Processes and Membranes

Abstract

A process for manufacturing an ion-conducting membrane comprising a recombination catalyst-containing membrane layer. The membrane layer when fabricated from an ink comprising a stabilized dispersion of recombination catalyst nanoparticles. Also provided is an ion-conducting membrane for an electrochemical device, such as a fuel cell or water electrolysis device, having a recombination catalyst-containing membrane layer comprising dispersed recombination catalyst nanoparticles, a nanoparticle stabilizer, and an ion-conducting polymer.

Description

[Technical Field]

[0001] The present invention relates to ion-conducting membranes suitable for use in electrochemical devices such as water electrolyzers and fuel cells, catalyst-coated membranes incorporating such ion-conducting membranes, and processes for their manufacture. [Background technology]

[0002] Electrolysis of water to produce high-purity hydrogen and oxygen can be carried out in both alkaline and acidic systems. These electrolyzers that use a solid proton-conducting polymer electrolyte membrane or proton exchange membrane (PEM) are known as proton exchange membrane water electrolyzers (PEMWE). Those that utilize a solid anion-conducting polymer electrolyte membrane or anion exchange membrane (AEM) are known as anion exchange membrane water electrolyzers (AEMWE).

[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 conducts protons, and protons generated at the anode are transported 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 comprise 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] In water electrolyzer applications, hydrogen evolution reaction (HER) catalysts, e.g., platinum-containing HER catalysts such as platinum on a carbon support, are used in such cathode catalyst layers. Oxygen evolution reaction (OER) catalysts are utilized in the electrolyzer anode catalyst layer. In PEMWE applications, suitable OER catalysts include iridium or iridium oxide (IrOx), or oxides containing both iridium and ruthenium. In AEMWE applications, non-platinum group metal OER catalysts, e.g., alloys and oxides of nickel, cobalt, iron, and copper, can also be used.

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

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

[0008] CCMs may be incorporated into membrane electrode assemblies (MEAs), which essentially consist of five layers. The central layer is a polymeric ion-conducting membrane. On either side of the ion-conducting membrane are electrocatalyst layers containing electrocatalysts designed for specific electrolysis reactions. Finally, adjacent to each electrocatalyst layer are gas diffusion layers or porous transport layers, depending on the final MEA application and stack configuration. These layers allow reactants to reach the electrocatalyst layers and products to leave.

[0009] It is desirable to reduce the thickness of such ion-conducting membranes to minimize electronic and ionic resistance, however, it is also important to minimize any hydrogen crossover through the membrane to avoid mixing of hydrogen and oxygen and related safety concerns.

[0010] For water electrolysis devices, it is beneficial to maintain low levels of hydrogen crossover even when the pressure difference across the membrane is high. The use of high pressure during electrolysis operation is advantageous because it reduces the requirement for compression of the produced hydrogen and reduces operating costs. This has led to the use of membranes with thicknesses greater than 125 μm, typically approaching 200 μm or even thicker. Examples of membranes currently in use include Nafion™ N115 (125 μm thick) or Nafion™ N117 (175 μm thick).

[0011] It is known to produce proton exchange membranes containing recombination catalysts that catalyze the reaction between hydrogen and oxygen to form water, thereby reducing the level of hydrogen crossover. For example, International Publication No. 2020 / 148545 (Johnson Matthey Fuel Cells Ltd.) describes the introduction of a catalyst containing platinum on a graphene support into a proton exchange membrane. In addition, Chinese Patent No. 114874475 (FOSHAN CLEANEST ENERGY TECH CO. LTD.) describes the use of platinum particles supported on hollow polydopamine microspheres in a proton exchange membrane. In this case, the platinum particles are localized in the membrane on the surface of the catalyst support.

[0012] It is also known to incorporate platinum nanoparticles into ion-conducting membranes. For example, U.S. Patent No. 10,476,094 (LG Chem Ltd.) describes a reinforced membrane comprising a porous polymer support. Platinum nanoparticles are provided on both sides of the porous polymer support and within the pores. To prepare the membrane, the porous support is immersed in a platinum precursor solution, and the precursor is then reduced by adding a reducing agent. This methodology has several manufacturing drawbacks. For example, immersing the porous polymer support in the platinum precursor solution can result in swelling and / or deformation of the support during impregnation, and this immersion and drying method is difficult to reproduce on a large manufacturing scale. Furthermore, the need to co-locate the reinforcing component and the platinum nanoparticles limits the available scope for optimizing performance by modifying the membrane configuration and limits the ability to disperse the nanoparticles.

[0013] A similar approach involving the local deposition of Pt nanoparticles on the pores of a PTFE film is described in the paper "Reinforced and self-humidifying composite membrane for fuel cell applications" by Liu et al., Journal of Membrane Science, 330 (2009) 357-362. Also, US Patent No. 2015 / 0236354 (Solvicore GmbH & Co. KG) describes that a colloidal dispersion containing nanosized precious metal particles and an ionomer component can be formed by dissolving a suitable precious metal precursor with a liquid acidic ionomer component, followed by a reduction step. Such a colloidal dispersion does not contain any other components.

[0014] Incorporating recombined catalyst particles into an ion-conducting membrane layer remains challenging. It is difficult to reproducibly manufacture consistent inks that contain both ion-conducting polymers and catalyst particles and that maintain their properties upon storage. Typically, this means that such inks must be prepared immediately prior to manufacturing, thus reducing manufacturing flexibility.

[0015] The inclusion of a recombination catalyst also increases the cost of the ion-conducting membrane, so it is desirable to maximize the reduction in hydrogen crossover for a given amount of recombination catalyst.

[0016] There remains a need to further develop efficient and scalable processes for the manufacture of ion-conducting membranes and to improve such membranes to enable efficient operation at high pressure differentials across the membrane. Summary of the Invention

[0017] The inventors have surprisingly found that stabilized dispersions of recombined catalyst nanoparticles can be advantageously combined with ion-conducting polymers to form inks suitable for ion-conducting membrane fabrication, and that such inks exhibit nanoparticle size stability over extended periods of time. The inventors have also found that ion-conducting membranes can be fabricated using such inks that exhibit excellent dispersion of the catalyst nanoparticles within the membrane layer.

[0018] Accordingly, in a first aspect of the present invention there is provided a process for producing an ion-conducting membrane comprising a recombination catalyst-containing membrane layer, the process comprising: (i) providing a stabilized dispersion of recombination catalyst nanoparticles; (ii) mixing the stabilized dispersion with an ion-conducting polymer to form an ink; (iii) fabricating a membrane layer from the ink.

[0019] In a second aspect of the present invention, there is provided an ink for use in producing an ion-conducting membrane, the ink comprising recombination catalyst nanoparticles, a nanoparticle stabilizer, and an ion-conducting polymer.

