Electrolyte membrane for fuel cell

Abstract

This electrolyte membrane for a fuel cell comprises a polybenzimidazole nanofiber and a polymer of monomers selected from N-vinylimidazoles, derivatives of N-vinylimidazoles, and mixtures thereof, and is characterized by, as a result of containing no fluorine atoms, causing little environmental loads, and undergoing little deterioration in high-temperature regions of 80°C or higher.

Description

Electrolyte membrane for fuel cells

[0001] The present invention relates to an electrolyte membrane for a fuel cell.

[0002] Fuel cells generate electricity directly by supplying fuel such as hydrogen and oxygen from the atmosphere to a cell and causing an electrochemical reaction between them to produce water. Because fuel cells are highly efficient at converting energy and have excellent environmental adaptability, they are being developed for a variety of uses, including small-scale local power generation, home power generation, simple power sources at campsites, mobile power sources for automobiles and small boats, and power sources for artificial satellites and space development.

[0003] Such fuel cells, particularly solid polymer fuel cells, are composed of a module in which a plurality of unit cells are arranged in parallel, each unit cell comprising a membrane electrode assembly made up of a solid polymer electrolyte membrane and an anode electrode and a cathode electrode disposed on either side of the membrane sandwiched between a pair of separators. Conventionally, fluorine-based polymers such as Nafion (registered trademark, the same applies hereinafter) and Aquivion (registered trademark, the same applies hereinafter) have been widely used as the electrolyte membrane (see, for example, Patent Document 1).

[0004] However, these electrolyte membranes containing fluorine-based polymers are expensive and suffer from the problem that their proton conductivity drops significantly under high-temperature, non-humidified conditions.Furthermore, because they all contain fluorine, they are undesirable from environmental and regulatory perspectives.

[0005] In this regard, Patent Document 2 discloses an electrolyte for high-temperature fuel cells using acid-doped polybenzimidazole nanofibers as an electrolyte membrane that is compatible with the power generation conditions of next-generation fuel cells that can be driven at high temperatures (120 to 150°C). However, the technology of Patent Document 2 contains ether side chains, which reduces the proportion of ion-exchange groups, leaving room for improvement in terms of proton conductivity.

[0006] International Publication No. 2017 / 141878 Japanese Patent Application Laid-Open No. 2020-181701

[0007] The present invention has been made in view of the above circumstances, and aims to provide an electrolyte membrane for fuel cells which does not contain fluorine atoms and therefore has a low environmental impact, and which is less susceptible to deterioration at high temperatures of 80°C or higher.

[0008] As a result of extensive research to achieve the above object, the present inventors have found that a membrane comprising a polybenzimidazole nanofiber and a polymer of a monomer selected from N-vinylimidazole, a derivative of N-vinylimidazole, and a mixture thereof is less susceptible to deterioration at high temperatures of 80°C or higher, and have completed the present invention.

[0009] That is, the present invention provides: 1. an electrolyte membrane for a fuel cell, comprising polybenzimidazole nanofibers and a polymer of a monomer selected from N-vinylimidazole, a derivative of N-vinylimidazole, and a mixture thereof; 2. an electrolyte membrane for a fuel cell according to claim 1, wherein the derivative of N-vinylimidazole is at least one of those represented by the following formulas (1) and (2): (In formula (1), X - In formula (2), Z represents an alkylene group having 1 to 20 carbon atoms, which may have an oxygen atom between its carbon-carbon bonds; - each independently represents a monovalent anion, and n represents an integer of 1 to 20. 3. - But HSO4 - 4. The electrolyte membrane for a fuel cell of 2, wherein Z is an alkylene group having 4 to 16 carbon atoms, which may have an oxygen atom interposed between its carbon-carbon bonds; 5. The electrolyte membrane for a fuel cell of 2, wherein Y is - However, both are HSO4 -6. A method for producing an electrolyte membrane for a fuel cell, comprising impregnating a fiber structure made of polybenzimidazole nanofibers with a monomer selected from N-vinylimidazole, an N-vinylimidazole derivative, and a mixture thereof, and then polymerizing the monomer; 7. A method for producing an electrolyte membrane for a fuel cell, comprising heat-pressing a fiber structure made of polybenzimidazole nanofibers under conditions of 80°C or higher and 0.5 MPa or higher, and then impregnating the fiber structure with the monomer; 8. A method for producing an electrolyte membrane for a fuel cell, comprising further treating the fiber structure made of polybenzimidazole nanofibers after the heat-pressing under conditions of oxygen plasma, and then impregnating the fiber structure with the monomer; 9. A method for producing an electrolyte membrane for a fuel cell, comprising polybenzimidazole nanofibers, wherein the fiber structure is produced by electrospinning; and 10. A method for producing an electrolyte membrane for a fuel cell, comprising polybenzimidazole nanofibers, wherein the fiber structure is produced by electrospinning from a solution of polybenzimidazole in dimethylacetamide.

