Gas diffusion layer for fuel cells, having a gradient of properties and low plastic deformability, and method for producing same

Abstract

The present invention relates to a method for producing a gas diffusion layer for a fuel cell, the gas diffusion layer having a gradient of properties and low plastic deformability (low setting behavior). 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 for fuel cells with property gradient and low plastic deformability and process for its production

[0002] The present invention relates to a method for producing a gas diffusion layer for a fuel cell with a property gradient and low plastic deformability (low settling behavior). 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 (H2-2H + + 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] 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. Furthermore, the GDL also acts as a mechanical balance between the macrostructured flow distribution plate and the catalyst layers. For this purpose, 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. Therefore, high demands are placed on the mechanical properties of the GDL.

[0007] Gas diffusion layers for fuel cells typically consist of a carbon fiber substrate that is hydrophobically treated with fluoropolymers (e.g., PTFE) and then 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 using carbon materials such as carbon black or graphite powder. Other approaches to producing MPLs use carbon black in a silicone- or PVA-based binder, for example.

[0008] The development of fuel cells suitable for everyday use is an important contribution to the energy transition from fossil to sustainable fuels. Therefore, there is currently a great demand for PEM fuel cells that are improved with regard to the complex property profile described above.

[0009] WO 2021 / 099129 A1 is based on the task of reducing or avoiding the disadvantages resulting from gradients in chemical composition and / or operating parameters between the supply and discharge of the operating media. To solve this problem, a gas diffusion layer is used that has a property gradient with respect to its base area (in the x,y plane), thereby improving the distribution of the operating media through the GDL. By specifically adapting the GDL properties to the gradients of the operating media, as determined by the ambient conditions of the half-cell reactions, the performance of the fuel cell can be significantly increased. In particular, the fluctuations in the current density across the active area can be reduced.

[0010] WO 2022 / 002932 A1 describes a gas diffusion layer for a fuel cell with a planar extent and a layer thickness measured perpendicular to the planar extent, wherein at least one of the physical properties, selected from hydrophobicity and permeability, changes from an initial value in an initial region to a final value in an end region in at least one direction along the planar extent. The gas diffusion layer can have a microporous layer applied thereto. 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 electrical contact resistance, the electrical volume resistance, the wetting angle, and the hydrophobicity can be adjusted, for example, by using different local densities or contents of a hydrophobic material. The porosity can also be adjusted using a variable drilling pattern when using a laser. The disclosure of this application is vague and lacks both information on the implementation of the described concepts and a reproducible embodiment and 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] 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 to the stack design may be necessary, such as the use of additional spring assemblies to compensate for the stress loss that occurs during assembly. 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 securing them, which increases manufacturing costs.

[0013] The invention is based on the object of avoiding or at least mitigating the disadvantages described above. Surprisingly, it has now been found that gas diffusion layers with a good property profile, especially with regard to their transport properties, and with significantly improved settling behavior can be achieved if the gas diffusion length is given a property gradient with regard to at least one chemical and / or physical property with respect to its base area (in the x,y plane) and subjected to post-treatment at elevated pressure and, if appropriate, elevated temperature. This allows the plastic deformation component of the GDL to be significantly reduced. Surprisingly, it has now been found that this post-treatment can also have a positive effect on the gradients. Especially with regard to transport properties, such as gas permeability and dry diffusion length, even more pronounced gradients can thus be achieved.

[0014] SUMMARY OF THE INVENTION

[0015] A first subject of the invention is a method for producing a gas diffusion layer for a fuel cell, comprising

[0016] A) a flat electrically conductive fiber material and

[0017] B) a microporous layer on at least one of the surfaces of the fiber material, wherein the microporous layer has at least one property gradient with respect to at least one chemical and / or physical property with respect to the base surface of the gas diffusion layer (in the x,y plane), 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 B), wherein the composition of the precursor is varied to produce a gradient, iii) the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and optionally elevated temperature.

