GAS DIFFUSION LAYER FOR FUEL CELLS WITH IMPROVED BENDING PROPERTIES

Description

[0001] The present invention relates to a gas diffusion layer with high flexural stiffness and high flexibility, a method for producing such a gas diffusion layer and a fuel cell containing such a gas diffusion layer. BACKGROUND OF THE INVENTION

[0002] Fuel cells utilize the chemical reaction of a fuel, particularly hydrogen, with oxygen to produce water, in order to generate electrical energy. In hydrogen-oxygen fuel cells, hydrogen or a hydrogen-containing gas mixture is supplied to the anode, where electrochemical oxidation takes place, releasing electrons (H₂ → 2 H⁺ + 2 e⁻). A membrane, which separates the reaction compartments gas-tight and provides electrical insulation, transports the protons from the anode compartment to the cathode compartment. The electrons supplied at the anode are then transferred to the cathode via an external circuit. Oxygen or an oxygen-containing gas mixture is supplied to the cathode, where the oxygen is reduced, accepting the electrons. The oxygen anions formed react with the protons transported across the membrane to form water (½ O₂ + 2 H⁺ + 2 e⁻ → H₂O).

[0003] For many applications, especially in automotive powertrains, low-temperature proton exchange membrane fuel cells (PEMFCs, also known as polymer electrolyte membrane fuel cells) are used. Their core component is a polymer electrolyte membrane (PEM) that is permeable only to protons (or hydronium ions, H₃O⁺) and water, and spatially separates the oxidizing agent, generally atmospheric oxygen, from the reducing agent. A catalyst layer, forming the electrodes, is applied to the gas-tight, electrically insulating, proton-conducting membrane on both the anode and cathode sides. This catalyst layer typically contains platinum as the catalytically active metal. The actual redox reactions and charge separations take place within the catalyst layers. The membrane and catalyst layers form a single unit, also known as a CCM (catalyst-coated membrane).On both sides of the CCM is a gas diffusion layer (GDL) that stabilizes the cell structure and performs transport and distribution functions for reaction gases, water, heat, and electricity. The membrane, electrodes, and gas diffusion layer form the membrane electrode assembly (MEA). Flow distribution plates (so-called bipolar plates) are arranged between the membrane electrode assemblies. These plates have channels for supplying the adjacent cathode and anode with process gases and typically also include internal cooling channels.

[0004] The gas diffusion layers located between the flow distributor plates and the catalyst layers are of crucial importance for the function and performance of the fuel cell. The process components consumed and produced in the electrode reactions must be transported through the gas diffusion layer and homogeneously distributed from the macroscopic structure of the flow distributor plates / bipolar plates to the microscopic structure of the catalyst layers. The electrons generated and consumed in the half-cell reactions must be conducted to the flow distributor plates with minimal voltage loss. The heat generated during the reaction must be dissipated to the coolant in the flow distributor plates, so the materials of the gas diffusion layer must also possess sufficient thermal conductivity.Furthermore, the GDL must also act as a mechanical buffer between the macrostructured flow distributor plate and the catalyst layers. This requires compensating for component tolerances and distributing the compression pressure. The GDL also serves as mechanical protection for the very thin membranes, which are subjected to high loads in the fuel cells. Therefore, particularly stringent requirements are placed on the mechanical properties of the GDL.

[0005] Gas diffusion layers for fuel cells typically consist of a carbon fiber substrate, which is usually made hydrophobic with fluoropolymers (e.g., PTFE) and then coated with a microporous layer (MPL). The MPL generally consists of a fluorine-containing polymer as a binder (e.g., PTFE) and a porous and electrically conductive carbon material (e.g., carbon black or graphite powder). The following three materials are currently used as carbon fiber substrates for GDLs: Carbon fiber papers (wet-laid and chemically bonded carbon fiber nonwovens with chemical binders that are carbonized), carbon fiber woven fabrics (e.g., from yarns of oxidized but not yet carbonized polyacrylonitrile fibers that are carbonized or graphitized after weaving), carbon fiber nonwovens (e.g., dried, carded, and hydroentangled nonwovens of oxidized polyacrylonitrile that are subsequently thickness-calibrated and carbonized).

[0006] Carbon fiber papers offer the technical advantage of high strength under various stresses (tension, bending, shear). This ensures good force transmission from the webs of the flow distributor plates / bipolar plates to the channel areas, resulting in homogeneous surface pressure and electrode bonding by the flow distributor. However, these advantages are always accompanied by significant material embrittlement, which manifests as paper breakage even under slight bending stress. The causes of this behavior lie in the product design and the manufacturing process. Specifically, the chemical binders used in the production of carbon fiber papers (usually phenolic resins) become highly embrittled during carbonization, leading to the described material behavior.

[0007] In structural mechanics, different types of stress are distinguished depending on the forces acting on a component. There is a need for materials for gas diffusion layers that, while possessing otherwise equivalent mechanical properties, exhibit advantageous behavior under geometric changes such as those occurring under bending loads (e.g., deflection, curvature). The materials used to date do not yet possess the optimal combination of properties. These materials are either stiff and brittle (papers) or flexible and pliable (nonwovens, woven fabrics). A combination of high flexural stiffness and high flexibility would be desirable.

[0008] WO 2015 / 118323 (DE 112015000657 T5) describes a porous gas diffusion substrate comprising the following components: (a) a porous nonwoven fabric comprising carbon fibers, (b) a carbon-containing residue embedded in the porous nonwoven fabric, (c) a fluorinated polymer and (d) inert particles, wherein at least some of the carbon fibers of the porous nonwoven fabric have a coating comprising the fluorinated polymer and the inert particles.

