Process for the manfacture of catalyst-coated polymer electrolyte membranes

Abstract

A method of manufacturing a catalyst-coated polymer electrolyte membrane for an electrochemical device is provided. The method comprises the steps of: (a) applying a catalyst composition to a support substrate to form a first catalyst layer, the catalyst composition comprising a noble metal-containing electrocatalyst and a catalyst layer sulfonic acid ionomer with a transition temperature Talpha; (b) heat treating the first catalyst layer at a temperature T, wherein T is in the range of and including Talpha+ 40 °C to Talpha + 120 °C; and (c) coating the first catalyst layer with one or more layers of a membrane ionomer to form a polymer electrolyte membrane.

Description

[0001] P102010W001

[0002] PROCESS FOR THE MANFACTURE OF CATALYST-COATED POLYMER ELECTROLYTE MEMBRANES

[0003] Field of the Invention

[0004] This invention relates to methods for the manufacture of catalyst-coated polymer electrolyte membranes for electrochemical devices, such as fuel cells and water electrolysers.

[0005] Background

[0006] Catalyst-coated polymer electrolyte (ion-conducting) membranes (CCMs) may be employed within electrochemical devices, such as electrolysers, fuel cells, electrochemical hydrogen pumps, and devices for the electrochemical reduction of carbon dioxide. Such CCMs comprise a solid polymeric ion-conducting membrane, such as a proton exchange membrane (PEM) or an anion exchange membrane (AEM), with an anode catalyst layer and / or a cathode catalyst layer applied to a face of the membrane, the anode catalyst layer and cathode catalyst layer being applied to opposite faces of the membrane.

[0007] For water electrolyser applications, hydrogen evolution reaction (HER) electrocatalysts are used in such cathode catalyst layers, for example HER catalysts comprising platinum, such as platinum on a carbon support. Oxygen evolution reaction (OER) electrocatalysts are utilised in electrolyser anode catalyst layers. For PEM water electrolyser applications, suitable OER catalysts comprise iridium and I or ruthenium, for example oxides of iridium, or oxides containing both iridium and ruthenium. For AEM water electrolyser applications, non-platinum group metal OER catalysts may also be used, such as alloys and oxides of nickel, cobalt, iron, and copper.

[0008] For fuel cell applications, oxygen reduction reaction (ORR) electrocatalysts are used in cathode catalyst layers and hydrogen oxidation reaction (HOR) electrocatalysts are utilised in anode catalyst layers. For PEM fuel cell applications, suitable cathode and anode catalyst materials comprise a platinum group metal or an alloy of a platinum group metal with one or more other metals, for example platinum, or an alloy of platinum with one or more other metals, typically provided on a carbon support.

[0009] Other electrochemical devices may also incorporate CCMs, with the catalyst layers being selected according to the desired electrochemistry. For example, for electrochemical hydrogen pump applications, CCMs may be provided with a first catalyst layer incorporating a hydrogen oxidation reaction (HOR) catalyst (such as a platinum catalyst) on one face of the ion-conducting membrane, and a second catalyst layer incorporating a hydrogen evolution P102010W001 reaction (HER) catalyst (such as a platinum catalyst) on the opposite face of the ionconducting membrane.

[0010] Separate seal material layers, typically formed from non-ion conducting polymers, may be positioned around the edge region of a CCM, for example on exposed surfaces of the polymer electrolyte membrane where no electrocatalyst is present (but will also often overlap on to the edge of the electrocatalyst layer) to provide a seal to prevent escape of reactant and product gases, to reinforce and strengthen the edge of the CCM and provide a suitable surface for supporting subsequent components such as sub-gaskets. An adhesive layer may be present on one or both surfaces of the seal material layer.

[0011] CCMs may be incorporated into a membrane electrode assembly (MEA), which is essentially composed of five layers. The central layer is the polymer electrolyte membrane. On either side of the polymer electrolyte membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer there is a transport layer, the composition and properties of which depend on the final MEA application and stack configuration. Such layers allow the reactants to reach the electrocatalyst layer and products to leave.

[0012] It is known to form CCMs by sequential deposition of layers. EP2774203B1 describes a method comprising the steps of a) preparing a first catalyst layer on a supporting substrate; b) coating the first catalyst layer with an ionomer dispersion to form an ionomer membrane in contact with the first catalyst layer; and c) applying a second catalyst layer on top of the ionomer membrane. In Example 1 , a catalyst ink comprising a short-side-chain PFSA ionomer Aquivion (RTM) D79-20BS (Solvay) and a Pt-alloy electrocatalyst is applied to a carrier substrate by knife-coating. The catalyst layer is then dried at 100 °C for five minutes.

[0013] There are significant manufacturing efficiency advantages associated with the manufacture of CCMs via sequential deposition of layers (such processes may be known as additive layer manufacturing), for example relating to the reduction in handling steps and the number of required backing materials. However, the formation of a polymer electrolyte membrane on, and in contact with, a catalyst layer is technically challenging, in particular across a range of catalyst layer properties, such as catalyst layer thickness and catalyst metal loading.

