Electrolytic membrane for fuel cell

Abstract

To provide an electrolytic membrane for a fuel cell that has low environmental impact due to the absence of fluorine atoms and exhibits minimal degradation at high temperatures of 80°C or above.SOLUTION: An electrolytic membrane for a fuel cell includes polybenzimidazole nanofibers and a polymer of monomers selected from, N-vinylimidazole, N-vinylimidazole derivatives, and mixtures thereof.SELECTED DRAWING: None

Description

[Technical Field]

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

[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 being formed by sandwiching a membrane electrode assembly, which is made up of a solid polymer electrolyte membrane and an anode electrode and a cathode electrode disposed on either side of the membrane, 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 that uses acid-doped polybenzimidazole nanofibers as an electrolyte membrane that is compatible with the power generation conditions of next-generation fuel cells that can be operated at high temperatures (120 to 150°C). However, in the technology of Patent Document 2, since ether side chains are contained, the proportion of ion exchange groups is reduced, and there is room for improvement in terms of proton conductivity. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2017 / 141878 [Patent Document 2] Japanese Patent Publication No. 2020-181701 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made in consideration of these circumstances, and aims to provide an electrolyte membrane for fuel cells that does not contain fluorine atoms, thereby reducing the environmental impact, and that is less susceptible to deterioration at high temperatures of 80°C or higher. [Means for solving the problem]

[0008] As a result of extensive research to achieve the above object, the present inventors discovered that a membrane composed of polybenzimidazole nanofibers and a polymer of a monomer selected from N-vinylimidazole, N-vinylimidazole derivatives, and mixtures thereof, is less susceptible to deterioration at high temperatures of 80°C or higher, and thus completed the present invention.

[0009] That is, the present invention is 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. The electrolyte membrane for a fuel cell according to 1, wherein the N-vinylimidazole derivative is at least one of those represented by the following formula (1) and formula (2): [ka] (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; -represent independently a monovalent anion, and n represents an integer of 1 to 20. 3. The above X - But HSO4 - 2. An electrolyte membrane for a fuel cell, 4. The electrolyte membrane for a fuel cell according to 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 - 2. An electrolyte membrane for a fuel cell, 6. A method for producing an electrolyte membrane for a fuel cell, which comprises 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. The method for producing an electrolyte membrane for a fuel cell according to 6, wherein a fiber structure made of polybenzimidazole nanofibers is heat-pressed under conditions of 80°C or higher and 0.5 MPa or higher, and then the monomer is impregnated therein. 8. The method for producing an electrolyte membrane for a fuel cell according to 7, further treating the fiber structure made of the polybenzimidazole nanofibers after the heat press treatment with oxygen plasma, and then impregnating the fiber structure with the monomer. 9. The method for producing an electrolyte membrane for a fuel cell according to 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 9, wherein the fiber structure made of polybenzimidazole nanofibers is produced by electrospinning from a solution of polybenzimidazole in dimethylacetamide. to provide. [Effects of the Invention]

[0010] The electrolyte membrane for fuel cells of the present invention exhibits little degradation at high temperatures of 80°C or higher, and because all of the monomers used as raw materials for the membrane have an imidazolium salt group, which is a proton-accepting group, and there are a large number of proton-accepting groups in the molecule, the membrane exhibits proton conductivity superior to that of fluoropolymers such as Nafion under high temperature and non-humidified conditions. Furthermore, the electrolyte membrane of the present invention contains polybenzimidazole nanofibers, which are heat-resistant polymeric materials, as a reinforcing material, and therefore exhibits higher strength properties than those that do not contain this material. Furthermore, the electrolyte membrane of the present invention has a fluorine-free structure, which is advantageous in terms of cost compared to commonly used fluorine-based polymers, and also has a small environmental impact. [Brief explanation of the drawings]

[0011] [Figure 1] 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 for 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. DETAILED DESCRIPTION OF THE INVENTION

[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 (hereinafter sometimes simply referred to as "monomer") selected from N-vinylimidazole, derivatives of N-vinylimidazole, and mixtures thereof.

