membrane

Abstract

The specification describes bilayer electrolyte membrane comprising: a first layer comprising a polymer electrolyte having particles of a recombination catalyst dispersed therein; and a second layer comprising a polymer electrolyte not having any recombination catalyst dispersed therein; wherein the thickness of the bilayer electrolyte membrane is 40-60 µm; the concentration of recombination catalyst in the first layer is 1-100 µg / cm2; and the bilayer electrolyte membrane is a single coherent polymer film. Also described is a method for preparing the bilayer electrolyte membrane, a catalyst coated membrane for an electrochemical device comprising the bilayer electrolyte membrane, and a fuel cell comprising the catalyst coated membrane.

Description

[0001] MEMBRANE

[0002] Field of the Invention

[0003] The present invention relates to electrolyte membranes, and their use in electrochemical devices, such as water electrolysers, and includes catalyst-coated membranes (CCMs) incorporating such membranes, and methods of their manufacture.

[0004] Background

[0005] The electrolysis of water to produce high purity hydrogen and oxygen can be carried out in both alkaline and acidic electrolyte systems. Suitably, such electrolysis is carried out in an electrolyser incorporating an ion-conducting membrane which acts as a polymer electrolyte. Those electrolysers that employ a solid proton-conducting polymer electrolyte membrane, or proton exchange membrane (PEM), are known as proton exchange membrane water electrolysers (PEMWEs). Those electrolysers that utilise a solid anion-conducting polymer electrolyte membrane, or anion exchange membrane (AEM), are known as anion exchange membrane water electrolysers (AEMWEs).

[0006] A catalyst-coated membrane (CCM) may be employed within the stack of a water electrolyser. CCMs comprises an electrolyte membrane, such as a PEM or AEM, with at least one of an anode catalyst layer and a cathode catalyst layer coated on a face of the membrane. Typically for PEMWEs, cathode catalyst materials comprise platinum. Anode catalysts for PEMWEs typically comprise iridium or iridium oxide (IrOx) materials, or oxides containing both iridium and ruthenium.

[0007] To form a water electrolyser, 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 an electrolyser system including power and control systems.

[0008] Electrolyte membranes, such as PEMs and AEMs, are also used in fuel cells. In proton exchange membrane fuel cells (PEM FC) the membrane is proton-conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.

[0009] It is desirable to reduce the thickness of membranes used in electrochemical devices, such as water electrolysers, to minimise electronic and ionic resistance. However it is also important to minimise any hydrogen crossover through the membrane, to avoid hydrogen mixing with oxygen and associated safety concerns.

[0010] For water electrolysers, it is beneficial to maintain low levels of hydrogen crossover even at high pressure differentials across the membrane. The use of high pressures during electrolyser operation is advantageous as it reduces the extent of compression required of the generated hydrogen and reduces operating costs. This has led to the use of membranes with a thickness of over 125 pm, and typically close to 200pm, or thicker. Examples of currently used membranes include Nation™ N115 (thickness 125 pm) or Nation™ N117 (thickness 175 pm).

[0011] It is also important that membranes are stable during long term electrochemical operation to minimise maintenance and the replacement of expensive components.

[0012] It is known to coat membrane components with a catalyst layer suitable for catalysing a recombination reaction of molecular oxygen and hydrogen (a “recombination catalyst”).

[0013] For example, WO2018 / 115821A1 (Johnson Matthey Fuel Cells Ltd) describes a CCM with a laminate structure comprising a first layer, an intermediate layer and a second layer. The intermediate layer is disposed between the first and second layers and has a recombination catalyst layer disposed on a first face thereof. The layers may be combined by laminating together first, second and third membrane components.

[0014] In addition, WO2023 / 172626A1 (Electric Hydrogen Co) describes a recombination layerwhich is placed externally to the membrane between a membrane (e.g. a CCM) and a porous transport layer (PTL), as shown in Figure 1.

[0015] It is also known to disperse a recombination catalyst within the membrane itself rather than coating it onto a membrane component.

[0016] For example, WO2018 / 185615A1 (3M) describes a membrane for a water electrolyser comprising at least one of metallic Pt or Pt oxide (as recombination catalyst), a cathode, and an anode. The recombination catalyst may be incorporated into the membrane by addition of Pt containing salts to the membrane followed by chemical reduction. Supported Pt can also be incorporated into the membrane via addition of the supported Pt, which has been prewetted with deionized water, to the liquid suspension of the ionomer, followed by casting a membrane from the resultant mixture. The examples describe mono-, bi- and tri-layer membranes comprising a recombination catalyst. In the examples the bi- and tri-layer membranes are formed by lamination. There remains a need to further enhance and develop membranes for electrochemical applications, for example for water electrolysis applications, which enable efficient operation at high pressure differentials across the membrane.