[0009] The production of the porous gas diffusion substrate comprises the following stages: (i) Providing a porous nonwoven fabric comprising carbon fibers; (ii) Providing a dispersion comprising a carbonizable resin; (iii) Impregnating the porous nonwoven fabric with the dispersion from step (ii); (iv) Curing the carbonizable resin at a temperature of 100°C to 300°C; (v) Heat-treating the carbonizable resin to carbonize / graphite it at a temperature of 900°C to 3000°C to provide a gas diffusion substrate precursor; (vi) Providing a dispersion comprising water, fluorinated polymer, and inert particles; (vii) Treating the gas diffusion substrate precursor with the dispersion prepared in step (vi); (viii) Drying; and (ix) Heating to a temperature of up to 400°C to provide the gas diffusion substrate.

[0010] The impregnation of the porous nonwoven fabric with a carbonizable resin and the subsequent carbonization lead to an embrittlement of the material, so that the resulting gas diffusion substrates have a high flexural stiffness, but not good flexibility.

[0011] WO 97 / 20358 concerns a gas diffusion electrode for polymer electrolyte membrane fuel cells with a gas diffusion layer containing a mechanically stable support material, which may be a nonwoven fabric, woven fabric, or paper containing carbon fibers, glass fibers, or fibers of organic polymers. To manufacture the gas diffusion electrode, a suspension containing an electrically conductive material, e.g., carbon black, and a binder is applied to or impregnated with the support material. Suitable binders include temperature-stable polymers such as perfluorinated polymers, polyetherketones, polyethersulfones, polysulfones, polybenzimidazoles, polyphenylene sulfides, polyimides, polyamides, and polyphenylene oxides. It is noted that several binders can be mixed, depending on the desired hydrophobicity.This document does not teach the use of a combination of special fluorinated polymers and special high-performance polymers to optimize mechanical properties, especially against bending stress.

[0012] The present invention is based on the objective of providing a planar electrically conductive material and a gas diffusion layer based thereon for a fuel cell, while avoiding the aforementioned disadvantages.

[0013] Surprisingly, it has now been found that this problem can be solved and a flexibly stiffened composite material can be achieved that is particularly advantageous for gas diffusion layers when a carbon fiber substrate in the form of a nonwoven or woven fabric is equipped with a hydrophobic fluoropolymer and with certain thermoplastic high-performance plastics. SUMMARY OF THE INVENTION

[0014] A first object of the invention is a gas diffusion layer for a fuel cell, which A) a planar electrically conductive material comprising a) at least one fiber material selected from carbon fiber nonwovens, carbon fiber woven fabrics and mixtures thereof, wherein the fiber material b1) contains at least one fluorine-containing polymer, and b2) contains at least one polymer other than b1), selected from polyaryletherketones, polyphenylene sulfides, polysulfones, polyethersulfones, semi-aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and mixtures thereof, applied thereon and / or incorporated therein, and B) optionally a microporous layer on one of the surfaces of the electrically conductive material A) includes.

[0015] A special embodiment is a gas diffusion layer, wherein the planar electrically conductive material A) is obtainable by coating and / or impregnating at least one fiber material a), selected from carbon fiber nonwovens, carbon fiber woven fabrics and mixtures thereof, with an aqueous composition, containing at least one fluorinated polymer b1), and at least one polymer b2) other than b1), selected from polyaryletherketones, polyphenylene sulfides, polysulfones, polyethersulfones, semi-aromatic (co)polyamides, polyimides, polyamideimides,

[0016] Polyetherimides and mixtures thereof, and subsequent drying and / or sintering of the treated fiber material, wherein the sintering is carried out by thermal treatment at a temperature in the range of 300 to 450 °C.

[0017] Another object of the invention is a method for producing a gas diffusion layer for a fuel cell, comprising a planar electrically conductive material A) in which one i) provides at least one fiber material selected from carbon fiber nonwovens, carbon fiber woven fabrics and mixtures thereof, ii) coats and / or impregnates the fiber material provided in step i) with a composition comprising at least one fluorinated polymer b1) and at least one polymer b2) other than b1) selected from polyetherketones, polyphenylene sulfides, polysulfones, polyethersulfones, semi-aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and mixtures thereof, iii) subjects the coated and / or impregnated fiber material to drying and / or sintering, wherein the sintering is carried out by thermal treatment at a temperature in the range of 300 to 450°C, and iv) optionally coats the dried and / or sintered fiber material with a microporous layer.

[0018] Another item is a fuel cell comprising at least one gas diffusion layer, as defined above and below, or obtainable by a process as defined above and below.

[0019] Another aspect of the invention is the use of a planar electrically conductive material A), comprising a) at least one fiber material selected from carbon fiber nonwovens, carbon fiber woven fabrics and mixtures thereof, wherein the fiber material b1) contains at least one fluorine-containing polymer, and b2) contains at least one polymer other than b1), selected from polyetherketones, polyphenylene sulfides, polysulfones, polyethersulfones, semi-aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and mixtures thereof, applied thereon and / or incorporated therein, as a gas diffusion layer for a fuel cell. DESCRIPTION OF THE INVENTION

[0020] The planar electrically conductive materials used according to the invention and the gas diffusion layers based thereon have the following advantages: They combine good application-related product properties with good processability; compared to the previously preferred stiffened carbon fiber papers, they exhibit improved bending behavior with otherwise comparable mechanical properties; for the first time, a combination of stiffness and flexibility is achieved; in particular, a significantly greater maximum deflection (either until breakage or until the specimens slip off the supports) or a significantly larger bending angle can be achieved in the 3-point bending test with comparable bending stiffnesses or bending moduli. Bending tests

[0021] The determination of bending properties can be carried out using standard methods known to those skilled in the art, such as those described in DIN EN ISO 178:2019-08 (Plastics - Determination of bending properties). The principle of the bending test is based on a bending fixture in which a specimen is positioned as a bending beam on two supports and loaded in the middle via an inner support (3-point bending test). The test is performed by measuring the central deflection at a constant speed until a predetermined deformation is reached or until the specimen fails (i.e., maximum deflection until breakage or slippage of the specimen from the supports).

