Separator assembly for fuel cell and method for manufacturing separator assembly for fuel cell

A separator assembly for a fuel cell includes a first separator, a second separator, and a joined portion. In the joined portion, the first separator and the second separator are joined to each other through laser welding. The first separator includes a first surface that is intended to be opposed to the membrane electrode assembly. The first surface of the first separator includes an exposed portion where the base of the first separator is exposed. The second separator includes a second surface that is intended to be opposed to the membrane electrode assembly. A film including conductive particles is arranged on the entire second surface of the second separator. The joined portion is formed by irradiating the exposed portion of the first separator with laser.

BACKGROUND

The present invention relates to a separator assembly for a fuel cell and a method for manufacturing a separator assembly for a fuel cell.

A polymer electrolyte fuel cell includes a fuel cell stack configured by single cells laminated with one another. Each single cell includes a membrane electrode assembly and two separators that hold the membrane electrode assembly in between. A fuel gas passage through which fuel gas is supplied is defined between the separator (hereinafter referred to as first separator) located on the anode side of the membrane electrode assembly and the membrane electrode assembly. An oxide gas passage through which oxidizing gas is supplied is defined between the separator (hereinafter referred to as second separator) located on the cathode side of the membrane electrode assembly and the membrane electrode assembly. The separators each include a base made of a metal plate such as a stainless steel plate. Films that enhance the corrosion resistance and conductivity of the bases are formed on the entire surfaces of the bases opposed to the membrane electrode assembly (refer to, for example, Japanese Laid-Open Patent Publication No. 2007-311074). Further, the fuel cell stack described in the document includes a separator assembly where the first separator and the second separator, which are adjacent to each other in the laminating direction of the single cells, are joined to each other through laser welding in order to reduce the contact resistance between the first separator and the second separator.

In the separator assembly of the document, when the first separator and the second separator are laser-welded to each other, laser irradiation is performed on the films, which respectively coat the bases. In this case, when part of the films is melted and then enters the welded parts of the bases, the bases may corrode easily. In addition, laser irradiation may cause evaporation (ablation) to occur locally in the films. In this case, when the laser energy is used to vaporize the films, the energy needed to weld the bases to each other may tend to lack, resulting in weld failure. Such inconveniences occur not only in the separator assembly, in which the separators each including the base made of a stainless steel plate are joined to each other, but also occur in the same manner in a separator assembly in which separators each having a base made of a metal plate other than a stainless steel plate are joined to each other.

SUMMARY

It is an object of the present invention to provide a separator assembly for a fuel cell and a method for manufacturing a separator assembly for a fuel cell capable of limiting corrosion of the separator and limiting the occurrence of weld failure.

A separator assembly for a fuel cell that solves the above-described problem is applied to a fuel cell stack configured by single cells laminated in a laminating direction. Each of the single cells includes a membrane electrode assembly, a first separator having a base made of a metal plate and arranged on an anode side of the membrane electrode assembly, and a second separator having a base made of a metal plate and arranged on a cathode side of the membrane electrode assembly. The separator assembly includes the first separator and the second separator. The first separator and the second separator are adjacent to each other in the laminating direction. The separator assembly also includes a joined portion where the first separator and the second separator are joined to each other through laser welding. The first separator includes a first surface that is intended to be opposed to the membrane electrode assembly. The first surface of the first separator includes an exposed portion where the base of the first separator is exposed. The second separator includes a second surface that is intended to be opposed to the membrane electrode assembly. A film including conductive particles is arranged on the entire second surface of the second separator. The joined portion is formed by irradiating the exposed portion of the first separator with laser.

A method for manufacturing a separator assembly for a fuel cell that solves the above-described problem is provided. The separator assembly is applied to a fuel cell stack configured by single cells laminated in a laminating direction. Each of the single cells includes a membrane electrode assembly, a first separator having a base made of a metal plate and arranged on an anode side of the membrane electrode assembly, and a second separator having a base made of a metal plate and arranged on a cathode side of the membrane electrode assembly. The first separator and the second separator, which are adjacent to each other in the laminating direction, are joined to each other through laser welding. The method includes a step of forming a film including conductive particles on an entire surface of the second separator that is intended to be opposed to the membrane electrode assembly and a step of laser-welding the first separator and the second separator to each other by irradiating, with laser, an exposed portion of the first separator where the base of the first separator is exposed.

