Gas diffusion layer with improved fibre bonding and method for producing same

Abstract

The present invention relates to a method for producing a gas diffusion layer for a fuel cell on the basis of a flat, electrically conductive fibre material with good bonding of the fibres in the fibre material. The invention furthermore relates to the gas diffusion layers which can be obtained according to this method, to a fuel cell which contains such a gas diffusion layer, and to a fuel cell stack which contains at least two such fuel cells.

Description

[0001] Gas diffusion layer with improved fiber integration and process for its production

[0002] Description

[0003] The present invention relates to a method for producing a gas diffusion layer for a fuel cell based on a flat, electrically conductive fiber material with good fiber integration into the fiber material. The invention further relates to the gas diffusion layers obtainable by this method, a fuel cell containing such a gas diffusion layer, and a fuel cell stack containing at least two such fuel cells.

[0004] BACKGROUND OF THE INVENTION

[0005] Fuel cells use the chemical reaction of a fuel, particularly hydrogen, with oxygen to produce water to generate electrical energy. In hydrogen-oxygen fuel cells, hydrogen or a hydrogen-containing gas mixture is fed to the anode, where electrochemical oxidation takes place with the release of electrons (H2 2 H + + 2 e _ ). The protons are transported from the anode compartment to the cathode compartment via a membrane that separates the reaction chambers from each other in a gas-tight manner and is electrically insulated. The electrons provided at the anode are conducted to the cathode via an external conductor circuit. Oxygen or an oxygen-containing gas mixture is supplied to the cathode, whereby a reduction of the oxygen takes place with the absorption of electrons. The oxygen anions formed react with the protons transported across the membrane to form water (1 / 2 O2 + 2 H + + 2

[0006] Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells (PEMFCs), are used for many applications, especially in automotive powertrains. A special type is water-oxygen fuel cells in the form of low-temperature proton exchange membrane fuel cells (LT-PEMFCs). The core of proton exchange membrane fuel cells is a polymer electrolyte membrane (PEM), which is permeable only to protons (or oxonium ions HsO) and water and spatially separates the oxidizing agent, generally atmospheric oxygen, from the reducing agent.A catalyst layer is applied to the anode and cathode sides of the gas-tight, electrically insulating, proton-conducting membrane, forming the electrodes. This layer typically contains platinum as the catalytically active metal. The actual redox reactions and charge separation take place in 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), which 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 units, which have channels for supplying the adjacent cathode and anode with process gases and usually also internal cooling channels.

[0007] Gas diffusion layers for fuel cells typically consist of a carbon fiber substrate that is hydrophobically treated with fluoropolymers (e.g., PTFE) and subsequently coated with a microporous layer (MPL). The MPL typically consists of a fluorine-containing polymer as a binder (e.g., PTFE) and an electrically conductive material, often consisting of carbon materials such as carbon black or graphite powder. The gas diffusion layers are crucial for the function and performance of the fuel cell. They transport the process components consumed and generated in the electrode reactions, and they also conduct the electrons generated and consumed in the half-cell reactions, as well as the heat generated during the reaction, to the flow distribution plates.In addition, the GDL also acts as a mechanical balance between the macrostructured flow distribution plate and the catalyst layers. To achieve this, component tolerances must be compensated and the compression pressure distributed. The GDL also serves as mechanical protection for the very thin membranes, which are subject to high loads in fuel cells. The sensitive membranes should be protected from damage by the gas diffusion layer and its components. Therefore, high demands are placed on the mechanical properties and surface properties of the GDL.

[0008] A major problem with fiber-based gas diffusion layers is the potential damage to the fuel cell membrane caused by loose fibers and fibers that are not embedded in the flat fiber substrate but in the MPL (i.e., the layer facing the membrane). Fibers that protrude from the MPL surface are particularly problematic. This includes fibers that protrude from the MPL surface even before the GDL is installed in the fuel cell stack and fibers that protrude from the MPL surface when the GDL is subjected to compressive stress, as occurs during fuel cell stack manufacturing. The fibers of the gas diffusion layers are typically very stiff and brittle, and the fiber thickness is often in the range of the thickness of the fuel cell membrane. Therefore, there is a risk that the membrane will be penetrated by fibers, causing a short circuit.In the worst case, a short circuit caused by fiber penetration of the membrane can lead to the failure of the entire fuel cell stack. This poses a very significant risk to the service life of the fuel cells. The membrane of a fuel cell is very thin, usually only a few microns thick. Typical thicknesses range from 8 to 50 microns, although membranes with thicknesses of 5 microns are already being tested in some cases. It is to be expected that with the increasing use of fuel cells in automotive applications, there will be a need for the thickness of all flat components (membranes, GDL / MPL, etc.) to continue to decrease. Thus, the performance problem caused by internal short circuits, which can be caused by protruding fibers, other raised structures or general pressure points of the GDLs resting on the membrane, will likely increase in the future.Avoiding such critical structures and / or smoothing the MPL surfaces of the GDL are therefore very important goals in the development of optimized gas diffusion layers.

[0009] To date, automated inspection systems have performed a full-surface optical inspection and follow-up check of the GDL surface to detect protruding fibers and other defects that could lead to membrane damage. Areas identified as defective are then removed during cutting. The disadvantage of this method is that detection accuracy is highly dependent on the incidence of light and the camera angle relative to the surface being inspected. There is a likelihood that a certain number of critical areas will not be captured and detected. The mechanical removal of critical areas of the GDL by cutting also leads to significant material loss and reduces yield. Cutting can also be a cause of defects, as improper cutting or the use of blunt punching tools can lead to frayed edges.

[0010] The development of fuel cells suitable for everyday use is an important contribution to the energy transition from fossil fuels to sustainable energy storage and energy converters. Therefore, there is currently a great need for PEM fuel cells whose failure probability due to membrane damage, and especially short circuits caused by loose fibers embedded in or protruding from the MPL, is substantially reduced. JP 2007242378 A (JP 5041459 B2) describes a gas diffusion layer consisting of porous sintered carbon particles and water-repellent particles. To produce this layer, carbon particles and water-repellent particles are dispersed in water in the presence of a non-ionic surfactant, concentrated under phase inversion, and sintered. This sintered film is peeled off, pulverized again, and the resulting sintered coarse particles are hot-pressed in a mold to form a GDL.This eliminates the need for a fiber-based substrate in the final GDL, thus eliminating the previously described problems caused by loose fibers located in the membrane-proximal layer. However, such an approach is likely to lead to disadvantages in further processing, cutting and manufacturing of the cell stacks, as well as in fuel cell stability.

[0011] US 2019 / 0344405 A1 (US 11141839 B2) describes an adhesive device for bonding a gas diffusion layer within a fuel cell. This device features a suction device and is intended to bind or remove fluffy or loose fibers from a gas diffusion layer. The use of an additional device increases manufacturing costs. Furthermore, it is questionable whether the use of this device solves the problem of internal short circuits caused by protruding fibers from the GDLs resting on the membrane.

[0012] EP 3276718 A1 describes a porous carbon electrode substrate that is said to be extremely low in short circuits when used in a fuel cell. This prevents the occurrence of carbon fibers that protrude from the substrate surface or are caused to protrude when the carbon electrode substrate is subjected to pressure, as well as short carbon fibers that are insufficiently bonded to the substrate surface. Short carbon fibers and a binder resin containing at least 35 wt.% carbon and carbonizing upon heating are used for production. The resulting GDL substrate is thus based on a fully resin-impregnated fiber material.WO 2020 / 165075 A1 (EP 3924536 A1 ) describes a process for producing a gas diffusion layer, which comprises the following steps: a) preparing a carrier-binder paste containing a solvent, a fluorinated binder and conductive carrier particles, b) preparing an adhesive composition comprising a solvent, a fluorinated binder and essentially no or at most 15 wt.-% conductive carrier particles, based on the total weight of the fluorinated binder and all conductive carrier particles; and c) combining a layer of the carrier material, a layer of the adhesive composition and a layer of the carrier-binder paste, wherein the layer of the adhesive composition is applied between the layer of the carrier material and the layer of the carrier-binder paste, and pressing the combination of carrier material, adhesive composition and carrier-binder paste at a pressure of at least 15 kilopascals (0.15 bar) and / or heating the combination of carrier material, adhesive composition and carrier-binder paste to a temperature of at least 300°C.

