.alpha.,.beta.,.beta.-trifluorostyrene-based and substituted.alpha.,.beta.,.beta.-trifluorostyrene-based composite membranes

CA 02240495 2002-09-10 a, fi, fi-TRIFLUOROSTYRENE-BASED AND SUBSTITUED oc, fi, fi-TRIFLUOROSTYRENE-BASED COMPOSITE MEMBRANES

Cross-Reference to Related Patents

This application corresponds with U.S. Patent No. 5,834,523, issued November 10, 1998, which is a continuation-in-part of U.S. Patent Application Serial No. 08/442,206 filed May 16, 1995, now U.S. Patent No. 5,498,639, issued March 12, 1996, which is a continuation of U.S. Patent Application Serial No. 08/124,924 filed September 21, 1993, now U.S. Patent No. 5,422,411 issued

June 6, 1995, entitled Trifluorostyrene and Substituted Trifluorostyrene Copolymeric Compositions and Ionexchange Membranes Formed Therefrom''. The '206 and '924 applications describe polymeric compositions derived from copolymers of α, β, β-trifluoi'ostyrene with a variety of substituted α, β, β-trifluorostyrenes. These compositions are suitable for use as membranes, particularly as ion-exchange membranes. This application is also related to U.S. Patent Application Serial No. 08/480,098 filed June 6, 1995, now U.S. Patent 5,602,185, issued February 11, 1997, entitled Substituted

Trifluorostyrene Compositions''. The latter application describes copolymers of α, β, β-trifluorostyrene and substituted α, β, β-trifluorostyrenes, including sulfonyl fluoride substituted α, β, β-trifluorostyrene monomeric units that are conveniently hydrolyzed to produce polymeric compositions wi.th ion-exchange moieties.

Field Of The Invention

This invention relates generally to composite membranes comprising a porous substrate and a polymeric composition comprising various combinations of α, β, βCA 02240495 2002-09-10

-2trifluorostyrene, substituted α, β, β-trifluorostyrene and ethylene-based monomeric units. Where the polymeric composition includes ion-exchange moiet.ies, the resultant composite membranes are useful in electrochemical applications, particularly as membrane electrolytes in electrochemical fuel cells.

Background Of The Invention

Dense Films can be obtained from solutions of polyα, β, β-trifluorostyrene. However, the brittleness of these films greatly limits their application. Films obtained from some sulfonated poly- α, β, βtrifluorostyrene can be used as ion-exchange membranes. However, such films often have unfavourable mechanical properties when wet, and are known to be very brittle in the dry state (see, for example, Russian Chemical Reviews, Vol.59, p. 583 (1998)). Such films are of little practical use in fuel cells due to their poor physical properties. Some improvements in mechanical properties have

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- 3 been achieved by blending sulfonated poly-α,β,βtrifluorostyrene with polyvinylidene fluoride and triethyl phosphate plasticizer, but these films remained unsatisfactory for application in electrochemical cells (see Fuel Cell Handbook, A.J. Appleby, published by Van Nostrand Reinhold, p. 286 (1989)) .

U.S. Patent No. 5,422,411 and the related patent applications mentioned above describe various polymeric compositions incorporating substituted a,β,β-trifluorostyrenes and some cases further incorporating substituted ethylenes. Typically these compositions, as membranes, possess favorable mechanical properties compared to polya, β,β-trifluorostyrene and sulfonated poly-α,β,βtrifluorostyrene, although some of the membranes have a tendency to become brittle in the fully dehydrated state, depending, for example, on the equivalent weight. This effect is most apparent at equivalent weights below approximately 380 g/mol. Ion-exchange membranes derived from these polymeric compositions are suitable for many applications, including use in electrochemical applications, such as fuel cells.

