WO2007034233A1 - Process for preparing a composite membrane - Google Patents

Process for preparing a composite membrane Download PDF

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Publication number
WO2007034233A1
WO2007034233A1 PCT/GB2006/050228 GB2006050228W WO2007034233A1 WO 2007034233 A1 WO2007034233 A1 WO 2007034233A1 GB 2006050228 W GB2006050228 W GB 2006050228W WO 2007034233 A1 WO2007034233 A1 WO 2007034233A1
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WO
WIPO (PCT)
Prior art keywords
composite membrane
preparing
membrane according
polymer
web
Prior art date
Application number
PCT/GB2006/050228
Other languages
French (fr)
Inventor
David Edward Barnwell
Peter Jamie Bouwman
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Johnson Matthey Public Limited Company
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Filing date
Publication date
Application filed by Johnson Matthey Public Limited Company filed Critical Johnson Matthey Public Limited Company
Publication of WO2007034233A1 publication Critical patent/WO2007034233A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0081—After-treatment of organic or inorganic membranes
    • B01D67/0088—Physical treatment with compounds, e.g. swelling, coating or impregnation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B01D69/1213—Laminated layers
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/20—Manufacture of shaped structures of ion-exchange resins
    • C08J5/22—Films, membranes or diaphragms
    • C08J5/2206—Films, membranes or diaphragms based on organic and/or inorganic macromolecular compounds
    • C08J5/2275—Heterogeneous membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/39—Electrospinning
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/40—Fibre reinforced membranes
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2323/04—Homopolymers or copolymers of ethene
    • C08J2323/06—Polyethene
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0088—Composites
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0088—Composites
    • H01M2300/0094—Composites in the form of layered products, e.g. coatings
    • Y—GENERAL 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
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the present invention relates to processes for preparing composite membranes that are suitable for use in fuel cells.
  • 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.
  • Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
  • the electrolyte is a solid polymeric membrane.
  • the membrane is electronically insulating but ionically conducting.
  • Proton-conducting membranes 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.
  • membranes used in the PEM fuel cell and other devices include perfluorinated sulphonic acid membranes sold under the trade names Nafion ® (E.I. DuPont de Nemours and Co.), Aciplex ® (Asahi Kasei) and Flemion ® (Asahi Glass KK).
  • Nafion ® E.I. DuPont de Nemours and Co.
  • Aciplex ® Aciplex ®
  • Flemion ® Adsahi Glass KK
  • the membranes are typically below 200 ⁇ m in thickness to provide a high level of proton conductivity. It is desirable to use increasingly thinner membranes of 50 ⁇ m and below, particularly for transportation applications, to raise the electrical efficiency of the MEA at higher power densities and to simplify the water management.
  • Thinner membranes have reduced strength and tear resistance and may also be subject to significant dimensional change as the water content of the membrane varies.
  • Composite membrane structures, wherein the membrane contains a reinforcing material have been prepared to overcome these problems. It has been suggested that it may be advantageous to coat the reinforcing material with a polymeric material prior to incorporation into the composite membrane.
  • EP 958 624 discloses a composite membrane comprising a microporous polymer sheet, wherein the pores of the polymer sheet are at least partially covered with a iunctional material.
  • the functional material may be an organic polymer different from the polymer sheet.
  • EP 875 524 discloses a composite membrane comprising a fibrous reinforcing material such as glass or quartz fibres. The fibres may be coated with polymers to change their surface characteristics.
  • the present inventors have developed an improved process for preparing a composite membrane wherein the reinforcing material is coated with a polymer prior to incorporation into the composite membrane.
  • the present invention provides a process for preparing a composite membrane comprising the steps of: a) coating the surface of a reinforcing material with a polymer layer using a chemical vapour deposition technique, thus providing a coated reinforcing material; and b) combining the coated reinforcing material with an ion-conducting polymer to form a composite membrane.
  • the polymer layer alters the surface characteristics of the reinforcing material.
