Catalyst coated membranes for water electrolysers.

Abstract

A catalyst coated membrane for a water electrolyzer, the catalyst coated membrane comprising: an ionomer membrane having a first surface and a second surface; an anode catalyst layer on the first surface of the ionomer membrane; and a cathode catalyst layer on the second surface of the ionomer membrane, the ionomer membrane including a reinforcing material that is asymmetrically distributed with respect to a mid-plane of the ionomer membrane such that an average position of the reinforcing material is closer to the anode catalyst layer than to the cathode catalyst layer.

Description

[Technical field]

[0001] The present specification relates to catalyst coated proton exchange membranes for green hydrogen producing water electrolysers including components thereof and methods of manufacture. [Background technology]

[0002] During the hydrolysis of water, hydrogen is produced at the cathode of the electrochemical cell and oxygen is produced at the anode. If the electrochemical cell is powered by a renewable electricity supply, the hydrogen produced in this way is known as green hydrogen.

[0003] In fact, there are several different types of electrolyzer configurations that can be used to generate green hydrogen. Electrolysis of water to produce high purity hydrogen and oxygen can be performed with both alkaline and acid electrolyte systems, and actual devices using both types of electrolyte systems exist as commercial products. Acid electrolyte-based electrolyzers typically use solid proton-conducting polymer electrolyte membranes or proton exchange membranes (PEMs) and are known as polymer electrolyte membrane water electrolyzers or proton exchange membrane water electrolyzers (PEMWEs). Catalyst-coated membranes (CCMs) can be used in cells of PEMWEs that contain proton exchange membranes coated on one side with a cathode catalyst for catalyzing the hydrogen evolution reaction and on the other side with an anode catalyst for catalyzing the oxygen evolution reaction. Typically, for PEMWEs, the cathode catalyst material comprises platinum. The anode catalyst typically comprises iridium or iridium oxide (IrOx) materials, or oxides containing both iridium and ruthenium.

[0004] Additional layers are added to both sides of the CCM to create an assembly sometimes called a membrane electrode assembly (MEA). These additional layers can include a porous transport layer (PTL) on the anode side of the CCM and a gas diffusion layer (GDL) on the cathode side. These layers may or may not be directly attached to the CCM. Other components may include bipolar plates and current collector plates. A stack of such assemblies makes up a PEMWE system, including power and control systems. Additional hardware is required to make the stack, including cell frames, seals, and compression plates. Multiple stacks make up a PEMWE system, also including thermal and fluid management, system control, power supply, and hydrogen conditioning systems.

[0005] Precious metals such as Pt and Ir are required for the electrodes, and these metals are used to make electrocatalysts. The chemical and physical form of the metals and any support materials that make up the catalyst can affect the amount of precious metal required and the performance characteristics of the catalyst in water electrolyzer applications. Proton exchange membranes formed from ionomers are coated with a catalyst layer to form a catalyst coated membrane (CCM). The catalyst can be formulated in an ink and deposited on the membrane to form a CCM, or transferred to the membrane from a decal. The ink formulation and / or the morphology of the catalyst layer can affect the functional performance. For example, the catalyst ink may contain ionomers or PTFE (polytetrafluoroethylene) or other fluoropolymers or polymers, which act as binders for the catalyst when the layer is dried. When the catalyst is bound to an ionomer, the ionomer also acts as an ion (proton) conducting medium to move protons towards (cathode) or away from (anode) the active sites on the electrocatalyst surface.

[0006] One problem with existing water electrolyser systems is that iridium is a key component in their production, but limited amounts of iridium are available worldwide and are expensive. Iridium, or more specifically iridium oxide materials (IrOx), are used as the anode catalyst in hydrogen-producing PEM water electrolysers. Iridium oxide materials are particularly advantageous in that they are active enough to catalyze the required oxygen evolution reaction, yet stable enough to withstand the harsh acidic and oxidizing environments encountered within PEM water electrolysers. However, the scarcity of iridium has led to a significant increase in the number of gigawatts (GW) of power per kilogram (kg) of iridium (or conversely, the number of GW per unit of power-kg). Ir There is a need to develop proton exchange membrane water electrolyzer systems that provide reduced amounts of iridium per gigawatt (GW) of water. Thus, improvements are needed in electrolyzer systems, and in particular in the catalyst coated membranes of such systems.

[0007] In an electrolyser, the cathode, also known as the hydrogen electrode, is the electrode where hydrogen is produced. The anode, also known as the oxygen electrode, is the electrode where oxygen is produced. The result of excessive hydrogen crossover is the combination of molecular H2 and molecular O2 at the anode side, a potentially explosive mixture that presents a significant safety hazard due to the wide explosion range of 5-95% H2 in O2. Therefore, for efficient operation and maximum performance of a PEMWE, it is important to keep the electronic and ionic resistance in the CCM as low as possible, but also to minimize hydrogen crossover through the membrane into the oxygen stream. Traditionally, membranes have been 125 microns or thicker due to the need to limit such hydrogen crossover. However, the use of thicker membranes increases the electronic and ionic resistance in the CCM. Hydrogen crossover is exacerbated by the use of thinner membranes in PEMWEs, so it is very typical to use membranes with thicknesses greater than 125 μm, typically approaching or even exceeding 200 μm.

[0008] Thus, improved performance can be obtained by using thinner PEMs in PEMWEs, but in practice this has not been possible due to increased hydrogen crossover and the resulting safety risks. Traditionally, the thickness of the PEM in PEMWEs is 125 μm or more to reduce the level of hydrogen crossover, but the concomitant increase in ionic resistance severely limits the PEMWE performance. Examples of membranes currently in use include Nafion™ N115 (125 μm thick) and Nafion™ N117 (175 μm thick).

[0009] Therefore, there is a need for high performance PEMWEs that limit hydrogen crossover to reduce safety risks in hydrogen producing water electrolyzer applications, and also have lower ionic resistance, e.g., by being of lower thickness or higher conductivity than conventional currently used membranes. By "high performance" we mean that the PEMWE can operate at the highest possible current density and with the highest possible electrical efficiency (i.e., low cell voltage).

[0010] Johnson Matthey has previously filed a patent application (published as WO 2018 / 115821) directed to addressing the aforementioned needs. WO 2018 / 115821 describes the fabrication of a CCM for a PEM water electrolyser by laminating together three membranes: a first membrane coated with a cathode catalyst layer (a hydrogen evolution reaction or HER catalyst, such as platinum black in an ionomer dispersion), a second membrane coated with an anode catalyst layer (an oxygen evolution reaction or OER catalyst, such as IrO2 black in an ionomer dispersion), a third membrane coated with a recombination catalyst (e.g., palladium supported on carbon black in an ionomer solution) to reduce hydrogen crossover, the third membrane being sandwiched between the first and second membranes.

[0011] An advantage of the lamination method described in WO 2018 / 115821 is that very thin membrane components can be utilized, resulting in thin CCMs.

[0012] In the examples disclosed in WO 2018 / 115821, each of the three membranes was a 17 μm thick membrane and contained 900 EW Flemion™ ionomer from Asahi Glass Group with PTFE reinforcement material. A ceria hydrogen peroxide scavenger catalyst was coated on one side of each of the three membranes. A cathode catalyst layer containing Pt black in a dispersion of ionomer was coated on one of the membrane components on the ceria layer, an anode catalyst layer containing IrO2 black in a solution of ionomer was coated on another of the membrane components on the ceria layer, and a recombination catalyst containing Pd supported on carbon black in a solution of ionomer was deposited on the final membrane component on the ceria layer. The three catalyst-coated membrane components were then positioned such that the membrane component with the recombination catalyst layer was in the center and sandwiched between the other two membrane components with the anode and cathode catalyst layers facing outwards. The central membrane component was oriented such that the recombination catalyst layer faced the membrane component carrying the anode catalyst layer. These three layers were then stacked to form the CCM.

[0013] While the lamination method described in WO 2018 / 115821 can produce CCMs with reduced levels of defects compared to traditional lamination methods, the method still produces CCMs with two lamination interfaces between the polymer films and does not completely avoid the problems associated with lamination.

[0014] There remains a need for alternative improved membranes and corresponding CCMs that meet one or more, and preferably all, of the following characteristics: A good balance between strength and high conductivity. low, or at least acceptable, levels of H2 crossover; and It is free, or at least substantially free, of interfacial defects.

[0015] It is also necessary to: Select or develop suitable catalytic materials for use in such CCMs. Improved catalyst ink formulations and deposition methods for producing improved catalyst layers are developed. Provides optimized levels and distribution of other additives such as radical scavengers.