For ease of handling, for example, in the preparation of membrane electrode assemblies for use in electrochemical fuel cells, the mechanical strength of the membrane in the dry state is important. In electrochemical applications, such as electrolytic cells and fuel cells, the dimensional stability (changes in the dimensions of the membrane due to changes in the degree of hydration) of the membrane during operation is also important. However, to improve performance, it is

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- 4 generally desirable to reduce membrane thickness and to decrease the equivalent weight (thereby increasing the water content) of the membrane electrolyte, both of which tend to decrease both the mechanical strength in the dry state and the dimensional stability in the wet state. One way to improve mechanical strength and dimensional stability in ionomeric membranes is through use of a substrate or support material, to give a composite membrane. The substrate is selected so that it imparts mechanical strength and dimensional stability to the membrane. The substrate material can be combined with the membrane polymeric material to form a composite membrane in a variety of ways. For example, if possible, an unsupported membrane can be preformed and then laminated to the porous substrate. Alternatively, a solution of the polymer can be impregnated into the porous substrate material, and the composite membrane subsequently dried. The formation of composite membranes via impregnation provides a more intimate contact between the two components, thus giving advantages over standard lamination approaches.

Composite ion-exchange membranes prepared by impregnating commercially available porous polytetrafluoroethylene film (Gore-tex®) with Nafion®, a perfluorosulfonate ionomer, have been described in Journal of the Electrochemical Society, Vol. 132, pp. 514-515 (1985). The major goal in the study was to develop a composite membrane with the desirable chemical and mechanical features of Nafion®, but which could be produced at low cost. Indeed, based on the polymer loadings necessary to produce these composite membranes,

CA 02240495 1998-06-16 ? · · -, · '

Λ * · 1 Ί ' ' ·> . > *

- 5 they are a low cost alternative to the costly perfluorosulfonic acid membranes. As indicated above, however, these perfluorosulfonate ionomers are known to form membranes suitable for use in electrochemical applications without the use of a substrate .

It has been discovered that polymers which have a tendency to become brittle in the dehydrated state can be rendered mechanically stable, even in the fully dehydrated state, by impregnation into suitable substrates.

Furthermore, it has been discovered that even polymers which are poor film formers, or polymers which form films with mechanical properties and dimensional stability which would preclude their use in electrochemical and other applications, can be made into composite membranes through incorporation into a suitable substrate. The resulting composite membranes have the desired physical properties for use in a wide range of applications.

Summary Of The Invention

A composite membrane comprises a flexible porous polymeric sheet material impregnated with a polymer comprising fragments of formula (I):

<img file="CA2240495C_D0001.tif" />

wherein m, n, p, g, r, and s are zero or an integer greater than zero, and at least one of m, n, p and q is an integer greater than zero, whereby the polymer comprises at least one type of substituted

AMENDED SHEET

CA 02240495 1998-06-16 or unsubstituted α,β,β-trifluorostyrene monomeric unit. The thickness of the flexible polymeric sheet material into which the polymer of formula (I) is impregnated is preferably 10-200 pm.

In the above formula:

X is selected from the group consisting of

S0₂F, SO₃H, P0₂H₂, PO₃H₂,

CH₂P0₃H₂, COOH, 0S0₃H, 0P0₂H₂,

OPO₃H_2z OArSO₃H where Ar is an aryl, NR₃⁺ (where R is selected, from the group consisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃⁺ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls);

A_T and A₂ are selected from the group consisting of halogens, C_xH_yF_z (where x is an integer greater than zero and y+z = 2x+l), CF=CF₂, CN, NO₂ and OH, OR (where R is selected from the group consisting of alkyls and perfluoroalkyls and aryls), and when m is an integer greater than zero, the group from which A_x and A₂ are selected further consists of SO₂F, SO₃H, po₂h₂, po₃h_2/ ch₂po₃h_2/ COOH, oso₃h, opo₂h₂, opo₃h₂, OArSO₃H where Ar is an aryl, NR₃⁺ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃⁺ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls);

Z is hydrogen or a non-hydrogen substituent whereby, when s is an integer greater than zero, the polymer further comprises styrene or substituted styrene monomeric units;

E is selected from the group consisting of

CH₂-CH₂, CH₂-CHF, CFH-CFH, cf₂-ch₂, cf₂-chf, cf₂-cf₂ whereby, when r is an integer greater than zero, the polymer further comprises ethylene, partially fluorinated ethylene or tetrafluoroethylene

CA 02240495 1998-06-16

- 7 monomeric units.

In some embodiments of the above composite membrane, at least p is an integer greater than zero, whereby the polymer comprises at least one type of A-substituted a,S,β-trifluorostyrene monomeric unit.

