KR20130103665A - Improved polymer electrolyte membrane based on polyazole - Google Patents

Abstract

The present invention is a proton conductive polymer electrolyte membrane based on a polyazole salt of an inorganic or organic acid doped with an acid as an electrolyte, wherein the polyazole salt of the inorganic or organic acid is more than the polyazole salt of an acid used as an electrolyte. A membrane electrode comprising a proton conductive polymer electrolyte membrane having a solubility in acid used as a low electrolyte, a process for preparing a proton conductive polymer electrolyte membrane of the present invention, and at least two electrochemically active electrodes separated by a polymer electrolyte membrane An assembly relates to a membrane electrode assembly wherein the polymer electrolyte membrane is a proton conductive polymer electrolyte membrane according to the invention, and a fuel cell comprising at least one membrane electrode assembly according to the invention.

Description

Improved polymer electrolyte membrane based on polyazoles {IMPROVED POLYMER ELECTROLYTE MEMBRANE BASED ON POLYAZOLE}

The present invention includes a proton conductive polymer electrolyte membrane based on a polyazole salt of an inorganic or organic acid doped with an acid as an electrolyte, a process for producing the proton conductive polymer electrolyte membrane, and a proton conductive polymer electrolyte membrane. A membrane electrode assembly, and a fuel cell comprising the membrane electrode assembly of the present invention.

Proton conductivity, ie acid doped polyazole membranes for use in polymer electrolyte membrane (PEM) fuel cells are known from the prior art. The basic polyazole film is generally doped with concentrated phosphoric acid or sulfuric acid, thus acting as a proton conductor and separator in a polymer electrolyte membrane fuel cell (PEM fuel cell). Because of the very good properties of the polyazole polymers, such polymer electrolyte membranes can be used in fuel cells at long term operating temperatures of 100 ° C. or higher, especially 120 ° C. or higher, when processed to produce a membrane-electrode assembly (MEA). have. This high long term operating temperature makes it possible to increase the activity of the noble metal-based catalyst included in the membrane electrode assembly. Especially when reformates derived from hydrocarbons are used, a significant amount of carbon monoxide is included in the reformate gas, which amount generally has to be removed by complex gas treatment or gas purification. The opportunity to increase the operating temperature makes it possible to tolerate significantly higher concentrations of carbon monoxide impurities for long periods of time.

The use of polymer electrolyte membranes based on polyazole polymers makes it possible, firstly, to partially eliminate the need for complex gas treatment or gas purification, and secondly to reduce the catalyst load in the membrane electrode assembly. Both are inevitable requirements for the high volume use of PEM fuel cells, because otherwise the cost of PEM fuel cell systems becomes too high.

J.S. Wainright et al., J. Electrochem. Soc, Vol. 142, No. 7, July 1995, L121-L123, relates to polybenzimidazole films which are doped with phosphoric acid and are potential polymer electrolytes for use in hydrogen / air fuel cells and direct tanol fuel cells. The electrolyte has a low permeability to methanol vapors, as a result of which the negative effects on methanol crossovers commonly observed in direct methanol fuel cells can be reduced.

DE 101 176 87 A1 relates to proton conductive polymer membranes which are based on polyazoles and have a high specific conductivity, in particular high conductivity at operating temperatures above 100 ° C., and are substituted without further fuel gas humidification. . The proton conductive polymer membrane according to DE 101 176 87 A1 comprises the following steps:

(A) reacting the at least one aromatic tetraamino compound with the at least one aromatic dicarboxylic acid or ester thereof at a temperature of 350 ° C.,

(B) dissolving the solid prepolymer obtained as is in step (A) in polyphosphoric acid,

(C) heating the solution obtainable as such in step (B) to a temperature of 300 ° C. under an inert gas to form a dissolved polyazole polymer,

(D) forming a film on the support using a solution of the polyazole polymer obtained as is in step (C), and

(E) treating the film formed in step (D) until it is self-supporting

It can be obtained by a process comprising a.

DE 10 2006 036019 A1 relates to a membrane electrode assembly comprising at least two electrochemically active electrodes separated by at least one polymer electrolyte membrane, the polymer electrolyte membrane at least partially passing through the polymer electrolyte membrane. (reinforcing element) The membrane electrode assembly

(i) molding the polymer electrolyte membrane in the presence of the reinforcing member,

(ii) assembling the membrane and the electrodes in the desired order

It is preferable that it is obtained by the process containing.

The membrane electrode assembly is particularly suitable for use in fuel cells.

Polymeric membranes based on the above mentioned polyazoles are generally operated in the presence of phosphoric acid as an electrolyte.

However, at high phosphoric acid content, the membrane is soft and therefore has only limited mechanical strength. In addition, its mechanical stability decreases with increasing temperature, and the solubility of the polymer backbone decreases. In the upper region of the typical operating region (160-180 ° C.) of the fuel cell, it can cause durability problems. In addition, the polymer electrolyte membrane may dissolve or flow out at relatively high temperatures under undesirable operating conditions. This result is a failure of the membrane electrode assembly comprising the polymer electrolyte membrane mentioned above.

It is therefore an object of the present invention to reduce the solubility of polymer electrolyte membranes based on polyazoles in acids used as electrolytes, preferred phosphoric acid, and to improve the mechanical stability of the membranes.

J.-P. Belieres et al., Chem. Commun., 2006, 4799-4801, describe polymer electrolyte membranes in which an ionic liquid is used instead of phosphoric acid as an electrolyte. However, J.-P. Belieres et al. Do not disclose polymer electrolyte membranes which contain phosphoric acid as an electrolyte, but which have a reduced solubility in phosphoric acid as compared to polymer electrolyte membranes which carry phosphoric acid as a commonly used electrolyte.

