WO2004114451A1

WO2004114451A1 - Sealed and gasketed membrane electrode assembly

Info

Publication number: WO2004114451A1
Application number: PCT/GB2004/002566
Authority: WO
Inventors: Christopher James Barnett; David Edward Barnwell; Silvain Buche; Adam John Hodgkinson; Peter Antony Trew
Original assignee: Johnson Matthey Public Limted Company
Priority date: 2003-06-20
Filing date: 2004-06-14
Publication date: 2004-12-29
Also published as: GB0314436D0

Abstract

A sealed and gasketed membrane electrode assembly comprising first and second gas diffusion substrates, a polymer electrolyte membrane, first and second electrocatalyst layers and an elastomeric, electrically insulating, mold processable material that encapsulates the edges of the first gas diffusion substrate, the second gas diffusion substrate and the polymer electrolyte membrane, and additionally forms one or more gasketing members.

Description

SEALED AND GASKETED MEMBRANE ELECTRODE ASSEMBLY

The present invention relates to a membrane electrode assembly suitable for use in a polymer electrolyte membrane fuel cell wherein the membrane electrode assembly comprises an integrated seal and gasket.

A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Fuel cells are a clean and efficient power source, and may replace traditional power sources such as the internal combustion engine in both stationary and automotive power applications.

In a polymer electrolyte membrane (PEM) fuel cell, the electrolyte is a solid polymer membrane which is electronically insulating but ionically-conducting. Proton- conducting membranes based on perfluorosulphonic acid materials are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.

The principle component of a polymer electrolyte fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer membrane. On either side of the membrane there is an electrocatalyst layer, containing an electrocatalyst, which is tailored for the different requirements at the anode and the cathode. Finally, adjacent to each electrocatalyst layer there is a gas diffusion substrate. The gas diffusion substrate must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the substrate must be porous and electrically conducting.

The MEAs can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion substrate to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of a membrane and laminated together to form the five-layer MEA. Alternatively, the electrocatalyst layer may be applied to both faces of the membrane to form a catalyst coated membrane. Subsequently, gas diffusion substrates are applied to both faces of the catalyst coated membrane. Finally, an MEA can be formed from a membrane coated on one side with an electrocatalyst layer, a gas diffusion substrate adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the membrane.

Typically tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Field flow plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs, removing products, providing electrical connections and providing physical support. The field flow plates and MEAs in the stack are compressed together at pressures typically from 50 to 200psi absolute, using for example a bladder or piston system or a series of bolts located in stack end plates. Typically, one of the stack end plates also contains the necessary ports to provide access and removal from the stack of the reactants, products and any associated humidification water. Ports are also required to provide access to and removal of the stack coolant from the stack cooling plates which are needed to remove the excess heat generated within the MEAs. From the ports in the stack end plate the gases and fluids are transported through the stack to each field flow plate. The porting design may either be internal to the MEA or external of the MEA.

In the fuel cell stack it is essential that any loss to the atmosphere or any potential mixing of the reactants is prevented. This would lead to a decrease in the overall system efficiency and is a potentially hazardous situation due to the risk of combustion from the mixing of the fuel and oxidant. Sealing and gasketing in the stack are used for the purpose of preventing such occurrences. For the purposes of this description, the term "sealing" is used to denote a method of preventing fluid diffusion out of or through a single component. For example, the perimeter of a gas diffusion substrate can be sealed by impregnating the perimeter with a sealant material. The term "gasketing" is used to denote a method of preventing fluid diffusion between components by placing a resilient material between the two components. In a well-known method of sealing and gasketing the components in a fuel cell stack the membrane protrudes beyond the gas diffusion substrates by a considerable margin, e.g. by as much as 25mm, so that gaskets can be positioned between the protruding membrane and the field flow plates. The gaskets are held in place by compression. This method can be problematic, particularly with the very thin membranes (approximately 30μm) that are increasingly being used, because the membrane is weak and may be damaged by the compressive forces. Additionally it is wasteful to use large amounts of expensive membrane material in regions outside the active area of the membrane electrode assembly.

