Catalyst-coated membranes for water electrolysis

Abstract

A catalyst-coated membrane for use in a water electrolyzer is provided. The catalyst-coated membrane comprises a thin film coating of iridium oxide on a first main surface of an electrolyte membrane. The thin film coating satisfies the following requirements: (i) the thin film coating has a first region adjacent to the electrolyte membrane and a second region distal to the electrolyte membrane; (ii) the porosity of the second region of the thin film coating is higher than the porosity of the first region of the thin film coating; and (iii) the thin film coating has a total porosity in the range of 20 vol% to 60 vol% and including 20 vol% and 60 vol%.

Description

Technical Field

[0001] This invention relates to catalyst-coated membranes and membrane electrode assemblies for water electrolysis. Specifically, this invention relates to catalyst-coated membranes and membrane electrode assemblies for proton exchange membrane (PEM) water electrolysis. Background Technology

[0002] Solid polymer electrolyte membranes (such as proton exchange membranes (PEMs) or anion exchange membranes (AEMs)) can be combined with anolyte and cathode catalyst layers positioned on opposite sides of the membrane for water electrolysis. In some cases, the anolyte and / or cathode catalyst layers are applied to the membrane surface to form a catalyst-coated membrane (CCM). In other cases, the corresponding catalyst layers can be applied to other components (such as transport layers), and the catalyst layers are pressed against the membrane during electrolyzer assembly and subsequent use.

[0003] Hydrogen evolution reaction (HER) catalysts, such as those containing platinum (e.g., platinum on a carbon support), are used in the cathode catalyst layer of such electrolyzers. Oxygen evolution reaction (OER) catalysts are used in the anode catalyst layer of the electrolyzers, where iridium-containing catalysts (e.g., iridium oxide (IrOx)) provide a particularly good balance between OER activity and stability under electrolysis conditions. These catalysts are typically applied to the membrane (or other components) in the form of catalyst ink and, after treatment (e.g., drying), form a wall-mounted layer comprising particulate catalyst and an ionically conductive polymer.

[0004] Because iridium is relatively scarce and expensive, it is desirable to reduce the amount of iridium present in the anode catalyst layer of water electrolyzers, for example, to less than 1.0 mg. Ir cm -2 However, reducing the amount of iridium in the anode layer may lead to problems such as limited conductivity of the catalyst layer and reduced oxygen evolution efficiency of the anode layer.

[0005] There is also a desire to improve the manufacturing efficiency of catalyst-coated membranes and other electrolyzer components, as the demand for hydrogen derived from water electrolysis is rapidly increasing in response to net-zero targets.

[0006] Vapor deposition techniques are known for forming iridium oxide layers for water electrolysis. For example, as described by Slavcheva, E. et al. in “Sputtered iridium oxide films as electrocatalysts for water splitting via PEM electrolysis”, Electrochimica Acta 52 (2007) 3889, sputtered iridium oxide films can be deposited on a titanium sublayer on a Toray paper substrate. SEM images show that the IrO2 layer consists of uniformly densely packed granular particles with a characteristic size of approximately 70 nm.

[0007] As further described by Slavcheva, E., in “Magnetron Sputtered Iridium Oxide an Anode Catalyst for PEM Hydrogen Generation”, Macedonian Journal or Chemistry and Chemical Engineering, Vol. 30, No. 1, pp. 45–54 (2011), iridium oxide films can be deposited onto a titanium substrate on hydrophobic carbon paper by reactive magnetron sputtering. Catalyst layers fabricated using what the authors consider to be optimal sputtering schemes (oxygen flow rates of 8 sccm to 12 sccm) were described as having a uniform microporous structure, and their electrochemical activity was tested by hot-pressing the layers onto a Nafion 117 film.

[0008] Further enhancements and development are still needed for anode layers used in water electrolyzers that address one or more previously identified challenges, particularly at low iridium loadings (<1 mg). Ir cm -2 It provides a highly conductive layer that can be manufactured efficiently. Summary of the Invention

[0009] The inventors have identified that thin-film coatings with hierarchical porous structures offer particular practicality as anode layers for water electrolysis. Such thin-film coatings can be deposited directly onto the electrolyte membrane and provide high lateral conductivity while maintaining OER catalytic activity at low iridium loadings.

