JP2006019028A - Solid polymer electrolyte film for fuel cell, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid polymer electrolyte film for a fuel cell in which high ion conductivity is shown, in which swelling against methanol is less, and which is superior in battery characteristics, and to provide its manufacturing method. <P>SOLUTION: This is the solid polymer electrolyte film for the fuel cell manufactured by graft-polymerizing a radical-reactive monomer after irradiation of radiation to a film formed by a copolymer of fluorocarbon based vinyl monomer and hydrocarbon based vinyl monomer. In the solid polymer electrolyte film for the fuel cell and in the manufacturing method of the solid polymer electrolyte film for the fuel cell wherein a heat-treatment is applied before the irradiation of radiation to the copolymer of the fluorocarbon based vinyl monomer and the hydrocarbon based vinyl monomer, after heat-treating the film formed by the copolymer of the fluorocarbon based vinyl monomer and the hydrocarbon based vinyl monomer, the radiation is irradiated and the radical-reactive monomer is graft-polymerized. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a solid polymer electrolyte membrane for a polymer electrolyte fuel cell and a method for producing the same.

  A fuel cell using a solid polymer electrolyte type ion exchange membrane is expected to be widely put into practical use as a power source for electric vehicles or a simple auxiliary power source because its operating temperature is as low as 100 ° C. or less and its energy density is high. In this fuel cell, there are important elemental technologies related to a solid polymer electrolyte membrane, a platinum-based catalyst, a gas diffusion electrode, and a polymer electrolyte membrane-electrode assembly. However, among these, development of a solid polymer electrolyte membrane having good characteristics as a fuel cell is one of the most important technologies.

  In a solid polymer electrolyte membrane fuel cell, gas diffusion electrodes are combined on both sides of the electrolyte membrane, and the membrane and the electrode have a substantially integrated structure. For this reason, the electrolyte membrane acts as an electrolyte for conducting protons, and also has a role as a diaphragm for preventing direct mixing of hydrogen or methanol as a fuel with an oxidizing agent even under pressure. Such an electrolyte membrane is required to have a high proton transfer rate and high ion exchange capacity as an electrolyte, and a constant and high water retention in order to keep electric resistance low. On the other hand, because of its role as a diaphragm, the mechanical strength of the membrane is large, its dimensional stability is excellent, its chemical stability with respect to long-term use is excellent, and hydrogen gas or methanol as fuel Further, it is required that the gas does not have excessive permeability with respect to oxygen gas which is an oxidizing agent.

  In early solid polymer electrolyte membrane fuel cells, ion exchange membranes of hydrocarbon resins produced by copolymerization of styrene and divinylbenzene were used as electrolyte membranes. However, this electrolyte membrane has poor durability because of its very low durability. Therefore, after that, fluororesin-based perfluorosulfonic acid membrane “Nafion (registered trademark of DuPont)” developed by DuPont is generally used. Has been used.

  However, conventional fluororesin-based electrolyte membranes such as “Nafion” are excellent in chemical durability and stability, but in direct methanol fuel cells (DMFC) using methanol as fuel, methanol is the electrolyte membrane. There was a problem that the crossover phenomenon passing through the slab occurred and the output decreased.

  Furthermore, since the fluororesin-based electrolyte membrane starts from the synthesis of the monomer, it has a problem that the manufacturing process is large and the cost is high, which has been a great obstacle to practical use.

  Therefore, efforts are being made to develop low-cost electrolyte membranes that can replace the “Nafion” and the like. Among them, the method of producing a solid polymer electrolyte membrane by irradiating a fluororesin film with radiation and graft polymerization of a reactive monomer such as styrene and then introducing a sulfone group is the same as or higher than that of Nafion. It is considered to be a promising method because ionic conductivity can be easily obtained. For example, Patent Document 1 (Japanese Patent Laid-Open No. 2001-348439), Patent Document 2 (Japanese Patent Laid-Open No. 2002-313364), and Patent Document 3 ( Japanese Patent Laid-Open No. 2003-82129).

  Fluoropolymers such as polytetrafluoroethylene (PTFE), poly [tetrafluoroethylene-hexafluoropropylene] (FEP), poly [tetrafluoroethylene-perfluoroalkyl vinyl ether] (PFA) are used as films for grafting reactive monomers. Has been studied mainly because of its excellent oxidation resistance. However, these films become brittle as the graft ratio of reactive monomers such as styrene increases, and an attempt to form a sulfonated electrolyte membrane-electrode assembly (Mebrane-Electrode-Assembly: MEA) by hot pressing. Then, there is a problem that the electrolyte membrane is easily cracked.

