Fuel cell stack

A fuel cell stack is provided in which a plurality of single cells each including a membrane electrode assembly are stacked in a stacking direction. The fuel cell stack includes a plurality of electrical insulation members each connected to an outer peripheral portion of a corresponding one of the membrane electrode assemblies. The fuel cell stack further includes a first displacement absorbing member disposed between each insulation member and an adjacent insulation member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2008-295450 filed Nov. 19, 2008, which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the structure of a fuel cell stack.

2. Description of the Related Art

A fuel cell, which directly converts chemical energy to electric energy by utilizing electrochemical reaction of reaction gases including an anode gas such as hydrogen and a cathode gas such as oxygen, has been well known.

Japanese Unexamined Patent Application Publication No. 2006-92924 discloses a solid polymer electrolyte fuel cell stack including a plurality of single cells. Each of the single cells includes a membrane electrode assembly (hereinafter referred to as “MEA”) and separators disposed on both sides of the MEA. The MEA has an anode electrode and a cathode electrode sandwiching an electrolyte membrane therebetween. In the outer periphery of the fuel cell stack, insulating resin members are formed so that the stacked single cells can be joined to each other and insulation from the outside can be ensured.

However, in the fuel cell stack described in Japanese Unexamined Patent Application Publication No. 2006-92924, the plurality of single cells and the resin members are integrally formed. Therefore, when the electrolyte membranes of the MEAs swell and the fuel cell stack expands in the direction in which the single cells are stacked (hereinafter referred to as “the stacking direction”), the resin members cannot follow displacement of the fuel cell stack, or in other words, displacements between the plurality of MEAs. This may cause resin members to crack. If the resin members crack, water vapor generated in the MEAs may leak out from the inside of the fuel cell stack to the outside, a liquid junction may be generated, and insulation performance of the fuel cell stack may deteriorate.

SUMMARY OF THE INVENTION

The present invention provides a fuel cell stack in which insulation performance is restrained from deteriorating.

In one embodiment, a fuel cell stack is provided in which a plurality of single cells each including a membrane electrode assembly are stacked in a stacking direction. The fuel cell stack includes a plurality of insulation members each connected to an outer peripheral portion of a corresponding one of the membrane electrode assemblies. The plurality of insulation members are electrically insulating. The fuel cell stack further includes a first displacement absorbing member disposed between each insulation member and an adjacent insulation member.

In another embodiment, a fuel cell stack is provided in which a plurality of single cells are stacked in a stacking direction. The fuel cell includes a plurality of membrane electrode assemblies each including an electrolyte membrane and outer peripheral members configured and arranged to absorb displacement between the plurality of membrane electrode assemblies. Each of the outer peripheral members is connected to an outer peripheral portion of a corresponding one of the plurality of membrane electrode assemblies.

In another embodiment, a fuel cell stack is provided including a plurality of membrane electrode assemblies and displacement absorbing means for absorbing displacements between each of the membrane electrode assembly and an adjacent membrane electrode assembly and for supporting the plurality of membrane electrode assemblies at outer peripheral portions thereof.

When the electrolyte membranes of the membrane electrode assemblies swell, the first displacement absorbing members, the outer peripheral members, or the displacement absorbing means deform so that the insulation members can follow displacement of the fuel cell stack in the stacking direction, whereby the insulation members are prevented from cracking.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

A fuel cell system directly converts chemical energy of fuel to electric energy. In a fuel cell system, an electrolyte membrane is sandwiched between an anode electrode and a cathode electrode. The anode electrode is supplied with anode gas including hydrogen, and the cathode electrode is supplied with cathode gas including oxygen. The following electrochemical reactions occur on the surfaces of the anode electrode and the cathode electrode in contact with the electrolyte membrane, so that electric energy is obtained from the electrodes.
Anode electrode reaction: 2H2→4H++4e−(1)
Cathode electrode reaction: 4H++4e−+O2→2H2O  (2)

FIG. 1shows a fuel cell stack100, which is a fuel cell system used for a mobile vehicle such as an automobile.

