MEMBRANE ELECTRODE ASSEMBLIES AND THEIR PRODUCTION Field of the Invention

This invention relates to membrane electrode assemblies and their production. Background of the Invention Electrochemical cells, including solid polymer fuel cells and electrolysers, are typically constructed of a membrane electrode assembly (MEA) which comprises an ionically conducting membrane contained in rigid electrode and manifold structures to deliver the fuel, and rigid metal or graphite bi-polar plates to separate the individual cells in a cell stack. The function of the membrane is to facilitate the transfer of electrically charged ions from one electrode to the other and to maintain separation of the fuel and oxidant streams. It has been assumed that the thickness of the operating membrane should be reduced, in order to maximise the efficiency of the resulting cell. This has been the objective of previous membrane technologies irrespective of the fuel to be used, e.g. hydrogen-oxygen, hydrogen-air, alcohol-air alcohol liquid oxidant, or sodium borohydride. For these reasons, the thickness of normal Nafion membranes is in the range 50 to 200 μm.

Fuel and/or oxidant cross-over effectively provides a parasitic loss within the cell, and significantly reduces overall efficiency of operation. While this effect occurs in all types of solid polymer cell operating on any combination of fuels and oxidants, it is of particular importance when using direct alcohol fuel, i.e. a fuel cell system in which alcohol or alcohol water mixture is introduced directly into the anode chamber. In this case, the efficiency of the fuel cell is normally reduced by the cross-over of alcohol through the membrane. Summary of the Invention

The present invention is based at least in part on the realisation that, provided that the ionic properties of the membrane can be controlled, a thick membrane may be particularly advantageous in that it reduces the cross-over of fuel and or oxidant from one side of the cell to the other. According to the invention, a membrane electrode assembly comprises a membrane, a catalyst and an electrode, in which the membrane is obtainable by the steps of

(i) forming a polymer film and

(ii) reacting the film with a material having strongly ionic groups, to form a

polymer film having the ionic groups grafted thereon. Description of the Invention

In this specification, reference to specific materials will be understood as illustrative only. A variety of polymers can be used, including LDPE and ETFE, and other materials, can be used in the invention, and are known to those of ordinary skill in the art. On irradiation, and grafting with a functional, intermediate molecule such as styrene, functionalisation can be achieved using a material that introduces ionic groups, e.g. by using sulphonic acid groups. They provide the membrane's ionic conductivity.

The production of an ionically conducting material in the form of a membrane can be achieved in several ways, including mutual grafting or post-irradiation grafting, e.g. using ionising radiation from a gamma source or high energy radiation from an electron beam device.

The processes described herein result in the production of a material which is ionically conductive by a two-step procedure, firstly the grafting of an intermediate moiety onto the polymer film, followed by sulphonation of the graft to provide the hydrophilic ion conductive group.

Two techniques have been used to produce the grafted polymer, mutual grafting, whereby the monomer is present during the irradiation and post-irradiation grafting, where the polymer is first irradiated to create radicals which are then subsequently reacted with a monomer. In each case, the grafted intermediate moiety is the polymer grafted with styrene, which in itself in hydrophobic. In order for it to become ion- conducting, it has to be further functionalised, e.g. sulphonated.

The ionic conductivity of the resulting membrane when made by post-irradiation grafting depends upon a number of factors. These include: (i) the chemical composition of the initial membrane,

(ii) the method of irradiation and grafting, (iii) the dose rate and total radiation dose applied, (iv) the treatment of the irradiated polymer membrane prior to exposure to the monomers or polymers that are to be grafted to the membrane, (v) the chemical composition of the graft materials,

(vi) precise details of the subsequent sulphonation process. It is normally found that the degree of grafting, which is related to ionic conductivity of a membrane, varies through the thickness of the membrane, being higher

at the surface and decreasing towards the centre. For this reason, thick membranes, e.g. over 75 μm in thickness, are not easily made with high and uniform levels of ionic conductivity as judged by uniformity of graft weight (GW). However, it will now be evident that, based on the knowledge of one of ordinary skill in the art, it is possible by selection of initial membrane material, highly developed post-irradiation graft technology and post-graft sulphonation processes to produce materials which exhibit high conductance as determined by uniformity of graft weight, throughout the thickness of membranes, from 75 μm to 1.0 mm or more in thickness.

For mutual grafting, parameters such as total dose, dose rate, and monomer concentration may be important in determining the properties of the resulting copolymer. For mutual grafting, typical total doses are 0.5 to 3 Mrad. Thus, over periods of typically 20 to 100 hours, then the dose rate range is 0.0005 to 0.20 Mrad/hr. Monomer concentrations can vary from 10% to 100%.

