Catalyst-coated membranes and processes for their manufacture

Abstract

A process for the manufacture of a catalyst-coated polymer electrolyte membrane for a water electrolyser comprising an iridium-containing anode layer is provided. The process comprises the steps of: (i) provision of a polymer electrolyte membrane with a first major surface and a second opposed major surface; (ii) plasma treatment of the first major surface of the polymer electrolyte membrane to form an etched polymer electrolyte surface layer; (iii) vapour deposition of an iridium-containing compound onto the etched polymer electrolyte surface layer to form the iridium-containing anode layer.

Description

[0001] P101958W001

[0002] CATALYST-COATED MEMBRANES AND PROCESSES FOR THEIR MANUFACTURE

[0003] Field of the Invention

[0004] This invention relates to catalyst-coated polymer electrolyte membranes for water electrolysis and to processes of manufacture of such catalyst-coated membranes. In particular this invention relates to catalyst-coated membranes for a proton-exchange membrane (PEM) water electrolyser.

[0005] Background

[0006] Solid polymer electrolyte membranes, such as proton exchange membranes (PEMs) or anion exchange membranes (AEMs), may be employed for water 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 is applied to a face of the membrane to form a catalyst-coated membrane (CCM). In other cases, the catalyst layers may be applied to other components, such as gas diffusion layers or porous transport layers, and the catalyst layers compressed against the membrane during assembly and use of the electrochemical device.

[0007] 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 utilised in electrolyser anode catalyst layers, with iridium-containing compounds, such as iridium oxide (IrOx) or iridium ruthenium oxide (IrRuOx), offering a particularly good balance between OER activity and stability under electrolysis conditions. Such catalysts are typically applied to membranes (or other components) in a catalyst ink to form a layer comprising the particulate catalyst and an ionconducting polymer.

[0008] Due to the relative scarcity and cost of iridium it is desirable to reduce the amount of iridium present in water electrolyser anode catalyst layers, for example to less than 0.5 mgircm-2. However, reducing the amount of iridium in an anode layer can lead to significant problems with poor catalyst layer conductivity and a reduction in catalytic activity.

[0009] It is also desirable to increase the manufacturing efficiency of catalyst-coated membranes and other electrolyser components as the demand for water electrolysis rapidly increases in response to net zero targets.

[0010] A process is described in ‘Sputter-etching treatment of proton-exchange membranes: Completely dry thin-film approach to low-loading catalyst-coated membranes for water electrolysis’, International Journal of Hydrogen Energy 45 (2020) 20776-20786 that involves simultaneously plasma etching a proton-exchange membrane (PEM) and depositing a cerium P101958W001 oxide layer during reactive magnetron sputtering, and then subsequently coating with a thin layer of iridium metal, forming an anode-side catalyst-coated membrane. A similar process is described in US2022 / 0045347 (Univerzita Karlova).

[0011] The inclusion of a cerium-containing compounds in electrolyser CCMs can, under certain electrolyser operating conditions, lead to dissolution and migration of cerium ions which can cause poisoning of the cathode and subsequent loss of performance.

[0012] It is further described in CN108950587A (STATE GRID CORPORATION OF CHINA; GLOBAL ENERGY INTERCONNECTION RESEARCH INSTITUTE CO., LTD.; STATE GRID SHANXI ELECTRIC POWER COMPANY) that solid polymer membranes may be prepared by a method including the steps: (1) coating a catalyst on a surface of the proton exchange membrane to disperse the catalyst on the surface; (2) etching the surface of the proton exchange membrane obtained in the step (1); (3) coating with catalyst particles. The method utilises a reactive ion etching process and requires two catalyst coating steps. A high loading of catalyst is required with a loading in the first coating step of 0.1 to 0.5 mg / cm2and a loading in the second coating step of 1 to 4 mg / cm2.

[0013] There remains a need to further enhance and develop catalyst-coated membranes for water electrolysis which address one or more previously identified challenges, in particular CCMs which provide high oxygen evolution activity at low iridium loadings (< 0.3 mgircm-2), and which can be efficiently manufactured.

[0014] Summary of the invention

[0015] The present inventors have identified that catalyst-coated membranes may be advantageously formed by plasma-etching the surface of a polymer electrolyte membrane prior to vapour deposition of an iridium-containing compound directly onto the membrane surface. Without being bound by theory, it is understood that such plasma-etched surfaces provide a suitable substrate for vapour deposition which offers an advantageous balance of layer porosity and conductivity at low iridium loadings (< 0.3 mgircm-2), and enhances the surface area of the deposited iridium-containing compound. Furthermore, the omission of an intermediate coating layer or catalyst support facilitates interfacial contact and transfer of protons from iridium sites to the membrane.

[0016] Therefore, in a first aspect of the invention, there is provided a process for the manufacture of a catalyst-coated polymer electrolyte membrane for a water electrolyser comprising an iridium- containing anode layer, the process comprising the steps of:

[0017] (i) provision of a polymer electrolyte membrane with a first major surface and a second opposed major surface; P101958W001

[0018] (ii) plasma treatment of the first major surface of the polymer electrolyte membrane to form an etched polymer electrolyte surface layer;

[0019] (iii) vapour deposition of an iridium-containing compound onto the etched polymer electrolyte surface layer to form the iridium-containing anode layer.