The fuel cell stack100includes a plurality of single cells10, a pair of collector plates20, a pair of insulation plates30, a pair of end plates40, and nuts50screwed into tension rods (not shown).

The single cells10, which generate electromotive force, are unit cells of a solid polymer electrolyte membrane fuel cell type. The fuel cell stack100includes a stack of the single cells10. The structure of the single cell10is described below in detail with reference toFIG. 2.

Each of the pair of collector plates20is disposed on an outer surface of the stack of the single cells10. The collector plates20are made of gas-impermeable electroconductive material such as compact carbon. Each of the collector plates20has an output terminal21on an upper side thereof. The fuel cell stack100outputs electrons generated in the single cells10through the output terminals21.

Each of the pair of insulation plates30is disposed on an outer surface of a corresponding one of the collector plates20. The insulation plates30are made of insulating rubber.

Each of the pair of end plates40is disposed on an outer surface of a corresponding one of the insulation plates30. The end plates40are made of metal or resin material having rigidity. One of the end plates40includes a cooling water inlet41A, a cooling water outlet41B, an anode gas inlet42A, an anode gas outlet42B, a cathode gas inlet43A, and a cathode gas outlet43B.

The nuts50are disposed on outer surfaces of the pair of end plates40at positions near the four corners of each of the end plates40. The nuts50are screwed into ends of each of the tension rods extending through the fuel cell stack100. The fuel cell stack100is fastened in the stacking direction by the tension rods and the nuts50. In order to prevent a short-circuit between the single cells10, the surfaces of the tension rods are insulated.

Alternatively, the fuel cell stack100may be fastened in the stacking direction by using tension plates.

Referring toFIG. 2, the structure of the single cell10is described.FIG. 2is a partial sectional view of adjacent single cells10in the stacking direction. Each of the single cells10includes an MEA60, an anode separator71and a cathode separator72sandwiching the MEA60therebetween, and an insulation member80integrally formed with the MEA60.

The MEA60is a layered stack including an electrolyte membrane61, an anode electrode62disposed on one surface of the electrolyte membrane61, and a cathode electrode63disposed on the other surface of the electrolyte membrane61.

The electrolyte membrane61is a proton-conductive ion exchange membrane made of fluorocarbon resin. The electrolyte membrane61is larger than the anode electrode62and the cathode electrode63, so that the electrolyte membrane61has an outer edge61A which extends past the outer edges of the anode electrode62and the cathode electrode63. Because the electrolyte membrane61conducts electricity well in a wet condition, the anode gas and the cathode gas are humidified in the fuel cell stack100.

The anode electrode62is a stack of layers including an electrode catalyst layer made of an alloy including platinum or the like, a water-repellent layer made of fluorocarbon resin or the like, and a gas diffusion layer made of a carbon cloth or the like, which are stacked on the electrolyte membrane61in this order.

As with the anode electrode62, the cathode electrode63is a stack of layers including an electrode catalyst layer, a water-repellent layer, and a gas diffusion layer, which are stacked on the electrolyte membrane61in this order.

The anode separator71is a corrugated panel of electroconductive material such as metal. The anode separator71is larger than the MEA60. On the side of the anode separator71which contacts the anode electrode62, an anode gas passage71A for supplying anode gas to the anode electrode62is formed between the anode separator71and the anode electrode62. On the opposite side of the anode separator71, a cooling water channel71B, through which cooling water for cooling the fuel cell stack100flows, is formed between the anode separator71and the cathode separator72.

The cathode separator72is a corrugated panel made of electroconductive material such as metal. The cathode separator72is larger than the MEA60. On the side of the cathode separator72which contacts the cathode electrode63, a cathode gas passage72A for supplying the cathode gas to the cathode electrode63is formed between the cathode separator72and the cathode electrode63. On the opposite side of the cathode separator72, a cooling water channel72B, through which cooling water for cooling the fuel cell stack100flows, is formed between the cathode separator72and the anode separator71.

