JP2004161956A - Fluorine-based ion-exchange membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fluorine-based ion-exchange membrane having excellent mechanical strength and thermal dimensional stability. <P>SOLUTION: The fluorine-based ion exchange membrane has 1-500 μm membrane thickness, ≥0.0005 plane orientation degree (▵P), ≥0.05 S/cm horizontal ion conductivity at 25°C and ≥1 shift (▵T) of α-dispersion temperature. <P>COPYRIGHT: (C)2004,JPO

Description

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【発明の効果】
本発明のフッ素系イオン交換膜は、高い強度と高い熱寸法安定性を同時に満足するものであり、産業上極めて有用である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fluorine-based ion exchange membrane, and more particularly to a fluorine-based ion exchange membrane having excellent performance as an electrolyte of a solid polymer electrolyte fuel cell and as a diaphragm.
[0002]
[Prior art]
2. Description of the Related Art A fuel cell is a kind of power generation device that extracts electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and has recently attracted attention as a clean energy supply source. Fuel cells are classified into phosphoric acid type, molten carbonate type, solid oxide type, solid polymer electrolyte type, etc., depending on the type of electrolyte used. Among these, solid polymer electrolyte type fuel cells are standard Because of its low operating temperature of 100 ° C. or lower and high energy density, it is expected to be widely used as a power source for electric vehicles and the like.
[0003]
The basic structure of a solid polymer electrolyte fuel cell is composed of an ion exchange membrane and a pair of gas diffusion electrodes joined to both sides of the membrane. Hydrogen is supplied to one electrode, oxygen is supplied to the other, and the space between the two electrodes is external. It generates power by connecting to a load circuit. More specifically, protons and electrons are generated at the hydrogen side electrode, and the protons move inside the ion exchange membrane and reach the oxygen side electrode, and then react with oxygen to generate water. On the other hand, the electrons that flow out of the hydrogen-side electrode through the conducting wire, after electric energy is extracted in the external load circuit, further travel through the conducting wire to reach the oxygen-side electrode, and contribute to the progress of the water generation reaction.
[0004]
As the required characteristics of the ion exchange membrane, firstly, high ion conductivity can be mentioned, but when protons move inside the ion exchange membrane, it is considered that water molecules are stabilized by hydration, Along with ion conductivity, high water content and water dispersibility are also important required characteristics. In addition, since the ion exchange membrane has a function as a barrier for preventing a direct reaction between hydrogen and oxygen, low permeability to gas is required. Other required characteristics include chemical stability for withstanding a strong oxidizing atmosphere during operation of the fuel cell and mechanical strength for withstanding further thinning.
[0005]
As a material of the ion exchange membrane used in the solid polymer electrolyte fuel cell, a fluorine-based ion exchange resin is widely used because of its high chemical stability. "Nafion (registered trademark)" manufactured by DuPont having a sulfonic acid group at a chain end is widely used. Such a fluorine-based ion exchange resin has generally well-balanced properties as a solid polymer electrolyte material. However, as the battery is put into practical use, further improvement in physical properties has been required.
[0006]
For example, thinning of ion-exchange membranes is expected to become even more important in the future in order to achieve a high level of current density and uniformity of moisture in the membranes. It is necessary to improve the mechanical strength. Similarly, demands for higher strength are increasing from the viewpoint of improving long-term durability. The stretching technique is one of the effective means for improving the mechanical strength of a membrane or a film, and a method of obtaining a high-strength ion exchange membrane by stretching is already known. Patent Literature 1 discloses that at least one plane direction is used for an ion exchange resin swollen with a liquid organic compound or a melt processable precursor of an ion exchange resin swollen with a fluorine-containing liquid organic compound. A production method for stretching the film is disclosed.
[0007]
In Example 1 of Patent Literature 1, the mechanical strength was 2.8 × 10 by stretching the fluorine-based ion exchange resin 2 × 2 times in the longitudinal and transverse directions at 125 ° C.⁷6.3 × 10 from Pa⁷It is disclosed that it rises to Pa. However, it has been revealed that the stretched film according to this example has a large heat shrinkage. For example, when the film is exposed to a temperature equivalent to a hot press at the time of producing a membrane electrode assembly (MEA), a large heat shrinkage occurs and the orientation is relaxed. And the film shrinks in hot water.
[0008]
Patent Document 2 discloses that heat shrinkage is reduced by heat-setting a stretched and oriented fluorine ion exchange membrane. However, there was a problem that the orientation was relaxed due to the heat fixation and the strength was reduced. In addition, since the ionic conductivity is reduced by the heat fixation, a washing process is required as a post-process, and the process is complicated.
As described above, the stretch-oriented film exhibits high mechanical strength, but in many cases, has a large heat shrinkage and the orientation is alleviated. There were restrictions.
[0009]
As described above, the prior art relating to the enhancement of strength is merely an attempt at simple stretching. In particular, since the stabilization of the stretching orientation is insufficient and the heat shrinkage is large, an industrially useful technique as an ion exchange membrane is used. Could not be disclosed.
[0010]
[Patent Document 1]
JP-A-60-149631
[Patent Document 2]
WO 02/062879 pamphlet
[0011]
[Problems to be solved by the invention]
An object of the present invention is to provide a fluorine-based ion exchange membrane having excellent mechanical strength and excellent thermal dimensional stability, and a method for producing the same.
[0012]
[Means for Solving the Problems]
By orienting the fluorine-based ion exchange membrane, its mechanical strength increases, but on the other hand, thermal shrinkage increases. The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, by appropriately setting the degree of plane orientation of the fluorine-based ion exchange membrane, α-dispersion temperature shift, etc., high strength and thermal dimensional stability Were found to be compatible, and the present invention was completed.
[0013]
That is, the present invention is as follows.
[1] When the film thickness is 1 to 500 μm, the degree of plane orientation (ΔP) is 0.0005 or more, the horizontal ionic conductivity at 25 ° C. is 0.05 S / cm or more, and the α dispersion temperature shift (ΔT) is 1 or more. A fluorine-based ion exchange membrane characterized by the following.
