Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
  • C08J5/20—Manufacture of shaped structures of ion-exchange resins
  • C08J5/22—Films, membranes or diaphragms
  • C08J5/2206—Films, membranes or diaphragms based on organic and/or inorganic macromolecular compounds
  • C08J5/2218—Synthetic macromolecular compounds
  • C08J5/2256—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions other than those involving carbon-to-carbon bonds, e.g. obtained by polycondensation
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
  • C08J5/20—Manufacture of shaped structures of ion-exchange resins
  • C08J5/22—Films, membranes or diaphragms
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
  • H01M8/1018—Polymeric electrolyte materials
  • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
  • H01M8/103—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having nitrogen, e.g. sulfonated polybenzimidazoles [S-PBI], polybenzimidazoles with phosphoric acid, sulfonated polyamides [S-PA] or sulfonated polyphosphazenes [S-PPh]
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2379/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing nitrogen with or without oxygen, or carbon only, not provided for in groups C08J2361/00 - C08J2377/00
  • C08J2379/04—Polycondensates having nitrogen-containing heterocyclic rings in the main chain; Polyhydrazides; Polyamide acids or similar polyimide precursors
  • C08J2379/06—Polyhydrazides; Polytriazoles; Polyamino-triazoles; Polyoxadiazoles
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M2300/00—Electrolytes
  • H01M2300/0017—Non-aqueous electrolytes
  • H01M2300/0065—Solid electrolytes
  • H01M2300/0082—Organic polymers
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• the present invention relates to a proton-conducting polymer electrolyte membrane based on a polyazole salt of an inorganic or organic acid which is doped with an acid as electrolyte, a process for producing the proton-conducting polymer electrolyte membrane, a membrane-electrode assembly comprising the proton-conducting polymer electrolyte membrane and a fuel cell comprising the membrane-electrode assembly of the invention.
• Proton-conducting, i.e. acid-doped, polyazole membranes for use in PEM fuel cells are known from the prior art.
• the basic polyazole films are generally doped with concentrated phosphoric acid or sulfuric acid and then act as proton conductors and separators in polymer electrolyte membrane fuel cells (PEM fuel cells).
• PEM fuel cells polymer electrolyte membrane fuel cells Due to the excellent properties of the polyazole polymer, such polymer electrolyte membranes can, when processed to produce membrane-electrode assemblies (MEAs), be used in fuel cells at long-term operating temperatures above 100°C, in particular above 120°C. This high long-term operating temperature allows the activity of the noble metal-based catalysts comprised in the membrane-electrode assembly to be increased.
• polymer electrolyte membranes based on polyazole polymers enables, firstly, the complicated gas treatment or gas purification to be partly dispensed with and, secondly, the catalyst loading in the membrane-electrode assembly to be reduced. Both are indispensible prerequisites for mass use of PEM fuel cells, since otherwise the costs of a PEM fuel cell system are too high.
• DE 101 176 87 A1 relates to proton-conducting polymer membranes which are based on polyazoles and have a high specific conductivity, in particular at operating temperatures above 100°C, and make do without additional fuel gas humidification.
• the proton-conducting polymer membranes according to DE 101 176 87 A1 can be obtained by a process comprising the following steps:
• step E treatment of the membrane formed in step D) until it is self-supporting.
• DE 10 2006 036019 A1 relates to a membrane-electrode assembly comprising at least two electrochemically active electrodes which are separated by at least one polymer electrolyte membrane, with the polymer electrolyte membrane having reinforcing elements which penetrate at least partly through the polymer electrolyte membrane.
• the membrane-electrode assembly is preferably obtained by a process in which
• the membrane-electrode assembly is particularly suitable for use in fuel cells.
• the abovementioned polymer membranes based on polyazoles are generally operated in the presence of phosphoric acid as electrolyte.
• the membranes are soft and therefore have only limited mechanical strength.
• the mechanical stability decreases with increasing temperature and the solubility of the polymer framework is increased. In the upper region of the typical operating window of a fuel cell (from about 160 to 180°C), this can lead to durability problems.
