Fuel cell

A fuel cell includes a membrane electrode assembly and first and second metal separators. The first metal separator has first outer protrusions provided outside an oxygen-containing gas flow field. The second metal separator has second outer protrusions provided outside a fuel gas flow field. The first and second protrusions sandwich outer edges of electrode catalyst layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell including a membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly. The membrane electrode assembly includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The anode and the cathode include electrode catalyst layers provided respectively on both surfaces of the electrolyte membrane.

2. Description of the Related Art

For example, a solid polymer fuel cell employs a polymer ion exchange membrane as an electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. Each of the anode and the cathode is made of an electrode catalyst layer and a gas diffusion layer (e.g., porous carbon). The membrane electrode assembly is sandwiched between separators (bipolar plates) to form a power generation cell. In use, generally, a predetermined number of power generation cells are stacked together to form a fuel cell stack.

In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. A gas chiefly containing oxygen such as the air (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. The electrode catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.

In this type of the fuel cell, for example, the structure as disclosed in Japanese Laid-Open Patent Publication No. 2002-373678 is adopted. In the conventional technique, as shown inFIG. 7, a unit cell1includes an electrolyte membrane2, catalyst electrodes3a,3bformed on both surfaces of the electrolyte membrane2, and gas diffusion electrodes4a,4bformed on the catalyst electrodes3a,3boppositely.

The gas diffusion electrodes4a,4bare sandwiched between separators5a,5b.A fuel gas flow field6afor supplying a fuel gas to the catalyst electrode3ais formed between the gas diffusion electrode4aand the separator5a,and an oxygen-containing gas flow field6bfor supplying an oxygen-containing gas to the catalyst electrode3bis formed between the gas diffusion electrode4band the separator5b.

In the unit cell1, at the time of power generation, water is likely to be produced at the catalyst electrode3bon the cathode side, and area of the electrolyte membrane2to which the catalyst electrode3bis applied is swelled. Therefore, a dimensional change may occur between the area of the electrolyte membrane2to which the catalyst electrodes3a,3bare applied, and the area of the electrolyte membrane2to which the catalyst electrodes3a,3bare applied. The dimensional change may cause stress generation undesirably. Further, edges of the catalyst electrodes3a,3bare in the outer boundary area to which the catalyst is applied. In the outer boundary area, the electrolyte membrane2may be damaged easily by the stress concentration.

Though the gas diffusion electrodes4a,4bare sandwiched by a plurality of protrusions7a,7bprovided on the separators5a,5b,the edges of the catalyst electrodes3a,3bare not sandwiched reliably. Thus, in the conventional technique, cracks or the like may be generated in the electrolyte membrane2undesirably.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide a fuel cell with a simple structure in which generation of stress in an electrolyte membrane is reliably prevented, and the desired power generation performance is achieved.

According to the present invention, a fuel cell comprises a membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly. The membrane electrode assembly comprises an electrolyte membrane, a cathode and an anode. The cathode and the anode include electrode catalyst layers provided respectively on both surfaces of the electrolyte membrane. An oxygen-containing gas flow field for supplying an oxygen-containing gas is provided between the cathode and one of the separators and a fuel gas flow field for supplying a fuel gas is provided between the anode and the other of the separators.

The one separator has a first outer protrusion provided outside the oxygen-containing gas flow field, and the other separator has a second outer protrusion provided outside the fuel gas flow field. Outer edges of the electrode catalyst layers are provided in the contact width where the first outer protrusion contacts the cathode and the second outer protrusion contacts the anode.

It is preferable that the contact width of the first outer protrusion which contacts the cathode is larger than the contact width of a first protrusion which is provided in the oxygen-containing gas flow field, and contacts the cathode, and it is preferable that the contact width of the second outer protrusion which contacts the anode is larger than the contact width of a second protrusion which is provided in the fuel gas flow field, and contacts the anode.

Further, it is preferable that an outer edge of the electrode catalyst layer of the anode and an outer edge of the electrode catalyst layer of the cathode sandwiching the electrolyte membrane are out of alignment with each other.