[0020] It has also been found that ion-conducting membranes can be produced that include nanoparticle stabilizers and recombination catalyst nanoparticles dispersed in an ion-conducting polymer layer. Such membranes have been found to provide improved reductions in hydrogen crossover levels during testing, as described in the Examples. Thus, in a third aspect of the present invention, there is provided an ion-conducting membrane, such as a proton exchange membrane or an anion exchange membrane, that includes a recombination catalyst-containing membrane layer, the membrane layer comprising dispersed recombination catalyst nanoparticles, a nanoparticle stabilizer, and an ion-conducting polymer.

[0021] Such membranes are particularly suitable for use in water electrolysis devices.

[0022] The ion-conducting membrane of the third aspect may be obtained or is obtainable by the process of the first aspect.

[0023] The ion-conducting membrane of the third aspect is particularly useful as a component of a catalyst-coated membrane (CCM). Accordingly, in a fourth aspect of the invention there is provided a CCM for an electrochemical device such as a water electrolyser or a fuel cell, comprising a membrane according to the third aspect.

[0024] Preferably, the CCM is for use in a water electrolysis device, such as a PEM water electrolysis device. In such cases, the CCM comprises a cathode catalyst layer for catalyzing the hydrogen evolution reaction and / or an anode catalyst layer for catalyzing the oxygen evolution reaction. Typically, the cathode catalyst layer comprises platinum and / or the anode catalyst layer comprises iridium.

[0025] The CCM may also be for use in a fuel cell, such as a PEM fuel cell, in which case the CCM includes a cathode catalyst layer for catalyzing the oxygen reduction reaction and / or an anode catalyst layer for catalyzing the hydrogen oxidation reaction.

[0026] In a fifth aspect of the present invention there is provided a water electrolyser or fuel cell comprising a membrane according to the third aspect or a catalyst coated membrane according to the fourth aspect. [Brief explanation of the drawings]

[0027] [Figure 1] 1 shows a schematic diagram of an exemplary configuration of an electrolyte membrane of the present invention. [Figure 2] 1 shows a schematic diagram of an exemplary configuration of a catalyst coated membrane of the present invention. [Figure 3] 1 shows the results of a stability test of an ink containing PVP-stabilized platinum nanoparticles and an ion-conducting polymer. [Figure 4-1] 1 shows the results of a scanning electron microscopy-energy dispersive X-ray (SEM_EDX) analysis of a film including a platinum-containing film layer. [Figure 4-2]1 shows the results of a scanning electron microscopy-energy dispersive X-ray (SEM_EDX) analysis of a film including a platinum-containing film layer. [Figure 5] 1 shows the results of hydrogen crossover tests on catalyst coated membranes. [Figure 6] 10 shows the results of further hydrogen crossover testing of catalyst coated membranes. DETAILED DESCRIPTION OF THE INVENTION

[0028] Preferred and / or optional features of the invention will now be described. Any aspect of the invention may be combined with any other aspect of the invention unless the context requires otherwise. Any preferred and / or optional feature of any aspect may be combined with any other feature of the invention, either singly or in any combination, unless the context requires otherwise.

[0029] The present invention provides a process for producing an ion-conducting membrane having a recombination catalyst-containing membrane layer, and an ion-conducting membrane including such a membrane layer. It may be preferred that the ion-conducting membrane is a proton exchange membrane (PEM), e.g., a PEM for a water electrolysis device. However, those skilled in the art will understand that the process and recombination catalyst-containing membrane layer described herein are also useful for other types of electrolyte membranes, such as PEMs for fuel cells and anion exchange membranes for water electrolysis devices, fuel cells, or other applications.

[0030] The recombination catalyst containing layer comprises recombination catalyst nanoparticles. As used herein, the term nanoparticles relates to particles having a particle size in the range of 1 to 100 nm, limits included.

[0031] A recombination catalyst is a catalyst that catalyzes the reaction between hydrogen gas and oxygen gas to form water. Thus, the recombination catalyst used in the ion conducting membrane of the present invention may be any catalyst that can catalyze the reaction between hydrogen gas and oxygen gas to form water, thereby reducing or preventing the crossover of either hydrogen or oxygen, or both, through the membrane.

[0032] Preferably, the recombination catalyst is selected from the list comprising: i) platinum group metals (i.e., the group of elements that includes platinum, palladium, iridium, rhodium, ruthenium, and osmium); ii) gold, iii) base metals such as iron, nickel, cobalt, or chromium; iv) alloys containing one or more of the above elements; v) A mixture of any of the above.

[0033] Preferably, the recombination catalyst comprises or consists essentially of platinum or palladium (i.e., the nanoparticles are platinum nanoparticles or palladium nanoparticles). Alternatively, the recombination catalyst may be a platinum alloy with one or more of the above elements, such as a platinum-palladium alloy, a platinum-iridium alloy, a platinum-cobalt alloy, or a platinum-ruthenium alloy.

[0034] Preferably, the recombined catalyst nanoparticles in the dispersion are unsupported. The term "unsupported" will be readily understood by those skilled in the art. For example, it will be understood that the recombined catalyst particles are not bound or immobilized to a solid catalyst support, such as a carbon support, by physical or chemical bonds, for example, by ionic or covalent bonds, or by non-specific interactions such as van der Waals forces. The use of unsupported nanoparticles provides increased membrane stability during electrochemical operation, avoids degradation pathways due to catalyst support corrosion or other chemical or electrochemical reactions, and allows for greater dispersion within the membrane layer.

[0035] The process includes the steps of (i) providing a stabilized dispersion of recombined catalyst nanoparticles. Those skilled in the art will appreciate that a stabilized nanoparticle dispersion comprises solid catalyst nanoparticles in a liquid phase that includes at least one nanoparticle stabilizer that interacts with the nanoparticles to prevent nanoparticle aggregation. Such stabilizer additionally acts as a capping agent during synthesis.

[0036] The processes described herein involve a first step of forming a stabilized nanoparticle dispersion prior to combining the stabilized dispersion with an ion-conducting polymer in a subsequent ink-forming step. Accordingly, those skilled in the art will appreciate that the or each stabilizer used to form the stabilized nanoparticle dispersion will not be the same as the ion-conducting polymer used in step (ii) to form the ink.

[0037] Suitable nanoparticle stabilizers are selected from agents that interact with the catalyst nanoparticle surface, prevent aggregation and coalescence of the catalyst nanoparticles, and enable the formation of a nanoparticle dispersion. Such stabilization of the nanoparticle surface is typically due to interactions between the nanoparticles and polar functional groups of the stabilizer. Typically, the stabilizer comprises an amide, carboxylic acid, sulfonic acid, amine, alcohol, or ether functional group. It may be preferred that the stabilizer comprises an amide or ether functional group. It may be even more preferred that the stabilizer comprises a tertiary amide group. In some embodiments, the stabilizer does not have an acid functional group. In some embodiments, the stabilizer does not have a sulfonic acid functional group. Preferably, the nanoparticle stabilizer is water-soluble, such as a water-soluble polymer. It is preferred that the nanoparticle stabilizer has a water solubility of at least 1 mg / mL, preferably at least 10 mg / mL, or more preferably at least 100 mg / mL at 25°C.