[0020] Preferably, the plasma treatment of the surface of the polymer electrolyte membrane in step (ii) is glow discharge plasma treatment. The use of a glow discharge plasma enables a high level of control over the etching process. This method also provides a simplified manufacturing process without the need for material deposition prior to plasma treatment, or concurrent material deposition during plasma treatment, and is suitable for larger scale manufacturing, for example in a roll-to-roll process.

[0021] In a second aspect of the invention, there is provided a catalyst-coated membrane obtainable or obtained by the process of the first aspect.

[0022] Preferably, the plasma-etched surface comprises a plurality of elongated polymer electrolyte protrusions. Such a surface structure advantageously enables porous layers to be formed facilitating reagent transfer whilst also assisting the maintenance of layer conductivity and, in combination with a thin-film coating of an iridium-containing compound on the surface of the protrusions, provides an enhanced electrochemical performance, in particular at low iridium loadings, such as < 0.3 mgircm ~2.

[0023] Therefore, in a third aspect of the invention, there is provided a catalyst-coated polymer electrolyte membrane for a water electrolyser, the catalyst-coated membrane comprising:

[0024] (i) a polymer electrolyte membrane with a first major surface and a second opposed major surface, the first major surface comprising a plurality of elongated polymer electrolyte protrusions;

[0025] (ii) an iridium-containing anode layer comprising a thin film of an iridium-containing compound coating the surface of the elongated polymer electrolyte protrusions and having an iridium loading of less than 0.30 mgircm-2, preferably in the range of and including 0.01 to 0.30 mgir cm-2.

[0026] Preferably, the iridium-containing compound partially coats the surface of the elongated polymer electrolyte protrusions.

[0027] The catalyst-coated membrane may be advantageously incorporated into a membraneelectrode assembly. Therefore, in a fourth aspect of the invention there is provided a membrane-electrode assembly (MEA) for a water electrolyser comprising a catalyst-coated membrane and a transport layer, wherein the catalyst-coated membrane is according to the P101958W001 first aspect, and the MEA is configured such that the iridium-containing anode layer is positioned between the polymer electrolyte membrane and the transport layer.

[0028] In a fifth aspect of the invention there is provided a water electrolyser, such as a protonexchange membrane (PEM) water electrolyser, comprising a catalyst-coated membrane obtainable or obtained according to the first aspect, comprising a catalyst-coated membrane according to the second or to the third aspect, or comprising a membrane-electrode assembly according to the fourth aspect.

[0029] Brief description of the Figures

[0030] Figure 1 shows a schematic representation of a water electrolyser.

[0031] Figure 2 shows a scanning-electron microscope (SEM) image of a catalyst-coated membrane formed in Comparative Example 1 .

[0032] Figure 3 shows focused ion beam-scanning electron microscope (FIB-SEM) image of a cross section of the CCM formed in Example 1 A.

[0033] Figure 4 shows focused ion beam-scanning electron microscope (FIB-SEM) image of a cross section of the CCM formed in Example 1 B.

[0034] Figure 5 shows focused ion beam-scanning electron microscope (FIB-SEM) image of a cross section of the CCM formed in Example 1C.

[0035] Figure 6 shows focused ion beam-scanning electron microscope (FIB-SEM) image of a cross section of the CCM formed in Example 1 D.

[0036] Figure 7 shows an X-ray diffraction analysis of an iridium-containing anode layer formed according to a process as described herein.

[0037] Figure 8 shows a focused ion beam-scanning electron microscope (FIB-SEM) image of a cross section of the CCM formed in Example 2.

[0038] Detailed Description

[0039] 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.

[0040] The present invention provides a catalyst-coated polymer electrolyte membrane (CCM) for a water electrolyser, such as a proton exchange membrane (PEM) water electrolyser or an anion exchange membrane (AEM) water electrolyser, and in particular for a PEM water electrolyser, and processes for their manufacture. The CCMs comprise an iridium-containing P101958W001 compound in the anode layer. The iridium-containing compound acts as the anode (oxygen evolving) catalyst during water electrolysis by facilitating the oxygen evolution reaction.

[0041] The CCM comprises a polymer electrolyte membrane with a first major surface and a second opposed major surface. Suitably, the polymer electrolyte membrane is a proton-exchange membrane (PEM) or an anion-exchange membrane (AEM). Preferably, the electrolyte membrane is a proton exchange membrane. Such electrolyte membranes are formed from ion-conducting polymers, such as proton-conducting polymers or anion-conducting polymers, such as a hydroxyl anion-conducting polymer. Such materials are known to those skilled in the art.

[0042] Suitably, the polymer electrolyte membrane is a PEM and the membrane is formed from ionconducting polymers comprising sulfonic acid groups. Suitably, the ion-conducting polymer is a perfluorinated sulfonic acid ionomer, or a partially-fluorinated or non-fluorinated hydrocarbon sulfonic acid ionomer. Examples of suitable proton-conducting polymers include partially- or fully-fluorinated sulfonic acid polymers, such as perfluorosulfonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aquivion™ (Synesqo), 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, Toyobo Corporation, and others. Suitably, the ion-conducting polymer has an equivalent weight of about 1100 or less, typically about 900 or less, suitably about 850 or less. Typically, the ion-conducting polymer has an equivalent weight of at least about 450.

