WO2023052750A1

WO2023052750A1 - Catalyst coated proton exchange membranes for hydrogen producing water electrolysers

Info

Publication number: WO2023052750A1
Application number: PCT/GB2022/052437
Authority: WO
Inventors: Peter Richard Ellis; Samuel Arthur French; Michael Ian Petch; Chris ZALITIS
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2021-09-28
Filing date: 2022-09-27
Publication date: 2023-04-06
Also published as: GB202113809D0

Abstract

A method of fabricating a proton exchange membrane for a hydrogen producing water electrolyser, the method comprising: depositing a plurality of proton conducting polymer layers on top of each other to form a multi-layer proton conducting membrane structure using a plurality of different dispersions of ionomer in liquid solvent, at least one of the dispersions comprising a reinforcement polymer in addition to ionomer, at least one of the dispersions comprising a recombination catalyst in addition to ionomer, and at least one of the dispersions comprising a radical reducing additive in addition to ionomer, wherein at least one of the proton conducting polymer layers comprises the reinforcement polymer in addition to ionomer, wherein at least one of the proton conducting polymer layers comprises the recombination catalyst in addition to ionomer for catalysing a recombination reaction of molecular oxygen and hydrogen, and wherein at least one of the proton conducting polymer layers comprises the radical reducing additive in addition to ionomer.

Description

CATALYST COATED PROTON EXCHANGE MEMBRANES FOR

HYDROGEN PRODUCING WATER ELECTROLYSERS

Field

The present specification relates to catalyst coated proton exchange membranes for water electrolysers including components thereof and methods of manufacture.

Background

During water electrolysis hydrogen is produced at the cathode of the electrochemical cell while oxygen is produced at the anode. If the electrochemical cell is powered by a renewable electricity supply, the hydrogen produced in this manner is known as green hydrogen.

In practice, there are several different types of electrolyser configuration which can be used to produce green hydrogen. Electrolysis of water to produce high purity hydrogen and oxygen can be carried out in both alkaline and acidic electrolyte systems and practical devices using both types of electrolyte systems exist as commercial products. Those electrolysers that are acid electrolyte-based typically employ a solid proton-conducting polymer electrolyte membrane or proton exchange membrane (PEM) and are known as polymer electrolyte membrane water electrolysers or proton exchange membrane water electrolysers (PEMWEs). A catalyst-coated membrane (CCM) is employed within the cell of a PEMWE, which comprises the proton exchange membrane with two catalyst layers (for the anode and cathode reactions) applied on either face of the proton exchange membrane.

The present specification is focused on proton exchange membrane water electrolysers (PEMWEs) based on a catalyst coated proton exchange membrane. The catalyst coated membrane (CCM) comprises a proton conducting polymer membrane coated on one side by a cathode catalyst for catalysing a hydrogen evolution reaction and coated on the other side by an anode catalyst for catalysing an oxygen evolution reaction. Typically, for PEMWEs, cathode catalyst materials comprise platinum. Anode catalysts typically comprise iridium or iridium oxide (IrOx) materials or oxides containing both iridium and ruthenium.

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

Precious metals such as Pt and Ir are required for the electrodes, these metals being used to fabricate the electrode catalysts. The chemical and physical form of the metal and any support material which make up the catalysts can affect the amount of precious metal which is required and the performance characteristics of the catalyst in a water electrolyser application. Proton exchange membranes formed from ionomer are coated with catalyst layers to form catalyst coated membranes (CCM). Catalysts can be formulated into inks and deposited on a membrane to form a CCM or transferred to the membrane from a decal. The ink formulation and/or form of catalyst layer can also affect functional performance. For example, the catalyst ink may contain ionomer or PTFE or other fluoropolymer or polymer, which act as binders for the catalyst when the layer is dried. If the catalyst is bound with ionomer, the ionomer additionally acts as an ion (proton) conducting medium to move protons towards (cathode) or away from (anode) the active sites on the electrocatalysts' surface.

One problem with existing water electrolyser systems is that iridium is a key component in their manufacture, yet a limited amount of iridium is available in the world and is 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 terms of having sufficient activity to catalyse the required oxygen evolution reaction yet sufficient stability to survive the harsh acidic and oxidising environment to which they are subjected in a PEM water electrolyser. However due to the scarcity of iridium there is a need to develop proton exchange membrane water electrolyser systems which provide a significant increase in gigawatt (GW) power per kilogram (kg) of iridium (or conversely a decrease in the amount of iridium per unit power - kgi_r / GW). Accordingly, improvements in the electrolyser systems, and in particular the catalyst coated membranes of such systems, are required.

