JP4332607B2 - High strength solid electrolyte membrane - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solid polymer electrolyte membrane having a high mechanical strength, particularly a high tensile strength, and relates to a solid electrolyte membrane useful for applications such as a polymer electrolyte fuel cell, water electrolysis, salt electrolysis, and various sensors.
[0002]
[Prior art]
The electrolyte membrane functions as a conductor that moves protons generated by dissociation of hydrogen to the oxygen side. Generally, the proton conductivity increases as the membrane exchange capacity increases, and the membrane resistance increases as the membrane thickness decreases. Is preferable. Conventionally, a perfluorocarbon sulfonic acid membrane has been used as an electrolyte membrane for a polymer electrolyte fuel cell, and exhibits relatively good performance. Actually, a membrane having an exchange capacity of about 1.25 meq / g and a perfluorocarbon-based electrolyte membrane having a thickness of about 50 μm are made and marketed, and a typical example thereof is Nafion (registered trademark) (DuPont, USA). Manufactured, hereinafter simply Nafion), Aciplex <registered trademark> (manufactured by Asahi Kasei Kogyo Co., hereinafter simply Aciplex), Flemion <registered trademark> (manufactured by Asahi Glass, hereinafter simply Flemion), and the like.
[0003]
However, conventional electrolyte membranes have the disadvantages of decreasing cell voltage and mechanical strength due to chemical shorts as the amount of permeation of gas components increases. Has been.
For example, in order to suppress a decrease in mechanical strength, a porous substrate such as a woven fabric made of a fluoropolymer such as polytetrafluoroethylene (PTFE) is inserted into the membrane (Japanese Patent Laid-Open No. 53-56192, JP-A-58-37186, JP-A-58-37187, etc.), impregnating a porous film with a solution of a fluorine-containing polymer having a functional group, followed by drying and heat treatment (JP-A-6-342666), Means for mixing a fibrillated fiber with a fluorinated cation exchange resin having a sulfonic acid group or a carboxylic acid group (Japanese Patent Laid-Open Nos. 53-14988, 54-1283, 54-107479, 54-107) 157777) has been attempted. In addition, a fluoropolymer having a sulfonic acid group or a precursor thereof and a carboxylic acid group or a precursor thereof is mechanically mixed (Japanese Examined Patent Publication No. 62-7217, Japanese Examined Patent Publication No. 60-17034, Japanese Unexamined Patent Publication No. Sho 62-53341, Japanese Patent Publication No. Shoko). 59-15934) Attempts have been made.
[0004]
However, each of the above examples has a drawback that the electrical resistance increases with the improvement of the mechanical strength of the membrane, and there is a strong demand for a means for achieving the maintenance or reduction of the electrical resistance along with the improvement of the strength.
[0005]
[Problems to be solved by the invention]
It is an object of the present invention to provide a solid electrolyte membrane having a low mechanical resistance and excellent mechanical strength, and a method for producing the same.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventor has found that two or more fluoropolymers having an acidic functional group or the precursor form a compatible state, and thereafter It has been discovered that the temperature shifts from a compatible state to a phase separation state, and the phase separation structure can be controlled by manipulating the temperature of the system. It was found that the mechanical strength of the film can be improved by reflecting it in the microstructure, and the present invention has been made.
[0007]
Hereinafter, the present invention will be described in detail.
The acidic functional group possessed by the fluoropolymer according to the present application is a sulfonic acid group, a carboxylic acid group, a phosphonic acid group, a hydroquinone group, a thiol group, etc., and the precursor is a chemical reaction such as hydrolysis or substitution reaction. Can be converted into an acidic functional group. As a typical example, one or more polymerizable monomers represented by the following formula (1) and one or more polymerizable monomers selected from the group of polymerizable monomers described below are used. And a copolymer composed of a body.
