JP2001185164A - Ion exchanger polymer solution and method of manufacturing electrode for solid polymeric electrolyte fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stabilized ion exchanger polymer solution for a solid polymeric electrolyte fuel cell for a long period, to obtain a highly durable electrode. SOLUTION: An ion exchanger polymer consisting of a fluorine contained polymer containing functional groups capable of cross-linkage with covalent bond and sulfonic acid groups and being a dry resin having an ion exchange capacity of 1.0-4.0 milli-equivalent/g is dissolved in a solvent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an ion exchanger polymer solution.

[0002]

2. Description of the Related Art A hydrogen / oxygen fuel cell has attracted attention as a power generation system whose reaction product is only water in principle and has almost no adverse effect on the global environment. Solid polymer electrolyte fuel cells were once mounted on spacecraft in the Gemini and Biosatellite programs, but the power density at that time was low. Since then, higher performance alkaline fuel cells have been developed, and up to the present space shuttle, alkaline fuel cells have been adopted for space applications.

However, in recent years, attention has been paid again to solid polymer electrolyte fuel cells due to technological advances. The reasons are as follows. (1) A highly conductive film was developed as a solid polymer electrolyte. (2) By supporting the catalyst used for the gas diffusion electrode layer on carbon and coating this with an ion-exchange resin, an extremely large activity can be obtained.

[0004] However, the ion exchange resins used for coating catalysts currently used under the operating conditions of a low operating temperature and a high gas utilization rate become excessively swelled or partially swelled, especially at the air electrode where water is generated. The structure was not stabilized due to melting or the like, and there was a problem in long-term stability of battery performance. Although it is conceivable to lower the water content of the ion exchange resin in order to stabilize the structure, the conductivity is lowered and the battery performance is lowered.
Furthermore, because the gas permeability of the ion exchange resin decreases,
The supply of gas supplied to the catalyst surface through the coated ion exchange resin is delayed. Therefore, the gas concentration at the reaction site decreases and the voltage loss increases, that is, the concentration overvoltage increases and the output decreases.

[0005] Therefore, a resin having a high ion exchange capacity is used as the ion exchange resin for coating the catalyst.
For example, polytetrafluoroethylene (hereinafter, PTFE)
That. ), A fluororesin such as a tetrafluoroethylene / hexafluoropropylene copolymer, a tetrafluoroethylene / perfluoro (alkyl vinyl ether) copolymer, or the like as a water-repellent agent in an electrode, in particular, in an air electrode. Attempts have been made to efficiently discharge water, suppress flooding, suppress overswelling of the coated ion exchange resin, and stabilize the structure (Japanese Patent Laid-Open No. 5-36418).

However, since water is generated only in the ion-exchange resin portion, the effect is small. If the amount of the water repellent in the electrode is increased in order to achieve sufficient water repellency, the water repellent becomes less effective. The electric resistance of the electrode increases due to the insulator. In addition, the inclusion of the water repellent increases the thickness of the electrode, thereby deteriorating gas permeability and conversely lowering the output.
Therefore, it was not easy to obtain a polymer electrolyte fuel cell having sufficient conductivity, high output, and long-term stability.

[0007]

That is, in order to increase the output, it is important that the ion exchange resin coating the catalyst has high conductivity and high gas permeability in order to enhance the battery performance. Ion exchange resins having a high exchange group concentration are preferred. However, when a conventional ion exchange resin having a high ion exchange group concentration is used, the fuel gas permeability and conductivity are high and the initial output of the fuel cell is high, but the output tends to decrease when used for a long time. was there.

Accordingly, the present invention provides a method for obtaining a highly durable electrode for a solid polymer electrolyte fuel cell whose performance is stable even after long-term use, and an ion exchanger polymer solution therefor. With the goal.

[0009]

According to the present invention, there is provided a fluoropolymer having a functional group capable of forming a crosslink by a covalent bond, a sulfonic acid group or a precursor group thereof, and having an ion exchange capacity of 1: 1. Provided is an ion exchanger polymer solution, which is a solution obtained by dissolving an ion exchanger polymer, which is 0.0 to 4.0 meq / g dry resin, in a solvent.

