B1_D0038.tif" /></chemistry><chemistry id="chem0039" num="0039"><img file="EP1518282B1_D0039.tif" /></chemistry><chemistry id="chem0040" num="0040"><img file="EP1518282B1_D0040.tif" /></chemistry><chemistry id="chem0041" num="0041"><img file="EP1518282B1_D0041.tif" /></chemistry><chemistry id="chem0042" num="0042"><img file="EP1518282B1_D0042.tif" /></chemistry><chemistry id="chem0043" num="0043"><img file="EP1518282B1_D0043.tif" /></chemistry><chemistry id="chem0044" num="0044"><img file="EP1518282B1_D0044.tif" /></chemistry><chemistry id="chem0045" num="0045"><img file="EP1518282B1_D0045.tif" /></chemistry><chemistry id="chem0046" num="0046"><img file="EP1518282B1_D0046.tif" /></chemistry>where n and m is an integer greater than or equal to 10, preferably greater than or equal to 100.

0055Further preferred polyazole polymers are polyimidazoles, polybenzimidazole ether ketone, polybenzthiazoles, polybenzoxazoles, polytriazoles, polyoxadiazoles, polythiadiazoles, polypyrazoles, polyquinoxalines, poly (pyridines), poly (pyrimidines) and poly (tetrazapyrenes).

0056Preferred polyazoles are distinguished by a high molecular weight. This applies in particular to the polybenzimidazoles. Measured as intrinsic viscosity, this is in the range from 0.3 to 10 dl / g, preferably 1 to 5 dl / g.

0057Celazole from Celanese is particularly preferred. The properties of the polymer film and polymer membrane can be determined by sieving the starting polymer, as described in German patent application no.<patcit id="pcit0018" dnum="DE10129458"><text>10129458</text></patcit> described, improved.

0058The polymer film based on basic polymers used for doping can also have further additions of fillers and / or auxiliaries. In addition, the polymer film can further modifications, for example by crosslinking as in German patent application no.<patcit id="pcit0019" dnum="DE10110752"><text>10110752</text></patcit> or in <patcit id="pcit0020" dnum="WO0044816A"><text>WO 00/44816</text></patcit> exhibit. In a preferred embodiment, the polymer film used for doping from a basic polymer and at least one blend component additionally contains a crosslinking agent as in German patent application no.<patcit id="pcit0021" dnum="DE10140147"><text>10140147</text></patcit> described. A major advantage of such a system is the fact that higher levels of doping and thus higher conductivity can be achieved with sufficient mechanical membrane stability.

0059In addition to the basic polymers mentioned above, a blend of one or more basic polymers with a further polymer 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 in German patent application no.<patcit id="pcit0022" dnum="DE10052242"><text>10052242</text></patcit> described.

0060The preferred polymers which can be used as the blen component include, inter alia, polyolefins, such as poly (chloroprene), polyacetylene, polyphenylene, poly (<i>p</i>-xylylene), polyarylmethylene, polyarmethylene, polystyrene, polymethylstyrene, polyvinyl alcohol, polyvinyl acetate, polyvinyl ether, polyvinylamine, poly (N-vinyl acetate), polyvinyl imidazole, polyvinyl carbazole, polyvinyl pyrrolidone, polyvinyl pyridine, polyvinyl chloride, polyvinylidene fluoride, polytetrafluorethylene with polytetrafluorethylene with polytetrafluorethylene with polytetrafluorethylene with polytetrafluorethylene with polytetrafluorethylene with Perfluoropropyl vinyl ether, with trifluoronitrosomethane, with sulfonyl fluoride vinyl ether, with carbalkoxy-perfluoroalkoxy vinyl ether, Polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, polyacrolein, polyacrylamide, polyacrylonitrile, polycyanoacrylates, polymethacrylimide, cycloolefinic copolymers, in particular from norbornene; Polymers with CO bonds in the main chain, for example polyacetal, polyoxymethylene, polyether, polypropylene oxide, polyepichlorohydrin, polytetrahydrofuran, polyphenylene oxide, polyether ketone, polyester, in particular polyhydroxyacetic acid, polyethylene terephthalate, polybutylene terephthalate, polyhydroxybenzoate, polyhydroxypropionic acid, polypivalonicolonactone, polypivalololonactone; Polymeric CS bonds in the main chain, for example polysulfide ether, polyphenylene sulfide, polyether sulfone; Polymeric CN bonds in the main chain, for example polyimines, polyisocyanides, polyetherimine, polyaniline, polyamides, polyhydrazides, polyurethanes, polyimides, polyazoles, polyazines; Liquid-crystalline polymers, in particular Vectra and inorganic polymers, for example polysilanes, polycarbosilanes, polysiloxanes, polysilicic acid, polysilicates, silicones, polyphosphazenes and polythiazyl.

0061For use in fuel cells with a continuous use temperature above 100 ° C., preference is given to those blend polymers which have a glass transition temperature or Vicat softening temperature VST / A / 50 of at least 100 ° C., preferably at least 150 ° C. and very particularly preferably at least 180 ° C. .

0062Polysulfones with a Vicat softening temperature VST / A / 50 of 180 ° C. to 230 ° C. are preferred.

0063The preferred polymers include polysulfones, especially polysulfones with aromatics 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 less than or equal to 40 cm³/ 10 min, in particular less than or equal to 30 cm³/ 10 min and particularly preferably less than or equal to 20 cm³/ 10 min measured according to ISO 1133.

0064To further improve the application properties, the flat material can have fillers, in particular proton-conducting fillers.

