Gas diffusion layer with low plastic deformability and high surface quality, and method for its production

Abstract

The present invention relates to a method for producing a gas diffusion layer for a fuel cell, the gas diffusion layer having low plastic deformability (low setting behavior) and good surface quality. The invention also relates to the gas diffusion layers obtainable according to the method and to a fuel cell that contains such a gas diffusion layer.

Description

[0001] Gas diffusion layer with low plastic deformability and high surface quality and process for its production

[0002] The present invention relates to a method for producing a gas diffusion layer for a fuel cell with low plastic deformability (low settling behavior) and a good surface finish. The invention further relates to the gas diffusion layers obtainable by this method and to a fuel cell containing such a gas diffusion layer.

[0003] BACKGROUND OF THE INVENTION

[0004] 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 electrically insulates them. 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 the oxygen is reduced and the electrons are absorbed. The oxygen anions formed react with the protons transported across the membrane to form water (1 / 2 O2 + 2 H + + 2 e- -> H2O).

[0005] 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, the core of which is a polymer electrolyte membrane (PEM) that is only suitable for protons (or oxonium ions H3O+) 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; this layer forms the electrodes and usually 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 unit, also known as a CCM (catalyst coated membrane). On both sides of the CCM there is a gas diffusion layer (GDL), which stabilizes the cell structure and takes on the 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. These plates have channels for supplying the adjacent cathode and anode with process gases and usually also have internal cooling channels.

[0006] 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.

[0007] A major problem with fiber-based gas diffusion layers is the potential for damage to the fuel cell membrane caused by an inhomogeneous surface of the gas diffusion layer or protruding fibers. These fibers are typically very stiff and brittle. Furthermore, the fiber thickness is often close to the thickness of the fuel cell membrane, posing a risk of fiber penetration into the membrane and causing a short circuit. In the worst case, a short circuit caused by fiber penetration of the membrane can lead to the functional failure of the entire fuel cell stack. Other sources of error that can lead to similar failures or a significant reduction in the stack's service life include, for example, a very rough MPL surface or impurities of varying hardness integrated into the MPL.Since the membrane can be exposed to considerable mechanical stress during operation of the fuel cell, the stack failure may occur at a later time.

[0008] The membrane of a fuel cell is very thin, usually just a few microns thick. Typical thicknesses range from 8 to 50 microns, although membranes as thin as 5 microns are already being tested. It is expected that with the increasing use of fuel cells in automotive applications, there will be a need to further reduce the thickness of all flat components (membranes, GDL / MPL, others). A very significant performance problem is caused by internal short circuits, which can be caused by protruding fibers from the GDLs resting on the membrane. Therefore, there is a need to prevent protruding fibers and / or smooth the MPL surfaces of the GDL.

[0009] GDLs are typically heavily compressed for use in fuel cells. The properties of GDLs resulting from compression can be characterized by the proportion of elastic and plastic deformation. During plastic deformation, the gas diffusion layer does not fully return to its original shape after loading, but rather undergoes a permanent change in shape. The property of a material to permanently change its shape when stress is applied, i.e., its deformability, is also referred to as "settling." Materials with low plastic deformability exhibit low settling behavior. The settling behavior of GDLs known from the state of the art still requires improvement. If the GDL is clamped in the fuel cell stack under high pressures, setting occurs due to the clamping forces and dynamic force changes during operation.This can lead to a loss of compression pressure in the fuel cell stack, which increases the material resistance of most components and, above all, their contact resistance within the stack. Furthermore, an adjustment of the stack design may be necessary, such as the use of additional spring assemblies to compensate for the loss of tension that occurs during setting. This can increase the length of the stacks and the space required for their installation. Furthermore, additional measures may be required during stack assembly, e.g., by repeatedly clamping and unclamping the stacks during assembly and only then fixing them, which increases manufacturing costs. The development of fuel cells suitable for everyday use is an important contribution to the energy transition from fossil to sustainable fuels. There is therefore currently a great demand for PEM fuel cells that are improved with regard to the complex property profile described.

[0010] It is known to use gas diffusion layers that have a gradient with respect to at least one physical or chemical property to adjust the application properties of fuel cells. WO 2022 / 002932 A1 describes a gas diffusion layer for a fuel cell, wherein at least one physical property, selected from hydrophobicity and permeability, changes in at least one direction along the largest areal extent. Specifically, it is described, for example, to control the hydrophobicity via the content of a hydrophobizing material (such as PTFE) and the permeability via the porosity of the gas diffusion layer. When a microporous layer is applied, its thickness can be influenced, which leads, among other things, to a changing local penetration depth into the carrier layer.The disclosure of this application is not very specific and lacks both information on the implementation of the described concepts and a reproducible embodiment as well as application-related data.

