JP2003297394A - Electrolyte membrane for solid high polymer fuel cell and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrolyte membrane for a solid high polymer fuel cell with superior mechanical strength and power generating characteristic, and its manufacturing method. <P>SOLUTION: The electrolyte membrane for the solid high polymer fuel cell is reinforced by a fluorine fiber sheet formed by coupling pieces of fluorine fiber, and it is a cation exchange membrane with a thickness of 5-95 μm comprising perfluorocarbon-based ion exchange resin having a sulfonic acid group. The manufacturing method of the electrolyte membrane for the solid high polymer fuel cell has a papermaking process of obtaining a fluorine fiber paper sheet by subjecting the fluorine fiber to wet type papermaking, a heating process of obtaining a fluorine fiber sheet by heating the fluorine fiber paper sheet and thermally fusing the pieces of fiber, and an integrating process of integrating the perfluorocarbon-based ion exchange resin having the sulfonic acid group with the fluorine fiber sheet. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrolyte membrane for polymer electrolyte fuel cells and a method for producing the same.

[0002]

2. Description of the Related Art Hydrogen / oxygen fuel cells are attracting attention as a power generation system that contributes to global environmental protection because the reaction products are only water. Among the fuel cells, a polymer electrolyte fuel cell using a cation exchange membrane as an electrolyte has a feature that it has a low operating temperature and can be miniaturized.
It is promising for applications such as home-use stationary power supplies, in-vehicle power supplies, mobile power supplies for mobiles, and research and development is proceeding. A proton conductive cation exchange membrane having a thickness of 100 μm to 200 μm is usually used as the polymer membrane of the polymer electrolyte fuel cell electrolyte membrane. Further, as the proton conductive cation exchange membrane, sulfonic acid is used. A typical example is a cation exchange membrane of a perfluorocarbon polymer having a group. However, in the polymer electrolyte fuel cell using the conventional cation exchange membrane, the power density sufficient for practical use cannot be obtained.

As a method for further increasing the power density of the polymer electrolyte fuel cell, for example, there is a method of lowering the electric resistance of the ion exchange membrane, and further as a method of lowering the electric resistance of the cation exchange membrane. For example, there is a method of reducing the film thickness. By the way, in a perfluorocarbon-based ion exchange membrane having a sulfonic acid group, the presence of water is indispensable for ion exchange, and when the membrane thickness is large (for example, the conventional thickness of 100 μm to 200 μm), the proton conductive cation exchange membrane is used. There is a problem that it is difficult to control the water content in the membrane. Therefore, the above-described method of reducing the film thickness has an advantage that the water content in the film can be easily managed in addition to the fact that the electric resistance can be reduced.

On the other hand, reducing the film thickness not only lowers the mechanical strength (tensile strength, burst strength, tear strength, etc.) of the film, but also reduces the dimensional stability when wet. There was a problem. Therefore, the mechanical strength is drastically reduced in the dry state to easily cause cracks, and the wet state is extremely expanded. Further, there is a problem that workability and handleability are low when the film is joined to the gas diffusion electrode. In addition, there is a problem that it cannot withstand external force due to hydrogen or air introduced when it is mounted on a fuel cell to generate power, and it is deformed to lower the power generation capacity.

[0005]

Therefore, as a method for solving the above problems, a method of impregnating a perfluorocarbon-based ion exchange resin having a sulfonic acid group into a polytetrafluoroethylene (hereinafter referred to as PTFE) porous film is proposed. (Japanese Patent Publication No. 5-75835). With this method, although the film thickness can be reduced to increase the mechanical strength, the porosity of the porous film is low, so the content of the ion exchange resin is insufficient, and the electrical resistance of the cation exchange membrane is sufficiently reduced. There was a problem not to do.

Further, there has been proposed a cation exchange membrane in which the cation exchange membrane is reinforced with a fibril-like, woven cloth-like, or non-woven cloth-like perfluorocarbon polymer (Japanese Patent Laid-Open No. 6-58242).
No. 231779). However, this cation exchange membrane has a thickness of 100 μm to 200 μm and is not sufficiently thin, and the electric resistance cannot be sufficiently reduced.