In other embodiments of the above composite membrane, at least m is an integer greater than zero, whereby the polymer comprises at least one type of X-substituted a,S,β-trifluorostyrene monomeric unit. In preferred embodiments, at least one of n, p and. q is also an integer greater than zero, whereby the polymer comprises more than one type of α,β,β-trifluorostyrene-based monomeric unit. In other preferred embodiments, at least one of p and q, as well as m, is an integer greater than zero, whereby the polymer comprises more than one type of substituted or,β,β-trifluorostyrene monomeric unit.

A preferred X-substituent is SO₂F, whereby the polymer comprises a,β,β-trifluorostyrene sulfonyl fluoride monomeric units. Again, at least one of n, p and q may also be an integer greater than zero. In particularly preferred embodiments, both n and p are integers greater than zero, and A_L is selected from the group consisting of fluorine, CF₃ and paraphenoxy .

Another preferred X-substituent is SO₃H. Again, n may also be an integer greater than zero, and at least one of p and q may also be an integer greater than zero. In particularly preferred embodiments, both n and p are integers greater than zero, and A_x is selected from the group consisting of fluorine, CF₃ and para-phenoxy.

AMENDED SHEET

CA 02240495 1998-06-16

- 8 In still further embodiments of the above composite membrane both m and p are integers greater than zero, and A_x is selected from the group consisting of S0₂F, SO₃H, P0₂H_2/ P0₃H₂, CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂, OPO₃H₂, OArSO₃H where Ar is an aryl, NR₃⁺ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃⁺(where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls). In these embodiments the polymer comprises more than one type of ion-exchange moiety.

In the composite membranes described above, preferred flexible polymeric sheet materials include those comprising polytetrafluorethylene, a polyolefin or à partially fluorinated polyolefin. Flexible polyermic sheet materials comprising a polymer selected from the group consisting of polyethylene, polypropylene, polyvinylidene fluoride, poly(ethylene-co-tetrafluoroethylene) and poly (tetrafluoroethylene-co-hexafluoropropylene) are particularly preferred for certain applications. Expanded polytetrafluoroethylene is another example of a preferred substrate.

For certain applications, such as fuel cell applications, the composite membrane is preferably substantially gas impermeable.

In the aspects and embodiments described above, the polymeric compositions can consist essentially of the described monomeric units.

In the aspects and embodiments described above, the A_x, A₂, A₃ substituents may be further elaborated by known means such as, for example, by hydrolysis of the CN group to form COOH or by reduction with common reducing agents (such as, for example, Raney

AMENDED sheet

CA 02240495 1998-06-16

<img file="CA2240495C_D0002.tif" />

nickel) to form a primary amine, therebytransforming the A_1Z A₂ and A₃ substituents into ionexchange moieties. The resulting polymeric composition may thus comprise one or more type of ion-exchange moiety, and may also comprise both cation-exchange and anion-exchange moieties.

The term monomeric unit as used herein indicates that the polymeric composition contains the described fragment or unit, and is obtained by a polymerization reaction involving the corresponding unsaturated monomer.

The substituents on the aromatic rings (including, for example, A_x, A₂, A₃, X, B and D) may each be located in the ortho, meta or para positions, as indicated in the formulas wherein the chemical bond drawn for the substituents intersects the aromatic ring. In preferred aspects of the described embodiments, the substituents are in the meta or para positions.

As used herein, the term aryl refers to a substituted or unsubstituted phenyl group. The formula C_xHyF_z (where x is an integer greater than zero and y+z = 2x+l) is used to indicate alkyl, perfluoroalkyl or partially fluorinated alkyl groups .

In accordance with convention in the art, the above chemical formulas for polymeric compositions containing more than two monomeric units (where at least three of m, n, p and q are greater than zero) are intended to indicate that the monomeric units are present in the polymeric composition, but are not limited to the particular order in which the monomeric units are set forth in each general formula. For example, random linear copolymers,

AMENDED SHEET

CA 02240495 1998-06-16

- 10 alternating copolymers and linear block copolymers, formed from the indicated monomeric units, are contemplated.

AMENDED SHEET

CA 02240495 1998-06-16 }

A - 19 Brief Description Of The Drawing's

FIG. 1 is a plot of cell voltage as a function of current density (expressed in milliamperes per square centimeter) in an electrochemical fuel cell employing a composite membrane of porous polyethylene impregnated with a sulfonated copolymer of ot,β,&amp;-trifluorostyrene and m-trifluoromethyla, β, β-trif lu’orostyrene, as the proton exchange membrane .