[2] The fluorinated ion exchange membrane according to [1], wherein the heat shrinkage at 160 ° C. is 45% or less.
[3] The method according to [1] or [2], wherein the counter ion of the ion exchange group is a proton and at least one cation selected from alkali metal ions, alkaline earth metal ions, and transition metal ions. The fluorinated ion exchange membrane according to the above.
[0014]
[4] A method for producing a fluorine-based ion-exchange membrane, comprising replacing a counter ion of an ion-exchange group of an oriented fluorine-based ion-exchange membrane with a proton and at least one kind of cation.
[5] The method for producing a fluorine-based ion-exchange membrane according to [4], wherein the oriented fluorine-based ion-exchange membrane according to [4] is produced by a process including the following.
(1) forming a fluorine-based ion-exchange resin precursor film from a fluorine-based ion-exchange resin precursor having an ion-exchange membrane precursor;
(2) a step of orienting the fluorine-based ion exchange resin precursor membrane, and
(3) a step of hydrolyzing the ion-exchange resin precursor under restraint while maintaining the orientation state of the fluorine-based ion-exchange resin precursor film obtained in (2).
[0015]
[6] The method for producing a fluorine-based ion-exchange membrane according to [4], wherein the oriented fluorine-based ion-exchange membrane according to [4] is produced by a process including the following.
(1) forming a fluorine-based ion-exchange resin precursor film from a fluorine-based ion-exchange resin precursor having an ion-exchange membrane precursor;
(2) hydrolyzing the fluorinated ion exchange resin precursor membrane to obtain a fluorinated ion exchange resin membrane; and
(3) A step of orienting the fluorine-based ion exchange resin membrane obtained in (2).
[7] A membrane electrode assembly comprising the fluorine-based ion exchange membrane according to any one of [1] to [3].
[8] A solid polymer electrolyte fuel cell comprising the fluorine-based ion exchange membrane according to any one of [1] to [3].
[0016]
Hereinafter, the present invention will be described in detail.
First, the fluorine-based ion exchange membrane of the present invention will be described.
(Thickness)
The thickness of the fluorine-based ion exchange membrane of the present invention is 1 μm or more, preferably 5 μm or more, more preferably 8 μm or more, and most preferably 10 μm or more. When the film thickness is less than 1 μm, inconveniences such as a direct reaction between hydrogen and oxygen are likely to occur, and the film may be damaged due to a difference in pressure or distortion during handling during fuel cell production or during fuel cell operation. It's easy to do. On the other hand, the upper limit of the film thickness is 500 μm or less, preferably 150 μm or less, more preferably 100 μm or less, and most preferably 50 μm or less. If the film thickness exceeds 500 μm, the ion permeability becomes low, so that sufficient performance as an ion exchange membrane cannot be exhibited.
[0017]
(Plane orientation degree)
A feature of the fluorine-based ion exchange membrane of the present invention is the degree of plane orientation. If this value does not show a certain value or more, it is merely a thin film, and improvement in mechanical strength cannot be expected.
The degree of plane orientation of the fluorine-based ion exchange membrane of the present invention is 0.0005 or more, preferably 0.0010 or more, more preferably 0.0015 or more, and most preferably 0.0020 or more. When the degree of plane orientation is less than 0.0005, the mechanical strength is insufficient and the ion exchange membrane does not have sufficient performance.
[0018]
(Horizontal ionic conductivity in 25 ° C water)
The horizontal ion conductivity of the fluorine-based ion exchange membrane of the present invention in water at 25 ° C. is 0.05 S / cm or more, preferably 0.06 S / cm or more, more preferably 0.07 S / cm or more, and further preferably 0 or more. 0.08 S / cm or more, most preferably 0.09 S / cm or more. When the horizontal ionic conductivity is less than 0.05 S / cm, the internal resistance increases when used as an ion exchange membrane for a fuel cell.
(Α dispersion temperature shift)
The α-dispersion temperature shift of the fluorine-based ion exchange membrane of the present invention is 1 or more, preferably 5 or more, more preferably 10 or more, still more preferably 15 or more, and most preferably 20 or more. When the α-dispersion temperature shift is less than 1, heat shrinkage occurs at a high temperature, and, for example, the production yield of the membrane electrode assembly decreases.
[0019]
(Thermal shrinkage at 160 ° C)
The heat shrinkage at 160 ° C. of the fluorine-based ion exchange membrane of the present invention is preferably 45% or less, more preferably 40% or less, further preferably 35% or less, further preferably 30% or less, and most preferably 25% or less. % Or less. If the heat shrinkage exceeds 45%, heat shrinkage may occur in applications involving high-temperature processing, which may hinder, for example, the production of MEAs.
(Equivalent weight)
The equivalent weight (EW) of the fluorine-based ion exchange membrane of the present invention is not limited, but is preferably 400 or more, more preferably 600 or more, and most preferably 700 or more. If the equivalent weight is too low, the strength may decrease. On the other hand, the upper limit of EW is preferably 1400 or less, more preferably 1200 or less, and most preferably 1000 or less. When the equivalent weight is increased, the mechanical strength is improved even in an unoriented film, but at the same time, the ion conductivity may be reduced due to the reduced density of ion exchange groups.
[0020]
(Salt substitution rate)
The salt substitution ratio of the fluorine-based ion exchange membrane of the present invention is not limited, but is preferably 0.1% or more, more preferably 1% or more, further preferably 5% or more, further preferably 10% or more, and most preferably 20% or more. % Or more. When the salt substitution ratio is less than 0.1%, the α-dispersion temperature shift is small, so that thermal shrinkage tends to occur at high temperatures, and for example, the production yield of the membrane / electrode assembly may decrease. The upper limit of the salt substitution rate is preferably 99% or less, more preferably 90% or less, further preferably 80% or less, further preferably 70% or less, and most preferably 60% or less. When the salt substitution ratio exceeds 99%, a membrane having a salt substitution ratio is generally considered to have low ionic conductivity, and thus may not have sufficient performance as an ion exchange membrane.
