Fuel cell membrane, fuel cell and process for manufacturing a fuel cell membrane

Abstract

Fuel cell membrane (10) comprising: - at least one fluoropolymer-based polymer (4), - at least one perfluorosulfonic acid-based ionomer (6) and - at least one organic antioxidant (5), wherein the fluoropolymer-based polymer (4) is covalently bonded to the organic antioxidant (5) and wherein the organic antioxidant (5) is 4-isopropenylphenol, 4-ethenylphenol, 1,2-dihydroxy-4-allylbenzene or a mixture of the phenols.

Description

The present invention relates to a fuel cell membrane, a fuel cell and two methods for producing a fuel cell membrane. Conventional fuel cell membranes, such as those sold under the trade names Nafion®, Aquivion®, or Fumapem, are perfluorosulfonic acid-based membranes with the high proton conductivity required for fuel cell applications. To prevent membrane dehydration, inorganic additives such as CeO2, TiO2, ZrO2, ZrP, SiO2, and the like are added before or during membrane fabrication. Furthermore, a reinforcing material, typically impregnated with the ionomer and the additive, can be used to improve mechanical stability.However, these conventional fuel cell membranes frequently experience ionomer loss, particularly when exposed to temperatures around 80 to 100 °C and high current densities. This loss can occur through leaching or decomposition, with leaching or decomposition being especially prevalent in short-side-chain ionomers (SSCs), which are preferred due to their particularly high proton conductivity. Furthermore, inorganic additives dispersed within the ionomer, often used as radical scavengers or antioxidants, can also be decomposed. These effects contribute to performance losses and ultimately fuel cell failure. German patent application DE 11 2005 002 052 T5 teaches the use of radical scavengers, such as phenols, in dispersed form within the membrane polymers or introduced as functional groups on these polymers.German patent application DE 11 2006 000 450 T5 describes graft polymerization under irradiation for the production of membranes. Based on this prior art, it is an object of the present invention to provide a fuel cell membrane and a fuel cell with such a fuel cell membrane, characterized by consistently good performance and high mechanical, chemical, and physical stability. Furthermore, it is an object of the present invention to provide methods for producing such a fuel cell membrane that are simple and can be implemented without significant technical effort. According to the invention, a fuel cell membrane (hereinafter referred to as "membrane" unless otherwise specified) is provided which is characterized by consistently good stability with regard to its physical, chemical and mechanical properties and provides a permanently high proton conductivity, so that a high-performance fuel cell with a long operating time can be manufactured with it. The fuel cell membrane according to the invention comprises at least one fluoropolymer-based polymer, at least one perfluorosulfonic acid-based ionomer, and at least one organic antioxidant. The fluoropolymer-based polymer is covalently bonded to the organic antioxidant and / or to the perfluorosulfonic acid-based ionomer. This results in a stable structure within the membrane, in which the fluoropolymer-based polymer acts as a reinforcing material. In particular, the use of an organic antioxidant that is covalently bonded at least to the fluoropolymer-based polymer ensures a permanently effective radical scavenging function and antioxidant effect, which primarily benefits the chemical stability of the membrane and secondarily its physical and mechanical stability. The organic antioxidant remains permanently within the membrane and therefore cannot be washed out. The stability of the membrane can be further improved by covalently linking all three components – the fluoropolymer-based polymer, the organic antioxidant, and the perfluorosulfonic acid-based ionomer – to each other. This also effectively prevents leaching, release, or detachment of the ionomer from the membrane. The membrane according to the invention thus contributes significantly to the consistently high performance of a fuel cell. A further advantageous embodiment provides that the fluoropolymer-based polymer is selected from expanded PTFE (polytetrafluoroethylene), expanded FEP (perfluoro(ethylene / propylene)), expanded ECTFE (polychlorotrifluoroethylene), expanded PVF (polyvinyl fluoride), expanded PFA (perfluoroalkoxy polymer), expanded ETFE (ethylene-tetrafluoroethylene copolymer), and