# Patent

## Title
Separator and separator seal for polymer electrolyte fuel cells

## Abstract

A rubber composition comprising (A) a liquid alkenyl-containing organopolysiloxane with Mw<100,000, (B) a gum-like organopolysiloxane with Mw≧150,000, (C) an organohydrogenpolysiloxane, (D) fumed silica, and (E) an addition reaction catalyst and having a viscosity of 20-200 Pa-s at a shear rate of 10 s−1 and 25° C. is effectively injection moldable into a cured product which is useful as a separator seal in PEFCs.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2007-317475 filed in Japan on Dec. 7, 2007, the entire contents of which are hereby incorporated by reference.
TECHNICAL FIELD
This invention relates to separators and separator seals for use in polymer electrolyte fuel cells (PEFC) featuring compactness and more particularly, to separators and separator seals for PEFC which have a long service life and ease of molding.
BACKGROUND ART
The fuel cell is capable of producing electricity without a substantial need for fossil fuel that poses concerns about resource depletion, without noise, and at a high energy recovery rate as compared with other energy-based power generating systems. Great efforts have been made to exploit the fuel cell as a power generating plant of relatively compact size in buildings and factories, with some cells having been commercially implemented. In particular, polymer electrolyte fuel cells (PEFC) can operate at lower temperature than fuel cells of other types. The PEFC then draws attention not only as a device for household co-generation, but also as the replacement power source for internal combustion engines on vehicles because of the minimized corrosion concern regarding the materials of which cell components are made and their ability to discharge relatively high current flow despite low temperature operation. The PEFC is constructed of electrolyte membranes, separators and other components. The separator is generally a plate which is provided with a plurality of parallel channels on one surface or both surfaces. The separator plays the role of conducting the electricity produced at the gas diffusion electrode within the fuel cell to the exterior, discharging water produced within the channels in the course of electricity generation, and securing the channels as a flow path for incoming reaction gas to the fuel cell. Such a fuel cell separator is required to be more compact in size. Since a multiplicity of separators are used in stack, there is a demand for a separator seal material having durability and long term service.
As the separator sealing material, packing materials based on various resins have been under study. Among them, sealing materials based on silicone rubber are often used for their moldability, heat resistance and elasticity. JP-A 11-129396, JP-A 11-309747 and JP-A 2001-58338 disclose liquid silicone rubber compositions of the addition cure type having a good flow and brief cure. These compositions are injection moldable to form seals. However, silicone rubbers obtained by curing the addition cure type compositions are still unsatisfactory in maintaining elasticity over a long term.
JP-A 2002-313373 and JP-A 2003-257455 disclose organopolysiloxane compositions with reduced compression set. For the seal on fuel cell separators, in particular, seal performance in acidic aqueous solution is important. JP-A 2002-309092 discloses how to reduce the compression set in acidic aqueous solution.
For the PEFC, several tens to several hundreds of unit cells wherein thin-film structures having an electrolyte membrane sandwiched between electrodes are alternated with separators and assembled into a stack. In order that the overall stack be compact, it is necessary that separator substrates have a thickness of up to 1 mm and seals formed thereon be as thin as 0.5 mm or less, for example. When such thin-wall seals are formed on separator substrates, the flow and rubber strength of the seal-forming material become important factors. At the same time, acid resistance and compression set are also requisite as the seal. However, JP-A 11-129396 and JP-A 11-309747 cited above merely describe an appropriate viscosity range of 1,000 to 10,000 poise, but refer nowhere to the detail of silicone rubber. While the flow at a high shear is a key for injection molding, these patents do not discuss the viscosity versus shear rate. In our experiment, the silicone resin KE1950-60A/B (Shin-Etsu Chemical Co., Ltd.) used in Examples was found to have a viscosity in excess of 200 Pa-s at a shear rate of 10 s−1, which is not regarded appropriate to form thin-wall seals. JP-A 2001-58338 describes an example in which a liquid silicone rubber material is injection molded to form a seal, but refers nowhere to the composition and flow of the material.
JP-A 2002-313373 discloses a composition comprising fumed silica treated with two different surface treating agents, but refers nowhere to the viscosity and moldability of the composition. JP-A 2003-257455 describes that rubber having improved compression set and strength is obtainable by the combined use of two different alkenyl-containing fluids, but refers nowhere to moldability.
SUMMARY OF THE INVENTION
An object of the invention is to provide a separator seal for use in polymer electrolyte fuel cells, which is made of cured rubber having ease of molding, reduced compression set, and satisfactory seal performance; and a separator having the seal formed at the periphery of a separator substrate.
The inventors have found that a liquid silicone rubber composition comprising as essential components (A) a liquid organopolysiloxane containing at least two alkenyl groups each attached to a silicon atom in a molecule and having a weight average molecular weight of less than 100,000, (B) a gum-like organopolysiloxane having a weight average molecular weight of at least 150,000, (C) an organohydrogenpolysiloxane containing at least three silicon-bonded hydrogen atoms in a molecule, (D) fumed silica having a BET specific surface area of 50 to 400 m2/g, and (E) an addition reaction catalyst is effectively moldable, especially injection moldable, when the composition has a viscosity of 20 to 200 Pa-s at a shear rate of 10 s−1 and 25° C., that the cured rubber has reduced compression set and exhibits excellent seal performance so that it becomes an effective seal on a separator in PEFCs.
Accordingly, in one aspect, the present invention provides a separator seal for use in polymer electrolyte fuel cells which is formed of a sealing composition in the cured state, said sealing composition comprising as essential components,
(A) 100 parts by weight of a liquid organopolysiloxane containing at least two alkenyl groups each attached to a silicon atom in a molecule and having a weight average molecular weight of less than 100,000,
(B) 3 to 30 parts by weight of a gum-like organopolysiloxane having a weight average molecular weight of at least 150,000,
(C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxane containing at least three hydrogen atoms each attached to a silicon atom in a molecule,
(D) 5 to 30 parts by weight of fumed silica having a BET specific surface area of 50 to 400 m2/g, and
(E) a catalytic amount of an addition reaction catalyst,
said composition having a viscosity of 20 to 200 Pa-s at 25° C. and a shear rate of 10 s−1.
In a preferred embodiment, the alkenyl-containing organopolysiloxane (A) has a weight average molecular weight of 10,000 to 80,000, at least 90 mol % of the entire organic groups attached to silicon atoms being methyl. Also preferably, a molar ratio of Si—H groups in component (C) to total alkenyl groups in components (A) and (B) is between 0.8:1 and 5.0:1. In a preferred embodiment, the composition at 25° C. has a viscosity V(10) at a shear rate of 10 s−1 and a viscosity V(0.9) at a shear rate of 0.9 s−1, which fall in the range: 1.0<V(0.9)/V(10)<2.5. Typically, the gum-like organopolysiloxane (B) contains alkenyl groups in an amount of 1.0×10−6 to 1.0×10−3 mol/g.
In another aspect, the present invention provides a separator for use in PEFCs comprising a substrate comprising a metal thin plate or a conductive powder and a binder and a seal formed at a periphery on at least one surface of the substrate, the seal comprising a cured product of the sealing composition.
BENEFITS OF THE INVENTION
The sealing composition of the invention is effectively moldable, especially injection moldable. The cured rubber has reduced compression set and exhibits excellent seal performance so that it becomes an effective separator seal for use in PEFCs.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective exploded view showing main components of a power-generating cell in a fuel cell stack according to one embodiment of the invention.
FIG. 2 is a cross-sectional view of the fuel cell stack taken along lines II-II in FIG. 1.
FIG. 3 is a cross-sectional view traversing a fuel gas inlet communication hole of the fuel cell stack.
FIG. 4 is a cross-sectional view traversing an oxidant gas inlet communication hole of the fuel cell stack.
FIG. 5 is a front view of a first metal separator constituting the power-generating cell.
FIG. 6 is a front view showing one surface of a second metal separator constituting the power-generating cell.
FIG. 7 is a front view showing the other surface of the second metal separator constituting the power-generating cell.



