Method of preparing a catalyst layer

Abstract

According to the invention there is a method of preparing a catalyst layer for a fuel cell or an electrolyser. The method comprises the steps of: (i) providing a donor substrate having opposing first and second surfaces and providing a catalyst ink disposed as a layer on the second surface, wherein the catalyst ink comprises an electrocatalyst, an ion-conducting polymer, and a solvent; (ii) providing an acceptor substrate, wherein the second surface of the donor substrate faces towards the acceptor substrate; and (iii) irradiating the catalyst ink with laser radiation at a wavelength which is absorbed by the catalyst ink so as to transfer the catalyst ink from the donor substrate to the acceptor substrate.

Description

[0001] Method of preparing a catalyst layer

[0002] Field of the Invention

[0003] The invention relates to a method of preparing a catalyst layer for an electrochemical device, such as a fuel cell or water electrolyser. The invention also relates to a method of manufacturing a catalyst-coated ion-conducting membrane for an electrochemical device, and associated catalyst layers and catalyst-coated ion-conducting membranes manufactured using such methods.

[0004] Background of the Invention

[0005] The electrolysis of water to produce high purity hydrogen and oxygen can be carried out in both alkaline and acidic electrolyte systems. Those electrolysers that employ a solid protonconducting polymer membrane, or proton exchange membrane (PEM), are known as proton exchange membrane water electrolysers (PEMWEs). Those electrolysers that utilise a solid anion-conducting polymer membrane, or anion exchange membrane (AEM), are known as anion exchange membrane water electrolysers (AEMWEs).

[0006] Ion-conducting membranes, such as PEMs and AEMs, are also used in fuel cells. In a proton exchange membrane fuel cell (PEMFC) the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.

[0007] Catalyst-coated (ion-conducting) membranes (CCMs) may be employed within electrochemical devices, such as electrolysers and fuel cells. Such CCMs comprise an ionconducting membrane, such as a PEM or 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.

[0008] For water electrolyser applications, hydrogen evolution reaction (HER) catalysts are used in such cathode catalyst layers, for example HER catalysts comprising platinum, such as platinum on a carbon support. Oxygen evolution reaction (OER) catalysts are utilised in electrolyser anode catalyst layers. For PEMWE applications, suitable OER catalysts comprise iridium or iridium oxide, or oxides containing both iridium and ruthenium. For AEMWE applications, non-platinum group metal OER catalysts may also be used, such as alloys and oxides of nickel, cobalt, iron, and copper.

[0009] For fuel cell applications, oxygen reduction reaction (ORR) catalysts are used in cathode catalyst layers and hydrogen oxidation reaction (HOR) catalysts are utilised in anode catalyst layers. For PEMFC 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.

[0010] Such cathode and anode catalyst layers that are used in fuel cells and electrolysers incorporating an ion-conducting membrane comprise an ion-conducting polymer in addition to the electrocatalyst. The ion-conducting polymer plays a key role in catalyst layer performance acting as a binder and transport medium. As such, the reproducible formation of high- performance catalyst layers incorporating such polymers is a key challenge during manufacture of fuel cell and electrolyser components.

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

[0012] The membrane electrode assembly can be constructed by a number of known methods. A common method involves depositing one or both of the catalyst layers on a decal transfer substrate and transferring the catalyst layers to either side of the ion-conducting membrane. Subsequently, a gas diffusion layer (or porous transport layer) is applied to the electrocatalyst layer. Alternatively, a catalyst layer can be applied to a gas diffusion layer (or porous transport layer) to form a gas diffusion electrode (or porous transport electrode), which is then combined with the ion-conducting membrane. As a further alternative, catalyst layers can be coated directly onto either side of an ion-conducting membrane. A membrane electrode assembly can be prepared by any combination of these methods e.g., one catalyst layer is applied to the ion-conducting membrane to form a catalyst-coated ion-conducting membrane, and the other catalyst layer is applied as a gas diffusion electrode.

[0013] The catalyst layers can be deposited using a catalyst ink which typically comprises an electrocatalyst, an ion-conducting polymer, solvents / dispersants and / or diluents, and any agents or additives desired to be included in the electrocatalyst layer. Known printing techniques, such as slot die, spray coating, screen printing, inkjet printing and gravure, require the catalyst inks to have strict physical properties (e.g. rheology) in order to manufacture acceptable catalyst layers. Such catalyst inks typically comprise volatile organic compounds, such as short chain alcohols, as a solvent or as part of a solvent mixture, which can lead to undesirable safety risks during manufacturing processes. It is desirable to develop deposition techniques that allow manufacture of catalyst layers using catalyst inks with a wider range of physical properties (e.g. rheological properties).

[0014] The demand for hydrogen-based solutions for the reduction of carbon emissions is expected to continue to grow rapidly in response to net-zero targets. In order to respond to this demand, rapid increases in the volume of production of key components, such as CCMs, are required. There remains a need to further enhance and develop methods for the production of catalyst-coated ion-conducting membranes, in particular methods which enable large scale manufacturing, whilst optimising the use and recycling of key raw materials and minimising waste. In particular, there is a desire to increase manufacturing throughput of key components, such as CCMs, whilst at least maintaining performance.

