JP4683181B2 - Method for producing solid polymer electrolyte membrane for fuel cell - Google Patents

Description

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

  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, the 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 as an electrolyte, a high ion exchange capacity, 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, since this electrolyte membrane has very low durability, it is poor in practicality. Therefore, after that, a fluororesin-based perfluorosulfonic acid membrane “Nafion (registered trademark of DuPont)” developed by DuPont is used. It has been used in general.

  However, conventional fluororesin electrolyte membranes such as “Nafion” have excellent chemical durability and stability, but in direct methanol fuel cells (DMFC) using methanol as fuel, the membrane swells with methanol. However, there is a problem that the joint between the membrane and the electrode is peeled off, and further, a crossover phenomenon in which methanol passes through the electrolyte membrane occurs, resulting in a decrease in output.

  Furthermore, since the fluororesin-based electrolyte membrane starts from the synthesis of the monomer, there are problems in that the number of manufacturing processes is increased and the cost is increased, and this is a major obstacle to practical use.

  For this reason, efforts are being made to develop low-cost electrolyte membranes that are less swelled with methanol instead of “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. Since ionic conductivity can be easily obtained, it is considered a promising method. This method is proposed in, for example, Japanese Patent Application Laid-Open No. 2001-348439 (Patent Document 1), Japanese Patent Application Laid-Open No. 2002-313364 (Patent Document 2), Japanese Patent Application Laid-Open No. 2003-82129 (Patent Document 3), and the like. .

  Examples of films for grafting reactive monomers include polytetrafluoroethylene (PTFE), poly [tetrafluoroethylene-hexafluoropropylene] (FEP), and poly [tetrafluoroethylene-perfluoroalkyl vinyl ether] (PFA). Has been mainly studied because of its superior oxidation resistance. However, these films become brittle as the graft ratio of reactive monomers such as styrene increases, and an attempt is made 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.

  For this reason, the film to which the reactive monomer is grafted is preferably a film in which a reactive monomer such as styrene is grafted to increase the elongation and breaking strength after sulfonation. 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). For a method of grafting a reactive monomer such as styrene to an ETFE film to obtain an electrolyte membrane, see, for example, JP-A-9-102322 (Patent Document 4), H.C. P. Black et al. , American Chemical Society Symposium Series Vol. 744, p. 174 (1999) (Non-Patent Document 1), A.I. S. Arico et al. , Journal of Power Sources 123, p. 107 (2003) (Non-Patent Document 2). 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, T. Hatanaka et al. , Fuel 81, p. 2173 (2002) (Non-patent Document 3).

  In this case, after irradiating the ETFE film with radiation, a reactive monomer such as styrene is graft polymerized, and the sulfonated film has high ionic conductivity while maintaining excellent mechanical properties by increasing the grafting ratio of styrene. Show. However, as the graft rate increases, the degree of swelling with respect to methanol also increases. For this reason, when a membrane having a high graft ratio, that is, a high ion conductivity is made into MEA and then immersed in methanol, there is a problem that the membrane and the electrode peel off or the membrane is broken due to methanol swelling of the membrane. .

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 provides a solid polymer electrolyte membrane produced by radiation-induced graft polymerization, while maintaining the high ionic conductivity, to provide a process for producing a solid polyelectrolyte film smaller than the conventional graft membrane methanol swelling Objective.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors have formed a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer as a base material for radiation graft polymerization. Thus, by using a film having a crystallinity of 33% or more, it was found that the degree of swelling with respect to methanol decreases while maintaining high ionic conductivity, and the present invention has been made.

Accordingly, the present invention provides the following method for producing a solid polymer electrolyte membrane .
Claim 1 :
A film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer is subjected to a heat treatment before irradiating with radiation, the crystallinity is set to 33% or more, and then irradiated with radiation. then by graft polymerizing a radical reactive monomer graft polymerized film introduced chlorosulfonic acid group, further a fuel cell for a solid polymer electrolyte, characterized in Rukoto was sulfonated by hydrolyzing chlorosulfonic acid A method for producing a membrane.
Claim 2 :
Production of tetrafluoroethylene copolymer in which claim 1 for a fuel cell polymer electrolyte membrane according - the fluorocarbon-based vinyl monomer and hydrocarbon-based film formed by the copolymer of vinyl monomers, ethylene Method.

  The solid polymer electrolyte membrane produced by the radiation grafting of the present invention exhibits high ionic conductivity and is less swelled with methanol, and is therefore suitable as an electrolyte membrane for fuel cells, particularly as an electrolyte membrane for direct methanol fuel cells. ing.