[0010] The electrolyte membrane for fuel cells of the present invention exhibits minimal degradation at high temperatures of 80°C or higher, and because all of the monomers used as raw materials for the membrane contain an imidazolium base as a proton-accepting group and have a large number of proton-accepting groups in the molecule, it exhibits proton conductivity superior to that of fluoropolymers such as Nafion at high temperatures and in an unhumidified state. Furthermore, the electrolyte membrane of the present invention contains polybenzimidazole nanofibers, a heat-resistant polymer material, as a reinforcing material, and therefore exhibits higher strength properties than membranes that do not contain such nanofibers. Furthermore, the electrolyte membrane of the present invention has a fluorine-free structure, which is advantageous in terms of cost compared to commonly used fluoropolymers, and also has a small environmental impact.

[0011] FIG. 1 is a graph showing the temperature change in proton conductivity in an unhumidified state for the electrolyte membranes obtained in Examples 1-1 to 1-3 and Comparative Example 1, and the Nafion membrane. In the graph, PIL / 11 wt% PBI-NF indicates the results for Example 1-1, PIL / 13 wt% PBI-NF indicates the results for Example 1-2, PIL / 33 wt% PBI-NF indicates the results for Example 1-3, PIL indicates the results for Comparative Example 1, and Nafion211 indicates the results for the Nafion membrane.

[0012] The present invention will be described in more detail below. The electrolyte membrane for a fuel cell according to the present invention is characterized by comprising a polymer of polybenzimidazole nanofibers and a monomer selected from N-vinylimidazole, a derivative of N-vinylimidazole, and a mixture thereof (hereinafter, sometimes simply referred to as "monomer").

[0013] The fiber diameter of the polybenzimidazole nanofibers (hereinafter referred to as "PBI nanofibers") used in the present invention may be in the nano range (less than 1000 nm), preferably 1 to 800 nm, and more preferably 10 to 500 nm. The fiber diameter of the nanofibers is an average value obtained by analyzing electron microscope images, and the specific method for obtaining the diameter is as described in the Examples below.

[0014] The PBI nanofiber used in the present invention may be any fiber produced by various conventionally known methods such as melt-blowing, flash spinning, electrospinning (ES), solution blow spinning (SBS), and centrifugal spinning, but fibers produced by electrospinning (ES) are preferred.

[0015] Electrospinning is a method of forming ultrafine fibers by drawing a charged polybenzimidazole solution in an electric field, causing the solution to burst due to the repulsive force of the electric charge. The basic configuration of an ES device consists of one electrode, which also serves as a nozzle for supplying the solution and applies a high voltage of several thousand to several tens of thousands of volts to the solution, and the other electrode (counter electrode) facing the first electrode. The solution ejected from one electrode is transformed into nanofibers by a high-speed jet in the electric field between the two opposing electrodes, followed by bending and stretching of the jet, and then deposited on the surface of the other electrode, resulting in a nanofiber mat. In the present invention, a known ES device can be used, and the spinning conditions of the ES device, such as the distance between the nozzle tip and the counter electrode, the applied voltage, and the solution flow rate, can be appropriately set depending on the raw material used and the desired fiber diameter.

