Binder for fuel cell

Abstract

To provide a binder for a fuel cell that has low environmental impact due to the absence of fluorine atoms and can be used under high-temperature conditions without humidification.SOLUTION: A binder for a fuel cell includes 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 a binder 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 comprising a membrane electrode assembly consisting 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. Furthermore, each of the anode and cathode electrodes has a catalyst layer on the electrolyte membrane side that is directly involved in the reaction.

[0004] The catalyst layer mainly comprises carbon as a catalyst support, platinum particles, and an ion-conductive polymer binder. As the ion-conducting polymer binder, conventionally, fluorine-based polymers such as Nafion (registered trademark, hereinafter the same) and Aquivion (registered trademark, hereinafter the same) have been widely used, similar to the materials that make up the electrolyte membrane. However, these polymers are materials that conduct protons upon hydration, and therefore are not suitable for fuel cells that aim to generate electricity at high temperatures without humidification.

[0005] In view of this, for example, Patent Document 1 discloses a vinylidene fluoride-hexafluoropropylene copolymer modified with a nitrogen-containing heterocyclic compound having two or more amino groups as a binder for fuel cells that has excellent proton conductivity and binding properties under high-temperature operating conditions, even in the absence or presence of low humidity. When the binder of Patent Document 1 is used, power generation is possible even at 150°C under unhumidified conditions. However, since the binder is a fluorine-based polymer like Nafion, there are problems in terms of environmental impact, safety, etc.

[0006] Furthermore, Non-Patent Documents 1 and 2 disclose a polymeric ionic liquid binder having a sulfo group and an imidazolium salt group in the side chain, but this binder does not show any improvement in the oxygen activation process at 80°C unless used in combination with Nafion. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-158372 [Non-patent literature]

[0008] [Non-Patent Document 1] ACS Energy Lett. 2020, 5 (6), 1726-1731. https: / / doi.org / 10.1021 / acsenergylett.0c00532. [Non-patent document 2] Macromolecules 2022, 55 (15), 6716-6729. https: / / doi.org / 10.1021 / acs.macromol.2c00468. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made in view of the above circumstances, and has as its object to provide a binder for fuel cells that does not contain fluorine atoms and therefore has a low environmental impact and can be used under non-humidified, high-temperature conditions. [Means for solving the problem]

[0010] As a result of extensive research to achieve the above object, the present inventors have found that a binder for fuel cells comprising a polymer of a monomer selected from N-vinylimidazole, a derivative of N-vinylimidazole, and a mixture thereof can be used under non-humidified, high-temperature conditions, and have completed the present invention.

[0011] That is, the present invention is 1. A binder for a fuel cell comprising a polymer of a monomer selected from N-vinylimidazole, a derivative of N-vinylimidazole, and a mixture thereof; 2. The binder for a fuel cell according to claim 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. Fuel cell binder, 4. The binder 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. Fuel cell binder, 6. A binder for a fuel cell according to any one of 1 to 5, which is used for a catalyst ink for a fuel cell. 7. A catalyst ink for a fuel cell comprising catalyst-supported carbon particles, a solvent, and the fuel cell binder of 6. 8. Fuel cell catalyst layer obtained from catalyst ink of 7 to provide. [Effects of the Invention]

[0012] The fuel cell binder of the present invention exhibits minimal degradation at high temperatures of 80°C or higher, and because all of the raw material monomers 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 binder exhibits proton conductivity superior to that of fluoropolymers such as Nafion under high temperature and non-humidified conditions. Furthermore, the binder for fuel cells of the present invention has a structure that does not contain fluorine, which is advantageous in terms of cost compared to commonly used fluorine-based polymers, and also has a small environmental impact. The fuel cell binder of the present invention having these properties is particularly suitable as a binder for forming a catalyst layer of a fuel cell, and a fuel cell produced using the binder can generate electricity even at high temperatures (120 to 150°C) and under unhumidified conditions. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 10 is a diagram showing the results of a constant voltage-current test in Example 3-1. [Figure 2] FIG. 1 shows the results of EIS measurements during a constant current test in Example 3-1. The upper left shows a Nyquist plot, the lower left shows an enlarged view of the high frequency region of the Nyquist plot, and the upper right and lower right show Bode plots. [Figure 3] FIG. 10 is a diagram showing the results of a constant voltage-current test in Comparative Example 2-1. [Figure 4] FIG. 1 shows the results of EIS measurements during a constant current test in Comparative Example 2-1. The upper left shows a Nyquist plot, the lower left shows an enlarged view of the high frequency region of the Nyquist plot, and the upper right and lower right show Bode plots. [Figure 5] FIG. 10 is a graph showing the change in cell voltage over time with temperature in Example 3-2 under non-humidified conditions. [Figure 6] FIG. 10 is a graph showing the change in cell voltage over time with temperature in Comparative Example 2-2 under non-humidified conditions. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in more detail below. The binder for fuel cells according to the present invention is characterized by comprising a polymer of a monomer (hereinafter sometimes simply referred to as "monomer") selected from N-vinylimidazole, a derivative of N-vinylimidazole, and a mixture thereof.

