JPWO1999053559A1 - Sealant for fuel cells - Google Patents

Abstract

(57) [Abstract] The separators (24) (25) of a single cell (20) of a fuel cell, a pair of electrodes (22) (23), and a solid electrolyte ion exchange resin (21) are sealed by forming a sealing layer through three-dimensional crosslinking of a liquid resin composition (26) comprising: A) an addition-polymerizable oligomer whose main chain in the molecule is a linear polyisobutylene structure or a perfluoropolyether structure and which has alkenyl groups at least on both ends; B) a curing agent whose molecule contains at least two hydrogen atoms bonded to at least one silicon atom; and C) a hydrosilylation reaction catalyst, thereby making it possible to make the single cell thinner and more compact and enabling a high output of the fuel cell.

Description

Detailed Description of the Invention

TECHNICAL FIELD The present invention relates to a liquid resin composition sealant that can be three-dimensionally crosslinked to form a low-gas-permeable elastic seal layer at the interface between a separator, a pair of electrodes, and a solid electrolyte ion-exchange resin membrane in a single fuel cell, thereby ensuring complete airtightness between these components. BACKGROUND ART A fuel cell is a device that directly converts the energy contained in a fuel into electrical energy. For example, a fuel gas containing hydrogen is supplied to the anode and an oxidizing gas containing oxygen is supplied to the cathode, generating electromotive force through electrochemical reactions occurring at both electrodes. This electrochemical reaction can be represented by the reaction of formula (1) at the anode, the reaction of formula (2) at the cathode, and the reaction of formula (3) occurring throughout the entire cell. _H2 → 2H ⁺ + ^2e- Equation (1) (1/2) _O2 + 2H+ ^2e- → _H2O Equation (2) _H2 + (1/2) _O2 → _H2O Equation (3) A typical fuel cell configuration consists of a pair of electrodes sandwiching a solid electrolyte membrane. A hydrogen-containing fuel gas is supplied to the anode, and an oxygen-containing oxidizing gas is supplied to the cathode. However, if the separation is insufficient and one gas mixes with the other, power generation efficiency decreases. Generally, fuel cells have a stack structure in which unit cells, each consisting of a pair of electrodes as a basic unit, are stacked. In a unit cell, a pair of electrodes form a sandwich structure sandwiching a solid electrolyte membrane, and a gas-impermeable separator further sandwiches this sandwich structure. The separator prevents gases from mixing between adjacent unit cells. The solid electrolyte membrane serves to separate the fuel gas and oxidizing gas supplied within the unit cell. As a conventional airtight method, a groove is provided at the end of the separator and an O-ring is placed in this groove to prevent the gases supplied to both sides of the solid electrolyte membrane from mixing.
These are disclosed in Japanese Patent Application Laid-Open No. 6-68884 and Japanese Patent Laid-Open No. 6-119930. Ion-exchange resin membranes are used as solid electrolytes in small fuel cells. Since these ion-exchange resin membranes behave electrically conductive in a wet state, moisture is supplied to the inside of the fuel cell during operation to maintain the ion-exchange resin membrane in a wet state. In other words, the ion-exchange resin membrane must have two functions: to separate the fuel gas from the oxidizing gas, and to maintain a wet state. A preferred ion-exchange resin membrane with these functions is a fluorine-based resin ion-exchange resin membrane. A technology for sealing these membranes with an adhesive instead of the O-ring described above is disclosed in Japanese Patent Application Laid-Open No. 7-249.
The technology for thermocompression bonding of ion exchange resin membranes is disclosed in Japanese Patent Application Laid-Open No. 6-1
However, the adhesive strength of such fluorine-based resin membranes is generally very low, and these techniques have not been able to provide reliable airtightness. Therefore, a technique for improving adhesive strength by previously subjecting the joining surface of the ion-exchange resin membrane to an ion-exchange treatment when bonding using an epoxy resin adhesive is disclosed in Japanese Patent Application Laid-Open No. 9-199145. However, although this pretreatment improves adhesive strength, it also reduces electrical conductivity, resulting in a decrease in the electromotive force of the fuel cell. A sealing agent composition that undergoes addition polymerization via a hydrosilylation reaction is disclosed in Japanese Patent Application Laid-Open No. 8-2000.
In Japanese Patent Application Laid-Open No. 69317, a perfluoropolyether composition was disclosed.
No. 79691 discloses polyisobutylene-based compositions. Disclosure of the Invention: If the individual components of a fuel cell are made thinner, the individual cells can be stacked in a small space, thereby increasing the fuel cell's output. However, when an O-ring is used to seal the separator and ion-exchange resin membrane, the separator must be thick enough to accommodate the O-ring groove, which hinders the achievement of thinner and higher-output fuel cells. Furthermore, the ion-exchange resin membrane must be compressed by the O-ring's clamping force to achieve airtightness, so the membrane dimensions must be larger than the O-ring groove. Conversely, O-rings cannot be clamped close to the electrodes. This is because the electrodes are porous and made of a very fragile material to allow gas to diffuse through them, and excessive clamping with an O-ring would damage the electrodes. Ultimately, the O-ring seal requires a larger ion-exchange resin membrane area, hindering the miniaturization of fuel cells. Furthermore, ion-exchange