As a film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer used in the solid polymer electrolyte membrane for fuel cells of the present invention, the elongation / breaking strength after reactive monomer grafting is used. Is more preferable than ethylene-tetrafluoroethylene copolymer (ETFE).
In this case, the ratio of ethylene to ethylene tetrafluoride is preferably 40:60 to 60:40 (molar ratio).

Here, in the present invention, a copolymer having a crystallinity of 33% or more is used as a copolymer of a fluorinated vinyl monomer such as ETFE and a hydrocarbon vinyl monomer (hereinafter simply referred to as a copolymer).
That is, according to the study of the present inventor, when using a copolymer such as ETFE as an electrolyte membrane of a direct methanol fuel cell, the degree of methanol swelling differs depending on the difference in crystallinity. It has been found that the degree of methanol swelling differs depending on the crystallinity of the ETFE film before irradiation. The degree of crystallinity of the ETFE film is determined according to the method of Mohamed Mahmud Nasef et al. , Rad. Phys. Chem. 68 (5), 875-883 (2003), it can be obtained from the DSC measurement result by the following equation.
Crystallinity (%) = (ΔHm / ΔHm100) × 100

  Here, ΔHm is the heat of fusion of the ETFE film, and ΔHm100 is the heat of fusion of 100% crystallized ETFE. The crystallinity can be obtained by using 113.4 J / g as the value of ΔHm100 (for example, Hans-Peter Black et al., J. Mater. Chem. 10, 1795-1803 (2000)).

  In this case, a copolymer of a fluorine-based vinyl monomer and a hydrocarbon-based vinyl monomer, particularly a film formed of ETFE is used, and after irradiation with radiation, a radical-reactive monomer is graft-polymerized. Production of a solid polymer electrolyte membrane for a battery has been conventionally performed, but the crystallinity of an ETFE film that has been conventionally used was about 32%. When a copolymer film having a crystallinity of 33% or more, preferably 35% or more, particularly an ETFE film is used, the degree of methanol swelling becomes small. The upper limit of the crystallinity is appropriately selected, but is usually 50% or less, particularly 40% or less.

  Therefore, in the present invention, as described above, the copolymer film, particularly the ETFE film, having a crystallinity of 33% or more, particularly 35% or more is used. Here, most of commercially available ETFE films have a crystallinity of about 32%, but an ETFE film having a crystallinity of 33% or more, preferably 35% or more may be searched for and used in advance. . Further, as a method for increasing the crystallinity, it is possible to control by changing the film composition, for example, the ratio of ethylene and tetrafluoroethylene or the addition of a third component, but the cost of the film increases. .

  As a simple method for increasing the crystallinity, it is possible to increase the crystallinity by, for example, performing a heat treatment before irradiation with radiation. At this time, 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 heat treatment time is preferably 1 minute to 2 hours, particularly preferably 30 minutes to 1 hour. If the treatment time is too short, the film will not be sufficiently heat treated and the crystallinity will hardly change. If the treatment time is too long, there arises a problem that the film is deformed.

Here, examples of the heat-treated film include ETFE films having a crystallinity of less than 33%, and commercially available ETFE films having a crystallinity of about 32% can be used. Examples of commercially available products include Norton ETFE manufactured by Saint-Gobain.
In addition, the thickness of the film to be used is preferably 10 to 300 μm, particularly preferably 25 to 200 μm.

  In the present invention, a copolymer having a crystallinity of 33% or more is used in this way, and after irradiating this with radiation, a radical reactive monomer is graft-polymerized to obtain a solid polymer electrolyte for a fuel cell. Although a film is obtained, a known method can be adopted as the radiation graft polymerization method.

That is, radiation graft polymerization is a method in which a radical is generated by irradiating a fluorine resin film with radiation, and the radical reactive monomer is grafted using the film as a grafting point. Examples include X-rays, electron beams, ion beams, and ultraviolet rays. Gamma rays and electron beams are preferred because of the ease of radical generation.
Irradiation is performed so that the absorbed dose of radiation is 1 kGy or more. If it is less than 1 kGy, the amount of radicals produced is small, and a graft rate sufficient to obtain the desired ionic conductivity cannot be obtained. When it exceeds 100 kGy, mechanical properties such as elongation and strength of the fluororesin are deteriorated. A desirable absorbed dose is 1 to 100 kGy, and a more desirable absorbed dose is 1 to 50 kGy.

  Examples of the radically polymerizable monomer to be grafted include styrene, α-methylstyrene, sodium styrenesulfonate, trifluorostyrene, and divinylbenzene. In graft polymerization of these organic compounds, an initiator such as azobisisobutylnitrile and an organic solvent such as toluene, xylene, hexane, and heptane may be used.

