Separator for fuel cells and method for producing same

The present invention relates to a fuel cell separator of a fuel cell and a method of producing the fuel cell separator (separator for fuel cells and method for producing same). In the fuel cell, a membrane electrode assembly is sandwiched between separators, and seal members are formed integrally with outer ends of the separators. The membrane electrode assembly includes electrodes and an electrolyte membrane interposed between the electrodes.

For example, in a solid polymer electrolyte fuel cell, an electrolyte membrane (polymer ion exchange membrane) is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between a pair of separators to form a unit cell (power generation cell). In use, in the fuel cell, generally, a predetermined number of unit cells are stacked together to form a fuel cell stack, e.g., mounted in a fuel cell vehicle.

In the fuel cell, a fuel gas flow field is provided in one surface of the separator facing an anode, for supplying a fuel gas to the anode, and an oxygen-containing gas flow field is provided in the other surface of the separator facing a cathode, for supplying an oxygen-containing gas to the cathode. Further, a coolant flow field is formed between the adjacent separators of each fuel cell, for supplying a coolant within an electrode area along surfaces of the separators.

Further, in the fuel cell, so called internal manifold structure is often adopted. In the internal manifold structure, fuel gas passages, oxygen-containing gas passages, and coolant passages extend through the unit cells in the stacking direction for allowing the fuel gas, the oxygen-containing gas, and the coolant to flow through the unit cells.

In this internal manifold type fuel cell, seals need to be provided in an air-tight manner or a liquid-tight manner for preventing leakage of the fuel gas, the oxygen-containing gas, and the coolant. In this regard, for example, a method for fabricating a seal-integrated separator disclosed in Japanese Patent No. 3532547 is known.

In this production method, a separator body is held between an upper mold and a lower mold. In this state, melted seal material is injected into each of grooves in the upper mold and the lower mold through separate gates, respectively, to form the seal material integrally and simultaneously on both surfaces of the separator body. Therefore, according to the disclosure, in comparison with the case of providing seal material separate from the separator body on both front and back surfaces of the separator body and the case of coating the seal material on the separator body, the seal material can be positioned highly accurately, and the number of assembling steps is reduced greatly.

Normally, in the state where the separator is held between the upper mold and the lower mold, when a melted seal member is supplied, burrs tend to be produced easily along surfaces of the molds facing each other. Consequently, after the seal-integrated separator is produced, working operation for removing the burrs at the outer end of the separator is required. Therefore, a larger number of working steps are required, and the production cost becomes high. Further, improvement in the overall work efficiency may not be achieved easily.

The present invention has been made to solve the problem of this type, and an object of the present invention is to provide a fuel cell separator and a method of producing the fuel cell separator in which, with the simple structure and steps, it is possible to reliably suppress production of burrs at the outer end of a seal member, achieve improvement in the work efficiency, and produce the fuel cell separator economically.

In a fuel cell separator according to the present invention, a membrane electrode assembly is sandwiched between the fuel cell separators. The membrane electrode assembly includes electrodes and an electrolyte membrane interposed between the electrodes. A seal member is formed integrally with an outer end of the fuel cell separator. A fluid passage is formed in a surface of the fuel cell separator for allowing at least a fuel gas, an oxygen-containing gas, or a coolant as fluid to flow in a stacking direction in which the fuel cell separator and the membrane electrode assembly are stacked together.

A resin member covering part of the resin member is provided at an outer end of the fuel cell separator, outside the fluid passage, over the entire perimeter of the fuel cell separator, and the seal member is provided on both surfaces of the fuel cell separator. The expression “over the entire perimeter” herein includes the case where the cutout is formed in only part of the outer end of the separator.

Further, a method of producing a fuel cell separator according to the invention of the present application includes the step of providing a resin member at an outer end of the fuel cell separator, outside the fluid passage, over the entire perimeter of the fuel cell separator. Further, this production method includes the step of supplying a melted seal member in a state where the resin member is held between a pair of molds to form the seal member covering part of the resin member, on both surfaces of the fuel cell separator.

In the present invention, the resin member is provided over the entire perimeter of the fuel cell separator, and the seal member covering part of the resin member is provided on both surfaces of the fuel cell separator. In the structure, in the state where the resin member is held between the pair of molds, the melted seal member is supplied to form the seal member on both surfaces of the fuel cell separator. Therefore, it becomes possible to prevent burrs of the seal member from being produced at the outer end of the fuel cell separator as much as possible.