[0010] Further, the present invention provides a coating solution obtained by dispersing a catalyst in the above ion exchange polymer solution, and heating the layer formed using the coating solution to crosslink the ion exchange polymer. Provided is a method for producing an electrode for a solid polymer electrolyte fuel cell, which comprises obtaining an electrode layer.

The fluoropolymer having a functional group capable of forming a crosslink by a covalent bond and a sulfonic acid group or its precursor group used in the present invention has substantially all hydrogen atoms replaced by fluorine atoms. Perfluorocarbon polymers (including those containing ether-bonding oxygen atoms) are preferred. Here, the precursor group of a sulfonic acid group, a group that can be converted into a sulfonic acid group by hydrolysis or the like, specifically a -SO ₂ F, -SO ₂ Cl or the like. Further, a copolymer obtained by copolymerizing a fluorine-containing vinyl ether having a sulfonic acid group or a precursor group thereof, a fluorine-containing olefin, and a fluorine-containing compound having a functional group serving as a crosslinking site (hereinafter, referred to as a crosslinkable group). Is preferred.

As the fluorine-containing vinyl ether having a sulfonic acid group or a precursor group thereof, various ones are widely used, and a preferable one is a compound represented by the general formula CF ₂ ＣＦCF (O
_{CF 2 CFX) m -O p -} (CF 2) n -SO 2 F ( where
X is a fluorine atom or a trifluoromethyl group, m is an integer of 0 to 3, n is an integer of 0 to 12, p is 0 or 1, and when n is 0, p is also 0 . )). Specific examples thereof include compounds of formulas 1 to 4. However, in the formula, a is an integer of 1 to 9, b is an integer of 1 to 8, c is an integer of 0 to 8, and d is 2 or 3.

[0013]

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The fluorovinyl ether having a —SO ₂ F group can be homopolymerized, but has a low radical polymerization reactivity. Therefore, a polymer copolymerized with a comonomer such as another olefin is usually used. In the present invention, an unsaturated compound having radical polymerizability, preferably a fluorine-containing unsaturated compound is selected as the comonomer.

Such comonomers include tetrafluoroethylene, chlorotrifluoroethylene, trifluoroethylene, vinylidene fluoride, vinyl fluoride, ethylene, perfluoro (3-butenyl vinyl ether), perfluoro (allyl vinyl ether), Perfluoro (2,2-dimethyl-1,3-dioxole),
Perfluoro (1,3-dioxole), perfluoro (2-methylene-4-methyl-1,3-dioxolane), perfluoro (3,5-dioxa-1,6-heptadiene), perfluoro (4-methoxy -1,3-dioxole) and the like. In particular, tetrafluoroethylene is preferably employed.

[0016] In addition to these comonomers, perfluoro- such as propylene and hexafluoropropylene may be used.
α-olefins, (perfluoroalkyl) ethylenes such as (perfluorobutyl) ethylene, (perfluoroalkyl) propenes such as 3-perfluorooctyl-1-propene, perfluoro (alkyl vinyl ether) s (alkyl group May have a linear or branched structure,
Further, it may contain an etheric oxygen atom. ) May be copolymerized.

Perfluoro (alkyl vinyl ether)
As a class, CF ₂ ＣＦCF— (OCF ₂ CFY) _q —O—
Perfluorovinyl ether represented by R ^f is preferred. However, in the formula, Y is a fluorine atom or a trifluoromethyl group, q is an integer of 0 to 3, and R ^f is a linear or branched perfluoroalkyl group having 1 to 12 carbon atoms.