[0014] For example, it has been found that coating a membrane ionomer dispersion onto a catalyst layer can lead membrane ionomer penetration into the catalyst layer. This is undesirable as such penetration can lead to variability in catalyst layer performance which can be a significant issue in particular during large scale manufacture with tight quality control standards. There remains a need to further enhance and develop methods for the production of catalyst-coated P102010W001 polymer electrolyte membranes, in particular methods which enable sequential deposition of CCM components whilst maintaining technical performance.

[0015] Summary of the invention

[0016] The present inventors have found that by adjusting the heat treatment temperature applied to a catalyst layer, manufacturing issues relating to penetration of membrane ionomers during additive layer manufacturing of CCMs can be significantly reduced or eliminated.

[0017] Therefore, in a first aspect of the invention there is provided a method of manufacturing a catalyst-coated polymer electrolyte membrane for an electrochemical device, such as a fuel cell or an electrolyser, the method comprising the steps of:

[0018] (a) applying a catalyst composition to a support substrate to form a first catalyst layer, the catalyst composition comprising a noble metal-containing electrocatalyst and a catalyst layer sulfonic acid ionomer with a transition temperature TaiPha;

[0019] (b) heat treating the first catalyst layer at a temperature T, wherein T is in the range of and including T alpha"*" 40 °C to Taipha + 120 °C;

[0020] (c) coating the first catalyst layer with one or more layers of a membrane ionomer to form a polymer electrolyte membrane.

[0021] In a second aspect of the invention there is provided a catalyst-coated polymer electrolyte membrane (CCM) for an electrochemical device, such as a fuel cell or electrolyser, obtainable or obtained by the method of the first aspect.

[0022] Brief description of the Figures

[0023] Figure 1 shows a schematic representation of patches of the first catalyst layer on a support substrate.

[0024] Figure 2 shows a schematic representation of a method of manufacturing a catalyst-coated polymer electrolyte membrane.

[0025] Figure 3 shows the results of an analysis of membrane ionomer penetration at different heat treatment temperatures.

[0026] Detailed Description P102010W001

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

[0028] The present invention provides a method of manufacturing a catalyst-coated polymer electrolyte membrane. Suitably, the catalyst-coated polymer electrolyte membrane is a catalyst-coated proton exchange membrane or a catalyst-coated anion exchange membrane. Preferably, the catalyst-coated polymer electrolyte membrane is a catalyst-coated proton exchange membrane. The catalyst-coated polymer electrolyte membrane is suitable for an electrochemical device, such as a fuel cell, a water electrolyser, an electrochemical hydrogen pump, or a device for the electrochemical reduction of carbon dioxide, and in particular a catalyst-coated proton exchange membrane for a fuel cell or for a water electrolyser.

[0029] As used herein the term “catalyst-coated polymer electrolyte membrane” refers to a polymer electrolyte membrane with a first face and a second face, and which has a first catalyst layer on the first face of the membrane and, optionally, a second catalyst layer on the second opposite face of the membrane.

[0030] The method comprises step (a) applying a catalyst composition to a support substrate to form a first catalyst layer. The support substrate provides support for the catalyst-coated polymer electrolyte membrane during manufacture and, if not immediately removed, can provide support and strength during any subsequent storage and / or transportation. The material from which the support substrate is made should provide the required support, be able to withstand the process conditions involved in producing the catalyst-coated polymer electrolyte membrane and be able to be easily removed without damage to the catalyst-coated polymer electrolyte membrane. Examples of materials suitable for use as a support substrate include a fluoropolymer, such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP - a copolymer of hexafluoropropylene and tetrafluoroethylene), polyolefins, such as biaxially oriented polypropylene (BOPP), polyesters, such as polyethylene naphthalate (PEN), and poly(phenylene sulfide) (PPS). 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- P102010W001

[0031] PI-PFA, PTFE-PI-PTFE, ETFE-PET. The layers may be bonded using an adhesive, such as acrylic or polyurethane.

[0032] The catalyst composition, and therefore the first catalyst layer, comprises a noble metalcontaining electrocatalyst. The noble metals are Ru, Rh, Pd, Os, Ir, Pt, and Au. Suitably, the electrocatalyst is a catalyst of the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), the oxygen reduction reaction (ORR) and I or the hydrogen oxidation reaction (HOR). Suitably, the electrocatalyst comprises Pt, such as Pt metal or an alloy of platinum with other noble metals or base metals. In some other embodiments, the electrocatalyst comprises iridium and I or ruthenium, such as an oxide of iridium and I or ruthenium. In some embodiments, the catalyst composition, and therefore the first catalyst layer, comprises an electrocatalyst comprising Pt, such as Pt metal or an alloy of platinum with other noble metals or base metals, in combination with an oxygen evolution catalyst, such as an oxide of iridium and I or ruthenium.

[0033] Preferably, the electrocatalyst is provided on a support material, such as a support metal comprising a metal oxide or carbon. Typically, the loading of catalyst on the support material is in the range of and including 10 to 90 wt%, preferably in the range of and including 15 to 70 wt%, of the total weight of the supported catalyst. Preferably, the catalyst in the first catalyst layer on the support substrate comprises platinum on a carbon support material, for example Pt / C, or an alloy of platinum with other noble metals or base metals on a carbon support.