[0013] The objective of this document is to provide mechanically stable gas diffusion electrodes in which the carrier-binder layer, preferably in the form of a microporous layer, is firmly bonded to the carrier material. This is achieved by the additional adhesive layer, which is free of electrically conductive particles or contains only a small amount of them. It is critical for the process that all three layers—carrier material, adhesive layer, and microporous layer—are pressed together at high pressure and a temperature of at least 300°C, using long treatment times of at least 15 minutes and preferably 1 to 4 hours.It is not described to first apply a first gas- and water-permeable polymer layer to the carrier material, then subject it to post-treatment at elevated pressure and optionally elevated temperature, and then coat the polymer layer of the resulting material with a microporous layer. EP 3396753 A1 describes a gas diffusion electrode that is said to be less susceptible to the occurrence of short-circuit currents when used in a fuel cell. The GDL substrate comprises short carbon fibers bonded with a carbon resin. The gas diffusion electrode has a multilayer structure with preferably at least two microporous layers that differ in their layer fill rate, and the microporous layer(s) must have sufficient thicknesses under compressive load.To reduce the probability of short circuits, a variety of measures have been described, such as pressure treatment of the precursor substrate before carbonizing the binder resin and increasing the temperature during the carbonization step. Only in cases where a further reduction in the short-circuit current density is desired is post-treatment by calendering, followed by blowing with air and extracting the air, described. A disadvantage of this process is that a resin binder in the fiber substrate is often undesirable.

[0014] EP 3957789 A1 describes a gas diffusion layer for a PEM fuel cell, comprising a carbon fiber nonwoven fabric containing fibers with an average fiber diameter of 5 to 20 μm. At least some of the carbon fibers have a flat region. In these flat regions, the maximum fiber diameter is 10 to 50% larger than the average fiber diameter in a planar view of the surface of the carbon fiber nonwoven fabric. Thus, the GDL has an enlarged contact area with the bipolar plates and improved thermal conductivity.

[0015] DE 10 2020 121 892 A1 describes a gas diffusion layer for fuel cells with improved flexural properties. The gas diffusion layer comprises a carbon fiber material and, applied thereto and / or incorporated therein, at least one fluorine-containing polymer and at least one polymer different therefrom, selected from polyether ketones, polyphenylene sulfides, polysulfones, polyether sulfones, partially aromatic (co)polyamides, polyimides, polyamide-imides, polyetherimides, and mixtures thereof.

[0016] KR 10 2022 0153522 A describes a gas diffusion layer for fuel cells based on recycled carbon fibers. To impart hydrophobic properties, the carbon fiber substrate is impregnated under pressure with an aqueous dispersion of a fluoropolymer. The application of a separate polymer layer to the fiber substrate is not described.

[0017] CN 112310413 A describes a gas diffusion layer and a method for its production. The GDL comprises a support layer and, superimposed thereon, a diffusion layer made of a carbon composite material (carbon material diffusion layer) and a microporous layer. The support layer is porous and is preferably selected from layers of carbon nanotubes (CNTs), graphene layers, and carbon fiber nonwovens. Specifically, carbon fibers are used as the framework material and carbon nanotubes as the filler to produce the support layer. This document does not describe the use of carbon fibers in the support layer; polyacrylonitrile fibers are used as the starting material for their production.

[0018] According to the teaching of CN 112310413 A, the diffusion layer must contain carbon nanotubes and carbon fibers. To produce the diffusion layer, the carrier layer can be coated with a composition containing a carbon fiber dispersion, a carbon nanotube dispersion, a binder, and a hydrophobic agent, which can be a fluorine-containing polymer such as PTFE. After application, the diffusion layer can be subjected to a post-treatment, which includes, among other things, treatment at elevated pressure and drying.

[0019] The invention is based on the object of providing a gas diffusion layer based on a flat, electrically conductive fiber material and a microporous layer, and a method for its production, wherein the proportion of fibers that can lead to membrane damage when the gas diffusion layer is used in a PEM fuel cell is significantly reduced compared to conventional gas diffusion layers. In particular, the overall proportion of loose fibers that are not sufficiently bonded to the fiber material, fibers that protrude from the MPL surface, and fibers that are embedded in the MPL but are caused to protrude when the gas diffusion layer is subjected to compressive stress should be significantly reduced. Thus, when the gas diffusion layer is used in a PEM fuel cell, the frequency of short circuits caused by fiber penetration of the membrane should be significantly reduced.

[0020] Surprisingly, it has now been found that this object is achieved if, in order to produce a gas diffusion layer for a fuel cell, a flat electrically conductive fiber material A) is first coated with a gas- and water-permeable polymer layer, the coated fiber material thus obtained is subjected to a post-treatment at elevated pressure and optionally elevated temperature, and then a microporous layer is applied to the polymer layer of the material thus obtained.

[0021] SUMMARY OF THE INVENTION

[0022] A first subject of the invention is a method for producing a gas diffusion layer for a fuel cell, comprising

[0023] A) a flat electrically conductive fiber material,

[0024] B) a gas and water permeable polymer layer on at least one of the surfaces of the fiber material,

[0025] C) a microporous layer on at least one of the gas- and water-permeable polymer layers, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder, in which i) a flat electrically conductive fiber material A) is provided, ii) the fiber material provided in step i) is coated with a coating agent to form a gas- and water-permeable polymer layer, iii) the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and optionally elevated temperature, and iv) the polymer layer of the material obtained in step iii) is coated with a precursor to form a microporous layer.

[0026] Another object of the invention is a gas diffusion layer for a fuel cell, which is obtainable by a process as defined above and below.

[0027] Another object of the invention is a gas diffusion layer for a fuel cell, which

[0028] A) a flat electrically conductive fiber material,

[0029] B) a gas and water permeable polymer layer on at least one of the surfaces of the fiber material,

[0030] C) a microporous layer on at least one of the gas and water permeable polymer layers, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder.

[0031] The invention further provides a fuel cell comprising at least one gas diffusion layer as defined above and below, or comprising at least one gas diffusion layer obtainable by a process as defined above and below. The invention further provides the use of a gas diffusion layer as defined above and below, or obtainable by a process as defined above and below, in a proton exchange membrane fuel cell.A specific embodiment is a proton exchange membrane fuel cell comprising a proton exchange membrane coated on both sides with a catalytically active electrode, on each side of the catalyst-coated membrane a gas diffusion layer whose inner side is in contact with the catalytically active electrode, on each side of the catalyst-coated membrane a flow distributor plate having channels for supplying reactant gases and removing products of the cell reaction, wherein the channels are in contact with the outer side of the gas diffusion layer.

[0032] Another object of the invention is a fuel cell stack comprising a plurality of fuel cells as defined above and below.

[0033] Another object of the invention is the use of a gas diffusion layer comprising

[0034] A) a flat electrically conductive fiber material,

[0035] B) a gas and water permeable polymer layer on at least one of the surfaces of the fiber material,

[0036] C) a microporous layer on at least one of the gas- and water-permeable polymer layers, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder, in a fuel cell for reducing the probability of failure during operation, specifically for reducing the probability of puncture of the fuel cell membrane, more specifically for reducing the shorting number.

[0037] DESCRIPTION OF THE INVENTION

[0038] The gas diffusion layers according to the invention and obtained by the process according to the invention have the following advantages:

[0039] The resulting gas diffusion layers have very good surface properties.

[0040] The fiber materials used to produce gas diffusion layers, especially nonwovens, can have rougher or finer surfaces depending on the production process. Rough fiber materials have a higher number of fibers not embedded in the fiber material than fine ones. Fiber materials also exist that have a rough side and a fine (less rough) side. The method according to the invention is suitable for improving the surface properties of both fiber materials with rough and fine surfaces, as well as one or both sides of fiber materials with a fine and a rough side.

[0041] In the gas diffusion layer according to the invention, the proportion of fibers that could lead to membrane damage when used in a PEM fuel cell is significantly reduced compared to conventional gas diffusion layers (without additional polymer coating and without post-treatment at elevated pressure, e.g., by pressing or calendering). In particular, the total proportion of loose fibers that are not sufficiently bonded to the fiber material, fibers that protrude from the MPL surface, and fibers that are embedded in the MPL but are caused to protrude under pressure are significantly reduced. When the gas diffusion layer is used in a PEM fuel cell, the frequency of short circuits caused by fiber penetration of the membrane is significantly reduced.

[0042] For the purposes of the invention, a nonwoven generally refers to a sheet-like structure consisting primarily of individual fibers whose cohesion is essentially achieved solely by their inherent adhesion. The conversion of a nonwoven into a nonwoven fabric by creating a stronger bond between the fibers than that present in the nonwoven is achieved by nonwoven bonding processes, which are usually divided into mechanical, chemical, and thermal processes. Nonwovens, nonwoven fabrics, and processes for their production are described in H. Fuchs and W. Albrecht, Vliesstoffe (Vlieses), 2nd edition, Wiley-VCH, Weinheim, Germany.

[0043] Gas diffusion layers are also referred to as GDL and microporous layers as MPL.

[0044] An orthogonal coordinate system can be used to describe the flat fiber material A), the intermediate products produced therefrom and the gas diffusion layer according to the invention, whereby the base area of ​​the fiber material lies in the plane spanned by the x-axis and the y-axis (also referred to as the x,y plane). The orthogonal z-axis serves to describe the thickness of the material or individual layers. In accordance with the usual description for fiber composite materials, the x-axis is also referred to as the machine direction (MD) and the y-axis as the cross-machine direction (CMD or CD). The mass transport between the flow distributor plate (bipolar plate) and the membrane essentially takes place in the z-axis direction.