[0022] As a result of the bending tests, the bending stiffness [N / mm] can be determined as the slope of the initially straight section of the load-deformation curve. This can be read from the force / displacement diagram, i.e., from the formula given below: (XH - XL ) / DL. The bending stiffness is a measure of the resistance of a component subjected to bending stress to deformation.

[0023] Furthermore, the bending modulus E [N / mm² or MPa] can be determined. The bending modulus is calculated as follows: E = I v 3 X H − X L / 4 D L ba 3 with E: Bending modulus in kN / mm² < Iv: Span in mm XH: End of bending modulus determination in kN XL: Start of bending modulus determination in kN DL: Deflection in mm between XH and XL b: Specimen width in mm a: Specimen thickness in mm

[0024] The maximum bending angle α can also be calculated from the maximum deflection and half the distance between the two support points: α = arctan max . Durchbiegung / halbe Stützweite

[0025] Preferably, the planar electrically conductive material A) or a sintered product thereof has a flexural stiffness, determined by central loading in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen of 50 mm width, 0.13 mm thickness and 100 mm length with a support span of 32 mm and a test speed of 2 mm / min, of 0.05 to 0.08 N / mm.

[0026] Preferably, the planar electrically conductive material A) or a sintered product thereof has a maximum bending angle α, determined by central loading in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen of 50 mm width, 0.13 mm thickness and 100 mm length at a support span of 32 mm and a test speed of 2 mm / min of 50 to 60°.

[0027] Preferably, the planar electrically conductive material A) or a sintered product thereof has a flexural modulus, determined by central stress in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen of 50 mm width, 0.13 mm thickness and 100 mm length at a support span of 32 mm and a test speed of 2 mm / min of 3000 to 6000 N / mm².

[0028] Preferably, a gas diffusion layer according to the invention, comprising a planar electrically conductive material and a microporous layer, has a bending stiffness, determined by central loading in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen of 50 mm width, 0.16 mm thickness and 100 mm length at a support span of 32 mm and a test speed of 2 mm / min, of 0.08 to 0.12 N / mm.

[0029] Preferably, a gas diffusion layer according to the invention, comprising a planar electrically conductive material and a microporous layer, has a maximum bending angle α, determined by central loading in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen of 50 mm width, 0.16 mm thickness and 100 mm length at a support span of 32 mm and a test speed of 2 mm / min, of 50 to 60°.

[0030] Preferably, a gas diffusion layer according to the invention, comprising a planar electrically conductive material and a microporous layer, has a bending modulus, determined by central loading in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen of 50 mm width, 0.16 mm thickness and 100 mm length at a support span of 32 mm and a test speed of 2 mm / min, of 3000 to 6000 N / mm². Gas diffusion layer

[0031] The planar electrically conductive material and the gas diffusion layer used according to the invention are planar structures with a substantially two-dimensional, planar extent and a correspondingly smaller thickness. The gas diffusion layer has a base area that generally corresponds substantially to the base area of ​​the adjacent membrane with the catalyst layers and the base area of ​​the adjacent flow distributor plate of the fuel cell. The shape of the base area of ​​the gas diffusion layer can be, for example, polygonal (n-sided with n ≥ 3, e.g., triangular, square, pentagonal, hexagonal, etc.), circular, segmented circular (e.g., semicircular), elliptical, or segmented ellipsoid. Preferably, the base area is rectangular or circular.

[0032] The gas diffusion layer comprises, as component A), a fiber-reinforced composite material consisting of at least one electrically conductive planar carbon fiber material a), at least one fluorine-containing polymer b1), and at least one polymer b2) different from b1), selected from polyetherketones, polyphenylene sulfides, polysulfones, polyethersulfones, semi-aromatic (co)polyamides, polyimides, polyamide-imides, polyether-imides, and mixtures thereof. As described in more detail below, component b1) can serve to increase the hydrophobicity of the fiber material a). Component b2) can serve to stiffen the planar electrically conductive material or the gas diffusion layer. The combination of components a), b1), and b2) achieves the advantageous mechanical properties described above, and in particular, advantageous flexural behavior.For this purpose, at least one fiber material can be a) equipped with the polymer components b1) and b2) and, if necessary, further additives by means of conventional coating and impregnation processes. Such processes are described in more detail below.

[0033] The at least one fiber material a) is selected from carbon fiber nonwovens, carbon fiber woven fabrics, and mixtures thereof, which contain the polymer components b1) and b2) applied to them and / or incorporated therein. Preferably, the fiber material a) comprises at least one carbon fiber nonwoven. In a particular embodiment, the fiber material a) consists of a carbon fiber nonwoven.

[0034] The fibers contained in the fiber material a) comprise carbon fibers (carbon fibers) and optionally various fibers thereof, preferably selected from glass fibers, fibers of organic polymers such as polypropylene, polyester, polyphenylene sulfide, polyetherketones, and mixtures thereof. Specifically, the fibers contained in the fiber material a) consist only of carbon fibers.

