JP2000188111A - Solid high polymer electrolyte fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid high polymer electrolyte fuel cell having a high output density and little deterioration of battery characteristics with age. SOLUTION: In this solid high polymer electrolyte fuel cell, an air electrode contains a catalyst and a carbon sulfonic acid type ion exchange resin containing fluoride, the ion exchange resin is made of a copolymer containing a polymerized unit based on tetrafluoroetylene, a polymerized unit based on per-fluorovinylethel having sulfonic acid group and a polymerized unit based on per-fuluorovinylethel containing no functional group.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a solid polymer electrolyte fuel cell.

[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] Many studies have been made on a method for manufacturing an electrode-solid polymer electrolyte membrane assembly (hereinafter simply referred to as an assembly) of a solid polymer electrolyte fuel cell. The solid polymer electrolyte fuel cells currently being studied have a drawback that the operating temperature is as low as 50 to 120 ° C., so that the exhaust heat cannot be used effectively for the auxiliary power of the fuel cell. To compensate for this, the polymer electrolyte fuel cell is required to have a particularly high output density. In addition, as a challenge for practical use, high energy efficiency, even under operating conditions with high fuel and air utilization rates,
There is a demand for the development of a joined body that can obtain a high power density.

[0005] Under the operating conditions of a low operating temperature and a high gas utilization rate, clogging (flooding) of the electrode porous body due to condensation of water vapor tends to occur particularly at the air electrode where water is generated. In order to obtain stable characteristics over a long period of time, it is necessary to ensure the water repellency of the electrode so that flooding does not occur. This is particularly important for a solid polymer electrolyte fuel cell capable of obtaining a high power density at a low temperature.

[0006] In order to ensure the water repellency of the electrode, it is effective that the ion exchange capacity of the ion exchange resin coating the catalyst in the electrode is small, that is, the content of ion exchange groups in the ion exchange resin is low. However, in this case, since the ion exchange resin has a low water content, the conductivity is reduced, and the battery performance is reduced. Further, since the gas permeability of the ion exchange resin is reduced, the supply of the 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.

[0007] For this reason, 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, particularly in an air electrode, to suppress flooding. (Japanese Patent Laid-Open No. 5-36418). In this specification, A / B
The copolymer is a copolymer composed of a polymer unit based on A and a polymer unit based on B.

However, when the amount of the water repellent in the electrode is increased in order to make the electrode sufficiently water repellent, the electric resistance of the electrode is increased because the water repellent is an insulator. In addition, since the thickness of the electrode increases, gas permeability deteriorates, and conversely, the output decreases. To compensate for the decrease in electrode conductivity,
For example, it is necessary to increase the conductivity of the carbon material that is the carrier of the catalyst and the ion conductivity of the ion exchange resin that coats the catalyst. However, it is difficult to obtain an electrode that satisfies both sufficient conductivity and sufficient water repellency at the same time, and it has not been easy to obtain a polymer electrolyte fuel cell that has high output and is stable over a long period of time.

Further, a method of mixing pitch fluoride (JP-A-7-212324) and a method of fluorinating a catalyst carrier (JP-A-7-192338) have been proposed. There is a problem that cannot be coated. Also, a method of imparting a gradient to the water repellency in the thickness direction of the electrode (Japanese Patent Application Laid-Open Nos. 5-251086 and 7-1)
34993) has been proposed, but the manufacturing method is complicated.

[0010]

In order to increase the output, the smaller the amount of the water repellent contained in the electrode, the better from the viewpoints of conductivity and gas permeability. It is important that the ion exchange resin covering the catalyst has high conductivity and high gas permeability in order to enhance battery performance, and an ion exchange resin having a high exchange group concentration is preferable. However, when an ion exchange resin having a high exchange group concentration is used, the permeability and conductivity of the fuel gas are high and the initial output of the fuel cell is high, but flooding is likely to occur, and the output tends to decrease when used for a long time. .

Accordingly, the present invention provides an air electrode having high conductivity and high gas permeability of the contained ion exchange resin and having high water repellency even when used for a long period of time, so that a high output can be obtained for a long period of time. It is an object of the present invention to provide a polymer electrolyte fuel cell that can be maintained.

[0012]

According to the present invention, a gas diffusion electrode containing a catalyst and an ion exchange resin is used as a fuel electrode and an air electrode, and the fuel electrode is provided on one surface of a membrane solid polymer electrolyte.
In a solid polymer electrolyte fuel cell in which the air electrode is disposed on the other surface, the ion-exchange resin contained in the air electrode includes the following polymerized units A, B, and C: A solid polymer electrolyte fuel cell characterized by being an ion exchange resin comprising a copolymer containing: Polymerized unit A: Polymerized unit based on tetrafluoroethylene, Polymerized unit B: Polymerized unit based on perfluorovinyl ether having a sulfonic acid group, Polymerized unit C: Based on perfluorovinyl ether having no ion exchange group or its precursor group Polymerized units.