[0015] Summary of the Invention

[0016] The present invention seeks to address at least some of the problems, desires and needs outlined above. The present inventors have surprisingly found that catalysts inks comprising an ion-conducting polymer and an electrocatalyst may be advantageously transferred using laser irradiation to yield fuel cell and electrolyser catalyst layers with good layer quality. In particular, the present invention provides a method of preparing a catalyst layer with a faster throughput than known methods, whilst at least maintaining performance. The method of the invention is also suitable for using inks with a wide range of physical (e.g. rheological) properties. For example, inks with high solids content can be used in the method, which can reduce the number of passes required to deposit a catalyst layer, thereby further improving manufacturing speed. Further still, inks with low VOC content (e.g. organic solvent content of <10 wt.%) are suitable for use in the method, which can improve the safety of the manufacturing process.

[0017] In a first aspect of the invention, there is provided a method of preparing a catalyst layer for a fuel cell or an electrolyser, the method comprising the steps of: providing a donor substrate having opposing first and second surfaces and providing a catalyst ink disposed as a layer on the second surface, wherein the catalyst ink comprises an electrocatalyst, an ion-conducting polymer, and a solvent; providing an acceptor substrate, wherein the second surface of the donor substrate faces towards the acceptor substrate; irradiating the catalyst ink with laser radiation at a wavelength which is absorbed by the catalyst ink so as to transfer the catalyst ink from the donor substrate to the acceptor substrate.

[0018] Preferably, the fuel cell or the electrolyser is a proton-exchange membrane fuel cell or a proton-exchange membrane electrolyser. The method can further comprise the step of drying the catalyst ink on the acceptor substrate so as to remove substantially all the solvent.

[0019] In a further aspect there is provided a catalyst layer obtained by a method of the first aspect.

[0020] In a further aspect there is provided a catalyst-coated ion-conducting membrane comprising a catalyst layer obtained by a method of the first aspect.

[0021] Brief Description of the Drawings

[0022] Figure 1 is a plot showing voltage versus current density for a membrane electrode assembly comprising a catalyst layer manufactured using a method of the invention, and a comparative membrane electrode assembly manufactured using a known process;

[0023] Figure 2 is a temperature sweep at 2.5 A / cm2for the same membrane electrode assemblies for which data is illustrated in Figure 1;

[0024] Figure 3 is a plot showing voltage versus current density for a membrane assembly comprising a catalyst layer manufactured using a method of the invention; and

[0025] Figure 4 is a photograph of a catalyst layer manufactured using a method of the invention.

[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 invention provides a method of preparing a catalyst layer for an electrochemical device, such as a fuel cell or an electrolyser. The method can be used to prepare an anode catalyst layer and / or a cathode catalyst layer, as desired. The method includes using a laser- induced forward transfer (LIFT) process. Methods of the present invention can be performed using an apparatus as described in WO2019 / 145300, for example.

[0029] The method comprises the step of providing a donor substrate having opposing first and second surfaces. The donor substrate is preferably transparent to the laser radiation. That is, the donor substrate absorbs the laser radiation to a substantially negligible degree. The donor substrate is an ink carrier, for example a circulating ribbon. A layer of catalyst ink is provided as a layer on the donor substrate. The catalyst ink can be applied to the donor substrate using an inking unit. Preferably, the catalyst ink is applied as a layer so as to uniformly coat the donor substrate substantially across its second surface. The layer of catalyst ink on the second surface can have a thickness of 100 m or less, preferably 75 pm or less, preferably 50 pm or less, more preferably 30 pm or less and most preferably 25 pm or less. The layer of catalyst ink disposed on the second surface of the donor substrate can have a thickness of 10 pm or more, 15 pm or more, and 20 pm or more. The thickness of the layer of catalyst ink disposed on the second surface of the donor substrate can be in a range comprising any combination of the aforementioned ranges, e.g. in a range of about 10 pm to 100 pm, preferably about 15 pm to 75 pm, and preferably about 20 pm to 50 pm.

[0030] The method further comprises the step of providing an acceptor substrate, wherein the second surface of the donor substrate faces the towards the acceptor substrate. The second surface of the donor substrate and the acceptor substrate are typically separated by a gap. The gap can be a distance of about 2 mm or less, 500 pm or less, preferably 300 pm or less, and more preferably 200 pm or less. The acceptor substrate can be a porous substrate. For example, the acceptor substrate can be an ion-conducting membrane, such as a polymer electrolyte membrane, a porous reinforcement material, a gas diffusion layer, a porous transport layer or a porous carrier sheet. The ion-conducting membrane can be a partially fluorinated or perfluorinated sulphonic acid polymer membrane, e.g. Nation® (E.l. DuPont de Nemours and Co.), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Speciality Polymers), or Flemion® (Asahi Glass Co.). The ion-conducting membrane can be a hydrocarbon-based ionconducting membrane, such as those 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. In other embodiments, the acceptor substrate can be a non-porous substrate. For example, the acceptor substrate can be a non-porous carrier sheet, such as a non-porous decal transfer substrate. In some embodiments the acceptor substrate is a hydrocarbon-based ion-conducting membrane and the catalyst ink comprises a partially fluorinated or perfluorinated sulphonic acid polymer.