[0016] The solvent used for preparing the polybenzimidazole solution to be used in electrospinning is not limited as long as it has the ability to dissolve polybenzimidazole, and examples thereof include N,N-dimethylacetamide, dimethyl sulfoxide, formic acid, and N-methylpyrrolidone. In the present invention, N,N-dimethylacetamide is preferred.

[0017] The polybenzimidazole may be a commercially available product, or may be produced by a known method using 3,3'-diaminobenzidine and isophthalic acid or 1,3-dicyanobenzene or the like.

[0018] The N-vinylimidazole derivative used in the present invention is not particularly limited, but is preferably an N-vinylimidazolium salt represented by the following formula (1) (hereinafter referred to as compound (1)) or a compound having N-vinylimidazolium bases at both ends represented by the following formula (2) (hereinafter referred to as compound (2)). When producing a polymer, N-vinylimidazole, compound (1), and compound (2) may be used alone or in combination of two or more, but in the present invention, it is preferable to use compound (1) and compound (2) in combination.

[0019]

[0020] In formula (1), X - represents a monovalent anion, and specific examples thereof include BF4 - , PF6 - , AsF6 - , SbF6 - , AlCl4 - , NbF6 - , HSO4 - , ClO - , CH3SO4 - , CH3SO3 - ,p-CH3C6H4SO3 - , CF3SO3 - , CF3CO2 - , (FSO2)2N - , (CF3SO2)2N - , (C2F5SO2)2N - , Cl - ,Br - , I - , O.H. - However, considering that it is used as an electrolyte for fuel cells and does not contain halogens, HSO4 - is preferred.

[0021] In formula (2), Z represents an alkylene group having 1 to 20 carbon atoms, preferably 4 to 16 carbon atoms, and more preferably 6 to 12 carbon atoms, which may have an oxygen atom interposed between its carbon-carbon bond. However, from the viewpoint of stability during high-temperature operation and long-term operation of a device such as a fuel cell to which the fuel cell electrolyte membrane of the present invention is applied, an alkylene group containing no oxygen atom is preferred. Specific examples of the alkylene group include methylene, ethylene, trimethylene, propylene, tetramethylene, pentamethylene, hexamethylene, heptamethylene, octamethylene, nonamethylene, and decamethylene, with a linear alkylene group being preferred.

[0022] Therefore, the compound (2) is more preferably one represented by the following formula (2-1).

[0023] (In the formula, Y - each independently represents a monovalent anion, and n represents an integer of 1 to 20, preferably 4 to 16, and more preferably 6 to 12.

[0024] Y- Each of the groups independently represents a monovalent anion, and specific and preferred examples thereof include the above-mentioned X - In this case, too, considering that the electrolyte is used in a fuel cell and does not contain halogen, two Y - are both HSO4 - is preferred.

[0025] The compounds (1) and (2) can be produced by known methods. Compound (1) can be easily obtained, for example, by mixing an amine such as vinylimidazole with a protonic acid that forms the desired anion and neutralizing the mixture. Compound (2) can be obtained, for example, by mixing a large excess of an amine such as vinylimidazole with a linear alkyl group having halides at both ends to obtain a salt with quaternized ends, converting the anion to a hydroxide anion through an anion exchange resin, and then mixing a protonic acid that forms the desired anion with an equimolar amount of hydroxide anion.

[0026] In the present invention, the polymerization method of the monomer may be appropriately selected from conventionally known polymerization methods, and for example, a polymer may be produced by reacting the monomers by radical polymerization. In this case, when compound (1) and compound (2) are used in combination as monomers, their use ratio is arbitrary, but in consideration of further increasing the proton conductivity of the resulting electrolyte membrane, the mass ratio of compound (1):compound (2) is preferably 1:1 to 10:1, more preferably 2:1 to 8:1, even more preferably 2:1 to 6:1, and even more preferably 3:1 to 5:1.