[0010] Such thin-film coatings can be advantageously deposited directly onto the surface of an electrolyte membrane to form a catalyst-coated film. In this case, the hierarchical porosity of the thin-film coating provides a region of lower porosity adjacent to the membrane and a region of higher porosity distal to the membrane. This configuration provides high lateral conductivity while also facilitating reagent transfer.

[0011] Therefore, in a first aspect of the invention, a catalyst-coated membrane for a water electrolyzer is provided, the catalyst-coated membrane comprising a thin film coating of iridium oxide on a first main surface of an electrolyte membrane, the thin film coating satisfying the following requirements:

[0012] (i) The thin film coating has a first region adjacent to the electrolyte membrane and a second region distal to the electrolyte membrane;

[0013] (ii) The porosity of the second region of the thin film coating is higher than that of the first region of the thin film coating;

[0014] (iii) The thin film coating has a total porosity in the range of 20% to 60% and including 20% ​​and 60% volume.

[0015] Thin-film coatings can be advantageously incorporated into membrane electrode assemblies. Therefore, in a second aspect of the invention, a membrane electrode assembly (MEA) for a water electrolyzer is provided, the MEA comprising a catalyst-coated membrane and a transport layer, wherein the catalyst-coated membrane is the same as described in the first aspect, and the MEA is configured such that a thin-film coating on the surface of an electrolyte membrane is positioned between the electrolyte membrane and the transport layer.

[0016] In a third aspect of the invention, a water electrolyzer (such as a proton exchange membrane (PEM) water electrolyzer) is provided, the water electrolyzer comprising a catalyst-coated membrane according to the first aspect or a membrane electrode assembly according to the second aspect. Attached Figure Description

[0017] Figure 1A and Figure 1B A SEM image of a cross-section of the CCM formed in Example 1A is shown.

[0018] Figure 2A and Figure 2B An SEM image of a cross-section of the CCM formed in Example 1B is shown.

[0019] Figure 3 An example analysis of the porosity of the first and second regions of the thin film coating of the CCM formed in Example 1A is shown. Detailed Implementation

[0020] Preferred and / or optional features of the invention will now be described. Unless the context otherwise requires, any preferred and / or optional feature of any aspect may be combined, alone or in combination, with any other preferred and / or optional feature of any aspect of the invention.

[0021] This invention provides a thin-film coating comprising iridium oxide for use in water electrolyzers, such as proton exchange membrane (PEM) or anion exchange membrane (AEM) water electrolyzers. The thin-film coating acts as an anode (oxygen evolution) catalyst during water electrolysis by promoting the oxygen evolution reaction.

[0022] The term "thin film" is used with its conventional meaning in the art, which will be understood by those skilled in the art. Suitablely, the thin film coating of the present invention has a thickness of no more than 2000 nm, typically no more than 1750 nm, or preferably no more than 1500 nm, or no more than 1000 nm. The thin film coating typically has a thickness of at least 60 nm, at least 100 nm, or preferably at least 200 nm. Therefore, the thin film coating of the present invention can have a thickness in the range of 60 nm to 2000 nm and includes 60 nm, 2000 nm, preferably 60 nm to 1750 nm, 100 nm to 1000 nm, or 200 nm to 1000 nm. The thickness of the thin film coating can be measured by scanning electron microscopy (SEM). SEM analysis is performed on a cross-section of the structure, and the thickness is measured at multiple (e.g., 10) points. The thickness value is then determined by calculating the arithmetic mean of the measurements.

[0023] Typically, thin-film coatings are provided via vapor deposition, meaning the coating is a vapor-deposited thin-film coating. Vapor deposition techniques (such as physical vapor deposition, for example, magnetron sputtering processes) offer proper control over the structure of the formed coating and are suitable for efficient large-scale production.

[0024] The thin-film coating has a porous structure comprising or substantially composed of iridium oxide. Suitably, the iridium oxide is an iridium oxide material or a mixture of iridium metal oxides, for example, a metal oxide material comprising iridium and metal M, where M = Ta, Nb, Ti, Rh, Ru, or Pt. Such materials may be doped with one or more additional elements or may be undoped. Preferably, the iridium oxide is a metal oxide material comprising iridium and ruthenium. Such materials provide high oxygen evolution reaction (OER) catalytic activity. Preferably, the iridium oxide is a metal oxide material comprising iridium and platinum. Such materials provide high layer conductivity.