CF ₂ ＣＦCF— (OCF ₂ CFY) _q —O—
Preferred examples of the perfluorovinyl ether represented by R ^f include compounds of formulas 5 to 7. However,
In Formulas 5 to 7, e is an integer of 1 to 8, and f is 1 to
An integer of 8; g is 2 or 3;

[0019]

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In the fluorine-containing polymer of the present invention,
The functional group capable of forming a crosslink is introduced by copolymerizing a radically polymerizable fluorine-containing unsaturated compound having a crosslink site. The polymer obtained by copolymerizing the fluorine-containing unsaturated compound can be crosslinked by heat treatment or the like. Moreover, you may mix a crosslinking agent as needed. Specific examples of the above fluorine-containing unsaturated compound include the following six types.

First, a perfluorounsaturated compound having two double bonds is mentioned, and specifically, compounds of formulas 8 to 15 are preferred. Among them, the compounds of formulas 13 to 15 have double bonds having different reactivities, and the polymerization reactivity of the other double bond is smaller even if the side of the perfluorovinyloxy group is polymerized. It can be easily introduced into the resin as a cross-linking site without reacting during polymerization.

Wherein h is an integer of 2 to 8,
i and j are each independently an integer of 1 to 5, and k is 0 to
And r is an integer from 0 to 5. S is an integer of 1 to 8, t is an integer of 2 to 5, and u is 0.
Is an integer of up to 5.

[0023]

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[0024]

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Secondly, perfluoroethene or perfluoro (alkyl vinyl ether) having a bromine atom (the alkyl chain may contain an oxygen atom having an ether bond) is mentioned. And the like are preferred. In Formulas 16 to 18 and Formulas 19 to 31 described later, v and w are independently 1 to 5
Is an integer.

[0026]

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Third, a carboxylic acid group, a carboxylic acid group or a carboxylic acid ester group (such as an alkoxycarbonyl group)
Or a polyfluoro (alkyl vinyl ether) having the formula (where the alkyl chain may contain an etheric oxygen atom), and specifically preferred are the compounds of the formulas 19 to 21 and the like. However,
In the formula, M is an alkyl group having 1 to 5 carbon atoms, a hydrogen atom, an alkali metal (lithium, potassium, sodium) atom or NH ₄ .

[0028]

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Fourth, hydroxyl group-containing polyfluoroethene or polyfluoro (alkyl vinyl ether) (the alkyl chain may contain an oxygen atom having an ether bond) is mentioned. And the like are preferred.

[0030]

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Fifth, there can be mentioned perfluoroethene or perfluoro (alkyl vinyl ether) having a cyano group (the alkyl chain may contain an oxygen atom having an ether bond). And the like are preferred.

[0032]

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Sixth, perfluoroethene or perfluoro (alkyl vinyl ether) having a cyanato group
(The alkyl chain may contain an oxygen atom having an ether bond), and specifically preferred are compounds of the formulas 30 to 31 and the like. Here, in the formula, Z is a hydrogen atom or a trifluoromethyl group.

[0034]

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In the present invention, the concentration of the sulfonic acid groups in the ion exchanger polymer, that is, the ion exchange capacity, is in the range of 1.0 to 4.0 meq / g dry resin. If the ion exchange capacity is smaller than this, the electrical resistance of the membrane increases, while if it is larger, the mechanical strength of the membrane becomes insufficient. More preferably, a resin having a dry resin of 1.3 to 3.0 meq / g is used.

The content of the polymerized unit constituting the crosslinked site in the ion exchanger polymer is 0.1 to 50 mol%.
It is preferred that If the amount is less than 0.1 mol%, the effect of improving the strength is small even if cross-linking is formed, and if it exceeds 50 mol%, the cross-linked resin becomes brittle. More preferably, it is 0.5 to 30 mol%.

In the present invention, the method for producing the ion-exchange polymer contained in the ion-exchange polymer solution may be any of emulsion polymerization, solution polymerization, suspension polymerization and bulk polymerization which are generally used for producing a fluorinated olefin polymer. Either can be preferably employed. The polymerization is carried out under conditions in which radicals are generated, and a method of irradiating radiation such as ultraviolet rays, γ rays, and an electron beam, and a method of adding a radical initiator used in ordinary radical polymerization are generally used. The polymerization temperature is usually -20 ° C
About 150 ° C.

