WO2007113589A1 - Membrane electrode assembly - Google Patents

Membrane electrode assembly Download PDF

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Publication number
WO2007113589A1
WO2007113589A1 PCT/GB2007/050135 GB2007050135W WO2007113589A1 WO 2007113589 A1 WO2007113589 A1 WO 2007113589A1 GB 2007050135 W GB2007050135 W GB 2007050135W WO 2007113589 A1 WO2007113589 A1 WO 2007113589A1
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WO
WIPO (PCT)
Prior art keywords
membrane
layer
layers
film layer
gas diffusion
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Application number
PCT/GB2007/050135
Other languages
French (fr)
Inventor
David Edward Barnwell
Peter Anthony Trew
Original Assignee
Johnson Matthey Public Limited Company
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Publication date
Application filed by Johnson Matthey Public Limited Company filed Critical Johnson Matthey Public Limited Company
Publication of WO2007113589A1 publication Critical patent/WO2007113589A1/en

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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/02Details
    • H01M8/0271Sealing or supporting means around electrodes, matrices or membranes
    • H01M8/0273Sealing or supporting means around electrodes, matrices or membranes with sealing or supporting means in the form of a frame
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30Hydrogen technology
    • Y02E60/50Fuel cells

Definitions

  • the present invention relates to a membrane electrode assembly suitable for use in a proton exchange membrane fuel cell.
  • a fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte.
  • a fuel e.g. hydrogen or methanol
  • an oxidant e.g. oxygen or air
  • 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.
  • 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.
  • MEA membrane electrode assembly
  • 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.
  • an electrocatalyst layer containing an electrocatalyst, which is tailored for the different requirements at the anode and the cathode.
  • an electrocatalyst layer containing an electrocatalyst, which is tailored for the different requirements at the anode and the cathode.
  • a gas diffusion layer adjacent to each electrocatalyst layer there is a gas diffusion layer.
  • the gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
  • the MEA can be constructed by several methods.
  • the electrocatalyst layer may be applied to the gas diffusion layer 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.
  • the electrocatalyst layer may be applied to both faces of the membrane to form a catalyst coated membrane.
  • gas diffusion layers are applied to both faces of the catalyst coated membrane.
  • an MEA can be formed from a membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the membrane.
  • MEAs 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.
  • WO 2005/020356 discloses MEAs wherein film layers are positioned around the edge region of the MEA.
  • the film layers are positioned between the edges of the membrane and the gas diffusion layers.
  • An adhesive layer is present on one or both surfaces of the film layer. The fihn layers reinforce the edge region of the MEA and prevent fibres from the gas diffusion layer from puncturing the membrane.
  • MEA failure as demonstrated by sudden performance loss, often occurs in the edge region of the MEA and incorporation of fihn layers into the edge region can help to increase MEA lifetime before significant performance loss is seen.
  • the present inventors have sought to provide an MEA wherein failure in the edge region is further decreased and serviceable lifetime is further increased before significant performance loss.
  • the present invention provides a membrane electrode assembly comprising an ion-conducting membrane, electrocatalyst layers disposed either side of the membrane and gas diffusion layers disposed adjacent to the electrocatalyst layers, wherein the membrane electrode assembly has an edge region and a central region, wherein a film layer, optionally with an adhesive layer on one or both faces of the fihn layer, is positioned between the membrane and a gas diffusion layer at the edge region of the membrane electrode assembly, and characterised in that the fihn layer and/or, if present, the adhesive layer comprises one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers.
  • the film layer and/or, if present, the adhesive layer comprises one or more additives which is an oxygen scavenger.
  • the film layer and/or, if present, the adhesive layer comprises one or more additives which is an antioxidant.
  • the film layer and/or, if present, the adhesive layer comprises one or more additives which is a free radical scavenger.
  • the MEA comprises two film layers, wherein both film layers optionally have an adhesive layer on one or both faces of the film layer.