[0027] Various known polymerization initiators can also be used during polymerization. Specific examples include persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate; peroxides such as benzoyl peroxide, cumene hydroperoxide, and t-butyl hydroperoxide; and azo compounds such as azobisisobutyronitrile, azobismethylbutyronitrile, azobisisovaleronitrile, 2,2'-azobis(isobutyrate)dimethyl, 2,2'-azobis(N-butyl-2-methylpropionamide), 4,4'-azobis(4-cyanopentanoic acid), 2,2'-azobis(2-amidinopropane)dihydrochloride, and 2,2'-azobis(N,N'-dimethyleneisobutylamidine)dihydrochloride. These polymerization initiators can be used alone or in combination of two or more. The amount of radical polymerization initiator added is usually preferably 0.01 to 50% by mass relative to the monomer.

[0028] The reaction temperature is preferably 60 to 120°C, more preferably 70 to 100°C. The reaction time is preferably 30 minutes to 24 hours, more preferably 1 to 18 hours. The polymerization reaction can be carried out in a solvent. Examples of solvents that can be used include water; hydrocarbon solvents such as pentane, hexane, cyclohexane, heptane, isooctane, toluene, xylene, and mesitylene; aprotic polar solvents such as acetonitrile, propionitrile, N,N-dimethylformamide, and N-methylpyrrolidone; halogenated hydrocarbon solvents such as dichloromethane, dichloroethane, and chlorobenzene; and ether solvents such as diethyl ether, tetrahydrofuran, dioxane, and dimethoxyethane. These solvents may be used alone or in combination.

[0029] Both of the compounds (1) and (2) are salts, and hydrogen sulfates, which are particularly suitable salts, are soluble in water. Therefore, when these compounds are used as raw materials, they are preferably polymerized using water as a solvent, and a water-soluble persulfate or azo compound is preferably used as a polymerization initiator.

[0030] After the reaction is completed, the reaction mixture is cooled to room temperature, and then subjected to known post-treatments such as filtration, washing, and drying to obtain a polymer.

[0031] The fuel cell electrolyte membrane of the present invention contains the above-mentioned PBI nanofibers and polymer, and although there are no particular limitations on the method for producing it, a preferred method is to impregnate a fiber structure made of PBI nanofibers with the above-mentioned monomer and then polymerize the monomer in the fiber structure. The fiber structure may be either a woven fabric or a nonwoven fabric, but nonwoven fabric is preferred considering that the nanofibers will be produced by electrospinning.

[0032] In particular, before the impregnation and polymerization of the monomer, it is preferable to heat-press the fiber structure made of PBI nanofibers at a temperature of 80° C. or higher, preferably 120° C. or higher, more preferably 150° C. or higher, and even more preferably 170° C. or higher, and at a pressure of 0.5 MPa or higher, preferably 0.7 MPa or higher, and more preferably 0.9 MPa or higher. The heat-pressing treatment increases the fiber diameter of the nanofibers that make up the fiber structure, resulting in a decrease in apparent porosity and a dense fiber structure.

[0033] The upper limit of the pressing temperature is not particularly limited as long as it is lower than the glass transition temperature of polybenzimidazole, but is preferably 250°C or lower, more preferably 200°C or lower. The upper limit of the pressing pressure is not particularly limited as long as the voids in the fiber structure are not eliminated, but is preferably 1.5 MPa or lower, more preferably 1.2 MPa or lower.

[0034] Furthermore, in order to improve the hydrophilicity of the fiber structure, it is preferable to subject the fiber structure made of PBI nanofibers after heat pressing to oxygen plasma treatment, and then impregnate and polymerize the monomer. The conditions for the oxygen plasma treatment are not particularly limited, and for example, the conditions can be an oxygen flow rate of 10 to 200 mL / min, 5 to 30 W, and a treatment time of about 10 to 120 seconds.

[0035] The method for impregnating the fibrous structure with the monomer may be either a method of immersing the fibrous structure in a monomer solution or dispersion, or a method of casting the monomer solution or dispersion onto a fibrous structure arranged on a substrate, but the latter method is preferred. Examples of solvents used in preparing the monomer solution or dispersion include those similar to those exemplified for the polymerization reaction above. When using the above compounds (1) and (2), it is preferable to use an aqueous solution in which these compounds are dissolved in water. After the fibrous structure is impregnated with the monomer, the monomer is polymerized by heating to the polymerization reaction temperature described above. The heating time is the same as the reaction time during the polymerization described above.

