Solid polymer membrane for fuel cell prepared by in situ polymerization

FIELD OF THE INVENTION

[0001] The present invention relates to a solid polymer electrolyte membrane, more particularly to a direct methanol fuel cell containing the solid polymer electrolyte membrane.

BACKGROUND OF THE INVENTION

[0002] Direct methanol fuel cells (DMFCs), fuel cells in which the anode is fed directly with liquid or vaporous methanol, have been under development for a considerable period of time, and are well-known in the art. See for example Baldauf et al, J. Power Sources, vol. 84, Pages 161-166. One essential component in a direct methanol, or any, fuel cell is the separator membrane.

[0003] It has long been known in the art to form ionically conducting polymer electrolyte membranes and gels from organic polymers containing ionic pendant groups. Well-known so-called ionomer membranes in widespread commercial use are Nafion® perfluoroionomer membranes available from E. I. du Pont de Nemours and Company, Wilmington Del. Nafion® is formed by copolymerizing tetrafluoroethylene (TFE) with perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride), as disclosed in U.S. Pat. No. 3,282,875. Other well-known perfluoroionomer membranes are copolymers of TFE with perfluoro (3-oxa-4-pentene sulfonyl fluoride), as disclosed in U.S. Pat. No. 4,358,545. The copolymers so formed are converted to the ionomeric form by hydrolysis, typically by exposure to an appropriate aqueous base, as disclosed in U.S. Pat. No. 3,282,875. Lithium, sodium and potassium are all well known in the art as suitable cations for the above cited ionomers.

[0004] Other fluorinated ionomer membranes are known in the art such as those described in WO 9952954, WO 0024709, WO 0077057, and U.S. Pat. No. 6,025,092.

[0005] DMFCs employing ionomeric polymer electrolyte membranes as separators are known to exhibit high methanol cross-over—the transport of as much as 40% of the methanol from the anode to the cathode by diffusion through the membrane. This methanol cross-over essentially represents a fuel leak, greatly decreasing the efficiency of the fuel cell. In addition, the presence of methanol at the cathode interferes with the cathode reaction, with the methanol itself undergoing oxidation, and, in sufficient volume, floods the cathode and shuts down the fuel cell altogether. Methanol cross-over occurs primarily as a result of the high solubility of methanol in the ionomeric membranes of the art.

[0006] It is of considerable interest in the art to identify ways to reduce methanol cross-over in ionomeric membranes while entailing as small as possible cost in conductivity.

[0007] Kyota et al, J P Sho 53(1978)-60388, describes a process for producing modified Nafion® membranes with reduced permeability to hydroxide ion by swelling with a solvent or liquid, diffusing a polymerizable vinyl monomer into the swollen matrix with an initiator, and polymerizing in situ. Also disclosed by reference is a process for diffusing the monomers without solvent-swelling, but the solvent-swelling process is said to be superior. Disclosed monomers include vinyl acetate, acrylics, vinylisocyanate, di-vinyls such as divinyl benzene, styrene, and fluorinated vinyl monomers though not expressly TFE itself. Methanol permeability is not discussed.

[0008] Seita et al, U.S. Pat. No. 4,200,538, disclose a cation exchange membrane prepared by swelling a fluorinated ionomer with an organic solvent, removing the solvent, immersing in a vinyl monomer, adding initiators and other additives, and polymerizing the monomer in situ. Improvements in hydroxyl ion permeability are noted. Suitable monomers include styrene and styrene derivatives; acrylic, methacrylic, and maleic acids and salts and esters thereof; vinyl acetate, vinyl isocyanate, acrylonitrile, acrolein, vinyl chloride, vinylidene chloride, vinylidene fluoride, vinyl fluoride; and numerous others. Methanol permeability is not discussed.

[0009] Fleischer et al, U.S. Pat. No. 5,643,689, disclose composite membranes which include combination of ionomeric polymers and numerous non-ionic polymers including polythyleneimine and polyvinylpyrrolidone. Metal oxides are always present in the composite. The composites are prepared by dissolving the respective polymers in a common solvent and then removing the solvent, and are said to be useful in hydrogen fuel cells.

[0010] Li et al, WO 98/42037, discloses polymer electrolyte blends in batteries. Disclosed are blends of polybenzimidazoles with Nafion® and other polymers in concentration ratios of ca. 1:1. Preferred are blends of polybenzimidazoles and polyacrylamides. Polyvinylpyrrolidone and polyethyleneimine are also disclosed.

SUMMARY OF THE INVENTION

[0011] In a first aspect, the invention provides a solid polymer electrolyte membrane comprising a fluorinated ionomer having imbibed therein the polymerization product of a composition comprising a non-fluorinated, non-ionomeric monomer, wherein the fluorinated ionomer comprises at least 6 mole % of monomer units having a fluorinated pendant group with a terminal ionic group. The invention also provides a composition further comprising a free radical initiator, and optionally a crosslinking agent.

[0012] In the first aspect, the monomer is a polar vinyl monomer that may be selected from the group consisting of vinyl acetate; vinyl isocyanate; acrylonitrile; acrylic acid; acrylate esters; and mixtures thereof.

[0013] In a second aspect, the invention provides a catalyst coated membrane comprising a solid polymer electrolyte membrane having a first surface and a second surface, an anode present on the first surface of the solid polymer electrolyte membrane, and a cathode present on the second surface of the solid polymer electrolyte membrane, wherein the solid polymer electrolyte membrane comprises a fluorinated ionomer having imbibed therein the polymerization product of a composition comprising a non-fluorinated, non-ionomeric monomer, wherein the fluorinated ionomer comprises at least 6 mole % of monomer units having a fluorinated pendant group with a terminal ionic group. The invention also provides a composition further comprising a free radical initiator, and optionally a crosslinking agent.