DETAILED DESCRIPTION

An embodiment will now be described with reference toFIGS. 1 to 3C.

As shown inFIG. 1, the fuel cell stack of a polymer electrolyte fuel cell includes single cells10laminated in a laminating direction L (vertical direction inFIG. 1). Each single cell10includes a membrane electrode assembly11, a first separator30, and a second separator40. The first separator30and the second separator40hold the membrane electrode assembly11in between. The membrane electrode assembly11includes an electrolyte membrane12made of a polymer electrolyte film and two electrode catalyst layers13that hold the electrolyte membrane12in between. A gas diffusion layer14made of carbon fiber is arranged between the membrane electrode assembly11and the first separator30. A gas diffusion layer15made of carbon fiber is arranged between the membrane electrode assembly11and the second separator40.

The first separator30includes a base30amade of a metal plate such as a stainless steel plate and is located on the anode side of the membrane electrode assembly11. The first separator30includes first projections31and first recesses32, both of which extend. The first projections31and the first recesses32are alternately arranged. The top surface (upper surface inFIG. 1) of each first projection31of the first separator30abuts the gas diffusion layer14, which is located on the anode side. The space defined by each first recess32of the first separator30and the gas diffusion layer14defines a gas passage through which fuel gas such as hydrogen gas flows.

The second separator40includes a base40amade of a metal plate such as a stainless steel plate and is located on the cathode side of the membrane electrode assembly11. The second separator40includes second projections41and second recesses42, both of which extend. The second projections41and the second recesses42are alternately arranged. The top surface (lower surface inFIG. 1) of each second projection41of the second separator40abuts the gas diffusion layer15, which is located on the cathode side. The space defined by each second recess42of the second separator40and the gas diffusion layer15defines a gas passage through which fuel gas such as oxidant gas flows.

The space defined by the back surface of each first projection31of the first separator30and the back surface of each second projection41of the second separator40configures a cooling passage through which coolant flows.

As shown inFIGS. 1 and 2, the surface (upper surface inFIGS. 1 and 2) of the first separator30opposed to the membrane electrode assembly11defines a first surface35. A film50is arranged on the part of the first surface35corresponding to the top surface of each first projection31that abuts the membrane electrode assembly11. The film50includes a first layer51applied to the top surface of the first projection31and a second layer52applied to the upper surface of the first layer51. The film50is not arranged on a portion of the first surface35of the first separator30other than the part corresponding to the top surface of the first projection31. Instead, the base30ais exposed at the part on which the film50is not arranged. The portion where the base30ais exposed is hereinafter referred to as an exposed portion36.

The surface (lower surface inFIGS. 1 and 2) of the second separator40opposed to the membrane electrode assembly11defines a second surface45. A film60is arranged on the second surface45. The film60includes a first layer61applied to the entire second surface45and a second layer62applied to the upper surface of the first layer61.

The first layers51and61include conductive particles made of titanium nitride and binder made of epoxy plastic.

The second layers52and62include graphite particles and binder made of polyvinylidene difluoride (PVDF) plastic. Polyvinylidene difluoride is a thermoplastic, and epoxy plastic is a thermosetting plastic. The thermosetting temperature of epoxy plastic is lower than the melting point of a polyvinylidene difluoride plastic.

The first separator30and the second separator40, which are adjacent to each other in the laminating direction L of the single cells10, are joined to each other through laser welding. The unit of the first separator30and the second separator40joined to each other is hereinafter referred to as a separator assembly20.

The separator assembly20includes a joined portion21formed by joining the bottom of each first recess32of the first separator30and the bottom of each second recess42of the second separator40to each other through laser welding. The joined portion21is formed by irradiating, with laser, a part of the exposed portion36of the first separator30corresponding to the bottom of each first recess32. The joined portion21, which is a nugget, does not reach the second surface45of the second separator40.

The method for manufacturing the separator assembly20will now be described.