  • the reinforcing material is a porous web and the coated reinforcing material is combined with an ion-conducting polymer by impregnating the coated porous web with an ion-conducting polymer. Impregnation of the coated porous web is easier than impregnation of a non-coated porous web. Easier impregnation helps to ensure that the composite membrane is uniform, without pin-holes or cracks.
  • One advantage of using a chemical vapour deposition technique to coat the porous web is that a thin and uniform polymer layer may be formed, and the porosity of the porous web is retained (a thicker or less uniform polymer layer could block some of the pores).
  • the chemical vapour deposition (CVD) technique is suitably any CVD technique known to the skilled person, but it preferably a plasma enhanced CVD technique.
  • Plasma enhanced CVD techniques are described in WO 98/58117.
  • a plasma may be generated in a deposition chamber by the application of a high voltage direct current, radio frequencies or microwaves to a gas or mixture of gases.
  • the plasma comprises a monomeric compound and preferably also comprises an inert gas such as argon.
  • the monomeric compound is suitably an alkene such as IH, IH, 2H- perfluoro-1-dodecene or an acrylate such as IH, IH, 2H, 2H-heptadecafluorodecyl acrylate.
  • Deposition of a polymer layer is achieved by applying a high voltage field.
  • the monomeric compounds in the plasma polymerise to form the polymer layer on a reinforcing material positioned in the deposition chamber.
  • the thickness of the applied polymer layer is proportional to the deposition time and rate, which depends on the particular design of the plasma chamber and the properties and concentration of the reactant chemicals.
  • the thickness of the layer can be controlled between 1 nm and 100 nm.
  • the deposition chamber is suitably pressure and temperature controlled. Suitable background argon pressures range from 10 ⁇ mbar to 10 "1 mbar, preferably between 10 "3 mbar and 10 "2 mbar. Water-cooling of the chamber and surfaces exposed to the plasma can ensure quick removal of heat and maintain the temperature around 40°C during deposition.
  • the reinforcing material may be discrete fibres or particles, but is preferably a porous web.
  • the term "porous web” is used to describe any continuous porous reinforcing material that may be incorporated into a composite membrane.
  • the porous web is an expanded polymer web and is preferably not a polytetrafluoroethylene web.
  • Preferred polymers include polyethylene and polyvinylidene fluoride, and an especially preferred polymer is high density polyethylene (HDPE).
  • HDPE high density polyethylene
  • the porous web is made up of randomly oriented individual fibres. This type of web is disclosed in EP 875 524.
  • the fibres are suitably glass, polymer, ceramic, quartz, silica, carbon or metal fibres. (If carbon or metal fibres are used, the polymer layer must be sufficiently thick to electrically insulate the fibres).
  • the fibres are preferably glass, quartz or amorphous silica fibres.
  • the porous web is an electrospun fibre web.
  • Electrospun webs are made by an electrospinning process wherein an electrically charged polymer solution or polymer melt is drawn from an orifice to a collector. This produces a web of randomly-oriented inter-tangled very fine fibres (the fibres typically have nanometer diameters). Such webs are described in US 2003/0195611.
  • the porosity of the porous web is suitably greater than 50%, preferably greater than 70%.
  • the thickness of the porous web is suitably between 3 ⁇ m and 40 ⁇ m, preferably between 5 ⁇ m and 25 ⁇ m. Thicker webs are not preferred as it is desirable to keep the thickness of the composite membrane less than 50 ⁇ m, preferably 30 ⁇ m or less. Thinner webs are not preferred because they will provide less reinforcement.
  • Discrete fibres that are suitable for use as a reinforcing material include glass, polymer, ceramic, quartz, silica, carbon or metal fibres. (If carbon or metal fibres are used, the polymer layer must be sufficiently thick to electrically insulate the fibres).
  • the fibres are preferably glass, quartz or amorphous silica fibres.
  • the fibres suitably have an average diameter between 0.2 ⁇ m and 50 ⁇ mm and suitably have an average length between 0.05mm to 300mm.
  • the polymer layer suitably coats substantially all of the surface of the reinforcing material (e.g. at least 90% of the surface area) and preferably coats the entire surface of the reinforcing material.