Several methods that avoid the problems associated with gasketing directly onto the membrane have been disclosed. WO 02/093669 discloses a method wherein gas diffusion substrates comprise gasket members on both planar faces. To form an MEA, the gasketed substrates may be catalysed and then combined with a membrane, or the gasketed substrates may be combined with a catalysed membrane. When the substrates and membranes are combined, a peripheral region of the membrane is sandwiched between the gasket members such that gas cannot diffuse between the two substrates. In this method, gasket members must be applied to both of the gas diffusion substrates before formation of the MEA. It would be advantageous to have a method of sealing and gasketing the components in a fuel cell stack wherein a single integrated component can be applied after the formation of the MEA, and the single component meets all the sealing and gasketing requirements of the fuel cell stack.

EP 1 018 177 describes a method of applying resilient seals to formed MEAs wherein the gas diffusion substrates and the membrane in the MEAs are coextensive. The present inventors have found that it is advantageous to apply elastomeric material to MEAs wherein the membrane extends beyond at least one of the gas diffusion substrates. If membrane is exposed beyond the gas diffusion substrates, there can be direct adhesion between the membrane and the elastomeric material and better sealing can be achieved.

Accordingly the present invention provides a sealed and gasketed membrane electrode assembly comprising a) a first gas diffusion substrate, having edges and first and second planar faces, wherein the first gas diffusion substrate has a central region and an edge region such that the edge region is adjacent to the edges; b) a second gas diffusion substrate, having edges and first and second planar faces, wherein the second gas diffusion substrate has a central region and an edge region such that the edge region is adjacent to the edges; c) a polymer electrolyte membrane, having edges and first and second planar faces, wherein the membrane is located between the first and second gas diffusion substrates such that the first planar face of the membrane faces the first planar face of the first gas diffusion substrate, the second planar face of the membrane faces the first planar face of the second gas diffusion substrate and the edges of the membrane extend beyond the edges of at least one of the first and second gas diffusion substrates; d) a first electrocatalyst layer, wherein the first electrocatalyst layer is located between the first planar face of the first gas diffusion substrate and the first planar face of the polymer electrolyte membrane, and wherein the first electrocatalyst layer is not present as a layer on the edge region of the first gas diffusion substrate; e) a second electrocatalyst layer, wherein the second electrocatalyst layer is located between the first planar face of the second gas diffusion substrate and the second planar face of the polymer electrolyte membrane, and wherein the second electrocatalyst layer is not present as a layer on the edge region of the second gas diffusion substrate; and f) an elastomeric, electrically insulating, mold processable material that encapsulates the edges of the first gas diffusion substrate, the second gas diffusion substrate and the polymer electrolyte membrane; wherein the elastomeric material forms a layer on the edge region on the second planar face of each of the first and second gas diffusion substrates; wherein the elastomeric material is not present as a layer on the first planar faces of each of the first and second gas diffusion substrates; and wherein the elastomeric material forms one or more gasketing members. The gas diffusion substrates may be any suitable gas diffusion substrates known to those skilled in the art. Typical substrates include substrates based on carbon paper (eg Toray® paper available from Toray Industries, Japan), woven carbon cloths (eg Zoltek® PWB-3 available from Zoltek Corporation, USA) or non-woven carbon fibre webs (eg Optimat 203 available from Technical Fibre Products, UK). The carbon substrate is typically modified with a particulate material either embedded within the substrate or coated onto the planar faces, or a combination of both. The particulate material is typically a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE). Suitably the gas diffusion substrates are between 150 and 300μm thick. In a preferred embodiment there is a layer of particulate material such as carbon black and PTFE on the first planar faces of the first and second gas diffusion substrates. Suitably these layers are present on the entire first planar face, but they may only be present on the central regions or they may only be present on the edge regions.

The first and second gas diffusion substrates both have a central region and an edge region extending through the thickness of the substrates. The edge region is adjacent to the edges of the gas diffusion substrates. Suitably the edge region comprises a volume within the substrate wherein all the region is within a distance of less than 20mm from the edges, preferably all the region is within a distance of less than 16mm from the edges. The planar faces of the gas diffusion substrates may be continuous so that the edges of the substrate are found only at the periphery of the substrates. Alternatively, the planar faces of the gas diffusion substrates may not be continuous and may comprise ports or holes. In this case, the substrates will have edges around the periphery of the substrate and also within the substrate at the ports. The edge region will encompass the volume within the substrate that is adjacent to the periphery of the substrate and the volume that is adjacent to the ports.