[0043] 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.

[0044] 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 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 P101958W001 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.

[0045] 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 vapour deposited iridium-containing compounds, 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.

[0046] 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).

[0047] 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.

[0048] Preferably, the first major surface of the polymer electrolyte membrane comprises a plurality of elongated polymer electrolyte protrusions. Suitably, such a surface structure may be formed by plasma etching. Such a configuration has been found to provide an advantageous configuration supporting an iridium-containing anode layer which maintains electrochemical performance at particularly low iridium loadings. Such elongated polymer electrolyte protrusions have a length (linear distance from the polymer electrolyte membrane bulk to the tip of the protrusion) and a width (suitably measured at 50% of the length). By elongated it is P101958W001 meant herein that the length is greater than the width, i.e. the aspect ratio (length: width) is > 1 :1 , preferably > 2: 1. Such distances may be measured using SEM analysis of a cross section of the catalyst-coated membrane.

[0049] It will be understood by the skilled person that there will be variation in dimensions of the elongated polymer electrolyte protrusions. Preferably, at least 50 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 1 :1 to 25:1 , at least 60 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 1 :1 to 25:1 , at least 70 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 1 :1 to 25:1 , at least 80 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 1 :1 to 25:1 , or at least 90 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 1 :1 to 25:1. It may be further preferred that at least 50 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 3:1 to 25:1 , at least 60 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 3:1 to 25:1 , at least 70 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 3:1 to 25:1 , at least 80 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 3:1 to 25:1 , or at least 90 % of the elongated polymer electrolyte protrusions have an aspect ratio in the range of 3:1 to 25:1.

[0050] Preferably, the elongated polymer electrolyte protrusions are present in a density of at least 5,000 protrusions per pm2of the x-y geometric area of the iridium-containing anode layer. It may be preferred that the elongated polymer electrolyte protrusions are present in a density of at least 10,000, or at least 20,000 protrusions per pm2of the x-y geometric area of the iridium-containing anode layer. The maximum density of protrusions is not particularly limited in the present invention, however typically there are no more than 1 ,000,000 protrusions per pm2of the x-y geometric area of the iridium-containing anode layer, such as no more than 500,000 or no more than 250,000. Suitably, the density of the elongated polymer electrolyte protrusions is in the range of and including 5,000 to 1 ,000,000 protrusions per pm2of the x-y geometric area of the iridium-containing anode layer, such as in the range of and including 10,000 to 500,000. The density of the polymer electrolyte protrusions may be determined by counting the number of protrusions per unit area in a series of (for example 10) scanning electron microscope images of the iridium-containing anode layer.

[0051] Preferably, the region of the first major surface which comprises the elongated polymer electrolyte protrusions, suitably the product of plasma treatment leading to an etched polymer electrolyte surface layer, has a thickness in the range of and including 25 to 2000 nm. The thickness of the region of the first major surface which comprises the elongated polymer electrolyte protrusions may be determined by scanning electron microscope (SEM) analysis of a cross section of the membrane after plasma treatment. SEM analysis is carried out on P101958W001 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. A thickness of less than 25 nm is typically insufficient to provide the desired layer porosity at the desired iridium loadings. A thickness of greater than 2000 nm can lead to a significant reduction of layer conductivity at the desired iridium loadings.

[0052] Preferably, the thickness of the region of the first major surface which comprises the elongated polymer electrolyte protrusions is in the range of and including 50 to 1500 nm, 50 to 1200 nm, 100 to 1000 nm, 150 to 800 nm, or 200 to 600 nm. It is considered that a thickness in the range of 200 to 600 nm provides a particularly suitable thickness in combination with an iridium loading in the range of and including 0.01 to 0.20 mgircm-2, preferably in the range of 0.01 to 0.10 mgircm-2, enabling a suitable balance of layer porosity and conductivity.

[0053] It may be preferred that the thickness of the region of the first major surface which comprises the elongated polymer electrolyte protrusions is in the range of and including 25 to 100 nm. It is considered that a thickness in the range of 25 to 100 nm provides a particularly suitable thickness in combination with an iridium loading in the range of and including 0.01 to 0.10 mgircnr2in circumstances in which layer anode conductivity is a key requirement of the formed catalyst-coated membrane, for example when combined with other, lower conductivity, electrolyser components.

[0054] The catalyst-coated membrane comprises an iridium-containing anode layer. This layer comprises a thin film of an iridium-containing compound coating the surface of the elongated polymer electrolyte protrusions.

[0055] The iridium-containing compound is provided in contact with the surface of the polymer electrolyte membrane without any intermediate surface layer. Such an arrangement offers advantages related to the low interfacial resistance and the facilitation of the transfer of protons from iridium sites to the membrane.