The above-mentioned object is achieved by a proton conductive polymer electrolyte membrane based on a polyazole salt of an inorganic or organic acid doped with an acid as an electrolyte, wherein the polyazole salt of the organic or inorganic acid is an acid used as an electrolyte. It has a solubility in acid used as an electrolyte lower than the polyazole salt of.

In order to achieve the proton conductivity of the polymer electrolyte membrane, the polymer electrolyte membrane based on the polyazole salt of the organic or inorganic acid is doped with an acid as the electrolyte. Here it is possible in principle to use all known Lewis acids and Brönsted acids, preferably inorganic Lewis acids and Brönsted acids as electrolytes.

In addition, it is also possible to use polyacids, in particular isopoly and heteropoly acids and also mixtures of various acids. For the purposes of the present patent application, heteropolyacids, as partially mixed anhydrides, have two or more different central atoms and in each case are metals such as Cr, Mo, V or W and nonmetals such as As, I, Inorganic polyacids formed from weak polybasic oxo acids of P, Se, Si or Te. Polyacids include, for example, 12-molybdate phosphoric acid and 12-tungstophosphoric acid. Preferably, the polyacid used is polyphosphoric acid.

For the purposes of this patent application, the term polyphosphoric acid means commercial polyphosphoric acid. Polyphosphate _{Η n + 2 Ρ n O 3n} +1 (n> 1) is normally held in the content (acid amount) calculated as P ₂ 0 ₅ as 83% or more.

Particular preference is given to using sulfuric acid and / or phosphoric acid or compounds which liberate these acids as electrolytes, for example during hydrolysis. Very particular preference is given to using phosphoric acid as the electrolyte. Here, highly concentrated acids are generally used. The concentration of phosphoric acid used very preferably is generally at least 50% by weight, preferably at least 80% by weight, based on the total weight of the electrolyte. The remaining amount up to 50% by weight, preferably up to 20% by weight, is generally water.

The conductivity of the polymer membrane can be influenced through the degree of doping. Here, the conductivity generally increases with increasing amount of electrolyte until the maximum value is reached. For the purposes of the present patent application, the amount of electrolyte (degree of doping) is given as mol of acid per mol of polymer repeat unit. According to the invention, the degree of doping is preferably given to a degree of doping of 3 to 80, particularly preferably 5 to 60, very particularly preferably 12 to 60.

Durability and mechanical membrane stability in acids used as electrolytes, particularly preferably phosphoric acid, can be significantly improved by the use of the proton conductive polymer electrolyte membrane according to the invention without adversely affecting the performance of the polymer electrolyte membrane in fuel cells.

Suitable organic or inorganic acids for forming the polyazole salts are all acids so long as these acids form a polyazole salt that is less soluble in the electrolyte, preferably phosphoric acid, than the polyazole salt of the electrolyte.

Suitable inorganic acids are for example HNO, sulfuric acid, sulfates such as K ₂ SO ₄ .

Suitable organic acids are aliphatic or aromatic acids which are preferably perfluorinated.

Thus, preferred organic and inorganic acids are perfluorinated phenols such as pentafluorophenol, perfluorinated phenyl alcohol, K ₂ S0 ₄ , HNO ₃ , FSO ₃ H, HPO ₂ F ₂ , H ₂ SO 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 and the like, 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 , CF ₃ CF ₂ P0 ₃ H ₂ , CF ₃ CF ₂ CF ₂ P0 ₃ H ₂ , and the like, and perfluoroalkylcarboxylic acids.

Particularly preferred organic and inorganic acids are pentafluorophenol, CH ₃ SO ₃ H, CF ₃ SO ₃ H, CF ₃ CF ₂ SO ₃ H, (CF ₃ ) ₂ S0 ₂ NH, (CF ₃ CF ₂ ) ₂ S0 ₂ NH , (CF ₃ CF ₂ CF ₂ ) ₂ SO ₂ NH, CF ₃ PO ₃ H ₂ , CF ₃ CF ₂ PO ₃ H ₂ and CF ₃ CF ₂ CF ₂ PO ₃ H ₂ .

Very particular preference is given to using salts of pentafluorophenol as polyazole salts.

The polyazole salts used according to the invention in proton conductive polymer electrolyte membranes are preferably based on at least one polyazole. Polyazoles which are preferably used are 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).

In the formula,

The radicals Ar are the same or different and are tetravalent aromatic or heteroaromatic groups, each of which may have one or more rings,

The radicals Ar ¹ are the same or different and are divalent aromatic or heteroaromatic groups, each of which may have one or more rings,

The radicals Ar ² are the same 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 the same or different and are each a trivalent aromatic or heteroaromatic group which may have one or more rings,

The radicals Ar ⁴ are the same or different and are each a trivalent aromatic or heteroaromatic group which may have one or more rings,

The radicals Ar ⁵ are the same or different and are each a tetravalent aromatic or heteroaromatic group which may have one or more rings,

The radicals Ar ⁶ are the same or different and are each a divalent aromatic or heteroaromatic group which may have one or more rings,

The radicals Ar ⁷ are the same or different and are each a divalent aromatic or heteroaromatic group which may have one or more rings,

The radicals Ar ⁸ are the same or different and are each a trivalent aromatic or heteroaromatic group which may have one or more rings,

The radicals Ar ⁹ are the same or different and are each a divalent or trivalent or tetravalent aromatic or heteroaromatic group which may have one or more rings,

The radicals Ar ¹⁰ are the same or different and are each a divalent or trivalent aromatic or heteroaromatic group which may have one or more rings,

The radicals Ar ¹¹ are the same or different and are each a divalent aromatic or heteroaromatic group which may have one or more rings,

The radicals X are the same or different and each is an oxygen, sulfur or amino group, wherein the amino group adds a hydrogen atom, a group having 1 to 20 carbon atoms, preferably a branched or unbranched alkyl or alkoxy group or an aryl group Holds as a radical,

The radicals R are the same or different and each is hydrogen, an alkyl group or an aromatic group, and is an alkylene group or an aromatic group in formula (XX), provided that in formula (XX) R is not hydrogen,

 n and m are each an integer of ≧ 10, preferably ≧ 100.