Y- (CF ₂ ) a- (CFRt) b- (CFRt ') cO- (C (CF ₂ X) F-CF ₂ -O) n-CF = CF ₂ [Formula 1]
(Wherein, -Y _{is, -SO 3 H, -SO 2 F} , -SO 3 Na, -SO 3 K, -SO 2 NH 2, -SO 2 NH 4, -COOH, -CN, -COF, - COOR (R is an alkyl group having 1 to 10 carbon atoms), - PO ₃ H ₂ or -PO ₃ H in which .a is an integer of 0 to 6, b is an integer of 0 to 6, c is 0 or 1 And a + b + c ≠ 0 and n is an integer of 0 to 6. X is one of Cl, Br or F, or a combination of two or more when n ≧ 1, and Rt and Rt ′ are F, Cl, a perfluoroalkyl group having 1 to 10 carbon atoms and a fluorochloroalkyl group having 1 to 10 carbon atoms.)
Examples of polymerizable monomers to be copolymerized therewith include tetrafluoroethylene, trifluoromonochloroethylene, trifluoroethylene, vinylidene fluoride, 1,1-difluoro-2,2-dichloroethylene, 1,1-difluoro-2. -Chloroethylene, hexafluoropropylene, 1,1,1,3,3-pentafluoropropylene, octafluoroisobutylene, ethylene, vinyl chloride, alkyl vinyl ester and the like.
[0008]
In the present invention, the two or more types of fluoropolymers are sufficiently compatible to form a homogeneous mixed melt from the polymers exemplified above, and below the melting point of the melt. Selected as a combination of those that cause phase separation under the following temperature conditions. For example, a combination of two or more kinds of fluoropolymers having different average molecular weights, a combination of two or more kinds of fluoropolymers having different functional groups, and the like, are easy to achieve phase separation of a mixed solution. Therefore, a combination of polymers having different functional groups is preferable, and among them, a polymer having a sulfonic acid functional group and a carboxylic acid functional group are preferably selected from the balance of mechanical properties and electric resistance of the finally obtained exchange membrane. A combination of polymers is particularly preferred.
[0009]
When the precursor of the acidic group is used, an acidic group, for example, a sulfonic acid group precursor is converted into a sulfonic acid group after film formation by a method described later.
The exchange capacity of the fluoropolymer having a sulfonic acid group or a precursor thereof of the present invention is 0.8 to 1.5 meq / g, preferably 0.9 to 1.3 meq / g. Alternatively, when the precursor is included, the exchange capacity is 0.6 to 1.2 meq / g, preferably 0.7 to 1.1 meq / g, from the viewpoint of ionic conductivity of the membrane. Particularly preferred. The fluorine-containing polymer having a sulfonic acid group has a smaller electric resistance at the same exchange capacity than the fluorine-containing polymer having a carboxylic acid group, and the mechanical strength of the fluorine-containing polymer having a carboxylic acid group is large. It is known to researchers in the field. Therefore, it is advantageous for obtaining the target solid electrolyte to impart the strength of the fluoropolymer having a carboxylic acid group to the fluoropolymer having a sulfonic acid group having a low electric resistance. Therefore, in the sponge-like microstructure solid electrolyte membrane of the present invention, the fluorinated polymer having a carboxylic acid group or its precursor is preferably 5 to 40% by weight.
[0010]
The microstructure of the solid polymer film of the present invention is sponge-like. The microstructure can be observed as a structure formed from a plurality of phases having different refractive indexes using a phase contrast microscope. Photo 1 shows the sponge-like microstructure of the electrolyte membrane obtained in Example 1. As is apparent from the photograph, it consists of two phases: a white network structure having a relatively small refractive index and a black spherical structure having a large refractive index. Both the network structure and the spherical structure are three-dimensionally continuous, and each forms one continuous phase uniformly distributed in the system. If the system does not form a sponge-like microstructure, at least one of the above structures is discontinuous, and one structure is divided and surrounded by the other structure as a fine drop-like structure within the system. A so-called sea-island structure is formed.
[0011]
The solid polymer membrane having the above-mentioned sponge-like microstructure is formed by phase separation of two or more kinds of fluoropolymers. In order to cause this phase separation in an actual exchange membrane manufacturing process, a film forming process is required as the first stage, a compatibility process as the second stage, and a phase separation process as the third stage.