[0014] In the second aspect, the monomer is a polar vinyl monomer that may be selected from the group consisting of vinyl acetate; vinyl isocyanate; acrylonitrile; acrylic acid; acrylate esters; and mixtures thereof. The anode and cathode comprise a catalyst, which may be supported or unsupported.

[0015] In a third aspect, the invention provides a fuel cell comprising a solid polymer electrolyte membrane having a first surface and a second surface, wherein the solid polymer electrolyte membrane comprises a fluorinated ionomer having imbibed therein the polymerization product of a composition comprising a non-fluorinated, non-ionomeric monomer, wherein the fluorinated ionomer comprises at least 6 mole % of monomer units having a fluorinated pendant group with a terminal ionic group. The invention also provides a composition further comprising a free radical initiator, and optionally a crosslinking agent.

[0016] In the third aspect, the monomer is a polar vinyl monomer that may be selected from the group consisting of vinyl acetate; vinyl isocyanate; acrylonitrile; acrylic acid; acrylate esters; and mixtures thereof.

[0017] In the third aspect, the fuel cell further comprises an anode and a cathode present on the first and second surfaces of the polymer electrolyte membrane.

[0018] In the third aspect, the fuel cell further comprises a means for delivering liquid or gaseous fuel to the anode, a means for delivering oxygen to the cathode, a means for connecting the anode and cathode to an external electrical load, methanol in the liquid or gaseous state in contact with the anode, and oxygen in contact with the cathode.

[0019] In the third aspect, the fuel is in the liquid or vapor phase and comprises methanol or hydrogen.

BRIEF DESCRIPTION OF DRAWINGS

[0020]
FIG. 1 depicts a typical direct methanol fuel cell.

DETAILED DESCRIPTION

[0021] Following the practice of the art, in the present invention, the term “ionomer” is used to refer to a polymeric material having a pendant group with a terminal ionic group. The terminal ionic group may be an acid or a salt thereof as might be encountered in an intermediate stage of fabrication or production of a fuel cell. Proper operation of the fuel cell of the invention requires that the ionomer be in acid form. The term “polymeric precursor” to an ionomer suitable for use in the present invention refers to the non-ionic form of a polymer which when subject to hydrolysis according to well-known methods in the art is converted into the ionomer suitable for use in the present invention, or a salt thereof.

[0022] The term “membrane precursor” refers to a membrane formed from the ionomer suitable for the practice of the invention, prior to the formation of a blend with another polymer which is not an ionomer in order to produce the composite ionomeric polymer electrolyte membrane of the invention. It is not necessary for the practice of the invention that a precursor membrane first be formed followed by incorporation of a polymer that is not an ionomer to form the composite membrane of the invention. For example, it is possible in some cases to melt blend the ionomeric precursor and the polymer which is not an ionomer followed by melt casting a film and hydrolysis. In other cases, it is possible to dissolve the ionomer or its precursor and the other polymer that is not an ionomer in a common solvent, and then solution cast a film. However, it is found in the practice of the invention that it is convenient to first fabricate a membrane precursor from the ionomer or its precursor followed by incorporation of an other polymer that is not an ionomer.

[0023] It is found that a solid polymer electrolyte membrane comprising a fluorinated ionomer having imbibed therein the polymerization product of a composition comprising a non-fluorinated, non-ionomeric monomer, provides a membrane having reduced methanol permeability, at relatively modest cost in conductivity, and provides an improved fuel cell, eg. a DMFC or hydrogen fuel cell.

[0024] One of ordinary skill in the art will understand that the membrane having a film or sheet structure will have utility in packaging, in non-electrochemical membrane applications. Membranes also have application as an adhesive or other functional layer in a multilayer film or sheet structure, and other classic applications for polymer films and sheets that are outside electrochemistry. For the purposes of the present invention, the term “membrane,” a term of art in common use in the fuel cell art is synonymous with the term “film” or “sheet ” which are terms of art in more general usage but refer to the same articles. Ionomers suitable for use in the present invention comprise at least 6 mol % of monomer units having a fluorinated pendant group with a terminal ionic group, preferably a sulfonic acid or sulfonate salt. A “polymeric precursor” to an ionomer suitable for use in the present invention preferably comprises a sulfonyl fluoride end-group, which when subject to hydrolysis under alkaline conditions, according to well-known methods in the art, is converted into a sulfonic acid or sulfonate salt.

[0025] Any fuel cell, and in particular a direct methanol fuel cell or a hydrogen fuel cell, known in the art, of the type provided with a solid polymer electrolyte membrane may be employed in the present invention. It is by the substitution of a membrane comprising a fluorinated ionomer having imbibed therein the polymerization product of a composition comprising a non-fluorinated, non-ionomeric monomer, wherein the fluorinated ionomer comprises at least 6 mole % of monomer units having a fluorinated pendant group with a terminal ionic group, according to the teachings of the present invention, for the ionomeric membrane of the art that the benefits of the present invention may be realized.

[0026] Ionomeric polymer electrolyte membranes have been prepared that are particularly well-suited for use in direct methanol fuel cells because of the surprisingly large decrease in methanol permeability achieved at relatively small sacrifice of conductivity.

Ionomeric Membrane Polymers

[0027] Some ionomeric polymer electrolyte membranes suitable for use in the present invention, and methods for preparing them, are variously described in Kyota et al, op. cit., and Fleischer et al, op.cit.

[0028] A membrane in accordance with the invention is a mixture of an ionomeric polymer or ionomer, and a non-ionomeric polymer combined therewith. The ionomer suitable for the practice of the invention has cation exchange groups that can transport protons across the membrane. The cation exchange groups are acids preferably selected from the group consisting of sulfonic, carboxylic, phosphonic, imide, methide, sulfonimide and sulfonamide groups. Various known cation exchange ionomers can be used including ionomeric derivatives of trifluoroethylene, tetrafluoroethylene, styrene-divinylbenzene, alpha, beta, beta-trifluorostyrene, etc., in which cation exchange groups have been introduced alpha, beta, beta-trifluorstyrene polymers useful for the practice of the invention are disclosed in U.S. Pat. No 5,422,411.

