Method and apparatus

Abstract

A method for producing an ion conducting membrane for a water electrolyser or fuel cell is provided. The method includes the step of in-line mixing a first liquid stream comprising an ionically conductive polymer and a second liquid stream comprising a cerium-containing compound to form the coating composition. The coating composition is then deposited onto a substrate to form a film layer. An apparatus for producing an ion conducting membrane for a water electrolyser or fuel cell is also provided.

Description

Technical Field

[0001] The present invention relates to a method for producing an ion-conducting membrane for an electrochemical device, such as a water electrolyser or a fuel cell, and to an apparatus suitable for producing such an ion-conducting membrane. Background Art

[0002] The electrolysis of water can be carried out in both alkaline electrolyte systems and acidic electrolyte systems to produce high-purity hydrogen and oxygen. Those electrolyzers that employ solid proton-conducting polymer membranes or proton exchange membranes (PEMs) are referred to as proton exchange membrane water electrolyzers (PEMWEs). Those electrolyzers that utilize solid anion-conducting polymer membranes or anion exchange membranes (AEMs) are referred to as anion exchange membrane water electrolyzers (AEMWEs).

[0003] Ion-conducting membranes such as PEMs and AEMs are also used in fuel cells. In a proton exchange membrane fuel cell (PEMFC), the membrane is proton-conducting and transports protons produced at the anode across the membrane to the cathode where they combine with oxygen to form water.

[0004] Catalyst coated membranes (CCMs) can be used in electrochemical devices such as electrolyzers and fuel cells. Such CCMs include an ion conducting membrane such as a PEM or AEM, with at least one of an anode catalyst layer and a cathode catalyst layer applied to a face of the membrane.

[0005] For water electrolyzer applications, hydrogen evolution reaction (HER) catalysts are used in such cathode catalyst layers, for example HER catalysts comprising platinum (such as platinum on a carbon support). Oxygen evolution reaction (OER) catalysts are used in electrolyzer anode catalyst layers. For PEMWE applications, suitable OER catalysts include iridium or iridium oxide (IrOx), or oxides containing both iridium and ruthenium. For AEMWE applications, non-platinum group metal OER catalysts, such as alloys and oxides of nickel, cobalt, iron and copper, may also be used.

[0006] For fuel cell applications, an oxygen reduction reaction (ORR) catalyst is used in the cathode catalyst layer, and a hydrogen oxidation reaction (HOR) catalyst is used in the anode catalyst layer. For PEMFC applications, suitable cathode catalyst materials and anode catalyst materials include platinum group metals or alloys of platinum group metals with one or more other metals, such as platinum or alloys of platinum with one or more other metals.

[0007] A separate membrane layer, typically formed of a non-ion-conducting polymer, can be positioned around the edge region of the CCM, for example on the exposed surface of the ion-conducting membrane where no electrocatalyst is present (but will often overlap the edge of the electrocatalyst layer as well), to provide a seal to prevent the escape of reactant and product gases, to strengthen and reinforce the edge of the CCM, and to provide a suitable surface for supporting subsequent components (such as a subgasket or elastomeric gasket). An adhesive layer may be present on one or both surfaces of the sealing membrane layer.

[0008] The CCM can be incorporated into a membrane electrode assembly (MEA), which is essentially composed of five layers. The middle layer is a polymer ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer containing an electrocatalyst designed for a specific electrolysis reaction. Finally, adjacent to each electrocatalyst layer is a gas diffusion layer or a porous transport layer, depending on the final MEA application and stack configuration. Such layers allow reactants to reach the electrocatalyst layer and allow products to leave.

[0009] Ion-conducting membranes (such as PEMs and AEMs) are key components of electrochemical devices (such as fuel cells and electrolyzers). However, they are subject to significant chemical and mechanical stresses during operation that can lead to membrane degradation, which can have an adverse impact on the performance and maintenance costs of fuel cell and electrolyzer stacks.