[0056] The term “thin film” takes its conventional meaning in the art, which will be understood by a skilled person. It will be further understood by the skilled person that the thickness of the thin film coating will depend on the selected iridium loading, and is readily varied by adjusting the deposition time. Suitably, the thin film coatings of the present invention have a thickness of no more than 2000 nm, no more than 1500 nm, more than 1000 nm, no more than 500 nm, or no more than 250 nm. The thin film coatings typically have a thickness of at least 1 nm, such as at least 5 nm, at least 10 nm, or at least 25 nm. Accordingly, the thin film coatings of the invention may have a thickness in the range of and including 1 to 2000 nm, such as in the range of 5 to 1000 nm, or 10 to 500 nm. The thickness of the thin film coating may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross P101958W001 sections of the structure and the 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 thickness of the thin film varies at different points of the polymer electrolyte protrusions, such as a greater thickness at the tips of the protrusions than present on the sides of the protrusions. It will be understood that the thickness values set out hereinbefore refer to an average thickness taking into account multiple points of measurement.

[0057] Preferably, the catalyst-coated membrane comprises an iridium-containing anode layer which comprises a thin film of an iridium-containing compound partially coating the surface of the elongated polymer electrolyte protrusions. Partial coating of the etched surface layer provides a porous structure facilitating transfer of the reactants required for oxygen evolution and the products formed. It will be understood by the skilled person that partial coating means that there are some parts of the surface of the elongated polymer electrolyte protrusions that do not have a thin film coating of an iridium-containing compound. Suitably, the thin film coating is present on at least 10 %, at least 20 %, at least 30 %, at least 40 %, at least 50 %, at least 60 %, at least 70 %, or at least 80 % of the surface of the elongated polymer electrolyte protrusions. Suitably, the thin film coating is present on less than 99 % of the surface of the elongated polymer electrolyte protrusions, such as less than 97 %, or less than 95 %. Preferably, the thin film coating is present in the range of and including 50 to 99 %, 60 to 97 %, or 70 to 95 % the surface of the elongated polymer electrolyte protrusions.

[0058] The iridium-containing compound is preferably an oxide of iridium. Such materials provide a suitable balance of OER catalytic activity and stability. Suitably the oxide of iridium is an iridium oxide (IrOx), or a mixed iridium metal oxide, for example a metal oxide material comprising iridium 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. It may be preferred that the oxide of iridium is a metal oxide material comprising iridium and ruthenium. Such materials offer high oxygen evolution catalytic activity. It may be preferred that the oxide of iridium is a metal oxide material comprising iridium and platinum. Such materials offer high layer conductivity.

[0059] Preferably, iridium-containing compound an iridium oxide (IrOx) material. Such iridium oxide (IrOx) materials may be amorphous, semi-amorphous, or may be crystalline (typically with a rutile crystal structure). Preferably, the iridium oxide (IrOx) material 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 iridium oxide, and in particular a peak in the 2-theta 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. P101958W001

[0060] Preferably, the iridium oxide (IrOx) material comprises a mixture of oxide and hydroxide groups, with both Ir (III) and Ir (IV) species present. Some lr(0) may be present in the iridium oxide material, although it may be preferred that no I r(0) present, for example that no I r(0) is observable by x-ray diffraction analysis.

[0061] In some embodiments, the iridium-containing anode layer comprises one or more additional materials. Suitably the one or more additional materials are selected from platinum, and oxides of tantalum, niobium, or zirconium. Such materials can provide benefits such as increased conductivity in the case of platinum, and increased dispersion of the iridium- containing compound within the layer. The one or more additional materials may be codeposited with the iridium-containing compound or as a subsequent layer. It may be preferred that the anode-containing layer comprises platinum, for example platinum in an amount in the range of and including 0.04 to 1.20 mgptcm-2of the geometric area of the iridium-containing anode layer.

[0062] In some embodiments, the iridium-containing anode layer comprises the iridium-containing compound but does not comprise any additional metal-containing compounds. In some embodiments, the iridium-containing anode layer consists essentially of the iridium-containing compound (preferably iridium oxide (IrOx)).

[0063] Preferably, the iridium-containing anode layer has an iridium loading of less than 0.30 mgircnr2, such as less than 0.25 mgircm-2, less than 0.20 mgircm-2, less than 0.15 mgircm-2, or less than 0.10 mgircm-2, in which the iridium loading is the amount of iridium per x-y geometric area of the anode layer. The lower limit of iridium loading is not particularly limited in the present invention and is dependent on the desired electrochemical performance. Suitably, the iridium loading is at least 0.001 mgircm-2, at least 0.002 mgircm-2, at least 0.003 mgircm-2, at least 0.004 mgircnr2, at least 0.005 mgircm-2, at least 0.006 mgircm’2, at least 0.007 mgircm-2, at least 0.008 mgircm-2, at least 0.009 mgircm-2, at least 0.01 mgircm-2, at least 0.02 mgircm-2, or at least 0.03 mgircnr2. Preferably, the iridium-containing anode layer has an iridium loading, in the range of and including 0.01 to 0.30 mgircm-2, in the range of and including 0.01 to 0.25 mgircm’2, 0.01 to 0.20 mgircnr2, 0.01 to 0.15 mgircm-2, 0.01 to 0.10 mgircm-2, 0.02 to 0.08 mgircm-2, or 0.03 to 0.06 mgircm-2. The iridium loading of the anode layer may be suitably determined by x-ray fluorescence (XRF) analysis.