  • Each film layer is positioned between the membrane and one of the two gas diffusion layers at the edge regions of the MEA.
  • each film layer, or if present the adhesive layer comprises one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers.
  • the MEA has an edge region and a central region. It is useful to describe the dimensions of the MEA by referring to x, y and z directions.
  • the x and y directions are within the plane of the layered MEA, and the z direction is through the thickness of the MEA (this is shown in figure 1).
  • the ion-conducting membrane, electrocatalyst layers and gas diffusion layers do not necessarily have the same dimensions in the x-y plane.
  • the ion-conducting membrane may be larger than the electrocatalyst layers and gas diffusion layers, so may protrude beyond these layers at the edge of the MEA.
  • the gas diffusion layers may be larger than the membrane, so there may be a gap between the two gas diffusion layers at the edge of the MEA.
  • the edge region is a region around the circumference of the MEA, encompassing the edge of the membrane, the edges of the gas diffusion layers and optionally encompassing the edges of the catalyst layers.
  • the edge region extends inwardly from the edges of the membrane and the gas diffusion layers, suitably by at least lmm, preferably by at least 5mm.
  • the central region is the remaining volume of the membrane electrode assembly that is not part of the edge region.
  • the film layer (or each film layer) is suitably a non-conducting self-supporting film material.
  • the film layer does not impregnate the gas diffusion layers.
  • the film layer is non-adhesive.
  • the melting temperature of the film layer must be higher than the temperatures used when manufacturing the membrane electrode assembly (e.g. lamination temperatures), and preferably the film layer shows no appreciable softening at the manufacturing temperatures.
  • the film layer does not shrink appreciably under the manufacturing conditions.
  • the film layer is suitably made of a material that does not leach contaminants into the fuel cell system, and that has low permeability to hydrogen, oxygen and water
  • the material of the film layer is preferably resistant to puncture by fibres from the gas diffusion substrate.
  • the film layer is preferably a polymeric material such as polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), ethylene-vinyl alcohol (EVOH), biaxially-oriented polypropylene (BOPP), polytetrafhioro ethylene (PTFE), ethylene tetrafluoroethylene (ETFE), polyether sulphone (PES), polyether ether ketone (PEEK), fluorinated ethylene-propylene (FEP), polyphenylene sulphide (PPS) or polyimide (PI).
  • PET polyethylene terephthalate
  • PVDF polyvinylidene fluoride
  • EVOH ethylene-vinyl alcohol
  • BOPP biaxially-oriented polypropylene
  • PTFE polytetrafhioro ethylene
  • ETFE ethylene tetrafluoroethylene
  • PES polyether sulphone
  • PEEK polyether ether ketone
  • FEP fluorinated ethylene
  • the thickness of the backing layer is suitably greater than l ⁇ m, preferably between 5 ⁇ m and 50 ⁇ m.
  • the film layer has an oxygen permeation rate of less than 50cm 3 /m 2 /day/bar, preferably less than 10cm 3 /m 2 /day/bar, and most preferably less than Icm 3 /m 2 /day/bar.
  • Film layers that have this property include ethylene- vinyl alcohol (EVOH) (l-5cm 3 /m 2 /day/bar), metallised PET (0.5-4cm 3 /m 2 /day/bar) and amorphous silica coated PET (0.2-0.7cm 3 /m 2 /day ⁇ ar).
  • the MEA comprises two film layers, and the fihn layer positioned between the membrane and the cathode gas diffusion layer has a lower oxygen permeation rate than the film layer positioned between the membrane and the anode gas diffusion layer.
  • the fihn layer positioned between the membrane and the cathode diffusion layer suitably has an oxygen permeation rate of less than 10cm 3 /m 2 /day/bar, and preferably less than lcm 3 /m 2 /day ⁇ ar, whereas the fihn layer positioned between the membrane and the anode gas diffusion layer may have an oxygen permeation rate of more than 10cm 3 /m 2 /day/bar.
  • the thickness of the film layer is suitably from l ⁇ m to 300 ⁇ m, preferably from 5 ⁇ mto 30 ⁇ m.
  • the adhesive layer may contain a hot-melt adhesive, a pressure-sensitive adhesive or a thermosetting adhesive.
  • the adhesive may be a polyethylene-based or polypropylene-based adhesive.
  • the adhesive may be a copolymer of ethylene and methacrylic acid or a copolymer of ethylene and vinyl acetate, as described in US 6,756,147.
  • the adhesive layer does not shrink appreciably under manufacturing conditions.
  • the adhesive layer is made of a material that does not leach contaminants into the fuel cell system.
  • the fihn layer may have one adhesive layer.
  • This adhesive layer can face the gas diffusion layer, and the adhesive can impregnate into the gas diffusion layer.