Further, it is preferable that adhesive layers are provided around the electrode catalyst layer of the anode and around the electrode catalyst layer of the cathode, respectively, and it is preferable that gas diffusion layers are provided to cover the electrode catalyst layers and the adhesive layers. Further, it is preferable that the pair of separators are metal separators or carbon separators.

In the present invention, the first outer protrusion of one separator and the second outer protrusion of the other separator reliably sandwich the outer edges of the electrode catalyst layers, i.e., the outer boundary area of the electrode catalyst layers. Therefore, no stress concentration occurs in the electrolyte membrane. Thus, with the simple structure, damage of the solid polymer electrolyte membrane is prevented, and the desired power generation performance can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a perspective view schematically showing main components of a fuel cell10according a first embodiment of the present invention.FIG. 2is a cross sectional view showing part of the fuel cell10. A plurality of the fuel cells10may be stacked together to form a fuel cell stack.

The fuel cell10includes a membrane electrode assembly14and first and second metal separators16,18sandwiching the membrane electrode assembly14. The first and second metal separators16,18are thin metal plates such as steel plates, stainless steel plates, aluminum plates, or plated steel sheets. The first and second metal separators16,18are formed by press forming to have a desired shape.

At one end of the fuel cell10in a horizontal direction indicated by an arrow B inFIG. 1, an oxygen-containing gas supply passage20afor supplying an oxygen-containing gas, a coolant supply passage22afor supplying a coolant, and a fuel gas discharge passage24bfor discharging a fuel gas such as a hydrogen-containing gas are arranged vertically in a direction indicated by an arrow C. The oxygen-containing gas supply passage20a,the coolant supply passage22a,and the fuel gas discharge passage24bextend through the fuel cell10in the direction indicated by the arrow A.

At the other end of the fuel cell10in the direction indicated by the arrow B, a fuel gas supply passage24afor supplying the fuel gas, a coolant discharge passage22bfor discharging the coolant, and an oxygen-containing gas discharge passage20bfor discharging the oxygen-containing gas are arranged in the direction indicated by the arrow C. The fuel gas supply passage24a,the coolant discharge passage22b,and the oxygen-containing gas discharge passage20bextend through the fuel cell10in the direction indicated by the arrow A.

The membrane electrode assembly14includes a cathode28, an anode30, and a solid polymer electrolyte membrane26interposed between the cathode28and the anode30. The solid polymer electrolyte membrane26is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example.

As show inFIG. 2, the cathode28and the anode30include electrode catalyst layers32a,32bfixed to both surfaces of the electrolyte membrane26and gas diffusion layers34a,34bsuch as carbon papers on the electrode catalyst layers32a,32b.

The electrode catalyst layers32a,32bare platinum alloy supported on porous carbon particles. The carbon particles are deposited uniformly on the surfaces of the gas diffusion layers34a,34b.The surface area of the electrode catalyst layer32aof the cathode28is smaller than the surface area of the electrode catalyst layer32bof the anode30. The surface areas of the gas diffusion layers34a,34bare larger than the surface areas of the electrode catalyst layers32a,32b.Outer edges of the gas diffusion layers34a,34bare adhered to the solid polymer electrolyte membrane26by adhesive layers35a,35b,respectively. As shown inFIG. 1, the area H where the electrode catalyst of the cathode28and the anode30is applied is inside the outer edges of the gas diffusion layers34a,34b.

As shown inFIGS. 1 and 3, the first metal separator16has an oxygen-containing gas flow field36on its surface16afacing the membrane electrode assembly14. The oxygen-containing gas flow field36is connected to the oxygen-containing gas supply passage20aat one end, and connected to the oxygen-containing gas discharge passage20bat the other end. The first metal separator16has a coolant flow field38on its surface16bopposite to the surface16a.The coolant flow field38is formed between the surface16band the second metal separator18. The coolant flow field38is connected to the coolant supply passage22aat one end, and connected to the coolant discharge passage22bat the other end (seeFIG. 4). The oxygen-containing gas flow field36and the coolant flow field38are formed on both surfaces16a,16bof the first metal separator16by press forming.