The cooling water channel71B, which is formed by the anode separator71of one of a pair of adjacent single cells10, and the cooling water channel72B, which is formed by the cathode separator72of the other one of the pair of adjacent single cells10, face each other. The cooling water channels71B and72B constitute a cooling water channel73.

The insulation member80, which is made of electrically insulating resin, is a frame-shaped member disposed along the outer periphery of the MEA60. The insulation member80includes a frame portion81integrally formed with the outer periphery of the MEA60, and a protruding portion82protruding from the frame portion81in the stacking direction.

The protruding portions82of the insulation member80jut out from an end of the frame portion81both ways in the stacking direction (vertical directions inFIG. 2). The protruding portion82of the insulation member80of one of a pair of adjacent single cells10and the protruding portion82of the insulation member80of the other one of the pair of single cells10are bonded to each other via a first displacement absorbing member90. In one embodiment, the first displacement absorbing members90are bonding members.

In the frame portion81of the insulation member80, a slot83is formed so that the outer edge61A of the electrolyte membrane61can be inserted therein. The frame portion81is sandwiched between the anode separator71and the cathode separator72of the single cell10, and bonded to the anode separator71and the cathode separator72via second displacement absorbing members92. In one embodiment, the second displacement absorbing member92are bonding members.

The first displacement absorbing members90, with which the space between the insulation members80is filled, and the second displacement absorbing members92, with which the insulation member80is bonded to the separators71and72, can be adhesives having a Young's modulus lower than that of the insulation member80when the adhesives are cured. It is preferable that the Young's modulus of the first displacement absorbing members90and the second displacement absorbing members92is equal to or lower than 20 MPa.

Since the fuel cell stack100includes the insulation members80covering the outer peripheries of the single cells10, insulation between the inside and the outside of the fuel cell stack100can be ensured.

As shown inFIG. 3A, the thickness t1of the protruding portion82of the insulation member80of the single cell10in the stacking direction is smaller than the thickness t2of the single cell10in the stacking direction. The thickness t2of the single cell10in the stacking direction is the sum of the thickness of the MEA60in the stacking direction, the thickness of the anode separator71in the stacking direction, and the thickness of the cathode separator72in the stacking direction. If, for example, the thickness t1of the protruding portion82of the insulation member80in the stacking direction is larger than the thickness t2of the single cell10in the stacking direction as shown inFIG. 3B, the insulation members80of adjacent single cells10interfere with each other, so that the contact pressure between the single cells10decreases, which may impair power generation efficiency. In the present embodiment, the thickness t1of the protruding portion82in the stacking direction is smaller than the thickness t2of the single cell10in the stacking direction. Therefore, the insulation members80of adjacent single cells10do not interfere with each other, whereby power generation efficiency is less likely to be impaired.

FIGS. 4A and 4Billustrate the sealing ability of a fuel cell stack against water vapor generated in an MEA.FIG. 4Ashows the fuel cell stack100of the present embodiment, andFIG. 4Bshows a fuel cell stack200of a comparative example.

In the fuel cell stack200shown inFIG. 4B, insulation members80are disposed so as to sandwich an outer edge61A of an electrolyte membrane61of a single cell10therebetween, whereby the outer periphery of an electrolyte membrane61is exposed to the outside. Thus, as shown by the arrow A, water vapor generated in an MEA60can be easily released to the outside from between the electrolyte membrane61and the insulation members80. Moreover, in the fuel cell stack200, rubber gaskets74are disposed between the insulation members80and the separators71and72. Because the rubber gaskets74are permeable to water vapor, water vapor generated in the MEA60may leak to the outside from between the insulation members80and the separators71and72as shown by the arrow B.

In contrast, in the fuel cell stack100of the present embodiment shown inFIG. 4A, the outer edge61A of the electrolyte membrane61of the single cell10is inserted into the slot83of the frame portion81of the insulation member80, and the frame portion81is bonded to the separators71and72via the second displacement absorption members92, whereby water vapor generated in the MEA60is restrained from passing between the insulation member80and the separators71and72. Moreover, in the fuel cell stack100, the space between the protruding portions82of the insulation members80of adjacent single cells10is filled with the first absorption displacement member90, so that the inside of the fuel cell stack100is separated from the outside by the insulation members80and the first displacement absorbing members90. Thus, even if water vapor passes between the insulation member80and the separators71and72, the water vapor is prevented from leaking to the outside.

