Polymer electrolyte membrane fuel cell, method for producing a diffusion layer, diffusion layer and use of a diffusion layer

Abstract

A polymer polymer electrolyte membrane fuel cell comprising a membrane-electrode assembly having an anode, a cathode and a polymer electrolyte membrane present between the anode and cathode, wherein a water-retaining diffusion layer is provided on the anode and a water-removing diffusion layer is provided on the cathode, which are designed in such a way that, during intended operation of the fuel cell, water produced in the cathode is removed by the water-removing diffusion layer, and a gas supply of hydrogen to the anode takes place via the water-retaining diffusion layer, wherein the hydrogen in the anode is split into protons and electrons and the protons are passed through an ionomer matrix included in the membrane-electrode assembly in a reaction direction, and the water-retaining diffusion layer has water-retaining properties such that, when the fuel cell is operating as intended, water is retained at least in the anode to provide humidification of the ionomer matrix and associated enhanced proton conduction, such that, in comparison to having a diffusion layer on the anode that is identical to the water-removing diffusion layer of the cathode, the fuel cell has at least one performance improvement measured on a polarization curve and preferably the performance improvement increases with increasing current density, wherein the ionomer matrix contains hydrocarbon ionomers.

Description

[0001] Polymer electrolyte membrane fuel cell, manufacturing process for a diffusion layer, diffusion layer and use of a diffusion layer

[0002] The invention relates to a polymer electrolyte membrane fuel cell, a diffusion layer, a manufacturing process for a diffusion layer, a diffusion layer and a use for a diffusion layer.

[0003] Gas diffusion layers (GDLs) are considered a key component in various types of hydrogen fuel cells, including direct methanol fuel cells, phosphoric acid fuel cells, but especially in polymer electrolyte membrane fuel cells (PEMFCs) – whose application area is primarily the automotive and heavy vehicle industries – as well as in other electrochemical devices such as electrolyzers.

[0004] GDLs represent the functional interface between the respective catalyst layers (anode and cathode) and so-called bipolar plates or gas flow plates. Within the catalyst layers, the corresponding electrochemical half-cell reactions of a fuel cell take place, while the gas flow plates with gas channels supply the respective reaction gases – hydrogen on the anode side and oxygen on the cathode side – for the operation of the fuel cell.

[0005] In this context, the gas flow regulator (GDL) performs several fundamental functions for the operation of a fuel cell or PEMFC. Its primary function is to optimally and homogeneously supply the reaction gases from the gas flow plate channels to the respective catalyst layer. In addition, GDLs also facilitate the transport of electrons and the removal of heat and liquid product water generated during fuel cell operation from the catalyst layer.

[0006] Conventionally, gas diffusion layers (GDLs) consist of two layers: the substrate or gas diffusion layer and a microporous layer. The gas diffusion layer is a carbon paper with randomly oriented 6-10 pm carbon fibers, coated on one side with the microporous layer based on carbon particles and a hydrophobic binder such as polytetrafluoroethylene to improve contact with the catalyst layer and optimize water removal. For automotive PEMFCs, the paper and nonwoven GDL concepts are known from the prior art, where, for example, polyacrylonitrile fibers are used for the GDLs. Manufacturing processes resulting in highly porous structures with a thickness of approximately 150 pm typically include fiber production, fiber laying, fiber consolidation, carbonization, and finishing.Depending on the manufacturing process, different microstructures and properties result, which are adapted for the application.

[0007] The microporous layer is intended to improve the contact between the GDL and the catalyst layer. In particular, the relatively rough GDL, with potentially protruding GDL fibers, can damage the catalyst layer, which can be partially suppressed by the microporous layer.

[0008] The development and deployment of gas discharge tubes (GDLs) is an optimization problem that depends significantly on factors such as the underlying fuel cell technology, the functional materials used, and the operating parameters and requirements determined by the technical application. Therefore, the development of GDLs involves considerable research and correspondingly high costs.

[0009] In particular, research in the development of GDLs focuses on commercially used and established technologies. Therefore, GDLs in the state of the art are primarily developed with regard to commercially used PEMFCs, which are based on perfluorosulfonic acid ionomers – for example, NATION – for proton conduction.

[0010] Furthermore, GDLs are specifically optimized for the cathode side. For example, they are made hydrophobic by impregnation with polytetrafluoroethylene to reduce the adhesive strength of water and thus the transport resistance of the water generated in the cathode. This is because water—generated by the reaction in the cathode—can potentially flood the cathode, particularly in cathodes with perfluorosulfonic acid ionomers, significantly impairing the reaction dynamics.

[0011] The optimization of gas discharge lines (GDLs) on the cathode side of the PEMFC is the primary objective here, as the cathode side of the PEMFC, with its associated cathode reaction—the so-called oxygen reduction reaction, which proceeds kinetically slower than the hydrogen oxidation reaction on the anode side and is therefore considered a limiting factor—is regarded by experts as the more technically relevant component of a PEMFC. The GDL is thus attributed a significant implicit influence on the fuel cell's performance, particularly due to the preferential water removal on the cathode side.

[0012] The optimization problem described is addressed in the prior art with regard to the commercially used perfluorosulfonic acid ionomers and, in particular, with consideration of the preferred properties on the cathode side - where more or less water is produced depending on the current density with which the fuel cell is operated.

[0013] From a technical, economic and ecological perspective, the aim is generally to reduce the use of perfluorinated materials and, in particular, to replace perfluorosulfonic acid ionomers in PEMFCs or to avoid their use as much as possible.

[0014] However, alternative ionomer materials are based, as explained below, on hydrocarbon-based compounds. These differ, as also explained below, in their chemical composition, which results in different requirements within PEMFC, particularly regarding the humidification or hydrogenation of the ionomer materials.

[0015] Since, as described above, research into novel GDL materials and alternatives is associated with considerable research effort and is expensive – with new alternatives having to be tested accordingly to meet the requirements of industry – the GDLs available in the state of the art are often simply transferred to the anode side of PEMFCs for reasons of cost and efficiency, and are also used for PEMFCs that are based on other ionomer materials, for example.

[0016] Accordingly, in light of the above-described preferred or “one-sided” development regarding gas supply in PEMFCs, it can be summarized that there is optimization potential and a need for development in the application of GDLs in PEMFC technology.

[0017] Therefore - but also in principle - the state of the art reveals a need to provide and research alternative measures for gas supply in electrochemical systems, especially in PEMFCs.

[0018] Therefore, the object of the invention is to provide teachings and means relating to gas supply in electrochemical systems, particularly in PEMFCs, and to improve their operation. At a minimum, the object of the invention is to provide alternative gas supply means compared to the prior art. The background of the invention is that the inventors have undertaken the task of exploring alternative PEMFC technologies to perfluorosulfonic acid-based PEMFC technologies – especially for automotive and heavy vehicle applications.

[0019] In particular, a surprising performance increase was observed in connection with the use of alternative diffusion layers on the anode side of a PEMFC based on novel hydrocarbon ionomers.

[0020] This prompted further research into the advantageous effects of using alternative diffusion layers, particularly with a suitable choice of diffusion layer on the anode side. These were initially tested on a PEMFC largely based on hydrocarbon ionomers. However, the aforementioned effects were also demonstrated for PEMFCs based on the established perfluorosulfonic acid ionomers.

[0021] In summary, as detailed below, device effects, manufacturing processes with diffusion layer compositions, diffusion layers and uses of alternative diffusion layers have been achieved, which offer advantages in particular for use on the anode side or for the anode arrangement of a PEMFC and especially for PEMFCs that primarily use hydrocarbon ionomers.

[0022] Furthermore, alternative diffusion layers were explored which are preferably suitable for application on the anode side of a PEMFC, but also potentially offer application potential for other applications in which the functions of a GDL are sought.

[0023] According to the invention, a polymer electrolyte membrane fuel cell comprises a membrane electrode arrangement with an anode, a cathode and a polymer electrolyte membrane located between the anode and cathode, wherein a water-retaining diffusion layer is provided on the anode and a water-removing diffusion layer is provided on the cathode, which are arranged such that, in the intended operation of the fuel cell, water generated in the cathode is removed by the water-removing diffusion layer, and a gas supply of hydrogen to the anode takes place through the water-retaining diffusion layer, wherein the hydrogen is split into protons and electrons within the anode and the protons are guided in a reaction direction by an ionomer matrix contained in the membrane electrode arrangement.The membrane electrode assembly is preferably a film-like polymer electrolyte membrane arranged between two catalyst layers, the anode and the cathode. The membrane electrode assembly is preferably a catalyst-coated polymer electrolyte membrane, which can be provided by a direct coating process. However, the catalyst-coated polymer electrolyte membrane is also preferably obtainable, for example, by a decal manufacturing process or a lamination process.

[0024] The basic functionality and structure of the PEMFC as well as the membrane electrode arrangement within the PEMFC are known from the prior art.

[0025] Subsequently, the characteristic partial reactions of a PEMFC take place in the membrane electrode assembly: splitting of hydrogen into protons and electrons, or a so-called hydrogen oxidation reaction, of hydrogen supplied – preferably during operation of the PEMFC at the anode – and an oxygen reduction reaction, or the reaction of protons and electrons with oxygen to form water, whereby the proton conduction from the protons in the reaction direction is decisive. The reaction direction preferably corresponds to a direction from the anode to the cathode, in which the protons diffuse through the membrane electrode assembly.

[0026] In particular, the hydrogen oxidation reaction is preferably assigned to the anode, the oxygen reduction reaction to the cathode, and the proton conduction primarily to the polymer electrolyte membrane.

[0027] However, proton conduction also preferably takes place within the anode, at least as far as the polymer electrolyte membrane, and also within the cathode in order to provide the protons for the described oxygen reduction reaction within the cathode.

[0028] Accordingly, in addition to the polymer electrolyte membrane, proton-conducting properties can also be attributed to the anode and the cathode, whereby ionomers are preferably present in the anode, cathode and especially also in the polymer electrolyte membrane of the membrane-electrode unit, which preferably provide the proton conduction in a major way.

[0029] In the context of the invention, the ionomers within the membrane electrode unit are combined to form the ionomer matrix, which generally preferably relates to the ionomers present in the anode, cathode and polymer electrolyte membrane and can be considered as a kind of proton-conducting network.

[0030] The basic structure and function of ionomers, which generally relate to thermoplastic polymers and can be obtained by copolymerization of a nonpolar with a polar monomer, are known to those skilled in the art. Ionomers are preferably present in the catalyst layers at a certain weight fraction, for example, preferably at a fraction of 1–30 wt.% (of the total weight of the catalyst layer, preferably the anode), whereas for the polymer electrolyte membrane, whose core function is preferably proton conduction, ionomers can preferably represent a base material or a material that is preferably substantially present. The ionomer in question can also preferably be considered fundamental or at least decisive for other chemical and physical properties of the catalyst layers and, in particular, the polymer electrolyte membrane.

[0031] In particular, preferably the anode, cathode and polymer electrolyte membrane each comprise only one type of ionomer, although these are not limited to this and, for example, the anode, cathode and polymer electrolyte membrane can comprise several different ionomers.

[0032] According to the invention, a water-retaining diffusion layer is provided on the anode and a water-removing diffusion layer on the cathode, preferably configured with regard to their mechanical, electrical, and thermal properties suitable for use in a PEMFC. Preferably, suitable properties of the diffusion layers relate, for example, to the contact pressure of the diffusion layers with the respective catalyst layer and to the compressibility of the diffusion layers, in order to advantageously influence the structural, electrical, thermal, and transport properties within the PEMFC.

[0033] The water-retaining diffusion layer is preferably obtained by a manufacturing process according to the invention, as described below, whereas the water-repellent diffusion layer is preferably a hydrophobically post-treated, non-woven GDL from the prior art based on carbon materials.

[0034] In particular, the preferred properties of the water-retaining diffusion layer are determined by the manufacturing process for the water-retaining diffusion layer. In contrast, the properties of the water-repellent diffusion layer are determined according to the prior art of the GDL used, which is optimized according to the prior art to remove water from the cathode, which is generated, for example, during the operation of the PEMFC.