[0011] EP 3957789 A1 describes a gas diffusion layer that, despite its low density, exhibits high thermal conductivity and good handling and cell performance. The GDL comprises a carbon fiber felt containing carbon fibers with an average fiber diameter of 5 to 20 μm, wherein at least a portion of the carbon fibers constituting the carbon fiber felt has a flat portion, in which, in a planar view of the surface of the carbon fiber felt, a maximum value of the fiber diameter is observed that is 10 to 50% larger than the average fiber diameter, and the frequency of the flat portions on the surface of the carbon fiber felt is 50 to 200 / mm 2 amounts.

[0012] WO 2020 / 165075 A1 describes a method 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 substantially no or equal to or less than 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 them only in small quantities. The layers are pressed together at a maximum pressure of 2.5 MPa and a temperature of at least 300°C, using long treatment times of at least 15 minutes and preferably 1 to 4 hours. The additional binder layer between the substrate and the MPL increases the number of required process steps, and the long pressing time greatly complicates industrial applicability.

[0014] JP 2007242378 A describes a gas diffusion layer consisting of porous sintered carbon particles and water-repellent particles. To produce the 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 removed, pulverized again, and the resulting sintered coarse particles are hot-pressed in a mold to form a GDL. This allows, if desired, the use of a fiber-based substrate in the final GDL to be omitted, so that it consists only of coarse carbon-based particles and water-repellent particles. This approach is likely to lead to disadvantages in further processing during cutting and manufacturing of the cell stacks, as well as in cell stability.

[0015] EP 3276718 A1 describes a porous carbon electrode substrate that hardly causes short circuits when used in a fuel cell. 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, are adequately removed. Production involves using short carbon fibers and a binder resin containing at least 35 wt.% carbon, which carbonizes upon heating. The resulting GDL substrate is thus based on a fully resin-impregnated fiber material.

[0016] EP 3396753 A1 describes a gas diffusion electrode that is less susceptible to the occurrence of a short-circuit current 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 thickness under compressive load. To reduce the probability of short circuits, a variety of measures are described, e.g., pressure treatment of the precursor substrate before carbonization of the binder resin and increasing the temperature in the carbonization step. Only if further reduction is desired is a post-treatment by calendering followed by air blowing and suction described.The disadvantages of this process are that pressure treatment prior to carbonization requires additional effort, resin binders in the fiber substrate are often undesirable, and the use of two or more MPL layers also increases the complexity. A multi-layer structure also always increases the risk of delamination during winding, warping, stretching, or under pressure.

[0017] US 2019 / 0344405 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 that can be caused by protruding fibers from the GDLs resting on the membrane.

[0018] The invention is based on the object of avoiding or at least reducing the disadvantages described above.

[0019] Surprisingly, it has now been discovered that gas diffusion layers with a good property profile, especially with very good surface properties and significantly improved settling behavior, can be achieved by subjecting the gas diffusion layer to post-treatment at elevated pressure and temperature. Hot-compressed gas diffusion layers are characterized in particular by a considerably smoother surface on the side(s) coated with a microporous layer. This results in a significantly lower probability of short circuits caused by protruding fibers and impurities on the surface of the MPL, by other causes of a rough surface, or by other effects that occur during fuel cell operation and can lead to membrane penetration. Furthermore, post-treatment at elevated pressure and temperature can significantly reduce the plastic deformation component of the GDL.Furthermore, it was surprisingly discovered that the post-treatment also allows the transport properties of the GDL to be controlled. Thus, properties such as gas permeability and dry diffusion length can be controlled independently of the material composition of the gas diffusion layer.

[0020] SUMMARY OF THE INVENTION

[0021] A first subject of the invention is a method for producing a gas diffusion layer for a fuel cell, comprising

[0022] A) a flat electrically conductive fiber material and

[0023] B) a microporous layer on at least one of the surfaces of the fiber material, comprising conductive particles in a matrix of a polymeric binder, in which i) a sheet-like electrically conductive fiber material A) is provided, ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer, iii) the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and optionally elevated temperature.

[0024] Preferably, the process according to the invention, especially the post-treatment in step iii), results in a gas diffusion layer with reduced plastic deformability compared to a non-post-treated gas diffusion layer. In particular, a gas diffusion layer with a reduced compression set value compared to a non-post-treated gas diffusion layer is achieved.

[0025] Preferably, the process according to the invention, especially the post-treatment in step iii), results in a gas diffusion layer with a smoother surface of the at least one microporous layer.

