JP2010103118A - Electrolyte film excellent in durability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrolyte film for a solid polymer fuel cell for maintaining battery characteristics after long hours of use (for example, several thousand hours), as is unattainable with a polymer ion exchange membrane including a fluorine system electrolyte film of prior art. <P>SOLUTION: In the electrolyte film for the solid polymer fuel cell and a membrane electrode assembly using the same with a monomer having a sulfone group as a cation exchange group grafted in a fluorine system polymer base material, a main chain of a graft chain is composed of hydrocarbon or hydrocarbon partially fluorinated and is combined with a sulfone group or a substituent having a sulfone group as a side chain, with an O/S value at least on one surface of the electrolyte film of 5.0 or more, and a surface element ratio of S of 0.4% or more and 5.0% or less in an element composition percentage by ESCA. An area change ratio of the electrolyte film at dipping in 40 wt.% methanol aqueous solution is to be 40% or less. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrolyte membrane characterized by exhibiting stable characteristics even in long-term use in a polymer electrolyte fuel cell.

Since the polymer electrolyte fuel cell has a high energy density, it is expected to be used in a wide range of fields such as a household cogeneration power source, a power source for portable devices, a power source for electric vehicles, and a simple auxiliary power source.
In the polymer electrolyte fuel cell, the electrolyte membrane functions as an electrolyte for conducting protons, and also has a role as a diaphragm for preventing direct mixing of hydrogen, methanol, and oxygen as fuels. As such an electrolyte membrane, it has a high ion exchange capacity as an electrolyte, is electrically and chemically stable and has a low electrical resistance because a current flows for a long time, has a strong mechanical strength, and hydrogen as a fuel. Gas, methanol, and oxygen gas are required to have low gas permeability.

  As this electrolyte membrane, a perfluorosulfonic acid membrane “Nafion (registered trademark of DuPont)” developed by DuPont has been generally used. However, conventional fluorine-based polymer ion exchange membranes such as “Nafion” have excellent chemical stability, but have low ion exchange capacity and insufficient water retention, so that the ion exchange membrane There was a problem that the proton conductivity was lowered due to the drying. When many sulfonic acid groups are introduced as a countermeasure, the membrane strength is extremely lowered by water retention, and the membrane is easily damaged. Therefore, it is difficult to achieve both proton conductivity and membrane strength. . In addition, fluorine polymer electrolyte membranes such as Nafion are very expensive because of the complicated synthesis of fluorine monomers as raw materials, and this is a major obstacle to the practical application of polymer electrolyte fuel cells. Yes.

  For this reason, development of low-cost and high-performance electrolyte membranes that replaces fluorinated electrolyte membranes such as Nafion is underway. As an example, an electrolyte membrane synthesized by introducing a styrene monomer into an ethylenetetrafluoroethylene copolymer (ETFE) membrane containing a hydrocarbon structure by a radiation graft reaction and then sulfonating (see Patent Document 1) is proposed. Has been.

  However, these and other conventional electrolyte membranes have a problem that the output is greatly reduced with long-term use. About this cause, the adhesiveness between the electrode and the electrolyte membrane is lowered by long-term use, that is, a space is formed between the electrode and the electrolyte membrane, so that proton conductivity in that portion is inhibited. there were.

  Here, as a technique for improving the adhesion between the electrolyte membrane and the electrode, for example, a method of increasing the contact area by attaching unevenness of about 1 to 5 μm to the surface of the electrolyte membrane by plasma etching treatment is disclosed. (See Patent Document 2). However, with this method, it is possible to increase the contact area with the electrode by making the electrolyte membrane surface uneven, but when considering long-term use, the amount of liquid retained in the electrolyte membrane and changes in temperature Along with this, stresses such as expansion and contraction of the membrane are continuously applied to the electrolyte membrane. Therefore, there is a problem that the membrane is damaged (breaks) particularly from the concave portion when the irregularity is too large. Moreover, in the case of long-term use, the effect was inadequate in terms of durability.