[0045] The thickness of the conductive flat fiber material and the gas diffusion layer can be determined according to DIN 53855-1:1993-08 "Determination of the thickness of textile fabrics".

[0046] The thickness of the gas- and water-permeable polymer layer B) can also be determined by measuring a cross-section of the GDL along the z-axis (i.e. measurement perpendicular to the surface of the GDL or in a plan view onto the x,z-plane or the y,z-plane). A scanning electron microscope (SEM), for example, can be used for this purpose. In a special version, this measurement is carried out at a magnification of 200x and an accelerating voltage of 15.0 kV. If the polymer layer B) is located partly inside and partly outside the fiber material A), the proportion of the polymer layer B) that is located inside the fiber material A) is also taken into account when determining the thickness of the polymer layer B). In general, the polymers used to produce layer B) accumulate around the fibers inside the fiber material A).The penetration depth can be well controlled by the amount of polymer applied, the viscosity of the polymer dispersion, and the pressure and temperature conditions of the post-treatment in step iii). Thus, the boundaries of the polymer layer B) are generally easy to detect, so that by performing multiple measurements at different locations and averaging, the thickness of the polymer layer B) can be determined with good accuracy.

[0047] The determination of the mass per unit area in g / m 2 can be carried out according to ISO 9073-1 or EN 29073-1:1992.

[0048] The porosity of the GDL can be determined using mercury porosimetry, as described in DIN ISO 15901-1:2019-03 and ISO 15901-1:2016- Part 1: Mercury porosimetry.

[0049] The roughness is determined using the stylus method as described in DIN 4768-1:1974-08 entitled "Determination of roughness measurements R a, Rz, Rmax with electrical stylus instruments; Basics". The mean roughness value R a the mean distance of a measuring point on the surface to the center line and R z refers to the average roughness depth. The measurements can be performed, for example, with a digital microscope suitable for determining roughness, such as the Keyence VHX-7000. The values ​​are each an average of six measurements: three in the machine direction (MD) and three perpendicular to the machine direction (CD). Step i) Providing a flat, conductive fiber material A)

[0050] In step i) of the process according to the invention, a flat electrically conductive fiber material A) is provided.

[0051] The sheet-like electrically conductive fiber material A) used according to the invention, the intermediate products produced therefrom, and the gas diffusion layer according to the invention are sheet-like structures that have a substantially two-dimensional, planar extension and a comparatively smaller thickness. They have a base area that generally essentially corresponds to the base area of ​​the adjacent membrane with the catalyst layers and the base area of ​​the adjacent flow distributor plate. The shape of the base area of ​​the fiber material A) and the gas diffusion layer can be, for example, polygonal (n-sided with n > 3, e.g., triangular, square, pentagonal, hexagonal, etc.), circular, circular-segment-shaped (e.g., semicircular), elliptical, or elliptical-segment-shaped. The base area is preferably rectangular or circular.

[0052] Component A) preferably comprises a fiber material selected from nonwovens, papers, woven fabrics, and combinations thereof. Suitable substrate materials are fiber materials that are themselves conductive or are made conductive by the addition of conductive additives, such as carbon or metal particles. Suitable substrate materials include, in principle, carbon fibers, glass fibers, fibers of organic polymers such as polypropylene, polyester, polyphenylene sulfide, polyether ketones, and mixtures thereof. The fibers contained in fiber material A) preferably comprise or consist of carbon fibers (carbon fibers). Such fiber materials particularly advantageously fulfill the GDL requirements for gas diffusivity, liquid-water permeability, electrical and thermal conductivity.

[0053] Carbon fibers can be produced in a conventional 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% by weight, based on the total weight of the monomers used for polymerization, of acrylonitrile. The resulting polymer solution is spun into filaments, e.g., by wet spinning and coagulation, and then gathered into tows. Before this PAN precursor is converted into carbon fibers at high temperatures, it is generally subjected to oxidative cyclization (also referred to as oxidation for short) in an oxygen-containing atmosphere at elevated temperatures of approximately 180 to 300°C. The resulting chemical crosslinking improves the dimensional stability of the fibers.The actual pyrolysis to produce carbon fibers then takes place at temperatures of at least 1200 °C. Depending on the desired fiber shape, either the starting fibers or a flat 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 a 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, produces so-called high-modulus or graphite fibers. These fibers are highly pure, lightweight, high-strength, and highly conductive to electricity and heat.

[0054] The fiber material A) is preferably selected from carbon fiber nonwovens, carbon fiber fabrics, carbon fiber papers, and combinations thereof. In a preferred embodiment, the fiber material A) comprises at least one carbon fiber nonwoven or the fiber material A) consists of a carbon fiber nonwoven. These are advantageous, among other things, because they are compression-elastic and can be easily produced on an industrial scale, e.g., in a roll-to-roll process.

[0055] In carbon fiber fabrics, the flat fiber material is produced by interlacing two thread systems: warp and weft. As with textiles, fiber bundles are flexibly but inextricably linked. Oxidized, but not yet carbonized or graphitized PAN fibers are preferably used to produce carbon fiber fabrics. Carbonization or graphitization, to impart electrical conductivity to the flat fiber material, occurs after weaving.

[0056] As described above, oxidized PAN fibers are generally used to produce carbon fiber paper. These are shredded into fiber fragments in a conventional manner, slurried, and, analogous to papermaking, a fiber mat is produced by separating the fibers from the vat using sieves and dried. In a preferred embodiment, at least one binder is additionally incorporated into the paper. Suitable binders include, for example, phenolic, furan, polyimide resins, etc. To incorporate the binder, the paper can be impregnated with it, and the binder can then be cured if necessary. After impregnation and curing, the carbon fiber paper is subjected to another carbonization / graphitization process to convert the binder into compounds with improved electrical conductivity. In another suitable embodiment, a filled carbon fiber paper is used to provide the fiber material A).The initial production process is as described above, but instead of introducing a binder and carbonization / graphitization, a filler consisting of a carbon material in a polymer binder is introduced into the still-moist paper. A carbon-PTFE filler is specifically used for this purpose. This filling increases the thermal and electrical conductivity to such an extent that carbonization / graphitization is no longer necessary.

[0057] Non-oxidized or oxidized PAN fibers can be used to produce carbon fiber nonwovens. These fibers can be dry-laid (carded) in a first step and then consolidated into a nonwoven. This can be achieved, for example, by hydroentangling, where the carbon fibers are oriented, entangled, and thus mechanically stabilized. If necessary, the thickness of the consolidated nonwoven can be calibrated to a desired value. Nonwovens based on non-oxidized PAN fibers are first subjected to oxidation at elevated temperature and in an oxygen atmosphere after the nonwoven laying and consolidation, followed by carbonization / graphitization in an inert gas atmosphere. Nonwovens based on oxidized PAN fibers are only subjected to carbonization / graphitization after the nonwoven laying and consolidation.Optionally, at least one binder can be incorporated into the nonwoven, which can then be cured if necessary. Suitable binders are those mentioned for carbon fiber paper, especially phenolic resins. The binder can be added, for example, after carbonization / graphitization, and the resulting impregnated nonwoven can then be carbonized / graphitized again.

[0058] If the flat, electrically conductive fiber material A) comprises at least one carbon fiber nonwoven fabric, it can have rougher or finer surfaces, or even a rough and a fine surface. Rough carbon fiber nonwoven fabrics have a higher number of fibers not embedded in the fiber material than fine ones. The method according to the invention is suitable for improving the surface properties of carbon fiber nonwoven fabrics with rough and / or fine surfaces.

[0059] In a specific embodiment, the fiber material A) is a fiber composite material. Specifically, the fiber composite material comprises at least one fiber material and, applied thereto and / or incorporated therein, a1) at least one polymeric additive, a2) optionally at least one conductivity-enhancing additive, and a3) optionally at least one further additive.

[0060] The polymeric additive a1) is preferably selected from fluorine-containing polymers a11), fluorine-free high-temperature-resistant polymers a12), polymers different therefrom a13) and mixtures thereof.

[0061] In order to improve the transport processes through the GDL and at the interfaces, it can be advantageous to increase the hydrophobicity of the fiber material A). Suitable polymeric additives a1), which act, for example, as binders and simultaneously increase the hydrophobicity, are fluorine-containing polymers a11). In principle, suitable as fluorine-containing polymers a11) are the fluorine-containing polymers b1) used in the polymer layer B) and / or the fluorine-containing polymers c1) used as polymeric binders of the microporous layer C). Preferably, the fiber material A) then contains at least one fluorine-containing polymer a11) applied thereto and / or incorporated therein. The fluorine-containing polymer a11) is preferably selected from polytetrafluoroethylenes (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), perfluoroalkoxy polymers (PFA) and mixtures thereof. Perfluoroalkoxy polymers are, for example,Copolymers of tetrafluoroethylene (TFE) and perfluoroalkoxyvinyl ethers, such as perfluorovinyl propyl ether. Polytetrafluoroethylene (PTFE) is preferably used as polymer a11). The fiber material can be impregnated with the fluorine-containing polymer a11 using conventional impregnation processes. For this purpose, for example, a PTFE dispersion can be applied in an immersion bath, the solvent evaporated, and the treated fiber material sintered at elevated temperatures, generally at least 300 °C.

