Fuel cell having coolant flow field wall

A power generation cell includes a membrane electrode assembly, with an anode side metal separator and a cathode side metal separator sandwiching the membrane electrode assembly. Flow field walls are provided within the coolant flow field for preventing coolant from flowing into an area corresponding to an oxygen-containing gas inlet buffer, while allowing the coolant to flow into an area corresponding to an oxygen-containing gas outlet buffer. Likewise, flow field walls contact each other for preventing the coolant from flowing into an area corresponding to a fuel gas inlet buffer, while allowing the fuel gas to flow into an area corresponding to a fuel gas outlet buffer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell formed by stacking electrolyte electrode assemblies and separators alternately in a stacking direction. Each of the electrolyte electrode assemblies includes a pair of electrodes and an electrolyte interposed between the electrodes. A reactant gas flow field for supplying a reactant gas is formed between the electrode electrolyte assembly and one of separators sandwiching the electrolyte electrode assembly in order to supply a reactant gas along a surface of the electrode. A coolant flow field is formed between adjacent separators, which are stacked together, for allowing a coolant to flow in a direction substantially perpendicular to the flow direction of the reactant gas.

2. Description of the Related Art

A polymer electrolyte fuel cell employs, for example, a membrane electrode assembly (MEA), which includes an anode, a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. Normally, a predetermined number of membrane electrode assemblies and separators are stacked together alternately to form a fuel cell stack.

In the power generation cell, a fuel gas flow field (reactant gas flow field) for supplying a fuel gas along the surface of the anode and an oxygen-containing gas flow field (reactant gas flow field) for supplying an oxygen-containing gas along the surface of the cathode are formed on a pair of separators sandwiching the membrane electrode assembly. Further, a coolant flow field for cooling power generation surfaces of the membrane electrode assembly is formed between adjacent separators which are stacked together.

For example, Japanese Laid-Open Patent Publication No. 2003-338300 proposes a fuel cell in which cooling is performed efficiently by supplying a coolant in a direction perpendicular to the flow direction of the reactant gases, i.e., the fuel gas and the oxygen-containing gas, whereby power generation can be performed efficiently.

As shown inFIG. 11, in the fuel cell, a first separator1and a second separator2are stacked together. The first separator1has a curved fuel gas flow field3, and the second separator2has a straight oxygen-containing gas flow field5on a surface thereof facing a cathode4.

The top surface of a straight portion6of the first separator1contacts the top surface of a ridge7of the second separator2. The top surface of a curved portion8of the first separator1is spaced away from the top surface of the ridge7. Thus, a coolant flow field9having the flow direction perpendicular to the flow direction of the oxygen-containing gas flow field5is formed between the first and second separators1and2.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide a fuel cell having a simple structure in which the flow direction of a reactant gas is substantially perpendicular to the flow direction of a coolant, in which simply by controlling the flow of the coolant within a coolant flow field, the temperature distribution on the electrode surface becomes uniform.

The present invention relates to a fuel cell formed by stacking electrolyte electrode assemblies and separators alternately in a stacking direction. Each of the electrolyte electrode assemblies includes a pair of electrodes with an electrolyte interposed between the electrodes. A reactant gas flow field for supplying a reactant gas is formed between the electrode electrolyte assembly and one of separators sandwiching the electrolyte electrode assembly, in order to supply a reactant gas along a surface of the electrode. A coolant flow field is formed between adjacent separators, which are stacked together, for allowing a coolant to flow in a direction substantially perpendicular to the flow direction of the reactant gas. A reactant gas supply passage and a reactant gas discharge passage for the reactant gas flow field extend through the fuel cell in the stacking direction.

One of the separators includes an inlet buffer connecting the reactant gas supply passage and the reactant gas flow field, and an outlet buffer connecting the reactant gas discharge passage and the reactant gas flow field. The coolant flow field has a flow field wall between adjacent separators for preventing the coolant from flowing into an area corresponding to the inlet buffer, while allowing the coolant to flow into an area corresponding to the outlet buffer.