In a preferred embodiment of the invention, the first and second gas diffusion substrates have the same dimensions. Preferably the central regions of the first and second gas diffusion substrates are the same size, and the edge regions of the first and second gas diffusion substrates are the same size. The polymer electrolyte membrane may be any type of ion-conducting membrane known to those skilled in the art. Suitably the membrane is proton-conducting. In state of the art membrane electrode assemblies, the membranes are often based on perfluorinated sulphonic acid materials such as Nation® (DuPont), Flemion® (Asahi Glass) and Aciplex® (Asahi Kasei). The membrane may be a composite membrane, containing the proton-conducting material and other materials that confer properties such as mechanical strength. For example, the membrane may comprise a proton-conducting membrane and a matrix of silica fibres, as described in EP 875 524. The membrane is suitably less than lOOμm thick, preferably less than 50μm thick.

The membrane is located between the first and second gas diffusion substrates so that each planar face of the membrane is adjacent to a planar face of the first or the second gas diffusion substrate. The planar faces of the membrane may be continuous so that the edges of the membrane are found around only at the periphery of the membrane. Alternatively, the planar faces of the membranes may not be continuous and may comprise ports or holes. In this case, the membranes will have edges around the periphery of the substrate and also within the membrane at the ports. The edges of the membrane extend beyond the edges of at least one of the first and second gas diffusion substrates. Preferably the edges of the membrane extend beyond the edges of both the first and second gas diffusion substrates, so that protruding membrane is present at all the edges of the membrane electrode assembly. Suitably the membrane extends beyond the edges of at least one of the gas diffusion substrates and preferably both by between 0.5mm and 5mm, suitably between 0.5mm and 3mm. The present invention does not require that the membrane extends significantly beyond the membrane, e.g. by 25mm, as required by prior art methods that place gaskets between the membrane and the field flow plates. Therefore, the present invention does not waste significant amounts of expensive membrane material beyond the active area of the fuel cell.

In the present invention, the membrane extends beyond at least one of the gas diffusion substrates and the sealing region encapsulates the membrane. If the membrane did not extend beyond at least one gas diffusion substrate (e.g. if the components in the membrane electrode assembly were coextensive) then the contact between the membrane and the sealing region would be reduced and there would be a greater risk of gas leakage. An additional advantage of the membrane extending beyond at least one gas diffusion substrate is that the risk of electrical shorting between the two gas diffusion substrates is reduced. If the membrane was coextensive with the two substrates, or inboard of the substrates, then there is an increased risk that stray fibres from opposing substrates can touch, leading to an electrical short.

The electrocatalyst layers comprise an electrocatalyst which may be a finely divided metal black, or may be a supported catalyst wherein small metal particles are dispersed on electrically conducting particulate carbon supports. The electrocatalyst metal is suitably selected from

(i) the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium),

(ii) gold or silver,

(iii) a base metal, or an alloy or mixture comprising one or more of these metals or their oxides. The preferred electrocatalyst metal is platinum, which may be alloyed with other precious metals such as ruthenium, or base metals such as molybdenum or tungsten. If the electrocatalyst is a supported catalyst, the loading of metal particles on the carbon support material is suitably in the range 10-100wt%, preferably 15-60wt%.

The electrocatalyst layers suitably comprise other components, such as ion-conducting polymer, which is included to improve the ionic conductivity within the layer. To incorporate the layers into the membrane electrode assembly, the layers can be formed on the gas diffusion substrates, or the layers can be formed on the membrane. The layers are applied such that in the final membrane electrode assembly, the electrocatalyst layers are not present as layers on the edge regions of the gas diffusion substrates and are only present on all or part of the central regions. In a particular embodiment the electrocatalyst layers are present on the entire central region, so that the active area of the cell (wherein the electrochemical reactions take place) is maximised. In an alternative embodiment, the electrocatalyst layers are only present on part of the central region such that there is a gap of at least 1mm between the edge of the central region and the edge of the electrocatalyst layer. The electrocatalyst layers should not be present on the edge regions of the substrates because the sealing region extends over the edge regions of the substrates and therefore the edge regions are outside the active area of the MEA. The electrocatalyst should not be outside the active area because this is a waste of expensive catalyst material and, moreover, if gaseous reactants can reach catalyst outside the active area there is a risk of heterogeneous gas phase reactions and resultant pin-holing of the membrane.