FIG. 2 is a plot of cell voltage as a function of current density in an electrochemical fuel cell employing a composite membrane, prepared by impregnation of porous polyethylene with sulfonated poly-a,β,β-trifluorostyrene, as the proton exchange membrane .

FIG. 3 is a plot of cell voltage as a function of current density in an electrochemical fuel cell employing a composite membrane, prepared by impregnation of porous polyethylene with a copolymer of α,β,β-trifluorostyrene, m-trif luoromethyl-οί, β, β-trif luorostyrene and p-sulfonyl fluoride-a,β,β-trifluorostyrene, and subsequent hydrolysis, as the proton exchange membrane .

AMENDED SHEET

CA 02240495 1998-06-16 ·&gt;'»·»·&gt; ·) i ) 1 ·, % 5 » 5 n η i • - ~ - -&gt; η -» η-»

- 20 FIG. 4 is a plot of cell voltage as a function of current density in an electrochemical fuel cell employing a composite membrane of expanded, polytetrafluoroethylene impregnated with a sulfonated, copolymer of α,β,β-trifluorostyrene and m-trifluoromethyl-a,β,β-trifluorostyrene, as the proton exchange membrane .

FIG. 5 is a plot of cell voltage as a function of current density in an electrochemical fuel cell employing a composite membrane of expanded polytetrafluoroethylene impregnated with a low equivalent weight sulfonated copolymer of α,β,βtrif luorostyrene and m-trif luoromethyl-a:, β, β-trifluorostyrene, as the proton exchange membrane.

Detailed Description Of The Preferred Embodiments

Methods for preparing the polymeric compositions described herein are described in the related applications or will be apparent to those skilled in the art.

The preferred substrate material is dependent on the application in which the composite membrane is to be used. The substrate material preferably has good mechanical properties, is chemically and thermally stable in the environment in which the composite membrane is to be used, is tolerant of the solvent used for impregnation, and in most applications is preferably flexible. Preferred substrates for electrochemical applications are porous polymeric materials . Preferred polymeric materials are, for example,

AMENDED SHEET

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- 21 hydrocarbons such as porous polyolefins, especially polyethylene and polypropylene . In some applications, a perfluorinated polymeric substrate may be preferred, for example, a preferred substrate material, when the composite membrane is to be used in an electrochemical fuel cell, is porous polytetrafluoroethylene, also known as expanded polytetrafluoroethylene. Porous polyolefins and polytetrafluoroethylenes typically have excellent mechanical strength, flexibility and do not swell in water. Polytetrafluoroethylene offers additional advantages in that it is also chemically inert, and porous polytetrafluoroethylene films with different characteristics are commercially available from various sources. It may be possible to obtain or prepare other suitable porous polymeric substrates from, such as, for example, polyvinylidene fluoride or polysulfones. Copolymeric substrates such as, for example, poly(ethylene-co-tetrafluoroethylene) and poly(tetrafluoroethylene-cohexafluoropropylene), may also be used.

The degree of porosity, pore size and thickness of the substrate used in the composite membrane can be selected to suit the application. For use of the composite membrane as an electrolyte in an electrochemical fuel cell, the substrate thickness is preferably 10-200 pm, and more preferably 25-50 pm, the preferable average pore diameter is 0.1-1.0 pm, and the preferable porosity is 50-98%, more preferably 75-90%.

Depending on the application the resultant composite membrane may be gas permeable or gas impermeable. The loading of the polymeric

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- 22 composition on the substrate can be varied in order to control the porosity of the resultant composite membrane. For fuel cell applications, the composite membrane is preferably substantially gas impermeable, thus the degree of impregnation and loading is such that the porosity of the composite membrane is reduced essentially to zero.

In a method for preparing composite membranes, the polymeric composition is dissolved in a solvent, typically an organic solvent, to form a solution. The solvent used will depend, for example, on both the nature of the polymeric composition and the substrate. For impregnation of porous polyolefins with the type of polymeric compositions described herein, suitable solvents include N,N-dimethylformamide, N-methylpyrrolidone, dimethylsulfoxide and N,N-dimethylacetamide. When polytetrafluoroethylene is the substrate, an alcohol or mixture of alcohols (chosen, for example, from methanol, ethanol and propan-2-ol) is often the preferred solvent. The concentration of the solution will depend on the loading desired, and whether the composite membrane is to be porous or not. For example, if the composite membrane is to be gas permeable a lower concentration is generally preferred.