Therefore, as a method of reducing the film thickness, sulfonic acid reinforced with fluorocarbon polymer fibril fibers characterized in that the number of fibrils having a fibril fiber diameter of 1 μm or less accounts for 70% or more of the total fibril number. A perfluorocarbon-based cation exchange membrane having a group has been proposed (JP 2001-345111 A). This cation exchange membrane has the same water content and ion exchange capacity as compared with the conventional perfluorocarbon-based cation exchange membrane, and has a high tensile fracture stress. However, this invention still has a problem. That is, the tensile yield stress without permanent set is the flow direction (MD: machine direction) and M when extruded into a sheet.
There was a problem that both the direction perpendicular to D (TD: lateral direction) was small, and it was easily deformed and not restored by an external force. This is presumably because the fibril fiber-reinforced cation exchange membrane is formed by kneading the fibril fibers and the ion exchange resin, and then subjected to film formation and heat treatment, so that the fibril fibers are not fixed to each other. An object of the present invention is to provide an electrolyte membrane for a polymer electrolyte fuel cell having excellent mechanical strength and power generation characteristics, and a method for producing the same.

[0008]

The electrolyte membrane for polymer electrolyte fuel cells of the present invention is made of a perfluorocarbon-based ion exchange resin having a sulfonic acid group, which is reinforced by a fluorofiber sheet in which fluorofibers are bonded to each other. Thickness is 5
It is characterized in that it is a cation exchange membrane having a thickness of ˜95 μm. Further, the method for producing an electrolyte membrane for a solid polymer electrolyte fuel cell according to the present invention comprises a papermaking step of wet-making a fluorofiber to obtain a fluorofiber paper sheet, and heating the fluorofiber paper sheet to produce a fluorofiber sheet. The present invention is characterized by including a heating step for obtaining and a step of integrating the fluorofiber sheet with a perfluorocarbon-based ion exchange resin having a sulfonic acid group.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION An example of the electrolyte membrane for a polymer electrolyte fuel cell of the present invention (hereinafter abbreviated as electrolyte membrane) will be described. This electrolyte membrane is a cation exchange membrane reinforced by a fluorofiber sheet and made of a perfluorocarbon-based ion exchange resin having a sulfonic acid group.

The fluorofiber sheet is one in which fluorofibers are bonded to each other, and examples thereof include those obtained by heat-treating dry non-woven fabric, wet non-woven fabric, and wet papermaking paper. Among them, usually, as the fluorofiber sheet, those in which fluorofibers are bonded by heat fusion are used, and in particular, fluorofiber short fibers containing a binder are formed into a sheet by a wet papermaking method. A fluorofiber sheet obtained by heat-sealing the sheet and removing the binder by thermal decomposition is preferable. Examples of the heat fusion method include a method using a heating furnace such as an electric furnace, a method using a thermal calendar, and a method using hot pressing. Such a fluorofiber sheet, the fluorofiber paper sheet is composed of short fibrous fluorofibers oriented in irregular directions, and has a structure in which the fibers are bonded by heat fusion, Since the yield stress is large and the fiber orientation is small, the directionality of the reinforcing electrolyte membrane can be reduced. Then, the mechanical strength of the electrolyte membrane can be increased.

Examples of the fluorine fiber used in the fluorine fiber sheet include polytetrafluoroethylene (PT).
FE), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene / hexafluoropropylene / perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene / hexafluoropropylene copolymer (FEP), polychloro Trifluoroethylene (PCTFE), ethylene / tetrafluoroethylene copolymer (ETFE) and ethylene / chlorotrifluoroethylene copolymer (EC
TFE) and the like. The above-mentioned fluorine-containing fibers may be used alone or in combination of two or more.
Such a fluorine fiber has high chemical resistance and acid resistance, and when it is used as a reinforcing material for an electrolyte membrane, leaching of impurity ions into the ion exchange resin is small. Among the above-mentioned fluorine fibers, it is preferable to use the PTFE fiber which is excellent in long-term stability and heat resistance during fuel cell operation.

Further, as the PTFE fibers, stretched or unstretched PTFE fine particles are dispersed in a binder such as viscose, carboxymethyl cellulose, polyvinyl alcohol, etc., and this dispersion is spun through a pore into a coagulation bath. Those listed are preferred. In the PTFE fiber containing such a binder, the fibers can be more firmly bound by the binder when the PTFE fiber is formed into a sheet. here,
The binder is a substance having a self-adhesive function.
The above stretched or unstretched PTFE fiber has an appropriate length,
For example, it can be cut into a length of 3 to 15 mm and dispersed in water together with a dispersant such as polyacrylamide, if necessary, to be used as a raw material for a fluorofiber paper sheet.