  • the "surface of the porous web” includes the interior of the pores of the web, as well as the exterior of the web.
  • the polymer layer is suitably between lnm and lOOnm thick, preferably between lOnm and 50nm thick.
  • the polymer layer is preferably sufficiently thick to guarantee a continuous layer without gaps, but is preferably sufficiently thin that if the reinforcing material is a porous web, less than 10% of the pores of the porous web are filled by the polymer layer, preferably less than 1%.
  • the polymer layer is suitably a fluoropolymer.
  • the polymer layer is an ion-conducting polymer such as a fluoropolymer with sulfonic acid side groups.
  • An ion-conducting polymer layer will provide enhanced interaction between the coated reinforcing material and the ion-conducting polymer and may also increase the overall ionic conductivity of the membrane.
  • the surface of the reinforcing material is coated with more than one polymer layer. Multiple coatings of the same polymer will increase the thickness of the polymer layer and may improve the uniformity of the layer. Multiple coatings of different polymers, e.g. a layer of PTFE and a layer of fluoropolymer with sulfonic acid side groups, may enhance proton conduction and adhesion.
  • the reinforcing material is a porous web
  • it may be combined with an ion-conducting polymer by impregnating the coated porous web with the ion- conducting polymer using techniques known to the skilled person and disclosed, e.g. in EP 958 624 and EP 875 524. Impregnation of the coated porous web is easier than impregnation of a non-coated porous web; the web is completely filled with the ion- conducting polymer quickly and simply.
  • the coated porous web is immersed in a dispersion of the ion-conducting polymer in a solvent, and the solvent is removed by drying the impregnated web, suitably at room temperature. It is preferred that the solvent is essentially aqueous, e.g. the solvent is at least 95% water.
  • the reinforcing material is discrete fibres, then it may be combined with an ion-conducting polymer by dispersing the coated fibres in a dispersion of the ion- conducting polymer in a solvent and then casting a film of the dispersion and drying to form a membrane.
  • the ion-conducting polymer is suitably a proton-conducting polymer.
  • proton-conducting polymers are well known to the skilled person and include perfluorinated sulphonic acid polymers such as Nafion®, Flemion® and Aciplex®.
  • the process of the invention comprises a further step, which is after step (b): c) coating a layer of ion-conducting polymer onto one or both surfaces of the composite membrane.
  • This step provides a composite membrane with two or three layers.
  • One layer of the membrane consists of ion-conducting polymer and the coated reinforcing material. On one or both faces of this layer, there is an additional layer of ion-conducting polymer. It is advantageous to have a non-reinforced ion-conducting layer on one and preferably both faces of the composite membrane because this reduces the likelihood of gas transfer across the membrane.
  • the process of the invention may also be used to prepare multi-layer composite membranes wherein more than layer comprises reinforcing material. These may be prepared by laminating together two or more composite membranes produced according to the process of the invention.
  • the present invention further provides a composite membrane prepared according to the process of the invention.
  • the thickness of the composite membrane is suitably less than 200 ⁇ m, preferably less than lOO ⁇ m and most preferably less than 50 ⁇ m.
  • the present invention further provides a catalyst coated membrane comprising a composite membrane according to the invention and an electrocatalyst layer deposited on the membrane.
  • Suitable electrocatalysts are well known to the skilled person and include platinum and platinum alloys deposited on conducting carbon support materials.
  • the present invention yet further provides a membrane electrode assembly comprising a composite membrane according to the invention.
  • Methods of preparing membrane electrode assemblies from membranes, catalyst inks and gas diffusion substrates are well known to the skilled person.
  • A4 size sheets of porous polyethylene (UHDPE) from Solupor (DSM, The Netherlands) were coated using a plasma chemical vapour deposition process.
  • the targeted film thickness was 15nm and this was set by adjustment of the deposition time.
  • the monomer used in the procedure was perfluoro decyl acrylate and this provided a hydrophobic coating on the polyethylene (the change in the surface properties was observed by a water droplet test).
