Catalyst ink for anode layer for polymer electrolyte membrane electrolysers

Abstract

A process for the manufacture of an anode layer for a polymer electrolyte membrane electrolyser, the process comprising the steps of: (i) forming a catalyst ink comprising an iridium- and / or ruthenium-containing OER catalyst, an ionomer, a solvent, and a cellulose compound; (ii) applying the catalyst ink to a substrate to form the anode layer; and (iii) drying the anode layer. Also described are anode layers for a polymer electrolyte membrane electrolyser, the anode layer comprising an iridium - and / or ruthenium-containing OER catalyst, an ionomer, and a cellulose compound.

Description

[0001] P102289W001

[0002] ANODE LAYERS FOR POLYMER ELECTROLYTE MEMBRANE ELECTROLYSERS

[0003] Field of the Invention

[0004] This invention relates to anode catalyst inks, anode catalyst layers, and catalyst-coated membranes for a polymer electrolyte membrane electrolyser, in particular for a proton exchange membrane (PEM) water electrolyser, and to processes for their manufacture.

[0005] Background to the invention

[0006] Solid polymer electrolyte membranes, such as proton exchange membranes (PEMs) or anion exchange membranes (AEMs), may be employed for electrolysis in combination with anode and cathode catalyst layers which are positioned on opposite sides of the membrane. In some cases, the anode catalyst layer and I or the cathode catalyst layer are applied to a face of the membrane to form a catalyst-coated membrane (CCM). In other cases, the respective catalyst layers may be applied to other components, such as transport layers, and the catalyst layers compressed against the membrane during assembly and subsequent use of the electrolysis cell.

[0007] For water electrolysis, hydrogen evolution reaction (HER) catalysts are used in such electrolyser cathode catalyst layers, for example, HER catalysts comprising platinum, such as platinum on a carbon support. Oxygen evolution reaction (OER) catalysts are used in electrolyser anode catalyst layers, with noble metal-containing catalysts, such as iridium and I or ruthenium-containing catalysts, offering a particularly good balance between OER activity and stability under electrolysis conditions. The OER reaction in acidic conditions is approximated by the following equation:

[0008] 2H2O O2+ 4H++ 4e-

[0009] Separate film 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 or elastomeric gaskets. An adhesive layer may be present on one or both surfaces of the seal film layer.

[0010] 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 features of which depend on the final MEA P102289W001 application and stack configuration. Such transport layers allow the reactants to reach the electrocatalyst layers and products to leave.

[0011] Catalyst-coated membranes are suitably produced at scale using roll-to-roll coating processes. Such processes utilise coating techniques, such as slot-die coating or gravure coating, to form a catalyst layer directly on the surface of a polymer electrolyte membrane, or on a decal transfer substrate with subsequent transfer of the layer onto the polymer electrolyte membrane. A description of a roll-to-roll coating process is provided in US2024 / 0290936A1 (W.L. Gore & Associates GmbH). Such processes typically utilise a catalyst ink comprising a catalyst and an ionomer dispersed in a solvent, or mixture of mixture of solvents, such as a mixture of an alcohol and water. The role of the ionomer is to act as a conductor extending proton conduction from the bulk of the membrane to the surface of the catalyst and to act as a binder providing structure to the catalyst layers.

[0012] Other anode layers for electrolysers are known. For example, it is described in W02024058401A (GWANGJU INST SCIENCE & TECH) that electrolyser anode layers may be formed using a combination of cellulose binders, suitably cross linked though I PA gelation. Such anode layers do not comprise an ionomer.

[0013] Due to the relative cost and scarcity of iridium and ruthenium it is desirable to reduce the loading of these metals in electrolyser anode layers, for example to less than 1.2 mg / cm2of the anode layer. There are however significant challenges associated with the formation of such low-metal content electrolyser anode layers by suitable coating techniques for roll-to-roll coating, such as slot-die or gravure coating, whilst maintaining catalyst layer performance. In particular it has been found a reduction in coating line speeds is required in order to successfully form a catalyst layer without significant defects, and in particular that intermittent- or patch-coated parts are of poor quality.

[0014] There remains a need to further enhance and develop anode layers for electrolysis with low iridium and I or ruthenium content, which can be efficiently formed using roll-to-roll coating processes.

[0015] Summary of the invention

[0016] The present inventors have identified that cellulose additives have particular benefits when incorporated into electrolyser anode layers incorporating an ionomer and iridium- and or ruthenium-containing compounds at low metal loadings. The incorporation of cellulose additives enables the efficient formation of such anode layers using roll-to-roll coating techniques at increased coating line speed whilst retaining anode layer performance. Furthermore, such low loaded anode layers may be produced via intermittent coating, P102289W001 optimising the use of the anode ink, and are suitable for decal transfer from a substrate onto a polymer electrolyte membrane.