[0015] 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.

[0016] [ka]

[0017] 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 a binder for fuel cells and does not contain halogens, HSO4 - is preferred.

[0018] 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.

[0019] Therefore, the compound (2) is more preferably one represented by the following formula (2-1).

[0020] [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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Generally, a solid polymer fuel cell is composed of a large number of unit cells arranged side by side, each unit cell comprising a pair of electrodes (membrane electrode assembly) sandwiching a solid polymer membrane and a pair of separators sandwiching these electrodes to form a gas supply / discharge flow path. The fuel cell binder of the present invention can be suitably used as a binder for forming layers constituting the membrane electrode assembly, particularly as a binder used in a catalyst layer. When the binder of the present invention is used in a catalyst, it may be used as a catalyst ink containing commonly used catalyst-supporting carbon particles, a solvent, and the binder of the present invention and the solvent.

[0029] The catalyst can be appropriately selected from catalysts that have been used in the catalyst layers of conventional fuel cells. Specific examples include metal catalysts such as transition metals such as platinum, gold, silver, palladium, iridium, rhodium, ruthenium, iron, cobalt, nickel, chromium, tungsten, manganese, and osmium, and alloys thereof, and among these, platinum and platinum alloys are preferred. Examples of carbon include carbon black. The amount of catalyst supported is not particularly limited, but is generally 10 to 70 mass % in terms of metal content.

[0030] Catalyst-supported carbon is commercially available, and specific examples thereof include platinum-supported carbon particles such as UNPC40-II (manufactured by Ishifuku Kinzoku Kogyo Co., Ltd., Pt 39% by mass supported carbon), TEC10E50E (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., Pt 50% by mass supported carbon), and TEC10V40E (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., Pt 40% by mass supported carbon).

[0031] Specific examples of the solvent include water; alcohol solvents such as methanol and ethanol; amide solvents such as N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide; ester solvents such as ethyl acetate; and ketone solvents such as acetone and ethyl methyl ketone, with water being preferred.

[0032] The solid content in the catalyst ink is not particularly limited, but in consideration of dispersion stability, workability, etc., it is preferably 1 to 40 mass %, more preferably 4 to 30 mass %, and even more preferably 5 to 25 mass %. Furthermore, in consideration of the stability of the composition, the binder / catalyst-supported carbon ratio (mass ratio) in the catalyst ink is preferably 0.1 to 1.5, more preferably 0.2 to 1.2, and even more preferably 0.3 to 1.0.

[0033] The order of mixing the components when preparing the catalyst ink is arbitrary, but a suitable method is to dissolve or disperse the binder in a solvent and then add the catalyst-supported carbon. There is no particular limitation on the thickness of the catalyst layer, and it can be, for example, about 1 to 500 μm, preferably 1 to 100 μm, and more preferably 1 to 20 μm.

[0034] In producing a membrane electrode assembly, a catalyst layer using the binder of the present invention may be formed on a gas diffusion layer or on a polymer electrolyte membrane. Examples of the gas diffusion layer include conductive and porous sheet-like materials such as carbon particle aggregates, carbon fiber fabric, carbon paper, carbon felt, and carbon nonwoven fabric.

[0035] The polymer electrolyte membrane may be appropriately selected from those conventionally used in solid polymer fuel cells, and specific examples thereof include perfluorosulfonic acid membranes such as Nafion (registered trademark) (Fujifilm Wako Pure Chemical Industries, Ltd., DE2020 CS), Aquivion (registered trademark) (Sigma-Aldrich, D72-25BS), and Flemion (registered trademark) (Asahi Glass Co., Ltd.); fluorine-based polymer electrolyte membranes such as ethylene-tetrafluoroethylene copolymer resin membranes and resin membranes with trifluorostyrene as the base polymer; and hydrocarbon-based resin membranes having sulfonic acid groups.

[0036] In addition, when considering application to a fuel cell used under non-humidified, high-temperature conditions, it is preferable that the electrolyte membrane is one that is less susceptible to deterioration under non-humidified, high-temperature conditions and that can maintain power generation performance. As such an electrolyte membrane, for example, a gel electrolyte membrane containing a polymer obtained by polymerizing an imidazolium salt having an alkenyl group and a compound having an imidazolium salt having an alkenyl group at both ends, as disclosed in JP 2022-090254 A, silica nanofibers, and a liquid, in which the polymer is swollen by the liquid, can be suitably used.

