Coating of cation exchange membranes

Abstract

The invention relates to the coating of cation exchange membranes with catalytically active substances. The catalytically actively coated cation exchange membranes are used in electrochemical cells, especially in fuel cells (proton exchange membrane fuel cells - PEMFC) or in electrolysers for water electrolysis (polymer electrolyte membrane water electrolysis - PEMWE). In order to counteract the disadvantages of conventional decal processes, an alterative process for coating cation exchange membranes was sought which enables the transfer of electrocatalysts without the need for high temperatures, high pressures and PFAS-based substrates. It was surprisingly found that catalyst layers which are treated, shortly before the transfer step, with a polymer-swelling solvent conducting the cations can be transferred far more easily.

Description

[0001] Coating of cation exchange membranes

[0002] The invention relates to the coating of cation exchange membranes with catalytically active substances. The catalytically active coated cation exchange membranes are used in electrochemical cells, particularly in fuel cells (Proton Exchange Membrane Fuel Cells - PEMFCs) or in electrolyzers for water electrolysis (Polymer Electrolyte Membrane Water Electrolysis - PEMWEs). In PEMFCs and PEMWEs, singly positively charged hydrogen atoms (H) are transferred across the membrane. + exchanged, which are also called protons. A cation exchange membrane within the meaning of the invention is therefore also a proton exchange membrane.

[0003] To increase efficiency, a thin layer of catalyst is required on the electrode or membrane surface in water electrolysis and fuel cells. The catalyst accelerates the electrochemical reaction in the cell and is therefore also called an electrocatalyst. The production of a catalyst-coated membrane, which forms the heart of an electrolyzer or fuel cell, significantly determines the costs and thus the economic viability of both technologies.

[0004] The coating of cation exchange membranes with electrocatalysts is now standardly carried out using the so-called "decal process." The particulate electrocatalyst is first mixed with a thermoplastic adhesion promoter to form a coating compound, which is then applied to a transfer substrate—usually a PTFE (polytetrafluoroethylene) film. The coated transfer substrate is then pressed onto the cation exchange membrane and heated. The thermoplastic melts and bonds the catalyst particles to the membrane. The transfer substrate is then peeled off the membrane like a decal and discarded. A coating consisting of the adhesion promoter and the immobilized catalyst particles remains on the membrane.

[0005] The decal process enables the production of a catalyst-coated membrane (CCM) via a roll-to-roll process, which is essential for industrial mass production. The catalyst layers are continuously coated onto the eponymous decal film using a printing process, dried, and then transferred to the polymer membrane by hot pressing. The membranes are made of cation-conducting polymers. These are typically sulfonated, perfluorinated polymers such as Nation® from The Chemours Company, Wilmington, Delaware (US). A comprehensive description of the decal process can be found in the dissertation by:

[0006] Fröhlich, Konstantin: The decal process for the production of catalyst-coated membranes for PEM fuel cells. KIT Scientific Publishing, 2015.

[0007] DOI: 10.5445 / KSP / 1000045306

[0008] However, the decal process requires harsh conditions: typical process parameters are transfer temperatures of around 150 °C, pressures of several megapascals and pressing times of several minutes.

[0009] Furthermore, per- and polyfluorinated alkyl compounds (PFAS), particularly polytetrafluoroethylene (PTFE), are used almost exclusively as transfer substrates. These are necessary due to their low surface energy and the resulting good transferability of the electrode layer, while simultaneously offering good temperature resistance.

[0010] An example of this practice is CN113745538A, in which a water-moistened catalyst layer is transferred from a PTFE transfer substrate to a proton exchange membrane at temperatures of 180°C and 0.1 MPa.

[0011] US 2010 / 183804 A1 describes a process for producing a membrane electrode assembly (MEA) in which a NafionO-containing catalyst ink is applied to a TeflonO transfer substrate and dried. The catalyst ink is then brushed with a swelling agent, and the MEA is pressed at a pressure of 0.25 to 3 MPa at temperatures below 150°C. The disadvantage of this process is again the harsh conditions and the need for a fluorine-containing transfer substrate.

[0012] The use of these process parameters enables a continuous process; however, the relatively high temperatures (necessary to reach the glass transition temperature of the polymer membrane), pressures, and residence times make this process inefficient. Furthermore, the PFAS-based substrates are expensive and virtually non-reusable. The use of perfluorinated polymers as "single-use" items is increasingly viewed with skepticism due to their potential environmental impact.

[0013] To counteract the disadvantages of the classic decal process, an alternative process for coating cation exchange membranes was sought that enables electrocatalyst transfer without requiring high temperatures, high pressures and PFAS-based substrates.

[0014] In the application EP23164605, which was still unpublished at the time of filing, a

[0015] A process for coating anion exchange membranes is described. This process involves processing positively charged ionomers, which naturally repel cations and are therefore unsuitable for the production of cation-conducting membranes.

[0016] It was surprisingly found that catalyst layers that were treated with a solvent that swells the cation-conducting polymer shortly before the transfer step can be transferred much more easily.