  • the coated polyethylene sheets were impregnated with a proton-conducting polymer (a fluorocarbon polymer with acidic sulfate groups attached) by dipping the sheets in a solution of the polymer and drying.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Inorganic Chemistry (AREA)
  • Materials Engineering (AREA)
  • Health & Medical Sciences (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
  • Organic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Sustainable Development (AREA)
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Abstract

A process for preparing a composite membrane, suitable for use in a proton exchange membrane fuel cell, is disclosed. The surface of a reinforcing material is coated with a polymer layer using a chemical vapour deposition technique, thus providing a coated reinforcing material, and the coated porous web is combined with an ion-conducting polymer.

Description

PROCESS FOR PREPARING A COMPOSITE MEMBRANE
The present invention relates to processes for preparing composite membranes that are suitable for use in fuel cells.
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. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
In proton exchange membrane (PEM) fuel cells, the electrolyte is a solid polymeric membrane. The membrane is electronically insulating but ionically conducting. Proton-conducting membranes 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.
Conventional membranes used in the PEM fuel cell and other devices include perfluorinated sulphonic acid membranes sold under the trade names Nafion® (E.I. DuPont de Nemours and Co.), Aciplex® (Asahi Kasei) and Flemion® (Asahi Glass KK). For application in the PEM fuel cell the membranes are typically below 200μm in thickness to provide a high level of proton conductivity. It is desirable to use increasingly thinner membranes of 50μm and below, particularly for transportation applications, to raise the electrical efficiency of the MEA at higher power densities and to simplify the water management.
Thinner membranes have reduced strength and tear resistance and may also be subject to significant dimensional change as the water content of the membrane varies. Composite membrane structures, wherein the membrane contains a reinforcing material, have been prepared to overcome these problems. It has been suggested that it may be advantageous to coat the reinforcing material with a polymeric material prior to incorporation into the composite membrane. EP 958 624 discloses a composite membrane comprising a microporous polymer sheet, wherein the pores of the polymer sheet are at least partially covered with a iunctional material. The functional material may be an organic polymer different from the polymer sheet. EP 875 524 discloses a composite membrane comprising a fibrous reinforcing material such as glass or quartz fibres. The fibres may be coated with polymers to change their surface characteristics.
The present inventors have developed an improved process for preparing a composite membrane wherein the reinforcing material is coated with a polymer prior to incorporation into the composite membrane. The present invention provides a process for preparing a composite membrane comprising the steps of: a) coating the surface of a reinforcing material with a polymer layer using a chemical vapour deposition technique, thus providing a coated reinforcing material; and b) combining the coated reinforcing material with an ion-conducting polymer to form a composite membrane.
The polymer layer alters the surface characteristics of the reinforcing material.
In a preferred embodiment of the invention, the reinforcing material is a porous web and the coated reinforcing material is combined with an ion-conducting polymer by impregnating the coated porous web with an ion-conducting polymer. Impregnation of the coated porous web is easier than impregnation of a non-coated porous web. Easier impregnation helps to ensure that the composite membrane is uniform, without pin-holes or cracks. One advantage of using a chemical vapour deposition technique to coat the porous web is that a thin and uniform polymer layer may be formed, and the porosity of the porous web is retained (a thicker or less uniform polymer layer could block some of the pores). It is desirable to retain high porosity in the reinforcing material because the ionic conductivity of the membrane may be compromised if there are significant regions of the membrane that do not contain ion- conducting polymer. Another advantage of chemical vapour deposition techniques is that they can be used at low temperatures (e.g. less than 6O0C) and can therefore be used with reinforcing materials that are unstable at higher temperatures, whereas other coating techniques may require high temperatures.