[0017] Summary of the invention

[0018] The present inventors have surprisingly found that thin electrolyte membranes with a thickness of 40-60 m may be produced with an excellent balance of low hydrogen crossover and high ionic conductivity. Such membranes may be produced by dispersing recombination catalyst particles in a membrane layer with a controlled thickness, and forming the membrane as a single coherent membrane without lamination interfaces. Such membranes enable the incorporation of a recombination catalyst whilst maintaining high ion-conductivity.

[0019] Therefore, in a first aspect of the invention there is provided a bilayer electrolyte membrane comprising: a first layer comprising a polymer electrolyte having particles of a recombination catalyst dispersed therein; and a second layer comprising a polymer electrolyte not having any recombination catalyst dispersed therein; wherein the thickness of the bilayer electrolyte membrane is 40-60 pm; the concentration of recombination catalyst in the first layer is 1-100 pg / cm2; and the bilayer electrolyte membrane is a single coherent polymer film.

[0020] Such membranes are particularly suitable for use in a water electrolyser. Providing the membrane as a single non-laminated component, rather than, for example, two or more membrane components laminated together, for instance as described in WO2018 / 185615A1 , additionally offers stability benefits and manufacturing process efficiencies on a large scale.

[0021] The membranes of the first aspect have particular utility as components of a catalyst coated membrane (CCM). It has been found that such CCMs offer an excellent balance between membrane resistance and low hydrogen cross-over during operation. Therefore, in a second aspect of the invention there is provided a CCM for an electrochemical device, comprising a membrane according to the first aspect.

[0022] Suitably, the CCM is for a water electrolyser, such as a PEM water electrolyser. In such cases the CCM comprises a cathode catalyst layer for catalysing a hydrogen evolution reaction and / or an anode catalyst layer for catalysing an oxygen evolution reaction. Typically, the cathode catalyst layer comprises platinum and / or the anode catalyst layer comprises iridium. The CCM may also be for a fuel cell, such as a PEM fuel cell. In such cases the CCM comprises a cathode catalyst layer for catalysing an oxygen reduction reaction and / or an anode catalyst layer for catalysing a hydrogen oxidation reaction.

[0023] In a third aspect of the invention there is provided a water electrolyser or a fuel cell comprising a membrane according to the first aspect, or a catalyst coated membrane according to the second aspect.

[0024] The present inventors have also advantageously found that electrolyte membranes comprising a recombination catalyst layer as described herein may be prepared by the deposition of suitable inks rather than by a lamination method. Therefore in a fourth aspect of the invention there is provided a method of manufacturing a bilayer electrolyte membrane according to the first aspect, the method comprising the steps of:

[0025] (i-a) applying an ink comprising an ionomer and not having any recombination catalyst dispersed therein, onto a substrate to form a precursor second layer;

[0026] (ii-a) applying an ink comprising an ionomer and having a recombination catalyst dispersed therein, onto said precursor second layer to form a precursor first layer; or

[0027] (i-b) applying an ink comprising an ionomer and having a recombination catalyst dispersed therein, onto a substrate to form a precursor first layer;

[0028] (ii-b) applying an ink comprising an ionomer and not having any recombination catalyst dispersed therein, onto said precursor first layer to form a precursor second layer; and

[0029] (iii) drying the product of step (ii-a) or (ii-b).

[0030] Brief description of the Figures

[0031] Figure 1 shows a schematic of an example arrangement of an electrolyte membrane of the invention.

[0032] Figure 2 shows a schematic of an example arrangement of a comparative catalyst coated membrane not according to the invention.

[0033] Figure 3 shows the degree of crossover (%H2 in O2) for CCM 1 and CCM 2 prepared in the examples.

[0034] Detailed Description

[0035] Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise. Any sub-headings are for convenience only and are not intended to limit the invention.

[0036] Bilayer membrane

[0037] The bilayer membranes described herein are particularly suitable as a proton exchange membrane (PEM), such as a PEM for a water electrolyser. It will however be understood by the skilled person that the layer structure described herein would have utility in other types of electrolyte membrane, such as proton exchange membranes for fuel cells, and anion exchange membranes for water electrolysers, fuel cells or other applications.

[0038] The membrane is a bilayer electrolyte membrane, by which it is meant that the only layers present are the first and second layers which will be more fully described in later sections. References to “the membrane” should be understood to mean the bilayer electrolyte membrane unless context requires otherwise.

[0039] The membrane is a single coherent polymer film. The term ‘coherent’ as used herein means that the membrane is free from internal lamination interfaces.

[0040] Lamination of ion conductive membranes comprises pressing and / or bonding at least two solid ion conductive membranes together, such membranes optionally being coated with a catalyst layer. 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 ion conductive polymer material. Microscopy and spectroscopy techniques can thus distinguish between lamination interfaces between layers of ion 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 non-laminated interface can be identified as being non-laminated in a membrane 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 / 1 H / 19F solid state NMR, neutron diffraction, and / or a combination of two or more of the aforementioned techniques.

