Proton-exchange membrane fuel cell with improved contact between the gas diffusion layer and the polymer-electrolyte membrane

Abstract

The invention relates to a proton-exchange membrane fuel cell with improved contact properties between the gas diffusion layers (GDL) and the catalyst-coated polymer-electrolyte membrane (CCM), to a gas diffusion layer, to a method for producing a gas diffusion layer, and to a method for producing a proton-exchange membrane fuel cell which exhibits an improved contact at the boundary surface between the gas diffusion layers and the catalyst-coated polymer-electrolyte membrane. The gas diffusion layer comprises a carbon fiber nonwoven material or a carbon fiber paper comprising carbon fibers which have a main orientation direction with respect to the base surface of the nonwoven material or paper, said main orientation direction not being parallel to the main orientation of the channels of an adjacent flow distributor plate.

Description

[0001] Proton exchange membrane fuel cell with improved contact between gas diffusion layer and polymer electrolyte membrane

[0002] DESCRIPTION

[0003] The present invention relates to a proton exchange membrane fuel cell with improved contact properties between the gas diffusion layers (GDL) and the catalyst-coated polymer electrolyte membrane (CCM). The invention further relates to a method for producing a gas diffusion layer and a method for producing a proton exchange membrane fuel cell that has improved contact at the interface between the gas diffusion layers and the catalyst-coated polymer electrolyte membrane. The invention also relates to the gas diffusion layers and proton exchange membrane fuel cells and fuel cell stacks obtainable by these methods. It also relates to the use of such a gas diffusion layer in a proton exchange membrane fuel cell to improve the contact between the catalyst-coated polymer electrolyte membrane and the gas diffusion layers.

[0004] BACKGROUND OF THE INVENTION

[0005] Fuel cells use the chemical reaction of a fuel, particularly hydrogen, with oxygen to produce water to generate electrical energy. In hydrogen-oxygen fuel cells, hydrogen or a hydrogen-containing gas mixture is fed to the anode, where electrochemical oxidation takes place with the release of electrons (H2A2H + + 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

[0006] For many applications, especially in automotive powertrains, proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells (PEMFCs), are used. A special type is water-oxygen fuel cells in the form of low-temperature proton exchange membrane fuel cells (LT-PEMFCs). The core of proton exchange membrane fuel cells is a polymer electrolyte membrane (PEM), which is only suitable for protons (or oxonium ions HaO). +) 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 single unit, also known as a CCM (catalyst coated membrane).

[0007] On both sides of the CCM, there is usually a gas diffusion layer (GDL), which stabilizes the cell structure and performs transport and distribution functions for reaction gases, water, heat, and electricity. Gas diffusion layers for fuel cells typically consist of a carbon fiber substrate, which is usually hydrophobically treated with fluoropolymers (e.g., PTFE) and then coated with a microporous layer (MPL). The MPL typically consists of a polymeric binder, often using fluorine-containing polymers (e.g., PTFE), and at least one porous and electrically conductive carbon material (e.g., carbon black, graphite, etc.). The following three materials are currently used as carbon fiber substrates for the GDL:

[0008] Carbon fiber fabrics (e.g. made from yarns of oxidized but not yet carbonized polyacrylonitrile fibers, which are carbonized or graphitized after weaving), carbon fiber papers (wet-laid and chemically bonded carbon fiber nonwovens, with chemical binders that are carbonized), carbon fiber nonwovens (e.g. dry-laid, carded and hydroentangled nonwovens made from oxidized polyacrylonitrile, which are subsequently thickness-calibrated and carbonized).

[0009] In a typical design, gas diffusion layers for proton exchange membrane fuel cells have a fiber side (=substrate side) and a side coated with an MPL (MPL side).

[0010] The membrane, electrodes, and gas diffusion layers form the membrane electrode assembly (MEA) of the proton exchange membrane fuel cell. Flow distribution plates are arranged at the terminal ends of the fuel cell stacks and between the individual membrane electrode assemblies. These plates contain channels for supplying the adjacent cathode and anode with process gases, as well as generally internal cooling channels. The terminal flow distribution plates are called end plates, and those arranged between the individual membrane electrode assemblies are called bipolar plates. The flow distribution plates are usually embossed metal plates or embossed graphitic carbon fiber mats. In a proton exchange membrane fuel cell (PEM fuel cell), the gas diffusion layers are usually in direct contact with the flow distribution plates (bipolar plates).The webs of the bipolar plates are usually placed directly on the fiber side (=substrate side) of the GDL and form the direct contact point with the gas diffusion layer, through which electrical conduction takes place. In this arrangement, the MPL side of the GDL is in contact with the catalyst-coated membrane (CCM). The material flow (reaction gases oxygen and hydrogen, as well as water) from the channels to the membrane and from the membrane back into the channels takes place via the channels and the pore structure of the gas diffusion layer. The gas diffusion layers located between the bipolar plates and the catalyst layers are therefore crucial for the function and performance of the fuel cell.The process components consumed and generated in the electrode reactions must be transported through the gas diffusion layer and homogeneously distributed from the macroscopic structure of the bipolar plates to the microscopic structure of the catalyst layers. The electrons generated and consumed in the half-cell reactions must be conducted to the bipolar plates with the lowest possible voltage loss. The heat generated during the reaction must be dissipated to the coolant in the bipolar plates, so the materials of the GDL must also have sufficient thermal conductivity. Furthermore, the GDL must act as a mechanical balance between the macrostructured flow distribution plates 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 the fuel cells.Therefore, high demands are placed on the mechanical properties of GDL.

[0011] The components of a fuel cell are typically joined under high pressure during production. Due to the surface structure of the flow distribution plates with their webs and channels, different areas of the fiber side of the gas diffusion layer in contact with the flow distribution plate experience different compressive stresses. This varying mechanical stress leads to the fiber layer changing its shape depending on the load, e.g., through penetration of the webs and compression of the gas diffusion layer at the contact points between the webs and the fiber layer of the GDL, or bending or corrugation of the gas diffusion layer in the area of ​​the channels of the flow distribution plates. The magnitude of both phenomena results from the mechanical properties of the gas diffusion layer, e.g., its strength, elasticity, or plasticity.The mechanical properties of a gas diffusion layer, in turn, depend heavily on the type of carbon fiber substrate. As previously mentioned, woven fabrics, papers, and nonwovens are used. In woven fabrics and dry-laid carbonized nonwovens, the fibers are mechanically bonded or intertwined by a water jet. In paper or wet-laid nonwovens, they are chemically bonded by an additional resin and then carbonized. While woven fabrics typically have an ordered structure of continuous fibers, nonwovens and papers use short-cut fibers that are chaotically and randomly oriented.

[0012] Thus, the different compressive stress within the fuel cell stack perpendicular to the base area (the x,y plane) of the gas diffusion layers has an important influence on the application properties of the fuel cells, e.g. the effectiveness of water drainage, the electrical contacting, the current density distribution and the achievable total electrical cell voltage, as well as the service life of the fuel cell stack.

[0013] For example, an inhomogeneous mechanical compressive stress distribution across the stack assembly surface also affects the contact between the MPL side of the gas diffusion layer and the catalyst-coated polymer electrolyte membrane (COM). For example, if the gas diffusion layer corrugates and / or bends into the channel structure in the area of ​​the flow distribution plates, this can result in partial or complete delamination of the GDL from the COM on the MPL side, thus increasing the electrical contact resistance to the COM. This negatively impacts the current density distribution across the base area and can significantly reduce the fuel cell's performance and service life (particularly if delamination progresses to the web areas).In the worst case, reducing the contact area between the GDL and the COM can lead to "bubbles" forming between these layers, which can fill with water, leading to what is known as "flooding" of the fuel cell. The electrical contact resistance between the COM and the GDL significantly determines the fuel cell's performance.

[0014] Since the properties of the bipolar plate webs are individually designed in terms of spacing, web width, angle, and curvature depending on requirements, a gas diffusion layer with specific (mechanical) properties can lead to different performance with different web profiles. The substrate of the gas diffusion layer is exposed to high stress, particularly at the web-to-channel transition. This point is particularly susceptible to damage to the microporous layer due to fiber breakage or other damage, depending on the radius and angle of the webs. Broken fibers have particularly sharp edges at the break point and can cause short circuits by penetrating the microporous layer and the membrane. This can significantly reduce the performance and service life of a fuel cell or fuel cell stack.

[0015] There continues to be a general need for proton exchange membrane fuel cells for fuel cell stacks that avoid the aforementioned disadvantages. The state of the art often addresses the problem of insufficient contact between the gas diffusion layer and the catalyst-coated membrane, and especially excessive electrical contact resistance, in the design of the flow distribution plates.

[0016] JP 2021 125356 A relates to a fuel cell separator with improved corrosion resistance and conductivity, comprising a substrate with a coating in contact with an electrode of a fuel cell. The fuel cell is, for example, a solid polymer fuel cell. The separator may have channels for gas supply. The coating contains Fe3O4. The separator has an electrode-side surface roughness (Sa) of 5-100 pm, preferably 10-50 pm. The coating is, for example, an electroplated layer or a sintered layer. The substrate is, for example, stainless steel, a Ti base material, or an Al base material. The electrode surface with which the coating layer is in contact is composed, for example, of a carbon base material. A GDL can be arranged between the catalyst-coated membrane and the separator.The contact surface pressure acting between the coating layer and the electrode surface is 5 MPa or less, preferably 3 MPa or less.

[0017] JP 2021 026909 A describes a fuel cell in which pressure loss on the cathode side is avoided. The fuel cell comprises a laminate that, on the cathode side, comprises a separator, a gas diffusion layer, and a catalyst-containing electrode layer, followed by an electrolyte layer. On the anode side, a catalyst-containing electrode layer, a gas diffusion layer, and a separator (in that order). Both separators have gas channels. The cathode gas diffusion layer has higher flexural strength and air permeability than the anode gas diffusion layer, relative to its thickness.

[0018] US 2021 / 0005905 A1 (DE 10 2018 202 561 A1) relates to a fuel cell with an ion-selective separator, a gas diffusion layer, and a separator plate. The separator plate, together with the gas diffusion layer, forms at least one gas-conducting flow field. At least one channel web of the separator plate has an end with a top side and a front side. The front side serves to divide a flow impinging on the front side of the channel web in a first direction into two partial flows. The front side serves to deflect a liquid in the flow impinging on the front side adjacent to the top side in such a way that the liquid is further away from the top side after the deflection than before the deflection.

[0019] WO 2022129533 A1 (DE 10 2020216 101 A1) relates to an arrangement of electrochemical cells comprising at least one gas diffusion layer, preferably with a microporous layer, a catalyst-coated membrane with a frame, and a bipolar plate. The gas diffusion layer is bonded to the catalyst-coated membrane and / or to the bipolar plate by means of a preferably electrically conductive adhesive. The adhesive is arranged on a surface of the frame of the catalyst-coated membrane, the bipolar plate, and / or optionally the microporous layer of the gas diffusion layer, and the surface is plasma-functionalized. Furthermore, the invention relates to a vehicle comprising the arrangement of electrochemical cells and a method for producing the arrangement of electrochemical cells.The plasma functionality of the surface, or rather, the pretreatment of the surface with plasma, achieves a covalent fixation of the adhesive to the frame of the membrane, the bipolar plate, or the microporous layer of the gas diffusion layer. This creates a stronger bond and thus serves to more firmly fix the gas diffusion layer to the bipolar plate or the catalyst-coated membrane. Furthermore, better wetting of the surface with the adhesive is possible. By using the adhesive, the pressing force typically required in the arrangement of electrochemical cells, which can particularly impair the gas diffusion layer, can be significantly reduced, thus improving gas distribution in the electrochemical cell. The electrically conductive adhesive also improves the electrical contact between the bipolar plate and the gas diffusion layer. The contact resistance is reduced.In addition, the adhesive prevents the gas diffusion layer or the catalyst-coated membrane from slipping on the bipolar plate when the arrangement is stacked.

[0020] WO 2022 / 094717 A1 relates to a fuel cell arrangement with improved contact pressure distribution. The fuel cell arrangement comprises a unit cell with a catalyst-coated proton exchange membrane and, on both sides thereof, a first and a second electrode and a first and a second gas diffusion layer. Adjacent to the first gas diffusion layer is the surface of a first flow field plate which comprises a plurality of first channels separated by webs, and wherein the first channels have a first channel length and a width which varies along at least a portion of the first channel length. Adjacent to the second gas diffusion layer is the surface of a second flow field plate. This configuration with different channel widths results in a situation where, when a substantially uniform compressive force is exerted on this unit cell (orwhen a non-uniform compressive force is applied to the active region of the unit cell), the contact pressure between the first gas diffusion layer and the webs of the first flow field plate is substantially uniformly distributed over the active region of the unit cell.

[0021] WO 2022 / 028998 A1 (DE 102020209 811 A1) relates to a method for reducing the contact resistance between a metallic separator plate, for example a monopolar plate or a bipolar plate, and a gas diffusion layer of a fuel cell, in which the separator plate is provided at least partially with a carbon-based coating. According to the invention, at least one elastomer is added to the carbon as a binder to form the coating.

