Gas diffusion layer for fuel cell having characteristic gradient and low plastically

Abstract

The present invention relates to a method for producing a gas diffusion layer for a fuel cell having a characteristic gradient and a low plastic deformability (low permanent deformation), to a gas diffusion layer obtained according to the method, and to a fuel cell comprising such a gas diffusion layer.

Description

Field of the Invention

[0001] The invention relates to a method for producing a gas diffusion layer for a fuel cell, the gas diffusion layer having a property gradient and low plastic deformability (low permanent deformation), a gas diffusion layer obtained according to the method, and a fuel cell comprising such a gas diffusion layer. Background Art

[0002] Fuel cells use a fuel (especially hydrogen) to react with oxygen to produce water to generate electricity. In a hydrogen-oxygen fuel cell, hydrogen or a gas mixture containing hydrogen is supplied to the anode, where it undergoes electrochemical oxidation and releases electrons (H2→2H + +2e - ). The protons are transported from the anode region to the cathode region via a membrane, which separates the reaction chambers from one another in a gas-tight manner and electrically insulates them. The electrons provided at the anode are transported to the cathode via an external conductor circuit. Oxygen or a gas mixture containing oxygen is supplied to the cathode, where the oxygen is reduced by absorbing electrons. The oxygen anions formed in the process react with the protons transported via the membrane and form water (1 / 2O2 + 2H + +2e - →H2O).

[0003] Many applications (especially automotive powertrains) use low-temperature proton exchange membrane fuel cells (PEMFCs), which are core components that allow only protons (or hydronium ions H3O + ) and water pass through and spatially separate the oxidant (generally atmospheric oxygen) from the reductant. Catalyst layers are applied on the anode and cathode sides of the gas-tight electrically insulating proton-conducting membrane, which form electrodes and usually contain platinum as a catalytically active metal. The actual redox reaction and charge separation take place in the catalyst layers. The membrane and the catalyst layer form a unit also known as CCM (catalyst coated membrane). Gas diffusion layers (GDL) are provided on both sides of the CCM, which stabilize the cell structure and assume the transport and distribution functions of the reaction gases, water, heat and current. The membrane, electrodes and gas diffusion layers form a membrane electrode unit (MEA). Distribution plates (so-called bipolar plates) are provided between the membrane electrode units. These distribution plates have channels for supplying process gases to the adjacent cathodes and anodes, and usually also have internal cooling channels.

[0004] The gas diffusion layer plays a crucial role in the function and performance of the fuel cell. On the one hand, the gas diffusion layer is used to transport the process components consumed and generated in the electrode reaction, and on the other hand, it conducts the electrons generated and consumed in the half-cell reaction and the heat generated in the reaction to the distribution plate. In addition, the GDL also plays a mechanical compensation role between the macro-structured distribution plate and the catalyst layer. To this end, the component tolerances must be compensated and the compression pressure must be distributed. The GDL also provides mechanical protection for the very thin membranes that are exposed to high loads in the fuel cell. This places high demands on the mechanical properties of the GDL.

[0005] Gas diffusion layers for fuel cells are generally made of a carbon fiber substrate that is made hydrophobic with a fluoropolymer (e.g., PTFE) and then coated with a microporous layer (MPL). MPL is usually made of a fluoropolymer (e.g., PTFE) as a binder and a conductive material, wherein carbon materials such as carbon black or graphite powder are usually used. Other ways to make MPL use carbon black in silicone-based or PVA-based binders.

[0006] The development of fuel cells suitable for everyday use makes an important contribution to the energy transition from fossil fuels to sustainable fuels. Therefore, there is currently a great demand for PEM fuel cells that have improved aspects of the above-mentioned complex characteristics.

[0007] The purpose of WO 2021 / 099129 A1 is to reduce or avoid disadvantages caused by gradients related to the chemical composition and / or operating parameters between the input and output of the working medium. To achieve this purpose, a gas diffusion layer is used, which has a characteristic gradient on its base surface (xy plane) so as to improve the distribution of the GDL to the working medium. By specifically adjusting the GDL characteristics according to the gradient of the working medium, as predetermined by the environmental conditions of the half-cell reaction, the performance of the fuel cell can be significantly improved. In particular, current density fluctuations on the active surface can be reduced.

[0008] WO 2022 / 002932 A1 describes a gas diffusion layer for a fuel cell, which has a planar extension and a layer thickness measured perpendicular to the planar extension, wherein at least one physical property (selected from hydrophobicity and permeability) changes from an initial value in an initial region to a final value in a final region in at least one direction extending along the plane. The gas diffusion layer may have a microporous layer applied thereon. Specifically, for example, hydrophobicity is controlled by the content of a hydrophobic material (such as PTFE), and permeability is controlled by the porosity of the gas diffusion layer. The thickness of the applied microporous layer may be affected, which mainly causes a change in the local penetration depth of the carrier layer. For example, by using hydrophobic materials with different local densities or contents, contact resistance, volume resistance, wetting angle and hydrophobicity can be adjusted. When using a laser, the porosity can also be set by a variable drilling mode. The disclosure of this application is not very specific, and lacks both instructions for executing the described scheme and repeatable embodiments and application data.

[0009] EP 3957789 A1 describes a gas diffusion layer having high thermal conductivity despite its low density, and having good operability and battery performance. The GDL comprises a carbon fiber felt containing carbon fibers having an average fiber diameter of 5 μm to 20 μm, wherein at least a portion of the carbon fibers forming the carbon fiber felt has a flat portion, wherein a maximum value of the fiber diameter observed from a plan view of the surface of the carbon fiber felt is 10% to 50% greater than the average fiber diameter, and a frequency of occurrence of the flat portion on the surface of the carbon fiber felt is 50 / mm 2 Up to 200 / mm 2 .

[0010] When used in fuel cells, the GDL is usually pressed (compressed) with high strength. Due to the compression, the properties of the GDL can be characterized by the ratio of elastic to plastic deformation. In plastic deformation, the gas diffusion layer cannot return to its original shape 100% after loading, and its shape is permanently changed. The property of a material to permanently change its shape when stress is applied, that is, deformability, is also expressed by the term "permanent deformation". Materials with low plastic deformability have low permanent deformation performance. The permanent deformation of the GDL known in the prior art still needs to be improved. If the GDL in the fuel cell stack is tensioned under high pressure, permanent deformation will occur due to the tension force and dynamic force changes in operation. This may lead to compression pressure losses in the fuel cell stack, thereby increasing the material resistance of most components in the stack, mainly its transition resistance. In addition, it may be necessary to adjust the stack design, such as using additional spring groups to compensate for the tension losses that occur during permanent deformation. As a result, the length of the stack and the required installation space may increase. In addition, additional measures may be required when installing the stack, such as tightening and loosening the stack several times during assembly before fixing it, which increases the manufacturing difficulty.