• the polymer electrolyte membranes can be dissolved or flow away at relatively high temperatures under unfavorable operating conditions. The consequence is failure of the membrane- electrode assembly comprising the abovementioned polymer electrolyte membrane. It is therefore an object of the present invention to reduce the solubility of polymer electrolyte membranes based on polyazoles in the acid used as electrolyte, preferably phosphoric acid, and to improve the mechanical stability of the membrane.
• the abovementioned object is achieved by a proton-conducting polymer electrolyte membrane based on a polyazole salt of an organic or inorganic acid which is doped with an acid as electrolyte, with the polyazole salt of the organic or inorganic acid having a lower solubility in the acid used as electrolyte than the polyazole salt of the acid used as electrolyte.
• the polymer electrolyte membrane based on a polyazole salt of an organic or inorganic acid is doped with an acid as electrolyte.
• an acid as electrolyte.
• heteropolyacids are inorganic polyacids which have at least two different central atoms and are formed in each case from weak, polybasic oxo acids of a metal, e.g. Cr, Mo, V or W, and a nonmetal, e.g. As, I, P, Se, Si or Te, as partially mixed anhydrides. They include, for example, 12-molybdophosphoric acid and 12-tungsto- phosphoric acid. A polyacid which is preferably used is polyphosphoric acid.
• polyphosphoric acid refers to commercial polyphosphoric acids.
• the polyphosphoric acids ⁇ ⁇ +2 ⁇ 3 ⁇ + ⁇ ( ⁇ >1 ) usually have a content, calculated as P 2 0 5 (acidimetric) of at least 83%.
• highly concentrated acids are generally used.
• the concentration of the phosphoric acid which is particularly preferably used is generally at least 50% by weight, preferably at least 80% by weight, based on the total weight of the electrolyte.
• the remaining up to 50% by weight, preferably up to 20% by weight, is generally water.
• the conductivity of the polymer membrane can be influenced via the degree of doping.
• the conductivity generally increases with increasing amount of electrolyte until a maximum value is reached.
• the amount of electrolyte degree of doping
• preference is given to a degree of doping of from 3 to 80, particularly preferably from 5 to 60, very particularly preferably from 12 to 60.
• the durability and the mechanical membrane stability in the acid used as electrolyte, particularly preferably phosphoric acid, can be significantly improved by use of the proton-conducting polymer electrolyte membranes according to the present invention without the performance of the polymer electrolyte membrane in the fuel cell being adversely affected.
• Organic or inorganic acids suitable for forming the polyazole salt are all acids as long as they form a polyazole salt which is less soluble in the electrolyte, preferably phosphoric acid, than is the polyazole salt of the electrolyte.
• Suitable inorganic acids are, for example, HN0 3 , sulfuric acid, sulfates such as K 2 S0 4 .
• Suitable organic acids are aliphatic or aromatic acids which are preferably perfluorinated.
• Preferred organic and inorganic acids are thus selected from the group consisting of perfluorinated phenols such as pentafluorophenol, perfluorinated phenyl alcohols, K 2 S0 4 , HNO 3 , FSO 3 H, HPO2F2, H2SO 3 , HOOC-COOH, sulfonic acids such as CH 3 SO 3 H, perfluorinated sulfonic acids such as CF 3 SO 3 H, CF 3 CF 2 S0 3 H, etc, perfluorosulfonamides such as (CF 3 ) 2 S0 2 NH, (CF 3 CF 2 ) 2 S0 2 NH, (CF 3 CF 2 CF 2 ) 2 S0 2 NH, etc., perfluorinated phosphonic acids such as CF 3 P0 3 H 2 , CF3CF2P03H2,CF3CF2CF 2 P03H2 etc.
• perfluorinated phosphonic acids such as CF 3 P0 3 H 2 ,
• organic and inorganic acids are pentafluorophenol, CH 3 SO 3 H, CF 3 SO 3 H, CF 3 CF2SO 3 H, (CF 3 ) 2 S0 2 NH, (CF 3 CF2)2S0 2 NH, (CFaCFzCFzkSCkNH, CF3PO3H2, CF3CF2PO3H2 and CF3CF2CF2PO3H2.
• polyazole salt used according to the invention in the proton electron-conducting polymer electrolyte membranes is preferably based on one or more polyazoles.
