Method of manufacturing a catalyst-coated ion-conducting membrane or a membrane electrode assembly

Abstract

Method According to the invention there is provided a method of manufacturing a catalyst-coated ion- conducting membrane or a membrane electrode assembly. The method comprises the steps of: providing an ion-conducting membrane comprising a first face and a second face; depositing a catalyst ink onto the first face of the ion-conducting membrane to form a wet catalyst layer, wherein the catalyst ink comprises a solvent, an electrocatalyst dispersed in the solvent, and an ion-conducting polymer or a blend of polymers; and drying the wet catalyst layer to form a dried catalyst layer, wherein the ion-conducting polymer or the blend of polymers has a stress relaxation time of 600 s or less.

Description

[0001] METHOD OF MANUFACTURING A CATALYST-COATED ION-CONDUCTING MEMBRANE OR A MEMBRANE ELECTRODE ASSEMBLY

[0002] Field of the Invention

[0003] This invention relates to a method of manufacturing a catalyst-coated ion-conducting membrane ora membrane electrode assembly. In particular, this invention relates to a method of preparing a catalyst layer for a catalyst-coated ion-conducting membrane or a membrane electrode assembly. This invention also relates to associated catalyst-coated ion-conducting membranes and membrane electrode assemblies, which can be used in an electrochemical device, such as a fuel cell or an electrolyser.

[0004] Background of the Invention

[0005] A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol such as methanol or ethanol, or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

[0006] Fuel cells are usually classified according to the nature of the electrolyte employed. Often the electrolyte is a solid polymeric membrane, in which the membrane is electronically insulating but ionically conducting. In the proton exchange membrane fuel cell (PEMFC) the ion-conducting membrane is proton conducting, and protons, produced at the anode, are transported across the ion-conducting membrane to the cathode, where they combine with oxygen to form water.

[0007] An electrolyser is an electrochemical device for electrolysing water to produce high purity hydrogen and oxygen. Electrolysers can operate in both alkaline and acidic systems. Those electrolysers that employ a solid proton-conducting polymer electrolyte membrane, or proton exchange membrane (PEM), are known as proton exchange membrane water electrolysers (PEMWEs). Those electrolysers that utilise a solid anion-conducting polymer electrolyte membrane, or anion exchange membrane (AEM), are known as anion exchange membrane water electrolysers (AEMWEs).

[0008] A principal component of a fuel cell or electrolyser is the membrane electrode assembly (MEA). The MEA is typically composed of five layers. The central layer is the polymer ionconducting membrane (also referred to as the electrolyte membrane). On either face of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer, and / or a porous transport layer. The gas diffusion layer and porous transport layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore, the gas diffusion layer and porous transport layer must be both porous and electrically conducting.

[0009] The electrocatalyst layers also generally comprise a proton conducting material, such as a proton conducting polymer, for example, to aid transfer of protons from the anode electrocatalyst of a fuel cell to the ion-conducting membrane and / or from the ion-conducting membrane to the cathode electrocatalyst of a fuel cell.

[0010] The membrane electrode assembly can be constructed by a number of known methods. For example, the methods can involve depositing one or both of the electrocatalyst layers on a decal transfer substrate and transferring the electrocatalyst layers to either side of the ionconducting membrane. Subsequently, a gas diffusion layer is applied to the electrocatalyst layer. Alternatively, an electrocatalyst layer can be applied to a gas diffusion layer to form a gas diffusion electrode, which is then combined with the ion-conducting membrane. A membrane electrode assembly can be prepared by a combination of these methods, e.g. one electrocatalyst layer is applied to the ion-conducting membrane to form a catalyst coated ionconducting membrane, and the other electrocatalyst layer is applied as a gas diffusion electrode. As a further alternative method, an electrocatalyst ink can be deposited directly onto an ion-conducting membrane and dried to form the electrocatalyst layer. However, ionconducting membranes can swell and contract upon changes in humidity, such as coating with a wet electrocatalyst ink and drying respectively. Therefore, such methods of coating the electrocatalyst ink directly onto the ion-conducting membrane can result in the formation of undesirable cracking defects in the electrocatalyst layer. It is therefore desirable to develop a method to reduce or eliminate cracking defects in the electrocatalyst layer when coating an electrocatalyst ink directly onto an ion-conducting membrane.

[0011] Summary of the Invention

[0012] It is an object of the present invention to provide an improved method of manufacturing a catalyst-coated ion-conducting membrane, suitably for use in an electrochemical device, such as a fuel cell or an electrolyser.

[0013] Accordingly, in a first aspect of the invention there is provided a method of manufacturing a catalyst-coated ion-conducting membrane, the method comprising the steps of: providing an ion-conducting membrane comprising a first face and a second face; depositing a catalyst ink onto the first face of the ion-conducting membrane to form a wet catalyst layer, wherein the catalyst ink comprises a solvent, an electrocatalyst dispersed in the solvent, and an ion-conducting polymer or a blend of polymers comprising a first ionconducting polymer; drying the wet catalyst layer to form a dried catalyst layer; wherein the ion-conducting polymer or the blend of polymers has a stress relaxation time of 600 s or less.

[0014] Using an ion-conducting polymer (or a blend of polymers) which has a stress relaxation time of 600 s or less, can surprisingly suppress the formation of cracking defects when depositing a catalyst ink directly onto an ion-conducting (electrolyte) membrane. In particular, using a catalyst ink comprising such an ion-conducting polymer (or blend of polymers) allows thicker catalyst layers of higher loading to be deposited in a single pass, whilst maintaining satisfactorily low levels of cracking defects. Consequently, methods of the invention can improve manufacturing efficiency and can increase throughput of catalyst-coated ionconducting membranes. Additionally, the presence of defects, such as cracking defects, in the catalyst layer can accelerate certain failures modes during use, which can degrade performance more quickly. Therefore, catalyst layers with a low percentage defect area are also expected to exhibit improved durability.