JP-A-9-102322 JP-A-4-220957

  The present invention is a solid polymer type that can maintain the battery characteristics even when used for a long period of time (for example, several thousand hours), which suppresses the drawbacks of polymer ion exchange membranes including conventional fluorine-based electrolyte membranes. An object is to provide an electrolyte membrane for a fuel cell.

  The present inventors have found that the deterioration of battery characteristics (for example, output, durability, etc.) associated with long-term use is caused by insufficient adhesion of the electrolyte membrane to the electrode material and dimensional change of the electrolyte membrane. .

  Specifically, the electrolyte membrane is in a state in which moisture or liquid such as methanol used as fuel in the direct methanol fuel cell is retained inside the battery. Depending on the change, dimensional changes (swelling and shrinking) of the electrolyte membrane occur. In addition, a dimensional change accompanying a temperature change during operation and stoppage also occurs. Such a phenomenon occurs repeatedly in long-term use, and even if the electrode and the electrolyte membrane are bonded in the initial stage, peeling occurs gradually at the interface, and the battery characteristics deteriorate accordingly. I found out.

Accordingly, the present invention provides an electrolyte membrane that can maintain adhesion to an electrode material even when used in a fuel cell, even when the usage situation or environment changes.
That is, the electrolyte membrane of the polymer electrolyte fuel cell of the present invention is an electrolyte membrane in which a monomer having a sulfone group as a cation exchange group is grafted to a fluorine-based polymer substrate, and the main chain of the graft chain is It is composed of a hydrocarbon or a partially fluorinated hydrocarbon, and has a sulfone group or a substituent having a sulfone group as its side chain, and at least one of the electrolyte membranes in terms of elemental composition ratio by ESCA The electrolyte membrane is characterized in that the surface O / S value is 5.0 or more and the surface element ratio of S is 0.4 or more and 5.0% or less.

  The electrolyte membrane is characterized in that the area change rate after being immersed in a 40 wt% methanol aqueous solution at a liquid temperature of 25 ° C. ± 2 ° C. for 24 hours is 40% or less.

  The polymer electrolyte membrane of the present invention is obtained by limiting the material of the base material and the graft chain, and further performing surface treatment after imparting ion conductivity to the membrane. By using this as an electrolyte membrane for a polymer electrolyte fuel cell, it is possible to improve adhesion to the electrode and to exhibit stable characteristics as a fuel cell over a long period of time (for example, several thousand hours).

  The electrolyte membrane of the present invention uses a fluoropolymer membrane or a fluoropolymer membrane to which a cross-linked structure is given by radiation reaction or the like as a base material, and a monomer having a cation exchange group is introduced into the base material by a graft reaction. The film can be produced by modifying at least one surface of the film by a discharge treatment. Thereby, it is possible to provide an electrolyte membrane capable of maintaining the characteristics even when the fuel cell is used for a long time (for example, several thousand hours).

  As a base material that can be used in the present invention, it is preferable to use a fluorine-based polymer that is highly durable against an electrochemical reaction or the like inside the battery. Specific examples of the fluorine-based polymer include polytetrafluoroethylene (hereinafter abbreviated as “PTFE”), tetrafluoroethylene-hexafluoropropylene copolymer (hereinafter abbreviated as “FEP”), tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer (hereinafter abbreviated as “PFA”), polyvinylidene fluoride (hereinafter abbreviated as “PVDF”), ethylene-tetrafluoroethylene copolymer (hereinafter abbreviated as “ETFE”), ethylene- A chlorotrifluoroethylene copolymer (ECTFE) etc. can be used. Moreover, when these base materials are previously crosslinked, the area change rate accompanying the liquid retention can be reduced. The cross-linking of the substrate can be performed by a known method. For example, the cross-linking method of PTFE is disclosed in Japanese Patent Laid-Open No. 6-164423, and the cross-linking method of FEP and PFA is disclosed in Japanese Patent Laid-Open No. 2001-348439. Yes. The form of the polymer substrate is a form of a film (film) from the request for use as an electrolyte membrane of a solid polymer fuel cell, and its size and thickness can be appropriately determined.

