WO2010047661A1

WO2010047661A1 - A novel acid-doped polymer electrolyte membrane

Info

Publication number: WO2010047661A1
Application number: PCT/SG2009/000388
Authority: WO
Inventors: Xinhui Zhang; Liang Hong; Zhaolin Liu; Siok Wei Tay
Original assignee: Agency For Science, Technology And Research
Priority date: 2008-10-20
Filing date: 2009-10-20
Publication date: 2010-04-29

Abstract

The present invention relates to a polymer electrolyte membrane (PEM). The PEM comprises a cross-linked polymer matrix doped with an acid that is dispersed substantially throughout the bulk of said polymer matrix. The present invention also relates to a method of making the same and to a fuel cell which comprises the PEM.

Description

A NOVEL ACID-DOPED POLYMER ELECTROLYTE MEMBRANE

Technical Field

The present invention generally relates to an acid- doped polymer electrolyte membrane and a method for producing the same.

Background

The use of hydrogen as an energy source has become imperative due to concerns of global warming and rising gasoline prices. Proton exchange (or electrolyte) membrane fuel cells (PEMFCs) have significant potential in replacing internal combustion engines , such as automobile engines. PEMFCs have a number of advantages, such as utilizing a clean energy source (i.e. hydrogen) to produce electricity, they are able to produce power at high density, they are able to start-up rapidly, are able to respond rapidly to varying loads and exhibit little noise generation.

As an electrochemical device, the energy utilization efficiency and the operating cost of the PEMFC is affected by the kinetics of electrode catalysts. The slow kinetics of the electrode (both anode and cathode) catalysts of PEMFC is typically associated with fuel-caused catalyst deactivation and electrochemical polarization.

In the pursuit of a suitable PEMFC (100 KW) capable of powering motor vehicles, the purity of hydrogen (H₂) used is an important aspect to consider. As industrial H₂ production is primarily derived from steam-reforming and partial oxidation -of light hydrocarbons (including CH₄ to C₄H₁0) , carbon monoxide (CO) is typically a by-product of H₂ production. While known purification techniques can reduce CO content in the H₂ to about 2 mol%, this level of CO is still higher than the acceptable limit of 20 ppm (i.e. about 0.05 mol%) , which is the concentration capable of causing deactivation of the Pt catalyst on the anode of fuel cells at low temperatures (<80°C) . Therefore, an additional purification step that uses pressure swing adsorption to further purify the H₂ is needed.

Another factor that lowers the efficiency of a PEMFC is the activation over-potential on the electrode/electrolyte interface. This kinetic factor limits the extent of high thermodynamic efficiency. Thus, in the current state of the art, fuel cells typically achieve efficiency of below 35%.

If these problems were overcome, the efficiency of the fuel cell could increase up to 50%. However, it is not easy to solve these two problems together because although a more reactive anode catalyst will be beneficial in achieving faster electrode kinetics at 70°C to 8O⁰C (preferably for mobile applications) , a more reactive catalyst is, at the same time, more vulnerable to CO poisoning, which leads to catalyst deactivation.

Hence, one proposed solution has been directed to a new type of polymer electrolyte membrane (PEM), which will permit cell operation at temperatures above 150⁰C to 200⁰C. Increasing the operation temperature to above 15O⁰C will largely promote CO-tolerance of the anode catalyst and therefore reduce or negate the need for purifying H₂. Additionally, the high operating temperature will further benefit electrode kinetics and reduce or prevent activation over-potential. However, commercially available PEMs, such as sulfonate perfluoropolymers (e.g. Nafion-117®) , show a significant loss in conductivity when the operating temperature is above 100⁰C. This can be largely attributed to the loss of matrix water. It is therefore necessary to provide a PEM that is capable of withstanding high temperature operation without a significant drop in conductivity.

An anhydrous PEM in the form of phosphoric acid

(H₃PO₄) -doped polybenzimidazole (PBI) has been developed as an anhydrous polymer electrolyte membrane (PEM) . It possesses a uniform mineral acid-polymer semi-gel structure, which allows for high acid-doping and is hence capable of exhibiting high ionic conductivity and stable mechanical properties in the temperature range of 15O⁰C to 200⁰C.

However, presently known H₃P0₄~doped PBI membranes can only be obtained under very stringent polymerization conditions. These stringent conditions include using highly pure monomers, using an anhydrous solvent for polymerization and a need for high monomer conversion before a polymer membrane can be successfully produced. Otherwise, a mechanically weak, paste-like membrane will result. In particular, upon a subsequent hydrolysis step, the membrane typically disintegrates as it lacks the necessary mechanical strength to entrain the hydrolyzed acid within the membrane.

Accordingly, the success rate of obtaining good, free-standing membranes has been quite low on the basis of repetitive preparations in the laboratory. Even in the rare instance where a membrane was successfully produced, the resulting product nevertheless fell short in terms of having sufficient mechanical strength. In particular, such membranes suffer from a low tensile stress breaking point, poor elongation properties and relatively low maximum loading (in N) .

There is therefore a need to provide for an acid- doped proton exchange membrane that can overcome, or at least ameliorate the disadvantages above. In particular, there is a need to provide an acid-doped polymer electrolyte membrane that is capable of substantially entraining an acid dopant within the membrane. There is also a need to provide a polymer electrolyte membrane that possesses superior mechanical qualities such as high maximal tensile stress, high maximum loading and good elongation properties. There is further a need to provide a method for producing an acid-doped polymer electrolyte membrane having the above mechanical qualities.

Summary

According to a first aspect, there is provided a polymer electrolyte membrane (PEM) comprising a cross- linked polymer matrix doped with an acid that is dispersed substantially throughout the bulk of said polymer matrix. Advantageously, the membrane is cross-linked by a cross-linker which may be selected to form a cross-linked network that permeates substantially uniformly throughout the membrane matrix. Advantageously, the substantially uniform cross-linked network enables the acid to be dispersed throughout the bulk of said polymer matrix. The cross-linker may be a polymeric cross-linker having plural hydroxyl functional groups thereon that have formed multiple cross-linkages with said polymer matrix. Advantageously, the polymer electrolyte membrane, after being cross-linked with the polymeric cross-linker, is mechanically strong enough to entrain the dopant acid within the membrane. Also advantageously, the polymer electrolyte membrane, having the acid dopant present throughout the bulk of the matrix, possesses good mechanical qualities such as high tensile strength and yield strength. Advantageously, the acid dopant is present at a content throughout the bulk of said polymer matrix to facilitate conduction of protons. More advantageously, the acid is a strong acid such as phosphoric acid.