[0041] Due to physical defects and / or chemical variations at lamination interfaces between ion conductive polymer membranes, such interfaces can increase the resistance of a multi-layer ion conductive membrane. As such, it has been found to be advantageous to fabricate a multilayer ion conductive membrane by depositing layers of ion conducting polymer dispersed in a liquid solvent to build up a multi-layer membrane structure rather than via lamination of individual solid layers / mem branes of ion conductive polymer.

[0042] Although the membrane is a single coherent polymer film, the first and second layers can be differentiated in the cross-section of the film because particles of a recombination catalyst are dispersed in the first layer (and are thus visible in cross-section) but not in the second layer.

[0043] The membrane has an overall a thickness of 40-60 .m. It is preferred that the membrane has an overall thickness of 45-55 .m. Despite this relatively low thickness, hydrogen crossover is kept low by the arrangement of recombination catalyst within the membrane.

[0044] The membrane thickness (and the thickness of the first and second layers) may be measured by scanning electron microscopy (SEM). The thickness of the first and second layers is calculated as the distance from one face of the layer to the boundary between the first and second layers. SEM analysis is carried out on cross sections of the membrane and the membrane and / or layer thickness measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values.

[0045] First layer

[0046] The first layer, which may also be referred to as the “recombination catalyst layer”, is a membrane layer which comprises particles of a recombination catalyst dispersed in an ion conducting polymer. By recombination catalyst it is meant a catalyst which catalyses the reaction between hydrogen and oxygen to form water. Accordingly, the recombination catalyst may be any catalyst capable of catalysing the reaction between hydrogen and oxygen to form water, thus reducing or preventing the crossover of either hydrogen or oxygen, or both, through the membrane. Suitably, the recombination catalyst is selected from one or more of platinum, palladium, and alloys or mixed oxides thereof. Preferably, the recombination catalyst is platinum, or a platinum alloy, such as platinum alloyed with one or more other platinum group metals (i.e. the group of elements comprising platinum, palladium, iridium, rhodium, ruthenium, and osmium) or alloyed with cobalt. It is particularly preferred that the particles of a recombination catalyst consist of platinum. The recombination catalyst may be supported or unsupported, preferably unsupported. The term unsupported will be readily understood by the skilled person. For example, it will be understood that the catalyst particles are not bound or fixed to a catalyst support, such as a carbon support, by physical or chemical bonds, e.g. by way of ionic or covalent bonds, or nonspecific interactions such as Van der Waals forces. It has been found that the use of an unsupported recombination catalyst facilitates ink processing prior to membrane formation, and offers increased membrane stability during electrochemical operation, avoiding routes of degradation via corrosion of the catalyst support.

[0047] In cases in which the membrane is for a PEM electrochemical device, the ion conducting polymer is suitably a proton conducting polymer, and in particular a partially- or fully-fluorinated sulphonic acid polymer. Examples of suitable proton-conducting polymers include perfluorosulphonic (PFSA) acid polymers, such as perfluorosulphonic acid polymers available from 3M Corporation or Aquivion (RTM) ion-conducting polymers available from Solvay. It may be preferred that the ion conducting polymer is a PFSA polymer and has an equivalent weight in the range of and including 350 to 1200 EW, such as 600 to 1000 EW, such as 700 to 900 EW. It may be preferred that the equivalent weight of the ion conducting polymer in the recombination catalyst layer is greater than the equivalent weight ion conducting polymer in any other layers of the membrane. It may be preferred that the ion conducting polymer in the second layer is the same as the ion conducting polymer in the first layer. It may alternatively be preferred that the ion conducting polymer in the first and second layers are different.

[0048] By dispersed in the ion conducting polymer it is meant herein that the particles of recombination catalyst are distributed throughout the recombination catalyst layer, i.e. they are not located in a discrete layer or region of the recombination catalyst layer.

[0049] Recombination catalyst

[0050] It is preferred that the particles of recombination catalyst have a particle size distribution such that the d90 is less than or equal to 3.0 .m. The use of particles with a d90 less than or equal to 3.0 .m offers improved mechanical stability in thin membrane layers (such as layers with a thickness less than 30 .m) and offers benefits associated with ink processability and the use of ink in coating equipment. The term d90 as used with regards to the particle size distribution in the membrane refers to the number distribution of particle size (the value of particle diameter at 90% in the cumulative number distribution, i.e. 90 % of the total particles in the sample have a diameter smaller than this value). The d90 of particles in the membrane may be determined by scanning electron microscopy (SEM), for example analysing a cross section of the membrane by SEM and, from the resulting image, measuring the diameter of a population of (e.g. 100) particles by image analysis and then calculating the d90.

