WO2008015478A1

WO2008015478A1 - Membrane production method

Info

Publication number: WO2008015478A1
Application number: PCT/GB2007/050459
Authority: WO
Inventors: Donald James Highgate; Hiu-Wan Yu
Original assignee: Itm Power (Research) Ltd.
Priority date: 2006-07-29
Filing date: 2007-07-30
Publication date: 2008-02-07
Also published as: GB0900433D0; GB2452225B; GB0615139D0; GB2452225A

Abstract

A method for producing a membrane suitable for use in a membrane electrode assembly comprises polymerising a liquid monomer on a substantially perfectly flat surface of a non-reactive material, wherein the non-reactive material and the liquid monomer are immiscible.

Description

MEMBRANE PRODUCTION METHOD Field of the Invention

This invention relates to the production of membranes of the type suitable for use in a membrane electrode assembly. Background of the Invention

Electrochemical cells require a membrane electrode assembly (MEA). A

MEA is made up of an ion-exchange membrane and one or more electrodes.

Commonly, the electrode is coated with a catalyst. In some cases, it is beneficial for the membrane to be made of a hydrophilic polymer. Membranes of this type are disclosed in WO03/023890 and WO2005/124915.

A current membrane production technique involves constraining the liquid monomer between smooth flat mould surfaces (usually of aluminium), and preventing adhesion between the polymerised membrane and the mould surfaces by the use of a plastics sheet. The use of plastics sheets to prevent adhesion has significant disadvantages. The use of engineering shims to define the separation of the mould surfaces, and a move from plastic to metal plates have improved the membrane uniformity and reproducibility; h owever, some issues with flatness and uniformity remain.

It is sometime conventient to form the plastics sheets into a bag, in order to contain the monomer and exclude oxygen from the process. Suitable bags are currently made by heat-sealing sheets, e.g. of 0.2 mm thick HDPE (high- density polyethylene) along three edges. The HDPE has a +/- 10 % tolerance, which means that a sheet having a nominal 0.2 mm thickness can be anywhere between 0.18-0.22 mm thick. As the bag is of two sheets put together, the total bag thickness can be anywhere between 0.36-0.44 mm thick. Therefore, when trying to cure a thick membrane of 100 μm, the actual membrane thickness may be anywhere between 60-140 μm.

The mould material should not adhere to the polymer, after polymerisation. This limits both the choice of materials and the quality of the resulting membrane surface. Summary of the Invention

The present invention is based on the discovery that thin flat membranes can be produced by supporting a layer of liquid monomer on a non-reactive fluid or substantially flat surface, and performing all or part of the polymerisation process. The surfaces of the formed polymer can therefore be uniformly flat. The fluid is preferably of greater density than the monomer mixture, immiscible therewith and/or of significantly different surface free energy.

The present invention is a method for producing a membrane suitable for use in a membrane electrode assembly, compris ing polymerising a liquid monomer on a substantially flat surface of a non -reactive material, wherein the non-reactive material and the liquid monomer are immiscible.

Where the non-reactive surface is a fluid, it should be of significantly greater density than the liquid monomer, and the interfacial tension present at the interface will ensure that the interface is substantially uniform. The shape of the interface will normally be defined by gravity and will in this case be horizontal. The thickness of the monomer layer can be very small (down to a mono-molecular layer), as is well known from the science of Langmuir films.

A further property of such interfaces is that the interfacial tension can act to distribute a third material, for example, a solid powder uniformly along the interface. This effect can be employed to distribute and maintain a catalyst material, electrically conducting elements or the components of a boundary "frame" into the interface between the supporting liquid and the liquid monomer mixture. Description of Preferred Embodiments

This method of the invention can be applied to form any desired membrane or structure by a single polymerisation stage, or in two or more stages, to produce a composite membrane having different properties as a function of thickness. The method has been found to be particularly suitable for the production of membranes and MEAs, for use in electrochemical cells.