[0018] Another object of the invention is a gas diffusion layer obtainable by a process as described above and below.

[0019] Another object of the invention is a gas diffusion layer for a fuel cell, comprising

[0020] A) a flat electrically conductive fiber material and

[0021] B) a microporous layer on at least one of the surfaces of the fiber material, wherein the gas diffusion layer has at least one property gradient with respect to its base area (in the x,y plane) with respect to at least one chemical and / or physical property and the gas diffusion layer has been subjected to a post-treatment at elevated pressure and optionally elevated temperature, so that it has a reduced plastic deformability compared to a non-post-treated gas diffusion layer.

[0022] In a special design, the gas diffusion layer has a reduced compression set value compared to a non-treated gas diffusion layer.

[0023] Another subject of the invention is a fuel cell comprising at least one gas diffusion layer as defined above and below.

[0024] 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.

[0025] DESCRIPTION OF THE INVENTION The gas diffusion layers according to the invention and obtained by the process according to the invention have the following advantages:

[0026] The gas diffusion layers exhibit significantly improved settling behavior. The inventive post-treatment at elevated pressure and, if necessary, elevated temperature can significantly reduce the plastic deformation component of the GDL.

[0027] 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 stress loss, can be reduced or even eliminated entirely. Surprisingly, it was found that post-treatment can also have a positive effect on the gradients. Especially with regard to transport properties, such as gas permeability and dry diffusion length, even more pronounced gradients can be achieved. Furthermore, the raw materials used to create the gradients lead to highly plastic deformation behavior without post-treatment. Different regions of the gradient can exhibit different settling behavior. This is not only significantly minimized by the post-treatment according to the invention, but the different regions of the gradient are generally also homogenized.

[0028] Thanks to the at least one property gradient exhibited by the gas diffusion layers according to the invention, the properties of the GDL can be specifically adapted to the operating conditions of the respective fuel half-cell. The GDL is characterized by improved properties regarding the distribution of operating materials. In particular, it is possible to control various transport processes independently of one another through the GDL. For example, the transport of liquid water and gaseous water can be adjusted separately. The oxygen transport through the GDL to the cathode can also be specifically controlled.

[0029] The gas diffusion layers according to the invention can be produced easily and cost-effectively.

[0030] Due to the property gradient of the gas diffusion layers according to the invention, fluctuations in the current density across the active area can be reduced in the resulting fuel cells.

[0031] 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 deformation. 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. Gas diffusion layers with low settling behavior are characterized by low compression set values. Compression set and methods for measuring it are described in detail below.

[0032] 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.

[0033] Regarding the fiber materials A) and the precursors and conditions for forming a microporous layer B), reference is made in full to the explanations below. iii) Post-treatment at elevated pressure and temperature

[0034] In a special 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 100°C.

[0035] Preferably, the treatment in step iii) is carried out at a pressure in the range from 5 to 100 bar (0.5 to 10.0 MPa), particularly preferably from 15 to 80 bar.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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).

[0040] 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.

[0041] 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.

[0042] In a further preferred embodiment, the treatment in step iii) takes place in a calender. In principle, known and commercially available calenders can be used in step iii) of the process according to the invention. For example, it is 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, an offset upper roll, or an offset lower roll.A four-roll calender can have an L-arrangement, an inverted L-arrangement, an S-arrangement, a Z-arrangement or any other arrangement of rolls.

[0043] 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.

[0044] Preferably, the calendering in step iii) is carried out at a web speed of 0.05 m / min to 30 m / min. Specifically, the treatment in step iii) is carried out 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. Fiber material A) and

[0045] 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.

[0046] 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.Within the scope of the invention, an orthogonal coordinate system is used to describe the GDL, with the base area 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). The mass transport between the flow distributor plate and the membrane essentially occurs in the z-axis direction.