When the fuel cell generates power, the electrolyte membranes61of the MEAs60swell, so that the fuel cell stack100expands in the stacking direction.FIGS. 5A to 5Eillustrate states of an insulation member when the fuel cell stack expands in the stacking direction.

FIG. 5Cshows a fuel cell stack300, which is a comparative example of the fuel cell stack100. In the fuel cell stack300, protruding portions82of insulation members80of adjacent single cells10are connected to each other via a first displacement absorbing member90having a Young's modulus higher than that of the insulation members80.FIG. 5Dshows a fuel cell stack300, which is a comparative example of the fuel cell stack100. In the fuel cell stack300, protruding portions82of insulation members80of single cells10are integrally formed with each other. With the fuel cell stack300shown inFIGS. 5C and 5D, the insulation members80and the first displacement absorbing members90may not be able to follow displacement of the fuel cell stack (displacements between a plurality of MEAs60) in the stacking direction, which may occur during power generation or on other occasions. Causes of displacement include, but are not limited to, swelling of the MEAs60and vibration of the fuel cell stack300for example in a moving automotive vehicle subjected to unevenness in the road. Therefore, the protruding portion82of the insulation member80, for example, may crack as shown inFIG. 5E.

In contrast, in the fuel cell stack100shown inFIG. 5A, the protruding portions82of the insulation members80of adjacent single cells10are bonded to each other via the first displacement absorbing member90having a lower Young's modulus than the insulation members80. As shown inFIG. 5B, the first displacement absorbing member90deforms so that the insulation member80can follow displacement of the fuel cell stack in the stacking direction, whereby the insulation member80is restrained from cracking.

Accordingly, the fuel cell stack100of the present embodiment has the following advantages.

In the fuel cell stack100, the space between the protruding portions82of the insulation members80of adjacent single cells10is filled with the first displacement absorbing member90. Even when the electrolyte membrane61of the MEA60swells, the first displacement absorbing member90deforms so that the insulation member80can follow the displacement of the fuel cell in the stacking direction, whereby the insulation member80is restrained from cracking. Therefore, water vapor generated in the fuel cell stack100does not leak to the outside and generation of a liquid junction is suppressed, whereby the insulation performance of the fuel cell stack100is restrained from deteriorating.

In the fuel cell stack100, the frame portion81of the insulation member80is integrally formed with the outer periphery of the MEA60. Thus, as compared with the fuel cell stack200shown inFIG. 4B, in which the outer edge61A of the electrolyte membrane61is sandwiched between the insulation members80, the area of the electrolyte membrane61that swells can be decreased. Therefore, with the fuel cell stack100, the displacement of the fuel cell stack in the stacking direction due to swelling of the electrolyte membrane61can be reduced as compared with the fuel cell stack200, whereby the insulation member80is more securely restrained from cracking.

In the fuel cell stack100, the space between the protruding portions82of the insulation members80of adjacent single cells10is filled with the first displacement absorbing member90, so that the inside of the fuel cell stack100is separated from the outside. Thus, water vapor generated in the MEA60does not leak out of the fuel cell stack100. Moreover, in the fuel cell stack100, the outer edge61A of the electrolyte membrane61of the single cell10is inserted into the slot83of the frame portion81of the insulation member80, and the frame portion81of the insulation member80is bonded to the separators71and72via the second displacement absorbing member92. Thus, water vapor generated in the MEA60is restrained from passing between the insulation member80and the separators71and72. In this manner, water vapor is restrained from leaking to the outside without using gaskets or the like, whereby the number of components and the size of the fuel cell stack100can be reduced.

Second Embodiment

FIG. 6is a partial sectional view of single cells10of a fuel cell stack100of a second embodiment in the stacking direction.