[0034] The catalyst composition, and therefore the first catalyst layer, comprises a catalyst layer sulfonic acid ionomer. Suitably, the catalyst layer sulfonic acid ionomer is a perfluorinated sulfonic acid ionomer, or a partially-fluorinated or non-fluorinated hydrocarbon sulfonic acid ionomer.

[0035] Typically, the catalyst layer sulfonic acid ionomer has an equivalent weight of about 1100 or less, typically about 900 or less, suitably about 850 or less. Typically, the catalyst layer sulfonic acid ionomer has an equivalent weight of at least about 450. The equivalent weight of the ionomer may be readily measured using an acid titration following a hydroxide exchange. For example, a sample may be vacuum dried at about 110 °C for 16 hours to obtain about 2g of the dried material. The material may then be immersed in about 30 mL of a 0.1 N NaOH solution to substitute sodium ions for protons in the sample. 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. P102010W001

[0036] Suitably, the catalyst layer ionomer comprises sulfonic acid groups and is a short side chain ionomer. By short side chain ionomer it is meant herein that the side chain has a length of 3 atoms connecting the backbone and the sulfonic acid group, for example the side chain is [polymer backbone]-O-CF2CF2-SC>3H, for example -CF(-O-CF2CF2-SC>3H)(CF2)n-. Such materials may be purchased, for example Aquivion (RTM) PFSA ionomers from Syensqo.

[0037] Suitably, the catalyst layer ionomer comprises sulfonic acid groups and is a long side chain ionomer. By long side chain ionomer it is meant herein that the side chain has a length of 4 or 5 atoms connecting the backbone and the sulfonic acid group, preferably the side chain is [polymer backbone]-O-CF2CF(CF3)OCF2CF2-SO3H, or [polymer backbone]- OCF2CF2CF2CF2SO3H. Such materials may be purchased, for example Nation (RTM) PFSA ionomers from Chemours, or Forblue (RTM) i-series ionomers from AGC.

[0038] Sulfonic acid ionomer materials exhibit a thermal transition between a state in which clusters of ionic groups are closely associated and a state in which the interactions between those clusters have been weakened therefore allowing for long range molecular motion. This transition is described as an alpha transition, and the transition temperature is Ta or T alpha. The transition temperature Ta of the sulfonic acid ionomer materials are measured by subjecting a sample of the material to dynamic mechanical analysis at a relative humidity of 0%, an oscillation frequency of 1 Hz, and a temperature sweep rate of 1 °C / min. The transition temperature Ta is suitably derived from a plot of Tan delta vs temperature and is the temperature at which the Tan delta value is at its maximum.

[0039] The first catalyst layer is provided on a first face of the support substrate. The first catalyst layer may cover substantially the entire first face of the support substrate, or it may be preferred that the first catalyst layer is provided in at least one region of the first face of the support substrate with the first catalyst layer being absent on other regions of the first face of the support substrate. In such cases it will be understood that the dimensions of the one or more regions coated with a first catalyst layer will depend on the configuration of the electrochemical device into which the formed catalyst-coated ion-conducting membrane is designed to be incorporated. Typically, the region(s) coated with the first catalyst layer corresponds to the active catalyst area of the formed catalyst-coated polymer electrolyte membrane. Suitably, such regions are in the shape of a quadrilateral, such as a rectangle or a square, or may be, for example, an oval or circle. It may be preferred that the regions coated with a first catalyst layer do not extend to the edges of the support substrate.

[0040] It will be understood by the skilled person that the present method may be used to prepare a single catalyst-coated polymer electrolyte membrane, in which case a single region (or patch) of catalyst layer may be provided on the support substrate, or may be used to manufacture P102010W001 multiple catalyst-coated polymer electrolyte membranes, for example using a roll-to-roll process. In such cases, multiple regions (or patches) of the first catalyst layer may be provided on the support substrate.

[0041] Preferably, the first catalyst layer is provided in the form of discrete patches on the support substrate. By discrete patches it is meant that the first catalyst layer is present on the first face of the support substrate in two or more regions which are not connected to each other. The provision of the first catalyst layer as discrete patches offers advantages associated with a reduction in the amount of catalyst material used in the manufacturing process and avoids the provision of catalyst layers in areas of the catalyst coated membrane that are not required to be electrochemically active.

[0042] The first catalyst layer may be prepared using, for example, coating methods such as a slotdie coating process, inkjet printing, gravure printing, curtain coating, spray coating, or a laser transfer process, such as laser induced forward transfer (LIFT). The catalyst ink may be applied to the support substrate continuously or discontinuously (intermittently). Preferably, the catalyst ink is applied to the substrate intermittently. An intermittent application process provides a pattern of discrete catalyst layer patches on the support substrate. Preferably, the catalyst composition is applied to the support substrate in a roll-to-roll coating process comprising a slot-die or a gravure roller, or comprising a laser transfer apparatus, configured for intermittent delivery of the catalyst composition, such as a roll-to-roll coating process comprising a slot-die configured for intermittent delivery of the catalyst composition. An example of an intermittent slot-die process is described in US2024 / 0290936 A1 , which is incorporated herein by reference.