[0047] According to the invention, the gas diffusion layer has at least one property gradient with respect to at least one chemical and / or physical property. This means that at least one property of the gas diffusion layer is location-dependent. The property gradient can extend over one, two, or all three spatial directions. It can extend over the entire length in one spatial direction or a specific section. The change in property can be sudden (i.e., the gas diffusion layer according to the invention has heterogeneity with respect to at least one property) or continuous (i.e., the gas diffusion layer according to the invention has inhomogeneity with respect to at least one property). A sudden change in property generally has at least 2, preferably at least 3, in particular at least 4 steps with respect to the property exhibiting the gradient.Both the sheet-like fiber material A) and the microporous layer B) or both can have at least one property gradient.

[0048] Preferably, at least the microporous layer B) has at least one property gradient. Preferably, at least the cathode-side gas diffusion layers of the fuel cells according to the invention have an MPL that has a property gradient with respect to the base area (in the x,y plane) of the GDL. It has been found that by using an MPL that has a property gradient with respect to the base area (in the x,y plane) of the GDL, a more uniform current density distribution of the fuel cell can be achieved. In a specific embodiment, only the microporous layer has one or more property gradients.

[0049] Preferably, the gas diffusion layer (i.e., the sheet-like fiber material A) and / or the microporous layer B)) has at least one property gradient that changes monotonically depending on the location. A monotonic property change is understood to mean that the function value representing the property change either always increases or always decreases when the value for the spatial coordinate increases. It is permissible for the function value representing the property change to remain constant over the course of the spatial coordinate, even over one or more sub-areas. However, it does not exhibit any local minima or maxima.

[0050] Preferably, the gas diffusion layer (ie only the sheet-like fiber material A) or only the microporous layer B) or the sheet-like fiber material A) and the microporous layer B) has only property gradients that change monotonically depending on the location.

[0051] Preferably, at least the microporous layer B) has at least one property gradient that changes monotonically depending on the location. In a specific embodiment, only the microporous layer has at least one property gradient that changes monotonically depending on the location. In a further specific embodiment, the microporous layer has only property gradients that change monotonously depending on the location. More specifically, the microporous layer has only a single property gradient, and this single property gradient changes monotonously depending on the location. 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, wovens, and combinations thereof.Suitable substrate materials are fiber materials that are conductive themselves 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 the 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.

[0052] Carbon fibers can be produced in the conventional manner, with polyacrylonitrile fibers (PAN fibers) preferably being used 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.

[0053] The fiber material A) is preferably selected from carbon fiber fabrics, carbon fiber papers, and carbon fiber nonwovens. 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 permanently bonded together. 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.

[0054] 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.

[0055] 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, whereby 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.

[0056] 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.

[0057] 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).

[0058] The fiber materials A) contained in the gas diffusion layer may contain conventional additives a3). These are preferably selected from hydrophobizing agents, conductivity-enhancing additives, surfactants, and mixtures thereof.

[0059] To improve transport processes through the GDL and at the interfaces, it may 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, typically at least 300 °C.

[0060] 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).

[0061] 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.

[0062] 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).

[0063] 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), ie, before the post-treatment in step iii) and before the incorporation of the GDL into a fuel cell.

[0064] The fiber material A) preferably has a porosity in the range of 10 to 90%, particularly preferably 20 to 85%. The porosity of the fiber material can be calculated from the measured thickness and the measured basis weight, given a known fiber density. For example, for a carbon fiber density of 1.8 g / cm 3 Porosity [%] = [(1.8 - area weight / thickness) / 1.8] x 100. It is also possible to determine the density of the gas diffusion layer using helium density measurement and the specific pore volume using mercury porosimetry. The porosity is then calculated as follows: Porosity [%] = specific pore volume / (specific pore volume + 1 / He density) x 100%.

[0065] 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.

[0066] 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 a microporous layer (MPL) B) on one of the surfaces of the fiber material A).

[0067] 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, more preferably from 10 to 100 nm. The average pore diameter can again be determined by 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 also be created. Depending on the composition, the average pore diameter is then, for example, greater than 1 pm.