First, as shown inFIG. 3A, a press die (not shown) is used to press the base30aof the first separator30to form the first projections31and the first recesses32on the base30a. Using the same method, the second projections41and the second recesses42are formed on the base40aof the second separator40.

Next, as shown inFIG. 3B, the first layer51is applied to the part of the first surface35of the base30athat abuts the membrane electrode assembly11, that is, the top surface of each first projection31. Subsequently, the second layer52is applied to the upper surface of the first layer51. Then, the first layer51and the second layer52are thermocompression-bonded to the base30a. In this manner, the first separator30is manufactured.

Afterwards, as shown inFIG. 3C, the first layer61is applied to the entire second surface45of the base40a. Subsequently, the second layer62is applied to the upper surface of the part of the first layer61corresponding to the top surface of each second projection41. Then, the first layer61and the second layer62are thermocompression-bonded to the base40a. In this manner, the second separator40is manufactured.

Subsequently, as shown inFIG. 2, the back surface of the first separator30located on the opposite side from the first surface35and the back surface of the second separator40located on the opposite side from the second surface45are abutted against each other. In this state, the exposed portion36is irradiated with laser from the first separator30to perform welding. This forms the joined portion21, where the first separator30and the second separator40are joined to each other through laser welding. In this manner, the separator assembly20is manufactured by joining the first separator30to the second separator40.

The advantages of the present embodiment will now be described.

(1) The separator assembly20for a fuel cell includes the first separator30, the second separator40, and the joined portion21. The first separator30and the second separator40are adjacent to each other in the laminating direction L. The first separator30is configured to be located on the anode side of the membrane electrode assembly11. The first separator30includes the first surface35, which is intended to be opposed to the membrane electrode assembly11. The first surface35of the first separator30includes the exposed portion36, where the base30aof the first separator30is exposed. The second separator40is configured to be located on the cathode side of the membrane electrode assembly11. The second separator40includes the second surface45, which is intended to be opposed to the membrane electrode assembly11. The film60, which includes conductive particles, is arranged on the entire second surface45of the second separator40. The joined portion21is formed by irradiating the exposed portion36of the first separator30with laser.

When the fuel cell is running, the cathode side of the membrane electrode assembly11has a higher potential than the anode side. Thus, corrosion resulting from potential difference tends to occur in the second separator40located on the cathode side of the membrane electrode assembly11.

In the above-described structure, the film60is formed on the entire second surface45of the second separator40. This limits corrosion of the second surface45resulting from potential difference.

The joined portion21, where the first separator30and the second separator40are joined to each other, is formed by irradiating the exposed portion36of the first separator30with laser. This prevents the films50and60from being irradiated with laser and thus limits the occurrence of weld failure resulting from laser irradiation on the films50and60.

(2) The film50, which includes conductive particles, is arranged on the part of the first surface35of the first separator30configured to abut the membrane electrode assembly11.

This structure limits increases in the contact resistance between the membrane electrode assembly11and the first separator30.

(3) The method for manufacturing the separator assembly20for a fuel cell includes a step of forming the film60, which includes conductive particles, on the entire second surface45of the second separator40, which is intended to be opposed to the membrane electrode assembly11. This method also includes a step of laser-welding the first separator30and the second separator40to each other by irradiating the exposed portion36of the first separator30, where the base30aof the first separator30is exposed, with laser.

Using the method, the same advantage as the above-described advantage (1) is obtained.

Modifications

Instead of titanium nitride of which the first layers51and61are made, other conductive particles such as titanium carbide or titanium boride may be used.

Instead of graphite particles of which the first layers51and61are made, other conductive particles such as carbon black may be used.

Instead of arranging the film50on the first surface35of the first separator30, the entire first surface35may be configured by the exposed portion36.

The exposed portion36may configure only the part of the first surface35of the first separator30corresponding to the joined portion21, and the film50may be arranged on a portion other than the exposed portion36. In this case, the first surface35of the first separator30is coated by the film50over a broader range. This effectively limits corrosion.

The material of the base of the first separator and the material of the base of the second separator may be changed to different metal materials other than stainless steel. The metal materials include, for example, pure titanium, titanium alloy, aluminum alloy, and magnesium alloy.