Further, preferably, a coolant supply passage and a coolant discharge passage for the coolant flow field extend through the separators, and the coolant discharge passage is spaced away from the outlet buffer. In such a structure, since the distance between the outlet buffer and the coolant discharge passage is relatively large, whereby the route for the coolant becomes longer, within the coolant flow field, the coolant is likely to be retained at a position near the outlet buffer. Thus, the temperature at the outlet of the reactant gas flow field is increased. Accordingly, water is mixed together with the gas from the reactant gas flow field, and it is possible to suitably prevent retention of condensed water.

Further, preferably, the inlet buffer and the outlet buffer are provided outside a reaction surface of the electrolyte electrode assembly. Since a relatively large amount of coolant flows in the outlet buffer, even if the outlet buffer is cooled, water condensation does not occur on the electrode reaction surface.

According to the present invention, flow field walls prevent the flow of the coolant into an area corresponding to the inlet buffer between the separators. Therefore, the coolant is primarily supplied to the power generation area, whereby heat exchange efficiency is improved.

Further, the coolant flows into an area corresponding to the outlet buffer. The flow rate of coolant flowing through the area corresponding to the outlet buffer is higher than the flow rate of coolant flowing through the coolant flow field. Therefore, flow of coolant is inhibited in an area corresponding to the position near the outlet buffer of the power generation area. Accordingly, in the reactant gas flow field, the temperature at the position near the outlet buffer is increased, and the reactant gas is warmed to vaporize condensed water or the like in the reactant gas flow field. The vaporized water becomes mixed together with the reactant gas. Accordingly, condensed water is not significantly retained in the reactant gas flow field, and it is possible to ensure that water is discharged smoothly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is an exploded perspective view showing the main components of a power generation cell10in the fuel cell according to an embodiment of the present invention.FIG. 2is a cross sectional view showing the power generation cell10taken along line II-II inFIG. 1.FIG. 3is a cross sectional view showing the power generation cell10taken along line III-III inFIG. 1. A plurality of power generation cells10are stacked together to form the fuel cell.

As shown inFIG. 1, each of the power generation cells10includes a membrane electrode assembly (electrolyte electrode assembly)12, with an anode side metal separator14and a cathode side metal separator16sandwiching the membrane electrode assembly12. For example, the anode side metal separator14and the cathode side metal separator16may be steel plates, stainless steel plates, aluminum plates, or plated steel sheets. The anode side metal separator14and the cathode side metal separator16may be made of metal plates having anti-corrosive surfaces formed by surface treatment.

At one end of the power generation cell10, in a longitudinal direction indicated by the arrow B inFIG. 1, an oxygen-containing gas supply passage18afor supplying an oxygen-containing gas, and a fuel gas discharge passage20bfor discharging a fuel gas such as a hydrogen containing gas are provided. The oxygen-containing gas supply passage18aand the fuel gas discharge passage20bextend through the power generation cell10in the direction indicated by the arrow A.

At the other end of the power generation cell10in the longitudinal direction, a fuel gas supply passage20afor supplying the fuel gas, and an oxygen-containing gas discharge passage18bfor discharging the oxygen-containing gas are provided. The fuel gas supply passage20aand the oxygen-containing gas discharge passage18bextend through the power generation cell10in the direction indicated by the arrow A.

At an upper end of the power generation cell10in a lateral direction, for example, two coolant supply passages22aare provided for supplying a coolant, and at a lower end of the power generation cell10in the lateral direction, for example, two coolant discharge passages22bare provided for discharging the coolant.

The membrane electrode assembly12includes an anode26, a cathode28, and a solid polymer electrolyte membrane (electrolyte)24interposed between the anode26and the cathode28. The solid polymer electrolyte membrane24is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. The surface area of the anode26is smaller than the surface area of the cathode28(seeFIGS. 1 to 3).

