Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-03-30
2004-02-17
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S309000, C029S623300
Reexamination Certificate
active
06692858
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electrolyte membrane for a polymer electrolyte fuel cell.
BACKGROUND OF THE INVENTION
As a power generation system, attention has been drawn to a hydrogen/oxygen fuel cell which gives substantially no adverse effect to the global environment since its reaction product is only water in principle. Polymer electrolyte fuel cells were once mounted on space ships in the Gemini Project and the Bio-satellite Project, but the cell power densities at that time were low. Thereafter, alkaline fuel cells having higher performance have been developed, and such alkaline fuel cells have been employed for space crafts including current space shuttles.
Whereas, along with the progress of technology in recent years, attention has been drawn again to polymer electrolyte fuel cells, for the following reasons. (1) As a polymer electrolyte, a highly conductive membrane has been developed. (2) Obtaining an extremely large activity has been made possible, since a catalyst to be used for a gas diffusion electrode layer is supported on carbon and coated with ion exchange resins.
For further improvement in the performance, it may be conceived to increase the concentration in sulfonic acid group density and to decrease the thickness of a solid polymer electrolyte membrane. However, a remarkable increase in the concentration in sulfonic acid group density will cause the following problems; deterioration in mechanical strength and tear strength of the electrolyte membrane and changes in the size during handling of the electrolyte membrane, and a decrease in durability due to creep phenomenon of the electrolyte membrane, etc. On the other hand, a decrease in its thickness will cause the following problems; deterioration in mechanical strength and tear strength of the electrolyte membrane, and lowering in e.g. workability and handling in such a case wherein a membrane is bonded with a gas-diffusion electrode.
As a method for resolving the above-mentioned problems, it has been proposed that a porous membrane made of polytetrafluoroethylene (hereinafter referred to as PTFE) is impregnated with a fluorine type ion exchange polymer having sulfonic acid groups (JP-B-5-75835). With the membrane reinforced with porous-like PTFE obtained by such a method, the electric resistance will not be sufficiently lowered, while the thickness of the membrane can be thinned. Further, with such a method, the interface of the PTFE porous membrane and the ion exchange polymer is not bonded completely. Accordingly, when the membrane is used as an electrolyte membrane in a polymer electrolyte fuel cell for a long period of time, a leak of hydrogen gas will increase to result in the deterioration of the cell performance due to its insufficient bonding.
As another method for resolving the problems, a cation exchange membrane has been proposed which is reinforced with a perfluorocarbon polymer of fibrilliform, fabric-form or unwoven fabric-form. (JP-A6-231779) The performance in the power generation of the fuel cell using such a membrane is considerably good due to its low electric resistance, but the thickness is from 100 to 200 &mgr;m at smallest, which is not thin enough and has lack of uniformity in thickness, and thus the power generation characteristics and the mass productivity is still insufficient.
The object of the present invention is to provide an electrolyte membrane for a polymer electrolyte fuel cell, which is a reinforced membrane with a thin and uniform thickness, excellent in the power generation performance, capable of mass production and also excellent in a hydrogen gas barrier property.
SUMMARY OF THE INVENTION
The present invention provides a method for producing an electrolyte membrane for a polymer electrolyte fuel cell, which comprises forming into a membrane a mixture of a perfluorocarbon polymer having sulfonic acid groups or their precursors and a fluorocarbon polymer which can be fibrillated, laminating a support film for stretching to at least one side of the resulting membrane, and stretching the laminate of the membrane and the support film under heating.
In accordance with such a method, an electrolyte membrane reinforced with fibril reinforcement can be produced which is excellent in the strength even with a thin thickness. Further, by adding to the above-mentioned method a step of laminating a non-reinforced membrane to the reinforced-film with fibril reinforcement, it is possible to produce an electrolyte membrane composed of a laminate comprising a cation exchange layer reinforced with a fibril reinforcement and a non-reinforced cation exchange layer.
Further, the present invention provides an electrolyte membrane for a polymer electrolyte fuel cell, and a polymer electrolyte fuel cell equipped with such an electrolyte membrane, wherein the membrane comprising a cation exchange membrane made of perfluorocarbon polymer having sulfonic acid groups, reinforced with reinforcement made of fibrilliform fluorocarbon polymer, the membrane having a thickness of from 3 to 70 &mgr;m and number of fibrils of the reinforcement having a fibril-fiber diameter of at most 1 &mgr;m accounting for at least 70% based on the total number of fibrils.
Further, the present invention provides a method for producing an electrolyte membrane for a polymer electrolyte fuel cell, wherein the membrane is a laminate comprising at least two cation exchange layers made of a perfluorocarbon polymer having sulfonic acid groups, of which at least one layer is reinforced with reinforcement made of fibrilliform fluorocarbon polymer, and at least one layer is not substantially reinforced with reinforcement, which comprises the following steps:
a step of preparing a precursor membrane of a fibril-reinforced membrane, comprising forming into a membrane a mixture of a perfluorocarbon polymer having precursors of sulfonic acid groups and a fluorocarbon polymer which can be fibrillated to obtain a precursor membrane of a fibril-reinforced membrane;
a step of preparing a non-reinforced membrane, comprising forming into a membrane a perfluorocarbon polymer having precursors of sulfonic acid groups to obtain a non-reinforced membrane;
a step of stretching the precursor membrane to obtain a fibril-reinforced membrane; and
a step of laminating the fibril-reinforced membrane or the precursor membrane and the non-reinforced membrane.
Furthermore, the present invention provides an electrolyte membrane for a polymer electrolyte fuel cell and a polymer electrolyte fuel cell equipped with such an electrolyte membrane, wherein the membrane composed of a laminate comprising at least two cation exchange layers made of a perfluorocarbon polymer having sulfonic acid groups, of which at least one layer is reinforced with reinforcement made of a fibrilliform fluorocarbon polymer, and at least one layer is not substantially reinforced with any reinforcement.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, a precursor of a sulfonic acid group means a group which can be converted to sulfonic acid group (—SO
3
H group) by e.g. hydrolysis, a treatment for converting to acid forms. As such an example, a —SO
2
F group, a —SO
2
Cl group, etc. may be mentioned.
A reinforcing material consisting of fibrilliform of a fluorocarbon polymer (hereinafter referred to as fibril reinforcement) in the present invention can attain reinforcing effects such as an increase in tear strength and improvement in dimensional stability even with its small contents in the cation exchange membrane. Further, such material is an effective reinforcing material since an increase in the electric resistance of the membrane can be suppressed when the fibril reinforcement is incorporated.
A perfluorocarbon polymer having sulfonic acid groups (hereinafter referred to as sulfonic acid type perfluorocarbon polymer) reinforced with fibril reinforcement is a useful membrane in view of its low electric resistance and high mechanical strength. However, such a membrane tends to have a high hydrogen gas permeability, comparing with a
Higuchi Yoshiaki
Hommura Satoru
Shimoda Hiroshi
Terada Ichiro
Asahi Glass Company Limited
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Parsons Thomas H.
Ryan Patrick
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