[0017] The invention therefore relates to a method for coating

[0018] Cation exchange membranes, comprising the following non-chronological steps: a) providing a flat cation exchange membrane which contains or consists of a membrane material; b) providing a first flat transfer substrate which is coated on at least one side with a first composition, wherein the first composition contains at least one first polymer and at least one first catalytically active or catalytically activatable substance; c) providing a first swelling agent which is at least partially in liquid form; d) applying the first swelling agent to the first composition and / or to the cation exchange membrane; g) swelling the first polymer with the first swelling agent; e) pressing the first composition with the cation exchange membrane in the presence of the first transfer substrate, wherein the temperature of the first composition during pressing is between -90°C and 100°C;f) peeling the first transfer substrate from the first composition to obtain a cation exchange membrane coated on one side with the first composition;

[0019] Such a method is the subject of the invention.

[0020] Due to swelling, the catalyst layer can be transferred even below the glass transition temperature of the polymer membrane. The temperature of the first composition and / or the second composition during compression is preferably between -30°C and 80°C or between -15°C and 70°C. In particular, the temperature of the first composition is at room temperature, i.e., approximately between 10°C and 30°C.

[0021] Furthermore, the transfer can be performed at lower pressures. The compression pressure should be between 0.001 MPa and 0.15 MPa. Compression is preferably performed at a pressure of 0.01 MPa to 0.1 MPa. This process also allows the transfer of catalyst layers printed onto conventional non-fluorinated films, such as PET films, which offers enormous economic and ecological advantages.

[0022] The swelling agent is a substance that is at least partially in liquid form and swells the polymer contained in the composition when it comes into contact with the polymer. Swelling means that the swelling agent penetrates the polymer and expands it. The volume of the polymer therefore increases continuously upon contact with the swelling agent as the swelling agent penetrates the polymer. Swelling is therefore a process that requires a certain amount of time. Swelling agents also include substances or mixtures of substances that dissolve the polymer. Contact between the polymer and the swelling agent occurs when the swelling agent is applied to the composition or during pressing. Swelling can therefore occur simultaneously with application and / or pressing, but can also take place earlier or later. The chronology of the sequence of steps is not important.In particular, swelling does not have to occur separately; it can also take place simultaneously with other process steps. The process can also include holding phases in which the swelling agent is given time to swell the polymer. A chemical reaction between the swelling agent and the polymer is not necessary during swelling, but is not excluded either. In particular, it is possible for the swelling agent to partially or completely dissolve the polymer. It is important that the swelling agent is at least partially in liquid form when it comes into contact with the polymer. This promotes swelling. The swelling agent can also be partially in the gas phase, but then has less effective contact with the polymer, which causes the polymer to swell more slowly. In the interest of high process efficiency, the use of a liquid swelling agent is therefore recommended.

[0023] As already described, the transfer of the catalyst-containing composition from the transfer substrate to the cation exchange membrane is facilitated by swelling immediately before compression. For this reason, the swelling agent is preferably applied at a time prior to compression, at a time interval sufficient for the swelling process, but not longer. The swelling agent is therefore applied as late as possible, but as early as necessary.

[0024] In any case, it is too early to provide the first swelling agent already in the composition. In an unfavorable case, this could lead to an interaction between the first swelling agent and the first polymer long before pressing. This could result in premature detachment of the composition from the transfer substrate or even dissolution of the first polymer. This is particularly to be feared if the coated transfer substrate is provided at a different production site than the pressing. According to the invention, the swelling agent is therefore provided outside the first composition, i.e. separately. An interaction between the first swelling agent and the first polymer can only occur once the first swelling agent has been applied to the first composition and / or to the cation exchange membrane.By choosing the time of application, the duration of the interaction between the first swelling agent and the first polymer can be optimally controlled.

[0025] In the simplest case, the initial composition is solid at the time the first swelling agent is applied. Applying the swelling agent to the dry composition softens the solid coating on the transfer substrate, facilitating transfer.

[0026] Apart from the swelling step and the processing temperature, the process according to the invention is similar to a decal process. However, due to the use of the partially liquid swelling agent, the process according to the invention can be considered wet. Nevertheless, a lengthy and energy-intensive drying step can be omitted because the swelling agent is used only in small quantities and evaporates over time under ambient conditions. This applies particularly to swelling agents with a low boiling point, such as ethanol.

[0027] In principle, the process according to the invention involves processing cation exchange membranes in a flat shape. "Flat" here means that the height (or thickness) of the cation exchange membrane is much smaller than its length and width. The cation exchange membrane thus has an essentially two-dimensional shape. Any embossing on the surface of the membrane does not change its flat shape. It is therefore a flat membrane. The transfer substrate is also flat in a similar manner.