[0038] It may be preferred that the stabiliser has a higher hydrophobicity and / or a lower water uptake value than the ion-conducting polymer used in step (ii).

[0039] Suitably, the stabilized dispersion comprises a polymeric nanoparticle stabilizer. It may be preferred that the stabilized dispersion comprises a polymeric nanoparticle stabilizer that has a higher hydrophobicity and / or a lower water uptake value than the ion-conducting polymer used in step (ii). It may be preferred that the polymeric nanoparticle stabilizer has a lower weight average molecular weight than the ion-conducting polymer used in step (ii).

[0040] It may further be preferred that the polymeric stabilizer comprises an amide functionality, e.g., a tertiary amide functionality, e.g., a pyrrolidone functionality (such as polyvinylpyrrolidone (PVP) or a copolymer comprising vinylpyrrolidone as the first polymerized unit).

[0041] Preferably, the stabilizer is polyvinylpyrrolidone (PVP). The use of PVP has been found to provide superior dispersion stability in the presence of perfluorosulfonic acid (PFSA) polymers, providing greater nanoparticle dispersion stability than nanoparticles with only PFSA.

[0042] Preferably, the stabilizer is PVP having a weight average molecular weight in the range of 5,000 to 50,000, inclusive. Such a range is believed to provide a good balance between dispersion stability and ease of polymer processability. It may be more preferred that the polymer stabilizer is polyvinylpyrrolidone having a weight average molecular weight in the range of 8,000 to 45,000, inclusive.

[0043] Typically, the stabilized dispersion is formed in an aqueous medium, such as water.

[0044] Preferably, the recombination catalyst nanoparticles are present in an amount of 0.5 to 10 g L -1 For example, if the recombination catalyst nanoparticles are platinum particles, such particles are typically present in the dispersion in an amount ranging from 0.5 to 10 g Pt L -1The nanoparticle concentration can be adjusted using techniques known to those skilled in the art, such as evaporation or cross-flow filtration.

[0045] Suitably, the dispersion formed in step (i) has a zeta potential more positive than +25 mV or more negative than -25 mV. Zeta potential may be measured using electrophoretic light scattering, for example using a Zetasizer Ultra (Malvern Panalytical).

[0046] Those skilled in the art will recognize methods for producing suitable catalyst nanoparticle dispersions. For example, the dispersions can be produced by continuous flow hydrothermal synthesis. Preferably, such synthesis can be carried out in a mixing reactor, such as that described in WO2015075439(A1) (The University of Nottingham), which is incorporated herein by reference.

[0047] Catalyst nanoparticle dispersions can also be prepared by mixing a suitable catalyst precursor with a stabilizer in a solvent such as water, followed by in situ nanoparticle formation. For example, in the case of platinum nanoparticles, a dispersion can be prepared by mixing a platinum precursor, such as chloroplatinic acid (HPtCl), platinum nitrate, or Pt(acac), with a stabilizer in a solvent such as water, followed by reduction of the platinum precursor, for example, using sodium borohydride or formaldehyde. An example of such a preparation is described in Du, YK, Journal of Applied Polymer Science, Vol. 99, 23-36 (2006), which is incorporated herein by reference.

[0048] The process includes (ii) mixing the stabilized dispersion with an ion-conducting polymer to form an ink. Typically, this is accomplished by forming a dispersion of the ion-conducting polymer and then mixing this dispersion with a stabilized dispersion of recombined catalyst nanoparticles.

[0049] Those skilled in the art will recognize suitable ion-conducting polymers for preparing ion-conducting membranes. The ion-conducting polymer can be a proton-conducting polymer or an anion-conducting polymer, such as a hydroxyl anion-conducting polymer. Examples of suitable proton-conducting polymers include perfluorosulfonic acid ionomers (e.g., Nafion® (Chemours Company), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Specialty Polymers), Flemion® (Asahi Glass Co.), or sulfonated hydrocarbon-based ionomers such as those available from FuMA-Tech GmbH as the fumapem® P, E, or K series of products (JSR Corporation, Toyobo Corporation, etc.). Examples of suitable anion-conducting polymers include A901 and A201 from Tokuyama Corporation, Fumasep FAA from FuMA-Tech GmbH, and Aemion polymers from Ionomr.

[0050] When the membrane is for use in a PEM electrochemical device, the ion-conducting polymer is preferably a proton-conducting polymer, specifically a partially or fully fluorinated sulfonic acid polymer. Examples of suitable proton-conducting polymers include perfluorosulfonic acid (PFSA) polymers. It may be preferred that the ion-conducting polymer is a PFSA polymer and has an equivalent weight (EW) of greater than 750 EW, greater than 760 EW, greater than 770 EW, or greater than 790 EW. For example, it may be preferred that the ion-conducting polymer is a PFSA polymer having an equivalent weight in the range of 750 to 1200 EW, inclusive, such as 770 to 1000 EW or 800 to 900 EW, inclusive.

[0051] The ion-conducting polymer is typically dispersed in a mixture of an organic solvent and water. For example, the solvent may be a mixture of an alcohol (e.g., ethanol or propanol) and water. The volume ratio of the organic solvent, such as ethanol, to water may be in the range of 95:5 to 60:40 (inclusive), for example, in the range of 90:10 to 70:30 (inclusive). The solvent is formulated to achieve the desired dispersion, coating, and drying characteristics.

[0052] The ink may also include a radical reducing additive (e.g., a peroxide radical reducing additive such as ceria). For example, the radical reducing additive (e.g., ceria) may be provided in the dispersion at a weight percentage in the range of 0.15 wt % to 0.35 wt %, inclusive, for example, 0.20 to 0.30 wt %, inclusive, based on the weight of the ion-conducting polymer. The radical reducing agent is typically added to the ink when the stabilized dispersion is mixed with the ion-conducting polymer.

[0053] The ink formed typically comprises or consists essentially of: (i) Ion-conducting polymers, such as proton-conducting polymers, e.g., PFSA ionomers. The ion-conducting polymers are typically provided in the ink in a weight percentage ranging from 5% to 25% by weight, inclusive, for example, from 10% to 20% by weight, inclusive, based on the total weight of the ink components. (ii) recombination catalyst nanoparticles, such as palladium or platinum nanoparticles. Typically, the catalyst nanoparticles are present in the ink in an amount ranging from 0.01 to 0.40 wt %, inclusive, based on the weight of the ink components, e.g., platinum nanoparticles are present in the ink in an amount ranging from 0.01 to 0.40 wt %, inclusive; (iii) a nanoparticle stabilizer, such as a polymeric nanoparticle stabilizer, e.g., PVP. Typically, the nanoparticle stabilizer is present in the ink in an amount ranging from 0.05 to 2% by weight, e.g., 0.05 to 0.50% by weight, inclusive, based on the total weight of the ink components; (iv) Optionally, a radical reducing additive such as ceria (CeO), typically present in an amount ranging from 0.15% to 0.35% by weight, inclusive, based on the total weight of the ink components. Components (i) to (iv) are dispersed in a solvent such as a mixture of alcohol (eg, ethanol or 1-propanol) and water, for example, in a volume ratio of alcohol:water of 95:5 to 60:40.