The elastomeric, electrically insulating, mold processable material is suitably a polymeric material chosen from a group including silicones, fluorosilicones, fiuoroelastomers (e.g. Viton), EPDM (ethylene propylene diene monomer) rubbers, thermoplastic elastomers (e.g. styrene-butadiene block copolymer) and liquid crystal polymer elastomers. The material must be chemically resistant in a fuel cell environment and must tolerate typical temperatures in a PEM fuel cell (up to 120°C for MEAs using perfluorinated membranes and potentially higher for other membranes). Suitably, the material is rapidly heat curable such that it can be employed in molding processes. Preferably the material should cure in less than 10 minutes at 120°C or below, most preferably the material should cure in seconds at 90°C or below. The viscosity of the material under molding conditions is suitably 2 to 200Pa.s., preferably 40 to 140Pa.s.

The elastomeric material must encapsulate the edges of the first gas diffusion substrate and the second gas diffusion substrate so that there cannot be fluid leakage through the edges of the substrates. The elastomeric material must encapsulate the edges of the polymer electrolyte membrane so that there is no route for fluid leakage along the interface of the sealing region and the membrane, and so that a single region of elastomeric material can seal the edges of both gas diffusion substrates.

The elastomeric material must be present as a layer on the edge region of the second planar face of each of the first and second gas diffusion substrates to prevent fluid egress through these edge regions. Preferably the elastomeric material is not present as a layer on any of the central region of the second planar face of each of the first and second gas diffusion substrates. The elastomeric material will probably impregnate the edge region of the gas diffusion substrate to a certain extent, and this will facilitate mechanical keying between the elastomeric material and the substrate. It is possible, but not necessary, that the elastomeric material will impregnate the entire thickness of the edge region of the gas diffusion substrate. Even if the elastomeric material impregnates the entire edge region, it does not form a layer on the first planar face of the gas diffusion substrate. Preferably, the elastomeric material does not impregnate any of the central region of the gas diffusion substrate because any part of the gas diffusion substrate that is impregnated with elastomeric material cannot take part in fuel cell reactions and will be outside the active area of the membrane electrode assembly. Electrocatalyst is present next to some or all of the central region and it is undesirable for electrocatalyst to be outside the active area. The elastomeric material is not present as a layer on the first planar faces of each of the first and second gas diffusion substrates so that the elastomeric material can be applied to a formed MEA.

The elastomeric material forms one or more gasketing members suitably in the form of ribs or ridges. It is advantageous that the elastomeric material has, the combined functions of sealing the edge of gas diffusion substrates and providing one or more gasketing members. The sealing and gasketing features of the membrane electrode assembly can be applied using a single material in a single step¹.

The gasketing members will ultimately be positioned adjacent to a co-operating field flow plate when the MEA is in a fuel cell stack. The gasketing members are compressible under fuel cell loads, and are compressed against the field flow plates so that gas cannot escape between the substrates and the plates. The gasketing member may compress against a flat portion of the field flow plate or may locate into a channel. The gasketing members may be present on one or both faces of the MEA, depending on the design of the field flow plates. The gasketing members may be located around the entire periphery of the MEA, or they may be located only in specific regions, e.g. along two sides of the MEA only. The gasketing members usually run parallel to the edges of the MEA, but they may also comprise cross-pieces wherein regions of the gasketing member are perpendicular to the edge of the MEA.