[0035] The carbon fibers can be produced in the usual manner, preferably using polyacrylonitrile fibers (PAN fibers) as the starting material. PAN fibers are produced by radical polymerization of a monomer composition that preferably contains at least 90 wt% acrylonitrile, based on the total weight of the monomers used for polymerization. The resulting polymer solution is spun into filaments, e.g., by wet spinning and coagulation, and bundled into ropes. Before this PAN precursor is converted into carbon fibers at high temperatures, it is generally subjected to oxidative cyclization (also referred to simply as oxidation) in an oxygen-containing atmosphere at elevated temperatures of approximately 180 to 300 °C. The resulting chemical cross-linking improves the dimensional stability of the fibers.The actual pyrolysis to carbon fibers then takes place at temperatures of at least 1200 °C. Depending on the desired fiber material shape, either the starting fibers or a sheet-like fiber material can be used for this pyrolysis. Depending on the temperature during pyrolysis, a distinction is made between carbonization and graphitization. Carbonization refers to treatment at approximately 1200 to 1500 °C under an inert gas atmosphere, which leads to the release of volatile products. Graphitization, i.e., heating to approximately 2000 to 3000 °C under an inert gas, yields so-called high-modulus or graphite fibers. These fibers are highly pure, lightweight, extremely strong, and very good conductors of electricity and heat.

[0036] Carbon fiber fabrics are produced by interlacing two yarn systems: warp (warp threads) and weft (weft threads). As with textiles, fiber bundles are flexibly but permanently bonded together. Carbon fiber fabrics are preferably made from oxidized, but not yet carbonized or graphitized, PAN fibers. Carbonization or graphitization, which imparts electrical conductivity to the fiber material, takes place after weaving.

[0037] For the production of carbon fiber nonwovens, either unoxidized or oxidized PAN fibers can be used. In a first preferred embodiment, the fibers are first laid down (carded) into a dry pile and then bonded to form a nonwoven. This can be achieved, for example, by hydroentangling, whereby the carbon fibers are oriented, interlocked, and thus mechanically stabilized. If necessary, the thickness of the bonded nonwoven can be calibrated to a desired value. Nonwovens based on unoxidized PAN fibers are, after laying down and bonding, first subjected to oxidation at elevated temperature and under an oxygen atmosphere, and then to carbonization / graphitization under an inert gas atmosphere. Nonwovens based on oxidized PAN fibers are subjected only to carbonization / graphitization after laying down and bonding.Carbon fiber nonwovens, for the production of which the fibers are laid dry into a pile in a first step, are a preferred embodiment of the invention.

[0038] If a bonded fiber material is used as fiber material a), it is specifically selected from mechanically bonded fiber materials. A chemical bond, especially with carbonizable polymeric binders, can adversely affect the bending properties of the gas diffusion layer. Specifically, the fiber materials used according to the invention do not contain any polymers other than polymers b1) and b2) added as binders.

[0039] In a further suitable embodiment, the fibers are wet-laid in a first step. In this embodiment, the term carbon fiber nonwoven also includes, for example, a wet-laid material consisting of short-cut carbon fibers, carbon black, at least one polymer b1), specifically PTFE, and at least one polymer b2), specifically PEEK. In contrast to carbon fiber papers known from the prior art, the wet-laid fiber materials used according to the invention contain no or only a very small proportion of phenolic resins as binders. Preferably, the mass fraction of phenolic resins is 0 to 10%, preferably 0 to 5%, and particularly 0 to 1%, based on the mass of the fiber material a). Specifically, the wet-laid fiber materials used according to the invention contain no phenolic resins as binders. Even more specifically, the wet-laid fiber materials used according to the invention contain no polymers other than polymers b1) and b2) as binders.

[0040] In a particular embodiment, the planar electrically conductive fiber material comprises a) at least one carbon fiber nonwoven fabric. These are advantageous, among other things, because they are compression-elastic and can be easily manufactured on an industrial scale, e.g., in a roll-to-roll process.

[0041] According to the invention, the fiber materials are equipped a) with at least one fluorine-containing polymer b1), at least one polymer b2) different from b1) and optionally at least one further additive.

[0042] Specifically, the fiber material contains a) at least one fluorine-containing polymer b1), at least one polymer b2) other than b1), selected from polyaryletherketones, polyphenylene sulfides, polysulfones, polyethersulfones, semi-aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and mixtures thereof, optionally at least one conductivity-enhancing additive b3), optionally at least one further additive b4), selected from polymeric binders b41) and b2) other than b42), surfactants b42), further additives and excipients b43). applied to it and / or incorporated into it.

[0043] In the following, the mass or total weight of the fiber material a) refers to the unfinished fiber material, i.e. without components b1), b2) and, if present, b3) and b4).

[0044] In a particular embodiment, the fiber material a) contains no added polymeric binders b41). This applies especially to polymeric binders b41), which would carbonize under the manufacturing conditions of the planar electrically conductive material A). Polymer b1)

[0045] Preferably, the fluorine-containing polymer b1) is selected from polytetrafluoroethylenes (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), perfluoroalkoxy polymers (PFA), and mixtures thereof. Perfluoroalkoxy polymers are, for example, copolymers of tetrafluoroethylene (TFE) and perfluoroalkoxy vinyl ethers, such as perfluorovinyl propyl ether. Preferably, a polytetrafluoroethylene is used as polymer b1).

[0046] Preferably, the mass fraction of the fluorinated polymer b1) is 0.5 to 40%, preferably 1 to 20%, and in particular 1 to 10%, based on the mass of the fiber material a). In a special embodiment, the fluorinated polymer b1) is PTFE and the mass fraction is 0.5 to 40%, preferably 1 to 20%, and in particular 1 to 10%, based on the mass of the fiber material a). Polymer b2)

[0047] In a preferred embodiment, the polymer b2) is selected from among so-called high-performance plastics, which are characterized by properties such as a high glass transition temperature, a high melting point, good temperature resistance, good chemical resistance, and good mechanical properties. Preferably, the polymers b2) have a continuous operating temperature (continuous service temperature) of at least 150°C.