Specifically, for example, the first metal separator16is formed to have a corrugated shape such that a plurality of grooves36aforming the oxygen-containing gas flow field36are provided on the surface16a,and a plurality of grooves38aforming the coolant flow field38are provided on the surface16b.As shown inFIGS. 3 and 4, first protrusions36bon the surface16aare formed by providing the grooves38aon the surface16b,and first protrusions38bon the surface16bare formed by providing the grooves36aon the surface16a.

On the surface16a,the grooves36aextend substantially straight in the direction indicated by the arrow B. On opposite sides of the grooves36ain the direction indicated by the arrow B, a plurality of projections40aare provided, e.g., by embossing. Likewise, on the surface16b,the grooves38aextend substantially straight in the direction indicated by the arrow B. On opposite sides of the grooves38ain the direction indicated by the arrow B, a plurality of projections40bare provided, e.g., by embossing.

Further, as shown inFIG. 3, on the surface16a,two first outer protrusions42aeach having a substantially L-shape for guiding the oxygen-containing gas from the oxygen-containing gas supply passage20ato the oxygen-containing gas discharge passage20bare provided outside the oxygen-containing gas flow field36. As shown inFIG. 2, at the first outer protrusions42a,the outer edge of the electrode catalyst layer32aof the cathode28is provided at a substantially middle position along the width of first outer protrusions42a.

The contact width L1of the first outer protrusion42a(the width of the first outer protrusion42awhich contacts the cathode28) is larger than the contact width L2of the first protrusion36b(the width of first protrusion36bwhich contacts the cathode28). Therefore, as describe later, it is possible to absorb the dimensional displacement of the electrode catalyst layer32asufficiently, and the outer edge of the electrode catalyst layer32ais reliably supported by the first outer protrusions42a.As shown inFIG. 4, on the surface16b,two first recesses42beach having a substantially L-shape is formed. The first recesses42bare formed by the back surfaces of the first outer protrusions42a.

A first seal member46is formed integrally on the surfaces16a,16bof the first metal separator16, e.g., by heat treatment, injection molding, or the like, to cover (sandwich) the outer edge of the first metal separator16. The first seal member46is made of seal material, cushion material or packing material such as EPDM (Ethylene Propylene Diene Monomer), NBR (Nitrile Butadiene Rubber), fluoro rubber, silicone rubber, fluoro silicone rubber, butyl rubber (Isobutene-Isoprene Rubber), natural rubber, styrene rubber, chloroprene rubber, or acrylic rubber.

The first seal member46includes a line seal46aprovided around the oxygen-containing gas flow field36on the surface16a.The line seal46ais not provided between the oxygen-containing gas supply passage20aand the oxygen-containing gas flow field36, and between the oxygen-containing gas discharge passage20band the oxygen-containing gas flow field36. Thus, the oxygen-containing gas flow field36is connected to the oxygen-containing gas supply passage20aand the oxygen-containing gas discharge passage20bon the surface16a(seeFIG. 3).

As shown inFIGS. 1 and 5, the second metal separator18has a fuel gas flow field48on its surface18afacing the membrane electrode assembly14. The fuel gas flow field48is connected to the fuel gas supply passage24aat one end, and connected to the fuel gas discharge passage24bat the other end.

As shown inFIG. 1, the second metal separator18has a coolant flow field38on its surface18bopposite to the surface18a.The coolant flow field38is formed between the surface18band the first metal separator16. The coolant flow field38is connected to the coolant supply passage22aat one end, and connected to the coolant discharge passage22bat the other end. The fuel gas flow field48and the coolant flow field38are formed on both surfaces18a,18bof the second metal separator18by press forming.

Specifically, for example, the second metal separator18is formed to have a corrugated shape such that a plurality of grooves48aforming the fuel gas flow field48are provided on the surface18a(seeFIG. 5), and a plurality of grooves48aforming the coolant flow field38are provided on the surface18b(seeFIG. 1). Second protrusions48bon the surface18aare formed by providing the grooves38aon the surface18b,and first protrusions38bon the surface16bare formed by providing the grooves48aon the surface18b.