[0036] Any casting method can be used, and various methods can be used, such as scraper coating, bar coating, brush coating, spraying, immersion, flow coating, roll coating, curtain coating, spin coating, knife coating, etc. The electrolyte membrane formed on the substrate can be used by peeling it off from the substrate.

[0037] The thickness of the fuel cell electrolyte membrane of the present invention is not particularly limited and can be, for example, about 1 to 300 μm, preferably 1 to 50 μm, and more preferably 1 to 25 μm. Furthermore, in consideration of the balance between the strength and proton conductivity of the electrolyte membrane, the content of the fibrous structure made of PBI nanofibers in the fuel cell electrolyte membrane is preferably 5 to 40 mass%, more preferably 10 to 35 mass%, and even more preferably 10 to 20 mass%.

[0038] Generally, a solid polymer fuel cell is composed of a large number of unit cells arranged side by side, each unit cell being composed of a pair of electrodes sandwiching a solid polymer membrane and a pair of separators sandwiching these electrodes to form a gas supply / discharge flow path. The fuel cell electrolyte membrane of the present invention can be used as part or all of the solid polymer membrane.

[0039] The present invention will be explained in more detail below by way of Synthesis Examples, Production Examples, Examples and Reference Examples, but the present invention is not limited to the following Examples.

[0040] [1] Synthesis of raw material monomers [Synthesis Example 1] Synthesis of N-vinylimidazolium hydrogen sulfate (A)

[0041] After thoroughly cooling 60 mL of ion-exchanged water in an ice bath, 21.7 g of concentrated sulfuric acid (manufactured by Kanto Chemical Co., Inc.) was gradually added thereto with stirring to avoid sudden heat generation. Subsequently, a solution of 20.0 g of 1-vinylimidazole (manufactured by Tokyo Chemical Industry Co., Ltd.) in 60 mL of ion-exchanged water was gradually added dropwise to the resulting aqueous sulfuric acid solution with stirring on ice to avoid sudden heat generation, and stirring was then continued for several hours. Most of the ion-exchanged water was first removed from this solution using an evaporator, and then a vacuum was applied for 5 hours using a vacuum pump. 43.2 g of the target product, N-vinylimidazolium hydrogensulfate (A), was obtained as a white solid containing a small amount of water (yield: quantitative).

[0042] [Synthesis Example 2] Synthesis of 3-vinyl-1-[12-(3-vinylimidazolidin-1-ium-1-yl)dodecyl]imidazolium disulfate (B)

[0043] 22.7 g of 1-vinylimidazole (Tokyo Chemical Industry Co., Ltd.) was added to a solution of 32.8 g of 1,12-dibromododecane (Tokyo Chemical Industry Co., Ltd.) in 400 mL of acetonitrile (Sanyo Chemical Industries, Ltd.), and the mixture was stirred at room temperature for at least two weeks. The precipitated crystals were filtered under reduced pressure using a Kiriyama funnel, and the solvent was removed using a vacuum pump to obtain 46.3 g of the intermediate, 3-vinyl-1-[12-(3-vinylimidazolidin-1-ium-1-yl)dodecyl]imidazolium dibromide, as a white solid (85% yield). 4.5 g of the resulting dibromo compound was dissolved in 20 mL of ion-exchange water and subjected to column treatment using 30 mL of ion-exchange resin DS-2 (Organo Corporation). The eluate containing the reaction product was subjected to the same column treatment several times to completely convert the bromide ions to hydroxide ions. 700 g of the final eluate containing the reaction product was cooled, and concentrated sulfuric acid (Kanto Chemical Co., Inc.) was added until the neutralization point was reached. The amount of concentrated sulfuric acid used was 1.6 g. This reaction solution was placed in an evaporator to remove water, and then dehydrated using a vacuum pump. 20 mL of a 1:1 (volume ratio) mixture of ion-exchanged water and methanol was added to the resulting jelly-like solid, and the mixture was stirred for several hours. After that, the insoluble matter was removed by filtration through a membrane filter, and the filtrate was placed in an evaporator to remove the solvent. The resulting solid was further evaporated using a vacuum pump, yielding 3.11 g of the target product, 3-vinyl-1-[12-(3-vinylimidazolidin-1-ium-1-yl)octyl]imidazolium disulfate (B), as a light brown solid (yield 65%).