[0016] This specification aims to address these needs. Summary of the Invention

[0017] A catalyst coated membrane (CCM) for a water electrolyser described herein is produced by a process comprising: forming an ionomer membrane having a first surface and a second surface, the ionomer membrane comprising a reinforcing material; forming an anode catalyst layer on the first surface of the ionomer membrane; and forming a cathode catalyst layer on the second surface of the ionomer membrane, the reinforcing material being asymmetrically distributed with respect to the mid-plane of the ionomer membrane such that its average location is closer to the anode catalyst layer than to the cathode catalyst layer.

[0018] Ionomer membranes can be formed by a multilayer process that includes depositing a dispersion of an ionomer in a liquid carrier onto a solid backing layer and at least partially drying to form a first ionomer layer, depositing a dispersion of an ionomer in a liquid carrier onto the first ionomer layer and at least partially drying to form a second ionomer layer on the first ionomer layer, and optionally sequentially depositing and drying one or more additional layers of ionomer on the first and second ionomer layers to build an ionomer membrane structure on the solid backing layer, wherein at least one of the ionomer layers includes a reinforcing material.

[0019] In connection with the above, it has been found that it can be advantageous to distribute the reinforcing material asymmetrically so that the average location of the reinforcing material is closer to the anode catalyst layer than to the cathode catalyst layer. For example, if a single reinforcing layer is provided in the ionomer membrane, it is located closer to the anode catalyst layer than to the cathode catalyst layer. Alternatively, if two reinforcing layers are provided in the ionomer membrane, they are located so that the location of the midpoint between the two reinforcing layers is closer to the anode catalyst layer than to the cathode catalyst layer.

[0020] One potential reason this asymmetric configuration is advantageous is that it has been found to be very important for CCMs in water electrolyzer applications to have a high quality anode catalyst layer (more important than the cathode catalyst layer, although the quality of that layer is also important). Grading the reinforcing material toward the anode side has been found to be advantageous for ensuring that the best quality anode catalyst layer is produced and maintained during operation, which is important for achieving better performance in water electrolyzer applications.

[0021] According to the above method, a catalyst coated membrane for a water electrolyser is provided, the catalyst coated membrane comprising an ionomer membrane having a first surface and a second surface, an anode catalyst layer on the first surface of the ionomer membrane, and a cathode catalyst layer on the second surface of the ionomer membrane, the ionomer membrane including a reinforcing material that is asymmetrically distributed with respect to a mid-plane of the ionomer membrane such that the average location of the reinforcing material is closer to the anode catalyst layer than to the cathode catalyst layer.

[0022] When multiple deposition and drying steps are utilized to build the ionomer membrane structure, the ionomer membrane can be formed with at least 3, 5, or 7 ionomer layers, up to 15, 12, or 10 ionomer layers, or with a number of ionomer layers within the range defined by any combination of the lower and upper limits above. At least one, and optionally at least two, of the ionomer layers may include a reinforcing material in addition to the ionomer. The reinforcing material may include a porous reinforcing polymer layer impregnated with the ionomer, and the reinforcing material is optionally expanded polytetrafluoroethylene (ePTFE).

[0023] In addition to the asymmetry of the reinforcing material distribution within the ionomer membrane as it is angled towards the anode side, another asymmetry arises in the quality of the surface of the ionomer membrane on which the catalyst layer is fabricated. In particular, it has been found that the side of the ionomer membrane formed by the interface with the backing layer (i.e., proximal to the backing layer) provides a better and less defective surface for the subsequent deposition of the anode catalyst layer compared to the side of the membrane that is distal to the backing layer (i.e., the surface formed by the final layer deposited to form the ionomer membrane). As mentioned earlier, it has been found that it is very important for CCMs in water electrolyzer applications to have a high quality anode catalyst layer. Thus, it is the anode catalyst layer, not the cathode catalyst layer, that should be formed on this better surface quality side of the ionomer membrane. This is especially the case when the ionomer membrane is built up to a target thickness via multiple deposition and drying steps, which can result in defects accumulating on the top surface of the ionomer membrane. Furthermore, the inclusion of additives such as reinforcing materials and radical reducing agents in the membrane during fabrication can also contribute to the formation of defects on the top surface of the ionomer membrane. Therefore, when preparing such multi-layer composite ionomer membranes, it is particularly important to use the lower defect exterior surface formed adjacent the backing layer as the substrate surface for the anode catalyst layer.

[0024] Furthermore, yet another advantageous asymmetry in the CCM structure relates to the type of ionomer used in the ionomer membrane, the cathode catalyst layer, and the anode catalyst layer. Advantageously, the ionomer of the anode catalyst layer differs from the ionomer of the membrane in that it has one or more of a higher equivalent weight than the membrane ionomer; a longer side chain than the membrane ionomer; and / or a different chemical group in the side chain compared to the membrane ionomer. In contrast, the ionomer used in the cathode catalyst layer may be the same or similar to that used in the ionomer membrane. Examples of different ionomers for use in different layers are provided later in this specification. However, as a general point, it should be noted that this is another example of the surprisingly advantageous asymmetry in the catalyst coated membrane described herein.

[0025] Most advantageously, the above mentioned asymmetries are combined to provide an optimized CCM for green hydrogen producing water electrolyzers, which has (i) a reinforcing material that is graded towards the anode side, (ii) a better quality interface between the ionomer membrane and the catalyst layer on the anode side, and (iii) a different ionomer in the anode catalyst layer.

[0026] Although certain aspects of the present specification have been summarized above, it should be noted that various aspects of catalyst coated membranes and components thereof are described herein, including aspects of ionomeric membranes including reinforcing layers, aspects of anode and cathode catalyst layers, aspects of catalysts disposed in the catalyst layers, aspects of inks used to manufacture the catalyst layers, and aspects relating to methods of manufacture. This specification is intended to provide a basis for claiming these different aspects and features thereof individually or in any combination. [Brief description of the drawings]

[0027] For a better understanding of the invention and to show how the same may be carried into effect, specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1]1 shows an example of the method steps involved in the manufacture of a proton exchange membrane including two reinforcing layers. [Diagram 2] 1 shows a schematic example of a CCM structure including a proton exchange membrane that includes two reinforcement layers. [Diagram 3] 1 shows a micrograph of a cross section of a proton exchange membrane including two reinforcing layers. [Figure 4] 1 shows performance data for membranes according to embodiments herein versus a commercially available Nafion™ 115 membrane, demonstrating the improvement in performance with increasing current density at a given voltage. [Diagram 5] It is shown that optimization of ionomer concentration can result in improved properties of CCMs versus PEMWE. [Figure 6] 1 shows the improvements in CCM performance achieved through modifications to the cathode catalyst layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] As discussed in the Background section with reference to the Summary section, the present disclosure relates to providing catalyst coated proton exchange membranes and components thereof for water electrolysers.

[0029] Key focus areas for achieving improved CCM performance include: (A) improvements in the anode catalyst layer, e.g., reducing the iridium content while maintaining the functional performance balance activity and stability; (B) improvements in the proton exchange membrane, such as reducing the membrane thickness to increase the proton exchange rate while including additives to retain mechanical stability, mitigate hydrogen crossover, and / or reduce radical concentration to improve chemical stability and lifetime; and (C) improvements in the cathode catalyst layer, such as reducing the platinum content while maintaining functional performance.

[0030] This specification addresses each of these three areas: improvements in the proton exchange membrane and overall CCM structure are described first, followed by improvements in the anode catalyst layer, and then improvements in the cathode catalyst layer.

[0031] Proton exchange membrane One aspect of the present disclosure relates to improvements in proton exchange membrane configurations, including, inter alia, reducing the thickness of the membrane to reduce its proton resistance while retaining mechanical stability, mitigating hydrogen crossover, reducing defects introduced during fabrication (especially at the interfaces between membrane sublayers), and including additives to improve chemical stability and lifetime.

[0032] The membrane in a PEMWE is required to allow protons to move easily between the anode and cathode (low resistance) while stopping gas (especially H2) crossover. Thinner membranes have less ionomer passing through and therefore less resistance to proton transfer, but allow for increased gas crossover. Thus, the membrane thickness is often chosen to balance resistance and crossover, the most common being Nafion™ 115 and 117 from Chemours™, 125 μm and 175 μm thick, respectively.

[0033] In fuel cells using thinner membranes, the reinforcing material has been shown to extend the pot life of the CCM by restraining the ionomer against swelling and preventing pinhole or crack formation. This has been found to carry over to electrolysers. However, N115 and N117 mentioned above do not include the reinforcing material. Cast Nafion™ 212 membranes can include the reinforcing material, but are currently considered too thin (too much hydrogen crossover) for electrolyser operation. There are no commercially available membranes within the 50-125 μm gap that would be optimal for certain PEM water electrolyser applications. Therefore, a membrane within this gap with the reinforcing material would be highly beneficial for electrolyser operation. The ideal membrane would also incorporate H2O2 capture catalyst(s) to protect against radical decomposition and recombination catalyst.