On the surface18a,the grooves38aextend substantially straight in the direction indicated by the arrow B. On opposite sides of the grooves48ain the direction indicated by the arrow B, a plurality of projections50aare provided, e.g., by embossing. Likewise, on the surface18b,the grooves38aextend substantially straight in the direction indicated by the arrow B. On opposite sides of the grooves38ain the direction indicated by the arrow B, a plurality of projections50bare provided, e.g., by embossing.

Further, as shown inFIG. 5, on the surface18a,two second outer protrusions52aeach having a substantially L-shape for guiding the fuel gas from the fuel gas supply passage24ato the fuel gas discharge passage24bare provided outside the fuel gas flow field48. As shown inFIG. 2, at the second outer protrusions52a,the outer edge of the electrode catalyst layer32bof the anode30is provided at a substantially middle position along the width of second outer protrusions52a.

The contact width L1of the second outer protrusion52a(the width of the second outer protrusion52awhich contacts the anode30) is larger than the contact width L2of the second protrusion48b(the width of second protrusion48bwhich contacts the anode30). Therefore, as describe later, it is possible to absorb the dimensional displacement of the electrode catalyst layer32bsufficiently, and the outer edge of the electrode catalyst layer32bis reliably supported by the second outer protrusions52a.As shown inFIG. 1, on the surface18b,two second recesses52beach having a substantially L-shape is formed. The second recesses52bare formed by the back surfaces of the second outer protrusions52a.

A second seal member54is formed integrally on the surfaces18a,18bof the second metal separator18, e.g., by heat treatment, injection molding, or the like, to cover (sandwich) the outer edge of the second metal separator18. The material used for the second seal member54is the same as the material used for the first seal member46. The second seal member54includes a line seal54aprovided around the fuel gas flow field48on the surface18a.The line seal54ais not provided between the fuel gas supply passage24aand the fuel gas flow field48, and between the fuel gas discharge passage24band the fuel gas flow field48. Thus, the fuel gas flow field48is connected to the fuel gas supply passage24aand the fuel gas discharge passage24bon the surface18a(seeFIG. 5).

A line seal54bis provided around the coolant flow field38on the surface18b.The line seal54ais not provided between the coolant supply passage22aand the coolant flow field38, and between the coolant discharge passage22band the coolant field38. Thus, the coolant flow field38is connected to the coolant supply passage22aand the coolant discharge passage22bon the surface18b(seeFIG. 1).

Next, operation of the fuel cell10will be described below.

As shown inFIG. 1, an oxygen-containing gas is supplied to the oxygen-containing gas supply passage20a,and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage24a.Further, a coolant such as pure water, an ethylene glycol, or an oil is supplied to the coolant supply passage22a.

As shown inFIGS. 1 and 3, the oxygen-containing gas flows from the oxygen-containing gas supply passage20ainto the oxygen-containing gas flow field36of the first metal separator16. The oxygen-containing gas flows along the cathode28of the membrane electrode assembly14for inducing an electrochemical reaction at the cathode28. Likewise, as shown inFIGS. 1 and 5, the fuel gas flows from the fuel gas supply passage24ainto the fuel gas flow field48of the second metal separator18. The fuel gas flows along the anode30of the membrane electrode assembly14for inducing an electrochemical reaction at the anode30.

Thus, in each of the membrane electrode assemblies14, the oxygen-containing gas supplied to the cathode28, and the fuel gas supplied to the anode30are consumed in the electrochemical reactions at catalyst layers of the cathode28and the anode30for generating electricity (seeFIG. 2).

Then, after the oxygen-containing gas is consumed at the cathode28, the oxygen-containing gas is discharged into the oxygen-containing gas discharge passage20b(seeFIG. 3). Likewise, after the fuel gas is consumed at the anode30, the fuel gas is discharged into the fuel gas discharge passage24b(seeFIG. 5).