[0051] It may be preferred that the d90 is less than or equal to 2.8 pm, 2.6 pm, 2.5 pm, 2.4 pm, 2.3 pm, 2.2 pm, 2.1 pm or 2.0 pm. It may be preferred that the particles of recombination catalyst have a particle size distribution such that the d90 is greater than or equal to 1.0 pm, 1.5 pm, 1 .7 pm, or 1.9 pm. It may be further preferred that the particles of recombination catalyst have a particle size distribution such that the d90 is in the range of and including 1.0 to 3.0 pm, or 1 .5 to 3.0 pm, such as 1.5 to 2.8 pm, or 1 .5 to 2.6 pm.

[0052] It may be preferred that the particles of recombination catalyst have an average particle size greater than or equal to 0.1 pm. The average particle size may be determined by scanning electron microscopy (SEM), for example analysing a cross section of the membrane by SEM, and from the resulting image measuring the diameter of a population of (e.g. 100) observable particles by image analysis and then calculating the average (mean) particle size. The use of particles greater than 0.1 pm offers advantages with regards to efficient ink preparation and their use has been shown to provide significant reduction in hydrogen crossover.

[0053] It may be preferred that the average particle size is greater than or equal to 0.2 pm, 0.3 pm, 0.4 pm, 0.5 pm, 0.6 pm, 0.7 pm, 0.8 pm, or 0.9 pm. It may be preferred that the particles of recombination catalyst have an average particle size less than or equal to 2.0 pm, 1 .8 pm, 1.6 pm, 1 .5 pm, 1.4 pm, 1.3 pm or 1.2 |im. It may be preferred that the particles of recombination catalyst have an average particle size in the range of and including 0.2 to 2.0 pm, 0.5 to 2.0 pm, such as 0.7 to 1.8 ^m, or 0.8 to 1.5 pm.

[0054] The recombination catalyst may be supported or unsupported. The preferred features of particle size distribution and average particle size apply to both supported and unsupported recombination catalyst particles.

[0055] The first layer has a recombination catalyst loading (e.g. platinum loading) in the range of 1- 100 pg / cm2, preferably in the range of 5-50 pg / cm2, more preferably in the range of 30-50 pg / cm2. It has been found that this range of catalyst loading provides a suitable balance between reducing the level of hydrogen crossover during use and the cost associated with the inclusion of catalyst in the membrane. The catalyst loading may be determined by inductively coupled plasma mass spectrometry (ICP-MS).

[0056] The recombination catalyst layer preferably has a thickness in the range of 10-30 pm. The dispersion of particles of a recombination catalyst in a membrane layer of 10-30 pm offers improved membrane stability benefits in comparison with the use of thinner catalyst layer, e.g. applied to a membrane surface such as described in WO2018 / 185615A1. The use of a recombination catalyst layer with a thickness greater than 30 .m is not required to substantially reduce hydrogen crossover and can provide manufacturing difficulties, in particular when forming non-laminated membrane structures. The thickness of the recombination catalyst layer may be determined by SEM analysis of a cross-section of the membrane. It is preferred that the recombination catalyst layer has a thickness in the range of 15-25 pm. Such thicknesses offer a suitable balance between the reduction of hydrogen crossover by the formed membrane and manufacturing efficiency.

[0057] It is preferred that the thickness of the first layer (recombination catalyst layer) is less than the thickness of the second layer (without recombination catalyst). This asymmetry means that the recombination catalyst is placed closer to the anode than the cathode in a water electrolyser configuration, which is considered beneficial for the reduction in hydrogen crossover.

[0058] Second layer

[0059] It is preferred that the second ion conducting polymer layer has a thickness in the range of 20 to 40 pm, such as in the range of 25 to 35 pm.

[0060] It is preferred that the membrane comprises a reinforcement polymer, such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). The first and / or second layers may contain a reinforcement polymer, and it is preferred that the second layer contains a reinforcement polymer. It is particularly preferred that the second layer contains reinforcement polymer and the first layer does not contain a reinforcement polymer. Where a reinforcement polymer is present, it is not considered as a separate layer but is considered a component of the first or second layers. The reinforcement material may comprise a porous reinforcement polymer sheet which is impregnated with ion conducting polymer, the reinforcement material optionally being expanded polytetrafluoroethylene (ePTFE). As typical reinforcement polymer materials are not conductive to ions, or not sufficiently conductive to ions, the reinforcement layer is thus formed using a porous reinforcement polymer which is impregnated with ion conducting polymer through the pores of the material to provide ion conductive paths from one side of the layer to the other side of the layer.

[0061] Preferred features of the membrane

[0062] Preferably, the membrane comprises a radical reducing additive (e.g. peroxide radical reducing additive, such as a cerium-containing compound, for example a cerium salt or cerium oxide (CeC>2)). 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. For the avoidance of doubt, the radical reducing additive is a component distinct from the recombination catalyst. Where present, it may be preferred that the radical reducing additive is dispersed within the recombination catalyst layer. In some embodiments, the membrane does not comprise a radical reducing additive.