[0022] US 2018 / 0006314 A1 describes a bipolar plate that can improve the efficiency of a battery by reducing the contact resistance to an electrode in contact with the bipolar plate. For this purpose, a bipolar plate is used whose surface is at least partially coated with a conductive thermoplastic, which morphologically adapts the bipolar plate to the electrode. In a specific embodiment, the invention relates to a fuel cell stack with at least one cell configured as described above, wherein the thermoplastic is melted by applying a voltage, thus morphologically adapting the bipolar plate to the electrode.

[0023] US 2010 / 0291464 A1 (DE 102010 020 168 A1) relates to a stainless steel-based bipolar plate that is in electrically conductive contact with a diffusion layer of a fuel cell. To reduce the electrical contact resistance between the plate and the diffusion layer, the contact is made at least partially via a nickel-based alloy.

[0024] KR 101320786 B1 relates to a device and a method for measuring the contact resistance of bipolar plates for fuel cells.

[0025] It is also already known that the properties of the gas diffusion layer can be adapted to the water management of PEM fuel cells in order to avoid flooding effects and achieve a consistently high current density under various operating conditions, such as fluctuating operating temperatures. The problem of contact resistance, and specifically the GDL-CCM contact, is only mentioned sporadically.

[0026] US 2023 / 0163314 A1 relates to a gas diffusion layer for an electrochemical device, e.g., a PEM fuel cell, having a side with an enlarged surface area that serves to accommodate a catalyst or to contact a catalyst layer. It is described that gas diffusion layers with mechanically machined surface roughness and thus an enlarged surface area increase the effective diffusivity of gas-phase reactants in electrochemical devices, such as, for example, a PEM fuel cell.

[0027] Polymer electrolyte membrane fuel cells, enlarge.

[0028] WO 2023 / 190153 A1 relates to a gas diffusion layer with a microporous layer on at least one side of a conductive porous base material, wherein the microporous layer contains carbon black and graphite particles with an aspect ratio of 10 or more, and the conductive porous base material is characterized in that the thickness of that part of the conductive porous base material impregnated with the microporous layer is 5% or more and 20% or less of the thickness of that part of the conductive porous base material not impregnated with the microporous layer. Within the gas diffusion layer, the electrical resistance is thereby reduced, while the gas diffusion capacity is not impaired. The use of such a gas diffusion layer in a fuel cell enables an improvement in its performance with regard to power generation.

[0029] DE 102020 202 433 A1 teaches the use of a gas diffusion layer to improve the electrical contact resistance of a fuel cell. This layer comprises an electrically conductive network containing electrically conductive fibers surrounded by an electrically insulating, hydrophobic material, and the removal of the electrically insulating material at least partially in the region of the bipolar plate. Specifically, the electrically conductive network can comprise carbon fibers surrounded by a hydrophobic material, such as PTFE, and the removal can be carried out, for example, using a laser.

[0030] DE 10 2016200 802 A1 relates to a flow body-gas diffusion layer unit for a fuel cell, in which a flow body and a gas diffusion layer are formed by a porous body and arranged between a membrane electrode assembly and the bipolar plates. The porosity of the body in the region in which it is formed as a flow body is greater than the porosity of the region of the body formed as a gas diffusion layer. This provides a layered structure for a fuel cell that is intended to improve the contact resistance between the components of the layered structure.

[0031] The members of the patent family KR 20110062552A (US 10,431,838 B2; US 10,511,043 B2; DE 10 2010 002 392 A1) describe a gas diffusion layer for a fuel cell in which penetration of the gas diffusion layer into the channels of the flow field of the flow distributor plate (bipolar plate) is prevented. To achieve this goal, the gas diffusion layer roll fabric is cut such that the machine direction of the high-stiffness GDL material and the main flow direction of the flow distributor plate are not oriented parallel to each other, resulting in increased stiffness of the gas diffusion layer in the direction transverse to the main flow field direction. In a specific embodiment, the gas diffusion layer comprises a dual layer structure with a microporous and a macroporous layer.

[0032] The members belonging to the patent family of KR 20120061232 A (US 9,847,535 B2, US 10,629,918 B2, DE 10 2011 006651 A1) describe a fuel cell stack with improved freeze-thaw stability. The fuel cell stack comprises a gas diffusion layer between a membrane electrode assembly and a flow distributor plate (bipolar plate), wherein the GDL has a structure that reduces contact resistance at transition points in a fuel cell and stiffness in the width direction of the GDL material perpendicular to the main flow direction of the flow distributor plate, which stiffness is increased by cutting the GDL material at a specific angle by aligning the machine direction of the GDL material (i.e., the direction of high stiffness) not parallel to the main flow field direction of the flow distributor plate.

[0033] It has now been found that by appropriately optimizing the mechanical properties of the gas diffusion layer, the different compressive stresses of the gas diffusion layers within the fuel cell stack can be reduced or avoided, so that the application properties of the fuel cells, especially the electrical contacting by reducing the contact resistance between GDL and CCM, can be significantly reduced and thus the current density distribution, the achievable total electrical cell voltage, the service life and other properties of the fuel cell stack can be significantly increased.

[0034] SUMMARY OF THE INVENTION

[0035] A first subject matter of the invention is a proton exchange membrane fuel cell, comprising a proton exchange membrane coated on both sides with a catalytically active electrode, on each side of the catalyst-coated membrane a gas diffusion layer, the inside of which is in contact with the catalytically active electrode, on each side of the catalyst-coated membrane a flow distributor plate having channels for supplying reactant gases and removing products of the cell reaction, wherein the channels are in contact with the outside of the gas diffusion layer, wherein the gas diffusion layer comprises a sheet-like electrically conductive fiber material A) selected from carbon fiber nonwovens, carbon fiber papers and combinations thereof, wherein the fiber material A) comprises one or more fiber layers, wherein the fibers of one fiber layer or, if the fiber material comprises several fiber layers,the fibers of at least one of the fiber layers are oriented and have a main orientation direction with respect to the base area of ​​the fiber layer (x,y plane), and wherein the channels of the flow distributor plate have a main orientation direction and the main orientation direction of the fibers is not aligned parallel to the main orientation direction of the channels.

[0036] A preferred embodiment of the invention is a proton exchange membrane fuel cell comprising a proton exchange membrane coated on both sides with a catalytically active electrode, on each side of the catalyst-coated membrane a gas diffusion layer, the inside of which is in contact with the catalytically active electrode, on each side of the catalyst-coated membrane a flow distributor plate having channels for supplying reactant gases and removing products of the cell reaction, wherein the channels are in contact with the outside of the gas diffusion layer, wherein the gas diffusion layer comprises a sheet-like electrically conductive fiber material A) which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric, wherein the carbon fiber nonwoven fabric comprises one or more consolidated fiber pile layers, wherein the fibers of one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers,the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of ​​the fiber pile layer (x,y plane), and wherein the channels of the flow distributor plate have a main orientation direction and the main orientation direction of the fibers is not aligned parallel to the main orientation direction of the channels.

[0037] In a preferred embodiment, the sheet-like electrically conductive fiber material A) is selected from carbon fiber nonwovens, carbon fiber papers and combinations thereof, and the main orientation direction of the fibers and the main orientation direction of the channels of the flow distributor plate in contact with the gas diffusion layer have an angle of 30° to 90°, preferably 45° to 90°, in particular 60° to 90°, to one another.

[0038] In a special embodiment, the flat electrically conductive fiber material A) consists of a carbon fiber nonwoven fabric.

[0039] In a special embodiment, the carbon fiber nonwoven fabric has several fiber pile layers, in particular 2, 3, 4, 5 or more than 5 fiber pile layers.

[0040] In a specific embodiment, the carbon fiber nonwoven fabric comprises a plurality of fiber pile layers, wherein the fibers of the fiber pile layer are oriented on the side facing the flow distributor plate (i.e., the fibers of the fiber pile layer in contact with the flow distributor plate) and the main orientation direction of the fibers of this fiber pile layer is not aligned parallel to the main orientation direction of the channels of the flow distributor plate. Preferably, the main orientation direction of the fibers of the fiber pile layer on the side facing the flow distributor plate and the main orientation direction of the channels of the flow distributor plate are at an angle of 30° to 90°, particularly preferably 45° to 90°, in particular 60° to 90°, to one another. A further subject matter of the invention is a proton exchange membrane fuel cell, wherein the gas diffusion layers

[0041] A) a sheet-like electrically conductive fiber material selected from carbon fiber nonwovens, carbon fiber papers and combinations thereof, and

[0042] B) a microporous layer on the side facing the catalytically active electrode, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder.

[0043] A special embodiment is a proton exchange membrane fuel cell, wherein the gas diffusion layers

[0044] A) a sheet-like electrically conductive fiber material comprising or consisting of a carbon fiber nonwoven fabric, and

[0045] B) a microporous layer on the side facing the catalytically active electrode, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder.

[0046] The invention further provides a method for producing a gas diffusion layer for a fuel cell, as defined above and below, in which i) a sheet-like electrically conductive fiber material A) is provided which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of ​​the fiber pile layer (x,y plane), ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer, iii) optionally the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and / or elevated temperature.

[0047] The invention further provides a process for producing a proton exchange membrane fuel cell, as defined above and below, in which i) a sheet-like electrically conductive fiber material A) is provided, which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of ​​the fiber pile layer (x,y plane), ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer, iii) optionally subjecting the coated fiber material obtained in step ii) to a post-treatment at elevated pressure and / or elevated temperature,iv) the coated fiber material obtained in step ii) or iii) is placed as a gas diffusion layer on both sides of a proton exchange membrane coated on both sides with catalytically active electrodes, wherein the microporous layers are located on the sides of the gas diffusion layers facing the catalytically active electrodes, v) a flow distributor plate is placed on each of the outer sides of the gas diffusion layers, which flow distributor plates have channels for supplying reactant gases and removing products of the cell reaction, which are in contact with the outer sides of the gas diffusion layers, vi) the flow distributor plates and the components located therebetween are pressed together.

[0048] Preferably, in the processes described above and below for producing a gas diffusion layer and for producing a proton exchange membrane fuel cell for producing the sheet-like electrically conductive fiber material A) i-1) a fiber composition is provided which comprises carbon fibers and / or precursors of carbon fibers, i-2) the fiber composition provided in step i-1) is subjected to carding to increase the parallel orientation in the fiber longitudinal direction, i-3) the fiber composition obtained in step i-2) is laid down to form a fiber web, i-4) optionally at least one further fiber web layer is laid down on the first fiber web layer, i-5) the fiber web layer(s) obtained in step i-3) or i-4) is consolidated to form a nonwoven fabric, i-6) optionally the nonwoven fabric obtained in step i-5) is subjected to finishing with at least one additive,i-7) if appropriate, the nonwoven fabric obtained in step i-5) or the finished nonwoven fabric obtained in step i-6) is subjected to a post-treatment at elevated pressure and optionally elevated temperature, i-8) if the fiber composition used in step i-1) comprises precursors of carbon fibers, the nonwoven fabric obtained in step i-5) or the finished nonwoven fabric obtained in step i-6) or the post-treated nonwoven fabric obtained in step i-7) is subjected to pyrolysis at a temperature of at least 1000 °C.,

[0049] Specifically, to produce the sheet-like electrically conductive fiber material A) in step i-4), at least one further fiber pile layer is deposited on the first fiber pile layer, wherein at least one of the further pile layers has a main orientation direction of the fibers which is substantially transverse to the main orientation direction of the fibers of the first pile layer.

[0050] Specifically, to produce the sheet-like electrically conductive fiber material A), in step i-5), the fiber pile layer(s) obtained in step i-3) or i-4) are consolidated into a nonwoven fabric by exposure to water-containing fluid jets.

[0051] A further subject of the invention is a gas diffusion layer obtainable by a process as defined above and below.

[0052] A further object of the invention is a proton exchange membrane fuel cell obtainable by a process as defined above and below.

[0053] A further object of the invention is a fuel cell stack comprising a plurality of proton exchange membrane fuel cells as defined above and below.

[0054] A further object of the invention is the use of at least one gas diffusion layer, obtainable by a process as defined above and below, in a proton exchange membrane fuel cell, for improving the contact between the catalyst-coated polymer electrolyte membrane and the microporous layers of the gas diffusion layers.

[0055] DESCRIPTION OF THE INVENTION

[0056] The proton exchange membrane fuel cells according to the invention and the gas diffusion layers used therein have the following advantages:

[0057] The proton exchange membrane fuel cell according to the invention has improved contact properties between the gas diffusion layers (GDL) and the catalyst-coated polymer electrolyte membrane (CCM).

[0058] The use of a gas diffusion layer comprising one or more fiber pile layers in which the fibers have a main orientation direction relative to the base surface (x,y plane) allows the fibers in one or more of the fiber pile layers of the GDL to be oriented in a fuel cell that is not parallel to the main orientation direction of the channels of the flow distributor plate. Surprisingly, this leads to a reduction in contact resistance at the interface between the GDL and the CCM compared to prior art fuel cells. In particular, the GDL has a microporous layer (MPL) on the side facing the catalytically active electrode.Then, said use leads to a reduction of the contact resistance at the interface of the microporous layer of the gas diffusion layer to the catalyst layer applied on the polymer electrolyte membrane of the proton exchange membrane fuel cell, especially in the region of the channels of the flow distribution plate of the fuel cell, especially in the region of the center of the channels.