In one embodiment of the invention, the one or more gasketing members are located such that they will be ultimately be compressed between a field flow plate and the membrane. Compression of the gasket in this position provides mechanical sealing at the membrane edge and ensures effective sealing. In an alternative embodiment the one or more gasketing members are located such that they will ultimately be compressed between a field flow plate and a gas diffusion substrate. The advantage of having the gasketing members in this position is that it is easier to form gasketing members that are located on the substrates (as opposed to on the exposed membrane) because this region of the MEA is more rigid.

The gasketing members may have any shape suitable for compressing against a field flow plate. This includes ribs or ridges with conical or round cross-sections. The height of the gasketing member above the first or second gas diffusion substrate is suitably between 0.25-5mm, preferably between 0.5- 1.5mm. Suitably there is more than one gasket member so that if one gasket member fails, there is still adequate gasketing between the substrate and the plate. Preferably there are between 2-5 gasketing members. With more gasketing members there must be greater compression of the fuel cell stack, so it is not desirable to have more than five. However, the skilled person will understand that the shape and number of the gasketing members will depend on the required degree of compression in the fuel cell stack and the load that can be applied.

In a particular embodiment of the invention where the MEAs are suitable for use in a fuel cell stack having ports that are external to the MEA, the elastomeric material extends beyond the edge of the MEA and provides gasketing means for the ports of the field flow plates. It is known in the art to extend the membrane beyond the edge of the gas diffusion substrates and to place gaskets on the surface of the membrane. Therefore, at external ports in known fuel cell stacks, the field flow plate would contact a gasket which was located on a membrane. On the other side of the membrane there would be another gasket which would contact the second field flow plate. Extending the membrane in this manner is a waste of expensive ion-conducting membrane material. In this embodiment of the present invention, the membrane does not extend as far as the external ports, and only the elastomeric material is present adjacent to the external ports. Therefore, at an external port the field flow plate contacts the elastomeric material, which contacts another field flow plate.

Two processes are particularly suitably for forming the sealed and gasketed MEA of the present invention. A first process is injection molding wherein the MEA is placed in a preheated mold, having an inlet and a vent. The MEA is clamped in the mold at a pressure that will not damage the MEA. The temperature in the mold should not be so high that it presents a risk of drying out the membrane or damaging the electrocatalyst layers. For commonly used perfluorosulphonic membranes, the temperature should not be above 100°C. The elastomeric, electrically insulating, mold processable material is injected through the inlet under pressure. The material will start to cure in the mold, but the MEA can be taken out of the mold before curing is complete. A second process is compression molding. The elastomeric material is applied to the MEA using any suitable dispensing process and is then placed in a preheated mold, having a vent. The MEA is clamped in the mold at a pressure that will not damage the MEA and a temperature that will not present a risk of drying out the membrane or damaging the electrocatalyst layers. In both molding processes, the mold is shaped such that the elastomeric material will encapsulate the edges of the gas diffusion substrates and the membrane, it will form a layer on the edge regions of the second planar faces of the gas diffusion substrates and it will form one or more gasketing members.

Before the elastomeric, electrically insulating, mold processable material is molded onto the MEA, it may be useful to pre-treat the MEA. The pre-treatment comprises impregnating part or all of the edge regions of the first and second gas diffusion substrates and/or coating part or all of the protruding edge of the membrane with a primer material such as a silicone, fluorosilicone, EVA copolymer, EAA copolymer, fluorosilane or silane (this may or may not be the same material as the elastomeric material that is used to form the seal on the MEA). Impregnation of a primer material into part or all of the edge regions may help to reinforce the edge regions of the substrate. If the primer material adheres strongly to the substrates or to the membrane, and the elastomeric material adheres strongly to the primer material, then the adhesion between the elastomeric material and the MEA can be improved. The pre-treatment may be carried out by any suitable process such as dip-coating, spraying, screen printing, robotic dispensing and pressing, or hot pressing.

In a further aspect, the present invention provides an integrated cell assembly comprising an MEA according to the invention joined to a field flow plate. In a yet further aspect, the present invention provides a fuel cell comprising an MEA according to the invention.

For a more complete understanding of the invention, reference is made to the schematic drawings wherein:

Fig. 1 is a cross-sectional side view of one end of a sealed and gasketed MEA according to one embodiment of the invention.

