JP2006156055A - Blend-cross-linked polymer electrolyte membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polymer electrolyte membrane that has low methanol permeability and high ionic conductivity suitable for a proton conducting membrane for a fuel cell. <P>SOLUTION: The polymer electrolyte membrane comprises a blend polymer including polyvinyl alcohol (PVA) that is a polymer having a hydroxyl group in a water-soluble polymer having an acid group, cross-linked with two types of difunctional and monofunctional cross linking agents. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a cross-linked polymer solid electrolyte of a proton conductive blend polymer using a water-soluble polymer as a raw material, which is useful as a polymer solid electrolyte membrane for fuel cells.

  As a polymer solid electrolyte membrane for a fuel cell, it has high proton conductivity and is sufficiently stable chemically, thermally, electrochemically and mechanically, mainly “Nafion (registered trademark)” manufactured by DuPont, USA. Perfluorocarbon sulfonic acid membranes and the like are known. However, in these conventional ones, the performance is lowered at a temperature of 100 ° C. or higher, and sufficient performance cannot be exhibited from the viewpoint of heat resistance, water content, and ion conductivity at a high temperature. It is pointed out that the cost is too high as an obstacle to the establishment of fuel cell technology.

  On the other hand, so-called hydrocarbon polymer electrolytes in which acidic groups such as sulfonic acid groups are introduced into engineering plastic polymers such as polyether ketone, polyether ether ketone, and polybenzimidazole have recently been actively studied. Compared to a sulfonic acid membrane, there is a problem that ion conductivity under low humidity conditions is small, and the cost is high.

  In view of the above, the present invention is suitable for a proton exchange membrane for a fuel cell and has excellent ion conductivity and mechanical properties, good water content, and performance in a wide temperature range from low to high temperatures. The objective is to provide a new polymer electrolyte membrane that is effective and inexpensive.

As a result of intensive studies to solve the above problems, the present inventors have added a polymer PVA having a hydroxyl group to a water-soluble polymer having an acidic group and a water-soluble polymer, mixed them, and then formed a film, and then played a role. It was found that a two-component blend crosslinked membrane obtained by chemically crosslinking using two different types of crosslinking agents gives a polymer electrolyte membrane excellent in mechanical properties, water content and ion conductivity. Completed.

That is, the present invention
(1) A blend cross-linked polymer electrolyte membrane characterized by introducing a cross-linked structure into a water-soluble polymer having an acidic group and a polymer PVA having a hydroxyl group by using two types of cross-linking agents that react with the hydroxyl group of PVA.
(2) The water-soluble polymer having an acidic group is a water-soluble polymer having poly-2-acrylamido-2-methylpropanesulfonic acid (PAMPS). Yes,
(3) The crosslinked polymer blend electrolyte membrane, wherein polyethylene glycol and a water-soluble polymer that is a derivative thereof are mixed,
(4) As two types of cross-linking agents that act on the blend polymer electrolyte membrane, one is a bifunctional reagent that reacts with the hydroxyl group of PVA such as terephthalaldehyde or glutaraldehyde,
Another is a cross-linked polymer electrolyte membrane characterized by being a monofunctional reagent that reacts with hydroxyl groups of PVA such as aldehydes having hydrophobic chains of various chain lengths,
(5) A fuel cell membrane / electrode assembly using the above-mentioned cross-linked polymer electrolyte membrane,
(6) A polymer electrolyte fuel cell using the cross-linked polymer electrolyte membrane,
(7) A direct methanol fuel cell using the cross-linked polymer electrolyte membrane.

  According to the present invention as described above, it is suitable for proton exchange membranes for fuel cells, has excellent ion conductivity and mechanical properties, has good water content, and exhibits performance in a wide range from low to high temperatures. In addition, a low-cost polymer electrolyte membrane is provided, and a fuel cell using the membrane is realized.

  The present invention will be described in detail below.

The basic structure of the polymer having an acidic group of the present invention is not particularly limited. Polyvinylsulfonic acid, polystyrenesulfonic acid, poly-2-acrylamido-2-methylpropanesulfonic acid (PAMPS), polyvinylphosphoric acid, polyacrylic A known or arbitrary structure such as acid or polymethacrylic acid that satisfies essential requirements such as water solubility can be used. Among them, PAMPS is preferable as a polymer having good compatibility with other water-soluble polymers and excellent ion conductivity.

In the present invention, the molecular weight of polyvinyl alcohol (PVA) is not particularly limited.
It is preferably in the range of 00 to 1,000,000, and more preferably in the range of 50,000 to 200,000 from the viewpoint of solubility in water.

