JP2009151938A - Solid polymer electrolyte membrane for fuel cell and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid polymer electrolyte membrane which is manufactured by a radiation graft polymerization method and has both high proton conductance and low methanol permeability, and a fuel cell. <P>SOLUTION: The solid polymer electrolyte membrane for the fuel cell is made by graft polymerization of a polymeric monomer on a resin film on which radiation is irradiated so that the number of unit of a graft chain may be 30 or less, and by sulfonating it. The solid polymer electrolyte membrane manufactured by radiation graft shows a high ion conductance and has a low permeability of methanol. Thereby, it is suitable for an electrolyte membrane of the fuel cell, in particular, for the electrolyte membrane for direct methanol type fuel cell. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a solid polymer electrolyte membrane for a fuel cell and a 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, 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 In addition, it is required to have no permeability to oxygen gas as an oxidant.

As such a solid polymer electrolyte membrane, a fluororesin-based perfluorosulfonic acid membrane “Nafion (registered trademark of DuPont)” developed by DuPont and the like has been generally 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. Further, since the fluororesin-based electrolyte membrane starts from the synthesis of the monomer, there are problems in that the number of manufacturing steps is increased and the cost is increased, which is a great obstacle when put to practical use.
For this reason, a technique for producing a solid polymer electrolyte membrane replacing “Nafion” or the like has been studied. As one of such techniques, a hydrocarbon monomer such as styrene or a hydrocarbon is added to a fluororesin. Preparation of an electrolyte membrane obtained by radiation graft polymerization of a fluorine-containing monomer containing a part has been studied (Patent Document 1: JP 2001-348439 A, Patent Document 2: JP 2002-313364 A, Patent Document 3: Special (See Kai 2003-82129).

  In these radiation graft polymerizations, styrene / divinylbenzene co-graft membranes prepared by co-grafting two or more graft raw materials such as styrene and divinylbenzene and fluororesin irradiated with radiation at the same time are equivalent to or more than “Nafion”. Although it is possible to obtain an electrolyte membrane having a proton conductivity exceeding that of Nafion and lower than that of Nafion, further reduction in methanol permeability is required. However, increasing the amount of divinylbenzene as a cross-linking agent or reducing the graft ratio in an attempt to reduce the methanol permeability of such a styrene / divinylbenzene co-graft membrane will reduce the methanol permeability, but at the same time, proton Since the conductivity is significantly lowered, there is a problem that a solid polymer electrolyte membrane having both high proton conductivity and low methanol permeability cannot be obtained.

JP 2001-348439 A JP 2002-313364 A JP 2003-82129 A

  Accordingly, an object of the present invention is to provide a solid polymer electrolyte membrane and a fuel cell which are produced by a radiation graft polymerization method and have both high proton conductivity and low methanol permeability.

  As a result of diligent studies to achieve the above object, the present inventors grafted a polymerizable monomer so that the number of graft chain units is 30 or less on a resin film irradiated with radiation, The inventors have found that a solid polymer electrolyte membrane having both high proton conductivity and low methanol permeability can be obtained by sulfonation, and the present invention has been made.

Accordingly, the present invention provides the following solid polymer electrolyte membrane for fuel cell and fuel cell.
Claim 1:
A solid polymer electrolyte membrane for a fuel cell, wherein a polymerizable monomer is graft-polymerized on a resin film irradiated with radiation so that the number of graft chain units is 30 or less and sulfonated.
Claim 2:
The resin film is at least a fluororesin film selected from polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer and ethylene-tetrafluoroethylene copolymer. The solid polymer electrolyte membrane according to claim 1, which is one type.
Claim 3:
The solid polymer electrolyte membrane according to claim 1 or 2, wherein the polymerizable monomer is a styrene monomer.
Claim 4:
4. The solid polymer electrolyte membrane according to claim 1, wherein the absorbed dose of radiation applied to the resin film is 10 kGy or more and the graft ratio is 60% or less.
Claim 5:
The solid polymer electrolyte membrane according to any one of claims 1 to 4, wherein the radiation is an electron beam.
Claim 6:
A fuel cell, wherein the solid polymer electrolyte membrane according to any one of claims 1 to 5 is provided between a fuel electrode and an air electrode.
Claim 7:
7. The fuel cell according to claim 6, wherein the fuel cell is a direct methanol type fueled with methanol.

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

  The solid polymer electrolyte membrane for a fuel cell of the present invention is obtained by grafting a polymerizable monomer onto a resin film irradiated with radiation so that the number of graft chain units is 30 or less and sulfonating the polymerized monomer. In this case, the resin film includes fluorine such as polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and ethylene-tetrafluoroethylene copolymer. Examples thereof include hydrocarbon resin films such as polyethylene resin films or polyethylene and polypropylene.