[0028] In the process according to the invention, the cation exchange membrane is coated on at least one side with a first composition. Since cation exchange membranes are usually used in electrochemical cells that comprise two compartments separated from the membrane, each filled with an electrocatalyst, it makes sense to coat the cation exchange membrane on both sides. More precisely, the cation exchange membrane is coated on one side with the first composition and on the other side with a second composition. The first composition then contains, for example, the catalyst for the cathode reaction and the second composition the catalyst for the anode reaction. This is possible in the process according to the invention, both simultaneously and serially. According to a preferred embodiment of the invention, the process additionally comprises the following,Non-chronological steps: h) Providing a second flat transfer substrate coated on at least one side with a second composition, wherein the second composition contains at least one second polymer and at least one second catalytically active or catalytically activatable substance; i) Providing a second swelling agent which is at least partially in liquid form; j) Applying the second swelling agent to the second composition and / or to the cation exchange membrane; k) Swelling the second polymer with the second swelling agent; l) Pressing the second composition with the cation exchange membrane in the presence of the second transfer substrate, wherein the temperature of the second composition during pressing is between -90°C and 100°C; m) Peeling the second transfer substrate from the second composition, so that a cation exchange membrane is obtained,which is coated on one side with the first composition and which is coated on the other side with the second composition.,

[0029] Such a process therefore aims at coating the cation exchange membrane on both sides: On one side, the coating is carried out with the first composition, and on the other side, the second composition is applied. The first and second compositions preferably have different formulations; the formulations can differ, in particular, in the catalyst they contain. However, the two compositions can also be essentially identical.

[0030] Preferably, the first composition is pressed onto the cation exchange membrane in the presence of the second transfer substrate, and the second composition is pressed onto the cation exchange membrane in the presence of the first transfer substrate. Thus, both compositions are pressed onto the membrane simultaneously. This increases the efficiency of the process.

[0031] The pressure during pressing should be between 0.001 MPa and 0.15 MPa. Preferably, pressing takes place at a pressure of 0.01 MPa to 0.1 MPa. The specified pressure ranges apply to both the pressing of the first composition with the cation exchange membrane and the pressing of the second composition with the cation exchange membrane. If both compositions are pressed simultaneously in the presence of their respective transfer substrates, the same pressure necessarily acts on the entire laminate. In this context, it should be noted that conventional decal processes operate with pressures of 0.3 MPa to 10 MPa. The process according to the invention therefore uses significantly lower pressures.

[0032] There are different options for applying the first swelling agent. It can be applied exclusively to the first composition, exclusively to the cation exchange membrane, or both. If a second swelling agent is used, the same options apply. The choice of application location depends on the swelling behavior of the swelling agent in the associated polymer. Ideally, the swelling agent is applied to a location where the transfer substrate and / or the cation exchange membrane are located immediately prior to compression. The time of application is chosen so that swelling can occur to the desired extent, but does not last longer than necessary.

[0033] A particular advantage of the process according to the invention is that the transfer substrate can be peeled off the swollen polymer particularly easily. Therefore, the process according to the invention allows the use of a transfer substrate made of a fluorine-free polymer. Known decal processes generally use fluoropolymer films, such as PTFE, as the transfer substrate to reduce adhesion and thus facilitate removal. Since the transfer substrate is generally discarded, the use of fluoropolymers is questionable for environmental reasons. In the process according to the invention, however, a transfer substrate made of a harmless, inexpensive, fluorine-free polymer such as polyethylene (PE) or polypropylene (PP) or polyethylene terephthalate (PET) can be used. A particular embodiment of the invention is therefore characterized in that at least one transfer substrate consists predominantly of a fluorine-free polymer.

[0034] If coating is carried out on both sides, it is advisable to use the same swelling agent for both sides. The first swelling agent therefore corresponds to the second swelling agent. In the simplest case, “corresponding” means that the first swelling agent and the second swelling agent are the same chemical substance. However, in operational practice the swelling agents are not used as pure substances, but in technical quality. This means that the swelling agent is used together with impurities or technically necessary accompanying substances, for example 98% ethanol and 2% water, because water cannot be completely removed from the ethanol without reasonable effort. If the first swelling agent is used in a different technical quality than the second swelling agent, but both cases contain the same substance, the first swelling agent corresponds to the second swelling agent within the meaning of the terminology used here.Thus, two batches of swelling agent with different water contents in the ethanol can be equivalent within the meaning of the invention, even though both batches are not exactly identical. The crucial factor is that the same substance, for example ethanol, is used. The different water contents are not important in this example. Analogously, it is advisable for the first polymer to be equivalent to the second polymer. Technically caused impurities in the polymer are not important in this case. Polymers can be equivalent even if they contain the same repeating unit but have different chain lengths. Preferably, the first polymer and the second polymer have the same repeating unit.

[0035] The polymer swollen with the swelling agent is preferably a cation-conducting polymer, i.e., an ionomer. Accordingly, the first polymer and / or the second polymer are cation-conducting. The use of non-ion-conducting polymers as adhesion promoters to immobilize the catalysts on the membrane is also conceivable; however, this is discouraged in the interest of the efficiency of the resulting cell.

