Sealing material for solid polymer fuel cell separator

Details Sealing material for solid polymer fuel cell separator Sealing material for solid polymer fuel cell separator

2002-04-16

2004-03-30

Dawson, Robert (Department: 1712)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C528S031000, C528S032000, C528S015000, C524S492000, C524S588000, C428S405000

Reexamination Certificate

active

06713205

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a separator of a solid polymer type fuel cell (or a solid polymer electrolyte fuel cell) used as a small scale fuel cell, and relates particularly to a sealing material for such a separator which can be used for extended periods and also offers superior moldability.
2. Description of the Prior Art
Fuel cells display many excellent properties, including being almost entirely free of natural resource depleting fossil fuels, generating very little noise during power generation, and displaying superior energy recovery rates when compared with other power generation sources. Consequently, the development of such fuel cells for comparatively small power generation plants for buildings or factories is being actively pursued, and in some cases this technology is already in use. Amongst fuel cells, solid polymer fuel cells operate at a lower temperature than other fuel cells and are consequently less likely to suffer corrosion on the surface of the component materials which make up the cell, and moreover are also capable of comparatively large current discharge considering their low operating temperature, and as such are attracting considerable attention, not only for household cogeneration, but also as potential energy sources to replace internal combustion engines in vehicles.
Amongst the components required to construct such a solid polymer fuel cell, a separator typically comprises a flat plate with a plurality of parallel channels formed in either both sides or one side of the plate, and this separator performs the function of transmitting the electricity generated at a gas diffusion electrode inside the fuel cell to an external point, as well as the function of draining water generated within the channels during the power generation process. Furthermore, the channels formed in the separator also act as fluid passages for the reactant gas flowing into the fuel cell. With the ongoing reductions in the size of fuel cells, a plurality of these type of fuel cell separators may be layered together, and in such cases, in order to seal the edges of the plurality of separators, a separator sealing material which displays excellent durability and can withstand prolonged use is required.
Packing materials formed from various types of resin are being investigated as potential separator sealing materials, although silicone rubber sealing materials, which offer superior properties of moldability, heat resistance and elasticity, are the most widely used. Amongst such silicone rubbers, addition curing type silicone resins are typically used due to their superior moldability, although they are somewhat unsatisfactory in terms of maintaining elasticity over prolonged periods. Particularly in the case of fuel cell separator packing materials, attempts have been made to ensure the maintenance of a good seal within an acidic aqueous solution by suppressing the compression set to a low value, but suppressing the compression set has proved difficult.
SUMMARY OF THE INVENTION
The present invention takes the above factors in consideration, with an object of providing a sealing material for a solid polymer fuel cell separator which displays excellent moldability, heat resistance and elasticity, has a compression set within air and within an acidic aqueous solution which can be suppressed to a low value, and produces an excellent seal, particularly in an acidic aqueous solution.
As a result of intensive research aimed at resolving the issues described above, the inventors of the present invention discovered that the above problems could be resolved by using a type of material described below, and were hence able to complete the present invention.
In other words, the present invention is a material for sealing the edges on at least one side of a solid polymer fuel cell separator, comprising
(A) 100 parts by weight of an organopolysiloxane with at least two alkenyl groups bonded to silicon atoms within each molecule,
(B) 0.1 to 30 parts by weight of an organohydrogenpolysiloxane with at least two hydrogen atoms bonded to silicon atoms within each molecule,
(C) 10 to 50 parts by weight of a fumed silica with a specific surface area of 50 to 250 m
2
/g which has undergone surface treatment with at least two different surface treatment agents, and
(D) an effective quantity of an addition reaction catalyst.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As follows is a more detailed description of the present invention.
A sealing material for a solid polymer fuel cell separator of the present invention incorporates the constituents (A) through (D), and each of these constituents is described in detail below.
[Constituent (A)]
In the present invention, the constituent (A) is an organopolysiloxane with at least two alkenyl groups bonded to silicon atoms within each molecule, and should preferably have a structure represented by an average composition formula (1) shown below.
R
1
a
SiO
(4−a)/2
  (1)
wherein, R
1
represents identical or different unsubstituted or substituted monovalent hydrocarbon groups of 1 to 10, and preferably 1 to 8 carbon atoms, and a represents a positive number of 1.5 to 2.8, and preferably 1.8 to 2.5, and even more preferably 1.95 to 2.05.
Suitable examples of the monovalent hydrocarbon groups represented by the aforementioned R
1
group include alkyl groups such as methyl groups, ethyl groups, propyl groups, isopropyl groups, butyl groups, isobutyl groups, tert-butyl groups, pentyl groups, neopentyl groups, hexyl groups, cyclohexyl groups, octyl groups, nonyl groups and decyl groups; aryl groups such as phenyl groups, tolyl groups, xylyl groups and naphthyl groups; aralkyl groups such as benzyl groups, phenylethyl groups and phenylpropyl groups; alkenyl groups such as vinyl groups, allyl groups, propenyl groups, isopropenyl groups, butenyl groups, hexenyl groups, cyclohexenyl groups, and octenyl groups; as well as groups in which either a portion of, or all of the hydrogen atoms in the above groups have been substituted with either a halogen atom such as fluorine, bromine or chlorine, or a cyano group, such as chloromethyl groups, chloropropyl groups, bromoethyl groups, trifluoropropyl groups and cyanoethyl groups. However, at least two of the R
1
groups must be alkenyl groups (preferably of 2 to 8 carbon atoms, and even more preferably of 2 to 6 carbon atoms), and examples of such alkenyl groups include those described above.
Within the R
1
groups of the constituent (A), the molar ratio of alkenyl groups bonded to silicon atoms relative to other non-alkenyl unsubstituted or substituted monovalent hydrocarbon groups bonded to silicon atoms, in other words, alkenyl groups
on-alkenyl unsubstituted or substituted monovalent hydrocarbon groups
should preferably be a ratio from 0.0001 to 0.02. If the molar ratio of alkenyl groups relative to other hydrocarbon groups is less than 0.0001, then the rubber hardness is insufficient and a satisfactory seal cannot be achieved, whereas at molar ratios exceeding 0.02, the cross linking density becomes too high, and the rubber becomes brittle. These alkenyl groups may be bonded to the silicon atoms at the terminals of the molecular chain, or bonded as pendent groups to non-terminal silicon atoms within the molecular chain, or alternatively bonded to both types of silicon atoms. The structure of the organopolysiloxane should be essentially a straight chain structure, although structures with partial branching or cyclic structures may also be used. There are no particular restrictions on the molecular weight, and materials from low viscosity liquid form materials through to high viscosity gums may be used, although in order to ensure a rubber like elastic body on curing, a viscosity at 25° C. of at least 100 mPa.s is required, with typical values being from 100 to 1,000,000 mPa.s, and values of 500 to 100,000 mPa.s being particularly desirable.
[Constituent (B)]
In the present invention, an organohydrogenpolysiloxane of the co

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