[0015] According to a second aspect there is provided a catalyst coated ion-conducting membrane obtainable using a method of the first aspect. Such a catalyst coated ionconducting layer can be characterised in that the dried catalyst layer has a percentage defect area of less than 2.0 %, preferably less than 1.5%, more preferably less than 1.0%, more preferably still less than 0.5%, and still more preferably less than 0.2%.

[0016] According to a third aspect there is provided a membrane electrode assembly (MEA) obtainable using a method of the first aspect. Such a membrane electrode assembly can be characterised in that the dried catalyst layer has a percentage defect area of less than 2.0 %, preferably less than 1.5%, more preferably less than 1.0%, more preferably still less than 0.5%, and still more preferably less than 0.2%.

[0017] According to a further aspect there is provided an electrochemical device comprising the catalyst-coated ion-conducting membrane of the second aspect. The electrochemical device is preferably a fuel cell or a water electrolyser, such as a proton exchange membrane (PEM) fuel cell or PEM water electrolyser.

[0018] Brief Description of the Drawings

[0019] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0020] Figure 1 is a schematic cross-sectional view of a catalyst-coated ion-conducting membrane obtainable by a method of the present disclosure;

[0021] Figure 2 is a schematic cross-sectional view of a catalyst-coated ion-conducting membrane obtainable by a method of the present disclosure;

[0022] Figure 3 is an optical microscope image of a dried catalyst layer on a backing sheet according to Comparative Example 1 at a magnification of 35x; Figure 4 is an optical microscope image of a dried catalyst layer on an electrolyte membrane according to Comparative Example 2 at a magnification of 35x;

[0023] Figure 5 is an optical microscope image of a dried catalyst layer on an electrolyte membrane according to Example 1 at a magnification of 35x;

[0024] Figure 6 is a plot of defect area (as a percentage of total area) as a function of metal loading in the catalyst layer;

[0025] Figure 7 is a plot of defect area (as a percentage of total area) as a function of stress relaxation time.

[0026] Detailed Description of the Invention

[0027] Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention, unless the context demands otherwise. Any of the preferred or optional features of any aspect may be combined, singly or in combination, with any aspect of the invention, unless the context demands otherwise.

[0028] The present disclosure provides a method comprising a step of depositing a catalyst layer directly onto an ion-conducting membrane. The method can be for manufacturing a catalyst-coated ion-conducting membrane, a membrane electrode assembly or other similar structure. A catalyst-coated ion-conducting membrane typically comprises an ion-conducting membrane layer which has a catalyst layer coated on at least one, suitably both, of its faces. The term “membrane electrode assembly” is used here to mean constructs comprising additional layers, such as but not limited to seals, gas diffusion layers and porous transport layers. The construct formed by the method is therefore dependent upon the form in which the ion-conducting membrane is provided. The resultant catalyst-coated ion-conducting membrane or membrane electrode assembly can be used in an electrochemical device, such as a fuel cell or a water electrolyser.

[0029] Figure 1 shows an exemplary catalyst-coated ion-conducting membrane 100 obtainable using a method of the present disclosure. The catalyst-coated ion-conducting membrane 100 comprises an ion-conducting membrane 110 and a first catalyst layer 120. The first catalyst layer 120 can be a cathode catalyst layer or an anode catalyst layer. Optionally, the ionconducting membrane 110 can be provided on a carrier support 140.

[0030] Figure 2 shows an exemplary catalyst-coated ion-conducting membrane 200 obtainable using a method of the present invention. The catalyst-coated ion-conducting membrane 200 comprises an ion-conducting membrane 210 disposed between two catalysts layers 220, 230. One of the catalyst layers is an anode catalyst layer, and the other catalyst layer is a cathode catalyst layer. One or both of the catalyst layers can be prepared using a method according to the present invention. In the method of the present disclosure, an ion-conducting membrane, such as an electrolyte membrane, is first provided. The ion-conducting membrane comprises a first face and a second face. A catalyst ink is deposited onto the first face of the ion-conducting membrane to form a wet catalyst layer. Preferably, the wet catalyst layer is formed in a single pass. The catalyst ink comprises a solvent, an electrocatalyst dispersed in the solvent, and an ion-conducting polymer or a blend of polymers comprising an ion-conducting polymer. The wet catalyst layer is subsequently dried to form a dried catalyst layer. Preferably, the dried catalyst layer is formed in a single deposition pass. The dried catalyst layer can be a dried first catalyst layer. The ion-conducting polymer or the blend of polymers has a stress relaxation time of 600 s or less. The stress relaxation time can be determined using the methods described below. Employing an ion-conducting polymer or blend of polymers (such as a blend of ion-conducting polymers) in the catalyst ink which has a stress relaxation time of less than 600 s, surprisingly and significantly reduces the area containing defects as a percentage of the total catalyst layer area (hereinafter “percentage defect area”). The percentage defect area can be determined using the methods described below. Moreover, using a catalyst ink comprising such an ion-conducting polymer or blend of polymers allows thicker catalyst layers of higher loading to be deposited in a single pass, whilst maintaining satisfactorily low levels of defects, such as cracking defects. Consequently, methods of the invention can improve manufacturing efficiency and can increase throughput of catalyst-coated ion-conducting membranes.