[0035] In comparison to the water-repellent diffusion layer, the water-retaining diffusion layer preferably has a thinner layer thickness and differs in terms of material composition, material properties, and microstructure properties, preferably in that it is more hydrophilic or has more hydrophilic water wetting properties.

[0036] The water-retaining diffusion layer is designed, among other things, to allow a gas supply of hydrogen to the anode during operation, wherein, preferably, further properties and functionalities preferred for GDLs are achieved for the water-retaining diffusion layer by the manufacturing process according to the invention. These relate in particular to the material properties and microstructure properties, thereby providing functionalities that are required in the operation of the PEMFC and relate, for example, to charge and heat transport.

[0037] According to the invention, the water-retaining diffusion layer also has water-retaining properties such that, in the intended operation of the fuel cell, water is retained at least in the anode in order to provide humidification of the ionomer matrix and an associated improved proton conductivity.

[0038] In this process, the water-retaining diffusion layer, or rather its water-retaining properties, retains water, particularly in the anode, during intended operation. These water-retaining properties are not limited to the anode and preferably also retain water in an anode side of the membrane-electrode unit, which comprises at least the anode, but also a polymer electrolyte membrane section, preferably adjacent to the anode, and which may also comprise the entire polymer electrolyte membrane. Furthermore, the water-retaining properties preferably extend, for example, to a section of the cathode adjacent to the polymer electrolyte membrane.

[0039] This contributes to the humidification according to the invention, wherein the humidification preferably describes a higher relative humidity, which relates at least to the anode, but preferably, as described, to the anode side. This allows – without being limited to – advantageously improved water management within the entire PEMFC or the membrane electrode assembly of the PEMFC.

[0040] In this context, it is assumed—without being limited by theory—that in the prior art, during operation, the anode or the anode side is generally drier than the cathode or the cathode side with respect to relative humidity. This can be explained, among other things, by the fact that water is produced in the cathode according to the partial reactions described above. Even if it can be assumed that the supplied reaction gases—hydrogen and oxygen—generally have a certain relative humidity, in the operation of the PEMFC, the anode is typically drier than the cathode, particularly in the prior art. Accordingly, the anode is generally described as drying out compared to the cathode, which, due to the low relative humidity in the anode or on the anode side, has a negative effect on the proton conductance of the affected ionomers.

[0041] The drying effect can also be intensified – without being limited to this by theory – particularly in the prior art, because within the PEMFC, especially at high current densities, water molecules are transported by electroosmosis from the anode to the cathode against a water concentration gradient, depending on the water content of the polymer membrane and temperature, proportional to the proton transport, which accordingly promotes a water flow in the reaction direction at high current densities.

[0042] Within the membrane electrode assembly, this preferably leads to a humidity gradient or an increasing relative humidity in the direction of the reaction and a higher concentration of water in the cathode or on the cathode side. This effect preferably results from a combination of the product water generated by the reaction and / or water accumulation due to electroosmotic pressure, which can accordingly intensify the drying out of at least the anode, but preferably also the anode side.

[0043] Without being limited by theory, during the operation of the PEMFC, a back-diffusion of water occurs in the opposite direction of the reaction towards the anode, which preferably counteracts the described drying out of the anode. In this context, according to the invention, the water-retaining properties of the water-retaining diffusion layer are preferably designed—without being limited by theory—such that the escape of water—in particular water from back-diffusion from the cathode, but also water supplied by reaction gases—from the anode is suppressed or reduced by the water-retaining diffusion layer.

[0044] According to the invention, a type of water retention or a stronger water retention is preferably achieved by the water-retaining diffusion layer, particularly in comparison to the water-transporting diffusion layer from the prior art, which is designed to transport water away. This is preferably related to the material and microstructure properties of the water-retaining diffusion layer and can preferably be attributed to the differences in the material composition or the material and microstructure properties of the water-retaining diffusion layer compared to the water-transporting diffusion layer.

[0045] In particular, the water-retaining diffusion layer preferably has such water-wetting properties that water is retained in the anode at an interface with the anode by the water-retaining diffusion layer.

[0046] Another effect, which can preferably be achieved by the water-retaining diffusion layer, relates to a possible promotion of the back-diffusion of water by the water-retaining diffusion layer, which is preferably more hydrophilic compared to the water-transporting diffusion layer.

[0047] Overall, it is advantageously achieved, without being limited by theory, that water is retained at least in the anode, or preferably more water on the anode side. Preferably, sufficient or enhanced wetting of at least the anode or the anode side is provided, thereby particularly preferably suppressing the drying out of the anode or the anode side. Preferably, a more uniform distribution of water or more uniform wetting in the ionomer matrix is ​​achieved overall.

[0048] Furthermore, partial flooding is preferably suppressed, at least within the membrane electrode unit, particularly the cathode, since it can be assumed, without being limited to this, that the water-retaining properties of the water-retaining diffusion layer preferentially distribute the water more evenly in the membrane electrode unit.

[0049] The humidification of the ionomer matrix preferably involves humidifying the respective ionomers within the membrane electrode assembly (PEMFC), and particularly preferably the ionomers of at least the anode or the anode side, with water. The water is preferably gaseous water in the PEMFC that is generated in the cathode during normal operation of the PEMFC, and / or water that is supplied to the PEMFC, preferably in gaseous form, via the gas supply with the respective reaction gas, and / or water that is already present in the PEMFC from a manufacturing process or previous operation. However, the water can also be partially present in liquid form in the PEMFC, for example, if water condenses out.

[0050] According to the invention, the improved proton conductivity is associated with humidification, with the respective humidified ionomer matrix preferably providing the improved proton conductivity. While not limited to this by the theory, the ionomers of the ionomer matrix preferably benefit from increased humidification with regard to their proton conductivity, so that protons can be conducted better, preferably at least within the anode, but more preferably within the anode side.

[0051] The ionomers of the ionomer matrix preferably comprise a hydrophobic backbone and hydrophilic side chains, wherein proton conduction is particularly preferably provided by the preferably moistened hydrophilic side chains, while the backbone is preferably associated with the mechanical stability of the ionomer. The ionomer preferably has repeating side chains that are bound to the backbone and preferably comprises electrically charged or ionic and electrically neutral functional groups. Side chains comprising ionic functional groups can be referred to as ionic side chains.

[0052] The ionomer preferably has a proportion of at least 10% ionic side chains (preferably in relation to the total number of ionic and non-ionic side chains), and particularly preferably a proportion of more than 30% ionic side chains. Preferably, the ionomer can also have a proportion of at least 10% (of the side chains) ionic side chains, for example, at least 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100% ionic side chains. Furthermore, the ionomer preferably has a proportion of 30% to 70% ionic side chains.

[0053] Examples of perfluorosulfonic acid ionomers used according to the invention are Nation, Acquivion or Flemion, which can contribute to proton conduction within the PEMFC.

[0054] According to the invention, the ionomer matrix includes hydrocarbon ionomers.

[0055] Since current hydrocarbon-based ionomers and membranes—without being theoretically limited to them—are considered more sensitive to a decrease in relative humidity, the humidification of the ionomer matrix is ​​even more important than with conventionally used perfluorosulfonic acid ionomers. In the context of the invention, the use of carbon hydrogen ionomers in combination with the water-retaining properties of the water-retaining diffusion layer has proven to be particularly advantageous.

[0056] In particular, it was recognized that when using carbon hydrogen ionomers, the performance of the PEMFC could be particularly increased by the water-retaining properties of the water-retaining diffusion layer within the PEMFC according to the invention.

[0057] While not limited to this by theory, hydrocarbon ionomers exhibit a stronger dependence of proton conductance on humidification, as they preferentially possess a higher number of acid functional groups—particularly sulfonated polyphenylenes—per repeating unit than perfluorosulfonic acid ionomers to provide comparable proton conductance. However, perfluorosulfonic acid ionomers also exhibit a comparable, though less pronounced, dependence on humidification.

[0058] Preferably, a hydrocarbon ionomer is an ionomer with a backbone comprising essentially carbon and hydrogen. The backbone of the hydrocarbon ionomer may further preferably include heteroatoms such as nitrogen, sulfur, oxygen, and / or phosphorus. The hydrocarbon ionomer may further preferably include a linker that forms a covalent bond between the backbone of the hydrocarbon ionomer and a side chain and / or between the individual units of the backbone, for example, but not limited to, sulfonyl, esters, ethers, amides, secondary amines, tertiary amines, and / or ammonium.

[0059] Examples of the hydrocarbon ionomer backbone include, but are not limited to, polyarylenes such as polyphenylenes, polysulfone, polyesters, polyethers, polyketones, polyimides, polyamides, polyimidazoles, polyurethanes, and polyalkylenes.

[0060] Preferably, the hydrocarbon ionomer comprises a backbone formed by a chain of carbon and hydrogen atoms. Preferably, the backbone includes aryl groups, such as phenyl groups. A hydrocarbon ionomer preferably also includes an ionic side chain comprising sulfonate SO3-X+, phosphoryl PO3-X+2, and / or carboxyl COO-X+, where X is a proton or another cation, such as a potassium or sodium ion. Preferred hydrocarbon ionomers can be found in WO 2018 / 187864 Al. For example, pemion can preferably be used as a hydrocarbon ionomer.

[0061] According to the invention, the fuel cell, in comparison to having a diffusion layer on the anode that is identical to the water-removing diffusion layer of the cathode, exhibits at least one performance improvement measured on a polarization curve, wherein the performance improvement preferably increases with an increasing current density.

[0062] Preferably, the performance improvement of PEMFCs (and PEMFCs in general) can be measured by their polarization curve, with the performance improvement preferably resulting from the described improved proton conduction or lower protonic resistance losses. Generally, the performance of the PEMFC is preferably considered better the higher the voltage remains at increasing current flow (electrical load). The load is preferably specified as current density relative to the cell area, from which a power density can be calculated, which preferably represents a measure of the achievable performance of the PEMFC.

[0063] The performance improvement is preferably achieved compared to a comparable PEMFC that, instead of having a water-retaining diffusion layer, has an additional water-repellent diffusion layer on the anode, identical to the water-repellent layer on the cathode. Preferably, in this case, the water-retaining properties are not achieved for the additional water-repellent diffusion layer instead of the water-retaining diffusion layer, which means that the advantageous properties of the PEMFC according to the invention cannot be achieved.

[0064] In particular, without being limited to this by the theory, the performance improvement of the PEMFC according to the invention compared to the comparison PEMFC preferably increases with increasing current density, since in this case a higher reaction rate is preferably present, resulting in more water being produced in the cathode, whereas in the comparison PEMFC the anode preferably dries out comparatively or is not sufficiently or less moistened.

[0065] The water-retaining properties of the water-retaining diffusion layer in the PEMFC according to the invention preferably result in increased water retention, at least in the anode, with increasing current density. More preferably, more water is retained from the cathode in the anode or on the anode side. This is preferably demonstrated by the fact that, with increasing current density, the measured voltage of the PEMFC according to the invention and the reference PEMFC differ more significantly from each other.

[0066] Preferably, the ionomer matrix of the membrane electrode arrangement is based at least partially or completely on hydrocarbon ionomers.

[0067] Sections of the ionomer matrix based on hydrocarbon ionomers preferably have a higher proportion of hydrocarbon ionomers than other optional chemical compounds used to construct the ionomers for proton conduction.

[0068] The ionomer matrix, preferably based section by section on hydrocarbon ionomers, is preferably formed by having hydrocarbon ionomers form the basis of the ionomer matrix at least section by section. In particular, the ionomer matrix of the anode or the anode side is preferably based on hydrocarbon ionomers.

[0069] Preferably, the ionomer matrix in the membrane electrode unit is based entirely on hydrocarbon ionomers, so that the ionomer matrix as a whole can be considered to be based on hydrocarbon ionomers.

[0070] In particular, the ionomer matrix may preferably comprise section by section or completely essentially hydrocarbon ionomers, preferably a proportion of more than 80 wt. %, more than 90 wt. %, more than 95 wt. %, more than 99 wt. %, £.

[0071] Furthermore, the ionomer matrix may preferably comprise sections or the entire matrix consisting exclusively of hydrocarbon ionomers.