[0011] The object of the present invention is to avoid or at least reduce the above-mentioned disadvantages. It has now surprisingly been found that if a gas diffusion layer has a property gradient with respect to its base surface (x, y plane) in at least one chemical and / or physical property and this gas diffusion layer is subjected to a post-treatment at high pressure and optionally high temperature, a gas diffusion layer with good properties (in particular with respect to its transport properties) and with a significantly improved permanent deformation can be obtained, which can significantly reduce the plastic deformation proportion of the GDL. It has surprisingly been found that this post-treatment also has a positive influence on the gradient. Thus, in particular with respect to the transport properties (such as permeability, dry diffusion length), a more pronounced gradient can be achieved. Summary of the invention

[0012] A first subject of the present invention is a method for producing a gas diffusion layer for a fuel cell, the gas diffusion layer comprising

[0013] A) a planar conductive fiber material, and

[0014] 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 the base surface of the gas diffusion layer (in the x, y plane) with respect to at least one chemical and / or physical property,

[0015] in

[0016] i) providing a planar conductive fiber material A),

[0017] ii) coating the fiber material provided in step i) with a precursor for forming the microporous layer B), wherein the composition of the precursor is varied to produce a gradient,

[0018] iii) post-treating the coated fiber material obtained in step ii) at high pressure and optionally high temperature.

[0019] Another subject of the invention is a gas diffusion layer obtained by a process as described above and below.

[0020] Another subject of the invention is a gas diffusion layer for a fuel cell, the gas diffusion layer comprising:

[0021] A) a planar conductive fiber material, and

[0022] B) a microporous layer on at least one of the surfaces of the fiber material,

[0023] The gas diffusion layer has at least one property gradient in at least one chemical and / or physical property relative to its base surface (in the x, y plane), and the gas diffusion layer is post-treated at high pressure and optionally high temperature so that the gas diffusion layer has reduced plastic deformability compared to a gas diffusion layer that has not been post-treated.

[0024] In a particular embodiment, the gas diffusion layer has a reduced compression set value relative to a gas diffusion layer that has not been post-treated.

[0025] Another subject matter of the invention is a fuel cell comprising at least one gas diffusion layer as described above and below.

[0026] A further subject of the invention is the use of a gas diffusion layer as described above and below or a gas diffusion layer obtained by a process as described above and below in a proton exchange membrane fuel cell.

[0027] Description of the invention

[0028] The gas diffusion layer of the present invention obtained by the method of the present invention has the following advantages:

[0029] - The gas diffusion layer has a significantly improved permanent deformation. By carrying out the post-treatment according to the invention at high pressure and optionally high temperature, the plastic deformation proportion of the GDL can be significantly reduced.

[0030] Design measures for adjusting the fuel cell stack to the consequences associated with permanent deformation of the GDL, such as reduced compression pressure, increased material resistance of installed components and voltage losses, can be reduced or can be omitted entirely.

[0031] - Surprisingly, it has been found that post-treatment also has a positive effect on the gradient. In particular, with regard to transport properties (e.g. air permeability, dry diffusion length), a more pronounced gradient can be achieved. Furthermore, the raw materials used to generate the gradient can lead to strong plastic deformation behavior without post-treatment. Different regions of the gradient can have different permanent deformation behaviors. By means of the post-treatment according to the invention, these cases are not only significantly reduced, but also the different regions of the gradient are generally homogenized.

[0032] - The at least one characteristic gradient of the gas diffusion layer according to the invention makes it possible to adjust the properties of the GDL in a targeted manner according to the operating conditions of the respective fuel half-cell. The GDL is characterized by improved properties in terms of the distribution of the operating material. In particular, different transport processes through the GDL can be controlled independently of one another. Thus, for example, the transport of liquid water and gaseous water can be set independently. The transport of oxygen through the GDL to the cathode can also be controlled in a targeted manner.

[0033] The gas diffusion layer according to the invention can be produced simply and cost-effectively.

[0034] The characteristic gradient of the gas diffusion layer according to the invention can reduce current density fluctuations on the active surface in the formed fuel cell.

[0035] Plastic deformation refers to the fact that a material (such as a gas diffusion layer) cannot return to 100% of its original shape after being loaded, but instead undergoes a permanent change of shape. Part of the deformation is elastic and therefore reversible, and only a certain part is plastic and remains permanent. The property of a material to permanently change its shape when a stress is applied, i.e., deformability, is also expressed by the term "permanent deformation". Materials with low plastic deformability exhibit low permanent deformation. Gas diffusion layers with low permanent deformation are characterized by low compression set values. Compression set and its measurement method are described in detail below.

[0036] The method of the present invention comprises the following steps:

[0037] i) providing a planar conductive fiber material A),

[0038] ii) coating the fiber material provided in step i) with a precursor for forming the microporous layer B), wherein the composition of the precursor is varied to produce a gradient,

[0039] iii) post-treating the coated fiber material obtained in step ii) at high pressure and optionally high temperature.

[0040] With regard to the fiber material A) and the precursors and conditions for forming the microporous layer B), please fully refer to the following description.

[0041] iii) Post-treatment under high pressure and high temperature

[0042] In a particular 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.

[0043] The treatment in step iii) is preferably carried out in a pressure range of 5 bar to 100 bar (0.5 MPa to 10.0 MPa), particularly preferably 15 bar to 80 bar.

[0044] Preferably, the treatment in step iii) is carried out in a temperature range of 100°C to 350°C, particularly preferably 120°C to 330°C, in particular 150°C to 320°C.

[0045] Preferably, said treatment in step iii) is carried out in a press for a period of time ranging from 5 seconds to 5 minutes, preferably from 10 seconds to 2 minutes.

[0046] Preferably, said treatment in step iii) is carried out in a calender for a period of time of more than 0 seconds to 10 seconds, preferably 0.1 seconds to 5 seconds.

