Semi-permeable membrane for membrane humidifier

Abstract

The invention relates to a semipermeable membrane (10), in particular for use in a membrane humidifier of a fuel cell system, 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), the capping layer comprises at least one organosilicon compound.

Description

Technical Field

[0001] The invention relates to a semipermeable membrane and a membrane humidifier comprising the corresponding semipermeable membrane, a fuel cell system comprising the corresponding membrane humidifier and a method for preparing the corresponding semipermeable membrane. Background Art

[0002] In recent years, the use of fuel cells in the field of vehicle technology has been considered a promising approach to reduce dependence on fossil raw materials such as oil. Fuel cells are one of the most important alternatives to the use of batteries, such as lithium-ion batteries. Fuel cell technology has certain advantages over battery technology, especially in terms of the actual handling of the fuel compared to electrochemical storage, in terms of storage potential and possibly short refueling times (long charging times can be avoided by short refueling times), as well as the potential to use the existing pipeline and storage infrastructure of conventional engine fuel supply without having to build a battery charging infrastructure. These advantages are particularly evident in applications in areas where long charging processes must be avoided, in particular in the aviation sector and in the field of large-scale commercial vehicle fleets.

[0003] In a fuel cell, oxygen reacts with a fuel such as hydrogen, methane or methanol under controlled reaction conditions to form water and, if necessary, other reaction products, wherein the reaction steps of the redox reaction are spatially separated. For this purpose, a fuel cell consists of an anode and a cathode, which are separated from each other by an electrolyte (e.g. an electrolyte membrane).

[0004] The reactants are usually continuously supplied to the fuel cell during operation. 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 setting of an optimal humidity, so that the electrolyte membrane can be prevented from drying out even at high operating temperatures. This usually requires complex guidance or control of the fluid flows and the use of efficient filter technology.

[0005] In order to ensure that the electrolyte membrane is adequately moistened even at high operating temperatures, it is generally advantageous to humidify the supplied process gas, in particular the cathode-side process gas. Moisture can be supplied to the process gas from a reservoir, such as humid air, by means of a suitable humidifying device. Since water is formed during the operation of the fuel cell, the exhaust gas of the fuel cell can also be used as a reservoir for moisture, for example. During humidification, it is usually sought that, apart from the moisture exchange, no contact or at least only a small contact occurs between the process gas and the reservoir, so that, for example, contamination of the process gas by the exhaust gas can be avoided.

[0006] In principle, various humidifiers are conceivable as humidifying devices. However, so-called membrane humidifiers are generally regarded as particularly efficient and advantageous forms of humidifying devices, in which one or more moisture-permeable membranes, such as hollow fiber membranes, are used, which are permeable to water vapor but which prevent further exchange of substances to the greatest extent possible.

[0007] If a humid gas stream and a dry gas stream are separated from each other by such a semipermeable membrane, water permeation from the humid gas stream to the dry gas stream occurs in a diffusion-driven manner due to the different partial pressures of water in the two gas streams.

[0008] PEM fuel cell systems, their construction and the use of membrane humidifiers in these PEM fuel cell systems are widely known to the person skilled in the art from the prior art and are described, for example, in DE 10 2015 202 089 A1, DE 10 2015 224 202 A1 or DE 10 2016 224 478 A1.

[0009] While small permeations of other components in addition to the desired water permeation in the storage liquid are generally considered unproblematic, for example in applications in the field of building air management, for applications in the field of fuel cells it is generally desirable to prevent excessive mixing of the inlet and outlet air flows as reliably as possible in addition to the water exchange. In addition to structural factors of the membrane humidifier, this generally depends to a large extent on the properties of the membranes used, so that there is a constant demand for further improvements of these membranes.

[0010] In this regard, membrane humidifiers have been proposed in the field of building air management, which use a semipermeable membrane as a water transport membrane, which includes a porous polyethylene substrate loaded with silica, which has a coating on the surface, which includes a cross-linked, water-permeable nonionic polyurethane-polyether polymer, as disclosed in EP 2435171 B1. According to EP 2435171 B1, these water transport membranes allow a large amount of water to pass through (vapor and liquid), but at the same time only a small amount of gas and impurities permeate, or even no gas and impurities permeate.

[0011] The inventors of the present invention have tested water transport membranes known from EP 2435171 B1 in the field of building air management and evaluated their performance in membrane humidifiers for fuel cells. In the tests, it was found that the water transport membranes known from the prior art do not fully meet the specific application requirements proposed in all respects, which the inventors assume is at least partly influenced by the increased temperatures compared to building air management and / or the air humidity that occurs in practice in the exhaust gas stream of fuel cells. Apart from the water permeation that can be achieved under process conditions and the observed permeation of exhaust gases or impurities, the durability of the water transport membranes in fuel cell systems is considered to be insufficient, especially at elevated temperatures or high air humidity, to ensure adequate durability under the mechanical loads that are expected, especially in vehicle applications. This durability, which the inventors of the present invention assess as insufficient for fuel cell applications, is primarily related to the bond strength between the substrate and the coating composed of a polyurethane-polyether polymer, where, according to the inventors' assessment, the effect that the coating may peel off during prolonged exposure to warm, humid environments (which may be tolerable for building air management applications) is also clearly documented in EP 2435171 B1 (see paragraph

[0063] ). Summary of the invention

[0012] The main object of the present invention is to eliminate or at least alleviate the above-mentioned disadvantages of the prior art.