The coolant supplied to the coolant supply passage22aflows into the coolant flow field38between the first and second metal separators16,18. After the coolant cools the membrane electrode assembly14, the coolant is discharged into the coolant discharge passage22b(seeFIG. 1).

In the first embodiment, the first metal separator16has the first outer protrusions42aoutside the oxygen-containing gas flow field36, and the second metal separator18has the second outer protrusions52aoutside the fuel gas flow field48.

As shown inFIG. 2, the first and second outer protrusions42a,52asandwich the outer edges, i.e., outer boundary areas of the electrode catalyst layers32a,32bof the membrane electrode assembly14. Therefore, even if the solid polymer electrolyte membrane26is swelled by the water produced in the power generation, stress concentration does not occur at outer edges of the electrode catalyst layers32a,32b.Further, the outer edge of the electrode catalyst layer32aand the outer edge of the electrode catalyst layer32bare provided at different positions, i.e., the position of the outer edge of the electrode catalyst layer32ais out of alignment with the position of the outer edge of the electrode catalyst layer32bin the stacking direction. Thus, it is possible to prevent stress concentration in the solid polymer electrolyte membrane26.

Therefore, in the first embodiment, damage of the solid polymer electrolyte membrane26is prevented. With the simple structure, the desired power generation performance can be achieved advantageously.

Further, in the first embodiment, the first and second outer protrusions42a,52aare wider than the first and second protrusions36b,48bin the oxygen-containing gas flow field36and the fuel gas flow field48. Specifically, as shown inFIG. 2, the contact width L1of the first and second outer protrusions42a,52ais larger than the contact length L2of the first and second protrusions36b,48b.

Positional displacement is likely to occur between the first and second metal separators16,18and the outer edges of the electrode catalyst layers32a,32b.Specifically, the positional displacement may occur at the time of applying the electrode catalyst on the solid polymer electrolyte membrane26, at the time of combining the solid polymer electrolyte membrane26and the gas diffusion layers34a,34btogether, at the time of combining the first and second metal separators16,18and the membrane electrode assembly14together, at the time of forming the first and second metal separators16,18by press forming, and at the time of stacking the first and second metal separators16,18together.

Therefore, in the first embodiment, the first and second outer protrusions42a,52aare wider than the first and second protrusions36b,48bfor effectively absorbing the positional displacement effectively, and reliably sandwiching the outer edges of the electrode catalyst layers32a,32bbetween the first and second outer protrusions42a,52a.Thus, damage of the solid polymer electrolyte membrane26is prevented, and the desired power generation performance can be maintained advantageously.

FIG. 6is a partial cross sectional view showing a fuel cell70according to a second embodiment of the present invention. The constituent elements that are identical to those of the fuel cell10according to the first embodiment are labeled with the same reference numeral, and description thereof will be omitted.

The fuel cell70includes first and second carbon separators72,74sandwiching the membrane electrode assembly14. The first carbon separator72has first protrusions76forming a plurality of grooves36aof an oxygen-containing gas flow field36. Further, a first outer protrusion78is provided outside the oxygen-containing gas flow field36. The contact width L3of the first outer protrusion78is larger than the contact width L4of the first protrusions76. The second carbon separator74has second protrusions80forming a plurality of grooves48aof a fuel gas flow field48. Further, a second outer protrusion82is provided outside the fuel gas flow field48. The contact width L3of the second outer protrusion82is larger than the contact width L4of the second protrusions80. Seal members84a,84bare interposed between outer edges of the solid polymer electrolyte membrane26and the first and second separators72,74.

In the second embodiment, the first and second outer protrusions78,82of the first and second carbon separators72,74reliably sandwich the outer edges of electrode catalyst layers32a,32b.The contact width L3of the first and second outer protrusions78,82is larger than the contact width L4of the first and second protrusions76,80. Thus, with the simple structure, the same advantages as with the first embodiment can be obtained. For example, damage of the solid polymer electrolyte membrane26is prevented, and the desired power generation performance can be obtained.

While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood that variations and modifications can be effected thereto by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.