In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As used herein, the singular forms “a,” “an” and “the” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The terms “first,” “second” and the like do not denote any order or importance, but rather are used to distinguish one element from another. It is also understood that terms such as “top,” “bottom,” “outward,” “inward” and the like are words of convenience and are not to be construed as limiting terms.
The sealing material or sealing rubber composition of the invention is used to form a seal at the periphery on at least one surface of a separator for use in polymer electrolyte fuel cells (PEFCs). The sealing composition comprises as essential components,
(A) 100 parts by weight of a liquid organopolysiloxane containing at least two alkenyl groups each attached to a silicon atom in a molecule and having a weight average molecular weight of less than 100,000,
(B) 3 to 30 parts by weight of a gum-like organopolysiloxane having a weight average molecular weight of at least 150,000,
(C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxane containing at least three hydrogen atoms each attached to a silicon atom in a molecule,
(D) 5 to 30 parts by weight of fumed silica having a BET specific surface area of 50 to 400 m2/g, and
(E) a catalytic amount of an addition reaction catalyst.
Component A
Component (A) is a liquid organopolysiloxane containing at least two alkenyl groups each attached to a silicon atom in a molecule and having a weight average molecular weight of less than 100,000. Most often, it is represented by the following average compositional formula (I):

R1aSiO(4-a)/2  (I)

wherein R1 which may be the same or different is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, and “a” is a positive number in the range of 1.5 to 2.8, preferably 1.8 to 2.5.

Examples of the substituted or unsubstituted monovalent hydrocarbon group represented by R1 include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl, and decyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl, as well as substituted forms of the foregoing groups in which some or all hydrogen atoms are replaced by halogen atoms (e.g., fluoro, bromo and chloro), cyano groups or the like, such as chloromethyl, chloropropyl, bromoethyl, trifluoropropyl and cyanoethyl. Preferably, at least 90 mol % of the entire R1 are methyl.
At least two of R1 should be alkenyl groups, preferably of 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms, and most preferably vinyl. The content of alkenyl groups is preferably 5.0×10−6 mol/g to 5.0×10−3 mol/g, more preferably 1.0×10−5 mol/g to 1.0×10−3 mol/g of the organopolysiloxane. An alkenyl content of less than 5.0×10−6 mol/g may give too low a rubber hardness to provide a satisfactory seal whereas an alkenyl content of more than 5.0×10−3 mol/g may result in a higher crosslinked density and hence, brittle rubber.
The alkenyl groups may be attached to a silicon atom at the end of the molecular chain or a silicon atom midway the molecular chain or both. The inclusion of at least alkenyl groups attached to silicon atoms at both ends of the molecular chain is preferred.
The preferred organopolysiloxane basically has a linear structure, but may partially have a branched or cyclic structure.
In the invention, the alkenyl-containing organopolysiloxane as component (A) should have a weight average molecular weight Mw of less than 100,000, and preferably in the range of 10,000 to 80,000. If Mw is less than 10,000, the resulting sealing composition may not have satisfactory rubber elasticity. If Mw is more than 100,000, the resulting sealing composition becomes too viscous to mold. As used herein, the weight average molecular weight is measured by gel permeation chromatography (GPC) versus polystyrene standards.
Component B
Component (B) is an organopolysiloxane having a weight average molecular weight of at least 150,000 which is gum-like at room temperature. Most often, it is represented by the following average compositional formula (II):

R2bSiO(4-b)/2  (II)

wherein R2 which may be the same or different is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, and “b” is a positive number in the range of 1.5 to 2.8, preferably 1.8 to 2.5.