[0013] One object of the present invention is in particular to provide a semipermeable membrane which has excellent water permeability while preventing air and other impurities from penetrating the membrane to the greatest extent possible. The semipermeable membrane to be provided should have a high mechanical load capacity and a durability that is improved compared to the prior art, in particular at elevated temperatures or high air humidity. In addition, the semipermeable membrane to be provided should be able to be produced in a manner that saves as much time and cost as possible, and in this case, ideally as few health-damaging and / or environmentally harmful materials as possible should be required in the production, wherein in particular the use of perfluorinated compounds should be abandoned.

[0014] A supplementary object of the present invention is to provide a method for preparing the corresponding semipermeable membrane.

[0015] A second object of the present invention is to provide a high-performance membrane humidifier for use in a fuel cell system and a corresponding fuel cell system.

[0016] The inventors of the present invention have now recognized that the above-mentioned objects can surprisingly be achieved by a semipermeable membrane comprising a carrier layer and a covering layer arranged on the carrier layer, the carrier layer comprising a specific composite material, the covering layer comprising an organosilicon compound, as defined in the claims, thereby obtaining a semipermeable membrane which is also excellently suitable in a preferred manner for many other applications in which the transfer of moisture and / or enthalpy from an air-humidified chamber to a drier chamber is important.

[0017] Therefore, the above-mentioned object is achieved by the subject matter of the present invention as defined in the claims. Preferred embodiments according to the present invention are derived from the dependent claims and the following embodiments.

[0018] In particularly preferred embodiments, these embodiments (which are hereinafter referred to as preferred) are combined with features of other embodiments referred to as preferred. Therefore, very particularly preferred is a combination of two or more embodiments referred to as particularly preferred below. Also preferred are embodiments in which features of one embodiment referred to as preferred to a certain extent are combined with one or more other features of other embodiments referred to as preferred to a certain extent. The features of the preferred membrane humidifier, method and fuel cell system will be derived from the features of the preferred semipermeable membrane.

[0019] The present invention particularly relates to a semipermeable membrane, in particular for use in a membrane humidifier of a fuel cell system, the semipermeable membrane comprising:

[0020] 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

[0021] b) A cover layer arranged on the carrier layer, the cover layer comprising at least one organosilicon compound.

[0022] The semipermeable membrane according to the invention is particularly suitable for installation in a membrane humidifier and for use in this form in a fuel cell system, in particular in a fluid line system. This suitability is particularly due to the advantageous specific properties of the semipermeable membrane according to the invention, in particular with regard to the excellent durability even at elevated temperatures of 80° C. or higher, or the resulting elevated air humidity that occurs in the exhaust gas stream of a fuel cell system. In addition, due to its advantageous water permeability and the reliable suppression of the permeation of air and other impurities, the semipermeable membrane according to the invention is also excellently suitable for other applications, for example in module assemblies for transferring water and / or enthalpy from a gas with a high partial pressure of water to a drier gas.

[0023] Compared to the prior art disclosed, for example, in EP 2435171 B1, advantageous properties, in particular good resistance and excellent bond strength between the layers, are achieved even without additional chemical crosslinking. In addition, advantageous transport properties are achieved without the need for additional post-processing steps after coating, in particular compared to PVA-based coatings, wherein in particular no intentional defects or intentional peeling in the covering layer are necessary, so that in the production of the semipermeable membrane according to the invention, a continuous, highly functional and durable covering layer is obtained solely by the coating step and the intermediate coating.

[0024] In accordance with the understanding of the skilled person, the term "semipermeable" means that the semipermeable membrane according to the invention does not have the same permeability to all substances, wherein the permeability to individual molecules and substances (optionally according to their aggregation state and / or their particulate form) can also be very small, so that these components are essentially impermeable to the semipermeable membrane. The skilled person understands that the semipermeable membrane according to the invention is a water transport membrane, which is accordingly permeable at least for gaseous water, preferably for gaseous and condensed water. In other words, it therefore relates to a semipermeable membrane according to the invention, wherein the semipermeable membrane is permeable to water.

[0025] Since the purpose of the semipermeable membrane according to the invention is also to prevent the permeation of other components, in particular air and other gases, as well as particulate impurities as well as water as well as possible, the semipermeable membrane according to the invention has a reduced permeability to these components. Preferred is a semipermeable membrane according to the invention in which the air permeation of the semipermeable membrane is 2.0 cm due to 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 here are semipermeable membranes according to the invention, wherein the semipermeable membrane is substantially air-tight, wherein the semipermeable membrane is in particular substantially impermeable to oxygen, nitrogen, carbon dioxide and mixtures of these gases. Correspondingly, semipermeable membranes that are substantially impermeable to particulate impurities are also particularly preferred.

[0026] Consistent with the understanding of those skilled in the art, a membrane is a thin flat structure whose extension in the XY plane is significantly greater than its extension in the Z direction, ie, it is a flat structure whose length and width are significantly greater than its thickness.