[0010] It is known to add cerium-containing compounds to ion-conducting membranes. Such additives act as scavengers of reactive oxygen species (ROS) that may cause membrane degradation.

[0011] WO2007120190 (3M Innovative Properties Company) describes the addition of cerium oxide compounds to fuel cell membranes and illustrates that incorporation of cerium oxide in amounts of 0.25 to 1 wt % may result in an increase in membrane life and a reduction in fluoride ion evolution (and an indication of membrane degradation).

[0012] The use of doped cerium-containing compounds is known. For example, Baker, AM et al., Journal of Materials Chemistry A, July 2017, 5(29), describe Ce 0.85 Zr 0.15 O2 exhibits excellent free radical scavenging activity. Venkateshkumar Prabhakaran and Vijay Ramani 2014 Journal of the Electrochemical Society (J. Electrochem. Soc.) 161F1 described that nitrogen-doped CeO2 has high ROS scavenging activity.

[0013] The addition of cerium-containing compounds may cause significant difficulties in the manufacturing process of ion-conducting membranes (particularly in achieving the high quality control standards required for ion-conducting membranes for use in fuel cell and electrolyzer applications) due to uneven distribution of the cerium-containing compound in the formed membrane and / or due to precipitation of cerium-containing compound particles during membrane manufacturing.

[0014] There remains a need to further enhance and develop methods for producing ion-conducting electrolytic membranes for fuel cells and electrolyzers, as well as equipment that facilitates the reproducible production of high-quality membranes and CCMs for electrochemical devices. Summary of the invention

[0015] The present inventors have recognised that in-line mixing of liquid streams comprising a cerium-containing compound and an ion-conducting polymer can be advantageously used during the production of ion-conducting membranes for fuel cells and electrolysers and that this provides a solution to one or more of the aforementioned problems.

[0016] Thus, in a first aspect of the invention there is provided a method for producing an ion conducting membrane (such as a PEM or AEM) for use in a water electrolyser or fuel cell, the ion conducting membrane comprising at least one membrane layer, the method comprising the steps of:

[0017] a) in-line mixing a first liquid stream comprising an ion-conductive polymer and a second liquid stream comprising a cerium-containing compound to form a coating composition; and

[0018] b) depositing the coating composition onto a substrate to form a film layer.

[0019] The use of in-line mixing provides an improvement in the reproducible quality of the produced films and reduces quality control problems associated with the formation of crystalline particles of cerium-containing compounds during the manufacturing process.

[0020] The method as described herein may have particular utility for producing catalyst coated membranes. Therefore, in a second aspect of the present invention, there is provided a method for manufacturing a catalyst coated membrane, the method comprising the steps of:

[0021] (i) providing an ion-conducting membrane produced using the method according to the first aspect, the membrane comprising a first side and a second side; and

[0022] (ii) forming a catalyst layer on the first surface and / or the second surface of the membrane.

[0023] Advantageously, the membrane produced by the method of the first aspect and the catalyst coated membrane produced by the method of the second aspect are suitable for use in a water electrolyser or a fuel cell.

[0024] In a third aspect of the invention there is provided an apparatus for producing an ion-conducting membrane for a water electrolyser or a fuel cell (such as a PEM or AEM), the apparatus comprising:

[0025] (i) a first source for providing a first liquid stream comprising an ion-conducting polymer;

[0026] (ii) a second source for providing a second liquid stream comprising a cerium-containing compound;

[0027] (iii) an in-line mixing device in fluid communication with the first source and the second source, the in-line mixing device being configured to mix the first liquid stream and the second liquid stream to form a coating composition;

[0028] (iv) a coating device configured to receive the coating composition from the in-line mixing device and coat a substrate with the coating composition to form an ion conductive membrane layer.