[0017] Therefore, in a first aspect of the invention there is provided a process for the manufacture of an anode layer for a polymer electrolyte membrane electrolyser, preferably a water electrolyser, such as a proton exchange membrane water electrolyser, the process comprising the steps of

[0018] (i) forming a catalyst ink comprising an iridium- and I or ruthenium-containing OER catalyst, an ionomer, a solvent, and a cellulose compound;

[0019] (ii) applying the catalyst ink to a substrate to form the anode layer;

[0020] (iii) drying the anode layer.

[0021] The addition of a cellulose additive provides catalyst inks with particular utility for the formation of anode layers at low iridium- and I or ruthenium- content and which are suitable for roll-to- roll coating techniques. It has been found by the present inventors that the cellulose additives offer variation in viscosity and advantageously shear thinning of low-iridium inks at relevant shear rates to facilitate roll-to-roll coating techniques. Therefore, in a second aspect of the invention there is provided a catalyst ink comprising an iridium- and I or ruthenium-containing OER catalyst, an ionomer, a solvent, and a cellulose compound.

[0022] In a third aspect of the invention there is provided a process for the manufacture of a catalyst ink according to the second aspect, the process comprising the step of adding the cellulose compound to a catalyst composition comprising the iridium- and I or ruthenium-containing OER catalyst, the ionomer, and the solvent.

[0023] The inclusion of a cellulose additive provides anode layers which may be formed by roll-to-roll coating techniques at low metal loadings and which testing in an electrolysis cell indicates have excellent electrochemical activity at low iridium and I or ruthenium loadings as set out in the Examples. Therefore, in a fourth aspect of the invention there is provided an anode layer for an electrolyser, preferably a water electrolyser, such as a proton exchange membrane water electrolyser, the anode layer comprising an iridium- and I or ruthenium-containing OER catalyst, an ionomer, and a cellulose compound. Such anode layers are obtainable by the process of the first aspect.

[0024] In a fifth aspect of the invention there is provided a catalyst-coated membrane (COM) for an electrolyser, preferably a water electrolyser, such as a proton exchange membrane water electrolyser, the COM comprising a polymer electrolyte membrane and an anode layer according to the fourth aspect. P102289W001

[0025] In a sixth aspect of the invention there is provided an electrolyser, preferably a water electrolyser, such as a proton exchange membrane water electrolyser, the electrolyser comprising an anode layer according to the fourth aspect or a CCM according to the fifth aspect.

[0026] Brief description of the Figures

[0027] Figure 1 shows a schematic representation of an example of a water electrolyser (20) incorporating a CCM.

[0028] Figure 2 shows the results of the electrochemical testing of anode layers formed in Example 2 and Comparative Example 2.

[0029] Detailed Description

[0030] 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.

[0031] The present invention provides an anode layer for a polymer electrolyte membrane electrolyser. Preferably, the electrolyser is a water electrolyser, such as a PEM water electrolyser or an AEM water electrolyser, and in particular a PEM water electrolyser. The anode layer comprises an iridium-containing and / or a ruthenium-containing oxygen evolution reaction (OER) catalyst. In an electrolyser the anode layer is the oxygen evolving layer. The anode layers may also be of utility in other electrolyser applications which utilise an oxygen evolving catalyst layer, for example an electrolyser used for the reduction of carbon dioxide.

[0032] The anode layer comprises an iridium- and I or ruthenium-containing OER catalyst. Preferably, the OER catalyst is an oxide of iridium and I or ruthenium. Such materials provide a suitable balance of OER catalytic activity and stability. Suitably the oxide of iridium is an iridium oxide (IrOx), ruthenium oxide (RuOx), iridium ruthenium oxide (IrRuOx), or a mixed iridium metal oxide or ruthenium metal oxide, for example a mixed metal oxide material comprising iridium and I or ruthenium and metal M, wherein M = Ta, Nb, Ti, Rh, Ru, or Pt. Such materials may be doped with one or more further elements or may be undoped.

[0033] Such oxides of iridium and I or ruthenium may be amorphous, semi-amorphous, or may be crystalline (typically with a rutile crystal structure). Preferably, the oxide of iridium and I or ruthenium is semi-amorphous. By semi-amorphous it is meant herein that broad Bragg peaks are observable in the X-ray diffraction pattern of the material which correspond to the crystalline oxide, for example an iridium oxide material with a broad Bragg peak in the 2-theta P102289W001 range 52 to 56 and a peak in the 2-theta range 32 to 36. By a broad peak it is meant herein that the peak height is less than the full width at half maximum.

[0034] Suitably, the oxides of iridium and I or ruthenium comprise a mixture of oxide and hydroxide groups, for example an iridium oxide (IrOx) material comprising a mixture of oxide and hydroxide groups, with both Ir (III) and Ir (IV) species present. Some I r(0) and I or Ru(0) may be present in the oxide material, although it may be preferred that no lr(0) and I or Ru(0) present, for example that no I r(0) and I or Ru(0) is observable by x-ray diffraction analysis.