In an electrolyser, the cathode is also known as the hydrogen electrode and is the electrode at which hydrogen is generated; the anode is also known as the oxygen electrode and is the electrode at which oxygen is generated. The result of any excessive crossover of hydrogen is a combination of molecular H2 and molecular O2 at the anode side, which is a potentially explosive mixture presenting a significant safety hazard, due to the wide explosive range of 5 - 95% H2 in O2. As such, although for efficient operation and maximum performance of a PEMWE it is important to keep the electronic and ionic resistances within the CCM as low as possible, it is also important to minimise any hydrogen crossover through the membrane into the oxygen stream. Traditionally, membranes have been 125 microns or thicker because of the need to limit such hydrogen crossover. However, the use of thicker membranes increases electronic and ionic resistances within the CCM. Since hydrogen crossover is exacerbated by the use of thinner membranes in PEMWEs it is quite typical to employ membranes with thicknesses of over 125 pm, and typically close to 200pm, or thicker.

Thus, although improved performance would be obtained by the use of a thinner PEM in PEMWEs, in practice this has not been possible because of the increased hydrogen crossover and the resultant safety risk. Traditionally, PEM thicknesses in PEMWEs are 125 pm or greater to reduce the level of hydrogen crossover, but the concomitant increase in ionic resistance severely limits PEMWE performance. Examples of currently used membranes include Nation™ N115 (thickness 125 pm) or Nation™ N117 (thickness 175 pm).

There is thus a need for a high-performance PEM which limits hydrogen crossover to reduce the safety risk in green hydrogen producing water electrolyser applications, yet has lower ionic resistance by for example, being of lower thickness or higher conductivity than the conventional, currently used membranes. By "high performance" is meant that the PEMWE can operate at as high a current density as possible, with as high an electrical efficiency (i.e. low cell voltage) as possible.

Johnson Matthey have previously filed a patent application (published as WO2018/115821) directed to addressing the aforementioned need. WO2018/115821 describes the manufacture of a CCM for PEM water electrolysers by laminating three membranes together: a first membrane coated with a cathode catalyst layer (a hydrogen evolution reaction or HER catalyst such as platinum black in a dispersion of ionomer); a second membrane coated with an anode catalyst layer (an oxygen evolution reaction or OER catalyst such as I rO₂ black in a dispersion of ionomer); and a third membrane coated with a recombination catalyst (such as palladium supported on carbon black in a solution of ionomer) to reduce hydrogen cross-over, the third membrane being sandwiched between the first and second membranes.

It is disclosed that the recombination catalyst is advantageously positioned closer to the anode catalyst layer than the cathode catalyst layer. In the three-membrane laminated structure disclosed in WO2018/115821, two of the membranes are disposed between the cathode catalyst and the recombination catalyst and one of the membranes is disposed between the anode catalyst and the recombination catalyst, such that the recombination catalyst is closer to the anode catalyst than the cathode catalyst. Orienting the three membrane components in the manner described means that, because the membrane components are of equal thickness, the recombination catalyst layer will be positioned closer to the anode catalyst layer in the final laminated CCM. As a result, the recombination catalyst layer will lie closer to the plane in which the levels of H2 and O2 are suitable for most effective recombination. This is because of the faster diffusion rate of H2 relative to O2 through the CCM such that the optimal plane is closer to the oxygen producing side of the CCM than the hydrogen producing side.

WO2018/115821 also discloses that each of the three membranes may have a thickness of 7 to 40 pm, 10 to 35 pm, 12 to 30 pm, 14 to 25 pm or 16 to 20 pm, with an overall thickness of the CCM being < 120 pm, < 100 pm, < 95 pm, < 90 pm, < 85 pm, < 80 pm, < 75 pm, < 70 pm, < 65 pm or < 60 pm. The provision of a thinner membrane in the CCM in combination with components such as a recombination catalyst to reduce hydrogen cross-over, enables the provision of a high performance CCM which simultaneously reduces hydrogen crossover.

WO2018/115821 also discloses that each of the three membranes may comprise a reinforcing component embedded into the membrane component, such as one or more planar reinforcing components. Suitable planar reinforcing components include those formed from expanded polymer networks (e.g. expanded PTFE (e-PTFE)). The reinforcing component provides structural support for the three membrane components, thereby increasing their mechanical strength and allowing thinner membrane components to be used.