[0047] Conventional equipment, such as a single-layer press or a multilayer press, a continuous belt press or a calender can be used for the post-treatment in step iii). In a particular embodiment, at least one double belt press is used for the post-treatment in step iii). In another particular embodiment, at least one calender is used for the post-treatment in step iii).

[0048] Single-layer or multi-layer presses are particularly suitable for discontinuous post-processing of segmented materials. Double-belt presses are suitable for processing both continuous web materials and segmented materials (sheets). Double-belt presses have two continuously circulating press belts between which the GDL web is post-processed while being transported in the forward direction under pressure and, if necessary, heat. The belts are parallel to each other and there is a gap between the upper and lower belts, which can be opened and closed to adjust to the thickness of the GDL material and to set the desired properties.

[0049] In a preferred embodiment, the treatment in step iii) is carried out in a double belt press. In particular, the treatment in step iii) is carried out in a double belt press at a pressure range of 1 MPa to 8 MPa (10 bar to 80 bar) and at a temperature range of 200° C. to 350° C.

[0050] In another preferred embodiment, the treatment in step iii) is carried out in a calender. In principle, a known commercially available calender can be used in step iii) of the method of the present invention. Therefore, a calender with 2, 3, 4 or more than 4 calender rollers can be used. In a simplest preferred embodiment, the calender used in the method of the present invention is a 2-roll calender. The gas diffusion layer can pass through the calender once or repeatedly, for example 1 time, 2 times, 3 times, 4 times, 5 times or more than 5 times. The calender rollers can be arranged in a geometric shape suitable for calendering the gas diffusion layer. A two-roll calender can have rollers arranged vertically, obliquely or horizontally. A three-roll calender can have a vertical layout, staggered upper rollers or staggered lower rollers. A four-roll calender can have rollers with an L layout, an inverted L layout, an S layout, a Z layout or other layouts.

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

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

[0053] In particular, the treatment in step iii) is carried out in a calender at a roller temperature range of 130° C. to 220° C., a line pressure range of 8 N / mm to 80 N / mm and a web speed of 1 m / min to 10 m / min.

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

[0055] In the context of the present invention, nonwoven fabrics / webs are generally understood to mean fabrics which consist largely of individual fibers, the cohesion of which is provided essentially only by their own adhesion. The conversion of a web into a nonwoven fabric by forming stronger bonds between the fibers than are present in the web is carried out by web consolidation processes, which are generally divided into mechanical, chemical and thermal processes. Webs, nonwovens and their production are described in H. Fuchs, W. Albrecht, "Nonwovens", 2nd edition (Wiley-VCH Publishers, Weinheim, Germany).

[0056] The planar conductive material A) and the gas diffusion layer used in the present invention are planar structures that are basically two-dimensionally extended and relatively thin. The base surface of the gas diffusion layer of the present invention generally corresponds to the base surface of the adjacent membrane with a catalyst layer and the base surface of the adjacent distribution plate in the fuel cell. The shape of the base surface of the gas diffusion layer can be, for example, a polygon (n-gon, n≥3, such as a triangle, a quadrilateral, a pentagon, a hexagon, etc.), a circle, a circle segment (such as a semicircle), an ellipse or an ellipse segment. The base surface is preferably rectangular or circular. Within the scope of the present invention, an orthogonal coordinate system is used to describe the GDL, wherein the base surface of the GDL is located in a plane described by the x-axis and the y-axis (also known as the xy plane). The z-axis orthogonal thereto is used to describe the material thickness. According to the conventional description applicable to fiber composite materials, the x-axis is also called the rolling direction (machine direction, machine direction, MD), and the y-axis is also called the rolling opposite direction (machine transverse direction, cross machine direction, CMD). Along the z-axis direction, material transmission is basically carried out between the distribution plate and the membrane.

[0057] According to the present invention, the gas diffusion layer has at least one characteristic gradient in at least one chemical and / or physical property. That is, at least one characteristic of the gas diffusion layer is position-dependent. The characteristic gradient can extend in one, two or all three spatial directions. The characteristic gradient can extend over the entire length along one spatial direction, or over a certain section. The characteristic change can be discontinuous (that is, the gas diffusion layer of the present invention has heterogeneity in at least one characteristic) or continuous (that is, the gas diffusion layer of the present invention has inhomogeneity in at least one characteristic). Discontinuous characteristic changes generally have at least 2, preferably at least 3, and in particular at least 4 stages relative to the characteristic having the gradient. The planar fiber material A), the microporous layer B), and both can have at least one characteristic gradient.

[0058] Preferably, at least the microporous layer B) has at least one characteristic gradient. Preferably, at least the cathode-side gas diffusion layer of the fuel cell of the present invention has an MPL having a characteristic gradient relative to the base plane (xy plane) of the GDL. The present invention has found that by using an MPL having a characteristic gradient relative to the base plane (xy plane) of the GDL, a more uniform fuel cell current density distribution can be achieved. In a specific embodiment, only the microporous layer has one or more characteristic gradients.

[0059] Preferably, the gas diffusion layer (i.e. the planar fiber material A) and / or the microporous layer B)) has at least one characteristic gradient that changes monotonically as a function of position. A monotonic characteristic change means that when the position coordinate value increases, the function value representing the characteristic change either always increases or always decreases. It is also possible for the function value representing the characteristic change to remain unchanged in one or more subareas during a change in the position coordinate. However, the function value does not have a local minimum or maximum value.

[0060] Preferably, the gas diffusion layer (ie only the planar fibrous material A) or only the microporous layer B) or both the planar fibrous material A) and the microporous layer B)) has only a property gradient which changes monotonically as a function of position.

[0061] Preferably, at least the microporous layer B) has at least one characteristic gradient that varies monotonically as a function of position. In a particular embodiment, only the microporous layer has at least one characteristic gradient that varies monotonically as a function of position. In another particular embodiment, the microporous layer has only a characteristic gradient that varies monotonically as a function of position. More particularly, the microporous layer has only a single characteristic gradient, and this one characteristic gradient varies monotonically as a function of position.