[0035] Preferably, the anode layer has a total iridium + ruthenium loading of less than 1 .20 mgir+RUcm'2, such as less than 1.00 mgir+RUcm'2, less than 0.90 mgir+RUcm'2, less than 0.80 mgir+RUcm'2, less than 0.70 mgir+RUcm'2, or preferably less than 0.60 mgir+RUcm'2, in which the total iridium + ruthenium loading is the amount of iridium per x-y geometric area of the anode layer added to the amount of ruthenium per x-y geometric area of the anode layer.

[0036] The lower limit of the total iridium + ruthenium loading in the anode layer is not particularly limited in the present invention and is dependent on the desired electrochemical performance. Suitably, the loading is at least 0.05 mgir+RUcm'2, at least 0.10 mgir+RUcm'2, at least 0.15 mgir+RuCm-2, at least 0.20 mgir+RUcm'2, at least 0.25 mgir+RUcm'2, or at least 0.30 mgir+RUcm'2. Preferably, the total iridium + ruthenium loading in the anode layer is in the range of and including 0.05 to 1.20 mgir+RUcm'2, in the range of and including 0.10 to 1.0. mgir+RUcm'2, 0.15 to 0.90 mgir+RuCm-2, 0.20 to 0.80 mgir+RUcm'2, 0.25 to 0.70 mgir+RUcm'2, or preferably 0.30 to 0.60 mgir+RuCnr2. The total iridium + ruthenium loading of the anode layer may be suitably determined by x-ray fluorescence (XRF) analysis.

[0037] The OER catalyst may be on a particulate catalyst support, for example an inorganic metal oxide support, for example a transitional metal oxide or an oxide of a main group metal, such as TiC>2, AI2O3, Nb2C>5, ZrC>2 or mixtures thereof. If the OER catalyst is a supported catalyst, the loading of catalyst metal (such as iridium and I or ruthenium) 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. It may be preferred that the OER catalyst is unsupported.

[0038] The anode layer comprises an ionomer. Suitably, the ionomer is a proton-conducting polymer or an anion-conducting polymer, such as a hydroxyl anion-conducting polymer. Preferably ionomer is a proton-conducting polymer. Suitably, the anode layer ionomer comprises sulfonic acid groups. Preferably, ionomer is a perfluorinated sulfonic acid ionomer, or a partially- fluorinated or non-fluorinated hydrocarbon sulfonic acid ionomer. Mixtures of ionomers may also be employed. Suitably, the ionomer is perfluorinated, partially fluorinated or hydrocarbon based. P102289W001

[0039] Preferably, the ionomer comprises sulfonic acid groups and has an equivalent weight of about 1100 or less, typically about 900 or less, suitably about 850 or less. Typically, the anode layer ionomer comprises sulfonic acid groups and 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

[0040] Preferably, the 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, preferably the side chain is [polymer backbone]-O-CF2CF2-SC>3H, for example -CF(-O-CF2CF2-SO3H)(CF2)n-. Such materials may be purchased, for example Aquivion (RTM) PFSA ionomers from Solvay Speciality Polymers.

[0041] Alternatively, it may be preferred that the 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 -OCF2CF2CF2CF2SO3H. Such materials may be purchased, for example Nation (RTM) PFSA ionomers from Chemours or Forblue (RTM) i-series ionomers from AGC.

[0042] The anode layer comprises a cellulose compound. Suitably, the cellulose compound is a cellulose ether. Preferably, the cellulose compound is a hydroxyalkyl ether cellulose. By hydroxyalkyl ether cellulose it is meant herein a cellulose compound in which some of the hydroxyl groups in the repeating glucose units have been hydroxyalkylated to form a cellulose ether. Preferably, the cellulose compound is a hydroxyalkyl cellulose in which the alkyl group is a C1.10 alkyl, such as a C1.5 alkyl group, such as hydroxypropyl cellulose or hydroxyethylcellulose. The hydroyxalkyl cellulose may also comprise alkyl ether groups, i.e. the cellulose compound may be a hydroxyalkylalkylcellulose, for example a hydroxyalkylmethylcellulose, such as a hydroxypropylmethylcellulose or a hydroxyethylmethylcellulose. Preferably, the cellulose compound is selected from a hydroxypropyl cellulose, a hydroxypropylmethylcellulose or a hydroxyethylcellulose. 2- hydroxypropyl cellulose may be particularly preferred.

[0043] Suitably, the cellulose compound has a weight average molecular weight of at least 40,000, preferably at least 70,000, at least 100,000, or at least 250,000. Suitably, the cellulose compound has a weight average molecular weight of less than or equal to 2,000,000, preferably less than or equal to 1 ,500,000, or less than or equal to 1 ,200,000. The cellulose P102289W001 compound may have a weight average molecular weight in the range of and including 40,000 to 2,000,000, preferably in the range of and including 100,000 to 1,500,000, or 250,000 to 1 ,200,000. The weight average molecular weight of the cellulose compound may be determined by size exclusion chromatography with multi angle light scattering (MALS) and refractive index detection (such as an OMNISEC GPC system, Malvern Panalytical).