0065Non-limiting examples of proton-conducting fillers are<tables id="tabl0001" num="0001"><table frame="none"><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="42mm" /><colspec colnum="2" colname="col2" colwidth="124mm" /><tbody><row><entry>Sulfates such as:</entry><entry>CsHSO₄, Fe (SO₄)₂, (NH₄)₃H (SO₄)₂, LiHSO₄, NaHSO₄, KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,</entry></row><row><entry>Phosphates like</entry><entry>Zr₃(PO₄)₄, Zr (HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄.3H₂O, H₈UO₂PO₄, Ce (HPO₄)₂, Ti (HPO₄)₂, KH₂PO₄, Close₂PO₄, LiH₂PO₄, NH₄H₂PO₄, CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₁₄, H₅Sb₅P₂O₂₀,</entry></row><row><entry>Polyacid like</entry><entry>H₃PW₁₂O₄₀.nH₂O (n = 21-29), H₃SiW₁₂O₄₀.nH₂O (n = 21-29), H_xWHERE₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃, HTiNbO₅, HTiTaO₅, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄</entry></row><row><entry>Selenite and arsenide like</entry><entry>(NH₄)₃H (SeO₄)₂, UO₂AsO₄, (NH₄)₃H (SeO₄)₂, KH₂AsO₄, Cs₃H (SeO₄)₂, Rb₃H (SeO₄)₂,</entry></row><row><entry>Oxides like silicates like</entry><entry>Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃ Zeolites, Zeolites (NH₄+), Layered silicates, framework silicates, H-natrolites, H-mordenites, NH₄-Analcine, NH₄-Sodalite, NH₄-Gallate, H-Montmorillonite</entry></row><row><entry>Acids like</entry><entry>HClO₄, SbF₅</entry></row><row><entry>Fillers like</entry><entry>Carbides, especially SiC, Si₃N₄, Fibers, in particular glass fibers, glass powders and / or polymer fibers, preferably based on polyazoles.</entry></row></tbody></tgroup></table></tables>

0066These additives can be contained in the proton-conducting polymer membrane in customary amounts, but the positive properties, such as high conductivity, long service life and high mechanical stability of the membrane, should not be adversely affected by the addition of excessive amounts of additives. In general, the membrane comprises at most 80% by weight, preferably at most 50% by weight and particularly preferably at most 20% by weight of additives.

0067To produce the polymer film, the polymer components are initially, for example, as in the applications cited above <patcit id="pcit0023" dnum="DE10110752"><text>DE No. 10110752</text></patcit> or <patcit id="pcit0024" dnum="WO0044816A"><text>WO 00/44816</text></patcit>, described dissolved or suspended and then used to produce the polymer films. Furthermore, the polymer films according to<patcit id="pcit0025" dnum="DE10052237"><text>DE No. 10052237</text></patcit> be produced continuously.

0068Alternatively, the film formation can be carried out according to the method described in Japanese Application No. <patcit id="pcit0026" dnum="JPHEI10125560B"><text>Hei 10-125560</text></patcit> described procedures take place.

0069The solution is poured into a cylinder with a cylindrical inner surface, and then the cylinder is rotated. At the same time, the solvent is allowed to evaporate by the centrifugal force caused by the rotation; wherein a cylindrical polymer film of largely uniform thickness forms on the inner surface of the cylinder.

0070With this method, the basic polymer can be formed with a uniform matrix.

0071This in the Japanese patent application <patcit id="pcit0027" dnum="JPHEI10125560B"><text>Hei 10-125560</text></patcit> described method is also part of the present description.

0072The solvent is then removed. This can be done by methods known to the person skilled in the art, for example by drying.

0073The film made of basic polymer or polymer blend is then impregnated or doped with a strong acid, preferably a mineral acid, the film being as described in German patent application no. <patcit id="pcit0028" dnum="DE10109829"><text>10109829</text></patcit> described previously can be treated. This variant is advantageous in order to exclude interactions of the residual solvent with the barrier layer.

0074For this purpose, the film made of basic polymer or polymer blend will be immersed in a strong acid, so that the film is impregnated with the strong acid and becomes a proton-conducting membrane. For this purpose, the basic polymer is usually immersed in a highly concentrated strong acid at a temperature of at least 35 ° C. over a period of from several minutes to several hours

0075Mineral acid, in particular phosphoric acid and / or sulfuric acid, is used as the strong acid.

0076In the context of the present description, "phosphoric acid" means polyphosphoric acid (H._{n + 2}P_nO_{3n + 1} (n> 1) usually have a content calculated as P₂O₅ (acidimetric) of at least 83%), phosphonic acid (H₃PO₃), Orthophosphoric acid (H₃PO₄), Pyrophosphoric acid (H₄P₂O₇), Triphosphoric acid (H₅P₃O₁₀) and metaphosphoric acid. The phosphoric acid, in particular orthophosphoric acid, preferably has a concentration of at least 80 percent by weight, particularly preferably a concentration of at least 85 percent by weight, even more preferably a concentration of at least 87 percent by weight and very particularly preferably a concentration of at least 89 percent by weight. The reason for this is that the basic polymer can be impregnated with a greater number of strong acid molecules as the strong acid concentration increases.

0077The polymer electrolyte membrane obtained, namely the complex of the basic polymer and the strong acid, is proton-conducting. After the doping, the degree of doping, expressed as moles of acid per repeating unit, should be greater than 6, preferably greater than 8 and most preferably greater than 9.

0078Instead of the polymer membranes based on basic polymers produced by means of classic processes, the polyazole-containing polymer membranes can also be used, as described in German patent applications no. <patcit id="pcit0029" dnum="DE10117686"><text>10117686</text></patcit>, <patcit id="pcit0030" dnum="DE10144815"><text>10144815</text></patcit>, <patcit id="pcit0031" dnum="DE10117687"><text>10117687</text></patcit> described, used. Such polymer electrolyte membranes provided with at least one barrier layer are also the subject of the present invention.

0079Accordingly, sheet materials according to the invention can be obtained by a method comprising the steps<ol id="ol0001" compact="compact"><li>i) comprising the preparation of a mixture Polyphosphoric acid, at least one polyazole and / or at least one or more compounds which are suitable for the formation of polyazoles under the action of heat according to step ii),</li><li>ii) heating the mixture obtainable according to step i) under inert gas to temperatures of up to 400 ° C.,</li><li>iii) applying a layer using the mixture according to step i) and / or ii) on a carrier,</li><li>iv) treatment of the membrane formed in step iii).</li></ol>

0080For this purpose, one or more compounds can be added to the mixture according to step i) which are suitable for the formation of polyazoles under the action of heat according to step ii).

0081Mixtures are suitable for this purpose which comprise one or more aromatic and / or heteroaromatic tetra-amino compounds and one or more aromatic and / or heteroaromatic carboxylic acids or their derivatives which comprise at least two acid groups per carboxylic acid monomer. Furthermore, one or more aromatic and / or heteroaromatic diaminocarboxylic acids can be used for the production of polyazoles.

0082The aromatic and heteroaromatic tetra-amino compounds include, inter alia, 3,3 ', 4,4'-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine, 1,2,4,5-tetraaminobenzene, 3,3', 4,4'-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. Of these, 3,3 ', 4,4'-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine and 1,2,4,5-tetraaminobenzene are particularly preferred.

