JP2005071694A - Electrolyte membrane for fuel cell and forming method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrolyte membrane for a fuel cell which is excellent in heat resistance and ion exchanging property and low in cost, and a forming method thereof. <P>SOLUTION: The electrolyte membrane for a fuel cell is formed by compounding a modified fluororesin made by irradiation by ionizing radiation at a temperature not lower than the melting point of the resin or a mixture of the modified fluororesin and the other polymer with a hybrid compound obtained by a reaction between a fluorine-containing hydrocarbon compound having a sulfonic group and tetraethoxysilane (TEOS). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrolyte membrane for fuel cells having excellent heat resistance and battery performance, and a method for producing the same.

  A polymer electrolyte fuel cell includes an electrolyte membrane made of a solid polymer, and a fuel electrode and an air electrode provided so as to sandwich the electrolyte membrane. As a solid polymer electrolyte membrane for a fuel cell, for example, there is one made of a perfluorosulfonic acid polymer known by a trade name “Nafion (registered trademark)” of DuPont.

  However, solid polymer electrolyte membranes made of perfluorosulfonic acid polymers are manufactured through a multi-step synthesis process and are expensive. In order to reduce costs, various electrolyte membranes using general-purpose hydrocarbon-based materials are available. It is being considered. However, hydrocarbon-based materials have poor heat resistance compared to perfluorosulfonic acid polymers, and when the operation is performed at a high temperature in order to improve the power generation efficiency of the fuel cell, the sulfonic acid groups are easily detached and ion exchange properties are improved. There was a problem of losing.

  Accordingly, the present inventors have provided a fuel cell comprising a modified fluororesin in which a component having a sulfonic acid group is grafted to a modified fluororesin that is crosslinked by irradiation with ionizing radiation at a temperature equal to or higher than the melting point of the resin. Proposed an electrolyte membrane (Patent Document 1).

JP 2002-313364 A

The modified fluororesin grafted with a component having a sulfonic acid group has a good ion exchange property, but has an SO ₂ decomposition peak temperature of about 300 ° C. and has a problem of poor heat resistance.

  An object of the present invention is to provide an electrolyte membrane for a fuel cell having excellent heat resistance and ion exchange properties, and a method for producing the same.

  In order to achieve the above object, the present invention has a sulfonic acid group in a modified fluororesin formed by irradiating ionizing radiation at a temperature equal to or higher than the melting point of the resin or a mixture of the modified fluororesin and another polymer. Provided is a fuel cell electrolyte membrane in which a hybrid compound obtained by reacting a fluorine-containing hydrocarbon compound and tetraethoxysilane (TEOS) is combined.

  Further, the present invention provides a method for producing an electrolyte membrane for a fuel cell, wherein ionizing radiation is dosed to a fluororesin under a condition where the oxygen concentration is 10 torr or less and the melting point of the fluororesin is heated. Sulfonic acid groups dispersed in alcohol under acidic conditions on a film made of a modified fluororesin irradiated by irradiation in the range of 0.1 kGy to 1 MGy or a mixture of the modified fluororesin and another polymer material There is provided a method for producing an electrolyte membrane for a fuel cell, characterized in that fluorine-containing hydrocarbon compounds having a chemical bond are chemically bonded to each other with tetraethoxysilane and combined with a modified fluororesin.

  As described above, the electrolyte membrane of the present invention has a sulfonic acid group dispersed in alcohol under acidic conditions in a modified fluororesin or a mixture of the modified fluororesin and another polymer irradiated with radiation. This is an electrolyte membrane produced by chemically bonding fluorine-containing hydrocarbon-based materials with tetraethoxysilane to form a composite with a modified fluorine resin, and has excellent heat resistance and ion exchange properties. Since a general-purpose and low-cost hydrocarbon-based material is used as a raw material, the cost can be reduced.

  According to the present invention, it is possible to realize an electrolyte membrane for a fuel cell having excellent ion exchange properties at low cost and excellent heat resistance.