[0031] The method further comprises the step of irradiating the catalyst ink (on the donor substrate) with laser radiation so as to transfer the catalyst ink from the donor substrate to the acceptor substrate. Preferably, the catalyst ink is irradiated with pulses of laser radiation. The laser radiation can be directed through the first surface of the donor substrate so as to impinge on the catalyst ink, preferably at or near the interface between the catalyst ink and the second surface of the donor substrate. The catalyst ink absorbs the laser radiation. For example, the electrocatalyst can absorb the laser radiation. In embodiments where the electrocatalyst comprises metal-containing particles supported on an electrically conductive support, the electrically conductive support can absorb the laser radiation. The laser radiation heats the catalyst ink in a targeted localised region so as to vaporise a portion of the catalyst ink in the targeted localised region. Such a process can induce an ejection of a droplet of the catalyst ink from the donor substrate to the acceptor substrate. That is, a liquid droplet of catalyst ink is transferred from the second surface of the donor substrate to the acceptor substrate in a precise manner.

[0032] The method can further comprise the step of drying the catalyst ink on the acceptor substrate so as to remove substantially all of the solvent. Such a drying step can include heating the acceptor substrate to a temperature of at least 80 °C, preferably at least 90 °C and more preferably at least 100 °C.

[0033] The process (optionally including the drying step) can be repeated as required to prepare a catalyst layer having a desired thickness and / or desired electrocatalyst loading. The catalyst layer can have a substantially uniform thickness. The catalyst layer can have a (dried) thickness of at least 1 pm, at least 2 pm, or at least 3 pm. The catalyst layer can have a (dried) thickness of 50 pm or less, 30 pm or less, 20 pm or less, or 15 pm or less. The catalyst layer can have a (dried) thickness in a range comprising any combination of the aforementioned lower and upper limits. In some embodiments, the catalyst layer can have a substantially non- uniform thickness or can be deposited in a pre-determined pattern. In such cases, the catalyst layer can comprise at least one first region having a first thickness and at least one second region having a second thickness, wherein the first and second thicknesses are different. In some embodiments, the catalyst layer can comprise at least one region having a first thickness and at least one region with no thickness (i.e. regions where the catalyst ink was not transferred). The first thickness can be at least 1 pm, at least 2 pm, or at least 3 pm. The first thickness can be 50 pm or less, 30 pm or less, 20 pm or less, or 15 pm or less. The second thickness can be >0 pm, at least 1 pm, at least 2 pm, or at least 3 pm. The second thickness can be less than 50 pm or less, 30 pm or less, 20 pm or less, 15 pm or less, or 5 pm or less. The first and second thicknesses can each be in a range comprising any combination of the aforementioned lower and upper limits.

[0034] The process (optionally including the drying step) can be repeated as required to prepare a catalyst layer having a desired catalyst loading. The catalyst layer can have a substantially uniform catalyst loading. The catalyst layer can have a catalyst loading in the range of 0.03 mg / cm2to 4.0 mg / cm2. For anode catalyst layers for fuel cells, the electrocatalyst loading can be in the range of 0.03 mg / cm2to 0.2 mg / cm-2. For anode catalyst layers for electrolysers, the electrocatalyst loading can be in the range of 0.4 mg / cm2to 4.0 mg / cm-2. For cathode catalyst layers for fuel cells, the electrocatalyst loading can be in the range of 0.05 mg / cm2to 1.0 mg / cm-2. For cathode catalyst layers for electrolysers, the electrocatalyst loading can be in the range of 0.05 mg / cm2to 1.0 mg / cm-2. In some embodiments, the catalyst layer can have a substantially non-uniform catalyst layer loading (e.g. as a gradient in the catalyst layer) or be deposited in a pre-determined pattern (e.g. continuous track having a serpentine path). In such cases, the catalyst layer can comprise at least one first region having a first catalyst layer loading and at least one second region having a second catalyst layer loading, wherein the first and second catalyst layer loadings are different. In some embodiments, the catalyst layer can comprise at least one region having a first catalyst layer loading and at least one region with a catalyst layer loading of zero (i.e. regions where the catalyst ink was not transferred). The first catalyst layer loading can be in a range of 0.03 mg / cm2to 4.0 mg / cm2. The second thickness can be in a range of >0 mg / cm2to <4.0 mg / cm2.

[0035] Suitable catalyst inks for the process of the invention are those used in the manufacture of catalyst layers for electrochemical devices, such as fuel cells and electrolysers. Typically, such inks include an electrocatalyst for a desired anode or cathode reaction, an ion-conducting polymer, a solvent and optionally other additives as desired, such as but not limited to radical scavengers, recombination catalysts, cell reversal tolerance additives, electrically conductive additives and / or fibrous substances as is known in the art.