[0036] In the process, a sulfonated, perfluorinated polymer is preferably used as the cation-conducting polymer, preferably as universally as possible. Preferably, the same cation-conducting polymer is used in the first and / or second composition as the first or second polymer for immobilizing the catalytically active substance. Cation-conducting polymers with the same repeating unit are particularly preferably used in the compositions and as the membrane material. The conductivity of the membrane for anions is intrinsically enabled by the cationic polymer. The chain length of the cationic polymers with the same repeating unit can vary.

[0037] It is particularly preferred to use the same cation-conducting polymer contained in the cation exchange membrane to immobilize the catalysts on the membrane. This reduces the number of substances to be processed. Accordingly, this preferred variant of the process is characterized in that the cation exchange membrane contains or consists of a cation-conducting polymer, and that this cation-conducting polymer is identical to the first polymer and / or the second polymer.

[0038] The use of the fluorine-free polymer as a transfer substrate is particularly advantageous when the transfer substrate is not reused and is therefore discarded after peeling.

[0039] Fluorine-free transfer substrates made of PP, PE, or PET are comparatively easy to dispose of and recycle and do not require fluorocarbons for their production. Furthermore, PP, PE, or PET films are more cost-effective than conventional transfer substrates made of PTFE.

[0040] In a particularly preferred embodiment, the method according to the invention is carried out as a roll-to-roll process. This means that the starting materials are at least partially provided in strip form on a spool and that the target product is also obtained in strip form on a spool. The strip-shaped materials move continuously from roll to roll through the process. The process is therefore particularly productive. Specifically, the roll-to-roll version of the process according to the invention provides that during operation at least one circulating spool is wound up or unwound, wherein the spool is selected from the group comprising the following spools: i) a spool on which the cation exchange membrane is provided in strip form; ii) a spool on which the coated cation exchange membrane is obtained in strip form; iii) a spool on which a transfer substrate is provided in strip form.

[0041] Preferably, two or three of the above-mentioned reels are wound or unwound. In the roll-to-roll process, the process steps toward the strip-shaped materials are performed sequentially along the feed direction; however, from a global perspective, the steps occur simultaneously because it is a continuous process.

[0042] Alternatively, the process can also be carried out in batches, using non-band-shaped, stationary materials.

[0043] In the roll-to-roll process, pressing is advantageously accomplished using a stationary pair of rollers, with at least one of the rollers rolling or rolling on a previously mentioned strip-shaped transfer substrate. This allows the compressive force required for pressing to be continuously applied to the transfer substrate, thus increasing production quality.

[0044] A particular advantage of the process is that, compared to conventional decal processes, it can be carried out at low temperatures. This eliminates the need for separate heating of the rollers. Conventional decal processes typically use heated rollers to soften the ionomer. This is not necessary here thanks to the swelling process. A preferred embodiment of the invention therefore provides for the rollers of the roller pair to be unheated.

[0045] In the roll-to-roll process, peeling is carried out using at least one stationary peeling roller, which rolls or rolls on a previously mentioned belt-shaped transfer substrate. This stabilizes the peeling process, thus increasing productivity and quality.

[0046] In the process according to the invention, the cation exchange membrane is coated with at least one composition containing an electrocatalyst. Electrocatalysts are substances that act catalytically in electrochemical reactions or can be activated to act catalytically. According to current research, the following elements can be processed in the process in pure form, as oxide, as hydroxide, or as oxide hydroxide: iridium (Ir), nickel (Ni), cobalt (Co), chromium (Cr), iron (Fe), ruthenium (Ru), copper (Cu), molybdenum (Mo), zinc (Zn), lead (Pb), manganese (Mn), tungsten (W), platinum (Pt), sulfur (S), tin (Sn), gold (Au), silver (Ag), palladium (Pd), rhenium (Re), and rhodium (Rh). These elements can be used as electrocatalysts in pure form, as oxides, as hydroxides, or as oxide hydroxides, individually or in combination.

[0047] Particularly preferred is a composition containing platinum or a platinum alloy supported on carbon (Pt / C) or an iridium-based supported or unsupported catalyst as the electrocatalyst. Preferred support materials for the iridium-based catalyst are antimony-doped tin oxide (ATO) and niobium- or tantalum-doped titanium oxide.

[0048] The swelling agent used in the process is selected to at least swell the polymer or ionomer in the coating composition. Since the ionomers used in water electrolysis and water synthesis must, by their very nature, exhibit high resistance to water absorption, aqueous swelling agents are generally unsuitable. Organic swelling agents are more suitable. The following swelling agents have proven particularly suitable: methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), isopropanol (IPA), methanol (MeOH), and ethanol (EtOH). These swelling agents can be used individually or in combination. Dimethyl sulfoxide (DMSO) and / or ethanol (EtOH) are particularly preferred. The latter swelling agents fulfill their purpose quite well and are also comparatively human-friendly, which simplifies their handling and processing.The swelling agent can also be mixed with substances that do not swell the polymer or do not swell sufficiently, such as water.