[0027] The semipermeable membrane according to the present invention comprises a carrier layer. Here, the carrier layer comprises a composite material. In accordance with the understanding of those skilled in the art, a composite material is a material consisting of two or more components, which are combined with each other and together form a material with different physicochemical properties from the individual components. The corresponding composite materials are sometimes also referred to as composite materials.

[0028] According to the invention, the carrier layer comprises a composite material, so that the carrier layer can also be formed at least partially from other materials. However, the person skilled in the art understands that it is preferred that the carrier layer not only only partially consists of a composite material, but at least predominantly or preferably substantially completely consists of a composite material. According to the inventors' assessment, embodiments in which the carrier layer is substantially completely formed from a composite material are preferred for substantially all applications.

[0029] 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 here for almost all embodiments is a semipermeable membrane according to the invention, in which the silicon-containing porous filler in the composite material is present in the form of a plurality of particles dispersed in the plastic. Even if it is conceivable that the silicon-containing porous filler is not uniformly distributed in the plastic matrix, preferred is such a semipermeable membrane according to the invention, in which the silicon-containing porous filler in the composite material is present in a manner that is essentially uniformly distributed in the plastic.

[0030] The plastic, which forms the carrier matrix for the embedded silicon-containing porous filler in the composite material, can in principle be any common polymer material, since the plastic matrix chemistry itself is not decisive for the application-related properties and in practice the plastic can be selected primarily with regard to the required mechanical properties.

[0031] According to the invention, the filler combined with the plastic is siliceous. In accordance with the understanding of the skilled person, this means that the filler consists of a compound containing silicon atoms in its molecular structure and / or crystal structure.

[0032] However, the filler used in the composite material is not only siliceous, but 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, that is, the porous filler or the particles of the porous filler have voids that may be connected to each other inside. The so-called open porosity or useful porosity is derived here from the total volume of the voids that are interconnected and connected to the environment. It is understood by those skilled in the art that for the siliceous porous filler used according to the present invention, the porosity should be at least partially open porosity, such as, for example, many zeolite materials and a large number of fillers composed of amorphous silicon dioxide (sometimes also called "silica") used in industry. For almost all embodiments, it is relevant to this semipermeable membrane according to the present invention, wherein the siliceous porous filler is microporous and / or mesoporous and / or macroporous. Preferred here is this semipermeable membrane according to the present invention, wherein the siliceous porous filler is mesoporous and / or macroporous, wherein the siliceous porous filler is particularly preferably constituted as a hierarchical porous filler, i.e., having a hierarchically structured pore system, wherein, for example, the mesopores are located at the edges of the macropores.

[0033] It is understood by those skilled in the art that the porosity of the silicon-containing porous filler determines or increases the porosity of the composite material, which in turn determines or increases the water permeability. Therefore, in almost all embodiments, it is relevant to the semipermeable membrane according to the invention, wherein the composite material is a porous composite material, preferably a microporous composite material. Preferred here is the semipermeable membrane according to the invention, wherein the composite material has a porosity in the range of 30 to 90%, preferably a porosity in the range of 40 to 80%, particularly preferably a porosity in the range of 50 to 60%.

[0034] In addition to the carrier layer, the semipermeable membrane according to the present invention also includes a cover layer. The cover layer is arranged on the carrier layer and at least partially covers the carrier layer, so that the cover layer can be understood as a coating of the carrier layer. Different from the prior art, the cover layer includes at least one organosilicon compound. Consistent with the understanding of those skilled in the art, organosilicon compounds, which are also referred to as silicon-containing organic compounds, are compounds that at least include silicon atoms and carbon atoms, wherein carbon can be directly or through heteroatoms (particularly oxygen) and silicon bonds, such as in siloxanes and polysiloxanes. The corresponding raw materials suitable for preparing the cover layer can be purchased by many manufacturers at this, such as from Evonik, Hubei or Wacker Chemie companies, some of which are sold for other purposes, such as as adhesion promoters. Exemplary goods are, for example, products sold under the trade name Dynasylan of Evonik, such as with specifications SIVO 110, SIVO 418, SIVO 850, VPS SIVO 608, Hydrosil 2909 or Triamo.

[0035] Although it is possible in principle to use other layers in the semipermeable membrane according to the invention, it is preferred in most cases, taking into account the performance characteristics and the thickness of the membrane that can be achieved, that the semipermeable membrane according to the invention consists only of the above two layers. Accordingly, preference is given to a semipermeable membrane according to the invention, wherein the semipermeable membrane consists of a carrier layer and a covering layer.

[0036] As mentioned above, the composite material can include a wide range of possible plastics, so that the person skilled in the art can select the plastic used, in particular in view of the mechanical requirements imposed on the semipermeable membrane and the available materials. However, in this regard, the inventors have succeeded in identifying some plastics that are particularly suitable for the semipermeable membrane according to the present invention due to their processing properties and mechanical properties. Preferred is the semipermeable membrane according to the present invention, wherein the plastic is selected from thermoplastics, preferably from polyvinyl chloride and polyolefins, particularly preferably from polyolefins, in particular polyethylene and polypropylene, wherein the plastic is very particularly preferably polyethylene, in particular ultra-high molecular weight polyethylene.