In the first-stage film-forming process, as will be described later, a film-like product is formed by a known method from a system comprising two or more kinds of fluoropolymers in an appropriate combination. For example, it can be performed by mixing using a general-purpose kneader such as a wheel shape, a plate shape or a roll shape. The mixing time is based on the time until two or more kinds of fluoropolymers are mixed in the state of each primary particle, but is preferably 10 minutes to 2 hours. Although there is no restriction | limiting in particular in mixing temperature, What is necessary is just temperature enough to show sufficient fluidity | liquidity for 2 or more types of polymers to achieve mixing. Furthermore, it can also serve as a second-stage compatibilizing step to be described later, in which case it is preferable to mix at a temperature necessary for compatibilization.
[0012]
Next, the mixture obtained by the above method is formed into a film. There is no restriction | limiting in particular in the film forming molding method, For example, it can shape | mold by normal methods, such as press molding, roll molding, and extrusion molding. Although there is no restriction | limiting in the film forming time, the time which can shape | mold in the film | membrane state without thickness unevenness is set, Preferably it is 5 to 10 minutes. In addition, when the above mixing is performed at a temperature necessary for the second-stage compatibility, the obtained film-like molded product is immediately processed in the third-stage phase separation step without going through the second stage described below. Can be performed.
[0013]
In the second step compatibility process, the film-like material obtained in the first process is heated at a temperature equal to or higher than the cloud point to homogenize two or more polymers constituting the film-like substance at the molecular level. Is for. The cloud point here is a temperature at which the system substantially starts phase separation in the temperature lowering process and can be determined relatively easily by a light scattering method. That is, in the process of applying a laser beam to a system that is in a molten state at a high temperature and decreasing the temperature while detecting the scattered light, the temperature at which the amount of scattered light that is detected increases rapidly is the cloud point. No cloud point is observed when the system is not phase separated.
[0014]
In the actual process, the film-like material obtained in the first process is maintained for a certain period of time at a temperature equal to or higher than the cloud point determined in advance by the above method. The holding temperature is preferably in the range of 1 to 200 ° C. from the cloud point, and the holding time is preferably in the range of 1 to 30 minutes.
The phase separation step in the third stage is performed by lowering the compatible material obtained in the second stage to a predetermined temperature below the cloud point and holding at that temperature. By controlling the holding temperature and holding time in this step, the sponge-like microstructure of the finally obtained exchange membrane can be controlled. The holding temperature and time depend on the combination of the fluorine-containing polymers used and the mixing ratio thereof. For example, in the case of an Aciplex polymer, the temperature is from 50 ° C. to the cloud point and is from 2 hours to 1 minute. By the sponge-like microstructure control method as described above, the spherical diameter of the spherical structure in the above-mentioned photograph 1 can be preferably 2.5 μm or less. The sphere diameter can be measured by a light scattering method.
[0015]
The sponge-like microstructure of the present invention is completed by rapidly cooling the film-like material having undergone phase separation through the above three steps. For the rapid cooling method, it is easiest to substantially cool rapidly from the cloud point to room temperature by quickly removing the film-like material from the heating device maintained at the cloud point in the third step. In addition, a solvent in which the polymer constituting the membrane is insoluble may be kept at a low temperature in advance, and the film-like material in a phase separation state may be quickly immersed therein. The quenching temperature is preferably 80 ° C. or lower than the cloud point.
[0016]
When the polymer functional group is a sulfonic acid group precursor or carboxylic acid precursor, the conversion of these precursors into sulfonic acid groups or carboxylic acid groups is usually achieved by hydrolysis with an acid or a base. Hydrolysis with a base, especially the use of hot solutions, for example solutions near the boiling point, is suitable for fast hydrolysis. The time required for hydrolysis increases with the thickness of the structure. It is also effective to include an organic compound miscible with water, such as dimethyl sulfoxide, in the hydrolysis bath.
[0017]
Since the solid electrolyte membrane of the present invention has various excellent performances, it can be widely applied to various purposes, fields and applications. Examples include electrodialysis, electrolytic reduction, fuel cell, salt electrolysis, various sensors, and the like. In such various uses, it may be used alone, or may be laminated and combined according to other known techniques.