[0062] The mass fraction of the polymeric additive a1) is preferably 0.5 to 50%, preferably 1 to 40%, based on the mass of the electrically conductive fiber material A). In a specific embodiment, the polymeric additive a1) comprises at least one fluorine-containing polymer a11). The mass fraction of the fluorine-containing polymer a11) is then preferably 0.5 to 40%, preferably 1 to 30%, based on the mass of the fiber material A).

[0063] The polymeric additive a1) may contain at least one fluorine-free, high-temperature-resistant polymer a12), such as can be used as component b2) in the polymer layer B) and / or as polymeric binder c2) of the microporous layer C). The fluorine-free, high-temperature-resistant polymer a12) is then preferably selected from polyaryletherketones, polyphenylene sulfides, polysulfones, polyethersulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides, and mixtures thereof.

[0064] In a specific embodiment, the polymeric additive a1) does not contain any fluorine-free, high-temperature-resistant polymer a12). In a further specific embodiment, the polymeric additive a1) contains at least one fluorine-free, high-temperature-resistant polymer a12). The mass fraction of the fluorine-free, high-temperature-resistant polymer a12) is then preferably 0.5 to 40%, preferably 1 to 30%, based on the mass of the fiber material A).

[0065] In a further specific embodiment, the binder a1) comprises a mixture of at least one fluorine-containing polymer a11) and at least one fluorine-free, high-temperature-resistant polymer a12). The total mass fraction of the polymers a11) and a12) is then preferably 0.5 to 40%, preferably 1 to 30%, based on the mass of the fiber material A).

[0066] The additive a1) can comprise at least one further polymer a13) different from a11) and a12). Suitable polymers a13) are, for example, selected from phenolic resins, furan resins, polyimide resins and mixtures thereof. Specifically, the polymeric additive a1) contains further polymers a13) different from the fluorine-containing polymers a11) and the polymers a12) in a weight fraction of at most 5%, preferably of at most 1%, particularly preferably of at most 0.5%, in particular of at most 0.1%, based on the total weight of the sheet-like electrically conductive fiber material A) applied thereto and / or incorporated therein. Even more specifically, the fiber material A) contains no additions of further polymers a13) that are different from the fluorine-containing polymers a11) and the polymers a12).

[0067] 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. To improve the electrical and thermal conductivity, however, the fiber material A) can additionally be treated with at least one conductivity-enhancing additive a2). Preferably, the conductivity-enhancing additive a2) is selected from metal particles, carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. A special embodiment is graphitic carbon nanofibers (GCNFs). The treatment of the fiber material A) with at least one conductivity-enhancing additive a2) can, for example, be carried out together with the polymeric additive a1) and / or other additives a3).

[0068] In a specific embodiment, the fiber material A) is free of added carbon nanotubes. In the context of the invention, “free of added carbon nanotubes” means that carbon nanotubes are not added to the fiber material A), specifically not applied thereto and / or incorporated therein. It is preferred that the fiber material A) is completely free of carbon nanotubes. Preferably, the fiber material A) contains carbon nanotubes in an amount in the range from 0 to 0.00005 wt.%, based on the total weight of the fiber material A), particularly preferably from 0 to 0.00001 wt.%, based on the total weight of the fiber material A), in particular 0 wt.%, based on the total weight of the fiber material A).

[0069] Preferably, the fiber material A) contains conductivity-enhancing additives in an amount of 0 to 40 wt.%, based on the total weight of the fiber material A). If the fiber material A) contains a conductivity-enhancing additive, then preferably in an amount of 0.1 to 40 wt.%, particularly preferably 0.5 to 30 wt.%, based on the mass of the fiber material A).

[0070] The fiber material A) may additionally contain at least one further additive a3). These include, for example, surface-active substances. The fiber material A) may be treated with at least one further additive a3) together with the polymeric additive a1) and / or conductivity-improving additives a2). The total mass fraction of further additives a3) is preferably 0 to 80%, preferably 0.1 to 50%, based on the mass of the fiber material A).

[0071] In a specific embodiment, the fiber material A), optionally equipped with components a1), a2), and / or a3), is subjected to a thermal treatment (drying and / or sintering). The thermal treatment of the fiber material A) is preferably carried out at a temperature of at least 250°C, preferably at least 300°C, in particular in a range from 300 to 450°C. A thermal treatment can also be carried out after the application of the polymer layer B) and / or the microporous layer C), as described in more detail below.

[0072] The fiber material A) preferably has a thickness in the range of 50 to 500 pm, particularly preferably 100 to 400 pm. This thickness refers to the uncompressed state of the fiber material A), ie, before the post-treatment in step iii) and before the incorporation of the GDL into a fuel cell.

[0073] The fiber material A) preferably has a porosity in the range of 10 to 90%, particularly preferably 20 to 85%. The porosity can be determined using mercury porosimetry according to DIN ISO 15901-1:2019-03 and ISO 15901-1:2016- Part 1.

[0074] The average pore diameter of the fiber material A) is preferably in a range from 5 to 60 pm, particularly preferably from 8 to 50 pm, in particular from 10 to 40 pm. The average pore diameter can be determined by mercury porosimetry.

[0075] Step ii) Coating with a gas and water permeable polymer layer

[0076] In step B) of the process according to the invention, a gas- and water-permeable polymer layer B) is applied to at least one of the surfaces of the fiber material A).

[0077] The polymer layer B) can be applied to one or both sides of the sheet-like electrically conductive fiber material A). In a specific embodiment, the polymer layer B) is applied to only one side of the sheet-like electrically conductive fiber material A). Preferably, the polymer layer B) is applied to only one side of the sheet-like electrically conductive fiber material A) in order to prepare it for installation on the membrane side of a fuel cell. In a specific embodiment, the polymer layer B) is applied to only one side of the sheet-like electrically conductive fiber material A), and this side is coated with a microporous layer C) (following post-treatment at elevated pressure and optionally elevated temperature in step iii)).

[0078] It was found that coating with a gas- and water-permeable polymer layer B) is suitable for improving the surface properties of both a fiber material with a fine surface and a fiber material with a rough surface. It was further found that additional post-treatment at elevated pressure and, if necessary, elevated temperature (= step iii)) significantly enhances this effect.

[0079] Preferably, the coating agent for forming a gas- and water-permeable polymer layer B) comprises at least one polymer selected from fluorine-containing polymers b1), fluorine-free, high-temperature-resistant polymers b2), polymers b3) different therefrom, and mixtures thereof.

[0080] Preferably, the fluorine-containing polymer b1) is selected from polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, perfluoroalkoxy polymers, and mixtures thereof. In a specific embodiment, the fluorine-containing polymer b1) comprises at least one tetrafluoroethylene-hexafluoropropylene copolymer (FEP). Specifically, the coating agent for forming the gas- and water-permeable polymer layer B) contains exclusively a tetrafluoroethylene-hexafluoropropylene copolymer (FEP) as the fluorine-containing polymer b1). Even more specifically, the coating agent for forming the polymer layer B) contains a tetrafluoroethylene-hexafluoropropylene copolymer (FEP) as the sole polymer component.

[0081] The polymer b2) is specifically selected from so-called high-performance plastics, which are characterized by properties such as a high glass transition temperature, a high melting temperature, good temperature resistance, good chemical resistance, and good mechanical properties. Preferably, the polymers b2) have a continuous operating temperature (continuous use temperature) of at least 150°C. In particular, the polymers b2) are thermoplastics.

[0082] Preferably, the fluorine-free, high-temperature-resistant polymer b2) is selected from polyaryletherketones, polyphenylene sulfides, polysulfones, polyethersulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and mixtures thereof.

[0083] Preferred polymers b2) are semi-aromatic and aromatic polymers.

[0084] The polymers b2) are preferably selected from polyaryletherketones (PAEK), polyphenylene sulfides (PPS), polysulfones (PSU), polyethersulfones (PES), partially aromatic (co)polyamides (high-temperature polyamides, HTPA), polyimides (PI), polyamideimides (PAI), polyetherimides (PEI) and mixtures (blends) thereof.

[0085] Suitable polymers b2) are also (semi-)aromatic polyesters, such as PET or PBT, polycarbonates (PC) and temperature-resistant melamines, such as melamine foams filled with nanoporous SiC aerogels.