resin membranes are very expensive, and the cost of machining the separator's O-ring grooves is also high, increasing the overall cost of the fuel cell. Next, airtight methods using epoxy resin adhesives require pretreatment of the ion-exchange resin membrane bonding surface, such as ion exchange treatment, to improve adhesive strength. Furthermore, the elution of impurity ions, such as chloride ions, from the epoxy resin is also a problem. Contamination of the membrane with impurity ions eluted from the epoxy resin reduces the membrane's electrical conductivity, resulting in a decrease in the electromotive force of the individual cells, and thus a decrease in the total electromotive force of the entire fuel cell stack, in which the individual cells are stacked in series. Similarly, when ion-exchange resin membranes are bonded by thermocompression, the membrane must be pre-treated with ion exchange, which results in the same problems described above. Furthermore, thermocompression bonding is prone to damage to the ion-exchange resin membrane, and a damaged membrane may cause an electrical short circuit due to internal pressure differences during fuel cell operation. Because the airtightness of fuel cells sealed with O-rings or epoxy resin adhesives is incomplete for the reasons described above, when used in vehicles, vibrations during operation can easily cause gas leakage. The sealing agent of the present invention minimizes the area of the ion exchange resin membrane of the single cell and makes it possible to make it thinner, thereby making it possible to reduce the size of the fuel cell and prevent a decrease in electromotive force.
Furthermore, the present invention provides a sealing agent that can be used to seal ion exchange resin membranes, separators,
The sealing agent of the present invention forms an elastic seal layer on the bonding surface of a pair of electrodes, thereby providing highly reliable airtightness and ensuring the maintenance of a moist state of the membrane. When the sealing agent of the present invention is used in the production of a fuel cell using an ion-exchange resin membrane, pretreatment such as ion exchange treatment to improve adhesion to the membrane or O treatment can be avoided.
This eliminates the need for separate sealing members such as rings. Furthermore, there is no problem with membrane contamination due to impurity ions leaching from epoxy resin adhesives. In other words, compared to conventional technologies, this method reliably bonds and airtightly the ion-exchange resin membrane to the separator or a pair of electrodes, enabling thinner and more compact designs compared to O-ring systems, and eliminating the need for pretreatment to improve adhesion compared to epoxy resin adhesive systems, thereby shortening the work process and reducing costs. The sealant of the present invention has the following characteristics: a) extremely low gas permeability; b) excellent adhesion and adhesion to the ion-exchange resin membrane; c) very little leaching of impurity ions after curing; and d) low moisture permeability. Therefore, the membrane can be reliably bonded and airtightly to the separator or a pair of electrodes without degrading the performance of the ion-exchange resin membrane. In the present invention, it is preferable that the surface of the separator or a pair of electrodes that is bonded to the ion-exchange resin membrane be roughened. This is because the larger adhesive area of the sealant that penetrates into the roughened surface provides even stronger adhesion. The sealing agent of the present invention is a liquid resin composition having low gas permeability and being reactive,
The sealant is applied to the joining surfaces of a separator, a pair of electrodes, and an ion-exchange resin membrane of a solid electrolyte in a single cell, and then three-dimensionally crosslinks the components to provide an airtight seal. The sealant comprises: (A) an addition-polymerizable oligomer having a linear polyisobutylene or perfluoropolyether main chain and alkenyl groups at at least both ends; (B) a curing agent containing at least two hydrogen atoms bonded to silicon atoms in the molecule; and (C) a hydrosilylation reaction catalyst. One of the two types of addition-polymerizable oligomers (A) has a linear polyisobutylene main chain and reactive groups at at least both ends, preferably with a molecular weight of 500 to 100,000, and with a total of 50% or more repeating units derived from isobutylene. The addition-polymerizable oligomer may be composed entirely of isobutylene units, or may be a copolymer of a polyolefin such as polyethylene or polypropylene or a polydiene such as polybutadiene or polyisoprene, with 50% or less of the molecule being isobutylene units. One of the addition-polymerizable oligomers A) has a main chain in the molecule that is a linear perfluoropolyether structure and has reactive groups at least at both ends, _and has 3 to 400 repeating units of the following structure: _CF2O- , _CF2CF2O- , _{CF2CF2CF2O- ,} CF( _{CF3 )}
CF ₂ O—, CF ₂ CF ₂ CF ₂ CF ₂ O—, C(CF ₃ ) ₂ O— The viscosity of such an addition-polymerizable oligomer having a perfluoropolyether structure is
The range of the viscosity is 25 to 1,000,000 ^mm2 /s. The curing agent of B) is not particularly limited as long as it contains at least two hydrogen atoms bonded to at least silicon atoms in the molecule. However, it is preferable that the curing agent itself is gas permeable and is a substance that is easily compatible with the addition polymerizable oligomer. In other words, when the main chain of the addition polymerizable oligomer is a polyisobutylene structure, the main chain of the curing agent is preferably a polyisobutylene structure, and when the main chain of the addition polymerizable oligomer is a perfluoropolyether structure, the main chain of the curing agent is also preferably a perfluoropolyether structure, which is the most preferable combination. It should be noted that the hydrosilyl group in the curing agent is a 1/2-thickness mixture of the alkenyl group of the addition polymerizable oligomer.