[0062] The gas diffusion layer comprises at least one planar conductive material as component A). Component A) preferably comprises a fiber material selected from a fiber mesh, paper, fabric and a combination thereof. Suitable substrate materials are fiber materials that are conductive themselves or that are made conductive by adding conductive additives (such as carbon particles or metal particles). In principle, fibers of carbon fibers, glass fibers, and organic polymers (such as polypropylene, polyester, polyphenylene sulfide, polyether ketone and mixtures thereof) are all suitable for use as substrate materials. The fibers contained in the fiber material A) preferably include carbon fibers or consist of carbon fibers (carbon fibers). Such carbon fibers are particularly advantageous in meeting the requirements for gas diffusion, liquid water permeability, electrical conductivity and thermal conductivity of the GDL. The fiber material A) is preferably selected from carbon fiber fabrics, carbon fiber paper and carbon fiber nonwovens. In a preferred embodiment, the fiber material A) includes at least one carbon fiber nonwoven, or the fiber material A) consists of a carbon fiber nonwoven.

[0063] Carbon fibers can be manufactured by conventional methods, wherein polyacrylonitrile fibers (PAN fibers) are preferably used as starting materials. PAN fibers are made by free radical polymerization of a monomer composition, which preferably contains at least 90% by weight of acrylonitrile, with reference to the total weight of the monomers used for polymerization. The resulting polymer solution is spun into filaments, for example, by wet spinning and coagulation spinning, and combined into ropes. Before the PAN precursor is converted into carbon fibers at high temperatures, the PAN precursor is usually subjected to oxidative cyclization (also referred to as oxidation for short) in an oxygen-containing atmosphere at elevated temperatures of about 180° C. to 300° C. The chemical crosslinking thus produced helps to improve the dimensional stability of the fibers. Subsequently, the actual pyrolysis into carbon fibers is carried out at a temperature of at least 1200° C. Depending on the shape of the desired fiber material, the initial fibers or the already planar fiber material can be used for this pyrolysis. Depending on the temperature during pyrolysis, a distinction is made between carbonization and graphitization. Carbonization refers to treatment under conditions of about 1200° C. to 1500° C. in an inert atmosphere to eliminate volatile products. So-called high modulus fibers or graphite fibers can be obtained by graphitization (i.e. heating to about 2000°C to 3000°C in an inert gas). These fibers are high in purity, light in weight, strong, and have excellent electrical and thermal conductivity.

[0064] The fiber material A) is preferably selected from the group consisting of carbon fiber woven fabrics, carbon fiber papers and carbon fiber nonwovens.

[0065] In the case of carbon fiber fabrics, the planar fiber material is made by interweaving two thread systems, warp (warp yarns) and weft (weft yarns). As in textiles, the fiber bundles are flexibly but inseparable. The carbon fiber fabric is preferably made of oxidized but not yet carbonized or graphitized PAN fibers. Carbonization or graphitization is carried out after weaving to make the planar fiber material conductive.

[0066] As mentioned at the beginning of this article, carbon fiber paper is usually produced from oxidized PAN fibers. These oxidized PAN fibers are comminuted into fiber fragments in a known manner, pulped, and formed into a fiber-based cloth by screening (pulpwood trough) in a manner similar to paper production and dried. In a preferred embodiment, at least one binder is additionally introduced into the paper. Suitable binders are, for example, phenolic resins, furan resins, polyimide resins, etc. In order to introduce the binder, the paper can be impregnated with this binder, and then, if necessary, the binder is cured. After impregnation and curing, the carbon fiber paper is carbonized / graphitized again in order to also convert the binder into a more conductive compound. In another suitable embodiment, the fiber material A is provided with filled carbon fiber paper. First, the production is carried out as described above, but instead of introducing the binder and carbonizing / graphitizing, a filler formed by a carbon material in a polymer binder is introduced into the still wet paper. For this purpose, in particular a carbon-PTFE filler is used. This filling increases the thermal conductivity and electrical conductivity, so that carbonization / graphitization can be omitted.

[0067] Non-oxidized or oxidized PAN fibers can be used to produce carbon fiber nonwovens. These fibers can be made into yarns (combed) in a dry-laid manner in a first step and then consolidated into a nonwoven. For example, this can be done by hydroentangling, in which the carbon fibers are oriented, entangled, and thus mechanically stabilized. The thickness of the cured nonwoven can be calibrated to the desired value as required. The nonwoven based on non-oxidized PAN fibers is first oxidized at elevated temperature and in an oxygen atmosphere after the nonwoven is webbed and cured, and then carbonized / graphitized in an inert atmosphere. The nonwoven based on oxidized PAN fibers is only carbonized / graphitized after the nonwoven is webbed and cured. Optionally, at least one binder can be additionally introduced into the fiber web, and then, as appropriate, this binder is hardened. Suitable binders are the binders mentioned for carbon fiber paper, in particular phenolic resins. For example, a binder can be introduced immediately after carbonization / graphitization, and finally the resulting impregnated fiber web is carbonized / graphitized again.

[0068] In a specific embodiment, the planar conductive fiber material A) comprises at least one carbon fiber nonwoven fabric. Advantages of carbon fiber nonwoven fabrics include, but are not limited to, compression elasticity, and are convenient for large-scale production, such as in a roll-to-roll process.

[0069] Fibrous materials A) generally refer to fiber composite materials, including:

[0070] a1) Carbon fiber,

[0071] a2) optionally at least one polymer binder and / or a pyrolysis product thereof,

[0072] a3) optionally at least one further additive different from a2).

[0073] The fiber material A) contained in the gas diffusion layer may contain customary additives a3). The additives are preferably selected from the group consisting of hydrophobizing agents, conductivity-improving additives, surface-active substances and mixtures thereof.

[0074] To improve the transport process through the GDL and at the interface, the hydrophobicity of the fiber material A) can advantageously be increased. Suitable hydrophobes are fluoropolymers, such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP). Preferably, PTFE is used as a hydrophobic agent. The fiber material can be finished with a hydrophobic agent by conventional impregnation methods. For this purpose, a PTFE dispersion is applied in an immersion tank, the solvent is evaporated, and the treated fiber material is sintered at a high temperature, usually at least 300° C.

[0075] Preferably, the hydrophobizing agent content of the fibrous material A) is 3 to 40% by weight relative to the total weight of the fibrous material A). In a specific embodiment, the hydrophobizing agent content of the fibrous material is 3 to 40% by weight relative to the total weight of the fibrous material A).

[0076] In order to improve the electrical and thermal conductivity, the fiber material A) can be finished with at least one conductivity-improving additive. The conductivity-improving additive is, for example, metal particles, carbon particles, etc. Preferably, the conductivity-improving additive is selected from carbon black, graphite, graphene, carbon nanotubes (CNT), carbon nanofibers and mixtures thereof. For example, the finishing of the fiber material A) with at least one conductivity-improving additive can be carried out together with the finishing with a hydrophobic agent, in particular a PTFE dispersion. Even without the conductivity-improving additive, the fiber material A) tends to have good electrical and thermal conductivity due to the carbon fibers used.

