Semipermeable membrane for membrane humidifier

Abstract

The invention relates to a semipermeable membrane (10), more particularly for use in membrane humidifiers for fuel cell systems, comprising a) a carrier layer (12) comprising a composite material, comprising at least one plastic and at least one silicon-containing porous filler embedded in the plastic, and b) a cover layer (14) arranged on the carrier layer (12) and comprising at least one organosilicon compound.

Description

Semipermeable membrane for membrane humidifiers Description The invention relates to a semipermeable membrane and a membrane humidifier comprising a corresponding semipermeable membrane, a fuel cell system comprising a corresponding membrane humidifier and a method for producing a corresponding semipermeable membrane. The use of fuel cells in automotive technology has been considered a promising option for reducing dependence on fossil fuels such as petroleum for several years. Fuel cells represent one of the most important alternatives to the use of batteries, such as lithium-ion batteries. Compared to battery technology, fuel cell technology offers specific advantages, particularly with regard to the practical handling of a fuel compared to electrochemical storage, its storage potential and potentially short fuel refill times, which avoid long charging times, as well as the potential for using the existing pipeline and storage infrastructure of the conventional fuel supply system instead of having to build a battery charging infrastructure.These advantages are particularly evident in applications where lengthy charging processes must be avoided, particularly in aviation and heavily used commercial vehicle fleets. In fuel cells, oxygen reacts with a fuel, such as hydrogen, methane, or methanol, to form water and, if necessary, other reaction products under controlled reaction conditions. The reaction steps of the redox reaction occur spatially separated. The fuel cell consists of an anode and a cathode, separated from each other by an electrolyte, such as an electrolyte membrane. The reactants are usually fed continuously into the fuel cell during operation. Fuel cells, especially polymer electrolyte membrane fuel cells (PEM fuel cells), place high demands on the purity of the process gases used and on the optimal humidity level to prevent the electrolyte membrane from drying out, even at high operating temperatures. This frequently requires complex fluid flow management and control and the use of high-performance filter technology. To ensure sufficient humidification of the electrolyte membrane even at high operating temperatures, it is generally advantageous to humidify the supplied process gas, particularly the process gas on the cathode side. This moisture can be added to the process gas via a suitable humidification device from a reservoir, for example, moist air. Since water is formed during operation of the fuel cell, the exhaust air from the fuel cell can also serve as a reservoir for the moisture. During humidification, the aim is generally to ensure that, apart from the moisture exchange, there is no contact between the process gas and the reservoir, or at least only a minimal contact, so that, for example, contamination of the process gas with the exhaust air can be avoided. Theoretically, various humidifiers can be used as humidification devices. However, membrane humidifiers, which use one or more moisture-permeable membranes, are generally considered to be particularly efficient and advantageous. Hollow fiber membrane that is permeable to water vapor, but prevents any further mass transfer as far as possible. If a moist and a dry gas stream are separated by such a semipermeable membrane, a diffusion-driven passage of water from the moist gas stream to the dry gas stream takes place due to the different partial pressures of the water in both gas streams. PEM fuel cell systems, their structure and the use of membrane humidifiers in these PEM fuel cell systems are well known to those skilled in the art from the prior art and are described, for example, in DE 102015202089 A1, DE 102015224202 A1 or DE 102016224478 A1. While a minor penetration of other components of the reservoir fluid, i.e., in addition to the desired penetration of moisture, is generally considered unproblematic for applications in building air management, for example, it is often desirable for fuel cell applications to prevent as reliably as possible any mixing of the supply and exhaust air streams beyond the moisture exchange. In addition to structural aspects of the membrane humidifiers, this is usually determined by the properties of the membranes used, so there is a constant need to further improve these membranes. In the field of building air management, membrane humidifiers have been proposed that use a semipermeable membrane as a water transport membrane. The membrane comprises a porous, silica-loaded polyethylene substrate with a surface coating comprising a cross-linked, water-permeable, non-ionic polyurethane-polyether polymer, as disclosed in EP 2435171 B1. According to EP 2435171 B1, these water transport membranes allow high water permeability (vapor and liquid), while simultaneously exhibiting little or no permeability of gas and contaminants. The inventors tested the water transport membranes known from EP 2435171 B1 in the field of building air management and evaluated their performance for use in membrane humidifiers for fuel cells. They found that the water transport membranes known from the prior art could not adequately meet the application-specific requirements in all areas. The inventors suspect that this was at least partly due to the increased temperatures and / or the air humidity in the exhaust air stream from fuel cells that are higher than those in building air management and / or the air humidity that occurs in practice. In addition to the achievable water permeation and the observed breakthrough of exhaust air or contaminants under process conditions, the durability of the water transport membranes in fuel cell systems was found to be inadequate, especially at elevated temperatures and / orhigh air humidity, in order to ensure sufficient durability, particularly under the mechanical stresses expected in vehicle use. This durability, which the inventors consider insufficient for fuel cell applications, primarily affects the bond strength between the substrate and the polyurethane-polyether polymer coating. This effect of potential detachment of the coating upon prolonged exposure to warm and humid environments, which may be acceptable for applications in building air management, is also described in EP 2435171 B1 (see paragraph ).