[0034] The catalyst coated membrane configuration according to one embodiment of the present disclosure differs in several ways from that described in WO 2018 / 115821.

[0035] Preferably, the ionomeric membrane component of the catalyst coated membrane is formed from a single coherent non-laminated polymer film and contains one polymer membrane instead of three. The polymer film may be conveniently prepared by sequentially printing, spraying, or coating multiple layers in a fluid deposition process, where the layers solidify during drying. At least one of the deposited layers is provided with a reinforcing polymer.

[0036] The single layer, reinforced polymer membrane has a high degree of stiffness that makes it easier to process to form a catalyst coated membrane, and can be used, for example, as a substrate for depositing both a cathode catalyst layer (on one side) and an anode catalyst layer (on the other side), either directly or via a decal.

[0037] While the prior WO 2018 / 115821 patent application discussed the advantages of processing three separate membranes such that each membrane only needed to be coated with a single catalyst layer, the present specification offers an alternative approach by providing a multi-layer reinforced polymer membrane that is more easily processed for deposition of catalyst layers on both sides thereof while avoiding lamination interfaces within the membrane, which is advantageous for reasons given below.

[0038] Lamination of proton conducting membranes involves pressing and / or bonding at least two solid proton conducting membranes together. A lamination interface is formed between the two membranes, where the solid surfaces of the individual membranes are pressed and / or bonded together. The lamination interface includes physical defects. Furthermore, the structural and / or chemical properties of the lamination interface are also different from that of the bulk polymeric material. This is because when a solid membrane is formed, the outer surface of the solid membrane has surface characteristics that differ from those of the bulk material. For example, a hydrophobic skin forms on the surface of the membrane at the air interface. Raman spectroscopy can detect this difference. Thus, when two solid membranes are pressed together, the lamination interface formed by the two solid surfaces is characteristic in chemical and / or structural morphology compared to the bulk of the proton conducting polymer material. Thus, microscopy and spectroscopic techniques can distinguish between lamination interfaces between layers of proton conducting polymers and interfaces formed by liquid deposition processes such as printing, spraying, or coating layers to build multilayer structures. That is, non-laminated interfaces are structurally and / or chemically distinct from laminated interfaces and are not merely a feature of the manufacturing process. Furthermore, non-laminated interfaces can be identified as non-laminated in the product CCM without prior knowledge of the manufacturing process. Examples of analytical techniques for detecting laminated interfaces include cross-sectional SEM. Changes in crystallinity at the interface can be detected using cross-sectional TEM. Other techniques for detecting laminated interfaces include 13C / 1H / 19F solid-state NMR, neutron diffraction, and / or a combination of two or more of the aforementioned techniques.

[0039] Due to physical defects and / or chemical variations at the lamination interfaces between proton conducting polymer membranes, such interfaces may increase the resistance of the multilayer proton conducting membrane. Therefore, it has been found to be advantageous to fabricate multilayer proton conducting membranes by depositing layers of ionomers dispersed in a liquid solvent to build a multilayer membrane structure, rather than through lamination of individual solid layers / membranes of proton conducting polymers.

[0040] Thus, the membranes herein are suitably made by casting, i.e. depositing multiple layers of proton conducting polymers on top of each other via liquid deposition processes such as printing, spraying or coating. This is not common for membranes used in electrolyzer CCMs, which are typically extruded or laminated together, as described in WO 2018 / 115821. Thus, the coherent non-laminated polymer film is composed of multiple proton conducting polymer layers, at least one of which comprises a porous reinforcing polymer impregnated with an ionomer.

[0041] The individual layers may be formed by preparing a dispersion of the ionomer in a liquid phase solvent and then depositing the dispersion to form a layer of the ionomer. The layer thus formed may be dried or at least partially dried before depositing a further layer of ionomer thereon. Depositing multiple thin layers in this manner is preferred to depositing one or several thicker layers, since it is easier to dry the thin layers and remove the liquid solvent from the layers compared to depositing and drying a thicker layer. If a significant amount of solvent remains trapped in the multilayer structure, the physical, chemical, and electronic properties of the resulting membrane may be compromised. Alternatively, if the membrane structure needs to be dried for an extended period of time to remove the solvent from the thick layer of deposited ionomer dispersion, the production time will increase significantly.

[0042] An example of sequential printing of layers is shown in Figure 1. In a first pass, a proton conducting polymer layer is applied onto a backing layer. The proton conducting polymer layer is then dried. In a second pass, an anode side reinforcing layer is applied onto the conductive polymer layer. The anode side reinforcing layer is then dried. This sequence of application and drying is continued during passes 3, 4, 6 and 7 to produce proton conducting polymer layers. The cathode side reinforcing layer is applied during pass 5.

[0043] The resulting film is referred to herein as "coherent," meaning that there are no internal lamination interfaces. The illustration in Figure 1 shows dashed lines between individual printed layers, but these are for illustrative purposes only and are not discernible in the final product. A schematic cross-section of the film formed after pass 7 is shown in the lower right corner of Figure 1.

[0044] Typically, the individual proton conducting polymer layers which together constitute a single coherent non-laminated polymer film have a thickness of from 6 to 12 μm, for example from 7 to 11 μm.

[0045] Typically, the anode-side and cathode-side reinforcing layers each have a thickness of 14 to 22 μm, for example 16 to 20 μm.

[0046] For the avoidance of doubt, these thicknesses refer to the thickness of the layer formed after drying of the layer and before application of the next layer. The thickness of the wet layer required to obtain a dry layer of the required thickness can be determined empirically. The membrane (composed of the individual proton conducting polymer layers, the anode side reinforcing layer and the cathode side reinforcing layer) may have a total thickness of, for example, 75 to 85 μm.

[0047] The membrane produced as shown in Figure 1 includes an anode side reinforcing layer and a cathode side reinforcing layer. The terms "anode side" and "cathode side" of the membrane should be understood as the sides of the membrane intended to apply the anode catalyst and the cathode catalyst, respectively. The reinforcing material may be formed from expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI), and the proton conducting polymer layer may be formed from a perfluorosulfonic acid (PFSA) polymer.

[0048] Alternatively, the membrane deposition process shown in Figure 1 can be terminated after pass 4, resulting in a thinner membrane product with a single reinforcing layer located closer to the anode side of the membrane. The membrane product thus has the following layer structure: 1. proton conducting polymer layer, 2. reinforcing layer, 3. proton conducting polymer layer, 4. proton conducting polymer layer. In this configuration, the membrane (composed of the individual proton conducting polymer layers and the anode side reinforcing layer) may have a total thickness of, for example, 45-55 μm.

[0049] In connection with the above, it is noted that the "anode side" of the membrane is the side of the membrane adjacent to the backing layer during fabrication. That is, the anode side is formed by the first proton conducting layer deposited on the backing layer during the multilayer fluid deposition process shown in FIG. 1. It has been found that the side of the membrane formed by the interface with the backing layer (i.e., proximal to the backing layer) provides a better surface for the subsequent deposition of the anode catalyst layer compared to the side of the membrane distal to the backing layer (i.e., the surface formed by the final layer deposited to form the membrane). Furthermore, it has been found that it is important for CCMs in water electrolyzer applications to have a high quality anode catalyst layer (the quality of that layer is also important, but more important than the cathode catalyst layer). Thus, it is the anode catalyst layer, rather than the cathode catalyst layer, that is preferably formed on this better surface quality side of the membrane.

[0050] Another point to note is that the reinforcing layers in the above examples are not located on the outer surface of the membrane, but rather centrally and / or symmetrically with respect to the central plane of the membrane. For example, in the above four coating pass example incorporating a single reinforcing layer, that reinforcing layer (deposited in the second coating pass) is located closer to the anode side of the membrane than to the cathode side of the membrane. In the seven coating pass example discussed above (and shown in FIG. 1 ) incorporating two reinforcing layers, the first reinforcing layer (deposited in the second coating pass) is located closer to the anode side of the membrane than to the central plane of the membrane, and the second reinforcing layer (deposited in the fifth coating pass) is located closer to the central plane of the membrane than to the cathode side of the membrane. In both cases, the reinforcing material is angled or shifted toward the anode side of the membrane compared to a symmetric arrangement with respect to the central plane of the membrane. Counterintuitively, this asymmetry may be advantageous for CCMs in water electrolyzer applications, possibly due to the asymmetry in the operating environment of CCMs in water electrolyzer applications. Membranes produced using this approach have been found to have beneficial mechanical and creep properties when compared to other commercially available electrolyzer membranes such as Nafion™ 115. The membranes also behave asymmetrically in the machine vs. cross direction, with the reinforcing material reducing swelling of the membrane during use, particularly in the x,y plane of the membrane. In this regard, when the membrane is produced using a roll-to-roll process, there are two directions: the machine direction, which is the longitudinal direction corresponding to the direction of movement of the printed material, and the cross direction, which is the left-right direction across the printed membrane strip. It has been found that there are several asymmetric properties related to the manufacturing process that have been observed in mechanical properties such as creep. One source of asymmetry results from the ePTFE being stronger in one direction than the other, which is not seen in non-reinforced membranes such as N115.

