JP2009206019A - Production method of polymer electrolyte membrane for fuel cell, electrolyte membrane thereof, and membrane electrode assembly for fuel cell using the membrane - Google Patents

Abstract

An object of the present invention is to provide a high proton conductivity at a low graft rate while suppressing a decrease in intrinsic properties of a polymer film substrate, and to suppress dimensional change, mechanical strength change, and permeation of methanol fuel. An object of the present invention is to provide a polymer electrolyte membrane for fuel cells having good heat resistance and oxidation resistance and a method for producing the same.
A polymer film substrate is irradiated with ultraviolet rays in a state in which a vinyl monomer capable of introducing a sulfonic acid group and a monomer solution containing water and a solvent containing a ketone solvent are brought into contact with the polymer film substrate to thereby remove the vinyl monomer. In the liquid phase system, it is formed by photografting polymerization from the surface of the polymer film substrate, and extending the graft chain in the internal direction from the substrate surface and the substrate surface. And a step of introducing a sulfonic acid group into a portion of the graft chain into which a sulfonic acid group can be introduced. The method for producing a polymer electrolyte membrane for a fuel cell is used.
As a result, radicals can be generated only on the substrate surface, vinyl monomers can be graft polymerized inward from the substrate surface, and sulfonic acid groups can be introduced. A polymer electrolyte membrane for a fuel cell having good physical properties while maintaining the physical properties can be provided.
[Selection figure] None

Description

  The present invention relates to a method for producing a polymer electrolyte membrane suitable for a fuel cell having excellent proton conductivity, dimensional stability, mechanical properties, and excellent oxidation resistance and heat resistance, a polymer electrolyte membrane, and the membrane The present invention relates to a fuel cell membrane electrode assembly.

  A fuel cell using a polymer electrolyte membrane is expected to be used as a power source for portable devices, home cogeneration, automobiles, and a simple auxiliary power source using fuel such as methanol and hydrogen because of its high energy density. In this fuel cell, the development of a polymer electrolyte membrane having excellent characteristics is one of the most important technologies.

  In a polymer electrolyte membrane fuel cell, the electrolyte membrane also has a role of conducting protons and a membrane that prevents hydrogen or methanol as a fuel and air (oxygen) as an oxidant from being directly mixed. . As an electrolyte membrane, since it conducts current for a long time, it has excellent chemical stability, especially resistance to hydroxyl radicals (oxidation resistance), which is the main cause of membrane degradation, and the operating temperature of the battery. It is required that the film has heat resistance at 80 ° C. or higher, and that the water retention of the membrane is constant in order to maintain high proton conductivity. On the other hand, due to its role as a diaphragm, the membrane has excellent mechanical strength, in particular, excellent elongation and dimensional stability, and excessive permeability to hydrogen gas, methanol or oxygen gas. It is required not to. If the elongation percentage of the membrane is low, the membrane is damaged when the membrane electrode assembly is molded.

  In early polymer electrolyte membrane fuel cells, a hydrocarbon polymer electrolyte membrane produced by copolymerization of styrene, which is a monomer capable of retaining a sulfonic acid group, and divinylbenzene, which is known as a chemical crosslinking agent, was used. However, this electrolyte membrane has poor durability due to oxidation resistance, so it is not practical. After that, a perfluorosulfonic acid membrane “Nafion (DuPont registered trademark)” developed by DuPont is generally used. Has been.

  However, although conventional fluorine-containing polymer electrolyte membranes such as Nafion are excellent in chemical durability and stability, the proton conductivity in the film thickness direction shows a value of around 0.06 S / cm. Insufficient water retention results in drying of the ion exchange membrane, resulting in a decrease in proton conductivity, and when methanol is used as fuel, membrane swelling and methanol crossover may occur. Furthermore, fluorine-containing polymer electrolyte membranes such as Nafion (registered trademark) are difficult and complicated to synthesize monomers, and the process of polymerizing them to produce polymer membranes is also complicated, so the product is very It is expensive and has become a major obstacle when a proton exchange membrane fuel cell is mounted on an automobile and put to practical use. Therefore, efforts have been made to develop a low-cost and high-performance electrolyte membrane that replaces the Nafion (registered trademark) and the like.

On the other hand, a monomer capable of introducing a sulfonic acid group into a polymer film in a high energy radiation pre-irradiation / post-graft polymerization method or a high energy radiation simultaneous graft polymerization method using γ rays and electron beams from a ⁶⁰ Co ray source Attempts have been made to fabricate solid polymer electrolyte membranes by grafting the polymer. The present inventors have repeatedly studied to develop these new solid polymer electrolyte membranes, and radiated styrene monomer on a polytetrafluoroethylene film imparted with a crosslinked structure by irradiation with radiation in a high temperature / inert gas atmosphere. A solid polymer electrolyte membrane and a method for producing the same have been filed (for example, patents) characterized in that the ion exchange capacity can be controlled in a wide range together with the property of being difficult to swell in methanol fuel by introducing by graft reaction and then sulfonating. Reference 1). However, since the polymer electrolyte membrane is irradiated with high-energy radiation in two stages for the purpose of imparting a crosslinked structure and introducing a graft chain, the mechanical strength of the membrane due to decomposition is lowered.

  On the other hand, polymer film base materials made of hydrocarbons, hydrocarbons / fluorinated carbons, and fluorocarbons are irradiated uniformly into the base material in the process of high-energy radiation irradiation. Due to the difference in chemical structure, irradiation atmosphere, temperature, pressure, etc., it was confirmed that not only the generation of initiation points (radicals) necessary for graft polymerization and the addition of a crosslinked structure but also decomposition occurred. It has been found that the inherent characteristic deterioration (mechanical strength, heat resistance, durability, etc.) is caused (see, for example, Non-Patent Document 1).

  In addition, in order to produce a polymer electrolyte membrane for a fuel cell having oxidation resistance and electrical conductivity without deteriorating the inherent properties of the polymer film substrate, it was formed on the polymer film substrate. The surface of the undercoat layer containing the photopolymerization initiator is grafted by irradiating with ultraviolet rays in a state where the vinyl monomer-containing mixed liquid capable of holding a sulfonic acid group is in contact with the surface, and then the grafted molecular chains and base material molecules Attempts have been made to produce polymer electrolyte membranes in which chains, grafted molecular chains and base molecular chains are multiple crosslinked by radiation and sulfonic acid groups are introduced (see, for example, Patent Document 2). In general, a photosensitizer is soluble in an organic solvent but insoluble in water. Therefore, the polymer film substrate is immersed in a solvent in which the photosensitizer is dissolved. It is necessary to apply the agent in advance. In this method, a polymer film base material is immersed in a solvent containing a photopolymerization initiator to form an undercoat layer containing the photopolymerization initiator on the polymer film base material.