The amount of hydrosilyl groups is preferably 0.5 to 5 moles per mole of the curing agent. The molecular weight of the curing agent is preferably in the range of 100 to 30,000. The hydrosilylation reaction catalyst C) may be any commonly used chloride of platinum, titanium, palladium, rhodium, etc. Specific examples of preferred catalysts include:
Examples of suitable sealants include platinum chloride, platinum-vinylsiloxane complexes, platinum-phosphine complexes, platinum-phosphite complexes, platinum-alcoholate complexes, and platinum-olefin complexes. The sealant of the present invention may optionally contain fillers, extender pigments, antioxidants, surfactants, and other well-known materials, provided that the elution of impurity ions is not a problem. The sealant of the present invention is used by applying the sealant in a liquid state to the joining surfaces of the separator, a pair of electrodes, and the ion exchange resin of the solid electrolyte to assemble a single cell. The sealant is then three-dimensionally crosslinked by heating or leaving at room temperature to form an elastic sealing layer of the sealant on the joining surfaces. The single cells thus manufactured are stacked to form a stack structure by applying a compressive force greater than the clamping force used for fixing in the crosslinking process. Since the sealant crosslinked by addition polymerization undergoes slight cure shrinkage, compressing the stack structure during stacking enhances its airtightness. The following describes an embodiment of the present invention based on examples. The fuel cell has a stack structure in which single cells are stacked. Fig. 1 shows a schematic cross section of a single cell (20). The single cell (20), which is the basic unit of a fuel cell, consists of an ion-exchange resin membrane (21), an anode (22), a cathode (23), and separators (24) and (25). The anode (22) and cathode (23) sandwich the ion-exchange resin membrane (21).
Separators (24) and (25) further sandwich the anode (22) and cathode (23) from both sides. Fuel gas and oxidant gas flow paths are formed on the surfaces of the anode (22) and cathode (23). A fuel gas flow path (24a) is formed between the anode (22) and separator (24), and an oxidant gas flow path (25a) is formed between the cathode (23) and separator (25). The separators (24) and (25) form gas flow paths between the electrodes and also serve to separate the fuel gas and oxidant gas between adjacent single cells. The ion-exchange resin membrane (21) is an ion-conductive membrane made of a solid electrolyte and a fluorine-based resin, and exhibits electrical conductivity in a wet state. In the experiments conducted in this invention, commercial Nafion (manufactured by DuPont) was used. Both the anode (22) and cathode (23) were made of carbon cloth woven with carbon fiber threads, and the ion-exchange resin membrane (21) was 120 to 130 mm thick.
The sealing agent (26) used in Example 1 is a heat-curable sealing agent (trade name: ) in which the main chains of both the addition polymerization oligomer and the curing agent are perfluoropolyether.
ThreeBond 11X-058, manufactured by ThreeBond Co., Ltd.) Anode (2
The sealant (26) is applied to the joining surfaces of the separator (24) to which the cathode (23) is attached and the separator (25) to which the cathode (23) is attached, and when the separators (24) and (25) are engaged at predetermined positions to assemble the unit cell (20), a non-crosslinked liquid sealing layer is formed covering the ion exchange resin membrane (21). Depending on the material, the ion-cured resin membrane (21) changes its properties when subjected to thermal treatment at 150°C or higher, becoming more hydrophobic and decreasing its conductivity.
The sealing agent (26) used was heated at 100°C or less, and the uncrosslinked sealing layer was three-dimensionally crosslinked to form an elastic sealing layer without thermal damage to the ion exchange resin membrane (21). The properties of the sealing agent (26) used were: A) an addition polymerizable oligomer having at least two alkenyl groups in the molecule and a perfluoropolyether structure in the main chain, and having a viscosity of 10,000 to 1,000,000 ^mm2 /s at 25°C; B) an addition polymerizable oligomer having at least two hydrogen atoms bonded to at least silicon atoms in the molecule;
The liquid resin composition comprises: (A) a curing agent containing at least one of the following groups, having a perfluoropolyether structure in its main chain, and having a viscosity of 10,000 to 500,000 ^mm2 /s at 25°C; (B) a curing agent containing a catalytic amount of a platinum group compound, and (C) a curing agent in an amount such that 0.5 to 5 moles of hydrosilyl groups are present per mole of alkenyl groups in the addition-polymerizable oligomer of (A). The sealant has the following properties: 1) it crosslinks in 30 to 60 minutes at a temperature range of 80 to 150°C, and the resulting rubbery elastic body has excellent elongation; 2) the crosslinked sealant has excellent gas barrier properties against fuel gases and oxidizing gases; 3) the crosslinked sealant has low moisture permeability; 4) the amount of impurity ions eluted from the crosslinked sealant is extremely small; 5) the crosslinked sealant has excellent resistance to methanol and the like; and 6) the crosslinked sealant has excellent adhesion to fluororesin-based ion-exchange resin membranes. In Example 2, the above-mentioned sealing agent 11X-058 was replaced with a heat-curable sealing agent (trade name: Th