• Polyazoles which are preferably used are polyazoles which comprise recurring azole units of the general formula (I) and/or (II) and/or (III) and/or (IV) and/or (V) and/or (VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X) and/or (XI) and/or (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).
• radicals Ar are identical or different and are each a tetravalent aromatic or heteroaromatic group which may have one or more rings
• the radicals Ar 1 are identical or different and are each a divalent aromatic or heteroaromatic group which may have one or more rings
• the radicals Ar 2 are identical or different and are each a divalent or trivalent aromatic or heteroaromatic group which may have one or more rings
• the radicals Ar 3 are identical or different and are each a trivalent aromatic or hetero- aromatic group which may have one or more rings
• the radicals Ar 4 are identical or different and are each a trivalent aromatic or hetero- aromatic group which may have one or more rings
• the radicals Ar 5 are identical or different and are each a tetravalent aromatic or hetero- aromatic group which may have one or more rings
• the radicals Ar 6 are identical or different and are each a divalent aromatic or hetero- aromatic group which may have one or more rings
• the radicals Ar 7 are identical or different and are each a divalent aromatic or hetero- aromatic group which may have one or
• Preferred aromatic or heteroaromatic groups are derived from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline, pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine, tetrazine, pyrole, pyrazole, anthracene, benzopyrrole, benzotriazole, benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazine, indolizine, quinolizine, pyridopyridine, imidazolepyrimidine, pyrazinopyrimidine, carbazole, azeridine, phenazine, benzoquinoline, phenoxazine
• Ar 1 , Ar 4 , Ar 6 , Ar 7 , Ar 8 , Ar 9 , Ar 10 and Ar 11 can have any substitution pattern; in the case of phenylene, for example, Ar 1 , Ar 4 , Ar 6 , Ar 7 , Ar 8 , Ar 9 , Ar 10 and Ar 11 can each be, independently of one another, ortho-, meta- or para-phenylene. Particularly preferred groups are derived from benzene and biphenyls, which may optionally be substituted.
• Preferred alkyl groups are alkyl groups having from 1 to 4 carbon atoms, e.g. methyl, ethyl, n-propyl, i-propyl and t-butyl groups.
• Preferred aromatic groups are phenyl or naphthyl groups.
• the alkyl groups and the aromatic groups can be monosubstituted or polysubstituted.
• Preferred substituents are halogen atoms, e.g. fluorine, amino groups, hydroxy groups or CrC 4 -alkyl groups, e.g. methyl or ethyl groups.
• the polyazoles can in principle have different recurring units which differ, for example, in their radical X. However, the respective polyazoles preferably have exclusively identical radicals X in a recurring unit.
• the polyazole salt is based on a polyazole comprising recurring azole units of the formula (I) and/or (II).
• the polyazoles used to form the polyazole salts are, in one embodiment, polyazoles comprising recurring azole units in the form of a copolymer or a blend which comprises at least two units of the formulae (I) to (XXII) which differ from one another.
• the polymers can be present as block copolymers (diblock, triblock), random copolymers, periodic copolymers and/or alternating polymers.
• the number of recurring azole units in the polymer is preferably an integer > 10, particularly preferably > 100.
• the polyazoles used to form the polyazole salt are polyazoles comprising recurring units of the formula (I) in which the radicals X within the recurring units are identical.
• Further preferred polyazoles on which the polyazole salts of the present invention are based are selected from the group consisting of polybenzimidazole, poly(pyridine), poly(pyrimidine), polyimidazole, polybenzothiazole, polybenzoxazole, polyoxadiazole, polyquinoxaline, polythiadiazole and poly(tetrazapyrene).
• the polyazole salt is based on a polyazole comprising recurring benzimidazole units. Suitable polyazoles which have recurring benzimidazole units are indicated below:
• n and m are integers > 10, preferably > 100.
• the polyazole on which the polyazole salt used according to the invention is based particularly preferably has repeating units of the following formula
• n is an integer > 10, preferably > 100.
• the polyazoles on which the polyazole salt used according to the invention is based have a high molecular weight.
• the molecular weight is at least 0.2 dl/g, preferably from 0.8 to 10 dl/g, particularly preferably from 1 to 10 dl/g.