[0031] Optionally, the method can comprise additional steps in which a second catalyst layer can be applied to the second face of the ion-conducting membrane opposite to the face on which the first catalyst layer is deposited. That is, the second catalyst layer and the first catalyst layer are on opposite faces of the ion-conducting membrane. The second catalyst layer can be applied before or after the first catalyst layer. The second catalyst layer can be applied by any suitable method. For example, the second catalyst layer can be applied by a decal transfer method or by coating a catalyst ink directly onto the second face of the ion-conducting membrane. Preferably, a method of depositing the second catalyst layer comprises the additional steps of: depositing a second catalyst ink onto the second face of the ion-conducting membrane to form a wet second catalyst layer, wherein the second catalyst ink comprises a solvent, an electrocatalyst dispersed in the solvent, and an ion-conducting polymer or a blend of polymers comprising an ion-conducting polymer; and drying the wet second catalyst layer to form a dried second catalyst layer. Preferably, the ion-conducting polymer or the blend of polymers of the second catalyst ink has a stress relaxation time of 600 s or less. The ion-conducting polymer or blend of polymers can be the same or different to the ion-conducting polymer or the blend of polymers of the first catalyst ink. The additional steps of depositing the second catalyst layer are preferably performed after the steps of depositing the first catalyst layer.

[0032] The catalyst layers formed by the method of the present disclosure typically exhibit a percentage defect area of less than 2.0 %, preferably less than 1.5%, more preferably less than 1.0%, more preferably less than 0.5% and still more preferably less than 0.1%.

[0033] Providing the ion-conducting membrane

[0034] The method is suitably part of a roll-to-roll manufacturing process. For example, the ionconducting membrane can be provided as a roll of ion-conducting membrane. The roll of ionconducting membrane can be unwound during the step of depositing the catalyst ink onto the first face of the ion-conducting membrane. The ion-conducting membrane can preferably be provided as an elongate strip.

[0035] The ion-conducting membrane is suitably an electrolyte membrane. The ion-conducting membrane comprises an ion-conducting polymer. The ion-conducting polymer is suitably a proton-conducting polymer. Preferred ion-conducting polymers are partially- or fully- fluorinated sulphonic acid polymers e.g. perfluorinated sulphonic acid polymers. For example, the ion-conducting polymer may be based on a perfluorinated sulphonic acid material such as Nation® (Chemours Company), Aquivion® (Syensqo), Flemion® (Asahi Glass Group) and Aciplex® (Asahi Kasei Chemicals Corp.), and perfluorosulphonic acid ionomer material supplied by 3M. Alternatively, the ion-conducting materials may be based on a sulphonated hydrocarbon polymer, such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, and others.

[0036] The ion-conducting membrane can comprise a reinforcing component, such as a planar reinforcing component. Preferably, the reinforcing component is porous (i.e. comprises pores). The reinforcing component can confer mechanical strength to the ion-conducting membrane. The reinforcing component can contain a porous reinforcing material, such as an expanded polytetrafluoroethylene (ePTFE) material or a nanofibre network, such as a network comprising polybenzimidazole (PBI) fibres or glass fibres. The network of fibres can be a nonwoven mat of fibres (e.g. nanofibres), such as an electrospun mat of fibres or nanofibres. In some other embodiments the electrolyte membrane comprises a reinforcing component in the form of a woven fabric, such as a woven fabric formed from polymer threads, such as ePTFE or PEEK threads. Suitable materials are described in US11742507B2 (AGC INC).

[0037] Typically, in the methods of manufacturing the catalyst coated ion-conducting membrane, the ion-conducting membrane is an elongate strip of ion-conducting membrane. For example, the ion-conducting membrane can be provided as a roll of ion-conducting membrane. The ion-conducting membrane can comprise a plurality of layers. For example, the ion-conducting membrane can comprise a first unreinforced ion-conducting membrane layer, a second unreinforced ion-conducting membrane layer, and a reinforced ion-conducting membrane layer disposed between the first and second unreinforced ion-conducting membrane layers, wherein the reinforced ion-conducting membrane layer comprises the reinforcing component.

[0038] The ion-conducting membrane can optionally be provided on a backing layer such that the face on which the catalyst ink is deposited is facing away from the backing layer. Put another way, the ion-conducting membrane can be provided with its second face facing or directly adjacent a backing layer. The backing layer can provide mechanical stability to the ion-conducting membrane during processing and if not immediately removed, can provide support and strength during any subsequent storage and / or transport. The material from which the backing layer is made should provide the required support, be able to withstand the process conditions involved in producing the catalyst-coated ion-conducting membrane and be able to be easily removed without damage to the ion-conducting membrane. Examples of materials suitable for use include a fluoropolymer, such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP - a copolymer of hexafluoropropylene and tetrafluoroethylene), and polyolefins, such as biaxially oriented polypropylene (BOPP). Other examples include laminates, multi-layer extrusions and coated films / foils capable of retaining their mechanical strength / integrity at elevated temperatures, for example temperatures up to 200 °C. Examples include laminates of: poly(ethylene-co-tetrafluoroethylene) and polyethylene naphthalate (PEN); polymethylpentene (PMP) and PEN; polyperfluoroalkoxy (PFA) and polyethylene terephthalate (PET) and polyimide (PI). The laminates can have two or more layers, for example ETFE-PEN-ETFE, PMP-PEN-PMP, PFA-PET-PFA, PEN-PFA, FEP-PI-FEP, PFA- PI-PFA and PTFE-PI-PTFE. The layers may be bonded using an adhesive, such as acrylic or polyurethane.