0083Mixture A) may further comprise aromatic and / or heteroaromatic carboxylic acids. These are dicarboxylic acids and tricarboxylic acids and tetracarboxylic acids or their esters or their anhydrides or their acid halides, in particular their acid halides and / or acid bromides. 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-diethylamino acid -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-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 2,7-diacarbonic acid, 2,7 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.

0084The heteroaromatic carboxylic acids are heteroaromatic dicarboxylic acids and tricarboxylic acids and tetracarboxylic acids or their esters or their anhydrides. Heteroaromatic carboxylic acids are understood as meaning aromatic systems which contain at least one nitrogen, oxygen, sulfur or phosphorus atom in the aromatic system. 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.

0085Mixture i) may also contain aromatic and heteroaromatic diaminocarboxylic acids. These include diaminobenzoic acid, 4-phenoxycarbonyl-3, '4'-diaminodiphenyl ether and their mono- and dihydrochloride derivatives.

0086The mixture produced in step i) preferably comprises at least 0.5% by weight, in particular 1 to 30% by weight and particularly preferably 2 to 15% by weight, of monomers for the production of polyazoles.

0087According to a further aspect of the present invention, the mixture prepared in step A) comprises compounds which, under the action of heat in step B), are suitable for the formation of polyazoles, these compounds by reaction of one or more aromatic and / or heteroaromatic tetraamino Compounds with one or more aromatic and / or heteroaromatic carboxylic acids or their derivatives, which contain at least two acid groups per carboxylic acid monomer, or of one or more aromatic and / or heteroaromatic diaminocarboxylic acids in the melt at temperatures of up to 400 ° C, in particular up to 350 ° C, preferably up to 280 ° C are. The compounds to be used to prepare these prepolymers have been set out above.

0088Furthermore, monomers can be used for the production of polyazoles, which comprise covalently bonded acid groups. These include aromatic and heteroaromatic dicarboxylic acids or their derivatives which have at least one phosphonic acid group, for example 2,5-dicarboxyphenylphosphonic acid, 2,3-dicarboxyphenylphosphonic acid, 3,4-dicarboxyphenylphosphonic acid and 3,5-dicarboxyphenylphosphonic acid; aromatic and heteroaromatic dicarboxylic acids or their derivatives which have at least one sulfonic acid group, in particular 2,5-dicarboxyphenylsulfonic acid, 2,3-dicarboxyphenylsulfonic acid, 3,4-dicarboxyphenylsulfonic acid and 3,5-dicarboxyphenylsulfonic acid; aromatic and heteroaromatic diaminocarboxylic acids which comprise at least one phosphonic acid group, for example 2,3-diamino-5-carboxyphenylphosphonic acid, 2,3-diamino-6-carboxyphenylphosphonic acid and 3,4-diamino-6-carboxyphenylphosphonic acid; aromatic and heteroaromatic diaminocarboxylic acids which comprise at least one sulfonic acid group, for example 2,3-diamino-5-carboxyphenylsulfonic acid, 2,3-diamino-6-carboxyphenylsulfonic acid and 3,4-diamino-6-carboxyphenylsulfonic acid.

0089A polyazole membrane made according to the method set forth above may contain the optional components set out above. This includes in particular blend polymers and fillers. Blend polymers can be dissolved, dispersed or suspended, among other things, in the mixture obtained in step i) and / or step ii). The weight ratio of polyazole to polymer (B) is preferably in the range from 0.1 to 50, preferably from 0.2 to 20, particularly preferably from 1 to 10, without any intention that this should impose a restriction. If the polyazole is not formed until step ii), the weight ratio can be calculated from the weight of the monomers to form the polyazole, taking into account the compounds released during the condensation, for example water.

0090To further improve the application properties, fillers, in particular proton-conducting fillers, and additional acids can also be added to the membrane. The addition can take place, for example, in step i), step ii) and / or step iii). Furthermore, these additives, if they are in liquid form, can also be added after the polymerization in step iv). These additives have been previously described.

0091The polyphosphoric acid used in step i) is a commercially available polyphosphoric acid such as is available, for example, from Riedel-de Haen. The polyphosphoric acids H_{n + 2}P_nO_{3n + 1} (n> 1) usually have a content calculated as P₂O₅ (acidimetric) of at least 83%. Instead of a solution of the monomers, a dispersion / suspension can also be produced. According to step ii), the mixture obtained in step i) is heated to a temperature of up to 400 ° C., in particular 350 ° C., preferably up to 280 ° C., in particular 100 ° C. to 250 ° C. and particularly preferably in the range of 200 ° C to 250 ° C heated. An inert gas, for example nitrogen or a noble gas, such as neon, argon, is used here.

0092The mixture produced in step i) and / or step ii) may additionally contain organic solvents. These can have a positive impact on processability. For example, the rheology of the solution can be improved so that it can be extruded or doctored more easily.

0093The formation of the flat structure according to step iii) takes place by means of measures known per se (casting, spraying, knife coating, extrusion) which are known from the prior art for polymer film production. Suitable carriers are all carriers which are inert under the conditions. These carriers include, in particular, films made from polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene, polyimides, polyphenylene sulfides (PPS) and polypropylene (PP). Furthermore, the membrane can also be formed directly on the electrode provided with a barrier layer.

0094The thickness of the flat structure according to step iii) is preferably between 10 and 4000 μm, preferably between 15 and 3500 μm, in particular between 20 and 3000 μm, particularly preferably between 30 and 1500 μm and very particularly preferably between 50 and 1200 μm.

0095The treatment of the membrane in step iv) is carried out in particular in the presence of temperatures in the range from 0 ° C. to 150 ° C., preferably at temperatures between 10 ° C. and 120 ° C., in particular between room temperature (20 ° C.) and 90 ° C. of moisture or water and / or water vapor. 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, whereby 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.

0096The partial hydrolysis of the polyphosphoric acid in step iv) leads to a solidification of the membrane and to a decrease in the layer thickness and formation of a membrane. The solidified membrane generally has a thickness between 15 and 3000 μm, preferably 20 and 2000 μm, in particular between 20 and 1500 μm.

0097The upper temperature limit of the treatment according to step iv) is usually 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.

0098The partial hydrolysis (step iv) 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.

0099Furthermore, the treatment time depends on the thickness of the membrane.