Each of the anode26and the cathode28includes a gas diffusion layer (not shown), such as a carbon paper, and an electrode catalyst layer (not shown) formed by a platinum alloy supported on porous carbon particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layer of the anode26and the electrode catalyst layer of the cathode28are fixed to both surfaces of the solid polymer electrolyte membrane24, respectively.

As shown inFIGS. 1 and 4, the anode side metal separator14has a fuel gas flow field (reactant gas flow field)30on a surface14athereof facing the membrane electrode assembly12. The fuel gas flow field30is connected to the fuel gas supply passage20aand to the fuel gas discharge passage20b. The fuel gas flow field30includes a plurality of flow grooves32extending in the direction indicated by the arrow B, and a plurality of embossed portions34a,34bprovided at opposite ends of the flow grooves32.

The flow grooves32are provided within an area corresponding to the power generation surface (electrode reaction surface) of the anode26. The embossed portions34aand the embossed portions34bare provided outside of the power generation surface of the anode26. In effect, the embossed portions34aform an inlet buffer36afor the fuel gas on the upper side, and the embossed portions34bform an outlet buffer36bfor the fuel gas on the lower side. The outlet buffer36bis spaced away from the coolant discharge passages22b.

As shown inFIG. 1, the cathode side metal separator16has an oxygen-containing gas flow field (reactant gas flow field)38on a surface16athereof facing the membrane electrode assembly12. The oxygen-containing gas flow field38is connected to the oxygen-containing gas supply passage18aand to the oxygen-containing gas discharge passage18b.

As with the fuel gas flow field30, the oxygen-containing gas flow field38includes a plurality of flow grooves40extending in the direction indicated by the arrow B, and a plurality of embossed portions42a,42bprovided at opposite ends of the flow grooves40. In effect, the embossed portions42aform an inlet buffer44afor the oxygen-containing gas on the upper side, and the embossed portions42bform an outlet buffer44bfor the oxygen-containing gas on the lower side. The outlet buffer44bis spaced away from the coolant discharge passages22b.

The flow grooves40are provided in an area corresponding to the power generation surface (electrode reaction surface) of the cathode28. The embossed portions42aand the embossed portions42bare provided outside of the power generation surface of the cathode28.

As shown inFIG. 5, a coolant flow field46is formed on a surface14bof the anode side metal separator14. The coolant flow field46is connected to the coolant supply passages22aand to the coolant discharge passages22b. The coolant flow field46is formed by stacking together the back surface of the fuel gas flow field30and the back surface of the oxygen-containing gas flow field38, and comprises grooves extending in the direction indicated by the arrow C.

The coolant flow field46has flow field walls48a,48bformed around the embossed portions34b,34aon the upper side, at respective positions of the inlet buffers44a,36a. Each of the flow field walls48a,48bprotrudes toward the surface14b, and comprises a ridge extending continuously in a rectangular shape. Flow field walls50a,50bprotrude on the surface16bof the cathode side metal separator16at respective positions of the inlet buffers44a,36a. The flow field walls50a,50bhave a structure which is the same as that of the flow field walls48a,48b.

When a plurality of the power generation cells10are stacked together, the anode side metal separator14of one of the adjacent power generation cells10is stacked on a cathode side metal separator16of another adjacent power generation cell10. Within an area corresponding to the inlet buffer44aof the coolant flow field46, the flow field walls48a,50acontact each other between the surface14bof the anode side metal separator14and the surface16bof the cathode side metal separator16(seeFIGS. 2 and 6).

The area surrounded by the flow field walls48a,50a, i.e., the area corresponding to the inlet buffer44a, is sealed from the coolant flow field46, and flow of the coolant into this area is prevented. Likewise, between the surfaces14b,16b, the flow field walls48b,50bcontact each other in the area corresponding to the inlet buffer36a, and flow of the coolant into this area is prevented.