  However, in this method, when a specific solvent containing a vinyl monomer (for example, water + acetone solvent) is used for light irradiation, the photosensitizer applied at a corner is dissolved in acetone, resulting in photosensitization. There is a problem in that the sensitizer is photoexcited to reduce the function of generating radicals on the substrate.

As a solution to this problem, an attempt has been made to coat and protect the surface of the applied photosensitizer with polyvinyl alcohol or the like. However, the applied photosensitizer and the polymer film substrate are solids, and it is difficult for the solids to generate radicals at the interface. is there.
JP 2001-348439 A JP 2007-258048 A J. Chen et al., J. Membrane Science, 277, 249 (2006) Y. Kimura et al., Nuclear Instruments and Methods in Physics Research B263, 463 (2007)

  In view of the above-mentioned problems, the present invention provides high proton conductivity at a low graft rate while suppressing deterioration of the inherent properties of a polymer film substrate, and has excellent dimensional stability and mechanical properties. Furthermore, it aims at providing the polymer electrolyte membrane for fuel cells with favorable heat resistance and oxidation resistance, and its manufacturing method.

  Another object of the present invention is to provide a membrane electrode assembly for a fuel cell in which the polymer electrolyte membrane is tightly bonded to a membrane electrode.

  Since the present invention has established a technique for extending a graft chain capable of introducing a sulfonic acid group from the membrane surface to the inside, it has high conductivity suitable for use in a fuel cell, excellent dimensional stability, excellent machine The present invention provides a polymer electrolyte membrane having a good characteristic, a low crossover of methanol fuel, and the like, and a method for producing the same.

  That is, a method for producing a polymer electrolyte membrane for a fuel cell according to an aspect of the present invention includes a monomer solution comprising a polymer film substrate containing a vinyl monomer capable of introducing a sulfonic acid group and a solvent containing water and a ketone solvent. UV grafting in a state of contact with the polymer, photo-grafting polymerization of the vinyl monomer from the surface of the polymer film substrate in the liquid phase system and extending the graft chain inward from the substrate surface and the substrate surface And a step of introducing a sulfonic acid group into a portion (for example, an aromatic ring) into which a sulfonic acid group can be introduced in the graft chain formed by the photograft polymerization step. The monomer solution may further contain a polyfunctional vinyl monomer. In the photograft polymerization step, the polymer film substrate is irradiated with ultraviolet rays in contact with the vapor of the monomer solution, and the vinyl monomer is photografted from the polymer film substrate surface in a gas phase system. Thus, the graft chain can be extended inward from the substrate surface and the substrate surface.

  Moreover, the manufacturing method of the polymer electrolyte membrane for fuel cells by other one aspect | mode of this invention is made into the range within 2 micrometers with respect to the depth direction on both surfaces of a polymer film base material through a polymer filter as one side. A polymer film substrate in a liquid phase system by contacting an electron beam irradiation process for performing an electron beam irradiation treatment of 5 to 30 kGy and a monomer solution containing a vinyl monomer capable of introducing a sulfonic acid group and an organic solvent. A graft polymerization step in which graft polymerization is performed from the surface and the graft chain is extended inward from the substrate surface and the substrate surface, and a sulfonic acid group-introducible portion in the graft chain formed by the graft polymerization step (for example, aromatic And a step of introducing a sulfonic acid group into the ring). The monomer solution may further contain a polyfunctional vinyl monomer. In the graft polymerization step, graft polymerization is performed in a state where the vapor of the monomer solution is in contact with the polymer film substrate, and the vinyl monomer is graft-polymerized from the surface of the polymer film substrate in the gas phase system, thereby grafting. Chains can also be extended inward from the substrate surface and substrate surface. In any one of the above-described methods for producing a polymer electrolyte membrane for a fuel cell, the polymer base material may be made of an olefin polymer material or a fluorine polymer material.

  Furthermore, the polymer electrolyte membrane for a fuel cell according to yet another aspect of the present invention is a polymer electrolyte membrane produced by any one of the above methods, wherein the graft chain is formed from the surface layer of the polymer film substrate. Furthermore, the graft ratio is 5% or more, the proton conductivity is 0.03 S / cm or more in the film surface direction, and the proton conductivity is 0.06 S in the film thickness direction. / Cm or more. In addition, this polymer electrolyte membrane can control each characteristic such as proton conductivity within an appropriate and wide range, and a single sulfonic acid group-introduced (retained) graft chain penetrates the cross section of the membrane. High proton conductivity, high mechanical properties, excellent dimensional stability, unique substrate characteristics, excellent methanol fuel permeation suppression, and oxidation resistance -It can be provided with at least one, preferably all, of excellent heat resistance.

A membrane electrode assembly for a fuel cell according to still another aspect of the present invention is characterized in that the polymer electrolyte membrane produced by any one of the above methods is closely bonded to the membrane electrode. This membrane / electrode assembly is composed of a polymer electrolyte membrane and a gas diffusion electrode having a pair of catalyst layers bonded to both sides thereof. The fuel cell is operated by supplying methanol or hydrogen as fuel to the gas diffusion electrode on the anode side and oxygen or air as oxidant to the other gas diffusion electrode on the cathode side. It consists of a system that connects circuits. At this time, protons (H ⁺ ) generated on the anode side move to the cathode side through the electrolyte membrane, and react with oxygen on the cathode side to generate water. What is important here is that the polymer electrolyte membrane functions as a proton transfer medium and functions as a diaphragm for fuel gas and oxygen gas.

  According to the polymer electrolyte membrane for a fuel cell and the method for producing the same of the present invention, since the reaction technology for extending the graft chain only from the surface of the polymer film substrate can be established, the inherent performance of the substrate can be achieved. While maintaining high proton conductivity at a low graft ratio, it has excellent dimensional stability and mechanical properties, suppresses methanol fuel permeation, and has excellent heat resistance and oxidation resistance, and is a polymer for fuel cells An electrolyte membrane can be provided.