[0072] For the preferred ionomer matrix, the influence of water has a particularly positive effect on the proton conductance of the anode arrangement, preferably on the water management within the PEMFC, and thus particularly preferably on the performance of the PEMC, since the ionomers of the ionomer matrix or the hydrocarbon ionomers benefit more from the humidification than perfluorosulfonic acid ionomers.

[0073] Preferably, the ionomer matrix based on hydrocarbon ionomers is free of perfluorinated sulfonic acids, and preferably essentially fluorine-free. Preferably, the fluorine-free ionomer matrix contains fluorine of at most 5 mol%, at most 2.5 mol%, or at most 1 mol%. The fact that the ionomer matrix is ​​essentially fluorine-free can particularly preferably mean that the ionomer matrix is ​​completely free of fluorine.

[0074] Preferably, the water-retaining diffusion layer comprises a polymer binder, wherein the polymer binder comprises polyvinylidene fluoride and / or preferably, the water-retaining diffusion layer comprises a polymer binder, wherein the polymer binder is a fluorine-free polymer binder, preferably polyolefins, polyaromatics or other hydrocarbon-based polymers, particularly preferably polybenzimidazole and / or an acrylic component.

[0075] The polymer binder preferably comprises a material or substance that holds or attracts other materials together, for example, mechanically, chemically, through adhesion or cohesion, thereby preferably providing a material bond within the diffusion layer. The polymer binder is particularly suitable, for example, for electrochemical applications, to integrate various components of an electrode into a cohesive unit and to maintain the electrode's physical structure. Preferably, the polymer binder is provided as a raw material within the water-retaining diffusion layer.

[0076] In the context of the invention, the polymer binder is particularly preferably provided for binding other raw materials in the water-retaining diffusion layer, wherein, in particular, the material properties and microstructural properties of the water-retaining diffusion layer are preferably adjusted by the amount and type of polymer binder in the manufacturing process according to the invention. This preferably affects functional properties such as mechanical stability, electrical conductivity, or permeability of the water-retaining diffusion layer.

[0077] In connection with the invention, polymer binders, for example polyvinylidene fluoride, have been preferably identified as suitable agents to be present in the water-retaining diffusion layer and preferably contribute to the described properties in the water-retaining diffusion layer. However, other polyvinylidene fluoride components or derivatives are also potentially suitable.

[0078] In particular, polyvinylidene fluoride preferably exhibits advantageous properties in the context of the invention, such as high thermal and electrochemical stability as well as advantageous functional properties such as adhesion properties for raw materials present in the water-retaining diffusion layer.

[0079] Furthermore, polyvinylidene fluoride is preferably used as a polymer binder in the invention, with which water-retaining properties are preferably achieved for the water-retaining diffusion layer, whereby the water-retaining diffusion layer is particularly more water-retaining than, for example, GDLs post-treated with polytetrafluoroethylene from the prior art or the water-repellent diffusion layer.

[0080] At least the use of polyvinylidene fluoride as a polymer binder in the water-retaining diffusion layer preferably leads to a more hydrophilic moistening behavior of the diffusion layer, which potentially has a preferentially complementary effect on the water-retaining properties of the water-retaining diffusion layer described above according to the invention.

[0081] Furthermore, using polyvinylidene fluoride as a polymer binder in the manufacturing process according to the invention, a variety of water-retaining diffusion layers with alternating properties and parameters, particularly as freestanding water-retaining diffusion layers, can preferably be obtained. These can be preferably verified by various experimentally measurable parameters.

[0082] Furthermore, in connection with the invention, other particularly preferred polymer binders, especially fluorine-free polymer binders, for example preferably polybenzimidazole and / or an acrylic component, for example polyacrylic acid, have been identified as suitable polymer binders to be present in the water-retaining diffusion layer and preferably contribute to the described properties in the water-retaining diffusion layer. However, other polybenzimidazole and / or acrylic components or derivatives are also potentially preferred.

[0083] For these preferred fluorine-free polymer binder agents, in the context of the invention, analogous considerations apply with regard to advantageous properties for the PEMFC according to the invention as for polyvinylidene fluoride.

[0084] Polybenzimidazoles can be obtained from aromatic polymers with benzimidazole units by polycondensation. They are preferably present as fine-grained powders, as filaments, but also in dissolved form.

[0085] Acrylic components are preferably characterized by an acrylic group and include substances such as acrylic acid, acrylic acid esters or polymers of this group, which can be supplied, for example, as a hygroscopic white powder, granules or in dissolved form.

[0086] In particular, polybenzimidazole and acrylic components are considered polymer binders within the scope of the invention, with which water-retaining properties for the water-retaining diffusion layer are preferably achieved. Furthermore, in the manufacturing process according to the invention, water-retaining diffusion layers are preferably provided with these polymer binders, which are particularly more water-retaining than, for example, GDLs post-treated with polytetrafluoroethylene from the prior art or the water-repellent diffusion layer, as well as water-retaining diffusion layers that use polyvinylidene fluoride as a polymer binder.

[0087] Furthermore, alternative fluorine-free polymer binders should be pursued from an ecological perspective.

[0088] In particular, a fluorine-free diffusion layer can be obtained by using fluorine-free polymer binders. According to the invention, the fluorine-free diffusion layer contains fluorine of at most 5 mol percent, at most 2.5 mol percent, or at most 1 mol percent. The fact that the diffusion layer is essentially fluorine-free can particularly preferably mean that the diffusion layer is completely free of fluorine.

[0089] Against the background of fluorine-free hydrocarbon ionomers, a reduction in fluorine use is preferably achieved in combination with preferred fluorine-free diffusion layers with fluorine-free polymer binders, which can prove to be particularly advantageous in view of ecological aspects.

[0090] The water-retaining diffusion layers according to the invention are preferably designed for use on the anode of the PEMFC according to the invention.

[0091] Preferably, the water-retaining diffusion layer comprises an electrically conductive filler, preferably a carbon-containing filler, wherein the binder distribution of the polymer-binder agent in the water-retaining diffusion layer is homogeneous, wherein the polymer-binder agent is preferably based on an acrylic component, or preferably, the water-retaining diffusion layer comprises an electrically conductive filler, preferably a carbon-containing filler, wherein the binder distribution of the polymer-binder agent within the water-retaining diffusion layer follows a gradient, whereby the binder distribution of the polymer-binder agent particularly preferably increases towards the anode, wherein the polymer-binder agent is preferably based on polyvinylidene fluoride or polybenzimidazole.

[0092] The electrically conductive filler is preferably a carbon-containing filler, for example, graphite powder or carbon black, to provide electrical conductivity, good gas permeability, and porosity within the water-retaining diffusion layer. The carbon-containing filler is preferably present in a particle size between 4 and 6 pm. The diffusion layer is preferably based on the filler, and the water-retaining diffusion layer is preferably also based on the polymer binder. The water-retaining diffusion layer preferably comprises, for example, additives or functional additives, and in particular hydrophilic additives, to enhance the water-retaining properties.

[0093] The binder distribution within the water-retaining diffusion layer is preferably described, for example, by a binder-agent ratio of the polymer-binder agent compared to the other raw material(s), preferably the filler.

[0094] According to the invention, the binder distribution is preferably homogeneous within the water-retaining diffusion layer. In the water-retaining diffusion layer with the homogeneous binder distribution, the binder-binder ratio is preferably largely homogeneous, or the polymer-binder agent is distributed largely uniformly within the water-retaining diffusion layer or between the filler, which is preferably porous and in the form of fibers or particles.

[0095] In this context, a uniform or homogeneous binder distribution is specifically mentioned when the polymer-binder agent is distributed evenly or in equal proportions on average within a section of the water-retaining diffusion layer or the entire layer.

[0096] In particular, the binder distribution is preferably linked to the micromorphology of the water-retaining diffusion layer, wherein the filler is preferably bonded by a binder network. It is preferably assumed that a higher binder-to-material ratio leads to a stronger bond of the filler and thus to finer pore properties, described by the pore size, pore distribution, and pore structures. Accordingly, the pore properties of the water-retaining diffusion layer are preferably linked to the binder distribution within the water-retaining diffusion layer.

[0097] The pore properties are therefore preferably homogeneous when the binder is distributed homogeneously, with the front and back surfaces of the water-retaining diffusion layer appearing similarly porous to an observer, for example, under a scanning electron microscope. This preferably leads to the conclusion that the micromorphology or pore properties are homogeneous within the layer, and that the binder distribution is also homogeneous. A homogeneous binder distribution preferably occurs when the polymer-binder is based on the acrylic component. However, this is not the only possible conclusion; such a binder distribution is preferably inferred experimentally from the observed pore properties of the front and back surfaces of exemplary water-retaining diffusion layers.

[0098] The homogeneous binder distribution and / or homogeneous pore properties within the water-retaining diffusion layer facilitate the supply of hydrogen gas to the anode. These homogeneous pore properties result in lower pressure drop and / or lower gas resistance within the water-retaining diffusion layer when gas flows through it. Other properties, such as electrical conductivity and heat dissipation within the water-retaining diffusion layer, can also be preferably homogeneous, characterized by a homogeneous binder distribution.

[0099] Furthermore, the homogeneous binder distribution preferably has a complementary effect on the water-retaining properties of the water-retaining diffusion layer, thereby advantageously influencing water management and / or moisture management in the PEMFC. Particularly when acrylic is used as the polymer binder, the water-retaining diffusion layer is preferably homogeneous to such an extent that actual contact, whether electrical, thermal, or compressional, between the water-retaining diffusion layer and the anode is preferably improved.

[0100] However, according to the invention, a binder distribution that follows a gradient or an inhomogeneous binder distribution of the water-retaining diffusion layer can preferably also be obtained. The inhomogeneous binder distribution can preferably be understood analogously to the homogeneous binder distribution.

[0101] In contrast to a water-retaining diffusion layer with a homogeneous binder distribution, a higher binder distribution – following the gradient – ​​is preferably present on the front or back side of the diffusion layer. This results in finer pore properties on one side (experimentally detectable using a scanning electron microscope), while coarser pore properties are preferably present on the other side. It can preferably be assumed that the pore size within the layer increases or decreases linearly, following the binder distribution.Preferably, a binder distribution follows a gradient if the polymer binder is based on polyvinylidene fluoride and / or polybenzimidazole, wherein, for example, such a binder distribution is preferably inferred experimentally from the observed pore properties of the front and back sides of exemplary water-retaining diffusion layers.

[0102] The pore properties and binder distribution observed in the respective water-retaining diffusion layers are—without being limited to this by the theory—preferably dependent on the polymer binders used and the quantity or material fraction of the polymer binder within the water-retaining diffusion layer. In particular, it is preferably assumed that when using polyvinylidene fluoride or polybenzimidazole, the polymer binder, in a manufacturing process or coating step in which a diffusion layer composition—as described below—is applied to a substrate, sinks due to gravity during a drying process.

[0103] With homogeneous binder distribution, as is preferably the case for acrylics, a settling effect – without being limited to this by the theory – preferably occurs only to a limited extent, is negligible or does not occur at all.

[0104] The binder distribution, which follows a gradient, preferably has a complementary effect on the water-retaining properties of the water-retaining diffusion layer. In particular, if the binder distribution, and thus preferably the fine porosity of the water-retaining diffusion layer, increases towards the anode, this preferably leads to improved actual contact, i.e., electrical, thermal, or even compression contact, between the diffusion layer and the anode. Simultaneously, the gas preferably enters on the more coarsely porous side of the water-retaining diffusion layer, i.e., the side with the coarser pore properties, and is then finely and homogeneously distributed towards the anode.

[0105] Due to the gradient, a higher binder distribution is particularly prevalent at the interface between the diffusion layer and the anode. This particularly favorably influences the water-retaining properties of the water-retaining diffusion layer.