[0068] The MPL contains conductive carbon particles, preferably carbon black or graphite, in a matrix of a polymeric binder. Preferred binders are the aforementioned fluorine-containing polymers, especially polytetrafluoroethylene (PTFE).

[0069] 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.

[0070] The presence of the MPL has a significant impact on the water balance of the fuel cell. Due to the high PTFE content and the smaller pores of the MPL, flooding of the GDL and the electrode is made more difficult, as the MPL acts as a liquid water barrier and thus promotes the mass transport of the gaseous reactants to the catalyst. It has been shown that it can be advantageous if the microporous layer in the gas diffusion layer according to the invention exhibits a property gradient with respect to its base area (in the x,y plane) of the GDL.

[0071] 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 75 to 1000 pm, particularly preferably from 100 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.

[0072] Furthermore, the gas diffusion layers preferably have a high total porosity. This is preferably in the range of 20% to 80%, determined, as previously described, by helium density measurement and mercury porosimetry.

[0073] Method for producing a gas diffusion layer

[0074] Step i

[0075] Regarding the suitable and preferred fiber materials A) used in step i), reference is made in full to the previous explanations. Step ii)

[0076] 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.

[0077] 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.

[0078] Preferably, the fiber material A) is coated with at least 2, preferably at least 3, in particular at least 4 laterally adjacent strips of precursors of different compositions to form a microporous layer. The MPL can be applied 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. The MPL layer thickness and penetration depth can be influenced by the coating process parameters as well as the viscosity of the coating. Finally, another thermal treatment follows, e.g., in a drying and sintering furnace. This can initially involve drying at a temperature of 100 to 200 °C, followed by sintering at a temperature of 300 to 500 °C.

[0079] Property gradient

[0080] As previously mentioned, both the sheet-like fiber material A) and the microporous layer B) as well as both can have at least one property gradient.

[0081] The property which exhibits the gradient is in principle selected from the chemical composition of the sheet-like fibre material A) and / or the microporous layer B), the mechanical properties of the sheet-like fibre material A) and / or the microporous layer B), the transport properties of the sheet-like fibre material A) and / or the microporous layer B), combinations thereof.

[0082] The chemical properties of the sheet-like fiber material A) and / or the microporous layer B), which may exhibit a gradient, include, for example, the content of hydrophobic agents, carbon particles, etc. This includes, in particular, the content of PTFE, carbon black, graphite, graphene, carbon nanotubes (CNT), carbon nanofibers, and mixtures thereof.

[0083] The mechanical properties of the sheet-like fiber material A) and / or the microporous layer B), which may exhibit a gradient, include, for example, the density, the area-related mass, the porosity and the average pore diameter.

[0084] Determining the density in g / m 3 can be done by helium density measurement as described previously.

[0085] 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.

[0086] The determination of porosity and pore size distribution can be carried out using mercury porosimetry, as described in DIN ISO 15901-1:2019-03 and ISO 15901-1:2016- Part 1: Mercury porosimetry.

[0087] To create a gradient in mechanical properties, for example, the compression behavior of the microporous layer can be gradient-induced by varying its composition with respect to at least one of its materials. This also changes the bonding to the electrode. Alternatively, a gradient in mechanical properties can be created by creating a gradient across the material width during nonwoven bonding using water jets. This influences mechanical properties and water transport.

[0088] The transport properties of the sheet-like fiber material A) and / or the microporous layer B), which may have a gradient, include: the gas permeability of the sheet-like fiber material A) and / or the microporous layer B), the liquid permeability of the sheet-like fiber material A) and / or the microporous layer B), the electrical resistance of the gas diffusion layer through the material plane, the thermal resistance of the gas diffusion layer through the material plane, the dry diffusion length.