[0026] Specifically, the post-treatment in step iii) is carried out at an elevated pressure of at least 0.5 MPa and an elevated temperature of at least 100°C.

[0027] A special embodiment is a method for achieving a gas diffusion layer with one, preferably two, particularly preferably three, in particular four of the following properties: a compression set value at 1.0 MPa of at most 5 pm, determined on a GDL with a basis weight of 95 to 100 g / m 2 and an MPL loading of 15 to 22 g / m 2on a ring-shaped sample with an inner diameter of 45 mm and an outer diameter of 56 mm, the sample being subjected to three loading cycles from 0.025 MPa to 1.0 MPa and the compression set value being the difference between the thickness measured at 1.0 MPa in the first loading cycle and the third loading cycle, a reduction in the mean roughness R a , determined according to DIN EN ISO 4288:1998-04 using the stylus method, compared to a non-treated gas diffusion layer of at least 10%, a reduction in the roughness depth R z , determined according to DIN EN ISO 4288:1998-04 using the stylus method, compared to a non-treated gas diffusion layer of at least 10%, 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 95 g / m 2 and an MPL loading of 15 g / m 2 .

[0028] Another object of the invention is a gas diffusion layer obtainable by a process as described above and below.

[0029] Another object of the invention is a gas diffusion layer for a fuel cell, comprising

[0030] A) a sheet-like electrically conductive fiber material and B) a microporous layer on at least one of the surfaces of the fiber material, wherein the gas diffusion layer has at least one of the following properties: a compression set value at 1.0 MPa of at most 5 pm, determined on a GDL with a basis weight of 95 to 100 g / m 2 and an MPL loading of 15 to 22 g / m 2on a ring-shaped sample with an inner diameter of 45 mm and an outer diameter of 56 mm, the sample being subjected to three loading cycles from 0.025 MPa to 1.0 MPa and the compression set value being the difference between the thickness measured at 1.0 MPa in the first loading cycle and the third loading cycle, a reduction in the mean roughness R a , determined according to DIN EN ISO 4288:1998-04 using the stylus method, compared to a non-treated gas diffusion layer of at least 10%, a reduction in the roughness depth R z , determined according to DIN EN ISO 4288:1998-04 using the stylus method, compared to a non-treated gas diffusion layer of at least 10%, 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 95 g / m 2 and an MPL loading of 15 g / m 2 .

[0031] Another subject of the invention is a fuel cell comprising at least one gas diffusion layer as defined above and below.

[0032] A further object of the invention is 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.

[0033] DESCRIPTION OF THE INVENTION

[0034] The gas diffusion layers according to the invention and obtained by the process according to the invention have the following advantages:

[0035] The resulting gas diffusion layers exhibit very good surface properties. Gas diffusion layers post-treated at elevated pressure and preferably elevated temperature are characterized, in particular, by a significantly smoother surface on the side(s) coated with a microporous layer.

[0036] The gas diffusion layers have a significantly lower probability of short circuits due to penetration of the proton exchange membrane, which can occur particularly due to protruding fibers, impurities on the surface of the MPL, or other causes of a rough surface.

[0037] The gas diffusion layers exhibit significantly improved settling behavior. The inventive post-treatment at elevated pressure and preferably elevated temperature can significantly reduce the plastic deformation component of the GDL.

[0038] Design measures to adapt the fuel cell stacks to the consequences associated with the settling of the GDL, such as reduced compression pressure, increased material resistance of the installed components and voltage loss, can be reduced or even eliminated altogether.

[0039] Surprisingly, it was discovered that the post-treatment according to the invention also allows the transport properties of the gas diffusion layer to be specifically influenced. Thus, properties such as gas permeability and dry diffusion length can be controlled independently of the material composition of the gas diffusion layer.

[0040] The gas diffusion layers according to the invention can be produced easily and cost-effectively.

[0041] The method according to the invention comprises the following steps: i) providing a flat electrically conductive fiber material A), ii) coating the fiber material provided in step i) with a precursor to form a microporous layer B), wherein the composition of the precursor is varied to produce a gradient, iii) post-treating the coated fiber material obtained in step ii) at elevated pressure and optionally elevated temperature.

[0042] With regard to the fiber materials A) and the precursors and conditions for the formation of a microporous layer B), reference is made in full to the explanations below. with increased pressure and increased

[0043] In a specific embodiment, the treatment in step iii) is carried out at an elevated pressure of at least 0.5 MPa and an elevated temperature of at least Preferably, the treatment in step iii) is carried out at a pressure in the range from 0.5 to 10.0 MPa (5 to 100 bar), particularly preferably from 1.5 to 8.0 MPa.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] For the post-treatment in step iii), conventional equipment such as single- or multi-opening presses, endless belt presses, or calenders can be used. 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).