[0064] Typically, the catalyst-coated membrane has a cathode catalyst layer on the second major face of the polymer electrolyte membrane. Such cathode catalyst layers comprise a hydrogen evolution reaction catalyst, such as a platinum-based catalyst, for example platinum on a carbon support (Pt / C). P101958W001

[0065] Advantageously, the configuration of the iridium-containing anode layers that are described herein have high lateral conductivity.

[0066] Preferably, the iridium-containing anode layer has a sheet resistance of less than or equal to 10,000 Q sq-1, less than or equal to 8,000 Q sq-1, less than or equal to 7,000 Q sq-1, less than or equal to 6,000 Q sq-1, less than or equal to 5,000 Q sq-1, less than or equal to 4,000 Q sq_1, less than or equal to 3,000 Q sq-1, less than or equal to 2,000 Q sq-1, less than or equal to 1 ,000 Q sq-1, or more preferably, less than or equal to 500 Q sq-1. The thin film coating may have a sheet resistance of at least 20 Q sq-1, at least 25 Q sq-1, at least 30 Q sq-1, or at least 35 Q sq-1. Preferably, the thin film coating has a sheet resistance in the range of and including 20 to 10,000 Q sq-1, 30 to 7000 Q sq’1, 40 to 3000 Q sq’1, 50 to 1000 Q sq’1, or 50 to 500 Q sq-1.

[0067] It will be understood by the skilled person that the iridium-containing anode layer may be present on the whole first major surface of the polymer electrolyte membrane or may be present in one or more patches which correspond to the active area of the catalyst-coated membrane when incorporated into an electrolysis cell.

[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 iridium-containing 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 thin film coating and the transport layer. P101958W001

[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 PEMWE applications, suitable PTLs comprise titanium. For AEMWE 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 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 iridium- containing anode layer (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 catalyst 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 application provides a process for the manufacture of a catalyst-coated membrane for a water electrolyser comprising an iridium-containing anode layer. The process comprises the step of (i) providing a polymer electrolyte membrane with a first major surface and a second opposed major surface. Suitable polymer electrolyte membranes are as described hereinbefore. P101958W001

[0075] The polymer electrolyte membrane may be provided in step (i) without a catalyst layer on the second major surface. In such cases the polymer electrolyte membrane is typically provided on a backing material that is removed later in the manufacturing process, for example prior to the formation of a catalyst cathode layer on the second major surface, for example through direct coating of the cathode catalyst layer onto the second major surface or by decal transfer of a catalyst layer onto the second major surface using an elevated temperature and pressure, such as by hot pressing. Preferably, the polymer electrolyte membrane provided in step (i) has a cathode catalyst layer on the second major surface of the polymer electrolyte membrane, such as a cathode catalyst layer comprising a platinum-on-carbon catalyst. Prior formation of the cathode catalyst layer is advantageous as it reduces the number of manufacturing steps that are required following deposition of the expensive iridium-containing compound, minimising potential waste.

[0076] Preferably, step (i) comprises the following sub-steps: a) providing a cathode catalyst layer on a support substrate; b) forming a polymer electrolyte membrane in contact with the first catalyst layer by depositing one or more layers of polymer electrolyte on the cathode catalyst layer.

[0077] In such cases, the CCM is manufactured through a series of deposition or coating passes, for example using a roll-to-roll manufacturing process, increasing efficiency and minimising the use of backing materials.

[0078] The process comprises the step of (ii), plasma treatment of the first major surface of the polymer electrolyte membrane to form an etched polymer electrolyte surface layer. Plasma treatment may be carried out on the whole of the first major surface of the electrolyte membrane. Preferably, the plasma treatment may be carried or in one or more regions, for example a series of patches which are surrounded by areas of membrane that are not plasma treated. Typically such patches are provided with dimensions which correspond to the active area of the catalyst-coated membrane when incorporated into an electrolysis cell.

[0079] The plasma is preferably a glow discharge. Suitably, the glow discharge is generated by applying a pulse de discharge to an inert gas, such as argon. The duration of plasma treatment may be varied in order to generate an etched polymer electrolyte surface layer of the desired thickness. Glow discharge plasma treatment refers to a non-thermal plasma technique wherein a low-pressure inert gas environment is subjected to a pulsed direct current (DC) or radio frequency (RF) electric field between two electrodes. P101958W001

[0080] The plasma treatment is step (ii) does not include inorganic material deposition, for example does not include cerium oxide deposition. This enables greater control of the surface treatment process, avoids depostion of material that could disrupt contact between the iridium- containing compound and the polymer electrolyte surface, and avoids materials that may migrate during use, for example cerium oxide as set out hereinbefore.

[0081] Preferably, the etched polymer electrolyte surface layer prior to vapour deposition in step (iii) has a thickness in the range of and including 25 to 2000 nm. The thickness of the etched polymer electrolyte surface layer may be determined by scanning electron microscope (SEM) analysis of a cross section of the membrane after plasma treatment. 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. A thickness of the etched polymer electrolyte surface layer of less than 25 nm is typically insufficient to provide the desired layer porosity after vapour deposition of the iridium-containing compound at the desired iridium loadings. A thickness of the etched polymer electrolyte surface layer of greater than 2000 nm can lead to a significant reduction of layer conductivity after vapour deposition of the iridium-containing compound at the desired iridium loadings.