Thus, with the simple structure and steps, it becomes possible to reliably suppress production of burrs at the end of the seal member, improve the work efficiency, and produce the fuel cell separator economically.

FIG. 1 is an exploded perspective view showing a unit cell including a fuel cell separator according to an embodiment of the present invention;

FIG. 2 is a cross sectional view showing the unit cell, taken along a line II-II in FIG. 1 ;

FIG. 3 is a partial perspective view showing a cathode side separator of the unit cell;

FIG. 4 is a front view showing a metal plate of the cathode side separator;

FIG. 5 is a perspective view showing a resin member forming apparatus for forming a resin member on the metal plate;

FIG. 6 is a front view showing a state where the metal plate and the resin member are joined together integrally; and

FIG. 7 is a view schematically showing a seal member forming apparatus for forming a seal member on the metal plate.

As shown in FIGS. 1 and 2 , a fuel cell stack 10 including a fuel cell separator according to an embodiment of the present invention is formed by stacking laterally elongated units of fuel cells 11 (hereinafter referred to as the unit cells 11 ), upright (such that electrode surfaces stand vertically in parallel), in a horizontal direction indicated by an arrow A. Alternatively, longitudinally elongated unit cells 11 standing upright may be stacked together in the direction indicated by the arrow A, or lying in the horizontal direction may be stacked in the gravity direction indicated by an arrow C.

Each of the unit cells 11 includes a membrane electrode assembly 12 , and a cathode side separator (fuel cell separator) 14 and an anode side separator (fuel cell separator) 16 sandwiching the membrane electrode assembly 12 .

The cathode side separator 14 and the anode side separator 16 are, for example, steel plates, stainless steel plates, aluminum plates, plated steel plates, or thin plate metal separators having anti-corrosive surfaces by surface treatment. These metal separators have rectangular surfaces, and formed by press forming to have a corrugated shape (ridges and grooves) in cross section. Instead of the metal separators for the cathode side separator 14 and the anode side separator 16 , for example, carbon separators may be used.

The cathode side separator 14 and the anode side separator 16 have a laterally elongated shape including short sides extending in a gravity direction indicated by an arrow C and long sides extending in a horizontal direction indicated by an arrow B. Alternatively, the short sides may extend in the horizontal direction and the long sides may extend in the gravity direction.

As shown in FIG. 1 , at one end of the unit cells 11 in a longitudinal direction indicated by the arrow B, an oxygen-containing gas supply passage (fluid passage) 18 a and a fuel gas discharge passage (fluid passage) 20 b are provided. The oxygen-containing gas supply passages 18 a and the fuel gas discharge passages 20 b extend through the unit cells 11 in the direction indicated by the arrow A. An oxygen-containing gas is supplied to the unit cells 11 through the oxygen-containing gas supply passage 18 a , and a fuel gas such as a hydrogen-containing gas is discharged from the unit cells 11 through the fuel gas discharge passage 20 b . For example, the oxygen-containing gas supply passage 18 a and the fuel gas discharge passage 20 b have a substantially rectangular opening shape. It should be noted that the oxygen-containing gas supply passage 18 a and the fuel gas discharge passage 20 b may have a triangular or polygonal opening shape.

At the other end of the unit cells 11 in the longitudinal direction, a fuel gas supply passage (fluid passage) 20 a for supplying the fuel gas and an oxygen-containing gas discharge passage (fluid gas passage) 18 b for discharging the oxygen-containing gas are provided. The oxygen-containing gas discharge passage 18 b and the fuel gas supply passage 20 a have a substantially rectangular opening shape. It should be noted oxygen-containing gas discharge passage 18 b and the fuel gas supply passage 20 a may have a triangular or polygonal opening shape.

A pair of coolant supply passages 22 a for supplying a coolant is provided at upper and lower marginal portions of the unit cell 11 in the lateral direction indicated by the arrow C. In the longitudinal direction, the marginal portions are provided on one side closer to the oxygen-containing gas supply passage 18 a . The coolant supply passages 22 a communicate with each other in the direction indicated by the arrow A. A pair of coolant discharge passages 22 b for discharging a coolant is provided at upper and lower marginal portions of the unit cell 11 in the lateral direction. In the longitudinal direction, the marginal portions are provided on one side closer to the fuel gas supply passage 20 a.