[0044] In the case that the cellulose compound is a hydroxyalkyl cellulose, the moles of substitution is suitably in the range of 1 to 4, preferably 2 to 4. The moles of substitution is the number of ether groups per monomer and may be determined by nuclear magnetic resonance spectrometry.

[0045] Suitably, the cellulose compound is present in the range of and including 0.02 to 2.0 wt% with respect to the combined weight of the catalyst, ionomer and cellulose compound in the anode layer. Preferably, the cellulose compound is present in the range of and including 0.05 to 1.0 wt%, 0.1 O to 1.0 wt %, or more preferably 0.15 to 0.70 wt%. Such an amount of the cellulose compound facilitates low defect layer formation at high line speeds whilst maintaining anode layer performance.

[0046] Suitably, the ionomer is present in the range of and including 5 to 50 wt% with respect to the combined weight of the catalyst, ionomer and cellulose compound in the anode layer, preferably from 10 to 30 wt%.

[0047] In some embodiments, the anode layer may comprise other components, for example the catalyst ink may comprise a radical reducing agent, for example a cerium-containing compound, such as cerium oxide or cerium metal oxide. In some embodiments, the anode layer consists essentially of the iridium- and I or ruthenium-containing OER catalyst, one or more ionomers, and the cellulose compound

[0048] The anode layers may suitably be formed from a catalyst ink comprising, or consisting essentially of, the iridium- and I or ruthenium-containing OER catalyst, the ionomer, the cellulose compound, one or more solvents, and optionally water. Suitably, the catalyst ink comprises water. Suitably the solvent is a water miscible solvent, such as an alcohol, preferably a linear or branched C1-C7 alcohol. Preferably, the solvent is selected from one or more of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, tert-butanol, or a glycol ether, preferably a glycol ether of formula HOCH2CH2OR in which R is a linear or branched C1-C4 alkyl, such as 2-butoxyethanol. More preferably, the solvent is 2- butoxyethanol. Use of a glycol ether, such as 2-butoxyethanol, offers advantages in terms of safety profile in comparison with alcohol solvents, such as methanol or ethanol, relating to a reduced flash point and / or a reduction in harmful oxidation products during catalyst layer manufacture. P102289W001

[0049] The catalyst ink may comprise other components, for example the catalyst ink may comprise a cerium-containing compound, such as cerium oxide or cerium metal oxide.

[0050] Suitably, the catalyst ink comprises water and I or solvent (55 to 75 wt%), ionomer (2 to 15 wt%), OER catalyst (20 to 40 wt%) and cellulose additive (0.02 to 1.0 wt%). It will be understood that other components may be present in the ink, such as a radical reducing agent (for example a cerium-containing compound, such as cerium oxide or cerium metal oxide), or additional metal-containing particles, (such as platinum). In some embodiments, the catalyst ink consists essentially of water and I or solvent (55 to 75 wt%), ionomer (2 to 15 wt%), OER catalyst (20 to 40 wt%) and cellulose additive (0.02 to 1.0 wt%) with the weight % of each component adding up to 100%.

[0051] The catalyst inks may be suitably prepared using a process comprising the step of adding the cellulose compound to a dispersion of the catalyst and the ionomer in the solvent and water (if present). Preferably, the cellulose compound is mixed with the solvent prior to addition of the cellulose compound to the dispersion. This pre-mixing step has found to significantly increase the viscosity of the catalyst ink in comparison with addition of the solid cellulose compound. It will be understood that the components of the catalyst ink may alternatively be combined in a different order, for example the OER catalyst may be added to a dispersion of ionomer and cellulose compound in the solvent and water (if present). Following addition of the cellulose compound the ink is suitably mixed, for example using a low shear mixer.

[0052] The anode layer is formed by applying the catalyst ink to a substrate. In some preferred embodiments, the catalyst ink is applied to the substrate using an apparatus comprising a slotdie or a gravure roller. In some preferred embodiments, the catalyst ink is applied to the substrate using a laser transfer process, such as a laser induced forward transfer (LIFT) process). Such a process is described in PCT / GB2024 / 051857 (Johnson Matthey Hydrogen Technologies Limited).

[0053] Preferably, the catalyst ink is applied to the substrate in a roll-to-roll coating process. More preferably, the catalyst ink is applied to the substrate in a roll-to-roll coating process using an apparatus comprising a slot-die or a gravure roller, or comprising a laser transfer apparatus, preferably a LIFT apparatus.