expanded PVDF (polyvinylidene fluoride). Mixtures of these polymers are also possible. According to the invention, it has been found that the CF or CH bonds of these polymers can be activated very readily, for example, by electron beams, so that covalent bonds between the organic antioxidant and the fluoropolymer-based polymer, and optionally also the perfluorosulfonic acid-based ionomer, can be formed particularly easily. Furthermore, these fluoropolymer-based polymers are characterized by high robustness in acidic environments.These advantageously used fluoropolymer-based polymers can thus improve the performance of the fuel cell. The perfluorosulfonic acid-based ionomer can advantageously be a short-side-chain or a long-side-chain ionomer. Mixtures of short-side-chain perfluorosulfonic acid (PFSA) ionomers (so-called SSC PFSA ionomers) and long-side-chain perfluorosulfonic acid ionomers (so-called LSC PFSA ionomers) are also possible. Short-side-chain perfluorosulfonic acid ionomers are particularly advantageous with regard to proton conductivity and thus offer a significant performance-enhancing effect. For examples of LSC PFSA and SSC PFSA ionomers, please refer to Fig. 1. Furthermore, the perfluorosulfonic acid-based ionomer with a short side chain advantageously has an equivalent weight of 700 to 1000 g / mol, and the perfluorosulfonic acid-based ionomer with a long side chain has an equivalent weight of more than 1000 to 1200 g / mol. Exemplary perfluorosulfonic acid-based ionomers with an equivalent weight of up to 1000 g / mol are available under the trade name Aquivion. Exemplary perfluorosulfonic acid-based ionomers with an equivalent weight of more than 1000 g / mol are available under the trade name Nafion. Equivalent weight is defined as the reciprocal of the ion exchange capacity (here: proton exchange capacity) in mol / g. According to the invention, the organic antioxidant is a phenol-based antioxidant. Phenol-based antioxidants are characterized by very good antioxidant properties, are good radical scavengers, and are also very readily activated with regard to forming a covalent bond with the fluoropolymer-based polymer and / or the perfluorosulfonic acid-based ionomer. These organic antioxidants for covalent bonding to the fluoropolymer-based polymer and / or the perfluorosulfonic acid-based ionomer are 4-isopropenylphenol, 4-ethenylphenol, and 1,2-dihydroxy-4-allylbenzene. Furthermore, according to the invention, a fuel cell is described which comprises a fuel cell membrane as disclosed above. By using the fuel cell membrane according to the invention, the fuel cell is also characterized by a consistently very high power output and power density with good stability even at high temperatures and current densities. A first method for producing a fuel cell membrane is also disclosed according to the invention. The fuel cell membrane obtained by the first method according to the invention comprises at least one fluoropolymer-based polymer, at least one perfluorosulfonic acid-based ionomer, and at least one organic antioxidant. In a first process step, a perfluorosulfonic acid-based ionomer precursor, comprising at least one carbon-carbon double bond, and an organic antioxidant precursor, also comprising at least one carbon-carbon double bond, are deposited onto a fluoropolymer-based polymer. This yields a membrane precursor assembly. The perfluorosulfonic acid-based ionomer precursor and the organic antioxidant precursor can each be deposited individually, optionally with one or more solvents, onto the fluoropolymer-based polymer, or they can be premixed and deposited as a mixture, which may also contain one or more suitable solvents.The use of a solvent-based mixture of perfluorosulfonic acid-based ionomer precursor and organic antioxidant precursor is advantageous with regard to a particularly uniform distribution of the perfluorosulfonic acid-based ionomer precursor and the organic antioxidant precursor. Suitable solvents can include, for example, water, isopropanol, and dichloromethane, as well as mixtures thereof. The organic antioxidant precursor is, in particular, a phenol-based antioxidant precursor whose OH group is protected by a protecting group that can be removed after the process. Particularly suitable antioxidant precursors with a protecting group are: 4-vinylphenyl acetate, trans-p-methoxypropenylbenzene, 4-methoxystyrene, 4-methoxycinnamonitrile, 4-(1-propenyl)-1,2-dimethoxybenzene, trans-1,2,4-trimethoxy-5-(1-propenyl)benzene, 1-acetoxy-2-methoxy-4-[(E)-1-propenyl]benzene / isoeugenyl