Polymer electrolyte fuel cell

Details Polymer electrolyte fuel cell Polymer electrolyte fuel cell

2002-10-15

2004-09-14

Maples, John S. (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000

Reexamination Certificate

active

06790552

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a solid polymer electrolyte fuel cell, and more particularly to improvements in sealing structure between a gasket disposed on the periphery of a membrane electrode assembly and conductive separator plates.
The most typical example of solid polymer electrolyte fuel cells comprises: an electrolyte membrane-electrode assembly (MEA) composed of a polymer electrolyte membrane, a gasket which is formed of a sealing material and supports the periphery of the electrolyte membrane, an anode attached to one face of the electrolyte membrane, and a cathode attached to the other face of the electrolyte membrane; an anode-side conductive separator plate and a cathode-side conductive separator plate sandwiching the MEA; and gas supply means for supplying a fuel gas and an oxidant gas to the anode and the cathode, respectively. The important problem with this kind of fuel cells is cross leakage of the gasses which takes place in the vicinity of gas manifold apertures. In the vicinity of oxidant gas manifold apertures, cross leakage of the gasses occurs because the gasket sags into a fuel gas flow channel of the conductive separator plate. The sagging consequently creates two leak paths leading to the oxidant gas manifold aperture from the anode. One of the leak paths is created by separation of the gasket from the anode side of the separator plate, and the other is created by separation of the gasket from the electrolyte membrane as the result of the sagging of the gasket. Likewise, in the vicinity of fuel gas manifold apertures, gas leakage occurs because the gasket sags into an oxidant gas flow channel of the conductive separator plate.
In order to solve this problem, the inventors of the present invention made the following proposal in WO 02/061869. The entire disclosure thereof including specification, claims, drawings and summary is incorporated herein by reference in its entirety. That is, the disclosure is a method in which a plurality of through holes
2
are arranged in the periphery of an electrolyte membrane
1
as illustrated in
FIG. 1
, and a gasket is integrally joined to the periphery of the electrolyte membrane by injection molding so as to include the through holes. In this method, the portion of the gasket covering one face of the electrolyte membrane is connected to the portion covering the other face thereof at a portion covering the edge of the electrolyte membrane and at the through holes, so that it is possible to eliminate the cross leakage of the gasses caused by separation of the gasket from the electrolyte membrane. Also in this method, the gasket is provided with ribs formed between the anode and the oxidant gas manifold apertures, and the ribs are fitted into grooves formed in the corresponding positions of a separator plate to prevent the separation of the gasket from the anode side of the separator plate. Likewise, ribs formed on the gasket are fitted into grooves of a separator plate to prevent the separation of the gasket from the cathode side of the separator plate. These ribs of the gasket not only mate the gasket to the separator plates but also function as flowing paths of molten resin in molding. Thus, the ribs are indispensable for gaskets that are thin and injection molded.
However, the prevention of the gas leakage by fitting the ribs of the gasket into the grooves of the separator plates has been found insufficient.
In injection molding, molded articles are inevitably subject to mold shrinkage. Since the degree of mold shrinkage varies depending on the molding materials and the shapes of the molded articles, it is normally difficult to predict beforehand. Thus, in case the degree of mold shrinkage has been beyond the prediction, there arises a problem of the ribs of the gasket not fitting into the grooves of the separator plates properly. Therefore, in the above-described structure of fitting the ribs of the gasket into the grooves of the separator plates, the gasket needs to be molded beforehand, and then the separator plates need to be designed based on the actual measurement of the mold shrinkage of the molded gasket. Since the separator plates are composed mainly of metal or carbon, even the molded separator plates are hardly subject to mold shrinkage. Thus, designing the separator plates so as to mate with the molded gasket is a rational process. This process, however, has a disadvantage that the design of the separator plates must be done each time the rate of mold shrinkage is changed, for example, by the change of the gasket material.
In the above-described structure, the sealing between the gasket and the separator plates is basically surface to surface sealing except for the mating portions, and both the gasket and the separator plates therefore need to have sufficient surface accuracy. However, on the surface of an injection molded article, gate marks and ejector pin marks are left inevitably. The heights of the marks are usually approximately a few tens of microns depending on the mold structures and materials. In the above-described structure of the fuel cell, when the gate marks and ejector pin marks are left on the rib portions or the standard thickness portion (the portion without the ribs) of the gasket, except for the case where the gasket is extremely elastic, clearances are produced between the separator plates and the gasket to cause cross leakage or outward leakage of the gases. This problem is common particularly in the case of using molded separator plates. Since the separator plates have almost no elasticity, such surface irregularities need to be compensated solely by the gasket. That is, it is necessary to use a highly elastic material for the gasket. However, such a highly elastic material has a problem in that it usually has poor mechanical strength and therefore tends to creep.
Further, in the above-described surface to surface sealing, a sufficient surface load needs to be applied onto both the gasket and the separator plates. Hence, another problem arises in that the clamping pressure of the cell stack must be heightened unnecessarily. This also involves a problem of requiring unnecessarily large-scale clamping members such as end plates, bolts, springs, etc., the large-scale clamping members giving a negative effect in terms of the volume.
BRIEF SUMMARY OF THE INVENTION
In one aspect of the present invention, small ribs for sealing are formed on the gasket in order to ensure the sealing between the gasket and the separator plates instead of sealing the gasket and the separator plates in a surface to surface manner.
In another aspect of the present invention, while ribs and other moldings of the conventional gasket are retained for ensuring moldability and mechanical strength, small ribs for sealing are formed on portions of the gasket which would conventionally come in contact with the separator plates in a surface to surface manner. The mechanical strength required for the gasket is bending strength, tortional strength or the like, and particularly a strength which allows the gasket not to sag into the gas flow channels of the separator plates. The former ribs are hereinafter referred to as dummy ribs since they make no direct contribution to the sealing, and the latter ribs are hereinafter referred to as seal ribs.
The present invention is directed to a polymer electrolyte fuel cell comprising a unit cell, the unit cell comprising: an electrolyte membrane-electrode assembly (hereinafter referred to as MEA) comprising a polymer electrolyte membrane, a gasket covering the periphery of the electrolyte membrane, an anode attached to one face of the electrolyte membrane, and a cathode attached to the other face of the electrolyte membrane; and an anode-side conductive separator plate and a cathode-side conductive separator plate sandwiching the MEA therebetween.
The gasket and the anode-side and cathode-side conductive separator plates have a pair of fuel gas manifold apertures, a pair of oxidant gas manifold apertures and a pair of cooling water manifold ape

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