[0054] The catalyst ink may be applied to the substrate continuously or discontinuously (intermittently). Preferably, the catalyst ink is applied to the substrate intermittently. An intermittent application process provides a pattern of discontinuous catalyst layer patches on the substrate. Such patches correspond to the active area in an electrolyser stack, increasing utilisation of catalyst ink and reducing waste. Preferably, the catalyst ink is applied to the substrate in a roll-to-roll coating process comprising a slot-die or a gravure roller, or comprising P102289W001 a laser transfer apparatus, configured for intermittent delivery of the catalyst ink, such as a roll-to-roll coating process comprising a slot-die configured for intermittent delivery of the catalyst ink. Such intermittent delivery at low iridium and I or ruthenium loading is facilitated through the use of the anode catalyst layers as described herein, offering a reduction in defects, in particular at the leading and trailing edges of the patches. An example of an intermittent slot die process is described in US2024 / 0290936 A1 which is incorporated herein by reference.

[0055] The process comprises the step (iii), drying the anode layer. Drying of the anode 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(s) used in the catalyst ink. Suitably, the anode layer is dried at a temperature of at least 60 °C, such as in the range of and including 60 to 180 °C.

[0056] Typically, the substrate is a decal transfer substrate or a polymer electrolyte membrane, i.e. the application of the catalyst ink in step (ii) to form the anode layer is either direct application onto a polymer electrolyte membrane, or application onto a decal transfer substrate with subsequent transfer of the anode layer onto a polymer electrolyte membrane.

[0057] The material from which a decal transfer substrate is made should provide the required support, preferably be compatible with the ink, preferably be impermeable to the ink, be able to withstand the process conditions involved in transferring the anode layer onto the membrane, and be able to be easily removed after decal transfer without damage to the anode layer. Examples of materials suitable for use include a fluoropolymer, such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP - a copolymer of hexafluoropropylene and tetrafluoroethylene), and polyolefins, such as biaxially oriented polypropylene (BOPP).

[0058] In cases in which the substrate is a decal transfer substrate, the process suitably comprises the additional step of transferring the anode layer from the decal transfer substrate to the surface of a polymer electrolyte membrane. Suitably, the transfer is carried out using a hot press. Suitably, the hot press transfer is carried out at a temperature in the range of and including 140 to 180 °C at a pressure in the range of and including of 400 to 800 PSI.

[0059] In cases in which the substrate is a polymer electrolyte membrane, the membrane is suitably provided on a backing material. It may be preferred that the membrane has a cathode catalyst on the opposite face of the membrane to which the catalyst ink is applied. In such cases the polymer electrolyte membrane and the cathode catalyst layer are as described herein in relation to the catalyst-coated membrane. P102289W001

[0060] The anode layers described herein may advantageously form part of a catalyst-coated membrane (CCM). Such CCMs have the anode layer on a first major face of a polymer electrolyte membrane. It will be understood by the skilled person that the anode layer may be present on the whole first major face of the polymer electrolyte membrane or may be present in one or more patches which correspond to the active areas of the catalyst-coated membrane when incorporated into an electrolysis cell.

[0061] The membrane comprises an ion-conducting polymer. Ion conducting polymers that are suitable for forming polymer electrolyte membranes are known to the skilled person and are available commercially. The ion-conducting polymer can be a proton-conducting polymer or an anion-conducting polymer, such as a hydroxyl anion-conducting polymer. Examples of suitable proton-conducting polymers include perfluorosulphonic acid ionomers (e.g. Nation (RTM) (Chemours), Aciplex® (Asahi Kasei), Aquivion (RTM) (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). Examples of suitable anion-conducting polymers include A901 and A201 made by Tokuyama Corporation, Fumasep FAA from FuMA- Tech GmbH, and Aemion polymers from lonomr. In cases in which the catalyst-coated membrane is for a PEM water electrolyser, the ion-conducting polymer is suitably a proton conducting polymer, and in particular a partially- or fully-fluorinated sulphonic acid polymer. Examples of suitable proton-conducting polymers include perfluorosulphonic (PFSA) acid polymers. It may be preferred that the ion-conducting polymer is a PFSA polymer and has an equivalent weight (EW) greater than 450 EW, greater than 550 EW, greater than 650 EW, or greater than 700 EW. For example, it may be preferred that the ion-conducting polymer is a PFSA polymer with an equivalent weight in the range of and including 600 to 1200 EW, such as in the range of and including 700 to 1000 EW.

[0062] The polymer electrolyte membrane may include additional components such as recombination catalysts, radical scavengers and reinforcement components. Recombination catalysts, such as platinum catalysts, for example Pt / C or platinum black, catalyse the reaction between hydrogen and oxygen and therefore help to reduce the cross-over of hydrogen through the membrane during electrolysis. Radical scavengers, such as oxides of cerium (for example CeO2), can help to increase membrane durability.

[0063] Suitable reinforcing components are porous polymer materials, for example a microporous web or fibres of a polymer material, such as polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ETFE), perfluoroalkyl alkane (PFA), or fluorinated ethylene propylene (FEP). For example, the planar reinforcing component may comprise electrospun PVDF or forcespun PVDF. In a P102289W001 preferred embodiment, the porous polymer material is expanded PTFE (ePTFE), for example the microporous web structures of ePTFE supplied by Donaldson Company, Inc., known as Tetratex®, or supplied by other manufacturers. In other preferred embodiments, the reinforcing component can comprise a network of fibres (e.g. nanofibres), such as a network comprising polybenzimidazole (PBI) fibres, or a woven fabric, for example a woven fabric formed from PTFE thread. The network of fibres can be a non-woven mat of fibres (e.g. nanofibres), such as an electrospun mat of fibres or nanofibres.