[0021]
(Converted puncture strength)
The converted puncture strength (the puncture strength in a dry state per 25 μm) of the fluorine-based ion exchange membrane of the present invention is not limited, but is preferably 300 g or more, more preferably 350 g or more, and most preferably 400 g or more. If the reduced puncture strength is less than 300 g, sufficient mechanical strength may not be obtained when the film is made thin. Although there is no particular upper limit on the converted piercing strength, a membrane having a strength of 3000 g or more is generally expected to have a low moisture content, and thus may not have sufficient performance as an ion exchange membrane.
[0022]
Next, a method for producing a fluorine-based ion exchange membrane of the present invention will be described.
(Raw polymer)
The fluorinated ion exchange resin precursor used in the present invention has a general formula CF₂= CF-O (CF₂CFXO)_n− (CF₂)_mA vinyl fluoride compound represented by the general formula CF₂= At least a binary copolymer with a fluorinated olefin represented by CFZ. Here, X is an F atom or a perfluoroalkyl group having 1 to 3 carbon atoms, n is an integer of 0 to 3, m is an integer of 1 to 3, Z is H, Cl, F or 1 carbon atom. To 3 perfluoroalkyl groups. W is converted to CO by hydrolysis.₂H or SO₃H is a functional group that can be converted to H. Such a functional group includes SO 2₂F, SO₂Cl, SO₂Br, COF, COCl, COBr, CO₂CH₃, CO₂C₂H₅Is usually preferably used.
[0023]
Such a fluorine-based ion exchange resin precursor can be synthesized by known means. For example, a method of dissolving the above-mentioned vinyl fluoride compound in a solvent such as chlorofluorocarbon and polymerizing it by reacting with a fluorinated olefin gas (solution polymerization), after emulsifying the vinyl fluoride compound in water together with a surfactant, , A method of polymerizing by reacting with a fluorinated olefin gas (emulsion polymerization), and further, a suspension polymerization and the like are known, all of which can be used as a suitable method.
[0024]
(Deposition process)
As a method of forming the fluorine-based ion exchange resin precursor into a film, any method generally known as a forming method such as a melt forming method (T-die method, inflation method, calendering method, etc.), a casting method, and the like can be used. Any of them can be suitably used. Examples of the casting method include a method in which a fluorinated ion exchange resin is dispersed in an appropriate solvent, and a method in which a polymerization reaction solution itself is formed into a sheet and the dispersion medium is removed.
[0025]
The resin temperature at the time of performing the melt molding by the T-die method is preferably 100 ° C. or higher, more preferably 200 ° C. or higher. In consideration of the heat resistance of the resin, the temperature is preferably 300 ° C. or lower, more preferably 280 ° C. or lower. The resin temperature at the time of performing the melt molding by the inflation method is preferably 100 ° C. or higher, more preferably 160 ° C. or higher. Considering the heat resistance of the resin, the temperature is preferably 300 ° C. or lower, more preferably 240 ° C. or lower.
[0026]
The sheet melt-formed by these methods is cooled to a temperature lower than the melting temperature by using a cooling roll or the like.
It is preferable to adjust the thickness of the precursor film to an optimum thickness in anticipation of a decrease in the thickness in the alignment step. Therefore, the film thickness is adjusted such that a film having a degree of plane orientation of 0.0005 or more and a film thickness of 1 to 500 μm is obtained when the film is oriented by stretching. For example, when performing 4 × 4 stretching in the alignment step, it is necessary to adjust the thickness of the precursor film to around 400 μm in order to make the thickness of the alignment film 25 μm.
[0027]
(Hydrolysis step)
As a hydrolysis method, for example, as described in Japanese Patent No. 2757331, an ion exchange group precursor of an alignment film is converted to an ion whose counter ion of an ion exchange group is a metal salt cation using an alkali hydroxide solution. The ion exchange group is converted to a proton exchange resin by using an acidic aqueous solution such as hydrochloric acid.₃H or COOH).
[0028]
In the hydrolysis, after converting the ion exchange group precursor into an ion exchange resin in which the counter ion of the ion exchange group is a metal salt cation, a part of the metal salt cation is replaced with a proton. At that time, by controlling the substitution rate with protons, ions having a horizontal ionic conductivity of 0.05 S / cm or more at 25 ° C. and an α dispersion temperature shift (ΔT) of 1 or more can be obtained by one hydrolysis. It can be converted to an exchange resin membrane. In addition, by adjusting the rate of substitution with protons, the cation is once converted to an ion exchange resin membrane having an α dispersion temperature shift (ΔT) of 1 or less by hydrolysis, and then converted to cations in a subsequent step of substituting with cations. By adjusting the substitution rate, it is possible to convert to an ion exchange resin membrane having a horizontal ionic conductivity at 25 ° C. of 0.05 S / cm or more and an α-dispersion temperature shift (ΔT) of 1 or more.
[0029]
Generally, an ion-exchange membrane is manufactured by forming an ion-exchange resin precursor into a membrane and then performing hydrolysis at a high temperature. Therefore, in the present invention, stretching can be roughly classified into “fluorine ion exchange resin precursor before hydrolysis” and “fluorine ion exchange resin after hydrolysis”. In the present invention, any film can be stretched according to the purpose.
[0030]
(Stretching of fluorinated ion exchange resin precursor)
The first preferred form of stretching in the present invention is performed on a fluorine-based ion exchange resin precursor. A particularly important point in the stretching of the fluorine-based ion exchange resin precursor is the prevention of the relaxation of the orientation accompanying the completion of the stretching. This is for the following reasons.
Generally, the stretching temperature of a film is often set with reference to the α dispersion temperature in viscoelasticity measurement. The α-dispersion temperature is a temperature at which the polymer main chain is considered to start thermal motion, and is widely used as an index when processing while giving a large strain to a polymer such as stretching.