0100As a rule, the treatment time is between a few seconds to minutes, for example under the action of superheated steam, or up 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.

0101If the partial hydrolysis is carried out at room temperature (20 ° C) with ambient air with a relative humidity of 40-80%, the treatment time is between 1 and 200 hours.

0102The membrane obtained in step iv) can be designed to be self-supporting, ie it can be detached from the support without damage and then, if necessary, further processed directly.

0103The treatment according to step iv) leads to a hardening of the coating. If the membrane is formed directly on the electrode, the treatment according to step D) is carried out until the coating has sufficient hardness to be able to be pressed into a membrane-electrode unit. The hardness is sufficient if a membrane treated accordingly is self-supporting. In many cases, however, a lower hardness is sufficient. The hardness determined in accordance with DIN 50539 (microhardness measurement) is generally at least 1 mN / mm², preferably at least 5 mN / mm² and very particularly preferably at least 50 mN / mm²without any limitation.

0104The concentration and the amount of phosphoric acid and thus the conductivity of the polymer membrane according to the invention can be set via the degree of hydrolysis, ie the duration, temperature and ambient humidity. According to the invention, the concentration of phosphoric acid is given as mole of acid per mole of repeat unit of the polymer. In the context of the present invention, a concentration (mol of phosphoric acid based on a repeating unit of the formula (III), ie Polybenzimidazole) between 10 and 80, in particular between 12 and 60, preferred. Such high degrees of doping (concentrations) are very difficult or even impossible to obtain by doping polyazoles with commercially available orthophosphoric acid.

0105The thickness of the barrier layer of a multilayer polymer electrolyte membrane according to the invention is generally not critical, as long as this layer has a sufficient barrier effect against mineral acids. The blocking effect can be determined via the amount of mineral acid that can be washed out by water. According to a particular aspect of the present invention, at most 10%, preferably at most 5%, of the mineral acid passes into the aqueous phase within one hour. These values ​​relate to the weight of mineral acid or the weight of the flat material with the mineral acid, the area in contact with water being taken into account in each case to calculate the value.

0106According to a particular embodiment of the present invention, the thickness of the barrier layer is less than 10 μm, preferably 1 to 8 μm and particularly preferably 2 to 6 μm. The advantage of such barrier layers is their relatively low resistance.

0107According to a further embodiment of the present invention, the thickness of the barrier layer is at least 10 μm, preferably the thickness is in the range from 10 μm to 30 μm. An advantage of such barrier layers is their particularly high barrier effect and their stability.

0108The thickness of the barrier layer can be measured using scanning electron microscopy (SEM). The thickness of the barrier layer results from the mean value of the thickness, which is obtained via the ratio of area to length of the barrier layer.

0109The barrier layer according to the invention is preferably a cation exchanger material. This cation exchange material allows the transport of the protons but not the anions such as phosphate anions. For better adhesion promotion, block copolymers consisting of components of the polymer electrolyte membrane and the cation exchange membrane can also be used at the interface between the polymer electrolyte membrane and the cation exchange material.

0110This barrier layer can be connected (laminated) to the doped polymer membrane or the doped polymer blend membrane in the form of a separate film, preferably self-supporting.

0111Furthermore, the barrier layer can also be made by applying a layer to the doped membrane and / or the electrode. For this purpose, for example, a mixture comprising a cation exchange material or a precursor material can be applied to the membrane and / or the electrode. Suitable processes include casting, spraying, knife coating and / or extrusion.

0112The barrier layer can furthermore have a gradient. For example, the concentration of acid groups can be varied. Such gradients can be measured, for example, using (energy-dispersive X-ray scattering (EDX), spatially resolved Raman spectroscopy and spatially resolved infrared spectroscopy.

0113In a variant of the present invention, insofar as the cation exchanger material is in the form of a self-supporting film, it can also be installed as a separate film in an MEU between the doped polymer electrolyte membrane and the catalyst layer or the electrode (also on both sides).

0114It 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. In addition to this embodiment, the barrier layer can also be applied on both sides.

0115As already stated, the cation exchange material is not subject to any significant restriction. Preferred materials are those whose cation exchange capacity is less than 0.9 meq / g, in particular less than 0.8 meq / g. According to a particular aspect of the present invention, the cation exchange capacity is at least 0.1 meq / g, in particular 0.2 meq / g, without any intention that this should impose a restriction. Preferred materials are those whose surface swelling in water at 80 ° C. is less than 20%, in particular less than 10%. Preferred materials are those whose conductivity at 80 ° C. in the humidified state is less than 0.06 S / cm, in particular less than 0.05 S / cm.

0116To measure the IEC, the sulfonic acid groups are converted into the free acid. To this end, the polymer is treated with acid in a known manner, excess acid being removed by washing. The sulfonated polymer is first treated in boiling water for 2 hours. Excess water is then dabbed off and the sample is dried for 15 hours at 160 ° C. in a vacuum drying cabinet at p &lt;1 mbar. Then the dry weight of the membrane is determined. The polymer dried in this way is then dissolved in DMSO at 80 ° C. for 1 h. The solution is then titrated with 0.1 M NaOH. The ion exchange capacity (IEC) is then calculated from the consumption of the acid up to the equivalent point and the dry weight.

0117At high current density and temperatures above 100 ° C, this thin layer is moistened by the product water produced on the cathode. When using hydrogen-rich reformate gas, the moisture present in the reformate is sufficient to moisten the barrier layer. The system therefore does not require additional humidification at temperatures above 100 ° C and high electrical output. Under certain circumstances, however, it may be necessary to additionally moisten the fuels at startup or at low temperatures or at low current densities. The barrier layer applied on the cathode side is preferably thicker than the barrier layer located on the anode side.

0118The barrier layer preferably comprises a cation exchange material. In principle, all cation exchange materials that can be processed into membranes can be used. These are preferably organic polymers with covalently bonded acid groups. The particularly suitable acid groups include carboxylic acid, sulfonic acid and phosphonic acid groups, with polymers containing sulfonic acid groups being particularly preferred. Methods for sulfonation of polymers are in <nplcit id="ncit0007" npl-type="s"><text>F. Kucera et. al. Polymer Engineering and Science 1988, Vol. 38, No 5, 783-792</text></nplcit> described.