Fig. 2 is a cross-sectional side view of one end of a sealed and gasketed MEA according to a further embodiment of the invention.

Fig. 3 is a cross-sectional side view of one end of a sealed and gasketed MEA according to a yet further embodiment of the invention.

Fig. 1 shows a portion of a membrane electrode assembly comprising a polymer electrolyte membrane (1) located between two gas diffusion substrates (2). The edge of the membrane (1) extends beyond the edges of the substrates (2). Electrocatalyst layers

(3) are located between the membrane (1) and the substrates (2). Elastomeric material

(4) is present on a region of the outside faces of the gas diffusion substrates (2) and envelops the edges of the membrane (1) and the substrates (2). In this embodiment, the elastomeric material does not impregnate the substrates (2). The elastomeric material (4) only covers an edge region of the substrates (2) and the electrocatalyst material (3) is not present as a layer on this edge region, but does cover the entire central region. The elastomeric material (4) forms several gasketing members (5). In this embodiment the gasketing members (5) are located such that they will ultimately be compressed between a field flow plate and the membrane (1), i.e. they are located above and below the protruding portion of the membrane.

Fig. 2 shows a portion of a membrane electrode assembly comprising a polymer electrolyte membrane (1) located between two gas diffusion substrates (2). The edge of the membrane (1) extends beyond the edges of the substrates (2). Electrocatalyst layers

(4) is present on a region of the outside faces of the gas diffusion substrates (2) and envelops the edges of the membrane (1) and the substrates (2). In this embodiment, the elastomeric material also impregnates an edge region of the substrates (2). The electrocatalyst material (3) is not present as a layer on this edge region. In this embodiment the gasketing members (5) are located such that they will ultimately be compressed between a field flow plate and a gas diffusion substrate, i.e. they are located above and below the substrates (2).

Fig. 3 shows a portion of a membrane electrode assembly comprising a polymer electrolyte membrane (1) located between two gas diffusion substrates (2). The edge of the membrane (1) extends beyond the edges of the substrates (2). Electrocatalyst layers

(4) is present on a region of the outside faces of the gas diffusion substrates (2) and envelops the edges of the membrane (1) and the substrates (2). The MEA has been preheated before the elastomeric material (4) was applied, by impregnating a primer material (6) into part of the edge regions of the substrates (2). The elastomeric material (4) forms several gasketing members (5).

The invention will now be described by reference to examples that are illustrative and not limiting of the invention.

EXAMPLE 1

A state-of-the-art MEA was prepared from two 200μm Toray® TGP-H-060 gas diffusion substrates (each with a carbon black/PTFE base layer across one entire face) and a 30μm Flemion® SH-30 membrane. Catalyst layers comprising carbon-supported Platinum catalysts and Flemion® ionomer were applied to the two substrates, on top of the base layer. The catalysed substrates and the membrane were combined in a lamination process. The two substrates had the same dimensions and the membrane extended beyond the edge of the substrates by 1mm. The catalyst layers were not present on the entire face of the substrates; there was a margin around the periphery of the substrates that was not coated with catalyst.

Silicone rubber was applied at various points around the periphery of the MEA using a pressurised syringe. The MEA was placed into a mold and heated to 92°C for 5 minutes at up to 250psi. The MEA was removed from the mold and allowed to cool. The mold was shaped such that the silicone rubber formed a layer on edge regions of the two substrates that is within 12.5mm of the edge of the substrates. The thickness of the layer was approximately 20-3 Oμm. The silicone rubber encapsulated the edges of the substrates and the membrane and extended 1.5mm beyond the edge of the membrane. The mold also formed the silicone rubber into several gasketing members, positioned above the substrate.

The MEA with the integrated seal and gasket was sandwiched between two field flow plates and placed in a fuel cell. It was operated for more than 300 hours. No leakage was observed after testing and no damage was evident when the MEA was removed from the cell.

EXAMPLE 2

A state-of-the-art MEA was prepared as described in Example 1. A mask was placed over the MEA, covering the active area. Silicone adhesive was dispensed around the perimeter of the MEA. The adhesive was pressed into the edge region of the substrates and was cured at 90°C for 10 minutes. This pre-treatment step provided an MEA wherein the edge regions of the substrates were impregnated with silicone.