[0029] In one embodiment of the invention, the ionomer comprises a polymer backbone and recurring side chains attached to the backbone with the side chains carrying the cation exchange groups. For example, ionomers are formed by copolymerization of a first fluorinated vinyl monomer and a second fluorinated vinyl monomer having a side cation exchange group or a fluorinated cation exchange group precursor (e.g., SO2F) which can be subsequently hydrolyzed to sulfonic acid groups. Possible first monomers include but are not limited to tetrafluoroethylene, hexafluoropropylene, vinyl fluoride, vinylidine fluoride, trifluorethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), and mixtures thereof. Possible second monomers include but are not limited to a variety of fluorinated vinyl ethers with fluorinated cation exchange groups or precursor groups.

[0030] In a further embodiment, the ionomer in accordance with the invention has a backbone which is substantially fluorinated and the ion exchange groups are sulfonic acid groups or alkali metal or ammonium salts thereof which are readily converted to sulfonic acid groups by ion exchange. “Substantially fluorinated” means that at least 60% of the total number of halogen and hydrogen atoms are fluorine atoms. In a further embodiment, the ionomer backbone and the side chains are highly fluorinated, particularly perfluorinated. The term “highly fluorinated” means that at least 90% of the total number of halogen and hydrogen atoms are fluorine atoms.

[0031] Some ionomers suitable for use in the present invention are variously described in U.S. Pat. No. 4,358,545, U.S. Pat. No. 4,940,525, WO 9945048, U.S. Pat. No. 6,025,092. Suitable ionomers as disclosed therein comprise a highly fluorinated carbon backbone having at least 6 mol % of a perfluoroalkenyl monomer unit having a pendant group comprising the radical represented by the formula

—(OCF2CFR)aOCF2(CFR′)bSO2X−(H+)[YZc]d (I)

[0032] wherein

[0033] R and R′ are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms, optionally substituted by one or more ether oxygens;

[0034] a=0,1 or 2;

[0035] b=0 to 6;

[0036] X is O, C or N with the proviso that d=O when X is O and d=1 otherwise, and c=1 when X is C and c=0 when X is N; when c=1, Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2Rf,SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)Rf, C(O)R3, and cycloalkenyl groups formed therewith wherein Rf is a perfluoroalkyl group of 1-10 carbons optionally containing one or more ether oxygens; R3 is an alkyl group of 1-6 carbons optionally substituted with one or more ether oxygens, or an aryl group optionally further substituted;

[0037] or, when c=0, Y may be an electron-withdrawing group represented by the formula —SO2Rf′ where Rf′ is the radical represented by the formula

—(Rf″SO2N—(H+)SO2)mRf′″

[0038] where m=0 or 1, and Rf″ is —CnF2n— and Rf′″ is —CnF2n+1 where n=1-10

[0039] Most preferably, the ionomer comprises a perfluorocarbon backbone and said pendant group is represented by the formula

—OCF2CF(CF3)—OCF2CF2SO3H

[0040] Ionomers of this type are disclosed in U.S. Pat. No. 3,282,875.

[0041] The equivalent weight (a term of the art defined herein to mean the weight of the ionomer in acid form required to neutralize one equivalent of NaOH) of the ionomer can be varied as desired for the particular application. Where the ionomer comprises a perfluorocarbon backbone and the side chain is represented by the formula

—[OCF2CF(CF3)]n—OCF2CF2SO3H

[0042] where n=0 or 1. The equivalent weight when n=1is preferably 800-1500, most preferably 900-1200. The equivalent weight when n=0 is preferably 600-1300.

[0043] In the manufacture of the preferred membranes wherein the ionomer has a highly fluorinated backbone and sulfonate ion exchange groups, a membrane precursor is conveniently initially formed from the polymer in its sulfonyl fluoride form since it is thermoplastic and conventional techniques for making films from thermoplastic polymers can be used. Alternatively, the ionomer precursor may be in another thermoplastic form such as by having —SO2X groups where X is alkoxy such as CH3O— or C4H9O—, or an amine. Solution film casting techniques using suitable solvents for the particular polymer can also be used if desired.

[0044] The ionomer precursor polymer in sulfonyl fluoride form can be converted to the sulfonate form (i.e, ionic form) by hydrolysis using methods known in the art. For example, the membrane may be hydrolyzed to convert it to the sodium sulfonate form by immersing it in 25% by weight NaOH for about 16 hours at a temperature of about 90° C. This is followed by rinsing the film twice in deionized 90° C. water using about 30 to about 60 minutes per rinse. Another possible method employs an aqueous solution of 6-20% of an alkali metal hydroxide and 5-40% of a polar organic solvent such as dimethyl sulfoxide with a contact time of at least 5 minutes at 50-100° C. followed by rinsing for 10 minutes. After hydrolyzing, the membrane can be converted if desired to another ionic form by contacting the membrane in a bath containing a 1% salt solution containing the desired cation or, to the acid form, by contacting with an acid and rinsing. For fuel cell use, the membrane is usually in the sulfonic acid form.

[0045] If desired, the membrane precursor may be a laminated membrane of two or more ionomeric precursors such as two highly fluorinated ionomers having different ion exchange groups and/or different ion exchange capacities. Such membranes can be made by laminating films or co-extruding a multi-layer film. In addition, the ionomeric component of the membrane suitable for use in the present invention may be itself a blend of two or more ionomers, such as two or more highly fluorinated ionomers preferred for the practice of the invention, that have different ion exchange groups and/or different ion exchange capacities. It is also possible to form a multilayer structure incorporating one or more layers of the composite membrane of the invention.