  • the adhesive layer can face the membrane and the adhesive can bond to the membrane. If the film layer has two adhesive layers, the adhesive layers may contain the same or different adhesives.
  • the thickness of the adhesive layer is suitably from l ⁇ m to 300 ⁇ m. If the adhesive layer will face the gas diffusion layer, the thickness is preferably from 20 ⁇ m to 300 ⁇ m. If the adhesive layer will face the membrane, the thickness of the adhesive layer is preferably from l ⁇ m to lO ⁇ m.
  • the one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers is present in the film layer or, if an adhesive layer is present, may be present in the adhesive layer, or may be present in the film layer and adhesive layer if both are present.
  • Oxygen scavenger additives will reduce the transport of oxygen across the film or adhesive layer and suitable oxygen scavengers include Amosorb® 3000 (a copoly ester resin from BP Amoco), Shelfplus® 2500 (Ciba Speciality Chemicals) and cobalt salts such as cobalt nitrate or cobalt octoate
  • Antioxidants can be classified as primary antioxidants, secondary antioxidants or multifunction antioxidants.
  • Primary antioxidants are hydrogen donors that react with peroxy radicals to form hydroperoxides. This prevents the peroxy radical from reacting with a polymer backbone and causing chain scission. Examples include phenolic stabilisers (available from the Irganox® range from Ciba) and secondary aromatic amines. Secondary antioxidants decompose hydroperoxide and prevent the formation of extremely reactive hydroxy and alkoxy radicals. Examples include trivalent phosphorus compounds (available from the Irgafos® range from Ciba) and thiosynergists (such as Irganox® PS 800 or 802 from Ciba). Multifunction antioxidants combine the functions of primary and second antioxidants. Examples include Irganox® 1520 or 1726 (Ciba).
  • Free radical scavengers can scavenge alkyl radicals but cannot scavenge alkoxy and hydroxy radicals. Examples include lactones (such as substituted benzofUranone) and hydroxylamines.
  • lactones such as substituted benzofUranone
  • hydroxylamines such as lactones (such as substituted benzofUranone)
  • the additives must be stable in the fuel cell environment and must not decompose at typical fuel cell operating temperatures (e.g. 70-100 0 C).
  • the film layer or the adhesive layer may comprise more than one type of additive.
  • the film layer and the adhesive layer may both comprise additives, which may be the same or different.
  • the preferred amount of additive present in the film layer or adhesive layer will depend on the nature of the film or adhesive material and on the nature of the additive. However, suitable levels are likely to be in the range of 0.01-1 wt% (based on the weight of the film layer or the adhesive layer), preferably in the range of 0.1 -0.25 wt%.
  • Incorporating additives into a film layer or an adhesive layer may be achieved by any suitable technique known to the skilled person.
  • the additives would be incorporated into the adhesive layer in a blending process. This could be done by taking adhesive resin pellets, heating and mixing with the additive, repelletising the blend and extrusion coating the blend onto the film layer. Alternatively, this could be done by mixing the additive in with the adhesive just prior to extruding the adhesive onto the film layer.
  • the ion-conducting membrane may be any type of ion-conducting membrane known to those skilled in the art.
  • the membrane is proton-conducting.
  • the membranes are often based on perfluorinated sulphonic acid materials such as Nafion® (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.
  • the membrane may comprise a proton-conducting membrane and a matrix of silica fibres, as described in EP 875 524 or the membrane may comprise an expanded PTFE substrate.
  • the membrane is suitably less than 200 ⁇ m thick, preferably less than 50 ⁇ m thick.
  • the electrocatalyst layers comprise an electrocatalyst which may be a finely divided metal powder (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
  • platinum group metals platinum, palladium, rhodium, ruthenium, indium and osmium
  • gold or silver platinum group metals
  • 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 15-75wt%.
  • the electrocatalyst layers suitably comprise other components, such as ion- conducting polymer, which is included to improve the ionic conductivity within the layer.
  • the layers can be formed on the gas diffusion substrates, or the layers can be formed on the membrane.
  • the gas diffusion layers may be any suitable gas diffusion layers known to those skilled in the art.
  • Typical gas diffusion layers include layers based on carbon paper (e.g. Toray® paper available from Toray Industries, Japan or Ul 05 or U 107 paper available from Mitsubishi Rayon, Japan), woven carbon cloths (e.g. the MK series of carbon cloths available from Mitsubishi Chemicals, Japan) or non- woven carbon fibre webs (e.g. ELAT series of non-woven substrates available from E-TEK Inc, USA; H2315 series available from Freudenberg FCCT oHG, Germany; or Sigracet® series available from SGL Technologies GmbH, Germany).