[0048] 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 adjust the desired properties.

[0049] 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 pressure in the range of 1 to 8 MPa (10 to 80 bar) and at a temperature in the range of 200 to 350°C.

[0050] 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.

[0051] Preferably, the treatment in step iii) is carried out in a calender at a line pressure in the range of 5 to 500 N / mm, preferably 10 to 100 N / mm.

[0052] Preferably, the calendering in step iii) is carried out at a speed of 0.05 m / min to 30 m / min.

[0053] 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.

[0054] Flat electrically conductive fiber material A) and gas diffusion layer (GDL)

[0055] 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.

[0056] The sheet-like electrically conductive material A) used according to the invention and the gas diffusion layer are sheet-like structures that have a substantially two-dimensional, planar extension and a comparatively smaller thickness. The gas diffusion layer according to the invention has 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 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.An orthogonal coordinate system can be used to describe the GDL, with the base of the GDL lying 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 material thickness. According to 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). Mass transport between the flow distributor plate and the membrane essentially occurs in the z-axis direction.

[0057] The gas diffusion layer comprises, as component A), at least one electrically conductive sheet-like fiber material. 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, carbon fibers). Such fiber materials particularly advantageously fulfill the GDL requirements for gas diffusivity, liquid water permeability, electrical and thermal conductivity.The fiber material A) is preferably selected from carbon fiber fabrics, carbon fiber papers, and carbon fiber nonwovens. 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.

[0058] The carbon fibers can be produced in the usual way, with polyacrylonitrile fibers (PAN fibers) preferably being used as the starting material. PAN fibers are produced by radical polymerization of a monomer composition which preferably contains at least 90% by weight, based on the total weight of the monomers used for the polymerization, of acrylonitrile. The resulting polymer solution is spun into filaments, e.g., by wet spinning and coagulation, and 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.

[0059] The fiber material A) is preferably selected from carbon fiber fabrics, carbon fiber papers and carbon fiber nonwovens.

[0060] 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.

[0061] 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 matrix is ​​produced by sieving (decking) 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.

[0062] 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.

[0063] In a specific embodiment, the sheet-like electrically conductive fiber material A) comprises 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., using a roll-to-roll process.

[0064] The fiber material A) is generally a fiber composite material comprising: a1) carbon fibers, a2) optionally at least one polymeric binder and / or a pyrolysis product thereof, a3) optionally at least one further additive different from a2).

[0065] The fiber materials A) contained in the gas diffusion layer can contain conventional additives a3). These are preferably selected from hydrophobizing agents, conductivity-enhancing additives, surface-active substances, and mixtures thereof. 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 hydrophobizing agents are fluorine-containing polymers, such as polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene copolymers (FEP). PTFE is preferably used as the hydrophobizing agent. The fiber material can be impregnated with the hydrophobizing agent using conventional impregnation processes. For this purpose, 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.

[0066] Preferably, the fiber material A) has a hydrophobizing agent content of 3 to 40 wt.%, based on the total weight of the fiber material A). In a specific embodiment, the fiber material has a PTFE content of 3 to 40 wt.%, based on the total weight of the fiber material A).

[0067] To improve electrical and thermal conductivity, the fiber material A) can be treated with at least one conductivity-enhancing additive. Suitable conductivity-enhancing additives include, for example, metal particles, carbon particles, etc. The conductivity-enhancing additive is preferably selected from carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. The treatment of the fiber material A) with at least one conductivity-enhancing additive can, for example, be carried out together with the hydrophobizing agent, especially a PTFE dispersion. Due to the carbon fibers used, the fiber material A) often has good electrical and thermal conductivity even without conductivity-enhancing additives.

[0068] Preferably, the fiber material A) contains conductivity-improving 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-improving additive, then preferably in an amount of 0.1 to 40 wt.%, particularly preferably 0.5 to 30 wt.%, based on the total weight of the fiber material A).

[0069] The fiber material A) preferably has a thickness in the range of 50 to 750 pm, particularly preferably 100 to 500 pm. This thickness refers to the uncompressed state of the fiber material A), i.e., before the post-treatment in step iii) and before the incorporation of the GDL into a fuel cell. The fiber material A) preferably has a porosity in the range of 10 to 90%, particularly preferably 20 to 85%, measured by mercury porosimetry according to DIN ISO 15901-1:2019-03.

[0070] 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, measured according to DIN ISO 15901-1:2019-03.