Examples of the substituted or unsubstituted monovalent hydrocarbon group represented by R2 include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl, and decyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl, as well as substituted forms of the foregoing groups in which some or all hydrogen atoms are replaced by halogen atoms (e.g., fluoro, bromo and chloro), cyano groups or the like, such as chloromethyl, chloropropyl, bromoethyl, trifluoropropyl and cyanoethyl. Preferably, at least 90 mol % of the entire R2 are methyl.
R2 may or may not contain alkenyl groups although the inclusion of alkenyl groups is preferred. The alkenyl groups, if present, preferably have 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms, and most preferably vinyl. The content of alkenyl groups is preferably 1.0×10−6 mol/g to 1.0×10−3 mol/g, more preferably 1.0×10−5 mol/g to 5.0×10−4 mol/g of the organopolysiloxane. An alkenyl content of more than 1.0×10−3 mol/g may result in a higher crosslinked density and hence, brittle rubber or aggravated compression set.
The alkenyl groups may be attached to a silicon atom at the end of the molecular chain or a silicon atom midway the molecular chain or both.
The preferred organopolysiloxane basically has a linear structure, but may partially have a branched or cyclic structure.
In the invention, the organopolysiloxane as component (B) should have a weight average molecular weight Mw equal to or more than 150,000. Although the upper limit need not be specified, it is preferred for viscosity and molding after mixing that Mw fall in the range of 200,000 to 1,000,000.
The gum-like organopolysiloxane (B) is blended in an amount of 3 to 30 parts, preferably 4 to 25 parts, and more preferably 5 to 20 parts by weight per 100 parts by weight of component (A).
Component C
Component (C) is an organohydrogenpolysiloxane containing at least three hydrogen atoms each attached to a silicon atom (i.e., Si—H groups) in a molecule. It serves as a crosslinking agent for curing the composition wherein Si—H groups in the molecule crosslink with silicon-bonded alkenyl groups in components (A) and (B) through hydrosilylation addition reaction.
Most often, the organohydrogenpolysiloxane (C) is represented by the following average compositional formula (III):

R3cHdSiO(4-c-d)/2  (III)

wherein R3 is each independently a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, “c” is a positive number of 0.7 to 2.1, “d” is a positive number of 0.001 to 1.0, and c+d is from 0.8 to 3.0. Preferred are those of formula (III) having at least three (typically 3 to 300), more preferably 3 to 100, most preferably 3 to 50 silicon-bonded hydrogen atoms in a molecule.