[0054] The process includes (iii) fabricating a film layer from the ink. The film layer is typically formed by depositing the ink onto a substrate to form a layer.

[0055] The coating composition can be deposited using a slot die coating process (whereby the dispersion is squeezed onto the substrate by gravity or under pressure through a slot), a knife coating process, a bar coating process, an inkjet printing process, a curtain coating process, a spray coating process, or a casting process. Preferably, the coating composition can be deposited using slot die coating, bar coating, or inkjet printing. Deposition using slot die coating may be particularly preferred.

[0056] The coating composition is deposited on a substrate to form a membrane layer. In some cases, the ion-conductive membrane is formed from a single membrane layer. Alternatively, the ion-conductive membrane may be formed from two or more layers, such as from two to seven layers. The number of layers is determined, for example, by the desired membrane thickness and the desired degree of compositional variation across the membrane (e.g., the membrane may include one or more layers containing a reinforcing polymer, such as ePTFE, or an additive, such as a radical-reducing additive).

[0057] Typically, the substrate is a backing sheet, an ion-conducting layer, a catalyst layer on a backing sheet, or a catalyst layer on a gas diffusion electrode. Those skilled in the art will understand that the choice of substrate depends on the structure and manufacturing stage of the membrane.

[0058] When the membrane is formed from a single membrane layer, or at the beginning of the manufacture of a multilayer membrane, the substrate is typically a backing layer. The backing layer provides support for the ion-conductive membrane during manufacture and, if not immediately removed, can provide support and strength during subsequent storage and / or transportation. The material from which the backing layer is made must provide the necessary support, be compatible with ink, preferably impermeable to ink, be able to withstand the process conditions involved in the manufacture of the ion-conductive membrane, and be easily removable without damaging the ion-conductive membrane. Examples of suitable materials for use include fluoropolymers such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymers (PFA), fluorinated ethylene propylene (FEP—a copolymer of hexafluoropropylene and tetrafluoroethylene), and polyolefins such as biaxially oriented polypropylene (BOPP).

[0059] In some cases where catalyst-coated membranes are produced, a catalyst layer is provided on a backing layer, for example by printing or using known coating techniques, and a coating composition may then be deposited on the catalyst layer such that the catalyst layer is disposed between the backing layer and the membrane layer formed by depositing the ink.

[0060] In some cases, typically when the thickness of the ion-conducting membrane is such that multiple passes are required to build the membrane structure, the substrate is a pre-formed membrane layer. It will be understood that the ion-conducting membrane may also be formed by sequential deposition of layers. For example, the ion-conducting membrane may be formed as follows: In a first pass, an ink containing an ion-conducting polymer may be deposited on a backing layer to form a first ion-conducting polymer layer, which is then dried. In a second pass, an ink is deposited on the first ion-conducting polymer layer to form a second ion-conducting polymer layer. The second ion-conducting polymer layer is then dried. This series of application and drying is continued to produce additional ion-conducting polymer layers as needed to form the desired membrane structure. Those skilled in the art will understand that the recombination catalyst-containing ink described above may be used in one or more of the coating passes required for the final membrane structure.

[0061] Membranes formed by the methods described herein can be used to manufacture catalyst-coated membranes. In such cases, the method can include forming a catalyst layer on the first and / or second surfaces of the membrane to form an anode and / or cathode. Those skilled in the art will understand that the specific types of catalysts for the cathode and anode will be selected depending, for example, on whether the membrane is for a fuel cell or an electrolyzer, and whether the membrane is a PEM or AEM, as described above. Furthermore, deposition methods can be modified; for example, the catalyst layer can be transferred to the membrane from a decal, e.g., by hot pressing, or the catalyst ink can be printed directly onto the membrane.

[0062] The present invention also provides an ion-conducting membrane, such as a PEM or AEM, comprising a recombination catalyst-containing membrane layer containing dispersed recombination catalyst nanoparticles. Such membranes are particularly suitable for electrolyzer applications. The ion-conducting membrane may be obtained or is obtainable by the process described above.

[0063] Typically, the ion-conducting membrane has a thickness of 100 μm or less. It may be preferred that the membrane has a thickness of 95 μm, 90 μm, or 85 μm or less. It may be preferred that the membrane has a thickness of at least 10 μm, e.g., at least 15 μm, at least 20 μm, at least 25 μm, at least 30 μm, or at least 40 μm. It may be more preferred that the membrane has a thickness in the range of 10 to 100 μm, e.g., 15 to 100 μm, 20 to 100 μm, 30 to 100 μm, 30 to 90 μm, or 40 to 90 μm (inclusive).

[0064] The thickness of the ion-conducting membrane (and the thickness of the membrane layers) can be measured by scanning electron microscopy (SEM) at 0% relative humidity. SEM analysis is performed on a cross-section of the membrane, measuring the membrane and / or layer thickness at multiple (e.g., 10) points. The thickness value is then determined by calculating the arithmetic mean of the measurements. Typically, SEM measurements are performed on cross-sections of membranes that have been embedded in resin, ground, and polished.

[0065] The ion-conducting membrane comprises a recombination catalyst-containing membrane layer. Those skilled in the art will understand that the membrane may comprise more than one recombination catalyst-containing membrane layer, for example, two or more recombination catalyst-containing membrane layers. It may be preferable for the membrane to have a single recombination catalyst-containing membrane layer.

[0066] The recombination catalyst nanoparticles are dispersed throughout the membrane layer. The membrane layer also includes an ion-conducting polymer and a nanoparticle stabilizer, each preferably as described above with respect to the present process. Dispersed throughout the membrane layer, as used herein, means that the nanoparticles are distributed throughout the membrane layer, i.e., they are not located in discrete areas of the layer, such as on the surface of a reinforcing component. Those skilled in the art will understand that the term "dispersed" does not exclude clustering of the nanoparticles, but in such cases, the clusters themselves are distributed throughout the membrane layer.

[0067] Suitably, the recombination catalyst nanoparticles have an average particle size of less than 50 nm, for example, in the range of 1 to 50 nm, inclusive. It may be preferred that the recombination catalyst nanoparticles have an average particle size in the range of 1 to 40 nm, 1 to 30 nm, 1 to 20 nm, or 1 to 10 nm, inclusive. The average particle size of the recombination catalyst nanoparticles in the film can be determined by transmission electron microscopy (TEM), for example, by analyzing a cross-section of the film by TEM, measuring the particle size of a population of particles (e.g., 100 particles) from the resulting image by image analysis, and then calculating the average (mean) value.