The pair of coolant supply passages 22 a have a substantially rectangular opening shape elongated in the flow direction of the coolant flow field 34 indicated by arrow B. The pair of coolant discharge passages 22 b have a substantially rectangular opening shape elongated in the flow direction of the coolant flow field 34 indicated by the arrow B.

As shown in FIGS. 1 and 2 , the membrane electrode assembly 12 includes a cathode 26 , an anode 28 , and a fluorine based or hydrocarbon based solid polymer electrolyte membrane 24 interposed between the cathode 26 and the anode 28 .

Each of the cathode 26 and the anode 28 has a gas diffusion layer (not shown) such as a carbon paper, and an electrode catalyst layer (not shown) that is made up from porous carbon particles supporting platinum alloy on the surface thereof. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layer of the cathode 26 and the electrode catalyst layer of the anode 28 are fixed to both surfaces of the solid polymer electrolyte membrane 24 , respectively.

As shown in FIG. 1 , the cathode side separator 14 has an oxygen-containing gas flow field 30 on its surface 14 a facing the membrane electrode assembly 12 . The oxygen-containing gas flow field 30 is connected to the oxygen-containing gas supply passage 18 a and the oxygen-containing gas discharge passage 18 b . The oxygen-containing gas flow field 30 includes a plurality of straight (or wavy) flow grooves extending in the horizontal direction indicated by the arrow B for allowing the oxygen-containing gas to flow toward one side in the longitudinal direction along the separator surface.

The anode side separator 16 has a fuel gas flow field 32 on its surface 16 a facing the membrane electrode assembly 12 . The fuel gas flow field 32 is connected to the fuel gas supply passage 20 a and the fuel gas discharge passage 20 b . The fuel gas flow field 32 includes a plurality of straight (or wavy) flow grooves extending in the horizontal direction indicated by the arrow B for allowing the fuel gas to flow toward the other side in the longitudinal direction along the separator surface.

A coolant flow field 34 is formed between the surface 16 b of the anode side separator 16 and the surface 14 b of the cathode side separator 14 that are adjacent to each other. The coolant flow field 34 is connected to the coolant supply passages 22 a and the coolant discharge passages 22 b . In the coolant flow field 34 , the coolant flows over the electrode area of the membrane electrode assembly 12 . In the anode side separator 16 , the coolant flow field 34 is the back of the fuel gas flow field 32 . In the cathode side separator 14 , the coolant flow field 34 is the back of the oxygen-containing gas flow field 30 .

As shown in FIG. 2 , a first resin member 36 A is formed integrally with the surfaces 14 a , 14 b of the cathode side separator 14 , covering the outer end of and extending along the edge of the outer end of a metal plate 14 P (see FIG. 2 ) of the cathode side separator 14 . The first resin member 36 A is formed over the entire perimeter of the cathode side separator 14 . The first resin member 36 A is provided outside the oxygen-containing gas supply passage 18 a , the oxygen-containing gas discharge passage 18 b , the fuel gas supply passage 20 a , the fuel gas discharge passage 20 b , the coolant supply passage 22 a , and the coolant discharge passage 22 b . Part of the outer end of the cathode side separator 14 alone may be cut off the first resin member 36 A.

As shown in FIGS. 2 and 3 , the first resin member 36 A has a slit 36 s at its center, and an end of the metal plate 14 P is inserted into the slit 36 s . The first resin member 36 A has a substantially T-like frame shape in cross section. The first resin member 36 A is positioned outside the outer end of the cathode side separator 14 , and includes a protrusion 36 t , e.g., having a substantially rectangular shape in cross section. A pair of overlapping portions 36 ka , 36 kb is formed integrally with the inside of the protrusion 36 t . The overlapping portions 36 ka , 36 kb extend in parallel along both surfaces 14 a , 14 b of the cathode side separator 14 , and the overlapping portions 36 ka , 36 kb are overlapped with seals 40 a , 40 b in the stacking direction. The seals 40 a , 40 b are described later.

The slit 36 s is terminated inside the protrusion 36 t at a length (depth) of T. The outermost circumference of the metal plate 14 P is placed inside the protrusion 36 t . As described later, at the time of forming the first seal member 40 by a seal member forming apparatus 60 , the first resin member 36 A is held between a pair of seal member forming molds 62 , 64 . However, since the metal plate 14 P is spaced inward by the distance T, the metal plate 14 P is not held between the seal member forming molds 62 , 64 . Therefore, the metal plate 14 P will never be deformed.