[0059] By optimizing the mechanical properties, different compressive stresses on the gas diffusion layers within the fuel cell stack can be reduced or eliminated. In particular, the electrical contact can be significantly improved by reducing the contact resistance between the GDL and the CCM, thus significantly increasing the current density distribution, the achievable total electrical cell voltage, the service life, and other properties of the fuel cell stack.

[0060] In particular, the use of multilayer nonwovens with a fiber orientation can help to avoid inhomogeneous compressive stress on the gas diffusion layers within the fuel cell stack and improve the GDL-CCM contact.

[0061] The gas diffusion layers exhibit good tensile properties, both in the x-axis direction (machine direction, MD), and in the y-axis direction, i.e., orthogonal to the roll direction (cross machine direction, CD or CMD). This results in very good values ​​for the ultimate tensile force and ultimate tensile elongation according to EN 29073-3:1992.

[0062] Compression-resistant materials bend into the channel of the bipolar plate while retaining their thickness in the z-direction. This leads to the side of the GDL facing the CCM, which usually has an MPL coating, at least partially losing contact with the CCM or delaminating. Especially with compression-resistant materials, an increase in contact resistance is observed in the region of the channels of the flow distributor plates. A further consequence can be a reduction in current density. Delamination at the channel locations can also continue over the entire surface, resulting in a further accelerated reduction in performance and a shorter service life of the fuel cell. Corrugation of the gas diffusion layer into the channel of the flow distributor plates can also reduce the contact area between the flow distributor plates and the gas diffusion layer, which can increase the electrical contact resistance between the two components.

[0063] Pressure-switchable, compressible, and flexible gas diffusion layers, on the other hand, are capable of responding flexibly to different pressure conditions across the surface. Although they are also compressed at the contact points with the webs of the flow distribution plates and may bend into the channel on the substrate side, they are flexible enough to maintain contact with the CCM on the MPL side and prevent delamination at this point. This ensures a sufficiently low contact resistance.

[0064] In addition to the fiber orientation, other properties, especially those of the gas diffusion layer, can be varied to optimize the mechanical properties of the GDL with regard to the task to be solved. This includes the selection of the components used to manufacture the gas diffusion layer, in particular the yarn thickness or the titer (weight per unit length), the fineness or running length (length per unit weight), the basis weight and the type of chemical binder of the carbon fiber nonwoven. The properties of the microporous layer of the GDL substrate can also contribute significantly to the interaction between COM and GDL (especially between COM and MPL). Furthermore, the manufacturing process of the GDL can have a significant influence on the mechanical properties of the GDL with regard to the task to be solved. This includes, in particular, the type of fiber orientation (e.g.in the preparation of the fibres and laying of the fibre piles as well as in the web consolidation), the type of laying of the various layers of the web (e.g. by using cross-layers or cross-layers), compaction, etc.

[0065] In the context of the invention, the term flow distribution plate includes both bipolar plates arranged in fuel cell stacks between two membrane electrode assemblies and end plates located at the ends of the fuel cell stack.

[0066] Flow distribution plates serve to ensure the most even supply of reaction gases to the membrane-electrode assemblies of proton exchange membrane fuel cells and to transport the reaction products—i.e., in hydrogen-oxygen fuel cells, the water formed during the cathode reaction. Flow distribution plates feature a channel structure open on one side between the inlet and outlet, the so-called flow field. The flow field serves to macroscopically distribute the reaction gases to the adjacent GDL, which then microscopically distributes them to the catalytically active areas of the membrane. The flow field also serves to transport the gaseous and liquid product water away.

[0067] The flow field is formed by webs and channels, the arrangement of which forms a characteristic design. The width of the channels is preferably in a range from 0.1 to 3.5 mm, particularly preferably from 0.2 to 1.5 mm. The width of the webs is preferably in a range from 0.1 to 2.5 mm, particularly preferably from 0.2 to 1.5 mm. The depth of the channels is preferably in a range from 0.1 to 2.0 mm, particularly preferably from 0.15 to 0.5 mm.

[0068] In a preferred embodiment, the transition from web to channel is not designed as a sharp edge, but rounded. Specifically, the transition from web to channel is designed in the shape of a circular segment. The central angle of the circular segment is then preferably in a range from 60° to 120°, particularly preferably 75° to 105°, in particular approximately 90° (quarter-circle segment). The radius of the circular segment is added to the web width within the scope of the invention. Figure 1 shows a section of a flow distributor plate with a rounded transition between web and channel. "B" stands for the width of the channel and "R" for the radius of the circular segment, which is adjoined on both sides by the webs S.

[0069] In a flow distributor plate used according to the invention, the flow field can have fixed values ​​with regard to the width of the channels, the width of the webs, the rounding of the transitions from web to channel, and the depth of the channels. Alternatively, at least one of these values ​​can vary. For example, the width of the channel can vary over its length and / or the flow field can have multiple channels of different widths. This applies analogously to the other parameters mentioned. Furthermore, in a fuel cell stack, the geometries of the individual flow distributor plates can be identical or vary. In a special embodiment, flow fields for the anode side of an MEA have a different geometry than flow fields for the cathode side of an MEA.

[0070] The main orientation direction of the channels is understood to be the direction in which the channels of a flow distributor plate, based on the total length of the channels, have the largest proportion of length exhibiting the same direction. Preferably, the channels of the flow distributor plate have an orientation corresponding to the main orientation direction for at least 25%, particularly preferably at least 40%, and in particular at least 50%, based on the total length. Specifically, the channels of the flow distributor plate have an orientation corresponding to the main orientation direction for at least 60%, even more specifically at least 75%, based on the total length.

[0071] In principle, it is possible for the channels of a flow distribution plate to have two main orientation directions. For example, in a pin-type flow field (see Figure 2 E), the channels can have two orientation directions at an angle of approximately 90° to each other, whereby the length of the channels in both orientation directions can be the same. In general, flow distribution plates with a net-like (pin- or grid-like) design can have two main orientation directions. These designs have two orthogonal sets of parallel channels that enable flow in two directions. According to the invention, the main orientation direction of the fibers is then not aligned parallel to a main orientation direction of the channels.In the case of flow distribution plates which have two main orientation directions, the carbon fibre nonwoven fabric preferably comprises at least two fibre pile layers, wherein the main orientation direction of the fibres of each of the fibre pile layers is not aligned parallel to one of the main orientation directions of the channels.

[0072] The main orientation direction refers to the spatial arrangement of the channels and not the direction of flow through them. Thus, parallel-oriented channels have the same main orientation direction, even if they are flowed through in the opposite direction over part of their length (e.g., in a meandering flow field).

[0073] Preferably, the following four flow field designs are used, in which the channels have a clearly identifiable main flow direction: parallel flow field with straight channels, serpentine flow field with one or more parallel channels with the same flow direction, interdigitated flow field with interrupted channels, and pin-type flow field. These flow field types are illustrated in Figure 2. A simple flow field design with a single channel is the single-channel meander flow field. Specifically, the flow distribution plates used according to the invention have a flow field selected from among parallel flow fields, meander flow fields, interdigitated flow fields, and pin-type flow fields.The papers, fiber pile layers, carbon fiber nonwovens, and gas diffusion layers used according to the invention are sheet-like structures that have an essentially two-dimensional, flat extension and a comparatively smaller thickness. An orthogonal coordinate system can be used to describe them, with the base area of ​​the papers, fiber pile layers, carbon fiber nonwovens, and gas diffusion layers 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 can be used to describe the material thickness. In accordance with the description customary 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 (CD). The mass transport between the flow distributor plate and the membrane essentially takes place in the z-axis direction.

[0074] In the context of the invention, a fibrous web (also called 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 fibrous web (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. Fiber webs, nonwovens, nonwoven fabrics, and processes for their production are described in H. Fuchs and W. Albrecht, Vliesstoffe (Vlieses), 2nd edition, Wiley-VCH, Weinheim, Germany.

[0075] Gas diffusion layers are also referred to as GDL and microporous layers as MPL.

[0076] The gas diffusion layer used 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 distribution 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.

[0077] According to the invention, consolidated fiber pile layers are used in the GDLs, wherein the fibers of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of ​​the fiber pile layer (in the x,y plane), wherein the main orientation direction of the fibers is not aligned parallel to the main orientation direction of the channels.

[0078] Processes for producing fiber webs (nonwovens) and nonwovens in which the fibers are oriented (in the sense of parallelized) are known in principle. Processes for producing paper in which the fibers are oriented are also known in principle. Fiber orientation influences the material properties of the nonwoven or paper and the resulting gas diffusion layers, e.g., strength, stiffness, and elongation properties. The properties of the gas diffusion layers, in turn, influence the degree of intrusion of the GDL into the channels of the flow distribution plates and the contact between the GDL and the MEA. Specifically, the orientation of the fibers relative to the orientation of the channels of the flow distribution plates influences the electrical contact, contact resistance, and current density distribution of the fuel cell.

[0079] Fiber orientation can be determined using well-known optical or mechanical measurement methods. For example, it is possible to take images of the cross-sectional area of ​​nonwovens, nonwoven fabrics, or papers and then evaluate them optically using light or scanning electron microscopy. The fibers are visible on the cross-sectional surface as circles or ellipses and can be analyzed using image analysis methods regarding their position (angle of inclination to the normal) and orientation angle.

[0080] In paper production, fiber orientation can be significantly influenced by the operating parameters at the headbox. The fiber suspension delivered from the stock feed system creates a fiber suspension jet in the machine direction as it exits the headbox nozzle. The orientation of the fiber suspension jet exiting the headbox and its speed influence the fiber orientation as they are deposited on the wire. If there is a speed difference between the jet and the wire (so-called leading or lagging), the fibers are oriented in the machine direction. A higher speed difference results in the fibers being more strongly oriented in the longitudinal direction, thus resulting in greater strength in the longitudinal direction. Cross flows in the jet can influence the cross-sectional profile of the fiber orientation.Such cross flows can be caused by appropriately arranged nozzles or the use of orifices in the headbox nozzle.

[0081] In longitudinal nonwovens (longitudinally laid nonwovens), the majority of the fibers are oriented in the longitudinal direction of the produced nonwoven (in the machine direction of the production line, or MD). In cross-laid nonwovens (cross-laid nonwovens), the majority of the fibers are oriented in the transverse direction of the produced nonwoven (perpendicular to the machine direction of the production line, or CD).

[0082] For higher basis weights or larger base areas, for example, multiple piles can be laid on top of each other or parallel to each other. So-called cross-layer nonwovens are obtained by superimposing individual fiber pile layers or nonwovens, each with a different primary fiber orientation. For this purpose, so-called cross-layers (also known as cross-layers) can be used in nonwoven production.

[0083] Multi-layer nonwovens and nonwoven fabrics obtained therefrom thus have different fiber layers, whereby the fibers of the different layers can each have the same or a different orientation. Preference is given to nonwovens and nonwoven fabrics that have several oriented fiber pile layers, with at least two fiber pile layers having different main orientation directions. A special embodiment is nonwovens and nonwoven fabrics that have several oriented fiber pile layers, with at least two adjacent fiber pile layers having different main orientation directions.

[0084] To produce a fiber web with oriented fibers, the fiber material can be carded and laid on a carding machine. The carding process determines the fiber orientation in the web, e.g., parallel, randomized, compacted, or a combination thereof. For example, the fibers can be oriented parallel during carding and the card web can be removed and laid down parallel to the machine direction, resulting in a longitudinal web in which the main orientation direction of the fibers is in the longitudinal direction of the produced nonwoven fabric (MD). These nonwovens have anisotropic properties, e.g., high strength in the longitudinal direction, which is lower in the transverse direction (CD). If the fiber web removed from the carding machine is laid down on a vertically arranged conveyor belt, a transverse web is obtained in which the main orientation direction of the fibers is in the transverse direction of the produced nonwoven fabric (CD). For higher basis weights, e.g.,several webs are laid on top of each other. For this purpose, for example, two or more cards can be connected in parallel and the webs laid down on a common conveyor belt. Cross-lappers can be used to produce multi-layer fiber webs (nonwovens) with fiber layers of different orientations. The desired isotropy of the fibers in the not yet consolidated web can be ensured using suitable control devices. This can then be subjected to consolidation using conventional processes, e.g. by thermal consolidation, mechanical consolidation or chemical consolidation to produce the nonwoven fabric. Thermal consolidation is preferably carried out by calendering or consolidation with hot air without the application of pressure, e.g. in a drum dryer or belt dryer. Mechanical consolidation is preferably carried out by hydroentanglement or needling.

[0085] During web consolidation, a (further) orientation of the fibers can occur. In a preferred embodiment, the fiber web layer(s) is / are subjected to a hydroentaglement (spunlacing) process for consolidation. The loose fiber web is passed through a series of high-pressure water jets that spray water onto the fiber material. As it passes through the rows of water jets, the web is densified and, at the same time, the fibers in the web are longitudinally oriented. After hydroentaglement, the residual water is removed from the resulting nonwoven. It has been found that it is advantageous for the mechanical properties of the gas diffusion layers and the resulting fuel cells if the water jet channels resulting from hydroentaglement are not aligned parallel to the channels of the flow distribution plates.According to the invention, it is therefore advantageous if the main orientation direction of the fibers and the water jet channels are not aligned parallel to the main orientation direction of the channels.