  On the other hand, in order to graft polymerize a polymerizable monomer so that the number of graft chain units is 30 or less on a resin film irradiated with radiation, the radiation dose irradiated to the resin film is increased and a large amount of radicals are generated. Thereafter, the graft ratio of the polymerizable monomer can be controlled to be a certain ratio (usually 60%) or less. Examples of the control of the graft ratio include a method of adjusting a polymerizable monomer concentration, an oxygen concentration and the like at the time of graft polymerization. As the polymerizable monomer, a monofunctional polymerizable monomer is preferable, and a styrene monomer such as styrene, α-methylstyrene, p-fluorostyrene, styrenesulfonic acid, sodium styrenesulfonate, and the like is preferable. Acrylic monomers can also be used alone or in appropriate combination. It is also possible to use a polyfunctional polymerizable monomer in combination.

  Radiation graft polymerization is a method in which a radical is generated by irradiating a resin film with radiation, and a polymerizable monomer is grafted using the radical as a grafting point. There are a pre-irradiation method in which a grafting reaction is performed by bringing a resin film into contact with a monomer after generating a radical as a starting point, and a simultaneous irradiation method in which radiation is irradiated in the presence of a polymerizable monomer and a resin film. Any method can be employed in the invention. In this case, the thickness of the resin film is not particularly limited, but is preferably 15 to 100 μm, particularly preferably 25 to 60 μm.

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

  It is preferable that the absorbed dose of radiation is 10 kGy or more, a desirable absorbed dose is 20 to 300 kGy, and a more desirable absorbed dose is 150 to 250 kGy. If it is less than 10 kGy, the amount of radicals produced is small, and the number of monomer units in the graft chain must be increased in order to obtain the desired ionic conductivity. If it exceeds 300 kGy, mechanical properties such as elongation and strength of the resin film 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, and particularly preferably 50 ppm or less. There is no.

  Here, the usage-amount of the polymerizable monomer grafted to the resin irradiated with radiation is 1,000 to 100,000 parts by weight, particularly 4,000 to 20,000 parts by weight of the polymerizable monomer with respect to 100 parts by weight of the resin film. It is preferable to use a part. If the amount of the monomer is too small, the contact may be insufficient. If the amount is too large, the monomer may not be used efficiently.

  In graft polymerization of these polymerizable monomers, a polymerization initiator such as azobisisobutyl nitrile may be appropriately used as long as the object of the present invention is not impaired.

  Furthermore, a solvent can be used during the grafting reaction, and the solvent is preferably one that uniformly dissolves the monomer, for example, ketones such as acetone and methyl ethyl ketone, esters such as ethyl acetate and butyl acetate, methyl alcohol and ethyl alcohol , Alcohols such as 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 -Aliphatic or alicyclic hydrocarbons such as hexane and cyclohexane, or a mixed solvent thereof can be used. The monomer / solvent (mass ratio) is preferably from 0.01 to 1. When the monomer / solvent (mass ratio) is larger than 1, it is difficult to adjust the number of monomer units of the graft chain, and when it is smaller than 0.01, the graft ratio may be too low. A more desirable range is 0.03 to 0.5.

  The reaction conditions for the graft polymerization are preferably 0 to 100 ° C., particularly 40 to 80 ° C. for 1 to 40 hours, particularly 4 to 20 hours. The reaction can be performed in an inert gas atmosphere such as nitrogen or argon, or at an oxygen concentration of 0.01 to 20 Vol%.

  As described above, a solid polymer electrolyte membrane can be obtained by graft polymerization of a polymerizable monomer to a resin film irradiated with radiation and further sulfonation.

  The grafted membrane can introduce chlorosulfonic acid groups by dipping 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 the separator, it is possible to obtain a fuel cell that is suitably used particularly as an electrolyte membrane for a direct methanol fuel cell, has no methanol crossover, and has 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.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated concretely, this invention is not restrict | limited to the following Example.

[Example 1]
Immediately after irradiating an ethylene-tetrafluoroethylene copolymer (ETFE, manufactured by Norton) having a length of 5 cm, a width of 6 cm, and a thickness of 50 μm with an electron beam of 20 kGy on both sides with an acceleration voltage of 100 kV in a nitrogen atmosphere at room temperature, styrene 6 After immersing in a 30 ml container equipped with a three-way cock containing 18 parts by mass of toluene and 18 parts by mass of toluene, bubbling with nitrogen for 15 minutes at room temperature and then heating at 60 ° C. for 16 hours to obtain a styrene graft membrane with a graft rate of 42% It was.
Graft ratio = [(film weight after grafting−film weight before grafting) / before grafting
Film mass] × 100 (%)
The weight of the film after grafting was the weight after the grafted film was washed once with toluene and three times with acetone and dried under reduced pressure at 60 ° C. for 2 hours.
The average number of styrene units per graft chain of the styrene graft chain determined by ¹³ C NMR was 20.
The graft polymerized membrane 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 2 hours, and then immersed in a 1N caustic potash aqueous solution at 90 ° C. for 2 hours. It was hydrolyzed and subsequently immersed in 2N hydrochloric acid at 90 ° C. for 2 hours and then washed 3 times with pure water to obtain a solid polymer electrolyte membrane containing sulfonic acid groups. The results of measuring the characteristics of the electrolyte membrane by the following method are shown in Table 1.