[0063] The thickness of the ion conducting polymer layers may be adjusted, for example, by varying the number of deposition passes of ion conducting polymer during manufacture of the membrane, or by variation in the pump speed during deposition of ion-conducting polymer.

[0064] In a preferred embodiment the bilayer membrane comprises or consists of: (i) a first layer having particles of a recombination catalyst dispersed therein and with a thickness of 10-30 .m; and (ii) a second layer with a thickness in the range of 20-40 .m; wherein the thickness of the bilayer electrolyte membrane is 40-60 .m, the concentration of recombination catalyst (such as platinum) in the first layer is 1-100 pg / cm2, and the bilayer electrolyte membrane is a single coherent polymer film. In such configurations it is preferred that the second layer comprises a reinforcement polymer, such as expanded polytetrafluoroethylene (ePTFE) or polybenzimidazole (PBI). Such a membrane structure has been found to provide a particularly suitable balance between membrane resistance and level of hydrogen crossover.

[0065] Catalyst coated membranes

[0066] The membranes as described herein may suitably be used as part of a catalyst coated membrane (CCMs). Such CCMs have an anode catalyst layer and / or a cathode catalyst layer applied to a face of the membrane.

[0067] Cathode catalyst layer

[0068] In the case of a CCM for a water electrolyser, a cathode catalyst layer may be applied to a surface of the membrane comprising a catalyst for catalysing the hydrogen evolution reaction. It is preferred that the cathode catalyst layer comprises platinum, for example a platinum-on- carbon catalyst. The 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 directly coated onto the membrane.

[0069] It is preferred that the cathode catalyst layer comprises platinum and has a platinum loading, provided by the platinum material (such as a platinum-on-carbon material), of less than 1 mgptcm-2. That said, there is also a lower limit to the amount of platinum which must be provided. As such, the platinum loading of the cathode layer is suitably more than 0.01 mgptcm-2, 0.04 mgpt cm-2, or 0.06 mgpt cm-2.

[0070] The cathode catalyst layer may comprise a platinum-on-carbon catalyst material which is between 20 and 60 wt% platinum, optionally between 40 and 60 wt% platinum. The platinum may be advantageously provided as nanoparticles on the carbon support material. The nanoparticles of platinum may have a crystallite size of: at least 1 nm, 2 nm, or 3 nm; no more than 15nm, 10 nm, or 6 nm; or within a range defined by any combination of the aforementioned lower and upper limits. Crystallite size can be measured through XRD and fitted using Rietveld analysis. X-ray diffraction data is collected using Cu Ka radiation (A = 1.5406 and 1.54439 A) on a Bruker AXS D8. Crystallite sizes are calculated from the Rietveld refinements using the LVol-IB method.

[0071] The cathode catalyst layer may comprise a platinum-on-carbon catalyst material in which the carbon support material is a partially graphitized carbon material (e.g., a heat-treated carbon black). Graphite material is more corrosion resistant. However, graphite support materials have a low surface area. As such, there is a compromise between the requirements of high surface area and high corrosion resistance. A partially graphitized material has been found to be a good compromise between surface area requirements and corrosion resistance requirements for the carbon support in this water electrolyser application.

[0072] Typically the cathode catalyst layer comprises both catalyst and an ion conducting polymer. The ion conducting polymer in the cathode catalyst layer may be an ionomer, such as a perfluorosulphonic (PFSA) acid polymer. The ion conducting polymer of the cathode layer may be the same or similar to that used in the membrane.

[0073] The cathode catalyst layer may have an ion conducting polymer I carbon weight ratio in the range of and including 0.6 and 1.0 (noting that this is the weight ratio between the ion conducting polymer and carbon, the platinum is not taken into account in this calculation). Furthermore, the cathode catalyst layer may have a thickness in the range of any including 1 to 15, 4 to 15, or 8 to 15 .m.

[0074] An example of such a cathode layer comprises the following features:

[0075] Nominal Pt Loading - 0.4mg Pt cm-2

[0076] Ion conducting polymer - Ionomer 800EW 3M C4 side chain

[0077] Ion conducting polymer / Carbon weight ratio - 0.8

[0078] Thickness - approximately 10 to 11 .m

[0079] Catalyst is 50wt% Pt-on-carbon

[0080] Carbon is a partially graphitized carbon support material

[0081] Anode catalyst layer

[0082] In the case of a CCM for a water electrolyser, an anode catalyst layer may be applied to a surface of the membrane comprising a catalyst for catalysing the oxygen evolution reaction. In the case that the CCM is for a PEMWE, it is preferred that the anode catalyst layer comprises iridium, such as iridium oxide or mixed oxides of iridium and another metal or metals.