[0029] The use of such equipment provides improvements in reproducible membrane quality and can be used to facilitate the operation of membrane production lines, in particular enabling such lines to be easily stopped and started without causing membrane quality control problems and thereby reducing waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic diagram of an example of an apparatus as described herein is shown. DETAILED DESCRIPTION

[0031] Preferred and / or optional features of the invention will now be set out.Unless the context requires otherwise, any preferred and / or optional feature of any aspect may be combined alone or in combination with any other preferred and / or optional feature of any aspect of the invention.

[0032] The present invention provides a method for producing an ion-conducting membrane, such as a proton exchange membrane or an anion exchange membrane, such as for use in an electrolyser or a fuel cell. It may be preferred that the membrane is a proton exchange membrane for use in an electrolyser or a fuel cell.

[0033] The method includes the steps of in-line mixing a first liquid stream comprising an ion-conducting polymer and a second liquid stream comprising a cerium-containing compound to form a coating composition.

[0034] The first liquid stream comprises an ion conducting polymer. Those skilled in the art will be aware of suitable ion conducting polymers for preparing ion conducting membranes. The ion conducting material may be a proton conducting polymer or an anion conducting polymer, such as a hydroxy anion conducting polymer. Examples of suitable proton conducting polymers include perfluorosulfonic acid ionomers (e.g. (E.I.DuPont de Nemours and Co.), (Asahi Kasei), Aquivion TM (Solvay Speciality Polymers), (Asahi Glass Co.) or ionomers based on sulfonated hydrocarbons, such as those available from FuMA-Tech GmbH ( P, E or K series products), those of JSR Corporation, Toyobo Corporation, etc. Examples of suitable anion conductive polymers include A901 manufactured by Tokuyama Corporation and Fumasep FAA from FuMA Technology Co., Ltd.

[0035] Suitably, the first liquid stream comprises an ion-conducting polymer dispersed in a mixture of water and a polar solvent other than water. The polar solvent may be a polar protic solvent. Preferably, the polar solvent is an alcohol such as methanol, ethanol, propan-1-ol, propan-2-ol, n-butanol, isobutanol, butan-2-ol and tert-butanol, or a mixture thereof. Preferably, the alcohol is ethanol and / or propan-1-ol. It may be preferred that the first liquid stream comprises (or is substantially composed of) at least one ion-conducting polymer, water and an alcohol such as at least one of ethanol or propan-1-ol.

[0036] Typically, the weight ratio of water: polar solvent (such as ethanol and / or propan-1-ol) is within the range of 9:1 to 1:1 (such as 9:1 to 7:3). It will be appreciated by those skilled in the art that the solvent ratio may vary depending on the properties of the selected ion-conducting polymer.

[0037] Typically, the first liquid stream comprises the ion-conducting polymer in an amount ranging from 5 wt % to 80 wt %, preferably from 10 wt % to 50 wt %, preferably from 15 wt % to 30 wt %, and most preferably from 15 wt % to 20 wt %, based on the total weight of the components of the first liquid stream and including 5 wt % and 80 wt %, preferably 10 wt % and 50 wt %, preferably 15 wt % and 30 wt %, and most preferably 15 wt % and 20 wt %.

[0038] The second liquid stream comprises a cerium-containing compound. Such compounds are selected as cerium-containing scavengers for reactive oxygen species. Suitably, the cerium-containing compound is a doped or undoped cerium oxide or a doped or undoped cerium salt. It may be preferred that the cerium-containing compound is a doped or undoped cerium oxide, such as doped or undoped cerium (IV) oxide (CeO 2 ). Suitable dopants may be selected from, for example, one or more transition metals, such as zirconium or nitrogen. It may be further preferred that the cerium-containing compound is cerium oxide (CeO 2 ).

[0039] Suitably, the second liquid stream comprises a cerium-containing compound in a solvent, such as water or a mixture of water and a polar solvent other than water, such as a mixture of water and an alcohol, for example a mixture of water and ethanol and / or propan-1-ol. It may be preferred that the solvent used in the first liquid stream and the second liquid stream is the same. It may be preferred that the second liquid stream comprises a cerium-containing compound in the form of a colloidal sol. The use of a cerium-containing compound in the form of a colloidal sol is conducive to dispersion during mixing. Such colloidal sols may include additives, such as acetic acid and / or nitric acid.