• the conversion to eta i is carried out according to the above relationship on the basis of the information in "Methods in Carbohydrate Chemistry", Volume IV, Starch, Academic Press, New York and London, 1964, page 127.
• the proton-conducting polymer electrolyte membranes according to the present invention have reinforcing elements. These reinforcing elements generally at least partly penetrate through the polymer electrolyte membrane, i.e. the reinforcing elements generally penetrate at least partly into the polymer electrolyte membrane.
• the reinforcing elements are particularly preferably predominantly embedded in the membrane and project, if at all, therefrom only in places.
• the reinforcing elements are at least partly joined to the membrane.
• a partial composite is considered to be a composite of reinforcing element and membrane in which the reinforcing elements advantageously take up such a force that in the force-elongation curve at 20°C, the reference force of the polymer electrolyte membrane having reinforcing elements compared to the polymer electrolyte membrane without reinforcing elements in the range from 0 to 1 % elongation differs at at least one point by at least 10%, preferably by at least 20% and very particularly preferably by at least 30%.
• the polymer electrolyte membrane having reinforcing elements is preferably fiber- reinforced.
• reinforcing elements which preferably comprise monofilaments, multifilaments, long and/or short fibers, hybrid yarns and/or bicomponent fibers.
• the reinforcing element can also form a textile sheet. Suitable textile sheets are nonwovens, woven fabrics, drawn-loop knits, formed-loop knits, felts, lay-ups and/or meshes, particularly preferably lay-ups, woven fabrics and/or nonwovens.
• Nonlimiting examples of the abovementioned woven fabrics are fabrics composed of polyphenylene sulfone, polyether sulfone, polyether ketone, polyether ether ketone, poly(acrylic), poly(ethylene terephthalate), poly(propylene), poly(tetrafluoroethylene), poly(ethylene-co-tetrafluoroethylene) (ETFE), 1 :1 alternating copolymer of ethylene and chlorotrifluoroethylene (ECTFE), polyvinylidene fluoride (PVDF), poly(acrylonitrile) and polyphenylene sulfide (PPS).
• the term woven fabrics refers to products composed of monofilament threads and/or multifilament threads which cross predominantly at right angles.
• the thread-to-thread distance of the textile sheets can usually be from 20 to 2000 ⁇ ; for the purposes of the present invention, textile sheets, in particular woven fabrics, lay-ups and meshes, having a thread-to-thread distance in the range from 30 to 300 ⁇ have been found to be particularly useful.
• the thread-to-thread distance can be determined, for example, by electronic image analysis of an optical photograph or transmission electron micrograph. Further details regarding suitable woven fabrics, lay-ups and meshes are disclosed in DE 10 2006 036019 A1 .
• Particularly preferably suitable woven fabrics are, for example, woven fabrics from SEFAR having the trade names SEFAR NITEX ® , SEFAR PETEX ® , SEFAR PROPYLTEX ® , SEFAR FLUORTEX ® and SEFAR PEAKTEX ® , woven fabrics from SAATI having the trade name Saati 90.30 and woven fabrics from DEXMET having the trade names Dexmet 2PTFE10-105, Dexmet 2PTFE5-105H and Dexmet 2PTFE2-50H.
• nonwovens refers to flexible, porous sheet-like structures which are not produced by classical methods of warp and weft weaving and by stitch formation, but rather by entanglement and/or cohesive and/or adhesive joining of fibers (e.g. spunbond or melt blown nonwovens).
• Nonwovens are loose materials made up of spinning fibers or filaments whose cohesion is generally produced by the intrinsic adhesion of the fibers or by means of mechanical after-consolidation.
• the individual fibers can have a preferential direction (oriented or cross nonwovens) or be unaligned (random nonwovens).
• the nonwovens can be mechanically strengthened by needling, intermeshing or by interlacing by means of strong water jets (known as spunlaced nonwovens).
• Suitable examples of preferred nonwovens are SEFAR PETEX ® , SEFAR FLUOROTEX ® and SEFAR PEEKTEX ®
• the composition of the reinforcing elements can be chosen freely and matched to the specific application.