[0086] The degree of fiber orientation can be characterized in longitudinal nonwovens or longitudinal nonwoven fabrics (longitudinally laid nonwovens or nonwovens) by specifying the MD / CD ratio. Similarly, the degree of fiber orientation in transverse nonwovens or transverse nonwoven fabrics (cross-laid nonwovens or nonwovens) can be characterized by specifying the CD / MD ratio. The gas diffusion layers used according to the invention can comprise a carbon fiber nonwoven fabric with a single consolidated fiber pile layer (single-layer nonwoven fabric) or with several consolidated fiber pile layers (multi-layer nonwoven fabric). It has been found that the use of multi-layer nonwoven fabrics can have a beneficial effect on the mechanical properties of the gas diffusion layer. Thus, the use of multi-layer nonwoven fabrics can help prevent inhomogeneous compressive stress on the gas diffusion layers within the fuel cell stack.Specifically, the GDL-CCM contact can be improved and the contact resistance between GDL and COM can be reduced. It has been found that the use of multilayer nonwovens with a fiber orientation of the fiber pile layers can be particularly advantageous.

[0087] A proton exchange membrane fuel cell is preferred, wherein the carbon fiber nonwoven fabric comprises 2, 3, 4, 5 or more than 5 fiber pile layers.

[0088] Particularly preferred is a proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the carbon fiber nonwoven fabric has a plurality of fiber pile layers and at least 2 of the fiber pile layers, preferably at least 3 of the fiber pile layers, in particular all fiber pile layers, have a fiber orientation.

[0089] A specific embodiment is a proton exchange membrane fuel cell, wherein the carbon fiber nonwoven fabric comprises several fiber pile layers, and at least two fiber pile layers have a fiber orientation with two different main orientation directions relative to one another. Preferably, the main orientation directions of the fibers of the pile layers with different main orientation directions are at an angle of 30° to 90°, particularly preferably 45° to 90°, in particular 60° to 90°, relative to one another.

[0090] A more specific embodiment is a proton exchange membrane fuel cell, wherein the carbon fiber nonwoven fabric comprises multiple fiber pile layers and at least two adjacent fiber pile layers have a fiber orientation with two different main orientation directions. Preferably, the main orientation directions of the fibers of the two adjacent pile layers with different main orientation directions are at an angle of 30° to 90°, particularly preferably 45° to 90°, in particular 60° to 90°, to each other.

[0091] A preferred embodiment of the invention is a proton exchange membrane fuel cell, wherein the gas diffusion layers comprise a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the fibers of one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers, the fibers of the fiber pile layer on the side facing the flow distributor plate have a main orientation direction that is not aligned parallel to the main orientation direction of the channels of the flow distributor plate.

[0092] Preferably, the main orientation direction of the fibers of at least one fiber pile layer and the main orientation direction of the channels are at an angle of 30° to 90°, particularly preferably 45° to 90°, in particular 60° to 90°, to one another. A first preferred embodiment is a gas diffusion layer comprising a carbon fiber nonwoven fabric with a single consolidated fiber pile layer (single-layer nonwoven fabric), wherein the main orientation direction of the fibers of the fiber pile layer and the main orientation direction of the channels of the flow distributor plate in contact with the gas diffusion layer are at an angle of 30° to 90°, particularly preferably 45° to 90°, in particular 60° to 90°, to one another.

[0093] A second preferred embodiment is a gas diffusion layer comprising a carbon fiber nonwoven fabric with 2, 3, 4, 5 or more than 5 fiber pile layers (multi-layer nonwoven fabric), wherein the main orientation direction of the fibers of at least one of the fiber pile layers and the main orientation direction of the channels of the flow distributor plate in contact with the gas diffusion layer have an angle of 30° to 90°, particularly preferably of 45° to 90°, in particular of 60° to 90°, to one another.

[0094] A particularly preferred embodiment is a gas diffusion layer comprising a carbon fiber nonwoven fabric with 2, 3, 4, 5 or more than 5 fiber pile layers (multi-layer nonwoven fabric), wherein the main orientation direction of the fibers of the fiber pile layer in contact with the flow distributor plate and the main orientation direction of the channels of this flow distributor plate have an angle of 30° to 90°, particularly preferably of 45° to 90°, in particular of 60° to 90°, to one another.

[0095] A preferred embodiment of the invention is a proton exchange membrane fuel cell, wherein the gas diffusion layer comprises a carbon fiber nonwoven fabric, for the production of which carbon fibers are provided, the provided carbon fibers are subjected to carding to increase the parallel orientation in the fiber longitudinal direction, the carded fibers are laid down to form a fiber web, optionally at least one further fiber web layer is laid down on the first fiber web layer and the fiber web layer(s) are consolidated to form a nonwoven fabric.

[0096] A proton exchange membrane fuel cell is preferred, wherein the gas diffusion layers comprise a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the main orientation direction of the fibers of one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers, the main orientation direction of the fibers of at least one of the fiber pile layers corresponds to the machine direction (MD) or the cross-machine direction (CD).

[0097] A particularly preferred embodiment of the invention is a proton exchange membrane fuel cell, wherein the gas diffusion layers comprise a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the main orientation direction of the fibers of one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers, the main orientation direction of the fibers of the fiber pile layer on the side facing the flow distributor plate corresponds to the machine direction (MD) or the cross direction to the machine direction (CD).

[0098] A preferred embodiment of the invention is a proton exchange membrane fuel cell wherein the gas diffusion layers comprise a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers, at least one of the fiber pile layers has an MD / CD ratio in the range of at least 2.0:1, preferably at least 5.0:1, in particular at least 10.0:1.

[0099] A further preferred embodiment of the invention is a proton exchange membrane fuel cell wherein the gas diffusion layers comprise a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers, at least one of the fiber pile layers has a CD / MD ratio in the range of at least 2.0:1, preferably at least 5.0:1, in particular at least 10.0:1.

[0100] A particularly preferred embodiment of the invention is a proton exchange membrane fuel cell wherein the gas diffusion layers comprise a carbon fiber nonwoven fabric comprising a plurality of consolidated fiber pile layers, wherein the fiber pile layer on the side facing the flow distributor plate has an MD / CD ratio in the range of at least 2.0:1, preferably at least 5.0:1, in particular at least 10.0:1.

[0101] A particularly preferred embodiment of the invention is a proton exchange membrane fuel cell wherein the gas diffusion layers comprise a carbon fiber nonwoven fabric comprising a plurality of consolidated fiber pile layers, wherein the fiber pile layer on the side facing the flow distributor plate has a CD / MD ratio in the range of at least 2.0:1, preferably at least 5.0:1, in particular at least 10.0:1.

[0102] Another particularly preferred embodiment of the invention is a proton exchange membrane fuel cell wherein the gas diffusion layers comprise a carbon fiber nonwoven fabric comprising a plurality of consolidated fiber pile layers, wherein at least one pile layer is oriented in the machine direction (MD) and at least one other pile layer is oriented in the cross-machine direction (CD).

[0103] Flat electrically conductive material A)

[0104] According to the invention, a sheet-like electrically conductive fiber material A) is used which is selected from carbon fiber nonwovens, carbon fiber papers, and combinations thereof. Preference is given to using a sheet-like electrically conductive fiber material A) which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric. Particular preference is given to using a sheet-like electrically conductive fiber material A) which consists of a carbon fiber nonwoven fabric. The carbon fiber nonwoven fabric can comprise one or more consolidated fiber pile layers. According to the invention, the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area (x,y plane) of the fiber pile layer or of the carbon fiber nonwoven fabric or of the sheet-like electrically conductive fiber material A).

[0105] Preferably, the fiber material A) has a basis weight of 15 to 400 g / m 2 , preferably 20 to 300 g / m 2, preferably 30 to 150 g / m 2 and in particular 40 to 120 g / m 2 , such as 50 to 100 g / m 2 , on. The determination of the basis weight (also called area-related mass or grammage) in g / m 2 can be carried out according to ISO 9073-1 or EN 29073-1:1992. For the purposes of the present invention, the basis weight refers to the untreated fiber material, i.e., without any components such as polymeric binders or other additives.

[0106] The fiber material A) preferably has a thickness in the range of 50 to 500 μm, particularly preferably 100 to 400 μm. This thickness refers to the untreated, uncompressed state of the fiber material, i.e., before the GDL is installed in a fuel cell. The thickness of the flat, electrically conductive fiber material A) and the gas diffusion layer can be determined according to DIN 53855-1:1993-08 "Determination of the Thickness of Textile Fabrics."

[0107] The fibers contained in the fiber material A) comprise carbon fibers (carbon fibers, carbon fibers) and optionally fibers different therefrom, preferably selected from glass fibers, fibers of organic polymers, such as polypropylene, polyester, polyphenylene sulfide, polyether ketones, and mixtures thereof.

[0108] Specifically, the fibers contained in fiber material A) consist only of carbon fibers.

[0109] Carbon fibers can be produced in a conventional manner, preferably using polyacrylonitrile fibers (PAN fibers) as the starting material. PAN fibers are produced by radical polymerization of a monomer composition that preferably contains at least 90% by weight, based on the total weight of the monomers used for polymerization, of acrylonitrile. The resulting polymer solution is spun into filaments, e.g., by wet spinning and coagulation, and 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 1000°C, preferably at least 1200°C. Depending on the shape of the desired fiber material, 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, very strong, and very good conductors of electricity and heat.

[0110] According to the invention, the sheet-like electrically conductive fiber material A) comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric. Preferably, the sheet-like electrically conductive fiber material A) consists of a carbon fiber nonwoven fabric. In an alternative embodiment, the sheet-like electrically conductive fiber material A) comprises a carbon fiber nonwoven fabric and at least one further sheet-like electrically conductive material, preferably selected from carbon fiber fabrics, carbon fiber papers, and combinations thereof.

[0111] In carbon fiber fabrics, the flat fiber material is produced by interlacing two thread systems: warp and weft. As with textiles, fiber bundles are flexibly but inextricably linked. Oxidized, but not yet carbonized or graphitized PAN fibers are preferably used to produce carbon fiber fabrics. Carbonization or graphitization, to impart electrical conductivity to the flat fiber material, occurs after weaving.

[0112] Oxidized PAN fibers are generally used to produce carbon fiber paper. These are shredded into fiber fragments in a conventional manner, slurried, and, analogous to papermaking, a fiber mat is produced by separating the fibers from the vat using sieves and dried. In a preferred embodiment, at least one binder is additionally incorporated into the paper. Suitable binders include, for example, phenolic, furan, polyimide resins, etc. To incorporate the binder, the paper can be impregnated with it, and the binder can then be cured if necessary. After impregnation and curing, the carbon fiber paper is subjected to another carbonization / graphitization process to convert the binder into compounds with improved electrical conductivity. In another suitable embodiment, a filled carbon fiber paper is used to provide the fiber material A).To produce the paper, a filler made 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 filler increases the thermal and electrical conductivity to such an extent that carbonization / graphitization is no longer necessary.

[0113] Non-oxidized or oxidized PAN fibers can be used to produce carbon fiber nonwovens. In a preferred embodiment, the fibers are carded, dry-laid into a nap, and then bonded to form a nonwoven. Bonding can be achieved using conventional methods, preferably by hydroentaglement (spunlacing). The loose fiber web is passed through a series of high-pressure water jets, which simultaneously compacts the web and longitudinally orients the fibers within the web. Optionally, the nonwoven can be treated with at least one additive. By treating it at elevated pressure and / or elevated temperature, the nonwoven can be compacted, and the thickness of the bonded 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 web 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 web laying and consolidation. Carbon fiber nonwovens, for the production of which the fibers are dry-laid into a pile in a first step, are a preferred embodiment of the invention.

[0114] In a preferred embodiment, electrically conductive fiber material A) contains at least one additive applied thereto and / or incorporated therein. For this purpose, the fiber material can be treated with polymer components and, if appropriate, other additives using conventional application and impregnation processes.

[0115] The additive is preferably selected from a1) at least one polymeric additive, a2) optionally at least one conductivity-improving additive, a3) optionally at least one further additive different from a1) and a2, and combinations thereof.

[0116] In a preferred embodiment, the electrically conductive fiber material A) contains at least one polymeric additive a1) applied thereto and / or incorporated therein. The polymeric additive a1) is preferably selected from fluorine-containing polymers a11), fluorine-free high-temperature-resistant polymers a12), polymers a13) different therefrom, and mixtures thereof.

[0117] The polymeric additive a1) preferably comprises at least one fluorine-containing polymer a11), preferably selected from polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), perfluoroalkoxy polymers (PFA), and mixtures thereof. Perfluoroalkoxy polymers are, for example, copolymers of tetrafluoroethylene (TFE) and perfluoroalkoxy vinyl ethers, such as perfluorovinyl propyl ether. The fluorine-containing polymer a11) particularly preferably comprises, and in particular is, polytetrafluoroethylene. Component a11) can serve to increase the hydrophobicity of the fiber material.