[0036] The electrocatalyst used will depend on the reaction it is intended to catalyse, and its selection is within the capability of the skilled person. The electrocatalyst may be a cathode or an anode electrocatalyst, preferably of a fuel cell or an electrolyser, more preferably a proton exchange membrane fuel cell or proton exchange membrane water electrolyser. The electrocatalyst can comprise catalytic particles (e.g. metal-containing particles, such as a first metal, an alloy of a first metal or an oxide thereof), which are preferably dispersed or otherwise supported on a particulate support material. Preferably, the particulate support material is electrically conductive. For example, the electrocatalyst can be a finely divided metal powder (metal black) or may be a supported catalyst wherein metal particles are dispersed on an electrically conducting particulate support. The support material can be carbon, metal oxide, nitride or carbide or other electrically conductive support. Preferably, the support material is a carbon-based support material. Suitable carbon-based support materials include those from the carbon black family, such as oil furnace blacks, extra-conductive blacks, acetylene blacks and graphitised versions thereof. Exemplary carbons include a commercially available carbon black (such as from Cabot Corp. (Vulcan XC72R) or Akzo Nobel (the Ketjen® black series)) or a graphitised version of these carbon blacks or other commercially available carbon blacks such as acetylene blacks (e.g. those available from Chevron Phillips (Shawinigan Black®) or Denka). The support material may also be one specifically designed for use in a fuel cell, such as those described in WO2013 / 045894.

[0037] The catalytic particles are suitably selected from: (i) the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium);

[0038] (ii) gold or silver;

[0039] (iii) a base metal; or

[0040] (iv) an alloy or mixture comprising one or more of these metals or their oxides.

[0041] 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) or gold. Preferred base metals are copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin.

[0042] Typically, the electrocatalyst comprises a platinum group metal or an alloy of a platinum group metal, preferably with a base metal, preferred base metals are as defined above. In particular, the electrocatalyst can comprise platinum or an alloy of platinum with a base metal, preferred based metals are as defined above, more preferably nickel or cobalt, most preferably nickel. The atomic ratio of platinum to alloying metal is typically in the range of and including 3:1 to 1 :3.

[0043] Where the electrocatalyst comprises catalytic particles supported on a support, the catalytic particles (i.e. metal-containing particles) can have a D50 particle size of no more than 50 nm, no more than 30 nm, no more than 20 nm, preferably no more than 10 nm, and more preferably no more than 5 nm. For example, the particles can have a D50 particle size of at least 1 nm. D50 particle size is measured by examination in a transmission electron microscope (TEM). The D50 particle size can be in a range comprising any combination of the aforementioned upper and lower limits.

[0044] The electrocatalyst (i.e. catalytic particles and, where present, the electrically conductive support) in the catalyst ink can have a D50 particle size (or aggregate size, as appropriate) of at least 0.1 pm, suitably at least 0.3 pm, preferably at least 0.5 pm, and typically at least 0.75 pm. The electrocatalyst in the catalyst ink can have a D50 particle size (or aggregate size, as appropriate) of no more than 20 pm, suitably not more than 10 pm, preferably no more than 5 pm, and typically no more than 3 pm. The electrocatalyst can have a D50 particle size (or aggregate size, as appropriate) in the range of and including 0.1 pm to 20 pm, suitably 0.3 pm to 10 pm, preferably 0.5 pm to 5 pm, and typically 0.75 pm to 3 pm. For example, a supported electrocatalyst can have a D50 particle size of about 1 pm. D50 particle size is measured by dynamic light scattering using a Malvern Mastersizer™.

[0045] The ion-conducting polymer can be a proton-conducting polymer or an anion-conducting polymer, such as a hydroxyl anion-conducting polymer. Preferably, the ion-conducting polymer is a proton-conducting polymer. Examples of suitable proton-conducting polymers include partially- or fully-fluorinated sulphonic acid polymers, such as perfluorosulphonic acid ionomers (e.g. Nation® (E.l. DuPont de Nemours and Co.), Aciplex® (Asahi Kasei), Aquivion™ (Solvay Speciality Polymers), 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. Suitably, the ionomer is a perfluorosulphonic acid, in particular the Nation® range available from Chemours company, especially Nation® 1100EW, and the Aquivion® range available from Solvay, especially Solvay® 830EW. Examples of suitable anion-conducting polymers include A901 made by Tokuyama Corporation and Fumasep FAA from FuMA-Tech GmbH.

[0046] The catalyst ink can comprise the electrocatalyst and the ion-conducting polymer in a combined amount of at least 5 wt.%, at least 10 wt.%, at least 15 wt.%, at least 16 wt.%, at least 17 wt.%, at least 20 wt.%, at least 25 wt.%, based on the total weight of the catalyst ink. The catalyst ink can comprise the electrocatalyst and the ion-conducting polymer in a combined amount of less than 50 wt.%, less than 40 wt.% or less than 30 wt.%, based on the total weight of the catalyst ink. The electrocatalyst and the ion-conducting polymer combined amount can be in a range comprising any combination of the aforementioned lower and upper limits, e.g. 5 wt.% to 50 wt.%.