[0049] Preferably, the swelling agent, its amount used and its contact time are selected so that it not only swells the respective polymer but also partially dissolves it. Dissolution is understood to mean the removal of the intermolecular bonding forces within the polymer. Every dissolution of the polymer is preceded by swelling. It is therefore often a question of the contact time when swelling turns into dissolution of the polymer. Dissolution is defined here as the dissolution of the polymer in a thin boundary layer of contact with the swelling agent. If the polymer is partially dissolved, the transfer tape can be removed particularly easily. A preferred development of the method therefore provides that the first swelling agent at least partially dissolves the first polymer and / or that the second swelling agent at least partially dissolves the second polymer.

[0050] Most preferably, a combination of a cation-conducting polymer and an organic substance is used as a swelling agent, namely a sulfonated, perfluorinated polymer and dimethyl sulfoxide and / or ethanol as swelling agent.

[0051] The mass ratio q / p between the swelling agent and the cation-conducting polymer is preferably between 0.01 and 2. Here, q is the mass of the swelling agent applied to the cation-conducting polymer, and p is the mass of the cation-conducting polymer. A low specific swelling agent usage allows processing without separate drying.

[0052] For the purposes of the method according to the invention, the cation exchange membrane does not necessarily have to be coated directly with the composition. It is also conceivable to use a cation exchange membrane that is already provided with a precoating. The composition is then applied to the precoating. The precoating is then located between the actual cation exchange membrane and the catalytically active coating. The precoating can improve the adhesion of the electrocatalyst to the membrane. A variant of the method according to the invention therefore provides that the cation exchange membrane is provided with at least one precoating, such that the composition is applied to the precoating.

[0053] The cation exchange membrane coated according to the invention can be installed in electrochemical cells, particularly as a catalyst coated membrane (CCM). The electrochemical cell can be a PEM electrolyzer or a PEM fuel cell. The electrolyzer can be used for the electrolysis of water. The PEM fuel cell serves to convert chemical energy into electrical energy by synthesizing hydrogen and oxygen to form water.

[0054] An embodiment of a process according to the invention will now be explained in more detail with reference to a drawing. It shows:

[0055] Figure 1: Schematic representation of a roll-to-roll process.

[0056] Figure 1 shows a schematic representation of the process according to the invention. The process shown is a roll-to-roll process.

[0057] A flat, ribbon-shaped cation exchange membrane 2 is provided on a first coil 1. The cation exchange membrane 2 consists entirely of a cation-conducting polymer (ionomer). The cation exchange membrane 2 is unwound from the first coil 1.

[0058] A first transfer substrate 4 is provided on a second coil 3. The first transfer substrate 4 is a flat strip made of polyethylene terephthalate (film) coated on one side with a first composition 5. The coating with the first composition 5 is dry, i.e. solid, in the provided state. The first composition 5 comprises the same cation-conducting polymer (ionomer) from which the cation exchange membrane 2 is made, and a first electrocatalyst is dispersed therein. The electrocatalyst is a substance that catalyzes, for example, the electrolysis or synthesis of water. More precisely, the electrocatalyst contained in the first composition 5 catalyzes the cathode reaction in an electrolyzer or a fuel cell.

[0059] The first transfer substrate 4 is unwound from the second reel 3 and contacted with the cation exchange membrane 2 by means of a first contact roller 6.

[0060] Similarly, a second transfer substrate 7 is unwound from a third reel 8 and contacted with the cation exchange membrane 2 by means of a second contact roller 9. The second transfer substrate 7 carries a second composition 10 containing the same ionomer but a different electrocatalyst. The other electrocatalyst catalyzes the anode reaction.

[0061] While the first transfer substrate 4 is in contact with the top side of the cation exchange membrane 2, the second transfer substrate 7 is in contact with the bottom side of the cation exchange membrane 2. Contact with the two rollers 6, 9 occurs simultaneously at the same point along the path of the cation exchange membrane.

[0062] Immediately before the two transfer substrates 4, 7 are contacted with the cation exchange membrane 2, the two compositions 5, 10 are each sprayed with a swelling agent 11. The swelling agent 11 is a liquid organic substance that swells the ionomer. The same swelling agent 11 is used for both compositions 5, 10. It is also conceivable to use a different swelling agent for the two compositions: a first swelling agent for the first composition 5 and a second swelling agent for the second composition 10. However, since both compositions contain the same cation-conducting polymer, it also makes sense to use the same swelling agent 11 for both compositions 5, 10.

[0063] The two compositions 5, 10 swell after spraying the liquid swelling agent 11. More precisely, the ionomer contained in both compositions swells because the swelling agent 11 penetrates the ionomer.