The porous substrate is then impregnated, for example, by constraining the substrate in a frame and dipping or soaking it in the solution. The contact time is dependent on the viscosity and' percentage solids of the solution. Other techniques known in the art, such as ultrasonication, may be used to facilitate impregnation. Also, multiple impregnations,

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- 23 possibly with different polymeric compositions, maybe desirable for some applications. The substrate is then removed from the solution and the composite membrane dried preferably in a humidity controlled atmosphere (generally at less than or equal to 2¾ relative humidity) at above ambient temperatures.

If the composite membrane includes protonexchange moieties and is to be used in, for example, a proton-exchange membrane fuel cell, it is removed from the frame, treated with 1 M hydrochloric acid and washed with deionized water prior to use.

The means by which the process described above could be modified for impregnation of non-membrane substrates, and also for a continuous composite membrane manufacturing process will be apparent to those skilled in the art.

In the preparation of composite ion-exchange membranes, the ion-exchange moieties can be:

(i) present in the polymeric composition prior to its impregnation into the substrate; or (ii) introduced post-impregnation through further reaction of the polymeric composition on the substrate; or (iii) introduced via conversion of precursor groups, present in the polymeric composition, after impregnation.

If the ion-exchange moieties are to be introduced via a post-impregnation conversion or 30 reaction, the substrate needs to selected such that it can withstand the post-impregnation treatment step. For example, in post-impregnation introduction of ion-exchange moieties, the ionexchange moieties may be introduced into polymeric

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- 24 compositions containing unsubstituted α,β,Β-trifluorostyrene units (so called base polymers) via aromatic substitution of at least a portion of those units, after preparation of a composite membrane. For example, pendant unsubstituted phenyl rings in the composite membrane can be conveniently sulfonated (see U.S. Patent 5,422,411) to produce a composite cation-exchange membrane. Similarly, such pendant unsubstituted phenyl rings may be phosphorylated, carboxylated, quaternaryaminoalkylated or chloromethylated, and further modified to include -CH₂PO₃H₂, -CH₂NR₃‘ where R is an alkyl, or -CH₂NAr₃” (where Ar is a substituted or unsubstituted phenyl group) and other substituents, to provide cation-exchange or anion-exchange composite membranes. Further still, the pendent phenyl moiety may contain a hydroxyl group which can be elaborated by known methods to generate OSOjH, -OPO₂H₂ and -OPO₃H₂ cation-exchange sites on the composite membrane.

The approach in which the ion-exchange functionality is introduced post-impregnation via conversion of a precursor using simple postimpregnation procedure, such as hydrolysis, can be advantageous. For example, composite membranes comprising polymers containing sulfonyl fluoride moieties (-SO₂F) can be hydrolyzed to generate -SO₃H cation-exchange sites. In a typical hydrolysis reaction, the sulfonyl fluoride is converted to the free sulfonic acid functionality by treatment of the composite membrane with concentrated aqueous alkali metal hydroxide at elevated temperatures. This and other procedures for the hydrolysis of SOjF to -SO₃H are well-known to those skilled in the

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- 25 art. The latter approach to the introduction of SO₃H moieties offers advantages over sulfonation of a base polymer in the composite membrane. For example, it permits greater control over the ionexchange capacity of the resultant composite membrane .

Membranes including sulfonyl fluoride substituted α,β,β-trifluorostyrene monomeric units are described in a related application.

Unsupported membranes containing a significant proportion of sulfonyl fluoride substituted α,β,βtrifluorostyrene monomeric units can be very fragile. The mechanical properties of these «

precursor ion-exchange membranes can be significantly enhanced through incorporation into a porous substrate.

It may be advantageous to introduce ionexchange moieties after preparation of the composite membranes, as described in (ii) and (iii) above. For example, in electrochemical applications where the preferred substrates are typically hydrophobic, the preparation of a composite membrane by first impregnating the substrate with a solution of a non-ionic polymer which is also essentially hydrophobic may lead to more facile and improved impregnation.