[0051] It is also important to note that the reinforcing layer(s) must be conductive to protons. Since typical reinforcing polymer materials are not conductive to protons or are not sufficiently conductive to protons, the reinforcing layers are formed using a porous reinforcing polymer impregnated with an ionomer through the pores of the material to provide a proton conducting path from one side of the layer to the other side of the layer. The porosity, thickness, strength of the reinforcing polymer, and the method used to ensure good impregnation of the ionomer into the pores are important to achieve the desired combination of mechanical and conductive requirements of the reinforcing layer, as well as drying characteristics to ensure that the solvent is removed during manufacture. For example, advantageously, each reinforcing layer has a thickness of 1.5 to 2.5 times the thickness of the non-reinforced proton conducting polymer layer. Furthermore, each reinforcing layer contains 7 to 15 weight percent of the porous polymer reinforcing material based on the total weight of the ionomer and the porous polymer reinforcing material in the reinforcing layer. For example, the reinforcing layer may be 4.7 g / m 2 of reinforcing material (e.g., ePTFE) and 36 g / m 2The reinforcing material therefore contributes 100 x 4.7 / (4.7+36) = 11.5 wt.%. In this regard, it should be noted that although the reinforcing layer is about twice as thick as the other non-reinforced proton conducting polymer layers, a significant amount of the layer is taken up by the reinforcing polymer. The ionomer dispersion loading of the layer therefore remains similar to that of the other non-reinforced layers. By balancing the thickness and loading of the reinforcing material and ionomer of the reinforcing layer in this way, a good combination of mechanical strength, electrical conductivity, drying and layer forming properties is obtained. The larger thickness helps to accommodate the reinforcing material. The important feature from a technical point of view is to obtain a good impregnation of the ionomer into the porous reinforcing material. Otherwise there would be holes, cavities or voids introducing places where there is no ionomer and therefore no conduction. One way to achieve this, in addition to choosing the appropriate materials and weight ratios for the ionomer and reinforcing material, is to deposit a wet layer of ionomer dispersion of sufficient thickness and then apply tension to the porous reinforcing material as it is lowered into the wet layer of ionomer dispersion. This layer is then dried to obtain the reinforcing layer. An example of a suitable reinforcing polymer is expanded polytetrafluoroethylene (ePTFE).

[0052] The nature of the porous reinforcing polymer, its location in the membrane, and the method of depositing and impregnating the ionomer are all important to obtain improved mechanical properties of the membrane while also achieving reduced proton resistance.

[0053] In addition to the reinforcing polymer, the ionomer itself provides the membrane with structural / mechanical integrity as well as conductive properties, and it is important to select a suitable ionomer and deposition method to achieve the desired combination of electrical conductivity and mechanical integrity. The ionomer may be a perfluorosulfonic acid ("PFSA") and may have an equivalent weight selected from greater than 750 EW, greater than 770 EW, or greater than 790 EW, less than 850 EW, 830 EW, or 810 EW, or within a range defined by any combination of the lower and upper limits set forth above.

[0054] An ionomer dispersion is formed that includes the ionomer dispersed in the solvent. The solvent may be a mixture of an organic solvent and water. For example, the solvent may be a mixture of an alcohol (e.g., ethanol or propanol) and water. The volume ratio of organic solvent to water may have an equivalent weight that is at least 60:40, 70:30, or 75:25, 95:5, 90:10, or up to 85:15, or within a range defined by any combination of the lower and upper limits set forth above. The solvent is formulated to achieve the desired dispersion, coating, and drying characteristics.

[0055] The ionomer may have an equivalent weight of at least 7%, 10%, 14%, or 16%, up to 22%, 20%, or 18% by weight; or within a range defined by any combination of the foregoing lower and upper limits. The ionomer content is selected to achieve the desired dispersion, coating, and drying characteristics.

[0056] The ionomer dispersion may also include a radical reducing additive (e.g., a peroxide radical reducing additive such as ceria). For example, the radical reducing additive may have an equivalent weight of at least 0.15 wt%, 0.20 wt%, or 0.23 wt%, or no more than 0.35 wt%, 0.30 wt%, or 0.28 wt%, based on the weight of the ionomer; or within a range defined by any combination of the lower and upper limits set forth above. The radical reducing agent may be dispersed in one or more of the proton conducting polymer layer and / or the reinforcing layer. In certain instances, the same ionomer dispersion composition is used for all layers. It is noted that peroxides can decompose to form a range of radicals (O, OH, OOH), and the radical reducing additive can reduce the amount of one, more, or all of these radicals.

[0057] catalyst coated membrane Catalyst coated membranes are prepared by applying an anode catalyst layer to the anode side of the membrane and a cathode catalyst layer to the cathode side of the membrane. An example of a catalyst coated membrane is shown in FIG. 2. The CCM configuration shown in FIG. 2 includes a membrane structure formed using the method of FIG. 1. In the figure, membrane 1 has an anode catalyst layer 2 on one side thereof and a cathode catalyst layer 3 on the opposite side thereof. The dashed lines indicate the ionomer layers deposited as shown in FIG. 1. Layers 4 and 5 are reinforcing layers that include reinforcing polymers 4a and 5a, respectively. As shown in FIG. 2, the reinforcing polymer has a thickness less than the thickness of the reinforcing layer in which it is disposed. That is, the ionomer in the reinforcing layer impregnates and surrounds the reinforcing polymer material.

[0058] As mentioned previously, the anode catalyst layer is applied to the side of the membrane that was adjacent (proximal) to the backing layer during the membrane deposition process. This is a better quality surface on which to fabricate the anode catalyst layer, and it has been found that good formation of the anode catalyst layer is important for water electrolyzer applications. Furthermore, the membrane is also oriented such that the reinforcing material is offset towards the anode side compared to a symmetrical arrangement about the central plane of the membrane.

[0059] The specific types of catalysts for the cathode and anode can vary. A cathode catalyst layer for catalyzing the hydrogen evolution reaction can include platinum, and / or an anode catalyst layer for catalyzing the oxygen evolution reaction can include iridium oxide or a mixed oxide of iridium and another metal(s). Various methods can be used to apply the anode and cathode catalyst layers. One method involves applying the catalyst layer as a decal. A typical procedure for providing a decal involves making an ink that includes a catalyst, an ionomer, an organic solvent, and water. The ink can be applied and bar coated onto a sheet of Teflon and dried to form a decal. The catalyst decal can be hot pressed with a membrane to form a CCM. Alternatively, the catalyst ink can be coated directly onto the membrane.

[0060] Low defect surface for anode catalyst layer One feature of a particular example according to the present specification focuses on the discovery that the surface of the membrane formed in close proximity to the backing layer on which the membrane is fabricated is more suitable for supporting the anode catalyst layer of a water electrolyzer CCM. As previously mentioned in the previous section, this surface has fewer defects than the surface of the membrane that is distal to the backing layer on which it is fabricated. It has been found that it is preferable to use the lower defect surface in a water electrolyzer CCM to fabricate the anode catalyst layer thereon. Thus, a catalyst coated membrane for a water electrolyzer is provided, the catalyst coated membrane comprising an ionomer membrane having a first surface and a second surface, an anode catalyst layer on the first surface of the ionomer membrane, and a cathode catalyst layer on the second surface of the ionomer membrane, the first surface of the ionomer membrane having fewer defects than the second surface of the ionomer membrane.

[0061] By "fewer defects" it is meant that the first surface can have one or more of a higher surface flatness than the second surface, a lower surface roughness than the second surface, a lower level of particulates and / or chemical contamination than the second surface, and / or a fewer number of cavities or voids than the second surface.

[0062] The ionomer membrane of the catalyst coated membrane may have a multilayer structure formed by at least 3, 5, or 7 ionomer layers; up to 15, 12, or 10 ionomer layers, or several ionomer layers within the range defined by any combination of the lower and upper limits mentioned above. At least one, and optionally at least two, of the ionomer layers may include a reinforcing material in addition to the ionomer. The reinforcing material may include a porous polymer layer impregnated with the ionomer, and the reinforcing material is optionally expanded polytetrafluoroethylene (ePTFE). Furthermore, as indicated above, the reinforcing material may be distributed asymmetrically with respect to the mid-plane of the ionomer membrane such that its average location is closer to the anode catalyst layer than to the cathode catalyst layer. Furthermore, the ionomer membrane may also include a radical reduction additive, optionally ceria.