0119The cation exchange materials, which are preferably used as barrier layers, cannot generally be used alone as cation exchange membranes in fuel cells, since their proton conductivity and swelling are too low and the mechanical stability cannot be guaranteed due to the small thickness. However, the cation exchange membranes described in the prior art were specifically developed with a high ion exchange capacity, high swelling, high proton conductivity and sufficient thickness to ensure sole use as a polymer electrolyte membrane in an MEE.

0120The most important types of cation exchange membranes which have gained commercial importance for use in fuel cells are listed below.

0121The most important representative is the perfluorosulfonic acid polymer Nafion^® (<patcit id="pcit0032" dnum="US3692569A"><text>US-A-3692569</text></patcit>). This polymer can be used as in<patcit id="pcit0033" dnum="US4453991A"><text>US-A-4453991</text></patcit> described in solution and then used as an ionomer. Cation exchange membranes are also obtained by filling a porous support material with such an ionomer. Expanded Teflon is preferred as the carrier material (<patcit id="pcit0034" dnum="US5635041A"><text>US-A-5635041</text></patcit>).

0122Another perfluorinated cation exchange membrane can be as in <patcit id="pcit0035" dnum="US5422411A"><text>US-A-5422411</text></patcit> described by copolymerization from trifluorostyrene and sulfonyl-modified trifuorostyrene. Composite membranes consisting of a porous carrier material, in particular expanded Teflon, filled with ionomers consisting of such sulfonyl-modified trifluorostyrene copolymers are in<patcit id="pcit0036" dnum="US5834523A"><text>US-A-5834523</text></patcit> described.

0123<patcit id="pcit0037" dnum="US6110616A"><text>US-A-6110616</text></patcit> describes copolymers of butadiene and styrene and their subsequent sulfonation for the production of cation exchange membranes for fuel cells.

0124Another class of partially fluorinated cation exchange membranes can be made by radiation grafting and subsequent sulfonation. As in<patcit id="pcit0038" dnum="EP667983A"><text>EP-A-667983</text></patcit> or <patcit id="pcit0039" dnum="DE19844645A"><text>DE-A-19844645</text></patcit> described on a previously irradiated polymer film, a grafting reaction is preferably carried out with styrene. The sulfonation of the side chains then takes place in a subsequent sulfonation reaction. Crosslinking can also be carried out at the same time as the grafting, and the mechanical properties can thus be changed.

0125In addition to the above membranes, another class of non-fluorinated membranes has been developed by sulfonation of high-temperature stable thermoplastics. Membranes made from sulfonated polyether ketones (<patcit id="pcit0040" dnum="DE4219077A"><text>DE-A-4219077</text></patcit>, <patcit id="pcit0041" dnum="WO9601177A"><text>WO96 / 01177</text></patcit>), sulfonated polysulfone (<nplcit id="ncit0008" npl-type="s"><text>J. Membr. Sci. 83 (1993) p.211</text></nplcit>) or sulfonated polyphenylene sulfide (<patcit id="pcit0042" dnum="DE19527435A"><text>DE-A-19527435</text></patcit>) known.

0126Ionomers made from sulfonated polyether ketones are in <patcit id="pcit0043" dnum="WO0015691A"><text>WO 00/15691</text></patcit> described.

0127Furthermore, acid-base blend membranes are known, which are as in <patcit id="pcit0044" dnum="DE19817374A"><text>DE-A-19817374</text></patcit> or <patcit id="pcit0045" dnum="WO0118894A"><text>WO 01/18894</text></patcit> described by mixtures of sulfonated polymers and basic polymers.

0128In order to adjust the ion exchange capacity for optimum acid retention, a cation exchange membrane known from the prior art can be mixed with a polymer which contains no or only a small amount of acid groups. Suitable polymers have been set out above as a blend component, with high-temperature-stable polymers being particularly preferred. The production and properties of cation exchange membranes consisting of sulfonated PEK and a) polysulfones (DEA-4422158), b) aromatic polyamides (<patcit id="pcit0046" dnum="DE42445264A"><text>DE-A-42445264</text></patcit>) or c) polybenzimidazole (<patcit id="pcit0047" dnum="DE19851498A"><text>DE-A-19851498</text></patcit>) are described. Alternatively, the sulfonation conditions can be selected so that a low degree of sulfonation occurs (<patcit id="pcit0048" dnum="DE19959289A"><text>DE-A-19959289</text></patcit>).

0129In addition to the cation exchange membranes based on organic polymers mentioned in the prior art, the cation exchange material can also consist of organic-inorganic composite materials. Such composite materials can preferably be produced using the sol-gel process. Mixtures of metal alkoxides, in particular siloxanes, are combined as starting compounds. These mixtures are characterized by a high purity of the starting materials and a low viscosity. These liquid precursor mixtures can be applied to very thin and uniformly covering layers on a substrate using known technologies such as spraying or spin coating. Solid films can then be produced on the surface by hydrolysis and condensation of the precursor mixtures. To adjust the proton conductivity, the organic residues of the alkoxides contain acidic groups, in particular sulfonic acid groups.

0130The precursor mixtures can also contain functional organic groups which bring about a crosslinking of the layer formed and thus a further reduction in the permeability of the mineral acid and the fuels. The crosslinking can take place after the layer formation both thermally and by irradiation (e-, UV, IR, NIR) or by means of a starter.

0131The production of such a composite material is, for example, in <nplcit id="ncit0009" npl-type="s"><text>Electrochimica Acta Volume 37, year 1992, pages 1615-1618 </text></nplcit>described. In addition, such composite materials are made<nplcit id="ncit0010" npl-type="b"><text>GW Scherer, CJ Brinker, Sol-Gel-Science, Academic Press, Bosten, 1990</text></nplcit>, known.

0132A group of preferred compounds can be represented by the formula (A)         (RO)_y(R¹)_e.g.MX_a     (A) wherein y is 1, 2 or 3, preferably 3, z is 0 or 1, preferably 0 and a is 1 or 2, preferably 1 and R and R¹ independently hydrogen, a linear or branched alkyl, alkenyl, cycloalkyl or cycloalkenyl radical having 1 to 20, preferably 1 to 8 carbon atoms, or an aromatic or heteroaromatic group having 5 to 20 carbon atoms, M is an element selected from Si, Zr, Ti, preferably Si and X independently represents a linear or branched alkylene or cycloalkylene group with 1 to 20, preferably 1 to 8 carbon atoms, or an aromatic or heteroaromatic group with 5 to 20 carbon atoms, which has at least one sulfonic or phosphonic acid.