[0047] The catalyst ink can comprise a total solids content (i.e. including the ion-conducting polymer, the electrocatalyst, and any other optional solid additives) of at least 5 wt.%, at least 10 wt.%, at least 15 wt.%, at least 16 wt.%, at least 17 wt.%, at least 20 wt.%, at least 25 wt.%, based on the total weight of the catalyst ink. The catalyst ink can have a total solids content of less than 50 wt.%, less than 40 wt.% or less than 30 wt.%, based on the total weight of the catalyst ink. The total solids content can be in a range comprising any combination of the aforementioned lower and upper limits, e.g. 5 wt.% to 50 wt.%. Methods of the present invention allow a wider range of inks to be used, including inks with a higher solids content, which would otherwise lead to defective layers using known techniques, such as slot die coating.

[0048] The solvent can have a boiling point of at least 95 °C, preferably at least 100 °C, and more preferably at least 110 °C. The solvent can comprise water, an organic solvent or a mixture of water and an organic solvent. The organic solvent is preferably miscible with water. The organic solvent is preferably a protic polar solvent, such as an alcoholic solvent. The organic solvent can be methanol, ethanol, 1-propanol, or iso-propanol, n-butanol, ethylene glycol, propylene glycol, dipropylene glycol, polyethylene glycol), polypropylene glycol), ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or any combination thereof. Preferably, the organic solvent is ethanol, n-propanol, iso-propanol, ethylene glycol, propylene glycol, ethylene carbonate, propylene carbonate, or any combination thereof. Preferably, the solvent comprises at least 50 wt.% water, preferably >60 wt.% water, more preferably >70 wt.% water, more preferably >80 wt.% water, >90 wt.% water, and more preferably 95 wt.% water, based on the total weight of the solvent. More preferably, the solvent consists essentially of or consists only of water. Using a solvent without volatile organic components can reduce the flammability of the ink resulting in a safer process with less chemical waste. When depositing catalyst inks onto highly hydrophobic substrates (e.g. PTFE carrier sheets, decal transfer substrates, and some fluorine-containing ionconducting membranes, such as membranes made from PFSA ionomers), organic solvents are typically added to inks to help prevent de-wetting. Methods of the present invention are unexpectedly suitable for printing catalyst inks with a solvent consisting essentially of (or consisting entirely of) water onto a variety of different acceptor substrates including hydrophobic substrates, whilst maintaining good catalyst layer properties and structure. For example, methods of the invention can allow catalyst inks, in particular aqueous-based catalyst inks, comprising a partially or per-fluorinated sulphonic acid ion-conducting polymer to be deposited onto a fluorinated or non-fluorinated acceptor substrate, such as a hydrocarbon-based ion-conducting membrane, and form an acceptable catalyst layer.

[0049] The catalyst inks of the present invention can have a far wider range of acceptable rheological properties compared to catalyst inks suitable for known coating techniques in the field, such as slot die, spray coating, screen printing, inkjet printing and gravure. For example, catalyst inks of the present invention can have a viscosity in a range of at least about 10 cP to about 1,000 cP or less, when measured at 25 °C and a shear rate of 100 s’1. Furthermore, methods of the present invention do not require passing the ink through a print head or nozzle. Consequently, methods of the present invention are not subject to the problem of blockages in the print head or nozzle, which can result in machine down-time.

[0050] Where the acceptor substrate is a decal transfer substrate, the catalyst layer produced by the method can be transferred from the acceptor substrate to an ion-conducting membrane via a decal process (e.g. using heat and pressure) to form a catalyst-coated ion-conducting membrane.

[0051] Where the acceptor substrate is an ion-conducting membrane, the product produced by the method is a catalyst-coated ion-conducting membrane. That is, a subsequent decal transfer step is not required. A gas diffusion layer and / or a porous transport layer can be applied to one or each face of such a catalyst coated membrane using methods known in the art to form a membrane electrode assembly. Where the acceptor substrate is a carrier sheet, the method can further comprise subsequent steps of forming an ion-conducting (electrolyte) membrane layer on the catalyst layer (in one or more passes), and then depositing a second catalyst layer on the ionconducting (electrolyte) membrane layer to form a catalyst-coated ion-conducting membrane. The second catalyst layer can also be deposited using methods of the present invention. The second catalyst layer is preferably different to the first catalyst layer. For example, the first and second catalyst layers typically comprise different electrocatalysts.