[0064] With the help of a stationary pair of rollers 12, 13, the first transfer substrate 4 with the first composition 5 and the second transfer substrate 7 with the second composition 10 are pressed together with the cation exchange membrane 2. The two rollers 12, 13 roll on the layer of the two transfer substrates 4, 7 and the cation exchange membrane 2 arranged between them. The two rollers 12, 13 press against each other with a force F, which presses the two swollen compositions 5, 10 together with the cation exchange membrane 2. The force F acts perpendicular to the plane in which the cation exchange membrane 2 extends. The force acts through the two transfer substrates 4, 7. The two transfer substrates 4, 7 insulate the two rollers of the roller pair 12, 13 from the composition 5, 10, so that they do not become contaminated. The pressing of the two compositions 5, 10 with the cation exchange membrane 2 takes place at room temperature.The rollers of the roller pair 12, 13 are not heated.

[0065] By pressing, a band-shaped sandwich 14 is obtained with the following layer structure (from top to bottom):

[0066] • First transfer substrate 4

[0067] • first composition 5

[0068] • Cation exchange membrane 2

[0069] • second composition 10

[0070] • second transfer substrate 7.

[0071] Due to the same ionomer in the two compositions 5 and 10 and in the cation exchange membrane 2, the transition from the first composition 5 into the cation exchange membrane 2 and further into the second composition 10 is smooth: In principle, the first electrocatalyst is enriched on the upper side of the cation exchange membrane 2, while the second electrocatalyst is enriched on the lower side of the

[0072] Cation exchange membrane 2 is enriched. The ionomer immobilizes the catalyst particles on both sides of membrane 2 and simultaneously creates a cation-conducting connection between the catalyst particles and through membrane 2.

[0073] After the two compositions 5, 10 have been pressed together with the cation exchange membrane 2, the two transfer substrates 4, 7 are no longer required. They are peeled off the sandwich 14 using two peeling rollers 15, 16, leaving a cation exchange membrane 17 coated on both sides with electrocatalyst. This is the target product of the process. The coated cation exchange membrane 17 is wound onto a fourth reel 18.

[0074] The two peeled-off transfer substrates 4, 7 are also wound onto a fifth or sixth reel 19, 20.

[0075] When the first spool 1 is empty, the process is stopped and the two spools 19, 20 with the peeled-off transfer substrates 4, 7 are discarded. Since these are PET films, they can be easily disposed of. The fourth spool 18 with the coated cation exchange membrane 17 is the target product in the container. Optionally, the coated cation exchange membrane 17 can be provided with a protective film when wound onto the fourth spool 18 (not shown). The protective film prevents damage to the coating. It is advisable to use one or both of the peeled-off transfer substrates 4, 7 as a protective film; this saves material and produces less waste. If, for example, the second peeled-off transfer substrate 7 is to be used as a protective film, it is not wound onto the sixth spool 20 but rather wound onto the fourth spool 18 together with the coated cation exchange membrane 17.

[0076] Then, a fresh first coil 1 with uncoated cation exchange membrane 2 is provided, as well as a fresh second coil 3 with fresh first transfer substrate 4 and fresh first composition 5, and a fresh third coil 8 with fresh second transfer substrate 7 and fresh second composition 10. The process starts from the new one.

[0077] By means of the roll-to-roll process described above, a flat, ribbon-shaped cation exchange membrane 2 is efficiently coated on both sides with electrocatalyst, so that a flat, ribbon-shaped, double-sided coated cation exchange membrane 17 is obtained.

[0078] This is cut to size and then installed into an electrolyzer or fuel cell such that the side of the coated cation exchange membrane 17, which catalyzes the anode reaction, is oriented toward the anode, while the other side is oriented toward the cathode (not shown). The resulting electrolyzer is used for the PEM-based electrochemical splitting of water into oxygen and hydrogen (PEMWE). A similarly manufactured fuel cell (PEMFC) is used, conversely, to convert chemical energy stored in hydrogen and oxygen into electrical energy.

[0079] Examples:

[0080] The following describes the transfer process for PEM applications using a platinum / carbon-based catalyst layer as an example. This experimentally demonstrates the effects achieved with the invention. Nation® was used as the cation- or proton-conducting (anionic) polymer. Nation® was used both as a membrane (Nation N115) and as an ionomer (D2021 Nation® solution).

[0081] 1. Providing a composition

[0082] A screw-cap jar was filled one-third with yttrium-stabilized zirconium oxide grinding beads (5 mm diameter). Then, 9 parts by weight of an ionomer solution (D2021 Nation® solution) were added to the screw-cap jar and diluted with 82 parts by weight of 1-propanol. Finally, 9 parts by weight of a platinum-on-carbon catalyst (Pt / C ~ 1 / 1) were added under oxygen exclusion and predispersed using a shaker. The ink was dispersed in the screw-cap jar using a roller-tilt mixer for 72 hours.

[0083] 2. Coating a transfer substrate with the composition

[0084] The composition provided under 1. was applied to a commercially available film made of fluoroethylene propylene (FEP - S1833-16, Bola) using a box doctor blade. Figure 2 shows a photo of this.