0133The residues R, R¹ and X can have further substituents, in particular halogens, such as fluorine atoms. The group X preferably represents a radical of the formulas Ph-SO₃H, C_nH_2n-SO₃H, C_nF_2n-SO₃H is where Ph is phenyl and n is an integer from 1 to 20. The group R preferably represents a radical of the formula C._nH_{2n + 1} where n is 1 to 3.

0134Preferred compounds are in particular hydroxysilyl acids, which are known per se and, for example, in <patcit id="pcit0049" dnum="DE10061920"><text>DE 100 61 920</text></patcit>, <patcit id="pcit0050" dnum="EP0771589A"><text>EP 0 771 589</text></patcit>, <patcit id="pcit0051" dnum="EP0765897A"><text>EP 0 765 897</text></patcit> and <patcit id="pcit0052" dnum="EP0582879A"><text>EP 0 582 879</text></patcit> are described.

0135Preferred hydroxysilyl acids can be represented by the formulas B or C.         [(RO)_y(R²)_e.g.Si- {R¹-SO₃}_a]_xM^{x +}     (B)         [(RO)_y(R²)_e.g.Si- {R¹-O_b-P (O_cR₃O₂^-}_a]_xM^{x +}     (C) in the M an H⁺ an NH₄⁺ or indicates a metal cation with a valence of x equal to 1 to 4, in which y = 1 to 3, z = 0 to 2 and a = 1 to 3, with the proviso that y + z = 4-a, in which b and c = 0 or 1, in which R and R²denote the same or different and denote methyl, ethyl, propyl, butyl or H and in the R³ is M or a methyl, ethyl, propyl or butyl radical, and in the R¹ a linear or branched alkyl or alkylene group with 1 to 12 carbon atoms, a cycloalkyl group with 5 to 8 carbon atoms or a unit of the general formulas<chemistry id="chem0047" num="0047"><img file="EP1518282B1_D0047.tif" /></chemistry><chemistry id="chem0048" num="0048"><img file="EP1518282B1_D0048.tif" /></chemistry>indicates in which n or m is a number from 0 to 6.

0136Preferred hydroxysilyl acids or their precursors (derivatives) are trihydroxysilylethylsulfonic acid, trihydroxysilylphenylsulfonic acid, trihydroxysilylpropylsulfonic acid, trihydroxysilylpropylmethylphosphonic acid, or dihydroxysilylpropylsulfonedioic acid or salts thereof.

0137The structure of the cation exchange material can be precisely adjusted by a suitable choice of the tri (network former), di (chain former) and monohydroxysilyl acid (chain link) as well as by adding other sol formers. Suitable sol formers are e.g. B. the hydrolyzed precursors of SiO₂, Al₂O₃, P₂O₅, TiO₂ or ZrO₂. The preferred compounds used include tetramethoxysilane, tetraethoxysilane, triethoxyvinylsilane, trimethoxyvinylsilane, triethoxypropenylsilane and trimethoxypropenylsilane.

0138A film of the basic polymer, a polymer electrolyte membrane doped with mineral acid or an electrode coated with a noble metal catalyst can optionally be used as substrates for the deposition of the barrier layer.

0139In a variant of the invention, the barrier layer is deposited on an electrode. ,

0140According to a particular aspect of the present invention, the material from which the barrier layer is produced is chemically compatible with the flat material doped with at least one mineral acid, so that good adhesion of the barrier layer to the flat material is achieved. Accordingly, when using a polyazole film, organic cation exchange polymers in particular are used, on which the polyazoles set out have high adhesion. These include, in particular, sulfonated polysulfones, polyether ketones and other polymers which have aromatic groups in the main chain. When using inorganic materials, high adhesion to the organic or inorganic supports can be achieved by selecting suitable functional groups.

0141Accordingly, when inorganic flat materials are used, the inorganic layers described above, which can be obtained, for example, by hydrolysis of hydroxysilyl acids, are preferably used.

0142The multilayer electrolyte membranes according to the invention show an excellent conductivity and performance taking into account the barrier layer.

0143The proton conductivity of preferred multilayer electrolyte membranes at temperatures of 120 ° C. is preferably at least 0.1 S / cm, in particular at least 0.11 S / cm, particularly preferably at least 0.12 S / cm. This conductivity is also achieved at temperatures of 80 ° C.

0144Here, a membrane according to the invention can be moistened at low temperatures. For this purpose, for example, the compound used as an energy source, for example hydrogen, can be provided with a proportion of water. In many cases, however, the water formed by the reaction is also sufficient to achieve humidification.

0145The specific conductivity is 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 sinking electrodes is 2 cm. The spectrum obtained is evaluated using a simple model consisting of a parallel arrangement of an ohmic resistor and a capacitor. The sample cross-section of the membrane doped with phosphoric acid is measured immediately before the sample assembly. 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.

0146The polymer membrane according to the invention has improved material properties compared to the previously known doped polymer membranes. Due to the low methanol permeability, the multilayer membranes can be used in particular in direct methanol fuel cells.

0147The passage current density when operating with 0.5 M methanol solution and 90 ° C. in a so-called liquid direct methanol fuel cell is preferably less than 100 mA / cm², in particular less than 70 mA / cm² particularly preferably less than 50 mA / cm² and most preferably less than 10 mA / cm². The passage current density is preferably less than 100 mA / cm when operated with a 2 M methanol solution and 160 ° C. in a so-called gaseous direct methanol fuel cell², in particular less than 50 mA / cm² very particularly preferably less than 10 mA / cm².

0148To determine the cross-over current density, the amount of carbon dioxide released at the cathode is determined using a CO₂Sensor measured. From the value of the CO thus obtained₂-Quantity will, as of <nplcit id="ncit0011" npl-type="s"><text>P. Zelenay, SC Thomas, S. Gottesfeld in S. Gottesfeld, TF Fuller "Proton Conducting Membrane Fuel Cells II" ECS Proc. Vol. 98-27 pp. 300-308</text></nplcit> described, the passage current density is calculated.

0149The invention further relates to the preferred use of the multilayer electrolyte membrane according to the invention or the coated electrode in a membrane electrode assembly (MEE) for a fuel cell.

0150The MEE contains at least one multilayer electrolyte membrane according to the invention and two electrodes, between which the multilayer electrolyte membrane is arranged in a sandwich-like manner.