  Further, according to the membrane electrode assembly for a fuel cell of the present invention, the polymer electrolyte membrane produced by the above production method is tightly joined to the membrane electrode, so that high proton conductivity is provided at a low graft rate. Thus, an electrode assembly for a fuel cell having excellent dimensional stability and mechanical properties, suppressing methanol fuel permeation, and having good heat resistance and oxidation resistance can be obtained.

  The method for producing a polymer electrolyte membrane for a fuel cell according to the present invention generates vinyl only on the surface layer (in the vicinity of the surface) of a polymer base material, and has a vinyl (system) having a portion capable of introducing (holding) a sulfonic acid group ) The monomer is graft-polymerized so as to extend not only on the surface of the polymer base material but also from the surface of the base material, and a sulfonic acid group is introduced into a portion where the sulfonic acid group can be introduced. That is, a method for producing a polymer electrolyte membrane for a fuel cell according to an embodiment of the present invention includes a monomer comprising a polymer film substrate, a vinyl monomer capable of introducing a sulfonic acid group, and a solvent containing water and a ketone solvent. Light that irradiates UV light in contact with the solution, photopolymerizes the vinyl monomer from the surface of the polymer film substrate in the liquid phase system, and extends the graft chain inward from the substrate surface and substrate surface. A graft polymerization step, and a step of introducing a sulfonic acid group into a portion (for example, an aromatic ring) into which a sulfonic acid group can be introduced in the graft chain formed by the photograft polymerization step. In addition, the method for producing a polymer electrolyte membrane for a fuel cell according to another embodiment of the present invention has a single-sided surface within a range of 2 μm with respect to the depth direction on both sides of the polymer film substrate via a polymer filter. A polymer film substrate in a liquid phase system by contacting an electron beam irradiation process for performing an electron beam irradiation treatment of 5 to 30 kGy and a monomer solution containing a vinyl monomer capable of introducing a sulfonic acid group and an organic solvent. A graft polymerization step in which graft polymerization is performed from the surface and the graft chain is extended inward from the substrate surface and the substrate surface, and a sulfonic acid group-introducible portion in the graft chain formed by the graft polymerization step (for example, aromatic And a step of introducing a sulfonic acid group into the ring). This will be described in detail below.

As used herein, the term “polymer film substrate” refers to a polymer material having a film-like form to be a substrate of an electrolyte membrane. For example, a film composed of polytetrafluoroethylene, polyvinylidene fluoride, ethylene / tetrafluoroethylene copolymer, polyethylene, polypropylene, polyethylene terephthalate, polyimide, acrylonitrile / butadiene / styrene copolymer, or the like can be used. Of these, base materials made of olefin polymer materials or fluorine polymer materials are made possible by imparting properties such as durability, cold resistance, heat resistance, and chemical resistance by making full use of the company's know-how technology. It is preferable because it is used in many fields as an engineer plastic. The present invention is particularly effective when the polymer film substrate is a fluorine-based material.
The thickness of the polymer film substrate is preferably 10 to 100 μm from the viewpoint of maintaining the mechanical strength of the electrolyte membrane.

In the present invention, the ketone solvent having a molecular structure that absorbs ultraviolet rays is a solvent having a ketone structure in the molecule, and is acetone, methyl ethyl ketone, 2-pentanone, 3-pentanone, 2-hexanone, 2-heptanone. 4-heptanone, methyl isopropyl ketone, diisobutyl ketone, acetonyl acetone, cyclohexanone, and the like can be used.
Among these, methyl ethyl ketone and acetone are preferable because they have a high radical generating ability with respect to a wide variety of substrates.

  The ketone solvent can be used as a solvent for the polymer solution together with water. The mixing ratio (volume ratio) of the ketone solvent and water can be determined as appropriate, and is, for example, ketone solvent: water = 1: 1 to 30, preferably 1: 3 to 10.

  Furthermore, any water-soluble organic solvent exemplified below can be mixed and used together with the ketone solvent and water. Examples of these organic solvents include alcohols such as methanol, ethanol, n-propanol, 2-propanol, n-butanol and 2-butanol, tetrahydrofuran, N, N-dimethylformamide, 1,4-dioxane, acetonitrile and the like. Can be mentioned.

  The amount of these organic solvents to be further mixed can be determined as appropriate. For example, 5 to 80% by volume, preferably 15 to 50% by volume, can be added on the basis of 100% by volume of the total amount of the ketone solvent and water.

  In the present invention, examples of a vinyl monomer that is graft-polymerized to a polymer film substrate with light from ultraviolet rays or an electron beam from a low energy accelerator, that is, a vinyl monomer having a moiety capable of introducing a sulfone group are shown below. As such a vinyl monomer, one type selected from the group consisting of a monofunctional vinyl monomer of Group A and a polyfunctional vinyl monomer of Group B can be used alone, or a mixture of two or more types can be used.

  Of these vinyl monomers, a vinyl monomer having an aromatic ring is preferable because it is easy to introduce a sulfonic acid group into the aromatic ring. Therefore, it is preferable to use at least one vinyl monomer having an aromatic ring among these vinyl monomers.

Group A:
Styrene; methylstyrenes (α-methylstyrene, vinyltoluene, etc.), ethylstyrenes, dimethylstyrenes, trimethylstyrenes, pentamethylstyrenes, diethylstyrenes, isopropylstyrenes, butylstyrenes (3-tert-butyl) Alkyl styrenes such as styrene and 4-tert-butylstyrene); chlorostyrenes, dichlorostyrenes, trichlorostyrenes, bromostyrenes (2-bromostyrene, 3-bromostyrene, 4-bromostyrene, etc.), fluorostyrene (2-fluorostyrene, 3-fluorostyrene, 4-fluorostyrene), etc .; methoxystyrenes, methoxymethylstyrenes, dimethoxystyrenes, ethoxystyrenes, vinylphenyl Alkoxy styrenes such as ethers; Hydroxy styrene derivatives such as hydroxy styrenes, methoxy hydroxy styrenes, acetoxy styrenes and vinyl benzyl alkyl ethers; Carboxy styrene derivatives such as vinyl benzoic acids and formyl styrenes; Nitro such as nitro styrenes A monofunctional vinyl monomer selected from the group consisting of styrene; aminostyrene derivatives such as aminostyrenes and dimethylaminostyrenes; and styrene derivatives containing ions such as vinylbenzyl sulfonic acids and styrenesulfonyl fluorides.