In a second aspect, there is provided a method of producing a polymer electrolyte membrane (PEM) , said method comprising the steps of:

(a) cross-linking a polymer-poly-acid solution to form a polymer-poly-acid-cross-linked gel matrix;

(b) forming a polymer membrane from said polymer- poly-acid-cross-linker gel matrix, wherein said polymer membrane is doped with said poly-acid; and

(c) hydrolyzing said polyacid to form an acid from said poly-acid and thereby form a cross-linked polymer matrix doped with an acid that is dispersed substantially throughout the bulk of said polymer matrix. Advantageously, the step of partial cross-linking the polymer-poly-acid solution occurs before the hydrolysis step (b) , resulting in the formation of a homogeneous syrup-like liquid, wherein the cross-linking occurs uniformly throughout the gel matrix. This in turn allows the acid to extend throughout the bulk of the polymer matrix after the hydrolysis step.

Also advantageously, the cross-linking between the polymer and cross-linker results in the formation of a mechanically strong, polymer-poly-acid-cross-linker gel matrix, that is capable of entraining the poly-acid therein, whilst maintaining its structural integrity during the subsequent hydrolysis step. As a consequence, the mechanically strong gel matrix further allows for a relatively high acid-doping level, thereby increasing the H⁺ conductivity of the polymer matrix.

In one embodiment of the second aspect, step (b) may be undertaken during step (a) .

In a third aspect, there is provided a fuel cell comprising, a polymer electrolyte membrane comprising a polymer matrix doped with an acid and cross-linked with a cross-linker selected to substantially entrain said acid within the bulk of said polymer matrix.

Definitions

The following words and terms used herein shall have the meaning indicated:

The term 'unsaturated' is to be interpreted broadly to include any hydrocarbon compound that comprises at least one or more C=C double bonds, or C≡C triple bonds.

As used herein, the terms "hydrolysis" and

"hydrolyzing" means, in the context of this specification, refers to a chemical reaction between two or more chemical species in which a chemical bond is split via the addition of water.

The term "gel matrix" means any polymer-containing material and the like that congeals or precipitatively solidifies when maintained at a gelation temperature such that gel formation occurs, but which may be liquid at non- gelation temperatures.

The term "polymer-poly-acid solution" means an aqueous solution and the like, when used in reference to a polymer and a poly-acid in such solution and which is maintained at non-gelation temperatures such that gel formation does not occur.

The term "polymer matrix" means a continuous phase in a material, where the continuous phase includes at least a type of polymer.

The term "bulk of said polymer matrix", in the context of this specification, refers to the volume occupied by the polymer matrix.

The term "substituted" is intended to indicate that one or more (e.g., 1, 2, 3, 4, or 5; in some embodiments 1, 2, or 3; and in other embodiments 1 or 2) hydrogen atoms on the group indicated in the expression using "substituted" is replaced with a selection from the indicated organic or inorganic group (s), or with a suitable organic or inorganic group known to those of skill in the art, provided that the indicated atom's normal valency is not exceeded, and that the substitution results in a stable compound. Suitable indicated organic or inorganic groups include, e.g., alkyl, alkenyl, alkynyl, alkoxy, halo, haloalkyl, hydroxy, hydroxyalkyl, aryl, heteroaryl, heterocycle, cycloalkyl, alkanoyl, alkoxycarbonyl, amino, alkylamino, dialkylamino, trifluoromethylthio, difluoromethyl, acylamino, nitro, trifluoromethyl, trifluoromethoxy, carboxy, carboxyalkyl, keto, thioxo, alkylthio, alkylsulfinyl, alkylsulfonyl, alkylsilyl, and cyano. Additionally, the suitable indicated groups can include, e.g., —X, —R, —0 - , —OR, — SR, -S-, -NR 2 , -NR 3 , -NR, -CX 3 , -CN, -OCN, -SCN, -N —C—0, -NCS, -NO, -NO 2 , —N 2 , -N 3 , NC (=0) R, -C ( — O)R, -C(=O)NRR -S(—0) 2 0 - , -S (—0) 2 OH, -S (-=0) 2 R, - 0S(—0) 2 OR, -S(—0) 2 NR, -S(—O)R, -OP (=0)0 2 RR, -P (— 0)0 2 RR-P (—0) (0 - ) 2 , -P(—0) (OH) 2 , -C(—O)R, -C( — O)X, -C(S)R, -C(O)OR, -C(O)O - , -C(S)OR, -C(O)SR, - C(S)SR, -C(O)NRR, -C(S)NRR, -C(NR)NRR, where each X is independently a halogen (or "halo" group) : F, Cl, Br, or I; and each R is independently H, alkyl, aryl, heterocycle. As would be readily understood by one skilled in the art, when a substituent is keto (i.e., =— 0) or thioxo (i.e., -=-S) , or the like, then two hydrogen atoms on the substituted atom are replaced. The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of portions of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value. Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Disclosure of Optional Embodiments

Exemplary, non-limiting embodiments of a polymer electrolyte membrane will now be disclosed. The polymer of the PEM may be a polyazole. In one embodiment, the polymer matrix of the PEM may comprise a plurality of polyazole blocks, each polyazole block being coupled with another by the cross-linker. The polyazole may be selected from the group consisting of polyimidazoles, polybenzimidazole, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (pyridines) , poly (pyrimidines) , poly (tetrazapyrenes) , and co-polymers thereof. In one embodiment, the polyazole is polybenzimidazole (PBI) .

The polybenzimidazole may be represented by one of the following exemplary formulae:

and

where n and m are each an integer greater than or equal to 10, preferably greater than or equal to 100.

The cross-linker may be a polymeric cross-linker. In one embodiment, the cross-linker may be an unsaturated organic polymer, or more preferably, an optionally substituted unsaturated polyester (UPE) . The optionally substituted unsaturated polyester may have an average molecular weight (Mw) in the range of from about 500 Da to about 1000 Da, from about 600 Da to about 900 Da, or from about 700 Da to about 800. In one embodiment, the unsaturated polyester may contain a plurality of macromolecular repeat units, each macromolecular repeat unit comprising substituted or unsubstituted monomers of aromatic acid anhydrides, glycolides, glycerides, styrene and their substituted analogues thereof. In another embodiment, the molar ratio of the aromatic acid anhydrides to the glycerides may be in a range of about 1.0 to about 0.5. In another embodiment, the molar ratio of the overall hydroxyl groups from the diols to the anhydrides is in a range from about 3 to about 4.