[0046] The thickness of the membrane can be varied as desired for a particular electrochemical cell application. Typically, the thickness of the membrane is generally less than about 250 μm, preferably in the range of about 25 μm to about 150 μm.

[0047] The membrane may optionally include a porous support for the purposes of improving mechanical properties, for decreasing cost and/or other reasons. The porous support of the membrane may be made from a wide range of components. The porous support of the present invention may be made from a hydrocarbon such as a polyolefin, e.g., polyethylene, polypropylene, polybutylene, copolymers of those materials, and the like. Perhalogenated polymers such as polychlorotrifluoroethylene may also be used. For resistance to thermal and chemical degradation, the support preferably is made of a highly fluorinated polymer, most preferably perfluorinated polymer.

[0048] For example, the polymer for the porous support can be a microporous film of polytetrafluoroethylene (PTFE) or a copolymer of tetrafluoroethylene with other perfluoroalkyl olefins or with perfluorovinyl ethers. Microporous PTFE films and sheeting are known which are suitable for use as a support layer. For example, U.S. Pat. No. 3,664,915 discloses uniaxially stretched film having at least 40% voids. U.S. Pat. Nos. 3,953,566, 3,962,153 and 4,187,390 disclose porous PTFE films having at least 70% voids.

[0049] Alternatively, the porous support may be a fabric made from fibers of the support polymers discussed above woven using various weaves such as the plain weave, basket weave, leno weave, or others. A membrane suitable for the practice of the invention can be made by coating the porous support fabric with a fluorinated ionomer having imbibed therein the polymerization product of a composition comprising a non-fluorinated, non-ionomeric monomer and a free radical initiator. The fluorinated ionomer comprises at least 6 mole % of monomer units having a fluorinated pendant group with a terminal ionic group. To be effective the coating must be on both the outside surfaces as well as distributed through the internal pores of the support. This may be accomplished by impregnating the porous support with a solution or dispersion of the ionomeric polymer and drying, followed by imbibing a composition comprising the non-ionomeric polymer, a free radical initiator and optionally a crosslonking agent into the ionomeric polymer and polymerizing to form the polymerization product of said composition. It is important to use a solvent which is not harmful to the polymer of the support under the impregnation conditions, and that can form a thin, even coating on the support.

[0050] It is preferred for the cation exchange ionomer to be present as a continuous phase within the membrane.

[0051] Non-ionomeric Polymers and Formation of Membranes

[0052] In accord with the present invention, the composite membrane of the invention further comprises a non-ionomeric polymer. The selection of non-ionomeric polymers and free radical initiators suitable for use in the ionomeric polymer electrolyte membrane composition is quite wide. It is desirable that the non-ionomeric polymer be chemically and thermally stable under conditions of use in a fuel cell. It is preferred that the non-ionomeric polymer comprises a relatively high frequency of dipolar monomer units but is not itself ionic. A “high frequency” of dipolar monomer units means that the mole percentage concentration of monomer units having a dipolar functionality should be at least 75%, and is preferably greater than 90%. A “high frequency” of dipolar monomer units also means that the monomer units of which the dipolar moiety is a part should be as short as possible to increase the frequency of occurrence of the dipolar moiety. Thus a vinyl monomer would be preferred over, for example a butenyl monomer.

[0053] Preferred for the non-ionomeric polymer are polymers or copolymers derived from polar vinyl monomers such as vinyl acetate, vinyl isocyanate, acrylonitrile, acrylic acid or acrylate esters. Preferred polymers include poly(vinyl acetate), polyacrylonitrile, or polyacrylates Also suitable for the practice of the invention is tetrafluoroethylene.

[0054] In one embodiment, the process for making the ionomeric polymer electrolyte membrane composition comprises three steps which may be performed concurrently by contacting an ionomer or polymeric precursor thereto with a solution of a free radical initiator; contacting the ionomer or polymeric precursor thereto with a solution of one or more polymerizable dipolar monomers, e.g. a dipolar vinyl monomers; and, carrying out the polymerization by heating the thus formed composite intermediate to a temperature sufficient to polymerize the vinyl monomers.

[0055] For monomers such as vinyl acetate or tetrafluoroethylene, the polymerization may be effected in the presence of the ionomer in proton or acid form. However, it is generally preferable to employ the ionomer in the salt form. After polymerization is completed the acid form may be regenerated by treatment in a mineral acid such as HNO3. It has been found in the practice of the present invention that the efficacy of the invention in bringing about a significant decrease in methanol permeability with relatively small sacrifice in conductivity, depends significanty upon the cation employed to form the ionomeric salt. Alkali metal cations, such as lithium, sodium and potassium are preferred over rubidium or cesium. It is also acceptable to employ the unhydrolyzed polymer precursor to the ionomer in place of the ionomer until after a composite polymer is formed, and then to hydrolyze the precursor to prepare the ionomer.

[0056] To enhance the rate of transport of monomers and initiators, or, in the alternative, of non-fluorinated, non-ionomeric polymer, into the precursor polymer or ionomer, it is useful to subject the precursor polymer or ionomer to swelling using a swelling agent, preferably one which also serves as a solvent for the monomers or non-fluorinated polymers. In the case of unhydrolyzed fluorinated polymer precursors, dimethylsulfoxide is an excellent swelling agent as are fluorinated solvents. In the case of ionomers, polar solvents such as MeOH, water or DMF are preferred. Usually the more swelled the ionomers or copolymers are, the more non-fluorinated, non-ionomeric polymer will be present in the final polymeric compositions. Other solvents may also be employed as may be effective for a particular polymer composition. The outcome of the practice of the invention is not highly dependent upon the solvent used for swelling the polymer except insofar as solvents which result in more swelling are in general preferred over those which result in less swelling. Extractability of the solvent after imbibition of the monomers or non-fluorinated, non-ionomeric polymer is also a desirable property.