Preferred aromatic or heteroaromatic groups are benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline, pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, tri Azine, tetrazine, pyrrole, pyrazole, anthracene, benzopyrrole, benzotriazole, benzooxadithiazol, benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazine, indole Lysine, quinolysine, pyridopyridine, imidazolpyrimidine, pyrazinopyrimidine, carbazole, azeridine, phenazine, benzoquinoline, phenoxazine, phenothiazine, aziridine, benzophteridine, phenanthrol Derived from lean and phenanthrene, which may also be optionally substituted.

Here, Ar ¹ , Ar ⁴ , Ar ⁶ , Ar ⁷ , Ar ⁸ , Ar ⁹ , Ar ¹⁰ and Ar ¹¹ may have any substitution pattern, and for phenylene, for example, Ar ¹ , Ar ⁴ , Ar ⁶ , Ar ⁷ , Ar ⁸ , Ar ⁹ , Ar ¹⁰ and Ar ¹¹ may each independently be orot-, meta- or para-phenylene. Particularly preferred groups are derived from benzene and biphenyls, which may be optionally substituted.

Preferred alkyl groups are alkyl groups having 1 to 4 carbon atoms, for example methyl, ethyl, n-propyl, i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl or naphthyl groups. Alkyl and aromatic groups may be monosubstituted or multiply substituted.

Preferred substituents are halogen atoms such as fluorine, amino groups, hydroxy groups or C ₁ -C ₄ alkyl groups such as methyl or ethyl groups.

The polyazoles may in principle have different repeating units, which repeating radicals differ, for example. However, it is preferred that each polyazole has only the radicals X which are wholly identical in the repeating units.

In a very preferred embodiment of the invention, the polyazole salts are based on polyazoles comprising repeating azole units of formula (I) and / or (II).

The polyazoles used to form the polyazole salts are polyazoles comprising repeating azole units in the form of copolymers or blends comprising at least two units of formulas (I) to (XXII) different from one another in one embodiment. The polymers may be present as block copolymers (diblock, triblock), random copolymers, periodic copolymers and / or alternating polymers.

The number of repeating azole units in the polymer is an integer of ≧ 10, particularly preferably ≧ 100.

In a further preferred embodiment, the polyazoles used to form the polyazole salts are polyazoles comprising repeating units of formula (I) in which the radicals X in the repeating unit are identical.

Further preferred polyazoles based on the polyazole salts of the present invention are polybenzimidazole, poly (pyridine), poly (pyrimidine), polyimidazole, polybenzothiazole, polybenzoxazole, polyoxadiazole , Polyquinoxaline, polythiadiazole, and poly (tetrazapyrene).

In a particularly preferred embodiment, the polyazole salts are based on polyazoles comprising repeat benzimidazole units. Suitable polyazoles having repeating benzimidazole units are represented as follows:

Wherein n and m are integers of ≥ 10, preferably ≥ 100.

It is particularly preferred that the polyazoles based on the polyazole salts used according to the invention have repeat units of the formula:

Wherein n is an integer of ≥ 10, preferably ≥ 100.

The polyazoles, preferably polybenzimidazoles based on the polyazole salts used according to the invention, have a high molecular weight. When measured as intrinsic viscosity, the molecular weight is at least 0.2 dl / g, preferably 0.8 to 10 dl / g, particularly preferably 1 to 10 dl / g. The viscosity eta i (also called intrinsic viscosity) is calculated from the relative viscosity eta rel according to the following equation: eta i = (2.303 x log eta rel) / concentration. The concentration is reported in g / 100 ml. The relative viscosity of the polyazole is measured by a capillary viscometer from the viscosity of the solution at 25 ° C., and the relative viscosity is the corrected running-out time for each of the solvent and the solution according to the following equation rel = t1 / t0. out time) t0 and t1. The conversion to eta i is performed according to the above relationship based on the information described in "Methods in Carbohydrate Chemistry", Volume IV, Starch, Academic Press, New York and London, 1964, page 127.

Preferred polybenzimidazoles are commercially available, for example under the trade name Celazole® PBI (PBI Performance Products Inc.).

In a preferred embodiment of the present invention, the proton conductive polymer electrolyte membrane according to the present invention has a reinforcing member. Such reinforcing members generally penetrate at least partially through the polymer electrolyte membrane, ie the reinforcing members generally at least partially penetrate into the polymer electrolyte membrane. It is particularly preferred that the reinforcing member be predominantly embedded in the membrane, and even if necessary, only protrude from the membrane in place.

According to the invention, the reinforcing member is at least partially bonded to the membrane. The partial composite has a reference force of the polymer electrolyte membrane with the reinforcing member compared to the polymer electrolyte membrane with the reinforcing member having no reinforcing member in the range of 0 to 1% elongation, in a force-elongation curve at 20 ° C. ) Is considered to be a composite of the reinforcing member and the membrane which advantageously occupies a force such that at least one point differs by at least 10%, preferably at least 20% and very particularly preferably at least 30%.