As shown inFIG. 3, the embossed portions34bof the anode side metal separator14and the embossed portions42aof the cathode side metal separator16contact each other in an area corresponding to the outlet buffer36bof the coolant flow field46, in order to form a flow channel52athat is connected to the coolant flow field46. The coolant flows from the coolant flow field46into the flow channel52a. Likewise, the embossed portions34aof the anode side metal separator14and the embossed portions42bof the cathode side metal separator16contact each other in an area corresponding to the outlet buffer44bof the coolant flow field46, in order to form a flow channel52bthat is connected to the coolant flow field46(seeFIG. 5).

A first seal member54is formed integrally on surfaces14a,14bof the anode side metal separator14, around an outer end of the anode side metal separator14. Likewise, a second seal member56is formed integrally on surfaces16a,16bof the cathode side metal separator16, around an outer end of the cathode side metal separator16.

A plurality of supply holes55aand discharge holes55bpass through the anode side metal separator14, at positions near the fuel gas supply passage20aand the fuel gas discharge passage20b.

Operation of the power generation cell10shall be described below.

First, as shown inFIG. 1, an oxygen-containing gas is supplied to the oxygen-containing gas supply passage18a, and a fuel gas, such as a hydrogen-containing gas, is supplied to the fuel gas supply passage20a. Further, a coolant, such as pure water, ethylene glycol, or oil, is supplied to the coolant supply passages22a.

Thus, the oxygen-containing gas flows through the oxygen-containing gas supply passage18a, and flows from the inlet buffer44ato the oxygen-containing gas flow field38of the cathode side metal separator16. The oxygen-containing gas flows in the direction indicated by the arrow B, and flows along the cathode28of the membrane electrode assembly12, for inducing an electrochemical reaction at the cathode28. The fuel gas flows through the fuel gas supply passage20ainto the supply holes55a, and flows from the inlet buffer36ato the fuel gas flow field30of the anode side metal separator14. The fuel gas flows through the fuel gas flow field30in the direction indicated by the arrow B along the anode26of the membrane electrode assembly12, for inducing an electrochemical reaction at the anode26.

Thus, in each of the membrane electrode assemblies12, the oxygen-containing gas supplied to the cathode28and the fuel gas supplied to the anode26are consumed in electrochemical reactions at respective catalyst layers of the cathode28and the anode26, thereby generating electricity.

Then, the oxygen-containing gas consumed at the cathode28flows through the outlet buffer44b, and is discharged into the oxygen-containing gas discharge passage18b. The oxygen-containing gas flows through the oxygen-containing gas discharge passage18bin the direction indicated by the arrow A. Likewise, the fuel gas consumed at the anode26flows through the outlet buffer36binto the discharge holes55b, and is discharged into the fuel gas discharge passage20b. The fuel gas flows through the fuel gas discharge passage20bin the direction indicated by the arrow A.

Further, coolant supplied to the coolant supply passages22aflows into the coolant flow field46formed between the anode side metal separator14and the cathode side metal separator16, and flows in the direction indicated by the arrow C. After the coolant has cooled the membrane electrode assembly12, the coolant is discharged into the coolant discharge passages22b.

In the present embodiment, within the coolant flow field46, flow field walls48a,50acontact each other for preventing the coolant from flowing into an area corresponding to the inlet buffer44afor the oxygen-containing gas, while allowing the coolant to flow into an area corresponding to the outlet buffer44bfor the oxygen-containing gas.

Thus, as shown inFIG. 5, when the coolant is supplied from the coolant supply passages22anear the oxygen-containing gas supply passage18aand into the coolant flow field46, the coolant does not flow into the area corresponding to the inlet buffer44a. Thus, the coolant flows primarily into the power generation area of the cathode28, whereby an improvement in heat exchange efficiency is advantageously achieved.