[0118] The mass fraction of the fluorine-containing polymer a11) is preferably 0.5 to 40%, preferably 1 to 20%, in particular 1 to 10%, based on the mass of the fiber material A).

[0119] In a specific embodiment, the fluorine-containing polymer a11) is PTFE and the mass fraction of PTFE is 0.5 to 40%, preferably 1 to 20%, in particular 1 to 10%, based on the mass of the fiber material.

[0120] Likewise preferably, the polymeric additive a1) comprises at least one polymer a12) different from a11) selected from fluorine-free, high-temperature-resistant polymers. Preferably, the polymer a12) is selected from so-called high-performance plastics, which are characterized by properties such as a high glass transition temperature, a high melting temperature, good temperature resistance, good chemical resistance, and good mechanical properties. Preferably, the polymers a12) have a continuous operating temperature (continuous use temperature) of at least 150°C. Preferred polymers a12) are semi-aromatic and aromatic polymers. The polymers a12) are preferably thermoplastics.

[0121] The polymer a12) is particularly preferably selected from polyaryletherketones (PAEK), polyphenylene sulfides (PPS), polysulfones (PSU), polyethersulfones (PES), partially aromatic (co)polyamides (high-temperature polyamides, HTPA), polyimides (PI), polyamideimides (PAI), polyetherimides (PEI) and mixtures (blends) thereof.

[0122] The use of such a component a12) in and / or on the electrically conductive fiber material A) of a gas diffusion layer can lead to a reduction of the contact resistance at the interface between the gas diffusion layer and the catalyst layer applied to the polymer electrolyte membrane of a polymer electrolyte membrane fuel cell, especially in the region of the channels of the flow distribution plate of the fuel cell, especially in the region of the center of the channels.

[0123] In particular, the polymer component a12) comprises at least one polyaryl ether ketone. Specifically, the polymer component a12) consists of at least one polyaryl ether ketone. Polyaryl ether ketones are semi-crystalline thermoplastics that have an alternating structure in which each aryl group is followed by a keto group (carbonyl group) or ether group. The proportions of the keto and ether groups are variable and can differ in the substitution pattern on the aryl rings. Suitable polyaryl ether ketones a12) are polyether ketones (PEK), polyether ether ketones (PEEK), polyether ketone ketones (PEKK), etc. Preferably, the polymer component a1) comprises at least one polyether ether ketone; in particular, the polymer component a1) consists of at least one polyether ether ketone.

[0124] Suitable partially aromatic (co)polyamides a12) are, in particular, the polymers known as high-temperature polyamides (HTPA). These are semi-crystalline or amorphous, thermoplastic, partially aromatic polyamides. These preferably contain at least one aromatic dicarboxylic acid in polymerized form, in particular selected from terephthalic acid, isophthalic acid, and mixtures of terephthalic acid and isophthalic acid. Preferred partially aromatic (co)polyamides a2) are selected from PA 6.T, PA 10.T, PA 12.T, PA 6.1, PA 10.1, PA 12.1, PA 6.T / 6.I, PA 6.T / 6, PA 6.T / 10T, PA 10.T / 6.T, PA 6.T / 12.T, PA12.T / 6.T, and mixtures thereof.

[0125] Another special embodiment of polyamides a12) is polyphthalamide (PPA).

[0126] Suitable polyimides a12) are polysuccinimide (PSI), polybismaleimide (PBMI), polyimidesulfone (PISO) and polymethacrylimide (PMI).

[0127] Suitable polymers a12) are also (semi-)aromatic polyesters, such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), polycarbonates (PC) and temperature-resistant melamines, such as melamine foams filled with nanoporous SiO2 aerogels.

[0128] The mass fraction of the polymer a12) is preferably 0.5 to 40%, preferably 1 to 20%, based on the mass of the fiber material a). In a specific embodiment, the polymer a12) is PEEK, and the mass fraction of the PEEK a12) is 0.5 to 40%, preferably 1 to 20%, based on the mass of the fiber material.

[0129] In a particularly preferred embodiment, polymer a1) comprises at least one fluorine-containing polymer a11) and at least one polymer a12) different from a11), as defined above, in order to simultaneously achieve a suitable hydrophobicity of the fiber material and a reduced contact resistance as explained above. Preferably, the proportion of fluorine-containing polymer a11), based on the total weight of polymers a11) and a12), is 10 to 100 wt. %, preferably 20 to 90 wt. %, more preferably 30 to 80 wt. %, even more preferably 40 to 75 wt. %, such as in particular 40 to 60 wt. % or 60 to 75 wt. %.

[0130] Suitable polymers a13) are, for example, selected from phenolic resins, furan resins, polyimide resins, and mixtures thereof. Specifically, the fiber material A) contains, in addition to the fluorine-containing polymers a11) and the fluorine-free, high-temperature-resistant polymers a12), other polymers a13) in a weight fraction of at most 5%, preferably of at most 1%, particularly preferably of at most 0.5%, in particular of at most 0.1%, based on the total weight of the fiber material A), applied thereto and / or incorporated therein. Even more specifically, the fiber material contains no additives of further polymers a13) that are different from the fluorine-containing polymers a11) and the polymers a12). This applies especially to polymeric binders a13) that would carbonize under the manufacturing conditions of the sheet-like, electrically conductive material A).

[0131] If a bonded fiber material is used as the fiber material, it is selected in particular from mechanically bonded fiber materials. Chemical bonding, especially with carbonizable polymeric binders, can adversely affect the flexural properties of the gas diffusion layer. Specifically, the fiber materials used according to the invention do not contain any polymers other than polymers a11) and a12) added as binders.

[0132] In a specific embodiment of the invention, the fibers are wet-laid in a first step. In this embodiment, the term carbon fiber nonwoven fabric then also includes, for example, a wet-laid material consisting of short-cut carbon fibers, carbon black, at least one polymer a11), especially PTFE, and at least one fluorine-free, high-temperature-resistant polymer a12), especially PEEK. In contrast to carbon fiber papers known from the prior art, the wet-laid fiber materials used according to the invention have no or only a very small proportion of phenolic resins as binders. The mass fraction of phenolic resins is preferably 0 to 10%, more preferably 0 to 5%, in particular 0 to 1%, based on the mass of the fiber material.

[0133] In many cases, the fiber material already possesses good electrical and thermal conductivity due to the carbon fibers used, even without conductivity-enhancing additives. To improve the electrical and thermal conductivity, however, the fiber material can additionally be treated with at least one conductivity-enhancing additive a2). Preferably, the conductivity-enhancing additive a2) is selected from metal particles, carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. A special embodiment is graphitic carbon nanofibers (GCNFs). Preferably, the conductivity-enhancing additive a2) comprises carbon black or consists of carbon black. The treatment of the fiber material with at least one conductivity-enhancing additive a2) can, for example, be carried out together with at least one polymeric additive a1) and / or further additives a3).An aqueous dispersion is preferably used to finish the fiber material. Preferably, the fiber material A) contains conductivity-improving additives a2) in a concentration of 0 to 50 wt.%, based on the total weight of the fiber material A).

[0134] If the fiber material A) contains a conductivity-improving additive a2), then it is preferably present in an amount of 0.5 to 45%, particularly preferably 1 to 25%, based on the mass of the fiber material. In a specific embodiment, the conductivity-improving additive a2) comprises carbon black or consists of carbon black, and the mass fraction is 0.5 to 45%, preferably 1 to 25%, based on the mass of the fiber material A).

[0135] In one embodiment, the fiber material A) further contains at least one additive a3) applied thereto and / or incorporated therein, which additive is selected, for example, from among surface-active substances other than polymeric additives a1) and conductivity-enhancing additives a2), and other additives and auxiliaries. The fiber material can be treated with at least one further additive a3), for example, together with a1) and / or a2).

[0136] The total mass fraction of further additives a3) is preferably 0 to 80%, preferably 0 to 50%, based on the mass of the fiber material A). If the fiber materials additionally contain at least one further additive a3), the total mass fraction of further additives a3) is 0.1 to 80%, preferably 0.5 to 50%, based on the mass of the fiber material A).

[0137] In a specific embodiment, the fiber material A), optionally equipped with components a1), a2), and / or a3), is subjected to a thermal treatment (drying and / or sintering). The thermal treatment of the fiber material A) is preferably carried out at a temperature of at least 250°C, preferably at least 300°C, in particular in a range from 250 to 500°C. A thermal treatment can also be carried out after the application of the microporous layer B), as described in more detail below.

[0138] The fiber material can be treated with the polymeric additives a1), conductivity-enhancing additives a2), and optionally further additives a3) by conventional methods. Suitable coating and impregnation methods are described in more detail below. The subsequent drying and / or sintering, preferably drying and sintering, of the coated and / or impregnated fiber material to produce the sheet-like electrically conductive material A) according to the invention is also described in more detail below. The binding effect of the polymer is achieved through sintering. The sintering step can also be carried out without a prior drying step. However, a prior drying step is preferably carried out, which promotes a more uniform binder distribution in the sintered product of the sheet-like electrically conductive material A).

[0139] Production of the flat electrically conductive material A)

[0140] The invention also relates to a process for producing the sheet-like electrically conductive fiber material A), wherein the fibers provided in at least one fiber pile layer are subjected to carding to increase the parallel orientation in the fiber longitudinal direction. The sheet-like electrically conductive fiber materials A) thus obtained are preferably suitable for producing gas diffusion layers for the proton exchange membrane fuel cells according to the invention.

[0141] Preferably, for the production of the sheet-like electrically conductive fiber material A) i-1) a fiber composition is provided which comprises carbon fibers and / or precursors of carbon fibers, i-2) the fiber composition provided in step i-1) is subjected to carding to increase the parallel orientation in the fiber longitudinal direction, i-3) the fiber composition obtained in step i-2) is laid down to form a fiber web, i-4) optionally at least one further fiber web layer is laid down on the first fiber web layer, i-5) the fiber web layer(s) obtained in step i-3) or i-4) is consolidated to form a nonwoven fabric, i-6) optionally the nonwoven fabric obtained in step i-5) is subjected to finishing with at least one additive, i-7) optionally the nonwoven fabric obtained in step i-5) or the finished nonwoven fabric obtained in step i-6) is subjected to post-treatment at elevated pressure and optionally elevated temperature,i-8) if the fiber composition used in step i-1) comprises precursors of carbon fibers, the nonwoven fabric obtained in step i-5) or the finished nonwoven fabric obtained in step i-6) or the post-treated nonwoven fabric obtained in step i-7) is subjected to pyrolysis at a temperature of at least 1000 °C.

[0142] In steps i-1) to i-5) of the method according to the invention, a flat, electrically conductive fiber material A) is provided, which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric, wherein the carbon fiber nonwoven fabric comprises one or more consolidated fiber pile layers. Preferably, at least one fiber material A) is provided which consists of a carbon fiber nonwoven fabric. With regard to suitable and preferred fiber materials and methods for nonwoven fabric production, reference is made in full to the above statements. The term "carbon fiber nonwoven fabric" also encompasses nonwoven fabrics that comprise carbon fiber precursors.

[0143] Preferably, in step i-4), at least one further fibrous pile layer is deposited on the first fibrous pile layer, wherein at least one of the further pile layers has a main orientation direction of the fibers which is substantially transverse to the main orientation direction of the fibers of the first pile layer.

[0144] Preferably, in step i-5), the fibrous web layer(s) obtained in step i-3) or i-4) are consolidated into a nonwoven fabric by exposure to water-containing fluid jets. In step i-6), the provided fibrous material A) is preferably coated and / or impregnated with an aqueous composition containing at least one polymeric additive a1), optionally at least one conductivity-improving additive a2), optionally at least one further additive a3) different from a1) and a2), and combinations thereof.

[0145] The aqueous composition can be in the form of a dispersion or a solution. Typically, it is in the form of an aqueous dispersion.

[0146] The finishing of the fiber materials by coating and / or impregnation is carried out using conventional application methods known to those skilled in the art. Preferably, a method selected from among padding, doctor blade coating, spraying, splashing, and combinations thereof is used for coating and / or impregnating the fiber materials. In the padding process, the fiber material is passed through a padder (dip tank) containing the additive-containing solution or dispersion and then squeezed out to the desired application rate of additive using a pair of rollers with adjustable pressure and, if necessary, gap.

[0147] In the squeegee process, a distinction is made between gravure and screen printing. In gravure printing, the squeegee used is a knife-like steel belt, with or without a support blade. It serves to wipe off excess additive-containing solution or dispersion from the webs of the printing cylinder (scraping). In screen printing, however, the squeegee is usually made of rubber or plastic with a sharp or rounded edge.

[0148] During spray application, the additive-containing solution or dispersion is applied to the fiber material to be finished using a slotted nozzle.

[0149] The kiss-roll coating process is used to coat the underside of horizontally running webs. The coating medium can be applied to the web in either a counter- or co-rotating direction. Transfer rollers allow for indirect coating with small application quantities.

[0150] In a particular embodiment, the padding process is used to finish the cleaning articles according to the invention.