[0031]
Since the α-dispersion temperature of the fluorine-based ion exchange resin precursor is around room temperature, when the constraint is removed from the stretched state, the α-dispersion often rapidly shrinks and loses the stretch orientation. The present inventors have found a stretching and fixing method that does not depend on the α-dispersion temperature with respect to the relaxation of the orientation of the fluorine-based ion-exchange resin precursor, focusing on hydrolysis, which is a production process unique to the precursor. That is, the present invention is characterized in that after the fluorine-based ion exchange resin precursor is stretched, the precursor is hydrolyzed in a state where the stretching orientation is restricted.
[0032]
Although it is not clear why stretching and fixing can be achieved by such a method, it is considered that the α-dispersion temperature of the fluorine-based ion exchange resin generated by hydrolysis is much higher than that of the precursor and exists around 120 ° C. Therefore, by performing the hydrolysis while maintaining the stretched orientation, the thermal motion of the main chain was reduced in the process of increasing the α dispersion temperature of the alignment film with the progress, and it was possible to achieve stretch fixation. Conceivable. In the present invention, such a stretching fixing method is referred to as “saponification fixing”.
[0033]
The reason why the saponification fixation can be achieved can be further considered as follows. Hydrolysis of the fluorinated ion-exchange resin precursor causes a large amount of water to be absorbed.However, such water does not exist uniformly inside the resin, but rather exists locally while forming microscopic water droplets. It is considered. Such water droplets are called clusters and can be specifically observed by small-angle X-ray diffraction or a transmission electron microscope. One cluster is expected to contain a plurality of side chain terminals. However, after stretching the fluorine-based ion-exchange resin precursor and forming a cluster while maintaining the constraint, these side chain terminals may be connected to each other. It can be expected to function as a kind of cross-linking point that is linked to each other via water. That is, in addition to the increase in the α dispersion temperature, the cluster formed after the stretching orientation functions as a pseudo-crosslinking point, so that the saponification fixation functions more favorably.
On the other hand, the orientation film without the saponification fixation is largely open when the constraint is released and when the film is exposed to a high-temperature saponification solution. Mechanical strength is reduced to the extent.
[0034]
(Stretching of fluorine ion exchange resin)
The second of the preferred modes of stretching in the present invention is for a fluorine-based ion exchange resin. As described above, since the α-dispersion temperature of the fluorine-based ion exchange resin is considered to be around 120 ° C., stretching and fixing by cooling is easy, and stretching orientation is easily maintained even after the constraint is released. In particular, since stretching to a fluorine-based ion exchange resin does not require a special treatment such as saponification fixing, a general stretching technique can be applied, and a viewpoint of improving the productivity of an ion exchange membrane for a fuel cell. Is preferred. That is, in the present invention, as a second preferred mode of stretching, the fluorine-based ion-exchange resin precursor is stretched after hydrolysis.
[0035]
(Orientation step)
As described above, the objects to be stretched can be broadly classified into “fluorine ion exchange resin precursor before hydrolysis” and “fluorine ion exchange resin after hydrolysis”. For any of the films, a method generally known as a film stretching method can be used. Among them, a transverse uniaxial stretching by a tenter, a sequential biaxial stretching by a tenter and a longitudinal stretching roll, Simultaneous biaxial stretching using a biaxial tenter and blow stretching using an inflation film forming apparatus are preferred, and simultaneous biaxial stretching and blow stretching are more preferred.
[0036]
Stretching in the present invention means elongation accompanied by generation of stretching stress, and elongation without generation of stretching stress is referred to as widening. For example, when the orientation step is not performed before the hydrolysis step, the film swells largely in the horizontal direction due to water absorption accompanying the hydrolysis, but when the film is stretched following this change, it can be considered as widening. .
Stretching is performed by appropriately selecting conditions such as a stretching ratio and a stretching temperature so that the degree of plane orientation (ΔP) of the film is 0.0005 or more and the film thickness is 1 to 500 μm.
[0037]
The stretching ratio is an area ratio. The lower limit is preferably 1.1 or more, more preferably 2 or more, most preferably 4 or more, and the upper limit is preferably 50 or less, more preferably 16 or less. , Most preferably 9 times or less. Of these, the stretching ratio in the transverse direction (direction perpendicular to the machine direction) is preferably at least 1.1 times, more preferably at least 1.5 times, most preferably at least 2 times, and most preferably at least 2 times. Is preferably 50 times or less, more preferably 9 times or less, and most preferably 3 times or less. A suitable stretching temperature is equal to or lower than the melting temperature of the precursor film, and the lower limit is preferably equal to or higher than (α dispersion temperature −100 ° C.), more preferably equal to or higher than (α dispersion temperature −80 ° C.), and most preferably equal to (α). The upper limit is preferably (α dispersion temperature + 100 ° C.) or less, more preferably (α dispersion temperature + 80 ° C.) or less, and most preferably (α dispersion temperature + 50 ° C.) or less.
[0038]
(Conversion of counter ion of ion exchange group by cation (salt substitution))
The term “salt substitution” used herein means that a counter ion of an ion exchange group is converted to an ion exchange resin that is a metal salt cation by hydrolysis of a fluorine-based ion exchange resin precursor membrane, and then a part of the metal salt cation is converted to a proton. This is a process that is performed on residual protons after producing an ion exchange resin membrane having an α dispersion temperature shift (ΔT) of 1 or less. By this salt substitution, the fluorinated ion exchange membrane of the present invention having a horizontal ionic conductivity at 25 ° C. of 0.05 S / cm or more and an α-dispersion temperature shift (ΔT) of 1 or more is produced.
[0039]
As the type of cation used in the present invention, any type of cation having an α-dispersion temperature shift of 1 or more, such as an alkali metal ion, an alkaline earth metal, and a transition metal ion, can be used. The α-dispersion temperature shift can be controlled by the type and amount of the cation to be replaced. In particular, the higher the ionic valence, the greater the effect.