[0025] Preferably, the porous structure comprises or is substantially composed of iridium oxide (IrOx) material. Such iridium oxide (IrOx) material can be amorphous, in which case the material may contain a mixture of oxide and hydroxide groups (where both Ir(III) and Ir(IV) substances are present), or it may be crystalline (such as iridium(IV) oxide having a rutile crystal structure). Some Ir(0) may be present in the porous structure, although it is preferable that Ir(0) is absent, for example, if Ir(0) is not observed by X-ray diffraction analysis.

[0026] The thin film coating has a total porosity ranging from 20% to 60% by volume, and including 20% ​​by volume and 60% by volume. Preferably, the thin film coating has a total porosity ranging from 30% to 50% by volume, and including 30% by volume and 50% by volume. The term "total porosity" as defined herein refers to the percentage of the total volume of the thin film coating not occupied by oxide structures. Total porosity can be determined by SEM analysis of a cross-section of the thin film coating. The portions of each image corresponding to porous or solid oxide structures can be classified, and the number of pixels in each target region can be counted and used to calculate the total porosity of the microstructure. Total porosity is measured at multiple (e.g., 10) points.

[0027] The thin film coating advantageously has a graded porosity. The thin film coating has a first region adjacent to the electrolyte membrane and a second region distal to the membrane. The thin film coating may consist of the first and second regions. The thin film coating may have a third region located between the first and second regions.

[0028] The porosity of the second region of the thin film coating is higher than that of the first region. The relative porosity of the first and second regions can be determined by SEM cross-sectional analysis as previously described regarding the total porosity. Providing the first region with lower porosity provides the thin film layer with high lateral electronic conductivity.

[0029] Suitably, the first region has a thickness in the range of 10 nm to 500 nm and including 10 nm and 500 nm, preferably in the range of 50 nm to 300 nm and including 50 nm and 300 nm. Suitably, the second region has a thickness in the range of 50 nm to 1500 nm and including 50 nm and 1500 nm, preferably in the range of 100 nm to 900 nm and including 100 nm and 900 nm.

[0030] Preferably, the second region comprises dendrites of iridium oxide, or has a dendritic structure. Such a structure provides a high surface area and facilitates bubble expulsion while maintaining a degree of flexibility.

[0031] Preferably, the iridium loading is 0.05 mg. Ir cm -2 Up to 0.70mg Ir cm -2 Within the range and including 0.05mg Ir cm -2 0.70mg Ir cm -2 Such as in 0.10mg Ir cm -2 Up to 0.70mg Ir cm -2 Or 0.30mg Ir cm -2 Up to 0.60 mg Ir cm -2 Within the range and containing 0.10mg Ir cm -2 0.70mg Ir cm -2 0.30mg Ir cm -2 0.60mg Ir cm -2 The iridium loading of the thin film coating can be appropriately determined by X-ray fluorescence (XRF) analysis.

[0032] Advantageously, the thin film coating described herein provides high lateral conductivity. Preferably, the thin film coating has a conductivity of less than 200 Ω sq. -1 Less than 150Ω sq -1 Or preferably less than 100Ω sq -1 The thin-film resistance. Thin-film coatings can have a resistance greater than 20 Ω sq. -1 Greater than 25Ω sq -1 Greater than 30Ω sq -1 or greater than 35Ω sq -1 The thin-film resistance is [value missing]. Preferably, the thin-film coating has a resistance of 20 Ω sq [value missing]. -1 Up to 200Ω sq -1 30Ω sq -1 Up to 200Ω sq -1 30Ω sq -1 Up to 100Ω sq -1 or 40Ω sq -1 Up to 100Ω sq -1 Within the range and including 20Ω sq -1 200Ω sq -1 30Ω sq -1 200Ω sq -130Ω sq -1 100Ω sq -1 40Ω sq -1 100Ω sq -1 Thin-layer resistance.

[0033] A thin film coating is applied to the surface of the polymer electrolyte membrane. Suitably, the electrolyte membrane is a proton exchange membrane (PEM) or 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 hydroxyl-containing anion-conducting polymers). Such materials are known to those skilled in the art.