[0039] The ion-conducting membrane can be provided with its second face adjacent to a preformed (second) catalyst layer. For example, the ion-conducting membrane can be provided with a pre-formed catalyst layer disposed between the second face of the ion-conducting membrane and a backing layer. In some embodiments, the ion-conducting membrane can be provided with the second face facing a gas diffusion layer (GDL) or a porous transport layer (PTL), optionally with a (second) catalyst layer disposed between the second face of the ionconducting membrane and the GDL or PTL. In some embodiments, the ion-conducting membrane can be provided with the second face adjacent and / or facing a gas diffusion electrode or a porous transport electrode.

[0040] Depositing the catalyst ink The method comprises depositing a catalyst ink onto a face (i.e. a first face) of the ionconducting membrane to form a wet catalyst layer. The catalyst ink can be deposited as a single continuous strip on the ion-conducting membrane along a length of the ion-conducting membrane (i.e. in a machine direction), for example, so as to extend substantially fully along a length of the ion-conducting membrane. The catalyst ink can be deposited as a plurality of parallel continuous strips in the machine direction, each continuous strip suitably extending substantially fully along a length of the ion-conducting membrane. Alternatively, the catalyst ink can be deposited as a plurality of spaced apart (i.e. discontinuous) patches, wherein each patch is spaced apart in the machine direction. The catalyst ink can be deposited as a plurality of adjacent columns of discrete patches (each column extending in the machine direction), for example, to form an array of discrete patches. Each continuous strip or patch can extend partially or fully across the width of the ion-conducting membrane (i.e. in a transverse direction). The wet catalyst layer can extend partially or fully across the width of the ionconducting membrane (i.e. in the transverse direction).

[0041] The catalyst ink can be deposited using a slot-die (slot, extrusion) coating process (whereby the coating is squeezed out under pressure via a slot onto the ion-conducting membrane substrate), gravure coating, screen printing, rotary screen printing, inkjet printing, spraying, painting, bar coating, pad coating, gap coating techniques such as knife or doctor blade over roll (whereby the coating is applied to the substrate then passes through a split between the knife and a support roller), metering rod application such as with a Meyer bar, and laser induced forward transfer (LIFT). Preferably, the catalyst ink is deposited using a slot die coating process, gravure coating, or laser induced forward transfer.

[0042] Catalyst ink

[0043] The catalyst ink comprises: a solvent, an electrocatalyst, and an ion-conducting polymer or a blend of polymers, wherein the blend of polymers comprises at least one ion-conducting polymer.

[0044] Solvent

[0045] The solvent is not particularly limited providing that the electrocatalyst and the ionconducting polymer or blend of polymers can be dispersed in the solvent. Preferably, the solvent can comprise water, ethanol, n-propanol, iso-propanol, or a mixture thereof. Preferably, the solvent comprises (or consists essentially of or consists of) water and ethanol, or water and n-propanol, and most preferably water and ethanol.

[0046] Electrocatalyst

[0047] The electrocatalyst comprises metal particles optionally supported on a catalyst support, such as an electrically conductive support. Preferably, the electrocatalyst comprises platinum group metal particles, optionally supported on a catalyst support such as an electrically conductive support. That is, the electrocatalyst can be unsupported metal particles (e.g. finely divided unsupported metal powder) or may be a supported electrocatalyst wherein metal particles (e.g. nanoparticles) are dispersed on an electrically conductive support, such as an electrically conducting particulate carbon support.

[0048] The metal particles of the electrocatalyst are suitably selected from:

[0049] (i) the platinum group metals (i.e. platinum, palladium, rhodium, ruthenium, iridium, and osmium),

[0050] (ii) gold or silver,

[0051] (iii) a base metal, or

[0052] (iv) an alloy or mixture comprising one or more of these metals or their oxides. Preferably, the metal in the metal particles of the electrocatalyst is a platinum group metal or an alloy of a platinum group metal. The most preferred electrocatalyst metal is platinum, which may be alloyed with other precious metals or base metals. A base metal is tin or a transition metal which is not a noble metal. A noble metal is a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium or osmium), silver or gold. Preferred base metals are copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin.

[0053] If the electrocatalyst is a supported catalyst, the loading of metal particles on the electrically conductive support material is suitably in the range 10 wt.% to 90 wt.%, or 30 wt.% to 75 wt.%, preferably 40 wt.% to 60 wt.%, and most preferably 45 wt.% to 55 wt.% based on the weight of the electrocatalyst. The loading of the metal particles can be determined using inductively coupled plasma mass spectrometry (ICPMS).

[0054] Preferably, the electrocatalyst comprises an electrically conductive support and metal particles supported on the electrically conductive support. The term “supported” will be readily understood by a skilled person. For example, it will be understood that the term “supported” includes the metal particles of the electrocatalyst being dispersed on (and / or in the pores of) the support material and bound or fixed to the support material by physical or chemical bonds. For instance, the catalyst may be bound or fixed to the support material by way of ionic or covalent bonds, or non-specific interactions such as van der Waals forces.

[0055] The electrically conductive support may be an electrically conductive carbon support material. Suitably, the electrically conductive carbon support material is a carbon powder which may be, for example, a carbon black or graphitised carbon black for example a commercially available carbon black (such as from Cabot Corp. (Vulcan® XC72R) or Akzo Nobel (the Ketjen® black series)). Another suitable carbon support material is an acetylene black (e.g. those available from Chevron Phillips (Shawinigan Black®) or Denka). The electrically conductive carbon support can be prepared by the method disclosed in WO2013 / 045894. Alternatively, the electrically conductive support can be a metal oxide or a mixed oxide, in particular a conductive mixed oxide such as niobia-doped titania, phosphorus- doped tin oxide and mixed platinum group metal oxides or mixed metal oxides (as disclosed in WO2012 / 080726), a carbide (e.g. tungsten carbide, molybdenum carbide or titanium carbide, suitably tungsten carbide or titanium carbide), a nitride, in particular a conductive nitride (e.g. titanium nitride or titanium aluminium nitride).