Group B:
Bis (vinylphenyl) ethane, divinylbenzene, 2,4,6-triallyloxy-1,3,5-triazine (triallyl cyanurate), triallyl-1,2,4-benzenetricarboxylate (triallyl trimellitate) ), Diallyl ether, triallyl-1,3,5-triazine-2,4,6- (1H, 3H, 5H) -trione, 2,3-diphenylbutadiene, 1,4-diphenyl-1,3-butadiene, 1,4-divinyloctafluorobutane, bis (vinylphenyl) methane, divinylacetylene, divinyl sulfide, divinyl sulfone, divinyl ether, divinyl sulfoxide, isoprene, 1,5-hexadiene, butadiene, 1,4-divinyl-2,3 , 5,6-tetrachlorobenzene selected from the group consisting of Polyfunctional vinyl monomer.

  These vinyl monomers can be dissolved in a solvent, for example, a mixed solution of the above ketone solvent and water to form a vinyl monomer solution. The blending amount (molar ratio) in the monomer solution composed of the vinyl monomer and the solvent (mixed solution) can be appropriately determined according to, for example, the blending amount (volume%) of the ketone solvent. For example, when the blending amount of the ketone solvent in the solvent (volume% based on 100% by volume of the whole solvent) is 5 to 30% by volume, the molar concentration of the vinyl monomer in the liquid phase by ultraviolet irradiation ( Mol / L) can be defined as 0.1 mol / L to 4 mol / L, preferably 0.3 mol / L to 1.0 mol / L. Further, the molar concentration of the vinyl monomer in the gas phase system by ultraviolet irradiation can be defined as 0.1 mol / L to 3 mol / L, preferably 0.3 mol / L to 1.5 mol / L. .

In addition, when using 2 or more types of said vinyl monomers, the mixture ratio (for example, volume ratio) can be determined suitably. In addition, when the B group polyfunctional vinyl monomer is blended in a predetermined ratio (for example, volume ratio) and used with the monofunctional vinyl monomer of Group A, good physical properties as a polymer electrolyte membrane for fuel cells are used. The oxidation resistance and heat resistance can be further improved by forming a crosslinked structure derived from a polyfunctional vinyl monomer while maintaining the above. At this time, the ratio (volume ratio) of the monofunctional vinyl monomer of group A and the polyfunctional vinyl monomer of B is, for example, monofunctional vinyl monomer: polyfunctional vinyl monomer = 1:
0.01-0.6, preferably 1: 0.02-0.3. In addition, for example, one or more vinyl monomers having an aromatic ring are selected from Group A monofunctional vinyl monomers, and one or more multifunctional vinyl monomers are selected from Group B multifunctional vinyl monomers. It is preferable.

  The photo-graft polymerization of the monomer onto the polymer film substrate can be performed in a liquid phase system or a gas phase system. In the case of a liquid phase system, the photograft polymerization uses a Pyrex glass ampoule with a cock that can be sealed, and a monomer solution consisting of a vinyl monomer and a solvent (mixed liquid) containing water and a ketone solvent is filled in it. Then, after immersing the polymer film substrate and bubbling an inert gas such as nitrogen gas, graft polymerization is performed at a temperature of 40 to 90 ° C. under irradiation of light (for example, ultraviolet rays). On the other hand, in the gas phase system, the polymer film substrate is not brought into direct contact with a monomer solution composed of a vinyl monomer and a solvent (mixture) containing water and a ketone solvent in a Pyrex glass ampoule with a stopcock that can be sealed. The vinyl monomer is vaporized by vaporizing a monomer solution composed of a vinyl monomer and a solvent (mixed solution) containing water and a ketone solvent under an inert gas atmosphere such as nitrogen gas at a temperature of 40 to 90 ° C. Steam is used to perform photografting polymerization.

  Ultraviolet rays are most preferable as the light beam for carrying out the graft polymerization, and examples of the ultraviolet light source include ultrahigh pressure mercury lamps, high pressure mercury lamps, metal halide lamps, xenon lamps, and low pressure sterilization lamps. When a 400 W high pressure mercury lamp is used, the irradiation time is preferably 30 to 600 minutes.

  On the other hand, in the present invention, electron beam irradiation from a low-energy accelerator can be used as a means for generating radicals only on the surface of the substrate as in photograft polymerization. In this case, in order to generate radicals by electron beam irradiation only on the surface layer of the polymer film substrate, only the surface layer of the polymer film substrate (for example, within 2 μm from the substrate surface) is passed through the polymer filter. Radicals are generated by electron beam irradiation, then brought into direct contact with a monomer solution composed of a vinyl monomer and an organic solvent bubbling with nitrogen gas, and sealed in a Pyrex glass ampoule with a stopcock in a nitrogen gas atmosphere under a nitrogen gas atmosphere. Graft polymerization is carried out at a temperature of ° C. In the gas phase system, the polymer film substrate irradiated with the electron beam is not directly brought into contact with a monomer solution composed of a vinyl monomer and an organic solvent previously bubbled with nitrogen gas in a sealable Pyrex glass ampoule with a stopcock. The vinyl monomer is vaporized by vaporizing a monomer solution composed of a vinyl monomer and an organic solvent under an inert gas atmosphere such as nitrogen gas at a temperature of 40 to 90 ° C., and graft polymerization is performed using the vapor.

The conditions of the electron beam irradiation treatment of the present invention are 5 to 30 kGy per one side of the substrate under an acceleration voltage of 25 to 50 keV and a current of 20 to 150 μA.
The depth at which radicals can be generated by irradiation from the surface of the polymer film substrate can be determined, for example, according to the thickness of the polymer film substrate, but the inherent property of the substrate is within 2 μm from the substrate surface. It is preferable in order to maintain the above characteristics. This depth can be controlled by the thickness and chemical structure of the polymer filter. It can also be controlled by the acceleration voltage, the distance between the substrate and the electron outlet of the accelerator, the degree of vacuum, and the like. The thickness of the polymer filter for controlling the depth is preferably 1 to 10 μm, and more preferably 2 to 5 μm. As the polymer filter, for example, a polymer filter made of a material such as polyethylene naphthalate, polyethylene terephthalate, polyimide, or polyethylene can be used. Among these, a polymer filter made of a polyethylene naphthalate material is preferable because it is easy to adhere to a base material and handle regardless of a thin film.