The polymer electrolyte membrane having the reinforcing member is preferably fiber reinforced. Here, reinforcing members are generally used which preferably comprise monofilament, multifilament, long and / or short fibers, hybrid yarns and / or bicomponent fibers. Apart from the reinforcing member comprising special fibers, the reinforcing member may also form a textile sheet. Suitable textile sheets are nonwovens, fabrics, draw-loop knits, form-loop knits, felts, lay-ups and / or meshes, particularly preferably Lay-up, woven or nonwoven. Non-limiting examples of the aforementioned fabrics include polyphenylene sulfones, polyether sulfones, polyether ketones, polyether ether ketones, poly (acryl), poly (ethylene terephthalate), poly (propylene), poly (tetrafluoro) Ethylene), 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 fabric means a product consisting mainly of monofilament threads and / or multifilament threads crossing at right angles. The thread-to-thread distance of the textile sheet can usually be 20 to 2000 μm, and for the purposes of the present invention, textile sheets, in particular fabrics, lay-ups and meshes, having a thread-to-thread distance in the range of 30 to 300 μm It turns out to be very useful. Here, the thread to thread distance can be measured, for example, by electronic image analysis of an optical photograph or transmission electron micrograph.

Further details regarding suitable fabrics, layups and meshes are disclosed in DE 10 2006 036019 A1.

Particularly preferred suitable fabrics include, for example, fabrics derived from SEFAR with the trade names SEFAR NITEX®, SEFAR PETEX®, SEFAR PROPYLTEX®, SEFAR FLUORTEX® and SEFAR PEAKTEX®, fabrics derived from SAATI with the trade name Saati 9.30, And fabrics derived from DEXMET having the trade names Dexmet 2PTFE10-105, Dexmet 2PTFE5-105H and Dexmet 2PTFE2-50H.

The term nonwoven is not produced by the classical method of warp and weft weaving and by stitch formation but rather by the entanglement and / or agglomeration and / or adhesive bonding of the fibers (eg a flexible porous sheet like structure (eg , Spunbond or melt blown nonwovens). Nonwovens are loose materials in which agglomeration is generally composed of spun fibers or filaments produced by the inherent adhesion of the fibers or by mechanical post-solidification.

According to the invention, the individual fibers may have a preferred direction (orientation or cross-woven) or may be unaligned (random non-woven). The nonwovens can be mechanically reinforced (known as spunlaced nonwovens) by needling, intermeshing or interlacing by powerful water jets.

Additional properties of suitable nonwovens can be found in DE 10 2006 036019 A1.

Suitable examples of preferred nonwovens are SEFAR PETEX®, SEFAR FLUOROTEX® and SEFAR PEEKTEX®.

In addition, the composition of the reinforcing member can be freely selected and matched according to the special use. However, the reinforcing members are 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 (SEFAR) and also DUOFIL®, EMMITEX GARN®. Square-weave braids, braids, twill braids or multiplex weaves (GDK) are likewise suitable.

In principle it is possible to use all kinds of materials as long as they are mainly inert and meet the mechanical requirements for strengthening under conditions prevailing during operation of the fuel cell.

The reinforcing member, optionally a component of a woven, draw-loop knit, foamed loop knit or nonwoven, may have a substantially rounded cross section or other shape, such as dumbbell shape, stretch shape, triangle or polygonal cross sectional area. Bicomponent fibers are also possible.

The reinforcing member preferably has a maximum diameter of 10 μm to 500 μm, particularly preferably 20 μm to 300 μm, very particularly preferably 20 μm to 200 μm, in particular 25 μm to 100 μm. Here, the maximum diameter means the longest dimension in cross section.

In addition, the reinforcing member preferably has a Young's modulus of at least 5 GPa, preferably 10 GPa, particularly preferably 20 GPa. The elongation at break of the reinforcing member is 0.5 to 300%, particularly preferably 1 to 60%.

 The proportion by volume of the reinforcing member based on the total volume of the polymer electrolyte membrane is preferably 5% to 95% by volume, particularly preferably 10 to 80% by volume, very particularly preferably 10 to 50% by volume, specifically 10 to 30% by volume. The volume fraction is usually measured at 20 ° C.

For the purposes of the present invention, the reinforcing member retains the reinforcing member in comparison with the polymer electrolyte membrane having no reinforcing member in the range of 0 to 1% elongation at one or more points in the force-elongation curve at 20 ° C. The forces are generally occupied such that the reference forces of the polymer electrolyte membranes differ by at least 0%, preferably at least 20%, particularly preferably at least 30%.

In addition, the reinforcement is measured at one or more points in the range of 0 to 1% elongation and has a reference force of 3 or less, preferably 2.5, at room temperature (20 ° C.) divided by the reference force of the support insert at 180 ° C. Hereafter, it is particularly advantageous to give an index of less than two.

The measurement of the reference force is performed according to EN29073, Part 3 on a 5 cm wide specimen with a measuring length of 100 mm. The numerical value of the prestressing force, reported in centinewtons [cN], corresponds to the numerical value of the mass per unit area of specimen recorded in grams per m ² .

The polymer electrolyte membrane can be prepared by methods known to those skilled in the art, and in one embodiment of the invention, the reinforcing member can be supplied directly during the preparation of the membrane.

Polymer electrolyte membranes of the present invention are usually prepared by first dissolving one or more polyazoles in one or more polar aprotic solvents such as dimethylacetamide (DMAc) and producing a polymer film (polymer membrane) in a classical process. Can be. In this case, the reinforcing member which may optionally be present may be introduced into the film, for example, during the manufacture of the film. In order to remove solvent residues, the film obtained in such a way can be treated with a cleaning liquid, for example as described in DE 10109829. Removing solvent residues from polyazole films as described in DE 10109829 will improve the mechanical properties of the film as compared to films without solvent residues removed in such a manner.