[0151] In step i-7) of the process according to the invention, the coated and / or impregnated fiber material is subjected to drying and / or sintering, preferably drying and sintering. Suitable processes for drying fiber materials coated and / or impregnated with additive-containing solutions or dispersions, such as nonwovens or woven fabrics, are known in principle. After application, for example, at least a portion of the solvent, especially the water, can be suctioned out of the fiber material, for example by passing it through a suction opening, from which the liquid is removed by means of an applied negative pressure. Alternatively or additionally, the fiber material can be dried at an elevated temperature. In addition, drying can take place at a reduced pressure. The fiber material is preferably dried at a temperature in the range of 20 to 250 °C, particularly preferably 40 to 200 °C. In addition, ieAfter, or alternatively to, drying, the coated and / or impregnated fiber material can be subjected to sintering. Sintering is preferably carried out by thermal treatment at a temperature of 250°C to 500°C, preferably 300°C to 450°C, and in particular 350°C to 450°C.

[0152] Alternatively or additionally, a compaction / thickness calibration treatment can be carried out in step i-7), preferably using a device 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. A particularly suitable calender is a felt belt calender.

[0153] Specifically, for the treatment in step i-7), a nonwoven fabric obtained by exposure to water-containing fluid jets in step i-5) is subjected to a post-treatment at elevated pressure and optionally elevated temperature, wherein preferably the nonwoven fabric still moist from the compaction with water-containing fluid jets is used.

[0154] If the fiber composition used in step i-1) comprises carbon fiber precursors, the nonwoven fabric is subjected to pyrolysis at a temperature of at least 1000 °C in step i-8). Depending on the temperature during pyrolysis, a distinction is made between carbonization and graphitization. Carbonization refers to a treatment at approximately 1000 to 1500 °C in an inert gas atmosphere, which leads to the elimination 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. The carbon content increases during pyrolysis, for example, from approximately 67 wt.% during treatment at temperatures below 1000 °C to approximately 99 wt.% during treatment at temperatures above 2000 °C. The fibers obtained by graphitization in particular are highly pure, lightweight, very strong, and very good conductors of electricity and heat.

[0155] Fabrication of a gas diffusion layer and a proton exchange membrane fuel cell

[0156] The invention relates to a method for producing a gas diffusion layer for a fuel cell, in which i) a flat, electrically conductive fiber material A) is provided which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric which comprises one or more consolidated fiber pile layers, wherein the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of ​​the fiber pile layer (x,y plane), ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer B), iii) optionally the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and / or elevated temperature.The invention also relates to a method for producing a proton exchange membrane fuel cell, in which, in addition to the previously described steps i) to iii), iv) the coated fiber material obtained in step ii) or iii) is placed as a gas diffusion layer on both sides of a proton exchange membrane coated on both sides with catalytically active electrodes, wherein the microporous layers are located on the sides of the gas diffusion layers facing the catalytically active electrodes, v) a flow distributor plate is placed on each of the outer sides of the gas diffusion layers, said flow distributor plate having channels for supplying reactant gases and removing products of the cell reaction, which are in contact with the outer sides of the gas diffusion layers, vi) the flow distributor plates and the components located therebetween are pressed together.

[0157] With regard to the provision of a sheet-like electrically conductive fiber material A) in step i) of the process, reference is made in full to the above-mentioned information on electrically conductive fiber materials A) which comprise a carbon fiber nonwoven fabric or consist of a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers with oriented fibers.

[0158] In step ii) of the process according to the invention, the fiber material provided in step i) is coated with a precursor to form a microporous layer B). The microporous layer B) preferably contains conductive particles in a matrix of a polymeric binder. The conductive particles are preferably selected from conductive carbon particles, in particular carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. Preference is given to using carbon black, graphite, or a mixture thereof.

[0159] In particular, the polymeric binder contains at least one fluorine-containing polymer. The fluorine-containing polymer is preferably selected from polytetrafluoroethylenes, tetrafluoroethylene-hexafluoropropylene copolymers, perfluoroalkoxy polymers, and mixtures thereof. Polytetrafluoroethylene (PTFE) is preferably used.

[0160] For producing the microporous layer B), the polymeric binder is preferably used in a weight amount of 0.5 to 50 wt.%, particularly preferably 1.0 to 40 wt.%, in particular 10 to 25 wt.%, based on the total weight of polymeric binders and conductive particles.

[0161] In contrast to the macroporous fiber material A), the MPL B) is microporous with pore diameters that are generally well below five micrometers, preferably well below one micrometer. The pore diameter of the MPL is preferably at most 900 nm, more preferably at most 500 nm, and in particular at most 300 nm. The average pore diameter of the MPL is preferably in a range from 5 to 200 nm, particularly preferably from 10 to 100 nm. The porosity and pore size distribution can be determined using mercury porosimetry, as described in DIN ISO 15901-1:2019-03: Mercury Porosimetry. The latter average pore diameters apply primarily to the use of carbon black as conductive particles in the MPL. By using graphite as conductive particles in the MPL or by using pore-forming agents, significantly larger MPL pores can also be created. Depending on the composition, the average pore diameter is e.g.larger than 1 pm. When using different conductive particles, the pore diameter can exhibit a bimodal or polymodal distribution curve.

[0162] In a special embodiment, the precursor used in step ii) to form the MPL contains at least one pore former. Suitable pore formers are commercially available plastic particles, e.g., made of polymethyl methacrylate (PMMA). A suitable particle size ranges from 10 to 100 pm.

[0163] 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 GDL is incorporated into a fuel cell.

[0164] The microporous layer preferably has a mean roughness R aof at most 10 pm, more preferably at most 5 pm, determined according to DIN 4768-1:1974-08. The microporous layer preferably has an average roughness R z of not more than 60 pm, preferably not more than 30 pm, determined according to DIN 4768-1:1974-08.

[0165] The presence of MPL has a significant impact on the water balance of the fuel cell. Due to the high PTFE content and the smaller pores of MPL, flooding of the GDL and the electrode is made more difficult by the MPL acting as a liquid water barrier, thus promoting the mass transport of gaseous reactants to the catalyst.

[0166] The application of the precursor to form the MPL B) onto the fiber material A) in step ii) can be carried out 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 and the viscosity of the coating.

[0167] In step iii) of the process according to the invention, the coated fiber material obtained in step ii) is optionally subjected to a post-treatment at elevated pressure and / or elevated temperature. In a specific embodiment, the treatment in step iii) is carried out at an elevated pressure of at least 0.5 MPa and an elevated temperature of at least 100°C.

[0168] Preferably, the treatment in step iii) is carried out at a pressure in the range from 0.5 to 10.0 MPa (5 to 100 bar), particularly preferably from 1.5 to 8.0 MPa.

[0169] The treatment in step iii) is preferably carried out at a temperature in the range from 100 to 400°C, particularly preferably from 120 to 370°C. The treatment in step iii) is preferably carried out in a press for a period of from 5 seconds to 5 minutes, preferably from 10 seconds to 2 minutes.

[0170] 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.

[0171] For the post-treatment in step iii), conventional devices such as single-daylight presses, multi-daylight presses, endless belt presses or calenders can be used.

[0172] In the process according to the invention for producing a proton exchange membrane fuel cell, a GDL obtained by the previously described steps i), ii) and optionally iii) is additionally subjected to steps iv), v) and vi).

[0173] Fuel cell and fuel cell stack

[0174] Another object of the invention is a proton exchange membrane fuel cell, comprising a proton exchange membrane coated on both sides with a catalytically active electrode, on each side of the catalyst-coated membrane a gas diffusion layer whose inner side is in contact with the catalytically active electrode, on each side of the catalyst-coated membrane a flow distributor plate having channels for supplying reactant gases and removing products of the cell reaction, wherein the channels are in contact with the outer side of the gas diffusion layer.

[0175] In principle, the gas diffusion layers according to the invention, which comprise a flat, electrically conductive fiber material A) comprising a carbon fiber nonwoven fabric or consisting of a carbon fiber nonwoven fabric comprising at least one fiber pile layer with oriented fibers, are suitable for all common proton exchange membrane fuel cell types. Reference is made in full to the above statements regarding the construction of proton exchange membrane fuel cells.

[0176] As described above, the gas diffusion layer according to the invention is suitable, compared to prior art gas diffusion layers, for reducing the contact resistance at the interface between the microporous layer of the gas diffusion layer and the catalyst layer applied to the proton exchange membrane in a proton exchange membrane fuel cell, especially in the region of the channels of the flow distribution plates of the fuel cell, particularly in the region of the center of the channels. The contact resistance of a GDL applied to a flow distribution plate at its interface with a catalyst layer applied to a proton exchange membrane can be simulated using the strip measurement described below.This applies both to the areas of the channels of the fuel cell flow distribution plate, particularly the areas of the center of the channels, and to the areas of the webs of the fuel cell flow distribution plate. The contact resistance in question is simulated by determining the contact resistance of the GDL applied to the flow distribution plate in the corresponding areas instead at the interface to the measuring electrodes. For details of the strip measurement, including the measuring apparatus, please refer to the following section on measuring methods. Since, after the individual components of a fuel cell have been pressed together, the pressure to which the GDL is exposed is reduced in the area of ​​the channels of the flow distribution plate, and particularly in the area of ​​the center of the channels, compared to the area of ​​the webs, the contact resistance at the interface to the catalyst layer is comparatively high there.Surprisingly, it was found that the GDL according to the invention leads to a significant reduction in contact resistance at the interface with the catalyst layer in the compressed fuel cell compared to prior art gas diffusion layers. This applies particularly in the area of ​​the channels, especially their center.

[0177] A further object of the invention is the use of at least one gas diffusion layer, obtainable by a process as defined above, in a proton exchange membrane fuel cell, for improving the contact between the catalyst-coated polymer electrolyte membrane and the microporous layers of the gas diffusion layers.

[0178] A specific embodiment of the invention is the use of at least one gas diffusion layer according to the invention to prevent a reduction in the contact area between the catalyst-coated polymer electrolyte membrane and the gas diffusion layers in regions opposite the channels of the flow distribution plates, in particular in regions opposite the center of the channels.

[0179] A further specific embodiment of the invention is the use of at least one gas diffusion layer according to the invention to prevent an increase in the contact resistance between the catalyst-coated membrane and the gas diffusion layers in regions opposite the channels of the flow distribution plates, in particular in regions opposite the center of the channels.

[0180] Measurement methods

[0181] The following describes in more detail the test methods underlying the present application and, in particular, the evaluation of the (comparative) examples:

[0182] Basis weight

[0183] The determination of the area-related mass (basis weight, grammage) in g / m 2 was carried out according to ISO 9073-1.

[0184] thickness

[0185] The determination of thickness in the uncompressed state can be carried out according to DIN 53855-1:1993-08 "Determination of the Thickness of Textile Fabrics." The determination of thickness at a specific compressive force (e.g., at 0.025 MPa or 6.0 MPa) is also described in DIN.

[0186] roughness

[0187] The determination of roughness, for example of the microporous layer, can be carried out using conventional stylus methods known to the person skilled in the art, such as those described in DIN 4768-1:1974-08 entitled "Determination of roughness measurement values ​​R a , R z, Rmax with electrical stylus instruments; Basics".

[0188] The mean roughness value R was determined a (mean distance of a measuring point on the surface to the center line) and the average roughness depth R z The measurements were performed using a Mahrsurf XCR20 measuring device with an MFW-250 free-surface probe. The values ​​are averages of six measurements: three in the machine direction (MD) and three perpendicular to the machine direction (CD).

[0189] Contact resistance by strip measurement

[0190] The contact resistance of a GDL applied to a bipolar plate at its interface with a catalyst layer applied to a polymer electrolyte membrane can be simulated using strip measurement. This applies both to the channel regions of the fuel cell's bipolar plate, particularly the channel center regions, and to the land regions of the fuel cell's bipolar plate. The respective contact resistance is simulated by determining the contact resistance of a circular GDL sample applied to the channel structure of the corresponding bipolar plate at its interface with measuring electrodes, specifically in the desired region (e.g., in the channel center region and / or land region).For this purpose, the circular GDL sample is clamped between the relevant channel structure at the top and the measuring electrodes 1-20 at the bottom at a compression pressure of 1.0 MPa, at the specified angle to the main flow field direction of the channel structure. The line of the 20 measuring electrodes is arranged in the apparatus perpendicular to the main flow field direction of the channel. The channels can, for example, have the geometry (B1, R1), (B1, R2), (B2, R1), or (B2, R2) as defined above. The circular GDL sample is large enough that its maximum width covers at least one channel and the two adjacent webs. The measuring electrodes 1 and 2, or 19 and 20, are arranged in the area of ​​the webs on the GDL sample; the measuring electrodes 10 and 11 are also arranged in the area of ​​the channel center on the GDL sample.Further details of the measurement method, including the measuring apparatus, can be found in the Journal of The Electrochemical Society, 159 (6) B709-B713 (2012), which is hereby incorporated by reference into the disclosure of the present application. This particularly concerns the experimental section of the cited publication, including the figures, such as Figure 1, to which reference is made therein (and in particular the explanations under "Measurement of the contact resistance").