[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, 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 this is as described in the Examples below.

[0014] The PBI nanofibers used in the present invention can be any fibers 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 in which a charged polybenzimidazole solution is drawn in an electric field, and the solution is ruptured by the repulsive force of the electric charges, thereby forming ultrafine fibrous materials. The basic structure 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 tens of thousands of volts to the solution, and the other electrode (counter electrode) facing the first electrode. The solution ejected from one electrode turns into nanofibers through a high-speed jet in the electric field between the two opposing electrodes, and the subsequent bending and stretching of the jet, which then deposits on the surface of the other electrode, resulting in a nanofiber mat. In the present invention, a known ES apparatus can be used, and the spinning conditions in the ES apparatus, such as the distance between the nozzle tip and the counter electrode, the applied voltage, and the flow rate of the solution, may be appropriately set depending on the raw material used, the desired fiber diameter, etc.

[0016] The solvent used to prepare 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-vinylimidazolium 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 an N-vinylimidazolium base at both ends represented by the following formula (2) (hereinafter referred to as compound (2)). In producing the polymer, N-vinylimidazole, compound (1), and compound (2) may be used singly or in combination of two or more. In the present invention, however, it is preferable to use compound (1) and compound (2) in combination.

[0019] [ka]

[0020] In formula (1), X - represents a monovalent anion, and a specific example is BF4 - , PF6 - , AsF6 - , SbF6 - , AlCl4 - , NbF6 - , HSO4 - , ClO4 - , 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 or 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 groups, with linear alkylene groups being preferred.

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

[0023] [ka] (In the formula, Y - represent each independently 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, considering that the electrolyte is used in fuel cells and does not contain halogen, - are both HSO4 - is preferred.

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

[0026] In the present invention, the method for polymerizing the monomers 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. However, 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] In addition, various known polymerization initiators can also be used during the polymerization. Specific examples thereof 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 either individually or in combination. The amount of the radical polymerization initiator to be added is usually preferably 0.01 to 50% by mass based on 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 of two or more.

[0029] Both of the above compounds (1) and (2) are salts, and hydrogen sulfate, which is a particularly suitable salt, is soluble in water. Therefore, when these compounds are used as raw materials, it is preferable to polymerize them using water as a solvent, and it is also preferable to use a water-soluble persulfate or an azo compound 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 production method thereof, a preferred production method involves impregnating a fiber structure made of PBI nanofibers with the above-mentioned monomer and then polymerizing 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 treatment to oxygen plasma treatment, and then impregnate and polymerize the monomer. There are no particular limitations on the conditions for the oxygen plasma treatment. For example, the conditions are 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 of 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 placed on a substrate, but the latter method is preferred. Examples of solvents used in preparing the monomer solution or dispersion include the same solvents as those exemplified for the polymerization reaction above. However, 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 fiber structure is impregnated with the monomer, the structure is heated to the polymerization reaction temperature described above to polymerize the monomer. The heating time is the same as the reaction time during the polymerization.

[0036] Any casting method can be used, and various methods such as scraper coating, bar coating, brush coating, spraying, dipping, flow coating, roll coating, curtain coating, spin coating, and knife coating can be used. The electrolyte membrane formed on the substrate may be peeled off from the substrate before use.