WO2018/115821 additionally discloses that each of the three membranes may comprise a hydrogen peroxide removal additive, such as a hydrogen peroxide decomposition catalyst, or a peroxyl radical scavenger additive, e.g. cerium dioxide (ceria). This may be dispersed through the bulk of one or more of the three membrane components or coated on the surfaces thereof. It is described that a common problem during the operation of water electrolysers is that the hydrogen and oxygen undergo an unwanted side-reaction to form hydrogen peroxide (H2O2), which in turn decomposes to form peroxide radicals. These radicals are highly reactive and attack the membrane components and other structures within the electrolyser, reducing the lifespan of the device. Providing a hydrogen peroxide decomposition or radical scavenger additive such as ceria reduces this problem.

In the examples disclosed in WO2018/115821, each of the three membranes was a 17 pm thick membrane and comprised a 900 EW Flemion™ ionomer from Asahi Glass Group with a PTFE reinforcement. A ceria hydrogen peroxide scavenger catalyst was coated on one side of each of the three membranes. A cathode catalyst layer comprising Pt black in a dispersion of ionomer was coated onto one of the membrane components over the ceria layer, an anode catalyst layer comprising I rO₂ black in a solution of ionomer was coated onto another of the membrane components over the ceria layer, and a recombination catalyst comprising Pd supported on carbon black in a solution of ionomer was deposited onto the final membrane component over the ceria layer. The three catalyst-coated membrane components were then arranged with the membrane component having the recombination catalyst layer in the middle, 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 which carried the anode catalyst layer. These three layers were then laminated to form a CCM.

The present specification builds on the work described in WO2018/115821 to provide improved CCMs for PEM water electrolyser applications.

Summary

According to one aspect of the present specification there is provided a method of fabricating a proton exchange membrane for a hydrogen producing water electrolyser, the method comprising: depositing a plurality of proton conducting polymer layers on top of each other to form a multi-layer proton conducting membrane structure using a plurality of different dispersions of ionomer in liquid solvent, at least one of the dispersions comprising a reinforcement polymer in addition to ionomer, at least one of the dispersions comprising a recombination catalyst in addition to ionomer, and at least one of the dispersions comprising a radical reducing additive in addition to ionomer, wherein at least one of the proton conducting polymer layers comprises the reinforcement polymer in addition to ionomer, wherein at least one of the proton conducting polymer layers comprises the recombination catalyst in addition to ionomer for catalysing a recombination reaction of molecular oxygen and hydrogen, and wherein at least one of the proton conducting polymer layers comprises the radical reducing additive in addition to ionomer.

This methodology differs from that described in the earlier WO2018/115821 patent application in that rather than requiring three separate membranes which are laminated together, the present method forms a single, multi-layer membrane structure in multiple deposition passes, the structure including at least one layer which comprises reinforcement polymer and at least one layer which comprises a recombination catalyst in addition to ionomer. The membrane is formed using a plurality of ionomer dispersions comprising different additives to build up the membrane structure. No lamination or bonding step is required to bond solid membranes together with a recombination catalyst therebetween.

The multi-layer, reinforced polymer membranes have a high degree of rigidity making them easier to process to form a catalyst coated membrane. While the earlier WO2018/115821 patent application discussed the benefits of processing three separate membranes such that each membrane only needs to be coated with a single catalyst layer, the present specification provides an alternative approach by providing multi-layer, reinforced printed polymer membranes which are more readily processed for printing of catalyst layers on either side thereof while avoiding lamination interfaces which is advantageous for the reasons given below.

Lamination of proton conductive membranes comprises pressing and/or bonding at least two solid proton conductive membranes together. A lamination interface is formed between the two membranes where solid surfaces of the individual membranes are pressed and/or bonded together. Lamination interfaces comprise physical defects. Furthermore, the structural and/or chemical nature of a lamination interface also differs from that of the bulk polymer material. This is because when a solid membrane is formed, the outer surfaces of the solid membrane have surface features which are distinct from those in the bulk material. For example, a hydrophobic skin forms on a surface of a membrane at an air interface. Raman spectroscopy can detect this difference. As such, when two solid membranes are pressed together, the lamination interface formed by the two solid surfaces is distinctive in chemical and/or structural form compared to the bulk of the proton conductive polymer material. Microscopy and spectroscopy techniques can thus distinguish between lamination interfaces between layers of proton conductive polymer and interfaces which have been formed via a liquid phase deposition process such as printing, spraying, or coating of layers to build up a multi-layer structure. That is, a non-laminated interface is structurally and/or chemically distinct from a laminated interface and is not just a feature of the manufacturing method. Furthermore, a nonlaminated interface can be identified as being non-laminated in a product CCM without prior knowledge of the manufacturing method. Examples of analysis techniques for detecting a laminated interface include cross-section SEM. Variations of crystallinity at interfaces can be detected using cross-section 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.