[0106] Preferably, the water-retaining diffusion layer comprises polyvinylidene fluoride as a polymer binder and has a layer thickness between 10 pm and 80 pm, preferably between 15 pm and 50 pm, particularly preferably between 18 pm and 30 pm, particularly preferably thinner than 30 pm, particularly preferably 20 pm, and / or a binder concentration between 10% and 60%, preferably between 20% and 45%, particularly preferably between 35% and 40%, particularly preferably 30% or 40%, and / or a contact angle between 50° and 100°, preferably between 60° and 90°, particularly preferably between 60° and 85°, particularly preferably between 60° and 70°, particularly preferably between 65° and 70°, and / or a Gurley permeability between 2 s and 100 s, preferably between 2 and 50 s, particularly preferably between 2 s and 10 s, particularly preferably 3 s or 4 s s or 5 s and / or a porosity measured by gravimetric measurement,between 50% and 90%, preferably between 70% and 86%, particularly preferably between 80% and 85%.

[0107] Preferably, the diffusion layer has a gas permeability for nitrogen, measured using a pressure drop method, between 1.5 x lO^mols^Pa^rrr 2 and 4 x 10-4 mols^Pa^nr 2 preferably between 2 x 10-4 mols^Pa^nr 2 and 3.6 x 10' 4 mols' ^a^rrr 2 and / or a porosity measured by gravimetric measurement.

[0108] The layer thicknesses, binder concentrations, contact angles, gas permeabilities and porosities present can be advantageous as preferably measurable parameters, both individually and in combination, for the water-retaining properties of the water-retaining diffusion layer according to the invention.

[0109] The parameters for the water-retaining diffusion layers are preferably obtained by the manufacturing process according to the invention described below, wherein these are preferably obtained based on the diffusion layer composition, preferably with the polymer-binder agent polyvinylidene fluoride, but preferably also with the other polymer-binder agents or combinations thereof described. Further preferred parameters for diffusion layers are described below.

[0110] The parameters preferably describe mean parameters of the investigated area of ​​the diffusion layer, whereby the investigated area and its dimensions may vary depending on the measurement method or diffusion layer.

[0111] The layer thickness here preferably describes a material thickness or an average material thickness of the preferably planar water-retaining diffusion layer through an expansion plane of the water-retaining diffusion layer and applies to a tested area of ​​the water-retaining diffusion layer, preferably to the entire water-retaining diffusion layer, including deviations due to manufacturing variations or defects. The layer thickness is preferably determined, for example, by a profilometric measurement method or a cross-sectional scanning electron microscope method.

[0112] Reducing the thickness of the water-retaining diffusion layer can—without being purely theoretical—promote its water-retaining properties and thus positively impact the performance of the corresponding PEMFC. By reducing the layer thickness, the water absorption effect of the diffusion layer can be diminished, thereby reducing water loss through the layer. Furthermore, reducing the layer thickness can also lower the electrical resistance of the diffusion layer, which likewise improves the performance of the PEMFC.An improvement in water retention properties and an improvement in performance is achieved, for example, with a preferred layer thickness of 30 pm for the water-retaining diffusion layer, and is particularly favored compared to preferred layer thicknesses of 60 pm or 90 pm.

[0113] The binder concentration preferably describes a ratio between the polymer binder agent and the filler of the water-retaining diffusion layer according to the invention or the diffusion layer composition for producing the water-retaining diffusion layer.

[0114] Particularly preferably, the binder concentration describes at least a mean volumetric proportion of the polymer binder relative to the filler in the dry water-retaining diffusion layer, especially in a section of the dry water-retaining diffusion layer, wherein the section preferably comprises an entire thickness of the diffusion layer.

[0115] The polymer binder preferably comprises one type of polymer binder, while, conversely, several different types of polymer binders preferably contribute to the binder concentration. This applies analogously to the filler, which preferably comprises one type or, alternatively, preferably several types of filler. The water-retaining diffusion layer preferably comprises essentially a polymer binder and a filler, wherein the proportion of the materials present within the diffusion layer is essentially determined by the binder concentration.

[0116] According to the invention, the binder concentration preferably has a significant effect on the material properties and microstructure properties of the diffusion layer, so that the binder concentration also preferably influences the other parameters of the water-retaining diffusion layer.

[0117] In the context of the invention, the contact angle of the water-retaining diffusion layer preferably describes an angle formed by the surface of a droplet of deionized water with the surface of the water-retaining diffusion layer. This preferably describes the wettability of the wetted surface of the diffusion layer with the wetting liquid. If water or deionized water is the wetting liquid, the surface under investigation is preferably described as hydrophilic at small contact angles of less than or equal to 90°, preferably as hydrophobic at larger contact angles above 90°, and as superhydrophobic at angles greater than 150°.

[0118] Means and methods for determining the contact angle are known from the prior art. Generally, the contact angle is preferably defined as the angle formed between the surface of a liquid droplet and the surface of a solid.

[0119] Preferably, the side of the water-retaining diffusion layer facing the anode is considered for wettability, since in the application according to the invention this represents the boundary layer to the anode and thus preferably contributes to the water-retaining properties of the water-retaining diffusion layer.

[0120] Particularly preferably, the side of the water-retaining diffusion layer facing the anode corresponds to a side of the water-retaining diffusion layer that was in contact with a substrate for providing the diffusion layer during the manufacturing process. Furthermore, preferably, in the case of the water-retaining diffusion layer with the inhomogeneous binder distribution, the side with the higher binder concentration or with the finer pore properties faces the anode.

[0121] In the context of the invention, the contact angle preferably describes a static contact angle that can be determined using the so-called Young's equation. The contact angle is preferably measured using static contact angle measurements with a Dataphysics OCA25 device and is preferably determined by averaging 10 measurements with 13 pL of deionized water.

[0122] In the context of the invention, the gas permeability of the water-retaining diffusion layers is preferably determined by so-called pressure drop measurements and is carried out using nitrogen gas. A Scribner 850e fuel cell test rig is preferably used as the mass flow controller. Preferably, the water-retaining diffusion layer is fixed between flow fields in a setup corresponding to a procedure for PEM single-cell tests proposed by the US Department of Energy.

[0123] Preferably, the water-retaining diffusion layers for measuring gas permeability are laminated with a 40 pm thick PAN film, which has a 4 cm window. 2 Leave open for gas flow. A nitrogen gas flow through the tested diffusion layer is preferably varied from 0.25 to 1 L min in increments of 0.05 L / min. 1 at room temperature. Preferably, the pressure drop is measured after a 2-minute stabilization period, for example with a Chauvin Arnoux CA852 manometer.

[0124] In this case, the gas permeability, which is preferably measured for nitrogen gas, is preferably related to a gas permeability of the water-containing diffusion layer for hydrogen, oxygen or (liquid) water, so that a higher nitrogen gas permeability preferably implicitly implies a higher hydrogen or oxygen gas permeability.

[0125] Porosity preferably describes the ratio of void volume to the total volume of the water-retaining diffusion layer. Preferably, the porosity is related to the actual voids present within the diffusion layer, such that the volumetric and gravimetric density of the diffusion layer decreases with higher porosity.

[0126] The porosity of the diffusion layer is preferably obtained by a gravimetric measurement, preferably performed on several 4 cm sections. 2 Diffusion layer measurements are performed using a Sartorious ME36S microbalance and calculated using state-of-the-art formulas.

[0127] The micromorphology properties are preferably investigated optically, in particular by in-plane and through-plane scanning electron microscope measurements, using, for example, a Tescan MAIA3 XMH electron microscope with 5 kV accelerating voltage, about 0.25 nA beam current and 7 mm working distance with a secondary electron detector on a scale of 50 pm.

[0128] In particular, the parameters preferably exhibit interdependencies, since, for example, porosity is generally related to gas permeability or the contact angle. Specifically, the contact angle or wetting properties of the water-retaining diffusion layer—preferably in connection with water—are preferably related to a surface structure, which is preferably characterized by the pore properties or porosity, but also by the material properties, especially at the interface.

[0129] Furthermore, the water-retaining diffusion layer preferably includes, in particular, flexible properties which preferably result - without being limited by the theory - from the diffusion layer (manufacturing) composition and the polymer binder used, as well as the binder concentration, and preferably contribute to good contact between the anode and the water-retaining diffusion layer.

[0130] The water-retaining diffusion layer has different parameters compared to the water-transporting diffusion layer, but preferably at least a smaller layer thickness and a different material composition.

[0131] Prior art water-transporting diffusion layers preferably have, as described above, a gas diffusion substrate with a microporous layer and have been post-treated with a hydrophobic coating. For example, the water-repellent diffusion layer preferably has a thickness of 185 pm and a Gurley permeability of 15 s. An alternative water-carrying diffusion layer preferably has a layer thickness of 180 pm or 185 pm and a Gurley permeability of 1 s. Further preferred parameter combinations for the water-carrying diffusion layer, consisting of layer thicknesses and / or Gurley permeabilities (according to ISO 5636-5), are, for example, 175 pm and / or 50 s, 180 pm and / or 30 s, 170 pm and / or 70 s, 195 pm and / or 27 s, 255 pm and / or 70 s, 120 pm and / or 10 s, 250 pm and / or 30 s, 250 pm and / or 2 s, 230 pm and / or 90 s, 290 pm and / or 35 s, 270 pm and / or 40 s.

[0132] In contrast, an experimentally measured water-transporting diffusion layer preferably has a layer thickness of 250 pm, a contact angle between 150° and 160°, and a gas permeability between 4 x 10' 4 mols^Pa^nr 2 and 5 x 10' 4 mols^Pa^rrr 2 and / or a porosity of 68%, preferably determined analogously to the parameters mentioned above. Other GDLs on the state of the art that can form water-carrying diffusion layers but exhibit different layer thicknesses and Gurley permeabilities can at least have comparable experimentally determinable parameters.

[0133] In particular, the differences in material properties and microstructure properties between the water-retaining diffusion layer and the water-repellent diffusion layer, or the differences in the described parameters or orders of magnitude, potentially contribute favorably to the water-retaining properties.

[0134] Furthermore, the ratios of the microstructure properties between the water-retaining diffusion layer and the water-transporting diffusion layer, which are preferably calculated from the orders of magnitude of the described parameters, potentially contribute to the water-retaining properties.

[0135] Furthermore, preferred thicknesses of the water-retaining diffusion layer containing the polymer binder polyvinylidene fluoride are between 5 pm and 15 pm, or between 45 pm and 55 pm, or between 25 pm and 35 pm. Preferred binder concentrations of the water-retaining diffusion layer are between 5% and 15%, or between 55% and 60%, or between 37% and 42%. Further preferred contact angles of the water-retaining diffusion layer are between 55° and 65°, or between 85° and 95°, or between 57° and 75°, or between 62° and 70°. Preferred gas permeabilities are between 1.8 x 10⁻⁶ 4 mols^Pa^m -2 and 2.2 x 10' 4 mols^Pa^m -2 or between 2.8 x 10' 4 mols^Pa^nr 2 and 4.1 x 10' 4 mols^Pa^nr 2 or between 1.3 x 10' 4 mols^Pa^nr 2 and 1.7 x 10' 4 mols^Pa^rrr 2 or between 2.5 x 10' 4 mols^Pa^nr 2 and 3.4 x 10'4 mols^Pa^nr 2 Preferred porosities are between 73% and 77%, or between 72% and 90%, or between 78% and 84%.

[0136] Preferably, the water-retaining diffusion layer comprises polybenzimidazole as a polymer binder and has a thickness between 10 pm and 50 pm, preferably 15 pm and 30 pm, particularly preferably 20 pm, and / or a binder concentration between 30% and 50%, preferably between 35% and 45%, particularly preferably 40%, and / or a Gurley permeability between 30 s and 110 s, preferably between 35 s and 60 s, particularly preferably between 45 s and 55 s, particularly preferably between 45 s and 50 s, particularly preferably 47 s or 48 s, and / or a porosity, measured by gravimetric analysis, between 45% and 72%, preferably between 50% and 65%, particularly preferably between 50% and 58%, particularly preferably 56% or 57%.or the water-retaining diffusion layer comprises an acrylic component as a polymer binder and has a thickness between 30 pm and 80 pm, preferably 40 pm and 60 pm, particularly preferably 50 pm, and / or a binder concentration between 30% and 50%, preferably between 35% and 45%, particularly preferably 40%, and / or a Gurley permeability between 1 s and 20 s, preferably between 2 s and 15 s, particularly preferably between 2 s and 1 s, particularly preferably 2 s or 3 s or 11 s, and / or a porosity, measured by gravimetric measurement, between 60% and 80%, preferably between 65% and 75%, particularly preferably 69% or 73%.