[0063] ) is already clearly documented. The primary object of the present invention was to eliminate or at least mitigate the disadvantages of the prior art described above. In particular, it was an object of the present invention to provide a semipermeable membrane that has excellent water permeability and simultaneously prevents the passage of air and other contaminants through the membrane as far as possible. The semipermeable membrane to be specified should have high mechanical strength and improved durability compared to the prior art, especially at elevated temperatures or high Humidities. Furthermore, the specified semipermeable membrane should be as time- and cost-efficient to manufacture as possible, while ideally requiring as few harmful and / or environmentally harmful materials as possible during production, in particular avoiding the use of perfluoro compounds. It was a supplementary object of the present invention to provide a process for producing corresponding semipermeable membranes. It was a secondary object of the present invention to provide a high-performance membrane humidifier for use in fuel cell systems and a corresponding fuel cell system. The inventors of the present invention have now recognized that the objects described above can surprisingly be achieved by a semipermeable membrane which contains a carrier layer comprising a specific composite material and a cover layer arranged on the carrier layer and comprising an organosilicon compound, as defined in the claims, thereby obtaining a semipermeable membrane which is also advantageously excellently suited for numerous other applications in which a moisture and / or enthalpy transfer from a humid compartment to a drier compartment is important. The above-mentioned objects are thus achieved by the subject matter of the invention as defined in the claims. Preferred embodiments of the invention emerge from the subclaims and the following statements. Such embodiments, which are designated as preferred below, are combined in particularly preferred embodiments with features of other embodiments designated as preferred. Combinations of two or more of the embodiments designated as particularly preferred below are therefore particularly preferred. Likewise preferred are embodiments in which one of the embodiments designated as preferred to any extent A designated feature of one embodiment is combined with one or more further features of other embodiments designated as preferred to some extent. Features of preferred membrane humidifiers, methods, and fuel cell systems result from the features of preferred semipermeable membranes. The invention relates in particular to a semipermeable membrane, in particular for use in membrane humidifiers for fuel cell systems, comprising: a) a carrier layer comprising a composite material, comprising at least one plastic and at least one silicon-containing porous filler embedded in the plastic, and b) a cover layer arranged on the carrier layer, comprising at least one organosilicon compound. The semipermeable membranes according to the invention are particularly suitable for installation in a membrane humidifier and, in this form, for use in a fuel cell system, particularly in the fluid line system. This suitability results in particular from the advantageous specific properties of the semipermeable membranes according to the invention, particularly with regard to their excellent durability even at elevated temperatures of 80°C or more, or the resulting increased air humidity levels that occur in the exhaust air stream of fuel cell systems.Furthermore, the semipermeable membranes according to the invention are also excellently suited for other applications due to their advantageous water permeability and the reliable suppression of the passage of air and other contaminants, for example in modules for the moisture and / or enthalpy transfer from a gas with a high partial pressure of water to a drier gas. Compared to the prior art, as disclosed, for example, in EP 2435171 B1, the advantageous properties, in particular the good durability and the excellent bond strength between the layers, are achieved even without additional chemical cross-linking. the favorable transport properties are achieved without the need for additional processing steps after coating, in particular in comparison with PVA-based coatings, whereby in particular no deliberate defects in the cover layer or deliberate detachments have to be created, so that in the production of semipermeable membranes according to the invention, a continuous, highly functional and durable cover layer can be obtained with just one coating step and one coating agent. In accordance with the expert understanding, the term "semipermeable" means that the semipermeable membrane according to the invention does not have the same permeability for all substances, whereby the permeability for individual molecules and substances, optionally depending on their state of aggregation and / or their particle shape, can also be so low that essentially no passage of these components through the semipermeable membrane can occur. The person skilled in the art understands that the semipermeable membrane according to the invention is a water transport membrane that is accordingly permeable at least to gaseous water, preferably to gaseous and condensed water. In other words, it is thus a semipermeable membrane according to the invention, wherein the semipermeable membrane is permeable to water. Since the task of the semipermeable membrane according to the invention is also to prevent, in addition to water transport, the passage of other components, in particular air and other gases, but also particulate contaminants, as best as possible, the permeability of the semipermeable membrane according to the invention for these components is reduced. A semipermeable membrane according to the invention is preferred, wherein the semipermeable membrane allows an air passage of 2.0 cm as a result of a pressure difference of 20 kPa applied between the side surfaces of the semipermeable membrane. 3 / (cm 2 min) or less, preferably 1 .0 cm 3 / (cm 2 min) or less, preferably 0.5 cm 3 / (cm 2 min) or less. Particularly preferred is a semipermeable membrane according to the invention, wherein the semipermeable membrane is substantially impermeable to air, wherein the A semipermeable membrane is essentially impermeable, particularly to oxygen, nitrogen, carbon dioxide, and mixtures of these gases. Accordingly, a semipermeable membrane that is essentially impermeable to particulate contaminants is also particularly preferred. In accordance with the expert understanding, a membrane is a thin, flat structure whose extension in the XY plane is significantly greater than the extension in the Z direction, i.e. a flat structure whose length and width are significantly greater than its thickness. The semipermeable membrane according to the invention comprises a support layer. This support layer comprises a composite material. According to the expert understanding, a composite material is a material consisting of two or more components that are bonded together and that together result in a material that has different physicochemical properties than the isolated components. Such composite materials are sometimes also referred to as composite materials. According to the invention, the carrier layer comprises the composite material, so that the carrier