In the present invention, the ion exchange resin contained in the air electrode is a copolymer having a sulfonic acid group as an ion exchange group. The copolymer is preferably obtained by subjecting a precursor (hereinafter, simply referred to as a precursor) composed of a resin having —SO ₂ F that can be converted to a sulfonic acid group to hydrolysis and acidification. That is, in order to incorporate the polymerized units B in the copolymer, a perfluorovinyl ether having a -SO ₂ F to synthesize a precursor by copolymerizing a raw material, the precursor for processing hydrolysis and acid form Is preferred.

The perfluorovinyl ether having —SO ₂ F includes CF ₂ ＝ CF— (OCF ₂ CFX)
_m -O _p - (CF ₂₎ a perfluorovinyl ether compound represented by _n -SO ₂ F (wherein, m is an integer of 0 to 3, n is an integer from 1 to 12, p is 0 or 1, X is F or CF
₃ Is preferred. Preferred examples of the perfluorovinyl ether compound include the following compounds. However, in the following formula, q is an integer of 1 to 8, r is 1
And an integer s represents an integer of 1 to 3.

[0015]

Embedded image CF ₂ ＣＦCFO (CF ₂ ) _q SO ₂ F, CF ₂ ＣＦCFOCF ₂ CF (CF ₃ ) O (CF ₂ ) _r SO
_{2 F, CF 2 = CF (} OCF 2 CF (CF 3)) s O (CF 2) 2 S
O ₂ F.

In the present invention, the polymerized unit C contained in the ion exchange resin contained in the air electrode is an ion exchange group such as a sulfonic acid group or a phosphonic acid group or a group capable of becoming an ion exchange group by hydrolysis or the like (an ion exchange group). (A precursor group of the exchange group). As the polymerized unit C, CF ₂ ＣＦCF—
_{(OCF 2 CFY) t -O-} R polymerized units based on a perfluorovinyl ether compound represented by the ^f is preferred. Here, in the formula, t is an integer of 0 to 3, and Y is F or CF.
₃ R ^f is a perfluoroalkyl group represented by C _u F _{2u + 1} linear or branched chain (1 ≦ u ≦ 12).

CF ₂ ＣＦCF— (OCF ₂ CFY) _t —O—
Preferred examples of the perfluorovinyl ether compound represented by R ^f include the following compounds. However, in the following formula, v is an integer of 1 to 8, w is an integer of 1 to 8,
x shows the integer of 1-3.

[0018]

Embedded image CF ₂ ＣＦCFO (CF ₂ ) _v CF ₃ , CF ₂ ＣＦCFOCF ₂ CF (CF ₃ ) O (CF ₂ ) _w CF ₃ , CF ₂ ＣＦCF (OCF ₂ CF (CF ₃ )) _× O (CF ₂ ) ₂ C
F _3.

In the present invention, the ion-exchange resin contained in the air electrode comprises a copolymer containing a polymerized unit A, a polymerized unit B and a polymerized unit C. And C, in addition to hexafluoropropylene,
Included if the polymerization units based on fluoroolefins such as chlorotrifluoroethylene and the polymerization units based on non-fluorinated olefins such as ethylene and vinylidene chloride are 10 mol% or less based on all the polymerization units constituting the ion exchange resin. It may be.

The precursor, after being hydrolyzed by an aqueous solution such as a -SO ₂ F, for example NaOH or KOH, is converted to the acid type resin is an acid form with an aqueous solution such as hydrochloric acid or sulfuric acid. For example, when hydrolyzed by KOH aqueous solution, -SO ₂
The desired ion exchange resin is obtained by converting F to —SO ₃ K and then replacing the K ion with a proton.
Further, the hydrolysis and the acidification treatment may be performed simultaneously.

In the ion exchange resin contained in the air electrode, the total amount (mol) of the polymerized units B and C is
Is preferably contained in an amount of 10 to 25 mol% with respect to the polymerization unit A. When the content is less than 10 mol%, the ion exchange resin has a low water content, and thus has low conductivity and low gas permeability. For this reason, the electrodes have a high resistance or an overvoltage. If it exceeds 25 mol%, the ion exchange resin has too high a water content, swells, and the wettability increases. Therefore, the pores of the air electrode are easily blocked by water, and flooding is likely to occur.