  The greater the amount of acidic groups in the acidic polymer of the present invention, the higher the conductivity and the better, but the solubility in water increases and the mechanical strength also decreases. The amount of the polymer having an acidic group with respect to polyvinyl alcohol can be selected in the range of 1: 0.5 to 2.0 by weight ratio, but about 1: 1 to 1.5 is most desirable.

  The structure of the first cross-linking agent of the present invention is not particularly limited, and bifunctional reagents such as terephthalaldehyde, glutaraldehyde, and suberoyl chloride are suitable, and bridge with PVA by reacting with the hydroxyl group of PVA. Various things can be used, such as the ability to provide a construction. That is, the role of the first crosslinking agent is to form a polymer chain having a mechanically strong film by forming a crosslinked PVA.

  The structure of the second cross-linking agent of the present invention is not particularly limited, and monofunctional reagents that react with the hydroxyl groups of PVA, such as aldehydes having hydrophobic chains of various chain lengths, are suitable. Those satisfying the essential requirements such as aldehydes having a linear or branched structure of 2 to 16 can be used. That is, the role of the second cross-linking agent is to provide a hydrophobic region in the blend polymer to give flexibility, and to adjust the water content so that a polymer having an acidic group stably forms an ion conductive region.

  The combination and composition of the two types of crosslinking agents of the present invention can be appropriately selected and optimized. If the first cross-linking agent is too much, a good flexible film cannot be formed, and if it is too little, the moisture content of the film becomes too high and the strength tends to be weakened. Usually, the composition ratio of the first crosslinking agent and the second crosslinking agent is variously selected in the range of 1: 0.1 to 10 in terms of mass ratio, but the range of 1: 0.5 to 4 is most desirable.

  As the solvent for the action of the two kinds of crosslinking agents of the present invention, those known per se, that is, aprotic solvents capable of slightly swelling the membrane, such as N, N′-dimethylformamide (DMF), dimethyl Those satisfying essential requirements such as sulfoxide (DMSO) and acetone can be used.

  The role of the two types of crosslinking agents in the present invention is as described above. However, if the addition amount is too small, there is no effect, and if it is too much, the effect is reduced. The amount of the crosslinking agent can be adjusted by the concentration of the crosslinking agent and the reaction time when a blend polymer film formed from a water-soluble polymer having an acidic group and a polymer having a hydroxyl group is immersed in a solvent containing two types of crosslinking agents. it can. Various cross-linking agent concentrations can be selected within the range of 0.1 to 20 wt%, preferably 1.0 to 6.0 wt%, and the cross-linking time can be selected within the range of 1 to 24 hours. About 1 to 3 hours is most desirable.

  In the polymer electrolyte of the present invention, for example, an acidic group-containing polymer and polyvinyl alcohol are each dissolved in distilled water, and these two types of aqueous solutions are mixed to form a film by a casting method, and then using two types of crosslinking agents. By crosslinking, a crosslinked polymer solid electrolyte membrane insoluble in water and having a low water content can be obtained.

  As the substrate to be cast, a glass plate, a Teflon (registered trademark) plate, a Teflon (registered trademark) sheet, or the like can be used. The thickness of the mixed solution at the time of casting is not particularly limited, but is preferably 10 to 1000 μm. If it is too thin, the shape of the film cannot be maintained, and if it is too thick, a non-uniform film is likely to be formed. More preferably, it is 50-300 micrometers. As a method for controlling the cast thickness of the mixed solution, a known method can be used. For example, the thickness can be controlled by the amount or concentration of the mixed solution with a constant thickness using an applicator, a doctor blade, or the like, or by using a glass petri dish or a Teflon (registered trademark) petri dish.

  The film of the present invention can have any film thickness depending on the purpose, but is preferably as thin as possible from the viewpoint of ion conductivity. Specifically, it is preferably 200 μm or less, and more preferably a film having a thickness of about 30 μm to 100 μm.

  Hereinafter, the present invention will be specifically described with reference to examples. The present invention is not limited to these examples.