• the reinforcing elements preferably comprise glass fibers, mineral fibers, natural fibers, carbon fibers, boron fibers, synthetic fibers, polymer fibers and/or ceramic fibers, in particular SEFAR CARBOTEX ® , SEFAR PETEX ® , SEFAR FLUORTEX ® , SEFAR PEEKTEX ® , SEFAR TETEX MONO ® , SEFAR TETEX DLW ® , SEFAR TETEX MULTI from SEFAR and also DUOFIL ® , EMMITEX GARN ® .
• the reinforcing elements which are optionally constituent of a woven fabric, drawn- loop knit, formed-loop knit or nonwoven, can have a virtually round cross section or else other shapes such as dumbbell-shaped, kidney-shaped, triangular or multilobal cross sections. Bicomponent fibers are also possible.
• the reinforcing elements preferably have a maximum diameter of from 10 ⁇ to 500 ⁇ , particularly preferably from 20 ⁇ to 300 ⁇ , very particularly preferably from 20 ⁇ to 200 ⁇ and in particular from 25 ⁇ to 100 ⁇ .
• the maximum diameter refers to the longest dimension in the cross section.
• the reinforcing elements preferably have a Young's modulus of at least 5 GPa, preferably at least 10 GPa, particularly preferably at least 20 GPa.
• the elongation at break of the reinforcing elements is preferably from 0.5% to 300%, particularly preferably from 1 % to 60%.
• the proportion by volume of the reinforcing elements is preferably from 5% by volume to 95% by volume, particularly preferably from 10% by volume to 80% by volume, very particularly preferably from 10% by volume to 50% by volume and in particular from 10% by volume to 30% by volume.
• the proportion by volume is usually measured at 20°C.
• the reinforcing elements usually take up such a force that in the force-elongation curve at 20°C, the reference force of the polymer electrolyte membrane having reinforcing elements compared to the polymer electrolyte membrane without reinforcing elements in the range from 0 to 1 % elongation at at least one point differs by at least 10%, preferably by at least 20%, particularly preferably by at least 30%.
• the reinforcement is advantageously such that the reference force of the polymer electrolyte membrane at room temperature (20°C) divided by the reference force of the support insert at 180°C, measured at at least one point in the range from 0 to 1 % elongation, gives a quotient of not more than 3, preferably not more than 2.5, particularly preferably ⁇ 2.
• the measurement of the reference force is carried out in accordance with EN29073, Part 3, on 5 cm wide specimens at a measurement length of 100 mm.
• the numerical value of the prestressing force, reported in centinewton [cN] corresponds to the numerical value of the mass per unit area of the specimen, reported in gram per m 2 .
• the polymer electrolyte membranes can be produced by methods known to those skilled in the art, and may, in one embodiment of the present invention, be provided with reinforcing elements directly during production of the membranes.
• the polymer electrolyte membranes of the present invention can usually be produced by firstly dissolving at least one polyazole in at least one polar, aprotic solvent, for example dimethylacetamide (DMAc), and producing a polymer film (polymer membrane) by means of a classical process.
• the reinforcing elements which may optionally be present can, for example, be introduced into the film during production of the film.
• the film obtained in this way can be treated with a washing liquid, for example as described in DE 10109829.
• the freeing of the polyazole film of solvent residues as described in DE 10109829 improves the mechanical properties of the film compared to films which have not been freed of solvent residues in this way.
• polymer film can have further modifications, for example by crosslinking, as described in DE 101 10752 and WO 00/44816.
• the thickness of the polyazole films can be within a wide range.
• the polyazole films before doping with acid as described below preferably have a thickness of generally from 5 ⁇ to 2000 ⁇ , preferably from 10 ⁇ to 1000 ⁇ , particularly preferably from 20 ⁇ to 1000 ⁇ .
• the abovementioned films are doped with an acid.
• Suitable acids electrophiles
• phosphoric acid H 3 P0 4
• polyazole salts of an organic or inorganic acid which are such that the polyazole salt of the organic or inorganic acid has a lower solubility in the acid used as electrolyte than the polyazole salt of the acid used as electrolyte are, according to the invention, used in the polymer electrolyte membranes.
• These polyazole salts are, according to the invention, obtained by treating the abovementioned polymer films which have been doped with the acid used as electrolyte, particularly preferably phosphoric acid, with at least one of the above- mentioned inorganic or organic acids.