[0083] The anode material can be formulated into an ink, suitably in an ion conducting polymer, printed ex-situ onto a PTFE sheet, and transferred onto the membrane by hot pressing. Alternatively, the ink can be directly coated onto the membrane.

[0084] The anode catalyst layer typically comprises both catalyst and an ion conducting polymer.

[0085] The anode catalyst layer may comprise between 5 and 20 wt% ion conducting polymer, for example between 8 and 15 wt%. Suitably, the amount of catalyst material in the anode catalyst layer can be 80 to 95 wt%, optionally between 85 and 92 wt%. The iridium loading of the anode catalyst layer is preferably less than 3 mg lr / cm2, optionally in a range 0.05 and 3 mg lr / cm2. The iridium containing catalyst material can be an iridium oxide catalyst material and the anode catalyst layer may have a thickness between 6 and 15 .m.

[0086] It is preferred that the anode catalyst layer is adjacent the first layer (containing recombination catalyst).

[0087] Manufacturing method

[0088] Also provided is a method of manufacturing an electrolyte membrane as described hereinbefore, the method comprising the steps of:

[0089] (i-a) applying an ink comprising an ionomer and not having any recombination catalyst dispersed therein, onto a substrate to form a precursor second layer;

[0090] (ii-a) applying an ink comprising an ionomer and having a recombination catalyst dispersed therein, onto said precursor second layer to form a precursor first layer; or

[0091] (i-b) applying an ink comprising an ionomer and having a recombination catalyst dispersed therein, onto a substrate to form a precursor first layer;

[0092] (ii-b) applying an ink comprising an ionomer and not having any recombination catalyst dispersed therein, onto said precursor first layer to form a precursor second layer; and

[0093] (iii) drying the product of step (ii-a) or (ii-b).

[0094] The term “first / second precursor layer” is used to refer to the layer which eventually becomes the first / second layer following step (iii).

[0095] The method involves carrying out steps (i-a), (ii-a) and (iii) or steps (i-b), (ii-b) and (iii) in sequence. When steps (i-a) and (ii-a) are followed, the precursor second layer is applied onto the substrate before the precursor first layer. When steps (i-b) and (ii-b) are followed, the precursor first layer is applied onto the substrate before the precursor second layer.

[0096] It may be preferred that the method involves carrying out steps (i-a), (ii-a) and (iii). This order is better suited to incorporating a reinforcement into the precursor second layer. In preferred embodiments the method comprises an additional step, carried out after step (i-a) and before step (ii-a), of adding a reinforcement onto the precursor second layer. In this way the reinforcement becomes incorporated into the precursor second layer.

[0097] It may be preferred to carry out a drying step after step (i-a) or (i-b). It may also be preferred to carry out a drying step after step (ii-a) or (ii-b). Any drying step(s) carried out between steps (i) and (ii), and between steps (ii) and (iii) should be sufficient to remove part or all of the solvent, but should not be high enough to cause annealing. The skilled person will be able to determine suitable temperatures and durations for the drying step.

[0098] In cases, in which the particles of a recombination catalyst are platinum particles, it is preferred that the particles of platinum are provided as platinum black. It has been found that platinum black may be processed efficiently to provide an ink suitable for use in the methods as described herein and shows reduced agglomeration during ink formation than other platinum sources, such as platinum on carbon.

[0099] Preferably, the particles of recombination catalyst in the ink have a d90 less than 3.0 .m. The particle size distribution may be determined by using a laser diffraction method. For example, the d90 may be determined by diluting the ink in 80:20 (v / v) ethanol: water and analysing the particle size distribution by laser diffraction, such as by using a Malvern Mastersizer 3000. The term d90 with reference to the particles in the ink refers to the volume-based particle size (the value of particle diameter at 90% in the cumulative volume distribution, i.e. 90 vol% of the particles in the sample have a diameter smaller than this value)

[0100] The desired particle size distribution may be suitably achieved by processing the ink using a high shear techniques, for example microfluidisation. It may be preferred that forming the ink comprises passing a dispersion of a platinum source, such as platinum black, and an ion conducting polymer through a microfluidizer.

[0101] The ink typically comprises the ion conducting polymer dispersed in a 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, such as ethanol, to water may be: at least 60: 40, 70: 30, or 75: 25; no more than 95: 5; 90: 10, or 85: 15; or within a range defined by any combination of the aforementioned lower and upper limits. The solvent is formulated for achieving the desired dispersion, coating, and drying characteristics. The ion conducting polymer may be provided in the ink at a weight percentage with respect to the total weight of recombination catalyst and ion conducting polymer: at least 7 wt%, 10 wt%, 14 wt%, or 16 wt%; no more than 22 wt%, 20 wt%, or 18 wt%; or within a range defined by any combination of the aforementioned lower and upper limits. The ion conducting polymer content is selected for achieving the desired dispersion, coating, and drying characteristics.