The aromatic acid anhydrides may be cyclic anhydrides which comprise a cyclic carbon ring having five or more carbon atoms. In one embodiment, the cyclic anhydrides may be selected from the group consisting of maleic anhydrides, phthalic anhydrides and their substituted analogues thereof. Advantageously, it has been surprisingly found that an unsaturated polyester produced using the above anhydrides, glycolides and glycerides is highly compatible with the acid-doped PBI membrane matrix. Other cross-linking agents, for example epoxy resin-amine or phenol aldehyde, tend to result in a severe phase separation during polymerization with PBI.

The glycolides may be products formed from trans- esterification reactions between diols and fatty acids; whilst the glycerides may be formed from trans- esterification reactions between triols and fatty acids.

The diol may be selected from the group consisting of: alkyl glycols, alkylene glycols, aliphatic or branched alkyl diols and cycloalkyl diols. Exemplary diols include, but are not limited to, ethylene glycol, propylene glycol, 1,3-propane diol, 1,4-butane diol, 1,4- cyclohexanedimethanol, diethylene glycol, triethylene glycol and 2, 2-dimethyl-1, 3-propanediol. The triols may be selected from aliphatic or branched alkyl triols, alkylene triols, alkoxy triols, and cycloalkyl triols. Exemplary triols include, but are not limited to, 1, 2, 3-propanetriol; 1, 2, 4-butanetriol; 1,2,10- decanetriol, 2, 2-bis (hydroxymethyl) -1-octanol and glycerol.

Exemplary fatty acids include, but are not limited to, butyric acid, hexanoic acid, octanoic acid, decanoic acid, dodecanoic acid, eicosanoic acid, docosanoic acid, arachidonic acid, eicosapentaenoic acid, docosahexaenoic acid, erucic acid, palmitic acid, stearic acid, myristic acid, oleic acid, linoleic acid and mixtures thereof.

In one embodiment, the optionally substituted unsaturated polyester may be synthesized from a polymerization reaction involving glyceride, glycol, maleic anhydride, phthalic anhydride and propylene glycol. The unsaturated polyester may have hydroxyl end groups, capable of forming cross linkages with polyazole blocks. In one exemplary embodiment, the unsaturated polyester may be represented by the following formula (I) :

(D In one embodiment, the ratio of x:y:z may be in a range of about 1:1:2 to about '1:1:4, such that the total molecular weight of the unsaturated polyester is in a range of 500 Da to about 1000 Da. Additionally, Ri and R₂ may be independently aliphatic, branched or cyclic alkyl, alkylene, alkoxy, alkyl alcohol, or substituted analogues thereof. In another embodiment, Rl and R2 are independently, -CH₂CH₂- or -CH₂CH(OH)-. The molecular weight of the unsaturated polyester should not be too high, else it would result in immiscibility between the unsaturated polyester and the polyazole. Advantageously, the inventors have surprisingly discovered that an unsaturated polyester having average molecular weight within the above defined range is suitable for use as a cross-linker for an acid-doped polyazole system. The weight proportion of the unsaturated polyester may be from about 2 to about 4 percent by weight based on the total weight of polyazole. The doping concentration of the acid may be in a range of about 20 moles to about 30 moles of acid per mole of polyazole. The acid may be a mineral acid. In one embodiment, the mineral acid is phosphoric acid. The use of phosphoric acid is advantageous because the pure form of this mineral acid does not release oxidative or corrosive gases. Also advantageously, phosphoric acid has a relatively high boiling point, which renders it suitable for high temperature applications.

The acid may also be an organic acid. The organic acid may be selected from carboxylic acids or halogenated carboxylic acids. In one embodiment, the acid is trichloroacetic acid.

The method of the second aspect may further comprise, before step (a) , a step of polymerizing a monomeric solution in the presence of a polyacid to form the polymer-poly-acid solution of step (a) . In one embodiment, the monomeric solution may comprise monomers of polyazoles, such as those defined in the first aspect. In yet another embodiment, the polyazole is polybenzimidazole. The polyacid may be a mineral acid. In one embodiment, the polyacid is polyphosphoric acid. In yet another embodiment, the polyacid may be an organic acid, selected from the group consisting of carboxylic acid and halogenated carboxylic acids. In one embodiment, the organic acid is trichloroacetic acid.

The depositing step (b) in the method of the second aspect may be a step of casting the polymer-poly-acid- cross-linker gel matrix onto an inert substrate to thereby form a layer of the polymer membrane doped with said poly- acid. In one embodiment, the polymer-poly-acid-cross- linker gel matrix is casted on a glass panel using a film applicator to form a membrane layer having a thickness in a range of about 100 μm to about 200 μm. In another embodiment, the thickness of the deposited membrane layer is about 150 μm.

The cross-linker used in step (a) of the method may be an unsaturated organic polymer, more preferably, an optionally substituted unsaturated polyester. In one embodiment, the cross-linker is an optionally substituted unsaturated polyester as defined in the first aspect.

The method of the second aspect may further comprise, before the step (a) , a step of polymerizing substituted or unsubstituted monomers of aromatic anhydrides, glycolides, glycerides, styrene, or their analogues thereof to form the cross-linker used in step (a) . In one embodiment, this polymerization step can be carried out at a temperature of about 100 ⁰C to about 150 ⁰C, and for about 3 to about 4 hours.

During the cross-linking step (a) , the cross-linker may be provided in an amount not more than four percent by weight based on the total weight of the polyazole. The cross-linking step (a) may be undertaken at a temperature ranging from about 100 ⁰C to about 200 ⁰C, preferably from about 150 ⁰C to about 200 ⁰C. The cross-linking step (a) may be undertaken from about 10 to about 16 hours. In one embodiment, the cross-linking (a) is undertaken for at least 12 hours. In yet another embodiment, the cross- linking step (a) is undertaken for at least 10 hours.

The depositing step (b) may be followed by a cooling step, whereby the temperature of the polymer membrane is allowed to drop to about 60 °C to about 70⁰C. It should be noted that the hydrolysis step (b) may typically occur during this cooling process.