[0057] In a preferred method, the initiator is conveniently added to the ionomers or precursor polymer dissolved in a solvent which also swells the ionomer or precursor polymer. Some preferred initiators include liquids, such as hexafluoropropylene oxide (HFPO) dimer peroxide; per fluoropropionyl peroxide, (CF3CF2COO)2; Lupersol® manufactured by Ashland, Covington, Ky.; 2,2-azobis(isobutyronitrile) and di-t-butyl peroxide. Optionally crosslinking agents may be present in the composition to be polymerized. Some useful crosslinking agents include divinyl benzene, dienes, fluorinated dienes, diacrylates, divinyl esters and divinylether. If the vinyl monomer is a liquid, it is preferably added in the same solution as the initiator. Preferably, the temperature is maintained well-below the initiation temperature to provide the most homogeneous composition. However, polymerizatiom may also occur as the vinyl monomer is diffusing into the ionomer or polymeric precursor thereto. If the vinyl monomer is a gas, it is preferred to contact the ionomer or polymeric precursor thereto to the gaseous vinyl monomer in a sealed vessel after the ionomer or polymeric precursor thereto has been treated with the initiator solution and, preferably, been swollen thereby. If the vinyl monomer is a solid, it is preferably first dissolved in a solvent and then the ionomer or polymeric precursor thereto is immersed in the monomer solution.

[0058] Monomers are present in the composition to be polymerized in the amount of greater than about 6%, more typically up to about 40%, free radical initiators are present in the amount of about less than about 0.1%, more typically about less than about 0.05%, and the optional crosslinking agent is present in the amount of about 0 to about 5%, more typically about 0.01 to about 5%, and still more typically about 0.5 to about 2%, based on the weight of the total composition to be polymerized.

[0059] Membrane Electrode Assemblies and Electrochemical Cell

[0060] One embodiment of a fuel cell suitable for the practice of the present invention is shown in FIG. 1. While the cell depicted represents a single-cell assembly such as that employed in determining some of the results herein, one of skill in the art will recognize that all of the essential elements of a direct methanol fuel cell are shown therein in schematic form.

[0061] A ionomeric polymer electrolyte membrane of the invention, 1, is used to form a membrane electrode assembly (MEA) by combining it with a catalyst layer, 2, comprising a catalyst, e.g. platinum, unsupported or supported on carbon particles, a binder such as Nafion®, and a gas diffusion backing, 3. The gas diffusion backing may comprise carbon paper which may be treated with a fluoropolymer and/or coated with a gas diffusion layer comprising carbon particles and a polymeric binder to form an membrane electrode assembly (MEA). The fuel cell is further provided with an inlet for liquid or gaseous methanol, 4, an anode outlet, 5, a cathode gas inlet, 6, a cathode gas outlet, 7, aluminum end blocks, 8, tied together with tie rods (not shown), a gasket for sealing, 9, an electrically insulating layer, 10, and graphite current collector blocks with flow fields for gas distribution, 11, and gold plated current collectors, 12.

[0062] The fuel cell utilizes a fuel source that may be in the liquid or gaseous phase, and may comprise an alcohol or ether. Typically a methanol/water solution is supplied to the anode compartment and air or oxygen supplied to the cathode compartment. The ionomeric polymer electrolyte membrane serves as an electrolyte for proton exchange and separates the anode compartment from the cathode compartment. A porous anode current collector, and a porous cathode current collector are provided to conduct current from the cell. A catalyst layer that functions as the cathode is in contact with and between the cathode-facing surface of the membrane and the cathode current collector. A catalyst layer that functions as the anode is disposed between and is in contact with the anode-facing surface of the membrane and anode current collector. The cathode current collector is electrically connected to a positive terminal and the anode current collector is electrically connected to a negative terminal.

[0063] The catalyst layers may be made from well-known electrically conductive, catalytically active particles or materials and may be made by methods well known in the art. The catalyst layer may be formed as a film of a polymer that serves as a binder for the catalyst particles. The binder polymer can be a hydrophobic polymer, a hydrophilic polymer or a mixture of such polymers. Preferably, the binder polymer is an ionomer and most preferably is the same ionomer as in the membrane.

[0064] For example, in a catalyst layer using a perfluorinated sulfonic acid polymer membrane and a platinum catalyst, the binder polymer can also be perfluorinated sulfonic acid polymer and the catalyst can be a platinum catalyst supported on carbon particles. In the catalyst layers the particles are preferably uniformly dispersed in the polymer to assure that a uniform and controlled depth of the catalyst is maintained, preferably at a high volume density with the particles being in contact with adjacent particles to form a low resistance conductive path through catalyst layer. The connectivity of the catalyst particles provides the pathway for electronic conduction and the network formed by the binder ionomer provides the pathway for proton conduction.

[0065] The catalyst layers formed on the membrane should be porous so that they are readily permeable to the gases/liquids that are consumed and produced in cell. The average pore diameter is preferably in the range of 0.01 to 50 μm, most preferably 0.1 to 30 μm. The porosity is generally in a range of 10 to 99%, preferably 10 to 60%.

[0066] The catalyst layers are preferably formed using an “ink”, i.e., a solution of the binder polymer and the catalyst particles, which is used to apply a coating to the membrane. The binder polymer may be in the ionomeric (proton) form or in the sulfonyl fluoride (precursor) form. When the binder polymer is in the proton form the preferred solvent is a mixture of water and alcohol. When the binder polymer is in the precursor form the preferred solvent is a perfluorinated solvent (FC-40 made by 3M).

[0067] The viscosity of the ink (when the binder is in the proton form) is preferably controlled in a range of 1 to 102 poises especially about 102 poises before printing. The viscosity may be controlled by:

[0068] (i) particle size selection,

[0069] (ii) composition of the catalytically active particles and binder,

[0070] (iii) adjusting the water content (if present), or typically

[0071] (iv) by incorporating a viscosity regulating agent such as carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and cellulose and polyethyleneglycol, polyvinyl alcohol, polyvinyl pyrrolidone, sodium polyacrylate and polymethyl vinyl ether.