The chemical vapour deposition (CVD) technique is suitably any CVD technique known to the skilled person, but it preferably a plasma enhanced CVD technique. Plasma enhanced CVD techniques are described in WO 98/58117. A plasma may be generated in a deposition chamber by the application of a high voltage direct current, radio frequencies or microwaves to a gas or mixture of gases. The plasma comprises a monomeric compound and preferably also comprises an inert gas such as argon. The monomeric compound is suitably an alkene such as IH, IH, 2H- perfluoro-1-dodecene or an acrylate such as IH, IH, 2H, 2H-heptadecafluorodecyl acrylate. Deposition of a polymer layer is achieved by applying a high voltage field. The monomeric compounds in the plasma polymerise to form the polymer layer on a reinforcing material positioned in the deposition chamber. The thickness of the applied polymer layer is proportional to the deposition time and rate, which depends on the particular design of the plasma chamber and the properties and concentration of the reactant chemicals. The thickness of the layer can be controlled between 1 nm and 100 nm.
The deposition chamber is suitably pressure and temperature controlled. Suitable background argon pressures range from 10^ mbar to 10"1 mbar, preferably between 10"3 mbar and 10"2 mbar. Water-cooling of the chamber and surfaces exposed to the plasma can ensure quick removal of heat and maintain the temperature around 40°C during deposition.
An additional advantage of the plasma technique is that the plasma effectively
"cleans" the surface of the reinforcing material prior to coating with the polymer layer, and this can enhance the adhesion of the polymer layer to the reinforcing material. If the reinforcing material is a porous web, cleaning the surface of the web can improve uniformity of the impregnation of the ion-conducting polymer. The reinforcing material may be discrete fibres or particles, but is preferably a porous web. The term "porous web" is used to describe any continuous porous reinforcing material that may be incorporated into a composite membrane. In one embodiment of the invention, the porous web is an expanded polymer web and is preferably not a polytetrafluoroethylene web. Preferred polymers include polyethylene and polyvinylidene fluoride, and an especially preferred polymer is high density polyethylene (HDPE). HDPE is preferred because the long polymer chains and absence of iunctional groups provide a durable polymer that is readily expanded to form a web structure. In a second embodiment of the invention, the porous web is made up of randomly oriented individual fibres. This type of web is disclosed in EP 875 524. The fibres are suitably glass, polymer, ceramic, quartz, silica, carbon or metal fibres. (If carbon or metal fibres are used, the polymer layer must be sufficiently thick to electrically insulate the fibres). The fibres are preferably glass, quartz or amorphous silica fibres. In a third embodiment of the invention, the porous web is an electrospun fibre web. Electrospun webs are made by an electrospinning process wherein an electrically charged polymer solution or polymer melt is drawn from an orifice to a collector. This produces a web of randomly-oriented inter-tangled very fine fibres (the fibres typically have nanometer diameters). Such webs are described in US 2003/0195611.
The porosity of the porous web is suitably greater than 50%, preferably greater than 70%. The thickness of the porous web is suitably between 3μm and 40μm, preferably between 5μm and 25μm. Thicker webs are not preferred as it is desirable to keep the thickness of the composite membrane less than 50μm, preferably 30μm or less. Thinner webs are not preferred because they will provide less reinforcement.
Discrete fibres that are suitable for use as a reinforcing material include glass, polymer, ceramic, quartz, silica, carbon or metal fibres. (If carbon or metal fibres are used, the polymer layer must be sufficiently thick to electrically insulate the fibres). The fibres are preferably glass, quartz or amorphous silica fibres. The fibres suitably have an average diameter between 0.2μm and 50μmm and suitably have an average length between 0.05mm to 300mm. The polymer layer suitably coats substantially all of the surface of the reinforcing material (e.g. at least 90% of the surface area) and preferably coats the entire surface of the reinforcing material. If the reinforcing material is a porous web, the "surface of the porous web" includes the interior of the pores of the web, as well as the exterior of the web. The polymer layer is suitably between lnm and lOOnm thick, preferably between lOnm and 50nm thick. The polymer layer is preferably sufficiently thick to guarantee a continuous layer without gaps, but is preferably sufficiently thin that if the reinforcing material is a porous web, less than 10% of the pores of the porous web are filled by the polymer layer, preferably less than 1%.