[0068] Typically, the recombination catalyst nanoparticles are substantially in the form of clusters of discrete nanoparticles. In such cases, the nanoparticles are present in the form of individual nanoparticles co-located within the cluster, and not in the form of nanoparticle aggregates or agglomerates in which the nanoparticles are bonded together via nanoparticle surface-to-nanoparticle surface interactions. Without being bound by theory, it is proposed that such an arrangement of nanoparticles may provide benefits related to higher accessibility for hydrogen to reach the catalytic sites. Typically, the clusters have an average particle size in the range of 100 to 500 nm (inclusive). The average particle size of the clusters of recombination catalyst nanoparticles in the film can be determined by transmission electron microscopy (TEM), for example, by analyzing a cross-section of the film by TEM, measuring the particle size of a population of clusters (e.g., 100) from the resulting image by image analysis, and then calculating the average (mean) value.

[0069] The membrane layer includes a polymeric nanoparticle stabilizer, such as polyvinylpyrrolidone, as described above. Typically, the recombination catalyst nanoparticles are at least partially coated with the nanoparticle stabilizer.

[0070] As mentioned above, it may be preferable for the nanoparticle stabilizer to have a higher hydrophobicity and / or lower water uptake value than the ion-conducting polymer used in the recombination catalyst-containing membrane layer. Without being bound by theory, the use of a nanoparticle stabilizer with a higher hydrophobicity and / or lower water uptake value than the ion-conducting polymer may potentially improve recombination catalyst efficiency by increasing the rate at which hydrogen gas can access the surface of the recombination catalyst nanoparticles and facilitating the removal of water formed from the catalyst surface. This increases the rate of recombination of hydrogen and oxygen, and therefore enhances protection from hydrogen crossover for a given catalyst loading in the membrane. The water uptake values ​​of the ion-conducting polymer and nanoparticle stabilizer can be determined by drying a sample of the material and then weighing the sample before and after immersion in water (e.g., at 23°C for 24 hours). For example, the water uptake value can be measured by (i) drying the sample in an oven until the weight stabilizes, (ii) cooling the sample in a desiccator, (iii) weighing the sample, (iv) immersing the sample in water (e.g., at 23°C for 24 hours), (iv) removing the sample and patting it dry with a lint-free cloth, and (v) reweighing the sample.

[0071] Preferably, the ion-conductive membrane has a density of 1 to 30 mg / cm -2 range (including boundary values), e.g., 5 to 25 mg / cm -2 range (including boundary values), or 8 to 15 mg / cm -2 range (including boundary values), or 1 to 10 mg / cm -2 or 1-5 mg / cm -2 The membrane has a recombination catalyst (e.g., platinum) loading in the range of 0.01 to 0.01. This catalyst loading range has been found to provide a favorable balance between reducing the level of hydrogen crossover during use and the costs associated with including a catalyst in the membrane. Catalyst loading can be determined by inductively coupled plasma mass spectrometry (ICP-MS).

[0072] Typically, the recombination-containing membrane layer has a thickness in the range of 5 to 30 μm, inclusive. Dispersion of nanoparticles within a membrane layer of at least 5 μm provides the advantage of improved membrane stability compared to the use of a thinner catalyst layer, for example, applied to the membrane surface. The use of a recombination-containing membrane layer with a thickness greater than 30 μm is not necessary to substantially reduce hydrogen crossover and may result in manufacturing difficulties, particularly when forming non-laminated membrane structures. The thickness of the membrane layer can be determined by SEM analysis of the membrane cross-section, as described above. It may be preferable for the recombination catalyst-containing membrane layer to have a thickness in the range of 5 to 20 μm, e.g., 7 to 15 μm, inclusive. Such a thickness provides a favorable balance between reducing hydrogen crossover through the formed membrane and manufacturing efficiency.

[0073] Preferably, the membrane is formed by depositing multiple layers of ion-conducting polymers onto each other via a liquid deposition process such as printing, spraying, or coating, by a method that does not require a lamination step to form the membrane.

[0074] Preferably, the membrane is a single coherent polymer film comprising multiple ion-conducting polymer layers. As used herein, the term "coherent" means that the membrane does not include internal laminate interfaces.

[0075] Lamination of ion-conducting membranes involves pressing and / or bonding at least two solid, ion-conducting membranes together, optionally coated with a catalyst layer. A lamination interface is formed between two membranes where the solid surfaces of the individual membranes are pressed and / or bonded together. The lamination interface contains physical defects. Furthermore, the structural and / or chemical properties of the lamination interface also differ from those of the bulk polymer material. This is because, when a solid membrane is formed, its outer surface has surface characteristics that differ from those of the bulk material. For example, at the air interface, a hydrophobic skin forms on the membrane's surface. Raman spectroscopy can detect this difference. Thus, when two solid membranes are pressed together, the lamination interface formed by the two solid surfaces has a distinctive chemical and / or structural morphology compared to the bulk of the ion-conducting polymer material. Therefore, microscopy and spectroscopic techniques can distinguish a lamination interface between layers of ion-conducting polymers from an interface formed by a liquid deposition process, such as printing, spraying, or coating layers to build a multilayer structure. That is, non-laminated interfaces are structurally and / or chemically distinct from laminated interfaces and are not merely a feature of the fabrication process. Furthermore, non-laminated interfaces can be identified as non-laminated in a film without prior knowledge of the fabrication process. Examples of analytical techniques for detecting laminated interfaces include cross-sectional SEM. Changes in crystallinity at interfaces can be detected using cross-sectional TEM. Other techniques for detecting laminated interfaces include 13C / 1H / 19F solid-state NMR, neutron diffraction, and / or a combination of two or more of the aforementioned techniques.

[0076] Due to physical defects and / or chemical variations at the lamination interfaces between ion-conducting polymer films, such interfaces may increase the resistance of the multilayer ion-conducting membrane. Therefore, it has been found advantageous to fabricate multilayer ion-conducting membranes by building up a multilayer membrane structure by depositing layers of ion-conducting polymers dispersed in a liquid solvent, rather than through the lamination of individual solid layers / films of ion-conducting polymers.

[0077] Preferably, the membrane comprises a reinforcing polymer such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). It may be preferred that the recombination-containing membrane layer does not comprise a reinforcing polymer.

[0078] The reinforcing material may include a porous reinforcing polymer sheet impregnated with an ion-conducting polymer, which is optionally expanded polytetrafluoroethylene (ePTFE). Because typical reinforcing polymer materials are not conductive to ions or are not sufficiently conductive to ions, the reinforcing layer is formed using a porous reinforcing polymer impregnated with an ion-conducting polymer through the pores of the material to provide an ion-conducting pathway from one side of the layer to the other side of the layer.

[0079] Preferably, the membrane includes a radical reducing additive (e.g., a peroxide radical reducing additive such as ceria). Note that peroxides can decompose to form a range of radicals (O, OH, OOH), and the radical reducing additive can reduce the amount of one, more, or all of these radicals. The radical reducing additive may be dispersed within the recombination-containing membrane layer.