As shown in FIG. 3 , a plurality of resin flow sections 36 g are provided at arbitrary positions of the first resin member 36 A. The resin flow sections 36 g are solidified in a gate 58 for forming the resin member which is described later. Between the adjacent resin flow sections 36 g , provided is an expansion 36 d that covers a welding line position and protrudes outward in a direction parallel to the separator surface. Preferably, the expansion 36 d is provided at an intermediate position between the resin flow sections 36 g , has a predetermined width, and is a trapezoid having slopes on both sides.

In the first resin member 36 A, resin knock sections 38 a , 38 b are formed integrally with the first resin member 36 A, between the oxygen-containing gas supply passage 18 a and the fuel gas discharge passage 20 b , and between the fuel gas supply passage 20 a and the oxygen-containing gas discharge passage 18 b . Knock holes 38 ah , 38 bh passes through the resin knock sections 38 a , 38 b . Knock pins (not shown) are inserted into the knock holes 38 ah , 38 bh . The resin knock sections 38 a , 38 b have cutouts 38 ak , 38 bk , if need be. The knock holes 38 ah , 38 bh pass through the solid polymer electrolyte membrane 24 .

For example, the first resin member 36 A is made of PPS (polyphenylene sulfide), PPA (polyphthalamide), PEN (polyethylene naphtalate), PES (polyethersulfone), LCP (liquid crystal polymer), PVDF (polyvinylidene difluoride), silicone resin, fluororesin, m-PPE (modified polyphenylene ether resin). Alternatively, the first resin member 36 A may be made of PET (polyethylene terephthalate), PBT (polybutylene terephthalate), or modified polyolefin. Preferably, the first resin member 36 A is made of thermoplastic resin.

A second resin member 36 B is formed integrally with the surfaces 16 a , 16 b of the anode side separator 16 , covering the outer end of and extending along the edge of the outer end of the metal plate 16 P (see FIG. 2 ) of the anode side separator 16 . The second resin member 36 B has the same structure as the first resin member 36 A. The constituent elements of the second resin member 36 B that are identical to those of the first resin member 36 A are labeled with the same reference numeral, and detailed description thereof is omitted.

A first seal member 40 is formed integrally with the cathode side separator 14 . The first seal member 40 includes a seal 40 a formed integrally with the surface 14 a of the cathode side separator 14 , and a seal 40 b formed integrally with the surface 14 b of the cathode side separator 14 . The outer end of the seal 40 a is overlapped with the overlapping portion 36 ka of the first resin member 36 A in the stacking direction, and the outer end of the seal 40 b is overlapped with the overlapping portion 36 kb of the first resin member 36 A in the stacking direction. No steps are present between the outer ends of the seals 40 a , 40 b and both end surfaces of the protrusion 36 t in the thickness direction. That is, the outer ends of the seals 40 a , 40 b and both end surfaces of the protrusion 36 t in the thickness direction are continuously flat along the separator surfaces.

The seals 40 a , 40 b are flat seals having a constant thickness, extending along the separator surfaces, and part of the flat seals includes ridge seals 40 at , 40 bt . As shown in FIG. 2 , the thickness h of the protrusion 36 t of the first resin member 36 A and the thickness H of the first seal member 40 are the same (h=H). The thickness h of the protrusion 36 t is the same as the thickness of the resin knock sections 38 a , 38 b.

A second seal member 42 is formed integrally with the anode side separator 16 . The second seal member 42 includes a seal 42 a formed integrally with the surface 16 a of the anode side separator 16 , and a seal 42 b formed integrally with the surface 16 b of the anode side separator 16 . The outer end of the seal 42 a is overlapped with the overlapping portion 36 ka of the second resin member 36 B in the stacking direction, and the outer end of the seal 42 b is overlapped with the overlapping portion 36 kb of the second resin member 36 B in the stacking direction. The seals 42 a , 42 b are flat seals extending along the separator surfaces with a constant thickness, and part of the flat seals includes ridge seals 42 at , 42 bt.

Each of the first seal member 40 and the second seal member 42 is, for example, an elastic seal member made of seal material, cushion material, or packing material such as an EPDM, an NBR, a fluoro rubber, a silicone rubber, a fluorosilicone rubber, a Butyl rubber, a natural rubber, a styrene rubber, a chloroprene rubber, or an acrylic rubber.