The following examples are for purposes of illustration, and are not intended to limit the invention. Examples 1-3 describe the preparation of composite ion-exchange membranes in which porous, high density polyethylene is used as the substrate. Examples 4 and 5 describe the preparation of composite ion-exchange membranes in which expanded polytetrafluoroethylene is used as

CA 02240495 1998-06-16 ' . i ·&gt; Ο Ί 1 Ί ·» ' 1 I ·) ' - · ·&gt;«-·. λ η .» ·&gt;

- 26 the porous substrate. In Examples 1, 2, 4 and 5 the ion-exchange moieties were present in the polymeric composition prior to its impregnation into the substrate. In Example 3 the ion-exchange moiety was generated by hydrolysis of sulfonyl fluoride moieties after preparation of the composite membrane. Example 6 sets forth the procedure used to test the composite ion-exchange membranes, prepared as described in Examples 1-5, as membrane electrolytes in an electrochemical fuel cell.

Example 1

Porous polyethylene impregnated with a sulfonated copolymer of a,β,β-trifluorostyrene and mtri fluoromethyl-a,β,β-tri fluoros tyrene (Composite Membrane A)

The substrate, .a 22.9 cm x 22.9 cm piece of high density polyethylene (obtained from 3M, product ID #43-9100-6770-1, 81% porosity, approximately 50 micrometer) was clamped in a frame and immersed in a N,N-dimethylformamide solution (7% w/w) of a sulfonated copolymer of a,β,β-trifluorostyrene and m-trifluoromethyl-a,β,β-trifluorostyrene (equivalent weight 384 g/mol) in a glass container. The container was covered to exclude moisture and particulate contaminants. After 1 hour excess polymer solution was removed and the transparent, wetted substrate was placed to dry in a chamber at approximately 2% relative humidity, at 50°C. After approximately 3 hours the dry composite membrane, now opaque, was a mechanically strong flexible film. On immersion in 1 M hydrochloric acid (to

AMENDED SHEET

CA 02240495 1998-06-16 . .T 1 . . .

» &gt; ·) ^Ί ' Ί rj η -ί - 1 &gt; -» 7 .·) , { &gt;- - ·» -η ·&gt; ' 1 ‘5 ‘ Ί ·» 1

- 27 ensure protonation of all the sulfonic acid moieties), and subsequent washing with deionized water, the composite membrane once again became transparent. The wet composite membrane (50-60 micrometer thick) was also strong and flexible.

Example 2

Porous polyethylene impregnated with sulfonated poly-α,β,β-trifluorostyrene (Composite Membrane B)

The substrate, a 25.4 cm x 25.4 cm piece of high density polyethylene (from 3M, product ID #439100-6770-1, 81% porosity, 50 micrometer) was clamped in a frame and immersed in a N,Ndimethylformamide solution (7% w/w) of a sulfonated polymer of α,β,β-trifluorostyrene (equivalent weight 430 g/mol) in a glass container. The container was covered to exclude moisture and particulate contaminants. After 2 hours excess polymer solution was removed and the transparent, wetted substrate was placed to dry in a chamber at approximately 2% relative humidity, at 50°C. After approximately 3 hours the dry composite membrane, now opaque, was a mechanically strong flexible film, in contrast to the analogous unsupported membrane which would be extremely fragile in the dry state. On immersion in 1 M hydrochloric acid (to ensure protonation of all the sulfonic acid moieties), and subsequent washing with deionized water, the composite membrane once again became transparent. The wet composite membrane (approximately 100 micrometer thick)was also strong and flexible.

AMENDED sheet f

CA 02240495 1998-06-16 &gt;

i 7 *5 *5

- 28 Example 3

Porous polyethylene impregnated with a copolymer of of, β, β- trif luoros tyrene, m- triflu.oromethyl-oi, β, β - tri f luoros tyrene and p-sulfonyl fluoride-α,β,β-trifluorostyrene, and subsequent hydrolysis (Composite Membrane C)