[0082] Preferably, the thickness of the etched polymer electrolyte surface layer prior to vapour deposition in step (iii) has a thickness in the range of and including 50 to 1500 nm, 50 to 1200 nm, 100 to 1000 nm, 150 to 800 nm, or 200 to 600 nm. It is considered that a thickness in the range of 200 to 600 nm provides a particularly suitable thickness in combination with an iridium loading in the range of and including 0.01 to 0.20 mgircm-2, preferably in the range of 0.01 to 0.10 mgircm-2, enabling a suitable balance of layer porosity and conductivity.

[0083] It may be preferred that the thickness of the etched polymer electrolyte surface layer prior to vapour deposition in step (iii) has a thickness in the range of and including 25 to 100 nm. It is considered that a thickness in the range of 25 to 100 nm provides a particularly suitable thickness in combination with an iridium loading in the range of and including 0.01 to 0.10 mgircnr2in circumstances in which layer anode conductivity is a key requirement of the formed catalyst-coated membrane, for example when combined with other, lower conductivity, electrolyser components.

[0084] Preferably, the etched polymer electrolyte surface layer comprises a plurality of elongated polymer electrolyte protrusions as described hereinbefore.

[0085] The process comprises the step of (iii), vapour deposition of an iridium-containing compound onto the etched polymer electrolyte surface layer to form the iridium-containing anode layer. P101958W001

[0086] Vapour deposition involves the deposition of the iridium-containing compound from a vapour phase. Suitably, the vapour deposition is physical vapour deposition, such as a magnetron sputtering process. Such methodology offers suitable control over the structure of the formed layer and are suitable for efficient large-scale production.

[0087] Preferably, the vapour deposition is a reactive magnetron sputtering process. Suitably, in cases in which the iridium-containing compound is an iridium-containing oxide the reactive magnetron process utilises an iridium metal target and oxygen as the reactive gas in order to from an iridium-containing oxide, such as iridium oxide (IrOx) or an iridium metal oxide, such as iridium ruthenium oxide (IrRuOx). Such iridium metal oxides may be formed, for example, using separate metal sputtering targets or from a mixed metal sputtering target.

[0088] The iridium-containing compound is vapour deposited directly onto the etched polymer electrolyte surface layer. It will be understood that this direct deposition provides the iridium- containing compound in contact with the polymer electrolyte without any intermediate surface layer.

[0089] Preferably, step (iii) provides a thin film of the iridium-containing compound coating the etched surface layer, and more preferably step (iii) provides a thin film of the iridium-containing compound partially coating the etched surface layer. Partial coating of the etched surface layer provides a porous structure facilitating transfer of reactants and products required for oxygen evolution. Preferably, the etched polymer electrolyte surface layer comprises a plurality of elongated polymer electrolyte protrusions and the step (iii) provides a thin film of the iridium-containing compound coating the surface of the polymer electrolyte protrusions, and more preferably (iii) provides a thin film of the iridium-containing compound partially coating the surface of the polymer electrolyte protrusions.

[0090] The process may also comprise in step (iii) the vapour deposition of one or more materials concurrently with the deposition of the iridium-containing compound. Such deposition can occur, for example, with the use of a second sputtering target during a reactive sputtering process. Suitably the one or more additional materials are selected from platinum, and oxides of tantalum, niobium, or zirconium.

[0091] Preferably, the process does not comprise the deposition of cerium, or an oxide of cerium. It has been found that cerium ions can migrate during certain electrolyser operating conditions leading to an impact on CCM performance.

[0092] Preferably, the only compound that is vapour deposited in step (iii) of the process is the iridium containing compound. P101958W001

[0093] The process may also comprise the additional step of deposition of platinum onto the iridium- containing anode layer. Suitably, the deposition is vapour deposition, such as physical vapour deposition, such as a magnetron sputtering process. Deposition of platinum onto the iridium- containing anode layer offers an increase in conductivity of the layer and can help to reduce problematic cross over of hydrogen through the catalyst-coated membrane. Suitably, the platinum is deposited such that the amount of platinum is in the range of and including 0.04 to 1 .20 mgptcm-2of the geometric area of the iridium-containing anode layer.

[0094] The degree of iridium loading may be suitably controlled by variation of the duration of the vapour deposition.

[0095] The process of the invention may comprise the additional step of (iv) applying a cathode catalyst layer to the second major surface of the polymer electrolyte membrane. Such cathode catalyst layers comprise a hydrogen evolution reaction catalyst, such as a platinum-based catalyst, for example platinum on a carbon support (Pt / C). Suitably, the cathode catalyst layer comprises a platinum catalyst and an ion-conducting polymer. The cathode catalyst layer may be applied by techniques known to those skilled in the art, for example application via transfer from a catalyst decal or by coating an ink comprising the catalyst and an ion-conducting polymer directly onto the membrane using a technique such as slot-die coating. The cathode catalyst layer may also be applied by other methodology, for example vapour deposition of a platinum-containing compound onto a second surface of the polymer electrolyte membrane.