[0037] Also, an electrolyte membrane for a fuel cell containing polybenzimidazole nanofibers and a polymer of a monomer selected from N-vinylimidazole, a derivative of N-vinylimidazole, and a mixture thereof can be suitably used. In this case, the fiber diameter of the polybenzimidazole nanofibers (hereinafter referred to as "PBI nanofibers") may be in the nano range (less than 1000 nm), preferably 1 to 800 nm, and more preferably 10 to 500 nm. The PBI nanofibers 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.

[0038] 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.

[0039] 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.

[0040] The N-vinylimidazole, N-vinylimidazole derivatives and mixtures thereof may be the same as those exemplified above as monomers used in the binder polymer of the present invention, and preferred examples thereof are also as described above. The method for polymerizing the monomer is the same as above.

[0041] Although there are no particular limitations on the method for producing an electrolyte membrane containing PBI nanofibers and a polymer, a preferred 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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%.

[0048] There is no particular limitation on the thickness of either electrolyte membrane, and it can be, for example, about 1 to 300 μm, preferably 5 to 100 μm.

[0049] There are no particular limitations on the method for applying the catalyst ink to the gas diffusion layer or electrolyte membrane, and any of the various application methods exemplified above for the casting method may be used, or spray application may be used. After application, the coating may be heated and dried as necessary to form a catalyst layer. [Example]

[0050] 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.

[0051] [1] Synthesis of raw material monomers [Synthesis Example 1] Synthesis of N-vinylimidazolium hydrogen sulfate (A) [ka]

[0052] 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).

[0053] [Synthesis Example 2] Synthesis of 3-vinyl-1-[12-(3-vinylimidazolidin-1-ium-1-yl)dodecyl]imidazolium disulfate (B) [ka]

[0054] 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%).

[0055] [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 obtained solid was analyzed using a nuclear magnetic resonance spectrometer (NMR, JNM-ECZ 400S / L1, manufactured by JEOL Ltd.). 1The 1H-NMR spectra were evaluated, and quantitative comparison with the NMR spectra of polybenzimidazoles 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 (hereafter referred to as PBI) was obtained. The NMR solvent used was TMS-containing d-DMSO, and chemical shifts were expressed in ppm relative to the TMS signal (0.00).

[0056] [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.

[0057] 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).

[0058] [4] Manufacturing of electrolyte membranes for fuel cells [Manufacturing Example 2-1] 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 (thickness 30 μm) with a PBI-NF structure content of 17% 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.

[0059] [Production Example 2-2] An electrolyte membrane was produced in the same manner as in Production Example 2-1, except that the solution was cast so that the PBI-NF structure content was 9% by mass.

[0060] [Manufacturing Example 2-3] An electrolyte membrane (thickness: 50 μm) was produced in the same manner as in Production Example 2-1, except that the solution was cast so that the PBI-NF structure content was 11% by mass.

[0061] [Manufacturing Example 2-4] An electrolyte membrane (thickness: 16 μm) was produced in the same manner as in Production Example 2-1, except that the solution was cast so that the PBI-NF structure content was 13 mass %.

[0062] [5] Manufacturing of binder polymers for fuel cells [Example 1-1] 5 g of the N-vinylimidazolium hydrogen sulfate (A) obtained in Synthesis Example 1 and potassium persulfate (Sigma-Aldrich, 1% by mass based on the N-vinylimidazolium hydrogen sulfate (A)) as an initiator were dissolved in 15 g of deionized water and stirred for 2 hours at 80° C. After stirring, the solution was filtered twice through absorbent cotton and then vacuum degassed to obtain an aqueous polymer dispersion with a solids content of approximately 25% by mass.

[0063] [6] Preparation of catalyst ink [Example 2-1] 0.554 g of water was added to 0.109 g of the aqueous polymer dispersion obtained in Example 1-1 and stirred, and then 0.1 g of Pt / C (TEC10E50E, manufactured by Tanaka Kikinzoku K.K.; the same applies hereinafter) was added and stirred at room temperature for 12 hours to prepare a catalyst ink (for coating) with a polymer / carbon ratio of 0.5 (mass ratio) and a solid content of 20 mass %.

[0064] [Example 2-2] To the catalyst ink prepared in Example 2-1, 1.91 g of water was added immediately before use to prepare a catalyst ink (for spray use) with a solid content of 5 mass %.

[0065] [Comparative Example 1-1] 0.217 g of 25 mass% Aquivion aqueous dispersion (Sigma-Aldrich, D72-25BS) was added to 0.1 g of Pt / C and stirred, after which 0.609 g of a butyl acetate / EtOH = 3 / 1 (w / w) mixed solvent was added dropwise and stirred at room temperature for 12 hours to produce a catalyst ink with a polymer / carbon ratio of 1.0 (mass ratio) and a solids content of 20 mass%.