[0077] Preferably, the content of the conductivity-improving additive of the fiber material A) is 0% to 40% by weight, relative to the total weight of the fiber material A). If the fiber material A) contains a conductivity-improving additive, the content is preferably 0.1% to 40% by weight, particularly preferably 0.5% to 30% by weight, relative to the total weight of the fiber material A).

[0078] The fiber material A) preferably has a thickness in the range of 50 μm to 750 μm, particularly preferably 100 μm to 500 μm. This thickness relates to the uncompressed state of the fiber material A), ie before the post-treatment in step iii) and before the GDL is installed in a fuel cell.

[0079] The fiber material A) preferably has a porosity in the range of 10% to 90%, particularly preferably 20% to 85%. When the fiber density is known, the porosity of the fiber material can be calculated from the measured thickness and the measured weight per unit area. Thus, for 1.8 g / cm 3 Carbon fiber density: porosity [%] = [(1.8-weight per unit area / thickness) / 1.8] × 100. In addition, the density of the gas diffusion layer can be determined by helium density measurement and the specific pore volume can be determined by mercury porosimetry. The porosity is calculated as follows: Porosity [%] = specific pore volume / (specific pore volume + 1 / He density) × 100%].

[0080] The average pore size of the fiber material A) is preferably in the range from 5 μm to 60 μm, particularly preferably from 8 μm to 50 μm, in particular from 10 μm to 40 μm. The average pore size can be determined by mercury porosimetry.

[0081] The gas diffusion layer of the present invention is composed of a double-layer or multi-layered composite based on a planar conductive fiber material A) and a microporous layer (MPL) B) on one surface of the fiber material A).

[0082] Compared to the macroporous fiber material A), the MPL B) is microporous, with a pore size generally well below one micrometer, preferably a maximum of 900 nm, particularly preferably a maximum of 500 nm, in particular a maximum of 300 nm. The average pore size of the MPL B) is preferably in the range of 5 nm to 200 nm, particularly preferably 10 nm to 100 nm. The average pore size can again be determined by mercury porosimetry. The last-mentioned average pore size applies primarily to the case where carbon black is used 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 produced. In this case, depending on the specific composition, the average pore size is, for example, greater than 1 μm.

[0083] MPL comprises conductive carbon particles, preferably carbon black or graphite, in a matrix consisting of a polymer binder. Preferred binders are the aforementioned fluoropolymers, in particular polytetrafluoroethylene (PTFE).

[0084] The microporous layer B) preferably has a thickness in the range of 5 to 150 μm, particularly preferably 10 to 100 μm. This thickness relates to the uncompressed state of the microporous layer B), ie before the post-treatment in step iii) and before the GDL is installed in a fuel cell.

[0085] The presence of the MPL has a significant impact on the water balance of the fuel cell. Due to the high PTFE ratio and small pores of the MPL, water influx into the GDL and electrodes becomes difficult because the MPL acts as a liquid barrier, thereby promoting bulk transport of gaseous reactants to the catalyst. It has been shown that, advantageously, in the gas diffusion layer of the present invention, the microporous layer has a characteristic gradient relative to the basal plane (xy plane) of the GDL.

[0086] The gas diffusion layer according to the invention preferably has a thickness (total thickness of fiber material A) and MPL B) in the range of 75 μm to 1000 μm, particularly preferably 100 μm to 500 μm. This thickness relates to the uncompressed state of the GDL, i.e. before post-treatment in step iii) and before installation in a fuel cell.

[0087] Furthermore, the gas diffusion layer preferably has a high total porosity, which is preferably in the range of 20% to 80%, as determined by helium density measurement and mercury porosimetry, as described above.

[0088] Method for manufacturing a gas diffusion layer

[0089] Step i)

[0090] With regard to suitable and preferred fiber materials A) used in step i), reference is made to the above explanations.

[0091] Step ii)

[0092] The precursor used in step ii) preferably comprises at least one fluoropolymer, at least one carbon material and optionally comprises at least one pore former. Fluoropolymer is preferably selected from carbon polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). PTFE is preferably used. Preferably, the carbon material is selected from carbon black, graphite, graphene, carbon nanotube (CNT), carbon nanofiber and mixture thereof. Carbon black or graphite is preferably used. In a particular embodiment, the precursor used in step b) comprises at least one pore former. Suitable pore formers are commercially available plastic particles, for example, formed by polymethyl methacrylate (PMMA). Suitable particle size is within the range of 10 μm to 100 μm.

[0093] Preferably, due to the use of the pore former, the volume ratio of the pores in the manufactured microporous layer is 0 volume % to 70 volume % relative to the total volume of the pores in the manufactured microporous layer.

[0094] Preferably, the fiber material A) is coated with at least 2, preferably at least 3, in particular at least 4 laterally adjacent precursor strips of different compositions for forming a microporous layer. The MPL can be applied in different ways. In discontinuous production, spraying, screen printing or Meyer-Rod processes are usually used, while in continuous coating, blade coating, slot nozzle and gravure roller processes are preferably used. The MPL layer thickness and penetration depth may be affected by the coating process parameters and the viscosity of the coating. Finally, heat treatment is performed again, for example in a drying furnace or a sintering furnace. Drying is first carried out at a temperature of 100°C to 200°C, and then sintering is carried out at a temperature of 300°C to 500°C.

[0095] Characteristic gradient

[0096] As already mentioned, both the planar fiber material A) and the microporous layer B) or both can have at least one property gradient.

[0097] The properties with gradient are in principle selected from

[0098] - the chemical composition of the planar fiber material A) and / or the microporous layer B),

[0099] - the mechanical properties of the planar fibrous material A) and / or the microporous layer B),

[0100] - the transport properties of the planar fibrous material A) and / or the microporous layer B),

[0101] - A combination of the above characteristics.

[0102] The chemical properties of the planar fiber material A) and / or the microporous layer B) that may have a gradient include, for example, the hydrophobic agent content, the carbon particle content, etc. In particular, they include the PTFE content, the carbon black content, the graphite content, the graphene content, the carbon nanotube (CNT) content, the carbon nanofiber content and the content of mixtures thereof.