acetate benzoic acid eugenyl esters. In a further process step, the membrane precursor assembly is irradiated with high-energy radiation. This irradiation induces a covalent bond between the forming fluoropolymer-based polymer and the organic antioxidant and / or the perfluorosulfonic acid-based ionomer. In other words, the high-energy radiation activates CF bonds and / or CH bonds of the fluoropolymer-based polymer precursor, thereby abstracting a fluorine atom or a hydrogen atom, respectively. The resulting radicals can then attack the respective double bonds of the perfluorosulfonic acid-based ionomer precursor or antioxidant precursor, generating covalent bonds. The high-energy radiation is selected according to the specific compounds used. The process enables the formation of a permanently stable covalent bond between the individual components of the fuel cell membrane, ideally between all three components of the membrane being produced, thus effectively preventing the individual components from dissolving, washing out, or detaching. Furthermore, the process is easily implemented without significant technical effort. The fuel cell membrane obtained according to the inventive process is characterized by very good physical, chemical, and mechanical stability, combined with a consistently high power density. Furthermore, a second method for producing a fuel cell membrane is also described according to the invention. As already disclosed with regard to the first method according to the invention, the fuel cell membrane produced according to the second method also comprises at least one fluoropolymer-based polymer, at least one perfluorosulfonic acid-based ionomer, and at least one organic antioxidant. In a first process step, at least one CH bond and / or one CF bond of a fluoropolymer-based polymer precursor is activated by irradiation with high-energy radiation. As in the first process, this generates radicals in the activated fluoropolymer-based polymer precursor, which can then react further. In a further process step, a perfluorosulfonic acid-based ionomer precursor, comprising at least one carbon-carbon double bond, and an organic antioxidant precursor, also comprising at least one carbon-carbon double bond, are deposited onto the activated fluoropolymer-based polymer precursor. These compounds react with the activated fluoropolymer-based polymer precursor to form a covalent bond between the fluoropolymer-based polymer and the organic antioxidant and / or the perfluorosulfonic acid-based ionomer. Ideally, all three components of the membrane to be produced are covalently linked. This prevents the detachment, separation, or leaching of the membrane components and thus significantly contributes to improving the physical, chemical, and mechanical stability of the membrane.A fuel cell manufactured with this membrane is therefore characterized by a consistently high power density. Regarding the components used in the second method according to the invention, reference is made to the explanations concerning the first method according to the invention. The second process according to the invention is also easy to implement without significant technical effort. As already explained above for the first process according to the invention, the perfluorosulfonic acid-based ionomer precursor and the organic antioxidant precursor can be applied separately or as a mixture, optionally with one or more solvents, which improves the uniform distribution of these components. To improve the formation of covalent bonds, especially between the perfluorosulfonic acid-based ionomer precursor and the organic antioxidant precursor, a further step of irradiation with high-energy radiation can advantageously be provided in the second process according to the invention after the step of applying the perfluorosulfonic acid-based ionomer precursor and the organic antioxidant precursor to the activated fluoropolymer-based polymer precursor. As mentioned above, the high-energy radiation can be selected according to the components or compounds to be activated. However, electron radiation is particularly suitable. Argon radiation can also be used if necessary, especially in the second method according to the invention. After formation of the (possibly covalently bonded) perfluorosulfonic acid-based ionomer, a further step of hydrolyzing the fluorosulfonic group(s) is advantageous, yielding an acid group. This activates and increases the proton conductance of the resulting membrane. According to the invention, the organic antioxidant is a phenol-based