[0063] The anode catalyst layer may comprise an iridium-containing, optionally iridium oxide-containing, catalyst material, and the cathode catalyst layer may comprise a platinum-on-carbon catalyst material. One or both of the anode and cathode catalyst layers may comprise both a catalytic material and an ionomeric material, with the catalytic material being dispersed in the ionomeric material, or vice versa. The ionomer in one or both of the anode and cathode catalyst layers may be different from the ionomer in the ionomeric membrane.

[0064] Also provided is a method of manufacturing a catalyst coated membrane for a water electrolyser as described above, comprising: depositing a dispersion of an ionomer in a liquid carrier onto a solid backing layer; drying the deposited dispersion to remove the liquid carrier and form an ionomer membrane disposed on the solid backing layer, the ionomer membrane having a first surface proximate to the solid backing layer and a second surface distal to the solid backing layer; forming an anode catalyst layer on the first surface of the ionomer membrane after removing the ionomer membrane from the solid backing layer; and forming a cathode catalyst layer on the second surface of the ionomer membrane, the cathode catalyst layer being formed on the second surface of the ionomer membrane either before or after removing the ionomer membrane from the solid backing layer.

[0065] The ionomer membrane can be formed on the solid backing layer by multiple deposition and drying steps, including depositing a dispersion of an ionomer in a liquid carrier on the solid backing layer and at least partially drying to form a first ionomer layer, depositing a dispersion of an ionomer in a liquid carrier on the first ionomer layer and at least partially drying to form a second ionomer layer on the first ionomer layer, and optionally sequentially depositing one or more additional layers of ionomer on the first and second ionomer layers and drying to build an ionomer membrane structure on the solid backing layer. The anode catalyst layer can be transferred onto the first surface of the ionomer membrane from a decal. Alternatively, the anode catalyst layer can be directly fabricated on the first surface of the ionomer membrane by a liquid deposition process.

[0066] Asymmetric distribution of reinforcing material Another feature of certain examples herein focuses on the discovery that providing an asymmetric distribution of reinforcing material within an ionomer membrane can be advantageous for water electrolyzer CCM performance. Accordingly, a catalyst coated membrane for a water electrolyzer is provided, the catalyst coated membrane comprising an ionomer membrane having a first surface and a second surface, an anode catalyst layer on the first surface of the ionomer membrane, and a cathode catalyst layer on the second surface of the ionomer membrane, the ionomer membrane including a reinforcing material, the reinforcing material being asymmetrically distributed with respect to a mid-plane of the ionomer membrane such that its average location is closer to the anode catalyst layer than to the cathode catalyst layer.

[0067] Again, the ionomer membrane may have a multilayer structure formed by at least 3, 5, or 7 ionomer layers, up to 15, 12, or 10 ionomer layers, or several ionomer layers within the range defined by any combination of the above lower and upper limits. At least one, and optionally at least two, of the ionomer layers contain a reinforcing material in addition to the ionomer. According to a particular example, only one of the ionomer layers contains a reinforcing material, said layer being closer to the anode catalyst layer than to the cathode catalyst layer. Alternatively, according to another example, two of the ionomer layers contain a reinforcing material, and the midpoint between the two ionomer layers is closer to the anode catalyst layer than to the cathode catalyst layer.

[0068] Other features of these examples are as described above. Such catalyst coated membranes are made by a process that includes forming an ionomer membrane having a first surface and a second surface, where the ionomer membrane includes a reinforcing material, forming an anode catalyst layer on the first surface of the ionomer membrane, and forming a cathode catalyst layer on the second surface of the ionomer membrane, where the reinforcing material is asymmetrically distributed with respect to the midplane of the ionomer membrane such that its average location is closer to the anode catalyst layer than to the cathode catalyst layer. As previously described, the ionomer membrane may be formed by depositing a dispersion of an ionomer in a liquid carrier onto a solid backing layer and at least partially drying to form a first ionomer layer, depositing a dispersion of an ionomer in a liquid carrier onto the first ionomer layer and at least partially drying to form a second ionomer layer on the first ionomer layer, and optionally sequentially depositing and drying one or more additional layers of ionomer onto the first and second ionomer layers to build an ionomer membrane structure on the solid backing layer, where at least one of the ionomer layers includes a reinforcing material. Advantageously, the anode catalyst layer is formed on the outer surface of the first ionomer layer after removing the ionomer membrane from the solid backing layer.

[0069] Reinforcement layer composition Another feature of certain examples according to the present specification focuses on the composition and structure of the reinforcing layer in the CCM. Thus, a catalyst coated membrane for a water electrolyzer is provided, the catalyst coated membrane comprising an ionomer membrane having a first surface and a second surface, an anode catalyst layer on the first surface of the ionomer membrane, and a cathode catalyst layer on the second surface of the ionomer membrane, the ionomer membrane comprising at least one reinforcing layer, the reinforcing layer comprising a porous polymer reinforcing material (e.g., expanded polytetrafluoroethylene-ePTFE) impregnated with an ionomer, or each reinforcing layer comprising 5-20 wt%, optionally 7-15 wt%, of a porous polymer reinforcing material based on the total weight of the ionomer and the porous polymer reinforcing material in the reinforcing layer. Each reinforcing layer may have a thickness in the range of 12-24 μm.

[0070] In addition to at least one reinforcing layer, the ionomeric membrane may include at least one non-reinforced ionomeric layer. Each non-reinforced ionomeric layer may have a thickness in the range of 6 to 12 μm. Advantageously, each reinforcing layer has a thickness 1.5 to 2.5 times the thickness of each non-reinforced ionomeric layer.

[0071] As described in the previous examples, the ionomer membrane is a multilayer structure formed by at least 3, 5, or 7 ionomer layers, up to 15, 12, or 10 ionomer layers, or several ionomer layers within the range defined by any combination of the above lower and upper limits, where at least one, and optionally at least two, ionomer layers contain a porous polymeric reinforcing material in addition to the ionomer. If only one of the ionomer layers contains a reinforcing material, said layer can be located closer to the anode catalyst layer than the cathode catalyst layer. Alternatively, if two of the ionomer layers contain a reinforcing material, they can be positioned such that the midpoint between the two ionomer layers is closer to the anode catalyst layer than the cathode catalyst layer.

[0072] Other features of these examples are as described above. The catalyst coated membranes are produced by a process comprising forming an ionomer membrane having a first surface and a second surface, forming an anode catalyst layer on the first surface of the ionomer membrane, and forming a cathode catalyst layer on the second surface of the ionomer membrane, wherein the ionomer membrane comprises at least one reinforcing layer, the reinforcing layer comprising a porous polymer reinforcing material impregnated with an ionomer, and the or each reinforcing layer comprises 5 to 20 weight percent porous polymer reinforcing material, based on the total weight of the ionomer and the porous polymer reinforcing material in the reinforcing layer. Again, the ionomer membrane can be formed by depositing a dispersion of an ionomer in a liquid carrier onto a solid backing layer and at least partially drying to form a first ionomer layer, depositing a dispersion of an ionomer in a liquid carrier onto the first ionomer layer and at least partially drying to form a second ionomer layer on the first ionomer layer, and optionally sequentially depositing and drying one or more additional layers of ionomer on the first and second ionomer layers to build an ionomer membrane structure on the solid backing layer, wherein at least one of the ionomer layers includes a reinforcing material.

[0073] The above examples have focused primarily on the characteristics of the ionomer membrane within the CCM structure, with details of the anode and cathode catalyst layers being provided in the following sections.

[0074] Anode catalyst layer A suitable catalytic material for the anode layer is electrochemical grade iridium oxide from J&J Materials Inc. This electrochemical grade iridium oxide from J&J Materials has been shown to provide good performance in water electrolyzer applications.

[0075] In addition to the iridium oxide catalytic material used in the anode layer, there are also several important aspects of how to fabricate the anode layer using such catalytic material to improve performance. Important aspects of the anode layer include: -Formulation of ink components, their amounts, choice of solvent, choice of ionomer (different from the ionomer used for the membrane). - Gradual dilution during the ink manufacturing process; - drying conditions to obtain a suitable layer structure; and -Distribution of iridium oxide catalyst across / through the final layer.