[0034] Suitablely, the electrolyte membrane is a PEM and is formed of an ion-conducting polymer containing sulfonic acid groups. Suitablely, 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 fluorinated or perfluorinated sulfonic acid polymers, such as perfluorosulfonic acid ionomers (e.g., Nafion). ® (Chemours), Aciplex ® (Asahi Kasei), Aquivion ™ (Solvay Specialty Polymers), Flemion ® (Asahi Glass Co.); or ionomers based on sulfonated hydrocarbons, such as those available from FuMA-Tech GmbH (in fumapem) ® Products from the P, E, or K series), and those obtained by Toyobo Corporation, etc. Appropriately, the ion-conducting polymer has an equivalent weight of about 1100 or less, typically about 900 or less, and suitably about 850 or less. Typically, the ion-conducting polymer has an equivalent weight of at least about 450.

[0035] Electrolyte membranes may include additional components such as composite catalysts, radical scavengers, and reinforcing components. Composite catalysts (such as platinum catalysts, e.g., Pt / C or platinum black) catalyze the reaction between hydrogen and oxygen, and thus help reduce hydrogen permeation through the membrane during electrolysis. Radical scavengers (such as cerium oxides (e.g., CeO2)) can help improve membrane durability.

[0036] Suitablely, the reinforcing component is a porous polymer material, such as a microporous mesh or fiber of a polymer material, such as polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ETFE), perfluoroalkyl alkanes (PFA), or fluorinated ethylene propylene (FEP). For example, a planar reinforcing component may comprise electrospun or force-spun PVDF. In a preferred embodiment, the porous polymer material is expanded polytetrafluoroethylene (ePTFE), for example, a microporous mesh ePTFE structure (referred to as Tetratex) provided by Donaldson Company, Inc. ® This can be a microporous mesh ePTFE structure provided by another manufacturer. In other preferred embodiments, the reinforcing component may comprise a network of fibers (e.g., nanofibers), such as a network comprising polybenzimidazole (PBI) fibers, or a woven fabric, such as a woven fabric formed from PTFE yarns. The fiber network may be a nonwoven fiber pad (e.g., nanofibers), such as an electrospun pad of fibers or nanofibers.

[0037] Advantageously, the catalyst-coated membrane exhibits limited swelling in water at elevated temperatures. This limitation contributes to improved durability of the catalyst-coated membrane with the thin-film coating. Such limited swelling can be provided, for example, by incorporating multiple polymer reinforcements into the membrane, or by using polymer reinforcements with high tensile strength, such as woven fabrics.

[0038] Therefore, in some embodiments, the electrolyte membrane has two or more reinforcing components, such as a microporous network of two or more layers of polymers, such as ePTFE. In some other embodiments, the electrolyte membrane comprises reinforcing components in the form of a woven fabric, such as a woven fabric formed from polymer yarns (such as ePTFE or PEEK yarns). Suitable materials are described in US11742507B2 (AGC INC).

[0039] Preferably, the electrolyte membrane has a thickness of 100 μm or less. More preferably, the membrane has a thickness of 95 μm, 90 μm, or 85 μm or less. More preferably, the membrane has a thickness of at least 10 μm, such as at least 15 μm, at least 20 μm, at least 25 μm, at least 30 μm, or at least 40 μm. More preferably, the membrane has a thickness in the range of 10 μm to 100 μm and includes 10 μm and 100 μm, such as 15 μm to 100 μm, 20 μm to 100 μm, 30 μm to 100 μm, 30 μm to 90 μm, or 40 μm to 90 μm. The membrane thickness can be measured by scanning electron microscopy (SEM). SEM analysis is performed on a cross-section of the membrane, and the membrane and / or layer thickness is measured at multiple (e.g., 10) points. The thickness value is then determined by calculating the arithmetic mean of the measurements. Typically, SEM measurements are performed on the cross-section of a catalyst-coated membrane embedded in resin and subjected to grinding and polishing.

[0040] A thin-film coating is applied to the surface of the electrolyte membrane (the first primary surface). Applying the coating directly to the membrane offers numerous advantages over applying it to other components, such as transport layers, including high manufacturing efficiency, low interfacial resistance, the ability to provide a uniform layer thickness in contact with the membrane, and the ability to use a variety of transport layers. Furthermore, applying the thin-film coating directly to the membrane allows a second region of the membrane to be positioned on the distal side of the membrane, thereby facilitating reagent transport to the catalyst surface.

[0041] Those skilled in the art will understand that the thin film coating may be present on the entire first primary surface of the electrolyte membrane, or may be present in one or more patches, which, when incorporated into the electrolyzer, correspond to the effective area of ​​the catalyst-coated membrane.