[0137] Preferably, the diffusion layer comprises the respective polymer binder agent as the sole polymer binder agent.

[0138] In particular, the Gurley permeabilities preferably describe Gurley permeabilities according to ISO 5635-5, which were determined, for example, using a Gurley 4340 smoothness and air permeability tester.

[0139] The Gurley permeabilities of the water-retaining diffusion layer preferably describe the Gurley permeabilities of the water-retaining diffusion layer in the uncompressed state.

[0140] Traditionally, compression is preferably applied when assembling individual PEMFC components, for example the membrane electrode assembly and the diffusion layers between bipolar plates to form a PEMFC, in order to reduce the contact resistance between the respective layers and to seal or make gas-tight a stack formed from the layers.

[0141] Preferably, stronger compression reduces the permeability of the diffusion layer, so that in general it can be assumed that higher Gurley permeabilities occur in the PEMFC for the respective diffusion layers, which may also depend on the compression described.

[0142] The compressibility of a diffusion layer can depend on the materials used, the manufacturing process, and especially on the polymer binder or whether it is a carrier-free diffusion layer, i.e., for example, a freestanding diffusion layer that is not coated onto a gas diffusion substrate. Carrier-free diffusion layers, for instance, can be irreversibly compressible.

[0143] Given that the PEMFC components are compressed between the bipolar plates and the reaction gases are supplied via channels for better distribution of the reaction gases, the actual permeability or Gurley permeability of the diffusion layers during operation is difficult or impossible to determine.

[0144] Preferably, the fuel cell comprises a further water-carrying diffusion layer, wherein the further water-carrying diffusion layer is provided on top of the water-retaining diffusion layer, and the gas diffusion layer is preferably a diffusion layer identical to the water-carrying diffusion layer. The water-retaining diffusion layer is preferably provided, in particular, as an intermediate layer or interface layer between the anode and a further water-carrying diffusion layer.

[0145] The additional water-removing diffusion layer is preferably configured similarly to or identically to the water-removing diffusion layer on the cathode. However, it can also preferably be a different water-removing diffusion layer.

[0146] In this context, but not limited to this, the water-retaining diffusion layer particularly preferably exhibits functions of a microporous layer comparable to microporous layers from the prior art, wherein, moreover, it provides the water-retaining properties in a manner essential to the invention.

[0147] In particular, a performance improvement is also preferred compared to the absence of a water-retaining diffusion layer or the presence of a water-repellent diffusion layer instead of a water-retaining diffusion layer. Furthermore, the performance improvement preferably increases with a higher current density during the intended operation of the PEMFC.

[0148] The fuel cell is preferably operated under dry operating conditions, preferably at a relative humidity between 20% and 60%, particularly preferably at 30% at the anode, and / or preferably at a relative humidity between 60% and 85%, particularly preferably 60% or 80%, and / or preferably at a temperature between 85°C and 130°C, preferably between 90°C and 110°C, particularly preferably at 95°C or 105°C or 110°C.

[0149] When operating a PEMFC, a distinction is preferably made between different operating conditions. In particular, the operating conditions are preferably characterized by the parameters temperature and relative humidity under which the PEMFC is operated. The temperature and relative humidity of the PEMFC are preferably controlled during operation, particularly via control systems that can, for example, regulate the cooling and the relative humidity of the supplied reaction gases.

[0150] In particular, the PEMFC is preferably operated with asymmetrical relative humidities at the anode and the cathode, so that gas with different relative humidities is supplied to each.

[0151] Furthermore, the reaction products water and heat generated within the PEMFC during operation have a preferential effect on the operating conditions. Additionally, the operating conditions under which the PEMFC existed before it began operating can also influence the current operating conditions.

[0152] According to the invention, the PEMFC is preferably operated at a higher temperature, particularly preferably between 85 °C and 130 °C, particularly preferably 120 °C (lower temperatures, however, are < 85 °C), which is particularly suitable for automotive applications. In a real automotive application, the hydrogen gas used typically has a low purity level, for example, below 99.9%. The PEMFC according to the invention preferably exhibits higher resistance to impurities, for example, carbon monoxide, at higher temperatures.

[0153] In particular, the PEMFC according to the invention is preferably operated under conditions that describe high-performance operation, such as in heavy vehicles.

[0154] The moisture is preferably a saturation of a gas mixture with a liquid, preferably water, wherein the maximum amount of moisture within a gas mixture (preferably air, oxygen, and / or hydrogen) is reached when the gas mixture can no longer absorb any more water. This amount preferably depends in particular on the temperature of the gas mixture.

[0155] Accordingly, relative humidity primarily describes the ratio of the existing to the maximum humidity of the gas mixture. Warmer gases can generally hold more water.

[0156] The PEMFC is preferably operated at a specific relative humidity, wherein the reaction gases supplied to the PEMFC are preferably configured to contain water and exhibit a specific relative humidity (at a given temperature). The actual relative humidity within the PEMFC is preferably further influenced by the reaction products of the PEMFC according to the invention.

[0157] According to the invention, the advantages are particularly evident under dry operating conditions, preferably comparable to the operating conditions in automotive applications, wherein in this context preferably, for example, when the PEMFC has not been operated in a vehicle for a longer period of time, relatively low relative humidity is present in the PEMFC or the PEMFC is dry, wherein, in particular, advantageous performance is achieved through the water-retaining properties of the water-retaining diffusion layer and, moreover, the components of the PEMFC are preferably protected.

[0158] According to the invention, the water-retaining diffusion layer preferably retains water supplied by the reaction gases and also preferably the water generated during operation of the PEMFC, at least in the anode or preferably in the anode side, thereby preferably favorably influencing the relative humidity of the ionomer matrix.

[0159] The PEMFC according to the invention is particularly preferably configured such that, under dry conditions, especially for high current densities with increased water formation through the water-retaining diffusion layer, a performance improvement of the PEMFC is achieved, since, particularly preferably under such operating conditions, the drying out of the anode or, more preferably, the anode side is suppressed by the water-retaining properties.

[0160] The water-retaining diffusion layer, which is arranged according to the invention to provide the water-retaining properties in the PEMFC, is preferably provided by the manufacturing process for a diffusion layer described below. Accordingly, the preceding explanations regarding the water-retaining diffusion layer preferably also apply to the diffusion layer obtained by the manufacturing process.

[0161] According to the invention, a manufacturing process for a diffusion layer for a membrane electrode assembly, a fuel cell, a hydrogen fuel cell, an electrolyzer or a water electrolyzer comprises the steps: a) providing a diffusion layer composition comprising a fluorine-free polymer binder, preferably polyolefins, polyaromatics or other hydrocarbon-based polymers, particularly preferably polybenzimidazole and / or an acrylic component, b) applying the diffusion layer composition to a substrate, c) drying the applied diffusion layer composition.

[0162] The manufacturing process according to the invention preferably provides that the diffusion layer according to the invention is obtained on the basis of the diffusion layer composition by the manufacturing process, which is preferably based, for example, on the principle of a non-solvent-induced phase separation.

[0163] According to the invention, in the step of providing a) the diffusion layer composition is preferably provided as a liquid material mixture or a dispersion, which preferably comprises raw materials or base materials of the diffusion layer as well as pore agents or surfactants, dissolved or dispersed by a solvent and is obtained, for example, by mixing or dispersing using a high-shear mixer or an ultrasonic high-shear mixer.

[0164] According to the invention, it is particularly preferred that the diffusion layer composition comprises the fluorine-free polymer binder, preferably polyolefins, polyaromatics, or other hydrocarbon-based polymers, most preferably polybenzimidazole, and / or an acrylic component. The diffusion layer composition preferably comprises exclusively the fluorine-free polymer binder as the polymer binder. The polymer binder is preferably included as a raw material in the diffusion layer composition.

[0165] According to the invention, in step b) the diffusion layer composition is applied to the substrate, wherein the substrate is preferably, for example, a rigid surface or a film. Preferably, the diffusion layer composition is applied to the substrate by an application device, preferably a doctor blade or a Mayer bar. More preferably, the diffusion layer composition can also be applied to the substrate, for example, by a slot nozzle device or by using several of the described devices combined into a single application device.

[0166] The diffusion layer composition is preferably configured, particularly with regard to the composition of the mixture, to adjust distribution properties, such as viscosity and flow behavior, such that the application device preferably distributes the mixture as a uniform and homogeneous layer on the substrate. Viscosity and flow behavior are preferably adjusted by selecting the solvent and / or the proportion of solvent in the diffusion layer composition.

[0167] The diffusion layer composition is preferably applied and distributed directly onto the substrate by the application device, wherein, alternatively, the diffusion layer composition is preferably first provided on the substrate and then distributed onto the substrate by the application device in a specific layer thickness, wherein the layer thickness of the liquid diffusion layer composition preferably differs from the layer thickness of the final diffusion layer. Preferably, the layer thickness of the final diffusion layer is thinner than that of the applied diffusion layer composition.

[0168] In particular, the application thickness or layer thickness of the diffusion layer composition layer is preferably determined by the amount of material deposited on the substrate and by adjusting the application device, wherein the application device is preferably moved across the substrate at a certain speed in a horizontal direction. Likewise, preferably when using a Mayer bar as the application device, the layer thickness is adjusted by parameters of the Mayer bar, wherein, for example, a weight, diameter, number of turns, and turn spacing of the Mayer bar are preferably such parameters.

[0169] Preferred layer thicknesses of the applied diffusion layer composition are, for example, 50 pm to 250 pm and are preferably set with regard to the final layer thickness of the diffusion layer to be obtained. In particular, an application temperature is preferably set for the application step, which especially preferentially affects the distribution properties of the diffusion layer composition. An application temperature between 20 °C and 130 °C is preferably selected.

[0170] Subsequently, the diffusion layer composition layer is dried in step c) according to the invention. In the context of the invention, drying preferably describes the removal or escape of liquids from the diffusion layer composition layer. In the context of the invention, the liquid is preferably the solvent(s) used in the diffusion layer composition layer, and the drying preferably takes place at an elevated temperature. The diffusion layer is preferably obtained, and the manufacturing process preferably comprises further steps as explained below.

[0171] Preferably, the diffusion layer is dried by drying the diffusion layer composition layer at 80 °C and 50 mbar or, even more preferably, at 80 °C and 1 bar.

[0172] The substrate is preferably formed by a rigid material or a film. In particular, the substrate is preferably glass, polytetrafluoroethylene, ethylene tetrafluoroethylene, or aluminum, which has been purified, for example, in a plasma furnace.

[0173] The diffusion layer obtained according to the invention can preferably be the water-retaining diffusion layer. Preferably, the diffusion layer – also as a water-retaining diffusion layer – can also be suitable and used for other applications.

[0174] The manufacturing process according to the invention is preferably designed to be transferable to a manufacturing process that is preferably carried out on a larger scale in order to preferably obtain a larger quantity or number of the diffusion layer.

[0175] For example, the manufacturing process can preferably be transferred to a roll-to-roll process, wherein the diffusion layer composition is applied continuously or section by section to a roll-to-roll substrate and the described manufacturing steps are carried out.

[0176] Preferably, the substrate is a membrane electrode assembly and the diffusion layer composition is applied directly to the membrane electrode assembly, preferably to an anode of the membrane electrode assembly, in the application step.

[0177] In particular, the substrate is preferably provided according to the catalyst-coated polymer electrolyte membrane described above, wherein the anode of the catalyst-coated polymer electrolyte membrane or another surface of an electrochemical system in which the diffusion layer according to the invention is provided as a substrate is particularly preferred. The diffusion layer is preferably obtained directly as a layer on the surface for intended use, wherein in particular a solvent of the diffusion layer composition can be used which is compatible with the coated surface.