  • carbon paper e.g. Toray® paper available from Toray Industries, Japan or Ul 05 or U 107 paper available from Mitsubishi Rayon, Japan
  • woven carbon cloths e.g. the MK series of carbon cloths available from Mitsubishi Chemicals, Japan
  • non- woven carbon fibre webs e.g. ELAT series of non-woven substrates available from E-TEK Inc,
  • the carbon paper, cloth or web 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).
  • PTFE polytetrafluoroethylene
  • the gas diffusion substrates are between 100 and 300 ⁇ m thick.
  • the film layer optionally with an adhesive layer on one or both faces of the film layer, is positioned between the membrane and a gas diffusion layer at the edge region of the MEA.
  • the electrocatalyst layer is not present in the edge region of the MEA so the film layer is not adjacent to the electrocatalyst layer. In another embodiment of the invention, the electrocatalyst layer is present in the edge region of the MEA so the film layer is adjacent to a limited region of the electrocatalyst layer.
  • the width of the electrocatalyst layer that overlaps the film layer is between 0.5 and 10mm, preferably between 1 and 5mm.
  • the film layer is positioned between the membrane and a gas diffusion layer, and if the film layer overlaps the electrocatalyst layer, the film layer can either be positioned between the membrane and the electrocatalyst layer, or can be positioned between the electrocatalyst layer and the gas diffusion layer. It is preferred that the film layer is positioned between the membrane and the electrocatalyst layer.
  • the MEAs of the invention may be prepared using the methods outlined in WO
  • the membrane, electrocatalyst layers, gas diffusion layers and film layers may be combined by a combination of coating and laminating processes.
  • electrocatalyst layers may be applied to gas diffusion layers by a printing process to form gas diffusion electrodes.
  • An assembly of two gas diffusion electrodes and a membrane may be prepared, with film layers interposed between the edges of the gas diffusion electrodes and the membranes, and the components may be laminated together.
  • the present invention provides a fuel cell comprising a membrane electrode assembly according to the invention.
  • Fig. 1 is a schematic diagram showing a membrane electrode assembly.
  • Fig. 2 is a schematic diagram showing a method of producing a membrane electrode assembly according to the invention.
  • Fig. 3 is a schematic diagram showing a method of producing a membrane electrode assembly according to the invention.
  • Figure 1 shows an MEA made up of a membrane (1) interposed between two gas diffusion layers (2).
  • the x, y and z directions of the MEA are shown.
  • the central region of the MEA is the region inside the hatched line (3) and the edge region is the remaining volume of the MEA.
  • the electrocatalyst layers and film layer are not shown.
  • Step (i) of Fig. 2 shows gas diffusion electrodes comprising gas diffusion layers (2) and electrocatalyst layers (4).
  • Film layers (5) having adhesive layers (6) are positioned on the edges of the inner faces of the gas diffusion layers (2).
  • the adhesive layers (6) comprise one or more additives chosen from the group consisting of oxygen scavengers, antioxidants and free radical scavengers.
  • the film layers are hot pressed (7) at the positions shown by arrows and the adhesive layers (6) impregnate into the gas diffusion layers (2).
  • a membrane (1) is positioned between the gas diffusion layers (2). The membrane (1) extends beyond the gas diffusion layers (2).
  • the gas diffusion layers (2) are laminated to the membrane (1) by hot pressing (7) at the positions shown by arrows.
  • Step (i) of Fig. 3 shows a membrane (1) between two gas diffusion electrodes comprising gas diffusion layers (2) and electrocatalyst layers (4).
  • the electrocatalyst layers (4) do not contact the entire substrate area.
  • the membrane (1) extends beyond the gas diffusion layers (2).
  • Film layers (5), having adhesive layers (6) on both faces are positioned on the edges of the inner faces of the gas diffusion layers (2).
  • the film layers (5) and/or the adhesive layers (6) comprise one or more additives chosen from the group consisting of oxygen scavengers, antioxidants and free radical scavengers.
  • the film layers are hot pressed (7) at the positions shown by arrows.
  • the gas diffusion electrodes are laminated to the membrane, the adhesive layers (6) impregnate into the gas diffusion layers (2) and adhere to the membrane (1).
  • the film layers (5) extend beyond the edge of the membrane, and the region between the film layers (5) is filled by adhesive.