In addition, the polymer film may have further modifications, for example by crosslinking, as described in DE 1010 10752 and WO 00/44816.

The thickness of the polyazole film (polyazole film) can be within a wide range. The polyazoles preferably have a thickness of generally 5 μm to 2000 μm, preferably 10 μm to 1000 μm, particularly preferably 20 μm to 1000 μm before doping with the acid as described below.

In order to obtain proton conductivity, the above mentioned film is doped with acid. Suitable acids (electrolytes) are mentioned above. Particular preference is given to using phosphoric acid (H ₃ PO ₄ ) as the acid.

In order to obtain improved membrane stability of the proton conductive polymer electrolyte membranes of the present invention in acids used as electrolytes, particularly preferably phosphoric acid, the polyazole salts of organic or inorganic acids have a lower electrolyte than the polyazole salts of acids used as electrolytes. Polyazole salts of organic or inorganic acids adapted to have solubility in the acids used are used in the polymer electrolyte membrane according to the invention. Such polyazole salts are obtained according to the invention by treating the aforementioned polymer film doped with an acid used as an electrolyte, particularly preferably with phosphoric acid, with one or more of the aforementioned inorganic or organic acids. This first washes the above-mentioned polymer film (polymer membrane), which is doped with an acid used as an electrolyte, particularly preferably with phosphoric acid, until it reaches neutral with water, after which the polyazole salt is in water or in phosphoric acid, This may be accomplished by treatment with one or more organic or inorganic acids that have a solubility in the acid used as the electrolyte lower than the polyazole salt of the acid used as the electrolyte. However, it is also possible that the above-mentioned polymer film (polymer film) which is doped with an acid, particularly preferably phosphoric acid, used as electrolyte can be directly treated with an inorganic or organic acid.

The polymer membranes based on the polyazole salts obtained in this way are dissolved in acids, particularly preferably phosphoric acid, used as electrolytes to a significantly lower degree than the corresponding polyazole membranes not based on the mentioned polyazoles.

Treatment of the above-mentioned polymer film (polymer membrane), which is doped with an acid used as an electrolyte, particularly preferably with phosphoric acid, with at least one inorganic or organic acid, is generally carried out in water or in phosphoric acid as mentioned above. . The treatment is generally carried out at room temperature. In general, the amount of inorganic or organic acid corresponds at least to the stoichiometric amount required to form the polyazole salt from the corresponding polyazole. The organic or inorganic acid may also be used in excess.

In a preferred embodiment, the proton conductive polymer electrolyte membrane of the present invention comprises the following steps:

(ia) dissolving at least one polyazole in phosphoric acid,

(iia) heating the solution obtainable as such in step (ia) to a temperature of up to 400 ° C., preferably from 100 to 250 ° C., under inert gas,

(iiia) providing a support and optionally arranging the reinforcing member on the support,

(iva) forming a film on the support of step (iii) using a solution of the polymer as obtained in step (ii),

(va) treating the film formed in step (iva) until the film is self-supporting,

(via) optionally washing the membrane obtained in step (va) with water until the membrane is neutral,

(viia) treating the membrane obtained in step (va) or in via with an organic or inorganic acid having _a lower pK _a in water than phosphoric acid,

(viiia) mixing the membrane obtained in step (viia) with phosphoric acid

It is obtained by the process containing.

Steps (ia), (iia), (iva) and (va) are described comprehensively in DE 10246461. This application is incorporated herein by reference.

A further process for preparing proton conductive polymer electrolytes based on polyazoles is described in DE 10 2006 036019 A1. In order to obtain a polymer electrolyte membrane according to the invention based on polyazole salts, the polymer electrolyte membrane obtained by the process described in DE 10 2006 036019 A1 can be treated according to steps (via), (viia) and (viiia). .

The addition of the reinforcing member as described in DE 10 2006 036019 A1 is optional.

The above-mentioned 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 that can be used in step (viia) are the acids mentioned above in this patent application.

Mixing the membrane obtained in step (viia) with phosphoric acid in step (viiia) is carried out to provide phosphoric acid as an electrolyte, the phosphoric acid being generally 30 to 99% by weight based on the amount of polymer electrolyte membrane obtained in step (viia) %, Preferably 40 to 90% by weight, particularly preferably 40 to 85% by weight.

In addition, the present invention provides a membrane electrode assembly comprising at least two electrochemically active electrodes separated by a polymer electrolyte membrane, wherein the polymer electrolyte membrane is a proton conductive polymer electrolyte membrane according to or made according to the present invention. to be.

Two or more electrochemically active electrodes are generally anodes and cathodes. The term "electrochemical activity" indicates that the electrode can catalyze the oxidation of hydrogen and / or one or more reformates and the reduction of oxygen. This property can be obtained by coating the electrode with a noble metal. Suitable precious metals are mentioned below. The term "electrode" means that the material is electrically conductive. The electrode can optionally carry a layer of precious metal. Such electrodes are known and are described, for example, in US Pat. No. 4,191,618, US Pat. No. 4,212,714 and US Pat. No. 4,333,805.

The electrode preferably comprises a gas diffusion layer in contact with the catalyst layer.

Sheet-like electrically conductive and acid resistant structures are commonly used as gas diffusion layers. Such structures include, for example, paper that becomes conductive by the addition of graphite fiber paper, carbon fiber paper, graphite fabric and / or carbon black. Fine dispersion of the gas and / or liquid streams is achieved by this layer.