0151The electrodes each have a catalytically active layer and a gas diffusion layer for supplying a reaction gas to the catalytically active layer. The gas diffusion layer is porous so that reactive gas can pass through.

0152The multilayer electrolyte membrane according to the invention can be used as an electrolyte membrane in electrochemical processes. In addition, the electrolyte membrane and a precursor for an MEU can be produced with one or both catalytically active layers. Furthermore, the MEE can also be produced by fixing the gas diffusion layer on the precursor.

0153Another object of the present invention is a fuel cell system with several different MEE's, of which at least one multilayer membrane according to the invention contains.

0154A membrane-electrode unit according to the invention shows a surprisingly high power density. According to a particular embodiment, preferred membrane electrode units have a current density of at least 0.1 A / cm², preferably 0.2 A / cm², particularly preferably 0.3 A / cm². This current density is in operation with pure hydrogen at the anode and air (approx. 20 vol.% Oxygen, approx. 80 vol.% Nitrogen) at the cathode at normal pressure (absolute 1013 mbar, with open cell outlet) and 0.6V Cell voltage measured. Particularly high temperatures in the range of 150-200 ° C, preferably 160-180 ° C, in particular 170 ° C can be used.

0155The aforementioned power densities can be achieved on both sides even with low stoichiometry of the fuel gases. According to a particular aspect of the present invention, the stoichiometry is less than or equal to 2, preferably less than or equal to 1.5, very particularly preferably less than or equal to 1.2.

Examples 1 to 6

0156Manufacture of cation exchange membranes:

The following stock solutions were prepared for the production of cation exchange membranes.

0157<ol id="ol0002" compact="compact"><li>a) 10 wt% PES (Ultrason E 7020 P) in NMP</li><li>b) 17 wt% sPEK (degree of sulfonation 50.3%) in NMP</li></ol>

0158The solutions were mixed together as shown in Table 1 and knife-coated with a doctor frame (50 μm). The films were then dried in an oven at 120 ° C. for 11 hours. The thickness of the films produced is 20-25 µm.

0159The polymers used to manufacture the membrane are set out in Table 1<tables id="tabl0002" num="0002"><table frame="all"><title>Table 1</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="18mm" /><colspec colnum="2" colname="col2" colwidth="18mm" /><colspec colnum="3" colname="col3" colwidth="18mm" /><thead><row rowsep="0"><entry morerows="1" rowsep="1" valign="top" /><entry valign="top">PES</entry><entry valign="top">sPEK</entry></row><row><entry valign="top">[% By weight]</entry><entry valign="top">[% By weight]</entry></row></thead><tbody><row><entry>example 1</entry><entry>0</entry><entry>100</entry></row><row><entry>Example 2</entry><entry>20</entry><entry>80</entry></row><row><entry>Example 3</entry><entry>30</entry><entry>70</entry></row><row><entry>Example 4</entry><entry>40</entry><entry>60</entry></row><row><entry>Example 5</entry><entry>50</entry><entry>50</entry></row><row><entry>Example 6</entry><entry>60</entry><entry>40</entry></row></tbody></tgroup></table></tables>

0160The specific conductivity is 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 evaluated using a simple model consisting of a parallel arrangement of an ohmic resistor and a capacitor. The sample cross-section of the sulfonated PEK membranes and sulfonated PEK blend membranes is measured after swelling in water at 80 ° C for 1 hour before sample assembly. To measure the temperature dependence and for humidification, the measuring cell is rinsed with tempered water. Before the start of the experiment, the cell is held at 80 ° C for 30 minutes and then the conductivity measurement is started. The cooling takes place at 1 K / min. Before each new measurement is started, it is kept at the desired temperature for 10 minutes.

0161Table 2 shows the result of the conductivity measurements of sulfonated PEK membranes and sulfonated PEK blend membranes.<tables id="tabl0003" num="0003"><table frame="all"><title>Table 2: Conductivity values ​​of sulfonated PEK membranes and sulfonated PEK blend membranes (percentage of PES blend component in percent by weight) for use as a barrier layer for phosphoric acid</title><tgroup cols="7"><colspec colnum="1" colname="col1" colwidth="24mm" /><colspec colnum="2" colname="col2" colwidth="23mm" /><colspec colnum="3" colname="col3" colwidth="24mm" /><colspec colnum="4" colname="col4" colwidth="24mm" /><colspec colnum="5" colname="col5" colwidth="24mm" /><colspec colnum="6" colname="col6" colwidth="24mm" /><colspec colnum="7" colname="col7" colwidth="24mm" /><thead><row><entry align="center" valign="top">T (° C)</entry><entry align="center" valign="top">Ex. 1</entry><entry align="center" valign="top">Ex. 2</entry><entry align="center" valign="top">Ex. 3</entry><entry align="center" valign="top">Ex. 4</entry><entry align="center" valign="top">Ex. 5</entry><entry align="center" valign="top">Ex. 6</entry></row></thead><tbody><row><entry align="center">80</entry><entry align="center">0,196</entry><entry align="center">0,160</entry><entry align="center">0,150</entry><entry align="center">0,149</entry><entry align="center">0,046</entry><entry align="center">0,035</entry></row><row><entry align="center">70</entry><entry align="center">0,181</entry><entry align="center">0,148</entry><entry align="center">0,139</entry><entry align="center">0,137</entry><entry align="center">0,042</entry><entry align="center">0,031</entry></row><row><entry align="center">60</entry><entry align="center">0,164</entry><entry align="center">0,136</entry><entry align="center">0,125</entry><entry align="center">0,125</entry><entry align="center">0,037</entry><entry align="center">0,028</entry></row><row><entry align="center">50</entry><entry align="center">0,150</entry><entry align="center">0,124</entry><entry align="center">0,113</entry><entry align="center">0,112</entry><entry align="center">0,032</entry><entry align="center">0,025</entry></row><row><entry align="center">40</entry><entry align="center">0,133</entry><entry align="center">0,110</entry><entry align="center">0,099</entry><entry align="center">0,098</entry><entry align="center">0,027</entry><entry align="center">0,022</entry></row><row><entry align="center">30</entry><entry align="center">0,116</entry><entry align="center">0,096</entry><entry align="center">0,086</entry><entry align="center">0,085</entry><entry align="center">0,023</entry><entry align="center">0,018</entry></row><row><entry align="center">22</entry><entry align="center">0,105</entry><entry align="center">0,086</entry><entry align="center">0,077</entry><entry align="center">0,074</entry><entry align="center">0,020</entry><entry align="center">0,016</entry></row></tbody></tgroup></table></tables>

0162The conductivity and barrier effect of the cation exchange membrane for phosphoric acid strongly depend on the content of acid groups expressed by the so-called ion exchange capacity (IEC).