[0071] The gas diffusion layer according to the invention consists of a two- or multi-layer composite based on a flat, electrically conductive fiber material A) and at least one microporous layer (MPL) B) on at least one of the surfaces of the fiber material A).

[0072] According to the invention, the microporous layer B) 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.

[0073] In particular, the polymeric binder contains at least one fluorine-containing polymer. The fluorine-containing polymer is preferably selected from polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, perfluoroalkoxy polymers, and mixtures thereof. Polytetrafluoroethylene (PTFE) is preferably used.

[0074] For the production of the microporous layer B), 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.

[0075] In contrast to the macroporous fiber material A), the MPL B) is microporous with pore diameters that are generally well below one micrometer, preferably at most 900 nm, more preferably at most 500 nm, in particular 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. The porosity and pore size distribution can be determined using mercury porosimetry, as described in DIN ISO 15901-1:2019-03: Mercury porosimetry. The latter average pore diameters apply primarily to the use of carbon black as conductive particles in the MPL. By using graphite as conductive particles in the MPL or by using pore formers, significantly larger MPL pores can be created. Depending on the composition, the average pore diameter is then, for example, B. larger 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.

[0076] The microporous layer B) 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.

[0077] The presence of MPL has a significant impact on the water balance of the fuel cell. Due to the high PTFE content 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 promoting the mass transport of gaseous reactants to the catalyst.

[0078] The gas diffusion layer according to the invention preferably has a thickness (total thickness of fiber material A) and MPL B)) 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.

[0079] Furthermore, the gas diffusion layers preferably have a high total porosity. This is preferably in the range of 20% to 80%, determined, as described above, by mercury porosimetry, measured using DIN ISO 15901-1:2019-03.

[0080] Procedure for one

[0081] Step i) With regard to the suitable and preferred fiber materials A) used in step i), reference is made in full to the previous explanations.

[0082] Step ii)

[0083] The precursors used in step ii) preferably contain at least one fluorine-containing polymer, at least one carbon material, and optionally at least one pore-forming agent. The fluorine-containing polymers are preferably selected from polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene copolymers (FEP). PTFE is preferably used. The carbon material is preferably selected from carbon black, graphite, graphene, carbon nanotubes (CNT), carbon nanofibers, and mixtures thereof. Carbon black or graphite is preferably used. In a specific embodiment, the precursors used in step b) contain at least one pore-forming agent. Suitable pore-forming agents are commercially available plastic particles, e.g., made of polymethyl methacrylate (PMMA). A suitable particle size is in the range of 10 to 100 pm.

[0084] Preferably, the volume fraction of the pores in the finished microporous layer, which is attributable to the use of a pore former, is 0 to 70 volume%, based on the total volume of the pores in the finished microporous layer.

[0085] The MPL can be applied to the fiber material 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 MPL layer thickness and penetration depth can be influenced by the coating process parameters and the viscosity of the coating. Finally, another thermal treatment follows, e.g., in a drying and sintering furnace. This can involve drying first at a temperature of 100 to 200 °C, followed by sintering at a temperature of 300 to 500 °C.

[0086] The post-treatment step iii) has been described in detail previously and is referred to here.

[0087] Compression Set Value: Plastic deformation occurs when a material, such as a gas diffusion layer, does not fully return to its original shape after being subjected to stress, but instead undergoes a permanent change in shape. Part of the deformation is elastic and thus reversible; only a certain part is plastic and remains permanently. The property of a material to permanently change its shape when stress is applied, i.e., its deformability, is also referred to as "settling." Materials with low plastic deformability exhibit low settling behavior.

[0088] The compression set value is a measure of how a material, in this case a GDL, behaves under compressive deformation and subsequent relaxation. GDLs are typically heavily compressed when used in fuel cells. The proportion of elastic and plastic deformation can be used to characterize the properties of a GDL resulting from compression. The compression set is the permanent deformation that remains after the applied force is removed. Gas diffusion layers with low settling behavior are characterized by low compression set values. The compression set value can be determined in the following way. It is possible to simultaneously determine the values ​​of other physical quantities, such as thickness, gas permeability, and electrical resistance, each at a specific compressive force and after single or multiple force applications.