[0080] Typically, the membrane is configured such that a recombination catalyst-containing membrane layer (1) is disposed between a first ion-conducting polymer layer (2) and a second ion-conducting polymer layer (3), as shown in Figure 1. In such a configuration, the second surface (4) of the first ion-conducting polymer layer (2) and the second surface (5) of the second ion-conducting polymer layer (3) each face inward toward the recombination catalyst-containing membrane layer (1). The first surface (6) of the first ion-conducting polymer layer (2) and the first surface (7) of the second ion-conducting polymer layer (3) are the outer surfaces of the membrane, i.e., they face toward the anode and cathode when incorporated into, for example, a water electrolysis device.

[0081] Preferably, the membrane comprises a recombination catalyst-containing membrane layer disposed between a first ion-conducting polymer layer and a second ion-conducting polymer layer. Those skilled in the art will appreciate that the first ion-conducting polymer layer and the second ion-conducting polymer layer may be formed from one or more sublayers that may be of the same or different composition.

[0082] When the membrane is for a PEM electrochemical device, the ion-conducting polymers present in the first and second ion-conducting polymer layers are preferably proton-conducting polymers, specifically partially or fully fluorinated sulfonic acid polymers. Examples of suitable proton-conducting polymers include perfluorosulfonic acid ionomers. It may be preferable for the ion-conducting polymer in the first and / or second ion-conducting layers to be the same as the ion-conducting polymer in the recombination catalyst-containing membrane layer. Alternatively, it may be preferable for the ion-conducting polymer in the first and / or second ion-conducting layers to be different from the ion-conducting polymer in the recombination catalyst-containing membrane layer.

[0083] Typically, a reinforcing polymer and / or a radical reducing agent (eg, a peroxide radical reducing additive such as ceria) is present in the first and / or second ion-conducting polymer layers.

[0084] It may be preferable for the thickness of the first ion-conducting polymer layer to be less than the thickness of the second ion-conducting polymer layer. This asymmetry allows the recombination catalyst-containing membrane layer to be located closer to the anode than to the cathode in a water electrolysis device configuration, which may be beneficial in reducing hydrogen crossover.

[0085] It may be preferred that the first ion-conducting polymer layer has a thickness in the range of 5 to 30 μm, inclusive, for example, 5 to 20 μm, or 5 to 15 μm, or 7 to 15 μm, inclusive. The inventors believe that such a thickness of the first ion-conducting polymer layer provides a suitable distance between the anode layer and the recombination catalyst in a CCM formed for a water electrolysis device, thereby providing a significant reduction in hydrogen crossover.

[0086] It may be preferred that the second ion-conducting polymer layer has a thickness in the range of 10 to 90 μm inclusive, for example in the range of 20 to 70 μm, 40 to 70 μm, or 25 to 45 μm inclusive.

[0087] The thickness of the ion-conducting polymer layer can be adjusted, for example, by varying the number of deposition passes of the ion-conducting polymer during the fabrication of the membrane or by varying the pump speed during the deposition of the ion-conducting polymer.

[0088] It may be preferred that the membrane comprises or consists of (i) a first ion-conducting layer having a thickness in the range of 5 to 15 μm (inclusive), (ii) a second ion-conducting layer having a thickness in the range of 25 to 45 μm (inclusive), and (iii) a recombination catalyst-containing membrane layer having a thickness in the range of 5 to 15 μm (inclusive) and disposed between the first and second ion-conducting layers. In such a configuration, it is preferred that the second ion-conducting layer comprises a reinforcing polymer such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). Such a membrane structure has been found to provide a particularly favorable balance between membrane resistance and hydrogen crossover levels.

[0089] It may be preferable for the membrane to comprise (i) a first ion-conducting layer having a thickness in the range of 5 to 15 μm (inclusive), (ii) a second ion-conducting layer having a thickness in the range of 40 to 70 μm (inclusive), and (iii) a recombination catalyst-containing membrane layer having a thickness in the range of 5 to 15 μm (inclusive) and disposed between the first and second ion-conducting layers. In such a configuration, it is preferable for the second ion-conducting layer to comprise a reinforcing polymer such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). It may be even more preferable for the second ion-conducting layer to comprise two regions of a reinforcing polymer, such as two sublayers containing a reinforcing polymer such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). This membrane structure allows operation even at particularly high gas pressure differentials across the membrane while maintaining low hydrogen crossover and low membrane resistance.

[0090] The membranes described herein may be suitably used as part of a catalyst-coated membrane (CCM). Such CCMs have an anode catalyst layer and / or a cathode catalyst layer applied to a face of the membrane. The membranes are also useful in systems in which one or more of the anode and cathode catalyst layers are applied to substrates located on either side of the membrane, such as gas diffusion layers or porous transport layers.

[0091] In the case of a CCM for a water electrolysis device, a cathode catalyst layer may be applied to the surface of the membrane containing a catalyst for catalyzing the hydrogen evolution reaction. It may be preferable for the cathode catalyst layer to contain platinum, e.g., a platinum-on-carbon catalyst. The catalyst material may be formulated into an ink, printed ex-situ on a PTFE sheet, and transferred onto the membrane by hot pressing. Alternatively, the ink may be coated directly onto the membrane.

[0092] In the case of a CCM for a water electrolysis device, an anode catalyst layer may be applied to the surface of the membrane containing a catalyst for catalyzing the oxygen evolution reaction. If the CCM is for a PEMWE, it may be preferred that the anode catalyst layer comprises iridium, e.g., iridium oxide, or a mixed oxide of iridium and another metal element.

[0093] The anode material can be formulated into an ink, preferably an ion-conducting polymer, printed ex-situ onto a PTFE sheet, and transferred onto the membrane by hot pressing. Alternatively, the ink can be coated directly onto the membrane.

[0094] Typically, a CCM includes a membrane comprising a first ion-conducting polymer layer and a second ion-conducting polymer layer, with a recombination catalyst-containing membrane layer disposed between the first and second ion-conducting polymer layers, as described above. Preferably, the CCM is configured so that the recombination catalyst-containing membrane layer is closer to the anode catalyst layer than to the cathode catalyst layer. It may further be preferable for the second ion-conducting polymer layer to be thicker than the first ion-conducting polymer layer. Such a configuration has been proposed to be advantageous in terms of reducing hydrogen crossover.

[0095] Preferably, the CCM is configured such that the second surface (4) of the first ion-conducting polymer layer (2) and the second surface (5) of the second ion-conducting polymer layer (3) each face inward toward the recombination catalyst-containing membrane layer (1), as shown in Figure 2. If present, the anode catalyst layer (8) is disposed on the first surface (6) of the first ion-conducting polymer layer (2). If present, the cathode catalyst layer (9) is disposed on the first surface (6) of the second ion-conducting polymer layer (3).