The substrate, a 25.4 cm x 25.4 cm piece of high density polyethylene (from. 3M, product ID #439100-6770-1, 81% porosity, approximately 50 micrometer) was clamped in a frame and immersed in a N,N-dimethylformamide solution (5% w/w) of a copolymer of a,B,B-trifluorostyrene, mtrifluoromethyl-a,B, β-trifluorostyrene and psulfonyl fluoride-α,B,B-trifluorostyrene (equivalent weight 480 g/mol after hydrolysis) in a glass container. The container was covered, to exclude moisture and particulate contaminants. After 2 hours excess polymer solution was removed and the transparent, wetted substrate was placed to dry in a chamber at approximately 2% relative humidity, at 50°C. After approximately 3 hours the dry composite membrane was a mechanically strong flexible film. The sulfonyl fluoride moieties were hydrolyzed by treatment of the composite membrane with potassium hydroxide solution (approximately 6% w/w, in 5:1 w/w water : 1-methoxy-2-propanol) at 60 °C (see U.S. Patent No. 5,310,765). The composite membrane was then immersed in 1 M hydrochloric acid to ensure protonation of all the sulfonic acid moieties in the composite membrane, and subsequently washed with deionized water. The wet, hydrolyzed composite membrane (50-70 micrometer thick) was also

AMENDED sheet

CA 02240495 1998-06-16 ·&gt;

&gt;

·&gt;

- 29 strong and. flexible.

Example 4

Expanded polytetrafluoroethylene impregnated with, a sulfonated copolymer of α,β,β-trifluorostyrene and m- trifluoromethyl-α,β,β-trifluorostyrene (Composite Membrane D)

The substrate, an 20.3 cm x 20.3 cm piece of expanded polytetrafluoroethylene (Tetratex® obtained from Tetratec Corporation, 80-90% porosity, approximately 38 micrometer, 0.45 micrometer pore size) was clamped in a frame and immersed in a methanol/propan-2-ol (3:1) solution (approximately 5% w/v) of a sulfonated copolymer of α,β,β-trifluorostyrene and m-trifluoromethyl-α,β,β-trifluorostyrene (equivalent weight 412 g/mol) in a glass container. The container was covered to exclude moisture and particulate contaminants.

After 18 hours excess polymer solution was removed and the transparent, wetted substrate was placed to dry in a chamber at approximately 2% relative humidity, at 5 0 °C. After approximately 1.5 hours the dry composite membrane, now opaque, was a mechanically strong flexible film. On immersion in 1 M hydrochloric acid (to ensure protonation of all the sulfonic acid moieties), and subsequent washing with deionized water, the composite membrane once again became transparent. The wet composite membrane (50-60 micrometer thick) was also strong and flexible.

AMENDED sheet

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- 30 Example 5

Expanded polytetrafluoroethylene impregnated with a sulfonated copolymer of a, β, β-trifluorostyrene and m-trifluoromethyl- α, β, β-trifluorostyrene (Composite Membrane E)

The composite membrane was prepared as described in Example 4, using a sulfonated copolymer of oc, β, β-trifluorostyrene and m-trifluoromethyl- α, β, β-trifluorostyrene with a lower equivalent weight (362 g/mol) and impregnating the substrate for 30 minutes. The resulting dry composite membrane was a mechanically strong flexible film, in contrast to the analogous unsupported membrane which, at this low equivalent weight, is extremely fragile and readily reduced to a powder on handling. The wet composite membrane (25-40 micrometer thick) was also strong and flexible, again in contrast to the unsupported membrane which is fragile and dimensionally unstable, and is therefore of limited use in electrochemical fuel cells.

Example 6

Each of the composite membranes prepared as described above was bonded to two catalyzed carbon fiber paper electrodes at room temperature under 3,402 kilograms of pressure. Each membrane electrode assembly was tested in a Ballard Mark IV single cell fuel cell (see U.S. Patent No's. 4,988,583; 5,108,849; 5,170,124;

5,176,966 and 5,200,278). The following operating conditions applied to the fuel cell in which the membranes were tested:

Temperature: 70°C

Reactant inlet pressure:

165 kiloNewtons/meter squared for both air and hydrogen

Reactant stoichiometries:

2.0 air and 1.15 hydrogen.

CA 02240495 2002-09-10

- 31 The membrane electrode assemblies incorporating the composite membranes were tested for 200-1400 hours, depending on availability of testing equipment.

FIGS. 1-5 are polarization plots of voltage as a function of 5 current density for composite membranes A-E, respectively, employed in membrane electrode assemblies in the electrochemical fuel cell. The data is comparable to data reported for unsupported membranes in related U.S. Patent No. 5,422,411.