antioxidant, since this binds very well covalently and exhibits high antioxidant activity and radical scavenging function. These phenol-based antioxidants are: 4-isopropenylphenol, 4-ethenylphenol, 1,2-dihydroxy-4-allylbenzene, and mixtures thereof. To avoid reducing these effects, a further advantageous formulation may provide for the hydroxyl group of the organic antioxidant precursor to be protected by a protecting group. Particularly suitable antioxidant precursors with a protecting group are: 4-vinylphenyl acetate, trans-p-methoxypropenylbenzene, 4-methoxystyrene, 4-methoxycinnamonitrile, 4-(1-propenyl)-1,2-dimethoxybenzene, trans-1,2,4-trimethoxy-5-(1-propenyl)benzene, 1-acetoxy-2-methoxy-4-[(E)-1-propenyl]benzene / isoeugenyl acetate benzoic acid eugenyl esters. Ester protecting groups are particularly preferred due to their ease of hydrolysis. In a further advantageous process step, the process then comprises a step of hydrolyzing the protecting group, in particular the ester protecting group. The advantages, beneficial effects, and further developments described for the fuel cell membrane, the fuel cell, and the methods according to the invention are mutually applicable. Furthermore, the methods according to the invention are suitable for producing the fuel cell membrane according to the invention, or the fuel cell membrane according to the invention can be obtained by applying one of the methods according to the invention. Further details, features, and advantages of the invention will become apparent from the following description and the figures. Figure 1 shows an exemplary LSC PFSA ionomer precursor and exemplary SSC PFSA ionomer precursors; Figure 2 shows a reaction scheme illustrating the first process according to the invention for producing a fuel cell membrane according to one embodiment; and Figure 3 shows a reaction scheme illustrating the second process according to the invention for producing a fuel cell membrane according to one embodiment. The figures represent only the essential details of the invention. All other details have been omitted for clarity. Furthermore, identical reference numerals denote identical components. Fig. 1 shows exemplary structures of preferred PFSA ionomer precursors with short side chains (see B and C in Fig. 1) and an exemplary structure of a preferred PFSA ionomer precursor with a long side chain (see A in Fig. 1). The PFSA ionomer precursors feature activatable C-C double bonds that can serve to form covalent compounds, thereby yielding the actual ionomers. In Fig. 1A, X can represent the repeating unit CF2=CF(CF2CF2)m(CF2CF(OCF2CFCF3OCF2CF2SO2F)X or the entire structure as a repeating structure, where m is an integer from 1 to 50 and in particular from 1 to 15. In Fig. 1B, Y can represent the repeating unit CF2=CF(CF2CF2)n(CF2CF(OCF2CF2CF2CF2SO2F)Y or the entire structure as a repeating structure, where n is an integer from 1 to 50 and in particular from 1 to 15. In Fig. 1C, Z can represent the repeating unit CF2=CF(CF2CF2)p(CF2CF(OCF2CF2SO2F)Z or the entire structure as a repeating structure, where p is an integer from 1 to 50 and in particular from 1 to 15. Fig. 2 shows a reaction scheme to illustrate the first process according to the invention for the production of a fuel cell membrane 10 comprising a fluoropolymer-based polymer 4, a perfluorosulfonic acid-based ionomer 6 and an organic antioxidant 5. First, a perfluorosulfonic acid-based ionomer precursor 3, comprising at least one carbon-carbon double bond, and an organic antioxidant precursor 2, comprising at least one carbon-carbon double bond, are applied to molecules of a fluoropolymer-based polymer precursor 1 to form a membrane precursor assembly 9. The application can be carried out by any conventional method, such as spraying, applying with a doctor blade, rolling, and the like. The perfluorosulfonic acid-based ionomer precursor 3 and the organic antioxidant precursor 2 can each be applied separately, each with a solvent, or as a mixture, optionally with one or more solvents. This improves the homogeneous distribution of the precursor compounds in the resulting membrane precursor assembly 9. The organic antioxidant precursor 2 can be a 4-isopropenylphenol, a 4-ethenylphenol or a 1,2-dihydroxy-4-allylbenzene. In a further process step, the membrane precursor assembly 9 is irradiated with high-energy radiation ΔE, in particular with electron radiation. The action of the high-energy radiation activates, for example, CH bonds and / or CF bonds of the molecules of the fluoropolymer-based polymer precursor 1 and, in