[0085] Figure 2: Coating of the transfer substrate with the composition

[0086] A box-type doctor blade with a squeegee gap of 50 μm was used. The resulting film thickness after drying (at 20 °C for 16 h) was approximately 7 μm.

[0087] 3. Transfer process

[0088] To selectively coat a specific geometric shape of the proton exchange membrane and thus potentially save catalyst, this shape can be separated from the coated substrate and then transferred to the membrane. In this example, 2 cm x 2 cm squares were cut from the substrate coated the day before to transfer the catalyst layer to a larger, rectangular piece of the membrane. For easier handling, the membrane piece was fixed to a stainless steel disc using transparent adhesive tape at the edges. Figure 3 shows a photo of this. Figure 3: Membrane (transparent film) fixed to a stainless steel disc.

[0089] The cut piece of coated FEP film is immersed in a container of ethyl alcohol for approximately 2 seconds, and the remaining swelling agent is then dripped off one of the corners of the transfer film. A photo of this is shown in Figure 4.

[0090] Figure 4: Wetting of the catalyst coating of the coated transfer substrate with swelling agent (here: ethyl alcohol)

[0091] The coated transfer substrate soaked in ethyl alcohol is placed on the cut membrane with the catalyst layer side facing up (photo in Figure 5).

[0092] Figure 5: Membrane with applied coated transfer substrate, before pressing

[0093] In this example, the contact pressure of approximately 0.07 MPa was achieved using the weight of planar stainless steel discs stacked on top of the membrane-transfer substrate assembly (photo in Figure 6). An absorbent fabric (more specifically, a cleanroom wipe) was inserted to absorb excess swelling agent. In this example, transfer was achieved within 60 seconds at 20 °C.

[0094] Through the combination of swelling agent and contact pressure, the catalyst layer is completely transferred to the membrane.

[0095] Figure 6: Transfer by pressing

[0096] As shown in Figure 7, after transferring the catalyst layer to the membrane, the FEP film was peeled off, leaving the catalyst layer on the membrane.

[0097] Figure 7: Left in the picture: Catalyst-coated membrane after successful transfer and removal of the transfer substrate

[0098] Right in the picture: Transfer substrate without catalyst, after peeling off the catalyst-coated membrane

[0099] 4. Comparison test: Transfer without swelling agent (not according to the invention)

[0100] Without wetting the catalyst layer with a swelling agent, no transfer of the catalyst layer occurred even under significantly higher pressure in a plate press (2 min at 60 MPa).

[0101] Figure 8: Largely unsuccessful transfer without source funds

[0102] Left image: Transfer film still almost completely coated with catalyst. Without swelling agent, >95% of the catalyst remains on the transfer film and is not transferred to the membrane. Right image: Membrane with insufficient transfer of the catalyst layer.

[0103] 5. Conclusion

[0104] From the comparison of the transfers shown in Figures 7 and 8 it can be seen that according to the invention the efficiency of the transfer of the catalyst layer at low temperatures can be significantly improved with the aid of swelling agents.

[0105] Reference symbol

[0106] 1 first coil

[0107] 2 cation exchange membrane

[0108] 3 second coil

[0109] 4 first transfer substrate

[0110] 5 first composition

[0111] 6 first contact roller

[0112] 7 second transfer substrate

[0113] 8 third coil

[0114] 9 second contact roller

[0115] 10 second composition

[0116] 11 swelling agents

[0117] 12 pairs of rollers

[0118] 13 pairs of rollers

[0119] 14 Sandwich

[0120] 15 first peeling roller

[0121] 16 second peeling roller

[0122] 17 coated cation exchange membrane

[0123] 18 fourth coil

[0124] 19 fifth coil

[0125] 20 six coils

[0126] F Force

Claims

Patent claims 1. A method for coating cation exchange membranes, comprising the following non-chronological steps: a) providing a flat cation exchange membrane which contains or consists of a membrane material; b) providing a first flat transfer substrate which is coated on at least one side with a first composition, wherein the first composition contains at least one first polymer and at least one first catalytically active or catalytically activatable substance; c) providing a first swelling agent which is at least partially in liquid form; d) applying the first swelling agent to the first composition and / or to the cation exchange membrane;e) pressing the first composition with the cation exchange membrane in the presence of the first transfer substrate; f) peeling the first transfer substrate from the first composition to obtain a cation exchange membrane coated on one side with the first composition; characterized in that the temperature of the first composition during pressing is between -90°C and 100°C, and that the method further comprises the following step: g) swelling the first polymer with the first swelling agent; 2. The method according to claim 1, additionally comprising the following, non-chronological steps: h) providing a second flat transfer substrate which is coated on at least one side with a second composition, wherein the second composition contains at least one second polymer and at least one second catalytically active or catalytically activatable substance; i) providing a second swelling agent which is at least partially in liquid form; j) applying the second swelling agent to the second composition and / or to the cation exchange membrane; k) swelling the second polymer with the second swelling agent; l) pressing the second composition with the cation exchange membrane in the presence of the second transfer substrate, wherein the temperature of the second composition during pressing is between -90°C and 100°C; m) peeling the second transfer substrate from the second composition to obtain a cation exchange membrane coated on one side with the first composition and coated on the other side with the second composition.