Abstract

A membrane electrode assembly comprising an ion-conducting membrane, electrocatalyst layers disposed either side of the membrane and gas diffusion layers disposed adjacent to the electrocatalyst layers is disclosed. The membrane electrode assembly has an edge region and a central region and a film layer, optionally with an adhesive layer on one or both faces of the film layer, is positioned between the membrane and a gas diffusion layer at the edge region of the membrane electrode assembly. The film layer and/or, if present, the adhesive layer comprises an additive selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers.

Description

MEMBRANE ELECTRODE ASSEMBLY
The present invention relates to a membrane electrode assembly suitable for use in a proton exchange membrane fuel cell.
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 proton exchange 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 layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
The MEA can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion layer 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 layers 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 layer 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.
WO 2005/020356 discloses MEAs wherein film layers are positioned around the edge region of the MEA. In one embodiment, the film layers are positioned between the edges of the membrane and the gas diffusion layers. An adhesive layer is present on one or both surfaces of the film layer. The fihn layers reinforce the edge region of the MEA and prevent fibres from the gas diffusion layer from puncturing the membrane.
MEA failure, as demonstrated by sudden performance loss, often occurs in the edge region of the MEA and incorporation of fihn layers into the edge region can help to increase MEA lifetime before significant performance loss is seen. The present inventors have sought to provide an MEA wherein failure in the edge region is further decreased and serviceable lifetime is further increased before significant performance loss.
Accordingly the present invention provides a membrane electrode assembly comprising an ion-conducting membrane, electrocatalyst layers disposed either side of the membrane and gas diffusion layers disposed adjacent to the electrocatalyst layers, wherein the membrane electrode assembly has an edge region and a central region, wherein a film layer, optionally with an adhesive layer on one or both faces of the fihn layer, is positioned between the membrane and a gas diffusion layer at the edge region of the membrane electrode assembly, and characterised in that the fihn layer and/or, if present, the adhesive layer comprises one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers.
In one embodiment, the film layer and/or, if present, the adhesive layer comprises one or more additives which is an oxygen scavenger.
hi a second embodiment, the film layer and/or, if present, the adhesive layer comprises one or more additives which is an antioxidant.
In a third embodiment, the film layer and/or, if present, the adhesive layer comprises one or more additives which is a free radical scavenger.
Preferably the MEA comprises two film layers, wherein both film layers optionally have an adhesive layer on one or both faces of the film layer. Each film layer is positioned between the membrane and one of the two gas diffusion layers at the edge regions of the MEA. Preferably each film layer, or if present the adhesive layer, comprises one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers.
Incorporation of oxygen scavengers, antioxidants or free radical scavengers into the firm layer or the adhesive layer is readily achieved and decreases membrane degradation in the edge region. Hydrogen peroxide formation and associated membrane degradation is minimised by decreasing the amount of oxygen that permeates through the film layer or adhesive layer. Free radical attack and associated membrane degradation is minimised by scavenging free radicals that permeate through the film layer or adhesive layer.
The MEA has an edge region and a central region. It is useful to describe the dimensions of the MEA by referring to x, y and z directions. The x and y directions are within the plane of the layered MEA, and the z direction is through the thickness of the MEA (this is shown in figure 1). The ion-conducting membrane, electrocatalyst layers and gas diffusion layers do not necessarily have the same dimensions in the x-y plane. The ion-conducting membrane may be larger than the electrocatalyst layers and gas diffusion layers, so may protrude beyond these layers at the edge of the MEA. Alternatively, the gas diffusion layers may be larger than the membrane, so there may be a gap between the two gas diffusion layers at the edge of the MEA. The edge region is a region around the circumference of the MEA, encompassing the edge of the membrane, the edges of the gas diffusion layers and optionally encompassing the edges of the catalyst layers. The edge region extends inwardly from the edges of the membrane and the gas diffusion layers, suitably by at least lmm, preferably by at least 5mm. The central region is the remaining volume of the membrane electrode assembly that is not part of the edge region.
The film layer (or each film layer) is suitably a non-conducting self-supporting film material. The film layer does not impregnate the gas diffusion layers. Suitably the film layer is non-adhesive. The melting temperature of the film layer must be higher than the temperatures used when manufacturing the membrane electrode assembly (e.g. lamination temperatures), and preferably the film layer shows no appreciable softening at the manufacturing temperatures. Preferably the film layer does not shrink appreciably under the manufacturing conditions.
The film layer is suitably made of a material that does not leach contaminants into the fuel cell system, and that has low permeability to hydrogen, oxygen and water