[0066] [7] Preparation of catalyst layer [Example 3-1] The catalyst ink prepared in Example 2-1 was applied to the electrolyte membrane prepared in Production Example 2-1 using a doctor blade, and dried at room temperature for 12 hours to form a catalyst layer.

[0067] [Example 3-2] The catalyst ink prepared in Example 2-2 was spray-coated onto the electrolyte membrane prepared in Production Example 2-2 using an airbrush, and dried at room temperature for 12 hours to form a catalyst layer.

[0068] [Comparative Example 2-1] A catalyst layer was formed in the same manner as in Example 3-1, except that the catalyst ink prepared in Comparative Example 1-1 and the electrolyte membrane produced in Production Example 2-3 were used.

[0069] [Comparative Example 2-2] A catalyst layer was formed in the same manner as in Example 3-1, except that the catalyst ink prepared in Comparative Example 1-1 and the electrolyte membrane produced in Production Example 2-2 were used.

[0070] (1) Fuel cell single cell test The power generation performance of the laminates of the electrolyte membrane and catalyst layer produced in Example 3-1 and Comparative Example 2-1 was measured by the following method. A gas diffusion layer (SGL Carbon Japan Co., Ltd., 22BB) with a microporous layer cut into 1 cm squares was laminated on the prepared catalyst layer, and the layer was tightly fastened to a measurement cell (FC-004, Chemix Co., Ltd.) using a 100 μm PTFE gasket to form a single cell. The power generation test was performed using an AutoPEM (Toyo Corporation). The humidifier temperatures were 80°C on the anode side and 75°C on the cathode side, and pure hydrogen and pure oxygen were supplied at 200 mL min with no back pressure at a humidity of 100% RH on the anode side and 81% RH on the cathode side. -1The fuel was blown through the air. A constant voltage-current test was performed using an electronic load (890e, Scribner). For EIS measurements, a current amplitude of 10% of the measurement voltage was applied, and the frequency was swept from 10 kHz to 0.1 Hz to measure the impedance. The measurement results are shown in Figures 1 to 4.

[0071] As shown in Figure 1, both the current and voltage are unstable up to 0.5V, but from 0.5V onwards, the voltage and current values ​​can be measured relatively stably. As shown in Figure 2, the minimum imaginary value between 1 MHz and 10 kHz was an average of 0.12 Ω, and the proton conductivity of the electrolyte membrane was 26 mS / cm. On the other hand, as shown in FIG. 3, when a current of 0.2 A or more is applied, the voltage value becomes unstable. As shown in FIG. 4, the average value of the minimum imaginary part of the impedance at around 10 kHz, which indicates the resistance of the electrolyte membrane, was 0.18 Ω at each current, and the proton conductivity was 12 mS / cm. As described above, it is clear that the proton conductivity of the electrolyte membrane of Example 3-1 is improved compared to that of Comparative Example 2-1.

[0072] (2) Fuel cell performance evaluation at temperatures above 100°C without humidification The single cells described in (1) above were used for the laminates of electrolyte membranes and catalyst layers prepared in Example 3-2 and Comparative Example 2-2. The power generation performance was measured in the same manner as in (1) above, except that the cathode and anode temperatures were set to 120, 130, 140, and 150°C under non-humidified conditions. All measurements were performed without aging. Figures 5 and 6 show the time-dependent changes in cell voltage with temperature for Example 3-2 and Comparative Example 2-2 under non-humidified conditions, and Table 1 summarizes the OCV values ​​and 1 kHz high-frequency resistance values ​​obtained from Figures 5 and 6.

[0073] [Table 1]

[0074] As shown in Table 1, the OCV of Example 3-2 was stable, rising to 0.92 V at 120° C. and 0.96 V at 150° C. On the other hand, the OCV of Comparative Example 2-2 decreased from 0.68 V at 120° C. to 0.63 V at 150° C. By using the binder of the present invention, a sufficiently high OCV was obtained even in the high temperature range, demonstrating that by applying a catalyst layer using the binder of the present invention to an electrolyte membrane that can generate electricity even under non-humidified, high-temperature conditions, it can function even under non-humidified, high-temperature conditions.

Claims

1. A binder for a fuel cell comprising a polymer of a monomer selected from N-vinylimidazole, a derivative of N-vinylimidazole, and a mixture thereof.

2. 2. The binder for fuel cells 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 binder for fuel cells according to claim 2, wherein

4. 3. The binder for fuel cells 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 Y - However, both are HSOs. 4 - 3. The binder for fuel cells according to claim 2, wherein

6. 6. The binder for fuel cells according to claim 1, which is used for catalyst ink for fuel cells.

7. A catalyst ink for a fuel cell, comprising catalyst-supporting carbon particles, a solvent, and the binder for a fuel cell according to claim 6.

8. A catalyst layer for a fuel cell obtained from the catalyst ink according to claim 7.

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