[0040] Typically, the cerium-containing compound (e.g., doped or undoped cerium oxide) is present in the second liquid stream in an amount of less than 1 wt % (e.g., less than 0.1 wt % or less than 0.01 wt %) based on the total weight of the components of the second liquid stream. Preferably, the cerium-containing compound (e.g., doped or undoped cerium oxide) is present in the second liquid stream in an amount in the range of 0.001 wt % to 0.1 wt % and including 0.001 wt % and 0.1 wt % (e.g., between 0.001 wt % and 0.01 wt %) based on the total weight of the components of the second liquid stream.

[0041] It should be understood that the second liquid stream may contain more than one cerium-containing compound, for example: a doped cerium-containing compound and an undoped cerium-containing compound; two doped cerium-containing compounds with different dopants; or a doped or undoped cerium oxide and a doped or undoped cerium salt. It should also be understood that the second liquid stream may contain the cerium-containing compound in more than one form, such as a dissolved cerium-containing compound and a cerium-containing compound in the form of a colloidal sol.

[0042] The first liquid stream and the second liquid stream are mixed online. Typically, such online mixing includes adding the second liquid stream to the flowing first liquid stream. Suitably, the second liquid stream is added at a controlled flow rate so as to obtain a desired amount of cerium-containing compound in the resulting coating composition. Such controlled addition can be provided, for example, by a metering pump.

[0043] In-line mixing can be performed using a static mixer, such as a tubular mixer or a plate static mixer. It should be understood that the static mixer may include an inlet for the first liquid stream and the second liquid stream, or the first liquid stream and the second liquid stream may be combined and the combined stream then flowed through the static mixer to mix the components.

[0044] It will be understood by those skilled in the art that in-line mixing of the first liquid stream and the second liquid stream is performed so that adequate mixing is achieved before the coating composition reaches a coating device or other equipment used for film preparation.

[0045] Suitably, the ratio of the flow rate of the first liquid stream to the flow rate of the second liquid stream at the in-line mixing point is in the range of 5:1 to 15:1 inclusive, for example in the range of 7:1 to 10:1 inclusive.

[0046] After in-line mixing of the first liquid stream and the second liquid stream to form the coating composition, the method includes the step of (b) depositing the coating composition onto a substrate to form a film layer.

[0047] The coating composition may be deposited using a slot die coating process (whereby the dispersion is extruded through a slot onto a substrate by gravity or under pressure), knife coating, rod coating, inkjet printing, gravure printing, curtain coating, spray coating or a casting process. Preferably, the coating composition may be deposited using slot die coating, rod coating, inkjet printing or gravure printing. Deposition using slot die coating may be particularly preferred.

[0048] The coating composition is deposited onto the substrate to form a membrane layer. In some cases, the ion conductive membrane is formed by a single membrane layer. Alternatively, the ion conductive membrane can be formed by two or more layers (such as between two and seven layers). The number of layers will be determined by, for example, the thickness of the desired film and the degree of variation of the desired composition over the entire film (e.g., the film may contain one or more layers comprising a reinforcing polymer such as ePTFE or an additive such as a composite catalyst).

[0049] Typically, the substrate is a backing sheet, a second membrane layer, a catalyst layer on a backing sheet, or a catalyst layer on a gas diffusion electrode. It will be appreciated by those skilled in the art that the choice of substrate will depend on the structure and production stage of the membrane.