For post-irradiation grafting, the temperature at which the grafting reaction proceeds may be important, since during this stage a finite number of radicals will be produced in the polymer, determined by the radiation dose and dose rate, the irradiation temperature and atmosphere. Typical temperatures and times used are 40 ⁰ C to 7O ⁰ C, over 3 to 24 hours.

In the pre-irradiation experiments exemplified, the irradiations were carried out in air at 23+1 ⁰ C in order to produce predominantly peroxy radicals. On heating, these radicals decompose, producing radicals able to react with the monomer. The grafting temperature and time should be chosen accordingly. A high temperature may result in the premature recombination of radicals without grafting, a low temperature will reduce the rate of radical interaction and lower the graft weight. The following Examples illustrate the invention.

All the polymers and reagents were used as received. Ethylene- tetrafluoroethylene film (ETFE) was supplied by Nowofol GmbH. Low density polyethylene film (LDPE) was supplied by Transatlantic Plastics Ltd. Styrene, 99% was supplied by Aldrich® (stabilised with 10-15 ppm 4-tert-butyl catechol). Toluene (SLR grade), methanol (SLR grade) and chlorosulphonic acid were supplied by Fisher Scientific UK. Demineralised water was from a mixed bed Elgastat®, with conductivity <50μS.

Example 1

Mutual Grafting

Pieces of polymer film (approx. 0.3 m x 0.5 m) were cut, weighed and rolled in a

'Swiss-Roll' configuration with a non-woven interlayer. The roll was placed in a glass vessel and the vessel filled with monomer solution. The solution was allowed to soak into the roll for approximately 10 minutes. The oxygen in the vessel was then either removed by purging with nitrogen or by evacuation using a water pump. The whole vessel was then placed in a Cobalt 60 gamma source to be irradiated for a predetermined time, the distance of the tube from the source determining the dose rate. The temperature of the irradiation cell was measured at 23 ± 1 ⁰ C. After irradiation, the vessel was emptied, the roll untied and the grafted films unwrapped from the interlayer.

The copolymerised film was washed in toluene for 24 h to remove unreacted or homopolymerised styrene, washed in methanol to remove the toluene, and dried to constant weight in an oven at 7O ⁰ C. No additional homopolymerisation inhibitor was used in any of the grafting experiments, as the inhibitor present in the styrene (4-tert-butyl catechol) was shown to prevent excessive homopolymerisation.

The degree of grafting of the membranes was calculated using the following formula:

W - W ₀

—5 x 100 = Degree of Grafting(%)

wherein W ₀ = weight of polymer film before grafting, and W ₉ = weight of grafted copolymer.

The degree of grafting therefore represents the grafted proportion of the copolymer and has an upper limit of 100%. On this basis, a copolymer with a graft weight of 50% comprises 50% of the original polymer and 50% graft.

The dose rates used were between 500 Gy. h ^"1 and 50 Gy. h ^"1 to a total dose of 10 kGy. The total dose range was 10 to 30 kGy. The monomer concentration ranged from 20% to 70% (V/V) in toluene. Sulphonation

A Standard sulphonation procedure was carried out on all the polymer films grafted and found to be suitable for all the copolymer types. The grafted polymers were immersed in a solution of chlorosulphonic acid in methylene chloride.

The concentration of the chlorosulphonic acid was 2%-5% (V/V) and the sulphonation time ranged from 1-5 h at ambient temperature. After sulphonation, the films were washed to neutrality with demineralised water. Heat-Treatment

Before being used, the membranes were subjected to a thermal annealing process, by being heated at 95 ⁰ C in demineralised water for 1 h and then dried in an oven at 4O ⁰ C. The process has been found to increase the hydrophilicity and thus ionic conductivity of the grafted membranes. Example 2 Post-irradiation Grafting

The polymer films were irradiated in air at 23 ± 1 ⁰ C at a known dose rate and to set total doses. The films were then stored in a freezer at -18 ⁰ C until required.

The irradiated films were prepared for grafting by placing pieces of cut and weighed film (approx. 0.3 m x 0.5 m) in a glass vessel and filling with monomer solution. As in Example 1 , no homopolymerisation inhibitor was used. The vessel was purged with nitrogen for 2 h, sealed and placed in a water bath at a set temperature for a known length of time. After grafting, the copolymers were retrieved, washed in toluene, dried to constant weight in the same manner as for the mutual grafts.

The polymer films were irradiated to total doses between 15 and 100 kGy. The monomer concentrations ranged from 20% to 80% (V/V). The grafting temperature ranged from 40 to 70 ± 1 ⁰ C. The grafting time ranged from 3 to 24 h. Sulphonation and heat-treatment were then conducted as in Example 1.

Results for Example 1 are shown in Table 1. Results for Example 2 are given in Tables 2A and 2B. PSSA refers to the polymer-styrene-sulphonic acid.

Table 1

Table 2A

Table 2B