[0056] Ion-conducting polymer and blend of polymers

[0057] The catalyst ink can comprise an ion-conducting polymer or a blend of polymers. The ion-conducting polymer or the blend of polymers is suitably dispersed in the solvent. The blend of polymers comprises a first ion-conducting polymer and at least one further polymer. The at least one further polymer can be a second ion-conducting polymer or a non-ion-conducting polymer. In some embodiments, the blend of polymers comprises a first ion-conducting polymer, a second ion-conducting polymer and at least one non-ion conducting polymer. In some preferred embodiments, the blend of polymers is a blend of ion-conducting polymers comprising (or consisting essentially of or consisting of) at least two ion-conducting polymers (e.g. first and second ion-conducting polymers).

[0058] In the following description, the term “ion-conducting polymer” can apply to a single ionconducting polymer and / or independently to each ion-conducting polymer present in the blend of polymers.

[0059] The ion-conducting polymer can be a proton-conducting polymer or an anion- conducting polymer, such as a hydroxyl anion-conducting polymer. Preferably a protonconducting polymer. Typically, the ion-conducting polymer comprises sulfonic acid groups. Suitably, the ion-conducting polymer is a perfluorinated sulfonic acid ionomer, or a partially- fluorinated or non-fluorinated hydrocarbon sulfonic acid ionomer. Examples of suitable protonconducting polymers include partially- or fully-fluorinated sulphonic acid polymers, such as perfluorosulphonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aquivion™ (Syensqo), Flemion® (Asahi Glass Co.), and from 3M; or ionomers based on a sulphonated hydrocarbon such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, and others. Typically, the ion-conducting polymer has an equivalent weight of about 1100 or less, typically about 900 or less, suitably about 850 or less. Typically, the ion-conducting polymer has an equivalent weight of at least about 450. The equivalent weight of the ion-conducting polymer may be readily measured using an acid titration following a hydroxide exchange. For example, a membrane sample may be vacuum dried at about 110 °C for 16 hours to obtain about 2g of the dried film. The film may then be immersed in about 30 ml_ of a 0.1 N NaOH solution to substitute sodium ions for protons in the membrane. Then titration by neutralisation is carried out, for example using 0.1 N hydrochloric acid, to determine the number of exchangeable protons, and therefore the EW may be calculated.

[0060] The at least one non-ion conducting polymer is suitably miscible with the ion-conducting polymer(s) in the blend of polymers. Examples of some suitable non-ion conducting polymers include, but are not limited to, copolymers of tetrafluoroethylene and perfluoro vinyl-ethers referred to as perfluoroelastomers (FFKM), or copolymers of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride referred as fluoroelastomers (FKM). Preferably, the at least one non-ion conducting polymer is present in minor amounts so that the ionic conductivity of the catalyst layer is not substantially reduced.

[0061] The ion-conducting polymer or the blend of polymers has a stress relaxation time of 600 s or less, preferably 500 s or less, and more preferably 400 s or less. The ion-conducting polymer or the blend of polymers can have a stress relaxation time of 200 s or more, and optionally 300 s or more. The ion-conducting polymer or the blend of polymers can have a stress relaxation time in a range comprising any of the aforementioned upper and lower limits, for example, the stress relaxation time can be in a range of from 200 s to 600 s. When the stress relaxation time exceeds about 600 s, the defect area as a percentage of the total area increases sharply when depositing catalyst layers directly onto an ion-conducting membrane.

[0062] Stress relaxation time is an intrinsic property of an ion-conducting polymer and can also apply to blends of polymers. The stress relaxation time can be determined for example using the method described in the examples section below.

[0063] The catalyst ink can comprise a single type of ion-conducting polymer. Alternatively, the catalyst ink can comprise a blend of polymers. The term “blend of polymers” is used here to mean an intimate mixture of two or more polymers without covalent bonds between them. The blend of polymers comprises a first ion-conducting polymer and at least one further polymer, such as a second ion-conducting polymer and / or a non-ion conducting polymer. Preferably, the blend of polymers comprises at least two ion-conducting polymers, i.e. at least a first ionconducting polymer and a second ion-conducting polymer. Preferably, the blend of polymers is a miscible polymer blend. Preferably, the blend of polymers can be a blend of ion-conducting polymers consisting essentially of (or consisting of) at least two ion-conducting polymers. For embodiments comprising a blend of polymers, the blend of polymers (or, as the case may be, the blend of ion-conducting polymers) has a stress relaxation time of 600 s or less, preferably 500 s or less, preferably 400 s or less. Suitably, the blend of polymers (or, as the case may be, the blend of ion-conducting polymers) can have a stress relaxation time of 200 s or more, and optionally 300 s or more. The stress relaxation time can be in a range comprising any of the aforementioned upper and lower limits. It is preferable that in embodiments comprising a blend of polymers, the first ion-conducting polymer has a stress relaxation time of 600 s or less, and the first ion-conducting polymer is present in the blend of polymers in an amount of at least 10 wt.%, preferably at least 20 wt.%, at least 30 wt.%, at least 50 wt.% and suitably at least 70 wt.%, based on the total weight of the blend of polymers. In embodiments where the further polymer in the blend (e.g. the second ion-conducting polymer) has a stress relaxation time of >600 s, the further polymer can be present in the blend of polymers in an amount of less than 90 wt.%, preferably less than 80 wt.%, less than 70 wt.%, less than 50 wt.%, and suitably less than 30 wt.%.