[0103] Mechanical properties of the planar fiber material A) and / or the microporous layer B) that may have a gradient include, for example, density, mass per unit area, porosity and average pore size.

[0104] As mentioned above, the density can be determined by helium densitometry, in g / m 3 As unit.

[0105] The mass per unit area can be determined according to ISO 9073-1 or EN 29073-1:1992 in g / m 2 As unit.

[0106] The determination of the porosity and the pore size distribution can be carried out by means of mercury porosimetry as described in DIN ISO 15901-1:2019-03 and ISO 15901-1:2016-Part 1: Mercury porosimetry.

[0107] To generate a gradient in mechanical properties, for example, a gradient in the compression behavior of the microporous layer can be provided by changing at least one material of the composition of the microporous layer. The connection to the electrodes is also changed thereby. Alternatively, a gradient in mechanical properties can be generated by generating a gradient across the material width during the consolidation of the fiber web by hydroentanglement. In this way, both the mechanical properties and the water transport are influenced.

[0108] The transport properties of the planar fiber material A) and / or the microporous layer B) that may have a gradient include:

[0109] - the air permeability of the planar fiber material A) and / or the microporous layer B),

[0110] - the liquid permeability of the planar fibrous material A) and / or the microporous layer B),

[0111] - the transition resistance of the gas diffusion layer through the material plane,

[0112] - the transition thermal resistance of the gas diffusion layer through the material plane,

[0113] -Stem diffusion length.

[0114] In a preferred embodiment, the microporous layer (MPL) has at least one property gradient in at least one chemical and / or physical property. This includes in particular mechanical properties and transport properties. The MPL has at least one property gradient relative to its base surface, i.e. in a top view or in the x, y plane. The MPL may also have a property gradient perpendicular to its base surface, i.e. in the z-axis direction, as the case may be.

[0115] Preferably, the microporous layer has at least 2, preferably at least 3, especially at least 4, especially at least 5, more especially at least 6 non-continuous regions that differ in at least one characteristic. In this embodiment, the characteristic change between the regions is non-continuous. Each region can be different in a certain same characteristic or (in the case of multiple characteristics) multiple same characteristics. This is a preferred embodiment. However, it can also be that two or more regions are different in different characteristics. In a specific embodiment, the microporous layer has at least 2, preferably at least 3, especially at least 4, especially at least 5, more especially at least 6 non-continuous regions that are different in the same characteristic.

[0116] In a particular embodiment, each individual region is substantially homogeneous in terms of its properties, wherein substantially homogeneous means that only property fluctuations occur within a region that would still occur if no gradient was intentionally generated (eg, due to manufacturing reasons).

[0117] In an alternative embodiment, the microporous layer has at least one continuous property gradient.

[0118] Preferably, the microporous layer has at least 2, preferably at least 3, in particular at least 4 transversely 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 transversely adjacent strips that differ in the same property. In particular, each individual strip is substantially homogeneous in terms of its properties.

[0119] Preferably, the gradient property of the microporous layer is selected from

[0120] -Gurley air permeability and

[0121] -Stem diffusion length.

[0122] Physical variables, measurement methods

[0123] The air permeability perpendicular to the plane of the material can be determined by Gurley measurement. For this purpose, an automated Gurley densitometer from Gurley Precision Instruments can be used. The measurement time is measured in seconds up to 100 cm 3 The air flows vertically through the sample at a constant pressure difference, with a flow area of ​​6.42 cm 2 The determination of air permeability according to the Gurley method is described in ISO 5636-5.

[0124] The air permeability can also be measured according to DIN EN ISO 9237:1995-12 for determining the air permeability of textile fabrics, expressed as l / m 2 The unit is s.

[0125] The permeability of a liquid (particularly liquid water) perpendicular to the plane of the material (liquid water permeability "through-plane") can be determined using a so-called "filtration cell" or according to the "Penn State" method [see citations ac]: [a] I S Hussaini and CY Wang, "Measurement of relative permeability of fuel cell diffusion media", Journal of Power Sources, Vol. 195, pp. 3830-3840, 2010; [b] J D Sole, "Study of water transport parameters and processes in PEMFC gas diffusion layers", Virginia Tech, 2008; [c] J. Benziger, J. Nehlsen, D. Blackwell, T. Brennan and J. Itescu, "Water flow in gas diffusion layers of PEM fuel cells", Journal of Membrane Science, Vol. 261, pp. 98-106, 2005.

[0126] The determination of the (specific) resistance through the plane (TP) can be carried out by a 4-point measurement method known from the literature.

[0127] The dry diffusion length indicates the actual distance a gas molecule travels through the planar fiber material A) and / or the microporous layer B) in μm. The dry diffusion length is determined with the aid of a stationary Wicke-Kallenbach cell.

[0128] There are two known test methods for determining through plane (TP) thermal resistance, namely the heat flow method or the laser scattering method.

[0129] The compression set value is a measure of how a material (here, a GDL) behaves during deformation under pressure and subsequent pressure relief. When used in fuel cells, the GDL is usually pressed (compressed) at high intensity. The compression properties of the GDL can be characterized by the ratio of elastic to plastic deformation (and optionally other physical variables such as permeability and electrical resistance). Plastic deformation is when a material does not return 100% to its original shape after loading, but instead undergoes a permanent change in shape. Compression set is an irreversible deformation that remains after the force has been removed.

[0130] The compression set value can be determined as follows: The values ​​of other physical variables (such as thickness, air permeability, electrical resistance) can be determined at a specific pressure and after single or multiple loadings.

[0131] Take three samples (left, right and center) across the width of the GDL to be tested and average them. If the material has a machine direction during manufacture, take the sample perpendicular to the machine direction (CMD). The sample is annular with an inner diameter of 45 mm and an outer diameter of 56 mm. The sample area is 8.72577 cm 2 . The sample is exposed to a time-varying pressure in a testing machine, which acts perpendicularly on the sample surface. The sensor determines the change in thickness of the GDL over time under the corresponding force. The sample is placed on a device for determining elastic and plastic deformations with the help of a force sensor, wherein the movement is transmitted to the sample via a spring. The distance moved until the maximum pressure is reached is measured by a displacement sensor. Since the deformation of the sample is nonlinear, the measurement curve is adapted to the relative change. A measuring cycle, that is, a loading until the maximum pressure and the subsequent pressure relief, lasts for 1 min. The sample undergoes three loading cycles. The initial value, to which only a small force is applied to the sample, is 0.025 MPa. Typical pressure values ​​for determining compression set values ​​(and other physical variables such as thickness, conductivity or resistivity per unit area, air permeability, etc.) are, for example, 0.6 MPa, 1.0 MPa and 2.4 MPa.