  Therefore, the film to which the reactive monomer is grafted is preferably a film that increases the elongation and breaking strength after grafting and sulfonation of a reactive monomer such as styrene. A film made of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer satisfies the requirement, and a typical example is poly [ethylene-tetrafluoroethylene] (ETFE). Regarding a method for obtaining an electrolyte membrane by grafting a reactive monomer such as styrene to an ETFE film, for example, Patent Document 4 (Japanese Patent Laid-Open No. 9-102322), Non-Patent Document 1 (HP Black et al., American Chemical) Society Symposium Series Vol. 744, p. 174 (1999)), Non-Patent Document 2 (AS Arico et al., Journal of Power Sources 123, p. 107 (2003)). The above document describes the cell characteristics when hydrogen is used as the fuel. As an example of the cell characteristics when methanol is used as the fuel, Non-Patent Document 3 (T. Hatanaka et al., Fuel 81, p. 2173 (2002)).

  However, after irradiating the ETFE film with radiation, a reactive monomer such as styrene is graft polymerized and the characteristics of the sulfonated film are evaluated. The ionic conductivity of the film alone is superior to that of Nafion. It was found that the battery characteristics after integrating the electrodes are often inferior to those of Nafion.

JP 2001-348439 A JP 2002-313364 A JP 2003-82129 A JP-A-9-102322 H. P. Black et al. , American Chemical Society Symposium Series Vol. 744, p. 174 (1999)), A. S. Arico et al. , Journal of Power Sources 123, p. 107 (2003)) T.A. Hatanaka et al. , Fuel 81, p. 2173 (2002))

  The present invention has been made in view of the above circumstances. In a solid polymer electrolyte membrane produced by a radiation graft polymerization method and a production method thereof, a solid polymer electrolyte membrane for a fuel cell excellent in battery characteristics and a production method thereof are provided. The purpose is to provide.

  As a result of intensive studies to achieve the above object, the present inventor has radiated a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer, followed by irradiation with a radical reactive monomer. In a solid polymer electrolyte membrane for fuel cells produced by graft polymerization, a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer is heated and then irradiated with radiation. The present inventors have found that a solid polymer electrolyte membrane having improved battery characteristics and excellent battery characteristics can be industrially advantageously produced by graft polymerization and sulfonation of a reactive monomer, compared to when heat treatment is not performed. It came to make.

Accordingly, the present invention provides the following method for producing a solid polymer electrolyte membrane and a fuel cell.
[Claim 1] For a fuel cell produced by irradiating a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer with radiation and then graft-polymerizing a radical reactive monomer. A fuel comprising a polymer electrolyte membrane, the film formed of a copolymer of the fluorocarbon vinyl monomer and the hydrocarbon vinyl monomer being subjected to a heat treatment before radiation irradiation. Solid polymer electrolyte membrane for batteries.
[Claim 2] The solid polymer electrolyte membrane for a fuel cell according to claim 1, wherein the copolymer of the fluorocarbon vinyl monomer and the hydrocarbon vinyl monomer is an ethylene-tetrafluoroethylene copolymer.
[Claim 3] The solid polymer electrolyte membrane for a fuel cell according to claim 1 or 2, wherein the heat treatment is a heat treatment at a temperature of 50 to 260 ° C.
[Claim 4] A solid polymer electrolyte membrane for a fuel cell, in which a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer is irradiated with radiation and then a radical reactive monomer is graft polymerized. In the production method, the film formed of the copolymer of the fluorocarbon vinyl monomer and the hydrocarbon vinyl monomer is heat-treated and then irradiated with radiation to graft polymerize the radical reactive monomer. A method for producing a solid polymer electrolyte membrane for a fuel cell.
[Claim 5] The method for producing a solid polymer electrolyte membrane for a fuel cell according to claim 4, wherein the copolymer of the fluorocarbon vinyl monomer and the hydrocarbon vinyl monomer is an ethylene-tetrafluoroethylene copolymer. .
[6] The method for producing a solid polymer electrolyte membrane for a fuel cell according to [4] or [5], wherein the heat treatment is a heat treatment at a temperature of 50 to 260 ° C.

  The solid polymer electrolyte membrane produced by the radiation grafting of the present invention exhibits high ionic conductivity, has little swelling with respect to methanol, and has excellent battery characteristics. Therefore, an electrolyte membrane for a fuel cell, particularly a direct methanol fuel Suitable as an electrolyte membrane for batteries. According to the production method of the present invention, the solid polymer electrolyte membrane can be advantageously produced industrially.