The fluorofibers used are fibrillated fibers or non-fibrillated fibers depending on the characteristics required for the fluorofiber sheet, specifically, the average pore diameter, the maximum pore diameter, the sheet strength, and the like. Fibers or mixtures thereof can be selected. For example, when stronger sheet strength is required, it is preferable to use fibers having an advanced degree of fibrillation. Also,
When using fibrillated fibers, the degree of fibrillation is preferably determined by the relationship between the average pore diameter, the maximum pore diameter and the sheet strength of the fluorofiber sheet. As a means for fibrillating fibers, for example, a general beating machine such as a ball mill, beater, lampen mill, PFI mill, SDR (single disc refiner), DDR (double disc refiner), and other refiners can be used. .

The length, diameter, etc. of the fluorofiber used are about 100 μm when the fluorofiber sheet is used.
It is preferable to select it so that it is uniform at m or less, and the fibers are uniformly dispersed and the orientation becomes small.

The porosity of the fluorine fiber sheet is 55 to 90%
Is preferred. When the porosity is 55 to 90%, a porous sheet with more voids can be obtained, more ion-exchange resin can be impregnated, and proton conductivity can be further enhanced. For example, if a fluorofiber sheet having a porosity of 70% or more is impregnated with a perfluorocarbon-based ion exchange resin liquid having a sulfonic acid group having an ion exchange capacity of about 1.0 mol / kg dry weight, 0.7
A cation exchange membrane having an ion exchange capacity of at least mol / kg dry weight can be obtained. If the porosity is less than 55%, the ion exchange resin may be insufficient and the ion exchange capacity may be insufficient. If the porosity exceeds 90%, the cation exchange membrane may have insufficient mechanical strength. Here, the porosity is the volume ratio of the space portion in the fluorofiber sheet.

The ion exchange capacity of the cation exchange membrane is preferably 0.7 mol / kg dry weight to 1.3 mol / kg dry weight. Ion exchange capacity is 0.7 mol /
If it is lower than the dry weight of kg, the electric resistance of the obtained cation exchange membrane may increase, and it may be 1.3 mol / k.
If it is higher than g dry weight, the cation exchange membrane may have insufficient mechanical strength.

Further, in the cation exchange membrane, the perfluorocarbon type ion exchange resin having a sulfonic acid group has a sulfonic acid group concentration, that is, an ion exchange capacity of 0.7 mol / kg dry weight to 2.0 mol / kg. It is preferably dry weight. When the ion exchange capacity is lower than 0.7 mol / kg dry weight, the resistance of the obtained cation exchange membrane may increase, and when it is higher than 2.0 mol / kg dry weight, the cation exchange membrane may be increased. May have insufficient mechanical strength. Since the perfluorocarbon-based ion exchange resin having a sulfonic acid group has high ionic conductivity, the power generation efficiency can be increased.

Further, the tensile yield stress in the longitudinal and transverse directions of the cation exchange membrane is both 12 MPa or more, and the ratio of the tensile yield stress in the longitudinal direction to the tensile yield stress in the transverse direction (longitudinal tensile stress). (Yield stress / Tensile yield stress in transverse direction)
Is preferably 2.0 or less. More preferably, the ratio of the tensile yield stress in the longitudinal direction / the tensile yield stress in the transverse direction is as close to 1.0 as possible. Here, the vertical direction is the longitudinal direction (MD) when formed into a sheet, and the lateral direction is the width direction when formed into a sheet, that is, the direction perpendicular to the longitudinal direction ( TD). When the tensile yield stress is less than 12 MPa, the tensile yield stress is easily damaged at the time of processing, and further, it is deformed when mounted in a fuel cell to generate electricity, and is difficult to restore. Further, when the ratio of the tensile yield stress in the longitudinal direction / the tensile yield stress in the lateral direction exceeds 2.0, when an external force is applied, the dimensional changes of MD and TD are different, so that the film may be deformed.

The cation exchange membrane has a thickness of 5 to 95.
μm, preferably 10 to 75 μm, and more preferably 15 to 65 μm. If the thickness is less than 5 μm, the mechanical strength of the electrolyte membrane will be insufficient and 95 μm
When it exceeds, the electric resistance of the obtained electrolyte membrane increases.