  In addition, as an organic solvent which comprises said monomer solution, the ketone solvent used by said photograft polymerization, Furthermore, the organic solvent which can be mixed is mentioned. That is, the organic solvent constituting the monomer solution does not need to contain a ketone solvent as an essential component, unlike the photografting polymerization solvent (mixed solution). However, for example, in a reaction in a gas phase system, an organic solvent containing a ketone solvent is preferably used because it participates in the control of the evaporation amount of the monomer, the control of the affinity at the interface between the substrate and the monomer, and the like. When the organic solvent contains a ketone solvent, the blending amount (volume ratio) can be appropriately determined. For example, the total amount of the organic solvent is 100% by volume, and the ketone solvent can be contained in an amount of 2 to 70% by volume, preferably 9 to 25% by volume.

The molar concentration (% by weight) of the vinyl monomer in the liquid phase system by electron beam irradiation is, for example, 10 to 60% by weight, preferably 20 to 20%, based on 100% by weight of the monomer solution (monomer + solvent total amount). It can be defined as 50% by weight. Further, the molar concentration (mol / L (solvent)) of the vinyl monomer in the gas phase system by electron beam irradiation is 0.1 mol / L to 3 mol / L, preferably 0.3 mol / L to 1. It can be defined as 5 mol / L.
In order to introduce a sulfonic acid group into the graft polymer film substrate, a dichloroethane solution or chloroform solution of chlorosulfonic acid is added to a phenyl group in the grafted molecular chain under a known condition, for example, a temperature of 40 to 80 ° C., 3 to It can carry out by making it react on the conditions for 20 hours.

  The polymer electrolyte membrane of the present invention can be applied to ultraviolet rays under a mixed solution composed of only water, a ketone solvent and an organic solvent without applying a photosensitizer which has been impossible in the past to the surface of the polymer film substrate. Irradiation generates radicals only on the surface of the base material, or radicals are generated only on the surface of the base material by irradiating with an electron beam through a polymer filter. And the graft chain is extended from the substrate surface to the inside.

  In other words, vinyl monomers that can introduce (hold) sulfonic acid groups into olefinic and fluorine polymer film substrates and polyfunctional vinyl monomers necessary for the introduction of chemical cross-linking structures can be used without using photosensitizers. The vinyl monomer graft polymerization reaction is performed by irradiating in a liquid phase system or a gas phase system using a mixed solution containing a ketone solvent having an ultraviolet absorbing ability found in the present invention. The polymer for the fuel cell of the present invention is obtained by extending the graft chain from the surface of the membrane to the inside which was impossible in the past with radicals generated on the surface of the substrate as the starting point, and finally introducing sulfonic acid groups. An electrolyte membrane can be manufactured. Since the present invention does not use a photosensitizer, the step of applying the photosensitizer and the step of further protecting the surface can be omitted. In general, since the polymer film substrate is hydrophobic, the polymer film substrate and water have no affinity. However, when acetone is added to water, water and acetone have high compatibility. Therefore, acetone in water retains a high affinity on the surface of a hydrophobic substrate, and also has a solid / liquid state at the interface. Regardless, a uniform wet state can be obtained. In the present invention, this uniform wet state can be obtained. This uniform wet state is very important, and in this state, the vinyl monomer is dissolved in acetone even if it is not dissolved in water, for example. Therefore, the acetone / monomer / base material can generate radical polymerization uniformly and continuously from the surface of the base material in the light irradiation step while maintaining high affinity, thereby generating graft polymerization. When a photosensitizer is used, the above uniform wet state cannot be created.

  On the other hand, in the case of using an electron beam from a low energy accelerator, radicals generated only on the surface layer (within 2 μm) of an olefin-based or fluorine-based polymer film substrate through the polymer filter found in the present invention. From the starting point, in the liquid phase or gas phase system, the vinyl monomer is subjected to a graft polymerization reaction, and the graft chain is extended from the surface of the membrane to the inside which was not possible before. By introducing, the polymer electrolyte membrane for fuel cells of the present invention can be produced.

  In addition, the polymer electrolyte membrane of the present invention uses the radicals generated in the surface layer of the substrate as described above to extend the graft chain from the membrane surface to the inside which was not possible before, and finally the sulfone. Since an acid group is introduced, an electrolyte membrane obtained by sulfonation because a “string-like” graft chain penetrates the substrate has a 0.03 S in the membrane surface direction at a graft ratio of 5% or more. In addition to exhibiting high electrical conductivity of 0.06 S / cm or more in the film thickness direction, the characteristics of the substrate itself can be maintained, so that dimensional changes of the electrolyte membrane and fuel crossover can be significantly suppressed. . Furthermore, a sulfonic acid group-retaining graft chain obtained by graft copolymerization of a vinyl monomer and a chemical crosslinking agent can impart excellent durability and heat resistance by introducing a crosslinked network structure.

  In the polymer electrolyte membrane of the present invention, the graft ratio of the vinyl monomer is preferably 5% or more and 24% or less. The proton conductivity is preferably 0.03 S / cm or more and 0.08 S / cm or less in the film surface direction, and preferably 0.06 S / cm or more and 0.13 S / cm or less in the film thickness direction. is there.

  By controlling the amount of grafting and the amount of sulfonation reaction, that is, the amount of sulfonic acid groups introduced, the ion exchange capacity of the resulting membrane can be changed. In the present invention, the graft ratio is preferably 5% or more with respect to the polymer film substrate, which corresponds to an ion exchange capacity of 0.4 mmol / g or more. Here, the ion exchange capacity is the amount of ion-exchange groups (mmol / g) per gram of dry electrolyte membrane. Although depending on the type of graft monomer, the ion exchange capacity is increased by increasing the graft ratio and introducing more ion exchange groups. However, if the amount of ion exchange groups is too high, the membrane swells when it contains water and the strength of the membrane decreases. In general, the water content increases as the ion exchange capacity increases, but the water content of the polymer electrolyte membrane of the present invention is preferably 4% or more, more preferably 6 to 25%.