The salt substitution rate is appropriately determined so that the horizontal ionic conductivity of the ion exchange resin membrane is 0.05 S / cm or more and the α dispersion temperature shift (ΔT) is 1 or more. And so on. If the salt substitution ratio is too low, heat shrinkage is large, which may hinder the production of the MEA. In order to reduce the heat shrinkage at 160 ° C. to 45% or less, the salt substitution rate is set to 5% or more. As for the salt replacement ratio, the higher the salt replacement ratio, the larger the α-dispersion temperature shift. However, if the salt replacement ratio is too large, the ionic conductivity is reduced, which may be undesirable as an electrolyte membrane. Therefore, it is preferable to use a polyvalent ion because good thermal dimensional stability can be improved at a low salt substitution rate. Specifically, when the counter ion is a monovalent cation, the salt substitution rate is preferably 10 to 90%, more preferably 20 to 80%, and most preferably 30 to 70%. In the case of a divalent cation, the salt substitution rate is preferably from 5 to 60%, more preferably from 10 to 50%, and most preferably from 20 to 40%.
[0040]
The salt replacement can be performed by immersing the fluorinated ion exchange membrane in a salt solution containing a predetermined concentration of the cation to be replaced, or by applying or spraying the salt solution.
The concentration of the salt aqueous solution can be adjusted by the following method. For example, the description is made on the assumption that the counter ion is an n-valent cation and the salt substitution rate is X%. First, a dry weight W (g) of a fluorine-based ion exchange membrane having a proton as a counter ion is measured. Next, the equivalent weight EW (g / eq) of the fluorine-based ion exchange membrane is measured. From these, the substituent amount A (mol) is determined using the following formula.
A = W / EW
[0041]
The amount A of the substituent is the total amount of ion exchange groups of the fluorine-based ion exchange membrane. The salt substitution ratio X% means that X% of counter ions in all ion exchange groups are replaced with one or more cations other than protons, and the remaining (100-X)% of counter ions are protons. . The weight C (g) of the cation salt required when the counter ion is replaced with an n-valent cation and the salt replacement ratio is changed to X% is determined using the following equation.
C = A × X / 100 × D / n
D: Molecular weight of cation salt (g / mol)
The concentration of the salt aqueous solution is adjusted by dissolving the cationic salt C in 500 ml of water.
[0042]
Salt substitution can be performed by immersing the fluorine-based ion exchange membrane in which the counter ion of the ion exchange group is a proton in the aqueous salt solution thus prepared. In many cases, the immersion temperature can be sufficiently replaced at room temperature, and the salt aqueous solution may be heated to shorten the salt replacement time. Although the immersion time depends on the concentration of the salt aqueous solution and the immersion temperature, the salt substitution can be suitably performed in a range of about 1 minute to 12 hours. After completion of the salt substitution, the membrane is thoroughly washed with water and dried.
In the same manner as described above, a salt-substituted ion exchange membrane can be obtained by immersing a fluoride ion exchange membrane in which a counter ion of an ion exchange group is composed of a proton and other cations in an acidic aqueous solution having a predetermined concentration. .
[0043]
(Swelling process)
When a higher ion conductivity is to be developed, the water content of the fluorinated ion exchange membrane can be improved by performing a swelling treatment after the hydrolysis step, if necessary. For example, as described in JP-A-6-342665, swelling treatment is performed by heating a fluorine-based ion-exchange membrane in water or a mixture of water and an organic solvent soluble in water, and then ion-exchanged. By returning the counter ion of the exchange group to a proton, a fluorine-based ion exchange membrane having a high water content can be obtained.
[0044]
(Method for producing membrane electrode assembly)
When a fluorine-based ion exchange membrane is used in a solid polymer electrolyte fuel cell, an anode and a cathode are used as an MEA in which two types of electrodes are joined to both sides of the membrane.
The electrode is composed of fine particles of a catalytic metal and a conductive agent supporting the fine particles, and contains a water repellent as needed. The catalyst used for the electrode is not limited as long as it is a metal that promotes the oxidation reaction of hydrogen and the reduction reaction by oxygen.Platinum, gold, silver, palladium, iridium, rhodium, ruthenium, iron, cobalt, nickel, chromium , Tungsten, manganese, vanadium or alloys thereof. Among these, platinum is mainly used.
[0045]
The conductive agent may be any material as long as it is an electron conductive material, and examples thereof include various metals and carbon materials. Examples of the carbon material include carbon black such as furnace black, channel black, and acetylene black, activated carbon, and graphite. These may be used alone or as a mixture of two or more.
As the water repellent, a fluorine-containing resin having water repellency is preferable, and one having excellent heat resistance and oxidation resistance is preferable. For example, polytetrafluoroethylene, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer and the like can be mentioned.
[0046]
In order to form an MEA from the electrode and the ion exchange membrane, for example, the following method is adopted. Platinum-supporting carbon serving as an electrode material is dispersed in a solution in which a fluorine-based ion exchange resin is dissolved in a mixed solution of alcohol and water to form a paste. A predetermined amount of this is applied to a polytetrafluoroethylene (PTFE) sheet and dried. Next, the application surfaces of the PTFE sheets are faced to each other, and an ion exchange membrane is interposed therebetween, and the sheets are joined by hot pressing. The hot pressing temperature depends on the type of the ion exchange membrane, but is usually 100 ° C. or higher, preferably 130 ° C. or higher, more preferably 150 ° C. or higher.
As a method other than the above, a method of directly applying an electrode catalyst to an ion exchange membrane to form a catalyst layer, a method of directly spraying an ion exchange membrane using a spray or the like to form a catalyst layer, or the like can also be used. .
[0047]
(Proton conversion of counter ion of ion exchange group by contact with acid)
When trying to develop higher ionic conductivity, after forming an electrode catalyst layer as necessary, by contacting with an acid, the substituted cation is washed away, and the counter ion of the ion exchange group becomes a proton, Higher ion conductivity can be exhibited. For example, a known method of dipping in an acidic aqueous solution such as hydrochloric acid or spraying the acidic aqueous solution can be used.