[0076] The ink can be deposited on a carrier substrate (e.g., a PTFE sheet) to form a decal and then transferred (e.g., by hot pressing) to the membrane. Alternatively, the ink can be coated directly onto the membrane. If deposited, selection of appropriate drying conditions is important to obtain the proper layer structure, including the proper distribution of the iridium oxide catalyst throughout the layer.

[0077] It has been found that optimizing the ionomer concentration and drying conditions results in significant improvement of CCM performance in water electrolyzer applications. Furthermore, it has been found that utilizing an ionomer in the anode catalyst layer that is different from that used in the ionomer membrane results in improved performance. Thus, a catalyst coated membrane for a water electrolyzer is provided, the catalyst coated membrane comprising an ionomer membrane having a first surface and a second surface, an anode catalyst layer on the first surface of the ionomer membrane, and a cathode catalyst layer on the second surface of the ionomer membrane, the anode catalyst layer comprising an iridium-containing catalytic material disposed in an ionomer, and the ionomer in the anode catalyst layer is different from the ionomer in the ionomer membrane.

[0078] The ionomer in the anode catalyst layer may preferably have an equivalent weight of 900 EW or more, 950 EW or more, 1000 EW or more, or 1050 EW or more, 1300 EW, 1200 EW, or 1150 EW or less; or within a range defined by any combination of the above lower and upper limits. The side chains of the ionomer each include a sulfonate group. Optionally, the side chains of the ionomer include an ether group in addition to the ether linkage to the backbone. Further, optionally, the side chains of the ionomer include a CF3 group. The side chains of the ionomer may have the structure: -CF2-CF(CF3)-O-CF2-CF2-SO3H. An example of such an ionomer is Nafion D-2021CS.

[0079] The ionomer of the anode catalyst layer differs from the ionomer of the membrane in that it has one or more of a higher equivalent weight than the membrane ionomer, longer side chains than the membrane ionomer, and / or different chemical groups in the side chains (e.g., ether groups and / or CF3 groups) compared to the membrane ionomer. In contrast, the ionomer in the membrane has an equivalent weight of 880 EW or less, 850 EW or less, or 830 EW or less, 750 EW, 770 EW, or 790 EW or more; or within a range defined by any combination of the above upper and lower limits. The side chains of the membrane ionomer also each contain a sulfonate group. However, the side chains are shorter than the side chains of the anode ionomer. The side chains of the membrane ionomer do not contain ether groups (except for the ether linkage to the backbone) and / or they do not contain CF3 groups. The side chains of the membrane ionomer may have the structure: -CF2-CF2-CF2-CF2-SO3H. An example of such an ionomer is 3M 825 ionomer.

[0080] Thus, according to one example, the ionomer in the anode catalyst layer is Nafion D-2021CS ionomer. Nafion D-2021CS is a high equivalent weight ionomer with long side chains with sulfonate end groups. In the presence of water, these sulfonic acid groups hydrate, solvate, and dissociate into protons, which allow the exchange of protons from the anode to the cathode. In contrast, the ionomer in the ionomer membrane can be 3M 800, 3M 825, or Asahi 800 ionomer. These ionomers have lower equivalent weights and shorter side chains.

[0081] The anode catalyst layer may comprise 5 to 20 wt%, for example 8 to 15 wt%, of an ionomer. The amount of catalyst material in the anode catalyst layer may be 80 to 95 wt%, optionally 85 to 92 wt%. The iridium loading of the anode catalyst layer is preferably 3 mg Ir / cm. 2 and optionally between 0.05 and 3 mg Ir / cm 2 The iridium-containing catalyst material can be an iridium oxide catalyst material, and the anode catalyst layer can have a thickness of 6 to 15 micrometers.

[0082] In addition to the above, the ionomer in the anode catalyst layer may be different from the ionomer in the cathode catalyst layer. For example, the cathode catalyst layer may include a platinum-containing catalytic material disposed in an ionomer, and the ionomer in the cathode catalyst layer may be different from the ionomer in the anode catalyst layer. The ionomer in the cathode layer may be the same or similar to the ionomer in the membrane.

[0083] In these examples, the CCM is produced by a method that includes forming an ionomer membrane having a first surface and a second surface, forming an anode catalyst layer on the first surface of the ionomer membrane, and forming a cathode catalyst layer on the second surface of the ionomer membrane, where the anode catalyst layer is produced by formulating an ink that includes a dispersion of an iridium-containing catalyst material and an ionomer in a liquid carrier, depositing the ink, and drying, where the ionomer in the anode catalyst layer is different from the ionomer in the ionomer membrane. The ink can be deposited directly on the ionomer membrane and dried to form the anode catalyst layer. Alternatively, the ink can be deposited on a carrier layer and dried to form a decal that includes the anode catalyst layer, and then transferred from the decal to the ionomer membrane.

[0084] The ink is advantageously prepared by mixing an iridium-containing catalyst material with water to form a slurry, and then mixing the slurry with an organic solvent and an ionomer. The ink may be formulated to have a solids content of 35-55% by weight. Furthermore, after deposition, the ink is dried at a temperature above the glass transition temperature of the ionomer in the anode catalyst layer to evaporate the solvent from the layer and promote flow of the ionomer.

[0085] In one example, 2 mg Ir / cm 2The anode catalyst layer can be produced using an ink formulation that includes IrO2 catalyst from J&J Materials and 12% high EW ionomer with long side chains (e.g., Nafion D-2021CS ionomer). The wet ink includes catalyst, ionomer, water, and 1-propanol. Water and propanol are added to adjust the solids content of the ink to be produced. The first step involves wetting the catalyst with the required amount of water. This slurry is mixed thoroughly to ensure that the catalyst is completely wetted. This is a critical step as the solvent is flammable, therefore, to prevent it from catching fire, the catalyst is pre-wetted before mixing with the ionomer / solvent. The next step involves the addition of the required amount of ionomer and propanol. After this addition, the slurry is mixed and then diluted to the target solids content for deposition. In this regard, it is advantageous to perform the final dilution just before the ink is to be used in the coating process. For example, after formulating an ink including a dispersion of an iridium-containing catalyst material and an ionomer in a liquid carrier, the ink is diluted to a target solids content within 24 hours, 12 hours, 8 hours, 4 hours, 2 hours, 1 hour, or 30 minutes prior to depositing the ink.

[0086] Once the ink is diluted to an appropriate solids content for deposition, it can be printed onto the PTFE and dried at a suitable temperature to evaporate the solvent and stabilize the ionomer in the layer.

[0087] The anode catalyst layer on PTFE can be transferred to an ionomer membrane to form a CCM by hot pressing. SEM images of the final CCM show that the resulting anode catalyst layer is 9-11.5 micrometers after hot pressing.

[0088] FIG. 5 shows an example of how modifications to the ionomer in the catalyst layer and to the method of making the catalyst layer can result in improved performance in terms of current density at a given operating potential.

[0089] Cathode catalyst layer The cathode electrode layer can be based on a particular type of platinum-on-carbon catalyst selected for its beneficial properties in PEMWE applications. The platinum-on-carbon catalyst material can be formulated into an ink, printed ex-situ onto a PTFE sheet, and transferred onto the membrane by hot pressing. Alternatively, the ink can be coated directly onto the membrane. The catalyst layer contains both a catalyst and an ionomer.

[0090] Provided herein is an improved cathode catalyst layer for use in a catalyst coated membrane for a water electrolyser. The catalyst coated membrane includes an ionomer membrane having a first surface and a second surface, an anode catalyst layer on the first surface of the ionomer membrane, and a cathode catalyst layer on the second surface of the ionomer membrane, the cathode catalyst layer including a platinum-on-carbon catalyst material disposed in an ionomer, the cathode catalyst layer having a 1 mg / kg electrode thickness provided by the platinum-on-carbon catalyst material. Pt cm -2 and a cathode catalyst layer having a platinum loading of less than 1000 nm.

[0091] Surprisingly, it has been found that reducing the platinum loading actually improves performance in terms of current density, especially when using platinum-on-carbon catalyst materials at certain ionomer / carbon weight ratios. That is, reducing the platinum loading using platinum-on-carbon catalyst materials surprisingly increases the current density for a given potential. This is shown in Figure 6, which shows that by optimizing the cathode catalyst layer, high performance can be achieved at low platinum loadings. That said, there is also a lower limit to the amount of platinum that must be provided. Thus, the platinum loading of the cathode layer can be as low as 1 mg. Pt cm -2 , 0.8 mg Pt cm -2 , 0.6 mg Pt cm -2 , 0.5 mg Pt cm -2、 0.4 mg Pt cm -2 , 0.3 mg Pt cm -2 , 0.2 mg Pt cm -2 , or 0.1 mgPt cm -2 Less than 0.01 mg Pt cm -2 Over 0.04 mg Pt cm -2 More than or equal to 0.06 mg Pt cm -2 More than or equal to 0.5 mg Pt Less than 0.01 mg Pt cm -2 More than or equal to 0.2 mg Pt Less than 0.01 mg Pt cm -2 Within a range defined by any of the preceding upper and lower limits, such as greater than or equal to 100%.