In the present invention, the graft chain can be obtained, for example, by using radiation and grafting the monomer to the substrate. The monomer that can be used in the present invention includes a monomer having a vinyl group, or a monomer in which a part of hydrogen bonded to the vinyl group is substituted with another atom or a functional group (hereinafter referred to as “vinyl”). System monomers)). The monomer may be a single kind or a mixture of plural kinds of monomers. When representing the monomers forming the graft chain in the general formula of _{H 2 C = CXR, H 2} C = CX- part of the present invention corresponds to the main chain, X is H, a hydrocarbon or F. R- corresponds to a side chain and is sulfonated by a sulfonation treatment described later. Specifically, it is preferable to use a monomer represented by the chemical formula (1).

Of the monomers represented by the chemical formula (1), it is more preferable to use an aromatic monomer in which R ₁ contains a benzene ring from the viewpoint that the sulfonation treatment described later can be easily performed.
Moreover, it is also possible to use a crosslinking agent having a plurality of unsaturated bonds having graft reactivity in the molecule as the vinyl monomer. Specific examples of vinyl monomers include, but are not limited to, 1,2-bis (p-vinylphenyl), divinyl sulfone, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, divinylbenzene, Cyclohexanedimethanol divinyl ether, phenylacetylene, diphenylacetylene, 1,4-diphenyl-1,3-butadiene, diallyl ether, 2,4,6-triallyloxy-1,3,5-triazine, triallyl-1,2 , 4-benzenetricarboxylate, triallyl-1,3,5-triazine-2,4,6-trione, butadiene, isobutene and the like. By using such a crosslinking agent, the graft chain can be crosslinked. By forming a crosslinked structure in the graft chain, swelling of the membrane can be suppressed.

  Graft polymerization of the above-mentioned monomer on the polymer substrate is a so-called pre-irradiation method in which the substrate is irradiated with radiation and then reacted with the monomer, or so-called simultaneous irradiation in which the substrate and the monomer are simultaneously irradiated with radiation to polymerize. It can be done by any of the methods. The pre-irradiation method is preferably used because the amount of homopolymer that does not graft onto the substrate is small. There are two types of pre-irradiation methods: a polymer radical method for irradiating a polymer substrate in an inert gas and a peroxide method for irradiating the substrate in an atmosphere in which oxygen is present. It can be used in the present invention.

  An example of the pre-irradiation method will be described below. First, after inserting the polymer substrate into a glass container, the container is vacuum degassed and replaced with an inert gas atmosphere. Next, the container containing the substrate is irradiated with an electron beam or γ-ray at 1 to 500 kGy at −10 to 80 ° C., preferably near room temperature. Next, the container is filled with a monomer to perform a graft reaction. The oxygen gas contained in the monomer is previously removed by bubbling with an inert gas not containing oxygen, freeze degassing, or the like. The monomer may be a single species or a mixture of plural species, or may be dissolved or diluted in a suitable solvent. When using a pre-crosslinked polymer substrate, the grafting reaction is usually carried out at 30 to 150 ° C, preferably 40 to 80 ° C.

  The graft ratio of the polymer thus obtained, that is, the weight percentage of the graft chain with respect to the polymer substrate before polymerization, is 8 to 70% by weight, more preferably 10 to 50% by weight. The graft ratio can be appropriately changed depending on the irradiation dose, polymerization temperature, polymerization time and the like.

  A cation exchange group such as a sulfone group is introduced into the polymer substrate into which the graft chain has been introduced as the next step. Introduction of a cation exchange group into the graft chain can be performed by a known method. For example, the conditions for introducing a sulfone group are disclosed in JP-A-2001-348439. Specifically, the grafted film is immersed in a chlorosulfonic acid solution having a concentration of 0.2 to 0.5 mol / L using 1,2-dichloroethane as a solvent, and reacted at 10 to 80 ° C. for 1 to 48 hours. After the reaction, the membrane is thoroughly washed with water. As a sulfonating agent necessary for the sulfonation reaction, concentrated sulfuric acid, fuming sulfuric acid, sulfur trioxide, sodium thiosulfate and the like can be used, and any kind can be used as long as it can introduce a sulfone group.