[0072] The area of the membrane to be coated with the ink may be the entire area or only a select portion of the surface of the membrane. The catalyst ink may be deposited upon the surface of the membrane by any suitable technique including spreading it with a knife or blade, brushing, pouring, metering bars, spraying and the like. The catalyst layer may also be applied by decal transfer, screen printing, or by application of a printing plate.

[0073] If desired, the coatings are built up to the thickness desired by repetitive application. The desired loading of catalyst upon the membrane can be predetermined, and the specific amount of catalyst material can be deposited upon the surface of the membrane so that no excess catalyst is applied. The catalyst particles are preferably deposited upon the surface of a membrane in a range from about 0.2 mg/cm2 to about 20 mg/cm2.

[0074] Typically a screen printing process is used for applying the catalyst layers to the membrane with a screen having a mesh number of 10 to 2400, more typically a mesh number of 50 to 1000, and a thickness in the range of 1 to 500 micrometers. The mesh and the thickness of the screen, and viscosity of the ink are selected to give electrode thickness ranging from 1 micron to 50 microns, more particularly 5 microns to 15 microns. The screen printing process can be repeated as needed to apply the desired thickness. Two to four passes, usually three passes, have been observed to produce the optimum performance. After each application of the ink, the solvent is preferably removed by warming the electrode layer to about 50° C. to 140° C., preferably about 75° C. A screen mask is used for forming an electrode layer having a desired size and configuration on the surface of the ion exchange membrane. The configuration is preferably a printed pattern matching the configuration of the electrode. The substances for the screen and the screen mask can be any materials having satisfactory strength such as stainless steel, poly(ethylene terephthalate) and nylon for the screen and epoxy resins for the screen mask.

[0075] After forming the catalyst coating, it is preferable to fix the ink on the surface of the membrane so that a strongly bonded structure of the electrode layer and the cation exchange membrane can be obtained. The ink may be fixed upon the surface of the membrane by any one or a combination of pressure, heat, adhesive, binder, solvent, electrostatic, and the like. Typically the ink is fixed upon the surface of the membrane by using pressure, heat or a combination of pressure and heat. The electrode layer is preferably pressed onto the surface of the membrane at 100° C. to 300° C., most typically 150° C. to 280° C., under a pressure of 510 to 51,000 kPa (5 to 500 ATM), most typically 1,015 to 10,500 kPa (10 to 100 ATM).

[0076] An alternative to applying the catalyst layer directly onto the membrane is the so-called “decal” process. In this process, the catalyst ink is coated, painted, sprayed or screen printed onto a substrate and the solvent is removed. The resulting “decal” is then subsequently transferred from the substrate to the membrane surface and bonded, typically by the application of heat and pressure.

[0077] When the binder polymer in the ink is in the precursor (sulfonyl fluoride) form, the catalyst coating after it is affixed to the membrane, either by direct coating or by decal transfer, is subjected to a chemical treatment (hydrolysis) where the binder is converted to the proton (or ionomeric) form.

[0078] The invention is illustrated in the following examples.

EXAMPLES

[0079] Glossary:

[0080] Lupersol® 11 t-Butyl Peroxypivalate, made by Ashland, Covington, Ky.

[0081] HFPO dimer peroxide having structure:

[0082] [CF3CF2CF2OCF(CF3)COO]2

[0083] AIBN 2,2′-Azobisisobutyronitrile

EXAMPLES

[0084] In the following specific embodiments conductivity, methanol absorption, and methanol permeability were determined, where indicated.

[0085] Methanol absorption was determined by first pre-drying the specimen and weighing in a closed container to get WD. The dried sample was then immersed in methanol at room temperature for 6-8 hours. The thus immersed specimen was then removed from the methanol, surface dried, placed in a closed container and weighed to get WW. The methanol absorption was determined from the equation:

% Absorption=[(WW−WD)/WD]×100

[0086] In order to determine methanol permeability, the membrane samples were loaded in permeation cells (316 SS, Millipore® high-pressure, 47 mm filters modified by the addition of liquid distribution plates). Each cell has a permeation area of 9.6 cm2. The cells (up to 4 per run) are located inside an insulated box kept at constant temperature. The insulated box was heated by two Chromalox®, 1100 watts, and finstrips heaters. The air within the box was mixed by a 7″ diameter, 5-blade propeller connected to a Dayton Model 4Z140 variable speed DC motor. The insulated box temperature was controlled by a Yokogawa UT320 Digital indicating temperature controller. 1 M methanol solution was circulated on the top side of the membrane at a flow rate of 5.7-9.6 cc/min (measured with Brook Instruments, Model 1355EYZQFA1G rotameters). The bottom of the membrane was swept with nitrogen at 1,000-5,000 SCCM (measured with mass flow controllers: 2 MKS type 1179 and 2 Tylan 2900 series mass flow meters connected by a Tylan RO-28 controller box). Both the methanol solution and the nitrogen were heated to the cell temperature by circulating through stainless steel coils before entering the permeation cells. Samples of the nitrogen sweeping the permeation cells were sent to a set of heated Valco® valves and then a 2 cc gas sample was injected into a HP 6890 Gas Chromatograph with a Thermal Conductivity Detector (TCD) and HP-PLOT Q GC Column to analyze the methanol and water. The GC was controlled by HP Chem Station software Revision A.06.03. The permeation rates (molar fluxes) of methanol and water through the membrane were calculated as follows:

Methanol Molar flux (mol/cm2 min)=gramsMeOH*F/(Vnitrogen*Ap*MWMeOH)

Water Molar flux (mol/cm2 min)=gramsWater*F/(Vnitrogen*Ap*MWWater)

[0087] Where:

[0088] gramsMeOH=MeOH Peak Area*MeOH Response Factor=Grams MeOH Injected in GC.