[0089] Three samples (left, right, and center) are taken from the GDL to be tested across the entire width, and an average value is calculated. If the material has a machine direction due to manufacturing, the samples are taken perpendicular to the machine direction (CMD). The samples are ring-shaped with an inner diameter of 45 mm and an outer diameter of 56 mm. The sample area is 8.72577 cm. 2. In a testing machine, the samples are subjected to a time-varying compressive force that acts perpendicular to the surface of the sample. A sensor determines the change in the thickness of the GDL over time under the respective applied pressure. The sample is mounted on a device for determining elastic and plastic deformation using force sensors, with the movement being transmitted to the sample via springs. The travel distance until the maximum compressive force is reached is determined using displacement sensors. Since the deformation of the sample is non-linear, the measurement curve is adapted to the relative change. One measuring cycle, i.e. a single load up to maximum pressure and the subsequent unloading, takes 1 minute. The sample goes through three loading cycles. The initial value, at which only a low force is exerted on the sample, is 0.025 MPa.Typical pressure values ​​for determining the compression set value (and other physical quantities such as thickness, electrical conductivity or sheet resistance, gas permeability, etc.) are 0.6 MPa, 1.0 MPa and 2.4 MPa.

[0090] The compression set value for a specific pressure is the difference between the thickness measured at that pressure in the first load cycle and the thickness measured at that pressure in the third load cycle.

[0091] Preferably, the GDL according to the invention has a compression set value at 10 bar (1 MPa), measured according to the method described above on an annular sample with an inner diameter of 45 mm and an outer diameter of 56 mm on a GDL with a basis weight of 90 to 95 g / m 2 and an MPL loading of 15.0 to 22.0 g / m 2 from 5 pm at the latest.

[0092] Roughness:

[0093] The roughness can be determined using standard stylus methods known to the expert, such as those described in DIN 4768-1:1974-08 entitled "Determination of roughness measurements R a , R z , R ma x with electrical stylus instruments; basics".

[0094] The mean roughness value R was determined a (mean distance of a measuring point on the surface to the center line) and the average roughness depth R z The measurements were performed using a Mahrsurf XCR20 measuring device with an MFW-250 free-surface caliper. The values ​​are averages from six measurements: three in the machine direction (MD) and three perpendicular to the machine direction (CD). Specific measurement conditions are described in the examples section, to which reference is made here.

[0095] Puncture measurement, Shorting Number:

[0096] 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.

[0097] 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.

[0098] 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).

[0099] The shorting number as a measurement to characterize the probability of a short circuit is defined as follows:

[0100] Shorting Number =

[0101] (Number of measuring points with penetration / total number of measuring points) x 100

[0102] 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.

[0103] Preferably, the gas diffusion layer according to the invention has a shorting number of at most 15%, 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 on.

[0104] Specific measurement conditions are described in the example section, to which reference is made here.

[0105] Other physical quantities:

[0106] The gas permeability perpendicular to the material plane can be determined by a Gurley measurement, which can be performed using an automated Gurley densometer from Gurley Precision Instruments. The measurement determines the time in seconds until 100 cm 3 Air at constant pressure difference vertically through the GDL sample with a flow area of ​​6.42 cm 2 The determination of air permeability according to Gurley is described in ISO 5636-5.

[0107] The dry diffusion length is the actual distance traveled by a gas molecule through the flat fiber material A) and / or the microporous layer B) in pm. It is determined using a stationary Wicke-Kallenbach cell.

[0108] The thickness of the gas diffusion layer can be determined according to DIN 53855-1:1993-08 "Determination of the Thickness of Textile Fabrics." The thickness can be determined at a specific compressive force (e.g., at 0.025 MPa or 1.0 MPa) using a compression set measuring device, as described in detail above.

[0109] The determination of the mass per unit area in g / m 2 can be carried out according to EN 29073-1 :1992.

[0110] The porosity of the GDL can be determined using mercury porosimetry, as described in DIN ISO 15901-1:2019-03: Mercury porosimetry.

[0111] fuel cell

[0112] Another subject of the invention is a fuel cell comprising at least one gas diffusion layer as defined above or obtainable by a process as defined above.

[0113] 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). Proton exchange membrane fuel cells are also referred to as polymer electrolyte fuel cells (PEFC) or low-temperature polymer electrolyte membrane fuel cells (LT-PEMFC). A specific embodiment of the invention is water-oxygen fuel cells in the form of low-temperature proton exchange membrane fuel cells (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. Preferably, a gas diffusion layer (GDL) is located on the anode and / or cathode side in contact with the catalyst layer. Specifically, the fuel cells have a polymer electrolyte membrane to which a catalyst layer is applied, which is in contact with the surface of the microporous layer B) of a gas diffusion layer according to the invention. Specifically, the fuel cells have 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 B) of the gas diffusion layer.More specifically, the fuel cells have 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 B) of a gas diffusion layer according to the invention.

[0114] An advantage of the invention is that the transport processes through the gas diffusion layer can be specifically adapted to the gradients of the operating media flowing through the fuel cell and / or the operating parameters of the fuel cell. For this purpose, at least one property gradient of the gas diffusion layer generally corresponds to at least one of the property gradients of the operating media flowing through the fuel cell and / or the operating parameters of the fuel cell.