• the abovementioned polymer films (polymer membranes) doped with the acid used as electrolyte, particularly preferably phosphoric acid to be treated directly with the inorganic or organic acid.
• the treatment of the abovementioned polymer films (polymer membranes) doped with the acid used as electrolyte, particularly preferably phosphoric acid, with the at least one inorganic or organic acid is, as mentioned above, generally carried out in water or in phosphoric acid. The treatment is usually carried out at room temperature.
• the amount of inorganic or organic acid corresponds to at least the stoichiometric amount necessary to form the polyazole salts from the corresponding polyazoles.
• the organic or inorganic acid can also be used in excess.
• the proton-conducting polymer electrolyte membranes of the present invention are obtained by a process which comprises the following steps: ia) dissolution of at least one polyazole in phosphoric acid,
• step iia heating of the solution which can be obtained as per step i) under inert gas to temperatures of up to 400°C, preferably from 100 to 250 °C,
• Step viiia mixing of the membrane obtained in step viia) with phosphoric acid.
• Steps ia), iia), iva) and va) have been described comprehensively in DE 10246461. This application is hereby incorporated by reference.
• steps via), viia) and viiia) are generally carried out at room temperature.
• Conventional deionized water is generally used as water in step via).
• Suitable organic or inorganic acids which can be used in step viia) are the acids mentioned above in the present patent application.
• step viia) The mixing of the membrane obtained in step viia) with phosphoric acid in step viiia) is carried out in order to provide phosphoric acid as electrolyte, with the phosphoric acid being used in an amount of generally from 30 to 99% by weight, preferably from 40 to 90% by weight, particularly preferably from 40 to 85% by weight, based on the amount of polymer electrolyte membrane obtained in step viia).
• the present invention further provides a membrane-electrode assembly comprising at least two electrochemically active electrodes which are separated by a polymer electrolyte membrane, wherein the polymer electrolyte membrane is a proton- conducting polymer electrolyte membrane according to the present invention or produced according to the present invention.
• the at least two electrochemically active electrodes are usually anode and cathode.
• electrochemically active indicates that the electrodes are able to catalyze the oxidation of hydrogen and/or at least one reformate and the reduction of oxygen. This property can be obtained by coating the electrodes with noble metals. Suitable noble metals are mentioned below.
• electrode means that the material is electrically conductive. The electrode can optionally have a layer of noble metal. Such electrodes are known and are described, for example, in US 4,191 ,618, US 4,212,714 and US 4,333,805.
• the electrodes preferably comprise gas diffusion layers which are in contact with a catalyst layer.
• Sheet-like, electrically conductive and acid-resistant structures are usually used as gas diffusion layers. These include, for example, graphite fiber papers, carbon fiber papers, woven graphite fabrics and/or papers which have been made conductive by addition of carbon black. Fine dispersion of the gas and/or liquid streams is achieved by means of these layers.
• gas diffusion layers which comprise a mechanically stable support material which is impregnated with at least one electrically conductive material such as carbon (for example carbon black).
• Support materials which are particularly suitable for these purposes comprise fibers, for example in the form of nonwovens, papers or woven fabrics, in particular carbon fibers, glass fibers or fibers comprising organic polymers, for example polypropylene, polyesters (in particular polyethylene terephthalate, polyphenylene sulfide or polyether ketones). Further details regarding such diffusion layers may be found, for example, in WO 97/20358.
• the gas diffusion layers preferably have a thickness of from 80 ⁇ to 2000 ⁇ , particularly preferably from 100 ⁇ to 1000 ⁇ , very particularly preferably from 150 ⁇ to 500 ⁇ .
• the gas diffusion layers preferably have a high porosity. This is usually in the range from 20% to 80%.
• the gas diffusion layers can comprise customary additives. These include, inter alia, fluoropolymers, e.g. polytetrafluoroethylene (PTFE) and surface-active substances.
• PTFE polytetrafluoroethylene
• at least one of the gas diffusion layers can comprise a compressible material.
• a compressible material has the property that the gas diffusion layer can be pressed to half, in particular a third, of its original thickness without loss of integrity. This property is generally displayed by gas diffusion layers composed of woven graphite fabrics and/or paper which has been made conductive by addition of carbon black.