(suitably lower than the permeability of the polymer electrolyte membrane).
Additionally, the material of the film layer is preferably resistant to puncture by fibres from the gas diffusion substrate.
The film layer is preferably a polymeric material such as polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), ethylene-vinyl alcohol (EVOH), biaxially-oriented polypropylene (BOPP), polytetrafhioro ethylene (PTFE), ethylene tetrafluoroethylene (ETFE), polyether sulphone (PES), polyether ether ketone (PEEK), fluorinated ethylene-propylene (FEP), polyphenylene sulphide (PPS) or polyimide (PI). The backing layer could also be a metallised film, i.e. a thin layer of metal coated with polymer. The thickness of the backing layer is suitably greater than lμm, preferably between 5μm and 50μm. In a preferred embodiment, the film layer has an oxygen permeation rate of less than 50cm3/m2/day/bar, preferably less than 10cm3/m2/day/bar, and most preferably less than Icm3/m2/day/bar. Film layers that have this property include ethylene- vinyl alcohol (EVOH) (l-5cm3/m2/day/bar), metallised PET (0.5-4cm3/m2/day/bar) and amorphous silica coated PET (0.2-0.7cm3/m2/dayΛar). Decreasing the oxygen permeability of the film layer improves the oxygen barrier properties of the film layer. Reducing the amount of oxygen that travels from the edge region of the cathode to the edge region of the anode should decrease degradation of the membrane in the edge region. In one embodiment of the invention, the MEA comprises two film layers, and the fihn layer positioned between the membrane and the cathode gas diffusion layer has a lower oxygen permeation rate than the film layer positioned between the membrane and the anode gas diffusion layer. The fihn layer positioned between the membrane and the cathode diffusion layer suitably has an oxygen permeation rate of less than 10cm3/m2/day/bar, and preferably less than lcm3/m2/dayΛar, whereas the fihn layer positioned between the membrane and the anode gas diffusion layer may have an oxygen permeation rate of more than 10cm3/m2/day/bar.
The thickness of the film layer is suitably from lμm to 300μm, preferably from 5μmto 30μm.
The adhesive layer may contain a hot-melt adhesive, a pressure-sensitive adhesive or a thermosetting adhesive. The adhesive may be a polyethylene-based or polypropylene-based adhesive. The adhesive may be a copolymer of ethylene and methacrylic acid or a copolymer of ethylene and vinyl acetate, as described in US 6,756,147. Preferably there is no softening of the adhesive layer at temperatures below 1000C. Preferably the adhesive layer does not shrink appreciably under manufacturing conditions. Suitably the adhesive layer is made of a material that does not leach contaminants into the fuel cell system.
The fihn layer may have one adhesive layer. This adhesive layer can face the gas diffusion layer, and the adhesive can impregnate into the gas diffusion layer. Alternatively, the adhesive layer can face the membrane and the adhesive can bond to the membrane. If the film layer has two adhesive layers, the adhesive layers may contain the same or different adhesives.
The thickness of the adhesive layer is suitably from lμm to 300μm. If the adhesive layer will face the gas diffusion layer, the thickness is preferably from 20μm to 300μm. If the adhesive layer will face the membrane, the thickness of the adhesive layer is preferably from lμm to lOμm.
The one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers is present in the film layer or, if an adhesive layer is present, may be present in the adhesive layer, or may be present in the film layer and adhesive layer if both are present. Oxygen scavenger additives will reduce the transport of oxygen across the film or adhesive layer and suitable oxygen scavengers include Amosorb® 3000 (a copoly ester resin from BP Amoco), Shelfplus® 2500 (Ciba Speciality Chemicals) and cobalt salts such as cobalt nitrate or cobalt octoate
(these are particularly effective when incorporated into Nylon MXD6 polymer).
Antioxidants can be classified as primary antioxidants, secondary antioxidants or multifunction antioxidants. Primary antioxidants are hydrogen donors that react with peroxy radicals to form hydroperoxides. This prevents the peroxy radical from reacting with a polymer backbone and causing chain scission. Examples include phenolic stabilisers (available from the Irganox® range from Ciba) and secondary aromatic amines. Secondary antioxidants decompose hydroperoxide and prevent the formation of extremely reactive hydroxy and alkoxy radicals. Examples include trivalent phosphorus compounds (available from the Irgafos® range from Ciba) and thiosynergists (such as Irganox® PS 800 or 802 from Ciba). Multifunction antioxidants combine the functions of primary and second antioxidants. Examples include Irganox® 1520 or 1726 (Ciba).
Free radical scavengers can scavenge alkyl radicals but cannot scavenge alkoxy and hydroxy radicals. Examples include lactones (such as substituted benzofUranone) and hydroxylamines. The additives must be stable in the fuel cell environment and must not decompose at typical fuel cell operating temperatures (e.g. 70-1000C).
The film layer or the adhesive layer may comprise more than one type of additive. The film layer and the adhesive layer may both comprise additives, which may be the same or different.
The preferred amount of additive present in the film layer or adhesive layer will depend on the nature of the film or adhesive material and on the nature of the additive. However, suitable levels are likely to be in the range of 0.01-1 wt% (based on the weight of the film layer or the adhesive layer), preferably in the range of 0.1 -0.25 wt%.
Incorporating additives into a film layer or an adhesive layer may be achieved by any suitable technique known to the skilled person. Typically the additives would be incorporated into the adhesive layer in a blending process. This could be done by taking adhesive resin pellets, heating and mixing with the additive, repelletising the blend and extrusion coating the blend onto the film layer. Alternatively, this could be done by mixing the additive in with the adhesive just prior to extruding the adhesive onto the film layer.