layer can also be formed at least partially from other materials. However, those skilled in the art will understand that it is preferable for the carrier layer not to consist only partially of the composite material, but rather to consist at least predominantly or, preferably, essentially entirely of the composite material. An embodiment in which the carrier layer is formed essentially entirely of the composite material is, in the opinion of the inventors, preferred for essentially all applications. The composite material itself is formed from at least one plastic and at least one silicon-containing porous filler, which is embedded in the plastic, so that the silicon-containing porous filler is at least partially dispersed in the plastic. Relevant for almost all embodiments is a semipermeable membrane according to the invention, wherein the silicon-containing porous filler is present in the composite material in a plurality of particles dispersed in the plastic. Even if an inhomogeneous distribution of the silicon-containing porous filler in the plastic matrix would be conceivable, a semipermeable membrane according to the invention is preferred, wherein the silicon-containing porous filler in the composite material is present substantially uniformly distributed in the plastic. The plastic that forms the carrier matrix for the embedded silicon-containing porous filler in the composite material can, in principle, be any common polymeric material, since the chemical nature of the plastic matrix itself is not decisive for the application-relevant properties and the choice of the plastic in practice will probably be made primarily from the point of view of the desired mechanical properties. According to the invention, the filler bonded to the plastic contains silicon. According to the expert's understanding, this means that the filler consists of a chemical compound that includes silicon atoms in its molecular and / or crystal structure. However, the filler used in the composite material not only contains silicon, but is also porous, which means that the ratio of the void volume of the voids inside the porous filler to the total volume of the porous filler is greater than zero, which expresses that the porous filler, or the particles of the porous filler, have voids inside that can be interconnected. The so-called open porosity or useful porosity results from the combined volume of the voids that are connected to each other and to the environment. The skilled person understands that for the silicon-containing porous fillers used according to the invention, the porosity should be at least partially open porosity, as is the case, for example, with many zeolite materials and also with numerous industrially used fillers made of amorphous silicon dioxide (sometimes also referred to as "silica").Relevant for almost all embodiments is a semipermeable membrane according to the invention, wherein the silicon-containing porous filler is microporous and / or mesoporous and / or macroporous. A semipermeable membrane according to the invention is preferred, wherein the silicon-containing porous filler is mesoporous and / or macroporous, wherein the silicon-containing porous filler is particularly preferably designed as a hierarchically porous filler, ie has a hierarchically structured pore system in which, for example, mesopores are located at the edges of macropores. Those skilled in the art understand that the porosity of the silicon-containing porous filler determines or promotes the porosity of the composite material, which in turn determines or promotes water permeability. Therefore, a semipermeable membrane according to the invention is relevant for almost all embodiments, wherein the composite material is a porous composite material, preferably a microporous composite material. A semipermeable membrane according to the invention is preferred, wherein the composite material has a porosity in the range of 30 to 90%, preferably in the range of 40 to 80%, particularly preferably in the range of 50 to 60%. In addition to the carrier layer, the semipermeable membrane according to the invention also comprises a cover layer. This cover layer is arranged on the carrier layer and at least partially covers it, so that the cover layer can be understood as a coating of the carrier layer. Unlike in the prior art, this cover layer comprises at least one organosilicon compound. In accordance with the expert understanding, organosilicon compounds, which are also referred to as organosilicon compounds, are compounds that comprise at least silicon and carbon atoms, where the carbon can be bonded to the silicon either directly or via a heteroatom, in particular oxygen, as is the case, for example, in siloxanes and polysiloxanes.Corresponding starting materials suitable for the production of topcoats are commercially available from numerous manufacturers, for example, Evonik, Hubei, or Wacker Chemie. Some are also offered for other purposes, such as adhesion promoters. Examples of commercial products include Evonik products sold under the trade names Dynasylan, for example, in the versions SIVO 110, SIVO 418, SIVO 850, VPS SIVO 608, Hydrosil 2909, or Triamo. Although it is in principle possible to use additional layers in semipermeable membranes according to the invention, in view of the performance properties and the achievable thickness of the membranes, it is preferred in most cases to form the semipermeable membrane according to the invention from only the two layers described above. Accordingly, a semipermeable membrane according to the invention is preferred, wherein the semipermeable membrane consists of the carrier layer and the cover layer. As explained above, the composite material can comprise a wide range of possible plastics, allowing the skilled person to select the plastic used, particularly with regard to the mechanical requirements placed on the semipermeable membrane and the available materials. However, the inventors have succeeded in identifying plastics that, due to their processing properties and mechanical properties, are particularly well suited for use in the semipermeable membranes according to the invention.A semipermeable membrane according to the invention is preferred, wherein the plastic is selected from the group consisting of thermoplastics, preferably selected from the group consisting of polyvinyl chlorides and polyolefins, particularly preferably selected from the group consisting of polyolefins, in particular polyethylene and polypropylene, wherein the plastic is very particularly preferably a polyethylene, in particular an ultra-high molecular weight polyethylene. In the same way, the inventors have succeeded in identifying particularly advantageous silicon-containing porous fillers with which composite materials can be obtained that are particularly advantageous when used in semipermeable membranes according to the invention. These silicon-containing porous fillers can generally be dispersed particularly well in conventional plastics, are available with suitable porosities, and show a beneficial interaction with the organosilicon compounds of the cover layer. A semipermeable membrane according to the invention is preferred, wherein the silicon-containing porous filler is selected from the group consisting of silicon-aluminum-phosphorus-oxygen compounds, silicon-containing metal-organic frameworks, zeolites, and amorphous Silicon dioxide, preferably