[0052] Where the acceptor substrate is a gas diffusion layer or porous transport layer, the product produced by the method is a gas diffusion electrode or porous transport electrode respectively. Such a gas diffusion electrode or porous transport electrode can be applied to a face of an ion-conducting membrane to form a membrane electrode assembly using methods known in the art. A combination of any of these methods can be used to make an anode catalyst layer and a cathode catalyst layer. For example, a method of the present invention can be used to prepare a cathode catalyst layer, whilst an anode catalyst layer is prepared using a conventional technique. In other embodiments, a method of the present invention can be used to prepare an anode catalyst layer, whilst a conventional technique is used to prepare the cathode catalyst layer. In a further embodiment, both anode and cathode catalyst layers are prepared using a method of the present invention.

[0053] An example of typical apparatus settings suitable for methods of the invention are detailed in Table 1 .

[0054] Table 1

[0055] Examples

[0056] Example 1

[0057] A cathede catalyst ink was prepared using the fcllcwing methcd. An aquecus dispersicn cf an icn-ccnducting pclymer (Naficn® 110OEW) and an electrocatalyst material (50 wt.% Pt / C prepared using a methcd analcgous to the general method of preparation of carbon support Pt catalysts described in WO 2013 / 045894) were mixed. This mixture was mechanically agitated using a stirrer until all of the catalyst had been wetted and dispersed in the liquid medium. The ink was then milled using an Eiger mill to form a well-dispersed ink. The solids content of the ink was ~25 wt.% based on the total weight of the ink. The ion-conducting polymer content in the ink was 90 wt.% with respect to the weight of carbon. The catalyst ink had a viscosity of 400 cP at a temperature of 25 °C and a shear rate of 100 s’1.

[0058] The cathode catalyst ink was used to form a cathode catalyst layer. The cathode catalyst ink was deposited onto a skived PTFE sheet (acceptor substrate) using a HelioSonic™ printing apparatus (commercially available from HelioSonic GmbH, Germany). The catalyst ink was applied as a 20 pm thick wet layer to an ink carrier (donor substrate) such that the layer of catalyst ink was facing towards the skived PTFE substrate. Pulses of laser radiation (70- 120 W) were directed through the ink carrier so as to impinge on the catalyst ink in a specific locality. The catalyst ink was heated in the locality so as to vaporise a portion of the catalyst ink and form a bulge. The catalyst ink is ejected as a liquid droplet so as to transfer from the ink carrier onto the skived PTFE substrate. The process was repeated at different predetermined localities to form a substantially uniform catalyst layer (area = 100 cm2) on the skived PTFE substrate. The deposited catalyst layer was heated to 100 °C for 10 minutes to remove any residual solvent. The catalyst layer had an average thickness of about 3.5 pm to 4.0 pm, when measured by FIB-SEM and taking an average of measurements from at least five different locations on the catalyst layer. The catalyst layer was deposited with a nominal platinum loading of 0.2 mgPt / cm2.

[0059] An anode catalyst ink was prepared using the following method. A PFSA ion-conducting polymer (3M, 800EW) and an electrocatalyst material (20 wt.% Pt / C - HiSPEC® 3000 available from Johnson Matthey Hydrogen Technologies Limited) in a mix of water and n- propanol. This mixture was mechanically agitated using a stirrer until all of the catalyst had been wetted and dispersed in the liquid medium. The ink was processed through an Eiger mill to form a well-dispersed ink. The anode catalyst ink was used to form an anode catalyst layer. The anode catalyst ink was coated onto a skived PTFE sheet using a slot die coating process with a nominal platinum loading of 0.1 mgPt / cm2The coating was dried to remove the solvent and form an anode catalyst layer.

[0060] Catalyst-coated ion-conducting membranes (with an active area of 50 cm2) were prepared by transferring the cathode and anode catalyst layers from their respective PTFE sheets to either side of a commercial reinforced PFSA ion-conducting membrane (15 pm thickness) respectively using a decal transfer process by lamination under pressure and at a temperature of between 150 °C and 200 °C.

[0061] A gas diffusion layer (Sigracet 22 BB, commercially available from SGL Carbon) was applied to each face of each catalyst coated ion-conducting membrane to form a complete membrane electrode assembly. The gas diffusion layer used was a carbon fibre paper with a hydrophobic microporous layer containing carbon and PTFE applied to the face in contact with the catalyst coated ion-conducting membrane.

[0062] Example 2

[0063] A membrane electrode assembly was prepared in the same way as Example 1, except the cathode catalyst ink was deposited directly onto a commercial reinforced PFSA ionconducting membrane (as the acceptor substrate) using the HelioSonic™ printing apparatus, rather than onto a skived PTFE substrate. As such, it was not necessary to perform the subsequent decal transfer step of Example 1 to form the cathode catalyst layer. The cathode catalyst layer had a nominal platinum loading of 0.3 mgPt / cm2. Figure 3 shows a polarisation curve of the membrane electrode assembly manufactured of Example 2.

[0064] Example 3

[0065] A membrane electrode assembly was prepared in the same was as Example 1, except the cathode catalyst ink was deposited directly onto a hydrocarbon-based sulfonic acid ionconducting membrane (as the acceptor substrate) using the HelioSonic™ printing apparatus. As such, it was not necessary to perform the subsequent decal transfer step of Example 1 to form the cathode catalyst layer. The cathode catalyst layer had a nominal platinum loading of 0.15 mgPt / cm2. The cathode catalyst layer exhibited good layer quality upon visual inspection.