The method of the second aspect may further comprise a step of curing the polymer matrix obtained from the hydrolysis step (c) . The curing step may include a step of curing the polymer via the application of heat, i.e. thermal curing. In one embodiment, the polymer matrix is cured at a temperature in a range of about 180⁰C to about 220⁰C for about 2 hours. In one embodiment, the curing step may be undertaken at 200⁰C.

Advantageously, the step of curing the polymer matrix may further aid in increasing the extent of cross-linking within the polymer matrix. Also advantageously, increased cross-linkage in the cured polymer matrix confers greater resistance against mechanical deformation and thermal decomposition. Furthermore, a more extensive cross- linking network in the matrix also serves to increase proton mobility and thereby increase conductivity of the polymer matrix.

Brief Description of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention. Fig. 1 is a graph showing the thermogravimetric analysis of PBI-polymer powder and PA-doped PBI-UPE membrane made according to Example 1.

Fig. 2 is a graph showing the thermogravimetric analysis of PBI-polymer powder and PA-doped PBI membrane made according to Comparative Example 1.

Fig. 3 is a graph showing the differential scanning calorimetry analysis of PBI-polymer powder according to comparative example 1 and PBI-UPE polymer powder according to Example 1.

Fig. 4 is a graph showing the differential scanning calorimetry analysis PA-doped PBI-UPE membrane before and after curing.

Fig. 5 is a graph showing the tensile-strain charge of the cured PA-doped PBI-UPE membrane made according to Example 1 and of the PA-doped PBI membrane made according to Comparative Example 1.

Fig. 6 is a graph showing the relationship between conductivity and temperature of PA-doped PBI-UPE membrane made according to Example 1 and of the PA-doped PBI membrane made according to Comparative Example 1.

Fig. 7 is a graph showing the electrochemical performance of the cured PA-doped PBI-UPE membrane in a single H₂-PEMFC at various temperatures of 95°C, 115⁰C and 15O⁰C.

Examples

A non-limiting example of the invention and a comparative example will be further described in greater detail, which should not be construed as in any way limiting the scope of the invention. Example 1

Synthesis of the unsaturated polyester (UPE) :

A three-necked, round-bottomed flask equipped with a mechanical stirrer, thermometer and a nitrogen gas inlet was used to create a ^reactor' for the preparation of the polyester resins. The reactor was then charged with 33.76 g (0.04 mol) of palm oil, 7.36 g (0.08 mol) of glycerol and 0.05 wt . % (with respect to the oil) lead (II) oxide (PbO) , under continuous stirring. The mixture was then heated to about 245°C under flushing of a mild nitrogen (N₂) stream and left for about 30 to about 40 minutes to allow the formation of a monoglyceride, which was then dissolved in methanol, where the volume ratio of resin:methanol is about 1:3. Subsequently, the reaction mixture was cooled to about 130⁰C, and 0.14 mol of acid anhydride (consisting of 6.87 g of maleic anhydride (0.07 mol) and 10.37 g of phthalic anhydride (0.07 mol) in the form of fine powder), an excess of glycerol (25%, 1.84 g) , and 0.2 mol of propylene glycol were added to this solution. Then the reaction mixture was stirred at this temperature for about 3 to about 4 hours until it reached acid value of about 20 to about 30. After being cooled down to room temperature, the resulting polyester mixture was then mixed with styrene (40 % by weight).

Preparation of PA-doped PBI-UPE membrane

Isophthalic acid (99%), 3, 3 ' -diaminobenzidine tetrahydrochloride dehydrate (97%) , and polyphosphoric acid (PPA, 115%) were obtained from Sigma-Aldrich of St. Louis of Missouri of the United States of America.

The general procedure for the preparation of H3PO4- doped PBI-UPE membrane is shown in Scheme 1 below: Scheme 1 : In-si tu synthesis of UPE-PBI in PPA

PBI segment capped by di-carboxylic ends

PBI block

As shown in Scheme 1, 4.60 g of 3, 3 ' -diaminobenzidine tetrahydrochloride dehydrate (97%) (11. β mmol) was first added into a three-neck reaction flask, followed by 90 g of polyphosphoric acid (PPA) . The mixture was stirred at 140⁰C for 1 hour to remove hydrochloride from the amine. After that, 1.922 g of isophthalic acid (11.6 mmol) was introduced into the flask and the reaction mixture was stirred using a mechanical overhead stirrer under a purge of a slow stream of nitrogen. The temperature of the reaction mixture was increased to the range of from about 170°C to about 190°C, for 24 hours to allow polymerization to take place. Through this course, the reaction mixture became more viscous and developed a dark brown color, which was due to the formation of polybenzimidazole (PBI) oligomer molecules in PPA.

Then, UPE obtained from the previous section was introduced into the pre-PBI PPA solution at an amount that was equivalent to 2 wt . % of PBI and the polymerization reaction was continued for an additional 12 hours at about 150⁰C to about 200⁰C to allow generation of a loose crosslinking network that consisted of rigid PBI blocks and UPE chains. A small amount of the polymerization syrup was withdrawn from the viscous solution and added into water to form a solid specimen, which was then soaked in a dilute NaHCC>₃ solution overnight. Finally, the specimen was washed thoroughly with water and methanol respectively, and dried in the vacuum oven for thermal and spectroscopy analysis. This is called the PBI-UPE powder.

It should be noted that in the absence of UPE, the above in-situ gel polymerization method could still occasionally- result in the average molecular weight of PBI sufficient to obtain a cast membrane consisting of PBI in PPA. However, this membrane falls to pieces upon hydrolysis of the PPA and as a result, a single piece of PA-doped PBI membrane cannot be achieved when UPE is absent.

A membrane was developed by casting the hot PBI-UPE- PPA solution directly on a flat glass panel using a film applicator with a gate thickness of 150 μm. The cast membrane was allowed to cool down from reaction temperature to about 6O⁰C to about 7O⁰C, and the hydrolysis of PPA to PA (H₃PO₄) took place at the same time during the cooling process. After that, the membrane formed was transferred to a chamber where the relative humidity and temperature were controlled at 25+5% and 60⁰C, respectively, for 24 hours. In the final step, the membrane was cured at 200⁰C for 2 hours and subsequently enclosed in a dry container to isolate it from moisture because the membrane would otherwise quickly absorb moisture from air which would result in loss of mechanical properties and proton conductivity.