[0048]
(Fuel cell manufacturing method)
Next, a method for manufacturing a solid polymer electrolyte fuel cell will be described. A solid polymer electrolyte fuel cell includes an MEA, a current collector, a fuel cell frame, a gas supply device, and the like. Among these, the current collector (bipolar plate) is a graphite or metal flange having a gas flow path on the surface or the like. In addition to transmitting electrons to an external load circuit, hydrogen or oxygen is transferred to the MEA surface. It has a function as a supply channel. By inserting MEAs between such current collectors and stacking a plurality of them, a fuel cell can be produced.
[0049]
Operation of the fuel cell is performed by supplying hydrogen to one electrode and oxygen or air to the other electrode. The higher the operating temperature of the fuel cell is, the higher the catalyst activity is, which is preferable. However, usually, the fuel cell is often operated at 50 ° C. to 100 ° C. where the water management is easy. On the other hand, a reinforced ion exchange membrane as in the present invention may be able to operate at 100 ° C. to 150 ° C. by improving the high temperature and high humidity strength. The higher the supply pressure of oxygen or hydrogen, the higher the output of the fuel cell is, which is preferable. However, since the probability of contact between the two due to breakage of the membrane increases, it is preferable to adjust the pressure to an appropriate pressure range.
[0050]
(Proton conversion of counter ion of ion exchange group by fuel cell operation)
In the present invention, the ion conductivity of the fluorinated ion exchange membrane reduced by the salt substitution is such that the protons moving inside the membrane during operation of the fuel cell wash out the substituted cations, and the counter ion of the ion exchange group. Becomes a proton, and may exhibit higher ionic conductivity.
[0051]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to the examples.
The property evaluation method used in the present invention is as follows.
(B) Film thickness
After leaving the ion exchange membrane in a constant temperature chamber at 23 ° C. and 65% for 1 hour or more, measurement is performed using a film thickness meter (Toyo Seiki Seisaku-sho, Ltd .: B-1).
[0052]
(B) Plane orientation degree
After leaving the ion exchange membrane in a constant temperature chamber at 23 ° C. and 65% for 12 hours or more, the refractive index in each of three directions is measured using an automatic birefringence meter KOBRA-21ADH manufactured by Oji Scientific Instruments.
nα: refractive index in the main orientation direction of the film
nβ: refractive index in the direction perpendicular to the main orientation direction of the film
nγ: refractive index in the thickness direction of the film
From these, the plane orientation degree ΔP is determined using the following equation.
ΔP = (nα + nβ) / 2−nγ
[0053]
(C) Horizontal ionic conductivity in 25 ° C water
After immersing the ion-exchange membrane in water at 25 ° C. for 30 minutes, it is cut into strips having a width of 1 cm, and six electrode wires having a diameter of 0.5 mm are brought into parallel contact with the surface at 1 cm intervals. After holding for 2 hours or more in a thermo-hygrostat adjusted to 25 ° C. and 98%, resistance is measured by an AC impedance method (10 kHz), and the resistance value per unit length is measured from the distance between the electrodes and the resistance. From this, the horizontal ionic conductivity Z (S / cm) at 25 ° C. is determined using the following equation.
Z = 1 / film thickness (cm) / film width (cm) / resistance value per unit length (Ω
/ Cm)
[0054]
(D) α dispersion temperature shift
After leaving the ion exchange membrane in a constant temperature chamber at 23 ° C. and 65% for 12 hours or more, a sample cut to a membrane width of 5 mm is set in a dynamic viscoelasticity measuring apparatus (ITA Measurement Control Co., Ltd .: DVA 200). Based on JIS K-7244, dynamic viscoelasticity was measured in a tensile mode at a chuck distance of 20 mm, a strain of 0.1%, a frequency of 35 Hz, and a heating rate of 5 ° C./min. Is the α dispersion temperature T_SAnd Next, the ion-exchange membrane is cut out to a membrane width of 5 mm, and immersed in 50 ml of 2N hydrochloric acid at 25 ° C. for 30 minutes or more to make the counter ion of the ion-exchange group a proton. The dynamic viscoelasticity of this ion exchange membrane was measured in the same manner as described above, and the peak temperature on the high temperature side among the tan δ peaks was changed to the α dispersion temperature T._HAnd From these, the α dispersion temperature shift ΔT is determined using the following equation.
ΔT = T_S−T_H
[0055]
(E) Heat shrinkage at 160 ° C
After leaving the ion exchange membrane in a constant temperature chamber at 23 ° C. and 65% for 12 hours or more, the membrane area before heating is measured. Then, after leaving it in an oven heated to 160 ° C. for 3 minutes, it is taken out of the oven and the film area after heating is measured while taking care not to absorb moisture. From these, the thermal shrinkage H at 160 ° C. is calculated using the following equation._a(%).
H_a= (1- (A₂/ A₁)^0.5) × 100
A₁: Film area before heating (cm²), A₂: Film area after heating (cm²)
[0056]
(F) Equivalent weight
Ion exchange group counter ion is proton ion exchange membrane about 2-10cm²Is immersed in 30 ml of a saturated aqueous solution of NaCl at 25 ° C., and left for 30 minutes with stirring. Next, the protons in the saturated aqueous NaCl solution are neutralized and titrated with a 0.01 N aqueous sodium hydroxide solution using phenolphthalein as an indicator. After the neutralization, the ion-exchange group obtained is a counter ion of a sodium ion, and the ion-exchange membrane is rinsed with pure water, dried in vacuum, and weighed. The amount of sodium hydroxide required for neutralization is represented by M (mmol), the weight of the ion exchange membrane in which the counter ion of the ion exchange group is sodium ion is represented by W (mg), and the equivalent weight EW (g / eq) is obtained from the following formula. Ask for.