selected from the group consisting of amorphous silicon dioxide, preferably selected from the group consisting of zeolites and amorphous silicon dioxide, particularly preferably selected from the group consisting of amorphous silicon dioxide, in particular aerogels, precipitated silicon dioxide and pyrogenic silicon dioxide, very particularly preferably selected from the group consisting of aerogels and precipitated silicon dioxide, in particular precipitated silicon dioxide. Those skilled in the art will understand that silicon-containing porous fillers, in particular the silicon-containing porous fillers described above as preferred, are generally hygroscopic and can therefore contribute beyond porosity to allowing water to pass through the composite material, or to the composite material having an advantageously high water absorption capacity. Accordingly, a semipermeable membrane according to the invention is also relevant for almost all embodiments, wherein the silicon-containing porous filler is a desiccant. The inventors have succeeded in identifying particularly advantageous mass fractions in the composite material for the two aforementioned components of the composite material. A semipermeable membrane according to the invention is preferred, wherein the combined mass fraction of the silicon-containing porous fillers in the composite material is in the range of 25 to 85%, preferably in the range of 45 to 80%, particularly preferably in the range of 60 to 75%, based on the mass of the composite material. Additionally or alternatively, a semipermeable membrane according to the invention is preferred, wherein the combined mass fraction of the plastics in the composite material is in the range of 15 to 75%, preferably in the range of 20 to 55%, particularly preferably in the range of 25 to 40%, based on the mass of the composite material. The inventors have also succeeded in identifying particularly suitable compounds for the organosilicon compounds with which particularly efficient semipermeable membranes according to the invention can be obtained, in particular with regard to advantageous water permeability, and the In particular, they exhibit high compatibility with the preferred silicon-containing porous fillers defined above. A semipermeable membrane according to the invention is preferred, wherein the organosilicon compound is selected from the group consisting of silyl ethers, silanes, siloxanes, and polysiloxanes, preferably selected from the group consisting of silyl ethers, siloxanes, and polysiloxanes, particularly preferably selected from the group consisting of siloxanes and polysiloxanes, and very particularly preferably selected from the group consisting of siloxanes and polysiloxanes. The person skilled in the art will understand that, although other compounds, such as binders, may potentially be present in the cover layer in addition to the organosilicon compounds provided according to the invention, it is preferred that the cover layer consists as largely as possible of the organosilicon compounds. Accordingly, a semipermeable membrane according to the invention is preferred, wherein the combined mass fraction of the organosilicon compounds in the cover layer is in the range from 50 to 100%, preferably in the range from 70 to 100%, particularly preferably in the range from 90 to 100%, very particularly preferably in the range from 95 to 100%, based on the mass of the cover layer, wherein the cover layer preferably consists essentially entirely of the organosilicon compounds. Furthermore, for certain applications, it may be preferable for the top layer to also comprise one or more additives in order to precisely adapt the physico-chemical properties of the top layer to the respective requirements of the application. In particular, the use of antioxidants to increase aging resistance is conceivable in this case. In addition, the top layer can be specifically modified physico-chemically in order to increase properties such as water wettability, oxidative and / or mechanical stability, selectivity or permeability, or to reduce susceptibility to fouling. This can be achieved, for example, by the use of ionizing radiation, for example electron, ion or gamma radiation, or by the introduction of suitable organic or inorganic components. Even though the semipermeable membranes according to the invention advantageously exhibit high resistance even without separate crosslinking, it may be advantageous, particularly for mechanically demanding applications, to additionally chemically crosslink the cover layer, for example, by means of thermal or radiation-induced crosslinking. Crosslinkers and / or corresponding initiators can be added to the cover layer as additives for this purpose. Coupling agents, for example, can also be considered as additives, which can further improve the bonding of the cover layer to the carrier layer. Since semipermeable membranes according to the invention offer high resistance to the passage of air and other contaminants, it is advantageously possible to make semipermeable membranes according to the invention particularly thin without introducing contaminants to an undesirable extent into the gas stream to be humidified. Since the thin membrane design is also particularly advantageous with regard to water permeability, material requirements, and manufacturing costs, it is accordingly expedient to utilize the favorable properties of semipermeable membranes according to the invention by also making them particularly thin.It can be seen as an advantage of the semipermeable membranes according to the invention that the advantageous properties with regard to the passage of air and other foreign particles through the semipermeable membrane with the specific cover layer can be achieved even if the latter is made relatively thin. Since a thin cover layer, in turn, results in material savings and increased water permeability, the inventors believe it is consequently also particularly advantageous to make the cover layer as thin as possible. In this respect, a semipermeable membrane according to the invention is preferred, wherein the cover layer has an average thickness in the range of 0.1 to 10 μm, preferably in the range of 0.5 to 8 μm, particularly preferably in the range of 1 to 5 μm.Additionally or alternatively, a semipermeable membrane according to the invention is preferred, wherein the carrier layer has an average thickness in the range from 10 to 500 pm, preferably in the range from 20 to 250 pm, particularly preferably. in the range of 40 to 190 pm, most preferably in the range of 80 to 160 pm. In this context, the inventors have also identified suitable basis weights for coating the carrier layer with the cover layer. A semipermeable membrane according to the invention is preferred, wherein the grammage of the cover layer is in the range of 0.25 to 15 g / m 2 , preferably in the range of 0.5 to 5 g / m 2 , lies. The performance of the specific cover layer in preventing the passage of air and other unwanted contaminants also advantageously allows for an asymmetric structure for the semipermeable membrane, in which only one cover layer is provided, which is accordingly applied only to one side of the carrier layer. This eliminates the need to provide the carrier layer with a cover layer on both sides, thus enabling material and weight savings as well as simpler production. Therefore, a semipermeable membrane