In addition, it is also possible to use a gas diffusion layer comprising a mechanically stable support material impregnated with one or more electrically conductive materials, such as carbon (eg carbon black). Suitable support materials for this purpose are fibers, such as those present in the form of nonwovens, paper or textiles, in particular carbon fibers, glass fibers or organic polymers, for example polypropylene, polyesters (especially polyethylene terephthalates, Polyphenylene sulfide or polyether ketone). Further details regarding such diffusion layers can be found, for example, in WO 97/20358.

The gas diffusion layer preferably has a thickness of 80 to 2000 μm, particularly preferably 100 to 1000 μm, very particularly preferably 150 to 500 μm.

In addition, the gas diffusion layer preferably has a high porosity. It is usually in the range of 20% to 80%.

The gas diffusion layer may comprise conventional additives. The additives include, among other things, fluoropolymers such as polytetrafluoroethylene (PTFE) and surface active materials.

In a preferred embodiment, one or more of the gas diffusion layers can comprise a compressible material. According to the invention, the compressible material possesses the property that the gas diffusion layer can be pressed to 1/2, in particular 1/3, of its original thickness without loss of its integrity.

This property is typically manifested by a gas diffusion layer consisting of a paper which becomes conductive by the addition of graphite fabric and / or carbon black.

The catalytically active layer comprises one or more catalytically active materials. Such materials include, among other things, precious metals, preferably platinum, palladium, rhodium, iridium and / or ruthenium. These materials can also be used in the form of alloys with each other. In addition, the two materials can also be used as alloys with base metals such as Cr, Zr, Ni, Co and / or Ti. In addition, oxides of the above-mentioned noble metals and / or base metals may also be used. The metals mentioned above are usually used after application of the carbon to the support material by known methods, usually in the form of nanoparticles and having a high specific surface area.

Catalytically active compounds, ie catalysts, are preferably used in the form of particles which particularly preferably have a size of 1 to 1000 nm, preferably 5 to 200 nm, particularly preferably 10 to 100 nm, wherein the size is Particle diameter. The weight ratio of the polyazole salts present in the proton conductive polymer electrolyte membrane of the present invention to a catalyst material comprising at least one precious metal and optionally at least one support material is generally in the range of greater than 0.05, preferably in the range from 0.1 to 0.6.

The thickness of the catalyst layer is generally 1 to 1000 μm, preferably 5 to 500 μm, particularly preferably 10 to 300 μm. These values represent averages that can be determined by measuring the layer thickness of the cross section in an image recorded by a scanning electron microscope (SEM).

The precious 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 . This value can be measured by elemental analysis of the sheet sample.

The catalyst layer is generally not self supporting but instead is usually applied to the gas diffusion layer and / or the membrane. Here, portions of the catalyst layer can for example diffuse into the gas diffusion layer and / or the membrane, as a result of which a transition layer is formed. This can also lead to a catalyst layer which is considered as part of the gas diffusion layer.

Generally, the surface of the polymer electrolyte membrane is in each case partly or wholly, preferably only partially, 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. It is in contact with the electrode. Here, the front side and the back side of the polymer electrolyte membrane are respectively observed from the side of the polymer electrolyte membrane facing or facing the observer, from the first electrode (front), preferably from the direction of the cathode, the second electrode (rear ), Preferably an anode.

Further information regarding the properties of the polymer electrolyte membrane and the properties of the electrodes is summarized for example in WO 01/188494 A2, DE 19509748, DE 19509749, WO 00/26982, WO 92/15121 and DE 19757492.

The manufacture of membrane electrode assemblies of the present invention is known to those skilled in the art. In general, the various components of the membrane electrode assembly are placed on top of each other by pressure and heat and bonded together, where the lamination is usually at a temperature of 10 to 300 ° C., preferably 20 to 200 ° C. and generally 1 to 1000 bar. , Preferably at a pressure of 3 to 300 bar.

The membrane electrode assembly of the present invention has significantly improved mechanical stability and strength, and therefore can be used to manufacture fuel cells and fuel cell stacks with very high performance. Here, only small power fluctuations of the resulting fuel cell or fuel cell stack occur, and high quality, reliability and reproducibility are achieved. This is also achieved as a result of the proton conductive membrane of the present invention based on the abovementioned polyazole salts used.

Due to the dimensional stability of the assembly in the case of varying ambient temperatures and atmospheric humidity, the membrane electrode assembly of the present invention can be stored and transported without problems. Even after prolonged storage or after transport to places with significantly different climatic conditions, the dimensions of the membrane electrode assembly are precisely maintained for trouble-free installation in the fuel cell or fuel cell stack. Thus, the membrane electrode assembly no longer needs to be conditioned for a location for outdoor installation, which simplifies the manufacture of fuel cells and saves time and money.

An advantage of the preferred membrane electrode assembly according to the invention is that this assembly allows the fuel cell to be operated at temperatures above 120 ° C. This applies in the case of gaseous fuels and liquid fuels, for example hydrogen containing gases, the hydrogen containing gases being produced from hydrocarbons in the preceding reforming step. As the oxidizing agent, it is possible to use oxygen or air, for example.

A further advantage of the preferred membrane electrode assemblies of the present invention is that they have a high resistance to carbon monoxide in operation above 120 ° C., even if they use pure platinum catalysts, ie no additional alloying components. At a temperature of 160 ° C., for example, it is possible that at least 1% of CO is present in the fuel gas without causing a significant reduction in the performance of the fuel cell.

Preferred membrane electrode assemblies can be operated in a fuel cell without humidifying the fuel gas or oxidant despite the possible high operating temperatures. Nevertheless, the fuel cell operates stably and the membrane does not lose its conductivity. This simplifies the overall fuel cell system and brings additional cost savings because the manipulation of the water cycle is simplified. In addition, the behavior of the fuel cell system at temperatures below 0 ° C. is thereby also simplified.