Next, an example of the method for producing the electrolyte membrane of the present invention will be described. In this manufacturing method, first, in a papermaking step, a prescribed amount of a fluorine fiber containing a binder is stirred and mixed in water, and a slurry whose concentration is preferably adjusted to a solid content concentration of 0.5% or less is prepared. Supply to wet paper making machines such as long-net type, cylinder type and short-net type. Then, it is dehydrated in a dehydration part having a continuous wire mesh, pressurized and squeezed. Then, it is heated and dried to obtain a paper-like porous fluorofiber paper sheet. Here, various additives such as a paper-strengthening agent and a dispersant used in ordinary papermaking can be appropriately added to the fluorofiber paper sheet, if necessary. The fluorofiber papermaking sheet obtained by such a wet papermaking method has an excellent feature that the fibers are dispersed uniformly and have a good formation as compared with the nonwoven fabric produced by the dry method.

Next, in the heating step, the obtained fluorofiber paper sheet is fired in an electric furnace. Firing is performed to thermally bond the fluorofibers together, and the binder in the fluorofibers is thermally decomposed and removed to produce a fluorofiber sheet. Further, it is preferable to have a cleaning step of cleaning and drying the fluorofiber sheet between the heating step and the integration step described later in order to further reduce the amount of impurities mixed.

Next, in the integration step, the ion exchange resin is integrated with the fluorine fiber sheet obtained as described above to obtain an electrolyte membrane. Examples of the method of integration include a method of impregnating a fluorine fiber sheet with an ion exchange resin solution, a method of laminating a fluorine fiber sheet and an ion exchange resin membrane, and adhering by pressure, but impregnation in terms of adhesion. Is preferred.

In the method of impregnating a fluorine fiber sheet with an ion exchange resin liquid, first, the fluorine fiber sheet is immersed in methanol and degassed in a vacuum, and the perfluorocarbon-based ion exchange resin liquid having a sulfonic acid group is added to the fluorine fiber sheet. Is uniformly impregnated to prepare an impregnated sheet. Then, this impregnated sheet is left at 25 ° C. for 2 days to evaporate and remove the solvent, and integrated to obtain an electrolyte membrane. Further, in order to obtain an electrolyte membrane having a uniform predetermined thickness, it is preferable to heat and pressurize the impregnated sheet with a hot press machine while vacuum degassing to mold the sheet. In this method, air bubbles existing in the film are removed by vacuum degassing, so that the electric resistance becomes lower and the strength further improves. Also, by heat and pressure treatment,
The thickness of the membrane becomes uniform, and the adhesion between the fluorofiber sheet and the ion exchange resin is further improved.

In order to obtain a cation exchange membrane having desired properties such as film thickness, ion exchange capacity and strength, the type of fluorofiber sheet (thickness, porosity, degree of fibrillation, etc.), perfluorocarbon This can be achieved by appropriately controlling the concentration, impregnation amount, heat treatment conditions, etc. of the system ion exchange resin solution.

In the above-mentioned manufacturing method, in the paper making step, the fluorine fiber containing the binder is made by the wet paper making method to obtain the fluorine fiber paper making sheet. However, the present invention is not limited to this. Alternatively, the fluorofiber sheet may be obtained by a dry method or the like.

[0026]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to these examples. <Example 1> First, in a papermaking step, unstretched PTFE fibers (trade name: Toray Fine Chemical Co., Ltd .; Toyofuron, fiber diameter 15 μm) obtained from a dispersion of PTFE powder using viscose as a binder matrix were prepared. It was cut to a length of 6 mm. Then, the papermaking raw material obtained by dispersing the 6 mm PTFE fiber in water was supplied to a cylinder paper machine and wet-processed to produce a fluorofiber papermaking sheet. Next, in a heating step, this fluorofiber paper sheet is heated to 400 ° C.
And heat it for 4 minutes to sinter and bond the fibers to each other, pyrolyze the viscose, and 325 ℃
Heat treated for 24 hours at a weight of 40 g / m ² and a thickness of 50
A fired fluorocarbon fiber sheet of μm was obtained (porosity 63.6).
%). Next, in the integration step, a perfluorocarbon-based ion exchange resin liquid having a sulfonic acid group that was vacuum degassed in a methanol solution for 10 minutes (manufactured by DuPont,
Nafion (trade name, 5% by weight liquid) was impregnated into a calcined fluorofiber sheet and left at 25 ° C. for 2 days to evaporate and remove the solvent. Next, the solvent-removed sheet is heat-treated under vacuum degassing with a hot press machine under the conditions of a temperature of 140 ° C. and a pressure of 5 MPa for 20 minutes (heating for 10 minutes, holding for 10 minutes), and a thickness of 50 μm. An ion exchange membrane was prepared.