according to the invention is preferred, in which a cover layer is arranged only on one side of the carrier layer. Even if, as explained above, it is possible and expedient to cover only one side of the carrier layer with the cover layer, in the opinion of the inventors, with a view to the intended application, it is expedient to cover the carrier layer with the cover layer on one side as completely as possible, so that no partial areas of the semipermeable membrane are created that are excessively permeable to air or unwanted contamination. Preference is given to a semipermeable membrane according to the invention, wherein the carrier layer on one side is covered by the cover layer to more than 60%, preferably to more than 75%, particularly preferably to more than 90%, most preferably essentially completely. Especially if a low coverage of the carrier layer by the cover layer is selected, it can be advantageous to instead cover the uncovered parts of the surface with a sealing, i.e.to be provided with a coating that is both ventilated and waterproof. To further avoid unwanted permeable sections of the semipermeable membrane, it is also preferable to use a coating that is as uniform as possible. Accordingly, a semipermeable membrane according to the invention is preferred, wherein the thickness of the cover layer varies by 50% or less, preferably 25% or less, particularly preferably 10% or less, and most preferably 5% or less, across the entire area of ​​the carrier layer covered by the cover layer. According to the inventors, a particular advantage of the semipermeable membranes according to the invention is that particularly advantageous bond strengths can be achieved between the cover layer and the carrier layer, which contribute to the long durability of the semipermeable membranes according to the invention, even at elevated temperatures and high humidity. According to the inventors, this effect is particularly pronounced when the organosilicon compounds of the cover layer are at least partially covalently bonded to the composite material. This bonding advantageously occurs at the silicon-containing porous filler of the composite material, which, due to its essentially homogeneous distribution in the plastic matrix, is also present on the surface of the carrier layer.Such a covalent bond can be achieved through a chemically and / or thermally induced reaction of the organosilicon compound with the silicon-containing porous filler, whereby the use of so-called "cross-linking agents" may be necessary to promote the covalent bond. A particularly advantageous effect here is that the silicon-containing porous fillers present on the surface of the composite material, due to their high porosity, can provide a large internal surface area available for the bonding of the organosilicon compound. A semipermeable membrane according to the invention is particularly preferred, wherein at least some of the organosilicon compounds of the cover layer are covalently bonded to the silicon-containing porous filler of the composite material. In particularly preferred embodiments, the covalent bonding of the organosilicon compound to the silicon-containing porous filler is achieved by a condensation reaction between an Si-OR group, where R is hydrogen or an organic radical, of the organosilicon compound with a silanol group present on the surface of the silicon-containing porous filler, as is the case, for example, on the surface of amorphous silicon dioxide. Preferred for this purpose is a semipermeable membrane according to the invention, wherein the organosilicon compound is selected from the group consisting of organosilicon compounds with at least one Si-OR group, where R is hydrogen or an organic radical, preferably an alkyl radical, particularly preferably an alkyl radical having 1 to 10 C atoms. Additionally or alternatively, preferred in this respect is a semipermeable membrane according to the invention, wherein the silicon-containing porous filler comprises Si-OH groups on the surface. In other words, a semipermeable membrane according to the invention is preferred, wherein the cover layer can be produced by coating the carrier layer with one or more organosilicon compounds selected from the group consisting of organosilicon compounds having at least one Si-OR group, wherein R is hydrogen or an organic radical, preferably an alkyl radical, particularly preferably an alkyl radical having 1 to 10 C atoms. While in semipermeable membranes, undesired delamination of the cover layer is often observed as a result of mechanical stress, which adversely affects performance and can even call into question their fundamental suitability in sensitive fuel cell systems, the advantageous bond strength of the semipermeable membranes according to the invention, especially when at least partial covalent bonding occurs, even allows, based on experiments by the inventors, the folding of the semipermeable membranes according to the invention. This can achieve great advantages with regard to space utilization and the effectively available surface area, making the semipermeable membranes according to the invention particularly suitable for use in fuel cell systems. Accordingly, a semipermeable membrane according to the invention is also preferred, wherein the semipermeable membrane is folded, preferably in a flat pleat arrangement, as is known, for example, from flat pleated filters. Particularly preferred, according to the inventors, is a semipermeable membrane for use in membrane humidifiers for fuel cell systems, comprising: aa) a carrier layer comprising a composite material, comprising at least one plastic and at least one silicon-containing porous filler embedded in the plastic, wherein the silicon-containing porous filler is selected from the group consisting of zeolites and amorphous silicon dioxide, in particular amorphous silicon dioxide, wherein the combined mass fraction of the plastics in the composite material is in the range of 15 to 75%, wherein the combined mass fraction of the silicon-containing porous fillers in the composite material is in the range of 25 to 85%, in each case based on the mass of the composite material, and bb) a cover layer arranged on the carrier layer, comprising at least one organosilicon compound,wherein the organosilicon compound is selected from the group consisting of siloxanes and polysiloxanes, wherein the combined mass fraction of the organosilicon compounds in the cover layer is in the range of 50 to 100%, based on the mass of the cover layer. The person skilled in the art will further understand that the invention also relates to a membrane humidifier, in particular for use in fuel cell systems, comprising at least one, preferably two or more, semipermeable membranes according to the invention. The invention also relates to a fuel cell system, in particular a polymer electrolyte membrane fuel cell system, comprising at least one fuel cell and at least one membrane humidifier according to the invention. The membrane humidifier according to the invention is arranged in the fluid line system of the fuel cell, preferably in the cathode-side fluid line system. The membrane humidifier is preferably arranged such that it simultaneously is arranged in the fluid supply line and the fluid discharge line so that it can be flowed through by the supplied process gas and the discharged process gas in such a way that the supplied process gas and