Examples of the monovalent hydrocarbon group represented by R3 are as exemplified above for R1 although groups free of aliphatic unsaturation (as in alkenyl groups) are preferred. Preferably, “c” is 0.8 to 2.0, “d” is 0.01 to 1.0, and c+d is from 1.0 to 2.5.
The molecular structure of the organohydrogenpolysiloxane may be straight, cyclic, branched or three-dimensional network. The number of silicon atoms per molecule or the degree of polymerization is preferably about 2 to about 300, especially about 4 to about 150. Differently stated, the preferred organohydrogenpolysiloxanes are those which are liquid at room temperature (25° C.) and specifically have a viscosity of up to 1,000 mPa-s, and more preferably 0.1 to 500 mPa-s at 25° C. as measured by a rotational viscometer.
The hydrogen atoms may be attached to a silicon atom at the end of the molecular chain or a silicon atom midway the molecular chain or both.
Exemplary of the organohydrogenpolysiloxane (C) are trimethylsiloxy-endcapped methylhydrogenpolysiloxane, trimethylsiloxy-endcapped dimethylsiloxane-methylhydrogensiloxane copolymers, dimethylhydrogensiloxy-endcapped dimethylpolysiloxane, dimethylhydrogensiloxy-endcapped dimethylsiloxane-methylhydrogensiloxane copolymers, trimethylsiloxy-endcapped methylhydrogensiloxane-diphenylsiloxane copolymers, trimethylsiloxy-endcapped methylhydrogensiloxane-diphenylsiloxane-dimethylsiloxane copolymers, copolymers composed of (CH3)2HSiO1/2 units and SiO4/2 units, and copolymers composed of (CH3)2HSiO1/2 units, SiO4/2 units and (C6H5)SiO3/2 units. As used herein, the term “endcapped” means that the polysiloxane is capped at both ends of its molecular chain with the indicated groups.
The amount of the organohydrogenpolysiloxane (C) blended is 0.5 to 20 parts, and preferably 0.6 to 15 parts by weight, per 100 parts by weight of component (A). The molar ratio of silicon-bonded hydrogen atoms (Si—H groups) in component (C) to total alkenyl groups in components (A) and (B), [Si—H/alkenyl], is preferably from 0.8:1 to 5.0:1, especially from 1.0:1 to 3.0:1. A molar ratio outside this range may lead to cured rubber with increased compression set, aggravating the seal performance.
Component D
Component (D) is fumed silica which is essential to impart satisfactory strength to silicone rubber. The fumed silica should have a specific surface area of 50 to 400 m2/g, and preferably 100 to 350 m2/g, as measured by the BET method. A surface area below 50 m2/g may compromise acid resistance whereas above 400 m2/g, compression set increases. The fumed silica may be used as such, but preferably after treatment with a surface hydrophobizing agent. Alternatively, a surface treating agent is added when the fumed silica is mixed with the silicone fluid, whereby the fumed silica is treated during the mixing step. Suitable surface treating agents are well-known agents including alkylalkoxysilanes, alkylchlorosilanes, alkylsilazanes, silane coupling agents, titanate treating agents, and fatty acid esters alone or in admixture. When two or more agents are used, they may be applied at the same time or different times.
The amount of fumed silica (D) blended is 5 to 30 parts, especially 10 to 28 parts by weight, per 100 parts by weight of component (A). The preferred amount of fumed silica is 15 to 28 parts when it has a surface area of 50 to 180 m2/g, and 10 to 20 parts when it has a surface area of 180 to 400 m2/g. Less than 5 parts of the fumed silica fails to provide satisfactory rubber strength whereas more than 30 parts increases compression set, aggravating the seal performance.
Component E
Component (E) is an addition reaction catalyst for promoting addition reaction between alkenyl groups in the organopolysiloxanes as components (A) and (B) and silicon-bonded hydrogen atoms (Si—H groups) in the organohydrogenpolysiloxane as component (C). Most often, the catalyst is selected from platinum group metal-based catalysts including platinum catalysts such as platinum black, platinic chloride, chloroplatinic acid, reaction products of chloroplatinic acid with monohydric alcohols, complexes of chloroplatinic acid with olefins, and platinum bisacetoacetate as well as palladium catalysts and rhodium catalysts, with the platinum catalysts being preferred.
The amount of the catalyst blended is a catalytic amount to promote addition reaction and usually about 0.5 to 1,000 ppm, especially about 1 to 500 ppm of platinum group metal based on the weight of component (A). Less than 0.5 ppm may be ineffective to promote addition reaction, leading to undercure. Amounts of more than 1,000 ppm may exert little further effect on reactivity and be uneconomical.
Other Components
If necessary, the composition may further contain other components, for example, fillers such as precipitated silica, ground quartz, diatomaceous earth and calcium carbonate; hydrosilylation reaction regulating agents such as nitrogen-containing compounds, acetylene compounds, phosphorus compounds, nitrile compounds, carboxylates, tin compounds, mercury compounds, and sulfur compounds; heat resistance improvers such as iron oxide and cerium oxide; internal parting agents such as dimethylsilicone fluid; tackifiers, and thixotropic agents.
Separator Seal
The separator seal is formed of the sealing material or addition reaction cure type silicone rubber composition comprising the above-described components in the cured state. The silicone rubber composition may be applied and cured by well-known techniques, forming a seal on a PEFC separator.
More particularly, when PEFC separator seals are manufactured using the cured rubber, the silicone rubber composition is molded into a seal shape by a compression molding, casting or injection molding technique, and the molded seal is combined with a separator substrate. Alternatively, integrated seal-separator substrate members are manufactured by dipping, coating, screen printing, or insert molding. Of these molding methods, the method involving holding a separator substrate in place in a mold and injection molding the rubber composition into the mold cavity on one side or both sides of the substrate, commonly referred to as insert molding method, is most preferred because a thin-wall seal can be briefly formed at a high accuracy. In the practice of insert molding, the fluidity of the rubber composition is important. In order that a seal having a thickness of up to 0.5 mm be insert molded at a high accuracy and efficiency, the rubber composition should have a fluidity corresponding to a viscosity of 20 to 200 Pa-s at a shear rate of 10 s−1 and 25° C., preferably 25 to 150 Pa-s, and more preferably 30 to 120 Pa-s at a shear rate of 10 s−1 and 25° C. If the viscosity is less than 20 Pa-s, it is difficult to control burrs. If the viscosity is more than 200 Pa-s, the material gives rise to such problems as scorching and weld-marks during molding. In a preferred embodiment, the composition at 25° C. has a viscosity V(10) at a shear rate of 10 s−1 and a viscosity V(0.9) at a shear rate of 0.9 s−1, which fall in the range: 1.0<V(0.9)/V(10)<2.5, and more preferably 1.0<V(0.9)/V(10)<2.0. If this value is less than 1.0 or more than 2.5, the composition may be difficult to mold. It is noted that the viscosity as used herein may be measured by a precision rotational viscometer Rotovisco RV1 by Eko Instruments Co., Ltd.