Due to physical defects and/or chemical variations at lamination interfaces between proton conductive polymer membranes, such interfaces can increase the resistance of a multi-layer proton conductive membrane. As such, it has been found to be advantageous to fabricate a multi-layer proton conductive membrane by depositing layers of ionomer dispersed in a liquid solvent to build up a multi-layer membrane structure rather than via lamination of individual solid layers/membranes of proton conductive polymer. The present specification also provides a catalyst coated proton exchange membrane for a hydrogen producing water electrolyser, the catalyst coated membrane comprising: a plurality of proton conducting polymer layers formed of ionomer and disposed on top of each other providing a non-laminated multi-layer proton exchange membrane structure, wherein at least one of the proton conducting polymer layers comprises a reinforcement polymer in addition to ionomer, wherein at least one of the proton conducting polymer layers comprises a recombination catalyst in addition to ionomer for catalysing a recombination reaction of molecular oxygen and hydrogen, wherein at least one of the proton conducting polymer layers comprises a radical reducing additive in addition to ionomer, and wherein a cathode catalyst layer is disposed on one side of the proton exchange membrane for catalysing a hydrogen evolution reaction and wherein an anode catalyst layer is disposed on the other side of the proton exchange membrane for catalysing an oxygen evolution reaction.

One or both of the cathode catalyst layer and anode catalyst layer may also be formed by depositing a dispersion of catalyst material in liquid solvent. The or each dispersion of catalyst material in liquid solvent may further comprise ionomer. As such, the entire CCM may be fabricated by depositing dispersions of component materials.

Thus according to the present specification, multi-layer membranes, and advantageously entire CCMs including anode and cathode layers, can be made by casting, i.e. depositing multiple layers of proton conductive polymer on top of each other via a liquid phase deposition process such as printing, spraying, or coating. This is unusual for membranes used in electrolyser CCMs which are typically extruded or alternatively laminated together as in WO2018/115821.

The individual layers are formed by preparing a dispersion of ionomer in a liquid phase solvent and then depositing the dispersion to form a layer of ionomer. The layer thus formed can be dried prior to deposition of a further layer of ionomer thereover. Depositing multiple thin layers in this manner is preferable to depositing one or several thicker layers as it is easier to dry a thin layer, removing liquid solvent from the layer, when compared to deposition and drying of a thicker layer. The physical, chemical, and electronic properties of a resultant membrane can be compromised if significant quantities of solvent remain trapped in the multi-layer structure. Alternatively, fabrication times are significantly increased if it is required to dry the membrane structure for extended periods to remove solvent from thick layers of deposited ionomer dispersion. As such, it is preferred that individual layers within each multi-layer membrane have a thickness of: no more than 30 pm, 25 pm, 20 pm, 15 pm, or 10 pm; no less than 1 pm, 2 pm, 3 pm, 4 pm, or 5 pm; and/or within a range defined by any combination of the aforementioned upper and lower limits.

The proton exchange membrane may have a total thickness in a range 40 to 115 micrometres. The layer comprising the recombination catalyst is advantageously disposed closer to one side of the membrane than the other but has at least one layer of proton conducting polymer thereover and thus does not form an external layer of the proton exchange membrane. This aids in protecting the recombination catalyst while locating it in a plane closer to the anode which achieves more efficient recombination of hydrogen and oxygen accounting for the faster diffusion rate of hydrogen through the membrane compared to oxygen.

Preferably the layer comprising the reinforcement polymer does not form an external layer of the proton exchange membrane. The one or more reinforcement layers can be located within the membrane to provide better mechanical stability. While the multi-layer structure comprises one or more layers which have reinforcement and one or more layers which have recombination catalyst, at least three of the layers in the membrane may not contain reinforcement polymer or recombination catalyst.

The reinforcement polymer may be formed of expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI) while the proton conducting polymer layers may be formed of perfluorosulfonic acid (PFSA) ionomer. The radical reducing additive (e.g. peroxide radical reducing additive such as Ceria) may be dispersed within every layer. It will be noted that peroxide can decompose to form a range of radicals (O, OH, OOH) and the radical reducing additive may reduce the amount of one, more, or all of these radicals.

The cathode catalyst layer for catalysing the hydrogen evolution reaction may comprise platinum, the recombination catalyst layer for catalysing the recombination reaction of molecular oxygen and hydrogen may comprise platinum or palladium, and/or the anode catalyst layer for catalysing the oxygen evolution reaction may comprises iridium oxide or mixed oxides of iridium and another metal or metals.