[0042] Typically, the cathode catalyst layer is disposed on the second main surface of the catalyst-coated membrane. Such cathode catalyst layers contain hydrogen evolution reaction catalysts, such as platinum-based catalysts, for example, platinum on a carbon support (Pt / C). Suitablely, the cathode catalyst layer comprises a platinum catalyst and an ion-conducting polymer.

[0043] A single film layer, typically formed of a nonionic conductive polymer, can be positioned around the edge region of the CCM, for example, on the exposed surface of the ion-conductive membrane where no electrocatalyst is present (but will also often overlap the edge of the electrocatalyst layer), to provide a seal to prevent the escape of reactant and product gases, to reinforce and strengthen the edges of the CCM, and to provide a suitable surface for supporting subsequent components such as sub-gaskets or elastic washers. An adhesive layer can be present on one or both surfaces of the sealing film layer.

[0044] In a water electrolyzer, additional transport layers are positioned on each side of the membrane to facilitate the transfer of reagents and products to and from the catalyst layer, and to provide electrical contact. The catalyst-coated membrane and transport layers together are referred to as a membrane electrode assembly (MEA). These additional transport layers may be referred to as porous transport layers or gas diffusion layers. These layers may or may not be directly attached to the CCM. Other components of the water electrolyzer may include bipolar plates and current collector plates. Stacks of such components constitute an electrolyzer system including power and control systems.

[0045] The MEA of the present invention is configured such that a thin film coating on the surface of the membrane is positioned between the electrolyte membrane and the transport layer, such that: (a) the thin film coating is in direct contact with the transport layer; or (b) the thin film coating is in contact with an intermediate conductive layer positioned between the thin film coating and the transport layer.

[0046] Suitable transport layers on the anode side of the CCM are known to those skilled in the art and are typically formed of metal-based porous structures. Such transport layers must be sufficiently conductive and in a form compatible with the CCM (e.g., without sharp edges or protrusions that would damage the membrane during use). These metal-based porous structures can take the form of, for example, felt or nonwoven fabric, mesh, foam, and sintered compacts containing metal particles. For PEMWE applications, suitable PTLs comprise titanium. For AEMWE applications, suitable PTLs comprise nickel or stainless steel.

[0047] Suitable transport layers on the cathode side of the CCM are known to those skilled in the art and are typically nonwoven paper or webs comprising a network of carbon fibers and thermosetting resin binders (e.g., TGP-H series carbon fiber paper available from Toray Industries Inc., Japan; H2315 series available from Freudenberg FCCT KG GmbH, Germany; or Sigracet available from SGL Technologies GmbH, Germany). ® The series, or AvCarb, may be available from Ballard Power Systems Inc. ® The carbon paper, fiber web, or cloth may be further treated before being incorporated into the MEA to make it more wettable (hydrophilic) or more waterproof (hydrophobic). The nature of any treatment will depend on the type of electrochemical device and the operating conditions to be used.

[0048] The thin film coatings described herein can be appropriately formed via physical vapor deposition. The hierarchical porosity of the film can be achieved using magnetron sputtering by selecting the total chamber pressure, oxygen partial pressure, and pulsed DC power. For example, a controllable oxygen partial pressure between 5 mTorr and 15 mTorr can be achieved by introducing Ar and O2 gases into the sputtering chamber and controlling the pumping speed via a variable-speed vacuum pump or throttle valve. In these cases, iridium oxide microstructures with high-porosity regions stacked on top of lower-porosity regions can be produced. Furthermore, controlling the power density can facilitate the synthesis of films with open-connected porosity. For example, a pulsed DC power of 100 W (power density of 13.1 W / cm²) can be maintained on a 7.62 cm (3.0 inch) diameter cathode under pulsed conditions of 100 kHz frequency and 2 µs reverse period (80% duty cycle). 2 This results in an average cathode voltage in the range of 250V to 400V. Under the aforementioned atmospheric conditions, these power settings can lead to open microstructures with layered porosity.

[0049] The present invention will now be described with reference to the following embodiments. These embodiments are provided to aid in understanding the invention and not to limit its scope.

[0050] Example

[0051] Test program

[0052] Scanning electron microscopy (SEM) analysis

[0053] Cross-sections of the CCM were analyzed by SEM. Samples were analyzed using a Zeiss Crossbeam 550 focused ion beam / field emission electron microscope.