[0048] Preferably, the polymers b2) are semi-aromatic and aromatic polymers. Preferably, the polymers b2) are thermoplastics.

[0049] The polymers b2) are preferably selected from polyaryletherketones (PAEK), polyphenylene sulfides (PPS), polysulfones (PSU), polyethersulfones (PES), semi-aromatic (co)polyamides (high-temperature polyamides, HTPA), polyimides (PI), polyamide-imides (PAI), polyetherimides (PEI), and blends thereof. Suitable polymers b2) also include (semi-)aromatic polyesters, such as PET or PBT, polycarbonates (PC), and temperature-resistant melamines, such as melamine foams filled with nanoporous SiO2 aerogels.

[0050] In a preferred embodiment, the polymer component b2) comprises at least one polyaryletherketone. Specifically, the polymer component b2) consists of at least one polyaryletherketone. Polyaryletherketones (PAEKs) are semi-crystalline thermoplastics with an alternating structure in which an aryl group is followed by either a keto group (carbonyl group) or an ether group, the proportions of keto and ether groups being variable and differing in the substitution pattern on the aryl rings. Suitable polyaryletherketones b2) are polyetherketones (PEK), polyetheretherketones (PEEK), polyetherketoneketones (PEKK), etc. Preferably, the polymer component b1) comprises at least one polyetheretherketone or consists of at least one polyetheretherketone.

[0051] Suitable semi-aromatic (co)polyamides b2) are the polymers designated as high-temperature polyamides (HTPA). These are semi-crystalline or amorphous, thermoplastic, semi-aromatic polyamides. Preferably, they contain at least one aromatic dicarboxylic acid polymerized within them, in particular selected from terephthalic acid, isophthalic acid, and mixtures of terephthalic acid and isophthalic acid. Preferred semi-aromatic (co)polyamides b2) are selected from PA 6.T, PA 10.T, PA 12.T, PA 6.I, PA 10.I, PA 12.I, PA 6.T / 6.I, PA 6.T / 6, PA 6.T / 10T, PA 10.T / 6.T, PA 6.T / 12.T, PA 12.T / 6.T, and mixtures thereof. Another special embodiment of the polyamides b2) is polyphthalamide (PPA).

[0052] Suitable polyimides b2) are polysuccinimide (PSI), polybismaleimide (PBMI), polyimide sulfone (PISO) and polymethacrylimide (PMI).

[0053] Preferably, the mass fraction of polymer b2) is 0.5 to 40%, preferably 1 to 20%, based on the mass of the fiber material a). In a particular embodiment, polymer b2) is selected from PEEK, PPS and mixtures thereof, and the mass fraction of polymer b2) is 0.5 to 40%, preferably 1 to 20%, based on the mass of the fiber material a). other additives

[0054] In many cases, the fiber material a) already possesses good electrical and thermal conductivity due to the carbon fibers used, even without conductivity-enhancing additives. However, to further improve its electrical and thermal conductivity, the fiber material a) can be additionally equipped with at least one conductivity-enhancing additive b3). Preferably, the conductivity-enhancing additive b3) is selected from metal particles, carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. Preferably, the conductivity-enhancing additive b3) comprises or consists of carbon black. The fiber material a) can be equipped with at least one conductivity-enhancing additive b3) for example, together with the polymer b1) and / or b2) and / or other additives. Preferably, an aqueous dispersion is used for the treatment of the fiber material a).

[0055] Preferably, the mass fraction of the conductivity-enhancing additive b3) is 0.5 to 45%, preferably 1 to 25%, based on the mass of the fiber material a). In a particular embodiment, the conductivity-enhancing additive b3) comprises or consists of carbon black and the mass fraction is 0.5 to 45%, preferably 1 to 25%, based on the mass of the fiber material a).

[0056] Preferably the total weight of components b1), b2) and b3) is 3 to 50 wt. %, preferably 5 to 35 wt. %, based on the total weight of the (untreated) fiber material a).

[0057] Preferably the fiber material contains a) applied to it and / or incorporated therein 10 to 50 wt.% of at least one fluorinated polymer b1), 5 to 40 wt.% of at least one polymer b2) different from b1) and 20 to 80 wt.% of at least one conductivity-enhancing additive b3), based on the total weight of components b1), b2) and b3).

[0058] The fiber materials a) may additionally contain at least one further additive b4). These include, for example, various polymeric binders b41) and surfactants b42) from components b1) and b2). Suitable binders b41) are, for example, furan resins, polyimide resins, etc. The fiber material a) may be finished with at least one further additive b4) together with the polymer b1) and / or b2) and / or other additives. The binders b41) may subsequently be cured. This may be done, for example, together with drying and / or sintering following the finishing with the polymers b1) and b2), or separately.

[0059] Preferably, the total mass fraction of further additives b4) is 0 to 80%, preferably 0 to 50%, based on the mass of the fiber material a). If the fiber materials a) additionally contain at least one further additive b4), the total mass fraction of further additives b4) is 0.1 to 80%, preferably 0.5 to 50%, based on the mass of the fiber material a).

[0060] Specifically, the fiber material a) contains various other polymers b41) in addition to the fluorinated polymers b1) and the polymers b2) in a weight fraction of at most 5%, preferably at most 1%, particularly preferably at most 0.5%, and especially at most 0.1%, based on the total weight of the fiber material a), applied to it and / or incorporated therein. More specifically, the fiber material a) contains no additions of other polymers b41) that are different from the fluorinated polymers b1) and the polymers b2).

[0061] The fiber material a) preferably has a thickness in the range of 50 to 500 µm, particularly preferably from 100 to 400 µm. This thickness refers to the unfinished, uncompressed state of the fiber material a), i.e., before the GDL is installed in a fuel cell.