Various measurements were performed as follows.
(Ionic conductivity) Self-made measurement cell (manufactured by Teflon (registered trademark)) was used, and conductivity was measured by AC impedance measurement by a single sine wave measurement method. The membrane sample is sandwiched between two Teflon (registered trademark) blocks with a 5 x 10 mm hole, and both ends of the membrane are contacted with platinum foil. The AC impedance is 0.02V AC voltage and the frequency is 0.001 to 10 ⁶ Hz. Was measured with a frequency response analyzer in the wet state.
(Measurement of moisture content) After immersing the membrane in pure water for 24 hours, the membrane is taken out and the surface of the membrane is gently wiped with tissue paper to measure the weight. Thereafter, the membrane is vacuum-dried at 110 ° C. for 24 hours, then weighed, and the water content is calculated by the following formula.

Moisture content (WU) = (weight after immersion in pure water / weight after vacuum drying) -1
(Ion exchange capacity) After the membrane was immersed in 20 mL of 2M NaCl for 24 hours, the solution was subjected to neutralization titration with 0.1M NaOH using a phenolphthalein indicator to obtain a sulfonic acid group equivalent per dry membrane weight. Measured as (meq / g).
<Example 1> A 6 wt% aqueous solution of PVA (Mw = 124,000 to 186,000) and a 15 wt% aqueous solution of PAMPS (Mw = 2,000,000) are prepared. The two are mixed so that the polymer weight ratio is PVA / PAMPS = 1: 2, 1: 1.5, 1: 1, 1: 0.5, and mixed and dissolved at 70 ° C. with a stirring motor until uniform. To do. After degassing under reduced pressure in a desiccator, removing the gas dissolved in the solution, the solution was poured onto a Teflon (registered trademark) sheet of a 15 cm diameter flat bottom petri dish with a Teflon (registered trademark) sheet, and water was added at room temperature. Evaporation gave a dry film. Next, the film was stirred for various times in a dimethylformamide solution containing 6 wt% of two kinds of crosslinking agents to form a crosslinked film. The membrane was taken out from the reaction solution, and the membrane was immersed in pure water all day and night to remove unreacted reaction reagent. The membrane was removed from the water and vacuum dried at 60 ° C. overnight. The moisture content, conductivity, and ion exchange capacity of the membrane were measured by the above methods. The results are shown in Table 1. At that time, terephthalaldehyde (T) is selected as the first crosslinking agent, and n-butyraldehyde (B), n-hexylaldehyde (H), and n-octylaldehyde (O) are selected as the second crosslinking agent, Used in various proportions. Hereinafter, for example, the abbreviation B2T5 indicates that the mass ratio of n-butyraldehyde to terephthalaldehyde is 2: 5.
<Comparative Example 1> Commercially available Nafion115 and Nafion117 membranes were immersed in pure water for a whole day and night and then vacuum dried at room temperature for a whole day and night. Table 1 shows the results of measuring the water content, conductivity, and ion exchange capacity of the membrane by the methods described in [0020].

<Example 2> When a PVA / PAMPS = 1: 1 blend film was reacted in a dimethylformamide solution under the condition that the mass ratio of n-octylaldehyde / terephthalaldehyde was 1: 1, the reaction time was FIG. 1 shows the changes in the ionic conductivity and water content of the membrane when the membrane was prepared in the same procedure as in Example 1 instead. It can be seen that the progress of the crosslinking reaction expels water in the membrane and increases the ionic conductivity. It has also been shown that good films can be obtained with a crosslinking reaction of about 2 hours.
<Example 3> Fig. 3 shows the ionic conductivity of a membrane obtained by reacting for 2 hours in various combinations and mass ratios as crosslinking agents in a dimethylformamide solution in a blend membrane having various PVA / PAMPS ratios. 2 (a) and (b). Good ionic conductivity is obtained when the PVA / PAMPS ratio is 1: 1 to 1.5. It can be seen that good ionic conductivity is exhibited when the mass ratio of the first crosslinking agent terephthalaldehyde and the second crosslinking agent is the same.
<Example 4> In a blend film having various PVA / PAMPS ratios, the first cross-linking agent terephthalaldehyde and the second cross-linking agent n-octylaldehyde were mixed in a dimethylformamide solution at a mass ratio of 1: 1. FIG. 3 shows the ion conductivity, water content, and ion exchange capacity of the membrane when reacted for a period of time. As the amount of PAMPS increases, the ion exchange capacity increases and the conductivity increases. However, if the water content increases too much, the ionic conductivity decreases and the optimum conditions for crosslinking exist.
Example 5 In a blend film having a PVA / PAMPS ratio of 1: 1.5, the mass ratio of the first crosslinking agent terephthalaldehyde and the second crosslinking agent n-octylaldehyde in the dimethylformamide solution was 5: 5,5. The temperature dependence of the ionic conductivity of the membrane when reacted for 2 hours under the conditions of: 10, 5:20 is shown in FIG. 4 (a) in comparison with the Nafion 117 membrane. Similarly, in a blend membrane with a PVA / PAMPS ratio of 1: 1, the first cross-linking agent terephthalaldehyde and the second cross-linking agent n-butyraldehyde have a mass ratio of 1: 1 and a PVA / PAMPS ratio in the dimethylformamide solution. In a 1: 1.5 blend membrane, the first cross-linking agent terephthalaldehyde and the second cross-linking agent n-hexyl aldehyde or n-octyl aldehyde were reacted in a dimethylformamide solution for 2 hours under the condition of a mass ratio of 1: 1. FIG. 4B shows the temperature dependence of the ionic conductivity of the film compared with that of the Nafion 117 film. It showed ionic conductivity over Nafion 117 membrane over a wide temperature range.