[0092] The platinum-on-carbon catalyst material may comprise 20-60% platinum by weight, optionally 40-60% platinum by weight. The platinum is provided as nanoparticles on the carbon support material. The platinum nanoparticles may have an equivalent weight that is at least 1 nm, 2 nm, or 3 nm, no more than 15 nm, 10 nm, or 6 nm, or within a range defined by any combination of the lower and upper limits set forth above. The crystallite size can be measured by XRD and fitted using Rietveld analysis. X-ray diffraction data is collected on a Bruker AXS D8 using Cu Kα radiation (λ=1.5406 and 1.54439 Å). The crystallite size is calculated from the Rietveld refinement using the LVol-IB method.

[0093] The platinum-on-carbon catalyst material is at least 50m 2 g -1 , 55m 2 g -1 , or 60m 2 g -1 , 120m 2 g -1 , 100m 2 g -1 , 80m 2 g -1 , 75m 2 g -1 , or 70m 2 g -1The metal surface area may be determined by pulsed CO chemisorption in helium carrier gas using a Micromeritics Autochem II chemisorption analyzer.

[0094] The platinum-on-carbon catalyst material may include a carbon support material that is a partially graphitized carbon material (e.g., heat-treated carbon black). Graphite materials are more corrosion resistant. However, graphite support materials have a low surface area. Thus, there is a compromise between the requirements for high surface area and high corrosion resistance. Partially graphitized materials have been found to be a good compromise between the surface area and corrosion resistance requirements for the carbon support in this water electrolyzer application.

[0095] The ionomer in the cathode catalyst layer may have an equivalent weight of 880 EW or less, 850 EW or less, or 830 EW or less, 750 EW, 770 EW, or 790 EW or more; or within a range defined by any combination of the above upper and lower limits. The side chains of the cathode ionomer each contain a sulfonate group. However, it is preferred that the side chains are shorter than the side chains of the anode ionomer. The side chains of the cathode ionomer may also not contain ether groups (except for the ether linkage to the backbone) and / or they may not contain CF3 groups. The side chains of the cathode ionomer may have the structure: -CF2-CF2-CF2-CF2-SO3H. An example of such an ionomer is an 800 EW 3M C4 side chain. The ionomer of the cathode layer may be the same or similar to that used in the membrane.

[0096] The cathode catalyst layer may have an ionomer / carbon weight ratio of 0.6 to 1.0 (note that this is the weight ratio of ionomer to carbon, platinum is not considered in this calculation.) Additionally, the cathode catalyst layer may have a thickness in the range of 1 to 15, 4 to 15, or 8 to 15 micrometers.

[0097] Examples of such cathode layers include the following features: - Nominal Pt loading: 0.4mgPtcm -2 -Ionomer-800 EW 3M C4 side chain -Ionomer / carbon weight ratio: 0.8 -Thickness - Approximately 10 to 11 micrometers The catalyst is 50 wt. % Pt on carbon. Carbon is a partially graphitized carbon support material.

[0098] Also provided is a method of making a catalyst coated membrane for a water electrolyzer, the method comprising forming an ionomer membrane having a first surface and a second surface, forming an anode catalyst layer on the first surface of the ionomer membrane, and forming a cathode catalyst layer on the second surface of the ionomer membrane, the cathode catalyst layer being fabricated by formulating an ink comprising a dispersion of a platinum on carbon catalyst material and an ionomer in a liquid carrier, depositing and drying the ink, the cathode catalyst layer comprising 1 mg of platinum on carbon catalyst material provided by the platinum on carbon catalyst material. Pt cm -2 and having a platinum loading of less than 1000 nm. The ink can be deposited directly onto the ionomer membrane and dried to form the cathode catalyst layer. Alternatively, the ink can be deposited onto a carrier layer and dried to form a decal including the cathode catalyst layer, and then transferred from the decal to the ionomer membrane.

[0099] Exemplary CCM Structure In accordance with the present specification, the inventors have developed several different versions of catalyst coated membranes (CCMs) for green hydrogen electrolyzer applications. These CCMs share a common structure: cathode catalyst layer / proton exchange membrane / anode catalyst layer. The cathode catalyst layer and the anode catalyst layer are substantially the same for each variant, with the cathode layer being based on a platinum-on-carbon catalyst as previously described herein, and the anode layer being based on an iridium oxide catalyst as previously described herein. Furthermore, the membranes of each variant are multi-layered structures including an ionomer layer with ceria additive and a reinforcing layer as previously described herein. The ionomer, ceria, and reinforcing materials are the same in all variants. The main differences between the variants are the membrane thickness and the specific multi-layer structure of the membrane. The membrane variants are as follows: 50 micron single reinforced membrane, 80 micron double reinforced membrane, 15 micron single reinforced membrane.

[0100] Each of these different variations is described in more detail below.

[0101] Example 1: 50 micron, single reinforced CCM product deformation The 50 micron membrane has the following structure:

[0102] [Table 1]

[0103] The layers are formed from an ionomer solution prepared by suspending ionomer (800 EW perfluorosulfonic acid "PFSA") and ceria (0.26 wt% ceria based on the weight of the ionomer) in an 80:20 ethanol:water mixture. ePTFE is expanded polytetrafluoroethylene. The layers are applied successively by printing / coating directly on top of each other, drying between each addition. Thickness refers to the thickness of the layer after drying.

[0104] The catalyst coated membrane is prepared by attaching an anode containing iridium oxide to one side of a 50 micron membrane and a cathode containing platinum on carbon to the other side of the 50 micron membrane. The catalyst layer is printed ex-situ on a PTFE sheet and transferred onto the membrane by hot pressing. The catalyst layer contains both catalyst and ionomer. The catalyst ink formulation, manufacturing process, and layer structure are as previously described herein.

[0105] The following table summarizes the materials and methods for the construction of a single reinforced 50 micron thick membrane containing ceria additive.

[0106] [Table 2]

[0107] The composition of the ionomer dispersion is as follows:

[0108] [Table 3]

[0109] The four coating passes to produce the membrane are as follows:

[0110] [Table 4]

[0111] For the coating process, the pump is calibrated to deliver a set weight of ionomer dispersion per minute to the die to achieve the required dry gsm coating. Factors include: target gsm; ionomer dispersion solids %; ionomer dispersion viscosity; and coating speed m / min. Process control determines whether each layer is thicker, gsm (measured by an ultrasonic thickness measurement system), Ce loading (μg / cm 2 -measured by in-line XRF), coating speed (m / min), and oven temperature (°C).

[0112] The 50 micrometer thick single-reinforced membrane is thinner than typical membranes used in electrolyzer applications, but has unusually high stiffness for its thinness. Because the membrane is thin, it has a lower proton resistance than a thicker membrane of the same equivalent weight (EW), and therefore a lower operating voltage at a given current density. For example, the membrane shows a >60% reduction in resistance compared to Nafion™ 115 (N115).

[0113] Example 2: 80 micron, double reinforced CCM product deformation The 80 micron membrane has the following structure:

[0114] [Table 5]

[0115] The layers are formed from an ionomer solution prepared by suspending ionomer (800 EW perfluorosulfonic acid "PFSA") and ceria (0.26 wt% ceria based on the weight of the ionomer) in an 80:20 ethanol:water mixture. ePTFE is expanded polytetrafluoroethylene. The layers are applied sequentially by printing directly on top of each other, drying between each addition. Thickness refers to the thickness of the layer after drying.

[0116] The catalyst coated membrane is prepared by attaching an anode containing iridium oxide to one side of an 80 micron membrane and a cathode containing platinum on carbon to the other side of the 80 micron membrane. The catalyst layer is printed ex-situ on a PTFE sheet and transferred onto the membrane by hot pressing. The catalyst layer contains both catalyst and ionomer. The catalyst ink formulation, manufacturing process, and layer structure are as previously described herein.

[0117] Making an 80 micrometer membrane is difficult as it is too thick to print / coat and too thin to extrude. Figure 1 shows how an 80 micrometer double-reinforced membrane is produced in a series of seven print / coat passes, with the reinforcing layer added in passes 2 and 5. The proton-conducting polymer layer is formed from a perfluorosulfonic acid (PFSA) ionomer (3M 800 EW PFSA ionomer), while the reinforcing polymer layer is formed from expanded polytetrafluoroethylene (ePTFE reinforcing material: 4.7 gsm). A substrate (PET with one side release layer) is used to fabricate the membrane.