[0064] Advantageously, the catalyst-coated membrane has restricted swelling in water at elevated temperatures. Such a restriction offers increased durability of the catalyst-coated membranes incorporating low iridium and I or ruthenium layers, which may suffer surface structure disruption with excessive swelling. Such restricted swelling may be provided, for example, by the incorporation of multiple polymeric reinforcements into the membrane, or by using a polymeric reinforcement with high tensile strength in both x and y dimensions, such as a woven fabric.

[0065] Therefore, in some embodiments, the polymer electrolyte membrane has two or more reinforcing components, such as two or more layers of a microporous web of polymer, such as ePTFE. In some other embodiments the electrolyte membrane comprises a reinforcing component in the form of a woven fabric, such as a woven fabric formed from polymer threads, such as ePTFE or PEEK threads. Suitable materials are described in US11742507B2 (AGC INC).

[0066] Preferably, the polymer electrolyte membrane has a thickness of less than or equal to 100 .m. It may be preferred that the membrane has a thickness of less than or equal to 95 .m, 90 .m, or 85 .m. It may be preferred that the membrane has a thickness of at least 10 .m, such as at least 15 .m, at least 20 .m, at least 25 .m, at least 30 .m or at least 40 .m. It may be further preferred that the membrane has a thickness in the range of and including 10 to 100 .m, such as 15 to 100 .m, 20 to 100 .m, 30 to 100 .m, 30 to 90 .m, or 40 to 90 .m. The membrane thickness may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the membrane and the membrane thickness measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values. Typically, the SEM measurement is carried out on a cross section of the catalyst-coated membrane (suitably dried at 0°C relative humidity), which is embedded in resin, ground and polished.

[0067] Typically, a cathode catalyst layer is provided on the second major face of the polymer electrolyte membrane (i.e. the opposite side of the membrane to the anode catalyst later). Such cathode catalyst layers comprise a hydrogen evolution reaction catalyst, such as a P102289W001 platinum-based catalyst, for example platinum on a carbon support (Pt / C). Suitably, the cathode catalyst layer comprises a hydrogen evolution reaction catalyst, such as a platinum- containing catalyst and an ion-conducting polymer. Such layers may be provided by directly coating the membrane or, for example, by decal transfer as hereinbefore described for the anode layer.

[0068] The catalyst-coated membrane may comprise a seal material on a first face and / or a second face of the CCM. Such seal materials are typically formed from non-ion conducting polymers, and may be positioned around the edge region of the 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 or elastomeric gaskets. An adhesive layer may be present on one or both surfaces of the seal material.

[0069] In a water electrolyser, additional transport layers are positioned each side of a membrane to facilitate reagent and product transfer to and from the catalyst layers, and to provide electrical contact. The catalyst-coated membrane and transport layer(s) are referred to together as a membrane electrode assembly (MEA). These additional transport layers may be known as porous transport layers or gas diffusion layers. These layers may or may not be directly attached to the CCM. Other components of a water electrolyser may include bipolar plates and current collector plates. Stacks of such assemblies make up an electrolyser system including power and control systems.

[0070] The MEAs of the present invention are configured such that the anode layer is positioned between the electrolyte membrane and a transport layer such that (a) it is in direct contact with the transport layer; or (b) it is in contact with an intermediate conductive layer positioned between the anode layer I thin film coating and the transport layer.

[0071] Suitable transport layers at the anode side of the CCM are known to the skilled person and 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 adjacent to the CCM (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 nonwoven cloths, mesh, foams and sintered compacts of metal-containing particles. For PEM water electrolyser applications, suitable PTLs comprise titanium. For AEM water electrolyser applications, suitable PTLs comprise nickel or stainless steel.

[0072] Suitable transport layers at the cathode side of the CCM are known to the skilled person and are typically non-woven papers or webs comprising a network of carbon fibres and a thermoset P102289W001 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 from Ballard Power Systems Inc., or woven carbon cloths. The carbon paper, web or cloth may be provided with a further treatment prior to being incorporated into as MEA 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] Figure 1 shows a schematic representation of an example of a water electrolyser (20) incorporating a CCM. A polymer electrolyte membrane (22) is provided with an anode layer comprising an iridium- and I or ruthenium-containing OER catalyst (24) and a cathode catalyst layer (26) incorporating a platinum-containing catalyst, the anode and cathode layers being provided on opposite faces of the polymer electrolyte membrane (22). Adjacent to the anode layer (24) is a transport layer (28) which is typically a metal-based porous structure and may be known as a porous transport layer (PTL). Adjacent to the cathode catalyst layer (26) is a transport layer (30) which is typically a non-woven paper or web comprising a network of carbon fibres and may be known as a gas diffusion layer (GDL). Adjacent to each transport layer (28, 30) is a bipolar plate (32) which provides flow channels for gases and uniformly distributes water participating in the reaction on the electrode surface. Some components, such as seal material layers and sub-gaskets are not shown in this representation but may be present as understood by the skilled person.