Preferred curing conditions for the silicone rubber composition include a temperature of 100 to 300° C. and a time of 10 seconds to 30 minutes and more preferably 120 to 200° C. and 15 seconds to 5 minutes. A time of less than 10 seconds is too short for rubber curing whereas more than 30 minutes is uneconomical. The cured rubber may be post-cured in an oven for the purposes of reducing compression set or the like. Preferred post-curing conditions include a temperature of 100 to 220° C. and a time of 30 minutes to 100 hours and more preferably 120 to 200° C. and 1 to 24 hours.
The separator substrate used herein may be a metal thin plate or a substrate manufactured by integral molding of an electrically conductive powder in a binder. A seal is formed from the silicone rubber composition along the periphery of this separator substrate by the above-described technique, whereupon a PEFC separator within the scope of the invention is available.
Examples of the conductive powder include natural graphite such as flake graphite, artificial graphite, and conductive carbon blacks such as acetylene black and Ketjen Black. Any powders may be used as long as they are electrically conductive. Suitable binders include epoxy resins, phenolic resins, and rubber-modified phenolic resins.
According to the invention, the silicone rubber composition is applied and cured to the periphery of a separator substrate by a suitable technique such as compression molding, casting, injection molding, transfer molding, dipping, coating or screen printing. Thus the cured silicone rubber composition forms a seal. This results in a separator for PEFCs in which a ring-like seal (separator seal) is formed on the substrate so as to circumferentially extend along the periphery of the substrate.
Preferably the seal has a thickness or height of 0.1 to 2 mm. A seal of less than 0.1 mm may be difficult to form and exert less sealing effects whereas a seal of more than 2 mm may be inconvenient for size reduction.
Now referring to the figures, some embodiments of the separator for PEFCs according to the invention are illustrated. The invention is not limited thereto.
FIG. 1 is a perspective exploded view showing main components of a power-generating cell 12 constituting a fuel cell stack 10 according to one embodiment of the invention. A plurality of power-generating cells 12 are stacked in the direction of arrow A to construct the fuel cell stack 10. FIG. 2 is a cross-sectional view of this fuel cell stack 10 taken along lines II-II in FIG. 1.
As shown in FIGS. 2 to 4, the fuel cell stack 10 includes a plurality of power-generating cells 12 stacked in the direction of arrow A and end plates 14a, 14b at opposite ends in the stacking direction. The end plates 14a, 14b are fixedly tied via tie rods (not shown) so that a fastening load is applied across the stacked cells 12 in the direction of arrow A.
As shown in FIG. 1, each power-generating cell 12 includes an electrolyte membrane-electrode assembly (MEA) 16 interposed between first and second metal separators 18 and 20. The first and second metal separators 18 and 20 are, for example, steel plates, stainless steel plates, aluminum plates, plated steel plates or such metal plates which have been surface treated to be corrosion resistant. Their thickness is set in the range of 0.05 to 1.0 mm, for example.
At one side edge of the power-generating cell 12 in the direction of arrow B (in FIG. 1, typically horizontal direction), an oxidant gas inlet communication hole 30a for feeding an oxidant gas such as oxygen-containing gas, a coolant outlet communication hole 32b for discharging a coolant medium, and a fuel gas outlet communication hole 34b for discharging a fuel gas such as hydrogen-containing gas, which are in fluid communication with corresponding holes in adjacent cells in the direction of arrow A or stacking direction, are arranged in the direction of arrow C (typically vertical direction).
At the other side edge of the power-generating cell 12 in the direction of arrow B, a fuel gas inlet communication hole 34a for feeding the fuel gas, a coolant inlet communication hole 32a for feeding the coolant medium, and an oxidant gas outlet communication hole 30b for discharging the oxidant gas, which are in fluid communication with corresponding holes in adjacent cells in the direction of arrow A, are arranged in the direction of arrow C.
Specifically, the MEA 16 includes a solid polymer electrolyte membrane 36 in the form of a perfluorocarbon sulfonic acid thin film impregnated with water, which is sandwiched between an anode (or first electrode) 38 and a cathode (or second electrode) 40. The anode 38 has a smaller surface area than the cathode 40.
The anode 38 and cathode 40 each include a gas diffusion layer formed of carbon paper or the like, and an electrocatalytic layer which is formed by uniformly applying porous carbon particles having a platinum alloy supported on their surfaces to the surface of the gas diffusion layer. The electrocatalytic layers are joined to the opposite surfaces of the solid polymer electrolyte membrane 36.
The first and second metal separators 18 and 20 have inner surfaces 18a and 20a facing MEA 16 and outer surfaces 18b and 20b, respectively. The inner surface 18a of first metal separator 18 is provided with oxidant gas flow channels (reaction gas flow channels) 42 which extend in a serpentine manner in the direction of arrow B and vertically upward (see FIGS. 1 and 5). As shown in FIG. 6, the inner surface 20a of second metal separator 20 is provided with fuel gas flow channels (reaction gas flow channels) 44 which are in fluid communication with fuel gas inlet communication hole 34a and fuel gas outlet communication hole 34b as will be described later, and extend in a serpentine manner in the direction of arrow B and vertically upward (in the direction of arrow C).
As shown in FIGS. 1 and 2, coolant flow channels 46 are defined between the surfaces 18b and 20b of first and second metal separators 18 and 20 and in fluid communication with coolant inlet and outlet communication holes 32a and 32b. The coolant flow channels 46 extend straight in the direction of arrow B.
As shown in FIGS. 1 to 5, a first seal member 50 extends circumferentially along the peripheral edge of first metal separator 18 and is integrally joined to the surfaces 18a and 18b of first metal separator 18. The first seal member 50 is formed by applying the rubber composition to the separator substrate by a technique such as compression molding, casting, injection molding, transfer molding, dipping, coating or screen printing, and curing.
The first seal member 50 includes a first planar portion 52 which is integrally joined to the surface 18a of first metal separator 18 and a second planar portion 54 which is integrally joined to the surface 18b of first metal separator 18. The second planar portion 54 extends longer than the first planar portion 52.