The polymer layer is suitably a fluoropolymer. In an embodiment of the invention, the polymer layer is an ion-conducting polymer such as a fluoropolymer with sulfonic acid side groups. An ion-conducting polymer layer will provide enhanced interaction between the coated reinforcing material and the ion-conducting polymer and may also increase the overall ionic conductivity of the membrane.
In an embodiment of the invention, the surface of the reinforcing material is coated with more than one polymer layer. Multiple coatings of the same polymer will increase the thickness of the polymer layer and may improve the uniformity of the layer. Multiple coatings of different polymers, e.g. a layer of PTFE and a layer of fluoropolymer with sulfonic acid side groups, may enhance proton conduction and adhesion.
If the reinforcing material is a porous web, then it may be combined with an ion-conducting polymer by impregnating the coated porous web with the ion- conducting polymer using techniques known to the skilled person and disclosed, e.g. in EP 958 624 and EP 875 524. Impregnation of the coated porous web is easier than impregnation of a non-coated porous web; the web is completely filled with the ion- conducting polymer quickly and simply. The coated porous web is immersed in a dispersion of the ion-conducting polymer in a solvent, and the solvent is removed by drying the impregnated web, suitably at room temperature. It is preferred that the solvent is essentially aqueous, e.g. the solvent is at least 95% water.
If the reinforcing material is discrete fibres, then it may be combined with an ion-conducting polymer by dispersing the coated fibres in a dispersion of the ion- conducting polymer in a solvent and then casting a film of the dispersion and drying to form a membrane.
The ion-conducting polymer is suitably a proton-conducting polymer. A wide variety of proton-conducting polymers are well known to the skilled person and include perfluorinated sulphonic acid polymers such as Nafion®, Flemion® and Aciplex®.
Preferably the process of the invention comprises a further step, which is after step (b): c) coating a layer of ion-conducting polymer onto one or both surfaces of the composite membrane.
This step provides a composite membrane with two or three layers. One layer of the membrane consists of ion-conducting polymer and the coated reinforcing material. On one or both faces of this layer, there is an additional layer of ion-conducting polymer. It is advantageous to have a non-reinforced ion-conducting layer on one and preferably both faces of the composite membrane because this reduces the likelihood of gas transfer across the membrane.
The process of the invention may also be used to prepare multi-layer composite membranes wherein more than layer comprises reinforcing material. These may be prepared by laminating together two or more composite membranes produced according to the process of the invention.
The inventors believe that the composite membrane produced according to the process of the invention has different physical properties to known composite membranes. This is because the CVD technique provides a thinner and more uniform polymer layer on the reinforcing material than could be achieved by other coating techniques. Therefore, the present invention further provides a composite membrane prepared according to the process of the invention.
The thickness of the composite membrane is suitably less than 200μm, preferably less than lOOμm and most preferably less than 50μm.
The present invention further provides a catalyst coated membrane comprising a composite membrane according to the invention and an electrocatalyst layer deposited on the membrane. Suitable electrocatalysts are well known to the skilled person and include platinum and platinum alloys deposited on conducting carbon support materials.
The present invention yet further provides a membrane electrode assembly comprising a composite membrane according to the invention. Methods of preparing membrane electrode assemblies from membranes, catalyst inks and gas diffusion substrates are well known to the skilled person.
The invention will now be described by reference to an example, which is intended to be illustrative and not limiting of the invention.
Example
Two A4 size sheets of porous polyethylene (UHDPE) from Solupor (DSM, The Netherlands) were coated using a plasma chemical vapour deposition process. The targeted film thickness was 15nm and this was set by adjustment of the deposition time. The monomer used in the procedure was perfluoro decyl acrylate and this provided a hydrophobic coating on the polyethylene (the change in the surface properties was observed by a water droplet test). The coated polyethylene sheets were impregnated with a proton-conducting polymer (a fluorocarbon polymer with acidic sulfate groups attached) by dipping the sheets in a solution of the polymer and drying.