[0086] In a preferred embodiment, polymer component b) comprises at least one polyaryl ether ketone. Specifically, polymer component b2) consists of at least one polyaryl ether ketone. Polyaryl ether ketones (PAEK) are semi-crystalline thermoplastics that have an alternating structure in which each aryl group is followed by a keto group (carbonyl group) or an ether group. The proportions of keto and ether groups are variable and can differ in the substitution pattern on the aryl rings. Suitable polyaryl ether ketones b2) are polyether ketones (PEK), polyether ether ketones (PEEK), polyether ketone ketones (PEKK), etc. Preferably, polymer component b2) comprises at least one polyether ether ketone or consists of at least one polyether ether ketone.

[0087] Suitable partially aromatic (co)polyamides b2) are the polymers known as high-temperature polyamides (HTPAs). These are semi-crystalline or amorphous, thermoplastic, partially aromatic polyamides. These preferably contain at least one aromatic dicarboxylic acid as polymerized units, in particular selected from terephthalic acid, isophthalic acid, and mixtures of terephthalic acid and isophthalic acid. Preferred partially aromatic (co)polyamides b2) are selected from PA 6.T, PA 10.T, PA 12.T, PA 6.I, PA 10.1, PA 12.1, PA 6.T / 6.I, PA 6.T / 6, PA 6.T / 10T, PA 10.T / 6.T, PA 6.T / 12.T, PA12.T / 6.T, and mixtures thereof. Another specific embodiment of the polyamides b2) is polyphthalamide (PPA).

[0088] Suitable polyimides b2) are polysuccinimide (PSI), polybismaleimide (PBMI), polyimidesulfone (PISO) and polymethacrylimide (PMI).

[0089] In a special embodiment, the coating agent for forming the gas- and water-permeable polymer layer B) comprises a mixture of at least one fluorine-containing polymer b1) and at least one fluorine-free, high-temperature-resistant polymer b2).

[0090] The polymer component b) may comprise at least one further polymer b3) different from b1) and b2). Suitable polymers b3) are selected, for example, from thermoplastics with a lower melting temperature and / or a lower permanent operating temperature (continuous use temperature) than the polymers b2). The polymers b3) are preferably selected from polyolefins, such as polyethylene, polypropylene, and copolymers and blends thereof. In a specific embodiment, the polymer b3) comprises or consists of at least one ultra-high molecular weight polyethylene (UHMWPE).

[0091] Specifically, polymer component b) contains further polymers b3) different from the fluorine-containing polymers b1) and the polymers b2) in a weight fraction of at most 5%, preferably of at most 1%, particularly preferably of at most 0.5%, in particular of at most 0.1%, based on the total weight of polymer component b). Even more specifically, polymer component b) contains no additions of further polymers b3) different from the fluorine-containing polymers b1) and the polymers b2). To improve the electrical and thermal conductivity, polymer layer B) can be provided with at least one conductivity-improving additive. Suitable conductivity-improving additives are those previously mentioned for fiber material A), e.g., metal particles, carbon particles, etc. The conductivity-improving additive is preferably selected from carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof.A special version is graphitic carbon nanofibers (GCNF). To achieve improved fiber integration, the polymer layer B) can generally be used in a thin layer and / or with a low coating weight relative to the polymer content. Therefore, the polymer layer B) often exhibits good electrical and thermal conductivity even without conductivity-enhancing additives. In a special embodiment, the polymer layer B) therefore contains no added conductivity-enhancing additives.

[0092] If the polymer layer B) contains at least one conductivity-improving additive, the content of conductivity-improving additives is preferably in a range from 0.1 to 40 wt.%, particularly preferably from 0.5 to 30 wt.%, based on the total weight of polymer B) and conductivity-improving additive.

[0093] In a specific embodiment, the polymer layer B) is free of added carbon nanotubes, carbon fibers, and mixtures thereof. In the context of the invention, “free of added carbon nanotubes, carbon fibers, and mixtures thereof” means that carbon nanotubes, carbon fibers, and mixtures thereof are not added to the coating agent for forming the polymer layer B). It is preferred that the polymer layer B) is completely free of carbon nanotubes, carbon fibers, and mixtures thereof. The polymer layer B) preferably contains carbon nanotubes, carbon fibers, and mixtures thereof in an amount in the range from 0 to 0.00005 wt.%, based on the total weight of the polymer layer B), particularly preferably from 0 to 0.00001 wt.%, based on the total weight of the polymer layer B), in particular 0 wt.%, based on the total weight of the polymer layer B).In a specific embodiment, the polymer layer B) is free of added components containing elemental carbon. In the context of the invention, “free of added components containing elemental carbon” means that components containing elemental carbon are not added to the coating agent for forming the polymer layer B). It is preferred that the polymer layer B) is completely free of components containing elemental carbon. The polymer layer B) preferably contains components containing elemental carbon in an amount in the range from 0 to 0.00005% by weight, based on the total weight of the polymer layer B), particularly preferably from 0 to 0.00001% by weight, based on the total weight of the polymer layer B), in particular 0% by weight, based on the total weight of the polymer layer B).

[0094] In a specific embodiment, the polymer layer B) is free of conductivity-improving additives. In the context of the invention, “free of conductivity-improving additives” means that conductivity-improving additives are not added to the coating agent for forming the polymer layer B). It is preferred that the polymer layer B) is completely free of conductivity-improving additives. The polymer layer B) preferably contains conductivity-improving additives in an amount in the range from 0 to 0.00005 wt. %, based on the total weight of the polymer layer B), particularly preferably from 0 to 0.00001 wt. %, based on the total weight of the polymer layer B), in particular 0 wt. %, based on the total weight of the polymer layer B).

[0095] To form the gas- and water-permeable polymer layer B), a liquid coating agent can preferably be used. The coating agent can contain at least one dispersing medium, which is selected, for example, from water, organic solvents, and mixtures thereof. Additionally, the coating agent can contain at least one additive, for example a dispersing agent. Suitable dispersing agents include surfactants, such as anionic and especially nonionic surfactants. Furthermore, the coating agent contains at least one polymer, as described above, optionally at least one conductivity-improving additive, and optionally at least one further, different additive.

[0096] The coating agent can be applied to the fiber material A) to form a gas- and water-permeable polymer layer B) in various ways. While spraying, screen printing, or Meyer rod processes are often used in discontinuous production, doctor blade, slot die, and gravure roller processes are preferred for continuous coating. The layer thickness and penetration depth can be influenced by the coating process parameters and the viscosity of the coating agent.

[0097] Preferably, for the production of the polymer layer B), the coating agent is applied in an amount such that the application weight, based on the polymer content, is 2 to 15 g / m 2 , preferably 4 to 7 g / m 2 , amounts.

[0098] The polymer layer B) preferably has a thickness of 3 to 50 pm, more preferably 5 to 30 pm. If the polymer layer B) is located partly inside and partly outside the fiber material A), the proportion of the polymer layer B) that is located inside the fiber material A) is also taken into account when determining the thickness of the polymer layer B). In this case, the polymer layer B) has a significantly greater thickness than would be expected if only the amount of polymer applied were taken into account. For example, if the longitudinal axis of the fibers is oriented essentially parallel to the x,y plane of the fiber material, the penetration depth of the polymer is generally 1 to 3 fiber layers, with a typical fiber diameter of e.g. 10 pm. In this case, the proportion of the polymer layer B) within the fiber material A) is approximately 12 to 32 pm.

[0099] The thickness of the polymer layer B) can be determined using a scanning electron microscope (SEM). This has been described in detail above. Before the post-treatment at elevated pressure in step iii), the polymer layer B) can be subjected to a thermal treatment. This can be carried out, for example, in a drying or sintering furnace. Drying is preferably carried out at a temperature of 80 to 300 °C, particularly preferably 100 to 200 °C. Subsequently, sintering can take place at a temperature of at least 300 °C, preferably about 300 to 500 °C. This thermal treatment is preferably not carried out under elevated pressure (i.e., it is carried out under the pressure that arises in the thermal treatment device at the treatment temperature without additional pressure-increasing measures).

[0100] Step iii) Post-treatment at elevated pressure and qqf. elevated temperature

[0101] In step iii) of the process according to the invention, the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and optionally elevated temperature.

[0102] The treatment in step iii) is preferably carried out at a surface pressure in the range from 0.5 to 10.0 MPa (5 to 100 bar), particularly preferably from 1.5 to 8.0 MPa and / or a line pressure of 3 to 500 N / mm, preferably 5 to 100 N / mm.

[0103] In a special embodiment, the treatment in step iii) is carried out at an increased surface pressure of at least 0.5 MPa and / or an increased line pressure of at least 3 N / mm and an increased temperature of at least 100°C.

[0104] The treatment in step iii) is preferably carried out at a temperature in the range from 100 to 350°C, particularly preferably from 120 to 330°C, in particular from 150 to 320°C.

[0105] For the post-treatment in step iii), conventional devices such as single- or multi-opening presses, endless belt presses, and / or calenders can be used. In a specific embodiment, at least one endless belt press, in particular a double-belt press, and / or at least one calender is used for the post-treatment in step iii).