A single cell (20) was fabricated in the same manner as above, using a sealant (TriBond 11X-066, manufactured by ThreeBond Co., Ltd.) instead of the sealant. The properties of the sealant 11X-066 used are: A) there are at least two
(B) an addition-polymerizable oligomer having at least two alkenyl groups and a polyisobutylene structure in the main chain, and having a viscosity of 25 to 100,000 mm ² /s at 25°C; (C) an addition-polymerizable oligomer having at least two hydrogen atoms bonded to at least one silicon atom in the molecule;
It also has a perfluoropolyether structure in the main chain, and has a viscosity of 10 to 10 at 25°C.
C) a curing agent having a viscosity of 1000000 mm ² /s, C) a catalytic amount of a platinum group compound, and A)
The ratio of hydrosilyl groups to alkenyl groups in the addition polymerizable oligomer is 0.5
The sealant has the following properties: 1) It crosslinks in 20 to 60 minutes at a temperature range of 70 to 100°C, and the resulting gel-like substance has excellent elongation. 2) The sealant after crosslinking has excellent gas barrier properties against fuel gases and oxidizing gases. 3) The sealant after crosslinking has low moisture permeability. 4) The amount of impurity ions eluted from the sealant after crosslinking is extremely small. 5) The sealant has low viscosity, making it easy to apply. 6) The sealant after crosslinking has excellent adhesion to the ion exchange resin membrane. For comparison, single cells were fabricated using two types of conventional sealants. One was an RTV silicone (trade name: ThreeBond1) instead of the sealant (26) of the present invention.
The other was a single cell (20) made using a one-component heat-curing epoxy resin (product name: ThreeBond 220D, manufactured by ThreeBond Co., Ltd.) at 25°C and 55% RH for 7 days.
The single cell (20) was fabricated under the conditions of 0°C and 60 minutes of curing. In both Example 1 and Example 2, in which the sealant of the present invention was used, it was possible to maintain airtightness against fuel gas and oxidizing gas, and a moist state inside the single cell.
In the single cell using the RTV silicone, it was not possible to maintain a sufficient moist state within the single cell. On the other hand, in the single cell using the heat-curing epoxy resin as a comparative example, the sealing and moist state were maintained, but discoloration of the ion exchange resin membrane (21) due to the elution of impurity ions was observed. When the single cell (20) was disassembled after operation was completed, the single cells using 11X-058 of Example 1 and 11X-066 of Example 2 had good adhesive strength to the ion exchange resin membrane (21) and underwent cohesive failure. On the other hand, the single cells using the RTV silicone and the one-component heat-curing epoxy resin as comparative examples underwent interfacial peeling. In the above-mentioned Examples 1 and 2, the thickness of the ion exchange resin membrane was 100 μm.
It is generally said that thinner ion exchange resin membranes improve conductivity and contribute to improved performance of single cells. Therefore, single cells were also produced using the sealants 11X-057 and 11X-066 for ion exchange resin membranes with a thickness of 50 μm, and tests similar to those in Examples 1 and 2 were carried out, confirming that similar effects were achieved. The method of thermocompression bonding ion exchange resin membranes often damages the membrane,
Usually, an ion exchange resin membrane having a thickness of 300 μm is used, and a membrane having a thickness of 100 μm is not used. This is because a damaged membrane may cause an electrical short circuit during the operation of the fuel cell. The sealing agent of the present invention is a sealing agent for sealing the ion exchange resin membrane (21) and the separator (24), (
Since an elastic sealant layer is formed between the separator (25) and the ion exchange resin membrane (21), electrical short circuits can be prevented without damaging the ion exchange resin membrane (21) during thermocompression bonding or fuel cell operation. Industrial Applicability As explained above, a fuel cell using the sealant of the present invention does not require pretreatment of the ion exchange resin membrane when fixing the ion exchange resin membrane to the separator, and can maintain a moist state during operation, isolating the fuel gas and the oxidizing gas, and providing an airtight seal. Therefore, when considering fuel cells for automobiles, it can exhibit good sealing performance against movements such as vibrations. In addition, an O sealant having a predetermined thickness of about 2 mm can be used.
Since there is no need to incorporate a ring, the entire fuel cell 10 can be made thinner.
Increasing the number of stacked unit cells makes it possible to increase battery capacity. Additionally, the adhesive used in the present invention can be applied automatically by machine, shortening the work process, improving efficiency, and reducing costs. The sealant of the present invention can bond and seal the ion-exchange resin membrane, which is a solid electrolyte, without contaminating it with ionic species, thereby eliminating the need for O-rings and enabling fuel cells to be made smaller or thinner. Furthermore, the sealant is gas-impermeable, providing excellent gas-tight sealing between the ion-exchange resin membrane and the separator body, thereby maintaining the electrolyte membrane in a moist state. As a result, a fuel cell in which the ion-exchange resin membrane, separator, and pair of electrodes are bonded and airtight with the sealant of the present invention is thin and compact, has high electromotive force, and is resistant to vibration, making it suitable for use as a fuel cell for automobiles.