The solvent of the ion exchanger polymer solution used in the present invention is not particularly limited, and examples thereof include the following solvents. Alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, n-butyl alcohol and isopropyl alcohol. 2,
2,2-trifluoroethanol, 2,2,3,3,3
-Pentafluoro-1-propanol, 2,2,3,3
-Tetrafluoro-1-propanol, 2,2,3
4,4,4-hexafluoro-1-butanol, 2,
2,3,3,4,4,4-heptafluoro-1-butanol, 1,1,1,3,3,3-hexafluoro-2-
Fluorinated alcohols such as propanol.

Perfluoro oxygen-containing or nitrogen-containing compounds such as perfluorotributylamine and perfluoro-2-n-butyltetrahydrofuran; chlorofluorocarbons such as 1,1,2-trichloro-1,2,2-trifluoroethane , 3,3-dichloro-1,1,1,2,2
-Pentafluoropropane, 1,3-dichloro-1,
In addition to hydrochlorofluorocarbons such as 1,2,2,3-pentafluoropropane, polar solvents such as N, N-dimethylformamide, N, N-dimethylacetamide, dimethylsulfoxide, and water can be used. These solvents may be used alone or as a mixture of two or more.

The concentration of the ion-exchange polymer solution is 0.3 to 30% of the total mass of the solution.
%. If it is less than 0.3%, it takes time to volatilize the solvent, and if the time is reduced, high-temperature heating is required, which is not preferable. If it exceeds 30%, the solution viscosity increases and the moldability deteriorates. In addition, when a gas diffusion electrode is produced by dispersing a catalyst in an ion exchanger polymer solution, the thickness of a resin film covering the catalyst is increased and battery performance is deteriorated, which is not preferable. 2-10%
Is particularly preferred.

When producing an electrode of a solid polymer electrolyte fuel cell using the ion exchanger polymer solution of the present invention,
It is preferable that the ion-exchange polymer is subjected to a crosslinking treatment after the formation of the electrode. As a cross-linking method, a heat, radiation, electron beam, photo-crosslinking method or the like, which is a method generally used for cross-linking a polymer material, is employed. Among them, the heat crosslinking method is preferred in view of the availability of the device and the ease of handling.

In the heat crosslinking method, crosslinking can be carried out simply by heating, but a radical initiator such as a peroxide and a crosslinking agent such as triallyl isocyanurate, bisphenol and bisphenol AF are added to promote the crosslinking reaction. A heating method can also be adopted. If necessary, auxiliaries such as sodium carbonate, potassium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, calcium hydroxide and magnesium hydroxide may be added.

In the case of using a copolymer containing a compound having a cyano group as represented by formulas 25 to 29 or a compound having a cyanato group as represented by formulas 30 to 31, a catalyst is separately added. Although not required, Lewis acids, protonic acids, tetraphenyl tin, triphenyl tin hydroxide, (C ₇ F ₁₅ COO) ₂ Zn may be used as a curing catalyst if necessary.
And the like. One or more selected from transition metal salts of carboxylic acids, ammonium salts of carboxylic acids, peroxides, amines, amidines, compounds having an imidoylamidine structure, and the like may be used. Further, a blocked amine compound in which part or all of active hydrogen of ammonia or an amine compound is protected by another functional group can be added as a catalyst.

When producing an electrode for a solid polymer electrolyte fuel cell using the ion exchanger polymer solution of the present invention,
A catalyst is dispersed in an ion exchanger polymer solution to form a coating solution, and a layer serving as an electrode layer is formed using the coating solution,
Preferably, the electrodes are formed by heating the shaped layer to crosslink the ion exchanger polymer.

More specifically, a method of manufacturing an electrode will be described. A conductive carbon black powder carrying platinum catalyst fine particles is mixed and dispersed according to a usual method.
It is preferable to obtain the membrane-electrode assembly by any of the following methods. The first method is to apply the above-mentioned dispersion liquid to one side of a cation exchange membrane, apply a coating liquid for forming a fuel electrode to the other side of the membrane, and after drying, use two carbon cloths or carbon cloths. This is a method of sandwiching and adhering paper. In the second method, a dispersion obtained by applying the above-mentioned dispersion on carbon cloth or carbon paper and a coating obtained by applying a coating liquid for forming a fuel electrode on carbon cloth or carbon paper are dried, and then dried. This is a method of adhering to both surfaces of the exchange membrane.