  In addition, the polymer electrolyte membrane of the present invention is excellent in mechanical properties. As the mechanical properties, the elongation is particularly important, and the polymer electrolyte membrane of the present invention is particularly excellent because of its high elongation. Of the mechanical properties required for the polymer electrolyte membrane, the elongation is particularly important. A “membrane / electrode assembly (MEA)” obtained by joining a polymer electrolyte membrane and a catalyst electrode (carbon atom, platinum, etc.) plays a central role in the fuel cell and is compatible with the polymer electrolyte membrane. Is produced by bonding an extremely poor inorganic electrode to a polymer electrolyte membrane by, for example, applying pressure heat treatment. The fact that the polymer electrolyte membrane has a high elongation rate means that the membrane is rich in elasticity, so that the pressure bonding by the pressure heat treatment can be improved.

  In addition, the higher the proton conductivity associated with the ion exchange capacity, the lower the electric resistance of the polymer electrolyte membrane, and the better the performance as an electrolyte membrane. However, when the proton conductivity of the ion exchange membrane at 25 ° C. is less than 0.05 S / cm, the output performance as a fuel cell is often significantly reduced. It is often designed to be 05 S / cm or more. In the polymer electrolyte membrane according to the present invention, the proton conductivity of the polymer electrolyte membrane at 25 ° C. is as high as that of Nafion even at a low grafting rate of 5%, compared with that synthesized using Nafion or radiation graft polymerization. Obtained. This is because the graft molecular chain that can hold sulfonic acid groups from the membrane surface by using radicals generated only on the surface of the substrate using ultraviolet rays or low-energy electron beams as the initiator for graft polymerization, Protons are easy to move because they exist as a single strand in the nano-order with respect to the direction, and since the multiple cross-linking structure is added, swelling of the membrane by water is suppressed, and as a result, adjacent sulfonic acid groups Proton transmission seems to have increased because the interaction between them becomes easier. One of the features is that the method of introducing a single chain in the film thickness direction is possible only by the graft polymerization method of the present invention in which the graft chain can be extended from the film surface to the inside.

  It is also conceivable to reduce the thickness of the polymer electrolyte membrane in order to increase the proton conductivity of the polymer electrolyte membrane. However, at present, an excessively thin polymer electrolyte membrane is easily damaged, and it is difficult to manufacture the membrane itself. Therefore, a polymer electrolyte membrane of 30 to 200 μm is usually used. In the present invention, a film thickness of 10 to 200 μm, preferably 10 to 100 μm is effective.

The polymer electrolyte membrane for a fuel cell of the present invention is closely bonded to a membrane electrode to constitute a fuel cell electrode assembly and can be used for a fuel cell.
The polymer electrolyte membrane for fuel cells of the present invention can also be used as an ion exchange membrane.

  Next, a membrane / electrode assembly using the polymer electrolyte membrane of the present invention will be described. The membrane / electrode assembly of the present invention comprises the above-described polymer electrolyte membrane of the present invention, an anode electrode, and a cathode electrode.

  The anode electrode and the cathode electrode are not particularly limited as long as they are usually used in the art. For example, the anode electrode can be carbon paper coated with platinum / ruthenium-supported carbon black as a catalyst, and the cathode electrode can be carbon paper coated with platinum-supported carbon black as a catalyst. . The amount of the catalyst supported on the anode and cathode can be determined as appropriate.

  In the membrane / electrode assembly of the present invention, the above-described polymer electrolyte membrane of the present invention is disposed between an anode electrode and a cathode electrode, and bonded using a method usually used in the art such as hot pressing. Can be manufactured.

  According to the polymer electrolyte membrane for a fuel cell and the method for producing the same of the present invention, the surface layer of the substrate is formed by photograft polymerization or low energy electron beam irradiation in a mixed solution containing a ketone solvent having a light absorbing ability. By establishing a method that only generates radicals, the graft chain can be extended not only to the membrane surface but also to the interior, which was impossible in the past, so it is low while maintaining the inherent properties of the polymer film substrate. In terms of graft ratio, it is possible to produce a polymer electrolyte membrane for fuel cells that gives high proton conductivity, suppresses dimensional changes, mechanical strength changes, and methanol fuel permeation, and also has good heat resistance and oxidation resistance. it can. Moreover, according to the polymer electrolyte membrane for fuel cells and the method for producing the same of the present invention, the physical properties of the polymer electrolyte membrane such as ion exchange capacity can be controlled in a wide range by adjusting the production conditions.

  Furthermore, according to the membrane electrode assembly for a fuel cell of the present invention, the polymer electrolyte membrane produced by the above production method is closely bonded to the membrane electrode, so that high proton conductivity is provided at a low graft rate. It is possible to obtain a fuel cell electrode assembly that suppresses dimensional changes, mechanical strength changes, and methanol fuel permeation, and also has good heat resistance and oxidation resistance.

Hereinafter, although an example and a comparative example explain the present invention, the present invention is not limited to this. In addition, each measured value was calculated | required by the following measurements.
(1) Graft rate (%)
When the film substrate is a main chain part, and the graft polymerized part of a fluorine monomer or a hydrocarbon monomer and the like is a graft chain part, the weight ratio of the graft chain part to the main chain part is the graft ratio (X _dg [wt%]).

(2) Ion exchange capacity (mmol / g)
The ion exchange capacity (Ion Exchange Capacity, IEC) of the membrane is expressed by the following equation.

For the measurement of [n (acid group) _obs ], the membrane was again immersed in a 1M sulfuric acid solution at 50 ° C. for 4 hours to obtain an acid type (H type). Then, it was immersed in a ₃ M NaCl aqueous solution at 50 ° C. for 4 hours to form —SO ₃ Na type, and the substituted proton (H ⁺ ) was neutralized and titrated with 0.2 M NaOH to determine the acid group concentration.

(3) Swelling rate (wt%)
Remove the H-type ion exchange membrane stored in pure water or 15M aqueous methanol solution at room temperature from pure water or aqueous methanol solution, and wipe it lightly (after about 1 minute). and W _s (g), followed, 16 hours the film at 60 ° C., the weight _W d (g) of the film when dried in vacuo and dry _weight, W s, swelling ratio from _{W d} by the following formula Desired.