Comparative Example 1 For comparison purposes, PA-doped PBI membrane was fabricated by applying the conventional impregnation/embedding method. PBI powder obtained from the PBI-PPA polymerization system was dissolved in N,N'- dimethylacetamide (DMAc) from Sigma Aldrich, U.S.A., at 150⁰C under stirring, to prepare a 5 wt . %-solution. Subsequently, the PBI solution was cast on a glass Petri dish and the solvent was slowly evaporated at 120⁰C for a period of 20 hours. The resultant membrane was further soaked in concentrated PA solution (85 wt%) for a few days at room temperature (that is, about 25⁰C) . The resultant PA-doped PBI membrane was thereafter dried in a vacuum oven at 100⁰C to undergo dehydration before the doping level of PA in the membrane was determined.

Results of Example 1 and Comparative Example 1

The structure and properties of each of the membranes formed in Example 1 and Comparative Example 1 were characterized as follows:

Inherent viscosity (ηi)

Inherent viscosity (ηi) of a polymer solution depends on concentration and size of the dissolved polymer molecules. A membrane sample was dissolved in concentrated sulfuric acid (96%) to make a solution of 0.2 g/dL and its viscosity (η) was measured using a viscometer (DV-II+Pro from Brookfield of Middleboro of Massachusetts of the United States of America) . Let η₀ be the viscosity of the pure solvent, η the viscosity of the solution in this solvent and c be the mass concentration of the polymer.

a. Relative viscosity η_r ≡ J—L Equation 1

⁷Io

b. Inherent Viscosity

In;?,. η_t ≡ Equation 2 c

Polymerization of 3, 3' -diaminobenzidine tetrahydrochloride dehydrate and isophthalic acid in PPA is a unique system because PPA is both a polar solvent and the precursor of PA (H₃PO₄) acid dopant. It was found that the PBI synthesized in PPA could often only reach a low level of average molecular weight according to its ru value (typically about 0.8 dL/g) in 96% sulfuric acid at 20⁰C although occasionally, higher molecular weight (-1.0-1.1 dL/g) could be obtained. The low molecular weight PBI in PPA that was obtained via the method described in Comparative Example 1 could hardly be converted to a membrane with high integrity through the membrane casting procedure.

In order to surmount this hurdle, 2 wt.% of UPE (prepared according to the method of Example 1) , based on the weight of PBI was incorporated into the polymerization process with the aim of connecting individual PBI short segments together to form a loosely cross-linked structure. Experimentally, about 4 wt% UPE is the highest possible dose that could be charged to the PBI-PPA polymerization system. If the amount of UPE exceeds this value, subsequent casting cannot proceed due to the formation of a highly viscous gum (PBI-UPE in PPA) . The effect of inherent viscosity on membrane development is shown in Table 1. The inherent viscosity of the PBI-UPE polymer obtained from Example 1 is 1.378 dL/g, which was greater than that of PBI alone obtained from Comparative Example 1 (0.802 dL/g) . Hence, a membrane with higher structural integrity was obtained after hydrolysis of PPA and curing of the PBI-UPE polymer.

Table 1

Doping level

The phosphoric acid (PA) -doping level of membrane was determined by the titration method. A pre-weighed piece of membrane sample was immersed in 0.1 M of sodium hydroxide solution for a few hours. The sample was then washed with water and dried overnight in a vacuum oven at 100⁰C to obtain the dry weight of polymer. The acid-doping level, X, expressed as moles of PA per mole of PBI repeat unit was calculated according to Equation 3 below:

X =(V_Na0H *C_Na0H) / (W_dry/M_w Equation 3

where V_Na0H and C_Na0H are the volume and the molarity of the sodium hydroxide titer, while W_dEy is the dry polymer weight and M_w is the formula weight of the repeating unit, respectively .

Table 2 is a comparison of PA doping levels in the PBI matrix. The doping level of PBI-UPE membrane was found to be unaffected by thermal curing after the hydrolysis of PPA. The PBI-UPE network and PA molecules constitute a uniform semi-gel structure, which is characterized by a large doping extent and is maintained by two levels of interactions. These two levels of interactions are: (1) the electrostatic interactions between the protonated imidazole ring and dihydrogen phosphate anions as well as hydrogen-bonding between the PA molecules and oxygen- containing segments of UPE; and (2) the interactions between the PA molecules in (1) and the rest of the PA molecules entrained in the membrane.

Table 2

Thermal and mechanical properties of the membrane

The thermal stability of the samples was measured using a High Resolution Thermogravimetric Analyzer (TA Instruments 2950 from TA Instruments of New Castle of Delaware of the United States of America) using 5 to 15 mg of sample. The weight-loss was recorded in the range from 25 ⁰C to 800 ⁰C using a constant heating rate of 10°C/min and N2 purge of 100 ml/min. The polymer segment motion behaviors of membrane were measured using a differential scanning calorimeter (DSC, DSC 822e from Mettler Toledo of Columbus of Ohio of the United States of America) equipped with a pressure DSC cell. The temperature scanning range from 25⁰C to 160°C was set for the first scan to erase the thermal history of a sample caused by preparation conditions. After the sample was cooled down to -50⁰C, a second scan was carried out and the temperature was set up to 160⁰C as well in order to obtain and record an energy- temperature profile from this scan. Both heating and cooling rates were fixed at 10°C/min in the above two scans. The mechanical properties of the membranes were tested on an Instron 5569 instrument (from Instron of Norwood of Massachusetts of the United States of America) using a 10 N load cell. Fig. 1 shows the comparison between different thermal properties of the PA-doped PBI-UPE membrane and the PBI- UPE powder. The dotted lines refer to the graphs of derivative weight loss (%) as a function of temperature (⁰C) while the continuous lines refer to the graphs of weight loss (%) as a function of temperature (⁰C) . Over the temperature range of investigation, the PA-doped PBI-UPE membrane as shown by the continuous line labeled as ^λλ (b) " exhibited four weight loss slope. The first slope (with peak value at 80⁰C) reports a mass loss of 35 wt.%, which is made up of mainly water due to the hygroscopic nature of PA and PA molecules residing in the bulk of the trapped PA phase. The removal of the strongly bound PA took place in the range from 160 °C to above 300 °C; which accounted for about 7 wt.% of mass loss. It should be noted that the removal of strongly bound PA occurred above the normal boiling point of absolute PA (which is 158°C) , indicative of the intermolecular, attractive interactions between PA and the polymer matrix. The last three mass loss slopes represent the decomposition of the matrix. On the contrary, for the PBI-UPE powder as shown by the continuous line labeled as "{a)", there were two weight loss-slopes below 800⁰C. A small dehydration peak appeared at near 100⁰C and the decomposition of PBI matrix started at 490⁰C. It is to be noted that the PA-doped PBI-UPE matrix displayed a far stiffer matrix disassociation slope than that of the PBI-UPE powder. This difference reflects the effect of thermal curing, which is a crucial step in completing the formation of a highly cross-linked membrane network. The higher the extent of cross-linkage in the membrane matrix, the stronger the tendency for it to shatter coincidently at its decomposition point.