[0050] In the case where the membrane is formed by a single membrane layer or at the beginning of the production of a multilayer membrane, the substrate is typically a backing layer. The backing layer provides support for the ion-conducting membrane during manufacture, and if not immediately removed, can provide support and strength during any subsequent storage and / or transport. The material made of the backing layer should provide the required support, preferably compatible with the coating composition, preferably impermeable to the coating composition, able to withstand the process conditions involved in the production of the ion-conducting membrane, and can be easily removed without damaging the ion-conducting membrane. Examples of materials suitable for use include fluoropolymers (such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymers (PFA), fluorinated ethylene propylene (FEP-copolymer of hexafluoropropylene and tetrafluoroethylene)) and polyolefins (such as biaxially oriented polypropylene (BOPP)).

[0051] In some cases of producing catalyst coated membranes, a catalyst layer is provided on a backing layer, for example by printing or using known coating techniques. Subsequently, the coating composition can be deposited onto the catalyst layer so that the catalyst layer is disposed between the backing layer and the membrane layer formed by depositing the coating composition. The use of the methods and apparatus as described herein can be particularly advantageous when the membrane layer is formed on a catalyst layer that enables a high quality membrane layer to be provided at the interface between the catalyst layer and the ion conducting membrane.

[0052] In some cases, usually when the ion conductive film thickness makes it necessary to have multiple processes in order to build the membrane structure, the substrate is the previously formed membrane layer.It should be understood that the ion conductive film can be formed by the sequential deposition of the layer.As an example, the ion conductive film can be formed as follows.In the first process, the coating composition described above can be deposited on the backing layer to form the first ion conductive polymer layer, which is then dried.In the second process, the coating composition is deposited on the first ion conductive polymer layer to form the second ion conductive polymer layer.Subsequently, the second ion conductive polymer layer is dried.Continue this application and dry order to produce other ion conductive polymer layers as required, thereby forming the desired membrane structure.

[0053] Typically, each of the layers in the ion-conducting membrane comprises a cerium-containing compound, however it will be appreciated that in some cases it may be desirable to include a membrane layer that does not contain a cerium-containing compound (or has a reduced content of a cerium-containing compound). In such cases, the use of the apparatus and methods as described herein is beneficial because it enables the operator to simply reduce or stop the flow of the second liquid stream during certain deposition steps.

[0054] The film formed by the method as described herein can be used in the production of catalyst coated membranes. In this case, the method may include forming a catalyst layer on the first and / or second side of the film to form a step of an anode and / or cathode. It will be appreciated by those skilled in the art that the specific type of catalyst for the cathode and anode is selected according to whether the film is used for a fuel cell or an electrolyzer and whether the film is a PEM or AEM as previously described. In addition, the deposition method can be changed, for example, the catalyst layer can be transferred from a decal to the film by, for example, hot pressing, or the catalyst ink can be directly printed on the film.

[0055] The present invention also provides an apparatus for producing an ion-conducting membrane.

[0056] The apparatus comprises a first source, the first source being in fluid communication with an in-line mixing device. The first source provides a first liquid stream comprising an ion-conducting polymer. Typically, the first source is a container containing a dispersion of at least one ion-conducting polymer in a solvent or solvent mixture. Preferably, the container is configured to agitate the contents, for example by stirring or shaking.

[0057] Suitably, the apparatus is provided with a first pump configured to transfer the contents of the first source as a first liquid stream at a controlled flow rate to the in-line mixing device. Preferably, the apparatus is further configured such that the first liquid stream passes through a filter before reaching the in-line mixing device.

[0058] The apparatus includes a second source, which is fluidically connected to the online mixing device. The second source provides a second liquid stream containing the cerium-containing compound. Typically, the second source is a container containing the cerium-containing compound in a solvent (such as water) or a solvent mixture. Preferably, the container is configured to agitate the contents, such as by stirring or shaking.

[0059] Suitably, the apparatus is provided with a second pump configured to transfer the contents of the second source as a second liquid stream at a controlled flow rate to the in-line mixing device. Preferably, the apparatus is further configured such that the second liquid stream passes through a filter before reaching the in-line mixing device.