[0089] gramswater=Water Peak Area*Water Response Factor=Grams water Injected in GC.

[0090] Vnitrogen=Vs−gramsMeOH/pMeOH−gramsWater/Pwater=Volume of nitrogen injected in GC (cm3)

[0091] Vs=Volume Gas Sample injected into GC (cm3)

[0092] Ts=Temperature of Gas sample=Temperature of sampling valve (K)

[0093] Ps=Pressure of gas sample (psia)

[0094]

Pnitrogen
=Density of nitrogen at Ts and Ps (g/cm3)

[0095]

PMeOH
=Density of Methanol at Ts and Ps (g/cm3)

[0096] ρWater=Density of Water at Ts and Ps (g/cm3)

[0097] Ap=Permeation Area of cells (cm2)

[0098] F=Flow of nitrogen sweeping membrane at Ts, Ps (cm3/min)

[0099] The methanol and water response factors were calculated by injecting known amounts of methanol and water into the GC. The methanol and water response factors were the ratio of grams of components injected /Peak area of methanol and water.

[0100] Conductivity of the subject membrane was determined by impedance spectroscopy by which is measured the ohmic (real) and capacitive (imaginary) components of the membrane impedance. Impedance wa determined using the Solartron model SI 1260 Impedance/Gain-phase Analyzer, manufactured by Schlumberger Technologies Ltd., Instrument Division, Farnborough, Hampshire, England, utilizing a conductivity cell having a cell constant of 202.09, as described in J. Phys. Chem., 1991, 95, 6040 and available from Fuel Cell Technologies, Albuquerque, N.M.

[0101] Prior to the conductivity measurement, a membrane was boiled in deionized water for one hour prior to testing. The conductivity cell was submersed in water at 25±1° C. during the experiment.

[0102] The impedance spectrum was determined from 10 Hz to 105 Hz at 0 VDC, and 100 mv (rms) AC. The real impedance that corresponded to the highest (least negative) imaginary impedance was determined.

[0103] Conductivity was calculated from the equation:

Conductivity=cell constant/[(real impedance)*(film thickness)]

Example 1

[0104] Four dried Nafion® 117 films (7.62×7.62 cm, 7.879 g) (E. I. duPont de Nemours and Co., Wilmington, Del.) were immersed in a solution of 50 mL of acrylonitrile (AN), and 0.6 g of Lupersol® 11, an initiator, at −5° C. for 3 hrs, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 70° C. for 18 hrs. to polymerize acrylonitrile to form a polacrylonitrile . After polymerization, the films were dried at 100° C. in full vacuum for 3 hrs to remove volatiles. The films weighed 11.00 g. The weight % of polymer formed in the Nafion® 117 film, as shown in Table 1 was determined by first weighing the dry Nafion® film, and re-weighing it following the steps of immersion, polymerization, and extraction of volatiles at 100° C. under full vacuum. The weight difference was divided by the initial weight to give the weight % increase in Table 1. After drying the films were treated with 10% HNO3 at 60° C. for 2 hrs, washed with water until pH=7was reached, and boiled in de-ionized water for 2 hrs. Conductivity and MeOH permeability are shown in Table 1.

Example 2

[0105] Four dried Nafion® 117 films (7.62×7.62 cm, 7.898 g) were immersed in a solution of 50 mL of acrylonitrile (AN), 2 mL of divinylbenzene (DVB), a crosslinking agent, and 0.6 g of Lupersol® 11 at −5° C. for 3 hrs, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 70° C. for 20 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 6 hrs to remove volatiles. The films weighed 10.668 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 3

[0106] Three dried Nafion® 117 films (7.62×7.62 cm, 5.93 g) were immersed in a solution of 50 mL of acrylonitrile (AN), 2 mL of divinylbeneze (DVB) and 0.6 g of Lupersol® 11 at −5° C. for 1.5 hrs, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 70° C. for 20 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 6 hrs to remove volatiles. The films weighed 10.668 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 4

[0107] Two dried Nafion® 117 films (7.62×7.62 cm, 3.977 g) were immersed in a solution of 50 mL of acrylonitrile (AN), 2 mL of divinylbeneze (DVB) and 0.6 g of Lupersol® 11 at −5° C. for 0.5 hrs, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 75° C. for 16 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 6 hrs to remove volatiles. The films weighed 4.361 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 5

[0108] Two dried Nafion® 117 films (7.62×7.62 cm, 4.018 g) were immersed a solution of 12 mL of THF and 2 mL of water for 30 min. and then 20 g of acrylonitrile (AN) and kept at 10° C. for 2 hrs, followed by addition of 0.2 g of Lupersole® 11. The films were transferred to a flask under N2 atmosphere. The sealed flask was heated at 70-80° C. for 15 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 3 hrs to remove volatiles. The films weighed 4.458 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 6

[0109] Three dried Nafion® 117 films (7.62×7.62 cm, 5.922 g) were boiled in de-ionic water for 1 hr. After being wiped with a paper towel, the films were immersed in a solution of 50 mL of acrylonitrile (AN) and 0.6 g of Lupersole® 11 at −5° C. for 2 hrs, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 70° C. for 16 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 6 hrs to remove volatiles. The films weighed 6.188 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 7

[0110] Four dried Nafion® 117 films (7.62×7.62 cm, 8.011 g) were immersed in a solution of 50 mL of methyl acrylate (MA), 2 mL of divinylbeneze (DVB) and 0.6 g of Lupersol® 11 at −5° C. for 3 hrs, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 75° C. for 16 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 6 hrs to remove volatiles. The films weighed 10.938 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 8

[0111] Four dried Nafion® 117 films (7.62×7.62 cm, 8.005 g) were immersed in a solution of 40 mL of methyl acrylate (MA), 15 mL of divinylbeneze (DVB) and 0.6 g of Lupersol® 11 at −5° C. for 2 hrs, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 70° C. for 11 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 5 hrs to remove volatiles. The films weighed 8.858 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 9