Preferred membrane electrode assemblies also make it possible to cool the fuel cell below room temperature without problems and then to be able to operate again without degrading performance.

Moreover, the membrane electrode assembly of the present invention exhibits very high long term stability. This makes it possible to provide a fuel cell with high long term stability as well. Moreover, the membrane electrode assembly of the present invention has very good heat and corrosion resistance and relatively low gas permeability, especially at high temperatures. Especially at high temperatures, a decrease in mechanical stability and structural integrity is reduced or avoided in the membrane electrode assembly of the present invention.

In addition, the membrane electrode assembly of the present invention can be manufactured inexpensively and simply.

In addition, the present invention provides a fuel cell comprising at least one membrane electrode assembly according to the present invention. Suitable fuel cells and components thereof are known to those skilled in the art.

Since the power of a single fuel cell is often too low for many applications, it is desirable for the purposes of the present invention to combine a plurality of single fuel cells through a separator plate to form a fuel cell stack. The separator plate must, optionally in combination with additional sealing material, seal the gas spaces of the cathode and anode from the outside and from each other. For this purpose, the separator plate is preferably arranged in parallel with the membrane electrode assembly. The sealing effect can be further improved by pressing the composite of the separator plate and the membrane electrode assembly.

The separator plates each have one or more gas channels for the reaction gas, which channels are advantageously aligned on the side facing the electrode. The gas channel should enable the dispersion of the reaction fluid.

Because of the high long term stability of the bay electrode assembly of the present invention, the fuel cell of the present invention also has high long term stability. The fuel cell of the present invention can generally be operated continuously at temperatures of 120 ° C. or higher using dry reaction gas for long periods of time, for example 5000 hours or more, without significant performance degradation being observed. The power density that can be achieved is still high after such a long time.

The fuel cell of the present invention exhibits a high open circuit voltage even after a long time, for example, after 5000 hours or more, and the open circuit voltage is preferably 900 mV or more after that time. To measure the open circuit voltage, the fuel cell is operated without current while water is supplied to the anode and air is supplied to the cathode. The measurement is performed by converting the fuel cell into a zero current state at a current of 0.2 A / cm ² and then recording its open circuit voltage for 5 minutes. The value after 5 minutes is each open circuit potential. The measured value of the open circuit voltage is at a temperature of 160 ° C. In addition, the fuel cell preferably exhibits low gas crossover after that time. To measure the crossover, 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 the reference electrode and the counter electrode, and the cathode serves as the working electrode. The cathode is set at a potential of 0.5 V, and hydrogen diffused through the membrane is oxidized in a mass transfer limited manner at the cathode. The resulting current is a measure of 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 value of the crossover of H ₂ is at a temperature of 160 ° C. or higher.

In addition, the present invention provides the use of the proton conductive polymer electrolyte membrane according to the invention comprising phosphoric acid as electrolyte in the membrane electrode assembly, and also the use of the proton conductive polymer electrolyte membrane according to the invention in a fuel cell. to provide.

Suitable polymer electrolyte membranes, membrane electrode assemblies and fuel cells are mentioned above.

The following examples illustrate the invention.

Example

In order to measure the solubility of the proton conductive polymer electrolyte membranes of the present invention based on polyazole salts in phosphoric acid, solubility tests are performed on each membrane (example according to the invention) in phosphoric acid, the solubility of which is known from the prior art. Was compared with the solubility of the polymer electrolyte membrane (comparative) based on the prepared polyazole.

Example  1: doping in water, Pentafluorophenol Polyazole  Preparation of Salt-Based Polymer Electrolyte Membranes

Membranes prepared as described in DE 10 2006 036019 A1 were thoroughly washed with deionized water. Subsequently, the film was sufficiently applied three times with a 0.1 M solution of pentafluorophenol in water without drying. The film was then doped with 85% strength H ₃ PO ₄ . 1 shows the solubility of the membrane of Example 1. Here, the dark spots in the figure were residual films that did not dissolve in 99% phosphoric acid.

Example  2: prepared in phosphoric acid Pentafluorophenol Polyazole  Preparation of Salt-Based Polymer Electrolyte Membranes

Membranes prepared as described in DE 10 2006 036019 A1 were thoroughly washed with deionized water. The film was then sufficiently applied three times with a 0.2 M solution of pentafluorophenol in H ₃ PO ₄ (85%) without drying. The film was then doped with 85% strength H ₃ PO ₄ . 2 shows the solubility of the membrane of Example 2. Here, the dark spots in the figure were residual films that did not dissolve in 99% phosphoric acid.

Example  3 ( Comparative example ):

A commercially available Celtec-P membrane (prepared as described in DE 10 2006 036019 A1) was used as a comparative membrane and heated in 120 g of 99% strength phosphoric acid at 230 ° C. for 3 hours [5 × 5 cm ]. 3 shows the solubility of the membrane of Example 3. Here, the dark spots in the figure were residual films that did not dissolve in 99% phosphoric acid.

1, 2 and 3 show the solubility of membranes derived from Examples 1, 2 and 3; Here, the dark spots in the figures were residual films that did not dissolve in 99% phosphoric acid.

As can be clearly understood from the figures, the polymer electrolyte membrane according to the invention based on polyazole salts with pentafluorophenol has a significantly lower solubility in phosphoric acid than that of membranes known from the prior art.

FIG. 4 shows the power achieved by the three above mentioned films derived from Examples 1, 2 and 3 in the current-voltage curve.

Voltage (mV) is plotted on the y axis and current density (A / cm ² ) is plotted on the x axis.