  The fluororesin used in the present invention includes tetrafluoroethylene polymer (PTFE), tetrafluoroethylene / perfluoro (alkyl vinyl ether) copolymer (PFA), and tetrafluoroethylene / hexafluoropropylene copolymer. (FEP) can be mentioned, and the shape is not particularly limited, but may be any of powder, sheet, film, block, and fiber, and between these materials or with these materials It may be a laminate or composite with other materials.

  The PTFE includes those containing 1 mol% or less of a polymer unit based on a copolymerizable monomer such as perfluoro (alkyl vinyl ether), hexafluoropropylene, (perfluoroalkyl) ethylene, or chlorotrifluoroethylene. It is. In the case of the copolymer type fluororesin, a small amount of the third component may be included in the molecular structure.

  The modified fluororesin in the present invention is irradiated with ionizing radiation in a dose range of 0.1 kGy to 1 MGy in an inert gas atmosphere having an oxygen concentration of 10 torr or less and heated to a melting point or higher. Can be obtained. In an atmosphere where the oxygen concentration exceeds 10 torr, a sufficient crosslinking effect cannot be achieved, and when the irradiation dose of ionizing radiation is less than 0.1 kGy, a sufficient crosslinking effect cannot be achieved, and when it exceeds 1 MGy, the elongation decreases significantly. Invite. The modified fluororesin molded body may be produced by irradiating ionizing radiation onto a fluororesin molded body having a shape such as a sheet or a block, or compression molding of a fluororesin powder irradiated with ionizing radiation. Thus, it may be formed into a sheet or block shape.

  As ionizing radiation for modifying the fluororesin, γ rays, electron beams, X rays, neutron rays, high energy ions, or the like are used. When irradiating with ionizing radiation, it is necessary to heat the fluororesin above its crystalline melting point. For example, when PTFE is used as the fluororesin, it is necessary to irradiate the ionizing radiation while the fluororesin is heated to a temperature higher than 327 ° C., which is the crystal melting point of this material, and PFA or FEP is used. In this case, the irradiation is performed by heating to a temperature higher than the melting point specified by 310 ° C. for the former and 275 ° C. for the latter. Heating the fluororesin beyond its melting point activates the molecular motion of the main chain constituting the fluororesin, and as a result, the cross-linking reaction between molecules can be efficiently promoted. However, excessive heating leads to breakage and decomposition of the molecular chain, so the heating temperature should be kept within a range of 10 to 30 ° C. higher than the melting point of the fluororesin.

  The modified fluororesin of the present invention is obtained by chemically bonding a fluorine-containing hydrocarbon-based material having sulfonic acid groups dispersed in alcohol under acidic conditions to each other with tetraethoxysilane. And can be obtained by compounding with a modified fluororesin.

  The compound to be complexed with the modified fluororesin is formed by reacting fluoroalkanoyl peroxide and 2-methacryloxyethanesulfonic acid (MES) alone, or further methyl methacrylate (MMA) or dimethylacrylamide (DMAA). Fluoroalkyl group-containing sulfonic acid oligomer produced by adding

  The fluoroalkyl group-containing sulfonic acid oligomer can be combined with a modified fluororesin by tetraethoxysilane, and by adding a large amount of these oligomers, ion exchange properties can be effectively improved. In addition, the above-mentioned material having a sulfonic acid group directly in the side chain is less likely to leave than a sulfonic acid group added to a benzene ring, so that excellent ion exchange can be maintained even at high temperature operation.

  In addition to general-purpose ethanol and propanol, the alcohol used for complexing is not limited to fluorine-based alcohols such as trifluoroethanol and hexafluoroisopropanol in order to increase the affinity between the modified fluorinated fat and the fluoroalkyl group-containing sulfonic acid oligomer. It may be used.

  In order to generate a chemical bond between the modified fluororesin and the fluoroalkyl group-containing sulfonic acid oligomer, a reactive site may be created by previously irradiating the modified fluororesin with radiation.