[0096] The process may comprise the additional step of applying a seal material to a first face and / or a second face of the catalyst-coated membrane.

[0097] The process may comprise the additional step(s) of applying a transport layer to a first face and / or a second face of the catalyst-coated membrane.

[0098] 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.

[0099] Examples

[0100] Test procedures

[0101] Scanning electron microscopy (SEM) analysis

[0102] Cross sections of the CCMs were analysed by SEM. The samples were analysed using a Zeiss Crossbeam 550 focussed ion beam / field emission electron microscope.

[0103] In-plane sheet resistivity measurement P101958W001

[0104] The in-plane sheet resistivity of the catalyst-coated membranes was measured using a Loresta-GX MCP-T700 with a LSP probe from NH instruments. 5 measurements were made over 30 seconds with the average reported.

[0105] Electrochemical testing

[0106] CCMs were prepared with a Pt / C-PFSA ionomer cathode catalyst layer (with a Pt loading of 0.4 mg cm-2of Pt). The electrical performance of the CCMs was tested by the following method. The CCM was first conditioned with water flowing across the anode at 80 °C for 12 hours. Then the polarisation measurement was performed. Anode and cathode pressures were kept equal at atmospheric pressure. The current density was increased from 0 A / cm2to 1 A / cm2in steps of 0.04 A / cm2and then from 1 A / cm2to 4 A / cm2in steps of 0.08 A / cm2. The current density was then decreased from 4 A / cm2to 1 A / cm2in steps of 0.08 A / cm2and then from 1 A / cm2to 0 A / cm2in steps of 0.04 A / cm2. The upward going measurement (low to high current) was used for further analysis.

[0107] Examples

[0108] General methodology

[0109] Iridium oxide thin films were produced by magnetron sputtering from an Ir metal target in low ionised plasma environment and at pulse de regime controlled by Energy plus power supply. The sputtering chamber of 50cm x 50cm dimension was equipped with a magnetron gun pointing to the substrate at an angle and connected to gas lines delivering Ar and O2 to the chamber via mass flow controllers.

[0110] The gas flow rates are measured in standard cubic centimetres per minute and controlled by digital mass flow. The total pressure in the chamber and the partial pressure of both gases depends on the effective pumping power of the turbo pumps and can be regulated by varying the opening of the throttle valve. The iridium oxide loading was varied by adjusting the deposition time and monitored by quartz microbalance using crystal density and Z- factor and further XRF measurement was performed to cross-check the final loading values.

[0111] A proton exchange membrane (PEM) of 80pm thickness (3M PFSA ionomer 800 EW, 2 x ePTFE reinforcement components was used. A section of the membrane was positioned in the vacuum chamber at 16cm to source. A designed mask was positioned on the top of the membrane to expose an area of 5cm x 5cm to coating.

[0112] The process consists of two stages, the membrane is exposed to a plasma discharge bias, in an activated plasma glow discharge of inert Ar gas at 10sccm flow rate and by applying a pulse de discharge of 100W at 150kHz and 2.7ps pulse-space and a working pressure in the region of 2-5E'3mbar. In the second stage, iridium oxide is directly deposited by reactive P101958W001 sputtering using sputtering conditions of 140W pulse de regime of 150khz, 1.5ps, flow rates at 30sccm for inert Ar and reactive O2 gases, a fixed substrate - target distance of 16cm and sputtering pressure 2 - 5E'3mbar, achieved at full pumping power of the turbo, 820HZ and 1000Hz.

[0113] Comparative example 1

[0114] An iridium-containing anode layer was sputter-deposited onto the surface of the PEM at a loading of 0.30 mglr / cm-2according to the general methodology however with the plasma treatment omitted. Figure 2 shows a FIB-SEM cross section of the formed layer which shows a continuous iridium oxide thin film.

[0115] Electrochemical testing indicated that the formed layer did not function as an anode layer in the test cell (the voltage limit exceeded the safety limit meaning that there is a high resistance from the cell).

[0116] Example 1

[0117] A series of samples were sputter-deposited onto the surface of the PEM at a loading of ~ 0.05 mglr / cm-2according to the general methodology with the duration of plasma pre-treatment varied to alter the etched electrolyte surface layer.

[0118] The CCM samples show promising electrochemical performance with a significant improvement in performance over the sample produced in Comparative Example 1 (without plasma etching) despite the significantly lower loading of iridium.

[0119] A FIB-SEM image of a cross section of the CCM formed in Example 1A is shown in Figure 3. This shows a thin film of iridium oxide partially coating an etched polymer electrolyte surface layer. The thickness of the layer is around 60 nm.

[0120] FIB-SEM images of CCMs formed in Example 1 B, 1C and 1 D are shown in Figures 4 to 6 respectively. This shows the surface is in the form of elongated polymer electrolyte protrusions with a partial coating of iridium oxide. Cross-sections of the surface layer indicated a thickness of around 300 - 1000 nm. P101958W001

[0121] X-ray diffraction analysis of an iridium-containing anode layer formed according to a process as described herein shows evidence of semi-amorphous structure, characterised by a broad Bragg peaks matching the reference diffraction pattern of iridium oxide (Figure 7).