[0060] The apparatus comprises an in-line mixing device configured to mix a first liquid stream and a second liquid stream to form a coating composition. Suitably, the in-line mixing device is configured such that the second liquid stream is added to the flowing first liquid stream.

[0061] It may be preferred that the in-line mixing device is a static mixer, such as a tubular mixer or a plate static mixer. The static mixer may include a first inlet for receiving a first liquid stream and a second inlet for receiving a second liquid stream. As an alternative, the apparatus may be configured such that the second liquid stream is added to the first liquid stream and then the combined first liquid stream and the second liquid stream pass through the static mixer.

[0062] The equipment includes a coating device, which is configured to utilize a coating composition to coat a substrate to form an ion conductive membrane. In order to avoid doubt, the equipment is configured so that the outlet of the online mixing device is communicated with the inlet fluid of the coating device. The coating device can be any equipment suitable for forming an ion conductive membrane on a substrate. Such equipment is well known to those skilled in the art, and includes a slot die coater, a rod coater, a roller blade coater, an inkjet printer or a gravure printer.

[0063] Preferably, the inline mixing device is located near the entrance of the coating device. It may be preferred that the device is configured so that the passage time of the coating composition between the outlet of the inline mixing device and the entrance of the coating device is less than 60 seconds, such as less than 30 seconds, less than 15 seconds or less than 10 seconds. It may be preferred that the device is configured so that the passage time of the coating composition between the outlet of the inline mixing device and the entrance of the coating device is within the range of 1 second and 60 seconds and includes 1 second and 60 seconds, preferably within 1 second and 30 seconds and includes 1 second and 30 seconds, or within the range of 1 second and 10 seconds and includes 1 second and 10 seconds.

[0064] Typically, the apparatus includes means for drying the film layer after it has been formed by the coating apparatus. Such means are known to those skilled in the art and include ovens and infrared dryers.

[0065] It may be preferred that the apparatus comprises an analytical device for analyzing the chemical composition of the film layer after formation, and in particular the cerium content in the film when the film is produced. Suitably, the device is an x-ray fluorescence (XRF) detector. Advantageously, the apparatus is configured so that the flow rate of the second liquid stream is automatically adjusted based on the cerium content detected in the film by the analytical device.

[0066] Preferably, the apparatus further comprises a purge line configured to enable removal of the coating composition from parts of the apparatus downstream of the static mixer (and upstream of the coating apparatus). This enables the coating composition to be discharged from the apparatus when the apparatus is shut down or between coating runs and avoids potential crystallization problems of cerium-containing compounds during periods when the apparatus is not in operation.

[0067] Figure 1An example of an apparatus (1) for producing an ion-conducting membrane is shown. The apparatus (1) comprises a first tank (2) serving as a source of a dispersion of an ion-conducting polymer in a solvent (e.g., a mixture of water and ethanol); and a second tank (3) serving as a source of cerium oxide in a solvent (e.g., a mixture of water and ethanol). A first pump (4) pumps the dispersion of the ion-conducting polymer from the tank (2) toward a static mixer (6), optionally through an in-line filter (5). A second pump (7) pumps a cerium-containing stream toward the static mixer (6), optionally through an in-line filter (8). The first pump (4) and the second pump (7) are preferably controlled by a human-machine interface (9), which can be used, for example, to adjust the flow rate. The apparatus may comprise a member (10) for transferring the dispersion stream of the ion-conducting polymer into the tank (2). The static mixer (6) mixes the two liquid feeds to form a coating composition, which is forwarded to a coating head (11) of a coating device (such as a slot die coater) which deposits the coating composition on a substrate to form a film, for example, onto a backing layer. The film is dried after formation to remove the solvent by, for example, passing through a heater ( Figure 1 ). Preferably, the device comprises a valve (12) connected to a purge line which enables the line between the static mixer and the coating head to be purged so that if the coating line is stopped, the mixed composition can be discarded before restarting the device. The film formed is analyzed in situ by an X-ray fluorescence analyzer (13) which determines the cerium content of the film. Advantageously, the device (1) is configured so that the speed of the first pump (7) is automatically adjusted based on data from the X-ray fluorescence analyzer (13).