Next, a method of producing the cathode side separator 14 will be described below. It should be noted that the anode side separator 16 is produced in the same manner as the cathode side separator 14 . Therefore, detailed description of the anode side separator 16 is omitted.

Firstly, as shown in FIG. 4 , the metal plate 14 P of the cathode side separator 14 is produced by press forming. Holes 18 ah , 18 bh , 20 ah , 20 bh , 22 ah , and 22 bh corresponding to the oxygen-containing gas supply passage 18 a , the oxygen-containing gas discharge passage 18 b , the fuel gas supply passage 20 a , the fuel gas discharge passage 20 b , the coolant supply passage 22 a , and the coolant discharge passage 22 b are formed in the metal plate 14 P. The oxygen-containing gas flow field 30 is formed in the metal plate 14 P by press forming.

Then, as shown in FIG. 5 , the metal plate 14 P is held between the pair of resin member forming molds 52 , 54 of the resin member forming apparatus 50 . When the resin member forming molds 52 , 54 are closed, a cavity 56 corresponding to the shape of the first resin member 36 A is formed between the resin member forming molds 52 , 54 . A plurality of gates 58 are connected to the cavity 56 for supplying melted resin to be formed into the resin member (hereinafter referred to as the melted resin member) in a direction parallel to surfaces of the resin member forming molds 52 , 54 facing each other, indicated by an arrow D. The cavity 56 includes a recess (not shown) corresponding to the shape of the expansion 36 d (see FIG. 3 ) between the gates 58 .

In the structure, by filling the melted resin member into the cavity 56 from the gates 58 , the first resin member 36 A is formed integrally with the edge of the outer end of the metal plate 14 P. The metal plate 14 P formed integrally with the first resin member 36 A is collected from the resin member forming apparatus 50 (see FIG. 6 ). Thereafter, the resin flow sections 36 g are removed from the first resin member 36 A, e.g., using a cutter or the like.

As shown in FIG. 7 , the first resin member 36 A is formed integrally with the metal plate 14 P, and the metal plate 14 P is held between the pair of seal member forming molds 62 , 64 of the seal member forming apparatus 60 . A cavity 66 a is formed between the seal member forming mold 62 and the metal plate 14 P. The seal 40 a of the first seal member 40 is formed by the cavity 66 a . A plurality of gates 68 a formed in the seal member forming mold 62 communicate with the cavity 66 a.

A cavity 66 b is formed between the seal member forming mold 64 and the metal plate 14 P. The seal 40 b of the first seal member 40 is formed by the cavity 66 b . A plurality of gates 68 b formed in the seal member forming mold 64 communicate with the cavity 66 b.

The metal plate 14 P is held between the seal member forming molds 62 , 64 , and the protrusion 36 t of the first resin member 36 A is pressed and held between the seal member forming molds 62 , 64 . Both end surfaces of the protrusion 36 t in the thickness direction contact the inner surfaces of the seal member forming molds 62 , 64 . A predetermined gap S is formed between the outer end surface of the protrusion 36 t and the inner surfaces of the seal member forming molds 62 , 64 .

In this state, the melted seal member poured from the gates 68 a , 68 b is filled into the cavities 66 a , 66 b . Therefore, the seal 40 a is formed integrally with the surface 14 a of the metal plate 14 P, and the seal 40 b is formed integrally with the surface 14 b of the metal plate 14 P. Consequently, the cathode side separator 14 is produced. The seals 40 a , 40 b are parts of the first seal member 40 . For example, as the seals 40 a , 40 b , elastic seal members of seal material, cushion material, or packing material such as an EPDM, an NBR, a fluoro rubber, a silicone rubber, a fluorosilicone rubber, a Butyl rubber, a natural rubber, a styrene rubber, a chloroprene rubber, or an acrylic rubber are used.

In the embodiment of the present invention, firstly, the first resin member 36 A is formed integrally with the outer end of the metal plate 14 P over the entire perimeter on both surfaces of the metal plate 14 P. Further, the seals 40 a , 40 b of the first seal member 40 are formed integrally with both surfaces 14 a , 14 b of the metal plate 14 P.