[0102] The ink may also comprise a radical reducing additive (e.g., a peroxide radical reducing additive such as ceria). For example, the radical reducing additive may be provided in the dispersion at a weight percentage, relative to the weight of ion conducting polymer, of : at least 0.15 wt%, 0.20 wt%, or 0.23 wt%; no more than 0.35 wt%, 0.30 wt%, or 0.28 wt%; or within a range defined by any combination of the aforementioned lower and upper limits.

[0103] The ink may be applied in steps (i) and (ii) by any suitable method, but typically by casting or printing the ink onto a substrate to form the layer.

[0104] 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. An example of a suitable cathode catalyst for a water electrolyser is a platinum on carbon catalyst, optionally provided as a decal. In the case of CCMs for PEMWEs, iridium oxide-based catalysts may be used for the anode. The iridium oxide-based catalyst can be prepared into an ink comprising ion conducting polymer, 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.

[0105] The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention and are not intended to limit its scope.

[0106] Examples

[0107] Formation of inks

[0108] An ion conducting polymer dispersion was created by mixing an ion conducting polymer (PFSA ionomer, 825EW, 3M Advanced Materials) with water and ethanol (-20:80). This dispersion was dosed with a second dispersion of radical scavenging additive (ceria), water and ethanol to achieve a target amount of radical scavenger (-0.3 wt% relative to weight of ionomer). This is referred to below as the first ink.

[0109] Particulate platinum black catalyst (Johnson Matthey pic) was added to a mixture of ion conducting polymer (PFSA ionomer, 825EW, 3M Advanced Materials) and water. The mixture was passed through a micro fluidiser (Microfluidics M-110P) using a z-type chamber at 30,000 psi until the ink until there was significant reduction in observable viscosity. A second ink was prepared by adding a quantity of platinum catalyst dispersion to the first ink with the amount of platinum adjusted to achieve a target Pt loading of 30 pg / cm2.

[0110] Formation of bilayer 50- pm proton exchange membrane (E1)

[0111] A bilayer membrane was prepared having the layer structure shown in Figure 1. The bilayer membrane included the following: a -21.0 pm thick layer with recombination catalyst at a concentration of 30 pg / cm2“first layer”; a -28.5 pm thick layer containing a reinforcement but without any recombination catalyst “second layer”.

[0112] The bilayer membrane was prepared as follows. The first ink was coated using slot-die coating onto a substrate to form the Pt-free layer (the “second layer”) with the inclusion of an ePTFE reinforcement and dried to remove solvent in an oven.

[0113] The second ink was coated onto the second layer following the same method to form the Ptcontaining layer (the “first layer”) and dried to remove solvent in an oven.

[0114] The overall membrane was then annealed.

[0115] Formation of trilayer 50- pm proton exchange membrane (CE2)

[0116] A bilayer membrane was prepared having the layer structure shown in Figure 2. The trilayer membrane included the following: a -10.5 pm thick layer without any recombination catalyst or reinforcement “first layer”; a -10.5 pm thick layer containing Pt recombination catalyst at a concentration of 30 pg / cm “second layer”; a -28.5 pm thick layer containing a reinforcement but without any recombination catalyst “third layer”.

[0117] The trilayer membrane was produced as above, except that the platinum-containing dispersion used to make the second layer had twice the concentration of platinum, and was coated into a layer half the thickness, to give identical platinum loadings. Following this, a first layer was applied using the first ink.

[0118] Measurement of membrane thickness and layer thickness

[0119] Two specimens of membrane (ca. 2 x 1 cm) were taken at different locations on the membrane sample. Specimens were embedded in epoxy resin and cured overnight at room temperature, and the block ground and polished, and coated with carbon. The cross section of each sample was then analysed by scanning electron microscope (SEM) at 2-3 locations and the membrane thickness and the membrane layer thickness were measured from the SEM image. An arithmetic mean of the membrane thickness and I or layer thickness was then calculated from the measurements (~10 measurements for each thickness parameter).

[0120] Preparation of CCMs

[0121] CCMs 1 and 2 were prepared from membranes E1 (bilayer) and CE2 (trilayer) with a Pt / C- PFSA ionomer cathode catalyst layer (with a Pt loading of 0.4 mg cm-2of Pt) and an IrOx- PFSA ionomer anode catalyst layer (with an Ir loading of 1.4 mg cm-2) with the anode layer applied to recombination catalyst-containing membrane layer.