[0074] The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention and are not intended to limit its scope.

[0075] Examples

[0076] Measurement methods

[0077] Viscosity

[0078] The viscosity of dispersions of additives is determined using a Discovery HR30 Rheometer, equipped with a cone-plate geometry and a temperature-controlled Peltier Plate. The cone diameter is 50 mm, and the cone angle is 59 ° 15’, with a truncation gap of 300 micron. The temperature is set at 25 °C for all tests. The procedure used is an eight-step hold at pre-fixed shear rates of 1 , 10, 25, 50, 100, 250, 500, 1000 1 / s: each hold lasts for 30 seconds, followed by a final recovery step lasting 1 minute at a shear rate of 1 / s; the corresponding viscosity values measured at each step are recorded and compared across samples. The viscosity of catalysts inks is determined with the same procedure, using a parallel plate geometry: the plate diameter is 40 mm, and the gap is set at 300 micron. P102289W001

[0079] Example 1 - Additive testing with a PFSA dispersion

[0080] Dispersions of additives (as set out in Table 1) in water (1.5 wt%) were roller mixed overnight and then ultrasonicated to remove trapped bubbles. PFSA ionomer inks was then prepared using 1 wt% additive, 5 wt% PFSA ionomer (AGC IC154), in ethanol: water (~40 wt% ethanol). The dispersions were roller-mixed overnight and then ultrasonicated to remove trapped bubbles. The viscosity of the dispersions was measured (in comparison with a PFSA dispersion without additive) and then retested after 5 days to assess ink stability. The results are described in Table 1. The results indicated that cellulose additives achieved an ink viscosity in the range compatible with roll-to-roll coating techniques and showed stability over a 5-day period.

[0081] Table 1 :

[0082] Example 2 - Preparation of inks and decal transfer using a hydroxypropyl cellulose additive

[0083] A stock solution of hydroxypropyl cellulose (Klucel M) was prepared by mixing the additive with ethanokwater (-40:60). P102289W001

[0084] A catalyst ink was prepared by addition of an aliquot of the stock solution of the additive to a dispersion of iridium oxide and AGC I Cl 54 PFSA (ink solids content - 40 wt%) in a mixture of ethanokwater (-40:60) to achieve a target additive wt % of 0.08 as a proportion of total ink weight. The ink was successfully coated onto by slot-die onto a PTFE decal transfer substrate at line speeds of 2 to 4 m / minute.

[0085] Catalyst layers were transferred from the decal transfer substrate to a polymer electrolyte membrane (800 EW, PFSA ionomer, approximately 80-micron thickness, with 2 ePTFE reinforcements) using a hot press.

[0086] A CCM was then formed by decal transfer of a Pt / C cathode layer onto the opposite face of the membrane with a Pt loading of 0.4 mgcm-2.

[0087] Comparative example 2 - Preparation of inks without a hydroxypropyl cellulose additive

[0088] A catalyst ink was prepared as described in Example 2 without the hydroxypropyl cellulose additive. During trials slot-die coating of the ink onto a decal transfer substrate was unsuccessful at line speeds greater than 1 m / minute. Patch coating of the ink onto the substrate was unsuccessful. Catalyst layers produced at a line speed of 1 m / minute were formed into a CCM as described for Example 2.

[0089] Electrochemical testing

[0090] Figure 2 shows the results of electrochemical testing of the CCMs formed in Example 2 and Example 4. This shows that the layers incorporating the cellulose additive perform at least as well as layers without the additive despite the significantly higher line speed.

[0091] Example 3 - Preparation of inks and decal transfer using a hydroxypropyl cellulose additive at low iridium loading

[0092] A stock solution of hydroxypropyl cellulose (Klucel M) at 1 .5 wt% was prepared by mixing the additive with 2-butoxyethanol.

[0093] A catalyst ink was prepared with by addition of an aliquot of the stock solution of the additive to a dispersion of iridium oxide and Aquivion SSC PFSA in a mixture of 2-butoxyethanol: water (50:50) to achieve a target additive wt % of 0.43. The ink was successfully coated onto by slot-die onto a decal transfer substrate, forming an anode layer with an iridium loading of around 0.7 mgcm-2.

[0094] Catalyst layers were transferred from the decal transfer substrate to a polymer electrolyte membrane (800 EW, PFSA ionomer, approximately 80-micron thickness, with 2 ePTFE reinforcements) using a hot press at a temperature on the range of 140 to 160 °C and a pressure of 500 to 700 PSI. P102289W001

[0095] A CCM was then formed by decal transfer of a Pt / C cathode layer onto the opposite face of the membrane with a Pt loading of 0.4 mgcm-2.