[0118] The following table summarizes the materials and methods for the construction of a doubly reinforced 80 micron thick membrane containing ceria additive.

[0119] [Table 6]

[0120] The composition of the ionomer dispersion is as follows:

[0121] [Table 7]

[0122] The seven coating passes to produce the membrane are as follows:

[0123] [Table 8]

[0124] For the coating process, the pump is calibrated to deliver a set weight of ionomer dispersion per minute to the die to achieve the required dry gsm coating. Factors include: target gsm; ionomer dispersion solids %; ionomer dispersion viscosity; and coating speed m / min. Process control determines whether each layer is thicker, gsm (measured by an ultrasonic thickness measurement system), Ce loading (μg / cm 2-Measured by in-line XRF), coating speed (m / min), and oven temperature (°C). Thickness was measured manually with a drop gauge for passes 2-7. Due to the fragile nature of the film after only one printing / coating pass, it is not possible to perform a gravimetric check on pass 1, and as a result the layer cannot be accurately cut from the backing. Basis weight / thickness is measured manually using gravimetric methods and with an in-line ultrasonic thickness measurement system (from Mesys™). Note that the calibration of the ultrasonic thickness measurement system starts to lose accuracy after pass 5.

[0125] Figure 3 shows a micrograph image of the 80 micron dual reinforced membrane. It is 80 micrometers thick and contains two reinforcing polymer layers. It also contains ceria, a free radical scavenger, to prevent membrane degradation and extend operational life. The 80 micron dual reinforced CCM product has improved performance in terms of increasing current density at a given voltage when compared to the Nafion™ 115 membrane, as shown in Figure 4.

[0126] The 80 micrometer thick double-reinforced membrane is thinner than typical membranes used in electrolyzer applications, but has unusually high stiffness for its thinness. Because the membrane is thin, it has a lower proton resistance than thicker membranes of the same equivalent weight (EW), and therefore a lower operating voltage at a given current density. For example, the membrane shows a 60% reduction in resistance compared to Nafion™ 115 (N115).

[0127] Example 3: 15 micron, single reinforced CCM product deformation This CCM product variant is based on a 15 micron single-reinforced membrane. The membrane is manufactured in a similar manner to the previous variant, but with only two printed layers, one of which includes the reinforcing material. Otherwise, the CCM is manufactured using the same method. The 15 micrometer thick single-reinforced membrane is much thinner than the other examples. Due to the thinness of the membrane, it has a much lower proton resistance, and therefore a much lower operating voltage at a given current density, than a thicker membrane of the same equivalent weight (EW). Nevertheless, this membrane structure is only suitable for certain applications, as it is too thin for many water electrolyzer applications.

[0128] Example 4: Improvement of the catalyst layer The previous three CCM examples were manufactured using the same catalyst ink formulation and catalyst layer processing method. As previously indicated, formulation and processing parameters have been developed to provide improved CCM performance in water electrolyzer applications. Optimizing the ionomer concentration and drying conditions of the catalyst layer, as previously described herein, was found to result in significant improvement in CCM performance in water electrolyzer applications. Figure 6 shows the results of the 0.4 mg catalyst ink formulation and the catalyst layer processing method. Pt / cm -2 (1A), 0.1 mg Pt / cm -2 (2A), and 0.08 mg Pt / cm -2 (3A) Electrochemical performance of the three catalyst layers.

[0129] Although the present invention has been particularly shown and described with reference to specific embodiments, it will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

1. 1. A catalyst coated membrane for a water electrolyzer, the catalyst coated membrane comprising: an ionomer membrane having a first surface and a second surface; an anode catalyst layer on the first surface of the ionomer membrane; a cathode catalyst layer on the second surface of the ionomer membrane; the ionomer membrane comprises a reinforcing material; A catalyst coated membrane, wherein the reinforcing material is asymmetrically distributed with respect to the central plane of the ionomer membrane such that its average position is closer to the anode catalyst layer than to the cathode catalyst layer.

2. 10. The catalyst coated membrane of claim 1, wherein the ionomer membrane is a multilayer structure formed with at least 3, 5, or 7 ionomer layers, and no more than 15, 12, or 10 ionomer layers, or a number of ionomer layers within a range defined by any combination of the aforementioned lower and upper limits, and wherein at least one, and optionally at least two, of the ionomer layers comprises a reinforcing material in addition to the ionomer.

3. 3. The catalyst coated membrane of claim 2, wherein only one of the ionomer layers includes a reinforcing material, said layer being closer to the anode catalyst layer than to the cathode catalyst layer.

4. 3. The catalyst coated membrane of claim 2, wherein two of the ionomer layers comprise a reinforcing material and a midpoint between the two ionomer layers is closer to the anode catalyst layer than to the cathode catalyst layer.

5. 5. The catalyst coated membrane of claim 1, wherein the reinforcing material comprises a porous polymer layer impregnated with an ionomer, and the reinforcing material is optionally expanded polytetrafluoroethylene (ePTFE).

6. 10. The catalyst coated membrane of claim 1, wherein the ionomer membrane further comprises a radical reduction additive, optionally ceria.

7. 10. The catalyst coated membrane of claim 1, wherein the anode catalyst layer comprises an iridium-containing catalytic material, optionally an iridium oxide-containing catalytic material.

8. The catalyst coated membrane of claim 1 , wherein the cathode catalyst layer comprises a platinum-on-carbon catalyst material.

9. 10. The catalyst coated membrane of claim 1, wherein one or both of the anode catalyst layer and the cathode catalyst layer comprises both a catalytic material and an ionomeric material.

10. 10. The catalyst coated membrane of claim 9, wherein the ionomer in one or both of the anode catalyst layer and the cathode catalyst layer is different from the ionomer in the ionomer membrane.

11. 10. The catalyst coated membrane of claim 1, wherein the first surface of the ionomer membrane has fewer defects than the second surface of the ionomer membrane.

12. 10. The catalyst coated membrane of claim 1, wherein the anode catalyst layer comprises an iridium-containing catalytic material disposed in an ionomer, and the ionomer in the anode catalyst layer differs from the ionomer in the membrane by having one or more of a higher equivalent weight than the membrane ionomer and / or longer side chains than the membrane ionomer.

13. 10. The catalyst coated membrane of claim 1, wherein the anode catalyst layer comprises an iridium-containing catalytic material disposed in an ionomer, and the ionomer in the anode catalyst layer has an equivalent weight of 900 EW or greater, 950 EW or greater, 1000 EW or greater, or 1050 EW or greater, 1300 EW, 1200 EW, or 1150 EW or less, or within a range defined by any combination of the foregoing lower and upper limits.

14. 10. The catalyst coated membrane of claim 1, wherein the membrane ionomer has an equivalent weight greater than 750 EW and less than 850 EW.

15. The cathode catalyst layer comprises a platinum-supported carbon catalyst material disposed in an ionomer, and the cathode catalyst layer is provided by the platinum-supported carbon catalyst material. Ｐｔ cm －２ less than, for example, 0.5 mg Ｐｔ cm －２ Less than and 0.01 mg Ｐｔ cm －２ 10. The catalyst coated membrane of claim 1 having a platinum loading of greater than 0.1 wt.

16. 10. The catalyst coated membrane of claim 1, wherein the cathode catalyst layer comprises a platinum-on-carbon catalyst material disposed in an ionomer, the cathode catalyst layer having an ionomer / carbon weight ratio of 0.6 to 1.

0.

17. 10. A method for manufacturing a catalyst coated membrane for a water electrolyzer according to claim 1, said method comprising: forming an ionomer membrane having a first surface and a second surface, the ionomer membrane comprising a reinforcing material; forming an anode catalyst layer on the first surface of the ionomer membrane; forming a cathode catalyst layer on the second surface of the ionomer membrane; The method, wherein the reinforcing material is asymmetrically distributed with respect to the central plane of the ionomer membrane such that its average position is closer to the anode catalyst layer than to the cathode catalyst layer.

18. The ionomer membrane is prepared by depositing a dispersion of an ionomer in a liquid carrier onto a solid backing layer and at least partially drying to form a first ionomer layer; depositing a dispersion of an ionomer in a liquid carrier onto the first ionomer layer and at least partially drying to form a second ionomer layer on the first ionomer layer; optionally, sequentially depositing and drying one or more additional ionomer layers on the first and second ionomer layers to build the ionomer membrane structure on the solid backing layer; The method of claim 17 , wherein at least one of the ionomer layers comprises the reinforcing material.

19. 20. The method of claim 18, wherein the anode catalyst layer is formed on the outer surface of the first ionomer layer after removing the ionomer membrane from the solid backing layer.

20. A water electrolysis cell comprising the catalyst coated membrane of claim 1.

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