  In order to impart ion exchange properties, it is necessary to secure a certain number or more of sulfonic acid groups, and the composite rate must be 10 wt% or more. Ion exchangeability is imparted by sulfonation following the grafting reaction, and the sulfonic acid equivalent weight, which is a measure thereof, must be 1100 g / eq or less.

  In the present invention, the electrolyte membrane can also be formed by a mixture of a modified fluororesin and another polymer material. Other polymeric materials include unmodified (not modified by irradiation with ionizing radiation) PTFE, PFA, FEP and other fluorine resins, polyethylene, polypropylene and other polyolefins, polyethersulfone, polyimide, Mention may be made of engineering plastics such as polyetheretherketone.

  Next, embodiments of the present invention will be described based on examples, but the embodiments of the present invention are not limited to these examples.

[Example 1]
A PTFE film having a thickness of 50 μm and a 6 cm square was irradiated with 120 kGy of an electron beam (acceleration voltage 800 keV) at a heating temperature of 340 ° C. in a nitrogen gas atmosphere having an oxygen concentration of 0.5 torr to obtain a modified PTFE film. Obtained. When the melting point and crystallization point of the PTFE film before and after the modification were measured by DSC, the melting point of the unmodified PTFE film was 330 ° C. and the crystallization point was 310 ° C., whereas the melting point of the modified PTFE film was The temperature was 315 ° C and the crystallization point was 290 ° C. Further, when the amount of residual radicals in the modified PTFE film was measured by ESR, no residual radicals were observed.

  On the other hand, fluoroalkanoyl peroxide and 2-methacryloxyethanesulfonic acid (MES) were reacted at 45 ° C. for 5 hours under a nitrogen stream, and the resulting crude product was purified by a reprecipitation method. The fluoroalkyl group-containing sulfonic acid oligomer shown in 1 was obtained.

The modified PTFE film was dipped in an ethanol solution of the fluorine-containing sulfonic acid oligomer and tetraethoxysilane (TEOS) dissolved in ethanol, and reacted at room temperature in the presence of 1N (normality) HCl for 24 hours. Thereafter, the film was vacuum-dried at room temperature for 24 hours. The composite ratio determined from the weight difference before and after the reaction of the PTFE film was 23%. Here, the normality is a unit indicating the concentration of the solution, and the concentration of a solution containing 1 gram equivalent of a solute in 1 dm ^{3 of} the solution is defined as 1N.

The composite modified PTFE film was hydrated at room temperature for 24 hours and immersed in a 0.1 mol / l KCl aqueous solution for 24 hours, then the film was taken out and 0.05 mol / l NaOH was dropped into the aqueous solution with a violet. The ion exchange capacity was measured by neutralization titration, and the sulfonic acid equivalent weight was calculated from the result, and it was 980 (g / eq). The ionic conductivity (25 ° C.) representing proton conductivity was 1.6 × 10 ⁻² (S / cm). Also performs TG-MS analysis, the results of evaluation of the heat resistance, decomposition peak temperature of SO ₂ was 430 ° C.. This peak temperature is the temperature at which the curve shows a peak in the TG-MS fragment gram of the horizontal axis temperature indicating the generation state of SO ₂ . Thus, it turned out that ion conductivity and heat resistance are favorable.

[Example 2]
A PTFE film having a thickness of 50 μm and a 6 cm square was irradiated with an electron beam in the same manner as in Example 1 to obtain a modified PTFE film. This modified PTFE film was irradiated with an electron beam at 30 kGy in air at room temperature to create a reactive site.

  On the other hand, fluoroalkanoyl peroxide, 2-methacryloxyethanesulfonic acid (MES), and dimethylacrylamide (DMAA) were reacted at 45 ° C. for 5 hours under a nitrogen stream, and the resulting crude product was purified by a reprecipitation method. Thus, a fluoroalkyl group-containing dimethylacrylamide co-oligomer having a sulfonic acid group represented by Chemical Formula 2 below was obtained.