In particular, in fuel cell applications, it can be used by being laminated with a porous film such as PTFE (Japanese Patent Laid-Open No. 6-342666, Japanese Patent Laid-Open No. 7-233267) or can be used integrally with a gas diffusion electrode (Japanese Patent Laid-Open No. 6-349498). . In salt electrolysis, it can be used as a film laminated with a perfluorocarbon carboxylic acid film, or can be reinforced with a cloth made of PTFE or the like, a woven cloth such as a net, a non-woven fabric, or a metal mesh or a porous body. Furthermore, the composite membrane using the electrolyte membrane of the present invention can be roughened on its surface, or a porous layer or a thin layer made of metal oxide particles can be formed on its surface.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Examples and comparative examples will be described below in more detail.
The method for measuring physical properties of the films obtained in Examples and Comparative Examples of the present application is as follows.
As the mechanical strength of the obtained film, the high elongation was measured using Tensilon (RTC-1210 manufactured by Orientec). The sample was a test piece cut to a width of 1 cm and a length of 10 cm in a wet state. Both ends were held and stretched at a speed of 100 mm / min at room temperature.
[0019]
Moreover, the direct current resistance value of the obtained film | membrane was measured on the electrolysis conditions of sodium chloride. Between the liquid level of the 12% sodium hydroxide (made by Wako Pure Chemicals, special grade) aqueous solution as the catholyte and the level of 3.5N sodium chloride (made by Wako Pure Chemicals) aqueous solution as the anolyte The solid electrolyte membrane was put in contact and fixed so as not to leak. The effective area of the solid electrolyte membrane is 1 cm2. A platinum electrode was used as the working electrode, platinum wires for measuring the potential difference were arranged on both sides in the immediate vicinity of the solid electrolyte membrane, and the voltage when a constant current was passed at 50 ° C. was measured. When the current is A ampere, the potential difference between both sides of the electrolyte membrane is V volts, and the thickness of the membrane is Tcm, the resistance value (R; Ω · cm) of the membrane is calculated by the equation (2).
R = V / (A · T) [Formula 2]
The cloud equivalent measurement and the spherical equivalent diameter corresponding to the pore portion of the sponge-like microstructure in the denatured film were performed with a light scattering device (manufactured by Otsuka Electronics Co., Ltd., DAYNA3000). In general, in the light scattering method, a sample is irradiated with laser light and scattered light is received. However, the cloud equivalent is detected by the temperature dependence of the scattered light intensity, and the spherical equivalent diameter is obtained by Debye-Buche analysis of the angle dependence of the scattered light. Asked. In addition, the sponge-like microstructure was directly observed with a phase contrast microscope (Nikon E4-PH-21).
[0020]
[Example 1]
Aciplex (manufactured by Asahi Kasei Kogyo Co., Ltd.), which is a carboxylic acid methyl ester type perfluorocarbon, is added to 70 g of Aciplex® (manufactured by Asahi Kasei Kogyo Co., Ltd., exchange capacity 1.05 meq / g) which is a sulfonyl fluoride type perfluorocarbon. 30 g of powder having an exchange capacity of 0.83 meq / g) was added, and the mixture was mixed for 1 hour at a rotation speed of 50 rpm and 200 ° C. using a plastmill (50MR manufactured by Toyo Seiki Seisakusho). Using the obtained mixed polymer, hot press film formation was performed at 240 ° C. and a pressure of 50 kg / cm ² for 5 minutes to obtain a uniform film having a thickness of 120 μm.
[0021]
The cloud point of the obtained film was measured and found to be 208 ° C. This film was heat-treated in an oven at 230 ° C. for 3 minutes, transferred to an oven at 100 ° C., held for 1 minute, and then taken out to room temperature. The spherical equivalent diameter of the sponge-like microstructure of the obtained film was 2.30 μm. The membrane was hydrolyzed to obtain a sodium sulfonate type and a sodium carboxylate type. As a result, the tensile strength at break was 1.39 kg / mm ² and the DC resistance value was 41 Ωcm ₋₁ .