[0064] Drying the wet catalyst layer

[0065] The wet catalyst layer is dried to form the dried catalyst layer. Suitably, the wet catalyst layer is heated (preferably in a single heating step) so as to remove substantially all of the solvent, and thereby form a dried catalyst layer. That is, the wet catalyst layer is suitably dried by a heating process. The catalyst layer can continue to be heated after the substantially all of the solvent has been removed. The heating process can anneal the (dried) catalyst layer.

[0066] The wet catalyst layer can be heated to a temperature in a range of 50 °C to 250 °C. The wet catalyst layer can be heated to a temperature that is above the boiling point of the solvent. The heating step can be performed using hot air impingement and / or infra-red drying treatments. Preferably, substantially all of the solvent is removed in a single heating step. Preferably, the ink is dried at a temperature at or above the glass transition temperature of the ion-conducting polymer (or each ion-conducting polymer) present in the wet catalyst layer in order to both evaporate solvent from the layer and also promote flow of the ionomer.

[0067] The present invention enables a catalyst layer having a higher metal loading per deposition pass to be deposited directly onto a face of ion-conducting membrane whilst maintaining an acceptably low percentage defect area. The dried catalyst layer suitably has an electrocatalyst metal loading of at least 0.20 mg / cm2, preferably at least 0.30 mg / cm2, at least 0.35 mg / cm2, and more preferably at least 0.40 mg / cm2. The dried catalyst layer can have an electrocatalyst metal loading of 1.0 mg / cm2or less, 0.9 mg / cm2or less, 0.8 mg / cm2or less, or 0.7 mg / cm2or less. The electrocatalyst metal loading in the dried catalyst layer can suitably be an electrocatalyst platinum group metal (PGM) loading. That is, the dried catalyst layer can have an electrocatalyst PGM loading of at least 0.20 mg / cm2, preferably at least 0.30 mg / cm2, at least 0.35 mg / cm2, and more preferably at least 0.40 mg / cm2. The dried catalyst layer can have an electrocatalyst PGM loading of 1 .0 mg / cm2or less, 0.9 mg / cm2or less, and 0.8 mg / cm2or less, or 0.7 mg / cm2or less. In this context, the electrocatalyst metal loading (or PGM loading) means the amount of metal (or PGM) of the electrocatalyst in the catalyst layer expressed as mg / cm2. The dried catalyst layer can have an electrocatalyst metal loading or platinum group metal loading in a range comprising any of the aforementioned upper and lower limits. Preferably, the dried catalyst layer is formed in a single deposition pass. The dried catalyst layer can be a dried first catalyst layer.

[0068] The (dried) catalyst layer can have a (through-plane) cross-sectional thickness of at least 2 pm, suitably at least 5 pm, and suitably at least 8 pm. The (dried) catalyst layer can have a (through-plane) cross-sectional thickness of 30 pm or less, suitably 25 pm or less and suitably 20 pm or less. The (dried) catalyst layer can have a (through-plane) thickness in a range comprising any of the aforementioned upper and lower limits.

[0069] The catalyst layer can be prepared in any number of deposition passes. A deposition pass comprises the step of depositing a wet catalyst layer. Preferably, the wet catalyst layer is dried in between successive deposition passes. For example, the steps of depositing the catalyst ink and drying the wet catalyst layer can be alternately repeated any number of times, for example two times or three times. In some embodiments, the catalyst layer is formed in more than one deposition pass, for example, two or three deposition passes. Preferably, the catalyst layer is formed in fewer than five deposition passes, preferably fewer than four deposition passes. Most preferably, the catalyst layer is formed in a single deposition pass. The catalyst ink of the present disclosure can be used for any or each deposition pass. For example, a first deposition pass may comprise depositing a catalyst ink not according to the present disclosure onto the first face of the ion-conducting membrane to form a wet catalyst layer, and drying the wet catalyst layer to form a defective catalyst layer comprising defects; and subsequently, as a second deposition pass, depositing the catalyst ink of the present disclosure onto the defective catalyst layer to form a wet catalyst layer, and drying the wet catalyst layer to form the dried catalyst layer of the present disclosure. In such embodiments, the catalyst ink of the present disclosure can fill any defects in the defective catalyst layer, thereby reducing the percentage defect area.

[0070] The invention will now be further described with reference to the following examples, which are illustrative and not limiting of the invention.

[0071] Examples

[0072] Percentage defect area measurement

[0073] The area of defects as a percentage of the total catalyst layer area (i.e. “percentage defect area”) is determined by focussing a backlit optical microscope at magnification of 35x and imaging the catalyst-coated ion-conducting membrane at 15 or more different locations. Defects are observable as bright regions. The area of bright regions (and hence the area of defects) as a percentage of the total area of each image is measured using image analysis techniques. The mean average value from all images is representative of the area of defects as a percentage of the total catalyst layer area (i.e. percentage defect area). Stress relaxation time measurements