  Next, the modified PTFE film in which the reaction active sites are formed is dipped into the trifluoroethanol solution of the fluorine-containing sulfonic acid oligomer and tetraethoxysilane (TEOS) dissolved in ethanol at room temperature in the presence of 1N HCl. The reaction was performed for 24 hours. Thereafter, the film was vacuum-dried at room temperature for 24 hours. The composite ratio determined from the weight difference before and after the reaction of the PTFE film was 28%.

The ion exchange capacity of the modified PTFE film to which ion conductivity was imparted by complexing was measured in the same manner as in Example 1, and the sulfonic acid equivalent weight was calculated from the result, which was 1008 (g / eq). The ionic conductivity (25 ° C.) was 1.2 × 10 ⁻³ (S / cm). The decomposition peak temperature of SO ₂ was 410 ° C. Both ionic conductivity and heat resistance are good.

[Example 3]
After firing PTFE molding powder (molecular weight 8 million, average particle size 25 μm) at 360 ° C. for 1 hour, electron beam (acceleration under a nitrogen gas atmosphere with an oxygen concentration of 0.5 torr and under a heating temperature of 335 ° C. A modified PTFE powder having an average particle diameter of 25 μm was obtained by irradiation with 120 kGy at a voltage of 800 keV and then grinding. This was blended 50% by weight with unmodified FEP pellets (MI = 25, average particle size 25 μm) to produce mixed pellets with a twin screw extruder. The obtained pellets were subjected to hot forming molding (preforming pressure: 500 kgf / cm ² , firing temperature: 360 ° C., cooling pressure: 350 kgf / cm ² ), and the resulting block was annealed at 200 ° C. for 10 hours, and then the thickness was increased. Skiving was performed on a 50 μm film.

  This unmodified FEP / modified PTFE film (thickness 50 μm, 6 cm square) was prepared by the same method as in Example 2, and a fluoroalkyl group-containing dimethylacrylamide co-oligomer having a sulfonic acid group and tetraethoxysilane (TEOS). ) Was added to the ethanol solution added, and reacted in the presence of 1N HCl at room temperature for 24 hours. Thereafter, the film was vacuum-dried at room temperature for 24 hours. The composite ratio determined from the weight difference before and after the reaction of the unmodified FEP / modified PTFE film was 33%.

The ion exchange capacity of the modified PTFE film to which ion conductivity was imparted by graft sulfonation was measured in the same manner as in Example 1. The sulfonic acid equivalent weight was calculated from the result, and it was 1060 (g / eq). . The ionic conductivity (25 ° C.) was 1.4 × 10 ⁻³ (S / cm). The decomposition peak temperature of SO ₂ was 380 ° C. In this case, both ionic conductivity and heat resistance are good.

[Comparative Example 1]
A PTFE film having a thickness of 50 μm and a 6 cm square was irradiated with an electron beam in the same manner as in Example 1 to obtain a modified PTFE film.

  On the other hand, when fluoroalkanoyl peroxide and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) were reacted at 45 ° C. for 5 hours under a nitrogen stream, the resulting crude product was purified by a reprecipitation method. A sulfobetaine segment-containing fluoroalkyl group oligomer represented by the following reaction formula was obtained.

  The modified PTFE film was dipped in an ethanol solution of the sulfobetaine segment-containing fluoroalkyl group oligomer and tetraethoxysilane (TEOS) dissolved in ethanol and reacted at room temperature in the presence of 1N HCl for 24 hours. Thereafter, the film was vacuum-dried at room temperature for 24 hours. The composite ratio determined from the weight difference before and after the reaction of the PTFE film was 88%.