[Brief explanation of the drawings]

FIG. 1 is a cross-sectional view of a single fuel cell. In the figure, (21) is an ion-cured resin film, (22) is an anode electrode, (2
3) is a cathode electrode, (24) and (25) are separators, (24a) is a fuel gas flow path, (25a) is an oxidizing gas flow path, and (26) is the sealing agent of the present invention.

[Procedure Amendment] Submission of a copy of amendments under Article 19 of the Patent Cooperation Treaty (Official)

[Submission date] September 13, 1999 (1999.9.13)

[Procedural Correction 1]

[Name of document to be corrected] Statement

[Item to be amended] Claims

[Correction method] Change

[Contents of the amendment]

[Claims]

───────────────────────────────────────────────────── (Note) This publication is based on the publication published internationally by the International Bureau of Patents (WIPO). The effect of the international publication of the Japanese patent application (Japanese utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act) and is unrelated to this publication.

Claims (3)

[Claims] [Claim 1] A sealant for fuel cells, characterized in that when a separator (24) (25), a pair of electrodes (22) (23), and a solid electrolyte ion exchange resin membrane (21) of a single cell (20) of a fuel cell are sealed by three-dimensional crosslinking with a low gas permeability and reactive liquid resin composition (26), the resin composition is three-dimensionally crosslinked by addition polymerization, and is composed of A) an addition-polymerizable oligomer whose main chain in the molecule is a linear polyisobutylene structure or a perfluoropolyether structure and has alkenyl groups at at least both ends, B) a curing agent whose molecule contains at least two hydrogen atoms bonded to at least one silicon atom, and C) a hydrosilylation reaction catalyst. [Claim 2] A sealing agent for fuel cells as described in Claim 1, characterized in that when the main chain of the addition-polymerizable oligomer A) is a polyisobutylene structure, the main chain of the curing agent B) is a polyisoprene structure. [Claim 3] A sealing agent for fuel cells as described in Claim 1, characterized in that when the main chain of the addition-polymerizable oligomer A) is a perfluoropolyether structure, the main chain of the curing agent B) is a perfluoropolyether structure.