[0178] Experimental studies have shown that, in the case of an acrylic component, polyethylene oxide or polyethylene glycol can be particularly preferably used as a pore medium in the diffusion layer composition. Since polyethylene oxide or polyethylene glycol is water-soluble, an aqueous solvent, particularly isopropanol, is especially suitable. In this preferred case, the diffusion layer composition is provided as an aqueous dispersion. The pore medium—unlike the polymer binder, which is intended to provide bonding within the diffusion layer during PEMFC operation—can be a water-soluble component. Specifically, the acrylic component used is an acrylic dispersion in water (not in solution form) that becomes stable after drying; that is, the acrylic particles of the acrylic component sinter together and create a cross-link within the diffusion layer.

[0179] The aqueous dispersion is particularly suitable for the direct application of the diffusion layer composition to the catalyst-coated polymer electrolyte membrane, since the aqueous dispersion is compatible with the components conventionally used for the catalyst-coated polymer electrolyte membrane and preferably does not dissolve or damage them. Furthermore, direct application to the catalyst-coated polymer electrolyte membrane potentially achieves a good bond between the diffusion layer and the catalyst-coated polymer electrolyte membrane, which has a particularly advantageous effect on the performance of the final PEMFC and also, in particular, on the water-retaining properties of the water-retaining diffusion layer used according to the invention, which is obtained by direct application.

[0180] Furthermore, the substrate is preferably, for example, a gas diffusion substrate, which is preferably a carbon paper with randomly oriented 6-10 pm carbon fibers, wherein the applied diffusion layer is preferably provided as a coated microporous layer on the gas diffusion substrate.

[0181] In particular, but preferably, the manufacturing process further comprises the steps: d) a washing step and e) a washing-drying step

[0182] In this context, the diffusion layer composition layer is particularly preferably dried by the drying step and is, for example, preferably present on the surface of the substrate to obtain a freestanding diffusion layer or as a diffusion layer. Alternatively, the diffusion layer composition layer or the diffusion layer is preferably present directly on the surface of the anode of the PEMFC or on a surface of an alternative electrochemical system or on the gas diffusion substrate. Furthermore, the diffusion layer composition layer is also preferably present as a freestanding diffusion layer.

[0183] According to the invention, in washing step d), the diffusion layer composition layer or the diffusion layer is preferably washed. For this purpose, a washing solution is preferably provided in a vessel or a basin, wherein the washing solution is preferably an aqueous solution and particularly preferably comprises deionized water and isopropanol in a preferred 50:50 ratio. Particularly preferably, the washing solution has a temperature between 70 °C and 100 °C, preferably 80 °C.

[0184] The washing step particularly favorably influences the microstructural properties of the final diffusion layer. For this purpose, a pore-enhancing agent is preferably provided in the diffusion layer composition or in the dried diffusion layer, which can impart the desired microstructural properties through a phase separation process in the washing step. The manufacturing process for the diffusion layer, particularly preferably, includes a delamination step of the diffusion layer from the substrate to obtain the free-standing diffusion layer, which is preferably carried out in the washing step.

[0185] The preferably freestanding, washed diffusion layer is then preferably transferred to a further substrate. Alternatively, it is preferably still present on the existing substrate. The manufacturing process preferably includes a washing-drying step e), in which the diffusion layer composition layer or the diffusion layer is preferably dried again from the washing solution. Drying is preferably carried out at a temperature of 120 °C - 150 °C. Particularly preferably, the diffusion layer can be detached from the substrate as a freestanding diffusion layer.

[0186] Preferably, the diffusion layer composition comprises an electrically conductive filler and a solvent, wherein the diffusion layer composition comprises a polybenzimidazole component as a polymer binder and preferably N-methyl-2-pyrrolidone, dimethylformamide and / or dimethylacetamide as an aprotic solvent, or the diffusion layer composition comprises an acrylic component as a polymer binder and preferably an aqueous solution, preferably water and / or isopropanol, wherein the diffusion layer composition is preferably obtained as an aqueous dispersion.

[0187] The manufacturing process is particularly preferably characterized by a composition of the diffusion layer composition, which preferably forms the basis for the final obtained diffusion layer and which, according to the invention, preferably comprises the electrically conductive filler and the solvent in addition to the fluorine-free polymer binder agent.

[0188] The diffusion layer composition preferably comprises polybenzimidazole as a polymer binder, enabling the electrically conductive filler, preferably a carbon-containing filler as described above, to be bound. Triton X100 is preferably suitable as a pore-forming agent, and the aprotic solvent is preferably used as a solvent to maintain the diffusion layer composition.

[0189] For acrylic components, polyethylene oxide or polyethylene glycol and an aqueous solvent, which may include water and isopropanol, are preferably suitable as pore mediums, whereby the diffusion layer composition can be provided as an aqueous dispersion.

[0190] As described above, polybenzimidazole preferentially results in an inhomogeneous binder distribution or porosity within the diffusion layer, whereas acrylic preferentially results in a homogeneous binder distribution or porosity.

[0191] Particularly preferably, the respective binder distribution is obtained in drying step c), wherein the polymer-binder agent preferably sinks within the diffusion layer composition layer to obtain the inhomogeneous binder distribution, or not to obtain the homogeneous binder distribution. According to the invention, a diffusion layer is obtainable by the described manufacturing process.

[0192] In particular, reference is preferably made to the above explanations regarding the structure and function of the diffusion layer, wherein, according to the invention, the diffusion layer preferably comprises in particular a fluorine-free polymer binder.

[0193] Furthermore, the diffusion layer is preferably essentially or exclusively fluorine-free, which preferably offers ecological advantages regardless of the intended application and the effects achieved.

[0194] The diffusion layer according to the invention can preferably be the water-retaining diffusion layer for the PEMFC. Preferably, the diffusion layer can be suitable and used for other applications.

[0195] Preferably, the diffusion layer is obtained as a directly coated diffusion layer on a membrane electrode unit, preferably an anode, by a preferred manufacturing process.

[0196] This refers in particular preferably to a diffusion layer composition as an aqueous dispersion with a water-soluble pore agent, such as polyethylene oxide or polyethylene glycol as described above, or a polymer binder agent suitable for obtaining an aqueous dispersion, such as acrylic components, for obtaining an aqueous system as a diffusion layer composition.

[0197] In this case, the diffusion layer composition can be applied directly to the catalyst-coated membrane electrode assembly, which is water-resistant due to the technology – water is produced during the operation of a PEMFC.

[0198] Preferably, the diffusion layer is configured as a water-retaining diffusion layer according to a preferred water-retaining diffusion layer as described above. Preferably, the diffusion layer has analogous polymer binders and microstructure properties or parameters.

[0199] For preferred quantities and numerical ranges, reference is made to the above descriptions of the preferred quantities and numerical ranges for the water-retaining diffusion layer. In the diffusion layer according to the invention, these are achieved with the preferred polymer binders, particularly preferably with polybenzimidazoles and / or the acrylic component.

[0200] According to the invention, the described diffusion layer is used as a water-retaining diffusion layer for an anode of a membrane electrode arrangement of a polymer electrolyte membrane fuel cell, wherein the membrane electrode arrangement comprises an ionomer matrix and preferably the ionomer matrix is ​​based at least partially or exclusively on hydrocarbon ionomers.

[0201] Reference is preferably made to the above descriptions of the PEMFC having the water-retaining diffusion layer.

[0202] Preferred embodiments of the invention are explained below by way of example with reference to the figures.

[0203] They show:

[0204] Figures 1a and 1b show a schematic diagram of a polymer electrolyte membrane fuel cell according to the invention. Figure 2 shows a schematic diagram of a manufacturing process according to the invention for obtaining a diffusion layer according to the invention.

[0205] Figures 3a-c Scanning electron microscope images of exemplary diffusion layers according to the invention and

[0206] Figure 3d shows a schematic representation of a binder distribution within a diffusion layer according to the invention.

[0207] Figure 4a, b Polarization curves of exemplary polymer electrolyte membrane fuel cells according to the invention and comparison polymer electrolyte membrane fuel cells

[0208] Figure 5 Table with exemplary water-retaining diffusion layers and current densities of the corresponding polymer electrolyte membrane fuel cells with the water-retaining diffusion layers

[0209] Figures 1a and 1b show schematic diagrams of a polymer electrolyte membrane fuel cell (PEMFC) 1 according to the invention, comprising a polymer electrolyte membrane 4 between two catalyst layers, an anode 3 and a cathode 5. A water-repellent diffusion layer 6a is provided on the cathode, and a water-retaining diffusion layer 2 according to the invention is provided on the anode. Gas flow plates 7a and 7b are also provided for gas supply, whereby, during intended operation of the PEMFC 1, hydrogen gas is supplied to the anode 3 and (atmospheric) oxygen to the cathode 5. In particular, Figure 1b shows a further water-repellent diffusion layer 6b on the diffusion layer 2.

[0210] With regard to material properties and microstructure properties as well as water-retaining properties of the water-retaining diffusion layer 2, particular reference is made to the descriptions and explanations of the PEMFC 1 according to the invention with the water-retaining diffusion layer in the preceding description.

[0211] During operation of the PEMFC 1, hydrogen is split into protons and electrons in the anode 3, with the protons being conducted through the polymer electrolyte membrane 4 to the cathode 5. The protons and electrons then react together with supplied (atmospheric) oxygen in the cathode 5 to form water, from which electrical power can be tapped in an external circuit.

[0212] The water-retaining properties according to the invention are achieved by the water-retaining diffusion layer 2 according to the invention on the anode 3, wherein the water that is present in the membrane electrode arrangement or that is generated in the cathode during the intended operation of the PEMFC 1 is retained at least in the anode.

[0213] This has a particularly beneficial effect on the performance of the PEMFC 1, as explained in more detail below using Figures 4a and 4b.

[0214] The improved water management, and in particular the suppression of the drying out of the anode 3, has a particularly beneficial effect on the performance, for example as measured by the polarization curve or by a current density at a certain voltage of the polarization curve of the PEMFC, since the proton conductivity within the PEMFC and in particular at least within the anode, as explained above, is improved.

[0215] Furthermore, flooding of the cathode is suppressed, since water is not only removed from the cathode by transport through the water-transporting diffusion layer 6a, but also preferentially retained on the side of the anode 3 due to the water-retaining properties of the water-retaining diffusion layer 2.

[0216] In this process, hydrocarbon ionomers are present in the catalyst layers and, in particular, in the polymer electrolyte membrane. These ionomers contribute to the proton conductivity of the membrane electrode assembly and, according to the invention, are combined as an ionomer matrix. In the exemplary embodiment, the ionomer matrix is ​​based on hydrocarbon ionomers. Their proton conductivity is particularly enhanced by the water-retaining properties of the water-retaining diffusion layer 2.

[0217] The water-retaining diffusion layer 2 comprises, depending on the manufacturing process, polyvinylidene fluoride, polybenzimidazole, or acrylic as a polymer binder, wherein the electrically conductive filler is a carbon-containing filler, graphite, or carbon black (for example, particles or flakes in the size range of 2–8 pm). The polymer binder and the electrically conductive filler are present in the water-retaining diffusion layer 2 in a specific ratio, the preferred ratios of which are essential to the invention and are described above.

[0218] The water-influencing properties are favored by the parameters according to the invention, such as polymer-binder concentration, thickness, contact angle, permeability and porosity as described above, with the diffusion layer having the preferred parameters in particular.

[0219] Typical thicknesses according to the embodiment of the invention are 10–25 pm for the polymer electrolyte membrane 4, 2–5 pm for the anode 3, and 5–12 pm for the cathode. The water-repellent diffusion layer 6a is typically 150–200 pm thick, while the water-retaining diffusion layer 2 is 15–50 pm thick. Particularly due to the small thickness of the anode 3 and the respective layers of the membrane-electrode unit, the water-retaining properties of the water-retaining diffusion layer 2, which retain at least the water in the anode 3, potentially also affect a polymer electrolyte membrane section adjacent to the anode 3 or the entire polymer electrolyte membrane, so that the ionomer matrix there also benefits from the water-retaining properties of the water-retaining diffusion layer 2 and / or from the humidification.