  The cation exchange group to be introduced is preferably a sulfone group which is a strong acid group because proton conductivity is improved, but is not limited thereto. The cation exchange group to be introduced may be a single species or a plurality of species. In the case of introducing a plurality of species, it is preferable to introduce a sulfone group and another cation exchange group, and examples of the other cation exchange group include a carboxyl group and a phosphone group.

  The elemental composition of the electrolyte membrane of the present invention is that the O / S (atomic percent of oxygen / atomic percent of sulfur) value of at least one surface of the membrane is 5.0 or more, and the surface element ratio of S is 0.2 to 5.0%, Preferably, the O / S value of at least one surface of the film is 7.0 or more, and the surface element ratio of S is 0.5 or more and 3.0% or less. Moreover, it is preferable that both surfaces of the film satisfy these characteristics. As described above, in the case of an electrolyte membrane having a sulfone group or a sulfone group and another ion exchange group in the graft chain, the amount of cation exchange groups introduced is increased by increasing the graft ratio, that is, O / It is possible to increase the S value. As the amount of cation exchange groups introduced is increased, the amount of hydrophilic groups introduced is increased, which has an advantageous effect on adhesiveness. On the other hand, the area change rate when water is absorbed and swollen is also proportionally increased. As a result, the stress acting on the interface between the electrode in the bonded state and the electrolyte membrane increases. Therefore, a hydrophilic functional group is introduced only on the surface of the electrolyte membrane that introduces a cation exchange group in an amount necessary for satisfying characteristics such as proton conductivity required for the electrolyte membrane and contributes to adhesion with the electrode. It is important to increase the amount of introduction. That is, for the purpose of achieving both proton conductivity and adhesion to the electrode, it is very important to define the element ratio of S derived from the sulfone group and to increase the O / S value. Become.

Here, as a method for increasing the O / S value after defining the element ratio of S, a method of using an oxygen-containing vinyl monomer as a monomer to be grafted, or a discharge treatment on at least one surface of the electrolyte membrane A method is mentioned. Here, the “oxygen-containing vinyl-based monomer” is a vinyl-based monomer in which a substituent bonded to the vinyl group contains oxygen. Specific examples include those containing oxygen in R ₁ of the monomer represented by the chemical formula (1). When the oxygen-containing vinyl monomer is grafted on the bulk of the electrolyte membrane, the membrane is easily hydrophilized due to the influence of the oxygen-containing functional group of the monomer, and as a result, the swelling property of the membrane tends to increase. Therefore, considering that the purpose of increasing the O / S value is to improve the adhesion of the electrolyte membrane to the electrode material, it is preferable to modify only the surface of the electrolyte membrane by discharge treatment. By introducing a hydrophilic functional group such as a hydroxyl group, a carbonyl group, or a carboxyl group on the surface of the film by performing a discharge treatment, the chemical bond strength with the components contained in the electrode can be increased, and the adhesion can be maintained over a long period of time. It becomes possible to hold.

The discharge treatment can be performed by plasma treatment using glow discharge, sputter etching treatment, atmospheric pressure plasma treatment, corona treatment, or the like. Among these, especially plasma processing by glow discharge and sputter etching processing can be performed in a specific gas atmosphere under reduced pressure, and stable and effective processing can be performed by selecting a gas type. preferable. Regarding the gas type, in the case of plasma treatment, it is preferably a gas containing oxygen atoms such as oxygen, water, carbon dioxide, or a mixed gas containing these. In the case of sputter etching, in addition to these, argon, nitrogen Etc. can also be used. In the case of sputter etching treatment, the charge frequency is 13.56 MHz which is an industrially assigned frequency, and the discharge energy calculated from the product of the treatment time and the discharge power is preferably 1 to 1000 J / cm ^2, preferably 5 to 200 J / cm ² is more preferred. Moreover, 0.05-200 Pa is preferable and the atmospheric pressure at the time of a process has more preferable 1-100 Pa.