[0122] X-ray photoelectron spectroscopy (XPS) analysis of the layer indicates Ir [IV] species as main species at the surface.

[0123] Example 2

[0124] A further sample was prepared in accordance with the general methodology as described hereinbefore. The membrane was etched using glow plasma discharge treatment at 100W for 15 mins, followed by PVD deposition of iridium oxide to yield a sample with an iridium loading of 111 pgirI cm2. Figure 7 shows a cross-section FIB - SEM image of the catalyst- coated polymer electrolyte membrane. For FIB-SEM preparation the samples were coated in a protective layer to support the surface layer structure for side imaging (the membrane is positioned at the bottom of the image of Figure 7, and the coating at the top of the image). The image shows a thin film of iridium oxide with a greater thickness iridium oxide coating at the tips of the elongated polymer electrolyte protrusions than at other points of the thin film coating.

Claims

P101958W001Claims1 . A process for the manufacture of a catalyst-coated polymer electrolyte membrane for a water electrolyser comprising an iridium-containing anode layer, the process comprising the steps of:(i) provision of a polymer electrolyte membrane with a first major surface and a second opposed major surface;(ii) plasma treatment of the first major surface of the polymer electrolyte membrane to form an etched polymer electrolyte surface layer;(iii) vapour deposition of an iridium-containing compound onto the etched polymer electrolyte surface layer to form the iridium-containing anode layer.

2. A process according to claim 1 , wherein the vapour deposition is sputter deposition.

3. A process according to claim 1 or claim 2, wherein the plasma treatment in step (ii) is glow discharge plasma treatment.

4. A process according to claim 3, wherein the glow discharge plasma treatment is carried out in argon.

5. A process according to any one of the preceding claims, wherein the etched surface layer has a thickness in the range of and including 25 to 2000 nm, preferably 200 to 600 nm.

6. A process according to any one of the preceding claims, wherein the iridium- containing compound is deposited as a thin film.

7. A process according to any one of the preceding claims, wherein the iridium- containing compound is iridium oxide or iridium ruthenium oxide.

8. A process according to any one of the preceding claims, wherein the process comprises in step (iii) the vapour deposition of one or more additional transition metals or transition-metal-containing compounds.

9. A process according to claim 8, wherein the additional transition metal is platinum.

10. A process according to any one of the preceding claims, wherein the etched surface layer comprises a plurality of elongated polymer electrolyte protrusions.

11. A process according to any one of the preceding claims, wherein the iridium- containing anode layer has an iridium loading in the range of and including 0.01 to 0.30 mgircm-2.P101958W00112. A process according to any one of the preceding claims, wherein the process comprises the additional step of (iv) applying a cathode catalyst layer to the second major surface of the polymer electrolyte membrane.

13. A process according to any one of claims 1 to 11 , wherein the polymer electrolyte membrane provided in step (i) has a cathode catalyst layer on the second major surface of the polymer electrolyte membrane.

14. A process according to claim 13, wherein step (i) comprises the following sub-steps: a) providing a cathode catalyst layer on a support substrate; b) forming a polymer electrolyte membrane in contact with the first catalyst layer by depositing one or more layers of polymer electrolyte on the cathode catalyst layer.

15. A process according to any one of claims 1 to 14, wherein the method comprises the additional step of applying a seal material to a first face and / or a second face of the catalyst-coated membrane.

16. A process according to any one of claims 1 to 15, wherein the method comprises the additional step of applying a transport layer to a first face and / or a second face of the catalyst-coated membrane.

17. A catalyst-coated polymer electrolyte membrane for a water electrolyser obtainable by a process according to any one of claims 1 to 16, the catalyst-coated membrane comprising:(i) a polymer electrolyte membrane with a first major surface and a second opposed major surface, the first major surface comprising a plurality of elongated polymer electrolyte protrusions;(ii) an iridium-containing anode layer comprising a thin film of an iridium- containing compound coating the surface of the elongated polymer electrolyte protrusions and having an iridium loading of less than 0.30 mgircm-2, preferably in the range of and including 0.01 to 0.30 mgircm-2.

18. A catalyst-coated polymer electrolyte membrane for a water electrolyser according to claim 17, wherein the thin film of an iridium-containing compound is partially coating the surface of the elongated polymer electrolyte protrusions.

19. A catalyst-coated membrane according to claim 17 or claim 18, wherein the iridium- containing anode layer has a thickness in the region of and including 25 to 2000 nm.P101958W00120. A catalyst-coated membrane according to any one of claims 17 to 19, wherein the elongated polymer electrolyte protrusions have an aspect ratio in the range of and including 3:1 to 25 :1.

21. A catalyst-coated membrane according to any one of claims 17 to 20, wherein the polymer electrolyte membrane comprises a woven reinforcement component.

22. A membrane-electrode assembly (MEA) fora water electrolyser comprising a catalyst- coated membrane and a transport layer, wherein the catalyst-coated membrane is according to any one of claims 17 to 21 and the MEA is configured such that the iridium-containing anode layer is positioned between the electrolyte membrane and the transport layer.

23. A water electrolyser comprising a catalyst-coated membrane according to any one of claims 17 to 21 , or an MEA according to claim 22.

Images