Fig. 2 shows the comparison between the PA-doped PBI membrane developed from Comparative Example 1 and the dry PBI powder. The dotted lines refer to the graphs of derivative weight loss (%) as a function of temperature

(⁰C) while the continuous lines refer to the graphs of weight loss (%) as a function of temperature (⁰C) . It can be seen that, after the initial weight loss (due to elimination of the liquid component) , the PA-doped PBI membrane displayed a similar TGA profile to that of PBI powder. Compared with a PA-doped PBI-UPE membrane, the PA-doped PBI membrane displayed slightly better thermal stability. However, this observation is consistent with the inference that increasing the extent of cross-linkage results in a faster decomposition rate.

Fig. 3 shows the DSC analysis of PA-doped PBI and PA- doped PBI-UPE powder. In Fig. 3, the PBI-UPE powder displayed slightly higher glass transition temperature (T₉) than the PBI powder. This indicates that the cross-linking extent in PBI-UPE was relatively mild prior to thermal curing. In particular, a lesser extent of cross-linking took place during the polymerization step. Fig. 4 shows the DSC analysis of PA-doped PBI-UPE before and after curing. A two-scan scheme was set to carry out the DSC analysis. For each sample (with identical mass) , the first scan (from room temperature to 16O⁰C) removed moisture and loosely bound PA molecules from the sample and a second scan (from room temperature to 550⁰C) was recorded for the analysis. Both DSC diagrams revealed a broad endothermic transition peak that spans from 15O⁰C to 36O⁰C. Analyzed in conjunction with the TGA profile of the cured PA-doped PBI-UPE membrane presented in Fig. 1, the endothermic response of DSC describes the evaporation of PA molecules from the PBI-UPE network.

The PA-doped PBI-UPE membrane before curing exhibited a stronger endothermic downturn (or specific heat) than its cured counterpart below 200⁰C, but the later one showed that a greater portion (about 6/7) of specific endothermic heat occurs in the upper temperature range (200⁰C to 36O⁰C) than that of the former. Besides this difference, the cured sample displayed a more symmetric endothermic contour than the un-cured sample. The above results suggest that the cured PBI-UPE network is superior in its ability to entrain PA molecules, in terms of binding affinity and uniformity of network distribution. The membrane obtained from Example 1, that is, PA- doped PBI-UPE membrane, exhibited yield strength of approximately 1.7 MPa at a yield strain of 9.11% and an ultimate tensile strength of about 10.05 MPa at an elongation of 146.7%. As shown in Fig. 5, this membrane, because of its viscoelastic network structure, can withstand a much higher tensile stress and strain than the PA-doped PBI membrane made by the conventional impregnating method (Comparative Example 1) . The mechanical properties such as tensile stress and Young' s Modulus data of these two membranes are given in Table 3.

Table 3

Without wishing to be bound by theory, it is postulated that the PA-doped PBI membrane showed significantly poorer mechanical properties primarily because the PBI molecular segments are not interconnected by chemical bonding as mentioned above. Accordingly, the relatively weaker hydrogen-bonding and Van der Waals forces existing between PBI segments and between PBI and PA were not able to provide the desired mechanical strength.

Proton Conductivity

Another useful property of PBI-based membranes lies in its humidity-independent proton conductivity in the temperature range from 12O⁰C to 150⁰C. To assess the proton conductivity of the two membranes obtained from Example 1 and Comparative Example 1, the membrane samples must be kept in an oven to maintain an anhydrous matrix before measurement. Proton conductivity of the membrane was measured using the normal four-point probe technique. The sample holder made of Teflon consists of two flat stainless steel ribbons as the outer current-carrying electrodes (placed 2 cm apart) and two gold (Au) wires as the inner potential- sensing electrodes (placed 1 cm apart) . A membrane 1 cm wide and 2 cm long was mounted on the holder. The impedance of the sample was determined using an electrochemical analyzer (Autolab Instrument of the Netherlands) at galvanostatic mode with an alternating current having amplitude of 0.1 mA and a frequency scanning range from 1 MHz to 50 Hz. On the Bode plot, there is a frequency range over which the impedance had a constant value, and the resistance corresponding to this frequency range could then be obtained from the Nyquist plot of this sample. The proton conductivity (C) is calculated according to the following expression:

F

C=-

ZWd Equation 4

where, R is the resistance of membrane specimen, L is the distance between potential-sensing electrodes, and (Wxd) is the cross section area of the specimen. The measurement setup was placed in a programmable furnace to control the temperature. The conductivity of the specimen was measured from 60 ⁰C to 180 ⁰C. Before the measurements were taken at each set temperature, the sample was held at a constant temperature for 30 minutes.

Fig. 6 illustrates the temperature-dependent proton conductivity trends, which were measured using the 4-probe method in a nil humidity condition as mentioned above. The PA-doped PBI membrane from Comparative Example 1 offers the maximum conductivity of 0.025S/cm at 110⁰C. In contrast, the PA-doped PBI-UPE membrane from Example 1 showed a conductivity of 0.072 S/cm at 60⁰C, which reached as high as 0.125 S/cm at 160⁰C, followed by a decrease in conductivity with increasing temperature.

The key factor responsible for the superior proton conductivity of the PA-doped PBI-UPE membrane (Example 1) over the PA-doped PBI membrane (Comparative Example 1) lies in its much higher PA doping level which is supported by the viscoelastic, cross-linked polymer membrane. Such high doping levels could not have been possible if the membrane lacked the requisite mechanical strength conferred by the cross-linkages.