At the air electrode, water is generated with the reaction of the fuel cell, and therefore, there is a problem in particular that the water content of the ion exchange resin increases. Further, since the gas continuously supplied to the air electrode is usually humidified in order to prevent excessive drying, it is necessary to consider an increase in the water content of the ion exchange resin due to the wet gas. The total amount of the polymerized unit B and the polymerized unit C with respect to the polymerized unit A is preferably 12 to 22 mol%, more preferably 15 to 20 mol%.

Further, in the ion exchange resin contained in the air electrode, the polymerization unit B is preferably at least 5 mol% based on the total amount of the polymerization units A and C. 5mo
If it is less than 1%, the content per unit weight of the ion exchange group having a role of conducting protons is small, and the resistance of the ion exchange resin is increased.

The ion exchange resin contained in the air electrode preferably contains the polymerized unit C in an amount of 2 mol% or more based on the total amount of the polymerized units A and B. The role of the polymerized unit C to enhance ionic conductivity is small.

However, it is considered that the gas that permeates the ion exchange resin coating the catalyst permeates not only the water-containing portion of the ion exchange resin but also the ether bond portion in the ion exchange resin. The polymerized unit C is contained in the ion exchange resin for the purpose of increasing the number of ether bonds in the ion exchange resin and increasing gas permeability. Therefore,
If the content is too low, the effect of reducing the concentration overvoltage by improving the gas permeability is not recognized. The polymerization unit C is particularly preferably at least 5 mol% based on the total amount of the polymerization units A and B.

On the other hand, the ion exchange resin contained in the fuel electrode in the present invention is not particularly limited, but a fluorine-containing carbon sulfonic acid type ion exchange resin is preferable. The polymerization unit A, the polymerization unit B and the polymerization unit C are used for the fuel electrode in the same manner as the air electrode.
It is also possible to use an ion exchange resin comprising a copolymer containing

At the fuel electrode, since water is not generated with the reaction of the fuel cell, the content of the polymerized units B and C may be higher than that of the air electrode. In order to ensure a smooth supply of fuel gas from the gas phase to the catalyst particles, the above content is preferably high, and particularly preferably the content of the polymerized unit B is high.

In the present invention, the catalyst contained in the fuel electrode and the air electrode and the ion exchange resin are in a weight ratio of catalyst: the above ion exchange resin = 0.40: 0.60 to 0.95: 0.05.
Is preferable from the viewpoint of the conductivity and water repellency of the electrode. In the case of a supported catalyst supported on a carrier such as carbon, the catalyst herein includes the weight of the carrier.

In the present invention, the air electrode and the fuel electrode (hereinafter collectively referred to as electrodes) are sprayed and coated with a liquid in which an ion exchange resin and a catalyst are dissolved or dispersed in a solvent (hereinafter referred to as electrode forming liquid). , Filtration and the like. The electrodes may be formed directly on the ion exchange membrane, or may be formed in layers on a current collector made of carbon paper or the like and then joined to the ion exchange membrane. Instead of carbon paper, a sheet in which a carbon layer made of carbon and fluororesin is formed on a carbon fiber woven fabric may be used. Also, forming electrodes on a separately prepared flat plate,
This may be transferred to an ion exchange membrane. When the electrode is not formed directly on the ion exchange membrane, a known hot pressing method and a bonding method (Japanese Patent Application Laid-Open Nos.
4420) and the like, and bonding with the ion exchange membrane.

The preferred range of the viscosity of the above liquid for forming an electrode varies depending on the method of forming the electrode, and ranges from a dispersion of about several tens cP to a paste of about 20,000 cP.
A wide viscosity range can be used. In order to adjust the viscosity, the liquid for forming an electrode may contain a thickener or a diluting solvent. Ethylcellulose, methylcellulose or cellosolve-based thickeners can be used as the thickener, but it is desirable not to use it because a removal operation is required. As the diluting solvent, alcohols such as methanol, ethanol and isopropyl alcohol, fluorocarbons, hydrofluorocarbons, hydrochlorofluorocarbons, water and the like can be used.

In the present invention, it is preferable that the electrode, particularly the air electrode, contain a water repellent since the effect of suppressing flooding is further enhanced. As the water repellent, for example, a tetrafluoroethylene / hexafluoropropylene copolymer, a tetrafluoroethylene / perfluoro (alkyl vinyl ether) copolymer, PTFE and the like can be used. Further, a fluorine-containing resin that can be dissolved in a solvent is preferable because the electrode is easily water-repellent.