After the pre-treatment step, silicone rubber was applied as for Example 1, and was molded as described in Example 1. The silicone rubber encapsulated the edges of the substrates and the membrane and extended 1.5mm beyond the edge of the membrane. The mold also formed the silicone rubber into several gasketing members, positioned above the substrate.

The MEA was sandwiched between two field flow plates and placed in a fuel cell. It was operated for more than 300 hours. No leakage was observed after testing and no damage was evident when the MEA was removed from the cell.

Claims

1. A sealed and gasketed membrane electrode assembly comprising a) a first gas diffusion substrate, having edges and first and second planar faces, wherein the first gas diffusion substrate has a central region and an edge region such that the edge region is adjacent to the edges; b) a second gas diffusion substrate, having edges and first and second planar faces, wherein the second gas diffusion subsfrate has a central region and an edge region such that the edge region is adjacent to the edges; c) a polymer electrolyte membrane, having edges and first and second planar faces, wherein the membrane is located between the first and second gas diffusion substrates such that the first planar face of the membrane faces the first planar face of the first gas diffusion substrate, the second planar face of the membrane faces the first planar face of the second gas diffusion substrate and the edges of the membrane extend beyond the edges of at least one of the first and second gas diffusion substrates; d) a first electrocatalyst layer, wherein the first electrocatalyst layer is located between the first planar face of the first gas diffusion substrate and the first planar face of the polymer electrolyte membrane, and wherein the first electrocatalyst layer is not present as a layer on the edge region of the first gas diffusion substrate; e) a second electrocatalyst layer, wherein the second electrocatalyst layer is located between the first planar face of the second gas diffusion substrate and the second planar face of the polymer electrolyte membrane, and wherein the second electrocatalyst layer is not present as a layer on the edge region of the second gas diffusion substrate; and f) an elastomeric, electrically insulating, mold processable material that encapsulates the edges of the first gas diffusion substrate, the second gas diffusion substrate and the polymer electrolyte membrane; wherein the elastomeric material forms a layer on the edge region on the second planar face of each of the first and second gas diffusion substrates; wherein the elastomeric material is not present as a layer on the first planar faces of each of the first and second gas diffusion substrates; and wherein the elastomeric material forms one or more gasketing members.

2. A membrane electrode assembly according to claim 1, wherein the edge regions of both the first gas diffusion substrate and the second gas diffusion substrate comprise a volume within the substrate that is within less than 20mm from the edges of the substrate.

3. A membrane electrode assembly according to claim 1 or claim 2, wherein the edges of the membrane extend beyond the edges of both the first and second gas diffusion substrates.

4. A membrane electrode assembly according to claim 3, wherein the membrane extends beyond the edges of the gas diffusion substrates by between 0.5mm and 5mm.

5. A membrane electrode assembly according to any preceding claim, wherein the one or more gasketing members are located such that they will be ultimately be compressed between a co-operating field flow plate and the membrane.

6. A membrane electrode assembly according to any one of claims 1 to 4, wherein the one or more gasketing members are located such that they will ultimately be compressed between a co-operating field flow plate and a gas diffusion substrate.

7. A membrane electrode assembly according to any preceding claim, which is suitable for use in a fuel cell stack having ports that are external to the membrane electrode assembly, wherein the elastomeric material extends beyond the edge of the membrane electrode assembly and provides gasketing means for the ports of the field flow plates.

8. A process for making a sealed and gasketed membrane electrode assembly according to any preceding claim, wherein injection molding is used.

9. A process for making a sealed and gasketed membrane electrode assembly according to any one of claims 1-7, wherein compression molding is used.

10. A process for making a sealed and gasketed membrane electrode assembly according to any one of claims 1-7, comprising a pre-treatment step that comprises impregnating part or all of the edge regions of the first and second gas diffusion subsfrates and/or coating part or all of the protruding edge of the membrane with a primer material.

11. An integrated cell assembly comprising a membrane electrode assembly according to any one of claims 1-7, and a field flow plate.

12. A fuel cell comprising a membrane electrode assembly according to any one of claims 1-7.