Specifically, as shown in FIG. 7 , the protrusion 36 t of the first resin member 36 A is pressed and held between the seal member forming molds 62 , 64 . In this state, the melted seal member is supplied into the cavities 66 a , 66 b . Therefore, the first seal member 40 is formed integrally with the metal plate 14 P. In the structure, it is possible to effectively prevent burrs of the first seal member 40 from being produced at the outer end of the metal plate 14 P.

In particular, the protrusion 36 t is held firmly such that the protrusion 36 t is collapsed between the seal member forming molds 62 , 64 . Therefore, at the time of forming the seal member, it becomes possible to prevent burrs from being produced as much as possible. Further, the front end of the metal plate 14 P, i.e., the protrusion 36 t can be held reliably. Therefore, the melted seal member can be filled into the cavities 66 a , 66 b at high pressure. Accordingly, improvement in the performance of forming the first seal member 40 is achieved. The melted seal member flows smoothly, and improvement in the work efficiency of forming the first seal member 40 is achieved easily.

Therefore, in the embodiment of the present invention, with the simple structure and steps, it becomes possible to reliably suppress production of burrs at the outer end of the first seal member 40 , achieve improvement in the work efficiency, and achieve economical production.

Further, the seals 40 a , 40 b of the first seal member 40 are overlapped with the overlapping portions 36 ka, 36 kb of the first resin member 36 A. In the structure, since the joining areas between the seals 40 a , 40 b and the overlapping portions 36 ka, 36 kb are large, the seals 40 a , 40 b and the overlapping portions 36 ka, 36 kb are securely and reliably adhered together. The first seal member 40 can securely hold the first resin member 36 A, and it becomes possible to prevent the first seal member 40 and the first resin member 36 A from being separated from each other.

Further, in the first resin member 36 A, the expansion 36 d covering the welding line position is provided and protrudes outward in a direction parallel to the separator surfaces, between the resin flow sections 36 g . In the structure, it becomes possible to reliably maintain the overall strength of the first resin member 36 A.

Furthermore, the outer end of the cathode side separator 14 is covered by the protrusion 36 t of the first resin member 36 A. In the structure, in comparison with the case of using rubber members, when an external load is applied to the cathode side separator 14 from the outside in a direction parallel to the separator surface, it is possible to receive the external load better than the rubber members and reliably protect the cathode side separator 14 .

Operation of the fuel cell stack 10 will be described below.

Firstly, as shown in FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage 18 a , and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage 20 a . Further, a coolant such as pure water, ethylene glycol or oil is supplied to the pair of coolant supply passages 22 a.

In the structure, the oxygen-containing gas is supplied from the oxygen-containing gas supply passage 18 a to the oxygen-containing gas flow field 30 of the cathode side separator 14 . The oxygen-containing gas flows in the horizontal direction indicated by the arrow B along the oxygen-containing gas flow field 30 , and the oxygen-containing gas is supplied to the cathode 26 of the membrane electrode assembly 12 .

In the meanwhile, the fuel gas is supplied from the fuel gas supply passage 20 a to the fuel gas flow field 32 of the anode side separator 16 . The fuel gas flows along the fuel gas flow field 32 in the horizontal direction indicated by the arrow B, and the fuel gas is supplied to the anode 28 of the membrane electrode assembly 12 .

Thus, in the membrane electrode assembly 12 , the oxygen-containing gas supplied to the cathode 26 and the fuel gas supplied to the anode 28 are consumed in the electrochemical reactions at catalyst layers of the cathode 26 and the anode 28 for generating electricity.

Then, the oxygen-containing gas supplied to, and partially consumed at the cathode 26 of the membrane electrode assembly 12 is discharged along the oxygen-containing gas discharge passage 18 b in the direction indicated by the arrow A. The fuel gas supplied to, and partially consumed at the anode 28 of the membrane electrode assembly 12 is discharged along the fuel gas discharge passage 20 b in the direction indicated by the arrow A.

Further, the coolant supplied to the pair of coolant supply passages 22 a flows into the coolant flow field 34 between the cathode side separator 14 and the anode side separator 16 . After the coolant temporarily flows inward in the gravity direction indicated by the arrow C, the coolant moves in the horizontal direction indicated by the arrow B, and cools the membrane electrode assembly 12 . After that, the coolant moves outward in the direction indicated by the arrow C, and then, the coolant is discharged to the pair of the coolant discharge passages 22 b.