The membrane configuration may be provided with anode and cathode catalyst layers to form a CCM for a water electrolyser. It is envisaged that such a CCM will be provided as a commercial product. However, it is also envisaged that for certain customers which use their own anode and cathode catalysts, the membrane configuration without the anode and cathode catalyst applied may be provided as a commercial product.

The present specification also provides for the use of a catalyst coated membrane as described above in a proton exchange membrane water electrolyser and provides a proton exchange membrane water electrolyser comprising the catalyst coated membrane.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 shows performance data for a membrane according to this specification verses a commercial Nation™ 115 membrane illustrating an improvement in performance in terms of increased current density at a given voltage;

Figure 2 shows a micrograph of a cross-section of a proton exchange membrane comprising two reinforcement layers;

Figure 3 shows how the double reinforced membrane is manufactured in a series of 7 coating passes with reinforcement layers coated in passes 2 and 5;

Figure 4 shows a schematic illustration of a membrane configuration according to the present specification including an 80 micrometre thick double reinforced membrane, a 15 micrometre thick single reinforced membrane, and a recombination catalyst disposed between the two membranes;

Figure 5 shows hydrogen cross-over performance for the 80 micrometre thick, double reinforced membrane (JM Membrane) of Figure 2 and two thicker commercial membranes - Nation™ 115 (N115) and Nation™ 117 (N117) - showing that a reduction in membrane thickness leads to an unwanted increase in hydrogen cross-over; a further data point (JM Membrane Plus) is for the membrane configuration illustrated in Figure 4 showing that hydrogen cross-over can be mitigated using this structure at a significantly reduced membrane thickness with an associated improvement in proton exchange rate; and

Figure 6 shows an alternative membrane configuration for use in PEMWE's, the configuration comprising two 80 micrometre thick double reinforced membranes with a recombination catalyst sandwiched therebetween and suitable for high pressure PEMWE's which are susceptible to higher hydrogen cross-over rates. Detailed Description

As described in the background section with reference to the summary section, the present specification is concerned with providing catalyst coated proton exchange membranes for green hydrogen producing water electrolysers.

The present specification is particularly directed towards improvements in the proton exchange membrane configuration, reducing the thickness of the membrane to reduce its protonic resistance while including additives to retain mechanical stability and mitigate against hydrogen cross-over.

A membrane in a PEMWE is required to allow protons to move between anode and cathode easily (low resistance) while stopping gas (especially H2) crossover. A thinner membrane has less ionomer to pass through and therefore has a lower resistance for proton transfer but allows increased gas crossover. Therefore, the thickness of the membrane is often chosen to balance the resistance with the crossover, with the most popular being Nation™ 115 and 117 from Chemours™ at 125 and 175 pm thick, respectively.

In fuel cells, which use thinner membranes, reinforcement has shown to extend the working life of a CCM by constraining the ionomer against swelling, preventing pinholes or cracks from forming. This has been found to carry over to electrolysers. However, the above mentioned N115 and N117 do not contain reinforcement. A Nation™ 212 membrane which is cast can contain reinforcement but is currently considered too thin for electrolyser operation (too much hydrogen crossover). There is no commercial membrane within a gap between 50 and 125 pm which appears to be optimal for certain PEM water electrolyser applications. Therefore, a membrane within this gap with reinforcement would be highly beneficial for electrolyser operation. An ideal membrane would also introduce H2O2 scavenging catalysts to protect against radical degradation and recombination catalysts.

A new proton exchange membrane has been fabricated along with a new CCM comprising said membrane. The new proton exchange membrane (referred to herein as the JM Membrane) has an improved performance in terms of increased current density at a given voltage when compared with a Nation™ 115 membrane as shown in Figure 1. Figure 2 shows a micrograph of the membrane. It is approximately 80 micrometres thick and comprises two reinforcement polymer layers. It also contains ceria which is a free radical scavenger, preventing membrane degradation and extending operational lifetime.

Making an 80 micrometre membrane is difficult as it rather too thick to be printed/coated and rather too thin to be extruded. Figure 3 shows how the 80 micrometre double reinforced membrane may be manufactured in a series of 7 printing/coating passes with reinforcement layers added in passes 2 and 5. The proton conductive polymer layers are formed from perfluorosulfonic acid (PFSA) ionomer (3M 800EW PFSA ionomer) while the reinforcement polymer layers are formed of expanded polytetrafluoroethylene (ePTFE reinforcement: 4.7gsm). A substrate (PET with 1 side release layer) is used for the fabrication of the membrane. The tables below summarize the materials and method for construction of the membrane.