The water repellent is usually contained in the electrode in an amount of 0.01 to 30.
Although it is preferable to include the water repellent in the air electrode of the present invention, even if the amount of the water repellent, which is an insulator, is small, the air repellency equivalent to the conventional one can be obtained. Therefore, the increase in resistance of the electrode due to the addition of the water repellent can be minimized, and the possibility that the pores of the electrode are crushed by the water repellent is also small.

The solid polymer electrolyte membrane in the present invention is not particularly limited, but is preferably made of a resin having, for example, a sulfonic acid group, a phosphonic acid group or a phenolic hydroxyl group as a cation exchange group. In particular, it is preferable to use a fluorine-containing carbon sulfonic acid type ion exchange resin. Specifically, a fluorinated carbon sulfonic acid type ion exchange resin composed of a copolymer containing the polymer units A and B is exemplified, and the copolymer may further contain the polymer units C.

The film-like solid polymer electrolyte is obtained by forming a precursor of the resin having thermal fluidity into a film by a known method such as hot press molding, roll molding, or extrusion molding, and subjecting the precursor to hydrolysis and acidification. Thus, a film-like solid polymer electrolyte is obtained. Further, it can be obtained by a solvent casting method from a solution in which a fluorine-based cation exchange resin is dissolved in a solvent such as alcohol.

Further, the membrane solid polymer electrolyte may be made of an ion exchange resin such as a hydrocarbon resin having a sulfonic acid group or a phosphonic acid group or a partially fluorinated hydrocarbon resin. Specifically, for example, a resin obtained by graft-polymerizing styrene onto an ethylene / tetrafluoroethylene copolymer and then introducing a sulfonic acid group into a polymerization unit based on styrene, a resin obtained by sulfonating polysulfone or polyetheretherketone, or the like. It may be composed of

Further, the membrane solid polymer electrolyte may be formed of a membrane in which the above cation exchange resin is combined with a reinforcing material. Examples of the reinforcing material include polyethylene, polytetrafluoroethylene, tetrafluoroethylene / perfluoro (propyl vinyl ether) copolymer, and tetrafluoroethylene / hexafluoropropylene copolymer. These reinforcing materials are used in the form of fibrils, woven fabrics, nonwoven fabrics or porous bodies.

The thickness of the membrane solid polymer electrolyte is, for example, 1
Those having a size of 0 to 300 μm are used. If the thickness is less than 10 μm, pinholes are likely to occur and a short circuit may occur. If the thickness is more than 300 μm, the electric resistance of the membrane increases, and the output characteristics of the fuel cell deteriorate. In particular, a thickness of 20 to 100 μm is preferable.

Since the ion exchange resin contained in the air electrode contains the polymerized unit C in addition to the polymerized unit B, the resin does not excessively swell even if water is produced by the reaction of the fuel cell. The wettability does not increase. Therefore, the generated water is easily discharged, so that a path for gas diffusion can be secured, and the ion exchange resin has a high gas permeability because it contains many ether bonds. Therefore, even when the fuel cell is used at a large current density, flooding of the electrode hardly occurs, and the concentration overvoltage can be reduced. That is, it is possible to reduce the voltage loss caused by the slow gas diffusion and supply and the decrease in the gas concentration at the reaction site.

[0039]

EXAMPLES The present invention will be described in detail with reference to Examples (Examples 1 to 5) and Comparative Examples (Examples 6 to 7), but the present invention is not limited to these.

The stainless steel autoclave, and CF ₂ ClCF ₂ CHClF as a polymerization solvent, and azobisisobutyronitrile as a polymerization initiator, CF ₂ = CF-O
_{CF 2 CF (CF 3) -OCF} 2 CF 2 SO 2 F ( perfluoro (3,6-dioxa-4-methyl-7-octenyl) sulfonyl fluoride, hereinafter referred to as PSVE) and CF ₂ = CF-OCF ₂ CF ₂ CF ₃ (perfluoro (propyl vinyl ether), hereinafter referred to as PPVE) was charged. Next, after the inside of the autoclave was sufficiently degassed with liquid nitrogen, tetrafluoroethylene (hereinafter referred to as TFE) was charged and polymerization was started at 70 ° C. During the polymerization, the pressure inside the autoclave was kept constant by introducing TFE from outside the system. Unreacted TFE after 10 hours
Was purged to terminate the polymerization, and the obtained polymer solution was coagulated with methanol, washed and dried to obtain TFE / PSV.
An E / PPVE copolymer was obtained.

30% by weight of dimethyl sulfoxide and KO
The copolymer was hydrolyzed in a mixed aqueous solution containing 15% by weight of H, washed with water, and then immersed in 1N hydrochloric acid to obtain a perfluorocarbon sulfonic acid type ion exchange resin.