particular, abstracts a hydrogen atom and / or a fluorine atom, so that the resulting radical reacts with the carbon-carbon double bond of the perfluorosulfonic acid-based ionomer precursor 3 and / or with the carbon-carbon double bond of the organic antioxidant precursor 2, forming a covalent bond G between the fluoropolymer-based polymer 4 and the organic antioxidant 5 and / or forming a covalent bond F between the fluoropolymer-based polymer 4 and the perfluorosulfonic acid-based ionomer 6.In addition, the organic antioxidant 5 can also be bound to the perfluorosulfonic acid-based ionomer 6 by a covalent bond H. Thus, in the resulting fuel cell membrane 10, a network of covalently bound fluoropolymer-based polymer 4, perfluorosulfonic acid-based ionomer 6 and organic antioxidant 5 is formed, wherein at least the fluoropolymer-based polymer 4 is covalently linked to the organic antioxidant 5 and / or to the perfluorosulfonic acid-based ionomer 6. Fig. 3 schematically illustrates an embodiment of a second method according to the invention for producing a fuel cell membrane 10. In a first process step, at least one CH bond and / or one CF bond of molecules of a fluoropolymer-based polymer precursor 1 is activated by irradiation with high-energy radiation ΔE1. ​​This yields an activated fluoropolymer-based polymer precursor 7. The activated fluoropolymer-based polymer precursor 7 can be of a radical nature, obtainable, for example, by abstraction of a hydrogen atom from a CH bond or by abstraction of a fluorine atom from a CF bond. Such radical sites are indicated by reference numeral 8. A perfluorosulfonic acid-based ionomer precursor 3, comprising at least one carbon-carbon double bond, and an organic antioxidant precursor 2, also comprising at least one carbon-carbon double bond, are subsequently deposited onto the activated fluoropolymer-based polymer precursor 7, forming a covalent bond between the fluoropolymer-based polymer 4 and the organic antioxidant 5 and / or forming a covalent bond between the fluoropolymer-based polymer 4 and the perfluorosulfonic acid-based ionomer 6.The radically activated fluoropolymer-based polymer precursor 7 then reacts with the carbon-carbon double bonds to form a covalent bond G between the fluoropolymer-based polymer 4 and the organic antioxidant 5 and / or to form a covalent bond F between the fluoropolymer-based polymer 4 and the perfluorosulfonic acid-based ionomer 6. In addition, the organic antioxidant 5 can also be bound to the perfluorosulfonic acid-based ionomer 6 by a covalent bond H. The formation of covalent bonds can optionally be improved by irradiation with high-energy radiation ΔE2. The second method for producing a fuel cell membrane 10, illustrated in Fig. 3, also results in a network of covalently bound fluoropolymer-based polymer 4, perfluorosulfonic acid-based ionomer 6 and organic antioxidant 5 in the obtained fuel cell membrane 10, wherein at least the fluoropolymer-based polymer 4 is covalently linked to the organic antioxidant 5 and / or the fluoropolymer-based polymer 4 is covalently linked to the perfluorosulfonic acid-based ionomer 6. Reference symbol list: 1 Fluoropolymer-based polymer precursor 2 Organic antioxidant precursor 3 Perfluorosulfonic acid-based ionomer precursor 4 Fluoropolymer-based polymer 5 Organic antioxidant 6 Perfluorosulfonic acid-based ionomer 7 Activated fluoropolymer-based polymer precursor 8 Radical sites 9 Membrane precursor assembly 10 Fuel cell membrane F Covalent bond between the fluoropolymer-based polymer and the perfluorosulfonic acid-based ionomer G Covalent bond between the fluoropolymer-based polymer and the organic antioxidant H Covalent bond between the organic antioxidant and the perfluorosulfonic acid-based ionomer

Claims

Fuel cell membrane (10) comprising: - at least one fluoropolymer-based polymer (4), - at least one perfluorosulfonic acid-based ionomer (6) and - at least one organic antioxidant (5), wherein the fluoropolymer-based polymer (4) is covalently bonded to the organic antioxidant (5) and wherein the organic antioxidant (5) is 4-isopropenylphenol, 4-ethenylphenol, 1,2-dihydroxy-4-allylbenzene or a mixture of the phenols. Fuel cell membrane (10) according to claim 1, wherein the fluoropolymer-based polymer (4) is selected from expanded PTFE, expanded FEP, expanded ECTFE, expanded PVF, expanded PFA, expanded ETFE and expanded PVDF. Fuel cell membrane (10) according to claim 1 or 2, wherein the perfluorosulfonic acid-based ionomer (6) is a short-side-chain ionomer or a long-side-chain ionomer, in particular a short-side-chain