(4) Proton conductivity in the membrane surface direction (S / cm) (for example, see Non-Patent Document 2)
An H-type polymer electrolyte membrane stored in water at room temperature is taken out and sandwiched between both platinum electrodes, and the resistance in the membrane surface direction due to impedance is measured. The proton conductivity of the polymer electrolyte membrane was calculated using the following formula.

(5) Oxidation resistance (conductive group elimination time)
Based on the weight of the electrolyte membrane saturated and swollen in an aqueous solution at 60 ° C., and then immersed in a 3% hydrogen peroxide solution at 60 ° C., the time when the weight of the electrolyte membrane starts to decrease (desorption of conductive group) Time) is a measure of oxidation resistance.

(6) Mechanical strength The mechanical strength of the polymer electrolyte membrane is as follows: tensile strength (MPa) and maximum elongation (%) at room temperature (about 25 ° C.) and humidity of 50% RH according to JIS K7127. Measurements were made using a mold specimen.

Example 1
A polyvinylidene fluoride film substrate (hereinafter abbreviated as “PVDF”) cut to 3 cm × 3 cm × 50 μm (made by Kureha Co., Ltd., trade name: Kureha KF Polymer) is put into a Pyrex glass ampoule with a cock, and then deaerated. Styrene consisting of a mixed solution (36 mL) of methyl ethyl ketone / isopropanol / water (5 / 28.5 / 66.5 v%) and styrene (2.4 mL, 0.546 mol / L) previously deaerated in a glass ampoule. After directly contacting the monomer solution (liquid phase graft polymerization solution system) and replacing with nitrogen gas, the cock was closed, and the liquid was irradiated at 60 ° C. using a high pressure mercury lamp (400 W) as a light source for 1 hour. Phase-based photograft polymerization was performed. The obtained photograft polymerized film was washed with toluene and dried. The graft rate was 12%.

  In order to sulfonate the graft polymerized membrane, it was immersed in 0.2 M chlorosulfonic acid diluted with 1,2-dichloroethane, reacted at 60 ° C. for 6 hours, and then hydrolyzed by washing with water. The ion exchange capacity, water content, proton conductivity, mechanical properties, and oxidation resistance of the polymer electrolyte membrane obtained in this example were measured. The results are shown in Table 1.

In Table 1, the oxidation resistance is expressed using time as a scale, and the longer the time, the better the oxidation resistance. The oxidation resistance of the present invention is determined by examining the change in weight due to the decomposition of the membrane as a function of time when the electrolyte membrane is immersed in a 3% hydrogen peroxide (H ₂ O ₂ ) aqueous solution and treated at a temperature of 60 ° C. It is a result. In Table 1, 200 hours in some examples and comparative examples show that no degradation occurs during this test period.

(Example 2)
In Example 1, an ethylene-tetrafluoroethylene copolymer (hereinafter abbreviated as “ETFE”) (trade name: Fluon, manufactured by Asahi Glass Co., Ltd.) was used as a base material. Other conditions are the same as those in Example 1. The graft rate was 23%. The results are shown in Table 1.

(Example 3)
PVDF cut to 3 cm × 3 cm × 50 μm was placed in a Pyrex glass ampoule with a cock and deaerated, and then a mixture of acetone / water (6 mL / 30 mL) previously deaerated in the glass ampoule and styrene (2.4 mL, 0.546 mol / L) and a styrene monomer solution (liquid phase graft polymerization solution system) directly contacted and replaced with nitrogen gas, then the cock was closed and a high pressure mercury lamp (400 W) was used as a light source at 60 ° C. Liquid phase photograft polymerization was performed by irradiating with ultraviolet rays for 1 hour. The obtained photograft polymerized film was washed with toluene and dried. The graft rate was 9%. The results are shown in Table 1.

Example 4
In Example 3, polytetrafluoroethylene (hereinafter, abbreviated as “PTFE”) (manufactured by Nitto Denko Corporation, trade name: NITOFLON) was used as a base material. UV irradiation is 3 hours. Other conditions are the same as those in Example 3. The graft rate was 5%. The results are shown in Table 1.

(Example 5)
In Example 4, 5 mol% of divinylbenzene, which is a polyfunctional vinyl monomer, was added to styrene in a liquid phase graft polymerization solution system (styrene monomer solution). Other conditions are the same as those in Example 4. The graft rate was 10%. The results are shown in Table 1.

(Example 6)
In Example 4, gas phase photografting polymerization was performed using vapor of styrene monomer solution (gas phase graft polymerization solution system) without directly contacting PTFE and styrene monomer solution. Other conditions are the same as in Example 4. The graft rate was 5%. The results are shown in Table 1.

(Example 7)
In Example 6, 5 mol% of bis (vinylphenyl) ethane, which is a polyfunctional vinyl monomer, was added to styrene in the gas phase graft polymerization solution system. Other conditions are the same as in Example 6. The graft rate was 15%. The results are shown in Table 1.

(Example 8)
The electron beam acceleration voltage is 25 keV, and under a current value of 99 μA, cut into 5 cm × 5 cm × 50 μm through a polyethylene naphthalate filter (thickness 5 μm) (trade name: Teonex, manufactured by Teijin DuPont Films Ltd.) in a nitrogen atmosphere. The irradiated PVDF was irradiated on both sides with 20 kGy per side at a depth of 2 μm from the substrate surface. In addition, the irradiation of 20 kGy per one side from the substrate surface to a depth of 2 μm was confirmed by a cellulose triacetate film dosimeter (FT-125) manufactured by Fuji Film Co., Ltd.

  Styrene monomer solution (liquid phase graft polymerization solution system) of styrene / isopropanol (40/60 w%; 60 g) previously degassed in a glass ampoule after putting irradiated PVDF in a glass ampoule with a cock Were directly contacted and replaced with nitrogen gas, the cock was closed, and graft polymerization was carried out at 60 ° C. for 6 hours. The obtained graft polymerized membrane was washed with toluene and dried. The graft ratio was 20%. The results are shown in Table 1.

Example 9
In Example 8, a styrene monomer solution composed of a mixed solution of methyl ethyl ketone / isopropanol / water (15 mL: 10/30/60 v%) and styrene (2.4 mL, 0.546 mol / L) was used as the styrene monomer solution. .

  Without direct contact between the irradiated PVDF and the styrene monomer solution, vapor phase graft polymerization was performed at 60 ° C. for 3 hours using vapor of the styrene monomer solution (gas phase graft polymerization solution system). Other conditions are the same as in Example 8. The graft rate was 19%. The results are shown in Table 1.