EW = (W / M) −22
[0057]
(G) Salt substitution rate
Ion exchange membrane about 2-10cm²Is immersed in 30 ml of a saturated aqueous solution of NaCl at 25 ° C., and left for 30 minutes with stirring. Next, the protons in the saturated aqueous NaCl solution are neutralized and titrated with a 0.01 N aqueous sodium hydroxide solution using phenolphthalein as an indicator. At this time, the amount of sodium hydroxide required for neutralization was M₂(Mmol). Next, the ion-exchange membrane after the neutralization titration is immersed in 50 ml of 2N hydrochloric acid at 25 ° C. for 30 minutes or more to turn the counter ion of the ion-exchange group into a proton. This ion-exchange membrane was immersed in 30 ml of a saturated NaCl aqueous solution at 25 ° C., and allowed to stand for 30 minutes with stirring. Then, protons in the saturated NaCl aqueous solution were concentrated using phenolphthalein as an indicator using a 0.01 N sodium hydroxide aqueous solution. Perform a Japanese titration. At this time, the amount of sodium hydroxide required for neutralization was M₁(Mmol). From these, the salt substitution rate S (%) is determined using the following equation.
S = (M₁-M₂) / M₁× 100
[0058]
Other methods include quantifying ions eluted by immersion in a saturated NaCl aqueous solution by inductively coupled plasma (ICP), or using an ion exchange membrane by proton nuclear magnetic resonance (H-NMR) analysis. A method of immersing in 50 ml of 2N hydrochloric acid for 30 minutes or more to determine the H content of the ion exchange membrane in which the counter ion of the ion exchange group has been made into a proton to determine the H content can also be used.
[0059]
(H) Melt index
Based on JIS K-7210, the melt index of the fluorine-based ion exchange resin precursor measured at a temperature of 270 ° C. under a load of 2.16 kg is defined as MI (g / 10 minutes).
(I) Actual stretching ratio
Thickness T of precursor film before stretching_bAnd the film thickness T when measuring the converted piercing strength_aThen, the actual stretching ratio is determined using the following equation.
[0060]
Actual stretch ratio = (T_b/ T_a)^0.5
(Nu) Converted puncture strength
After leaving the ion exchange membrane in a constant temperature chamber at 23 ° C. and 65% for 12 hours or more, the radius of curvature of the needle tip is 0.5 mm and the piercing speed is 2 mm / using a handy compression tester (KES-G5, manufactured by Kato Tech). A piercing test was performed under the conditions of sec, and the maximum piercing load was defined as piercing strength (g). Further, the converted piercing strength (g / 25 μm) is obtained by multiplying the piercing strength by 25 (μm) / film thickness (μm).
[0061]
Embodiment 1
(Low magnification stretching with respect to ion exchange resin precursor, Ca: salt substitution rate 27%)
General formula CF₂= CF-O (CF₂CFXO)_n− (CF₂)_mA vinyl fluoride compound represented by the general formula CF₂= Copolymer with a fluorinated olefin represented by CFZ (where X is CF₃  , N is 1, m is 2, Z is F, W is SO₂  F. ) Was formed by using a T-die method to form a precursor film having a thickness of 110 μm. The precursor film was simultaneously biaxially stretched 2.0 × 2.0 times at a stretching temperature of 25 ° C. using a simple small stretching machine to form an alignment film. After stretching, the alignment film is immersed in a hydrolysis bath (DMSO: KOH: water = 5: 30: 65) heated to 95 ° C. for 15 minutes while being restrained by a simple small-sized stretching machine, and the ion-exchange group is removed. A fluorinated ion-exchange membrane having a counter ion of potassium ion was obtained.
[0062]
This was thoroughly washed with water and immersed in a 2N hydrochloric acid bath heated to 65 ° C. for 15 minutes to obtain a fluorine-based ion exchange membrane having a proton as a counter ion of an ion exchange group. After thoroughly washing this, the film was dried. The dried membrane was removed from the constraints and immersed in an aqueous solution of calcium chloride at a predetermined concentration to obtain a fluorine-based ion exchange membrane having a 27% salt substitution rate in which the counter ion of the ion exchange group was replaced with calcium ions. After thoroughly washing this with water, the film was dried to obtain a dry film having a thickness of 28.8 μm. Various property tests of the obtained fluorine-based ion exchange membrane were performed. Table 1 shows the results.
[0063]
Embodiment 2
(Low magnification stretching with respect to ion exchange resin precursor, Ca: salt substitution rate 37%)
A fluorine-based ion exchange membrane having a thickness of 25.3 μm was obtained in the same manner as in Example 1 except that the salt substitution rate was 37%. Table 1 shows the measurement results of various properties of the obtained film.
[0064]
Embodiment 3
(Low magnification stretching with respect to ion exchange resin precursor, K: salt substitution rate 43%)
A fluorine-based ion exchange membrane having a thickness of 26.0 μm was obtained in the same manner as in Example 1 except that the cation was potassium ion and the salt substitution rate was 43%. Table 1 shows the measurement results of various properties of the obtained film.
[0065]
Embodiment 4
(Low magnification stretching with respect to ion exchange resin precursor, K: salt substitution rate 59%)
A fluorine-based ion exchange membrane having a thickness of 23.2 μm was obtained in the same manner as in Example 1 except that the cation was potassium ion and the salt substitution rate was 59%. Table 2 shows the measurement results of the properties of the obtained film.
[0066]
Embodiment 5
(Low magnification stretching with respect to ion exchange resin precursor, Fe: salt substitution rate 7%)
A fluorine-based ion-exchange membrane having a thickness of 25.0 μm was obtained in the same manner as in Example 1 except that the cation was iron ion and the salt substitution rate was 7%. Table 2 shows the measurement results of the properties of the obtained film.
[0067]
Embodiment 6
(High magnification stretching with respect to ion exchange resin precursor, Ca: salt substitution rate 38%)
As shown in Table 2, as shown in Table 2, a fluorine-based ion exchange membrane having a thickness of 11.2 μm was obtained in the same manner as in Example 1 except that the salt substitution rate was 38%. Table 2 shows the measurement results of the properties of the obtained film.