[0115] A further object of the invention is the use of a gas diffusion layer as defined above or obtainable by a process as defined above in a proton exchange membrane fuel cell.

[0116] FIGURE DESCRIPTION

[0117] 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.

[0118] Figure 2 shows the plastic deformation properties (settling behavior) based on the compression set values ​​at 1 MPa for 5 pairs each consisting of a comparison GDL (left bar) and a GDL according to the invention (right bar).

[0119] Figure 3a shows the mean roughness R a (MD) determined according to the stylus method as described in DIN 4768-1:1974-08, for 2 pairs each consisting of a comparison GDL (left bar) and a GDL according to the invention (right bar).

[0120] Figure 3b shows the mean roughness R a (CD) determined according to the stylus method as described in DIN 4768-1:1974-08, for 2 pairs each consisting of a comparison GDL (left bar) and a GDL according to the invention (right bar).

[0121] Figure 4a shows the average roughness R z (MD) determined according to the stylus method as described in DIN 4768-1:1974-08, for 2 pairs each consisting of a comparison GDL (left bar) and a GDL according to the invention (right bar).

[0122] Figure 4b shows the average roughness R z (CD) determined according to the stylus method as described in DIN 4768-1:1974-08, for 2 pairs each consisting of a comparison GDL (left bar) and a GDL according to the invention (right bar).

[0123] The following examples serve to illustrate the invention without limiting it in any way.

[0124] EXAMPLES

[0125] Production example 1:

[0126] Production of gas diffusion layer without (Example V1) and with inventive post-treatment (Example 1) at elevated pressure and elevated temperature

[0127] To produce a flat electrically conductive material, a nonwoven fabric made of 100% carbon fibers with a surface weight of 100 g / m 2 used. To finish the nonwoven fabric, an impregnation composition was mixed containing 80% carbon black and 20% 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 15 g / m 2). This was followed by drying for 2 minutes at 80 °C and sintering for 2 minutes at 400 °C. An MPL was then applied to the resulting substrate to produce the gas diffusion layers. For the MPL coating, an MPL paste containing 2.0 wt.% PTFE and 7.8 wt.% carbon in distilled water was applied to the fiber material. The fiber material was then dried for 2 minutes at 160 °C and sintered for 2 minutes at 400 °C. The resulting MPL loading was 15 g / m 2 The GDLs according to the invention were subjected to post-treatment in a double-belt press at a pressure of 25 bar and a temperature of 320 °C for 20 s. The non-post-treated GDL serves as a comparison.

[0128] The following materials were used to determine the application properties:

[0129] 1) Example 1 / V1

[0130] GDL from Production Example 1

[0131] 2) Example 2 / V2

[0132] GDL analogous to production example 1 with a basis weight of 100 g / m 2 .

[0133] 3) Example 3 / V3

[0134] GDL analogous to production example 1 with a basis weight of 132 g / m 2 .

[0135] 4) Example 4 / V4

[0136] GDL analogous to production example 1 with a basis weight of 135 g / m 2 .

[0137] 5) Example 5 / V5

[0138] GDL analogous to production example 1 with a basis weight of 96.5 g / m 2 .

[0139] 6) Example 6 / V6

[0140] GDL analogous to production example 1 with a basis weight of 94 g / m 2 .

[0141] Compression set

[0142] The compression set values ​​and thicknesses were determined using the procedure described in detail above. The values ​​are shown in Table 1 below.

[0143] Roughness The roughness was determined using the stylus method as described in DIN 4768-1:1974-08.

[0144] The mean roughness value R was determined a (mean distance of a measuring point on the surface to the center line) and the average roughness depth R z The measurements were performed using a Mahrsurf XCR20 measuring device with an MFW-250 free-surface probe. The values ​​are averages of six measurements: three in the machine direction (MD) and three perpendicular to the machine direction (CD).

[0145] The following conditions were chosen for the measurement: Probe = MFW-250. Probe diamond radius 2 pm, cone angle 60° LC (GS) = 2.5 mm = Cut Off = LT+LM

[0146] LT = 17.5 mm = sensing distance = 2.5 mm pre-travel and 2.5 mm lag for the Gaussian filter's oscillation. These 2 x 2.5 mm travels are not included in the measurement.

[0147] LM = 12.5 mm = measurement length used to determine the roughness value. Z = 5 = number of individual measurements for the Rz value. The measurement path (profile) is divided into 5 symmetrical individual sections (= LM / 5). The mean value is calculated from each individual section. The Rz value is then averaged from the 5 average values.

