This invention relates to a sealing rubber composition for use with separators in polymer electrolyte fuel cells which are typical small size fuel cells, a seal member formed from the rubber composition, and a polymer electrolyte fuel cell separator.
Fuel cells have the great advantages that there is in a substantial sense no need for the fossil fuel which requires to pay attention to the depletion of resource, no or little noise is produced upon power generation, and the recovery of energy is high as compared with other energy power generation devices. The construction of fuel cells as relatively small size power generation plants installed in buildings and factories has been under development and some are used in practice. In particular, polymer electrolyte fuel cells operate at low temperature as compared with other types of fuel cell. They eliminate in a material aspect a concern about the corrosion of cell constituent parts and can discharge a relatively high current flow for low-temperature operation. They are thus of great interest not only as household cogeneration devices, but also as substitute power supplies for internal combustion engines on vehicles.
The polymer electrolyte fuel cells are constructed of several components including a separator which is generally in the form of a plate having a plurality of parallel channels in one or both surfaces thereof. The separator plays the roles of conducting to the exterior the electricity generated on the gas diffusing electrode within the fuel cell, discharging the water formed in the channels during the electricity generation process and maintaining the channels as a flowpath for reactant gas to flow into the fuel cell. The fuel-cell separators are required to be reduced in size. Additionally, since a plurality of separators are used in stack, there is a need for a separator sealing material which remains fully durable in long-term service.
As the separator seal, sealing materials based on various resins have been under study in the art. Sealing materials based on silicone rubber are often employed for their moldability, heat resistance and elasticity. Typical silicone rubbers are cured products of silicone rubber compositions of the addition curing type which are more effectively moldable. These silicone rubbers generally contain non-functional oligomers, also known as low molecular weight siloxanes, which can leach out to raise a problem. When the separator is cooled with a coolant such as water or a mixture of water and ethylene glycol, low molecular weight siloxanes are leached out in the coolant. The negative influence of this leaching becomes a concern, especially for automotive and home use.
Another drawback is that silicone rubber may have insufficient chemical resistance. In the application of sealing polymer electrolyte fuel-cell separators where the seals are always in contact with acidic gases, the silicone rubber can be degraded and increased in compression set.
An object of the invention is to provide a sealing rubber composition for use with polymer electrolyte fuel-cell separators, which forms a seal member featuring little or no leaching of low molecular weight organopolysiloxanes having a degree of polymerization of up to 10, especially non-functional organopolysiloxanes having a degree of polymerization of up to 10 which can be readily leached in water or a mixture of water and ethylene glycol; a seal member formed from the rubber composition; and a polymer electrolyte fuel cell separator.
Another object of the invention is to provide a rubber composition for use in sealing of polymer electrolyte fuel-cell separators, which composition comprises siloxane bonds, fumed silica for reinforcement and an acid resistant filler and has improved acid resistance and minimized compression set; a seal member formed from the rubber composition; and a polymer electrolyte fuel cell separator.
It has been found that a rubber composition comprising (A) an organopolysiloxane having at least two alkenyl radicals per molecule, (B) an organohydrogenpolysiloxane having at least three Sixe2x80x94H radicals per molecule, and (E) an addition reaction catalyst, wherein the content of low molecular weight organopolysiloxanes is minimized is useful to form a seal member for a polymer electrolyte fuel-cell separator because the seal member allows for little or no leaching of low molecular weight organopolysiloxanes.
It has also been found that a rubber composition comprising components (A), (B) and (E) as described above, (C) fumed silica and (D) an acid resistant inorganic filler is useful to form a seal member for a polymer electrolyte fuel-cell separator because the seal member has improved acid resistance and minimized compression set.
Specifically, a first embodiment of the invention provides a rubber composition for sealing a periphery of at least one side of a polymer electrolyte fuel-cell separator, comprising
(A) 100 parts by weight of an organopolysiloxane having at least two alkenyl radicals each bonded to a silicon atom per molecule,
(B) 0.5 to 20 parts by weight of an organohydrogenpolysiloxane having at least three hydrogen atoms each bonded to a silicon atom per molecule, and
(E) a catalytic amount of an addition reaction catalyst,
wherein the content of low molecular weight organopolysiloxanes having a degree of polymerization of up to 10 is up to 0.2% by weight.
A second embodiment of the invention provides a rubber composition for sealing a periphery of at least one side of a polymer electrolyte fuel-cell separator, comprising
(A) 100 parts by weight of an organopolysiloxane having at least two alkenyl radicals each bonded to a silicon atom per molecule,
(B) 0.5 to 20 parts by weight of an organohydrogenpolysiloxane having at least three hydrogen atoms each bonded to a silicon atom per molecule,
(C) 5 to 40 parts by weight of fumed silica having a specific surface area of 50 to 400 m2/g,
(D) 10 to 200 parts by weight of an acid resistant inorganic filler other than fumed silica having an average particle size of up to 50 xcexcm, and
(E) a catalytic amount of an addition reaction catalyst.
In a preferred embodiment of the polymer electrolyte fuel-cell separator sealing rubber compositions, the organopolysiloxane (A) has an average degree of polymerization of 100 to 2,000, and at least 90 mol % of the entire organic radicals bonded to silicon atoms are methyl. Also preferably, the total amount of alkenyl radicals in component (A) and the total amount of silicon-bonded hydrogen atoms (i.e., Sixe2x80x94H radicals) in component (B) are such that the molar ratio of silicon-bonded hydrogen atoms to alkenyl radicals is in a range from 0.8/1 to 5.0/1.
The acid resistant inorganic filler (D) is typically selected from among quartz flour, diatomaceous earth, mica, talc, clay, alumina, aluminum hydroxide, barium sulfate, titanium oxide, and iron oxide and mixtures thereof.
In a further aspect, the invention provides a seal member for a polymer electrolyte fuel-cell separator, comprising the rubber composition in the cured state.
In a still further aspect, the invention provides a polymer electrolyte fuel-cell separator comprising a metal plate or a substrate including an electrically conductive powder and a binder, and a seal member formed along a periphery on at least one side of the metal plate or substrate by compression molding, casting, injection molding, transfer molding, dipping, coating or screen printing the rubber composition, and curing thereto.