[0112] Three dried Nafion® 117 films (7.62×7.62 cm, 5.914 g) were immersed in a solution of 50 mL of methyl acrylate (MA), 2 mL of divinylbeneze (DVB) and 0.6 g of Lupersol® 11 at −5° C. for 0.5 hr, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 70° C. for 16 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 4 hrs to remove volatile. The films weighed 6.248 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 10

[0113] Four dried Nafion® 117 films (7.62×7.62 cm, 8.069 g) were immersed in a solution of 30 mL of acrylonitrile (AN), 20 mL of methyl acrylate (MA), 4 mL of divinylbeneze (DVB) and 0.6 g of Lupersole® 11 at −5° C. for 0.5 hr, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 70-80° C. for 16 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 3 hrs to remove volatiles. The films weighed 8.548 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

Example 11

[0114] Four dried Nafion® 117 films (7.62×7.62 cm, 5.964 g) were immersed in a solution of 40 mL of methyl methacrylate (MMA), 7 mL of divinylbeneze (DVB) and 0.6 g of Lupersol® 11 at −5° C. for 2 hrs, and then transferred to a flask under N2 atmosphere. The sealed flask was heated at 70° C. for 16 hrs. After polymerization, the films were dried at 100° C. in full vacuum for 6 hrs to remove volatiles. The films weighed 6.318 g. The weight % of polymer formed in the Nafion® 117 film was determined as described in Example 1. After drying, the films were treated as in Example 1. Conductivity and MeOH permeability are shown in Table 1.

1TABLE 1Mono-Immmer-% Decrease in% Decreasemer(s)*VolumeVolumesionMeOHMeOHConductivityinExType(ml)DVB (ml)solventTime (h)Wt %permeabilityPermeability(S/cm).ConductivityControl1.85E−050.11AN500none3402.23E−0688%0.040759%2AN502none3354.92E−0673%0.039860%3AN502none1.516.26.36E−0665%0.085614%4AN502none0.59.60.073327%5AN200THF/H2O = 12/22111.48E−0520%0.089211%6AN500none34.58.67E−0653%0.086913%7MA502none336.67.53E−0659%0.059441%8MA307none210.68.15E−0656%0.083317%9MA502none0.55.61.36E−0525%0.091 9%10MA/AN20/304none0.55.91.39E−0525%0.0907 9%11MMA407none25.91.50E−0519%0.087313%*AN is CH2═CHCN, MA is methyl acrylate, and MMA is methyl methacrylate.

Example 12

[0115] Two Nafion® 117 films, (3.05×2.79 cm, 0.552 9 and 0.475 g, respectively) were treated with 0.1 M LiOH in water for 20 min. to form the lithium salt. The film was removed and washed with water and briefly dried in air and then immersed in a solution of 7.01 g of vinyl acetate, 0.2 g of divinylbenzene and 5 drops of Lupersol®D 11 at 0° C. to 5° C. for 3.5 hours. The film was removed, wiped off and transferred in a flask in N2 atmosphere. After being kept in an oven at 70° C. overnight, the film was converted back to the acid form by treatment with 8% HNO3 for 4 hours and washed with water. IR indicated that the film contained poly(vinylacetate): 2924, 2853, 1725 cm−1. Absorption of methanol was 34.1%, and conductivity was 0.098 S/cm. Absorption of MeOH for untreated Nafion® 117 was 66%, and conductivity was 0.103 S/cm.

Example 13

[0116] A Nafion® 117 film was treated with 0.1 M LiOH in water for 20 min. The film were removed and washed with water and briefly dried in air and then immersed in a solution of styrene, divinylbenzene and Lupersol® 11 in a ratio of 10.5 to 3.5 to 0.34 at 0° C. to −15° C. for 20 hours. The films were removed, wiped off and transferred in a flask in N2 atmosphere. After being kept in oven at 70° C. overnight, the films were treated with 8% HNO3 for 4 hours and washed with water. IR indicated that the films contained polystyrene. Absorption of methanol was 73.2%, and conductivity was 0.095 S/cm.

Example 14

[0117] A Nafion® 117 film was treated in dilute LiOH for 20 minutes. It was then immersed in 6.6 weight % of HFPO dimer peroxide in CF3CF2CF2OCF(CF3)CF2OCFHCF3 (E-2) for 2 hours. Upon removal from the solution, the films were briefly air dried and transferred into a dry-ice prechilled shaker tube. The tube was evacuated and loaded with 25 g of tetrafluoroethylene (TFE), shaken at 25° C. to 35° C. for 10 hrs, causing the pressure drop from 319 psi to 160 psi. Loose Poly TFE (PTFE) was observed on the surface of the film. Once the PTFE was wiped off with MeOH, the film was washed with water and treated with 8% HNO3 overnight, and then washed with water. Absorption of methanol was 51.1% and conductivity was 0.096 S/cm.

Example 15

[0118] A Nafion® 117 film (0.42 g) was treated with 2% Cs2CO3 in water for 18 hrs at room temperature to form Nafion® -Cs salt. After being removed, washed with water and dried in air at room temperature for 2 hrs, the film was immersed in 3% HFPO dimer peroxide in E-2 at −15° C. overnight. The films were removed, wiped off and transferred in a 240 mL of shaker tube at −10° C. After charging with 40 g of TFE, the tube was shaken at 30° C. for 10 hrs. After being wiped off to remove loose PTFE, the film was dried at 110° C. in full vacuum overnight. Weight of the film increased 2%. The film was treated with 8% HNO3 overnight, and then washed with water. Absorption of water and methanol were 31.8% and 50.5%, respectively, and conductivity was 0.063 S/cm.

Solid polymer membrane for fuel cell prepared by in situ polymerization

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