[0043] Suitably, step (a) further comprises the sub-steps of: i) applying a catalyst composition comprising the noble metal-containing catalyst and the catalyst layer sulfonic acid ionomer to the support substrate to form a wet layer; and (ii) drying the wet layer to form the first catalyst layer. Typically, the catalyst composition comprises the noble metal-containing catalyst and the catalyst layer sulfonic acid ionomer dispersed in a solvent, such as water, a polar solvent (other than water), or a mixture of water and a polar solvent (other than water). The polar solvent can be a polar protic solvent. Preferably, the polar solvent is an alcohol, more preferably a C1.4 alcohol, such as be methanol, ethanol, propan-1-ol or propan-2-ol. Drying of the first catalyst dispersion to form the first catalyst layer removes solvent from the layer and it will be understood by the skilled person that the drying temperature may be adjusted depending on the boiling point of the solvent used in the first catalyst dispersion.

[0044] The method comprises step (b) heat treating the first catalyst layer at a temperature T. Temperature T is in the range of and including TaiPha+ 40 °C to TaiPha + 120 °C. It has been P102010W001 found that a heat treatment temperature less than Talpha + 40°C can lead to membrane ionomer penetration at certain catalyst layer thicknesses and I or catalyst composition solids contents. A heat treatment temperature greater than TaiPha + 120 °C can lead to damage to the ionomer structure. Preferably, temperature T is in the range of and including TaiPha+ 40 °C to TaiPha + 80 °C. Such a range offers a balance between minimising ionomer penetration and minimising energy consumption during manufacture. In some embodiments the catalyst layer sulfonic acid ionomer has a transition temperature TaiPha in the range of and including 90 °C to 110 °C and the temperature T is in the range of and including 130 °C to 190 °C.

[0045] It will be understood in cases in which step (a) further comprises the sub-steps of: i) applying a catalyst composition comprising the noble metal-containing catalyst and the catalyst layer sulfonic acid ionomer to the support substrate to form a wet layer; and (ii) drying the wet layer to form the first catalyst layer; the drying step (a) ii) may form part of a heating profile also incorporating the heat treatment step. For example, in a roll-to-roll manufacturing process the catalyst layer may move through at least two heating zones in a single pass (for example at least two oven zones), one of which is configured to heat at a temperature for drying and a subsequent one configured to heat treat the catalyst layer at temperature T.

[0046] Suitably, the heat treatment of the first catalyst layer comprises heating at a temperature T in the range of TaiPha+ 40 °C to TaiPha+ 120 °C for a period between 10 seconds to 20 mins, preferably 1 minute to 10 minutes.

[0047] Typically, the first catalyst layer after heat treatment has a thickness in the range of and including 2 to 20 .m, such as in the range of and including 2 to 10 .m.

[0048] Preferably, the first catalyst layer on the support substrate has a noble metal (such as platinum) loading of less than or equal to 1 .0 mg / cm2such as less than or equal to 0.50 mg I cm2. Suitably, the first catalyst layer has a loading of noble metal of at least 0.01 mg / cm2. In some embodiments, the first catalyst layer on the support substrate has a noble metal (such as platinum) loading in the range of and including 0.03 to 0.50 mg / cm2, such as in the range of and including 0.03 to 0.20 mg / cm2. The loading is the amount of noble metal (s) per x-y geometric area of the first catalyst layer.

[0049] The method comprises step (c) coating the first catalyst layer with one or more layers of a membrane ionomer to form a polymer electrolyte membrane. It will be understood by the skilled person that the polymer electrolyte membrane is formed on the opposite face of the first catalyst layer to the face of the first catalyst layer in contact with the support substrate. It will also be understood that the polymer electrolyte membrane is formed in direct contact with the first catalyst layer. P102010W001

[0050] Suitably, the coating in step (c) is carried out by dispersion casting. The dispersion of membrane ionomer comprises the membrane ionomer dispersed in a solvent, or a mixture of solvents. Advantageously, the solvent (or mixture of solvents) is selected such that rapid solvent evaporation may be achieved. It may be preferred that the solvent (or each solvent in a mixture of solvents) has a boiling point at 1 bar of pressure in the range of and including 60 to 110 °C. It may be preferred that the solvent is water, a polar solvent (other than water), or a mixture of water and a polar solvent (other than water). The polar solvent can be a polar protic solvent. Preferably, the polar solvent is an alcohol, more preferably a C1.4 alcohol, such as methanol, ethanol, or propan-1-ol. It may be preferred that the solvent is a mixture of water and a C1.4 alcohol (such as methanol, ethanol, or propan-1-ol).

[0051] Typically, the dispersion of membrane ionomer used for coating comprises ionomer in an amount in the range of and including 5 to 25 wt% based on the total weight of components in the dispersion. It will be understood that the amount of ionomer in the dispersion can readily be varied to adjust the viscosity of the dispersion to facilitate coating.