As shown in FIGS. 2 and 3, the first planar portion 52 extends circumferentially at a position outward spaced apart from the peripheral edge of MEA 16, and the second planar portion 54 extends circumferentially over a region overlying a certain portion of cathode 40. As shown in FIG. 5, the first planar portion 52 is formed such that oxidant gas inlet and outlet communication holes 30a and 30b are in fluid communication with oxidant gas flow channels 42, and the second planar portion 54 is formed such that coolant inlet communication hole 32a is in fluid communication with coolant outlet communication hole 32b. 
A second seal member 56 extends circumferentially along the peripheral edge of second metal separator 20 and is integrally joined to the surfaces 20a and 20b of second metal separator 20. On the surface 20a side of second metal separator 20, the second seal member 56 is provided with an outside seal 58a which is disposed on surface 20a in proximity to the peripheral edge of second metal separator 20, and an inside seal 58b which is inwardly spaced apart from the outside seal 58a. The outside and inside seals 58a and 58b are provided on one side of second seal member 56 facing the anode 38.
The outside and inside seals 58a and 58b may have any desired shape selected from a tapered (or lip), trapezoid and semicylindrical shape. The outside seal 58a is in contact with first planar portion 52 of first metal separator 18, and the inside seal 58b is in direct contact with solid polymer electrolyte membrane 36 of MEA 16.
As shown in FIG. 6, the outside seal 58a circumscribes oxidant gas inlet communication hole 30a, coolant outlet communication hole 32b, fuel gas outlet communication hole 34b, fuel gas inlet communication hole 34a, coolant inlet communication hole 32a and oxidant gas outlet communication hole 30b. The inside seal 58b circumscribes fuel gas flow channels 44. The peripheral edge of MEA 16 is disposed between outside and inside seals 58a and 58b. 
On the surface 20b side of second metal separator 20, the second seal member 56 is provided with an outside seal (coolant seal) 58c which corresponds to outside seal 58a, and an inside seal 58d which corresponds to inside seal 58b (see FIG. 7). The outside and inside seals 58c and 58d have the same shape as outside and inside seals 58a and 58b. 
As shown in FIG. 6, the outside seal 58a is provided with an inlet manifold 60 which establishes fluid communication between oxidant gas inlet communication hole 30a and oxidant gas flow channels 42, and an outlet manifold 62 which establishes fluid communication between oxidant gas outlet communication hole 30b and oxidant gas flow channels 42.
The inlet manifold 60 is constructed by a plurality of supports 64 which are formed by cutting off outside seal 58a at positions spaced apart in the direction of arrow C and extend in the direction of arrow B. Communication paths for oxidant gas are defined between supports 64. The outlet manifold 62 is similarly constructed by a plurality of supports 66 which are formed by partially cutting off outside seal 58a and extend in the direction of arrow B. The supports 66 are in contact with first planar portion 52 to define communication paths for oxidant gas therebetween.
The supports 64 of inlet manifold 60 overlie seal laps 68 of outside seal 58c while being on the opposite surfaces 20a, 20b of second metal separator 20. Notably, the seal laps 68 are portions of outside seal 58c that overlie supports 64 of outside seal 58a, with second metal separator 20 interposed therebetween.
The outlet manifold 62 is constructed as is the inlet manifold 60. The supports 64 and seal laps 70 of outside seal 58c that overlie each other on the opposite surfaces 20a, 20b of second metal separator 20 are set such that the deformation of seals in the stacking direction under the load applied in the stacking direction is substantially equalized (see FIG. 6).
As shown in FIG. 7, the surface 20b of second metal separator 20 is provided with an inlet manifold 72 which establishes fluid communication between coolant inlet communication hole 32a and coolant flow channels 46, and an outlet manifold 74 which establishes fluid communication between coolant outlet communication hole 32b and coolant flow channels 46. The inlet manifold 72 is constructed by a plurality of supports 76 which are spaced apart in the direction of arrow C, extend in the direction of arrow B, and constitute outside and inside seals 58c and 58d. The outlet manifold 74 is similarly constructed by a plurality of supports 78 which are spaced apart in the direction of arrow C, extend in the direction of arrow B, and constitute outside and inside seals 58c and 58d. 
The inlet manifold 72 overlies seal laps 80a and 80b constituting outside and inside seals 58a and 58b on surface 20a, with second metal separator 20 interposed therebetween.
Similarly, supports 78 constituting outlet manifold 74 overlie seal laps 82a and 82b of outside and inside seals 58a and 58b, while being on opposite surfaces 20a and 20b of second metal separator 20, as shown in FIG. 7.
As shown in FIG. 7, on the surface 20b of second metal separator 20, an inlet manifold 84 and an outlet manifold 86 are provided in proximity to fuel gas inlet communication hole 34a and fuel gas outlet communication hole 34b, respectively. The inlet manifold 84 is provided with a plurality of supports 88 arranged in the direction of arrow C, and the outlet manifold 86 is similarly provided with a plurality of supports 90 arranged in the direction of arrow C.
The supports 88 of inlet manifold 84 overlie seal laps 92a and 92b of outside and inside seals 58a and 58b, with second metal separator 20 interposed therebetween. Similarly, the supports 90 of outlet manifold 86 overlie seal laps 94a and 94b of outside and inside seals 58a and 58b, with second metal separator 20 interposed therebetween.
The inlet manifold 84 and seal laps 92a, 92b, and the outlet manifold 86 and seal laps 94a, 94b are set such that the deformation of seals in the stacking direction under the load applied in the stacking direction is substantially equalized. Specifically, the inlet manifold 84 is constructed as is the inlet manifold 72. A plurality of feed holes 96 and discharge holes 98 are formed in proximity to inlet and outlet manifolds 84 and 86 and disposed outward of inside seal 58d. The feed holes 96 and discharge holes 98 are formed throughout the separator inward of inside seal 58b on the surface 20a side of second metal separator 20 and at the inlet and outlet sides of fuel gas flow channels 44 (see FIG. 6).
Although outside seal 58c is formed as a coolant seal on the surface 20b of second metal separator 20 in the illustrated embodiment, the invention is not limited thereto. Such a coolant seal may be formed on the surface 18b of first metal separator 18.
EXAMPLE
Examples and Comparative Examples are given below for further illustrating the invention, but the invention is not limited thereto. All parts are by weight. Mw is weight average molecular weight.