Power is at a temperature of 160 ℃ and measured in the fuel cells (H ₂ / air), in which the anode is coated with platinum of 1 mg per cm ^2, and the cathode was coated with a platinum and nickel of 1 mg per cm ^2.

In Fig. 4, the diamondoid represents the power of the film as it is in Comparative Example 3, the triangle represents the power of the film as it is in Example 2 according to the present invention, and the solid line is achieved by the film as in Example 1 according to the present invention. Power is shown.

As can be clearly seen from FIG. 4, the power was not adversely affected by the use of polyazole salts having a solubility in acid used as electrolyte lower than that of the polyazole salt of acid used as electrolyte.

Claims (15)

A proton-conducting polymer electrolyte membrane based on polyazole salts of organic or inorganic acids doped with an acid as an electrolyte, wherein the polyazole salts of organic or inorganic acids are acids used as electrolytes. A proton conductive polymer electrolyte membrane having a solubility in acid used as an electrolyte lower than the polyazole salt of. The proton conductive polymer electrolyte membrane according to claim 1, wherein the acid used as the electrolyte is phosphoric acid. 3. The inorganic or organic acid or inorganic and organic acid according to claim 1, wherein the inorganic or organic acid or inorganic and organic acid is perfluorinated phenol, perfluorinated phenyl alcohol, K ₂ S0 ₄ , HN0 ₃ , FS0 ₃ H, HP0 ₂ F ₂ , H ₂ S0. ₃ , a proton conductive polymer electrolyte membrane selected from the group consisting of HOOC-COOH, sulfonic acid, perfluorinated sulfonic acid, perfluorosulfonamide, perfluorophosphonic acid, and perfluoroalkylcarboxylic acid. 4. The inorganic or organic acid of claim 1, wherein the inorganic or organic acid is pentafluorophenol, CH ₃ S0 ₃ H, CF ₃ S0 ₃ H, CF ₃ CF ₂ S0 ₃ H, (CF ₃ ) ₂ S0 ₂ NH, (CF ₃ CF ₂ ) ₂ S0 ₂ NH, (CF ₃ CF ₂ CF ₂ ) ₂ S0 ₂ NH, CF ₃ P0 ₃ H ₂ , CF ₃ CF ₂ P0 ₃ H ₂ and CF ₃ CF ₂ CF ₂ P0 _A proton conductive polymer electrolyte membrane selected from the group consisting of ₃ H ₂ . 5. The proton conductive polymer electrolyte according to claim 1, wherein the polyazole salt is based on a polyazole comprising repeating azole units of the general formulas (I) and / or (II) membrane:

In the formula,
The radicals Ar are the same or different and are tetravalent aromatic or heteroaromatic groups, each of which may have one or more rings,
The radicals Ar ¹ are the same or different and are divalent aromatic or heteroaromatic groups, each of which may have one or more rings,
The radicals Ar ² are the same or different and are each a divalent or trivalent aromatic or heteroaromatic group which may have one or more rings,
The radicals X are the same or different and each is an oxygen, sulfur or amino group, wherein the amino group adds a hydrogen atom, a group having 1 to 20 carbon atoms, preferably a branched or unbranched alkyl or alkoxy group or an aryl group Holds as a radical,
n is an integer of 10 or more, preferably 100 or more. The method of claim 1, wherein the electrolyte comprises phosphoric acid, and the polyazole salt is polybenzimidazole, poly (pyridine), poly (pyrimidine), polyimidazole, polybenzothiazole. And a polyazole selected from the group consisting of polybenzoxazole, polyoxadiazole, polyquinoxaline, polythiadiazole, and poly (tetrazapyrene). The proton conductive polymer electrolyte membrane of claim 6, wherein the polyazole has repeating benzimidazole units of one of the formulas:

Wherein n is an integer of ≥ 10, preferably ≥ 100. 8. The proton conductive polymer electrolyte membrane according to any one of claims 1 to 7, wherein the polymer electrolyte membrane has a reinforcing element. The proton conductive polymer electrolyte membrane of claim 8, wherein the polymer electrolyte membrane is fiber reinforced. 10. The proton conductive polymer electrolyte membrane according to claim 8 or 9, wherein the proportion by volume of the reinforcing member is from 5% to 95% by volume based on the total volume of the polymer electrolyte membrane. A method for producing a proton conductive polymer electrolyte membrane according to any one of claims 1 to 10,
(ia) dissolving at least one polyazole in phosphoric acid,
(iia) heating the solution which can be obtained as such in step (ia) to a temperature of 400 ° C. or less under inert gas,
(iiia) providing a support, and optionally arranging the reinforcing member on the support,
(iva) forming a film on the support of step (iii) using a solution of the polymer as obtained in step (ii),
(va) treating the film formed in step (iva) until the film is self-supporting,
(via) optionally washing the membrane obtained in step (va) with water until the membrane is neutral,
(viia) treating the membrane obtained in step (va) or in step (via) with an organic or inorganic acid having _a lower pK _a than the acid used as electrolyte in water, particularly preferably phosphoric acid,
(viiia) mixing the membrane obtained in step (viia) with phosphoric acid
&Lt; / RTI &gt; A membrane-electrode assembly comprising two or more electrochemically active electrodes separated by a polymer electrolyte membrane, wherein the polymer electrolyte membrane is according to any one of claims 1 to 10 or 11. Membrane electrode assembly which is a proton conductive polymer electrolyte membrane prepared according to the invention. A fuel cell comprising at least one membrane electrode assembly according to claim 12. Use of a proton conductive polymer electrolyte membrane according to any one of claims 1 to 10 or made according to claim 11 in a membrane electrode assembly. Use of a proton conductive polymer electrolyte membrane according to any one of claims 1 to 10 or made according to claim 11 in a fuel cell.

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