The ionomer dispersion composition (ION0046) is as follows:

The seven coating passes for fabricating the membrane are as follows:

For the coating process, a pump is calibrated to deliver a set weight of ionomer dispersion per minute to a die to achieve the required dry gsm coating. Factors include: target gsm; ionomer dispersion %solids; ionomer dispersion viscosity; and coating speed m/min. Process controls are such that each layer will have a target for thickness gsm (measured by ultrasound thickness measurement system), Ce loading (pg/cm² - measured by in-line XRF), coating speed (m/min), and oven temperatures (°C). For the data shown in Figure 3, thickness was measured manually using a drop gauge for passes 2 to 7. It is not possible to conduct a gravimetric check on pass 1 due to the fragile nature of the membrane after only one printing/coating pass such that the layer cannot be cut off the backing accurately. Basis weight is measure manually using a gravimetric method and using an in-line ultrasound thickness measurement system (from Mesys™). It was noted that calibration of the ultrasound thickness measurement system starts to lose accuracy after pass 5.

The 80 micrometre thick double reinforced membrane is thinner than typical membranes used in electrolyser applications but has an unusually high rigidity for its thinness. Due to the thinness of the membrane, it has a lower protonic resistance and therefore a lower operating voltage at a given current density than thicker membranes of the same equivalent weight (EW), but exhibits a higher hydrogen cross-over than thicker membranes. In order to reduce hydrogen cross-over, the membrane has been combined with a recombination catalyst and a thin, single reinforced membrane. Figure 4 shows a schematic illustration (10) of such a membrane configuration (referred to herein as JM Membrane Plus) including an 80 micrometre thick membrane (11) with two reinforcement layers (12), a 15 micrometre thick membrane (13) with a single reinforcement layer (12), and a recombination catalyst layer (14) disposed between the two membranes (11, 13). The 15 micrometre thick single reinforced membrane (13) can be fabricated in a similar manner to the 80 micrometre thick double reinforced membrane (11) as described previously. The recombination catalyst layer (14) can be formed on either the 80 micrometre thick membrane (11) or the 15 micrometre thick membrane (13) and then the two membranes (11,13) laminated together with the recombination catalyst layer (14) disposed therebetween. For example, to laminate the membranes together, the two membranes can be hot-pressed at 170 °C, 800 PSI for 2-3 minutes.

The overall thickness of this membrane structure is still significantly lower than typical commercial membranes for electrolysers and thus still has good current density performance characteristics. Furthermore, due to the presence of the recombination catalyst disposed close to an anode side of the membrane, hydrogen cross-over is significantly reduced. In this regard, Figure 5 shows hydrogen cross-over performance for the 80 micrometre thick, double reinforced membrane (JM Membrane) and two thicker commercial membranes - Nation™ 115 (N115) and Nation™ 117 (N117) - showing that a reduction in membrane thickness leads to an unwanted increase in hydrogen cross-over. The JM Membrane has about double the hydrogen crossover of N115. However, the JM Membrane also shows a 60% reduction in resistance compared to N115. A further data point (JM Membrane Plus) is for the membrane configuration illustrated in Figure 4 comprising an 80 micrometre thick double reinforced membrane, a 15 micrometre thick single reinforced membrane, and a recombination catalyst disposed between the two membranes. The datapoint shows that hydrogen cross-over can be mitigated using this structure at a significantly reduced membrane thickness with an associated improvement in proton exchange rate. Accordingly, the JM Membrane Plus membrane configuration has a better combination of low hydrogen cross-over and low protonic resistance compared with the other membrane configurations.

The specific type of recombination catalyst incorporated into the membrane structure can be varied. Several different examples have been fabricated including recombination catalysts comprising palladium on carbon, platinum on carbon, varied metal loadings, and metals supported on carbons with varying amounts of graphitisation. Further examples include platinum on ceria and platinum on graphene. Of the materials tested, Pt based catalysts were found to be the better recombination catalysts for this hydrogen producing water electrolyser application. The recombination catalyst layers can be made with a Nation™ to carbon (support material) ratio of 120 wt.% or higher. This catalyst can be made into an ink for processing at 13.6% solids then diluted to 7% solids for spray coating. The recombination catalyst layer can be made to a target loading of, for example, between 0.01 - 0.02 mgpGM cm'² which gives a layer of about 5 pm thickness. However, loading and layer thickness can be varied. In addition, other catalyst supports have been trialled such as sulphonated carbon (to make the carbon more hydrophilic), graphite, CeCh and ZrCh. Furthermore, a range of deposition techniques can be used including bar coating, knife coating and spray coating.