0163To measure the IEC, the sulfonated polymer or the sulfonated blend membrane is first treated in boiling water for 2 hours. Excess water is then dabbed off and the sample is dried for 15 hours at 160 ° C. in a vacuum drying cabinet at p &lt;1 mbar. The dry weight of the membrane is then determined. The polymer dried in this way is then dissolved in DMSO at 80 ° C. for 1 h. The solution is then titrated with 0.1 M NaOH. The ion exchange capacity (IEC) is then calculated from the consumption of the acid up to the equivalent point and the dry weight.

0164To determine the swelling behavior, the sulfonated membranes or blend membranes are swollen at 80 ° C. for 2 hours and the increase in area is determined.

0165Table 3 shows the ion exchange capacity of sulfonated PEK membrane (0% PES) and blend membranes made of sulfonated PEK and various levels of PES.<tables id="tabl0004" num="0004"><table frame="none"><title>Table 3: Ion exchange capacity and swelling at 80 ° C of sulfonated PEK membrane (0% PES) and blend membranes made of sulfonated PEK and various levels of PES</title><tgroup cols="3" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="55mm" /><colspec colnum="2" colname="col2" colwidth="56mm" /><colspec colnum="3" colname="col3" colwidth="56mm" /><thead><row><entry valign="top" /><entry align="center" valign="top" /><entry align="center" valign="top">T = 80 ° C</entry></row><row><entry valign="top" /><entry align="center" valign="top">IEC (meg / g)</entry><entry align="center" valign="top">Swelling (%)</entry></row></thead><tbody><row><entry>example 1</entry><entry align="center">2,06</entry><entry align="center">156</entry></row><row><entry>Example 2</entry><entry align="center">1,71</entry><entry align="center">124,6</entry></row><row><entry>Example 3</entry><entry align="center">1,34</entry><entry align="center">61,6</entry></row><row><entry>Example 4</entry><entry align="center">1,03</entry><entry align="center">41,7</entry></row><row><entry>Example 5</entry><entry align="center">0,8</entry><entry align="center">8,6</entry></row><row><entry>Example 6</entry><entry align="center">0,59</entry><entry align="center">2</entry></row></tbody></tgroup></table></tables>

0166To measure the barrier effect of the cation exchange membranes using the example of phosphoric acid-doped membranes, the procedure is as follows:<ul id="ul0002" list-style="none" compact="compact"><li>First, a cation exchange membrane with a diameter of 7 cm is punched out in the dry state. This membrane is then immersed in 300 ml of water and the pH change is measured as a function of time. These membranes may experience a drop in pH due to residual acid residues from the sulfonation reaction due to the material selected. Since each membrane contains different amounts of acid groups, this blank value must be measured for each individual membrane.</li></ul>

0167Such a membrane is then clamped back into the measuring device and a membrane with acid-doped membrane is placed on it. For doping, a PBI film with an initial thickness of 50 µm is placed in 85% phosphoric acid at room temperature for at least 72 hours. A piece with a diameter of 3 cm is punched out of this acid-doped membrane and placed directly on the cation exchange membrane. The sandwich produced in this way is then placed in a beaker filled with 300 ml of water and the pH change is measured over 15 hours at room temperature (20 ° C.). A schematic structure of the measuring device is shown in<figref idref="f0001">Figure 1</figref> reproduced. The result thus obtained is in<figref idref="f0001">Figure 2</figref> represented graphically.

0168The negative values ​​in <figref idref="f0002">Figure 3</figref> After correcting the blank value, it can be explained that the loss of acid from the cation exchange membrane (blank value) itself is greater than the passage of phosphoric acid through the cation exchange membrane.

0169In <figref idref="f0003">Figure 4</figref> the measurement of the amount of acid which has passed through the barrier layer and which has been retained by the barrier layer is beyond doubt proven.

0170The results show that the use of cation exchange materials as a barrier leads to a surprisingly significant reduction in the release of mineral acid.

0171The results obtained surprisingly show that preferred cation exchange membranes according to the invention have a conductivity of &lt;0.06 S / cm, in particular &lt;0.05 S / cm, in the moistened state at 80 ° C.

0172Preferred cation exchange membranes according to the invention have an IEC value of less than 0.9 meq / g. The swelling of preferred cation exchange membranes is less than 20% at 80 ° C. It was surprisingly found that the use of the inventive membrane provided with a barrier layer with an ion exchange capacity of less than 0.9 meq / g and swelling in water of less than 10% at 80 ° C. leads to a particularly significant reduction in the passage of phosphoric acid and the acid concentration does not exceed 0 within 15 hours, 0005 mol / l increases.

Example 7

0173Production of an ultra-thin cation exchange membrane as a barrier layer on the membrane surface:<ul id="ul0003" list-style="none" compact="compact"><li>PBI film production: A 50 μm thick film was knife-coated from a 15 wt% polybenzimidazole (PBI) solution in DMAc and dried in the oven at 120 ° C. for 12 hours.</li><li>Preparation of the spray solution: A 10 wt% solution of PES (Ultrason E 7020) and sPEK (degree of sulfonation 50.3%) in DMAc was prepared, the weight ratio of PES to sPEK being 60:40.</li><li>Coating: For the coating, a glass plate was placed on a hot plate and heated to 150 ° C. After the temperature had been reached, the PBI film was placed on the glass plate. As soon as the film pulls flat on the glass plate, a metal stencil was put on. The spray solution was sprayed onto the film surface several times using an airbrush. The solvent was evaporated between each spray. The metal template was then removed and the sprayed area cut out. The thickness of the coating was 4-5 µm.</li></ul>

0174The coated polyazole film is with the coated side up as in <figref idref="f0001">Figure 1</figref> shown clamped and then immersed in a beaker filled with 100 ml of water. In this configuration, the underside is in contact with water, while 0.5 ml of phosphoric acid is applied to the opposite side.

0175The change in pH was observed over a period of 50 hours. For comparison, a polyazole film without a barrier layer was subjected to the same test.

0176The results obtained are in <figref idref="f0003">Figure 5</figref> shown, the effectiveness of the thin barrier layer is evident.