the discharged process gas are separated from one another in sections by the semipermeable membrane according to the invention. The invention further relates to a method for producing a semipermeable membrane according to the invention, comprising the method steps: u) producing or providing a carrier layer made of a composite material, comprising at least one plastic and at least one silicon-containing porous filler embedded in the plastic, v) applying a coating composition comprising at least one organosilicon compound and at least one solvent to the surface of the carrier layer, and w) evaporating the solvent to obtain a cover layer arranged on the carrier layer, comprising at least one organosilicon compound. A method according to the invention is preferred, wherein the carrier layer is produced by extrusion of a plastic mixture comprising the silicon-containing porous filler, preferably with an extruder. With regard to the achievable quality of the coating, particularly in the case of a partial covalent bond between the cover layer and the carrier layer, a process according to the invention is preferred, wherein the evaporation of the solvent takes place at a temperature in the range of 40 to 120 °C, preferably in the range of 50 to 100 °C, particularly preferably in the range of 60 to 90 °C. Thus, a process according to the invention is also preferred, wherein the evaporation of the solvent takes place such that at least some of the organosilicon compounds of the cover layer are covalently bonded to the silicon-containing porous filler of the composite material. With a view to efficient process control, a process according to the invention is preferred, wherein the solvent is selected from the group consisting of water, alcohols, in particular methanol, ethanol and isopropanol, aqueous acids, aqueous bases and mixtures of these solvents, in particular from the group consisting of water, aqueous bases, in particular aqueous solutions of alcoholates or hydroxides, and mixtures of these solvents, wherein the solvent preferably comprises water, alcohol and a base. The invention and preferred embodiments of the invention are explained and described in more detail below with reference to the accompanying figures. The figures show: Fig. 1 is a schematic cross-sectional view of a semipermeable membrane according to the invention in a preferred embodiment; Fig. 2 Four scanning electron microscope cross-sectional images of semipermeable membranes according to the invention after ten days of storage in boiling water; and Fig. 3 Two scanning electron microscope cross-sectional images of semipermeable membranes according to the invention after storage for ten days in a drying oven at 105 °C. Fig. 1 shows a schematic cross-sectional view of a semipermeable membrane 10 according to the invention in a preferred embodiment. The exemplary semipermeable membrane 10 is intended for use in a membrane humidifier, which in turn is used in a polymer electrolyte membrane fuel cell system. The semipermeable membrane 10 of Fig. 1 consists of a carrier layer 12 and a cover layer 14, which is arranged on one side of the carrier layer 12 and essentially completely covers the surface of the carrier layer 12 on this side. The carrier layer 12 is formed entirely from a composite material comprising polyethylene as a plastic matrix, in which porous amorphous silicon dioxide, namely precipitated silicon dioxide, is essentially uniformly dispersed. The carrier layer 12 has a substantially constant thickness of approximately 120 pm, whereas the cover layer 14 has a substantially constant thickness of approximately 3 pm. The presence of the silicon-containing porous filler in the plastic matrix of the composite material is visualized in Fig. 1 by the stars indicated in the carrier layer 12. The resulting semipermeable membrane 10 of Fig. 1 is permeable to water, but largely impermeable to air, to the extent that the semipermeable membrane 10 can be regarded as essentially impermeable to air according to the standards relevant in practical application. The silicon-containing porous filler has mesopores and macropores, so that the composite material itself is a meso- and macroporous material due to the embedded porous filler. In the composite material of the carrier layer 12, the mass fraction of the silicon-containing porous filler is approximately 65%, while the remaining mass fraction is made up of the polyethylene. In the example shown, the cover layer 14 consists essentially entirely of a polysiloxane which has Si-OR groups (R = H or alkyl) and is at least partially covalently bonded to the silicon-containing porous filler of the composite material, namely by reaction of the Si-OR groups with the silanol groups located on the surface of the filler particles. A corresponding semipermeable membrane 10 can be produced starting from a carrier layer 12, the composite material of which consists of the plastic and the silicon-containing porous filler and which can be produced, for example, by extrusion of a plastic mixture comprising these two components. A coating composition is then applied to the carrier layer 12, which contains the organosilicon compounds and at least one solvent, for example an aqueous solution of alcohols, which can be prepared by mixing water, alcohol, and a base. The formation of the cover layer 14 arranged on the carrier layer 12 then takes place at elevated temperatures of for example 80 °C by evaporating the solvent, wherein the conditions in this process step are preferably adjusted so that the covalent linking of the organosilicon compound of the cover layer 12 with the silicon-containing porous filler of the carrier layer 14 is promoted. Figures 2 and 3 each show scanning electron microscope cross-sectional images through semipermeable membranes according to the invention (for production see below) after different aging conditions, namely for Figure 2 after ten days of storage in moving, boiling water (100°C) and for Figure 3 after ten days of storage in a drying cabinet at 105°C. To obtain the fracture edges shown and to ensure representative images, the samples were immersed in liquid nitrogen and measured with a scanning electron microscope (SEM) immediately after the temperature-induced fracture (“cryogenic breaking”). The SEM images clearly show the structure of the composite material in the carrier layer, in whose carrier matrix made of polyolefin particulate porous amorphous silicon dioxide is clearly visible and dispersed. In all SEM images it can be seen that the respective cover layer, which is shown below in Figure 2 and above in Figure 3.3 is arranged above the carrier layer, despite the aging tests, which were geared to the loads to be expected in a fuel cell, not only is it clearly visible, but also a substantially continuous coating without defects has been maintained. In the following, the invention and preferred embodiments of the invention are further explained and described with reference to experiments. A. Preparation of semipermeable membranes according to the invention: Four semipermeable membranes E1 to E4 according to the invention were produced. For this purpose, a support layer made of composite material was provided, which is commercially available under the name Teslin SP600 from PPG Industries. The support layer consists of a polyolefin support matrix in which particulate porous amorphous silica is dispersed. The support layer