The fuel cell stack100of the second embodiment, which is similar to that of the first embodiment, differs from that of the first embodiment in that the stacked state of the single cells10is firmly held in the second embodiment. The difference is mainly described below.

The fuel cell stack100includes a stack of the single cells10each including an insulation member80, and the stack of the single cells are sandwiched between end plates40in the stacking direction. Thus, the rigidity of the fuel cell stack100in a direction perpendicular to the stacking direction is lower than the rigidity of the fuel cell stack100in the stacking direction. Therefore, when a force is applied to the fuel cell stack100from the outside in a direction perpendicular to the stacking direction, the single cells10may be moved in the direction perpendicular to the stacking direction. If the single cells10are moved by a large distance, the insulation member80and the first displacement absorbing member90may not be able to follow the movement of the single cells10, whereby the insulation member80and the first displacement absorbing member90may crack.

In order to prevent this, as shown inFIG. 6, the fuel cell stack100includes a pair of tie rods84so that the single cell10can be restrained from moving.

The tie rods84extend in the stacking direction along the outer peripheral surfaces of the protruding portions82of the insulation members80of the stack of the single cells10. The tie rods84are fixed to the end plates40. The pair of tie rods84are disposed so as to face each other and sandwich the single cells10therebetween from outer sides of the insulation members80.

The fuel cell stack100of the second embodiment includes the pair tie rods84, which are disposed on the outer sides of the stack of the single cells10and extend in the stacking direction. Thus, even when a force is applied from the outside in a direction perpendicular to the stacking direction, the single cells10are restrained from moving in the direction perpendicular to the stacking direction. Since the single cells10are restrained from moving in the direction perpendicular to the stacking direction, the first displacement absorbing member90can deform so that the insulation members80can follow movement of the single cells10. Thus, the insulation members80and the first displacement absorbing members90are restrained from cracking. Therefore, water vapor generated in the fuel cell stack100does not leak to the outside, generation of a liquid junction is suppressed, and the insulation performance of the fuel cell stack100is restrained from deteriorating.

Third Embodiment

FIG. 7is a partial sectional view of single cells10of a fuel cell stack100of a third embodiment in the stacking direction.

The fuel cell stack100of the third embodiment, which is similar to that of the first embodiment, differs from that of the first embodiment in the structure of insulation members80of the single cells10. The difference is mainly described below.

In the third embodiment, a protruding portion82of the insulation member80of the single cell10protrudes from an end of the frame portion81one way in the stacking direction (upward inFIG. 7). In order to absorb displacement of the fuel cell stack, a space between the protruding portion82of the insulation member80of one of a pair of adjacent single cells10and an end of the frame portion81of the insulation member80of the other one of the pair of adjacent single cells10is filled with a first displacement absorbing member90.

Also with the fuel cell stack100of the third embodiment, the first displacement absorbing member90deforms so that the insulation member80can follow the displacement of the fuel cell stack in the stacking direction, whereby the advantage similar to that of the first embodiment can be gained.

Moreover, in the fuel cell stack100of the third embodiment, the first displacement absorbing member90, which deforms in accordance with the displacement of the fuel cell stack, is disposed farther from ends of the anode separator71and the cathode separator72in the stacking direction than in the first embodiment. Therefore, when the first displacement absorbing member90deforms so that the insulation member80can follow the displacement of the fuel cell stack, the insulation member80and the ends of the separators71and72are prevented from colliding with or shifting with respect to each other, whereby the insulation member80is restrained from cracking.

While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the invention, as defined in the appended claims and equivalents thereof.

For example, the advantages of the above-described embodiments can be obtained by using displacement absorbing means such as an elastic member made from rubber or other like material instead of the bonding member as the first displacement absorbing90. Moreover, in each of the first to third embodiments, the fuel cell stack100is made by simultaneously stacking all the single cells10. However, some of the single cells10may be stacked beforehand to form a cell module, and a plurality of cell modules may be stacked so as to form the fuel cell stack100. With this structure, the number of man-hours required for assembling the fuel cell stack can be reduced.

Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.