The ion-conducting 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 Nafion® (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 or the membrane may comprise an expanded PTFE substrate. The membrane is suitably less than 200μm thick, preferably less than 50μm thick.
The electrocatalyst layers comprise an electrocatalyst which may be a finely divided metal powder (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, indium 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 15-75wt%.
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 gas diffusion layers may be any suitable gas diffusion layers known to those skilled in the art. Typical gas diffusion layers include layers based on carbon paper (e.g. Toray® paper available from Toray Industries, Japan or Ul 05 or U 107 paper available from Mitsubishi Rayon, Japan), woven carbon cloths (e.g. the MK series of carbon cloths available from Mitsubishi Chemicals, Japan) or non- woven carbon fibre webs (e.g. ELAT series of non-woven substrates available from E-TEK Inc, USA; H2315 series available from Freudenberg FCCT oHG, Germany; or Sigracet® series available from SGL Technologies GmbH, Germany). The carbon paper, cloth or web 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 100 and 300μm thick. Preferably there is a layer of particulate material such as carbon black and PTFE on the faces of the gas diffusion substrates that contact the electrocatalyst layers. The film layer, optionally with an adhesive layer on one or both faces of the film layer, is positioned between the membrane and a gas diffusion layer at the edge region of the MEA. In one embodiment of the invention, the electrocatalyst layer is not present in the edge region of the MEA so the film layer is not adjacent to the electrocatalyst layer. In another embodiment of the invention, the electrocatalyst layer is present in the edge region of the MEA so the film layer is adjacent to a limited region of the electrocatalyst layer. Suitably the width of the electrocatalyst layer that overlaps the film layer is between 0.5 and 10mm, preferably between 1 and 5mm. The film layer is positioned between the membrane and a gas diffusion layer, and if the film layer overlaps the electrocatalyst layer, the film layer can either be positioned between the membrane and the electrocatalyst layer, or can be positioned between the electrocatalyst layer and the gas diffusion layer. It is preferred that the film layer is positioned between the membrane and the electrocatalyst layer.
The MEAs of the invention may be prepared using the methods outlined in WO
2005/020356. The membrane, electrocatalyst layers, gas diffusion layers and film layers (with optional adhesive layers) may be combined by a combination of coating and laminating processes. For example, electrocatalyst layers may be applied to gas diffusion layers by a printing process to form gas diffusion electrodes. An assembly of two gas diffusion electrodes and a membrane may be prepared, with film layers interposed between the edges of the gas diffusion electrodes and the membranes, and the components may be laminated together.
In a further aspect, the present invention provides a fuel cell comprising a membrane electrode assembly according to the invention.
For a more complete understanding of the invention, reference is made to the schematic drawings wherein:
Fig. 1 is a schematic diagram showing a membrane electrode assembly.
Fig. 2 is a schematic diagram showing a method of producing a membrane electrode assembly according to the invention. Fig. 3 is a schematic diagram showing a method of producing a membrane electrode assembly according to the invention.
Figure 1 shows an MEA made up of a membrane (1) interposed between two gas diffusion layers (2). The x, y and z directions of the MEA are shown. The central region of the MEA is the region inside the hatched line (3) and the edge region is the remaining volume of the MEA. The electrocatalyst layers and film layer are not shown.
Step (i) of Fig. 2 shows gas diffusion electrodes comprising gas diffusion layers (2) and electrocatalyst layers (4). Film layers (5) having adhesive layers (6) are positioned on the edges of the inner faces of the gas diffusion layers (2). The film layers
(5) and/or the adhesive layers (6) comprise one or more additives chosen from the group consisting of oxygen scavengers, antioxidants and free radical scavengers. In step (ii) the film layers are hot pressed (7) at the positions shown by arrows and the adhesive layers (6) impregnate into the gas diffusion layers (2). A membrane (1) is positioned between the gas diffusion layers (2). The membrane (1) extends beyond the gas diffusion layers (2). In step (iii) the gas diffusion layers (2) are laminated to the membrane (1) by hot pressing (7) at the positions shown by arrows.
Step (i) of Fig. 3 shows a membrane (1) between two gas diffusion electrodes comprising gas diffusion layers (2) and electrocatalyst layers (4). The electrocatalyst layers (4) do not contact the entire substrate area. The membrane (1) extends beyond the gas diffusion layers (2). Film layers (5), having adhesive layers (6) on both faces are positioned on the edges of the inner faces of the gas diffusion layers (2). The film layers (5) and/or the adhesive layers (6) comprise one or more additives chosen from the group consisting of oxygen scavengers, antioxidants and free radical scavengers. In step (ii) the film layers are hot pressed (7) at the positions shown by arrows. The gas diffusion electrodes are laminated to the membrane, the adhesive layers (6) impregnate into the gas diffusion layers (2) and adhere to the membrane (1). The film layers (5) extend beyond the edge of the membrane, and the region between the film layers (5) is filled by adhesive.