[0096] Electrochemical testing indicated excellent CCM OER performance despite the low iridium loading.

Claims

P102289W001Claims1. A process for the manufacture of an anode layer for a polymer electrolyte membrane electrolyser, the process comprising the steps of:(i) forming a catalyst ink comprising an iridium- and I or ruthenium-containing OER catalyst, an ionomer, a solvent, and a cellulose compound;(ii) applying the catalyst ink to a substrate to form the anode layer;(iii) drying the anode layer.

2. A process according to claim 1 , wherein the OER catalyst is iridium oxide, an iridium metal oxide, or a ruthenium metal oxide.

3. A process according to claim 1 or claim 2, wherein the anode layer has a total iridium and I or ruthenium loading of less than or equal to 1.2 mgcm-2, less than or equal to 1.0 mgcm-2, less than or equal to 0.8 mgcm-2, or preferably less than or equal to 0.6 mgcm-2.

4. A process according to any one of claims 1 to 3, wherein the cellulose compound is a hydroxyalkylcellulose.

5. A process according to any one of the preceding claims, wherein the cellulose compound is selected from a hydroxypropylcellulose, a hydropropylmethylcellulose, or a hydroxyethylcellulose.

6. A process according to any one of the preceding claims, wherein the cellulose compound has a weight average molecular weight in the range of and including 100,000 to 2,000,000.

7. A process according to any one of the preceding claims, wherein the ink comprises 0.02 to 1.0 wt% of the cellulose compound based on the total weight of ink components.

8. A process according to any one of the preceding claims, wherein the substrate is a polymer electrolyte membrane.

9. A process according to any one of claims 1 to 7, wherein the substrate is a decal transfer substrate.

10. A process according to claim 9, wherein the process comprises the additional step of transferring the anode layer from the decal transfer substrate to the surface of a polymer electrolyte membrane.

11. A process according to any one of the preceding claims, wherein the drying in step (iii) is carried out at a temperature in the range of and including 60 to 150 °C.P102289W00112. A process according to any one of the preceding claims, wherein the solvent is a glycol ether, preferably 2-butoxyethanol.

13. A process according to any one of the preceding claims, wherein the catalyst ink is applied to the substrate in a roll-to-roll coating process.

14. A process according to any one of the preceding claims, wherein the catalyst ink is applied to the substrate using an apparatus comprising a slot-die or a gravure roller, preferably wherein the apparatus is configured for intermittent delivery of the catalyst ink.

15. A process according to any one of claims 1 to 13, wherein the catalyst ink is applied to the substrate using a laser transfer process, preferably a LIFT process.

16. A catalyst ink comprising an iridium- and I or ruthenium-containing OER catalyst, an ionomer, a solvent, and a cellulose compound.

17. A catalyst ink according to claim 16, wherein the cellulose compound is a hydroxyalkylcellulose, preferably a hydroxypropylcellulose, a hydropropylmethylcellulose, or a hydroxyethylcellulose.

18. A catalyst ink according to claim 16 or claim 17, wherein the cellulose compound has a weight average molecular weight in the range of and including 100,000 to 2,000,000.

19. A catalyst ink according to any one of claims 16 to 18, wherein the wherein the ink comprises 0.02 to 1 .0 wt% of the cellulose compound based on the total weight of ink components.

20. A process for the manufacture of a catalyst ink according to any one of claims 16 to 19, the process comprising the step of adding the cellulose compound to a catalyst composition comprising the iridium- and I or ruthenium-containing OER catalyst, the ionomer, and the solvent.

21. A process according to claim 20, comprising the step of mixing the cellulose compound with the solvent prior to addition of the cellulose compound to the catalyst composition.

22. An anode layer for a polymer electrolyte membrane electrolyser, the anode layer comprising an iridium- and I or ruthenium-containing OER catalyst, an ionomer, and a cellulose compound.

23. An anode layer according to claim 22, wherein the total iridium and I or ruthenium - loading is less than or equal to 1.2 mgcm-2, less than or equal to 1.0 mgcm-2, less than or equal to 0.8 mgcm-2, or preferably less than or equal to 0.6 mgcm-2.P102289W00124. An anode layer according to claim 22 or claim 23, wherein the cellulose compound is a hydroxyalkylcellulose, preferably a hydroxypropylcellulose, a hydropropylmethylcellulose, or a hydroxyethylcellulose.

25. An anode layer according to any one of claims 22 to 24, wherein the cellulose compound is present in the range of and including 0.02 to 2.0 wt% with respect to the combined weight of the catalyst, ionomer and cellulose compound in the anode layer.

26. A catalyst-coated membrane (CCM) for an electrolyser, the CCM comprising a polymer electrolyte membrane and an anode layer according to any one of claims 22 to 25.

27. An electrolyser comprising an anode layer according to any one of claims 22 to 25, or a catalyst-coated membrane according to claim 26.

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