Before polymerisation, the liquid monomer may be mixed with other raw materials, a catalyst, an ele ctrode layer or one or more different monomer materials (batch mixing). Other materials that may be used include, for example, an ionic component, a cross-linker or water. Preferably the ionic group is a strongly ionic group. A batch may be stored for a period of time, depending on the storage conditions. Prior to use, the batch may be re-mixed, avoiding the introduction of air bubbles, and an initiator may be added. Where the monomer is polymerised by UV irradiation, a photo-initiator may be added. Photo-initiators suitable for use in the invention will be known to those skilled in the art. Preferred photo-initiators include Darocur 1 173 (2-hydroxy-2- methyl-1 -phenyl-1 -propanone), Darocur TPO ( diphenyl (2,4,6-trimethylbenzoyl)- phosphine oxide), Genoc ure EPD ( ethyl 4-dimethylaminobenzoate), Genocure ITX ( thioxanthone), Genocure MBB ( methyl o-benzoylbenzoate), lrgacure 184 (1 -hydroxycyclohexyl phenyl ketone) and lrgacure 651 (α,α-dimethoxy-α- phenylacetophenone).

Alternatively or in addition, a thermal initiator may be used, and such materials are known to those skilled in the art. Preferred thermal initiators include 1 ,1 -azobis(cyclohexanecarbonitrile) and α-azo-isobutyronitrile.

A preferred liquid monomer mixture is of the type descrbed in WO03/023890, and comprises hydrophilic and hydrophobic monomers, a monomer providing a strongly ionic group such as a sulphonic acid group, and water. Illustrative materials and amounts, for a cationic exchange membrane, are given in the OR material of the Examples.

In a preferred embodiment, the liquid monomer mixture flows (or may be sprayed, for example) from a "charging" area (the area where the raw materials are mixed) onto a substantially flat surface comprising a non-reactive material. The monomer mixture and the non-reactive material do not mix, i.e. they are immiscible.

In one embodiment, the substantially flat surface is in the form of a "float bath" of a non-reactive, high density fluid, e.g. liquid, such as Flutec PP6 (perfluorodecalin), water or mercury. Alternatively, the surface is solid but easily removed, e.g. of glass or, preferably, ice. Ice allows for catalyst and/or electrode mesh structures to be frozen into the ice, and used in "one-shot" production techniques, i.e. the catalyst and/or electrode may be polymerised into the membrane in situ. After the membrane is formed, an ice mould can be melted away. The use of water or ice has the added advantage of hydrating the membrane. A powder or liquid non -reactive material may also be placed on a mesh structure (electrode). The liquid monomer may then be poured onto the non-reactive surface and cured, incorporating the mesh into the polymer. If the mesh has a suitable cross-section, the result is a series of individual membranes of shape and size defined by the spaces in the mesh. A catalyst and/or electrode may be floated onto a float bath, enabling one-sided polymerisation in situ of the additional components, to give one half of a MEA. Two of these may be pressed together (and optionally polymerised together by thermal or gamma polymerisation), to create a fully integrated MEA. A further embodiment involves the use of a second, non-reactive material, e.g. fluid, added on top of the monomer (which is lying on the substantially flat surface). Preferably, the second, non-reactive material has a density less than that of the liquid monomer and is surface free energy such that the interfacial tension between the monomer and the non -reactive material ensures that the interface between the monomer and the non-reactive material is substantially flat. The non-reactive material may be capable of transmitting the polymerising radiation. The monomer mixture flow and/or the irradiation may be continuous or intermittent (to enable either continuous or batch manufacture).

The monomer mixture will be in the form of a flat thin film lying on top of, or between two, substantially flat surfaces. In a preferred embodiment, polymerisation occurs under the influence of suitable radiation (UV, gamma or electron beam). Radiation may be applied to the reverse side of the film, after the materials have been sufficiently polymerised to acquire a mechanically stable form. Alternatively, if the non-reactive material forming the flat surface is able to transmit radiation, both sides of the membrane can be irradiated simultaneously.

In a preferred embodiment, the liquid monomer is irradiated horizontally. In this embodiment, there is no need for a top non -reactive layer, but the atmosphere may need to be free from oxygen. Argon and nitrogen are suitable non-reactive gases.

In another embodiment, a barrier is placed between the liquid monomer and the top and/or bottom non-reactive layers. This may be in the form of a solid. Preferably, the solid is made of a flexible plastics material, such as polyethene. This may allow for the top layer to be more dense than the monomer layer.

In another embodiment, the polymerisation can be conducted in such a way as to create a membrane which has "sticky" surfaces (known as "coating"); the application of a catalyst and/or electrode to this may aid the adhesion and surface contact of an MEA. The membrane coating step may be conducted before the monomers are fully polymerised, but a final polymerisation will be required.