[0106] In a special embodiment, at least one double-belt press is used for the post-treatment in step iii). In another special embodiment, at least one calender is used for the post-treatment in step iii).

[0107] Single-opening presses or multi-opening presses are particularly suitable for the discontinuous post-treatment of sectioned materials. Double-belt presses are suitable for the treatment of both continuous web-like materials and sectioned materials (sheet material). Double-belt presses have two continuously rotating press belts, between which the GDL web is post-treated under the influence of pressure and, if necessary, heat, while simultaneously being transported in the forward direction. The belts are aligned parallel to each other, and there is a gap between the upper and lower belts that can be opened and closed to adapt to the thickness of the GDL material and to set the desired properties. It is also conceivable for the press belts to not rotate during the pressing process.

[0108] In a preferred embodiment, the treatment in step iii) is carried out in a double-belt press. Specifically, the treatment in step iii) is carried out in a double-belt press at a surface pressure in the range of 0.5 to 10.0 MPa (5 to 100 bar), preferably 1 to 8 MPa (10 to 80 bar), and at a temperature in the range of 200 to 350°C.

[0109] In principle, known and commercially available calenders can be used in step iii) of the process according to the invention. It is thus possible to use calenders with 2, 3, 4, or more than 4 calender rolls. In the simplest preferred embodiment, the calender used in the process according to the invention is a 2-roll calender. The gas diffusion layer can be passed through the calender once or repeatedly, e.g., 1, 2, 3, 4, 5, or more than 5 times. The calender rolls can be arranged in a geometry suitable for calendering the gas diffusion layers. A two-roll calender can have a vertical, inclined, or horizontal arrangement of the rolls. A three-roll calender can have a vertical arrangement, a staggered top roll, or a staggered bottom roll. A four-roll calender can have an L-arrangement, an inverted L-arrangement, an S-arrangement, a Z-arrangement, or another arrangement of the rolls.

[0110] Preferably, the treatment in step iii) is carried out in a calender at a line pressure in the range of 3 to 500 N / mm, preferably 5 to 100 N / mm.

[0111] Preferably, the calendering in step iii) is carried out at a speed of 0.05 m / min to 30 m / min.

[0112] In a preferred embodiment, the treatment in step iii) takes place in a calender. Specifically, the treatment in step iii) takes place in a calender at a roll temperature in the range of 130 to 220°C, a line pressure in the range of 8 to 80 N / mm, and a web speed of 1 to 10 m / min.

[0113] Preferably, the treatment in step iii) is carried out in a press for a period of 5 seconds to 5 minutes, preferably 10 seconds to 2 minutes.

[0114] Preferably, the treatment in step iii) is carried out in a calender over a period of time from greater than 0 seconds to 10 seconds, preferably from 0.1 seconds to 5 seconds.

[0115] Step iv) Coating with a microporous layer C)

[0116] The gas diffusion layer according to the invention consists of a three- or multi-layer composite based on a flat, electrically conductive fiber material A), at least one gas- and water-permeable polymer layer B), and at least one microporous layer C). In step iv) of the process according to the invention, the polymer layer of the material obtained in step iii) is coated with a precursor to form a microporous layer C).

[0117] According to the invention, the microporous layer C) comprises conductive particles in a matrix of a polymeric binder. The conductive particles are preferably selected from conductive carbon particles, in particular carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. Carbon black, graphite, or a mixture thereof is preferably used.

[0118] The polymeric binder preferably comprises at least one polymer selected from fluorine-containing polymers c1), fluorine-free, high-temperature-resistant polymers c2), polymers b3) different therefrom, and mixtures thereof.

[0119] In particular, the polymeric binder contains at least one fluorine-containing polymer. Suitable fluorine-containing polymers c1) are the previously described fluorine-containing polymers b1) contained in the coating composition for forming the polymer layer B), which are incorporated herein by reference in their entirety. The fluorine-containing polymer c1) is preferably selected from polytetrafluoroethylenes (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), ethylene-tetrafluoroethylene copolymers (ETFE), perfluoroalkoxy polymers (PFA), and mixtures thereof. Polytetrafluoroethylene (PTFE) is preferably used.

[0120] In a specific embodiment, the polymeric binder contains at least one fluorine-containing polymer c1) as the sole polymer component. Specifically, the polymeric binder contains or consists of PTFE.

[0121] In a further specific embodiment, the polymeric binder contains at least one high-temperature-resistant polymer c2). Suitable high-temperature-resistant polymers c2) are the previously described high-temperature-resistant polymers b2) contained in the coating composition for forming the polymer layer B). These are incorporated herein by reference in their entirety. The polymers c2) are preferably selected from polyaryletherketones (PAEK), polyphenylene sulfides (PPS), polysulfones (PSU), polyethersulfones (PES), semi-aromatic (co)polyamides (high-temperature polyamides, HTPA), polyimides (PI), polyamideimides (PAI), polyetherimides (PEI), and blends thereof.

[0122] In a further specific embodiment, the polymeric binder contains a mixture of at least one fluorine-containing polymer c1) and at least one fluorine-free, high-temperature-resistant polymer c2).

[0123] The polymeric binder may comprise at least one further polymer c3) different from c1) and c2). Suitable polymers c3) are, for example, the aforementioned polymers b3).

[0124] Specifically, the polymeric binder used to produce the microporous layer C) contains further polymers c3) other than the fluorine-containing polymers c1) and the polymers c2) in a weight fraction of at most 5%, preferably of at most 1%, particularly preferably of at most 0.5%, in particular of at most 0.1%, based on the total weight of the polymeric binder. Even more specifically, the polymeric binder used to produce the microporous layer C) contains no additions of further polymers c3) other than the fluorine-containing polymers c1) and the polymers c2).

[0125] For producing the microporous layer C), the polymeric binder is preferably used in a weight amount of 0.5 to 50 wt.%, particularly preferably 1.0 to 40 wt.%, in particular 10 to 25 wt.%, based on the total weight of polymeric binders and conductive particles.

[0126] The MPL can be applied to the polymer layer B) in various ways. While spraying, screen printing, or Meyer rod processes are often used in discontinuous production, doctor blade, slot die, and gravure roll processes are preferred for continuous coating. Finally, another thermal treatment can be performed, e.g., in a drying and sintering furnace. This can initially involve drying at a temperature of 100 to 200 °C, followed, if necessary, by sintering at a temperature of 300 to 500 °C.

[0127] In contrast to the macroporous fiber material A), the MPL C) is microporous with pore diameters that are generally well below one micrometer, preferably of at most 900 nm, particularly preferably of at most 500 nm, in particular of at most 300 nm. The average pore diameter of the MPL B) is preferably in a range from 5 to 200 nm, particularly preferably from 10 to 100 nm.

[0128] Porosity and pore size distribution can be determined using mercury porosimetry, as described in DIN ISO 15901-1:2019-03 and ISO 15901-1:2016 - Part 1: Mercury porosimetry. The latter average pore diameters apply primarily to the use of carbon black as conductive particles in MPL. By using graphite as conductive particles in MPL or by using pore-forming agents, significantly larger MPL pores can be created. Depending on the composition, the average pore diameter is then, for example, greater than 1 pm. When using different conductive particles, the pore diameter can exhibit a bimodal or polymodal distribution curve. For example, when using a mixture of carbon black and graphite, a pore diameter distribution with two pore peaks (one carbon black and one graphite peak) can be obtained.

[0129] The microporous layer C) preferably has a thickness in the range of 5 to 150 pm, particularly preferably 10 to 100 pm. This thickness refers to the uncompressed state of the microporous layer B), ie, before the post-treatment in step iii) and before the incorporation of the GDL into a fuel cell.

[0130] The presence of MPL has a significant impact on the water balance of the fuel cell. Due to the high proportion of polymeric binder, especially PTFE, and the smaller pores of MPL, flooding of the GDL and the electrode is made more difficult by the MPL acting as a liquid water barrier, thus favoring the mass transport of gaseous reactants to the catalyst.

[0131] Gas diffusion layer

[0132] Another object of the invention is a gas diffusion layer for a fuel cell, comprising

[0133] A) a flat electrically conductive fiber material,

[0134] B) a gas and water permeable polymer layer on at least one of the surfaces of the fiber material,

[0135] C) a microporous layer on at least one of the gas and water permeable polymer layers, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder.

[0136] In particular, the invention relates to a gas diffusion layer obtainable by the method described above.

[0137] The gas diffusion layer according to the invention preferably has a thickness (total thickness of fiber material A), at least one polymer layer B), and at least one microporous layer C)) in the range from 50 to 1000 pm, particularly preferably from 75 to 500 pm. This thickness refers to the uncompressed state of the GDL, ie, before the post-treatment in step iii) and before its incorporation into a fuel cell.

[0138] Furthermore, the gas diffusion layers preferably have a high total porosity. This is preferably in the range of 20% to 80%, determined by mercury porosimetry, as described above.