had an average thickness of about 150 pm and an average basis weight of about 97 g / m 2 . The topcoat was applied to the carrier layer by coating using an Easycoater coating device from Coatema. To prepare the coating composition, an organofunctionalized siloxane oligomer was provided, which is commercially available from Evonik Operations under the trade name Dynasylan Hydrosil 2909. The organofunctionalized siloxane oligomer was diluted with a suitable solvent to obtain a coating composition (mass fraction of siloxane oligomer approximately 10%), which was uniformly applied to the carrier layer. Evaporation of the solvent (T = 70 °C; t = 5 min) resulted in the solid topcoat from the coating composition. For membrane E1, the coating composition was applied with an average layer thickness of approximately 22 μm.For membrane E2, the substrate prepared analogously to E1 was coated again, with the coating composition being applied again with an average layer thickness of approximately 22 pm. For membrane E3, a one-step coating process was again chosen, with the coating composition being applied with an average layer thickness of approximately 44 pm. Membrane E4 was produced largely analogously to membrane E1, but the vacuum fixation of the coating device was omitted and instead the substrate was fixed with an adhesive strip in order to exclude any influence of the vacuum fixation on the infiltration of the coating composition into the substrate. The layer thickness of the top layer in the dry state was calculated to be 2.2 pm (E1, E4) and 4.4 pm (E2, E3). B. Comparison membranes: The uncoated substrate, which was also used in membranes E1 to E4, was initially used as reference membrane V1. Commercially available membranes were also used as reference membranes. which are currently used in fuel cell applications, namely V2 (Nation 110; Chemours; chemically stabilized copolymers of perfluorosulfonic acids with polytetrafluoroethylene; average thickness approx. 127 pm), V3 (Fumasep F10120-PK; Fumatech; perfluorosulfonic acid-based membrane with long side chains (LSC), reinforced with PEEK fabric; average thickness approx. 120 pm) and V4 (Fumasep F-950; Fumatech; unreinforced perfluorosulfonic acid-based membrane with long side chains (LSC); average thickness approx. 50 pm). C. Experiments: The water transmission rate (WTR in kilograms per square meter per day) was determined for the membranes tested. The experimental setup was based on the apparatus disclosed for this purpose in EP 2435171 B1, which comprises two fluid pathways Z and A separated by the membrane to be tested, each with an inlet and an outlet. The properties of the test gases (air) introduced into the fluid pathways could be monitored and the changes analyzed. In the setup used, the area of ​​each membrane between the fluid pathways was 0.04 m 2The pressure of the gases introduced into both fluid lines was 200 kPa and the temperature was approximately 80 °C. The first gas stream Z is dried to an air humidity of approximately 0%, whereas the air humidity of the second gas stream A is set to approximately 90%, which corresponds to the conditions of a saturated air flow for the downstream side typical for fuel cell applications. The volume flow of the test gases used has a significant influence on the determined water transmission rate. Since EP 2435171 B1 does not provide any information on the volume flow, the inventors set the volume flow of the test gases used to Q = 20 NL / min, since the inventors believe that reliable results can be obtained with this value. The measured values ​​were each obtained as the average of three measurements. For the selected membranes, the gas crossover was also determined, for which the experimental setup described above only requires slight adjustments. Under otherwise identical conditions, The inlet of the second fluid path is closed, and the test gas is only directed through the first fluid path (Q = 70 NL / min). In this setup, the gas flow can be measured as the volume flow at the outlet of the second fluid path. D. Results The water transmission rate (WTR) for the membranes according to the invention and the comparison membranes was determined as described above. The results are summarized in Table 1. Table 1 The data presented in Table 1 clearly demonstrate that excellent water transmission rates can be achieved with the semipermeable membranes according to the invention, which are even higher at higher layer thicknesses than with some comparable prior art membranes. The water transmission rates are at a similar level to the water transmission rate of the most efficient prior art membranes, particularly when using a single-stage coating process, without having to rely on the use of perfluorosulfonic acid-based materials, so that the semipermeable membranes according to the invention represent a promising alternative to perfluorosulfonic acid-based membranes. systems without having to fear the disadvantages associated with perfluorinated substances, particularly with regard to environmental and health aspects. The comparison sample V1 showed an air permeation of 1.3 NL / min. The membranes according to the invention also showed an air permeation of 0.01 NL / min even at the smallest coating thickness (E1), which essentially corresponds to 0% within the measurement uncertainty and is in any case significantly higher than the typical market specification of 0.5 NL / min. To simulate aging behavior, the membranes according to E1 were aged under various conditions. Subsequently, the aged membranes were tested for their water transmission rate (WTR) compared to the uncoated substrate. Particularly striking was the fact that no delamination of the cover layer was observed in any of the aging tests, which speaks for the excellent durability. In aging test A1, the samples were functionally aged, with the aging conditions adapted to the stresses expected in a fuel cell. For this purpose, a water-filled sample vessel was sealed with the membrane and heated in an oven at 90 °C during aging. This allowed the membrane to experience continuous water vapor permeation at a temperature typical for fuel cell operation. The results are summarized in Table 2. Table 2 In aging test A2, the samples were aged without the presence of steam at an elevated temperature of 110 °C. The results are summarized in Table 3. Table 3 In aging test A3, the samples were stored in a boiling water bath (temperature approximately 100 °C) to specifically test their susceptibility to the coating delamination process disclosed in EP 2435171 B1. The results are summarized in Table 4. Table 4 The results of the aging tests clearly demonstrate that the favorable water transmission rate is maintained over long periods under the aging conditions. At the same time, however, the difference in water transmission rate compared to the uncoated substrate is maintained, which, together with the lack of delamination of the top layer, indicates a high durability of the coating and very good longevity. The advantageous durability even under mechanical stress was further demonstrated by a folding test. For this purpose, a membrane was folded into a flat fold arrangement and then tested for water transmission rate. The change in the water transmission rate from 51 (kg / (m 2 * d) to 54 (kg / (m 2 * d) is within the limits of measurement inaccuracy. Reference symbol Semipermeable membrane Carrier layer Cover layer