  When an electrolyte membrane is prepared by this procedure, both the base material and the graft chain are treated by the discharge treatment, but in particular, the graft chain modification effect consisting of hydrocarbons or partially fluorinated hydrocarbons. Therefore, a markedly higher treatment effect can be obtained by the graft chain modification than the modification effect of the fluorine-based substrate itself. Especially in the case of so-called perfluoro-based substrates such as PTFE, PFA, and FEP, it becomes more prominent and enables higher adhesion than when modifying perfluoro-based electrolyte membranes such as Nafion. can do. In the present invention, since it has a graft chain made of hydrocarbon or partially fluorinated hydrocarbon, the reforming effect at this portion can be obtained more effectively.

  With respect to this surface form, an effect can be obtained if only at least one surface of the electrolyte membrane is treated, but it is preferable to treat both surfaces because the effect is enhanced by treating both surfaces.

  The electrolyte membrane according to the present invention needs to have an area change rate of 40% or less after being immersed in a 40 wt% methanol aqueous solution at a liquid temperature of 25 ° C. ± 2 ° C. for 24 hours. Here, the “area change rate” is the rate of change of the area of the film when the liquid retention reaches a saturated state as compared to before the liquid retention. This is because if the area change rate exceeds 40%, it may be difficult to maintain the adhesion to the electrode obtained by defining the element ratio on the electrolyte membrane surface. This characteristic can be controlled by the graft ratio of the electrolyte membrane, the amount of ion exchange groups such as sulfone groups introduced, the degree of crosslinking (crosslinking of the substrate, the amount of crosslinking agent added), and the like.

  In addition, when the area change rate is 40% or less, in addition to being able to maintain adhesion with the electrode, it is possible to suppress swelling, and in the case of a direct methanol fuel cell, the effect of suppressing cross-leakage of methanol as a fuel Is preferable.

The polymer electrolyte membrane according to the present invention preferably has an electric conductivity at 25 ° C. is 0.03Ω ^-1 cm ^-1 or more, and more preferably 0.1 [Omega ^-1 cm ^-1 or more. This is because if the electric conductivity is less than 0.03Ω ⁻¹ cm ⁻¹ , the membrane resistance is large and it becomes difficult to obtain a sufficient output.

  Here, as the characteristic relating to the membrane resistance, the thickness of the electrolyte membrane can be cited, and in order to reduce the membrane resistance, it is preferable to reduce the thickness. However, if the thickness is too thin, the film strength is reduced and easily broken, and it also leads to problems such as pinholes and other film defects. Therefore, the thickness is preferably 5 to 300 μm, More preferably, it is 150 μm.

  In addition, Japanese Patent Application Laid-Open No. 04-220957 and Japanese Patent Application Laid-Open No. 2001-229936 propose to positively form irregularities on the electrolyte surface for the purpose of improving adhesion, as already described. In addition, when used for a long time, it leads to damage of the electrolyte membrane and local variations in film thickness, etc., so it is not preferable that the unevenness is too large. Specifically, Ra (arithmetic mean roughness; JIS B 0601 ) Is preferably 1 μm, more preferably 0.5 μm or less.

Examples and Comparative Examples of the present invention are shown below, but the present invention is not limited to these.
Example 1
Cut PTFE film (Nitto Denko, product number: No.900; thickness: 50μm) into 10cm square, put it in a SUS autoclave irradiation container (inner diameter 4cm, height 30cm) with a heater, and place 1 × ^10-2 Torr ( After degassing to 1.3 Pa), the container was filled with argon gas so that the internal pressure was 1 atm. Next, the container heater was heated to 340 ° C., and ⁶⁰ Co-γ rays were irradiated at a dose rate of 3 kGy / hr and a dose of 120 kGy. After irradiation, the container was cooled, and the film was taken out to obtain crosslinked PTFE.