  In this case, as graft copolymerization conditions, it is preferable to perform polymerization at 40 to 80 ° C., particularly 50 to 70 ° C. for 5 to 30 hours, particularly 10 to 20 hours under an inert atmosphere such as nitrogen. The graft ratio is preferably 20 to 100%, particularly preferably 30 to 60%.

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

  The membrane thus obtained is used as an electrolyte membrane for a fuel cell, particularly as an electrolyte membrane for a direct methanol fuel cell. In this case, the configuration of such a fuel cell is the electrolyte membrane of the present invention as an electrolyte membrane. A known configuration can be used except that is used.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention, these are mentioned for the purpose of illustration, This invention is not limited to the specific matter shown by these Examples. .

[Example 1]
As the ETFE film, a commercially available film (Norton ETFE manufactured by Saigoban Co., Ltd.) having a thickness of 50 μm was used. The crystallinity was measured by DSC (Perkin Elmer, DSC7). It was measured at a temperature elevation rate of 10 ° C./min in a nitrogen atmosphere, and the crystallinity was 35.2% from the peak area at a melting peak temperature of 268 ° C.
The ETFE film having a size of 6 cm × 6 cm 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 charged. 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 electron beam absorbed dose was applied to 3 types of 2 kGy, 3 kGy, and 5 kGy, and the graft ratio was calculated from the following formula.
Graft rate (wt%) = (W−W0) / W0 × 100
The graft ratio at each absorbed dose was as shown in (Table 1). W0 is the substrate mass before grafting, and W is the substrate mass after grafting. The post-grafting mass W 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 obtained membrane was measured for ionic conductivity and methanol swelling. The ionic conductivity was measured by measuring the surface conductivity at room temperature after being immersed in pure water for 1 hour by the alternating current impedance method. The degree of swelling of methanol was calculated from the change in mass before and after immersion after the membrane was vacuum dried at 60 ° C. for 2 hours and then immersed in methanol at room temperature for 16 hours. The film mass before immersion was W1, the film mass after immersion was W2, and the degree of swelling was calculated by the following equation.
Swelling degree (wt%) = (W2-W1) / W1 × 100
Table 1 shows the ionic conductivity and methanol swelling degree of the obtained solid polymer electrolyte membrane.

[Example 2]
As the ETFE film, a film having a thickness of 50 μm and commercially available from another manufacturer (ETFE manufactured by Sampratec) was used. The crystallinity of this film was measured by the same method as described in Example 1, and the crystallinity was 32.4%. The ETFE film was placed in an oven, held at 100 ° C. for 1 hour, and cooled to room temperature. The crystallinity of the ETFE film after heating was measured and found to be 33.5%.
The heat-treated ETFE film (size 6 cm × 6 cm) 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 charged. The flask was placed in a 500 cc separable flask and heated at 60 ° C. for 16 hours under a nitrogen atmosphere to conduct graft polymerization. The electron beam absorbed dose was carried out for 3 types of 2 kGy, 3 kGy, and 5 kGy, and the graft ratio at each absorbed dose was as shown in Table 1. Sulfonation was carried out as in Example 1. The ionic conductivity and methanol swelling degree of the obtained membrane were measured in the same manner as in Example 1. The measurement results are as shown in Table 1.

[Comparative Example 1]
A conventional method is shown as a comparative example. The ETFE film used had a crystallinity of 32.4% as shown in Example 2 and was used as it was without heat treatment. 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 as in Example 1. The ionic conductivity and methanol swelling degree of the obtained membrane were measured in the same manner as in Example 1. The measurement results are as shown in Table 1.

  Depending on the crystallinity of the ETFE film used, the graft ratio with respect to the electron beam absorbed dose varies, but sample No. 1 having an ionic conductivity of 0.10 S / cm. Comparing the degree of methanol swelling for 1-3, 2-2 and 3-1, it can be seen that the degree of swelling is smaller in the film having a higher degree of crystallinity. That is, it was found that the degree of swell of methanol was small in spite of the same degree of ionic conductivity with a crystallinity of 33% or more, preferably 35% or more.

Claims (2)

A film formed of a copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl monomer is subjected to a heat treatment before irradiating with radiation, the crystallinity is set to 33% or more, and then irradiated with radiation. then by graft polymerizing a radical reactive monomer graft polymerized film introduced chlorosulfonic acid group, further a fuel cell for a solid polymer electrolyte, characterized in Rukoto was sulfonated by hydrolyzing chlorosulfonic acid A method for producing a membrane. Production of tetrafluoroethylene copolymer in which claim 1 for a fuel cell polymer electrolyte membrane according - the fluorocarbon-based vinyl monomer and hydrocarbon-based film formed by the copolymer of vinyl monomers, ethylene Method.