[0037] There is no particular limitation on the thickness of the electrolyte membrane for fuel cells of the present invention, and it 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 electrolyte membrane for fuel cells 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. [Example]

[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) [ka]

[0041] After thoroughly cooling 60 mL of ion-exchanged water in an ice bath, 21.7 g of concentrated sulfuric acid (Kanto Chemical Co., Ltd.) was gradually added to the resulting aqueous sulfuric acid solution while stirring, taking care not to generate heat suddenly. Subsequently, a solution of 20.0 g of 1-vinylimidazole (Tokyo Chemical Industry Co., Ltd.) in 60 mL of ion-exchanged water was gradually added dropwise to the resulting aqueous sulfuric acid solution while stirring on ice, taking care not to generate heat suddenly. Stirring was then continued for several hours. Most of the ion-exchanged water was removed from this solution using an evaporator, and then the solution was evacuated for 5 hours using a vacuum pump. 43.2 g of the target product, N-vinylimidazolium hydrogen sulfate (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) [ka]

[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 Co., 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 (yield 85%). The resulting dibromo compound (4.5 g) 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. The final eluate (700 g) 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 evaporated to remove water, and then dehydrated using a vacuum pump. 20 mL of a 1:1 (volume ratio) mixture of ion-exchange water and methanol was added to the resulting jelly-like solid and stirred for several hours. After stirring, the insoluble matter was removed by filtration through a membrane filter, and the filtrate was evaporated to remove the solvent. The obtained solid was further evacuated by vacuum pumping, and 3.11 g of the target product, 3-vinyl-1-[12-(3-vinylimidazolidin-1-ium-1-yl)octyl]imidazolium disulfate (B), was obtained as a light brown solid (yield 65%).

[0044] [2] Synthesis of polybenzimidazole [Synthesis Example 3] 6.26 mg (29 mmol, Sigma-Aldrich, 98%) of 3,3'-diaminobenzidine, 4.88 g (29 mmol, Kanto Chemical, >99.0%) of isophthalic acid, and 216 g of polyphosphoric acid were placed in a two-neck flask and purged with argon. The mixture was then heated and stirred overnight in an oil bath at 150°C. The temperature was then raised 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 then neutralized in 1 L of a 30% by weight aqueous solution of sodium bicarbonate (Kanto Chemical, special grade) and heated and stirred at 50°C for 1 day. The mixture was then suction filtered using a large amount of water, thoroughly washed to remove residual salts, and vacuum dried at 130°C for 1 day to obtain 8.87 g of a brown solid. The chemical structure of the solid was characterized by 1H-NMR spectroscopy using a nuclear magnetic resonance spectrometer (NMR, JNM-ECZ 400S / L1, JEOL Ltd.). Comparison of the NMR spectrum with that of polybenzimidazole reported in the literature (Yang, J.; Aili, D.; Li, Q.; Xu, Y.; Liu, P.; Che, Q.; Jensen, J.O.; Bjerrum, N.J.; 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 made of PBI nanofibers (hereinafter referred to as PBI-NF structure) [Manufacturing Example 1] The PBI obtained in Synthesis Example 3 was added to N,N-dimethylacetamide (DMAc) and stirred at room temperature at 500 rpm for one day to prepare a 13% by 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 (0.19 nm inner diameter) stainless steel needle was used as the spinning nozzle. Spinning was performed under conditions of an applied voltage of 11 kV, a nozzle-to-collector distance of 10 cm, and a spinning solution supply flow rate of 1.3 L / min, yielding a PBI nanofiber nonwoven sheet (apparent porosity 84%) with a thickness of 32 μm and made of PBI-NF with an average fiber diameter of 190 nm.

[0046] The obtained sheet was heat-pressed at 180°C and 1 MPa for 30 minutes, and then subjected to oxygen plasma treatment at an oxygen flow rate of 100 mL / min and 20 W for 30 seconds to obtain a PBI-NF structure (apparent porosity 61%) with a thickness of 16 μm and consisting of PBI-NF with a diameter of 220 nm. The nanofiber diameter was determined as the average value of 100 points in the measurement sample from secondary electron images obtained by observation at 15 kV using a scanning electron microscope (JCM-7000, JEOL Ltd.) using the length measurement function of image analysis software (ImageJ, National Institutes of Health, USA). The measurement sample was platinum-coated using an autofine coater (JFC-1600, 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 (the same applies hereinafter).