[0052] Suitably, the membrane ionomer is a proton-conducting polymer, such as an ionomer which comprises sulfonic acid groups. The membrane ionomer preferably comprises a perfluorinated sulfonic acid (PFSA) ionomer, a partially-fluorinated sulfonic acid ionomer, a non-fluorinated sulfonic acid ionomer (such as a non-fluorinated hydrocarbon sulfonic acid ionomer), or mixtures thereof. The membrane ionomer may comprise a blend of protonconducting polymers. Examples of suitable proton-conducting polymers include perfluorosulphonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aquivion® (Syensqo), Flemion® (Asahi Glass Co.), 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, JSR Corporation, Toyobo Corporation, and others.

[0053] In the case that the catalyst-coated polymer electrolyte membrane comprises an AEM, the ionomer is an anion-conducting polymer, such as a hydroxy-conducting polymer, for example ionomers comprising quaternary ammonium functional groups. Suitable ionomers include Fumion FAA-3 (Fumatech), Aemion ionomers (lonomr), and PiperlON ionomers (Versogen).

[0054] The first (and any subsequent) polymer electrolyte membrane layers are preferably deposited by slot-die coating, knife-coating, bar coating, inkjet printing, gravure printing, a curtain coating process, or a laser transfer process, such as laser induced forward transfer (LIFT). Preferably, the first (and any subsequent) polymer electrolyte membrane layers are deposited by slot-die coating.

[0055] It is preferred that each polymer electrolyte membrane layer is dried (or partially dried) before the next layer is deposited. Evaporation of solvent may be achieved by heating the polymer P102010W001 electrolyte membrane layer, for example by passing the layer through an oven. Suitably, the polymer electrolyte membrane layer may be dried at a temperature, for example, in the range of and including 50 °C to 100 °C, such as in the range of and including 60 °C to 80 °C.

[0056] Once dried, the first-deposited polymer electrolyte membrane layer and the optional one or more additional layers together form the polymer electrolyte membrane. The polymer electrolyte membrane layer is suitably ion-conducting and electrically insulating. Typically, the process comprises the step of: (d) heat-treating the polymer electrolyte membrane formed in step (c). Heat treatment of the polymer electrolyte membrane improves the mechanical strength and dissolution resistance of the membrane and can increase the adhesion of the first and second (if present) catalyst layers to the membrane. Typically, the heat treatment is carried out a temperature in the range of and including 110 to 250 °C, such as 140 to 220 °C, or in the range of and including 150 to 180 °C.

[0057] Preferably, the polymer electrolyte membrane has a thickness at least 5 pm. It may be preferred that the polymer electrolyte membrane has a thickness of at least about 8 pm, at least about 10 pm, at least about 25 pm, at least about 30 pm or at least about 35 pm. Typically, the thickness of the polymer electrolyte membrane formed in step (b) is less than or equal to about 200 pm, such as less than or equal to 150 pm, or preferably less than or equal to 100 pm. Preferably, the polymer electrolyte membrane formed in step (b) has a thickness in the range of including 5 to 100 pm. The thickness of the membrane may be determined by analysis of scanning electron microscope (SEM) images of a cross sections of the membrane (suitably dried at 0% relative humidity) and measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values.

[0058] In some embodiments, the thickness of the polymer electrolyte membrane formed in step (b) may be less than or equal to 30 pm, less than or equal to 25 pm, less than or equal to 20, pm, or less than or equal to 15 pm. It may be preferred that the the polymer electrolyte membrane formed in step (b) has a thickness in the range of including 5 to 25 pm, 5 to 20 pm, 5 to 15 pm, 8 to 15 pm.

[0059] The polymer electrolyte membrane formed in step (c) may comprise one or more polymeric reinforcement components. Such components provide strength and restrict the swelling of the CCM. Typically, such reinforcement components are provided as part of the process of forming a membrane layer, for example by embedding the reinforcement component into one or more membrane layers, for example after a dispersion of membrane ionomer is cast and prior to drying the layer. P102010W001

[0060] Suitably, the reinforcement components are a porous polymer material, which has the membrane ionomer impregnated within the pores in the formed membrane. The reinforcement may be non-woven or woven. The porous polymer material may be a fluoropolymer. The porous polymer material may be selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethersulfone (PES), poly(vinylidene fluoride-co- hexafluoropropylene) (PVDF-HFP), polyimides (PI), polyetherimide (PEI), poly(aryl ether ketone) (PAEK), poly(aryl ether sulfone), poly(phenylene sulfide) (PPS), polyvinylpyrrolidone (PVP) and polyether ether ketone (PEEK). The porous polymer material may be expanded polytetrafluoroethylene (ePTFE). The porous polymer material may also comprise a polymer backbone based on a nitrogen-containing heterocycle, such as polybenzimidazole.

[0061] The polymer electrolyte membrane formed in step (c) may comprise one or more additives, for example a radical scavenger (such as ceria), and / or a recombination catalyst. Such additives may be introduced by inclusion in one of the dispersions used for the formation of a polymer electrolyte membrane layer. Suitable radical scavengers are known to those in the art and include metal oxides, such as cerium oxides, manganese oxides, titanium oxides, beryllium oxides, bismuth oxides, tantalum oxides, niobium oxides, hafnium oxides, vanadium oxides and lanthanum oxides, suitably cerium oxides, manganese oxides or titanium oxides, preferably cerium dioxide (ceria). A recombination catalyst catalyses the reaction of H2 and O2 to form H2O. Suitable recombination catalysts can comprise a metal (such as platinum) which may be supported, for example on carbon, or may be unsupported.