(Example 10)
In Example 9, 5 mol% of bis (vinylphenyl) ethane, which is a polyfunctional vinyl monomer, was added to the styrene in the gas phase graft polymerization solution system. The other conditions are the same as in Example 9. The graft rate was 24%. The results are shown in Table 1.

(Example 11)
The electron beam acceleration voltage is 25 keV, and under a current value of 149 μA, in a nitrogen atmosphere, cut into 5 cm × 5 cm × 50 μm through a polyethylene naphthalate filter (thick 3 μm) (Teijin DuPont Films, trade name: Teonex). The irradiated PTFE was irradiated on both sides with 30 kGy per side at a depth of 2 μm from the substrate surface.

  Styrene monomer solution (mixture: liquid phase) of styrene / n-butanol (40/60 w%; 60 g), which has been degassed in a glass ampule after degassing by putting irradiated PTFE into a glass ampule with a cock The graft polymerization solution system) was directly contacted and replaced with nitrogen gas, the cock was closed, and graft polymerization was performed at 60 ° C. for 8 hours. The obtained graft polymerized membrane was washed with toluene and dried. The graft rate was 11%. The results are shown in Table 1.

(Comparative Example 1)
The results of ion exchange capacity electrical conductivity, water content, and oxidation resistance measured for Nafion 112 (manufactured by DuPont) shown in Table 1 below are shown in Comparative Example 1 in Table 1.

(Comparative Example 2)
In Example 8, the PVDF was directly irradiated with an electron beam without passing through a polyethylene naphthalate filter. In order to obtain the same graft ratio as in Example 8, graft polymerization was carried out for 3 hours. The graft rate was 19%. The results are shown in Comparative Example 2 in Table 1.

(Comparative Example 3)
In Example 11, the PTFE was directly irradiated with an electron beam without passing through a polyethylene naphthalate filter. In order to obtain the same graft ratio as in Example 11, graft polymerization was carried out for 4 hours. The graft rate was 12%. The results are shown in Comparative Example 2 in Table 1.

As understood from Table 1 above, the polymer electrolyte membrane of the present invention has various characteristics, for example, proton conductivity, for example, 0.03 S / cm or more and 0.08 S / cm in the membrane surface direction. In the following range and film thickness direction, adjustment can be made within a desired range, for example, a range of 0.06 S / cm to 0.13 S / cm. Further, by introducing a polyfunctional group, the oxidation resistance is improved, and therefore the durability and heat resistance are improved.
Further, the polymer electrolyte membrane of the present invention exhibits a particularly high elongation and is excellent in mechanical properties. Furthermore, the polymer electrolyte membrane of the present invention exhibits a low swelling rate, particularly a low swelling rate in methanol, and therefore has excellent dimensional stability.

  The electrolyte membrane of the present invention provides high proton conductivity at a low graft ratio and has dimensional stability, high mechanical strength, fuel permeation suppression, heat resistance, and oxidation resistance. It is used as a polymer electrolyte membrane suitable for a fuel cell expected as a power source for portable devices, home cogeneration, automobiles, and a simple auxiliary power source.

Claims (9)

A polymer film substrate is irradiated with ultraviolet rays in a state where a vinyl monomer capable of introducing a sulfonic acid group and a monomer solution containing water and a solvent containing a ketone solvent are brought into contact with each other, and the vinyl monomer is subjected to a liquid phase system. A photo-graft polymerization step of photo-grafting polymerization from the surface of the polymer film substrate, and extending a graft chain in an internal direction from the surface of the substrate and the surface of the substrate;
A method for producing a polymer electrolyte membrane for a fuel cell, comprising the step of introducing a sulfonic acid group into a portion where the sulfonic acid group can be introduced in the graft chain formed by the photograft polymerization step.   The method for producing a polymer electrolyte membrane for a fuel cell according to claim 1, wherein the monomer solution further contains a polyfunctional vinyl monomer. In the photograft polymerization step, the polymer film substrate is irradiated with ultraviolet rays in contact with the vapor of the monomer solution, and the vinyl monomer is photografted from the polymer film substrate surface in a gas phase system. 3. The method for producing a polymer electrolyte membrane for a fuel cell according to claim 1 or 2, wherein the graft chain is polymerized to extend from the substrate surface and the substrate surface in an internal direction. An electron beam irradiation step of applying an electron beam irradiation treatment of 5 to 30 kGy as one side in a range of 2 μm or less with respect to the depth direction on both surfaces of the polymer film substrate through the polymer filter;
A monomer solution comprising a vinyl monomer capable of introducing a sulfonic acid group and an organic solvent is brought into contact, and the vinyl monomer is graft polymerized from the surface of the polymer film substrate in a liquid phase system, and the graft chain is formed on the surface of the substrate. And a graft polymerization step of extending from the surface of the base material in the internal direction;
A method for producing a polymer electrolyte membrane for a fuel cell, comprising a step of introducing a sulfonic acid group into a portion of the graft chain formed by the graft polymerization step into which a sulfonic acid group can be introduced.   The method for producing a polymer electrolyte membrane for a fuel cell according to claim 4, wherein the monomer solution further contains a polyfunctional vinyl monomer. In the graft polymerization step, graft polymerization is performed in a state where the vapor of the monomer solution is in contact with the polymer film substrate, and the vinyl monomer is graft-polymerized from the surface of the polymer film substrate in a gas phase system. 6. The method for producing a polymer electrolyte membrane for a fuel cell according to claim 4 or 5, wherein the graft chain is extended from the substrate surface and the substrate surface in an internal direction.   The method for producing a polymer electrolyte membrane for a fuel cell according to any one of claims 1 to 6, wherein the polymer film substrate is made of an olefin polymer material or a fluorine polymer material.   A polymer electrolyte membrane produced by the method according to any one of claims 1 to 6, wherein a graft chain extends only from a surface layer of a polymer film substrate, and further a graft ratio Is 5% or more, the proton conductivity is 0.03 S / cm or more in the film surface direction, and the proton conductivity is 0.06 S / cm or more in the film thickness direction. Polymer electrolyte membrane.   An electrode assembly for a fuel cell, comprising a polymer electrolyte membrane produced by the method according to any one of claims 1 to 6 in close contact with a membrane electrode.