[0037] The inventors have successfully identified in the same manner particularly advantageous silicon-containing porous fillers, through which composite materials can be obtained, which are particularly advantageous when used in the semipermeable membrane according to the present invention. These silicon-containing porous fillers can usually be particularly well dispersed in conventional plastics, can be obtained with a suitable porosity, and show a favorable interaction with the organosilicon compound of the covering layer. Preferred is the semipermeable membrane of the present invention, wherein the silicon-containing porous filler is selected from silicon-aluminum-phosphorus-oxygen compounds, silicon-containing organometallic framework compounds, zeolites and amorphous silica, preferably from amorphous silica, preferably from zeolites and amorphous silica, particularly preferably from amorphous silica, in particular aerogels, precipitated silica and pyrogenic silica, very particularly preferably from aerogels and precipitated silica, in particular precipitated silica.

[0038] It is understood by those skilled in the art that the silicon-containing porous filler, in particular the preferred silicon-containing porous filler mentioned above, is generally hygroscopic and therefore, in addition to the porosity, can also contribute to the passage of water through the composite material or to the composite material having an advantageously high water absorption capacity. It is relevant for almost all embodiments that there is also such a semipermeable membrane of the present invention, wherein the silicon-containing porous filler is a desiccant.

[0039] In this regard, the inventors have succeeded in determining particularly favorable mass fractions in the composite material for the two above-mentioned components of the composite material. That is, the semipermeable membrane according to the present invention is preferred, wherein the total mass fraction of the silicon-containing porous filler in the composite material is in the range of 25 to 85%, preferably in the range of 45 to 80%, and particularly preferably in the range of 60 to 75%, based on the mass of the composite material. Additionally or alternatively, the following semipermeable membrane according to the present invention is also preferred, wherein the total mass fraction of the plastic in the composite material is in the range of 15 to 75%, preferably in the range of 20 to 55%, and particularly preferably in the range of 25 to 40%, based on the mass of the composite material.

[0040] Furthermore, the inventors have succeeded in identifying compounds which are particularly suitable for organosilicon compounds, with which particularly efficient semipermeable membranes according to the invention can be obtained, in particular with regard to advantageous water permeability, and which are particularly highly compatible with the preferred silicon-containing porous fillers defined above. Preferred are semipermeable membranes according to the invention, in which the organosilicon compound is selected from the group consisting of silyl ethers, silanes, siloxanes and polysiloxanes, preferably from the group consisting of silyl ethers, siloxanes and polysiloxanes, particularly preferably from the group consisting of siloxanes and polysiloxanes, very particularly preferably from the group consisting of siloxanes and polysiloxanes.

[0041] In this regard, it is understood by the person skilled in the art that, in addition to the organosilicon compound provided according to the invention, further compounds, such as binders, may be present in the covering layer, but it is preferred that the covering layer consists essentially of the organosilicon compound as much as possible. Accordingly, preference is given to a semipermeable membrane according to the invention, wherein the total mass fraction of the organosilicon compound in the covering layer is in the range of 50 to 100%, preferably in the range of 70 to 100%, particularly preferably in the range of 90 to 100%, very particularly preferably in the range of 95 to 100%, based on the mass of the covering layer, wherein the covering layer preferably consists essentially completely of the organosilicon compound.

[0042] In addition, it may be preferred for a specific application that the cover layer also includes one or more additives so that the physico-chemical properties of the cover layer are matched with the corresponding requirements of the application in a target-accurate manner. Here, it is particularly conceivable to use an antioxidant to improve aging resistance. In addition, the cover layer can be modified in a targeted physico-chemical aspect, for example, to improve performance (such as water wettability, oxidation stability and / or mechanical stability, selectivity or permeability) or to reduce easy fouling. This can be achieved, for example, by using ionizing radiation (such as electronic radiation, ion radiation or gamma rays) or by introducing suitable organic or inorganic components. Although the semipermeable membrane according to the present invention also advantageously shows high tolerance even without separate crosslinking, it may be meaningful, especially for the application with high mechanical requirements, to carry out additional chemical crosslinking to the cover layer, for example, by heat or radiation-induced crosslinking, wherein a crosslinking agent and / or a corresponding initiator can be added to the cover layer as an additive to achieve this purpose. In addition, for example, a coupling agent can also be considered as an additive, and the coupling agent can be used to further improve the combination of the cover layer and the carrier layer.

[0043] Because the semipermeable membrane according to the present invention has a high resistance to the penetration of air and other impurities, it is advantageously possible to design the semipermeable membrane according to the present invention to be particularly thin without bringing impurities into the airflow to be moistened to an undesirable degree. Because the embodiment designed as a thin membrane is also particularly advantageous in terms of water permeability and in terms of material requirements and manufacturing costs, it is accordingly suitable to also utilize its advantageous properties by designing the semipermeable membrane according to the present invention to be particularly thin. Here, when a specific cover layer is designed to be relatively thin, the advantageous properties of air and other foreign particles passing through the semipermeable membrane can be achieved with the help of this specific cover layer, which can be regarded as an advantage of the semipermeable membrane according to the present invention. Since the thin cover layer saves material and increases water permeability, it is also particularly advantageous to design the cover layer to be as thin as possible according to the inventor's assessment. In this regard, it is preferred that the semipermeable membrane according to the present invention has an average thickness in the range of 0.1 to 10 μm, preferably in the range of 0.5 to 8 μm, and particularly preferably in the range of 1 to 5 μm. Additionally or alternatively, preference is given to semipermeable membranes according to the invention in which the average thickness of the carrier layer is in the range of 10 to 500 μm, preferably in the range of 20 to 250 μm, particularly preferably in the range of 40 to 190 μm, very particularly preferably in the range of 80 to 160 μm.