[0054] Compositional analysis and low-resolution general imaging Accelerating voltage: 20kV; Aperture used: 30 to 60 micrometers; Working distance (WD): 7 to 8 mm; Detector: Standard secondary electron and standard backscattered electron detectors.

[0055] High-resolution low-acceleration voltage imaging Accelerating voltage: 1.6kV; Aperture used: 20 to 30 micrometers; Working distance (WD): 2 to 3 mm; Detector: In-lens secondary electron and in-lens backscattered electron detectors.

[0056] Determination of total porosity

[0057] Grayscale color charts were obtained from SEM analysis, where pixel intensity was proportional to the density and atomic number of the scanned sample. In the target region, pixel intensity was used to classify all pixels associated with the membrane and pores of the thin film layer. The total porosity (volume %) of the thin film layer was determined by counting the number of pixels classified as pores and the number of pixels classified as solid IrOx regions. The total porosity was calculated as follows:

[0058] (Number of pixels in the porous region / Total number of pixels in the solid IrOx region + porous region) × 100%

[0059] In-plane thin-layer resistivity measurement

[0060] The in-plane thin-film resistivity of the catalyst-coated film was measured using a Loresta-GX MCP-T700 with an LSP probe from NH Instruments. Five measurements were performed over 30 seconds, and the average value was reported.

[0061] Electrochemical testing

[0062] A Pt / C-PFSA ionomer cathode catalyst layer (Pt loading of 0.4 mg cm⁻¹) was used. -2 CCM was prepared using Pt. The electrical properties of the CCM were tested using the following method: First, the CCM was conditioned by flowing water through the anode at 80°C for 12 hours. Then, polarization measurements were performed. The anode and cathode pressures were kept equal at atmospheric pressure. The current density was 0.04 A / cm². 2 The step size is from 0A / cm 2 Increased to 1A / cm 2 And then at 0.08 A / cm 2 The step size is from 1A / cm 2 Increased to 4A / cm 2 Then, the current density was set at 0.08 A / cm². 2 The step size is from 4A / cm 2 Reduced to 1A / cm 2 And then at 0.04 A / cm 2 The step size is from 1A / cm 2 Reduced to 0A / cm 2 The measurements taken upwards (from low current to high current) are used for further analysis.

[0063] Preparation of Examples

[0064] Example 1 - IrO with thin film x Preparation of catalyst-coated membranes

[0065] Iridium oxide films were deposited onto a sample of polymer electrolyte membrane (800 EW PFSA, 80 μm thick, with two ePTFE reinforcements) using magnetron sputtering (a physical vapor deposition technique). The substrate was placed on a flat substrate holder and masked with a metal sheet having perforated windows that allowed for selective masking of the substrate. The holder was loaded into the sputtering chamber and rotated beneath a confocal magnetron mounted in a downward sputtering configuration. The base pressure of the sputtering chamber was <5 × 10⁻⁷ Torr, achieved using a Pfeiffer HiPace 1200 turbomolecular pump supported by an Edwards nXDS15i dry vortex vacuum pump.

[0066] Prior to film deposition, the substrate was plasma etched in pure argon gas, which flowed into the sputtering chamber at 50 sccm and was maintained at 10 mTorr by a throttle valve located directly in front of the turbomolecular pump. The substrate fixture was plasma etched for 5 minutes using a Seren R601 RF generator running at 100 W.

[0067] To deposit the thin film, Ar and O2 gases are introduced into the sputtering chamber and maintained between 5 and 15 mTorr. The cathode is powered by a pulsed DC at 100 W, 100 kHz, and a reverse cycle of 2.0 μs (80% duty cycle), resulting in a cathode voltage of 250 V to 400 V. The substrate holder is rotated at a distance of approximately 7.62 cm (3.0 inches) below a confocal magnetron. Film thickness is controlled by the deposition time. The substrate holder is maintained at room temperature (approximately 21 °C), which is monitored by thermocouples adjacent to the deposition zone.