[0089] In a preferred embodiment, the microporous layer (MPL) has at least one property gradient with respect to at least one chemical and / or physical property. This includes, in particular, the mechanical properties and transport properties. The MPL has the at least one property gradient with respect to its base area, i.e., in the plan view or in the x,y plane. Optionally, the MPL can additionally have a property gradient perpendicular to its base area, i.e., in the direction of the z-axis.

[0090] The microporous layer preferably has at least 2, preferably at least 3, in particular at least 4, especially at least 5, more specifically at least 6 discrete regions which differ in at least one property. In this embodiment, the change in properties between the regions is abrupt. The individual regions can all differ in terms of the same property or (in the case of several different properties) the same properties. This is preferred. However, it is also possible for two or more regions to differ in terms of different properties. In a specific embodiment, the microporous layer has at least 2, preferably at least 3, in particular at least 4, especially at least 5, more specifically at least 6 discrete regions which all differ in terms of one and the same property.

[0091] In a special design, each individual region is essentially homogeneous in terms of its properties. Essentially homogeneous means that within a region, only those property fluctuations occur that would also occur (e.g., due to manufacturing) without intentionally creating a gradient.

[0092] In an alternative embodiment, the microporous layer has at least one continuous property gradient. Preferably, the microporous layer has at least 2, preferably at least 3, in particular at least 4 laterally adjacent strips that differ in at least one property. In a specific embodiment, the microporous layer has at least 2, preferably at least 3, in particular at least 4 laterally adjacent strips that all differ in one and the same property. Specifically, each individual strip is substantially homogeneous with respect to its properties.

[0093] Preferably, the property of the microporous layer having a gradient is selected from the Gurley gas permeability and the dry diffusion length.

[0094] Physical quantities, measurement methods

[0095] 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.

[0096] The measurement of gas permeability in l / m 2 It can also be carried out according to DIN EN ISO 9237:1995-12 to determine the air permeability of textile fabrics.

[0097] The permeability for liquids, especially for liquid water, perpendicular to the material plane (liquid-water permeability "through-plane") can be determined using a so-called "filtration cell" or according to the "Penn State" method [see references a - c]: [a] IS Hussaini and CY Wang, "Measurement of relative permeability of fuel cell diffusion media," Journal of Power Sources, vol. 195, pp. 3830-3840, 2010; [b] JD Sole, "Investigation of water transport parameters and processes in the gas diffusion layer of PEMFCs," Virginia Polytechnic Institute, 2008; [c] J. Benziger, J. Nehlsen, D. Blackwell, T. Brennan, and J. Itescu, "Water flow in the gas diffusion layer of PEM fuel cells," Journal of Membrane Science, vol. 261 , pp. 98-106, 2005.

[0098] The (specific) electrical resistance through the plane (TP) can be determined using a four-point measurement in a manner well-known in the literature. The dry diffusion length refers to 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.

[0099] Two well-known test methods can be used to determine the through-plane (TP) thermal resistance: the heat flow method or the laser flash method.

[0100] The compression set value is a measure of how a material, in this case a GDL, behaves under compression and subsequent relaxation. GDLs are typically heavily compressed when used in fuel cells. The proportion of elastic and plastic deformation (as well as, if necessary, the specification of other physical parameters such as gas permeability and electrical resistance) can be used to characterize the properties of a GDL resulting from compression. Plastic deformation occurs when a material does not fully return to its original shape after a load, but instead undergoes a permanent change in shape. The compression set is the permanent deformation that remains after the applied force is removed.

[0101] The compression set value can be determined as follows. It is possible to simultaneously determine the values ​​of other physical quantities, such as thickness, gas permeability, and electrical resistance, at a specific compressive force and after single or multiple applications.

[0102] 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.

[0103] 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.

[0104] Preferably, the GDL according to the invention has a compression set value at 6 bar (0.6 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 6 pm at the latest.

[0105] 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.

[0106] 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.