[0139] In the gas diffusion layer according to the invention, the proportion of fibers that could lead to membrane damage when used in a PEM fuel cell is significantly reduced compared to conventional gas diffusion layers. This leads to a significant reduction in short circuits caused by membrane penetration when used in a fuel cell.

[0140] Puncture measurement, Shorting Number:

[0141] Figure 1 shows a device for puncture measurement to determine the shorting number as a measured value to characterize the probability of a short circuit.

[0142] During the puncture measurement, a PP foil (4 μm thick) is mounted between two GDL samples (GDL sheet) and a spacer layer with a defined thickness (0.1 to 1.0 mm) and defined gaps. The materials lie on an electrically conductive and smooth metal plate. During the measurement, a metal stamp (12.7 mm diameter) slowly presses the upper GDL into the gap between the spacer layer and onto the PP foil. The electrically conductive pressure stamp and the metal plate are connected to a resistance measurement. The measurement at a test point is finished when the maximum pressure is reached. A puncture through the PP foil occurs if the threshold resistance falls below 10 kΩ. The associated pressure is documented. Since the GDL itself is conductive, this measurement detects damage to the PP foil caused by the indented GDL.During one measurement run, 117 measuring points are usually covered over an area of ​​approximately 300 x 400 mm.

[0143] Tests with PP films of varying thicknesses (4-14 μm) also showed that with decreasing PP film thickness, the probability of a puncture through the film / membrane increases, or that the number of punctures increases for a certain number of measurements using the same material / measurement parameter combination. At least 117 measurements were performed for each combination (standard: 4 runs of 117 measurements each).

[0144] The Shorting Number as a measurement value to characterize the probability of a short circuit is defined as follows: Shorting Number =

[0145] (Number of measuring points with penetration / total number of measuring points) x 100

[0146] In other words, the shorting number is defined as the percentage ratio between the number of measurements below the threshold resistance and the total number of measurements. The lower the number of measurements below the threshold resistance, the lower the shorting number, and the lower the probability of membrane penetration.

[0147] The gas diffusion layer according to the invention preferably has a shorting number of at most 50%, determined by means of puncture measurement on a GDL of 297 x 420 mm base area with a basis weight of 95 g / m 2 and an MPL loading of 15 g / m 2 Specific gas diffusion layers according to the invention may have a lower shorting number of, for example, at most 40% or at most 30%.

[0148] Specific measurement conditions are described in the example section, to which reference is made here.

[0149] Fuel cell and

[0150] Another subject of the invention is a fuel cell which comprises at least one gas diffusion layer as defined above or which comprises at least one gas diffusion layer obtainable by a process as defined above.

[0151] In principle, the gas diffusion layer according to the invention is suitable for all common fuel cell types. The fuel cell according to the invention is preferably a proton exchange membrane fuel cell (PEMFC). A preferred embodiment is water-oxygen fuel cells in the form of low-temperature proton exchange membrane fuel cells (LT-PEMFC). Reference is made in full to the above statements regarding the structure of fuel cells. The fuel cells according to the invention preferably comprise a polymer electrolyte membrane, to which a catalyst layer is applied on the anode and cathode sides, forming the electrodes. A gas diffusion layer (GDL) is preferably located on the anode and / or cathode side in contact with the catalyst layer.Specifically, the fuel cells comprise a polymer electrolyte membrane to which a catalyst layer is applied, which is in contact with the surface of the microporous layer C) of a gas diffusion layer according to the invention. Specifically, the fuel cells comprise a gas diffusion layer according to the invention on the cathode side, wherein the catalyst layer is in contact with the surface of the microporous layer C) of the gas diffusion layer. More specifically, the fuel cells comprise a gas diffusion layer according to the invention on the cathode side and on the anode side, wherein both the cathode layer and the anode layer are each in contact with the surface of the microporous layer C) of a gas diffusion layer according to the invention.

[0152] A specific embodiment of the invention is a proton exchange membrane fuel cell comprising a proton exchange membrane coated on both sides with a catalytically active electrode, on each side of the catalyst-coated membrane a gas diffusion layer whose inner side is in contact with the catalytically active electrode, on each side of the catalyst-coated membrane a flow distributor plate having channels for supplying reactant gases and removing products of the cell reaction, wherein the channels are in contact with the outer side of the gas diffusion layer.

[0153] It is an advantage of the invention that when the gas diffusion layer according to the invention is used in a PEM fuel cell, the frequency of short circuits caused by fiber penetration of the membrane is significantly reduced.

[0154] A further object of the invention is the use of a gas diffusion layer comprising A) a flat electrically conductive fiber material,

[0155] B) a gas and water permeable polymer layer on at least one of the surfaces of the fiber material,

[0156] C) a microporous layer on at least one of the gas- and water-permeable polymer layers, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder, in a fuel cell for reducing the probability of failure during operation, specifically for reducing the probability of puncture of the fuel cell membrane, more specifically for reducing the shorting number.

[0157] The invention further relates to a fuel cell stack comprising a plurality of fuel cells, as defined above. To achieve the desired electrical voltage and power, individual fuel cells are interconnected to form fuel cell stacks. The structure of fuel cell stacks and their manufacture are known to those skilled in the art. A typical fuel cell stack comprises a first and a second end plate, fuel cells arranged therebetween, and a clamping device for clamping the fuel cells between the end plates. The stack structure generally comprises several stacked, planar individual cells arranged in an electrical series circuit. The stack structure is provided at both ends with so-called current collector plates for collecting the electrical current and is electrically separated from the so-called end plates by an insulation - usually plate-shaped - that is part of the stack structure.The end plates are arranged at both ends of the stack structure and are connected to one another by a bracing device, e.g., in the form of tie rods. This bracing device enables the application of a bracing force or a compressive force to the stack structure. It has now been found that the gas diffusion layers according to the invention are advantageously suitable for use in fuel cells arranged in braced fuel cell stacks. In particular, they enable high compression of the individual cells, whereby the frequency of short circuits caused by fiber penetration of the membrane can be significantly reduced compared to conventional gas diffusion layers.

[0158] FIGURE DESCRIPTION

[0159] Figure 1 shows a device for puncture measurement to determine the shorting number as a measured value to characterize the probability of a short circuit.

[0160] Figure 2 a shows a scanning electron micrograph of a fiber substrate impregnated with a composition of carbon black and PTFE with a FEP polymer layer applied on one side and post-treatment by hot pressing

[0161] Figure 2 b shows a scanning electron micrograph of the substrate from Figure 2 a with additionally applied MPL.

[0162] Figure 3 shows the results of the puncture measurement according to Table 2.

[0163] The following examples serve to illustrate the invention without limiting it in any way.

[0164] EXAMPLES

[0165] I) Production of gas diffusion layers

[0166] Gas diffusion layers were produced from a carbon fiber nonwoven fabric. To better assess the influence of the various processing measures on the extent of fiber integration, a rougher nonwoven fabric (material A) and a finer nonwoven fabric (material B) were used. The rougher material A had a higher number of fibers not integrated into the fiber material than material B. Material A served as a model system for the performance of the inventive process with difficult fiber materials, even if products optimally suited for use on the membrane side of a fuel cell were not obtained in every case. According to the invention, a gas- and water-permeable polymer layer (B) was applied, a post-treatment by pressing or calendering, and a microporous layer (C) was applied. In the comparative tests, one or more of the following measures were omitted:

[0167] Application of a gas and water permeable polymer layer B), post-treatment by pressing or calendering, application of a microporous layer

[0168] Manufacturing example:

[0169] To produce a flat electrically conductive material, a nonwoven fabric made of 100% carbon fibers with a basis weight of 63 g / m 2 used. To finish the nonwoven fabric, an impregnation composition was mixed containing 70% carbon black and 30% PTFE based on the solids. The finishing was carried out by padding with an aqueous dispersion with 15% finish weight based on the mass of the GDL substrate (corresponding to 9.5 g / m 2 ).

[0170] This was followed by drying for 1 minute at 180 °C and sintering for 1 minute at 440 °C.

[0171] The rougher material A had a mean roughness value R a of 11 .28 pm and an average roughness Rz of 74.67 pm. The finer material B had a mean roughness R a of 7.40 pm and an average roughness depth R z of 55.05 pm. Roughness was determined according to DIN 4768-1:1974-08 using a Keyence VHX-7000 digital microscope. The values ​​are averages of six measurements: three in the machine direction (MD) and three perpendicular to the machine direction (CD).

[0172] A coating agent containing an aqueous dispersion of a tetrafluoroethylene-hexafluoropropylene copolymer (FEP coating) was then applied to the resulting substrate. The coating weight, based on the polymer content, was 7 g / m 2 After application, the coating was dried again for 2.5 minutes at 120 °C and sintered for 2.5 minutes at 400 °C.

[0173] The FEP-coated nonwovens were either subjected to post-treatment in a double-belt press at 60 bar pressure and a temperature of 250 °C for 30 s. Alternatively, calendering was carried out in a two-roll calender with a steel roll and a rubber roll (hardness 82 Shore D) at a line pressure of 40 N / mm, a steel roll temperature of 150 °C, and a web speed of 5 m / min.