[0068] Two catalyst-coated membrane samples were prepared with different iridium loadings, as determined by X-ray fluorescence (XRF) analysis:

[0069] Example 1A: 0.38mg Ir cm -2

[0070] Example 1B: 0.54mg Ir cm -2

[0071] Comparative Example 2 (CEx2) - Preparation of a catalyst-coated membrane with a low-loaded particulate IrOx coating

[0072] A sample of polymer electrolyte membrane (800 EW PFSA, 80 μm thick, with two ePTFE reinforcements) was coated with an ink containing a mixture of particulate IrOx and PFSA ionomer in ethanol:water and dried to form two catalyst-coated membrane samples, wherein the anode layer contained particulate IrOx dispersed in the ionomer, and its iridium loading was determined by X-ray fluorescence (XRF) analysis as follows:

[0073] Comparative Example 2A: 0.34 mg Ir cm -2

[0074] Comparative Example 2B: 0.55 mg Ir cm -2

[0075] result

[0076] Figure 1A and Figure 1B A cross-sectional SEM image of the catalyst-coated film formed in Example 1A is shown, and Figure 2A and Figure 2B SEM images of cross-sections of the catalyst-coated membranes formed in Example 1B are shown. These images show that, in each case, the formed IrOx films are porous with hierarchical porosity. Lower porosity is observed in the film coating region adjacent to the membrane (located at the bottom of the image). Dendritic microstructures are present in the region distal to the membrane.

[0077] Figure 3 An example analysis of the porosity of a first and a second region of the thin film coating of Example 1A is shown. The first region was measured to have a porosity of 25% by volume, and the second region was measured to have a porosity of 35% by volume.

[0078] Table 1 provides a summary of the test results for Examples 1A and 1B and Comparative Examples 2A and 2B.

[0079]

[0080] Data shows that the thin-film IrOx coating provides significantly lower sheet resistance than the comparative example, while maintaining high layer porosity and OER activity.

Claims

1. A catalyst-coated membrane for a water electrolyzer, the catalyst-coated membrane comprising a thin film coating of iridium oxide on a first main surface of an electrolyte membrane, the thin film coating satisfying the following requirements: (i) The thin film coating has a first region adjacent to the electrolyte membrane and a second region distal to the electrolyte membrane; (ii) The porosity of the second region of the thin film coating is higher than the porosity of the first region of the thin film coating; (iii) The thin film coating has a total porosity in the range of 20% to 60% and including 20% ​​and 60% by volume.

2. The catalyst-coated membrane for a water electrolyzer according to claim 1, wherein the thin film coating has a thickness in the range of 60 nm to 2000 nm and including 60 nm and 2000 nm.

3. The catalyst-coated film according to claim 1 or claim 2, wherein the second region comprises dendrites of the iridium oxide.

4. A catalyst-coated membrane according to any one of the preceding claims, wherein the thin film coating has a total porosity in the range of 30 vol% to 50 vol% and including 30 vol% and 50 vol% of total porosity.

5. A catalyst-coated film according to any one of the preceding claims, wherein the film coating has a concentration of 0.05 mg Ir cm -2 Up to 0.70mg Ir cm -2 Within the range and including 0.05mg Ir cm -2 0.70mg Ir cm -2 Iridium load.

6. A catalyst-coated film according to any one of the preceding claims, wherein the thin film coating has a strength of 30 Ω·sq. -1 Up to 100Ω sq -1 Within the range and including 30Ω sq -1 100Ω sq -1 Thin-layer resistance.

7. A catalyst-coated membrane according to any one of the preceding claims, wherein the electrolyte membrane has a thickness of less than or equal to 100 μm.

8. A catalyst-coated membrane according to any one of the preceding claims, wherein the electrolyte membrane comprises two polymer-reinforcing components.

9. A catalyst-coated membrane according to any one of the preceding claims, wherein the electrolyte membrane comprises a polymer reinforcement component in the form of a woven fabric.

10. A catalyst-coated membrane according to any one of the preceding claims, wherein a cathode catalyst layer comprising a hydrogen evolution reaction (HER) catalyst is located on a second main surface of the electrolyte membrane.

11. A membrane electrode assembly (MEA) for a water electrolyzer, the membrane electrode assembly comprising a catalyst-coated membrane and a transport layer, wherein the catalyst-coated membrane is according to any one of claims 1 to 10, and the MEA is configured such that the thin film coating on the surface of the electrolyte membrane is positioned between the electrolyte membrane and the transport layer.

12. A water electrolyzer comprising a catalyst-coated membrane according to any one of claims 1 to 10 or a membrane electrode assembly according to claim 11.

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