[0096] The catalyst-coated membrane may preferably comprise or consist of: (i) a first ion-conducting layer having a thickness in the range of 5 to 15 μm (inclusive); (ii) a second ion-conducting layer having a thickness in the range of 25 to 45 μm (inclusive); and (iii) a recombination catalyst-containing membrane layer having a thickness in the range of 5 to 15 μm (inclusive) and disposed between the first and second ion-conducting layers, wherein the second surfaces of the first and second ion-conducting polymer layers each face inward toward the recombination catalyst-containing membrane layer, and wherein the anode catalyst layer described above is disposed on the first surface of the first ion-conducting layer and / or the cathode catalyst layer described above is disposed on the first surface of the second ion-conducting layer. In such a configuration, the second ion-conducting layer preferably comprises a reinforcing polymer such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI).

[0097] The catalyst-coated membrane may preferably comprise or consist of: (i) a first ion-conducting layer having a thickness in the range of 5 to 15 μm (inclusive); (ii) a second ion-conducting layer having a thickness in the range of 40 to 70 μm (inclusive); and (iii) a recombination catalyst-containing membrane layer having a thickness in the range of 5 to 15 μm (inclusive) and disposed between the first and second ion-conducting layers, wherein the second surfaces of the first and second ion-conducting polymer layers each face inward toward the recombination catalyst-containing membrane layer, and wherein the anode catalyst layer described above is disposed on the first surface of the first ion-conducting layer and / or the cathode catalyst layer described above is disposed on the first surface of the second ion-conducting layer. In such a configuration, the second ion-conducting layer preferably comprises a reinforcing polymer such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). It may further be preferred that the second ion-conducting layer comprises two regions of a reinforcing polymer, such as two sub-layers comprising a reinforcing polymer, such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI).

[0098] The present invention will now be described with reference to the following examples, which are provided to aid in the understanding of the invention and are not intended to limit its scope. [Example]

[0099] Example 1 - Formation of a stabilized dispersion of platinum nanoparticles using PVP. A PVP-stabilized dispersion of platinum nanoparticles in water was prepared from an aqueous solution of platinum nitrate and 2 wt% PVP (MW 10,000) using a continuous-flow hydrothermal reactor operating at high temperature and pressure. The Pt loading in the dispersion was measured to be 3.93 g / L using ICP.

[0100] Dynamic light scattering (DLS) analysis showed that the z-average diameter of the PVP-Pt clusters in the dispersion was 280 nm, and small-angle X-ray scattering (SAXS) analysis showed that the particle size of the platinum nanoparticles was in the range of 1–10 nm.

[0101] Example 2 - Formation of an ink containing PVP-stabilized platinum nanoparticles and an ion-conducting polymer The aqueous dispersion prepared in Example 1 was mixed with additional water and EtOH to create an ethanol-in-water mixture (4:1 weight percent ratio). Dry PFSA ionomer (3M Corporation, EW-800) was added to create an ink of approximately 17 weight percent ionomer and 0.08 weight percent Pt. The ink was mixed using a roller mixer.

[0102] Analysis by dynamic light scattering (DLS) showed that the z-average diameter of the Pt-PVP clusters in the dispersion was 1200 nm.

[0103] The stability of the ink during storage was assessed by measuring the z-average diameter after 1 day and 3 months. The results are shown in Figure 3, which shows no change in the DLS analysis profile, indicating high stability and resistance to aggregation.

[0104] Example 3 - Preparation of a membrane containing a platinum-containing membrane layer A membrane comprising a platinum-containing membrane layer was prepared by knife coating a layer of the ink prepared according to the method of Example 2 onto a 15 μm thick PFSA membrane and allowing the formed layer to dry at room temperature.

[0105] A cross-section of the formed film was analyzed by scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX). This showed that the platinum-containing film layer had a thickness of approximately 30 μm. An enlarged cross-section of the SEM-EDX image of the platinum-containing film layer is shown in Figure 4. This shows that platinum nanoparticles are distributed throughout the film layer.

[0106] Example 4 - Preparation of a membrane containing a platinum-containing membrane layer and analysis by cryo-TEM The ink prepared according to Example 2 was frozen in a cryomicrotome using liquid nitrogen and cut into sections using a diamond knife. One section was placed on a copper grid and thawed to form a film less than 100 nm thick.

[0107] The formed films were analyzed by cryo-transmission electron microscopy (cryo-TEM), which shows that the platinum nanoparticles in the films are in the form of clusters of discrete Pt nanoparticles, with an average particle size in the range of 1–10 nm.

[0108] Using an algorithm, we measured the average nearest-neighbor distance between platinum particles from cryo-TEM images. This data was compared to an analysis of a control membrane incorporating a platinum-containing membrane layer prepared from an ink containing Pt particles derived from a platinum-black source with the same Pt ​​loading but without any stabilizers. This showed a significant decrease in interparticle distance when the membrane was formed from the stabilized nanoparticle dispersion, thus indicating increased Pt dispersion in the membrane layer.

[0109] Example 5 - Formation and hydrogen crossover testing of CCMs incorporating membranes with platinum-containing membrane layers. A composite membrane approximately 105 μm thick was prepared by casting a 10 μm recombination catalyst layer (using the method described in Example 3) onto an 80 μm PFSA membrane, and then laminating the product onto a 15 μm thick PFSA membrane, so that the recombination catalyst layer was located between the two PFSA membranes. The CCM was prepared by casting a Pt / C cathode catalyst layer (0.4 mg cm) onto a 15 μm thick PFSA membrane, so that the recombination catalyst layer was located between the two PFSA membranes. -2 Pt loading of 1000 mg cm) and an IrOx anode catalyst layer (2 mg cm -2 The composite membrane was prepared by laminating two Ir-containing films (having an Ir loading of 10 ...

[0110] Comparative examples of CCM were also prepared using (i) an 80 micron PFSA membrane without a recombination catalyst ("80 μm Control" in FIG. 5 ) and (ii) the same catalyst layer formed on the membrane prepared in Example 5, but with a recombination catalyst layer formed from an ink loaded with Pt particles (without the use of a stabilized nanoparticle dispersion, "Pt" in FIG. 5 ).

[0111] Catalyst Coated Membrane (CCM) Testing Hydrogen Crossover The level of hydrogen crossover for each CCM was measured at different pressures using the following method.

[0112] A water electrolysis cell incorporating the catalyst-coated membrane to be tested was prepared. The cell temperature was maintained at 80°C, and the anode and cathode pressures were set to 2 bar. Then, a current density of 2 A / cm was applied. 2 The cathode pressure was increased stepwise from 2 bar to 6 bar to 10 bar, with a minimum duration of 45 minutes for each step. The % H in oxygen at the anode gas outlet was measured using a Compact GC 4.0 Gas Chromatograph (GC) from Global Analysis Solutions. The CCM with PVP-Pt particles in the membrane did not reach equilibrium in the 2 bar test before the pressure change, so data for this value are not presented, but hydrogen crossover remained below 0.4%.

[0113] Figure 5 shows the results of hydrogen crossover testing of the CCM. These results indicate that the membrane formed from PVP-Pt particles provides the greatest reduction in hydrogen crossover.