[0022]
[Comparative Example 1]
Aciplex (manufactured by Asahi Kasei Kogyo Co., Ltd.), which is a carboxylic acid methyl ester type perfluorocarbon, is added to 70 g of Aciplex® (manufactured by Asahi Kasei Kogyo Co., Ltd., exchange capacity 1.05 meq / g) which is a sulfonyl fluoride type perfluorocarbon. 30 g of powder having an exchange capacity of 0.83 meq / g) was added, and the mixture was mixed for 1 hour at a rotational speed of 50 rpm and 170 ° C. using a plastomill (50MR manufactured by Toyo Seiki Seisakusho). Using the obtained mixed polymer, hot press film formation was performed at 170 ° C. and a pressure of 50 kg / cm ² for 5 minutes to obtain a uniform film having a thickness of 120 μm. The obtained film was hydrolyzed and then measured, and the tensile breaking strength was 1.30 kg / cm ² and the DC resistance was 42 Ωcm ₋₁ .
[0023]
[Example 2]
Aciplex (manufactured by Asahi Kasei Kogyo Co., Ltd.), which is a carboxylic acid methyl ester type perfluorocarbon, is added to 70 g of Aciplex® (manufactured by Asahi Kasei Kogyo Co., Ltd., exchange capacity 1.05 meq / g) which is a sulfonyl fluoride type perfluorocarbon. 30 g of powder having an exchange capacity of 0.83 meq / g) was added, and the mixture was mixed for 1 hour at a rotation speed of 50 rpm and 200 ° C. using a plastmill (50MR manufactured by Toyo Seiki Seisakusho). Using the obtained mixed polymer, hot press film formation was performed at 240 ° C. and a pressure of 50 kg / cm ² for 5 minutes to obtain a uniform film having a thickness of 120 μm.
[0024]
The cloud point of the obtained film was measured and found to be 208 ° C. This film was heat-treated in an oven at 230 ° C. for 3 minutes, then transferred to an 80 ° C. oven, held for 1 minute, and then taken out to room temperature. The spherical diameter of the sponge-like microstructure of the obtained film was 1.74 μm. The membrane was hydrolyzed to obtain a sodium sulfonate type and a sodium carboxylate type. As a result, the tensile strength at break was 1.42 kg / mm ² and the DC resistance value was 42 Ωcm ₋₁ .
[0025]
【The invention's effect】
A solid electrolyte membrane having both low electrical resistance and high mechanical strength, which has been difficult to achieve in the past, can be provided.
[Brief description of the drawings]
1 is a phase contrast micrograph showing the particle structure of a sponge-like microstructure of Example 1. FIG.

Claims (4)

A fluoropolymer 1 having a sulfonic acid group or a precursor thereof,
Unlike the fluoropolymer 1, the fluoropolymer 2 having a carboxylic acid group or a precursor thereof,
A mixture of
A solid electrolyte membrane characterized in that the microstructure has a sponge structure formed of a plurality of phases having different refractive indexes, and the spherical equivalent diameter in the sponge structure is 2.5 μm or less . The exchange capacity of the fluoropolymer 1 is 0.8 to 1.5 milliequivalent / g ,
The exchange capacity of the fluoropolymer 2 is 0.6 to 1.2 milliequivalent / g,
2. The solid electrolyte membrane according to claim 1 , wherein the fluoropolymer 2 is contained in a proportion of 5 to 40% by weight. It is a manufacturing method of the solid electrolyte membrane according to claim 1 or 2,
A fluorinated polymer 1 having a sulfonic acid group or its precursor, unlike the fluoropolymer 1, the fluorinated polymer 2 having a carboxylic acid group or a precursor thereof, a mixture of fogging of the mixture after holding the temperature above points, manufacturing method manufactured by you, characterized in that quenching to a temperature below the cloud point. The manufacturing method according to claim 3, wherein when rapidly cooling to a temperature lower than the cloud point, the product is rapidly cooled to a temperature of 80 ° C. or lower than the cloud point.

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