[0044] In this regard, the inventors have also determined a suitable unit area weight for coating the carrier layer with the cover layer. That is, a semipermeable membrane according to the present invention is preferred, wherein the grammage of the cover layer is between 0.25 and 15 g / m 2 In the range of 0.5 to 5 g / m 2within the range.

[0045] The properties of the specific covering layer also advantageously allow the selection of an asymmetric structure of the semipermeable membrane with respect to preventing the permeation of air and other undesirable impurities, in which only a covering layer is provided, which is accordingly applied only on one side of the carrier layer, so that it is not necessary to provide a covering layer on both sides of the carrier layer, thereby saving material and reducing weight and making the manufacture simpler. Therefore, preferred is a semipermeable membrane according to the invention, in which the covering layer is arranged only on one side of the carrier layer.

[0046] Although it is possible and expedient to cover only one side of the carrier layer with the covering layer, according to the inventors' assessment, it is expedient to cover the carrier layer with the covering layer on one side as far as possible over the entire surface, taking into account the intended application, so that no partial area of ​​the semipermeable membrane is produced that is excessively permeable to air or undesired impurities. Preference is given to the semipermeable membrane according to the invention, in which the carrier layer is covered on one side by more than 60%, preferably more than 75%, particularly preferably more than 90%, very particularly preferably substantially completely with the covering layer. Especially when a lower degree of coverage of the carrier layer with the covering layer is chosen, it may be advantageous to provide the uncovered surface part with a sealing (i.e. not only air-impermeable but also water-impermeable) coating instead.

[0047] In order to further avoid undesired permeable sections of the semipermeable membrane, it is also preferred to use a coating that is as uniform as possible. Therefore, preference is given to semipermeable membranes according to the invention in which the thickness of the covering layer varies by 50% or less, preferably 25% or less, particularly preferably 10% or less, very preferably 5% or less over the entire area of ​​the carrier layer covered by the covering layer.

[0048] According to the inventor's assessment, a special advantage of the semipermeable membrane according to the present invention is that a particularly favorable bonding strength can be achieved between the cover layer and the carrier layer, which contributes to the long-term durability of the semipermeable membrane according to the present invention, even in the case of elevated temperature and high air humidity. According to the inventor's assessment, this effect is particularly prominent in the following case: when the organosilicon compound of the cover layer is at least partially applied to the composite material in a covalent bonded manner, wherein this combination suitably occurs on the silicon-containing porous filler of the composite material, which is also present on the surface of the carrier layer due to being substantially uniformly distributed in the plastic matrix. This covalent bonding can be achieved by a chemical and / or thermally induced reaction of the organosilicon compound with the silicon-containing porous filler, wherein the so-called "crosslinking agent" that promotes covalent bonding may be used if necessary. Here, it is particularly advantageous that the silicon-containing porous filler present on the surface of the composite material can provide a larger internal surface that can be used to combine the organosilicon compound due to its high porosity. Particularly preferred is the semipermeable membrane of the present invention, wherein at least a portion of the organosilicon compound of the cover layer is covalently bonded to the silicon-containing porous filler of the composite material.

[0049] In a particularly preferred embodiment, the covalent bonding of the organosilicon compound to the silicon-containing porous filler is achieved by a condensation reaction between Si-OR groups of the organosilicon compound (where R is hydrogen or an organic group) and silanol groups present on the surface of the silicon-containing porous filler (such as is the case on the surface of amorphous silicon dioxide). Preference is given to this aspect to a semipermeable membrane according to the invention in which the organosilicon compound is selected from organosilicon compounds having at least one Si-OR group, in which R is hydrogen or an organic group, preferably an alkyl group, particularly preferably an alkyl group having 1 to 10 C atoms. In this regard, preference is additionally or alternatively given to a semipermeable membrane according to the invention in which the silicon-containing porous filler comprises Si-OH groups on the surface.

[0050] In other words, preference is given to semipermeable membranes according to the invention in which the cover layer can be prepared by coating the support layer with one or more organosilicon compounds selected from organosilicon compounds having at least one Si-OR group, wherein R is hydrogen or an organic group, preferably an alkyl group, particularly preferably an alkyl group having 1 to 10 C atoms.

[0051] Although in the case of semipermeable membranes, due to the presence of mechanical loads, undesirable delamination of the cover layer is observed in many cases, which has an adverse effect on the performance and can even make the basic suitability in sensitive fuel cell systems problematic, based on the experiments of the inventors, the favorable bonding strength of the semipermeable membrane according to the invention, in particular when at least partial covalent bonding occurs, even allows the folding of the semipermeable membrane according to the invention. As a result, great advantages can be achieved in terms of space utilization and effectively available surface, so that the semipermeable membrane according to the invention is particularly suitable for use in fuel cell systems. Accordingly, also preferred is a semipermeable membrane according to the invention, wherein the semipermeable membrane is folded, preferably arranged in flat folds, such as known from flat folded filters.