[0220] Since the water-retaining diffusion layer 2 has a lower roughness compared to the water-repellent diffusion layer 6a, the probability of damage to the catalyst-coated membrane, especially the anode 3 and the polymer electrolyte membrane 4, is reduced. This is because, if used on the anode 3, the water-repellent diffusion layer 6a could penetrate the anode 3 (which is thin compared to the cathode 5) via protruding carbon fibers, or even pass through it, potentially leading to a short circuit of the PEMFC 1 in the worst case scenario.

[0221] Since this risk is reduced, the water-retaining diffusion layer 2 provides a further indirect advantage, as potentially thinner and / or monolithic polymer electrolyte membranes 4 can be used, which preferably results in a potentially low high-frequency resistance of the PEMFC 1.

[0222] Figure 1b shows a PEMFC 1, which is analogous to the PEMFC 1 of Figure 1a, wherein, in addition, a further water-repellent diffusion layer 6b is provided between the gas flow field 7b and the water-retaining diffusion layer 2, which is preferably identical to the water-repellent diffusion layer 6a. In this embodiment as well, with the water-retaining diffusion layer 2 as an intermediate layer, the water-retaining diffusion layer 2 according to the invention achieves the advantageous effects during operation. In particular, the water-retaining diffusion layer 2 improves contact (physical, electrical, thermal, etc.) with the anode 3, protects the anode 3 from potential damage by the gas flow field 6b, and provides the water-retaining properties according to the invention.Since, in this embodiment as described below, an improvement in the performance of the PEMFC 1 is also achieved with the additional water-removing diffusion layer 6b, this can be cited to validate the advantageous effects of the water-retaining diffusion layer 2.

[0223] Furthermore, it becomes apparent that the water-retaining diffusion layer 2 can replace the water-transporting diffusion layer 6b, but the water-retaining diffusion layer 2 can also be provided as an intermediate layer between GDL 6b and anode 3.

[0224] Figure 2 shows a schematic representation of a manufacturing process according to the invention. First, a diffusion layer composition 21 is provided in step a). The diffusion layer composition 21 comprises raw materials, for example, the electrically conductive filler, the polymer binder, the pore-forming agent, and the solvent.

[0225] For the water-retaining diffusion layer, which comprises polyvinylidene fluoride as a polymer binder, SFG6L graphite powder, for example, is used as an electrically conductive filler, polyvinylidene fluoride pellets as the polymer binder, and N-methyl-2-pyrrolidone as the solvent. This mixture is then blended at 80 °C for approximately eight hours. The solution is subsequently enriched with N-methyl-2-pyrrolidone so that—regardless of the binder concentration—a solids content of 25 wt% is present in the diffusion layer composition 21.

[0226] For the water-retaining diffusion layer, which uses acrylic as a polymer binder, the described carbon-containing fillers can be used as electrically conductive fillers. Polyethylene glycol, polyethylene oxide, or Triton X100, for example, is used as a pore-forming agent, with water and / or isopropanol preferably being used as the solvent or dispersion agent, particularly in the case of polyethylene glycol or polyethylene oxide. The diffusion layer composition 21 can be obtained, in particular, using a similar process, preferably a high-shear mixer or an ultrasonic high-shear mixer. For the diffusion layer, which uses polybenzimidazole as a polymer binder, similar carbon-containing fillers can be used as electrically conductive fillers as for the polymer binders described above.In particular, dimethylformamide, dimethylacetamide and / or N-methyl-2-pyrrolidone can be used as solvents, and Triton X100 can be used as a pore-enhancing agent. The diffusion layer composition 21 can be obtained in a similar process, but preferably using a high-shear mixer or an ultrasonic high-shear mixer.

[0227] The diffusion layer composition 21 is applied to a substrate 8 in step b) of the application process using an application device 9. The application device 9 is a doctor blade device, but can also be a Mayer bar. The substrate 8 is made of polytetrafluoroethylene and has been previously purified, for example, in a plasma purification step. The substrate 8 can also be made of glass, ethylene tetrafluoroethylene copolymer, or aluminum foil. Furthermore, as described above, the substrate 8 can be a gas diffusion support.

[0228] In particular, the substrate 8 can also be an anode 3 of a catalyst-coated membrane, so that the diffusion layer can be obtained by directly applying the diffusion layer composition 21 to the catalyst-coated membrane. For example, the diffusion layer composition 21 is based on acrylic as a polymer binder, with polyethylene glycol or polyethylene oxide as a pore-enhancing agent and an aqueous solution as a solvent or dispersing agent to obtain the diffusion layer composition 21 as an aqueous dispersion.

[0229] The diffusion layer composition 21 is applied by the application device 9 with an application thickness between 50 and 250 pm and at an application temperature between 20 °C and 120 °C. The application thickness can be selected depending on the layer thickness of the final diffusion layer, which can be, for example, 15 to 50 pm. The application thickness is greater than the layer thickness because functional agents, especially solvents, escape during the drying process.

[0230] In drying step c), the diffusion layer composition 21 is dried to obtain the diffusion layer, for example at 80 °C and 50 mbar or at 100 °C and 1 mbar. If the substrate 8 is the catalyst-coated polymer electrolyte membrane, the manufacturing process can already be completed at this point, with the diffusion layer already present directly on the catalyst-coated polymer electrolyte membrane. However, a washing step d) is then carried out in Figure 2, in which the substrate 8, including the diffusion layer, is immersed in a washing solution 11, which is an aqueous solution, for example, deionized water and / or isopropanol. In the case of water, the temperature of the washing solution 11 is set to 80 °C, whereas in the case of a 50:50 water and isopropanol mixture, a temperature of 25 °C is set.

[0231] The diffusion layer is left in the washing solution for two minutes, for example, to obtain a washed diffusion layer 22. During this time, the washed diffusion layer 22 can be detached or delaminated from the substrate 8. Afterwards, the washed diffusion layer 22 can be removed from the washing solution and placed on the substrate 8 or another substrate 8 for the washing-drying step.

[0232] In washing and drying step e), the washed diffusion layer 22, which is now a free-standing diffusion layer detached from the substrate 8, is dried at a temperature of 120 °C to 150 °C to obtain the diffusion layer according to the invention without substrate 8. This is a water-retaining diffusion layer 2 according to the invention.

[0233] Figures 3a to 3c show scanning electron microscope images taken with a Tescan MAIA3 XMH electron microscope, of a front side 2a and a back side 2b of water-retaining diffusion layers 2 according to the invention. In Figure 3a, the diffusion layer has a homogeneous binder distribution, while in Figures 3b and 3c, the water-retaining diffusion layers 2 have an inhomogeneous binder distribution. It should be noted that Figure 3a, unlike Figures 3b and 3c, uses different scales: a scale of 10 pm was used to obtain the images in Figure 3a, and a scale of 50 pm was used to obtain the images in Figures 3b and 3c.

[0234] Further settings of the scanning electron microscope are familiar to those skilled in the art and can also be seen in Figures 3a to 3c. These include, for example, an accelerating voltage (HV) of 5 kV, a view field of 50 pm (Figure 3a) and 200 pm (Figures 3b and 3c), and a working distance (WD) of 7.12 mm (Figure 3a) and 10 mm (Figures 3b and 3c).

[0235] The water-retaining diffusion layer 2 in Figure 3a was obtained by the manufacturing process described above according to the invention, wherein the polymer binder and graphite powder (for example, commercially available SFG6L) dissolved in an aqueous solution were used as raw materials. The polymer binder was provided in the water-retaining diffusion layer 2 in a preferred binder concentration as described above, relative to the electrically conductive filler. The binder concentration was 40% PAA, with the dry film containing approximately 40% PAA binder and approximately 60% graphite.

[0236] In particular, it can be seen from Figure 3a that the front surface 2a of the water-retaining diffusion layer 2 with the polymer binder acrylic appears visually identical under the scanning electron microscope, especially with regard to surface structure and porosity. This, in particular, suggests a homogeneous binder distribution of the polymer binder acrylic in the water-retaining diffusion layer 2.

[0237] Figures 3b and 3c, on the other hand, show water-retaining diffusion layers 2, which were obtained by the manufacturing process described above according to the invention using a solvent selected from the group consisting of dimethylformamide, dimethylacetamide and / or N-methyl-2-pyrrolidone. Polybenzimidazole was used as the polymer binder with a binder concentration of 40%, with the dry film containing approximately 40% PBI binder and approximately 60% graphite.

[0238] In particular, Figures 3b and 3c, in contrast to Figure 3a, show clear differences in surface structure and porosity between the front side 2a and back side 2b of the respective water-retaining diffusion layer 2. This suggests, in particular, an inhomogeneous binder distribution that increases gradient-wise towards the front side 2a of the water-retaining diffusion layer.

[0239] This is illustrated schematically in particular by Figure 3c, which shows a water-retaining diffusion layer with an inhomogeneously distributed polymer-binder agent 2p, leading to a finer pore structure or to a densification of the carbon fibers 2f (as an electrically conductive filler). The water-retaining diffusion layer 2 is located on a substrate 8, after which, for example in drying step c) of the manufacturing process according to the invention, the polymer-binder agent 2p sinks due to gravity in the direction of the arrow and densifies towards the front 2a of the water-retaining diffusion layer.

[0240] In particular, the water-retaining diffusion layer 2 according to the invention is arranged with its front surface 2a on the anode 3, since this surface has an advantageously lower roughness due to its contact with the substrate 8. Furthermore, refining a coarser pore structure to a finer pore structure on the front surface facing the anode 3 leads to an advantageous homogeneous distribution of the supplied hydrogen gas to the anode 3.

[0241] Furthermore, slight differences in the apparent surface structure and porosity between the respective water-retaining diffusion layers 2 can be seen in Figures 3b and 3c. Different pore-enhancing agents were used in the manufacturing process for the water-retaining diffusion layers 2 in Figures 3b and 3c, with polyethylene glycol or polyethylene oxide being used for the water-retaining diffusion layer 2 in Figure 3b and Triton X100 being used for the water-retaining diffusion layer 2 in Figure 3c.

[0242] The explanations for Figures 3b and 3c with the polymer binder polybenzimidazole, in particular the inhomogeneity of the binder distribution, can be understood in a similar way for the use of polyvinylidene fluoride as a polymer binder, with which an inhomogeneous binder distribution can also be obtained.

[0243] Figure 4a and Figure 4b show polarization curves, with polarization curves being considered one of the most common methods for characterizing a fuel cell.

[0244] Reading and interpreting the polarization curve is known to those skilled in the art. Specifically, the polarization curve is recorded galvanostatically by drawing constant currents from the fuel cell and recording the system's voltage response. The data were obtained using a Scribner 850e test station.

[0245] In general, the PEMFC 1 exhibits better performance the higher the voltage remains at increasing current flow (electrical load). The load is specified relative to the cell area as current density, and power density is the product of cell voltage and current density and a measure of the achievable power of the PEMFC 1.

[0246] Figure 4a shows three measurement curves per diagram, where the measurement curves kl, k2, kl2 and k22 relate to the water-retaining diffusion layer 2 according to the invention, which is equipped with an acrylic component polymer binder agent (see Figure 3a and associated description) with a binder concentration of 40%.

[0247] The measurement curves rl and r2 serve for comparison, whereby these curves rl and r2 can be assigned to a reference polymer electrolyte membrane fuel cell that uses water-repellent diffusion layers 6a, 6b from the prior art. In this case, a water-repellent diffusion layer from the prior art was used on the anode 3 instead of the water-retaining diffusion layer 2 according to the invention, as was the case for the water-repellent diffusion layer 6a on the cathode.

[0248] The measurement curves kl and k2, on the other hand, can be assigned to a PEMFC 1 according to the invention, in which the water-retaining diffusion layer 2 according to the invention is used on the anode 3. Accordingly, this is a PEMFC 1 that can be schematically compared to the PEMFC 1 from Figure 1a.

[0249] In contrast, the measurement curves kl2 and k22 describe PEMFCs 1 according to the invention, in which an additional water-repellent diffusion layer 6b from the prior art is arranged on the diffusion layer 2. Accordingly, this is a PEMFC 1 that can be understood schematically as the PEMFC 1 from Figure 1b.