<Example 2> The same PTFE fiber (unbeaten fiber) as in Example 1 and this PTFE fiber were beaten to obtain a freeness (measured according to JIS P 8121) of 300 m.
A fibrillated product (beaten fiber) was prepared in l.
Next, 50% by weight of unbeaten fiber and 50% by weight of beaten fiber
The papermaking raw material obtained by dispersing the above in water was supplied to a cylinder paper machine and wet papermaking was carried out to produce a fluorofiber papermaking sheet. All the subsequent heating steps were performed in the same manner as in Example 1 to obtain a fired fluorofiber sheet having a weight of 20 g / m ² and a thickness of 22 μm (porosity 58.7%). Then, the integration step was performed in the same manner as in Example 1 to produce a cation exchange membrane having a thickness of 20 μm.

<Example 3> A calcined fluorofiber sheet obtained in the same manner as in Example 1 was uniaxially stretched in the machine direction, and the sheet had a sulfonic acid group in the same procedure as in the integration step of Example 1. A perfluorocarbon-based ion exchange resin solution was impregnated into a cation exchange membrane having a thickness of 50 μm.

<Comparative Example 1> Without using a fluorine fiber sheet,
Only the perfluorocarbon-based ion exchange resin solution having a sulfonic acid group used in Example 1 was cast at 25 ° C.
After being left for 2 days to remove the solvent by evaporation, the same heat treatment as in Example 1 was performed using a hot press machine to prepare a cation exchange membrane having a thickness of 50 μm. <Comparative Example 2> The thickness is 20 μm by the same procedure as in Comparative Example 1.
The cation exchange membrane of was prepared. Comparative Example 3 A commercially available cation exchange membrane (manufactured by Asahi Glass Co., Ltd., trade name: Flemion, a sulfonic acid type perfluorocarbon polymer reinforced with PEFE fibrils).

<Evaluation Results> Table 1 shows the results of evaluation of the cation exchange membranes of Examples and Comparative Examples by the evaluation method described later.

[0031]

[Table 1]

<Evaluation Method> (1) Tensile Yield Stress: Tensile yield stress was measured for the cation exchange membranes of Examples and Comparative Examples according to JIS K7161. Measurement environment: 25 ° C., 65% RH. Measuring instrument: Shimadzu AG5000D. Span between sample chucks: 10
mm. Tensile speed: 50 mm / min. (2) Ion exchange capacity: The cation exchange membranes of Examples and Comparative Examples were immersed in a 1N sodium hydroxide aqueous solution for 12 hours and then immersed in a 1N hydrochloric acid aqueous solution for 24 hours. Then, after washing with water, add 1N sodium hydroxide solution to 1
It was immersed for 2 hours, and hydrogen ions were leached into the solution. This solution was neutralized and titrated with an aqueous sodium hydroxide solution to calculate the ion exchange capacity.

As shown in Table 1, Example 1 and Example 2
Since the cation exchange membrane of (1) was reinforced by the fluorine fiber sheet in which the fibers were bonded to each other, it had a tensile yield stress of 12 MPa or more in both the longitudinal and transverse directions. In the cation exchange membrane of Example 3, the tensile yield stress in the longitudinal direction, which is the uniaxial stretching direction, was extremely excellent, and the average in the longitudinal direction and the transverse direction was the same as in Examples 1 and 2. . On the other hand, in the cation exchange membranes of Comparative Example 1 and Comparative Example 2, the tensile yield stress was low because they were not reinforced with the fluorine fiber sheet. Moreover, in the cation exchange membrane of Comparative Example 3, since the fluorine fibers were not bonded to each other, the reinforcing effect was weak and the tensile yield stress was low.

In addition, in the cation exchange membranes of Examples 1 to 3, Comparative Example 1 and Comparative Example which were composed only of perfluorocarbon type ion exchange resin, though the fluorine fiber sheet was included as a reinforcing material. The cation exchange membrane of Example 2 and the cation exchange membrane of Comparative Example 3 reinforced with PTFE fibrils had substantially the same ion exchange capacity. The ion exchange capacity of the cation exchange membranes of Examples 1 to 3 is 0.7 mol / kg dry weight, which is a preferable range.
It was between 1.3 mol / kg dry weight and had a sufficient ion exchange capacity.