[0052] According to the inventors' evaluation, particularly preferred is a semipermeable membrane for use in a membrane humidifier for a fuel cell system, the semipermeable membrane comprising:

[0053] aa) comprising a carrier layer of a composite material, the 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 zeolites and amorphous silicon dioxide, in particular amorphous silicon dioxide, wherein the total mass fraction of the plastic in the composite material is in the range of 15 to 75%, wherein the total mass fraction of the silicon-containing porous filler in the composite material is in the range of 25 to 85%, each based on the mass of the composite material, and

[0054] bb) a cover layer arranged on the carrier layer, the cover layer comprising at least one organosilicon compound, wherein the organosilicon compound is selected from siloxanes and polysiloxanes, wherein the total mass fraction of the organosilicon compound in the cover layer is in the range of 50 to 100%, based on the mass of the cover layer.

[0055] Furthermore, it is understood by those skilled in the art that the present invention also relates to a membrane humidifier, in particular for use in a fuel cell system, wherein the membrane humidifier comprises at least one, preferably two or more, semipermeable membranes according to the present invention.

[0056] The invention further 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.

[0057] Here, the membrane humidifier according to the present invention is arranged in the fluid pipeline system of the fuel cell, preferably in the fluid pipeline system on the cathode side. Here, the membrane humidifier is preferably arranged so that it is arranged in the fluid supply pipeline and the fluid discharge pipeline at the same time, so that the supplied process gas and the discharged process gas can flow through it, so that the supplied process gas and the discharged process gas are separated from each other in sections by the semipermeable membrane according to the present invention.

[0058] The present invention also relates to a method for preparing the semipermeable membrane of the present invention, the method comprising the following steps:

[0059] u) producing or providing a carrier layer consisting of a composite material comprising at least one plastic and at least one silicon-containing porous filler embedded in the plastic,

[0060] v) applying a coating composition comprising at least one organosilicon compound and at least one solvent on the surface of the carrier layer, and

[0061] w) evaporating the solvent to obtain a cover layer disposed on the carrier layer, the cover layer comprising at least one organosilicon compound.

[0062] Preference is given here to a method according to the invention in which the carrier layer is produced by extrusion of a plastics mixture comprising the silicon-containing porous filler, preferably using an extruder.

[0063] With regard to the achievable coating quality, in particular in the case of partially covalent bonding of the cover layer to the carrier layer, preference is given to a method according to the invention in which the evaporation of the solvent is carried out at a temperature in the range from 40 to 120° C., preferably in the range from 50 to 100° C., particularly preferably in the range from 60 to 90° C. Preference is therefore also given to a method according to the invention in which the evaporation of the solvent is carried out such that at least a portion of the organosilicon compound of the cover layer is covalently bonded to the silicon-containing porous filler of the composite material.

[0064] In view of effective process control, preference is given to the process according to the invention, wherein the solvent is selected from water, alcohols (especially methanol, ethanol and isopropanol), aqueous acids, aqueous bases and mixtures of these solvents, in particular selected from water, aqueous bases (especially aqueous solutions of alkoxides or hydroxides) and mixtures of these solvents, wherein the solvent preferably comprises water, alcohols and bases.

[0065] The present invention and preferred embodiments of the present invention are explained and described in more detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] In this figure:

[0067] Figure 1 A schematic cross-sectional view showing a preferred embodiment of the semipermeable membrane of the present invention;

[0068] Figure 2 shows four scanning electron microscope cross-sections through a semipermeable membrane of the present invention after storage in boiling water for ten days; and

[0069] Figure 3 Shown are two scanning electron microscope cross-sections through a semipermeable membrane of the present invention after storage in a drying oven at 105°C for ten days. DETAILED DESCRIPTION

[0070] Figure 1 A schematic cross-sectional view of a preferred embodiment of a semipermeable membrane 10 according to the present invention is shown. 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. Figure 1 The semipermeable membrane 10 is composed of a carrier layer 12 and a cover layer 14, which is arranged on one side of the carrier layer 12 and substantially completely covers the surface of the carrier layer 12 on this side. The carrier layer 12 is completely composed of a composite material, which contains polyethylene as a plastic matrix, in which porous amorphous silicon dioxide, i.e., precipitated silicon dioxide, is substantially uniformly dispersed. The carrier layer 12 has a substantially constant thickness of about 120 μm, while the cover layer 14 has a substantially constant thickness of about 3 μm.

[0071] exist Figure 1The presence of the silicon-containing porous filler in the plastic matrix of the composite material is shown in FIG. 1 by the star-shaped structure shown in the carrier layer 12 .

[0072] Figure 1 The semipermeable membrane 10 obtained in the process is water permeable but substantially air-impermeable. In this regard, the semipermeable membrane 10 can be regarded as substantially air-impermeable according to relevant standards in practical applications.