[0174] To produce gas diffusion layers according to the invention, an MPL was subsequently applied to the post-treated FEP coating. For the MPL coating, an MPL paste containing 3.3 wt.% PTFE and 13.3 wt.% carbon in distilled water was applied to the fiber material. The fiber material was then dried for 2.5 minutes at 120 °C and sintered for 2.5 minutes at 400 °C. The resulting MPL loading was 23 g / m². 2 .

[0175] Analogously to this preparation procedure, the following inventive GDL and comparative GDL shown in Table 1 were prepared.

[0176] Table 1 II) Application examples

[0177] Puncture measurement, Shorting Number:

[0178] The puncture measurement to determine the shorting number was performed as previously described in detail. During one measurement run, 117 measurement points were covered over an area of ​​approximately 300 x 400 mm. The results can be found in Table 2. The shorting number and the integrity probability each add up to 100% for an example. The pressure-dependent puncture probability indicates the probability of a short circuit in the respective pressure range. The sum of all pressure-dependent puncture probabilities for an example corresponds to the shorting number.

[0179] Table 2

[0180] Summary of results:

[0181] Comparative Example V1 shows that for a rough fiber material without FEP and MPL coating and without post-treatment at elevated pressure and temperature, very low pressures during the puncture measurement are sufficient to cause damage. Post-treatment of Material A only by pressing at elevated temperature (not shown by an example in Table 1) results in slightly higher pressures of at least 1-2 MPa being required during the puncture measurement to achieve 100% punctures.

[0182] Comparative example V2 shows that for a rough fiber material with only MPL coating and without any post-treatment, pressures of at least 2-3 MPa must be applied during the puncture measurement in order to achieve 100% punctures.

[0183] Example 3 according to the invention shows that the combination of FEP and MPL coating with a post-treatment by pressing no longer achieves 100% penetration even at significantly higher pressures.

[0184] Comparably good results were achieved when the post-treatment was performed by calendering. Example 3 demonstrates that the combination of all measures can significantly improve the puncture properties, even when using a rougher fiber material. Such a material is suitable, for example, as GDL for installation on the bipolar plate side of a fuel cell.

[0185] Comparative example V4 shows that with a fine fiber material (material B), the probability of puncture is lower due to the lower number of protruding fibers, i.e. without FEP and MPL coating and without post-treatment at increased pressure and temperature.

[0186] Comparative example V5 shows that on material B with only MPL coating and without any post-treatment, the shorting number is already reduced to 59% (corresponding to a probability of integrity of 41%).

[0187] Additional pressing (not demonstrated by an example) reduces the number of punctures by approximately 20%. Example 6 according to the invention demonstrates that the combination of all measures results in a material with excellent suitability as a GDL and for membrane-side incorporation into a fuel cell.

Claims

Patent claims 1 . A method for producing a gas diffusion layer for a fuel cell, comprising A) a flat electrically conductive fiber material, B) a gas and water permeable polymer layer on at least one of the surfaces of the fiber material, C) a microporous layer on at least one of the gas- and water-permeable polymer layers, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder, in which i) a flat electrically conductive fiber material A) is provided, ii) the fiber material provided in step i) is coated with a coating agent to form a gas- and water-permeable polymer layer, iii) the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and optionally elevated temperature, and iv) the polymer layer of the material obtained in step iii) is coated with a precursor to form a microporous layer.

2. Method according to one of claims 1, wherein the fiber material A) is selected from carbon fiber nonwovens, carbon fiber fabrics, carbon fiber papers and combinations thereof, wherein preferably the fiber material A) comprises a carbon fiber nonwoven or consists of a carbon fiber nonwoven.

3. The method according to claim 1 or 2, wherein the fibers contained in the fiber material A) comprise carbon fibers or consist of carbon fibers for the production of which polyacrylonitrile fibers are used as starting material.

4. The method according to any one of the preceding claims, wherein the fiber material A) contains carbon nanotubes in an amount in the range from 0 to 0.00005 wt.%, based on the total weight of the fiber material A), particularly preferably from 0 to 0.00001 wt.%, based on the total weight of the fiber material A), in particular from 0 wt.%, based on the total weight of the fiber material A).

5. Method according to one of claims 1 or 2 or 3 or 4, wherein the polymer layer B) is located partly inside and partly outside the fiber material A).

6. Process according to one of the preceding claims, wherein, for the production of the polymer layer B), a coating agent is applied in an amount such that the application weight, based on the polymer content, is 2 to 15 g / m 2 , preferably 4 to 7 g / m 2 , amounts.

7. Method according to one of the preceding claims, wherein the polymer layer B) has a thickness (based on the portions located inside and outside the fiber material A)) of 3 to 50 pm, preferably 5 to 30 pm.

8. The method according to any one of the preceding claims, wherein the polymer layer B) comprises at least one polymer selected from fluorine-containing polymers b1), fluorine-free, high-temperature-resistant polymers b2), polymers b3) different therefrom, and mixtures thereof.

9. Method according to one of the preceding claims, wherein the polymer layer B) comprises carbon nanotubes, carbon fibers and mixtures of which in an amount in the range from 0 to 0.00005 wt.%, based on the total weight of the polymer layer B), particularly preferably from 0 to 0.00001 wt.%, based on the total weight of the polymer layer B), in particular from 0 wt.%, based on the total weight of the polymer layer B).

10. The method according to any one of the preceding claims, wherein the microporous layer C) comprises at least one polymer selected from fluorine-containing polymers c1), fluorine-free, high-temperature-resistant polymers c2), polymers c3) different therefrom, and mixtures thereof.

11. A process according to claim 8 or 10, wherein the fluorine-containing polymer is selected from polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, perfluoroalkoxy polymers, and mixtures thereof.

12. The process according to claim 8 or 10, wherein the fluorine-free, high-temperature-resistant polymer is selected from polyaryletherketones, polyphenylene sulfides, polysulfones, polyethersulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and mixtures thereof.

13. The method according to any one of the preceding claims, wherein the treatment in step iii) is carried out at a surface pressure in the range of 0.5 to 10.0 MPa, preferably 1.5 to 8.0 MPa and / or a line pressure of 3 to 500 N / mm, preferably 5 to 100 N / mm.

14. The process according to any one of the preceding claims, wherein the treatment in step iii) is carried out at a temperature in the range from 100 to 350°C, preferably from 120 to 330°C, particularly preferably from 150 to 320°C.

15. Method according to one of the preceding claims, wherein for the treatment in step iii) a device is used selected from single-daylight presses, multi-daylight presses, endless belt presses, calenders and combinations thereof, preferably selected from double-belt presses, calenders and combinations thereof.

16. Process according to one of the preceding claims, wherein the treatment in step iii) is carried out in a calender at a line pressure in the range of 3 to 500 N / mm, preferably 5 to 100 N / mm.

17. A gas diffusion layer obtainable by a process as defined in any one of claims 1 to 16.

18. Gas diffusion layer for a fuel cell, comprising A) a flat electrically conductive fiber material, B) a gas and water permeable polymer layer on at least one of the surfaces of the fiber material, C) a microporous layer on at least one of the gas and water permeable polymer layers, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder.

19. Gas diffusion layer according to claim 18 with a shorting number of at most 25%, determined by means of puncture measurement on a GDL of 297 x 420 mm base area with a basis weight of 73 g / m 2 and an MPL loading of 23 g / m 2 .

20. A fuel cell comprising at least one gas diffusion layer as defined in any one of claims 18 or 19, or obtainable by a process as defined in any one of claims 1 to 16.

21. Fuel cell according to claim 20 in the form of a proton exchange membrane fuel cell, comprising a proton exchange membrane coated on both sides with a catalytically active electrode, on each side of the catalyst-coated membrane a gas diffusion layer whose inner side is in contact with the catalytically active electrode, on each side of the catalyst-coated membrane a flow distribution plate having channels for supplying reactant gases and removing products of the cell reaction, wherein the channels are in contact with the outer side of the gas diffusion layer.

22. Fuel cell according to claim 20 or 21, comprising at least one proton exchange membrane to which a catalyst layer is applied, wherein the catalyst layer is in contact with the surface of the microporous layer C) of the gas diffusion layer.

23. A fuel cell stack comprising a plurality of fuel cells as defined in any one of claims 20 to 22.

24. Use of a gas diffusion layer comprising A) a flat electrically conductive fiber material, B) a gas and water permeable polymer layer on at least one of the surfaces of the fiber material, C) a microporous layer on at least one of the gas- and water-permeable polymer layers, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder, in a fuel cell for reducing the probability of failure during operation, specifically for reducing the probability of puncture of the fuel cell membrane, more specifically for reducing the shorting number.

25. Use according to claim 24, wherein the gas diffusion layer is obtainable by a process as defined in any one of claims 1 to 16.

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