Claims

1. A method for producing an ion-conducting membrane for a water electrolyser or a fuel cell, the ion-conducting membrane comprising at least one membrane layer, the method comprising the following steps: (a) in-line mixing a first liquid stream comprising an ion-conductive polymer and a second liquid stream comprising a cerium-containing compound to form a coating composition; as well as (b) depositing the coating composition onto a substrate to form a film layer. 2 . The process of claim 1 , wherein the first liquid stream comprises a dispersion of an ionically conductive polymer in a mixture of water and a polar solvent other than water.

3. The process according to any one of claims 1 to 2, wherein the second liquid stream comprises the cerium-containing compound in the form of a colloidal sol.

4. The method according to any one of claims 1 to 3, wherein the cerium-containing compound is a doped or undoped cerium oxide.

5. The method according to any one of claims 1 to 4, wherein the substrate is selected from a backing sheet, a second membrane layer, or a catalyst layer on a backing sheet.

6. The method according to any one of claims 1 to 5, wherein the in-line mixing in step (a) is performed using a static mixer.

7. The process of any one of claims 1 to 6, wherein the ratio of the flow rate of the first liquid stream to the flow rate of the second liquid stream is in the range of 7:1 to 15:1, inclusive.

8. The method according to any one of claims 1 to 7, further comprising the step of (c) drying the film layer.

9. The method according to any one of claims 1 to 8, further comprising the following steps: (d) depositing the coating composition onto the film layer to form an additional film layer; and (e) drying the additional film layer.

10. The method of claim 9, wherein steps (d) and (e) are repeated 1 to 6 times.

11. A method according to any one of the preceding claims, wherein the ion-conducting membrane is a proton exchange membrane.

12. A method for manufacturing a catalyst coated membrane, the method comprising the following steps: (i) providing an ion-conducting membrane produced according to any one of claims 1 to 11, the membrane comprising a first side and a second side; as well as (ii) forming a catalyst layer on the first surface and / or the second surface of the membrane.

13. The method according to claim 12, further comprising the step of applying a sealing material to the first side and / or the second side of the catalyst coated membrane.

14. A method according to claim 12 or claim 13, further comprising the step of applying a gas diffusion layer and / or a porous transport layer to the first side and / or the second side of the catalyst coated membrane.

15. An apparatus for producing an ion-conducting membrane for a fuel cell or an electrolyser, the apparatus comprising: (i) a first source for providing a first liquid stream comprising an ion-conducting polymer; (ii) a second source for providing a second liquid stream comprising a cerium-containing compound; (iii) an in-line mixing device in fluid communication with the first source and the second source, the in-line mixing device being configured to mix the first liquid stream and the second liquid stream to form a coating composition; and (iv) a coating device configured to receive the coating composition from the in-line mixing device and coat a substrate with the coating composition to form an ion-conducting membrane layer.

16. The apparatus of claim 15, wherein the in-line mixing device is a static mixer.

17. The apparatus according to claim 15 or claim 16, wherein the coating apparatus is a slot die coater, a rod coater, an inkjet printer or a gravure printer.

18. The apparatus according to any one of claims 14 to 17, further comprising an analysis device for analyzing a cerium content in the film layer when the film layer is produced.

19. The apparatus according to claim 18, wherein the apparatus is configured such that the flow rate of the second liquid stream is automatically adjusted based on the cerium content detected in the membrane layer by the analyzing device.

20. Apparatus according to any one of claims 15 to 19, wherein the apparatus is configured such that the transit time of the coating composition between the outlet of the in-line mixing device and the inlet of the coating apparatus is less than 60 seconds, such as less than 30 seconds or less than 10 seconds.

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