When an ion exchanger polymer solution containing a fluorine-containing polymer having a sulfonic acid group precursor group is used, the precursor group may be converted to an acid form after forming the layer and before crosslinking. After the crosslinking, the precursor group may be converted to an acid form.

The obtained membrane-electrode assembly is provided with a groove serving as a passage for a fuel gas or an oxidizing gas, is sandwiched between separators made of a conductive carbon plate or the like, and is incorporated into a cell. In a hydrogen gas fuel cell, hydrogen gas is supplied to the anode side, and oxygen or air is supplied to the cathode side. The reaction is H ₂ → 2H ⁺ + 2e ^{− at the} anode and 1 at the cathode.
With / 2O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O, chemical energy is converted to electric energy.

[0048]

EXAMPLES [Example 1 (Example)] [Preparation of ion exchanger polymer solution] Using azobisisobutyronitrile as an initiator, 0.4 mol of tetrafluoroethylene and 0.134 mol of CF ₂ = CFOCF _{2 are used.}
CF (CF ₃ ) O (CF ₂ ) ₂ SO ₂ F and 0.02 mol of CF ₂ ＣＦCFO (CF ₂ ) ₃ COOCH ₃ were copolymerized at 70 ° C. for 5 hours to have an ion exchange capacity of 1.27. A copolymer of milliequivalent / gram dry resin was obtained. This copolymer is
It is hydrolyzed in an aqueous solution containing 30% of dimethyl sulfoxide of the total weight of the solution and potassium hydroxide of 15% of the total weight of the solution, immersed in 1 mol / L hydrochloric acid at room temperature for 16 hours, and acid-form (sulfonic acid). ), Washed with water and dried.

This was prepared by adding ethanol and 1,3-dichloro-
Dissolved in a mixed solvent of 80/20 with a mass ratio of 1,1,2,2,3-pentafluoropropane to form a sulfonic acid group and a crosslinkable carboxylic acid group having a solute concentration of 10% of the total mass of the solution An ion exchanger polymer solution comprising a perfluorocarbon polymer was obtained.

[Preparation of Fuel Cell and Evaluation of Performance] Using the above solution (ion exchange capacity: 1.27 meq / g dry resin), the mass ratio of the copolymer to platinum-supported carbon was 1: 3. Thus, a platinum-supported carbon was mixed and dispersed to obtain a coating liquid. The coating liquid was applied on a carbon cloth by a die coating method, and dried to obtain a gas diffusion electrode having a thickness of 10 μm and a gas diffusion electrode layer having a platinum carrying amount of 0.5 mg / cm ² .

On the other hand, tetrafluoroethylene and CF ₂ =
A copolymer of CFOCF ₂ CF (CF ₃ ) O (CF ₂ ) ₂ SO ₂ F (ion exchange capacity: 1.1 meq / g dry resin) is extruded to form a film, which is subjected to hydrolysis, acidification, and washing with water. Thus, an ion exchange membrane having a sulfonic acid group and a thickness of 50 μm was obtained. The film was sandwiched between two gas diffusion electrodes and pressed using a flat plate press. Then, the mixture was heated at 180 ° C. for 2 hours to crosslink the ion-exchange polymer in the electrode, thereby producing a membrane-electrode assembly.

A current collector made of titanium was placed outside the membrane-electrode assembly, a gas supply chamber made of PTFE was placed outside the current collector, and a heater was placed outside the current collector. A fuel cell having an effective membrane area of 9 cm ² was assembled. . The temperature of the fuel cell was maintained at 80 ° C., and oxygen was supplied to the oxidant electrode and hydrogen was supplied to the fuel electrode at 2 atm. When the terminal voltage at a current density of 1 A / cm ² was measured, the terminal voltage was 0.58 V. Also,
The terminal voltage after operation for 1000 hours was 0.57V.