[0107] fuel cell

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] FIGURE DESCRIPTION

[0113] Figure 1 shows a top view of a GDL material, the production of which is described in Example 1. Four laterally adjacent strips of four different MPL pastes (pastes 1 to 4), each 7 to 8 cm wide, were applied longitudinally to a nonwoven fabric in DIN A3 format (29.7 x 42 cm, where md denotes the machine direction). Gas diffusion layers measuring 274.8 x 96.5 mm were punched out of the dried and sintered material with the long side perpendicular to the machine direction. Figure 1 shows three alternative punching positions (GDLs 1 to 3). The resulting GDLs exhibit a property gradient along the x-axis, with GDL 1 having four strips, each with different properties, and GDLs 2 and 3 each having three strips with different properties. Of course, other punching positions and other formats are also possible, which can also have an angle other than 90° to the x-axis in order to vary the gradients.The GDL can also be punched into other shapes, such as squares, ovals, or circles, to create additional GDL variants with individual gradients. The GDLs according to the invention were subjected to post-treatment at elevated pressure and temperature. The untreated GDLs serve as a comparison.

[0114] Figure 2 shows the plastic deformation properties (settling behavior) based on the compression set values ​​at 6 bar for the four strips of the comparison GDL (left bars) and the inventive GDL (right bars).

[0115] Figure 3a shows the dry diffusion length in pm (triangles), the thickness of the GDL at 2 MPa in pm (squares) and the Gurley values ​​in s (circles) for the four strips of the comparison GDL.

[0116] Figure 3b shows the dry diffusion length in pm (triangles), the thickness of the GDL at 2 MPa in pm (squares) and the Gurley values ​​in s (circles) for the four strips of the inventive GDL.

[0117] Figure 4 shows the changes in the dry diffusion length (triangles) and Gurley values ​​(circles) for the four strips as a result of post-treatment at elevated pressure and temperature. The graphs show the differences in the values ​​(A values) for the inventive GDL (GDL 1) and the comparison GDL (GDL V1).

[0118] The following examples serve to illustrate the invention without limiting it in any way.

[0119] EXAMPLES

[0120] Production example 1:

[0121] Production of a gas diffusion layer according to the invention and a comparison diffusion layer with property gradient in the x-direction

[0122] From a commercially available electrically conductive fiber fleece with a thickness of 0.145 mm, a basis weight of 60 g / m 2 and a through plane resistance at 1 MPa compression of 6.6 mQcm2 Sheets in DIN A3 format (29.7 x 42 cm) were punched lengthwise (machine direction, md) from the GDL roll and individually coated. To create a microporous layer with a property gradient, four laterally adjacent strips of MPL pastes, each 7 to 8 cm wide, were applied lengthwise to the nonwoven fabric (see Figure 1). The pastes had a composition as shown in Table 1. To produce them, PTFE, various carbons, and plastic particles were dispersed in distilled water as pore formers and applied to the nonwoven fabric by doctor blade coating with a doctor blade gap of 240 μm. The sheets were then dried at 160 °C and sintered at 400 °C. The resulting MPL loadings ranged from 15 to 22 g / m², depending on the strip. 2 .

[0123] Table 1

[0124] 1 ) each based on the total weight of the sintered paste

[0125] Gas diffusion layers measuring 274.8 x 96.5 mm were punched from the resulting sheets with the long side perpendicular to the machine direction. Figure 1 shows three alternative punching positions. The resulting GDLs exhibit a property gradient along the x-axis, with GDL 1 having four strips, each with different properties, and GDLs 2 and 3 each having three strips with different properties.

[0126] To manufacture a fuel cell, the GDLs can, for example, be installed so that the x-direction (long side) is in the direction of the direct connection between the supply and discharge of the operating fluids to the flow distributor plate. In a flow distributor plate with straight channels, the long side of the GDL is thus parallel to the gas channels. However, even with a different flow field design, the installation can be such that the O2-rich fuel supplied on the cathode side (air side) of the fuel cell first comes into contact with the MPL formed by Paste 1 (layer 1), and the O2-poor fuel discharged comes into contact with the MPL formed by Paste 4 (layer 4).