Claims

Claims
1. A process for preparing a composite membrane comprising the steps of: a) coating the surface of a reinforcing material with a polymer layer using a chemical vapour deposition technique, thus providing a coated reinforcing material; and b) combining the coated reinforcing material with an ion-conducting polymer to form a composite membrane.
2. A process for preparing a composite membrane according to claim 1, wherein the chemical vapour deposition technique is a plasma enhanced chemical vapour deposition technique.
3. A process for preparing a composite membrane according to claim 1 or claim 2, wherein the reinforcing material is a porous web.
4. A process for preparing a composite membrane according to any preceding claim, wherein the porous web is an expanded polymer web.
5. A process for preparing a composite membrane according to claim 4, wherein the expanded polymer web is a high density polyethylene web.
6. A process for preparing a composite membrane according to claim 3, wherein the porous web is made up of randomly oriented individual fibres.
7. A process for preparing a composite membrane according to claim 6, wherein the fibres are glass, quartz or amorphous silica fibres.
8. A process for preparing a composite membrane according to claim 3, wherein the porous web is an electrospun fibre web.
9. A process for preparing a composite membrane according to any one of claims 3 to 8, wherein the thickness of the porous web is between 5μm and 25μm.
10. A process for preparing a composite membrane according to any preceding claim, wherein the polymer layer is between IOnm and 50nm thick.
11. A process for preparing a composite membrane according to any preceding claim, wherein the polymer layer is a fluoropolymer.
12. A process for preparing a composite membrane according to any one of claims 1 to 10, wherein the polymer layer is an ion-conducting polymer.
13. A process for preparing a composite membrane according to any preceding claim, comprising a further step, which is after step (b): c) coating a layer of ion-conducting polymer onto one or both surfaces of the composite membrane.
14. A composite membrane prepared according to the process of any preceding claim.
15. A catalyst coated membrane comprising a composite membrane according to claim 14 and an electrocatalyst layer deposited on the membrane.
16. A membrane electrode assembly comprising a composite membrane according to claim 14.
PCT/GB2006/050228 2005-09-23 2006-08-01 Process for preparing a composite membrane WO2007034233A1 (en)

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WO2011149732A3 (en) * 2010-05-25 2012-03-01 3M Innovative Properties Company Reinforced electrolyte membrane
WO2016142857A1 (en) * 2015-03-08 2016-09-15 University Of Tartu Ionic capacitive laminate and method of production

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EP1263066A2 (en) * 2001-05-25 2002-12-04 Ballard Power Systems Inc. Composite ion exchange membrane
US20030142901A1 (en) * 2001-11-02 2003-07-31 Joerg Lahann Switchable surfaces
US20040241520A1 (en) * 2003-05-31 2004-12-02 Korea Institute Of Science And Technology Method to manufacture composite polymer electrolyte membranes coated with inorganic thin films for fuel cells

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EP0875524A2 (en) * 1997-04-25 1998-11-04 Johnson Matthey Public Limited Company Composite membranes
EP1263066A2 (en) * 2001-05-25 2002-12-04 Ballard Power Systems Inc. Composite ion exchange membrane
US20030142901A1 (en) * 2001-11-02 2003-07-31 Joerg Lahann Switchable surfaces
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FR2937468A1 (en) * 2008-11-19 2010-04-23 Commissariat Energie Atomique Proton conductive membrane, useful for proton exchange membrane fuel cell, comprises a multilayer stack
WO2011149732A3 (en) * 2010-05-25 2012-03-01 3M Innovative Properties Company Reinforced electrolyte membrane
CN103004001A (en) * 2010-05-25 2013-03-27 3M创新有限公司 Reinforced electrolyte membrane
EP3147982A1 (en) * 2010-05-25 2017-03-29 3M Innovative Properties Company Reinforced electrolyte membrane
US9893373B2 (en) 2010-05-25 2018-02-13 3M Innovative Properties Company Reinforced electrolyte membrane
WO2016142857A1 (en) * 2015-03-08 2016-09-15 University Of Tartu Ionic capacitive laminate and method of production

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