[0074] The stress relaxation time was determined using the following method. A membrane consisting (only) of the ion-conducting polymer or the blend of polymers was prepared. A rectangular strip of the membrane consisting (only) of the ion-conducting polymer or the blend of polymers that is 6 mm in width and no less than 40 mm in length was cut using a punch die and removing the membrane strip from its backing, if present. The thickness of the resulting sample can then be measured using a low force high precision gauge instrument (e.g. a Mitutoyo VL-50-B micrometer), in which a motorised spindle is used to take measurement readings with a measuring force of 0.01 N and at a temperature of 20 °C ± 3 °C, and a relative humidity (RH) of 30-50%. The sample was then installed into a dynamic mechanical analyser (DMA) (e.g. a Q800 available from TA Instruments) equipped with a relative humidity controlling chamber and tensile (film) clamps, which can be set approximately 16 mm apart and tightened with 3 in lbs (-0.34 N m) of torque. Using automation of the DMA the sample length can be determined and an experiment can be performed comprising of the following parameters / steps: pre-force 0.002 N; measure length; set relative humidity to 70% and temperature to 80°C; hold for 120 mins; increase relative humidity to 80%; hold for a further 30 minutes; measure length; set data sampling interval to 0.1 s and hold for 0.1 minutes; set strain to 2.00%; measure stress at current sampling rate for 0.1 minutes; change data sampling interval to 1.0 s and hold for 10 minutes; change data sampling interval to 2.0 s and hold for 100 minutes; change data sampling interval to 5.0 s and hold for 100 minutes . Where ‘measure length’ sets the length of the sample to 0% strain.

[0075] The obtained data can be analysed to give a stress relaxation time by defining two times: ti corresponding to the time at which the peak stress omax is recorded, and t2corresponding to the time at which the stress has decayed to a value less than or equal to amax / e, where e is the mathematical constant approximately equal to 2.72. The relaxation time is equal to t2- ti.

[0076] Comparative Example 1

[0077] A 50 wt.% Pt / C catalyst was prepared using methods according to the general method for preparing carbon-supported platinum catalysts as described in WO2013 / 045894.

[0078] A catalyst ink was prepared comprising of 50 wt.% platinum catalyst supported on a high surface area carbon support, and a perfluorinated sulphonic acid (PFSA) ionomer (800EW, supplied by 3M) dispersed in a solvent mix comprising water and 1 -propanol. The ionomer had a stress relaxation time of 920 s.

[0079] The catalyst ink was coated onto a skived PTFE decal transfer substrate using a slot die coating process to form a wet catalyst layer in a single pass. The wet catalyst layer was dried at an elevated temperature not exceeding 250 °C to form a dried catalyst layer. The dried catalyst layer had a platinum loading of 0.4 mgpt / cm2.

[0080] Figure 3 shows an optical microscope image of the dried catalyst layer formed at a magnification of 35x. The catalyst layer had a percentage defect area of 0.0%.

[0081] Comparative Example 2

[0082] A catalyst layer was prepared in the same way as Comparative Example 1 , except that the catalyst ink was coated directly onto an electrolyte membrane rather than a skive PTFE decal transfer substrate. The electrolyte membrane was provided on a PET backing sheet with release layer (available from Daicel). The electrolyte membrane comprised an ion-conducting polymer (800 EW, supplied by 3M) and an expanded polytetrafluoroethylene (ePTFE) reinforcing component. The reinforcing component was disposed centrally in the through- plane (z) direction and extended to the edge of the electrolyte membrane in the in-plane (xy) direction.

[0083] Figure 4 shows an optical microscope image of the dried catalyst layer at a magnification of 35x. The catalyst layer at a platinum loading of 0.4 mgPt / cm2had a very high percentage defect area.

[0084] Comparative Example 3

[0085] Catalyst layers were prepared in the same way as Comparative Example 2, except that the solvent mix used in the catalyst ink comprised water and up to 50 wt.% ethanol. The nominal platinum loading in the catalyst layer was varied from about 0.2 mgPt / cm2to about 0.5 mgFt / cm2.

[0086] Comparative Example 4

[0087] Catalyst layers were prepared in the same way as Comparative Example 3, except that the ionomer used in the catalyst ink was a PFSA ionomer (790EW, supplied by Solvay). The ionomer had a stress relaxation time of 727 s. The nominal platinum loading in the catalyst layer was varied from about 0.35 mgPt / cm2to about 0.6 mgPt / cm2.

[0088] Example 1

[0089] Catalyst layers was prepared in the same way as Comparative Example 3, except that the ionomer used in the catalyst ink was a PFSA ionomer (800 EW, supplied by AGC). The ionomer had a stress relaxation time of 341 s. The nominal platinum loading in the catalyst layer was varied from about 0.3 mgPt / cm2to about 0.7 mgPt / cm2. Figure 5 shows an optical microscope image of a dried catalyst layer according to Example 1 with a platinum loading of about 0.4 mgpt / cm2at a magnification of 35x. The catalyst layer had a percentage defect area of 0.0%.

[0090] Examples 2 and 3, and Comparative Example 5: Blends of ion-conducting polymers

[0091] Catalyst layers were prepared in the same way as Example 1, except that a blend of ionomers was used in the catalyst ink. The blend of ionomers comprised the ionomer used in Example 1 (Ionomer 1), and the ionomer used in Comparative Example 1 (Ionomer 2), in amounts as detailed in Table 1 below. For ionomer blends, the stress relaxation measurements were performed on membranes consisting only of the relevant blend of ionomers.

[0092] Table 1 :

[0093] Results and discussion

[0094] Figure 6 shows a plot of percentage defect area as a function of platinum loading in the catalyst layer for Comparative Examples 3 and 4 and Example 1. For Comparative Examples 3 and 4, the percentage defect area increases sharply as the metal loading in the catalyst layer increases. The percentage defect area becomes unacceptable at higher loadings for the comparative examples. Surprisingly, using an ionomer (or blend of ionomers) that has a lower stress relaxation time beneficially reduces the extent of defects observed in the catalyst layer, even at high metal loadings. This can enable catalyst layers with a higher loading to be manufactured in a single pass while still maintaining acceptable layer quality and low level of defects, which can improve manufacturing throughput for higher loading catalyst layers.

[0095] Figure 7 and Table 2 show how the percentage defect area varied as a function of stress relaxation time for Examples 1-3 and Comparative Examples 3-5, for catalyst layer having a nominal platinum loading of 0.4 mgpt / cm2.