The composite modified PTFE film was hydrated at room temperature for 24 hours, immersed in a 0.1 mol / l KCl aqueous solution for 24 hours, the film was taken out, and 0.05 mol / l NaOH was dropped into the aqueous solution with a violet. The ion exchange capacity was measured by neutralization titration, and the sulfonic acid equivalent weight was calculated from the result, and it was 1600 (g / eq). The ionic conductivity (25 ° C.) representing proton conductivity was 0.6 × 10 ⁻² (S / cm). Also performs TG-MS analysis, the results of evaluation of the heat resistance, decomposition peak temperature of SO ₂ was 290 ° C.. In this case, the proton conductivity is good, but the heat resistance is not so good.

[Comparative Example 2]
A PTFE film having a thickness of 50 μm and a 6 cm square was irradiated with an electron beam in the same manner as in Example 1 to obtain a modified PTFE film. On the other hand, fluoroalkanoyl peroxide and 2-methacryloxyethanesulfonic acid (MES) were reacted in the same manner as in Example 1, and a fluoroalkyl group-containing sulfonic acid oligomer was obtained from the resulting crude product.

  The modified PTFE film was dipped in an ethanol solution of the fluorine-containing sulfonic acid oligomer dissolved in ethanol and reacted at room temperature in the presence of 1N HCl for 24 hours. Thereafter, the film was vacuum-dried at room temperature for 24 hours. The composite ratio determined from the weight difference before and after the reaction of the PTFE film was 96%.

The composite modified PTFE film was hydrated at room temperature for 24 hours, immersed in a 0.1 mol / l KCl aqueous solution for 24 hours, the film was taken out, and 0.05 mol / l NaOH was dropped into the aqueous solution with a violet. The ion exchange capacity was measured by neutralization titration, and the sulfonic acid equivalent weight was calculated from the result, and was 1000 (g / eq). The ionic conductivity (25 ° C.) representing proton conductivity was 1.5 × 10 ⁻² (S / cm). Also performs TG-MS analysis, the results of evaluation of the heat resistance, decomposition peak temperature of SO ₂ was 330 ° C.. Again, the proton conductivity is good, but the heat resistance is not as good as the current Nafion® membrane.

Claims (6)

  A fluorine-containing hydrocarbon compound having a sulfonic acid group and tetraethoxy are added to a modified fluororesin obtained by irradiating ionizing radiation at a temperature higher than the melting point of the resin or a mixture of the modified fluororesin and another polymer material. An electrolyte membrane for a fuel cell, wherein a hybrid compound obtained by reacting silane (TEOS) is combined.   2. The fluororesin is at least one selected from a tetrafluoroethylene polymer, a tetrafluoroethylene / perfluoro (alkyl vinyl ether) copolymer, and a tetrafluoroethylene / hexafluoropropylene copolymer. An electrolyte membrane for a fuel cell.   The fluorine-containing hydrocarbon material having a sulfonic acid group is produced by reacting fluoroalkanoyl peroxide with 2-methacryloxyethanesulfonic acid (MES) alone, or further methyl methacrylate (MMA) or dimethylacrylamide ( 2. The electrolyte membrane for a fuel cell according to claim 1, which is a fluoroalkyl group-containing sulfonic acid oligomer produced by adding DMAA). The electrolyte membrane for fuel cells according to claim 1, wherein the peak temperature of SO ₂ decomposition is 350 ° C. or higher.   The electrolyte membrane for fuel cells according to claim 1, wherein the sulfonic acid equivalent weight is 1100 g / eq or less. A modified fluororesin formed by irradiating a fluororesin with ionizing radiation in a dose range of 0.1 kGy to 1 MGy under an oxygen concentration of 10 torr or less and heated above the melting point of the fluororesin A membrane comprising an admixture of the modified fluororesin and another polymer material, and a fluorine-containing hydrocarbon compound having a sulfonic acid group dispersed in alcohol under an acidic condition is chemically bonded to each other with tetraethoxysilane; A method for producing an electrolyte membrane for a fuel cell, comprising combining with a modified fluororesin.