Example 2 (Example) In synthesizing the copolymer of Example 1, 0.02 mol of CF ₂に CFO (CF ₂ ) ₃ CO
Instead of using OCH ₃ , 0.01 mol CF ₂ =
An ion exchanger polymer solution was prepared in the same manner as in Example 1, except that CFOCF ₂ CF (CF ₃ ) O (CF ₂ ) ₂ CF = CF ₂ was used. The resulting solution (ion exchange capacity 1.2
(8 meq / g dry resin), a membrane-electrode assembly was prepared in the same manner as in Example 1, a fuel cell was assembled, and evaluation was performed in the same manner as in Example 1. When the terminal voltage at a current density of 1 A / cm ² was measured, the terminal voltage was 0.59 V.
The terminal voltage after operation for 1000 hours was 0.57V.

Example 3 (Comparative Example) In synthesizing the copolymer of Example 1, 0.02 mol of CF ₂ ＣＦCFO (CF ₂ ) ₃ CO
An ion exchanger polymer solution was prepared in the same manner as in Example 1 except that OCH ₃ was not used. Using the resulting solution (ion exchange capacity: 1.34 meq / g dry resin), a membrane-electrode assembly was prepared in the same manner as in Example 1, and a fuel cell was assembled. Current density 1A / cm
When the terminal voltage at ² was measured, the terminal voltage was 0.1.
It was 60V. However, after 1000 hours of operation, the terminal voltage dropped to 0.42V.

[0055]

When an electrode for a solid polymer electrolyte fuel cell is produced using the ion exchanger polymer solution of the present invention,
The electrode can easily contain a crosslinked fluorine-containing polymer containing a sulfonic acid group as a catalyst coating resin. The ion-exchange polymer in the electrode thus obtained has little swelling of the resin due to the operation of the fuel cell even with a high ion-exchange capacity. Therefore, a polymer electrolyte fuel cell having high output and excellent long-term durability can be obtained.

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Claims (4)

[Claims] 1. A fluoropolymer having a functional group capable of forming a crosslink by a covalent bond and a sulfonic acid group or its precursor group, and having an ion exchange capacity of 1.0 to 4.
An ion exchanger polymer solution characterized by being a solution in which an ion exchanger polymer which is 0 meq / g dry resin is dissolved in a solvent. 2. The basic skeleton of the fluoropolymer is CF ₂ ＣC.
Polymerized unit based on F ₂ and CF ₂ = CF (OCF ₂ CFX) _m
-O _p - (CF ₂₎ based on the _n A polymerized units (wherein X is a fluorine atom or a trifluoromethyl group, A is a sulfonic acid group or a precursor group, m is an integer of 0 to 3 And n is an integer of 0 to 12, p is 0 or 1,
When n is 0, p is also 0. The ion exchange polymer solution according to claim 1, which is a copolymer consisting of: 3. The copolymer further comprises the following (A) to
3. The ion exchanger polymer solution according to claim 2, wherein any one of the compounds (F) is copolymerized as a comonomer. (A) A perfluorounsaturated compound having two double bonds. (B) Perfluoroethene or perfluoro (alkyl vinyl ether) having a bromine atom. (C) Polyfluoroethene or polyfluoro (alkyl vinyl ether) having a carboxylic acid group, a carboxylic acid group or a carboxylic acid ester group. (D) Polyfluoroethene or polyfluoro (alkyl vinyl ether) having a hydroxyl group. (E) Perfluoroethene or perfluoro (alkyl vinyl ether) having a cyano group. (F) Perfluoroethene or perfluoro (alkyl vinyl ether) having a cyanato group. 4. A coating solution obtained by dispersing a catalyst in the ion-exchanger polymer solution according to claim 1, and heating the layer formed using the coating solution to convert the functional group. A method for producing an electrode for a solid polymer electrolyte fuel cell, characterized in that an electrode layer is obtained by reacting and crosslinking an ion exchanger polymer.