The final membrane, catalyst-coated membrane or MEA can be provided as a rolled sheet; alternatively, it may be cut to the desired length. Membranes produced using the method of the invention can be of uniform thickness. This can be achieved if the supporting, non-reactive material surface is flat and/or can be maintained horizontally. Other membrane configurations can be obtained, e.g. by the use of a solid, textured non -reactive material whose surface is maintained other than in the horizontal plane. They will have a good surface finish, and may additionally have an incorporated electrode and/or catalyst. The membranes can also be very thin.

The invention will now be illustrated by the following Examples. The "OR" mixture comprised: 30 g AMPSA (2-acrylamido-2-methyl-1 -propanesulfonic acid); 30 g HPLC (high performance liquid chromatography) grade water; 2 g allyl methacrylate; 75 g acrylonitrile; 75 g vinylpyrrolidone; and 3 g 2-hydroxy-2- methyl-1 -phenyl-propan-1 -one. Example 1

A thin sheet of water ice (>1 mm) was created within a plastic bag on a cold plate. This was kept chilled (-2 C) throughout the filling and curing process. Chilled OR (initially at ~0C) was then poured into the bag, which was sealed and placed under a uv lamp (Dymax PC2000 lamp 400W) for curing. After 5 minutes, the curing was complete and a membrane was formed. The bag was removed from the cold plate and the ice was allowed to melt, hydrating the membrane. Successful membrane production was also achieved by curing directly on an ice layer (not in a bag). The environment was free of oxygen; the curing box was filled with argon or nitrogen. Example 2

Flutec PP6 (perfluorodecalin ; ISC Chemicals Ltd.) is a fully-fluorinated, odourless, colourless, non-volatile liquid. It is a mixture of cis and trans-isomers, C-ιoF-₁8. Flutec PP6 was placed in a small glass container. OR was poured gently down onto the Flutec, and the separation interface was allowed to stabilise under the action of the interfacial tension between the liquids. The glass container was then placed into an argon environment, and cured for 5 minutes using either a Dymax PC2000 lamp 400W or gamma radiation. A polymerised membrane was successfully formed in both cases.

Example 3

Thin layers of OR were cured onto a horizontal glass layer. Polymerisation was achieved using a Dymax PC2000 lamp 400W, or by gamma radiation.

Thin layers of OR were also cured onto glass, where the glass was inside a polythene bag. This reduced the errors obtained when curing using the complete polythene bag method, by half. The monomer addition and polymerisation stages may be repeated a number of times, to create layers of composite membrane, containing layers of polymer with different properties.

Example 4

A layer of OR mixture was carefully spread onto a layer of Flutec PP6, and the interface allowed to stabilise to form a substantially perfectly flat horizontal interface. A second, liquid hexane layer, having a lower density than the liquid monomer mixture and an adequate interfacial tension, was carefully poured onto the liquid monomer and the second interface allowed to stabilise.

The system was then irradiated using UV light from a Dymax PC2000 400W lamp, and the monomer layer successfully cured to form a membrane.

Claims

1. A method for producing a membrane suitable for use in a membrane electrode assembly, comprising polymerising liquid monomer on a substantially flat surface of a non-reactive material, wherein the non-reactive material and the liquid monomer are immiscible.

2. A method according to claim 1 , wherein the non-reactive material comprises water or ice.

3. A method according to claim 1 , wherein the non-reactive material comprises glass.

4. A method according to any preceding claim, wherein the top surface of the non-reactive material exhibits a variation in thickness of no more than

0.02 mm.

5. A method according to claim 4, wherein the variation in thickness is no more than 0.01 mm.

6. A method according to any preceding claim, additionally comprising the step of polymerising another liquid monomer on the surface of the at least partially polymerised liquid monomer.

7. A method according to any preceding claim, additionally comprising the step of placing a second non-reactive material on top of the, or each, liquid monomer during polymerisation, wherein the interface between the liquid monomer and the second non-reactive material is substantially flat.

8. A method according to any preceding claim, wherein an ionic component is added to the liquid monomer before polymerisation.

9. A method according to any preceding claim, wherein the electrode and/or a catalyst are polymerised into the liquid monomer in situ.

10. A method according to claim 9, wherein the surface of the non-reactive material includes an electrode and/or the catalyst.

1 1. A method according to any preceding claim, wherein the liquid monomer is polymerised by irradiation.

12. A method according to claim 1 1 , wherein the irradiation is directed horizontally.

13. A method according to any of claims 1 to 10, wherein the liquid monomer is polymerised by the application of heat, in the presence of a thermal initiator.