1. Ion exchange capacity After immersing the H-type electrolyte membrane in a 0.02M aqueous sodium hydroxide solution for 24 hours, the membrane was taken out, and the solution was determined by neutralization titration with 0.02M hydrochloric acid.
2. Moisture content The moisture content was determined from the difference between the mass of the hydrous film after immersion in room temperature pure water and the mass of the dried film after drying at 100 ° C under reduced pressure.
Water content = (Hydrohydrate film mass−Dry film mass) / Dry film mass × 100 (%)
3. Ionic conductivity Using an impedance analyzer (Solartron 1260), the resistance in the longitudinal direction of the strip-shaped sample (width 1 cm) was measured at room temperature by the 4-terminal AC impedance method.
4). Methanol Permeability Coefficient 10M methanol water and pure water were separated by a membrane, and the amount of methanol that permeated the membrane from the methanol water side at room temperature and emerged on the pure water side was determined by gas chromatography.

[Comparative Example 1]
An ethylene-tetrafluoroethylene copolymer (ETFE, manufactured by Norton) having a length of 5 cm, a width of 6 cm, and a thickness of 50 μm was irradiated with an electron beam of 2 kGy at an acceleration voltage of 100 kV on both sides in a nitrogen atmosphere at room temperature. After immersing in a 30 ml container equipped with a three-way cock charged with parts by mass and bubbling with nitrogen at room temperature for 15 minutes, it was heated at 60 ° C. for 16 hours to obtain a styrene graft membrane having a graft ratio of 45%.
Graft ratio = [(film weight after grafting−film weight before grafting) / before grafting
Film mass] × 100 (%)
The weight of the film after grafting was the weight after the grafted film was washed once with toluene and three times with acetone and dried under reduced pressure at 60 ° C. for 2 hours.
The average number of styrene units per graft chain of the styrene graft chain determined by ¹³ C NMR was about 170.
The graft polymerized membrane 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 2 hours, and then immersed in a 1N caustic potash aqueous solution at 90 ° C. for 2 hours. It was hydrolyzed and subsequently immersed in 2N hydrochloric acid at 90 ° C. for 2 hours and then washed 3 times with pure water to obtain a solid polymer electrolyte membrane containing sulfonic acid groups. The results of measuring the characteristics of this electrolyte membrane by the method of Example 1 are shown in Table 1.

  From the results of Example 1 and Comparative Example 1, when the graft ratio is almost the same, it is confirmed that the graft membrane having a high absorbed dose of electron beams has a small number of grafted monomer units, the same ionic conductivity, and a small methanol permeability coefficient. did it.

[Comparative Example 2]
The ethylene-tetrafluoroethylene copolymer used in Example 1 was irradiated with an electron beam of 2 kGy at an acceleration voltage of 100 kV on both sides in a nitrogen atmosphere at room temperature, and immediately after that, 20.0 parts by mass of styrene and 2.4 parts by mass of divinylbenzene. The sample was immersed in a 30 ml container equipped with a three-way cock charged with a part, bubbled with nitrogen at room temperature for 15 minutes, and then heated at 60 ° C. for 16 hours. The graft ratio determined in the same manner as in Example 1 was 52%.
The graft polymerized membrane 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 2 hours, and then immersed in a 1N caustic potash aqueous solution at 90 ° C. for 2 hours. It was hydrolyzed and subsequently immersed in 2N hydrochloric acid at 90 ° C. for 2 hours and then washed 3 times with pure water to obtain a solid polymer electrolyte membrane containing sulfonic acid groups. The results of measuring the characteristics of this electrolyte membrane by the method of Example 1 are shown in Table 1.

[Comparative Example 3]
Table 1 shows the results of measuring the properties of Nafion 112 (manufactured by DuPont) by the method of Example 1.

Claims (7)

  A solid polymer electrolyte membrane for a fuel cell, wherein a polymerizable monomer is graft-polymerized on a resin film irradiated with radiation so that the number of graft chain units is 30 or less and sulfonated.   The resin film is at least a fluororesin film selected from polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer and ethylene-tetrafluoroethylene copolymer. The solid polymer electrolyte membrane according to claim 1, which is one type.   The solid polymer electrolyte membrane according to claim 1 or 2, wherein the polymerizable monomer is a styrene monomer.   4. The solid polymer electrolyte membrane according to claim 1, wherein the absorbed dose of radiation applied to the resin film is 10 kGy or more and the graft ratio is 60% or less.   The solid polymer electrolyte membrane according to any one of claims 1 to 4, wherein the radiation is an electron beam.   A fuel cell, wherein the solid polymer electrolyte membrane according to any one of claims 1 to 5 is provided between a fuel electrode and an air electrode.   7. The fuel cell according to claim 6, wherein the fuel cell is a direct methanol type fueled with methanol.