[0066] Example 4

[0067] The cathode catalyst ink of Example 1 was deposited onto a skived PTFE sheet (acceptor substrate) using a HelioSonic™ printing apparatus (commercially available from HelioSonic GmbH, Germany) using the same conditions as Example 1. However, the catalyst ink was transferred in a pre-determined pattern in a non-uniform layer. In this example the pre- determined pattern comprised continuous tracks 401 following a tortuous path separated by regions without any catalyst 402. The tracks had an average width of about 1 mm and were spaced apart by an average spacing of about 0.5 mm. The catalyst layer comprised first regions having a nominal platinum loading of 0.2-0.3 mgPt / cm2and second regions without any platinum being deposited. Figure 4 shows a photograph of the patterned catalyst layer.

[0068] Example 5

[0069] An electrolyser anode catalyst layer was produced by firstly preparing an ink containing iridium oxide (IrOx, 45 wt%) and ion-conducting polymer (PFSA, 5 wt%) in water (11 wt%) and propan-1-ol (39 wt%), and then diluting to a solids content of 38 wt% with additional solvent. The ink was deposited onto a polyimide film supported on a glass substrate (the transfer substrate) using knife coating. The transfer substrate was inverted and irradiated with a pulsed laser (1050-1070 nm wavelength, 200 mW at 30-60 kHz) to transfer the ink to form a catalyst layer on a PFTE substrate. Microscopy indicated that a good quality layer had been formed.

[0070] Example 6

[0071] A catalyst-coated membrane was prepared in the same was as Example 1, except the cathode catalyst ink was deposited directly onto a hydrocarbon-based anion-exchange membrane (as the acceptor substrate) using the HelioSonic™ printing apparatus. As such, it was not necessary to perform the subsequent decal transfer step of Example 1 to form the cathode catalyst layer. The cathode catalyst layer had a nominal platinum loading of 0.1 to 0.2 mgPt / cm2. The visual inspection indicated successful transfer of the layer onto the AEM.

[0072] Comparative Example 1

[0073] The cathode catalyst ink of Example 1 was deposited onto a skived PTFE sheet using a k-bar coating process to form a substantially uniform wet catalyst layer (area = 100 cm2). The wet catalyst layer was dried and annealed (150-200 °C). The nominal platinum layer loading was 0.2 mgPt / cm2. The k-bar coating process of Comparative Example 1 was slower than the printing process of Example 1.

[0074] An anode catalyst layer was prepared on a skived PTFE sheet using the same method as described in Example 1.

[0075] A catalyst-coated ion-conducting membrane (with an active area of 50 cm2) was prepared using the same method as described in Example 1 , that is, by transferring the cathode and anode catalyst layers from their respective PTFE sheets to either side of a commercial reinforced PFSA ion-conducting membrane (15 pm thickness) respectively using a decal transfer process by lamination under pressure and at a temperature of between 150 °C and 200 °C.

[0076] Comparative Example 2

[0077] The cathode catalyst ink of Example 1 was deposited directly onto a hydrocarbon-based ion-conducting polymer membrane using a k-bar coating process to form a substantially uniform wet catalyst layer (area = 100 cm2). The wet catalyst layer was dried and annealed (150-200 °C). Upon drying, the cathode catalyst layer cracked severely and was poorly adhered to the hydrocarbon-based ion-conducting membrane substrate. A satisfactory membrane electrode assembly could not be prepared.

[0078] Cell testing

[0079] The polarisation (current vs voltage) performance of the 50 cm2membrane electrode assemblies was measured in H2 / air at 80°C under fully humidified and pressurised (100 % RH, 100 kPag or 170 kPag inlet) conditions using H2and air flows both at a stoichiometry of 2.0. To check for any differences in the kinetic behaviour of the cathode catalysts, polarisation curves were also recorded using pure oxygen as the oxidant on the cathode side under the same temperature and pressure and hydrogen stoichiometry, but the oxygen stoichiometry used was 10.0. In all measurements, the cell humidity (RH) and pressure was controlled at the anode and cathode inlets.

[0080] Temperature sweeps were measured in H21 air (170 kPag inlet) using H2and air flows both at a stoichiometry of 2. Temperature sweeps were performed by fixing the dew point of the hydrogen and air at the cell inlets at 53°C and controlling the cell temperature at different points between about 40°C to 90 °C. For Figure 2, points were recorded at a current density of 2.5 A / cm2.