The flow channel52bis formed in the area corresponding to the outlet buffer44bfor the oxygen-containing gas, near the oxygen-containing gas discharge passage18b. Thus, the coolant supplied to the coolant flow field46flows through the flow channel52b, and is discharged into the coolant discharge passages22b. Accordingly, within the power generation area of the cathode28, flow of coolant is inhibited in the area near the outlet buffer44b.

Hereinafter, a specific explanation shall be given with reference toFIGS. 7 to 9. First, inFIG. 7, the coolant flow field46is provided in an area corresponding to the power generation surface60. A buffer62is provided within the power generation surface60, at one end thereof near the oxygen-containing gas discharge passage18b.

The flow rate of the coolant was measured at positions1to7, in a case in which the buffer62was closed to prevent entry of the coolant, wherein the results of the flow rate at positions1to7is shown inFIG. 8. Further, the flow rate of the coolant was measured at positions1to7, and a to c, when flow of coolant into the buffer62was allowed, wherein the results of the flow rate at positions1to7and a to c are shown inFIG. 9.

As can be seen fromFIG. 9, in the case in which the flow of coolant in the buffer62was allowed, since the coolant flows toward the buffer62, the flow rate of the coolant at position7near the buffer62becomes significantly lower in comparison with the case shown inFIG. 8.

Thus, within the oxygen-containing gas flow field38, cooling performance is significantly lower near the outlet buffer44bof the power generation surface60, so that the temperature at the position near the outlet buffer44bincreases. On the power generation surface60, at the position near the outlet buffer44b, it is likely that the amount of the water produced during the reaction is largest. Therefore, at this position, the consumed oxygen-containing gas is warmed by limiting the amount of supplied coolant. Consequently, water produced in the oxygen-containing gas following the reaction is vaporized, and mixed into the oxygen-containing gas. Thus, it is possible to prevent condensed water from being retained within the oxygen-containing gas flow field38, and it is possible to ensure that water is discharged smoothly.

Further, in the coolant flow field46, flow grooves extending in the direction indicated by the arrow C are provided in an area corresponding to the power generation surface60. The area corresponding to the outlet buffer44bis provided outside of the power generation surface60. Therefore, even if a large amount of coolant flows into the area corresponding to the outlet buffer44b, it is possible to prevent condensation of water on the reaction surface of the cathode28.

Further, the coolant discharge passages22bare spaced away from the outlet buffer44b. Thus, the distance between the area corresponding to the outlet buffer44band the coolant discharge passages22bis large, and moreover, the coolant flowing through the area corresponding to the outlet buffer44bcan flow over a long distance at a relatively high speed, while flowing around the coolant that flows through the area corresponding to the power generation surface60, which by contrast flows at a relatively low speed.

Thus, within the coolant flow field46, coolant is likely to be retained at an area near the outlet buffer44b, whereby the temperature at the position near the outlet of the oxygen-containing gas flow field38, corresponding to this area, is further increased.

Accordingly, the amount of water mixed into the consumed oxygen-containing gas is effectively increased, making it possible to inhibit condensed water from being retained within the oxygen-containing gas flow field38.

Further, within the coolant flow field46, the flow field walls48a,50bcontact each other to prevent the coolant from flowing into the area corresponding to the inlet buffer36afor the fuel gas, while allowing the coolant to flow into the area corresponding to the outlet buffer36bfor the fuel gas.

Therefore, coolant is reliably supplied to the upstream side of the fuel gas flow field30, thus achieving an improvement in heat exchange efficiency. Further, the consumed fuel gas at the position near the outlet buffer36bcan be suitably warmed. Accordingly, condensed water in the fuel gas flow field30is vaporized desirably and mixed together with the consumed fuel gas. Thus, the same advantages as in the case of the oxygen-containing gas flow field38can be achieved.FIG. 10shows the flow rate distribution of the coolant within the power generation surface of the coolant flow field46.

While the invention has been particularly shown and described with reference to a preferred embodiment, 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 in the appended claims.