perfluorosulfonic acid-based ionomer, wherein, if the perfluorosulfonic acid-based ionomer (6) is a short-side-chain ionomer, it is formed from a perfluorosulfonic acid-based ionomer precursor according to the following structure B or C, where in structure BY for the repetition unit CF 2 =CF(CF 2 CF 2 ) n (CF 2 CF(OCF 2 CF 2 CF 2 CF 2 SO 2 F)Y or represents the entire structure as a repeating structure, where n is an integer from 1 to 50 and in particular from 1 to 15 and where in structure CZ for the repetition unit CF 2 =CF(CF 2 CF 2 ) p (CF 2 CF(OCF 2 CF 2 SO 2 F)Z or stands for the entire structure as a repeating structure, where p is an integer from 1 to 50 and in particular from 1 to 15, wherein, if the perfluorosulfonic acid-based ionomer (6) is a long-side-chain ionomer, it is formed from a perfluorosulfonic acid-based ionomer precursor according to the following structure A, where in structure AX represents the repetition unit CF 2 =CF(CF 2 CF 2 ) m (CF 2 CF(OCF 2 CFCF 3 OCF 2 CF 2 SO 2 F)X or represents the entire structure as a repeating structure, where m is an integer from 1 to 50 and in particular from 1 to 15. Fuel cell membrane (10) according to claim 3, wherein the perfluorosulfonic acid-based ionomer (6) with short side chain has an equivalent weight of 700 to 1000 g / mol and the perfluorosulfonic acid-based ionomer with long side chain has an equivalent weight of more than 1000 to 1200 g / mol. Fuel cell membrane (10) according to one of the preceding claims, wherein the fluoropolymer-based polymer (4) is covalently linked to the perfluorosulfonic acid-based ionomer (6). Fuel cell comprising a fuel cell membrane (10) according to one of the preceding claims. A method for producing a fuel cell membrane (10) comprising: - at least one fluoropolymer-based polymer (4), - at least one perfluorosulfonic acid-based ionomer (6) and - at least one organic antioxidant (5), wherein the organic antioxidant (5) is 4-isopropenylphenol, 4-ethenylphenol, 1,2-dihydroxy-4-allylbenzene or a mixture of the phenols, the method comprising the steps of: - applying a perfluorosulfonic acid-based ionomer precursor (3), comprising at least one carbon-carbon double bond, and an organic antioxidant precursor (2), comprising at least one carbon-carbon double bond, to a fluoropolymer-based polymer (1) to produce a membrane precursor assembly (9) and - irradiating the membrane precursor assembly (9) with high-energy radiation (ΔE) to form covalent bonds (G) between the fluoropolymer-based polymer (4) and the organic antioxidant (5). A process for producing a fuel cell membrane (10) comprising: - at least one fluoropolymer-based polymer (4), - at least one perfluorosulfonic acid-based ionomer (6), and - at least one organic antioxidant (5), wherein the organic antioxidant (5) is 4-isopropenylphenol, 4-ethenylphenol, 1,2-dihydroxy-4-allylbenzene, or a mixture of the phenols, the process comprising the steps of: - activating at least one CH bond and / or at least one CF bond of a fluoropolymer-based polymer precursor (1) by irradiation with high-energy radiation (ΔE1), and - applying at least one perfluorosulfonic acid-based ionomer precursor (3), comprising at least one carbon-carbon double bond, and one organic antioxidant precursor (2), comprising at least one carbon-carbon double bond.on the activated fluoropolymer-based polymer precursor (7) forming covalent bonds (G) between the fluoropolymer-based polymer (4) and the organic antioxidant (5). Method according to claim 7 or 8, further comprising a step of forming covalent bonds (F) between the fluoropolymer-based polymer (4) and the perfluorosulfonic acid-based ionomer (6). Method according to claim 8 or 9, further comprising a further step of irradiation with high-energy radiation (ΔE2) after the step of applying the perfluorosulfonic acid-based ionomer precursor (3) and the organic antioxidant precursor (2) to the activated fluoropolymer-based polymer precursor (7). Method according to any one of claims 7 to 10, wherein the high-energy radiation (ΔE, ΔE1, ΔE2) is electron radiation. A method according to any one of claims 7 to 11 further comprising a step of hydrolyzing the fluorosulfonic groups of the perfluorosulfonic acid-based ionomer (6). Method according to any one of claims 7 to 12, wherein the hydroxyl group of the organic antioxidant precursor (2) is protected by a protecting group, in particular by an ester protecting group. The method according to claim 13, further comprising a step of hydrolyzing the protecting group, in particular the ester protecting group.

Images