Assessment of membrane in a single H₂-PrOtOn Exchange Membrane Fuel Cell (H₂-PEMFC) On the basis of conductivity measurement, only the PA-doped PBI-UPE membrane from Example 1 is capable of being assessed to determine its fuel cell performance. For the polarization measurements, a single cell was operated at 95°C, 115°C and 150⁰C and the pressures of both H₂ and O₂ streams was set at 1 bar without humidification. The measurement was carried out using an Arbin Electronic Fuel Cell Testing System loaded with MITS system (from Arbin Instruments of College Station of Texas of the United States of America) . The anode and cathode sheets used were carbon paper (SGL Group of Germany) coated with a layer of carbon-supported Pt catalyst (20 wt.%), which was supplied by E-TEK of Natick of Massachusetts of the United States of America. The Pt catalyst loading at the anode and cathode was 2-3 mg/cm², respectively. The effective electrode area was 5 cm². The gas flow rate was kept at a fixed stoichiometry whereby the molar ratio of H₂ to O₂ was 1.15:2, at a current density of 1 A/cm².

Fig. 7 displays the electrochemical performance of the cured PA-doped PBI-UPE membrane in a single H₂-PEMFC at the various temperatures of 95°C, 115°C and 15O⁰C, without humidification of either electrode. The membrane was evaluated at these three temperature points and the power density was increased with increasing of temperature. At 150⁰C and the current density of 0.9 A/cm², the highest power output (0.3W/cm²) of the cell was achieved. This outcome confirms that the PA molecules constitute a continuous phase in the PBI-UPE network, which permits transport of protons across the anhydrous membrane. For this conducting mechanism, an operation temperature in the range of 150⁰C to 200⁰C is imperative in order to achieve a high power output.

As can be concluded from the examples and figures, the inclusion of UPE into the PBI polymerization system allows for individual PBI segments to be interconnected and cross-linked via UPE macromer chains. The PBI-UPE network thus formed is capable of entraining PA molecules after hydrolysis of PPA and is greatly reinforced in the subsequent thermal curing step that promotes the extent of cross-linking. Sustained by this particular network, the resulting membrane can score a much higher PA doping level than conventional PA-doped PBI membranes, such as those made by the impregnating method (Comparative Example 1) .

Advantageously, such a high PA doping level brings about a proton conductivity of 10^"1 S/cm at 160 °C at zero matrix humidity. Furthermore, a promising performance of the membrane in a single H₂ fuel cell was accomplished at 150 ⁰C without humidifying either electrode. Besides possessing the desired high-temperature proton conductivity, the PA-doped PBI-UPE membrane also exhibited relatively better mechanical properties and thermal stability. It further appears that no arbitrary macromer can be used in place of the described UPE because the UPE is highly compatible with the PBI-PPA polymerization due to its thermodynamic affinity with both PBI and PPA, as well as its ability to undergo a condensation reaction with PBI to form a viscoelastic network.

Applications

The disclosed PA-doped PBI-UPE membrane is capable of being used in a hydrogen fuel cell at a high temperature of about 150⁰C without humidifying either electrode. The disclosed PA-doped PBI-UPE membrane has a proton conductivity of 10^"1 S/cm at 160⁰C at zero matrix humidity. Besides possessing the desired high-temperature proton conductivity, the PA-doped PBI-UPE membrane also exhibited sound mechanical properties and thermal stability.

The disclosed method of producing a PA-doped PBI-UPE membrane results in a much higher PA-doping level in the PBI-UPE membrane as compared to PA-doped PBI membranes made by conventional impregnation methods because in the disclosed method of forming the PA-doped PBI membranes of the present invention, the PA is doped throughout the bulk of the membrane structure. In contrast to the disclosed method of forming the PA-doped PBI membranes of the present invention, known PA-doped PBI membranes are doped only at their surface and hence are "surface-only-doped" PBI membranes. Unlike surface-only-doped PBI membranes, PBI membranes of the present invention have PA molecules that are effectively confined in the bulk of the membrane structure because the crosslinking network formed penetrates uniformly within PBI and throughout the membrane. The membrane of the present invention is stronger than surface-only-doped PBI membranes.

This increased level in PA-doping of the present invention may be due to the presence of the UPE, serving as a cross-linker in the PBI-UPE network. The cross- linked PBI-UPE network can satisfactorily entrain the PA molecules after the hydrolysis of PPA. The PBI-UPE network is further reinforced in the subsequent thermal curing step that promotes the extent of crosslinking. Accordingly, the resulting PBI-UPE membrane is mechanically stronger than conventional PA-doped PBI membranes which rely on the physical cohesive associations of PBI oligomer molecules.

Further, the formation of the PBI-UPE membrane makes it capable of holding almost 100% of the PA hydrolyzed from PPA, such that the entrained PA forms a continuous phase, thereby facilitating the transport of protons. This is in contrast to conventional PA-doped PBI membranes, whereby weak, physical cohesive interactions between the PBI oligomer molecules are not able to form a matrix to entrain the hydroyzed PA molecules. Consequently, conventional PBI membranes cannot sustain a high doping level, resulting in impaired conductivity.

In surface-only-doped PBI membranes, hydrolysis and crosslinking of the membrane using a cross-linker are conducted at the same time after the PBI membrane had been formed. Hence, the cross-linking between the PBI, acid and cross-linker only occurs on the surface of the membrane .

In contrast, in the disclosed method of the present invention, a selected cross-linker, such as UPE crosslinker, is provided in a liquid mixture of a polymer, such as PBI, and an acid, such as polyphosphoric acid (PPA) , to form a homogeneous syrup-like liquid. Cross- linking is then initiated in the cast PBI-PPA-UPE cast membrane before hydrolysis via a curing process, which leads to an interpenetration matrix between PBI and UPE crosslinker in the entire membrane, and then hydrolysis is conducted to form the phosphoric acid. Hence, the cross- linker substantially entrains said acid within the bulk of said polymer matrix embedded throughout the membrane structure.

Accordingly, the disclosed PA-doped PBI-UPE membrane may be used as a polymer electrolyte membrane in a fuel cell, such as one which utilizes hydrogen alone or in one that utilizes carbon-based fuels, such as natural gas, gasoline, methanol or biomass. In particular, the disclosed membrane may be used in a hydrogen fuel cell or in a direct methanol fuel cell. Advantageously, the PA- doped PBI-UPE membrane may be used in a fuel cell at elevated temperatures of above 15O⁰C to 200⁰C without significant loss of conductivity and mechanical properties.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person , skilled in the art after reading the foregoing disclosure without departing from ^"the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. A polymer electrolyte membrane (PEM) comprising a cross-linked polymer matrix doped with an acid that is dispersed substantially throughout the bulk of said polymer matrix.