  Hereinafter, the present invention will be described in more detail. The solid polymer electrolyte membrane for a fuel cell of the present invention is obtained by subjecting a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer to a heat treatment. Followed by irradiation with radiation followed by graft polymerization of a radical reactive monomer.

  As a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer used in the present invention, it has excellent elongation and breaking strength after reactive monomer grafting. Ethylene chloride copolymer (ETFE) is desirable.

  In the present invention, the film formed of the above copolymer is subjected to a heat treatment before radiation irradiation. In this case, the heat treatment is preferably performed at 50 to 260 ° C, particularly 100 to 200 ° C. If the heat treatment temperature is less than 50 ° C., the effect of the heat treatment may not be obtained. If the temperature is higher than 260 ° C., the melting point of ETFE may be exceeded and the shape of the film may not be maintained. The heating time is preferably 1 to 4 hours, particularly 1 to 2 hours. The heat treatment atmosphere may be air or an inert atmosphere such as nitrogen. Note that after the above heat treatment, irradiation with radiation can be performed by cooling to room temperature.

  In the present invention, in order to graft the reactive monomer in this way, by performing heat treatment before irradiating the fluororesin film with radiation, as is clear from the examples described later, current-voltage depending on the presence or absence of the heat treatment. Differences in the characteristics (IV characteristics) are recognized, and a higher voltage can be obtained at the same current density when the heat treatment is performed. Regarding the cause, it is considered that the crystal structure or crystallinity of the film changes depending on the presence or absence of heat treatment, and the difference influences the magnitude of the contact resistance with the electrode.

Next, the heat-treated film is cooled to room temperature and irradiated with radiation.
Radiation graft polymerization is a method in which a radical is generated by irradiating a fluorine resin film with radiation, and a radical-reactive monomer is grafted using the film as a grafting point. In this case, in the grafting method using radiation, Radiation is applied to the main chain of the fluororesin in advance to generate radicals that are the starting points of grafting, and then the pre-irradiation method in which the fluororesin is brought into contact with the monomer to carry out the graft reaction, and the monomer and fluororesin Although there is a simultaneous irradiation method of irradiating radiation in the coexistence, any method can be adopted in the present invention. In this case, the film thickness of the fluororesin is not particularly limited, but is preferably 25 to 100 μm, particularly preferably 25 to 80 μm.

  Examples of the radiation irradiated for graft polymerization in the present invention include γ-rays, X-rays, electron beams, ion beams, ultraviolet rays, and the like. In particular, γ-rays and electron beams are preferable because of the ease of radical generation.

  The absorbed dose of radiation is preferably 1 kGy or more, preferably 1 to 100 kGy, and more preferably 1 to 50 kGy. If it is less than 1 kGy, the amount of radical generation is small, and a graft rate sufficient to obtain the desired ionic conductivity may not be obtained. If it exceeds 100 kGy, mechanical properties such as elongation and strength of the fluororesin may be deteriorated.

  Further, the irradiation with radiation is preferably performed in an inert gas atmosphere such as helium, nitrogen, or argon gas, and the oxygen concentration in the gas is preferably 100 ppm or less, particularly preferably 50 ppm or less, but it is always necessary to be performed in the absence of oxygen. There is no.

  Examples of the radical polymerizable monomer to be grafted include substituted styrene derivatives such as styrene, α-methylstyrene, sodium styrenesulfonate, trifluorostyrene, and divinylbenzene.

  Here, the amount of the radical reactive monomer grafted to the fluororesin irradiated with radiation is 1,000 to 100,000 parts by mass, particularly 4,000, with respect to 100 parts by mass of the fluororesin. It is preferable to use ~ 20,000 mass parts. If the amount of the radical reactive monomer is too small, the contact may be insufficient. If the amount is too large, the radical reactive monomer may not be used efficiently.

  In graft polymerization of these organic compounds, a polymerization initiator such as azobisisobutylnitrile may be appropriately used as long as the object of the present invention is not impaired.

  Further, a solvent can be used during the grafting reaction, and the solvent is preferably one that uniformly dissolves the reactive monomer, for example, ketones such as acetone and methyl ethyl ketone, esters such as ethyl acetate and butyl acetate, methyl alcohol, Alcohols such as ethyl alcohol, propyl alcohol and butyl alcohol, ethers such as tetrahydrofuran and dioxane, aromatic hydrocarbons such as N, N-dimethylformamide, N, N-dimethylacetamide, benzene, toluene and xylene, n-heptane , N-hexane, cyclohexane and other aliphatic or alicyclic hydrocarbons, or a mixed solvent thereof can be used.