The membrane structure as described above can be coated with a cathode catalyst and an anode catalyst to form a catalyst coated membrane (CCM) for a water electrolyser. The specific type of catalysts for the cathode and anode can be varied. Furthermore, the method of deposition can be varied. For example, the cathode and anode catalyst can be formed on the first and second membranes prior to laminating the two membranes together or they can be deposited after lamination on either side of the composite membrane comprising the two membranes and the recombination catalyst. An example of a suitable cathode catalyst is a platinum on carbon catalyst, optionally provided as a decal. Iridium oxide-based catalysts are used for the anode. The iridium oxide-based catalyst can be prepared into an ink comprising ionomer, 1-propanol and water, and bar coated onto a sheet of Teflon and dried to form a decal. The catalyst decals can be hot pressed with the membrane to form a CCM.

Figure 6 shows an alternative membrane configuration (20) for use in high pressure PEM water electrolysers. The configuration of Figure 6 comprises two 80 micrometre thick double reinforced membranes (11) (as previously described) with a recombination catalyst layer (14) sandwiched therebetween. The configuration is similar to that previously described in that two multi-layer non- laminated reinforced membranes (11) are bonded together with a recombination catalyst layer (14) therebetween. The recombination catalyst can be deposited onto either of the membranes prior to laminating the two membranes together. As in the previously described example, the membrane configuration is based on an 80 micrometre thick, multi-layer double reinforced membrane. However, while in the previous example the second membrane was a thin 15 micrometre thick single reinforced membrane, giving a total thickness of about 100 - 115 pm, for this example the second membrane is another 80 micrometre thick, multi-layer double reinforced membrane give a total layer thickness of about 165 - 180 pm. This thicker membrane configuration is better for higher pressure or differential pressure electrolysers, where hydrogen crossover is exacerbated, the previous thinner membrane configuration being better for lower pressure operation where the main focus is energy efficiency of the CCM. In other respects, the thicker membrane configuration can be manufactured in an analogous manner to the previously described example. In one method, the two membranes can be bonded together by applying ionomer (e.g. Nation™ ionomer 1100EW at ~12% solids) onto one membrane using a k-bar. The second membrane is then aligned and placed directly on top. The double membrane assembly is then passed under the k-bar again to even and flatten the assembly which is then placed between two PTFE sheets and pressed at 800psi and 173°C for 2min to form the two membrane configuration with a layer of recombination catalyst therebetween. Applying a dispersion of ionomer in liquid solvent between both membranes improved the bonding of the membranes by acting as a glue when pressed at high temperature. No delamination is seen after testing compared to an assembly with no ionomer dispersion applied between the two membranes. Anode and cathode catalyst can be applied on either side of the membrane configuration or they can be applied prior to bonding together of the two membranes.

While the membrane configuration is thicker than the previous example and thus will have a lower current density for a given operating voltage, it still provides a lower operating voltage alternative for high pressure applications when compared to other available membranes which are suitable for such high pressure applications. The reinforcement layers and radical reducing additive (e.g. ceria) improve mechanical stability and damage by chemical attack. In contrast, equivalent membranes for high pressure applications are thicker and result in higher resistance.

The above described membrane configurations involve fabricating two separate multi-layer, reinforced membrane configurations by printing two different multi-layer structures and then laminating those two separate membranes together with a recombination catalyst layer disposed therebetween. However, an alternative approach according to the present specification is to deposit a single multilayer, reinforced membrane in which at least one of the layers comprises a recombination catalyst such that no second membrane or lamination step is required. For example, the 80 micrometer membrane as previously described may have an additional layer of ionomer with recombination catalyst printed thereon and then one or more further layers of ionomer, and optionally one or more further layers of reinforcement printed thereover. In this way, a structure of the type illustrated in Figure 4, or a variant thereof, can be fabricated as a single membrane via multiple printing steps. As such, this method can be used to fabricate a single, coherent multi-layer printed membrane which comprises both reinforcement and recombination catalyst layers without requiring an additional membrane to be laminated over the recombination catalyst layer.

Another example is a 50 micrometre thick membrane fabricated with five deposited layers: (1) ionomer; (2) ionomer; (3) ionomer with ePTFE reinforcement; (4) ionomer with recombination catalyst; (5) ionomer. A cathode catalyst layer can be desposited onto layer (1) and an anode catalyst layer can be deposited onto layer (5) to form a CCM for a water electrolyser. Ceria or another radical reducing additive can be included in one, more, or all of the layers.