[0073] The silicon-containing porous filler has mesopores and macropores, so that the composite material itself is a mesoporous 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 about 65%, and the rest of the mass fraction is composed of polyethylene.

[0074] In the example shown, the covering layer 14 consists essentially entirely of polysiloxane having Si-OR groups (R=H or alkyl) and being at least partially covalently bonded to the silicon-containing porous filler of the composite material, i.e. by reaction of the Si-OR groups with silanol groups located on the surface of the filler particles.

[0075] The corresponding semipermeable membrane 10 can be produced starting from a carrier layer 12, the composite material of which consists of plastic and silicon-containing porous filler, and which can be produced, for example, by extruding a plastic mixture containing these two components. Subsequently, a coating composition is applied to the carrier layer 12, the coating composition comprising an organosilicon compound and at least one solvent, for example an aqueous solution of an alkoxide, which can be produced by mixing water, alcohol and base. Then, at an elevated temperature of, for example, 80° C., a cover layer 14 arranged on the carrier layer 12 is formed by evaporating the solvent, wherein the conditions in this method step are preferably adjusted so as to promote covalent bonding of the organosilicon compound of the cover layer 12 with the silicon-containing porous filler of the carrier layer 14.

[0076] Figure 2 and 3 Scanning electron microscope cross-sectional photographs of the semipermeable membrane according to the present invention (for preparation, see below) after different aging conditions are shown respectively, namely Figure 2 After storage in running boiling water (100°C) for ten days, and Figure 3 After ten days of storage at 105° C. in a drying cabinet. In order to obtain the fracture edges shown and to ensure that the photographs are representative, the specimens were immersed in liquid nitrogen and measured with a scanning electron microscope (REM) immediately after temperature-induced fracture (“cold brittle fracture”). The REM photographs clearly show the structure of the composite material in the carrier layer, in which the carrier matrix of the composite material, composed of polyolefin, is dispersed with clearly visible granular porous amorphous silicon dioxide. In all REM photographs, it is shown that despite the aging tests for the loads expected in a fuel cell, the Figure 2are respectively arranged below the carrier layer and in Figure 3 The corresponding cover layer arranged above the carrier layer can not only be clearly seen, but also substantially maintains a continuous coverage without defect points.

[0077] The present invention and preferred embodiments of the present invention are further illustrated and described below with additional reference to experiments.

[0078] A. Preparation of the semipermeable membrane according to the present invention:

[0079] Four semipermeable membranes E1 to E4 according to the invention were prepared. For this purpose, a carrier layer consisting of a composite material was provided, which is commercially available from PPG Industries under the trade name Teslin SP600. The carrier layer consists of a carrier matrix consisting of polyolefin, in which particulate porous amorphous silicon dioxide is dispersed. The average thickness of the carrier layer is about 150 μm, and the average weight per unit area is about 97 g / m 2 .

[0080] The cover layer was applied to the carrier layer by coating using an Easycoater coating device from Coatema. To prepare the coating composition, an organofunctional siloxane oligomer was prepared, which is commercially available from Evonik Operations under the trade name Dynasylan Hydrosil 2909. The organofunctional siloxane oligomer was diluted with a suitable solvent to obtain a coating composition (mass fraction of siloxane oligomer about 10%), which was applied uniformly to the carrier layer. A solid cover layer was obtained from the coating composition by evaporation of the solvent (T=70° C.; t=5 min). For film E1, a coating composition with an average layer thickness of about 22 μm was applied. For film E2, a substrate prepared in a manner similar to E1 was coated again, wherein the coating composition was applied again with an average layer thickness of about 22 μm. For film E3, a single-step coating method was selected again, wherein the coating composition was applied with an average layer thickness of about 44 μm. Film E4 was produced in a manner similar to film E1, but the negative pressure fixation using the coating device was omitted and the substrate was fixed with adhesive tape instead, in order to be able to exclude possible influences of the negative pressure fixation on the penetration of the coating composition into the substrate. The layer thickness of the cover layer in the dry state was calculated to be 2.2 μm (E1, E4) and 4.4 μm (E2, E3), respectively.

[0081] B. Comparison film:

[0082] First, an uncoated substrate was used as a comparative membrane V1, which was also processed into membranes E1 to E4. In addition, commercially available membranes currently used for fuel cell applications were used as comparative membranes, namely V2 (Nafion 110; Chemours; chemically stabilized copolymer formed from perfluorosulfonic acid and polytetrafluoroethylene; average thickness of about 127 μm), V3 (Fumasep F10120-PK; Fumatech; perfluorosulfonic acid-based membrane with long side chains (LSC), reinforced with PEEK fabric; average thickness of about 120 μm) and V4 (Fumasep F-950; Fumatech, unreinforced perfluorosulfonic acid-based membrane with long side chains (LSC), average thickness of about 50 μm).