[0132] The compression set value for a certain pressure is obtained by the difference between the thickness measured at this pressure in the first loading cycle and the thickness measured at this pressure in the third loading cycle.

[0133] Preferably, the GDL of the present invention has a compression set value of up to 6 μm at 6 bar (0.6 MPa), and the compression set value is 90 g / m2 according to the above method. 2 Up to 95g / m 2 And the MPL load is 15.0g / m 2 Up to 22.0g / m 2 The GDL is measured using a ring sample with an inner diameter of 45 mm and an outer diameter of 56 mm.

[0134] The thickness of the gas diffusion layer can be determined according to DIN 53855-1:1993-08 "Determination of thickness of textile fabrics". The thickness at a certain pressure (eg 0.025 MPa or 1.0 MPa) can be determined in an apparatus for measuring compression set as described in detail above.

[0135] The mass per unit area can be determined according to ISO 9073-1 or EN 29073-1:1992 in g / m 2 As unit.

[0136] Fuel Cells

[0137] Another subject of the invention is a fuel cell comprising at least one gas diffusion layer as described above or at least one gas diffusion layer obtained by a method as described above.

[0138] In principle, the gas diffusion layer of the present invention is applicable to all common fuel cell types. The fuel cell of the present invention preferably refers to a proton exchange membrane fuel cell (PEMFC) in particular. A proton exchange membrane fuel cell is also known as a polymer electrolyte fuel cell (PEFC) or a low temperature polymer electrolyte membrane fuel cell (LT-PEMFC). A specific embodiment of the present invention is a hydrogen and oxygen fuel cell in the form of a low temperature proton exchange membrane fuel cell (PEMFC). Please fully refer to the description of the structure of the fuel cell in the foregoing text.

[0139] The fuel cell of the present invention preferably comprises a polymer electrolyte membrane, to which a catalyst layer is applied on the anode side and the cathode side, the catalyst layer forming an electrode. Preferably, a gas diffusion layer (GDL) in contact with the catalyst layer is provided on the anode and / or cathode side. The fuel cell in particular has a polymer electrolyte membrane, to which a catalyst layer is applied, the catalyst being in contact with the surface of the microporous layer B) of the gas diffusion layer of the present invention. In particular, the fuel cell has the gas diffusion layer of the present 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 particularly, the fuel cell has the gas diffusion layer of the present invention on the cathode side and on the anode side, wherein both the cathode layer and the anode layer are in contact with the surface of the microporous layer B) of the gas diffusion layer of the present invention.

[0140] One advantage of the invention is that the transport process through the gas diffusion layer can be specifically adapted to the gradient of the working medium flowing through the fuel cell and / or the operating parameters of the fuel cell. To this end, at least one characteristic gradient of the gas diffusion layer generally corresponds to at least one characteristic gradient of the working medium flowing through the fuel cell and / or the operating parameters of the fuel cell.

[0141] Another subject of the present invention is the use of a gas diffusion layer as described above or obtained by a process as described above in a proton exchange membrane fuel cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0142] Figure 1A top view of a GDL material is shown, the production of which is described in Example 1. Four transversely adjacent strips were applied in the longitudinal direction to a fiber web of DIN A3 format (29.7 cm×42 cm, where md stands for machine direction) using four different MPL coatings (coatings 1 to 4) with a width of 7 cm to 8 cm in each case. A gas diffusion layer of 274.8 mm×96.5 mm was punched out of the dried and sintered material, with its long side running transversely to the machine direction. Figure 1 Three alternative punching positions (GDL1 to 3) are shown. The resulting GDL has a characteristic gradient along the x-axis, wherein GDL 1 has 4 strips with different characteristics, and GDL 2 and 3 each have 3 strips with different characteristics. Of course, other punching positions and other specifications can also be used, which can also be at an angle different from 90° to the x-axis to change the gradient. GDLs of other shapes can also be punched out, such as squares, ovals, circles, to produce other GDL technical variants with separate gradients. The GDL of the present invention is post-treated under high pressure and high temperature. The GDL without post-treatment is used as a comparison.

[0143] Figure 2 The plastic deformation properties (permanent set performance) of four strips according to the compression set values ​​at 6 bar are shown for a comparative GDL (left column) and a GDL according to the invention (right column).

[0144] Figure 3a Shown are the dry diffusion length in μm (triangles), the GDL thickness at 2 MPa in μm (squares), and the Gurley value in s (circles) for four strips of comparative GDL.

[0145] Figure 3b The dry diffusion length in μm (triangles), the GDL thickness in μm at 2 MPa (squares) and the Gurley value in s (circles) for four strips of the GDL of the present invention are shown.

[0146] Figure 4 The characteristic changes of the dry diffusion length (triangles) and the Gurley value (circles) of four strips by post-treatment at high pressure and temperature are shown. The graph shows the difference (Δ value) between the values ​​of the GD of the invention (GDL 1) and the comparative GDL (GDL V1).

[0147] The following examples are used to illustrate the present invention in detail, but these examples do not limit the present invention in any way. DETAILED DESCRIPTION

[0148] I) Fabrication of Gas Diffusion Layer

[0149] Manufacturing Example 1:

[0150] Manufacturing the gas diffusion layer of the present invention and the comparative gas diffusion layer having the property gradient along the x-direction

[0151] The thickness is 0.145mm and the weight per unit area is 60g / m 2 、Transition resistance (throughplane) at 1MPa pressure is 6.6mΩcm 2 A commercially available conductive fiber web was obtained by punching sheets of DIN A3 format (29.7 cm×42 cm) from a GDL roll in the longitudinal direction (machine direction, md) and coating them one by one. To form a microporous layer with a characteristic gradient, four transversely adjacent MPL coating strips were applied in the longitudinal direction of the fiber web, each with a width of 7 cm to 8 cm (see Figure 1 ). The coatings had the composition shown in Table 1. To produce the coatings, PTFE, various carbons and plastic particles were dispersed as pore formers in distilled water and applied to the fiber web using a doctor blade with a doctor blade gap of 240 μm. The sheets were subsequently dried at 160° C. and sintered at 400° C. Depending on the strip, the resulting MPL loading was 15 g / m 2 Up to 22g / m 2 .