[0081] Results and discussion

[0082] The method of depositing the cathode catalyst layer in Example 1 was significantly faster than the k-bar coating method used in Comparative Example 1. For example, the 100 cm2cathode catalyst layer of Example 1 was deposited in less than 1 second, whereas the 100 cm2cathode catalyst layer of Comparative Example 1 was deposited in about 5-10 seconds. Figure 1 shows polarisation curves for Example 1 and Comparative Example 1 using the cell testing method described above (at 170 kPag inlet). Figure 2 shows a temperature sweep measured using the method described above. There was not a statistically significant difference in performance between Example 1 and Comparative Example 1. Therefore, the method of the invention can be used to manufacture catalyst layers at a significantly faster speed whilst maintaining performance. Figure 3 shows a polarisation curve for Example 2. In Example 2, the cathode layer was deposited directly onto an ion-conducting membrane. This method further increases the speed and efficiency of manufacturing because it does not require a decal transfer step. As such, the method of Example 2 can help to reduce the number of processing steps, which can further help to increase throughput.

[0083] Example 3 and Example 6 indicate that catalyst layers can be successfully formed on a hydrocarbon-based ion-conducting membrane using the method as described herein, with visual inspection of the layer formed in Example 3 indicating a higher layer quality than that observed using a prior art method in Comparative Example 2. Example 4 indicated that laser irradiation may be readily used to form a catalyst layer in a pre-determined pattern which would be challenging to prepare using prior art coating methods.

[0084] Example 5 shows that an electrolyser anode layer incorporating an IrOx oxygen evolution reaction catalyst may be prepared on using the hereinbefore described process.

Claims

Claims1. A method of preparing a catalyst layer for a fuel cell or an electrolyser, the method comprising the steps of: providing a donor substrate having opposing first and second surfaces and providing a catalyst ink disposed as a layer on the second surface, wherein the catalyst ink comprises an electrocatalyst, an ion-conducting polymer, and a solvent; providing an acceptor substrate, wherein the second surface of the donor substrate faces towards the acceptor substrate; and irradiating the catalyst ink with laser radiation at a wavelength which is absorbed by the catalyst ink so as to transfer the catalyst ink from the donor substrate to the acceptor substrate.

2. A method according to claim 1 , wherein the method further comprises the step of drying the catalyst ink on the acceptor substrate so as to remove substantially all the solvent.

3. A method according to claim 1 or 2, wherein the solvent has a boiling point of at least 95 °C, preferably at least 100 °C, at least 110 °C.

4. A method according to claim 1 or 2, wherein the solvent comprises water, ethanol, n- propanol, iso-propanol, n-butanol, methanol, ethylene glycol, propylene glycol, dipropylene glycol, poly(ethylene glycol), polypropylene glycol), ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, or a combination thereof.

5. A method according to claim 4, wherein the solvent consists essentially of water.

6. A method according to any previous claim, wherein the catalyst ink comprises the electrocatalyst and the ion-conducting polymer in a combined amount of at least 5 wt.% based on the total weight of the catalyst ink.

7. A method according to any previous claim, wherein the ion-conducting polymer is a partially fluorinated sulphonic acid ionomer or a perfluorinated sulphonic acid ionomer.

8. A method according to any one of claims 1 to 6, wherein the ion-conducting polymer is a proton-conducting polymer or an anion-conducting polymer.

9. A method according to any previous claim, wherein the acceptor substrate comprises a porous substrate.

10. A method according to any of claims 1 to 7, wherein the acceptor substrate comprises a non-porous substrate.SUBSTITUTE SHEET (RULE 26)11. A method according to any of claims 1 to 8, wherein the acceptor substrate comprises an ion-conducting membrane.

12. A method according to claim 11, wherein the ion-conducting membrane is a hydro- carbon-based ion-conducting membrane.

13. A method according to any of claims 1 to 8, wherein the acceptor substrate comprises a gas diffusion layer or porous transport layer.

14. A method according to any of claims 1 to 10, wherein the acceptor substrate comprises a decal transfer substrate.

15. A method according to any previous claim, wherein the layer of catalyst ink disposed on the second surface of the donor substrate has a thickness in the range of 10 pm to100 pm.

16. A method according to any previous claim, wherein the catalyst layer has a substantially uniform thickness.

17. A method according to any of claims 1 to 15, wherein the catalyst layer comprises at least one region having a first thickness and at least one region having a second thickness, wherein the first and second thicknesses are different.

18. A method according to any previous claim, wherein the catalyst layer has a substantially uniform catalyst loading.

19. A method according to any of claims 1 to 17, wherein the catalyst layer has a substantially non-uniform catalyst loading.

20. A method according to any of claims 1 to 17, wherein the catalyst layer is deposited in a pre-determined pattern.

21. A method according to any previous claim, wherein the electrocatalyst comprises metal-containing particles, which are optionally supported on a particulate support material.

22. A method according to claim 21, wherein the metal-containing particles comprise a platinum group metal or an alloy of a platinum group metal.

23. A method according to any of claims 21 or 22, wherein the particulate support material comprises a carbon-based material, metal oxide, nitride, carbide, or other electrically conductive support material.

24. A catalyst layer obtained using a method according to any previous claim.

25. A catalyst-coated ion-conducting membrane comprising the catalyst layer according to claim 24.SUBSTITUTE SHEET (RULE 26)

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