By adjusting the amount of the polymerization initiator, the pressure during the polymerization, and the PPVE concentration, seven types of ion exchange resins having the content of each polymerization unit shown in Examples 1 to 7 in Table 1 were synthesized. did. In addition, the ion exchange resin used on the fuel electrode side is a PPV in the polymerization method of the copolymer used for the air electrode.
In the same way except that E was not charged,
/ PSVE copolymer (83:17 in molar ratio) was synthesized. This was hydrolyzed in the same manner as the copolymer used for the air electrode, and then subjected to acid-form treatment before use.

Next, a catalyst comprising platinum supported on carbon black powder so as to contain 40% by weight of platinum and the ion exchange resin obtained as described above were mixed in a weight ratio of 3: 1.
Thus, a catalyst paste for forming an air electrode and a fuel electrode was prepared.

As the current collector for both the fuel electrode and the air electrode, carbon paper (trade name: TGP-H-060, manufactured by Toray Industries, Inc.) was used after water repellency treatment. As a gas diffusion layer,
After kneading a mixture of 60% by weight of carbon black powder and 40% by weight of PTFE powder, the mixture was rolled to a thickness of 10%.
A sheet in which PTFE was fibrillated at 0 μm and a porosity of 70% was used.

For both the fuel electrode and the air electrode, a catalyst paste was applied to the gas diffusion layer and dried to form an electrode sheet. At this time, the catalyst paste was applied such that the amount of platinum contained in the electrode sheet was 0.5 mg / cm ² . The electrode sheet was cut out to have an effective electrode area of 28 cm ^{2 for} both the fuel electrode and the air electrode.

As a solid polymer electrolyte, a perfluorocarbon sulfonic acid type ion exchange membrane (trade name: Flemion H)
R, manufactured by Asahi Glass Co., Ltd., ion exchange capacity: 1.1 meq / g dry resin, film thickness: 50 μm) was used. The air electrode and the fuel electrode are opposed to each other with the surface coated with the catalyst paste facing inward, and the electrode sheet and the membrane are joined by hot pressing with the ion exchange membrane sandwiched therebetween,
A joined body was produced.

The above joined body was placed in a measuring cell with two carbon papers as current collectors interposed therebetween, and was placed under normal pressure (1 at.
a) The gas was a continuous operation at a constant current of 0.5 A / cm ^{2 at} a hydrogen / air system and a cell temperature of 80 ° C., and the output voltage of the cell was measured at any time, and the output voltage of the cell was 50 mV lower than the initial value. The number of days required to decrease was measured. Table 1 shows the results.

[0048]

[Table 1]

[0049]

According to the present invention, the ion exchange resin contained in the air electrode is hardly swelled by the wetting gas or the generated water, has a low water content, and has high gas permeability. Is suppressed. Therefore,
A solid polymer electrolyte fuel cell having a high output density and little deterioration of output characteristics over time can be obtained.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4J100 AC26P AE38Q AE38R AE39R BA02Q BA02R BA57Q BB12Q BB13Q BB13R BB18R CA05 CA06 JA16 5H018 AA02 AA06 AS02 AS03 BB06 BB08 DD06 DD08 EE03 EE05 A05 EE05 A05 C05 EE19 HH05

Claims (4)

[Claims] A gas diffusion electrode containing a catalyst and an ion exchange resin is used as a fuel electrode and an air electrode. The fuel electrode is provided on one surface of the solid polymer electrolyte membrane, and the air electrode is provided on the other surface. In each of the arranged solid polymer electrolyte fuel cells, the ion exchange resin contained in the air electrode is an ion exchange resin comprising a copolymer containing the following polymerized units A, B and C. A solid polymer electrolyte fuel cell characterized by the following. Polymerized unit A: Polymerized unit based on tetrafluoroethylene, Polymerized unit B: Polymerized unit based on perfluorovinyl ether having a sulfonic acid group, Polymerized unit C: Based on perfluorovinyl ether having no ion exchange group or its precursor group Polymerized units. 2. The copolymer contains the polymerized unit B and the polymerized unit C in a total amount of 10 to 2 with respect to the polymerized unit A.
The solid polymer electrolyte fuel cell according to claim 1, which is contained in an amount of 5 mol%. 3. The copolymer according to claim 1, wherein the polymer unit B is 5 mol based on the total amount of the polymer unit A and the polymer unit C.
The solid polymer electrolyte fuel cell according to claim 2, which is contained in an amount of 1% or more. 4. In the copolymer, the polymer unit C is 2 mol based on the total amount of the polymer unit A and the polymer unit B.
The solid polymer electrolyte fuel cell according to claim 3, which is contained in an amount of 1% or more.