[0062] Suitably, the polymer electrolyte membrane formed in step (c) is a single coherent polymer film. The term ‘coherent’ as used herein means that the membrane is free from internal interfaces between ion-conducting polymer layers, for example formed through lamination or through annealing of membrane layers prior to subsequent membrane layer deposition. Lamination of ion-conducting membranes comprises pressing and / or bonding at least two solid ion-conducting layers together, such membranes optionally being coated with a catalyst layer. Such interfaces may be suitably detected by cross-section SEM. Preferably, polymer electrolyte membrane formed in step (c) is a single coherent polymer film with no internal interfaces between polymer electrolyte layers detectable by cross-section SEM. Due to physical defects and / or chemical variations at interfaces between polymer electrolyte layers, such interfaces can increase the resistance of a multi-layer polymer electrolyte membrane. As such, it is advantageous to fabricate a multi-layer polymer electrolyte membrane by depositing layers of ion-conducting polymer dispersed in a liquid solvent to build up a multi-layer membrane structure rather than via lamination of individual solid layers / membranes of ionomer. P102010W001

[0063] The formed polymer electrolyte membrane has a first face adjacent to the first catalyst layer and a second, opposite face. The method comprises the optional step of: (e) applying a second catalyst layer to the second face of the polymer electrolyte membrane. It will be understood by the skilled person that the second catalyst layer is applied to the opposite face of the polymer electrolyte membrane from the first catalyst layer such that, once the second catalyst layer is applied, the polymer electrolyte membrane is positioned between, and typically in direct contact with, the first and the second catalyst layers.

[0064] Suitably, the second catalyst layer comprises a second electrocatalyst and a second catalyst layer sulfonic acid ionomer. Such electrocatalysts and ionomers are suitably as described for the first catalyst layer.

[0065] In cases in which the catalyst-coated polymer electrolyte membrane is for a water electrolyser, the second catalyst layer preferably comprises an oxygen evolution reaction (OER) catalyst. The type of OER catalyst is not particularly limited in the current invention so long as the catalyst materials are sufficiently stable under PEM or AEM electrolyser operating conditions. Such materials are known to the skilled person. Typically, the OER catalyst comprises a transition metal, for example a noble metal (Rh, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au) or one or more of Ni, Fe, Cu and Co. Preferably, for PEM applications, the OER catalyst comprises iridium and I or ruthenium. Such materials have a particularly good combination of catalyst activity and stability under PEMWE operating conditions. Suitably, the OER catalyst is a doped or undoped oxide, for example a doped or undoped oxide of iridium and I or ruthenium, or an iridium metal oxide, for example a metal oxide material comprising iridium and metal M, wherein M = Ta, Nb, Ti, Rh, Ru, or Pt. Preferably, for AEM applications, the OER catalyst comprises non-noble transition metals, such as Ni, Co, Cu and Fe, for example alloys and oxides of one or more non-noble metal transition metals, such as alloys and oxides of Ni, Co, Cu and Fe. It may be preferred that the OER catalyst (preferably an iridium-containing catalyst) is on a catalyst support, for example an inorganic metal oxide support, for example a transitional metal oxide or oxide of a main group metal, such as TiO2, AI2O3, ZrO2 or mixtures thereof. If the OER catalyst is a supported catalyst, the loading of catalyst metal (such as iridium) on the support material is suitably in the range of and including 10 to 90 wt%, preferably in the range of and including 15 to 75 wt% of the total weight of the supported catalyst.

[0066] In cases in which the catalyst-coated polymer electrolyte membrane is for a fuel cell, the second catalyst layer comprises an oxygen reduction reaction (ORR) electrocatalyst or a hydrogen oxidation reaction (HOR) electrocatalyst, such as a catalyst comprising platinum on a carbon support. P102010W001

[0067] Preferably, the second catalyst layer is applied to regions of the polymer electrolyte membrane which correspond to the regions of the polymer electrolyte membrane with the first catalyst layer applied to the opposite face. Preferably, the regions of the second catalyst layer are centrally positioned over the regions of the first catalyst layer (but on the opposite face of the polymer electrolyte membrane).

[0068] The second catalyst layer may be applied to the membrane using, for example, coating methods such as a slot-die coating process, inkjet printing, gravure printing, curtain coating, a spray coating process, or a laser transfer process, such as laser induced forward transfer (LIFT). The catalyst layer may be applied by directly coating the membrane, or the catalyst layer may be formed on a suitable backing material and then applied to the membrane using a decal process. Preferably, the catalyst layer is applied directly to the membrane enabling efficient manufacturing and a reduction in the amount of backing material required during the CCM manufacturing process.