[0062] The fiber material a) can be treated with components b1), b2) and, if applicable, b3) and / or b4) using conventional methods. Suitable coating and impregnation processes are described in more detail below.

[0063] In a special embodiment, the fiber material equipped with components b1), b2) and optionally b3) and / or b4) is subjected to a thermal treatment (sintering).

[0064] A preferred embodiment is a gas diffusion layer comprising a sintered product of A) obtained by thermal treatment at a temperature of at least 250 °C, preferably at least 300 °C, in particular in a range of 300 to 450 °C, specifically from 350 to 450 °C.

[0065] In a preferred embodiment, the gas diffusion layer according to the invention consists of a two-layer composite based on a planar, electrically conductive material A) or a sintering product thereof and a microporous layer (MPL) B) on one of the surfaces of the fiber material A).

[0066] In contrast to the macroporous fiber material A), the MPL is microporous with pore diameters that are generally significantly less than one micrometer, preferably at most 900 nm, particularly preferably at most 500 nm, and especially at most 300 nm. The mean pore diameter of the MPL B) is preferably in the range of 5 to 200 nm, particularly preferably from 10 to 100 nm. The mean pore diameter can again be determined by mercury porosimetry. The MPL contains conductive carbon particles, preferably carbon black or graphite, in a matrix of a polymeric binder. Preferred binders are the aforementioned fluorine-containing polymers, especially polytetrafluoroethylene (PTFE).

[0067] The microporous layer b) preferably has a thickness in the range of 10 to 100 µm, particularly preferably 20 to 50 µm. This thickness refers to the uncompressed state of the microporous layer b), i.e., before the GDL is installed in a fuel cell.

[0068] The gas diffusion layer according to the invention preferably has a thickness (total thickness of fiber material A) and MPL B)) in the range of 80 to 1000 µm, particularly preferably from 100 to 500 µm. This thickness refers to the uncompressed state of the GDL, i.e., before its installation in a fuel cell. Manufacturing process

[0069] In step i) of the process according to the invention, at least one fiber material is provided, selected from carbon fiber nonwovens, carbon fiber woven fabrics and mixtures thereof. Regarding suitable and preferred fiber materials, reference is made in full to the preceding descriptions.

[0070] The fiber material provided in step i) is coated and / or impregnated in step ii) with an aqueous composition containing at least one fluorinated polymer b1) and at least one polymer other than b2), selected from polyaryletherketones, polyphenylene sulfides, polysulfones, polyethersulfones, semi-aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and mixtures thereof.

[0071] With regard to suitable and preferred polymers b1) and b2), full reference is made to the statements made above.

[0072] The finishing of the fiber materials by coating and / or impregnation is carried out according to conventional application methods known to those skilled in the art. Preferably, a method is used for coating and / or impregnating the fiber materials which is selected from fouling, doctoring, spraying, spattering and combinations thereof.

[0073] In the foulard process, the fiber material is passed through a foulard (immersion basin) containing the additive-containing solution or dispersion and then squeezed to the desired amount of additive via a pressure- and, if necessary, gap-adjustable pair of rollers.

[0074] In gravure printing, a distinction is made between intaglio and screen printing. In intaglio printing, a doctor blade, for example, a knife-like ground steel strip with or without a support blade, is used. It serves to scrape off the excess additive-containing solution or dispersion from the ribs of the printing cylinder (squeegeeing). In screen printing, on the other hand, the doctor blade is usually made of rubber or plastic with a sharp or rounded edge.

[0075] In spray application, the additive-containing solution or dispersion is applied to the fiber material to be treated using a slot nozzle.

[0076] The kiss-roll coating process is used to coat the underside of horizontally running webs. The coating medium can be applied to the web in either the opposite or opposite direction. Indirect coating with small application quantities can be achieved using transfer rollers.

[0077] In a particular embodiment, the foulard method is used to equip the cleaning articles according to the invention.

[0078] In step iii) of the process according to the invention, the coated and / or impregnated fiber material is subjected to drying and / or sintering. Suitable methods for drying nonwovens or fabrics coated and / or impregnated with additive-containing solutions or dispersions are known in principle. After application, for example, at least a portion of the solvent, especially the water, can be extracted from the fiber material by passing it, for example, over an extraction port from which the liquid is removed by means of an applied vacuum. Alternatively or additionally, the fiber material can be dried at an elevated temperature. Additionally, drying can be carried out at a reduced pressure.

[0079] Preferably, the fiber material is dried at a temperature in the range of 20 to 250 °C, particularly preferably 40 to 200 °C.

[0080] In addition to or as an alternative to drying, the coated and / or impregnated fiber material can be subjected to sintering in step iii). Sintering is preferably carried out by thermal treatment at a temperature of at least 250 °C, preferably at least 300 °C, particularly in the range of 300 to 450 °C, especially from 350 to 450 °C.

[0081] Finally, in step iv), the finished and dried and / or sintered fiber material can be coated with a microporous layer. Regarding suitable and preferred designs of the microporous layer, reference is made in full to the previously stated descriptions. Fuel cell

[0082] Another object of the invention is a fuel cell comprising at least one gas diffusion layer as defined above, or obtainable by a method as defined above.

[0083] In principle, the gas diffusion layer according to the invention is suitable for all common fuel cell types, especially low-temperature proton exchange membrane fuel cells (PEMFCs). Reference is made in full to the previously given explanations regarding the construction of fuel cells.

[0084] An advantage of the invention is that the gas diffusion position can be specifically adapted to the structural conditions of the fuel cell, the operating media flowing through it, and / or the operating parameters of the fuel cell.