This cross-linked PTFE film was put in a glass separable container with a cock (inner diameter 3 cm, height 20 cm) and deaerated, and then filled with 1 atm of argon gas. In this state, ⁶⁰ Co-γ rays were again irradiated at a dose rate of 10 kGy / hr and a dose of 60 kGy at room temperature. Next, about 100 g of a styrene / toluene mixed liquid (mixed liquid of 50% by volume of styrene and 50% by volume of toluene) that had been degassed in advance was put into this container under an argon atmosphere. Here, the film was completely immersed in the mixed solution. After introducing the mixed solution, the vessel was heated at 60 ° C. for 15 hours to carry out a graft reaction. After the reaction, the vessel was sufficiently washed with toluene and dried to obtain a graft membrane.

  This graft-polymerized crosslinked PTFE film was immersed in a 0.3M chlorosulfonic acid solution diluted with 1,2-dichloroethane, heated in a sealed state at 60 ° C. for 24 hours, washed with water, dried, and sulfonated graft. A membrane, ie an electrolyte membrane, was obtained.

Furthermore, this electrolyte membrane was placed on the electrode surface of a sputtering apparatus having parallel plate electrodes, and the pressure was reduced in this state, and H ₂ O gas was introduced into the system to adjust to 13 Pa. In this atmosphere, the sputtering process was performed under conditions of a frequency of 13.56 MHz and a processing energy of 20 J / cm ² , and once the inside of the processing tank was returned to atmospheric pressure, the film was turned over and fixed, and the same operation as above was performed. An electrolyte membrane 1 subjected to double-sided sputtering treatment was obtained.
(Example 2)
The above except that PVDF film (thickness 50 μm) was used as the substrate, the film was not crosslinked before polymerization, and heated at 60 ° C. for 2 hours after charging the treatment mixture during the grafting reaction. In accordance with the procedure of Example 1, an electrolyte membrane 2 was obtained.
(Example 3)
An electrolyte membrane 3 was obtained according to the procedure of Example 2 except that 4-methylstyrene was used instead of styrene as the monomer.
Example 4
An electrolyte membrane 4 was obtained according to the procedure of Example 2 except that the processing energy of the sputtering treatment was 3 J / cm ² .
(Example 5)
The electrolyte membrane 5 was obtained according to the procedure of Example 2 except that the treatment mixture was charged at the time of grafting reaction and heated at 60 ° C. for 5 hours.
(Example 6)
The electrolyte membrane 6 was obtained according to the procedure of Example 1 except that the treatment mixture was charged at 60 ° C. for 2 hours after the grafting reaction.
(Comparative Example 1)
An electrolyte membrane 11 was obtained according to the procedure of Example 2 except that the sputtering treatment was not performed.
(Comparative Example 2)
An electrolyte membrane 12 was obtained according to the procedure of Example 5 except that the processing energy at the time of sputtering was 3 J / cm ² .
(Comparative Example 3)
An electrolyte membrane 13 was obtained according to the procedure of Example 1 except that the treatment mixture was charged at the time of grafting reaction and heated at 60 ° C. for 30 minutes.
(Comparative Example 4)
An electrolyte membrane 14 was obtained according to the procedure of Example 2 except that the graft reaction treatment was not performed.
(Characteristic evaluation method)
(1) graft ratio (X _ds)
The graft ratio was calculated by the following formula.

X _ds = (W2-W1) x 100 / W1
W1: Weight of polymer base material before grafting (g)
W2: Weight of polymer base material after grafting (g)
(2) Electrical conductivity (κ)
The electrical conductivity of the electrolyte membrane was measured by the AC method (New Experimental Chemistry Course 19, Polymer Chemistry <II>,
p992, Maruzen) measured the membrane resistance (Rm) using a normal membrane resistance measuring cell and an LCR meter (E-4925A; manufactured by Hewlett Packard). The cell was filled with 1M aqueous sulfuric acid solution, the resistance between the platinum electrodes (distance 5 mm) with or without the membrane was measured, and the electrical conductivity (specific conductivity) of the membrane was calculated using the following equation.