[0044] [2] Synthesis of Polybenzimidazole [Synthesis Example 3] 6.26 mg of 3,3'-diaminobenzidine (29 mmol, Sigma-Aldrich, 98%), 4.88 g of isophthalic acid (29 mmol, Kanto Chemical, >99.0%), and 216 g of polyphosphoric acid were placed in a two-necked flask, and after purging with argon, the mixture was heated and stirred overnight in an oil bath at 150°C. The temperature was further increased to 190°C and the mixture was heated and stirred for 3 days. The black viscous reaction solution was added dropwise to 2 L of water, followed by reprecipitation and suction filtration to obtain a wire-like black-green solid. This precipitate was placed in 1 L of a 30% by mass aqueous solution of sodium bicarbonate (Kanto Chemical, special grade) to neutralize the solution, and the mixture was heated and stirred at 50°C for 1 day. Thereafter, the mixture was suction filtered using a large amount of water, the remaining salt was thoroughly washed away, and the mixture was vacuum dried at 130°C for 1 day to obtain 8.87 g of a brown solid. The chemical structure of the obtained solid was evaluated by H-NMR spectrum measurement using a nuclear magnetic resonance spectrometer (NMR, JNM-ECZ 400S / L1, manufactured by JEOL Ltd.). Comparison with the NMR spectrum of polybenzimidazole reported in the literature (Yang, J.; Aili, D.; Li, Q.; Xu, Y.; Liu, P.; Che, Q.; Jensen, JO; Bjerrum, NJ; He, R. Benzimidazole Grafted Polybenzimidazoles for Proton Exchange Membrane Fuel Cells. Polymer Chemistry 2013, 4 (17), 4768-4775. https: / / doi.org / 10.1039 / c3py00408b.) confirmed that polybenzimidazole (hereinafter referred to as PBI) was quantitatively obtained. The solvent used for NMR measurements was d-DMSO containing TMS, and chemical shifts were expressed in ppm scales with the TMS signal (0.00) as the reference.

[0045] [3] Production of a fiber structure (hereinafter referred to as PBI-NF structure) composed of PBI nanofibers [Production Example 1] The PBI obtained in Synthesis Example 3 was added to N,N-dimethylacetamide (hereinafter referred to as DMAc), and the mixture was stirred at room temperature and 500 rpm for one day to prepare a 13 mass% PBI / DMAc solution (spinning solution). The spinning solution was filled into a syringe, and nanofibers were produced on aluminum foil using an electrospinning apparatus. A 27G (inner diameter 0.19 nm) stainless steel needle was used as the spinning nozzle, and spinning was performed under conditions of an applied voltage of 11 kV, a nozzle-collector distance of 10 cm, and a spinning solution supply flow rate of 1.3 L / min. A PBI nanofiber nonwoven sheet (apparent porosity 84%) with a thickness of 32 μm composed of PBI-NF with an average fiber diameter of 190 nm was obtained.

[0046] The obtained sheet was subjected to a heat press treatment for 30 minutes under conditions of 180 ° C. and 1 MPa, and further subjected to oxygen plasma treatment for 30 seconds under conditions of an oxygen flow rate of 100 mL / min and 20 W. A PBI-NF structure (apparent porosity 61%) having a thickness of 16 μm and consisting of PBI-NF with a diameter of 220 nm was obtained. The fiber diameter of the nanofiber was obtained from the secondary electron image obtained by observation at 15 kV using a scanning electron microscope (JCM-7000, manufactured by JEOL Ltd.) using the length measurement function of image analysis software (ImageJ, manufactured by the National Institutes of Health) as the average value of 100 points in the measurement sample. The measurement sample was platinum-coated using an autofine coater (JFC-1600, manufactured by JEOL Ltd.). The film thickness was measured at five or more points using a film thickness meter (ABSOLUTE T310111, manufactured by Mitutoyo Corporation), and the average value was used (hereinafter the same).

