Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1998-09-30
2001-06-05
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S047000, C429S047000
Reexamination Certificate
active
06242123
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid polyelectrolyte membrane for fuel cells, and to a method for producing it.
2. Discussion of the Background
A polyelectrolyte-type fuel cell comprises a gas-diffusing cathode electrode, a gas-diffusing anode electrode and a solid polyelectrolyte membrane. Oxygen gas and hydrogen gas are supplied to the cathode and the anode, respectively, to generate electricity between the electrodes. The function of the polyelectrolyte membrane is to transport hydrogen ions, formed around the gas-diffusing anode electrode, to the gas-diffusing cathode electrode. Around the gas-diffusing cathode electrode, the thus-transported hydrogen ions react with oxygen gas and electrons to produce water.
The performance of the cell depends on the catalytic activity of the catalyst used in each gas-diffusing electrode, the gas-diffusing ability of each electrode, the hydrogen ion conductivity of the solid polyelectrolyte membrane, etc. Therefore, the internal resistance of the cell could be smaller when the solid polyelectrolyte membrane in the cell has a higher hydrogen ion conductivity, resulting in superior performance of the cell.
The solid polyelectrolyte membrane is produced by forming a resin having an ion-exchanging function into films; a membrane having a higher degree of ion-exchanging capacity having a higher degree of ion conductivity. Compared with other cells, the solid polyelectrolyte-type fuel cell is more compact and produces more power. Therefore, in the future, this type of fuel cell will be widely used as the power source for electric cars.
In solid polyelectrolyte-type fuel cells produced in the early years, an ion-exchange membrane used was prepared by infiltrating monomers of styrene and divinylbenzene into a reinforcing cloth followed by copolymeriztion, as the electrolyte membrane. However, since its durability is extremely poor, this ion-exchange membrane is impractical. Since then, perfluorosulfonic acid membranes (trade name, NAFION) developed by DUPONT have been generally used.
The perfluorosulfonic acid membranes have good ion conductivity and durability. However, since they are formed from a fluorine resin, they are extremely expensive, which is a serious bar to the commercialization of solid polyelectrolyte-type fuel cells for electric cars. For these reasons, various studies have heretofore been made for developing inexpensive electrolyte membranes capable of being substituted for NAFION. However, it has been presumed that electrolyte membranes essentially comprising a hydrocarbon resin will decompose at the hydrocarbon polymer chains by the action of peroxides or active radicals formed as intermediates in the electrode reaction, and it has been reported that, when such electrolyte membranes are used in cells, the cell output is lowered immediately after the cells have started.
SUMMARY OF THE INVENTION
The present inventors have developed ion-exchange polymer membranes. The membranes are produced by introducing styrene, which is a hydrocarbon monomer, into an ethylene-tetrafluoroethylene copolymer resin film, which is a popular inexpensive film, through radiation grafting polymerization, followed by sulfonating the resulting film. The membranes function well as solid electrolyte membranes for polyelectrolyte-type fuel cells. In fact, it has been verified that the cells comprising the ion-exchange polymer membrane of the present invention have higher power-generating capabilities than those comprising NAFION, and in durability tests for continuous operation, the cells lasted for about 600 hours. Thus, the ion-exchange polymer membranes are inexpensive and have both high ion conductivity and high durability.
However, the polyelectrolyte membranes as produced by grafting such an ethylene-tetrafluoroethylene copolymer resin film with styrene through radiation grafting polymerization followed by introducing sulfonic acid groups thereinto have an extremely high water content. It has been found that, when the polyelectrolyte of that type is used in the polyelectrolyte membrane in a fuel cell and when the catalyst layers of the gas-diffusing electrodes in the cell do not have satisfactory water repellency, the electrodes in the cell, especially the cathode at which water is formed through fuel cell reaction, are much wetted, causing the problem of output depression. In order to avoid this problem, it will be effective to add a water-repellent resin, such as TEFLON or the like, to the electrode catalyst layers; however, this is unfavorable, because it sacrifices the cell performance. This is because the resin added interferes with the diffusibility of the gas supplied to the electrodes and therefore increases the resistance of the electrodes.
The present invention has been made in consideration of the matters noted above, and its object is to provide an inexpensive, solid polyelectrolyte membrane for fuel cells, of which the water content is controlled so as to fall within a range which does not cause too much wetting of electrode catalysts.
The solid polyelectrolyte membrane for fuel cells of the invention is filmy and is made of a synthetic resin which comprises main chains having a copolymer structure of a first olefin hydrocarbon and an olefin perfluorocarbon, and side chains of a sulfonic acid group-having crosslinked polymer of a second olefin hydrocarbon and a diolefin hydrocarbon.
The first olefin hydrocarbon and olefin perfluorocarbon may contain 2 to 10 carbon atoms. The second olefin hydrocarbon may contain 8 to 20 carbon atoms, and the diolefin may contain 10 to 22 carbon atoms.
Preferably, the main chains are of an ethylene-tetrafluoroethylene copolymer, and the side chains are of a sulfonic acid group containing copolymer of styrene and divinylbenzene or a sulfonic acid group containing copolymer of styrene, &agr;-methylstyrene and divinylbenzene.
The solid polyelectrolyte membrane of the invention may also be described as a crosslinked polymer, comprising main chains and side chains extending from the main chains, where the main chains comprise alkylene and perfluoroalkylene units, and the side chains comprise hydrocarbon sulfonic acid units. The alkylene units may contain 2 to 10 carbon atoms, preferably 2 or 3 carbon atoms. The perfluoroalkylene units may contain 2 to 10 carbon atoms, preferably
2
carbon atoms. The hydrocarbon sulfonic acid units preferably are arylalkylene sulfonic acid units, with the sulfonic acid moiety attached to the aryl group. The hydrocarbon sulfonic acid units may contain 8 to 20 carbon atoms, preferably 8 or 9 carbon atoms.
Also preferably, in the synthetic resin, the moiety of the side chains is from 10 to 150 parts by weight relative to 100 parts by weight of the moiety of the main chains.
Still preferably, the solid polyelectrolyte membrane has an ion exchange capacity of from 1.0 to 3.5 milliequivalents/gram, and has a water content of from 30 to 200%.
The invention also provides a method for producing the solid polyelectrolyte membrane for fuel cells, which comprises a grafting step of exposing a filmy copolymer of an olefin hydrocarbon and an olefin perfluorocarbon to radiation, followed by contacting and reacting the thus-irradiated copolymer film with a polymerizing olefin hydrocarbon and a polymerizing diolefin hydrocarbon, thereby forming side chains of crosslinked grafted polymer chains of said olefin hydrocarbon and said diolefin hydrocarbon in the copolymer, and a sulfonic acid group-introducing step of introducing sulfonic acid groups into said side chains of the resulting crosslinked grafted polymer.
Preferably, in the method, the main chains of the copolymer are of an ethylene-tetrafluoroethylene copolymer or a propylene-tetrafluoroethylene copolymer, and the side olefins are of a copolymer of styrene and divinylbenzene or a copolymer of styrene, &agr;-methylstyrene and divinylbenzene.
Also preferably, the dose of radiations is from 1 to 100 KGy.
Still preferably, the grafting step is effected at a temperature not higher than
Asukabe Michio
Ito Naoki
Kato Michiaki
Nezu Shinji
Yamada Chiaki
Aisin Seiki Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Weiner Laura
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