[0148] VB = +-250pm = measuring range of the probe

[0149] Profile resolution per measuring section = 100,000 steps Linearity =< 1%

[0150] Touch force (measuring force) = 0.8 mN Touch speed 0.5 mm / sec.

[0151] Puncture measurement, Shorting Number:

[0152] The penetration measurement to determine the shorting number was performed as previously described in detail. In one measurement run, 117 measurement points were taken over an area of ​​approximately 300 x 400 mm. The results are shown in Table 1.

[0153] The Gurley gas permeability was determined perpendicular to the material plane using a Gurley densometer from Gurley Precision Instruments according to ISO 5636-5. The results can also be found in Table 1.

[0154] The dry diffusion length was determined using a stationary Wicke-Kallenbach cell. The results can also be found in Table 1. Table 1

[0155] (MD) = in machine direction, (CD) = perpendicular to the machine direction

Claims

Patent claims Method for producing a gas diffusion layer for a fuel cell, comprising A) a flat electrically conductive fiber material and B) a microporous layer on at least one of the surfaces of the fiber material, comprising conductive particles in a matrix of a polymeric binder, in which i) a sheet-like electrically conductive fiber material A) is provided, ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer, iii) the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and optionally elevated temperature. The method according to claim 1 for obtaining a gas diffusion layer having one or more of the following properties: a compression set value at 1.0 MPa of at most 5 μm, determined on a GDL with a basis weight of 95 to 100 g / m 2 and an MPL loading of 15 to 22 g / m2 on a ring-shaped sample with an inner diameter of 45 mm and an outer diameter of 56 mm, the sample being subjected to three loading cycles from 0.025 MPa to 1.0 MPa and the compression set value being the difference between the thickness measured at 1.0 MPa in the first loading cycle and the third loading cycle, a reduction in the mean roughness R a , determined according to DIN EN ISO 4288:1998-04 using the stylus method, compared to a non-treated gas diffusion layer of at least 10%, a reduction in the roughness depth R z , determined according to DIN EN ISO 4288:1998-04 using the stylus method, compared to a non-treated gas diffusion layer of at least 10%, a shorting number of not more than 25%, determined by means of a puncture test 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 .

3. The method according to claim 1 or 2, wherein the fiber material A) is selected from carbon fiber nonwovens, carbon fiber wovens and mixtures thereof.

4. A process according to any one of the preceding claims, wherein the treatment in step iii) is carried out at a pressure in the range of 0.5 to 10.0 MPa, preferably 1.5 to 8.0 MPa.

5. 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.

6. The method according to any 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.

7. The method according to any one of the preceding claims, wherein 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.

8. The method according to any one of the preceding claims, wherein the treatment in step iii) is carried out in a calender over a period of time of greater than 0 seconds to 10 seconds, preferably from 0.1 seconds to 5 seconds.

9. 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 5 to 500 N / mm, preferably 10 to 100 N / mm.

10. A gas diffusion layer obtainable by a process as defined in any one of claims 1 to 9.

11. Gas diffusion layer for a fuel cell, comprising A) a flat electrically conductive fiber material and B) a microporous layer on at least one of the surfaces of the fiber material, wherein the gas diffusion layer has at least one of the following properties: a compression set value at 1.0 MPa of at most 5 pm, determined on a GDL with a basis weight of 95 to 100 g / m 2 and an MPL loading of 15 to 22 g / m 2 on a ring-shaped sample with an inner diameter of 45 mm and an outer diameter of 56 mm, the sample being subjected to three loading cycles from 0.025 MPa to 1.0 MPa and the compression set value being the difference between the thickness measured at 1.0 MPa in the first loading cycle and the third loading cycle, a reduction in the mean roughness R a , determined according to DIN EN ISO 4288:1998-04 using the stylus method, compared to a non-treated gas diffusion layer of at least 10%, a reduction in the roughness depth R z, determined according to DIN EN ISO 4288:1998-04 using the stylus method, compared to a non-treated gas diffusion layer of at least 10%, 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 95 g / m 2 and an MPL loading of 15 g / m 2 .

12. A fuel cell comprising at least one gas diffusion layer as defined in any one of claims 10 or 11, or obtainable by a process as defined in any one of claims 1 to 9.

13. A fuel cell according to claim 12, comprising a polymer electrolyte membrane to which a catalyst layer is applied, wherein the catalyst layer is in contact with the surface of the microporous layer B) of the gas diffusion layer.

14. Use of a gas diffusion layer as defined in any one of claims 10 or 11, or obtainable by a process as defined in any one of claims 1 to 9, in a proton exchange membrane fuel cell.

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