  As the reaction conditions for the graft polymerization, the reaction time is 1 to 40 hours, particularly 4 to 20 hours at 0 to 100 ° C., particularly 40 to 80 ° C. in an inert gas atmosphere such as nitrogen or argon. Is preferred.

  As described above, a solid polymer electrolyte membrane can be obtained by graft polymerization of a radical reactive monomer to a fluorine-based resin irradiated with radiation, and further sulfonated as necessary.

  The grafted membrane can introduce chlorosulfonic acid groups by immersing in chlorosulfonic acid-dichloroethane. The membrane reacted with chlorosulfonic acid can be sulfonated by reacting in an aqueous solution of potassium hydroxide or sodium hydroxide to form an alkali salt of sulfonic acid, followed by acid treatment with hydrochloric acid or the like.

  The solid polymer electrolyte membrane of the present invention can be used as a solid polymer electrolyte membrane provided between a fuel electrode and an air electrode of a fuel cell, and has a catalyst layer, a fuel diffusion layer, and a catalyst layer on both sides of the solid polymer electrolyte membrane. By disposing a separator, it is possible to obtain a fuel cell having no cell crossover and excellent cell characteristics. Note that the configurations and materials of the fuel electrode and the air electrode and the configuration of the fuel cell can be known.

  The solid polymer electrolyte membrane of the present invention has excellent battery characteristics even after electrodes are integrated on both sides of the solid polymer electrolyte membrane. The battery characteristics were evaluated by preparing a MEA by hot pressing a sulfonated solid polymer electrolyte membrane between an anode electrode and a cathode electrode, and using a 1-10 M aqueous methanol solution on the fuel electrode side and oxygen on the air electrode side. Or it can investigate by flowing air and seeing a current-voltage characteristic (IV characteristic).

  EXAMPLES Hereinafter, the present invention will be specifically described by way of examples and comparative examples. However, these are given for illustrative purposes, and the present invention is limited to the specific items shown in these examples. It is not a thing.

[Example 1]
An ETFE film (manufactured by Saint-Gobain) having a size of 6 cm × 6 cm and a thickness of 50 μm was placed in an oven, held at 100 ° C. for 1 hour, and cooled to room temperature. The film was irradiated with an electron beam at 25 ° C., placed in a 500 cc separable flask charged with 40 parts by mass of styrene, 2 parts by mass of divinylbenzene, 40 parts by mass of hexane, and 0.01 parts by mass of azobisisobutylnitrile, and a nitrogen atmosphere Under heating at 60 ° C. for 16 hours, graft polymerization was performed. The electron beam absorbed dose was carried out for three types of 2, 3, 5 kGy, and the graft ratio was calculated from the following formula.
Graft rate (%) =
{(Film mass after grafting-mass before grafting) / mass before grafting} × 100

  The graft ratio at each absorbed dose was as shown in Table 1. The mass after grafting was the mass after the grafted film was washed three times with acetone and vacuum-dried at 60 ° C. for 2 hours.

The film is immersed in a mixed solution of 30 parts by mass of chlorosulfonic acid and 70 parts by mass of 1,2-dichloroethane, heated at 50 ° C. for 1 hour, and then hydrolyzed by being immersed in a 1N aqueous solution of caustic potassium at 90 ° C. for 1 hour. Subsequently, after being immersed in 2N hydrochloric acid at 90 ° C. for 1 hour, it was washed with pure water three times to obtain a solid polymer electrolyte membrane containing sulfonic acid groups.
The ionic conductivity of the surface of the obtained solid polymer electrolyte membrane after being immersed in pure water for 1 hour was as shown in Table 1.

[Example 2]
An ETFE film having a size of 6 cm × 6 cm and a thickness of 50 μm was placed in an oven, held at 200 ° C. for 1 hour, and cooled to room temperature. The film was irradiated with an electron beam at 25 ° C., placed in a 500 cc separable flask containing 40 parts by mass of styrene, 2 parts by mass of divinylbenzene, 40 parts by mass of hexane, and 0.01 parts by mass of azobisisobutylnitrile, and a nitrogen atmosphere Under heating at 60 ° C. for 16 hours, graft polymerization was performed. The electron beam absorbed dose was carried out for three types of 2, 3, 5 kGy, and the graft ratio was calculated from the following formula. The graft ratio at each absorbed dose was as shown in Table 1. Sulfonation was carried out in the same manner as in Example 1, and the ionic conductivity of the surface after immersion in pure water for 1 hour was as shown in Table 1.