[0114] Example 6 - Formation of a stabilized dispersion of platinum nanoparticles using PVP and formaldehyde. Pt(NO3)4 (equivalent to 1 g of Pt) was added to water (500 mL) and stirred. PVP10 (average molecular weight 10,000, 8.5 g) was added, followed by formaldehyde (37% in water, 20.8 g). The mixture was heated to 68°C, then allowed to cool to room temperature and stirred overnight to form a dispersion.

[0115] Example 7 - Formation of an ion-conducting polymer ink containing PVP-stabilized nanoparticles A stabilized aqueous dispersion of Pt nanoparticles (formed according to a method similar to Example 6) was mixed with ethanol and water to make a mixture with an ethanol:water weight ratio of 80:20. Dry ionomer (3M Corp, 800EW) was added to the mixture to make a dispersion with an ionomer solids content of approximately 17 wt%.

[0116] Example 8 - Hydrogen crossover test with various Pt loadings A series of catalyst-coated membranes were prepared with different loadings of stabilized Pt nanoparticles and the following structures: (1) Pt / C-containing cathode layer (2) Approximately 60 micron PFSA membrane (with two ePTFE reinforcements) (3) A Pt-containing recombination catalyst layer of approximately 10 microns (4) PFSA membrane layer of approximately 10 microns (5) Iridium oxide (IrOx)-containing anode layer Layers 2, 3, and 4 were applied using a film applicator (slot die and baker coater). Layers 1 and 5 were attached to composite layer 2-3-4 using lamination above the ionomer transition temperature (160°C).

[0117] Recombination catalyst layers were prepared using either (i) an ion-conducting polymer ink containing PVP-stabilized Pt nanoparticles prepared using a method similar to that of Example 7 (“PVP-Pt” in FIG. 6 ), or (ii) an ion-conducting polymer ink containing unsupported Pt particles (without the use of PVP) (“Pt particles” in FIG. 6 ).

[0118] The CCM was tested for hydrogen crossover, as shown in Figure 6. This shows that the use of the stabilized nanoparticle dispersion (Pt-PVP) allowed for nearly complete crossover of hydrogen across the membrane under the test conditions, even at low platinum loadings, demonstrating significantly improved performance when considered alongside a comparative CCM not prepared with the stabilized nanoparticle dispersion.

Claims

1. 1. A process for producing an ion-conducting membrane comprising a recombination catalyst-containing membrane layer, said process comprising: (i) providing a stabilized dispersion of recombination catalyst nanoparticles; (ii) mixing the stabilized dispersion with an ion-conducting polymer to form an ink; (iii) fabricating said membrane layer from said ink.

2. The process of claim 1 , wherein the recombination catalyst nanoparticles comprise platinum and / or palladium.

3. 3. The process of claim 1 or 2, wherein the stabilized dispersion of recombination catalyst nanoparticles comprises a nanoparticle stabilizer having a higher hydrophobicity and / or a lower water uptake value than the ion-conducting polymer used in step (ii).

4. The process of any one of claims 1 to 3, wherein the stabilized dispersion comprises a polymeric nanoparticle stabilizer.

5. 5. The process of claim 3 or 4, wherein the nanoparticle stabilizer is polyvinylpyrrolidone (PVP).

6. The process of any one of claims 1 to 5, wherein step (iii) comprises depositing the ink on a substrate such as a backing sheet, an ion-conducting polymer layer, or a catalyst layer on a backing sheet.

7. 7. The process of claim 6, wherein the substrate is a first ion-conducting polymer layer, and the process includes step (iv) of adding the second ion-conducting polymer layer such that the recombination catalyst-containing membrane layer is disposed between the first ion-conducting polymer layer and the second ion-conducting polymer layer.

8. The process of any one of claims 1 to 7, further comprising forming a catalyst layer on at least one side of the membrane to form a catalyst-coated membrane.

9. 10. The process of claim 8, further comprising applying a seal material to at least one surface of the catalyst coated membrane.

10. An ink for use in manufacturing an ion-conducting membrane, said ink comprising recombination catalyst nanoparticles, a nanoparticle stabilizer, and an ion-conducting polymer.

11. The ink of claim 10, wherein the nanoparticle stabilizer is polyvinylpyrrolidone.

12. 12. An ion-conducting membrane for an electrochemical device such as a fuel cell or a water electrolyzer, said ion-conducting membrane being produced by the process of any one of claims 1 to 9 or using the ink of claim 10 or 11.

13. An ion-conducting membrane for an electrochemical device, such as a fuel cell or a water electrolyzer, comprising a recombination catalyst-containing membrane layer, the membrane layer comprising dispersed recombination catalyst nanoparticles, a nanoparticle stabilizer, and an ion-conducting polymer.

14. 14. The ion-conducting membrane of claim 13, wherein the nanoparticle stabilizer has a higher hydrophobicity and / or a lower water uptake value than the ion-conducting polymer.

15. 15. The ion-conducting membrane of claim 13 or 14, wherein the recombination catalyst nanoparticles are in the form of clusters of discrete nanoparticles.

16. The ion-conducting membrane according to any one of claims 13 to 15, wherein the nanoparticle stabilizer is a polymer such as polyvinylpyrrolidone.

17. The ion-conducting membrane according to any one of claims 13 to 16, wherein the recombination catalyst nanoparticles are at least partially coated with the nanoparticle stabilizer.

18. The ion-conductive membrane according to any one of claims 13 to 17, wherein the nanoparticles have an average particle size of less than 50 nm.

19. An ion-conducting membrane according to any one of claims 13 to 18, wherein the membrane has a thickness in the range of 30 to 90 μm, inclusive.

20. 20. The ion-conducting membrane according to any one of claims 13 to 19, wherein the recombination catalyst-containing membrane layer has a thickness in the range of 5 to 30 μm, inclusive.

21. 21. The ion-conducting membrane according to any one of claims 13 to 20, wherein the membrane is a single coherent polymer film comprising multiple ion-conducting polymer layers.

22. 22. The ion-conducting membrane according to any one of claims 13 to 21, comprising a first ion-conducting polymer layer and a second ion-conducting polymer layer, and the recombination catalyst-containing membrane layer is disposed between the first ion-conducting polymer layer and the second ion-conducting polymer layer.

23. 23. An ion-conducting membrane according to claim 22, wherein the first ion-conducting polymer layer has a thickness in the range of 5 μm to 30 μm inclusive, preferably in the range of 5 μm to 20 μm inclusive.

24. An ion-conducting membrane according to claim 22 or 23, wherein the second ion-conducting polymer layer has a thickness in the range of 10 μm to 90 μm, preferably in the range of 40 μm to 70 μm, limits included.

25. A catalyst coating film for an electrochemical device such as a fuel cell or a water electrolysis device, comprising the ion-conductive film according to any one of claims 12 to 24.

26. A water electrolysis device comprising the ion-conductive membrane according to any one of claims 12 to 24 or the catalyst-coated membrane according to claim 25.

27. A fuel cell comprising the ion-conducting membrane according to any one of claims 12 to 24 or the catalyst-coated membrane according to claim 25.

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