[0069] Optionally, the method comprises the step of applying a seal material layer to the first face and / or the second face of the catalyst-coated polymer electrolyte membrane. Typically, the seal material layer is applied after the removal of the catalyst-coated polymer electrolyte membrane from the support substrate, but the seal material layer may also be applied to one face of the catalyst-coated polymer electrolyte membrane prior to removal from the support substrate, for example after heat-treatment of the membrane and I or after application of the second catalyst layer.

[0070] Optionally, the method comprises the step of applying a transport layer to a first face and / or a second face of the catalyst-coated polymer electrolyte membrane. Suitable transport layers are known by the skilled person.

[0071] For water electrolyser applications, at the anode side of the catalyst-coated polymer electrolyte membrane, suitable transport layers are typically formed from a metal-based porous structure. Such transport layers must be sufficiently conducting and in a form that is compatible with positioning in close proximity to the catalyst-coated membrane (without, for example, sharp edges or protrusions that would damage the membrane during use). Such metal-based porous structures may be in the form of, for example, felts or non-woven cloths, mesh, foams and sintered metal-containing particles. For PEMWE applications, suitable metal-based porous structures comprise titanium. For AEMWE applications, suitable metal-based porous structures comprise nickel or stainless steel.

[0072] For water electrolyser and fuel cell applications, suitable transport layers at the cathode (water electrolyser) side of the catalyst-coated polymer electrolyte membrane, or the anode and P102010W001 cathode sides of a fuel cell catalyst-coated polymer electrolyte membrane, are known to the skilled person and are typically non-woven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the H2315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SGL Technologies GmbH, Germany or AvCarb® series, or woven carbon cloths). The carbon paper, web or cloth may be provided with a further treatment either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of electrochemical device and the operating conditions that will be used.

[0073] It will be understood by the skilled person that the support substrate is removed from the formed catalyst-coated polymer electrolyte membrane prior to application of a transport layer on the first catalyst layer side of the catalyst-coated polymer electrolyte membrane.

[0074] Figures 1 and 2 depict exemplary methods of the present invention. The dimensions (e.g. thickness) of each layer are not drawn to scale for the sake of clarity.

[0075] It will be clear to the skilled person that although the process described below is with reference to the manufacture to a continuous roll of multiple catalyst-coated polymer electrolyte membrane components, the basic process could be applied to the manufacture of an individual catalyst-coated polymer electrolyte membrane component.

[0076] Figure 1 shows a plan view of a section of a support substrate (1) provided with patches of the first catalyst layer (2). The first catalyst layer (2) comprises a noble metal-containing electrocatalyst and a catalyst layer sulfonic acid ionomer.

[0077] Figure 2 shows a schematic of a process of forming a catalyst-coated polymer electrolyte membrane viewed as a cross section along line a-a indicated on Figure 1. In step (i) the support substrate (1) is provided with patches of the first catalyst layer (2). In step (ii), the first catalyst layer (2) is coated with a dispersion of a membrane ionomer to form a first polymer electrolyte membrane layer (3). In step (iii) a polymer electrolyte membrane (4) is formed by depositing one or more additional polymer electrolyte membrane layers on the first polymer electrolyte membrane layer (3) (noting that the line indicating the interface between layer (3) and (4) is for representative purposes only and an interface is typically not visible between the membrane layers). In step (iv) patches of a second catalyst layer (5) are applied to the opposite face of the polymer electrolyte membrane (4) to the first catalyst layer (2). The product may be further processed, for example though removal of the support substrate (1), cutting the coated membrane to form individual CCMs, the application of seals, etc. P102010W001

[0078] It will be understood that the first catalyst layer (2) may be provided as patches as shown in Figures 1 and 2 or may, for example, be provided in other configurations, such as provided over substantially all of the surface of the support substrate (1), or as a central continuous strip in the longitudinal direction of the support substrate (1). The first polymer electrolyte membrane layer (3) may be provided as a continuous layer over substantially all of the surface of the first catalyst layer (2) and any regions of the surface of the support substrate (1) where the first catalyst layer (2) is absent or may, for example, only be provided in regions where the first catalyst layer (2) is present. One or more polymer reinforcements may be incorporated into the membrane (not shown in Figure 2), for example by introducing an ePTFE web during the deposition of an additional polymer electrolyte membrane layer. As an alternative to the process shown in Figure 2, no second catalyst layer (5) may be provided (for example for applications where the catalyst-coated membrane is used together with a catalyst coated substrate).

[0079] Examples

[0080] The following measurement techniques were used:

[0081] Measurement of Talpha

[0082] A TA instruments Q800 DMA equipped with film tension clamps was used to measure the Ta. Relative humidity was set to 0% by using dry gasses and an in-line moisture trap. A sample of membrane formed from the ionomer (6mm wide) was installed vertically between two clamps c.a. 16mm apart. The instrument was set to the following settings:

[0083] Oscillation frequency: 1 Hz

[0084] Mode: ‘Multi-Frequency - Strain

[0085] Amplitude: 10pm

[0086] Static F: 0.001 N

[0087] Force Track: 125%

[0088] Minimum Osc F: 0.0001 N

[0089] Stabilisation cycles: 4

[0090] Average cycles: 3

[0091] The sample was equilibrated at 30°C for an hour. The temperature was then ramped to at least 120°C at a rate of 1.00°C / min. P102010W001

[0092] Using either TA instruments ‘Universal Analysis’ or ‘Trios’ software the data was analysed by plotting Tan(b) vs T. The plot was then smoothed using a ‘region width’ of 0.5°C and the Ta was taken to be the temperature at which the smoothed Tan(b) is at its maximum.