When the PAMPS / PVA ratio was 1: 1 and the amounts of cross-linking agents O-octylaldehyde (O) and terephthalaldehyde (T) were 5% to 5%, changes in membrane ionic conductivity at 25 ° C. were shown depending on the crosslinking time. FIG. (A) PAMPS / PVA ratio versus membrane at 25 ° C. when the amount of cross-linking agents O-octylaldehyde (O) and terephthalaldehyde (T) is 5% to 5%, 10% to 5%, 20% to 5% It is the figure which showed the ionic conductivity. (B) The amount of the cross-linking agents n-butyraldehyde (B) and terephthalaldehyde (T) is 5% to 5%, the amount of n-hexylaldehyde (H) and terephthalaldehyde (T) is 5% to 5%, O -PAMPS / PVA ratio when the amount of octylaldehyde (O) and terephthalaldehyde (T) is 5% to 5% versus membrane ion conductivity at 25 ° C. The crosslinking time is 2 hours. PAMPS / PVA ratio when the amounts of cross-linking agents O-octylaldehyde (O) and terephthalaldehyde (T) are 5% to 5% versus membrane ionic conductivity, water content and ion exchange capacity at 25 ° C. is there. The crosslinking time is 2 hours. (A) PAMPS / PVA ratio of 1.5 vs. when the amount of cross-linking agents O-octylaldehyde (O) and terephthalaldehyde (T) is 5% to 5%, 10% to 5%, 20% to 5% FIG. 2 is a diagram showing temperature changes in membrane ion conductivity at 25 ° C. of the membrane generated in 1 and the Nafion 117 membrane. (B) Membrane formed with a cross-linking agent of n-butyraldehyde (B) and terephthalaldehyde (T) in an amount of 5% to 5% and a PAMPS / PVA ratio of 1: 1, n-hexylaldehyde (H) and terephthalaldehyde ( A membrane formed with a PAMPS / PVA ratio of 1.5 to 1 when the amount of T) is 5% to 5% and the amount of O-octylaldehyde (O) and terephthalaldehyde (T) is 5% to 5%; and It is the figure which showed the temperature change of the membrane ion conductivity in 25 degreeC of a Nafion117 film | membrane. The crosslinking time is 2 hours.

Claims (7)

  A blended cross-linked polymer electrolyte membrane comprising a water-soluble polymer having an acidic group and a polymer PVA having a hydroxyl group introduced with a cross-linked structure of two types of cross-linking agents that react with the hydroxyl group of PVA. The blend-crosslinked polymer electrolyte membrane according to claim 1, wherein the water-soluble polymer having an acidic group is a water-soluble polymer having poly-2-acrylamido-2-methylpropanesulfonic acid (PAMPS). .   The blend cross-linked polymer electrolyte membrane according to claim 1 or 2, wherein polyethylene glycol and a water-soluble polymer that is a derivative thereof are mixed. The two types of crosslinking agents that act on the blend polymer electrolyte membrane according to any one of claims 1 to 3 include at least a bifunctional reagent that reacts with a hydroxyl group of PVA, such as terephthalaldehyde and glutaraldehyde, and hydrophobic groups having various chain lengths. A blend-crosslinked polymer electrolyte membrane, which is a monofunctional reagent that reacts with a hydroxyl group of PVA, such as an aldehyde having a functional chain.   A membrane / electrode assembly for a fuel cell using the cross-linked polymer electrolyte membrane according to any one of claims 1 to 4.   A solid polymer fuel cell using the cross-linked polymer electrolyte membrane according to claim 1. A direct methanol fuel cell using the crosslinked polymer electrolyte membrane according to any one of claims 1 to 4.

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