3. The method according to claim 2, characterized in that the pressing of the first composition with the cation exchange membrane takes place in the presence of the second transfer substrate and the pressing of the second composition with the cation exchange membrane takes place in the presence of the first transfer substrate.

4. The method according to any one of claims 1 to 3, characterized in that the first swelling agent is applied exclusively to the first composition, or that the first swelling agent is applied exclusively to the cation exchange membrane, or that the first swelling agent is applied both to the first composition and to the cation exchange membrane.

5. A method according to any one of claims 1 to 4, characterized in that the first swelling agent is provided outside the first composition.

6. A method according to any one of claims 1 to 5, wherein the first composition is solid at the time of application of the first swelling agent.

7. Method according to one of claims 1 to 6, characterized in that at least one said transfer substrate consists predominantly of a fluorine-free polymer.

8. Method according to one of claims 2 to 7, characterized in that the first swelling agent corresponds to the second swelling agent.

9. Process according to one of claims 2 to 8, characterized in that the first polymer and the second polymer have the same repeating unit.

10. The method according to any one of claims 1 to 9, characterized in that the first polymer and / or the second polymer is cation-conductive.

11. The method according to any one of claims 1 to 10, characterized in that at least one of the following substances is a sulfonated, perfluorinated polymer: o the membrane material; o the first polymer; o the second polymer.

12. The method according to any one of claims 1 to 11, characterized in that at least one of said swelling agents is an organic substance, or that the swelling agent is selected from the group consisting of the following swelling agents: methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), isopropanol (IPA), methanol (MeOH), ethanol (EtOH).

13. Process according to claims 11 and 12, wherein the mass of the sulfonated, perfluorinated polymer is p, wherein the mass of the swelling agent applied to the sulfonated, perfluorinated polymer is q, and wherein the ratio q / p is: 0.01 < q / p < 2 14. The method according to any one of claims 1 to 13, characterized in that the first swelling agent at least partially dissolves the first polymer and / or that the second swelling agent at least partially dissolves the second polymer.

15. A process according to any one of claims 1 to 14, characterized in that at least one of said swelling agents is present in a mixture.

16. The method according to any one of claims 7 to 15, characterized in that at least one of said carrier substrates is discarded after peeling or is used as a protective film for the coated cation exchange membrane.

17. A method according to any one of claims 1 to 16, characterized by at least one rotating coil which is wound or unwound during operation of the method, the coil being selected from the group comprising the following coils: i) a coil on which the cation exchange membrane is provided in ribbon form; ii) a coil on which the coated cation exchange membrane is obtained in ribbon form; iii) a coil on which said transfer substrate is provided in ribbon form.

18. The method according to claim 17, characterized in that the pressing is carried out by means of a stationary pair of rollers, wherein at least one roller of the pair of rollers rolls or rolls on an already mentioned band-shaped transfer substrate.

19. Method according to claim 18, characterized in that the rollers of the roller pair are unheated.

20. Method according to one of claims 17 to 19, characterized in that the peeling is carried out with the aid of at least one stationary peeling roller, the peeling roller rolling or rolling on an already mentioned band-shaped transfer substrate.

21. The method according to any one of claims 1 to 20, characterized in that at least one said catalytically active or catalytically activatable substance contains at least one element selected from the group consisting of the following elements: iridium (Ir), nickel (Ni), cobalt (Co), chromium (Cr), iron (Fe), ruthenium (Ru), copper (Cu), molybdenum (Mo), zinc (Zn), lead (Pb), manganese (Mn), tungsten (W), platinum (Pt), sulfur (S), tin (Sn), gold (Au), silver (Ag), palladium (Pd), rhenium (Re), rhodium (Rh), wherein the element is present in its pure form or as an oxide or as a hydroxide or as an oxide hydroxide.

22. The method according to claim 21, characterized in that the catalytically active or catalytically activatable substance is platinum or a platinum alloy supported on carbon (Pt / C) or an iridium-based, supported or unsupported catalyst, in particular antimony-doped tin oxide (ATO) or niobium-doped or tantalum-doped titanium oxide.

23. Method according to one of claims 1 to 22, characterized in that the temperature of the first composition and / or the second composition during pressing is between -30°C and 80°C or that the temperature of the first composition and / or the second composition during pressing is between -15°C and 70°C or that the temperature of the first composition and / or the second composition during pressing is between 15°C and 30°C.

24. The method according to any one of claims 1 to 23, characterized in that the cation exchange membrane is provided with at least one precoating, such that the composition is applied to the precoating 25. Method according to one of claims 1 to 24, characterized in that the pressing takes place at a pressure of 0.001 MPa to 0.15 MPa or that the pressing takes place at a pressure of 0.01 MPa to 0.1 MPa.

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