Claims

1. A membrane electrode assembly comprising an ion-conducting membrane, electro catalyst layers disposed either side of the membrane and gas diffusion layers disposed adjacent to the electrocatalyst layers, wherein the membrane electrode assembly has an edge region and a central region, wherein a film layer, optionally with an adhesive layer on one or both faces of the film layer, is positioned between the membrane and a gas diffusion layer at the edge region of the membrane electrode assembly, and characterised in that the film layer and/or, if present, the adhesive layer comprises one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers.
2. A membrane electrode assembly according to claim 1, wherein the film layer has an adhesive layer on one or both faces of the film layer, and the adhesive layer comprises one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers.
3. A membrane electrode assembly according to claim 1 or claim 2, wherein the film layer comprises one or more additives selected from the group consisting of oxygen scavengers, antioxidants or free radical scavengers.
4. A membrane electrode assembly according to any preceding claim, comprising two film layers, wherein both film layers optionally have an adhesive layer on one or both faces of the film layer, and wherein each film layer is positioned between the membrane and one of the two gas diffusion layers at the edge regions of the MEA.
5. A membrane electrode assembly according to any preceding claim, wherein the film layer has an oxygen permeation rate of less than 10cm3/m2/day/bar.
6. A fuel cell comprising a membrane electrode assembly according to any preceding claim.
PCT/GB2007/050135 2006-03-31 2007-03-21 Membrane electrode assembly WO2007113589A1 (en)

Applications Claiming Priority (2)

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GB0606422.4 2006-03-31

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US7989115B2 (en) 2007-12-14 2011-08-02 Gore Enterprise Holdings, Inc. Highly stable fuel cell membranes and methods of making them
US8637205B2 (en) 2009-12-22 2014-01-28 3M Innovative Properties Company Fuel cell subassemblies incorporating subgasketed thrifted membranes
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GB2555126B (en) * 2016-10-19 2019-05-15 Univ Cape Town A method of securing a gas diffusion layer to a catalyst coated membrane

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