Example 1
70 parts of dimethylpolysiloxane #1 capped with dimethylvinylsiloxy at both ends and having a Mw of 26,000 and a vinyl content of 0.000088 mol/g and 10 parts of gum-like dimethylpolysiloxane #2 capped with dimethylvinylsiloxy at both ends, containing vinyl on side chains, and having a Mw of 370,000 and a vinyl content of 0.00052 mol/g were mixed with 30 parts of fumed silica having a BET specific surface area of 200 m2/g (Aerosil 300, Nippon Aerosil Co., Ltd.), 6 parts of hexamethyldisilazane, and 2.0 parts of water at room temperature for 30 minutes. The mixture was heated at 150° C., agitated at the temperature for 3 hours, and cooled. The mixture was further combined with 50 parts of dimethylpolysiloxane #1 and milled for 30 minutes, yielding a silicone rubber base. To 160 parts of the silicone rubber base were added 3.3 parts (giving [Si—H/vinyl]=1.5 in molar ratio) of methylhydrogenpolysiloxane #3 capped with dimethylsiloxy at both ends, containing Si—H groups at ends and on side chains, and having a degree of polymerization of 20 and a Si—H content of 0.0072 mol/g as a crosslinking agent and 0.05 part of ethynyl cyclohexanol as a reaction regulator. Continued milling for 15 minutes gave a mixture. On measurement at 25° C., the mixture had a viscosity of 46 Pa-s at 10 s−1 and 59 Pa-s at 0.9 s−1.
A silicone rubber composition was prepared by combining 100 parts of the mixture with 0.1 part of a platinum catalyst (Pt concentration 1 wt %), press cured at 120° C. for 10 minutes, and post cured in an oven at 200° C. for 4 hours. The cured sample was measured for hardness according to JIS K6249 and for compression set after heating at 150° C. for 70 hours, with the results shown in Table 1. A molding test was performed by insert molding the silicone rubber composition on a stainless steel plate to form a seal having the structure shown in FIG. 5.
Example 2
43 parts of dimethylpolysiloxane #4 capped with dimethylvinylsiloxy at both ends and having a Mw of 18,000 and a vinyl content of 0.00011 mol/g, 20 parts of dimethylpolysiloxane #5 capped with trimethylsiloxy at both ends, containing vinyl on side chains and having a Mw of 42,000 and a vinyl content of 0.00007 mol/g and 12 parts of gum-like dimethylpolysiloxane #6 capped with dimethylvinylsiloxy at both ends, containing vinyl on side chains and having a Mw of 450,000 and a vinyl content of 0.000022 mol/g were mixed with 35 parts of fumed silica surface treated to be hydrophobic and having a BET specific surface area of 260 m2/g (Rheorosil DM30S, Tokuyama Co., Ltd.), 5 parts of hexamethyldisilazane, and 1.0 part of water at room temperature for 30 minutes. The mixture was heated at 150° C., agitated at the temperature for 3 hours, and cooled. The mixture was further combined with 50 parts of dimethylvinylsiloxy-endcapped dimethylpolysiloxane #4 and milled for 30 minutes, yielding a silicone rubber base. To 160 parts of the silicone rubber base were added 3.3 parts (giving [Si—H/vinyl]=2.0 in molar ratio) of methylhydrogenpolysiloxane #3 (in Example 1, degree of polymerization 20 and Si—H content 0.0072 mol/g) as a crosslinking agent and 0.05 part of ethynyl cyclohexanol as a reaction regulator. Continued milling for 15 minutes gave a mixture. On measurement at 25° C., the mixture had a viscosity of 81 Pa-s at 10 s−1 and 109 Pa-s at 0.9 s−1.
A silicone rubber composition was prepared by combining 100 parts of the mixture with 0.1 part of a platinum catalyst (Pt concentration 1 wt %) before it was measured for hardness and compression as in Example 1, with the results shown in Table 1. A molding test was performed by insert molding the silicone rubber composition on a stainless steel plate to form a seal having the structure shown in FIG. 5.
Comparative Example 1
80 parts of dimethylpolysiloxane #7 capped with dimethylvinylsiloxy at both ends, containing vinyl on side chains, and having a Mw of 49,000 and a vinyl content of 0.000081 mol/g was mixed with 30 parts of fumed silica having a BET specific surface area of 200 m2/g (Aerosil 300, Nippon Aerosil Co., Ltd.), 6 parts of hexamethyldisilazane, and 2.0 parts of water at room temperature for 30 minutes. The mixture was heated at 150° C., agitated at the temperature for 3 hours, and cooled. The mixture was further combined with 50 parts of dimethylpolysiloxane #7 and milled for 30 minutes, yielding a silicone rubber base. To 160 parts of the silicone rubber base were added 2.2 parts (giving [Si—H/vinyl]=1.5 in molar ratio) of methylhydrogenpolysiloxane #3 (in Example 1, degree of polymerization 20 and Si—H content 0.0072 mol/g) as a crosslinking agent and 0.05 part of ethynyl cyclohexanol as a reaction regulator. Continued milling for 15 minutes gave a mixture. On measurement at 25° C., the mixture had a viscosity of 52 Pa-s at 10 s−1 and 65 Pa-s at 0.9 s−1.
A silicone rubber composition was prepared by combining 100 parts of the mixture with 0.1 part of a platinum catalyst (Pt concentration 1 wt %), press cured at 120° C. for 10 minutes, and post cured in an oven at 200° C. for 4 hours. The cured sample was measured for hardness according to JIS K6249 and for compression set after heating at 150° C. for 70 hours, with the results shown in Table 1. A molding test was performed by insert molding the silicone rubber composition on a stainless steel plate to form a seal having the structure shown in FIG. 5.
Comparative Example 2
60 parts of dimethylpolysiloxane #8 capped with dimethylvinylsiloxy at both ends, containing vinyl on side chains, and having a Mw of 58,000 and a vinyl content of 0.000092 mol/g and 20 parts of gum-like dimethylpolysiloxane #6 capped with dimethylvinylsiloxy at both ends, containing vinyl on side chains and having a Mw of 450,000 and a vinyl content of 0.000022 mol/g were mixed with 40 parts of fumed silica surface treated to be hydrophobic and having a BET specific surface area of 260 m2/g (Rheorosil DM30S, Tokuyama Co., Ltd.), 5 parts of hexamethyldisilazane, and 1.0 part of water at room temperature for 30 minutes. The mixture was heated at 150° C., agitated at the temperature for 3 hours, and cooled. The mixture was further combined with 50 parts of dimethylpolysiloxane #4 and milled for 30 minutes, yielding a silicone rubber base. To 160 parts of the silicone rubber base were added 3.3 parts (giving [Si—H/vinyl]=2.0 in molar ratio) of methylhydrogenpolysiloxane #3 (in Example 1, degree of polymerization 20 and Si—H content 0.0072 mol/g) as a crosslinking agent and 0.05 part of ethynyl cyclohexanol as a reaction regulator. Continued milling for 15 minutes gave a mixture. On measurement at 25° C., the mixture had a viscosity of 215 Pa-s at 10 s−1 and 335 Pa-s at 0.9 s−1.
A silicone rubber composition was prepared by combining 100 parts of the mixture with 0.1 part of a platinum catalyst (Pt concentration 1 wt %) before it was measured for hardness and compression as in Example 1, with the results shown in Table 1. A molding test was performed by insert molding the silicone rubber composition on a stainless steel plate to form a seal having the structure shown in FIG. 5.