• the catalytically active layer comprises at least one catalytically active substance.
• catalytically active substance include, inter alia, noble metals, preferably platinum, palladium, rhodium, iridium and/or ruthenium. These substances can also be used in the form of alloys with one another. Furthermore, these substances can also be used in alloys with base metals, for example Cr, Zr, Ni, Co and/or Ti. In addition, the oxides of the above- mentioned noble metals and/or base metals can also be used.
• the abovementioned metals are usually used after application by known methods to a support material, usually carbon having a high specific surface area in the form of nanoparticles.
• the catalytically active compounds i.e. the catalysts
• the weight ratio of the polyazole salt present in the proton-conducting polymer electrolyte membrane of the present invention to catalyst material comprising at least one noble metal and optionally one or more support materials is generally greater than 0.05, preferably in the range from 0.1 to 0.6.
• the thickness of the catalyst layer is generally from 1 to 1000 ⁇ , preferably from 5 to 500 ⁇ , particularly preferably from 10 to 300 ⁇ . This value represents an average which can be determined by measuring the layer thickness in cross section in images recorded by means of a scanning electron microscope (SEM).
• SEM scanning electron microscope
• the noble metal content of the catalyst layer is generally from 0.1 to 10 mg/cm 2 , preferably from 0.2 to 6.0 mg/cm 2 , particularly preferably from 0.2 to 3.0 mg/cm 2 . These values can be determined by elemental analysis of a sheet specimen.
• the catalyst layer is generally not self-supporting but is instead usually applied to the gas diffusion layer and/or the membrane. Here, part of the catalyst layer can, for example, diffuse into the gas diffusion layer and/or the membrane, as a result of which transition layers are formed. This can also lead to the catalyst layer being considered to be part of the gas diffusion layer.
• the surfaces of the polymer electrolyte membrane are in contact with the electrodes in such a way that the first electrode covers the front side of the polymer electrolyte membrane and the second electrode covers the rear side of the polymer electrolyte membrane in each case partly or completely, preferably only partly.
• the front and rear sides of the polymer electrolyte membrane are the side of the polymer electrolyte membrane facing or facing away from, respectively, the viewer, with the view being from the first electrode (front), preferably the cathode, in the direction of the second electrode (rear), preferably the anode.
• the membrane-electrode assemblies of the invention have a significantly improved mechanical stability and strength and can therefore be used for producing fuel cells and fuel cell stacks having particularly high performance.
• only small power fluctuations of the resulting fuel cells or fuel cell stacks occur and high quality, reliability and reproducibility are achieved.
• This is also achieved as a result of the proton- conducting membranes of the invention which are based on the abovementioned polyazole salts being used.
• the membrane-electrode assemblies of the invention can be stored and transported without problems. Even after prolonged storage or after transport to places having significantly different climatic conditions, the dimensions of the membrane-electrode assemblies remain accurate for installation without problems in fuel cells or fuel cell stacks. The membrane-electrode assembly then no longer has to be conditioned on site for outdoor installation, which simplifies production of the fuel cells and saves time and money.
• An advantage of the preferred membrane-electrode assemblies according to the present invention is that they make it possible for the fuel cell to be operated at temperatures above 120°C. This applies in the case of gaseous and liquid fuels, e.g. hydrogen-comprising gases, which are produced, for example, from hydrocarbons in a preceding reforming step. As oxidant, it is possible to use, for example, oxygen or air.
• a further advantage of the preferred membrane-electrode assemblies of the present invention is that they have a high tolerance to carbon monoxide in operation above 120°C even with pure platinum catalysts, i.e. without a further alloying constituent.
• Preferred membrane-electrode assemblies can be operated in fuel cells without the fuel gases and the oxidants having to be humidified, despite the possible high operating temperatures.
• the fuel cell nevertheless operates stably and the membrane does not lose its conductivity. This simplifies the overall fuel cell system and brings additional cost savings since operation of the water circuit is simplified.
• the behavior of the fuel cell system at temperatures below 0°C is also simplified thereby.
• Preferred membrane-electrode assemblies also allow the fuel cell to be cooled to room temperature and below without problems and then be taken into operation again without a drop in performance.