[0250] To ensure that the results of the PEMFCs 1 according to the invention were comparable with the reference PEMFC, only the diffusion layer configuration on the anode 3 was modified. Apart from this, identical catalyst-coated membranes, including polymer electrolyte membrane 4, anode 3, and cathode 5, as well as a water-repellent GDL 6a for the cathode, were used in the same measurement setup for all measurement curves. Measurements were also performed under various operating conditions. For the left-hand diagrams, an operating temperature of 95 °C, a relative humidity of 35% for the anode, and a relative humidity of 80% for the cathode were set. In the right-hand diagram, the operating temperature was set to 105 °C and the relative humidity of the cathode to 60%.

[0251] The polarization curves show that the PEMFCs 1 according to the invention exhibit better performance than the reference PEMFC, since the voltage is higher relative to the current density for all current densities shown. This performance improvement is attributed to the water-retaining properties of the water-retaining diffusion layer 2 according to the invention, as this can be considered a key distinguishing feature. In particular, a performance improvement is also evident when the water-retaining diffusion layer 2 is provided only as an intermediate layer between the further water-carrying diffusion layer 6b and the anode 3.

[0252] Particularly at high current densities, improved performance is observed, which can be attributed to the fact that more water is produced within the PEMFC 1 or the cathode 5 under high load, thereby allowing the properties of the PEMFC 1 according to the invention, or the water-retaining properties of the water-retaining diffusion layer 2, to develop more fully. Furthermore, improved performance is evident under different operating conditions (diagram left and diagram right).

[0253] Figure 4b can be understood analogously, wherein in this case polybenzimidazole was used as a polymer binder agent with a binder concentration of 40% for the diffusion layer 2 according to the invention, and the measurement curve k3 relates to a PEMFC 1 according to the invention in which the water-retaining diffusion layer 2 according to the invention (and not the further water-transporting diffusion layer 6b) was used (see Figure 1a).

[0254] Analogous performance improvements of the polarization curve k3 can also be seen here, with this diffusion layer 2 according to the invention achieving the best performance and, for example, for a current density of 2.0 A / cm². 2 or 2000 mA / cm 2 exhibits a voltage value above 0.6V (cf. k2 with below 0.6V at 2000 mA / cm) 2 in Figure 4a).

[0255] Figure 5 shows a table relating to further preferred PEMFCs 1 according to the invention with water-retaining diffusion layers 2 according to the invention. In particular, the PEMFC 1 is based on hydrocarbon ionomers. The table refers to a further series of tests in which comparable experiments, as described above in connection with the polarization curves, were carried out and further particularly preferred embodiments for water-retaining diffusion layers 2 were determined with respect to the polymer binder used, the binder concentration, the porosity, the thickness and, in particular, the Gurley permeability.

[0256] Performance was measured by current density in mA / cm². 2 The voltage specified is 0.6 V for the PEMFC 1.

[0257] The current density values ​​represent test measurements of the PEMFCs 1 for dry operating conditions, with measurements performed under two different operating ranges. For operating range 1, a relative humidity of 35% at the anode and 80% at the cathode was set at an operating temperature of 95 °C. For operating range 2, a relative humidity of 35% at the anode and 60% at the cathode was set at an operating temperature of 105 °C.

[0258] In addition, in the case of the polymer binder agents polybenzimidazole and the acrylic component PEMFCs 1, PEMFCs 1 were tested according to the embodiment in Figure 1a, in which the water-retaining diffusion layer 2 replaces the GDL from the prior art, and according to the embodiment in Figure 1b, in which the water-retaining diffusion layer 2 is provided as an intermediate layer.

[0259] In this test series as well, a performance increase was observed in terms of current density compared to the reference PEMFC. The reference PEMFC was provided analogously to PEMFC 1, where it was configured as follows:

[0260] In each case, a water-repellent diffusion layer was applied. In this test series, a current density of 1425 mA / cm² was obtained for the comparison PEMFC in operating range 1. 2 and for operating range 2 a current density of 1150 mA / cm² 2 .

[0261] Reference symbol list

[0262] 1 Polymer electrolyte membrane fuel cell

[0263] 2 Water-retaining diffusion layer

[0264] 21 Diffusion layer - composition layer

[0265] 3 Anode

[0266] 4 Polymer electrolyte membrane

[0267] 5 Cathode

[0268] 6a, b Water-transporting diffusion layers

[0269] 7a, b Gasflussfeld

[0270] 8 Substrat

Claims

Patent claims 1. Polymer electrolyte membrane fuel cell (1) comprising a membrane electrode arrangement with an anode (3), a cathode (5) and a polymer electrolyte located between the anode (3) and cathode (5) Membrane (4), wherein a water-retaining diffusion layer (2) is provided on the anode (3) and a water-removing diffusion layer (6a) is provided on the cathode (5), which are configured such that, during the intended operation of the fuel cell (1), water generated in the cathode (5) is removed by the water-removing diffusion layer (6a), and a gas supply of hydrogen to the anode (3) takes place through the water-retaining diffusion layer (2), wherein the hydrogen is split into protons and electrons within the anode (3) and the protons are directed in a reaction direction by an ionomer matrix contained in the membrane-electrode assembly, and the water-retaining diffusion layer (2) has water-retaining properties such that, during the intended operation of the fuel cell (1), water is retained at least in the anode (3),to provide humidification of the ionomer matrix and an associated improved proton conductivity, such that the fuel cell (1), compared to having a diffusion layer on the anode (3) that is identical to the water-repellent diffusion layer (6a) of the cathode (5), exhibits at least one performance improvement as measured on a polarization curve, and preferably the performance improvement increases with increasing current density, wherein the ionomer matrix contains hydrocarbon ionomers.

2. Fuel cell according to claim 1, wherein the ionomer matrix is ​​based at least partially or completely on hydrocarbon ionomers.

3. Fuel cell (1) according to claim 1 or 2, wherein the water-retaining diffusion layer (2) comprises a polymer binder and the polymer binder comprises polyvinylidene fluoride.

4. Fuel cell (1) according to claim 1 or 2, wherein the water-retaining diffusion layer (2) comprises a polymer binder and the polymer binder is a fluorine-free polymer binder, preferably polyolefins, polyaromatics or other hydrocarbon-based polymers, particularly preferably polybenzimidazole and / or an acrylic component.

5. Fuel cell (1) according to claim 4, wherein the water-retaining diffusion layer (2) comprises an electrically conductive filler, preferably a carbon-containing filler, wherein the binder distribution of the polymer binder agent in the water-retaining diffusion layer (2) is homogeneous, wherein the polymer binder agent is preferably based on an acrylic component.

6. Fuel cell (1) according to claims 3 and 4, wherein the water-retaining diffusion layer comprises an electrically conductive filler, preferably a carbon-containing filler, wherein a binder distribution of the polymer-binder agent within the water-retaining diffusion layer (2) follows a gradient, wherein the binder distribution of the polymer-binder agent particularly preferably increases towards the anode (3), wherein the polymer-binder agent is preferably based on polyvinylidene fluoride or polybenzimidazole 7. Fuel cell (1) according to claim 3, wherein the water-retaining diffusion layer (2) comprises polyvinylidene fluoride as a polymer binder and has a layer thickness between 10 pm and 80 pm, preferably between 15 pm and 50 pm, particularly preferably between 18 pm and 30 pm, particularly preferably thinner than 30 pm, particularly preferably 20 pm, and / or a binder concentration between 10% and 60%, preferably between 20% and 45%, particularly preferably between 35% and 40%, particularly preferably 30% or 40%, and / or a contact angle between 50° and 100°, preferably between 60° and 90°, particularly preferably between 60° and 85°, particularly preferably between 60° and 70°, particularly preferably between 65° and 70°, and / or a Gurley permeability between 2 s and 100 s, preferably between 2 and 50 s, particularly preferably between 2 s and 10 s and / or a porosity measured with a gravimetric measurement, between 50% and 90%, preferably between 70% and 86%, particularly preferably between 80% and 85%.

8. Fuel cell (1) according to claim 4, wherein the water-retaining diffusion layer (2) comprises polybenzimidazole as a polymer binder and has a thickness between 10 pm and 50 pm, preferably 15 pm and 30 pm, particularly preferably 20 pm, and / or a binder concentration between 30% and 50%, preferably between 35% and 45%, particularly preferably 40%, and / or a Gurley permeability between 30 s and 110 s, preferably between 35 s and 60 s, particularly preferably between 45 s and 55 s, and / or a porosity, measured by gravimetric measurement, between 45% and 72%, preferably between 50% and 65%, particularly preferably between 50% and 58%, particularly preferably 56% or 57%.or the water-retaining diffusion layer (2) comprises acrylic components as a polymer-binder agent and has a thickness between 30 pm and 80 pm, preferably 40 pm and 60 pm, particularly preferably 50 pm, and / or a binder concentration between 30% and 50%, preferably between 35% and 45%, particularly preferably 40%, and / or a Gurley permeability between 1 s and 20 s, preferably between 2 s and 15 s, and / or a porosity, measured by gravimetric measurement, between 60% and 80%, preferably between 65% and 75%, particularly preferably 69% or 73%.

9. Fuel cell (1) according to one of claims 1 to 8, wherein the membrane electrode assembly comprises a gas diffusion layer (6b) provided on the water-holding diffusion layer (2) and the gas diffusion layer (6b) is preferably a diffusion layer identical to the water-removing diffusion layer (6a).

10. Fuel cell (1) according to any one of claims 1 to 9, wherein the fuel cell (1) is operated under dry operating conditions, preferably at a relative humidity between 20% and 60%, particularly preferably 30% at the anode and / or between 60% and 85%, preferably 60% or 80% at the cathode and / or preferably at a temperature between 85°C and 130°C, particularly preferably between 90°C and 110°C, particularly preferably at 95°C or 105°C or 110°C.

11. Manufacturing process for a diffusion layer for a membrane electrode assembly, a fuel cell, a hydrogen fuel cell, an electrolyzer or a water electrolyzer comprising the steps of: a) providing a diffusion layer composition (21) comprising a fluorine-free polymer binder, preferably polyolefins, polyaromatics or other hydrocarbon-based polymers, particularly preferably polybenzimidazole and / or an acrylic component, b) applying the diffusion layer composition (21) to a substrate (8), c) drying the applied diffusion layer composition (21).

12. Manufacturing method according to claim 11, wherein the substrate (8) is a membrane electrode assembly and the diffusion layer composition (21) is applied directly to the membrane electrode assembly, preferably to an anode (3) of the membrane electrode assembly, in the application step.

13. Manufacturing process according to claim 11, wherein the manufacturing process further comprises the steps: d) a washing step and e) a washing-drying step.

14. Manufacturing process according to claim 13, wherein the diffusion layer composition (21) comprises an electrically conductive filler and a solvent, wherein the diffusion layer composition (21) comprises polybenzimidazole as a polymer binder and preferably comprises N-methyl-2-pyrrolidone, dimethylformamide and / or dimethylacetamide as an aprotic solvent.

15. Manufacturing process according to claim 12 or 13, wherein the diffusion layer composition (21) comprises an electrically conductive filler and a solvent, the diffusion layer composition (21) comprises an acrylic component as a polymer binder and an aqueous solution preferably water and / or isopropanol as a solvent, wherein the diffusion layer composition (21) is preferably obtained as an aqueous dispersion.

16. Diffusion layer, wherein the diffusion layer is obtainable by a manufacturing process according to any one of claims 11 to 15.

17. Diffusion layer according to claim 16, wherein the diffusion layer is obtainable as a freestanding diffusion layer by a manufacturing process according to claims 11 to 15.

18. Diffusion layer according to claim 16, wherein the diffusion layer is obtainable as a directly coated diffusion layer on a membrane electrode assembly, preferably an anode (3), by a manufacturing process according to claim 12.

19. Diffusion layer according to one of claims 16 to 18, wherein the diffusion layer is configured as a water-retaining diffusion layer according to one of claims 7 or 8.

20. Use of a diffusion layer according to one of claims 17 to 19 as a water-retaining diffusion layer (2) for an anode of a membrane electrode arrangement of a polymer electrolyte membrane fuel cell (1), wherein the membrane electrode arrangement comprises an ionomer matrix and preferably the ionomer matrix is ​​based at least partially or exclusively on hydrocarbon ionomers.

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