[0093] Formation of Pt / C catalyst layer

[0094] A series of catalyst layers (see Table 1) were prepared by depositing inks comprising 20 wt% Pt / C catalyst, and PSFA ionomer (Syensqo Aquivion D79-25BS, Talpha 100 °C) in propanol: water (-20:80) onto an eTFE-PET substrate. The layers were dried (80 °C) and then heat treated at a peak temperature T (80, 120 or 160 °C) for 10 minutes.

[0095] Deposition of a membrane ionomer to form a polymer electrolyte membrane layer.

[0096] A polymer electrolyte membrane layer was formed on the Pt / C catalyst layers by coating a dispersion of PSFA ionomer (AGC IQ171) in ethanokwater at an ionomer solids amount in the dispersion between (10 and 20 wt%, see Table 1) and then drying at a temperature of 80 °C. P102010W001

[0097] Ionomer penetration

[0098] The penetration of the membrane ionomer into the catalyst layer was assessed for each of the materials prepared by imaging the reverse side of the catalyst layer (after removal of the backing sheet) using optical microscopy.

[0099] Figure 3 shows the results of membrane ionomer penetration analysis. The average ionomer penetration at a peak temperature of 80 °C (Talpha - 20°C) is around 21%, at a peak temperature of 120 °C (Talpha + 20°C) is around 4%, and no penetration was observed with a peak temperature of 160 °C (Talpha + 60°C). This study shows that increasing the heat (annealing) treatment temperature to at least 40 °C above the catalyst layer ionomer Talpha leads to a significant reduction in the membrane ionomer penetration across a range of Pt loadings, catalyst layer thicknesses and membrane ionomer dispersion concentrations.

Claims

P102010W001Claims1. A method of manufacturing a catalyst-coated polymer electrolyte membrane for an electrochemical device, the method comprising the steps of:(a) applying a catalyst composition to a support substrate to form a first catalyst layer, the catalyst composition comprising a noble metal-containing electrocatalyst and a catalyst layer sulfonic acid ionomer with a transition temperature TaiPha ;(b) heat treating the first catalyst layer at a temperature T, wherein T is in the range of and including T alpha"*" 40 °C to Taipha + 120 °C;(c) coating the first catalyst layer with one or more layers of a membrane ionomer to form a polymer electrolyte membrane.

2. A method according to claim 1 , wherein the noble metal-containing electrocatalyst is provided on a metal oxide or carbon support.

3. A method according to claim 1 or claim 2, wherein the noble metal is platinum.

4. A method according to any one of the preceding claims, wherein the catalyst layer sulfonic acid ionomer is a perfluorinated sulfonic acid (PFSA) ionomer, a partially-fluorinated sulfonic acid ionomer, or a non-fluorinated sulfonic acid ionomer.

5. A method according to any one of the preceding claims, wherein the catalyst layer sulfonic acid ionomer is a short side chain (SSC) ionomer.

6. A method according to any one of the preceding claims, wherein temperature T is in the range of and including Taipha+ 40 °C to Taipha + 80 °C.

7. A method according to any one of the preceding claims, wherein catalyst composition is an aqueous solvent composition.

8. A method according to any one of the preceding claims, wherein the first catalyst layer is subjected to a drying step prior to the heat treatment in step (b).

9. A method according to any one of the preceding claims, wherein the first catalyst layer is heated at temperature T for a period in the range of and including 10 seconds to 20 minutes.P102010W00110. A method according to any one of the preceding claims, wherein the catalyst layer sulfonic acid ionomer has a transition temperature TaiPha in the range of and including 90 °C to 110 °C and the temperature T is in the range of and including 130 °C to 190 °C11. A method according to any one of the preceding claims, wherein the method comprises the additional step of heat-treating the polymer electrolyte membrane, for example at a temperature in the range of and including 110 to 250 °C, such as 140 to 220 °C, or preferably 150 to 180 °C.

12. A method according to any one of the preceding claims, wherein the polymer electrolyte membrane has a first face adjacent to the first catalyst layer and a second opposite face, and the method comprises the additional step of: (d) applying a second catalyst layer to the second face of the polymer electrolyte membrane.

13. A method according to any one of the preceding claims, wherein the first catalyst layer is provided in the form of discrete patches on the support substrate.

14. A method according to any one of the preceding claims, wherein coating the first catalyst layer with one or more layers of a membrane ionomer is carried out by dispersion casting.

15. A method according to any one of the preceding claims, wherein the method comprises the additional step of removing the support substrate.

16. A method according to any one of the preceding claims, wherein the method comprises the additional step of applying a seal material layer to a first face and / or a second face of the catalyst-coated polymer electrolyte membrane.

17. A method according to any one of the preceding claims, wherein the method comprises the additional step of applying a transport layer to a first face and / or a second face of the catalyst-coated polymer electrolyte membrane.

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