[0035]

INDUSTRIAL APPLICABILITY In the electrolyte membrane for polymer electrolyte fuel cells of the present invention, a cation exchange made of a perfluorocarbon-based ion exchange resin having a sulfonic acid group, which is reinforced by a fluorine fiber sheet in which fluorine fibers are bonded to each other. Since it is a film, it is excellent in mechanical strength even though it is as thin as 5 to 95 μm, and can be prevented from being deformed by an external force. At the same time, since the thickness is as thin as 5 to 95 μm, the electric resistance is small and the power generation efficiency is high. Further, in the method for producing an electrolyte membrane for a polymer electrolyte fuel cell of the present invention, the fibers of the fluorofiber papermaking sheet obtained in the papermaking step are heat-sealed in the heating step, so that the fibers are bonded to each other. A fiber sheet can be obtained. Furthermore, in the integration step, since the perfluorocarbon-based ion exchange resin having a sulfonic acid group is integrated with the fluorofiber sheet, the above-mentioned electrolyte membrane for polymer electrolyte fuel cells can be obtained.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) // C08L 27:12 C08L 27:12 (72) Inventor Ippei Osaki 2-5-21 Marunouchi, Okayama City, Okayama Prefecture No. (72) Inventor Takanori Suzuki No. 3 Munehamacho for Shizuoka City, Shizuoka Prefecture Tomagawa Paper Mill Technical Research Institute Co., Ltd. (72) Nozomi Tsuda No. 3 Munezawa Town for Shizuoka City, Shizuoka Prefecture F-term in paper mill technical laboratory (reference) 4F071 AA26 AA26C AA27C AD03C AF01 AF13 AH12 AH15 BB13 BC02 BC12 5G301 CA30 CD01 5H026 AA06 BB01 BB02 BB03 BB08 CX02 CX04 CX05 EE19 HH03 HH04 HH05 H09

Claims (10)

[Claims] 1. A thickness of 5 to 95 made of a perfluorocarbon-based ion exchange resin having a sulfonic acid group, which is reinforced by a fluorofiber sheet in which fluorofibers are bonded to each other.
An electrolyte membrane for polymer electrolyte fuel cells, which is a cation exchange membrane having a thickness of μm. 2. The porosity of the fluorofiber sheet is 55.
% To 90%, The electrolyte membrane for polymer electrolyte fuel cells according to claim 1. 3. The electrolyte membrane for a polymer electrolyte fuel cell according to claim 1 or 2, wherein the fluorofiber sheet is made of fluorofiber short fibers by a wet papermaking method. 4. The ion exchange capacity of the cation exchange membrane is
The electrolyte membrane for polymer electrolyte fuel cells according to any one of claims 1 to 3, which has a dry weight of 0.7 to 1.3 mol / kg. 5. The cation exchange membrane has a tensile yield stress in the longitudinal direction and in the transverse direction of 12 MPa or more, and
The ratio of the longitudinal tensile yield stress to the transverse tensile yield stress (longitudinal tensile yield stress / transverse tensile yield stress) is
It is 2.0 or less, The electrolyte membrane for polymer electrolyte fuel cells in any one of Claims 1-4 characterized by the above-mentioned. 6. The cation exchange membrane is characterized in that a fluorine fiber sheet is impregnated with a perfluorocarbon-based ion exchange resin having a sulfonic acid group, whereby the ion exchange resin is reinforced by the fluorine fiber sheet. The electrolyte membrane for polymer electrolyte fuel cells according to any one of claims 1 to 5. 7. A papermaking step of wet-making fluorocarbon fiber to obtain a fluorofiber paper sheet; a heating step of heating the fluorofiber paper sheet to obtain a fluorofiber sheet in which fibers are thermally fused to each other; A method for producing an electrolyte membrane for a polymer electrolyte fuel cell, comprising an integration step of integrating a perfluorocarbon-based ion exchange resin having a sulfonic acid group with a fiber sheet. 8. The method for producing an electrolyte membrane for a polymer electrolyte fuel cell according to claim 7, wherein in the paper making step, the fluorine fiber containing a binder is made by a wet paper making method. 9. The electrolyte membrane for a polymer electrolyte fuel cell according to claim 7, further comprising a cleaning step of cleaning the fluorine fiber sheet between the heating step and the integrating step. Manufacturing method. 10. In the integration step, an impregnated sheet is prepared by impregnating a fluorofiber sheet with a perfluorocarbon-based ion exchange resin liquid having a sulfonic acid group, and the impregnated sheet is heated and pressed while being deaerated in vacuum. Thickness of 5 to 95
The method for producing an electrolyte membrane for polymer electrolyte fuel cells according to any one of claims 7 to 9, wherein the electrolyte membrane is molded to have a thickness of µm.