[0191] Maximum tensile force (HZK) in Newtons and maximum tensile force elongation (HZD). The determination of the maximum tensile force (HZK) in Newtons and the maximum tensile force elongation (HZD) was carried out according to EN 29073-3:1992 on test specimens measuring 200 mm x 50 mm. The tensile force / elongation behavior was measured in the longitudinal direction (roll direction, MD) and in the transverse direction (counter-roll direction, CD) on a dry material. For each individual measurement, three samples were punched, and an average value was calculated.

[0192] FIGURE DESCRIPTION

[0193] Figure 1 schematically illustrates a section of a prior art bipolar plate, such as can also be used in a fuel cell according to the invention (sectional drawing). "B" represents the width and "R" the radius of the curves of the channel K shown, which is adjoined by webs S on both sides.

[0194] Figure 2 shows flow field designs of flow distribution plates suitable for use in proton exchange membrane fuel cells according to the invention. Figure 2A shows a single-channel serpentine flow field. Figure 2B shows a five-channel meander flow field. Figure 2C shows an interdigitated flow field. Figure 2D shows a parallel flow field with straight channels. Figure 2D shows a pin-type flow field.

[0195] PREFERRED EMBODIMENTS OF THE INVENTION

[0196] 1. A proton exchange membrane fuel cell comprising a proton exchange membrane coated on both sides with a catalytically active electrode, a gas diffusion layer on each side of the catalyst-coated membrane, the inside of which is in contact with the catalytically active electrode, a flow distributor plate on each side of the catalyst-coated membrane, which has channels for supplying reactant gases and removing products of the cell reaction, wherein the channels are in contact with the outside of the gas diffusion layer, wherein the gas diffusion layer comprises a sheet-like, electrically conductive fiber material A) which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric, wherein the carbon fiber nonwoven fabric comprises one or more consolidated fiber pile layers, wherein the fibers of one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers,the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction relative to the base area of ​​the fiber pile layer (x,y plane), and wherein the channels of the flow distribution plate have a main orientation direction, and the main orientation direction of the fibers is not aligned parallel to the main orientation direction of the channels. A proton exchange membrane fuel cell according to embodiment 1, wherein the flow distribution plates have a flow field selected from the group consisting of parallel flow fields, meander flow fields, interdigitated flow fields, and pillar flow fields. A proton exchange membrane fuel cell according to embodiment 1 or 2,wherein the carbon fiber nonwoven fabric comprises a plurality of fiber pile layers, and the fibers of the fiber pile layer are oriented on the side facing the flow distributor plate, and the main orientation direction of the fibers of this fiber pile layer is not aligned parallel to the main orientation direction of the channels of the flow distributor plate. A proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the carbon fiber nonwoven fabric comprises 2, 3, 4, 5, or more than 5 fiber pile layers. A proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the carbon fiber nonwoven fabric has a plurality of fiber pile layers, and at least 2 of the fiber pile layers, preferably at least 3 of the fiber pile layers, in particular all fiber pile layers, have a fiber orientation. A proton exchange membrane fuel cell according to one of the preceding embodiments,wherein the carbon fiber nonwoven fabric comprises a plurality of fiber pile layers, and at least two fiber pile layers have a fiber orientation with two different main orientation directions, preferably at least two adjacent fiber pile layers have a fiber orientation with two different main orientation directions. A proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the main orientation direction of the fibers of at least one fiber pile layer and the main orientation direction of the channels of the flow distributor plate in contact with the gas diffusion layer have an angle of 30° to 90°, preferably 45° to 90°, in particular 60° to 90°, to each other. A proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the gas diffusion layer comprises a carbon fiber nonwoven fabric, for the production of which carbon fibers are provided,the provided carbon fibers are subjected to carding to increase the parallel orientation in the fiber longitudinal direction, the carded fibers are laid down to form a fiber web, optionally at least one further fiber web layer is laid onto the first fiber web layer, and the fiber web layer(s) are consolidated to form a nonwoven fabric. Proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the main orientation direction of the fibers of one fiber web layer or, if the carbon fiber nonwoven fabric comprises several fiber web layers, the main orientation direction of the fibers of at least one of the fiber web layers corresponds to the machine direction (MD) or the cross-machine direction (CD), preferably the carbon fiber nonwoven fabric comprises several fiber web layers and at least the main orientation direction of the fibers of the fiber web layer on the,

[0197] Flow distributor plate facing side, the machine direction (MD) or the cross-machine direction (CD). Proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the carbon fiber nonwoven fabric has a basis weight of at least 20 g / m 2 and the maximum tensile force in the machine direction is at least 5 N, preferably at least 7 N and / or the carbon fiber nonwoven fabric has a basis weight of at least 20 g / m 2 and the maximum tensile force in the transverse direction to the machine direction is at least 5 N, preferably at least 7 N. Proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the gas diffusion layers

[0198] A) a flat electrically conductive fiber material comprising a carbon fiber nonwoven fabric or consisting of a carbon fiber nonwoven fabric and

[0199] B) a microporous layer on the side facing the catalytically active electrode, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder. Proton exchange membrane fuel cell according to one of the preceding embodiments, wherein the flow distributor plates have at least one of the following properties: the width of the channels is in a range from 0.1 to 3.5 mm, preferably from

[0200] 0.2 to 1.5 mm, the width of the webs is in a range of 0.1 to 2.5 mm, preferably 0.2 to 1.5 mm, the depth of the channels is in a range of 0.1 to 2.0 mm, particularly preferably 0.15 to 0.5 mm. Method for producing a gas diffusion layer for a

[0201] Proton exchange membrane fuel cell as in any one of embodiments 1 to

[0202] 12, in which i) a sheet-like electrically conductive fiber material A) is provided, which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction relative to the base area of ​​the fiber pile layer (x,y plane), ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer, iii) optionally the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and / or elevated temperature. A method for producing a proton exchange membrane fuel cell as defined in any one of embodiments 1 to 12, in which i) a sheet-like electrically conductive fiber material A) is provided, which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric,which comprises one or more consolidated fiber pile layers, wherein the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of ​​the fiber pile layer (x,y plane), ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer, iii) optionally the coated fiber material obtained in step ii) is subjected to a post-treatment at elevated pressure and / or elevated temperature, iv) the coated fiber material obtained in step ii) or iii) is placed as a gas diffusion layer on both sides of a proton exchange membrane coated on both sides with catalytically active electrodes, wherein the microporous layers are located on the sides of the gas diffusion layers facing the catalytically active electrodes, v) a flow distributor plate is placed on each of the outer sides of the gas diffusion layers,which has channels for supplying reactant gases and removing products of the cell reaction, which are in contact with the outer sides of the gas diffusion layers, vi) the flow distribution plates and the components located therebetween are pressed together. The process according to embodiment 13 or 14, wherein, to produce the sheet-like electrically conductive fiber material A), i-1) a fiber composition comprising carbon fibers and / or carbon fiber precursors is provided, i-2) the fiber composition provided in step i-1) is subjected to carding to increase the parallel orientation in the fiber longitudinal direction, i-3) the fiber composition obtained in step i-2) is laid down to form a fiber web, i-4) optionally depositing at least one further fiber web layer onto the first fiber web layer, i-5) the fiber web layer(s) obtained in step i-3) or i-4) are consolidated to form a nonwoven fabric,i-6) optionally subjecting the nonwoven fabric obtained in step i-5) to finishing with at least one additive, i-7) optionally subjecting the nonwoven fabric obtained in step i-5) or the finished nonwoven fabric obtained in step i-6) to post-treatment at elevated pressure and optionally elevated temperature, i-8) if the fiber composition used in step i-1) comprises carbon fiber precursors, subjecting the nonwoven fabric obtained in step i-5) or the finished nonwoven fabric obtained in step i-6) or the post-treated nonwoven fabric obtained in step i-7) to pyrolysis at a temperature of at least 1000°C. Process according to embodiment 15, wherein in step i-4) at least one further fiber batt layer is deposited on the first fiber batt layer, wherein at least one of the further batt layers has a main orientation direction of the fibers,which is substantially transverse to the main orientation direction of the fibers of the first pile layer. Process according to embodiment 15 or 16, wherein in step i-5), the fiber pile layer(s) obtained in step i-3) or i-4) are consolidated into a nonwoven fabric by exposure to water-containing fluid jets. Process according to any one of embodiments 15 to 17, wherein for the treatment in step i-7), 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. Process according to any one of embodiments 15 to 18, wherein for the treatment in step i-7), a nonwoven fabric obtained by exposure to water-containing fluid jets in step i-5) is subjected to a post-treatment at elevated pressure and optionally elevated temperature.wherein the nonwoven fabric obtained from compaction with water-containing fluid jets is preferably used.

[0203] 20. A gas diffusion layer obtainable by a process as defined in any one of embodiments 13 or 15 to 18.

[0204] 21. A proton exchange membrane fuel cell obtainable by a process as defined in any one of embodiments 15 to 20.

[0205] 22. A fuel cell stack comprising a plurality of proton exchange membrane fuel cells as defined in any one of embodiments 1 to 11 and 20.

[0206] 23. Use of at least one gas diffusion layer obtainable by a process as defined in any one of embodiments 12 or 14 to 18 in a proton exchange membrane fuel cell for improving the contact between the catalyst-coated polymer electrolyte membrane and the microporous layers of the gas diffusion layers.

[0207] 24. Use according to embodiment 22 for preventing a reduction in the contact area between the catalyst-coated polymer electrolyte membrane and the gas diffusion layers in regions opposite the channels of the flow distribution plates, in particular in regions opposite the center of the channels.

[0208] 25. Use according to any one of embodiments 22 or 23 for preventing an increase in the contact resistance between the catalyst-coated membrane and the gas diffusion layers in regions opposite the channels of the flow distribution plates, in particular in regions opposite the center of the channels.

[0209] The invention is explained in more detail below using examples.

[0210] EXAMPLES

[0211] Example 1 :

[0212] Production of a flat electrically conductive fiber material from a single-layer fiber pile with oriented fibers

[0213] A pre-oxidized PAN fiber with a fiber titre of 1.2 dtex and fiber lengths of up to 60 mm is carded into a fiber web with a basis weight of 50 g / m 2 laid crosswise. The fiber web is fed to a bonding unit where the fibers are swirled and entangled on both sides using high-energy water jets at pressures of approximately 100 bar in the first stage and approximately 170 bar in a second stage. The nonwoven fabric is dried to a residual moisture content of 1 to 15% and subjected to thickness calibration, which reduces the thickness of the hydroentangled nonwoven fabric. The compacted nonwoven fabric is then fed to a carbonization unit where carbonization takes place in a nitrogen atmosphere at approximately 1000 to 1400°C. After carbonization, the nonwoven fabric has a thickness of 100 μm.

[0214] Example 2:

[0215] Production of a flat electrically conductive fiber material from a two-layer fiber pile with oriented fibers

[0216] A two-layer fiber pile consisting of a first cross-laid pile layer with a basis weight of 40 g / m 2 made of pre-oxidized PAN fibers with a fiber titre of 1.2 dtex and a fiber length of 52 mm and a second longitudinally oriented pile layer with a basis weight of 20 g / m 2Made from pre-oxidized PAN fibers with a fiber titer of 1.2 dtex and a fiber length of 52 mm, which was laid lengthwise on a carding machine, are fed to a bonding unit. The fibers are intermingled and entangled with each other using high-energy water jets on both sides at pressures of approximately 120 bar in the first stage and approximately 190 bar in a second stage. The nonwoven fabric is dried to a residual moisture content of approximately 5% and subjected to thickness calibration, which reduces the thickness of the hydroentangled nonwoven fabric. The nonwoven fabric is then fed to a carbonization unit where carbonization takes place in a nitrogen atmosphere at approximately 1000 to 1400°C. After carbonization, the nonwoven fabric has a thickness of 114 pm.

[0217] Example 3:

[0218] Example 3 was produced analogously to Example 2. However, the fiber titer of both pile layers was 2.2 dtex. The cross pile had a basis weight of 63 g / m 2 , the longitudinal pile has a surface weight of 32 g / m 2 After carbonization, the nonwoven fabric has a thickness of 151 pm.

[0219] Example 4:

[0220] Example 4 was produced analogously to Example 2. However, the fiber titer of both pile layers was 2.2 dtex. The cross pile had a basis weight of 110 g / m 2 , the longitudinal pile has a surface weight of 40 g / m 2 After carbonization, the nonwoven fabric has a thickness of 211 pm.

[0221] Example 5:

[0222] A single-layer carbonized carbon fiber paper with fiber orientation and a basis weight of 44.5 g / m 2 After carbonization, the carbon fiber paper substrate had a thickness of 192 pm measured at 0.25 bar.

[0223] Examples I to IV:

[0224] To finish the nonwoven fabric from Examples 1 to 4 (base nonwoven fabric), an impregnation composition was mixed containing 70 wt.% carbon black and 30 wt.% PTFE, based on the solids. Finishing was carried out by padding with an aqueous dispersion containing 15% finish weight based on the mass of the GDL substrate. This was followed by drying for 5 minutes at 160 °C and sintering for 10 minutes at 400 °C. An MPL was then applied. The paste had a composition as shown in Table 1. To prepare it, PTFE, various carbons, and plastic particles as pore formers were dispersed in distilled water and applied to the nonwoven fabric by doctor blade coating. The sheets were then dried at 160 °C and sintered at 400 °C. The resulting MPL loadings were 15 g / m 2 .