[0047] [4] Production of an electrolyte membrane for fuel cells [Example 1-1] 0.4 g of N-vinylimidazolium hydrogensulfate (A) obtained in Synthesis Example 1 was dissolved in 0.4 mL of deionized water, and 3-vinyl-1-[12-(3-vinylimidazolidin-1-ium-1-yl)dodecyl]imidazolium disulfate (B) obtained in Synthesis Example 2 (mass ratio (A):(B) = 5:1) and potassium persulfate (manufactured by Sigma-Aldrich, 1% by mass relative to N-vinylimidazolium hydrogensulfate (A)) were added to prepare a solution. The PBI-NF structure produced in Production Example 1 was placed on a silicon wafer, and the previously prepared solution was cast in a glove box (HO<0.1 ppm, O<0.1 ppm, UNIlab, manufactured by MBRAUN) filled with argon gas. A PTFE-processed PET sheet was placed on top and polymerized at 80°C for 12 hours while applying a load, to produce an electrolyte membrane with a PBI-NF structure content of 11% by mass. After polymerization, the wafer was removed from the glove box, and the resulting electrolyte membrane was peeled off from the PET sheet to obtain an electrolyte membrane.

[0048] Example 1-2 An electrolyte membrane was prepared in the same manner as in Example 1-1, except that the solution was cast so that the PBI-NF structure content was 17% by mass.

[0049] Example 1-3 An electrolyte membrane was prepared in the same manner as in Example 1-1, except that the solution was cast so that the PBI-NF structure content was 33% by mass.

[0050] Comparative Example 1 N-vinylimidazolium hydrogensulfate (A) obtained in Synthesis Example 1, 3-vinyl-1-[8-(3-vinylimidazolidin-1-ium-1-yl)octyl]imidazolium disulfate (B) obtained in Synthesis Example 2 (mass ratio (A):(B) = 5:1), and the initiator potassium persulfate (Sigma-Aldrich, 1% by mass relative to N-vinylimidazolium hydrogensulfate (A)) were dissolved in deionized water and stirred for 2 hours at room temperature. After stirring, the solution was filtered twice through absorbent cotton and then degassed under vacuum. The degassed solution was then spread on a glass slide and cast using a Teflon (registered trademark) scraper. The cast glass slide was placed in a glove box filled with argon gas and polymerized at 80°C for 12 hours. After polymerization, the glass slide was removed from the glove box to produce a polymer film that had absorbed moisture in the atmosphere. The resulting polymer membrane was peeled off from the slide glass to obtain an electrolyte membrane with a thickness of about 60 μm.

[0051] [4] Measurement of Proton Conductivity The proton conductivity of the electrolyte membranes obtained in Examples 1-1 to 1-3 and Comparative Example 1, as well as a commercially available Nafion membrane (manufactured by DuPont), was measured by the following method. The results are shown in Figure 1. [Proton Conductivity] As electrochemical impedance spectroscopy (EIS) measurement equipment, a Solartron 1255B frequency response analyzer and a Solartron SI 1287 potentiostat manufactured by Solartron were used, and an AC voltage of 5 mV amplitude was swept at frequencies from 1 MHz to 0.1 Hz to measure the impedance at each frequency. Specifically, a homemade bipolar cell was used, and a sample membrane (each electrolyte membrane, area 0.28 cm) sandwiched between platinum electrodes was measured. 2) were measured for resistance in the membrane direction at 20 to 150°C without external humidification. The sample membranes for recording data were stabilized for one hour under each test condition before measurement. Both the real and imaginary components of the impedance were measured, and proton conductivity was measured based on the assumption that the real axis intercept provides membrane resistance. Temperature dependence of ionic conductivity at 100°C or higher was measured by sweeping the temperature from a high temperature of 150°C to a low temperature using a Peltier module (FPH1-7106NC, Z-max) and PID control (TDU-5000AR) with a DC voltage source. σ = l / SR (where σ is S cm -1 where ℓ is the proton conductivity, ℓ is the membrane thickness (cm), and S is the active area (cm 2 ), R is the membrane resistance (Ω) obtained from EIS analysis.