• the membrane-electrode assemblies of the present invention display a very high long-term stability. This makes it possible to provide fuel cells which likewise have a high long-term stability. Furthermore, the membrane-electrode assemblies of the invention have excellent heat and corrosion resistance and a comparatively low gas permeability, in particular at high temperatures. A decrease in the mechanical stability and the structural integrity, in particular at high temperatures, is reduced or avoided in the membrane-electrode assemblies of the invention.
• the membrane-electrode assemblies of the invention can be produced inexpensively and simply.
• the present invention further provides a fuel cell comprising at least one membrane- electrode assembly according to the present invention. Suitable fuel cells and the components thereof are known to those skilled in the art.
• separator plates should, optionally in combination with further sealing materials, seal the gas spaces of the cathode and the anode from the outside and from one another.
• the separator plates are preferably juxtaposed in a sealing fashion with the membrane- electrode assembly. The sealing effect can be increased further by pressing the composite of separator plates and membrane-electrode assembly.
• the separator plates preferably each have at least one gas channel for reaction gases, which are advantageously arranged on the sides facing the electrodes.
• the gas channels should make dispersion of the reactant fluids possible.
• the fuel cell of the invention also has a high long-term stability.
• the fuel cell of the invention can usually be operated continuously at temperatures of more than 120°C using dry reaction gases for long periods, e.g. more than 5000 hours, without an appreciable degradation in performance being observed. The power densities which can be achieved are still high after such a long time.
• the fuel cells of the invention display a high open-circuit voltage even after a long time, for example more than 5000 hours; the open-circuit voltage is preferably at least 900 mV after this time.
• the fuel cell is operated without current with water being supplied to the anode and air being supplied to the cathode. The measurement is carried out by switching the fuel cell from a current of 0.2 A/cm 2 to the zero-current state and then recording the open-circuit voltage for five minutes. The value after five minutes is the respective open-circuit potential.
• the measured values of the open-circuit voltage are at a temperature of 160°C.
• the fuel cell preferably displays a low gas crossover after this time.
• the anode side of the fuel cell is operated using hydrogen (5 l/h), and the cathode is operated using nitrogen (5 l/h).
• the anode serves as reference electrode and counterelectrode, and the cathode serves as working electrode.
• the cathode is set to a potential of 0.5 V and the hydrogen diffusing through the membrane is oxidized at the cathode in a mass transfer-limited manner.
• the resulting current is a measure of the hydrogen permeation rate.
• the current is ⁇ 3 mA/cm 2 , preferably ⁇ 2 mA/cm 2 , particularly preferably ⁇ 1 mA/cm 2 , in a 50 cm 2 cell.
• the measured values of the H 2 crossover are at a temperature of 160°C.
• the present invention further provides for the use of the proton-conducting polymer electrolyte membrane of the invention comprising phosphoric acid as electrolyte in a membrane-electrode assembly and also the use of the proton-conducting polymer electrolyte membrane of the invention in a fuel cell.
• Example 1 Production of a polymer electrolyte membrane based on a polyazole salt of pentafluorophenol, with doping being carried out in water
• FIG. 3 shows the solubility of the membrane of Example 3.
• the dark spots in the figures are the remaining membrane which has not dissolved in 99% phosphoric acid.
• Figures 1 , 2 and 3 show the solubility of the membranes from Examples 1 , 2 and 3.
• the dark spots in the figures are the remaining membrane which has not dissolved in 99% phosphoric acid.
• FIG. 4 shows the power achieved by the three abovementioned membranes from Examples 1 , 2 and 3 in a current-voltage curve. The voltage in mV is shown on the y axis and the current density in A cm 2 is shown on the x axis.
• the power is measured in a fuel cell (H 2 /air) at temperatures of 160°C, with the anode being coated with one mg of platinum per cm 2 and the cathode being coated with 1 mg of platinum and nickel per cm 2 .
• the diamonds represent the power of a membrane as per Comparative Example 3
• the triangles represent the power of a membrane as per Example 2 according to the invention
• the solid line represents the power achieved by a membrane as per Example 1 according to the invention.

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• 2011
  • 2011-04-20 CN CN2011800268838A patent/CN102918693A/zh active Pending
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