[0047] [4] Manufacturing of electrolyte membranes for fuel cells [Example 1-1] A solution was prepared by dissolving 0.4 g of the N-vinylimidazolium hydrogensulfate (A) obtained in Synthesis Example 1 in 0.4 mL of deionized water, and further adding 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 the N-vinylimidazolium hydrogensulfate (A)) as an initiator. The PBI-NF structure prepared in Production Example 1 was placed on a silicon wafer, and the previously prepared solution was cast in a glove box filled with argon gas (HO<0.1 ppm, O<0.1 ppm, UNIlab, manufactured by MBRAUN). A PTFE-treated 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 the polymerization, the membrane was taken out of the glove box and peeled off from the PET sheet to obtain an electrolyte membrane.

[0048] [Example 1-2] An electrolyte membrane was produced 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] [Examples 1-3] An electrolyte membrane was produced in the same manner as in Example 1-1, except that the solution was cast so that the PBI-NF structure content was 33 mass %.

[0050] [Comparative Example 1] N-vinylimidazolium hydrogen sulfate (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 hydrogen sulfate (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 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 moisture-absorbed polymer film in the atmosphere. The resulting polymer membrane was peeled off from the slide glass to obtain an electrolyte membrane with a thickness of approximately 60 μm.

[0051] [4] Proton conductivity measurement 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 FIG. [Proton Conductivity] The electrochemical impedance spectroscopy (EIS) measurement equipment used was a Solartron 1255B frequency response analyzer and a Solartron SI 1287 potentiostat manufactured by Solartron. The impedance at each frequency was measured by sweeping an AC voltage of 5 mV amplitude from 1 MHz to 0.1 Hz. Specifically, a homemade bipolar cell was used, and a sample membrane (each electrolyte membrane, area 0.28 cm) was sandwiched between platinum electrodes. 2 The resistance in the membrane thickness direction was measured at 20 to 150°C without external humidification. The sample membranes from which data was recorded were allowed to stabilize for 1 hour under each test condition before measurement. Both the real and imaginary components of the impedance were measured, and the proton conductivity was calculated based on the following equation, assuming that the intercept on the real axis represents the membrane resistance. Temperature dependence of ionic conductivity above 100°C was measured by sweeping the temperature from a high of 150°C to a low of 150°C using a Peltier module (FPH1-7106NC, Z-max) and a PID control (TDU-5000AR) with a DC voltage source. σ=l / SR (where σ is S cm -1 where 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 FIG. 1, the electrolyte membranes prepared in each Example and Comparative Example 1 showed a decrease in membrane resistance with increasing temperature, and exhibited the highest proton conductivity at 150°C, whereas the membrane resistance of the Nafion membrane increased with increasing temperature. The reason why the proton conductivity of the non-humidified Nafion membrane decreases with increasing temperature between 90 and 150 °C is that the water trapped in the membrane dehydrates with increasing temperature, and its proton conduction mainly depends on the vehicle mechanism. The electrolyte membrane of Example 1-1 exhibited proton conductivity comparable to that of Comparative Example 1, whereas the proton conductivity decreased in Examples 1-2 and 1-3 as the PBI nanofiber content increased. 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 the mechanical strength of the 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 for the measurements was 15 mm x 5 mm. A strain was applied to each sample at room temperature at a rate of 1 mm / min, 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] [Table 1]

[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, a derivative of N-vinylimidazole, and a mixture thereof.

2. 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): 【Chemical 1】 (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 X - But HSO 4 - 3. The electrolyte membrane for a fuel cell according to claim 2, wherein

4. 3. The electrolyte membrane for a fuel cell in accordance with claim 2, wherein said 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 Y - However, both are HSOs. 4 - 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. 7. The 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. 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. 7. 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. 10. The method for producing an electrolyte membrane for a fuel cell according to claim 9, wherein the fiber structure made of the polybenzimidazole nanofibers is produced by electrospinning from a solution of polybenzimidazole in dimethylacetamide.

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