[0127] II) Application examples

[0128] For the measurements described below, the comparison GDL V1 and the inventive GDL 1 were used, each of which has 4 strips.

[0129] The compression set values ​​were determined using the method described in detail above. This method was also used to determine the thickness, gas permeability, rock diffusion length, and electrical resistance, each at a specific compressive force without prior application of force / during the first loading process. The values ​​are listed in Table 1 below.

[0130] 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 2.

[0131] The dry diffusion length was determined using a stationary Wicke-Kallenbach cell. The results can also be found in Table 2.

[0132] The determination of the (specific) electrical resistance through the plane (TP) was carried out by 4-point measurement.

[0133] The inventive post-treatment of the gas diffusion layers at elevated pressure and temperature leads to a significant reduction in the proportion of plastic deformation. Figure 2 demonstrates this using the compression set values ​​at 6 bar. The deformation of the post-treated GDL is predominantly elastic. Furthermore, a clear homogenization of the settling behavior of the various strips is also evident. The variance in settling behavior from strip to strip decreases significantly.

[0134] Figures 3a, 3b, and 4 show the influence of the post-treatment according to the invention on the transport properties of the GDL and their gradients based on the dry diffusion length and the Gurley values. Depending on the composition of the strips, the transport properties change significantly (strips 1 and 2) or only slightly (strips 3 and 4) due to treatment at elevated pressure and temperature. This can be used to control the gradient characteristics as needed.

[0135] Tabelle 2

[0136] TP = through plane

Claims

Patent claims 1. A 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, wherein the microporous layer has at least one property gradient with respect to at least one chemical and / or physical property with respect to the base surface of the gas diffusion layer (in the x,y plane), 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, wherein the composition of the precursor is varied to produce a gradient, iii) the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and optionally elevated temperature.

2. Process according to 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.

3. The process according to claim 1 or 2, 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. The method according to any one of the preceding claims, wherein the composition of the precursor is varied during coating such that the microporous layer has at least one monotonic property gradient with respect to the base area of ​​the gas diffusion layer (in the x,y plane).

9. A gas diffusion layer obtainable by a process as defined in any one of claims 1 to 8.

10. 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 property gradient with respect to its base area (in the x,y plane) with respect to at least one chemical and / or physical property and the gas diffusion layer has been subjected to a post-treatment at elevated pressure and optionally elevated temperature, so that it has a reduced plastic deformability compared to a non-post-treated gas diffusion layer.

11. Gas diffusion layer according to claim 9 or 10, which has a reduced compression set value compared to a non-post-treated gas diffusion layer.

12. Gas diffusion layer according to one of claims 9 to 11, wherein the microporous layer has at least one property gradient, preferably a continuous or discontinuous property gradient which changes monotonically depending on the location. The gas diffusion layer according to any one of claims 9 to 12, wherein the microporous layer has at least 2, preferably at least 3, in particular at least 4 regions that differ in at least one property. The gas diffusion layer according to any one of claims 9 to 13, wherein the property that has a gradient is selected from the chemical composition of the sheet-like fiber material A) and / or the microporous layer B), the mechanical properties of the sheet-like fiber material A) and / or the microporous layer B), the transport properties of the sheet-like fiber material A) and / or the microporous layer B), and combinations thereof. The gas diffusion layer according to any one of claims 9 to 14, which has a gradient of the dry diffusion length and / or the Gurley gas permeability.A fuel cell comprising at least one gas diffusion layer as defined in any one of claims 9 to 15, or obtainable by a process as defined in any one of claims 1 to 8. A fuel cell according to claim 16, 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. Use of a gas diffusion layer as defined in any one of claims 9 to 15, or obtainable by a process as defined in any one of claims 1 to 8, in a proton exchange membrane fuel cell.

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