TABLE 1









Example
Comparative Example












1
2
1
2















Hardness, Durometer A
42
48
43
49


Compression set in air, %
5.5
6.2
9.1
8.8


Molding test, voided samples
0%
0%
0%
80%









Below described is the molding test for inspecting whether an integral seal-separator having the structure shown in FIG. 5 is moldable from the silicone rubber composition. A stainless steel substrate having a primer coated thereon (Primer No. 101A/B, Shin-Etsu Chemical Co., Ltd., air drying+150° C.×30 min baking) was furnished as a separator. By insert molding, the silicone rubber composition was molded and cured around the substrate in a mold at a temperature of 150° C. for 5 minutes. After removal from the mold, the sample was post cured at 200° C. for 4 hours. The molded seal was inspected for the presence of voids and other unfilled defects. Provided that samples without such voids pass the test, a percentage of voided samples is shown in Table 1.
Japanese Patent Application No. 2007-317475 is incorporated herein by reference.
Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

## Claims

1. A separator seal for use in polymer electrolyte fuel cells which is formed of a sealing composition in the cured state, said sealing composition comprising
(A) 100 parts by weight of a liquid organopolysiloxane containing at least two alkenyl groups each attached to a silicon atom in a molecule and having a weight average molecular weight of less than 100,000,
(B) 3 to 30 parts by weight of a gum-like organopolysiloxane having a weight average molecular weight of at least 150,000,
(C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxane containing at least three hydrogen atoms each attached to a silicon atom in a molecule,
(D) 5 to 30 parts by weight of fumed silica having a BET specific surface area of 50 to 400 m2/g, and
(E) a catalytic amount of an addition reaction catalyst, said composition having a viscosity of 20 to 200 Pa-s at 25° C. and a shear rate of 10 s−1.

2. The separator seal of claim 1 wherein the organopolysiloxane (A) has a weight average molecular weight of 10,000 to 80,000, at least 90 mol % of the entire organic groups attached to silicon atoms being methyl.

3. The separator seal of claim 1 wherein a molar ratio of Si—H groups in component (C) to total alkenyl groups in components (A) and (B) is between 0.8:1 and 5.0:1.

4. The separator seal of claim 1 wherein said composition at 25° C. has a viscosity V(10) at a shear rate of 10 s−1 and a viscosity V(0.9) at a shear rate of 0.9 s−1, which fall in the range: 1.0<V(0.9)/V(10)<2.5.

5. The separator seal of claim 1 wherein the gum-like organopolysiloxane (B) contains 1.0×10−6 to 1.0×10−3 mol/g of alkenyl groups.

6. A separator for use in polymer electrolyte fuel cells comprising a substrate comprising a metal thin plate or a conductive powder and a binder and a seal formed at a periphery on at least one surface of the substrate, said seal comprising the separator seal of any one of claims 1 to 5.

7. The separator seal of claim 1, wherein the organohydrogenpolysiloxane (C) is represented by the following average compositional formula (III):


R3cHdSiO(4-c-d)/2  (III)

wherein R3 is each independently a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, “c” is a positive number of 0.7 to 2.1, “d” is a positive number of 0.001 to 1.0, and c+d is from 0.8 to 3.0.

8. The separator seal of claim 1, wherein the fumed silica (D) is surface-treated with a member selected from the group consisting of alkylalkoxysilanes, alkylchlorosilanes, alkylsilazanes, silane coupling agents, titanate treating agents, fatty acid esters, and mixtures thereof.

9. The separator seal of claim 1, wherein the addition reaction catalyst (E) is a member selected from the group consisting of platinum catalysts, palladium catalysts, and rhodium catalysts.

10. The polymer electrolyte fuel cell separator of claim 6, formed by holding said separator substrate in place in a mold and injection molding said sealing composition into a mold cavity on one or both sides of the substrate.