[0122] CCM performance testing

[0123] Hydrogen crossover

[0124] The level of hydrogen crossover for each CCM was measured at different current densities using the following method. A water electrolysis cell was prepared incorporating the catalyst- coated membrane to be tested. The cell temperature was held at to 80 °C and the anode and cathode pressure were set to 2 bar and 6 bar respectively. Next, the current density was set to 3 A / cm2, and this was maintained for 2.5 hours. The current density was then set to 2 A / cm2for 2 hours, then 1 A / cm2for 2.5 hours, then 0.5 A / cm2for 4.5 hours. The % of H2 in the oxygen at the anode gas outlet was measured by a sensor which measures thermal conductivity.

[0125] Electrical performance

[0126] The electrical performance of the CCMs was tested by the following method. The CCM was first conditioned with water flowing across the anode at 80 °C for 12 h. Then the polarisation measurement was performed. Anode and cathode pressures were kept equal at 0 bar. The current density was increased from 0 A / cm2to 1 A / cm2in steps of 0.04 A / cm2and then from 1 A / cm2to 4 A / cm2in steps of 0.08 A / cm2. The current density was then decreased from 4 A / cm2to 1 A / cm2in steps of 0.08 A / cm2and then from 1 A / cm2to 0 A / cm2in steps of 0.04 A / cm2. The upward going measurement (low to high current) was used for further analysis. The results are shown in Table 1.

[0127] Table 1.

[0128] Figure 3 shows the % H2 in O2 as a function of current density for CCMs 1 and 2. The % H2 in O2 is an indication of the degree of hydrogen crossover. CCM1 showed much lower H2 crossover compared to CCM2. Both CCMs show low resistance values consistent with a single coherent polymer film structure.

Claims

Claims1. A bilayer electrolyte membrane comprising: a first layer comprising a polymer electrolyte having particles of a recombination catalyst dispersed therein; and a second layer comprising a polymer electrolyte not having any recombination catalyst dispersed therein; wherein the thickness of the bilayer electrolyte membrane is 40-60 pm; the concentration of recombination catalyst in the first layer is 1-100 pg / cm2; and the bilayer electrolyte membrane is a single coherent polymer film.

2. A bilayer electrolyte membrane according to claim 1, wherein the recombination catalyst is selected from one or more of platinum, palladium, and alloys or mixed oxides thereof.

3. A bilayer electrolyte membrane according to any of claims 1 to 2, wherein the first layer has a thickness of 10-30 pm.

4. A bilayer electrolyte membrane according to any of claims 1 to 3, wherein the first layer has a thickness of 15-25 pm.

5. A bilayer electrolyte membrane according to any of claims 1 to 4, wherein the second layer has a thickness of 20-40 pm.

6. A bilayer electrolyte membrane according to any of claims 1 to 5, wherein the second layer has a thickness of 25-35 pm.

7. A bilayer electrolyte membrane according to any of claims 1 to 6, wherein the thickness of the bilayer electrolyte membrane is 45-55 pm.

8. A bilayer electrolyte membrane according to any of claims 1 to 7, wherein the concentration of recombination catalyst in the first layer is 5-50 pg / cm2.

9. A bilayer electrolyte membrane according to any of claims 1 to 8, wherein the concentration of recombination catalyst in the first layer is 30-50 pg / cm2.

10. A bilayer electrolyte membrane according to any of claims 1 to 9, wherein the first and / or second layer contains a reinforcement polymer.

11. A bilayer electrolyte membrane according to claim 10, wherein the second layer contains reinforcement polymer and the first layer does not contain a reinforcement polymer.

12. A bilayer electrolyte membrane according to any of claims 1 to 11 , wherein the thickness of the first layer is less than the thickness of the second layer.

13. A catalyst coated membrane for an electrochemical device, comprising a bilayer electrolyte membrane according to any one of claims 1 to 12.

14. A fuel cell or a water electrolyser comprising a membrane according to any of claims 1 to 12 or a catalyst coated membrane according to claim 13.

15. A method of manufacturing a bilayer electrolyte membrane according to any of claims 1 to 12, the method comprising the steps of:(i-a) applying an ink comprising an ionomer and not having any recombination catalyst dispersed therein, onto a substrate to form a precursor second layer;(ii-a) applying an ink comprising an ionomer and having a recombination catalyst dispersed therein, onto said precursor second layer to form a precursor first layer; or(i-b) applying an ink comprising an ionomer and having a recombination catalyst dispersed therein, onto a substrate to form a precursor first layer;(ii-b) applying an ink comprising an ionomer and not having any recombination catalyst dispersed therein, onto said precursor first layer to form a precursor second layer; and(iii) drying the product of step (ii-a) or (ii-b).

16. A method according to claim 15, comprising steps (i-a) and (ii-a) and an additional step, carried out after step (i-a) and before step (ii-a), of adding a reinforcement onto the precursor second layer.

17. A method according to claim 15 or claim 16, wherein a drying step is carried out after step (i-a) or (i-b).

18. A method according to any of claims 15 to 17, wherein a drying step is carried out after step (ii-a) or (ii-b).

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