2. The polymer electrolyte membrane of claim 1, wherein a polymeric cross-linker cross-links said polymer matrix.

3. The polymer electrolyte membrane of claim 2, wherein said polymeric cross-linker is an unsaturated organic polymer.

4. The polymer electrolyte membrane of claim 3, wherein said unsaturated organic polymer is an optionally substituted unsaturated polyester.

5. The polymer electrolyte membrane of claim 4, wherein said optionally substituted unsaturated polyester has an average molecular weight in the range of 500 Da to 1000 Da.

6. The polymer electrolyte membrane of claim 4 or claim 5, wherein said unsaturated polyester contains plural macromolecular repeat units, each of said macromolecular repeat units comprising substituted or unsubstituted monomers of aromatic acid anhydrides, glycolides, glycerides, styrene and their substituted analogues thereof.

IA

7. The polymer electrolyte membrane of claim 6, wherein the molar ratio of said aromatic acid anhydrides to said glycerides is in a range of 1 to 0.5.

8. The polymer electrolyte membrane of claim β or claim 7, wherein said aromatic acid anhydrides are cyclic anhydrides comprising a cyclic carbon ring having five or more carbon atoms .

9. The polymer electrolyte membrane of claim 8, wherein said cyclic anhydride is selected from the group consisting of: phthalic anhydrides, maleic anhydrides, and their substituted analogues thereof.

10. The polymer electrolyte membrane of claim 6, wherein said glycolides and glycerides are products of a trans-esterification reaction between diols and triols with fatty acids respectively.

11. The polymer electrolyte membrane of claim 10, wherein said diol is selected from the group consisting of: aliphatic or branched alkyl diols, alkylene glycols, cycloalkyl diols.

12. The polymer electrolyte membrane of claim 10, wherein said triol is selected from the group consisting: aliphatic and branched alkyl triols, alkylene triols, alkoxy triols, and cycloalkyl triols.

^•3S

13. The polymer electrolyte membrane of claim 10, wherein said fatty acid is selected from the group consisting of: butyric acid, hexanoic acid, octanoic acid, decanoic acid, dodecanoic acid, eicosanoic acid, docosanoic acid, arachidonic acid, eicosapentaenoic acid, docosahexaenoic acid, erucic acid, palmitic acid, stearic acid, myristic acid, oleic acid, linoleic acid and mixtures thereof

14. The polymer electrolyte membrane of any of the preceding claims, wherein .said polymer matrix comprises plural polyazole blocks, each of said polyazole block being coupled with another polyazole block by said cross-linker.

15. The polymer electrolyte membrane of claim 14, wherein said polyazole is selected from the group comprising of polyimidazoles, polybenzimidazole, polybenzothiazoles , polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (pyridines) , poly (pyrimidines) , poly (tetrazapyrenes) , and co-polymers thereof.

16. The polymer electrolyte membrane of claim 14, wherein said polyazole is polybenzimidazole

(PBI) .

17. The polymer electrolyte membrane of claim 14, wherein said cross-linker is in a weight proportion of up to about 4 weight percent with respect to said polyazole.

3G

18. The polymer electrolyte membrane of claim 14, wherein the doping concentration of said acid is about 20 moles to about 30 moles of acid per mole of said polyazole.

19. The polymer membrane electrolyte of any one of the preceding claims, wherein said acid is phosphoric acid.

20. The polymer membrane electrolyte of any one of the claims 1 to 18, wherein said acid is an organic acid selected from a carboxylic acid, a halogenated carboxylic acids, and mixtures thereof.

21. A method of producing a polymer electrolyte membrane (PEM) , said method comprising the steps of: (a) cross-linking a polymer-poly-acid solution to form a polymer-poly-acid-cross- linked gel matrix;

(b) forming a polymer membrane from said polymer-poly-acid-cross-linker gel matrix, wherein said polymer membrane is doped with said poly-acid; and

(c) hydrolyzing said polyacid to form an acid from said poly-acid and thereby form a cross-linked polymer matrix doped with an acid that is dispersed substantially throughout the bulk of said polymer matrix.

11

22. The method according to claim 21, further comprising, before step (a) , a step of polymerizing a monomeric solution in the presence of the poly-acid to form said polymer- poly-acid solution.

23. The method according to any one of claims 21 or 22, wherein said depositing step (b) is a step of casting said polymer-poly-acid-cross-linker gel matrix to form the polymer membrane doped with said poly-acid.

24. The method according to any one of claims 21 to 23, comprising the step of providing said poly- acid as polyphosphoric acid.

25. The method according to any one of claims 21 to 23, comprising the step of providing said poly- acid as an organic acid.

26. The method according to any one of claims 21 to 25, further comprising, during said cross- linking step (a) , the step of providing the cross-linker as an optionally substituted unsaturated polyester.

27. The method according to any one of claims 21 to 26, comprising the step of providing said polymer as a polyazole.

28. The method according to any of claims 21 to 27, further comprising, before reacting step (a) , a step of polymerizing substituted or

.

unsubstituted monomers of aromatic acid anhydrides, glycolides, glycerides and styrene to form said cross-linker.

29. The method according to claim 27, comprising during step (a) , the step of providing the cross-linker in not more than 4 weight percent of the total weight of said polyazole.

30. The method according to any one of claims 21 to 29, comprising the step of carrying out step (a) at a temperature from 100°C to 200°C.

31. The method according to claim 30, comprising the step of carrying out step (a) at a temperature from 150⁰C to 200⁰C.

32. The method according to any one of claims 21 to 31, comprising the step of carrying out step (a) from 10 hours to 16 hours.

33. The method according to any one of claims 21 to 33, wherein said deposited polymer membrane doped with said poly-acid layer has a thickness of 100 μm to 200 μm.

34. The method according to claim 33, wherein said deposited polymer membrane doped with said poly-acid layer has a thickness of 150 μm.

35. The method according to any of one of claims 21 to 34, wherein during said hydrolysis step (c) , said membrane is cooled to a temperature in a range of 60°C to 7O⁰C.

36. The method according to any one of claims 21 to 36, further comprising a step of curing said polymer matrix.

37. The method according to claim 36, wherein said curing step is thermal curing.

38. The method according to claim 38, wherein said thermal curing is carried out at a temperature of between 18O⁰C to 220°C.

39. A fuel cell comprising a _.polymer electrolyte membrane (PEM) comprising a cross-linked polymer matrix doped with an acid that is dispersed substantially throughout the bulk of said polymer matrix.