[0225] Example V:

[0226] For finishing, the nonwoven fabric from Example 5 (base nonwoven carbon fiber paper) was impregnated by padding with an aqueous PTFE dispersion at a weight of 10% based on the mass of the GDL substrate. This was followed by drying for 5 minutes at 160 °C and sintering for 10 minutes at 400 °C. An MPL was then applied. The paste had a composition shown in Table 1. To prepare the paste, PTFE, various carbons, and plastic particles as pore formers were dispersed in distilled water and applied to the carbon fiber paper by doctor blade. The sheets were then dried at 160 °C and sintered at 400 °C. The resulting MPL loadings were 15 g / m 2 .

[0227] Table 1

[0228] 1) each based on the total weight of the paste

[0229] Application properties

[0230] The contact resistance was determined by strip measurement as previously described. The measurement was carried out at a compression pressure of 1.0 MPa. The orientation of the GDL sample on the flow distribution plate was such that the direction opposite to the machine direction (CD) was parallel to the main orientation direction of the channels (i.e., the GDL is arranged on the flow distribution plate such that the machine direction (MD) of the GDL forms an angle of 90° to the main flow field direction of the flow distribution plate). In the two-layer base nonwovens, the longitudinal pile is always oriented towards the bipolar plate and the transverse pile is always oriented towards the electrode. The channels had the geometries (B1, R1), (B1, R2), (B2, R1), and (B2, R2) during the measurements. "B1" stands for a channel width of 0.3 mm, and "B2" for a channel width of 0.6 mm. “R1” stands for a channel rounding radius of 0.13 mm, and “R2” stands for a channel rounding radius of 0.26 mm.The measurement was taken in the center of the gas channel. The results are shown in Table 2.

[0231] The determination of the ultimate tensile force (TDF) in Newtons and the ultimate tensile elongation (TDE) was carried out according to EN 29073-3:1992, as described above. The results are shown in Table 3. Table 2

[0232] Table 3 a) Base nonwoven fabric

Claims

PATENT CLAIMS 1. Proton exchange membrane fuel cell, comprising a proton exchange membrane coated on both sides with a catalytically active electrode, on each side of the catalyst-coated membrane a Gas diffusion layer, the inside of which is in contact with the catalytically active electrode, on each side of the catalyst-coated membrane a A flow distributor plate having channels for supplying reactant gases and removing cell reaction products, the channels being in contact with the outside of the gas diffusion layer, the gas diffusion layer comprising a sheet-like, electrically conductive fiber material A) selected from carbon fiber nonwovens, carbon fiber papers, and combinations thereof, the fiber material A) comprising one or more fiber layers, the fibers of one fiber layer or, if the fiber material comprises multiple fiber layers, the fibers of at least one of the fiber layers being oriented and having a main orientation direction with respect to the base area of the fiber layer (x,y plane), and the channels of the flow distributor plate having a main orientation direction, and the main orientation direction of the fibers not being aligned parallel to the main orientation direction of the channels.

2. Proton exchange membrane fuel cell, comprising a proton exchange membrane coated on both sides with a catalytically active electrode, on each side of the catalyst-coated membrane a Gas diffusion layer, the inside of which is in contact with the catalytically active electrode, on each side of the catalyst-coated membrane a Flow distributor plate having channels for supplying reactant gases and removing products of the cell reaction, wherein the channels are in contact with the outside of the gas diffusion layer, wherein the gas diffusion layer comprises a flat electrically conductive fiber material A) which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric, wherein the carbon fiber nonwoven fabric comprises one or more consolidated fiber pile layers, wherein the fibers of one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers, the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of the fiber pile layer (x,y plane), and wherein the channels of the flow distributor plate have a main orientation direction and the main orientation direction of the fibers is not aligned parallel to the main orientation direction of the channels.

3. A proton exchange membrane fuel cell according to claim 1 or 2, wherein the flow distribution plates have a flow field selected from the group consisting of parallel flow fields, meander flow fields, interdigitated flow fields, and post flow fields.

4. Proton exchange membrane fuel cell according to claim 2, wherein the carbon fiber nonwoven fabric comprises a plurality of fiber pile layers and the fibers of the fiber pile layer are oriented on the side facing the flow distributor plate and the main orientation direction of the fibers of this fiber pile layer is not aligned parallel to the main orientation direction of the channels of the flow distributor plate.

5. A proton exchange membrane fuel cell according to any one of claims 2 to 4, wherein the carbon fiber nonwoven fabric comprises 2, 3, 4, 5 or more than 5 fiber pile layers.

6. Proton exchange membrane fuel cell according to one of claims 2 to 5, wherein the carbon fiber nonwoven fabric has a plurality of fiber pile layers and at least 2 of the fiber pile layers, preferably at least 3 of the fiber pile layers, in particular all fiber pile layers, have a fiber orientation.

7. Proton exchange membrane fuel cell according to one of claims 2 to 6, wherein the carbon fiber nonwoven fabric has a plurality of fiber pile layers and at least 2 fiber pile layers have a fiber orientation with two different main orientation directions, especially at least 2 adjacent fiber pile layers have a fiber orientation with two different main orientation directions.

8. Proton exchange membrane fuel cell according to claim 1, wherein the main orientation direction of the fibers and the main orientation direction of the channels of the flow distributor plate in contact with the gas diffusion layer have an angle of 30° to 90°, preferably of 45° to 90°, in particular of 60° to 90°, to one another.

9. Proton exchange membrane fuel cell according to one of claims 2 to 7, wherein the main orientation direction of the fibers of at least one fiber pile layer and the main orientation direction of the channels of the flow distributor plate in contact with the gas diffusion layer have an angle of 30° to 90°, preferably of 45° to 90°, in particular of 60° to 90°, to one another.

10. Proton exchange membrane fuel cell according to one of the preceding claims, wherein the gas diffusion layer comprises a carbon fiber nonwoven fabric, for the production of which carbon fibers are provided, the provided carbon fibers are subjected to carding to increase the parallel orientation in the fiber longitudinal direction, the carded fibers are laid down to form a fiber web, optionally at least one further fiber web layer is laid down on the first fiber web layer and the fiber web layer(s) are consolidated to form a nonwoven fabric.

11. Proton exchange membrane fuel cell according to one of the preceding claims, wherein the main orientation direction of the fibers of one fiber pile layer or, if the carbon fiber nonwoven fabric comprises several fiber pile layers, the main orientation direction of the fibers of at least one of the fiber pile layers corresponds to the machine direction (MD) or the cross-machine direction (CD).

12. Proton exchange membrane fuel cell according to one of the preceding claims, wherein the carbon fiber nonwoven fabric comprises a plurality of fiber pile layers and at least the main orientation direction of the fibers of the fiber pile layer on the side facing the flow distributor plate corresponds to the machine direction (MD) or the cross-machine direction (CD).

13. Proton exchange membrane fuel cell according to one of the preceding claims, wherein the carbon fiber nonwoven fabric has a basis weight of at least 20 g / m 2and the maximum tensile force in the machine direction is at least 5 N, preferably at least 7 N.

14. Proton exchange membrane fuel cell according to one of the preceding claims, wherein the carbon fiber nonwoven fabric has a basis weight of at least 20 g / m 2 and the maximum tensile force in the transverse direction to the machine direction is at least 5 N, preferably at least 7 N.

15. Proton exchange membrane fuel cell according to claim 1 or 8, wherein the gas diffusion layers A) a sheet-like electrically conductive fiber material selected from carbon fiber nonwovens, carbon fiber papers and combinations, and B) a microporous layer on the side facing the catalytically active electrode, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder.

16. Proton exchange membrane fuel cell according to one of claims 2 to 7 and 9 to 14, wherein the gas diffusion layers A) a flat electrically conductive fiber material comprising a carbon fiber nonwoven fabric or consisting of a carbon fiber nonwoven fabric and B) a microporous layer on the side facing the catalytically active electrode, wherein the microporous layer contains conductive particles in a matrix of a polymeric binder.

17. Proton exchange membrane fuel cell according to one of the preceding claims, wherein the flow distributor plates have at least one of the following properties: the width of the channels is in a range from 0.1 to 3.5 mm, preferably from 0.2 to 1.5 mm, the width of the webs is in a range from 0.1 to 2.5 mm, preferably from 0.2 to 1.5 mm, the depth of the channels is in a range from 0.1 to 2.0 mm, particularly preferably from 0.15 to 0.5 mm.

18. A method for producing a gas diffusion layer for a proton exchange membrane fuel cell, as defined in any one of claims 2 to 7 and 9 to 17, wherein i) a sheet-like electrically conductive fiber material A) is provided which comprises a carbon fiber nonwoven fabric or consists of a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of the fiber pile layer (x,y plane), ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer, iii) optionally subjecting the coated fibre material obtained in step ii) to a post-treatment at elevated pressure and / or elevated temperature.

19. A process for producing a proton exchange membrane fuel cell as defined in any one of claims 2 to 7 and 9 to 17, comprising: i) providing a sheet-like electrically conductive fiber material A) comprising a carbon fiber nonwoven fabric or consisting of a carbon fiber nonwoven fabric comprising one or more consolidated fiber pile layers, wherein the fibers of at least one of the fiber pile layers are oriented and have a main orientation direction with respect to the base area of the fiber pile layer (x,y plane); ii) coating the fiber material provided in step i) with a precursor to form a microporous layer; iii) optionally subjecting the coated fiber material obtained in step ii) to a post-treatment at elevated pressure and / or elevated temperature;iv) the coated fiber material obtained in step ii) or iii) is placed as a gas diffusion layer on both sides of a proton exchange membrane coated on both sides with catalytically active electrodes, wherein the microporous layers are located on the sides of the gas diffusion layers facing the catalytically active electrodes, v) a flow distributor plate is placed on each of the outer sides of the gas diffusion layers, which flow distributor plates have channels for supplying reactant gases and removing products of the cell reaction, which are in contact with the outer sides of the gas diffusion layers, vi) the flow distributor plates and the components located therebetween are pressed together.

20. The method according to claim 18 or 19, wherein for the production of the sheet-like electrically conductive fiber material A) i-1) a fiber composition is provided which comprises carbon fibers and / or precursors of carbon fibers, i-2) the fiber composition provided in step i-1) is subjected to carding to increase the parallel orientation in the fiber longitudinal direction, i-3) laying down the fiber composition obtained in step i-2) to form a fiber web, i-4) optionally laying down at least one further fiber web layer on the first fiber web layer, i-5) consolidating the fiber web layer(s) obtained in step i-3) or i-4) to form a nonwoven fabric, i-6) optionally subjecting the nonwoven fabric obtained in step i-5) to finishing with at least one additive, i-7) optionally subjecting the nonwoven fabric obtained in step i-5) or the finished nonwoven fabric obtained in step i-6) to a post-treatment at elevated pressure and optionally elevated temperature, i-8) if the fiber composition used in step i-1) comprises precursors of carbon fibers, subjecting the nonwoven fabric obtained in step i-5) or the finished nonwoven fabric obtained in step i-6) or the post-treated nonwoven fabric obtained in step i-7) to pyrolysis at a temperature of at least 1000 °C.

21. The method according to claim 20, wherein in step i-4) at least one further fibrous batt layer is deposited on the first fibrous batt layer, wherein at least one of the further batt layers has a main orientation direction of the fibers which is substantially transverse to the main orientation direction of the fibers of the first batt layer.

22. The method according to claim 20 or 21, wherein in step i-5) the fibrous web layer(s) obtained in step i-3) or i-4) are consolidated into a nonwoven fabric by the action of water-containing fluid jets.

23. The method according to any one of claims 20 to 22, wherein for the treatment in step i-7) 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.

24. The method according to any one of claims 20 to 23, wherein for the treatment in step i-7), a nonwoven fabric obtained by exposure to water-containing fluid jets in step i-5) is subjected to a post-treatment at elevated pressure and optionally elevated temperature, wherein preferably the nonwoven fabric obtained from the compaction with water-containing fluid jets is used.

25. A gas diffusion layer obtainable by a process as defined in any one of claims 18 or 20 to 24.

26. A proton exchange membrane fuel cell obtainable by a process as defined in any one of claims 19 to 24.

27. A fuel cell stack comprising a plurality of proton exchange membrane fuel cells as defined in any one of claims 1 to 17 and 26.

28. Use of at least one gas diffusion layer obtainable by a process as defined in any one of claims 18 or 20 to 24 in a proton exchange membrane fuel cell for improving the contact between the catalyst-coated polymer electrolyte membrane and the microporous layers of the gas diffusion layers.

29. Use according to claim 28 for preventing a reduction in the contact area between the catalyst-coated proton exchange membrane and the gas diffusion layers in regions opposite the channels of the flow distribution plates, in particular in regions opposite the center of the channels.

30. Use according to claim 28 or 29 for preventing an increase in the contact resistance between the catalyst-coated membrane and the gas diffusion layers in regions opposite the channels of the flow distribution plates, in particular in regions opposite the center of the channels.

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