[0096] Table 2:

[0097] Unexpectedly a sharp decrease in percentage defect area was observed when the catalyst layers comprised an ion-conducting polymer or blend of polymers exhibiting a stress relaxation time of less than about 600 s. When the stress relaxation time was greater than about 600 s the percentage defect area increased sharply. Using an ion-conducting polymer or blend of polymers having a stress relaxation time of less than 600 s can improve the layer quality of the catalyst layers formed, significantly reducing the number of defects observed, which can reduce the number of defective parts and therefore increase product yield and reduce waste.

Claims

Claims1. A method of manufacturing a catalyst-coated ion-conducting membrane or a membrane electrode assembly, the method comprising the steps of: providing an ion-conducting membrane comprising a first face and a second face; depositing a catalyst ink onto the first face of the ion-conducting membrane to form a wet catalyst layer, wherein the catalyst ink comprises a solvent, an electrocatalyst dispersed in the solvent, and an ion-conducting polymer or a blend of polymers comprising a first ionconducting polymer; drying the wet catalyst layer to form a dried catalyst layer; wherein the ion-conducting polymer or the blend of polymers has a stress relaxation time of 600 s or less.

2. A method according to claim 1 , wherein the ion-conducting polymer or the blend of polymers has a stress relaxation time of 500 s or less, preferably 400 s or less.

3. A method according to claim 1 or 2, wherein the ion-conducting polymer or the blend of polymers has a stress relaxation time of 200 s or more, and optionally 300 s or more.

4. A method according to any previous claim, wherein the blend of polymers further comprises a second ion-conducting polymer.

5. A method according to any previous, wherein the blend of polymers further comprises at least one non-ion conducting polymer.

6. A method according to any previous claim, wherein the first ion-conducting polymer has a stress relaxation time of 600 s or less, and the first ion-conducting polymer is present in the blend of polymers in an amount of at least 10 wt.%, preferably at least 20 wt.%, based on the total weight of the blend of polymers.

7. A method according to any previous claim, wherein the electrocatalyst comprises a platinum group metal, preferably supported on an electrically conductive support.

8. A method according to any previous claim, wherein the dried catalyst layer has a platinum group metal loading in a range of 0.20 mg / cm2to 1.0 mg / cm2, suitably 0.4 mg / cm2to 0.8 mg / cm2.

9. A method according to any previous claim, wherein the solvent comprises water or a mixture of water and at least one of ethanol, 1 -propanol or iso-propanol.

10. A method according to any previous claim, wherein the method is part of a roll-to-roll manufacturing process.

11. A method according to any previous claim, wherein the ion-conducting membrane is provided with the second face facing a backing layer, optionally wherein a second catalyst layer is disposed between the second face of the ion-conducting membrane and the backing layer.

12. A method according to any previous claim, wherein the ion-conducting membrane is provided with the second face adjacent to a second catalyst layer.

13. A method according to any of claims 1 to 10 or 12, wherein the ion-conducting membrane is provided with the second face adjacent to a gas diffusion electrode or porous transport electrode or facing a gas diffusion layer or porous transport layer, optionally wherein a second catalyst layer is disposed between the second face of the ion-conducting membrane and the gas diffusion layer or porous transport layer.

14. A method according to any previous claim, wherein the dried catalyst layer is formed in a single deposition pass.

15. A method according to any previous claim, wherein the catalyst ink is deposited as a single continuous strip along a length of the ion-conducting membrane.

16. A method according to any previous claim, wherein the catalyst ink is deposited as a plurality of spaced apart patches on the ion-conducting membrane.

17. A method according to any previous claim, wherein the catalyst ink is deposited onto the first face of the ion-conducting membrane using slot die coating, spray coating, knife coating, bar coating, inkjet printing, gravure printing, curtain coating, or laser induced forward transfer (LIFT).

18. A method according to any previous claim further comprising the steps of: depositing a second catalyst ink onto the second face of the ion-conducting membrane to form a wet second catalyst layer, wherein the second catalyst ink comprises asolvent, an electrocatalyst dispersed in the solvent, and an ion-conducting polymer or a blend of polymers comprising a first ion-conducting polymer; drying the wet second catalyst layer to form a dried second catalyst layer; wherein the ion-conducting polymer or the blend of polymers has a stress relaxation time of 600 s or less.

19. A method according to any previous claim, wherein the ion-conducting membrane comprises a reinforcing component.

20. A catalyst coated ion-conducting membrane obtainable using a method according to any of the preceding claims, wherein the catalyst layer has a percentage defect area of less than 2.0 %, and preferably less than 1.5 %.21 . A catalyst coated ion-conducting membrane according to claim 20, wherein the catalyst layer has a has a platinum group metal loading in a range of 0.20 mg / cm2to1.0 mg / cm2, suitably 0.30 mg / cm2to 0.80 mg / cm2, and suitably 0.35 mg / cm2to 0.70 mg / cm2.

22. A membrane electrode assembly obtainable using a method according to any of claims 1 to 19, wherein the catalyst layer has a percentage defect area of less than 2.0 %, and preferably less than 1.5 %.

23. A membrane electrode assembly according to claim 22, wherein the catalyst layer has a has a platinum group metal loading in a range of 0.20 mg / cm2to 1.0 mg / cm2, suitably 0.30 mg / cm2to 0.8 mg / cm2, and suitably 0.35 mg / cm2to 0.70 mg / cm2.

24. An electrochemical device comprising the catalyst-coated ion-conducting membrane according to claim 20 or the membrane electrode assembly according to claim 21.

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