[0085] The invention is explained using the following examples, which are not to be understood as limiting. FIGURE DESCRIPTION

[0086] Figure 1 Figure 1 shows the bending stiffness and the maximum deflection until breakage or slippage of the test specimens from the support points for three GDLs according to the invention and for 7 comparison GDLs, as specified in more detail below. Figure 2The figure shows the bending stiffness and the maximum bending angles for the three GDLs according to the invention and for the 7 comparison GDLs. Figure 3 Figure 1 shows the bending modulus and the maximum deflection until breakage or slippage of the test specimens from the support points for the three GDLs according to the invention and for the 7 comparison GDLs. Figure 4 The figure shows the bending modulus and the maximum deflection until breakage or slippage of the test specimens from the support points for three GDLs according to the invention and for 7 comparison GDLs. EXAMPLES

[0087] The bending properties were determined according to DIN EN ISO 178:2019-08. As described below, planar electrically conductive materials were produced according to the invention and for comparison. Some of the materials were additionally provided with a microporous layer (MPL). Test specimens with a width of 50 mm and a length of 100 mm were taken from these materials.

[0088] The bending stress was applied in a three-point bending test with a support span of 32 mm and a test speed of initially 2 mm / min and later 50 mm / min until failure occurred or the specimen slipped off the supports. The bending stiffness [N / mm] and the bending modulus E [N / mm²< ] were determined from the measurement of the mid-deflection.

[0089] The maximum bending angle α was calculated from the maximum deflection and half the distance between the two support points: α = arctan max . Durchbiegung / halbeStützweite I) Production of gas diffusion layers Production example 1

[0090] To produce a planar electrically conductive material, a nonwoven fabric made of 100% carbon fibers with a basis weight of 63 g / m² was used. For finishing the nonwoven, an impregnation composition was mixed, containing 60% carbon black, 22% PTFE, and 18% PEEK (based on the solids content). Finishing was carried out by foulard impregnation with an aqueous dispersion at 15% finishing weight based on the mass of the GDL substrate (corresponding to 9.5 g / m²). This was followed by drying for 5 minutes at 160 °C and sintering for 10 minutes at 400 °C. An MPL was then applied to the resulting stiffened substrate to produce the materials 2) and 3) according to the invention, prior to determining the application-related properties. Production example 2

[0091] To produce a sheet-like, electrically conductive material, a nonwoven fabric made of 100% carbon fibers with a basis weight of 40 g / m² was used. For finishing the nonwoven, an impregnation composition was mixed, containing, based on the solids content, 40% carbon black, 20% PTFE, and 40% PPS (Fortron® < 0205B4 from Celanese). Finishing was carried out by foulard impregnation with an aqueous dispersion at 15% finishing weight based on the mass of the GDL substrate (corresponding to 6.0 g / m²). This was followed by drying for 5 minutes at 150 °C and sintering for 10 minutes at 400 °C. II) Measurement of bending properties:

[0092] The following materials were used to determine the application-related properties: 1) GDL according to the invention from production example 1 without MPL. 2) GDL according to the invention analogous to production example 1 with MPL. For the MPL coating, an MPL paste containing 2 wt% FEP (tetrafluoroethylene hexafluoropropylene copolymer) and 7.8 wt% carbon in distilled water was applied to the fiber material. The fiber material was then dried at 160 °C and sintered at 400 °C. The resulting MPL loading was 21 g / m². 3) GDL according to the invention from production example 1 with MPL. For the MPL coating, an MPL paste containing 2.7 wt% PTFE and 10.8 wt% carbon in distilled water was applied to the fiber material. The fiber material was then dried at 160 °C and sintered at 400 °C. The resulting MPL loading was 15 g / m².V4) Comparison: Nonwoven fabric made of 100% carbon fibers with a basis weight of 63 g / m²< according to manufacturing example 1 without impregnation. V5) Comparison: Nonwoven fabric made of 100% carbon fibers with a basis weight of 63 g / m²< according to manufacturing example 1 with PTFE / carbon black impregnation without polymer stiffening. V6) Comparison: Carbon fiber paper GDL with MPL coating, total thickness 120 µm, basis weight 103 g / m²< CeTech GDL 120 (CeTech Co. Ltd., Taiwan). V7) Comparison: Carbon fiber paper GDL without PTFE impregnation and without MPL coating, not a rollable product but sheet material, total thickness 120 µm, Toray TGP-H030 (Toray Industries Inc.)., Japan) V8) Comparison: Carbon fiber paper GDL with MPL coating and PTFE impregnation, not a rollable product but sheet material, total thickness 220 µm, basis weight of 91 g / m² < SGL 29BC (SGL Carbon SE, Germany) V9) Comparison: Carbon fiber paper GDL with MPL coating and PTFE impregnation, not a rollable product but sheet material, total thickness 215 µm, basis weight: 74 g / m² < SGL 22BB (SGL Carbon SE, Germany) V10) Comparison: Carbon fiber paper GDL with MPL coating and PTFE impregnation, not a rollable product but sheet material, total thickness 190 µm, basis weight: 56 g / m² < AVCarb MB030 (AvCarb Material Solutions, USA) .

[0093] The results of the application-related measurements are in the Figures 1 to 4It is shown that the gas diffusion layers equipped according to the invention exhibit an optimal combination of properties, namely high flexural stiffness and high flexibility. Thus, the maximum deflection in the gas diffusion layers according to the invention does not result from material breakage but from the ends sliding off the support points. They exhibit at least comparable and often higher flexural stiffnesses and flexural moduli than carbon fiber papers known from the prior art. Nonwovens without any equipment or gas diffusion layers without additional polymer stiffening, as demonstrated by polyetherketones and polyphenylene sulfides, exhibit low flexural stiffnesses. In particular, the absence of brittle fracture behavior and the significantly larger maximum bending angle constitute a particularly advantageous combination of properties.