[0152] Table 1

[0153]

[0154] 1 ) are based on the total weight of the sintered coating

[0155] A gas diffusion layer having a size of 274.8 mm×96.5 mm was punched out from the obtained sheet, with its long side being transverse to the machine direction. Figure 1 Three alternative punching positions are shown. The resulting GDLs have a gradient of properties along the x-axis, with GDL 1 having four bands of different properties, and GDLs 2 and 3 each having three bands of different properties.

[0156] For the manufacture of fuel cells, the GDL can be installed, for example, in such a way that the x-direction (long side) is in the direction of the direct connection between the operating material input and output to the manifold. Thus, in a manifold with straight channels, the long sides of the GDL are parallel to the gas channels. However, in other flow field designs, it is also possible, for example, to install it in such a way that the oxygen-rich fuel supplied to the cathode side (air side) of the fuel cell first contacts the MPL (layer 1) formed by coating 1, and the oxygen-depleted fuel output contacts the MPL (layer 4) formed by coating 4.

[0157] II) Examples of applied technology

[0158] The comparative GDL V1 and the inventive GDL 1 each having 4 strips were used for the measurements to be described below.

[0159] The compression set values ​​were determined according to the method described in detail above. According to the method, the thickness, air permeability, dry diffusion length and electrical resistance at a certain pressure without prior force application / first loading process were also determined. The values ​​are given in Table 1 below.

[0160] The Gurley air permeability was measured perpendicular to the plane of the material using a Gurley densitometer from Gurley Precision Instruments according to ISO 5636-5. The results are also shown in Table 2.

[0161] The dry diffusion length was determined with the aid of a stationary Wicke-Kallenbach cell. The results are also shown in Table 2.

[0162] The through plane (TP) resistance was measured by a four-point measurement method.

[0163] The post-treatment of the gas diffusion layer at high pressure and high temperature of the present invention greatly reduces the proportion of plastic deformation. Figure 2 This is shown by the compression set values ​​at 6 bar. The deformation of the post-treated GDL is mainly elastic. In addition, it is shown that the permanent deformation of different strips is significantly similar. The variance of the permanent deformation between strips is significantly reduced.

[0164] Figure 3a , Figure 3b and Figure 4 The effect of the post-treatment of the invention on the transport properties of the GDL and its gradient is shown in terms of dry diffusion length and Gurley value. Depending on the specific composition of the strip, the transport properties are significantly changed (strips 1 and 2) or only slightly changed (strips 3 and 4) by post-treatment at high pressure and temperature. This can be used to control the behavior of the gradient as desired.

[0165] Table 2

[0166]

[0167] TP = through plane

Claims

1. A method for manufacturing a gas diffusion layer for a fuel cell, the gas diffusion layer comprising A) a planar 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 the base surface of the gas diffusion layer (in the x, y plane) with respect to at least one chemical and / or physical property, in i) providing a planar conductive fiber material A), ii) coating the fiber material provided in step i) with a precursor for forming a microporous layer, wherein the composition of the precursor is varied to produce a gradient, iii) post-treating the coated fiber material obtained in step ii) at high pressure and optionally high temperature.

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

3. The method according to claim 1 or 2, wherein the treatment in step iii) is carried out in a temperature range of 100°C to 350°C, preferably 120°C to 330°C, particularly preferably 150°C to 320°C.

4. The method according to any one of the preceding claims, wherein an apparatus is used for the treatment in step iii), the apparatus being selected from a single-layer press, a multilayer press, a continuous belt press, a calender and a combination thereof, preferably from a double-belt press, a calender and a combination thereof.

5. The method according to any of the preceding claims, wherein the treatment in step iii) is carried out in a press for a period of time of 5 seconds to 5 minutes, preferably 10 seconds to 2 minutes.

6. The method according to any of the preceding claims, wherein the treatment in step iii) is carried out in a calender for a period of time of more than 0 seconds to 10 seconds, preferably 0.1 seconds to 5 seconds.

7. The method according to any of the preceding claims, wherein the treatment in step iii) is carried out in a calender at a linear pressure in the range of 5 N / mm to 500 N / mm, preferably 10 N / mm to 100 N / mm.

8. A method according to any one of the preceding claims, wherein the composition of the precursor is varied during the coating such that the microporous layer has at least one monotonic characteristic gradient relative to the base surface (in the x, y plane) of the gas diffusion layer.

9. A gas diffusion layer obtained by the method according to any one of claims 1 to 8.

10. A gas diffusion layer for a fuel cell, the gas diffusion layer comprising A) a planar conductive fiber material, and B) a microporous layer on at least one of the surfaces of the fiber material, The gas diffusion layer has at least one property gradient in at least one chemical and / or physical property relative to its base surface (in the x, y plane), and the gas diffusion layer is post-treated at high pressure and optionally high temperature so that the gas diffusion layer has reduced plastic deformability compared to a gas diffusion layer that has not been post-treated.

11. The gas diffusion layer according to claim 9 or 10, having a reduced compression set value relative to a gas diffusion layer that has not been post-treated.

12. The gas diffusion layer according to any one of claims 9 to 11, wherein the microporous layer has at least one property gradient, preferably a continuous or discontinuous property gradient that varies monotonically as a function of position. 13 . The gas diffusion layer according to claim 9 , wherein the microporous layer has at least 2, preferably at least 3, in particular at least 4 regions which differ in at least one property.

14. A gas diffusion layer according to any one of claims 9 to 13, wherein the gradient property is selected from - the chemical composition of the planar fiber material A) and / or the microporous layer B), - the mechanical properties of the planar fibrous material A) and / or the microporous layer B), - the transport properties of the planar fibrous material A) and / or the microporous layer B), - A combination of the above characteristics. 15 . The gas diffusion layer according to claim 9 , having a dry diffusion length gradient and / or a Gurley permeability gradient.

16. A fuel cell comprising at least one gas diffusion layer according to any one of claims 9 to 15, or at least one gas diffusion layer obtained by the method according to any one of claims 1 to 8.

17. The fuel cell according to claim 16, comprising a polymer electrolyte membrane on which a catalyst layer is applied, wherein the catalyst layer is in contact with a surface of the microporous layer B) of the gas diffusion layer.

18. Use of a gas diffusion layer as claimed in any one of claims 9 to 15 or obtained by a method as claimed in any one of claims 1 to 8 in a proton exchange membrane fuel cell.

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