[0052] As shown in Figure 1, the electrolyte membranes prepared in each Example and Comparative Example 1 exhibited a decrease in membrane resistance with increasing temperature, exhibiting the highest proton conductivity at 150°C, while the Nafion membrane exhibited an increase in membrane resistance with increasing temperature. The reason why the proton conductivity of the non-humidified Nafion membrane decreased with increasing temperature between 90 and 150°C is because the water trapped in the membrane dehydrated with increasing temperature, and its proton conduction mainly depended on the vehicle mechanism. Note that while the electrolyte membrane of Example 1-1 exhibited proton conductivity comparable to that of Comparative Example 1, the proton conductivity decreased in Examples 1-2 and 1-3 with increasing PBI nanofiber content. This is thought to be due to a decrease in the number of carriers contained in the membrane per unit volume due to the composite.

[0053] [5] Measurement of Mechanical Strength of Electrolyte Membrane The stress-strain curves of the electrolyte membranes obtained in Examples 1-1 to 1-3 and Comparative Example 1 were measured using a universal testing machine (STA-1150, manufactured by A&D Co., Ltd.). The size of the samples used in the measurement was 15 mm x 5 mm. A strain was applied to each sample at a rate of 1 mm / min at room temperature, and Young's modulus was evaluated from the stress-strain curve. The tensile strength was evaluated as the stress value at the maximum value of the curve. Table 1 shows Young's modulus, tensile strength, and elongation at break.

[0054]

[0055] As shown in Table 1, the electrolyte membrane prepared in Example 1 had both a Young's modulus and a breaking strength that were more than twice as high as those of the electrolyte membrane prepared in Comparative Example 1, confirming the sufficient reinforcing effect of the PBI nanofibers. It was also found that the breaking strength of the electrolyte membrane improved as the PBI nanofiber content increased.

Claims

1. An electrolyte membrane for a fuel cell comprising polybenzimidazole nanofibers and a polymer of a monomer selected from N-vinylimidazole, derivatives of N-vinylimidazole, and mixtures thereof.

2. The electrolyte membrane for a fuel cell according to claim 1, wherein the N-vinylimidazole derivative is at least one of those represented by the following formulas (1) and (2): (In formula (1), X - In formula (2), Z represents an alkylene group having 1 to 20 carbon atoms, which may have an oxygen atom between its carbon-carbon bonds; - each independently represents a monovalent anion, and n represents an integer of 1 to 20.

3. The above X - But HSO4 - 3. The electrolyte membrane for a fuel cell according to claim 2, wherein 4. The electrolyte membrane for a fuel cell according to claim 2, wherein Z is an alkylene group having 4 to 16 carbon atoms which may have an oxygen atom interposed between its carbon-carbon bonds.

5. The above Y - However, both are HSO4 - 3. The electrolyte membrane for a fuel cell according to claim 2, wherein 6. A method for producing an electrolyte membrane for a fuel cell, comprising impregnating a fiber structure made of polybenzimidazole nanofibers with a monomer selected from N-vinylimidazole, N-vinylimidazole derivatives, and mixtures thereof, and then polymerizing the monomer.

7. A method for producing an electrolyte membrane for a fuel cell according to claim 6, wherein a fiber structure made of polybenzimidazole nanofibers is heat-pressed at 80°C or higher and 0.5 MPa or higher, and then impregnated with the monomer.

8. The method for producing an electrolyte membrane for a fuel cell according to claim 7, wherein the fiber structure made of the polybenzimidazole nanofibers after the heat press treatment is further treated with oxygen plasma and then impregnated with the monomer.

9. The method for producing an electrolyte membrane for a fuel cell according to claim 6, wherein the fiber structure made of the polybenzimidazole nanofibers is produced by an electrospinning method.

10. The method for producing an electrolyte membrane for a fuel cell according to claim 9, wherein the fiber structure made of polybenzimidazole nanofibers is produced by electrospinning from a solution of polybenzimidazole in dimethylacetamide.

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