Claims

1. Gas-diffusion layer for a fuel cell, said layer comprising A) a planar, electrically conductive material comprising a) at least one fibre material selected from carbon fibre nonwovens, carbon fibre fabrics and blends thereof, wherein the fibre material comprises b1) at least one fluorine-containing polymer, and b2) at least one polymer different from b1), selected from polyaryl ether ketones, polyphenylene sulfides, polysulfones, polyether sulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and blends thereof, applied thereto and / or incorporated therein, and B) optionally a microporous layer on one of the surfaces of the electrically conductive material A).

2. Gas-diffusion layer according to Claim 1, wherein the planar, electrically conductive material A) is obtainable by coating and / or impregnating at least one fibre material a), selected from carbon fibre nonwovens, carbon fibre fabrics and blends thereof, with an aqueous composition comprising at least one fluorine-containing polymer b1) and at least one polymer b2) different from b1) that is selected from polyaryl ether ketones, polyphenylene sulfides, polysulfones, polyether sulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and blends thereof, and subsequent drying and / or sintering of the treated fibre material, wherein the sintering is carried out by thermal treatment at a temperature within a range from 300 to 450°C.

3. Gas-diffusion layer according to Claim 1 or 2, wherein the fibre material a) comprises - at least one fluorine-containing polymer b1), - at least one polymer b2) different from b1), selected from polyaryl ether ketones, polyphenylene sulfides, polysulfones, polyether sulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and blends thereof, - optionally at least one conductivity-improving additive b3), - optionally at least one further additive b4) that is selected from - polymeric binders b41) different from b1) and b2), - surface-active substances b42), - further additives and auxiliaries b43), applied thereto and / or incorporated therein.

4. Gas-diffusion layer according to any of the preceding claims, wherein the fluorine-containing polymer b1) is selected from polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, perfluoroalkoxy polymers and blends thereof.

5. Gas-diffusion layer according to any of the preceding claims, wherein the polymer b2) is selected from polyaryl ether ketones, polyphenylene sulfides, polysulfones, polyether sulfones and blends thereof.

6. Gas-diffusion layer according to any of the preceding claims, wherein the fibre material a) additionally comprises b3) at least one conductivity-improving additive selected from metal particles, carbon black, graphite, graphene, carbon nanotubes (CNT), carbon nanofibres and mixtures thereof, applied thereto and / or incorporated therein.

7. Gas-diffusion layer according to Claim 6, wherein the fibre material a), based on the total weight of components b1), b2) and b3), comprises - 10% to 50% by weight of at least one fluorine-containing polymer b1), - 5% to 40% by weight of at least one polymer b2) different from b1), and - 20% to 80% by weight of at least one conductivity-improving additive b3), applied thereto and / or incorporated therein.

8. Gas-diffusion layer according to Claim 6 or 7, wherein the total weight of components b1), b2) and b3) is 3% to 50% by weight, preferably 5% to 35% by weight, based on the total weight of the fibre material a).

9. Gas-diffusion layer according to any of the preceding claims, wherein the fibre material a) comprises further polymers b41) different from the fluorine-containing polymers b1) and the polymers b2) in a proportion by weight of not more than 5%, preferably not more than 1%, more preferably not more than 0.5%, in particular not more than 0.1%, based on the total weight of the fibre material a), these being applied thereto and / or incorporated therein.

10. Gas-diffusion layer according to any of the preceding claims, which comprises a sintering product of A) obtained by thermal treatment at a temperature within a range from 300 to 450°C.

11. Process for producing a gas-diffusion layer for a fuel cell, said layer comprising a planar, electrically conductive material A) in which i) at least one fibre material selected from carbon fibre nonwovens, carbon fibre fabrics and blends thereof is provided, ii) the fibre material provided in step i) is coated and / or impregnated with a composition that comprises at least one fluorine-containing polymer b1) and at least one polymer b2) different from b1), selected from polyaryl ether ketones, polyphenylene sulfides, polysulfones, polyether sulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and blends thereof, (iii) the coated and / or impregnated fibre material is subjected to drying and / or sintering, wherein the sintering is carried out by thermal treatment at a temperature within a range from 300 to 450°C, and iv) optionally the thermally treated fibre material is coated with a microporous layer.

12. Fuel cell comprising at least one gas-diffusion layer as defined in any of Claims 1 to 10 or obtainable by a process as defined in Claim 11.

13. Use of a planar, electrically conductive material A) comprising a) at least one fibre material selected from carbon fibre nonwovens, carbon fibre fabrics and blends thereof, wherein the fibre material comprises b1) at least one fluorine-containing polymer, and b2) at least one polymer different from b1), selected from polyetherketones, polyphenylene sulfides, polysulfones, polyethersulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and blends thereof, applied thereto and / or incorporated therein, as or in a gas-diffusion layer for a fuel cell.

14. Use according to Claim 13, wherein the planar, electrically conductive material A) has at least one of the following properties: - a bending stiffness, determined by centre loading in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen 50 mm wide, 0.13 mm thick and 100 mm in length with a span of 32 mm and a test speed of 2 mm / min, of 0.05 to 0.08 N / mm, - a maximum bending angle α, determined by centre loading in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen 50 mm wide, 0.13 mm thick and 100 mm in length with a span of 32 mm and a test speed of 2 mm / min, of 50 to 60°, - a flexural modulus, determined by centre loading in a 3-point bending test according to DIN EN ISO 178:2019-08 on a rectangular test specimen 50 mm wide, 0.13 mm thick and 100 mm in length with a span of 32 mm and a test speed of 2 mm / min, of 3000 to 6000 N / mm2.