[Comparative Example 1]
An ETFE film having a size of 6 cm × 6 cm and a thickness of 50 μm is irradiated with an electron beam at 25 ° C., and 40 parts by mass of styrene, 2 parts by mass of divinylbenzene, 40 parts by mass of hexane, and 0.01 parts by mass of azobisisobutylnitrile are prepared. The mixture was placed in a 500 cc separable flask and heated at 60 ° C. for 16 hours under a nitrogen atmosphere to perform graft polymerization. The graft ratio at each absorbed dose was as shown in Table 1. Sulfonation was carried out in the same manner as in Example 1, and the ionic conductivity of the surface after immersion in pure water for 1 hour was as shown in Table 1.

  From the results in Table 1, it was recognized that the graft rate at the same electron beam absorbed dose decreased as the temperature of the heat treatment before the electron beam irradiation increased.

  Next, in order to confirm the effect of the heat treatment before the electron beam treatment on the battery characteristics, Sample No. in Table 1 showing the same ionic conductivity at each treatment temperature in the above results. About 1-2, 2-2, and 3-1, MEA was produced and the battery characteristic was evaluated by the following method.

The electrode is coated with a water-repellent paste made of FEP and carbon black (Vulcan XC-72, manufactured by Cabot) on carbon paper (TGP-H-060, manufactured by Toray), and treated at 340 ° C. to form a water-repellent layer. After forming, the paste which consists of a catalyst and a 20% Nafion solution was apply | coated on it, and it produced by drying at 60 degreeC. PtRu catalyst HiSPEC6000 (manufactured by Johnson Matthey) is used for the anode electrode catalyst, Pt catalyst HiSPEC1000 (manufactured by Johnson Matthey) is used for the cathode electrode catalyst, the catalyst loading amount of the anode electrode is 3 mg / cm ² , and the catalyst loading amount of the cathode electrode It was 3 mg / cm ² .

The size of the electrode was set to 2.2 cm × 2.2 cm, the sulfonated membrane was sandwiched, and MEA was produced by hot pressing at 150 ° C. and 30 kgf / cm ² for 5 minutes. The characteristic evaluation cell used was FC05-01SP manufactured by Electrochem. A 1M methanol aqueous solution was flowed at 0.86 ml / min on the fuel electrode side, air was flowed at 0.22 SLM on the air electrode side, and IV characteristics were evaluated at a cell temperature of 30 ° C. An evaluation device made by Chino was used for evaluation of the IV characteristics.

  Sample No. described above in FIG. The IV characteristic measured about MEA of 1-2, 2-2, 3-1 is shown. From the results of FIG. 3-1 uses an ETFE film that has not been heat-treated, but the radiation-grafted film obtained after heat-treating ETFE obtains a higher voltage in the region where the current density is larger, and the battery characteristics are improved by the heat treatment. It was confirmed.

Sample No. of Example and Comparative Example. It is a graph which shows the IV characteristic measured about MEA of 1-2, 2-2, 3-1.

Claims (6)

  A solid polymer electrolyte membrane for fuel cells produced by irradiating a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer with radiation and then graft-polymerizing a radical reactive monomer A solid polymer for a fuel cell, wherein the film formed of a copolymer of the fluorocarbon vinyl monomer and the hydrocarbon vinyl monomer is subjected to a heat treatment before radiation irradiation. Electrolyte membrane.   2. The solid polymer electrolyte membrane for a fuel cell according to claim 1, wherein the copolymer of the fluorocarbon vinyl monomer and the hydrocarbon vinyl monomer is an ethylene-tetrafluoroethylene copolymer. The solid polymer electrolyte membrane for a fuel cell according to claim 1 or 2, wherein the heat treatment is a heat treatment at a temperature of 50 to 260 ° C.   In a method for producing a solid polymer electrolyte membrane for a fuel cell in which a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer is irradiated with radiation and then a radical reactive monomer is graft polymerized. A fuel cell solid characterized by heat-treating a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer, and then irradiating with radiation to graft polymerize a radical reactive monomer. A method for producing a polymer electrolyte membrane.   The method for producing a solid polymer electrolyte membrane for a fuel cell according to claim 4, wherein the copolymer of the fluorocarbon vinyl monomer and the hydrocarbon vinyl monomer is an ethylene-tetrafluoroethylene copolymer. The method for producing a solid polymer electrolyte membrane for a fuel cell according to claim 4 or 5, wherein the heat treatment is a heat treatment at a temperature of 50 to 260 ° C.