[0068]
Embodiment 7
(Low draw ratio for low EW ion exchange resin precursor, Ca: salt substitution rate 25%)
(Raw polymer) Copolymer of vinyl fluoride compound of general formula and fluorinated olefin (where X is CF₃, N is 0, m is 2, Z is F, W is SO₂F. ), Except that the stretching temperature was 90 ° C., the stretching ratio was 1.9 × 1.9, and the salt replacement ratio was 25%, using a fluorine-based ion exchange resin precursor (EW: 720, MI: 3.6) consisting of A fluorine-based ion exchange membrane having a thickness of 32.2 μm was obtained in the same manner as in Example 1. Table 3 shows the measurement results of various characteristics of the obtained film.
[0069]
Embodiment 8
(Low draw ratio to ion exchange resin, Ca: salt substitution rate 33%)
(Raw polymer) Copolymer of vinyl fluoride compound of general formula and fluorinated olefin (where X is CF₃, N is 1, m is 2, Z is F, W is SO₂F. ) Was formed using a T-die method to form a precursor film having a thickness of 110 μm. This precursor membrane was immersed in a hydrolysis bath (DMSO: KOH: water = 5: 30: 65) heated to 95 ° C. for 1 hour to obtain a fluorine-based ion exchange membrane in which the counter ion of the ion exchange group was potassium ion. Was. After washing sufficiently with water, it was immersed in a 2N hydrochloric acid bath heated to 65 ° C. for 16 hours or more to obtain a fluorine-based ion exchange membrane in which the counter ion of the ion exchange group was proton. After thoroughly washing this, the film was dried. The dried film was simultaneously biaxially stretched 2 × 2 times at a stretching temperature of 125 ° C. using a simple small stretching machine to obtain an oriented film. After stretching, after detaching from the simple compact stretching machine, the alignment film is immersed in an aqueous solution of calcium chloride having a predetermined concentration, and the counter ion of the ion-exchange group is replaced by calcium ion, and the fluorine ion having a salt substitution rate of 33%. An exchange membrane was obtained. After thoroughly washing this with water, the film was dried to obtain a dried film having a thickness of 36.2 μm. Table 3 shows the measurement results of various characteristics of the obtained film.
[0070]
[Comparative Example 1]
(Unoriented film)
As in Example 1, a fluorine-based ion exchange resin precursor (EW: 950, MI: 20) was formed into a film using a T-die method, and was hydrolyzed in a non-oriented state to form a film having a thickness of 31.7 μm. A fluorinated ion exchange membrane was obtained. Table 4 shows the measurement results of various properties of the obtained film.
[0071]
[Comparative Example 2]
(Low magnification stretching to ion exchange resin precursor, salt substitution rate 0%)
A fluorine-based ion exchange membrane having a thickness of 25.2 μm was obtained in the same manner as in Example 1 except that the salt substitution treatment was not performed. Table 4 shows the measurement results of various properties of the obtained film.
[0072]
[Comparative Example 3]
(Unoriented film, Ca: salt substitution rate 35%)
A fluorine-based ion-exchange resin precursor (EW: 950, MI: 20) was formed by a T-die method in the same manner as in Example 1, and hydrolysis was performed in an unoriented state to obtain a fluorine-based ion-exchange membrane. Was. The membrane was immersed in an aqueous solution of calcium chloride having a predetermined concentration to obtain a fluorine-based ion-exchange membrane having a salt exchange rate of 35% in which counter ions of ion-exchange groups were replaced with calcium ions. After thoroughly washing this with water, the film was dried to obtain a dry film having a thickness of 31.2 μm. Table 4 shows the measurement results of various properties of the obtained film. In addition, "-" in a table | surface shows unmeasured.
[0073]
[Table 1]

[0074]
[Table 2]

[0075]
[Table 3]

[0076]
[Table 4]

[0077]
【The invention's effect】
The fluorine-based ion exchange membrane of the present invention satisfies both high strength and high thermal dimensional stability at the same time, and is extremely useful in industry.

Claims (8)

The film thickness is 1 to 500 μm, the degree of plane orientation (ΔP) is 0.0005 or more, the horizontal ionic conductivity at 25 ° C. is 0.05 S / cm or more, and the α dispersion temperature shift (ΔT) is 1 or more. Characterized fluorine-based ion exchange membrane. The fluorine-based ion exchange membrane according to claim 1, wherein the heat shrinkage at 160 ° C is 45% or less. 3. The fluorine system according to claim 1, wherein a counter ion of the ion exchange group is a proton and at least one cation selected from an alkali metal ion, an alkaline earth metal ion and a transition metal ion. Ion exchange membrane. A method for producing a fluorine-based ion exchange membrane, comprising replacing a counter ion of an ion exchange group of an oriented fluorine-based ion exchange membrane with a proton and at least one kind of cation. The method for producing a fluorine-based ion-exchange membrane according to claim 4, wherein the oriented fluorine-based ion-exchange membrane according to claim 4 is produced by steps including the following.
(1) forming a fluorine-based ion-exchange resin precursor film from a fluorine-based ion-exchange resin precursor having an ion-exchange membrane precursor;
(2) a step of orienting the fluorine-based ion-exchange resin precursor membrane, and (3) a step of subjecting the fluorine-based ion-exchange resin precursor membrane obtained in (2) to the ion-exchange resin precursor while maintaining the alignment state. Hydrolyzing the body. The method for producing a fluorine-based ion-exchange membrane according to claim 4, wherein the oriented fluorine-based ion-exchange membrane according to claim 4 is produced by steps including the following.
(1) forming a fluorine-based ion-exchange resin precursor film from a fluorine-based ion-exchange resin precursor having an ion-exchange membrane precursor;
(2) a step of hydrolyzing the fluorinated ion exchange resin precursor membrane to obtain a fluorinated ion exchange resin membrane, and (3) a step of orienting the fluorinated ion exchange resin membrane obtained in (2). A membrane electrode assembly comprising the fluorine-based ion exchange membrane according to claim 1. A solid polymer electrolyte fuel cell comprising the fluorine-based ion exchange membrane according to claim 1.