[0083] C. Experiment:

[0084] The water permeability (WTR) was determined for the membranes under investigation in kilograms per square meter per day. The experimental design followed the device configuration disclosed for this purpose in EP 2435171 B1 and comprising two fluid line paths Z and A separated from one another by the membrane to be tested, each with an inlet and an outlet, wherein the properties of the test gas (air) supplied to the fluid line paths can be controlled and changes analyzed. In the design adopted, the area of ​​the respective membrane between the fluid line paths was 0.04 m 2 , the pressure of the gas supplied to the two fluid line paths is 200 kPa and the temperature is about 80° C. The first air flow Z is dried to approximately 0% air humidity, while the air humidity of the second air flow A is adjusted to about 90%, which corresponds to the conditions of a saturated air flow on the exhaust side typical for fuel cell applications. The volume flow rate of the test gas used has a significant influence on the determined water permeability. Since EP 2435171 B1 does not provide data on the volume flow rate, the inventors set the volume flow rate of the test gas used to Q=20 NL / min, because according to the inventors' estimation, reliable results can be obtained with this value. The measured values ​​are respectively taken as the average of three measurements.

[0085] For selected membranes, the gas permeability ("Gas Crossover") was also determined, for which only a slight adjustment to the experimental design described above was required. Under the same other conditions, the inlet of the second fluid line path was closed and the test gas (Q=70 NL / min) was guided only through the first fluid line path. In this design, the gas permeability can be measured as the volume flow at the outlet of the second fluid line path.

[0086] D. Results

[0087] The water transmission rate (WTR) was determined for the films of the invention and the comparative films in the manner described above. The results are summarized in Table 1.

[0088] Table 1

[0089]

[0090] The data shown in Table 1 clearly show that excellent water permeabilities can be achieved with the semipermeable membranes according to the invention, which are higher than some comparative membranes from the prior art even in the case of greater layer thicknesses. Here, in particular when using a single-step coating process, the water permeabilities are at a similar level to those of the best performing membranes from the prior art, without having to rely on the use of perfluorosulfonic acid-based materials, so that the semipermeable membranes according to the invention can be a very promising alternative to perfluorosulfonic acid-based systems without having to worry about the disadvantages associated with perfluorination, in particular with regard to the environment and health.

[0091] Comparative sample V1 shows an air permeability of 1.3 NL / min. Even in the case of the smallest coating thickness (E1), the air permeability of the film according to the invention is only 0.01 NL / min, which is essentially equivalent to 0% within the measurement error range and is in any case significantly lower than the typical market specified value of 0.5 NL / min.

[0092] In order to simulate the aging behavior, the membranes according to the invention according to E1 were aged under various conditions. Subsequently, the water permeability (WTR) of the aged membranes was investigated in each case in comparison with an uncoated substrate. It is particularly evident here that no delamination of the cover layer was observed in any of the aging tests, which indicates excellent durability.

[0093] In the aging experiment A1, the samples were functionally aged, wherein the aging conditions were matched to the loads to be expected in the fuel cell. For this purpose, the sample container filled with water was sealed with a membrane and temperature-treated in an oven at 90° C. during the aging process. As a result, the membrane experienced a continuous water vapor permeation at temperatures typical for fuel cell operation. The results are summarized in Table 2.

[0094] Table 2

[0095]

[0096] In aging experiment A2, the samples were aged at an elevated temperature of 110° C. in the absence of water vapor. The results are summarized in Table 3.

[0097] Table 3

[0098]

[0099] In aging experiment A3, the samples were stored in a boiling water bath (temperature about 100° C.) in order to test, inter alia, the susceptibility to peeling of the coating disclosed in EP 2 435 171 B1. The results are summarized in Table 4.

[0100] Table 4

[0101]

[0102] The results of the aging tests clearly show that favorable water permeability is maintained over a long period of time under aging conditions. At the same time, however, the difference in water permeability relative to the uncoated substrate is also maintained, so that in combination with no observed delamination of the cover layer, a very good service life and high durability of the coating can be concluded.

[0103] Furthermore, the favorable durability even under mechanical load was demonstrated by folding tests. For this purpose, the membrane was folded into a flat folded arrangement and then checked for water permeability. The water permeability ranged from 51 (kg / (m 2 *d) changes to 54(kg / (m 2 *d), this change is here within the measurement error.

[0104] Reference numerals

[0105] 10 Semipermeable membrane

[0106] 12 Carrier layer

[0107] 14 Covering

Claims

1. A semipermeable membrane (10), in particular for use in a membrane humidifier of a fuel cell system, the semipermeable membrane 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), the cover layer comprising at least one organosilicon compound.

2. The semipermeable membrane (10) according to claim 1, wherein the plastic is selected from thermoplastics, preferably from polyvinyl chloride and polyolefins.

3. The semipermeable membrane (10) according to claim 1 or 2, wherein the silicon-containing porous filler is selected from silicon-aluminum-phosphorus-oxygen compounds, silicon-containing metal organic framework compounds, zeolites and amorphous silica.

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

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

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

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

8. 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 claim 7.

9. A method for preparing a semipermeable membrane (10) according to any one of claims 1 to 6, comprising the following method steps: u) preparing or providing a carrier layer (12) consisting 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 on the surface of the carrier layer (12), and w) evaporating the solvent to obtain a cover layer (14) disposed on the carrier layer (12), the cover layer comprising at least one organosilicon compound.

10. The method according to claim 9, wherein the evaporation of the solvent is performed such that at least a portion of the organosilicon compound of the cover layer (14) is covalently bonded to the silicon-containing porous filler of the composite material.

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