Compared with commercially available membranes for hydrogen producing water electrolysers, this 50 micrometre membrane it is thinner and is loaded with three additives: reinforcement, ceria and recombination catalyst (e.g. a platinum based catalyst). The thinner membrane leads to better energy efficiency while the additives mitigate the negative effects of thinner membranes, mechanical strength, crossover, and susceptibility to degradation through peroxide attack.

While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims. For examples, the precise thickness of the constituent membranes, type and concentration of ionomer, and type of additives can be tailored for a given application and operating conditions.

Claims

1. A method of fabricating a proton exchange membrane for a hydrogen producing water electrolyser, the method comprising: depositing a plurality of proton conducting polymer layers on top of each other to form a multi-layer proton conducting membrane structure using a plurality of different dispersions of ionomer in liquid solvent, at least one of the dispersions comprising a reinforcement polymer in addition to ionomer, at least one of the dispersions comprising a recombination catalyst in addition to ionomer, and at least one of the dispersions comprising a radical reducing additive in addition to ionomer, wherein at least one of the proton conducting polymer layers comprises the reinforcement polymer in addition to ionomer, wherein at least one of the proton conducting polymer layers comprises the recombination catalyst in addition to ionomer for catalysing a recombination reaction of molecular oxygen and hydrogen, and wherein at least one of the proton conducting polymer layers comprises the radical reducing additive in addition to ionomer.

2. A method according to claim 1, wherein the radical reducing additive is provided in every layer.

3. A method according to claim 1 or 2, wherein the layer comprising the recombination catalyst is disposed closer to one side of the membrane than the other but has at least one layer of proton conducting polymer thereover and thus does not form an external layer of the proton exchange membrane.

4. A method according to any preceding claim, wherein the layer comprising the reinforcement polymer does not form an external layer of the proton exchange membrane.

5. A method according to any preceding claim, wherein at least three of the layers do not contain reinforcement polymer or recombination catalyst.

6. A method according to any preceding claim, wherein each layer has a thickness, after drying, of: no more than 25 pm, 20 pm, 15 pm, or 10 pm; no less than 1 pm, 2 pm, 3 pm, 4 pm, or 5 pm; and/or within a range defined by any combination of the aforementioned upper and lower limits.

7. A method according to any preceding claim, wherein the proton exchange membrane has a total thickness in a range 40 to 115 micrometres.

8. A method according to any preceding claim, wherein the reinforcement polymer is expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI).

9. A method according to any preceding claim, wherein the proton conducting polymer layers are formed of perfluorosulfonic acid (PFSA) ionomer.

10. A method according to any preceding claim, wherein the radical reducing additive is ceria.

11. A method according to any preceding claim, further comprising forming a cathode catalyst layer on one side of the proton exchange membrane for catalysing a hydrogen evolution reaction and an anode catalyst layer on the other side of the proton exchange membrane for catalysing an oxygen evolution reaction thereby forming a catalyst coated membrane for a hydrogen producing water electrolyser.

12. A method according to claim 11, wherein one or both of the cathode catalyst layer and anode catalyst layer are formed by depositing a dispersion of catalyst material in liquid solvent.

13. A method according to claim 12, wherein the or each dispersion of catalyst material in liquid solvent further comprises ionomer.

14. A method according to any one of claims 11 to 13, wherein the cathode catalyst layer comprises platinum.

15. A method according to any one of claims 11 to 14, wherein the anode catalyst layer comprises an iridium oxide-based catalyst.

16. A method according to any preceding claim, wherein the recombination catalyst layer for catalysing the recombination reaction of molecular oxygen and hydrogen comprises platinum.

17. A catalyst coated proton exchange membrane for a hydrogen producing water electrolyser, the catalyst coated membrane comprising: a plurality of proton conducting polymer layers formed of ionomer and disposed on top of each other providing a non-laminated multi-layer proton exchange membrane structure, wherein at least one of the proton conducting polymer layers comprises a reinforcement polymer in addition to ionomer, wherein at least one of the proton conducting polymer layers comprises a recombination catalyst in addition to ionomer for catalysing a recombination reaction of molecular oxygen and hydrogen, wherein at least one of the proton conducting polymer layers comprises a radical reducing additive in addition to ionomer, and wherein a cathode catalyst layer is disposed on one side of the proton exchange membrane for catalysing a hydrogen evolution reaction and wherein an anode catalyst layer is disposed on the other side of the proton exchange membrane for catalysing an oxygen evolution reaction.

18. Use of a catalyst coated membrane according to claim 17 in a proton exchange membrane water electrolyser.

19. A proton exchange membrane water electrolyser comprising the catalyst coated membrane according to claim 17.

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