Claims

Claims 1. Semipermeable membrane (10), in particular for use in membrane humidifiers for fuel cell systems, comprising: a) a carrier layer (12) comprising a composite material, comprising at least one plastic and at least one silicon-containing porous filler embedded in the plastic, and b) a cover layer (14) arranged on the carrier layer (12), comprising at least one organosilicon compound.

2. Semipermeable membrane (10) according to claim 1, wherein the plastic is selected from the group consisting of thermoplastics, preferably selected from the group consisting of polyvinyl chlorides and polyolefins.

3. Semipermeable membrane (10) according to one of claims 1 or 2, wherein the silicon-containing porous filler is selected from the group consisting of silicon-aluminum-phosphorus-oxygen compounds, silicon-containing metal-organic framework compounds, zeolites and amorphous silicon dioxide.

4. Semipermeable membrane (10) according to one of claims 1 to 3, wherein the organosilicon compound is selected from the group consisting of silyl ethers, silanes, siloxanes and polysiloxanes.

5. Semipermeable membrane (10) according to one of claims 1 to 4, wherein at least a part of the organosilicon compounds of the cover layer (14) is covalently bonded to the silicon-containing porous filler of the composite material.

6. Semipermeable membrane (10) according to one of claims 1 to 5, wherein the cover layer (14) has an average thickness in the range of 0.1 to 10 pm.

7. Membrane humidifier, in particular for use in fuel cell systems, comprising at least one semipermeable membrane (10) according to one of claims 1 to 6.

8. Fuel cell system, in particular polymer electrolyte membrane fuel cell system, comprising at least one fuel cell and at least one membrane humidifier according to claim 7.

9. A method for producing a semipermeable membrane (10) according to one of claims 1 to 6, comprising the method steps: u) producing or providing a carrier layer (12) from a composite material, comprising at least one plastic and at least one silicon-containing porous filler embedded in the plastic, v) applying a coating composition comprising at least one organosilicon compound and at least one solvent to the surface of the carrier layer (12), and w) evaporating the solvent to obtain a cover layer (14) arranged on the carrier layer (12) and comprising at least one organosilicon compound. Method according to claim 9, wherein the evaporation of the solvent is carried out in such a way that at least a part of the organosilicon compounds of the cover layer (14) are covalently bonded to the silicon-containing porous filler of the composite material.

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