κ = 1 / Rm · d / S (Ω ^-1 cm ^-1 )
(3) Area change rate (S)
The electrolyte membrane was cut to 50 mm x 50 mm, and after leaving it in a dryer to dry sufficiently, the area was S1, and this sample was immersed in a 40 wt% methanol aqueous solution at a liquid temperature of 25 ° C ± 2 ° C for 24 hours The subsequent area was set to S2, and based on these values, the area change rate L was calculated by the following equation.

L = (S2-S1) x 100 / S1
(4) O / S value, surface element ratio of S Using ESCA (X-ray photoelectron spectrometer) as an analytical instrument, the surface element ratio was measured under the following conditions, and the calculation was performed based on the results.

Measuring instrument: ULVAC-PHI Quantum2000
Measurement area: φ200μm
X-ray output: 30W (15kV)
X-ray source: Monochrome AlK _α
Photoelectron extraction angle: 45 °
Neutralization condition: Combined use of neutralizing gun and ion gun (neutralization mode) (O / S value) = (O surface element ratio; Atomic%) / (S surface element ratio; Atomic%)
(5) Adhesiveness The adhesiveness test with respect to the electrode of an electrolyte membrane was done by the method shown below.

First, a membrane / electrode laminate was produced using the electrolyte membranes obtained in Examples and Comparative Examples. Specifically, 5 g of platinum-supporting carbon was dispersed in 100 ml of a Nafion solution (5% by weight) dissolved in isopropanol. Next, this dispersion was applied to the surface of the electrolyte membrane by screen printing and then dried at 100 ° C. for 20 minutes. In the same manner, the dispersion was applied to the back surface and dried to form electrode components on both surfaces of the electrolyte membrane. This was further held at 120 ° C. and 100 kg / cm ² for 2 minutes to produce an electrolyte membrane / electrode laminate.

Next, this laminate was immersed in a 40% by weight aqueous methanol solution and heated in a sealed container at 60 ° C. for 30 minutes, so that the electrolyte membrane was swollen. Next, the laminate was taken out and dried by blowing with air in an atmosphere of 60 ° C. for 30 minutes to return to a room temperature atmosphere. This was regarded as one cycle, and a total of 10 cycles were tested, and the number of cycles until the electrode and the electrolyte membrane were completely peeled and the adhesion state at the end of the 10 cycles were confirmed.
(Evaluation results)
The results are shown in the following table.

  As shown in the table, in Examples 1 to 6, the adhesion state was substantially maintained after 10 cycles of the adhesion test, whereas in Comparative Examples 1 and 2, the electrode and the electrolyte membrane were completely in the test. As a result, it was peeled off. Moreover, about Comparative Examples 3 and 4, although adhesiveness was favorable, it was confirmed that electroconductivity is very low and it does not function as an electrolyte membrane. From the above evaluation results, the effectiveness of the electrolyte membrane of the present invention was confirmed.

Claims (2)

An electrolyte membrane of a polymer electrolyte fuel cell in which a graft chain having a sulfone group as a cation exchange group is introduced to a fluorine-based polymer base material made of polytetrafluoroethylene or polyvinylidene fluoride, and an electrode are joined A membrane electrode assembly,
The graft chain is:

A sulfone group is introduced into a graft chain formed by graft-reacting at least one vinyl monomer selected from the formula represented by
A discharge treatment is applied to at least one surface of the membrane;
In the elemental composition ratio by ESCA, the O / S value of at least one surface of the electrolyte membrane is 5.0 or more, the surface element ratio of S is 0.4 or more and 5.0% or less, and the surface of the electrolyte membrane subjected to the discharge treatment is A membrane electrode assembly comprising an electrode and an electrode.   2. The membrane / electrode assembly according to claim 1, wherein an area change rate of the electrolyte membrane after being immersed in a 40 wt% methanol aqueous solution at a liquid temperature of 25 ° C. ± 2 ° C. for 24 hours is 40% or less. .